Antibody 2G12 Recognizes Di-Mannose Equivalently in Domain- and Nondomain-Exchanged Forms but Only Binds the HIV-1 Glycan Shield if Domain Exchanged

The broadly neutralizing anti-human immunodeficiency virus type 1 (HIV-1) antibody 2G12 targets the high-mannose cluster on the glycan shield of HIV-1. 2G12 has a unique V_H domain-exchanged structure, with a multivalent binding surface that includes two primary glycan binding sites. The high-mannose cluster is an attractive target for HIV-1 vaccine design, but so far, no carbohydrate immunogen has elicited 2G12-like antibodies. Important questions remain as to how this domain exchange arose in 2G12 and how this unusual event conferred unexpected reactivity against the glycan shield of HIV-1. In order to address these questions, we generated a nondomain-exchanged variant of 2G12 to produce a conventional Y/T-shaped antibody through a single amino acid substitution (2G12 I19R) and showed that, as for the 2G12 wild type (2G12 WT), this antibody is able to recognize the same Manα1,2Man motif on recombinant gp120, Candida albicans, and synthetic glycoconjugates. However, the nondomain-exchanged variant of 2G12 is unable to bind the cluster of mannose moieties on the surface of HIV-1. Crystallographic analysis of 2G12 I19R in complex with Manα1,2Man revealed an adaptable hinge between V_H and C_H1 that enables the V_H and V_L domains to assemble in such a way that the configuration of the primary binding site and its interaction with disaccharide are remarkably similar in the nondomain-exchanged and domain-exchanged forms. Together with data that suggest that very few substitutions are required for domain exchange, the results suggest potential mechanisms for the evolution of domain-exchanged antibodies and immunization strategies for eliciting such antibodies.The broadly neutralizing anti-human immunodeficiency virus type 1 (HIV-1) human monoclonal antibody 2G12 recognizes a highly conserved cluster of oligomannose residues on the glycan shield of the HIV-1 envelope glycoprotein gp120 (9, 10, 36, 39, 44, 45). The antibody binds terminal Manα1,2Man-linked sugars of high-mannose glycans (Man_8-9GlcNAc₂) with nanomolar affinity using a unique domain-exchanged structure in which the variable domains of the heavy chains swap to form a multivalent binding surface that includes two conventional antigen-combining sites and a third potential noncanonical binding site at the novel V_H/V_H′ interface (10). gp120 is one of the most heavily glycosylated proteins identified to date, with approximately 50% of its mass arising from host-derived N-linked glycans (24). These glycans play an important role in shielding the virus from the host immune system (34). Carbohydrates are generally poorly immunogenic, and the dense covering of glycans is often referred to as the “silent face” (52). The oligomannose glycans on gp120 in particular are closely packed, forming a tight cluster, and the unique domain-exchanged structure of 2G12 has been proposed as a means to recognize this cluster (10).The attraction of 2G12 as a template for HIV-1 vaccine design has recently been highlighted in a study that showed the antibody can protect macaques against simian-human immunodeficiency virus (SHIV) challenge at remarkably low serum neutralizing titers (18, 30, 43). When using 2G12 as a template for design of a carbohydrate immunogen, some important considerations must be taken into account. First, 2G12 is unusual in its specificity (targeting host cell-derived glycan motifs presented in a “nonself” arrangement), and although the 2G12 epitope is common to many HIV-1 envelopes, 2G12-like antibodies are rarely elicited (5, 38). Second, due to inherently weak carbohydrate-protein interactions (49, 50), it can be assumed that in order for a carbohydrate-specific antibody to achieve the affinity required to neutralize HIV-1, the avidity of the interaction must be enhanced by both Fab arms of the IgG-contacting glycan motifs simultaneously on the HIV-1 envelope. Third, the unique domain-exchanged structure of 2G12 has not been described for any other antibody (10). These considerations raise a number of questions. Which antigen or sequence of antigens elicited 2G12? Is domain exchange the only solution for recognition of highly clustered oligomannoses? If so, can domain exchange be elicited by immunization with clustered oligomannose motifs (38)?Efforts to design immunogens that elicit responses to the glycan shield of HIV-1 and neutralize the virus have to date been unsuccessful (2, 3, 14, 20, 21, 28, 29, 32, 46-48). Immunogen design strategies that mimic the 2G12 epitope have focused on both chemical and biochemical methods to generate multivalent and clustered displays of both high-mannose sugars (Man_8-9GlcNAc₂) (13, 15, 20, 21, 27-29, 32, 47) and truncated versions of such sugars (Man₉ and Man₄ linked via a 5-carbon linker) (3, 46). These constructs typically bind 2G12 with a lower affinity (on the order of 1 to 3 logs) than recombinant gp120. Although mannose-specific antibodies have been elicited by these immunogens, no HIV-1-neutralizing activities have been described. In a study by Luallen et al., antibodies against recombinant gp120 were generated by immunization with yeast cells that had been mutated to display only Man₈GlcNAc₂ glycans (27, 29). However, no neutralization activity against the corresponding pseudovirus was noted. It was proposed that this was due to either the low abundance of the gp120-specific antibodies in the serum or the antibodies elicited being against carbohydrate epitopes that differed from the 2G12 epitope (27, 29).To gain a better understanding of the importance of domain exchange for glycan recognition and how 2G12 may have been induced, we analyzed the binding characteristics of a nondomain-exchanged (conventional Y/T-shaped) 2G12 variant antibody. This variant was generated by a single point mutation, I19R, that disrupts the V_H/V_H′ interface. We show that the mutant is still able to recognize the Manα1,2Man motif arrayed on yeast, synthetic glycoconjugates, and recombinant gp120 in enzyme-linked immunosorbent assay (ELISA) format but is unable to recognize the discrete, dense mannose clusters found on the surface of the HIV-1 envelope (as measured by neutralization activity and binding to HIV-1-transfected cells). We further show that a major conformational change in the elbow region between V_H and C_H1 in this nondomain-exchanged variant of 2G12 allows the variable domains to assemble in similar orientations with respect to each other, as in the 2G12 wild type (WT), with an identical primary binding site, although with dramatically different orientations with respect to the constant domains. Thus, we conclude that 2G12 recognizes Manα1,2Man motifs in an identical manner in both conventional and domain-exchanged configurations, and the 2G12 specificity for Manα1,2Man likely first arose in a conventional IgG predecessor of 2G12. Subsequent domain exchange was the key event that then enabled high-affinity recognition of the tight oligomannose clusters on HIV-1.     

HIV-1 gp120 Determinants Proximal to the CD4 Binding Site Shift Protective Glycans That Are Targeted by Monoclonal Antibody 2G12

HIV-1 R5 envelopes vary considerably in their capacities to exploit low CD4 levels on macrophages for infection and in their sensitivities to the CD4 binding site (CD4bs) monoclonal antibody (MAb) b12 and the glycan-specific MAb 2G12. Here, we show that nonglycan determinants flanking the CD4 binding loop, which affect exposure of the CD4bs, also modulate 2G12 neutralization. Our data indicate that such residues act via a mechanism that involves shifts in the orientation of proximal glycans, thus modulating the sensitivity of 2G12 neutralization and affecting the overall presentation and structure of the glycan shield.The trimeric envelope (Env) spikes on HIV-1 virions are comprised of gp120 and gp41 heterodimers. gp120 is coated extensively with glycans (9, 11, 15) that are believed to protect the envelope from neutralizing antibodies. The extents and locations of glycosylation are variable and evolving (15). Thus, while some glycans are conserved, others appear or disappear in a host over the course of infection. Such changes may result in exposure or protection of functional envelope sites and can result from selection by different environmental pressures in vivo, including neutralizing antibodies.We previously reported that HIV-1 R5 envelopes varied considerably in tropism and neutralization sensitivity (3, 4, 12-14). We showed that highly macrophage-tropic R5 envelopes were more frequently detected in brain than in semen, blood, and lymph node (LN) samples (12, 14). The capacity of R5 envelopes to infect macrophages correlated with their ability to exploit low levels of cell surface CD4 for infection (12, 14). Determinants within and proximal to the CD4 binding site (CD4bs) were shown to modulate macrophage infectivity (3, 4, 5, 12, 13) and presumably acted by altering the avidity of the trimer for cell surface CD4. These determinants include residues proximal to the CD4 binding loop, which is likely the first part of the CD4bs contacted by CD4 (1). We also observed that macrophage-tropic R5 envelopes were frequently more resistant to the glycan-specific monoclonal antibody (MAb) 2G12 than were non-macrophage-tropic R5 Envs (13).Here, we investigated the envelope determinants of 2G12 sensitivity by using two HIV-1 envelopes that we used previously to map macrophage tropism determinants (4), B33 from brain and LN40 from lymph node tissue of an AIDS patient with neurological complications. While B33 imparts high levels of macrophage infectivity and is resistant to 2G12, LN40 Env confers very inefficient macrophage infection and is 2G12 sensitive (12-14).     

Fundamental Difference in the Content of High-Mannose Carbohydrate in the HIV-1 and HIV-2 Lineages

The virus-encoded envelope proteins of human immunodeficiency virus (HIV) and simian immunodeficiency virus (SIV) typically contain 26 to 30 sites for N-linked carbohydrate attachment. N-linked carbohydrate can be of three major types: high mannose, complex, or hybrid. The lectin proteins from Galanthus nivalis (GNA) and Hippeastrum hybrid (HHA), which specifically bind high-mannose carbohydrate, were found to potently inhibit the replication of a pathogenic cloned SIV from rhesus macaques, SIVmac239. Passage of SIVmac239 in the presence of escalating concentrations of GNA and HHA yielded a lectin-resistant virus population that uniformly eliminated three sites (of 26 total) for N-linked carbohydrate attachment (Asn-X-Ser or Asn-X-Thr) in the envelope protein. Two of these sites were in the gp120 surface subunit of the envelope protein (Asn244 and Asn460), and one site was in the envelope gp41 transmembrane protein (Asn625). Maximal resistance to GNA and HHA in a spreading infection was conferred to cloned variants that lacked all three sites in combination. Variant SIV gp120s exhibited dramatically decreased capacity for binding GNA compared to SIVmac239 gp120 in an enzyme-linked immunosorbent assay (ELISA). Purified gp120s from six independent HIV type 1 (HIV-1) isolates and two SIV isolates from chimpanzees (SIVcpz) consistently bound GNA in ELISA at 3- to 10-fold-higher levels than gp120s from five SIV isolates from rhesus macaques or sooty mangabeys (SIVmac/sm) and four HIV-2 isolates. Thus, our data indicate that characteristic high-mannose carbohydrate contents have been retained in the cross-species transmission lineages for SIVcpz-HIV-1 (high), SIVsm-SIVmac (low), and SIVsm-HIV-2 (low).The envelope proteins of human immunodeficiency virus (HIV) and simian immunodeficiency virus (SIV) are heavily glycosylated. N-linked carbohydrate is attached to the nascent protein at the asparagine of the consensus sequence N-X-S or N-X-T, where X is any amino acid except a proline (31, 52, 53). The number of potential N-linked carbohydrate attachment sites in the surface subunit of Env (gp120) ranges from 18 to 33, with a median of 25 (34, 65). There are typically 3 or 4 potential N-linked sites in the ectodomain of the Env transmembrane protein (gp41) (34).N-linked glycosylation of a protein consists of the en bloc transfer of the carbohydrate core oligosaccharide (two N-acetylglucosamines, nine mannoses, and three glucoses) from dolichol to the asparagine of the N-linked attachment site (8, 60). Initially the attached carbohydrate is processed into the high-mannose type (8). In the Golgi complex, high-mannose carbohydrate may be further processed into complex or hybrid oligosaccharides (58). Incomplete processing of N-linked carbohydrate results in the production of high-mannose carbohydrate chains, which terminate in mannose (58). Fully processed complex carbohydrate chains terminate in galactose, N-acetylglucosamine, sialic acid, or glucose (33, 57). Hybrid carbohydrate chains have two branches from the core, one that terminates in mannose and one that terminates in a sugar of the complex type (63).Glycoproteins exist as a heterogeneous population, exhibiting heterogeneity with respect to the proportion of potential glycosylation sites that are occupied and to the oligosaccharide structure observed at each site. Factors that influence the type of carbohydrate chain that is attached at any one N-linked site are the accessibility of the carbohydrate chain to processing enzymes (49), protein sequences surrounding the site (5, 40), and the type of cell from which the protein is produced (19).The N-linked carbohydrate chains of HIV and SIV Env are critical for the proper folding and cleavage of the fusion-competent envelope spike (20, 59, 61). After Env is assembled, enzymatic removal of N-linked carbohydrate does not dramatically affect the functional conformation (2, 6, 7, 13, 24, 38). It is generally accepted that the carbohydrate attached to Env limits the ability of the underlying protein to be recognized by B cells (11, 48, 62). This carbohydrate also shields protein epitopes that would otherwise be the direct targets of antibodies that neutralize viral infection (41, 48, 62, 64). Furthermore, the high-mannose carbohydrates of HIV and SIV Env bind dynamically to an array of lectin proteins that are part of the host lymphoreticular system. The interaction of viral high-mannose carbohydrate with host lectin proteins has been associated with the enhancement (9, 16, 17, 43-45) or suppression (42, 56) of viral infection of CD4-positive T cells. The high-mannose carbohydrate of Env is also known to activate the release of immune-modulatory proteins from a subset of host antigen-presenting cells (12, 54).The plant lectin proteins from Galanthus nivalis (GNA) and Hippeastrum hybrid (HHA) specifically bind terminal α-1,3- and/or α-1,6-mannose of high-mannose oligosaccharides but not hybrid oligosaccharides (28, 55). GNA and HHA inhibit the replication of HIV-1 and SIVmac251, and uncloned, resistant populations of virus have been selected (3, 14). In this report, we define two N-linked sites in the external surface glycoprotein gp120 and one in the transmembrane glycoprotein gp41 whose mutation imparts high-level resistance to the inhibitory effects of GNA and HHA to cloned SIVmac239. Furthermore, using a GNA-binding enzyme-linked immunosorbent assay (ELISA), we show that assorted HIV-1 and SIVcpz gp120s consistently are considerably higher in mannose content than assorted gp120s from SIVmac, SIVsm, and HIV-2. These results shed new light on the impact of virus-host evolutionary dynamics on viral carbohydrate composition, and they may have important implications for the mechanisms by which long-standing natural hosts such as sooty mangabeys can resist generalized lymphoid activation and disease despite high levels of SIV replication.     

Role of Complex Carbohydrates in Human Immunodeficiency Virus Type 1 Infection and Resistance to Antibody Neutralization

Complex N-glycans flank the receptor binding sites of the outer domain of HIV-1 gp120, ostensibly forming a protective “fence” against antibodies. Here, we investigated the effects of rebuilding this fence with smaller glycoforms by expressing HIV-1 pseudovirions from a primary isolate in a human cell line lacking N-acetylglucosamine transferase I (GnTI), the enzyme that initiates the conversion of oligomannose N-glycans into complex N-glycans. Thus, complex glycans, including those that surround the receptor binding sites, are replaced by fully trimmed oligomannose stumps. Conversely, the untrimmed oligomannoses of the silent domain of gp120 are likely to remain unchanged. For comparison, we produced a mutant virus lacking a complex N-glycan of the V3 loop (N301Q). Both variants exhibited increased sensitivities to V3 loop-specific monoclonal antibodies (MAbs) and soluble CD4. The N301Q virus was also sensitive to “nonneutralizing” MAbs targeting the primary and secondary receptor binding sites. Endoglycosidase H treatment resulted in the removal of outer domain glycans from the GnTI- but not the parent Env trimers, and this was associated with a rapid and complete loss in infectivity. Nevertheless, the glycan-depleted trimers could still bind to soluble receptor and coreceptor analogs, suggesting a block in post-receptor binding conformational changes necessary for fusion. Collectively, our data show that the antennae of complex N-glycans serve to protect the V3 loop and CD4 binding site, while N-glycan stems regulate native trimer conformation, such that their removal can lead to global changes in neutralization sensitivity and, in extreme cases, an inability to complete the conformational rearrangements necessary for infection.The intriguing results of a recent clinical trial suggest that an effective HIV-1 vaccine may be possible (97). Optimal efficacy may require a component that induces broadly neutralizing antibodies (BNAbs) that can block virus infection by their exclusive ability to recognize the trimeric envelope glycoprotein (Env) spikes on particle surfaces (43, 50, 87, 90). Env is therefore at the center of vaccine design programs aiming to elicit effective humoral immune responses.The amino acid sequence variability of Env presents a significant challenge for researchers seeking to elicit broadly effective NAbs. Early sequence comparisons revealed, however, that the surface gp120 subunit can be divided into discrete variable and conserved domains (Fig. ​(Fig.1A)1A) (110), the latter providing some hope for broadly effective NAb-based vaccines. Indeed, the constraints on variability in the conserved domains of gp120 responsible for binding the host cell receptor CD4, and coreceptor, generally CCR5, provide potential sites of vulnerability. However, viral defense strategies, such as the conformational masking of conserved epitopes (57), have made the task of eliciting bNAbs extremely difficult.Open in a separate windowFIG. 1.Glycan biosynthesis and distribution on gp120 and gp41. (A) Putative carbohydrate modifications are shown on gp120 and gp41 secondary structures, based on various published works (26, 42, 63, 74, 119, 128). The gp120 outer domain is indicated, as are residues that form the SOS gp120-gp41 disulfide bridge. The outer domain is divided into neutralizing and silent faces. Symbols distinguish complex, oligomannose, and unknown glycans. Generally, the complex glycans of the outer domain line the receptor binding sites of the neutralizing face, while the oligomannose glycans of the outer domain protect the silent domain (105). Asterisks denote sequons that are unlikely to be utilized, including position 139 (42), position 189 (26, 42), position 406 (42, 74), and position 637 (42). Glycans shown in gray indicate when sequon clustering may lead to some remaining unused, e.g., positions 156 and 160 (42, 119), positions 386, 392, and 397 (42), and positions 611 and 616 (42). There is also uncertainty regarding some glycan identities: glycans at positions 188, 355, 397, and 448 are not classified as predominantly complex or oligomannose (26, 42, 63, 128). The number of mannose moieties on oligomannose glycans can vary, as can the number of antennae and sialic acids on complex glycans (77). The glycan at position 301 appears to be predominantly a tetra-antennary complex glycan, as is the glycan at position 88, while most other complex glycans are biantennary (26, 128). (B) Schematic of essential steps of glycan biosynthesis from the Man₉GlcNAc₂ precursor to a mature multiantennary complex glycan. Mannosidase I progressively removes mannose moieties from the precursor, in a process that can be inhibited by the drug kifunensine. GnTI then transfers a GlcNAc moiety to the D1 arm of the resulting Man₅GlcNAc₂ intermediate, creating a hybrid glycan. Mannose trimming of the D2 and D3 arms then allows additional GlcNAc moieties to be added by a series of GnT family enzymes to form multiantennary complexes. This process can be inhibited by swainsonine. The antennae are ultimately capped and decorated by galactose and sialic acid. Hybrid and complex glycans are usually fucosylated at the basal GlcNAc, rendering them resistant to endo H digestion. However, NgF is able to remove all types of glycan.Carbohydrates provide a layer of protection against NAb attack (Fig. ​(Fig.1A).1A). As glycans are considered self, antibody responses against them are thought to be regulated by tolerance mechanisms. Thus, a glycan network forms a nonimmunogenic “cloak,” protecting the underlying protein from antibodies (3, 13, 20, 29, 39, 54, 65, 67, 74, 85, 96, 98, 117, 119, 120). The extent of this protection can be illustrated by considering the ways in which glycans differ from typical amino acid side chains. First, N-linked glycans are much larger, with an average mass more than 20 times that of a typical amino acid R-group. They are also usually more flexible and may therefore affect a greater volume of surrounding space. In the more densely populated parts of gp120, the carbohydrate field may even be stabilized by sugar-sugar hydrogen bonds, providing even greater coverage (18, 75, 125).The process of N-linked glycosylation can result in diverse structures that may be divided into three categories: oligomannose, hybrid, and complex (56). Each category shares a common Man₃GlcNAc₂ pentasaccharide stem (where Man is mannose and GlcNAc is N-acetylglucosamine), to which up to six mannose residues are attached in oligomannose N-glycans, while complex N-glycans are usually larger and may bear various sizes and numbers of antennae (Fig. ​(Fig.1B).1B). Glycan synthesis begins in the endoplasmic reticulum, where N-linked oligomannose precursors (Glc₃Man₉GlcNAc₂; Glc is glucose) are transferred cotranslationally to the free amide of the asparagine in a sequon Asn-X-Thr/Ser, where X is not Pro (40). Terminal glucose and mannose moieties are then trimmed to yield Man₅GlcNAc₂ (Fig. ​(Fig.1B).1B). Conversion to a hybrid glycan is then initiated by N-acetylglucosamine transferase I (GnTI), which transfers a GlcNAc moiety to the D1 arm of the Man₅GlcNAc₂ substrate (19) (Fig. ​(Fig.1B).1B). This hybrid glycoform is then a substrate for modification into complex glycans, in which the D2 and D3 arm mannose residues are replaced by complex antennae (19, 40, 56). Further enzymatic action catalyzes the addition of α-1-6-linked fucose moiety to the lower GlcNAc of complex glycan stems, but usually not to oligomannose glycan stems (Fig. ​(Fig.1B)1B) (21, 113).Most glycoproteins exhibit only fully mature complex glycans. However, the steric limitations imposed by the high density of glycans on some parts of gp120 lead to incomplete trimming, leaving “immature” oligomannose glycans (22, 26, 128). Spatial competition between neighboring sequons can sometimes lead to one or the other remaining unutilized, further distancing the final Env product from what might be expected based on its primary sequence (42, 48, 74, 119). An attempt to assign JR-FL gp120 and gp41 sequon use and types, based on various studies, is shown in Fig. ​Fig.1A1A (6, 26, 34, 35, 42, 63, 71, 74, 119, 128). At some positions, the glycan type is conserved. For example, the glycan at residue N301 has consistently been found to be complex (26, 63, 128). At other positions, considerable heterogeneity exists in the glycan populations, in some cases to the point where it is difficult to unequivocally assign them as predominantly complex or oligomannose. The reasons for these uncertainties might include incomplete trimming (42), interstrain sequence variability, the form of Env (e.g., gp120 or gp140), and the producer cell. The glycans of native Env trimers and monomeric gp120 may differ due to the constraints imposed by oligomerization (32, 41, 77). Thus, although all the potential sequons of HXB2 gp120 were found to be occupied in one study (63), some are unutilized or variably utilized on functional trimers, presumably due to steric limitations (42, 48, 75, 96, 119).The distribution of complex and oligomannose glycans on gp120 largely conforms with an antigenic map derived from structural models (59, 60, 102, 120), in which the outer domain is divided into a neutralizing face and an immunologically silent face. Oligomannose glycans cluster tightly on the silent face of gp120 (18, 128), while complex glycans flank the gp120 receptor binding sites of the neutralizing face, ostensibly forming a protective “fence” against NAbs (105). The relatively sparse clustering of complex glycans that form this fence may reflect a trade-off between protecting the underlying functional domains from NAbs by virtue of large antennae while at the same time permitting sufficient flexibility for the refolding events associated with receptor binding and fusion (29, 39, 67, 75, 98, 117). Conversely, the dense clustering of oligomannose glycans on the silent domain may be important for ensuring immune protection and/or in creating binding sites for lectins such as DC-SIGN (9, 44).The few available broadly neutralizing monoclonal antibodies (MAbs) define sites of vulnerability on Env trimers (reviewed in reference 52). They appear to fall into two general categories: those that access conserved sites by overcoming Env''s various evasion strategies and, intriguingly, those that exploit these very defensive mechanisms. Regarding the first category, MAb b12 recognizes an epitope that overlaps the CD4 binding site of gp120 (14), and MAbs 2F5 and 4E10 (84, 129) recognize adjacent epitopes of the membrane-proximal external region (MPER) at the C-terminal ectodomain of gp41. The variable neutralizing potencies of these MAbs against primary isolates that contain their core epitopes illustrate how conformational masking can dramatically regulate their exposure (11, 118). Conformational masking also limits the activities of MAbs directed to the V3 loop and MAbs whose epitopes overlap the coreceptor binding site (11, 62, 121).A second category of MAbs includes MAb 2G12, which recognizes a tight cluster of glycans in the silent domain of gp120 (16, 101, 103, 112). This epitope has recently sparked considerable interest in exploiting glycan clusters as possible carbohydrate-based vaccines (2, 15, 31, 70, 102, 116). Two recently described MAbs, PG9 and PG16 (L. M. Walker and D. R. Burton, unpublished data), also target epitopes regulated by the presence of glycans that involve conserved elements of the second and third variable loops and depend largely on the quaternary trimer structure and its in situ presentation on membranes. Their impressive breadth and potency may come from the fact that they target the very mechanisms (variable loops and glycans) that are generally thought to protect the virus from neutralization. Like 2G12, these epitopes are likely to be constitutively exposed and thus may not be subject to conformational masking (11, 118).The above findings reveal the importance of N-glycans both as a means of protection against neutralization as well as in directly contributing to unique neutralizing epitopes. Clearly, further studies on the nature and function of glycans in native Env trimers are warranted. Possible approaches may be divided into four categories, namely, (i) targeted mutation, (ii) enzymatic removal, (iii) expression in the presence of glycosylation inhibitors, and (iv) expression in mutant cell lines with engineered blocks in the glycosylation pathway. Much of the available information on the functional roles of glycans in HIV-1 and simian immunodeficiency virus (SIV) infection has come from the study of mutants that eliminate glycans either singly or in combination (20, 54, 66, 71, 74, 91, 95, 96). Most mutants of this type remain at least partially functional (74, 95, 96). In some cases these mutants have little effect on neutralization sensitivity, while in others they can lead to increased sensitivity to MAbs specific for the V3 loop and CD4 binding site (CD4bs) (54, 71, 72, 74, 106). In exceptional cases, increased sensitivity to MAbs targeting the coreceptor binding site and/or the gp41 MPER has been observed (54, 66, 72, 74).Of the remaining approaches for studying the roles of glycans, enzymatic removal is constrained by the extreme resistance of native Env trimers to many common glycosidases, contrasting with the relative sensitivity of soluble gp120 (67, 76, 101). Alternatively, drugs can be used to inhibit various stages of mammalian glycan biosynthesis. Notable examples are imino sugars, such as N-butyldeoxynojirimycin (NB-DNJ), that inhibit the early trimming of the glucose moieties from Glc₃Man₉GlcNAc₂ precursors in the endoplasmic reticulum (28, 38, 51). Viruses produced in the presence of these drugs may fail to undergo proper gp160 processing or fusion (37, 51). Other classes of inhibitor include kifunensine and swainsonine, which, respectively, inhibit the trimming of the Man₉GlcNAc₂ precursor into Man₅GlcNAc₂ or inhibit the removal of remaining D2 and D3 arm mannoses from the hybrid glycans, thus preventing the construction of complex glycan antennae (Fig. ​(Fig.1B)1B) (17, 33, 76, 104, 119). Unlike NB-DNJ, viruses produced in the presence of these drugs remain infectious (36, 76, 79, 100).Yet another approach is to express virus in insect cells that can only modify proteins with paucimannose N-glycans (58). However, the inefficient gp120/gp41 processing by furin-like proteases in these cells prevents their utility in functional studies (123). Another option is provided by ricin-selected GnTI-deficient cell lines that cannot transfer GlcNAc onto the mannosidase-trimmed Man₅GlcNAc₂ substrate, preventing the formation of hybrid and complex carbohydrates (Fig. ​(Fig.1B)1B) (17, 32, 36, 94). This arrests glycan processing at a well-defined point, leading to the substitution of complex glycans with Man₅GlcNAc₂ rather than with the larger Man₉GlcNAc₂ precursors typically obtained with kifunensine treatment (17, 32, 33, 104). With this in mind, here we produced HIV-1 pseudoviruses in GnTI-deficient cells to investigate the role of complex glycan antennae in viral resistance neutralization. By replacing complex glycans with smaller Man₅GlcNAc₂ we can determine the effect of “lowering the glycan fence” that surrounds the receptor binding sites, compared to the above-mentioned studies of individual glycan deletion mutants, whose effects are analogous to removing a fence post. Furthermore, since oligomannose glycans are sensitive to certain enzymes, such as endoglycosidase H (endo H), we investigated the effect of dismantling the glycan fence on Env function and stability. Our results suggest that the antennae of complex glycans protect against certain specificities but that glycan stems regulate trimer conformation with often more dramatic consequences for neutralization sensitivity and in extreme cases, infectious function.     

Crystallographic Definition of the Epitope Promiscuity of the Broadly Neutralizing Anti-Human Immunodeficiency Virus Type 1 Antibody 2F5: Vaccine Design Implications

The quest to create a human immunodeficiency virus type 1 (HIV-1) vaccine capable of eliciting broadly neutralizing antibodies against Env has been challenging. Among other problems, one difficulty in creating a potent immunogen resides in the substantial overall sequence variability of the HIV envelope protein. The membrane-proximal region (MPER) of gp41 is a particularly conserved tryptophan-rich region spanning residues 659 to 683, which is recognized by three broadly neutralizing monoclonal antibodies (bnMAbs), 2F5, Z13, and 4E10. In this study, we first describe the variability of residues in the gp41 MPER and report on the invariant nature of 15 out of 25 amino acids comprising this region. Subsequently, we evaluate the ability of the bnMAb 2F5 to recognize 31 varying sequences of the gp41 MPER at a molecular level. In 19 cases, resulting crystal structures show the various MPER peptides bound to the 2F5 Fab′. A variety of amino acid substitutions outside the ⁶⁶⁴DKW⁶⁶⁶ core epitope are tolerated. However, changes at the ⁶⁶⁴DKW⁶⁶⁶ motif itself are restricted to those residues that preserve the aspartate''s negative charge, the hydrophobic alkyl-π stacking arrangement between the β-turn lysine and tryptophan, and the positive charge of the former. We also characterize a possible molecular mechanism of 2F5 escape by sequence variability at position 667, which is often observed in HIV-1 clade C isolates. Based on our results, we propose a somewhat more flexible molecular model of epitope recognition by bnMAb 2F5, which could guide future attempts at designing small-molecule MPER-like vaccines capable of eliciting 2F5-like antibodies.Eliciting broadly neutralizing antibodies (bnAbs) against primary isolates of human immunodeficiency virus type I (HIV-1) has been identified as a major milestone to attain in the quest for a vaccine in the fight against AIDS (12, 28). These antibodies would need to interact with HIV-1 envelope glycoproteins gp41 and/or gp120 (Env), target conserved regions and functional conformations of gp41/gp120 trimeric complexes, and prevent new HIV-1 fusion events with target cells (21, 57, 70, 71). Although a humoral response generating neutralizing antibodies against HIV-1 can be detected in HIV-1-positive individuals, the titers are often very low, and virus control is seldom achieved by these neutralizing antibodies (22, 51, 52, 66, 67). The difficulty in eliciting a broad and potent neutralizing antibody response against HIV-1 is thought to reside in the high degree of genetic diversity of the virus, in the heterogeneity of Env on the surface of HIV-1, and in the masking of functional regions by conformational covering, by an extensive glycan shield, or by the ability of some conserved domains to partition to the viral membrane (24, 25, 29, 30, 38, 39, 56, 68, 69). So far, vaccine trials using as immunogens mimics of Env in different conformations have primarily elicited antibodies with only limited neutralization potency across different HIV-1 clades although recent work has demonstrated more encouraging results (4, 12, 61).The use of conserved regions on gp41 and gp120 Env as targets for vaccine design has been mostly characterized by the very few anti-HIV-1 broadly neutralizing monoclonal antibodies (bnMAbs) that recognize them: the CD4 binding-site on gp120 (bnMAb b12), a CD4-induced gp120 coreceptor binding site (bnMAbs 17b and X5), a mannose cluster on the outer face of gp120 (bnMAb 2G12), and the membrane proximal external region (MPER) of gp41 (bnMAbs 2F5, Z13 and 4E10) (13, 29, 44, 58, 73). The gp41 MPER region is a particularly conserved part of Env that spans residues 659 to 683 (HXB2 numbering) (37, 75). Substitution and deletion studies have linked this unusually tryptophan-rich region to the fusion process of HIV-1, possibly involving a series of conformational changes (5, 37, 41, 49, 54, 74). Additionally, the gp41 MPER has been implicated in gp41 oligomerization, membrane leakage ability facilitating pore formation, and binding to the galactosyl ceramide receptor on epithelial cells for initial mucosal infection mediated by transcytosis (2, 3, 40, 53, 63, 64, 72). This wide array of roles for the gp41 MPER will put considerable pressure on sequence conservation, and any change will certainly lead to a high cost in viral fitness.Monoclonal antibody 2F5 is a broadly neutralizing monoclonal anti-HIV-1 antibody isolated from a panel of sera from naturally infected asymptomatic individuals. It reacts with a core gp41 MPER epitope spanning residues 662 to 668 with the linear sequence ELDKWAS (6, 11, 42, 62, 75). 2F5 immunoglobulin G binding studies and screening of phage display libraries demonstrated that the DKW core is essential for 2F5 recognition and binding (15, 36, 50). Crystal structures of 2F5 with peptides representing its core gp41 epitope reveal a β-turn conformation involving the central DKW residues, flanked by an extended conformation and a canonical α-helical turn for residues located at the N terminus and C terminus of the core, respectively (9, 27, 45, 47). In addition to binding to its primary epitope, evidence is accumulating that 2F5 also undergoes secondary interactions: multiple reports have demonstrated affinity of 2F5 for membrane components, possibly through its partly hydrophobic flexible elongated complementarity-determining region (CDR) H3 loop, and it has also been suggested that 2F5 might interact in a secondary manner with other regions of gp41 (1, 10, 23, 32, 33, 55). Altogether, even though the characteristics of 2F5 interaction with its linear MPER consensus epitope have been described extensively, a number of questions persist about the exact mechanism of 2F5 neutralization at a molecular level.One such ambiguous area of the neutralization mechanism of 2F5 is investigated in this study. Indeed, compared to bnMAb 4E10, 2F5 is the more potent neutralizing antibody although its breadth across different HIV-1 isolates is more limited (6, 35). In an attempt to shed light on the exact molecular requirements for 2F5 recognition of its primary gp41 MPER epitope, we performed structural studies of 2F5 Fab′ with a variety of peptides. The remarkable breadth of possible 2F5 interactions reveals a somewhat surprising promiscuity of the 2F5 binding site. Furthermore, we link our structural observations with the natural variation observed within the gp41 MPER and discuss possible routes of 2F5 escape from a molecular standpoint. Finally, our discovery of 2F5''s ability to tolerate a rather broad spectrum of amino acids in its binding, a spectrum that even includes nonnatural amino acids, opens the door to new ways to design small-molecule immunogens potentially capable of eliciting 2F5-like neutralizing antibodies.     

Evidence for Mucin-Like Glycoproteins That Tether Sporozoites of Cryptosporidium parvum to the Inner Surface of the Oocyst Wall

Cryptosporidium parvum oocysts, which are spread by the fecal-oral route, have a single, multilayered wall that surrounds four sporozoites, the invasive form. The C. parvum oocyst wall is labeled by the Maclura pomifera agglutinin (MPA), which binds GalNAc, and the C. parvum wall contains at least two unique proteins (Cryptosporidium oocyst wall protein 1 [COWP1] and COWP8) identified by monoclonal antibodies. C. parvum sporozoites have on their surface multiple mucin-like glycoproteins with Ser- and Thr-rich repeats (e.g., gp40 and gp900). Here we used ruthenium red staining and electron microscopy to demonstrate fibrils, which appear to attach or tether sporozoites to the inner surface of the C. parvum oocyst wall. When disconnected from the sporozoites, some of these fibrillar tethers appear to collapse into globules on the inner surface of oocyst walls. The most abundant proteins of purified oocyst walls, which are missing the tethers and outer veil, were COWP1, COWP6, and COWP8, while COWP2, COWP3, and COWP4 were present in trace amounts. In contrast, MPA affinity-purified glycoproteins from C. parvum oocysts, which are composed of walls and sporozoites, included previously identified mucin-like glycoproteins, a GalNAc-binding lectin, a Ser protease inhibitor, and several novel glycoproteins (C. parvum MPA affinity-purified glycoprotein 1 [CpMPA1] to CpMPA4). By immunoelectron microscopy (immuno-EM), we localized mucin-like glycoproteins (gp40 and gp900) to the ruthenium red-stained fibrils on the inner surface wall of oocysts, while antibodies to the O-linked GalNAc on glycoproteins were localized to the globules. These results suggest that mucin-like glycoproteins, which are associated with the sporozoite surface, may contribute to fibrils and/or globules that tether sporozoites to the inner surface of oocyst walls.Cryptosporidium parvum and the related species Cryptosporidium hominis are apicomplexan parasites, which are spread by the fecal-oral route in contaminated water and cause diarrhea, particularly in immunocompromised hosts (1, 12, 39, 47). The infectious and diagnostic form of C. parvum is the oocyst, which has a single, multilayered, spherical wall that surrounds four sporozoites, the invasive forms (14, 27, 31). The outermost layer of the C. parvum oocyst wall is most often absent from electron micrographs, as it is labile to bleach used to remove contaminating bacteria from C. parvum oocysts (27). We will refer to this layer as the outer veil, which is the term used for a structure with an identical appearance on the surface of the oocyst wall of another apicomplexan parasite, Toxoplasma gondii (10). At the center of the C. parvum oocyst wall is a protease-resistant and rigid bilayer that contains GalNAc (5, 23, 43). When excysting sporozoites break through the oocyst wall, the broken edges of this bilayer curl in, while the overall shape of the oocyst wall remains spherical.The inner, moderately electron-dense layer of the C. parvum oocyst wall is where the Cryptosporidium oocyst wall proteins (Cryptosporidium oocyst wall protein 1 [COWP1] and COWP8) have been localized with monoclonal antibodies (4, 20, 28, 32). COWPs, which have homologues in Toxoplasma, are a family of nine proteins that contain polymorphic Cys-rich and His-rich repeats (37, 46). Finally, on the inner surface of C. parvum oocyst walls are knob-like structures, which cross-react with an anti-oocyst monoclonal antibody (11).Like other apicomplexa (e.g., Toxoplasma and Plasmodium), sporozoites of C. parvum are slender, move by gliding motility, and release adhesins from apical organelles when they invade host epithelial cells (1, 8, 12, 39). Unlike other apicomplexa, C. parvum parasites are missing a chloroplast-derived organelle called the apicoplast (1, 47, 49). C. parvum sporozoites have on their surface unique mucin-like glycoproteins, which contain Ser- and Thr-rich repeats that are polymorphic and may be modified by O-linked GalNAc (4-7, 21, 25, 26, 30, 32, 34, 35, 43, 45). These C. parvum mucins, which are highly immunogenic and are potentially important vaccine candidates, include gp900 and gp40/gp15 (4, 6, 7, 25, 26). gp40/gp15 is cleaved by furin-like proteases into two peptides (gp40 and gp15), each of which is antigenic (42). gp900, gp40, and gp15 are shed from the surface of the C. parvum sporozoites during gliding motility (4, 7, 35).The studies presented here began with electron microscopic observations of C. parvum oocysts stained with ruthenium red (23), which revealed novel fibrils or tethers that extend radially from the inner surface of the oocyst wall to the outer surface of sporozoites. We hypothesized that at least some of these fibrillar tethers might be the antigenic mucins, which are abundant on the surface of C. parvum sporozoites. To test this hypothesis, we used mass spectroscopy to identify oocyst wall proteins and sporozoite glycoproteins and used deconvolving and immunoelectron microscopy (immuno-EM) with lectins and anti-C. parvum antibodies to directly label the tethers.     

Utilization of Immunoglobulin G Fc Receptors by Human Immunodeficiency Virus Type 1: a Specific Role for Antibodies against the Membrane-Proximal External Region of gp41

Receptors (FcγRs) for the constant region of immunoglobulin G (IgG) are an important link between humoral immunity and cellular immunity. To help define the role of FcγRs in determining the fate of human immunodeficiency virus type 1 (HIV-1) immune complexes, cDNAs for the four major human Fcγ receptors (FcγRI, FcγRIIa, FcγRIIb, and FcγRIIIa) were stably expressed by lentiviral transduction in a cell line (TZM-bl) commonly used for standardized assessments of HIV-1 neutralization. Individual cell lines, each expressing a different FcγR, bound human IgG, as evidence that the physical properties of the receptors were preserved. In assays with a HIV-1 multisubtype panel, the neutralizing activities of two monoclonal antibodies (2F5 and 4E10) that target the membrane-proximal external region (MPER) of gp41 were potentiated by FcγRI and, to a lesser extent, by FcγRIIb. Moreover, the neutralizing activity of an HIV-1-positive plasma sample known to contain gp41 MPER-specific antibodies was potentiated by FcγRI. The neutralizing activities of monoclonal antibodies b12 and 2G12 and other HIV-1-positive plasma samples were rarely affected by any of the four FcγRs. Effects with gp41 MPER-specific antibodies were moderately stronger for IgG1 than for IgG3 and were ineffective for Fab. We conclude that FcγRI and FcγRIIb facilitate antibody-mediated neutralization of HIV-1 by a mechanism that is dependent on the Fc region, IgG subclass, and epitope specificity of antibody. The FcγR effects seen here suggests that the MPER of gp41 could have greater value for vaccines than previously recognized.Fc receptors (FcRs) are differentially expressed on a variety of cells of hematopoietic lineage, where they bind the constant region of antibody (Ab) and provide a link between humoral and cellular immunity. Humans possess two classes of FcRs for the constant region of IgG (FcγRs) that, when cross-linked, are distinguished by their ability to either activate or inhibit cell signaling (69, 77, 79). The activating receptors FcγRI (CD64), FcγRIIa (CD32), and FcγRIII (CD16) signal through an immunoreceptor tyrosine-based activation motif (ITAM), whereas FcγRIIb (CD32) contains an inhibitory motif (ITIM) that counters ITAM signals and B-cell receptor signals. It has been suggested that a balance between activating and inhibitory FcγRs coexpressed on the same cells plays an important role in regulating adaptive immunity (23, 68). Moreover, the inhibitory FcγRIIb, being the sole FcγR on B cells, appears to play an important role in regulating self-tolerance (23, 68). The biologic role of FcγRs may be further influenced by differences in their affinity for immunoglobulin G (IgG); thus, FcγRI is a high-affinity receptor that binds monomeric IgG (mIgG) and IgG immune complexes (IC), whereas FcγRIIa, FcγRIIb, and FcγRIIIa are medium- to low-affinity receptors that preferentially bind IgG IC (10, 49, 78). FcγRs also exhibit differences in their relative affinity for the four IgG subclasses (10), which has been suggested to influence the balance between activating and inhibitory FcγRs (67).In addition to their participation in acquired immunity, FcγRs can mediate several innate immune functions, including phagocytosis of opsonized pathogens, Ab-dependent cell cytotoxicity (ADCC), antigen uptake by professional antigen-presenting cells, and the production of inflammatory cytokines and chemokines (26, 35, 41, 48, 69). In some cases, interaction of Ab-coated viruses with FcγRs may be exploited by viruses as a means to facilitate entry into FcγR-expressing cells (2, 33, 47, 84). Several groups have reported FcγR-mediated Ab-dependent enhancement (ADE) of HIV-1 infection in vitro (47, 51, 58, 63, 94, 96), whereas other reports have implicated FcγRs in efficient inhibition of the virus in vitro (19, 21, 29, 44-46, 62, 98) and possibly as having beneficial effects against HIV-1 in vivo (5, 27, 28, 42). These conflicting results are further complicated by the fact that HIV-1-susceptible cells, such as monocytes and macrophages, can coexpress more than one FcγR (66, 77, 79).HIV-1 entry requires sequential interactions between the viral surface glycoprotein, gp120, and its cellular receptor (CD4) and coreceptor (usually CCR5 or CXCR4), followed by membrane fusion that is mediated by the viral transmembrane glycoprotein gp41 (17, 106). Abs neutralize the virus by binding either gp120 or gp41 and blocking entry into cells. Several human monoclonal Abs that neutralize a broad spectrum of HIV-1 variants have attracted considerable interest for vaccine design. Epitopes for these monoclonal Abs include the receptor binding domain of gp120 in the case of b12 (71, 86), a glycan-specific epitope on gp120 in the case of 2G12 (13, 85, 86), and two adjacent epitopes in the membrane-proximal external region (MPER) of g41 in the cases of 2F5 and 4E10 (3, 11, 38, 93). At least three of these monoclonal Abs have been shown to interact with FcRs and to mediate ADCC (42, 43).A highly standardized and validated assay for neutralizing Abs against HIV-1 that quantifies reductions in luciferase (Luc) reporter gene expression after a single round of virus infection in TZM-bl cells has been developed (60, 104). TZM-bl (also called JC53BL-13) is a CXCR4-positive HeLa cell line that was engineered to express CD4 and CCR5 and to contain integrated reporter genes for firefly Luc and Escherichia coli β-galactosidase under the control of the HIV-1 Tat-regulated promoter in the long terminal repeat terminal repeat sequence (74, 103). TZM-bl cells are permissive to infection by a wide variety of HIV-1, simian immunodeficiency virus, and human-simian immunodeficiency virus strains, including molecularly cloned Env-pseudotyped viruses. Here we report the creation and characterization of four new TZM-bl cell lines, each expressing one of the major human FcγRs. These new cell lines were used to gain a better understanding of the individual roles that FcγRs play in determining the fate of HIV-1 IC. Two FcγRs that potentiated the neutralizing activity of gp41 MPER-specific Abs were identified.     

Potent and Broadly Reactive HIV-2 Neutralizing Antibodies Elicited by a Vaccinia Virus Vector Prime-C2V3C3 Polypeptide Boost Immunization Strategy

Human immunodeficiency virus type 2 (HIV-2) infection affects about 1 to 2 million individuals, the majority living in West Africa, Europe, and India. As for HIV-1, new strategies for the prevention of HIV-2 infection are needed. Our aim was to produce new vaccine immunogens that elicit the production of broadly reactive HIV-2 neutralizing antibodies (NAbs). Native and truncated envelope proteins from the reference HIV-2ALI isolate were expressed in vaccinia virus or in bacteria. This source isolate was used due to its unique phenotype combining CD4 independence and CCR5 usage. NAbs were not elicited in BALB/c mice by single immunization with a truncated and fully glycosylated envelope gp125 (gp125t) or a recombinant polypeptide comprising the C2, V3, and C3 envelope regions (rpC2-C3). A strong and broad NAb response was, however, elicited in mice primed with gp125t expressed in vaccinia virus and boosted with rpC2-C3. Serum from these animals potently neutralized (median 50% neutralizing titer, 3,200) six of six highly divergent primary HIV-2 isolates. Coreceptor usage and the V3 sequence of NAb-sensitive isolates were similar to that of the vaccinating immunogen (HIV-2ALI). In contrast, NAbs were not reactive on three X4 isolates that displayed major changes in V3 loop sequence and structure. Collectively, our findings demonstrate that broadly reactive HIV-2 NAbs can be elicited by using a vaccinia virus vector-prime/rpC2-C3-boost immunization strategy and suggest a potential relationship between escape to neutralization and cell tropism.Human immunodeficiency virus type 2 (HIV-2) infection affects 1 to 2 million individuals, most of whom live in India, West Africa, and Europe (17). HIV-2 has diversified into eight genetic groups named A to H, of which group A is by far the most prevalent worldwide. Nucleotide sequences of Env can differ up to 21% within a particular group and by over 35% between groups.The mortality rate in HIV-2-infected patients is at least twice that of uninfected individuals (26). Nonetheless, the majority of HIV-2-infected individuals survive as elite controllers (17). In the absence of antiretroviral therapy, the numbers of infected cells (39) and viral loads (36) are much lower among HIV-2-infected individuals than among those who are HIV-1 infected. This may be related to a more effective immune response produced against HIV-2. In fact, most HIV-2-infected individuals have proliferative T-cell responses and strong cytotoxic responses to Env and Gag proteins (17, 31). Moreover, autologous and heterologous neutralizing antibodies (NAbs) are raised in most HIV-2-infected individuals (8, 32, 48, 52), and the virus seems unable to escape from these antibodies (52). As for HIV-1, the antibody specificities that mediate HIV-2 neutralization and control are still elusive. The V3 region in the envelope gp125 has been identified as a neutralizing target by some but not by all investigators (3, 6, 7, 11, 40, 47, 54). Other weakly neutralizing epitopes were identified in the V1, V2, V4, and C5 regions in gp125 and in the COOH-terminal region of the gp41 ectodomain (6, 7, 41). A better understanding of the neutralizing determinants in the HIV-2 Env will provide crucial information regarding the most relevant targets for vaccine design.The development of immunogens that elicit the production of broadly reactive NAbs is considered the number one priority for the HIV-1 vaccine field (4, 42). Most current HIV-1 vaccine candidates intended to elicit such broadly reactive NAbs are based on purified envelope constructs that mimic the structure of the most conserved neutralizing epitopes in the native trimeric Env complex and/or on the expression of wild-type or modified envelope glycoproteins by different types of expression vectors (4, 5, 29, 49, 58). With respect to HIV-2, purified gp125 glycoprotein or synthetic peptides representing selected V3 regions from HIV-2 strain SBL6669 induced autologous and heterologous NAbs in mice or guinea pigs (6, 7, 22). However, immunization of cynomolgus monkeys with a subunit vaccine consisting of gp130 (HIV-2BEN) micelles offered little protection against autologous or heterologous challenge (34). Immunization of rhesus (19, 44, 45) and cynomolgus (1) monkeys with canarypox or attenuated vaccinia virus expressing several HIV-2 SBL6669 proteins, including the envelope glycoproteins, in combination with booster immunizations with gp160, gp125, or V3 synthetic peptides, elicited a weak neutralizing response and partial protection against autologous HIV-2 challenge. Likewise, vaccination of rhesus monkeys with immunogens derived from the historic HIV-2ROD strain failed to generate neutralizing antibodies and to protect against heterologous challenge (55). Finally, baboons inoculated with a DNA vaccine expressing the tat, nef, gag, and env genes of the HIV-2UC2 group B isolate were partially protected against autologous challenge without the production of neutralizing antibodies (33). These studies illustrate the urgent need for new vaccine immunogens and/or vaccination strategies that elicit the production of broadly reactive NAbs against HIV-2. The present study was designed to investigate in the mouse model the immunogenicity and neutralizing response elicited by novel recombinant envelope proteins derived from the reference primary HIV-2ALI isolate, when administered alone or in different prime-boost combinations.     

Very Few Substitutions in a Germ Line Antibody Are Required To Initiate Significant Domain Exchange

2G12 is a broadly neutralizing anti-HIV-1 monoclonal human IgG1 antibody reactive with a high-mannose glycan cluster on the surface of glycoprotein gp120. A key feature of this very highly mutated antibody is domain exchange of the heavy-chain variable region (V_H) with the V_H of the adjacent Fab of the same immunoglobulin, which assembles a multivalent binding interface composed of two primary binding sites in close proximity. A non-germ line-encoded proline in the elbow between V_H and C_H1 and an extensive network of hydrophobic interactions in the V_H/V_H′ interface have been proposed to be crucial for domain exchange. To investigate the origins of domain exchange, a germ line version of 2G12 that behaves as a conventional antibody was engineered. Substitution of 5 to 7 residues for those of the wild type produced a significant fraction of domain-exchanged molecules, with no evidence of equilibrium between domain-exchanged and conventional forms. Two substitutions not previously implicated, A^H14 and E^H75, are the most crucial for domain exchange, together with I^H19 at the V_H/V_H′ interface and P^H113 in the elbow region. Structural modeling gave clues as to why these residues are essential for domain exchange. The demonstration that domain exchange can be initiated by a small number of substitutions in a germ line antibody suggests that the evolution of a domain-exchanged antibody response in vivo may be more readily achieved than considered to date.Protein oligomers are able to exchange or swap an element of their secondary structure or an entire protein domain. The functional unit in domain-exchanged proteins thereby stays preserved, as only the linking hinge loop changes conformation significantly (4, 17, 27). Analogous to other domain-swapped proteins, antibodies can exchange an entire domain, in this case the heavy-chain variable region (V_H), with an equivalent heavy-chain variable region of an adjacent Fab (V_H′) within the same immunoglobulin (Ig) molecule (11). The advantages of domain-exchanged proteins, including antibodies, are higher local concentrations of active sites, a larger binding surface, and a potential secondary active site at the new subunit interface (27, 45). The one and only antibody shown to be domain exchanged to date is 2G12 (7, 11), but this arrangement is potentially possible for any Ig and could have been overlooked at least in some instances.2G12 is one of only a few high-affinity monoclonal antibodies with broad neutralizing activity against different subtypes of HIV-1 (5, 30, 40, 43). The antibody binds a dense cluster of N-linked high-mannose glycans (Man_8-9GlcNAc₂) on the envelope surface glycoprotein gp120 (10, 35, 36, 41). The domain-exchanged arrangement forms a multivalent binding site composed of two primary binding sites in close proximity and a proposed secondary binding site formed by the novel V_H/V_H′ interface (11). 2G12 provides protection against infection in animal models (19, 31) and has been shown to induce neutralization escape following passive immunization in humans (39).Consensus has grown that a successful HIV-1 vaccine will need to include a component that elicits broadly neutralizing antibodies (8, 18, 21, 26, 32, 42). All attempts to elicit 2G12-like antibodies with the desired specificity and neutralization activity have failed to date (22, 29, 44), conceivably due to difficulties in generating adequate mimicry of the glycan cluster and tolerance mechanisms or, very likely, the inability to induce domain exchange (1). Unraveling the mechanism of domain exchange and how this conformation might have evolved is highly desirable to direct future HIV-1 vaccine design to elicit 2G12-like antibodies.By comparison with other domain-exchanged proteins (27), the following three mechanisms have been proposed to contribute to the unique structure of 2G12 compared to the structure of a conventional antibody: destabilization of the “closed” V_H/V_L interface, conformational change in the elbow between V_H and C_H1, and an energetically favorable “open” V_H/V_H′ interface (11). Key residues involved in promoting domain exchange were predicted based on examination of interacting residues at the two interfaces and by the effects of alanine substitutions on the binding of wild-type 2G12 to gp120. However, the importance of these key residues for domain exchange was not directly demonstrated experimentally (11).Here, we explored the minimal requirements for domain exchange of 2G12, starting with a germ line version of the antibody that adopts a conventional antibody structure. Although wild-type 2G12 is heavily somatically mutated, only five to seven substitutions in the germ line version of the antibody were shown to produce a significant fraction of domain-exchanged molecules. The results suggest the evolution of domain-exchanged antibody responses may be more facile than considered to date.     

Envelope-Modified Single-Cycle Simian Immunodeficiency Virus Selectively Enhances Antibody Responses and Partially Protects against Repeated,Low-Dose Vaginal Challenge

Immunization of rhesus macaques with strains of simian immunodeficiency virus (SIV) that are limited to a single cycle of infection elicits T-cell responses to multiple viral gene products and antibodies capable of neutralizing lab-adapted SIV, but not neutralization-resistant primary isolates of SIV. In an effort to improve upon the antibody responses, we immunized rhesus macaques with three strains of single-cycle SIV (scSIV) that express envelope glycoproteins modified to lack structural features thought to interfere with the development of neutralizing antibodies. These envelope-modified strains of scSIV lacked either five potential N-linked glycosylation sites in gp120, three potential N-linked glycosylation sites in gp41, or 100 amino acids in the V1V2 region of gp120. Three doses consisting of a mixture of the three envelope-modified strains of scSIV were administered on weeks 0, 6, and 12, followed by two booster inoculations with vesicular stomatitis virus (VSV) G trans-complemented scSIV on weeks 18 and 24. Although this immunization regimen did not elicit antibodies capable of detectably neutralizing SIV_mac239 or SIV_mac251_UCD, neutralizing antibody titers to the envelope-modified strains were selectively enhanced. Virus-specific antibodies and T cells were observed in the vaginal mucosa. After 20 weeks of repeated, low-dose vaginal challenge with SIV_mac251_UCD, six of eight immunized animals versus six of six naïve controls became infected. Although immunization did not significantly reduce the likelihood of acquiring immunodeficiency virus infection, statistically significant reductions in peak and set point viral loads were observed in the immunized animals relative to the naïve control animals.Development of a safe and effective vaccine for human immunodeficiency virus type 1 (HIV-1) is an urgent public health priority, but remains a formidable scientific challenge. Passive transfer experiments in macaques demonstrate neutralizing antibodies can prevent infection by laboratory-engineered simian-human immunodeficiency virus (SHIV) strains (6, 33, 34, 53, 59). However, no current vaccine approach is capable of eliciting antibodies that neutralize primary isolates with neutralization-resistant envelope glycoproteins. Virus-specific T-cell responses can be elicited by prime-boost strategies utilizing recombinant DNA and/or viral vectors (3, 10, 11, 16, 36, 73, 77, 78), which confer containment of viral loads following challenge with SHIV_89.6P (3, 13, 66, 68). Unfortunately, similar vaccine regimens are much less effective against SIV_mac239 and SIV_mac251 (12, 16, 31, 36, 73), which bear closer resemblance to most transmitted HIV-1 isolates in their inability to utilize CXCR4 as a coreceptor (18, 23, 24, 88) and inherent high degree of resistance to neutralization by antibodies or soluble CD4 (43, 55, 56). Live, attenuated SIV can provide apparent sterile protection against challenge with SIV_mac239 and SIV_mac251 or at least contain viral replication below the limit of detection (20, 22, 80). Due to the potential of the attenuated viruses themselves to cause disease in neonatal rhesus macaques (5, 7, 81) and to revert to a pathogenic phenotype through the accumulation of mutations over prolonged periods of replication in adult animals (2, 35, 76), attenuated HIV-1 is not under consideration for use in humans.As an experimental vaccine approach designed to retain many of the features of live, attenuated SIV, without the risk of reversion to a pathogenic phenotype, we and others devised genetic approaches for producing strains of SIV that are limited to a single cycle of infection (27, 28, 30, 38, 39, 45). In a previous study, immunization of rhesus macaques with single-cycle SIV (scSIV) trans-complemented with vesicular stomatitis virus (VSV) G elicited potent virus-specific T-cell responses (39), which were comparable in magnitude to T-cell responses elicited by optimized prime-boost regimens based on recombinant DNA and viral vectors (3, 16, 36, 68, 73, 78). Antibodies were elicited that neutralized lab-adapted SIV_mac251_LA (39). However, despite the presentation of the native, trimeric SIV envelope glycoprotein (Env) on the surface of infected cells and virions, none of the scSIV-immunized macaques developed antibody responses that neutralized SIV_mac239 (39). Therefore, we have now introduced Env modifications into scSIV that facilitate the development of neutralizing antibodies.Most primate lentiviral envelope glycoproteins are inherently resistant to neutralizing antibodies due to structural and thermodynamic properties that have evolved to enable persistent replication in the face of vigorous antibody responses (17, 46, 47, 64, 71, 75, 79, 83, 85). Among these, extensive N-linked glycosylation renders much of the Env surface inaccessible to antibodies (17, 48, 60, 63, 75). Removal of N-linked glycans from gp120 or gp41 by mutagenesis facilitates the induction of antibodies to epitopes that are occluded by these carbohydrates in the wild-type virus (64, 85). Consequently, antibodies from animals infected with glycan-deficient strains neutralize these strains better than antibodies from animals infected with the fully glycosylated SIV_mac239 parental strain (64, 85). Most importantly with regard to immunogen design, animals infected with the glycan-deficient strains developed higher neutralizing antibody titers against wild-type SIV_mac239 (64, 85). Additionally, the removal of a single N-linked glycan in gp120 enhanced the induction of neutralizing antibodies against SHIV_89.6P and SHIV_SF162 in a prime-boost strategy by 20-fold (50). These observations suggest that potential neutralization determinants accessible in the wild-type Env are poorly immunogenic unless specific N-linked glycans in gp120 and gp41 are eliminated by mutagenesis.The variable loop regions 1 and 2 (V1V2) of HIV-1 and SIV gp120 may also interfere with the development of neutralizing antibodies. Deletion of V1V2 from HIV-1 gp120 permitted neutralizing monoclonal antibodies to CD4-inducible epitopes to bind to gp120 in the absence of CD4, suggesting that V1V2 occludes potential neutralization determinants prior to the engagement of CD4 (82). A deletion in V2 of HIV-1 Env-exposed epitopes was conserved between clades (69), improved the ability of a secreted Env trimer to elicit neutralizing antibodies (9), and was present in a vaccine that conferred complete protection against SHIV_SF162P4 (8). A deletion of 100 amino acids in V1V2 of SIV_mac239 rendered the virus sensitive to monoclonal antibodies with various specificities (41). Furthermore, three of five macaques experimentally infected with SIV_mac239 with V1V2 deleted resisted superinfection with wild-type SIV_mac239 (51). Thus, occlusion of potential neutralization determinants by the V1V2 loop structure may contribute to the poor immunogenicity of the wild-type envelope glycoprotein.Here we tested the hypothesis that antibody responses to scSIV could be improved by immunizing macaques with strains of scSIV engineered to eliminate structural features that interfere with the development of neutralizing antibodies. Antibodies to Env-modified strains were selectively enhanced, but these did not neutralize the wild-type SIV strains. We then tested the hypothesis that immunization might prevent infection in a repeated, low-dose vaginal challenge model of heterosexual HIV-1 transmission. Indeed, while all six naïve control animals became infected, two of eight immunized animals remained uninfected after 20 weeks of repeated vaginal challenge. Relative to the naïve control group, reductions in peak and set point viral loads were statistically significant in the immunized animals that became infected.     

Ten Years of Global Evolution of the Human Respiratory Syncytial Virus BA Genotype with a 60-Nucleotide Duplication in the G Protein Gene

The emergence of natural isolates of human respiratory syncytial virus group B (HRSV-B) with a 60-nucleotide (nt) duplication in the G protein gene in Buenos Aires, Argentina, in 1999 (A. Trento et al., J. Gen. Virol. 84:3115-3120, 2003) and their dissemination worldwide allowed us to use the duplicated segment as a natural tag to examine in detail the evolution of HRSV during propagation in its natural host. Viruses with the duplicated segment were all clustered in a new genotype, named BA (A. Trento et al., J. Virol. 80:975-984, 2006). To obtain information about the prevalence of these viruses in Spain, we tested for the presence of the duplicated segment in positive HRSV-B clinical samples collected at the Severo Ochoa Hospital (Madrid) during 12 consecutive epidemics (1996-1997 to 2007-2008). Viruses with the 60-nt duplication were found in 61 samples, with a high prevalence relative to the rest of B genotypes in the most recent seasons. Global phylogenetic and demographic analysis of all G sequences containing the duplication, collected across five continents up until April 2009, revealed that the prevalence of the BA genotype increased gradually until 2004-2005, despite its rapid dissemination worldwide. After that date and coinciding with a bottleneck effect on the population size, a relatively new BA lineage (BA-IV) replaced all other group B viruses, suggesting further adaptation of the BA genotype to its natural host.Human respiratory syncytial virus (HRSV), a member of the Pneumovirus genus within the Paramyxoviridae family, is recognized as the leading agent responsible for severe respiratory infections in the pediatric population (31, 34, 35) and a pathogen of considerable importance in vulnerable adults (23, 24). The global respiratory syncytial virus (RSV) disease burden is estimated at 64 million cases and 160,000 deaths every year (70). This virus causes regular seasonal epidemics which take place during the winter months in temperate countries or during the rainy season in tropical areas (12). A peculiar aspect of HRSV is that the immune response produced by infection does not confer long-lasting protection, which is why reinfections are common throughout life (30).Neutralization tests performed with hyperimmune serum (16) and reactivity with specific monoclonal antibodies (4, 45) were used to classify HRSV isolates into two antigenic groups, A and B, which correlated with genetically distinct viruses (18). The main differences between these two groups are located in the major attachment G protein. This protein is a type II glycoprotein that shares neither sequence nor structural features with the attachment proteins (HN or H) of other paramyxoviruses (69), and it represents one of the targets of the immune response (27, 43). The full-length membrane-bound G protein (Gm) of 292 to 319 amino acids (depending on the viral strain) is also expressed in a secreted version (Gs) that lacks the transmembrane domain due to alternative initiation of translation at a second in-frame AUG codon in the G open reading frame (M48) (52). The G protein is the viral gene product with the highest degree of antigenic and genetic diversity among viral isolates (4, 18, 28, 45). Most changes are concentrated in two hypervariable regions that flank a highly conserved central region of the G protein ectodomain, which includes a cluster of four cysteines and the putative receptor binding site (43). It has been suggested that antigenic differences within this protein could facilitate repeated HRSV infections (37, 59). In addition, positive selection of amino acid changes was observed in the two hypervariable regions of the G protein ectodomain (7, 43, 71, 73, 74). One of the hypervariable regions, located in the C-terminal one-third of the G molecule, contains multiple epitopes recognized by monoclonal antibodies (43), suggesting that immune selection of new variants by antibodies may contribute to generation of HRSV diversity.Phylogenetic studies based on sequence analysis of the G protein have identified numerous genotypes in the antigenic groups A and B that show complex circulation patterns, since multiple genotypes of both antigenic groups may circulate within the same season and community, with one or two dominant genotypes being replaced in successive years (13, 14, 26, 27, 32, 49, 50). Each community shows a seasonal circulation pattern of genotypes, probably determined by local factors, such as the level of herd immunity to certain strains (3, 14, 49).The capacity of the G protein to accommodate drastic sequence changes was illustrated best by three antigenic group B viruses isolated in Buenos Aires, Argentina, in 1999 that contained a duplication of 60 nucleotides (nt) in the C-terminal third of the G protein gene (63). The global dissemination of these viruses allowed us to use the duplicated segment as a natural tag to reexamine the evolution of HRSV during propagation in its natural host. Phylogenetic analysis of G sequences revealed that all viruses with the duplicated segment clustered in a new genotype, named BA, and this finding supported the idea of a common ancestor for all viruses with the 60-nt duplication, dated about 1998 (64). The limited information about the molecular epidemiology of HRSV in Spain, together with an increase in G sequences with the duplicated segment reported worldwide, prompted us to conduct both a local search in Madrid for these viruses and a global phylogenetic analysis of HRSV with the 60-nt duplication from the time that these viruses were first detected, taking into account the geographic and temporal distribution of each isolate.     

Bocavirus Infection Induces Mitochondrion-Mediated Apoptosis and Cell Cycle Arrest at G2/M Phase

Bocavirus is a newly classified genus of the family Parvovirinae. Infection with Bocavirus minute virus of canines (MVC) produces a strong cytopathic effect in permissive Walter Reed/3873D (WRD) canine cells. We have systematically characterized the MVC infection-produced cytopathic effect in WRD cells, namely, the cell death and cell cycle arrest, and carefully examined how MVC infection induces the cytopathic effect. We found that MVC infection induces an apoptotic cell death characterized by Bax translocalization to the mitochondrial outer membrane, disruption of the mitochondrial outer membrane potential, and caspase activation. Moreover, we observed that the activation of caspases occurred only when the MVC genome was replicating, suggesting that replication of the MVC genome induces apoptosis. MVC infection also induced a gradual cell cycle arrest from the S phase in early infection to the G₂/M phase at a later stage, which was confirmed by the upregulation of cyclin B1 and phosphorylation of cdc2. Cell cycle arrest at the G₂/M phase was reproduced by transfection of a nonreplicative NS1 knockout mutant of the MVC infectious clone, as well as by inoculation of UV-irradiated MVC. In contrast with other parvoviruses, only expression of the MVC proteins by transfection did not induce apoptosis or cell cycle arrest. Taken together, our results demonstrate that MVC infection induces a mitochondrion-mediated apoptosis that is dependent on the replication of the viral genome, and the MVC genome per se is able to arrest the cell cycle at the G₂/M phase. Our results may shed light on the molecular pathogenesis of Bocavirus infection in general.The Bocavirus genus is newly classified within the subfamily Parvovirinae of the family Parvoviridae (21). The currently known members of the Bocavirus genus include bovine parvovirus type 1 (BPV1) (17), minute virus of canines (MVC) (57), and the recently identified human bocaviruses (HBoV, HBoV2, and HBoV3) (4, 7, 36).MVC was first recovered from canine fecal samples in 1970 (10). The virus causes respiratory disease with breathing difficulty (14, 32, 49) and enteritis with severe diarrhea (11, 39), which often occurs with coinfection with other viruses (39), spontaneous abortion of fetuses, and death of newborn puppies (14, 29). Pathological lesions in fetuses in experimental infections were found in the lymphoid tissue of the lung and small intestine (14). MVC was isolated and grown in the Walter Reed/3873D (WRD) canine cell line (10), which is derived from a subdermoid cyst of an irradiated male dog (10). The full-length 5.4-kb genome of MVC was recently mapped with palindromic termini (60). Under the control of a single P6 promoter, through the mechanism of alternative splicing and alternative polyadenylation, MVC expresses two nonstructural proteins (NS1 and NP1) and two capsid proteins (VP1 and VP2). Like the NS1 proteins of other parvoviruses, the NS1 of MVC is indispensable for genome replication. The NP1 protein, which is unique to the Bocavirus genus, appears to be critical for optimal viral replication, as the NP1 knockout mutant of MVC suffers from severe impairment of replication (60). A severe cytopathic effect during MVC infection of WRD cells has been documented (10, 60).The HBoV genome has been frequently detected worldwide in respiratory specimens from children under 2 years old with acute respiratory illnesses (2, 34, 55). HBoV is associated with acute expiratory wheezing and pneumonia (3, 34, 55) and is commonly detected in association with other respiratory viruses (34, 55). Further studies are necessary, however, to identify potential associations of HBoV infection with clinical symptoms or disease of acute gastroenteritis (7, 36). The full-length sequence of infectious MVC DNA (GenBank accession no. {"type":"entrez-nucleotide","attrs":{"text":"FJ214110","term_id":"219665308","term_text":"FJ214110"}}FJ214110) that we have reported shows 52.6% identity to HBoV, while the NS1, NP1, and VP1 proteins are 38.5%, 39.9%, and 43.7% identical to those of HBoV, respectively (60).The cytopathic effect induced during parvovirus infection has been widely documented, e.g., in infections with minute virus of mice (MVM) (13), human parvovirus B19 (B19V) (58), parvovirus H-1 (25, 52), and BPV1 (1). In Bocavirus, cell death during BPV1 infection of embryonic bovine tracheal cells has been shown to be achieved through necrosis, independent of apoptosis (1). B19V-induced cell death of primary erythroid progenitor cells has been shown to be mainly mediated by an apoptotic pathway (58) in which the nonstructural protein 11kDa plays a key role (16). In contrast, the MVM-induced cytopathic effect has been revealed to be mediated by NS1 interference with intracellular casein kinase II (CKII) signaling (22, 44, 45), a nonapoptotic cell death. Oncolytic parvovirus H-1 infections can induce either apoptosis or nonapoptotic cell death, depending on the cell type (25, 40). Therefore, the mechanisms underlying parvovirus infection-induced cell death vary, although NS1 has been widely shown to be involved in both apoptotic and nonapoptotic cell death. The nature of the cytopathic effect during Bocavirus MVC infection has not been studied.Parvovirus replication requires infected cells at the S phase. Infection with parvovirus has been revealed to accompany a cell cycle perturbation that mostly leads to an arrest in the S/G₂ phase or the G₂/M phase during infection (30, 33, 42, 47, 65). MVM NS1 expression induces an accumulation of sensitive cells in the S/G₂ phase (6, 46, 47). Whether MVC infection-induced cell death is accompanied by an alternation of cell cycle progression and whether the viral nonstructural protein is involved in these processes have not been addressed.In this study, we found, in contrast with other members of the family Parvoviridae, expression of both the nonstructural and structural proteins of MVC by transfection did not induce cell death or cell cycle arrest. However, the cytopathic effect induced during MVC infection is a replication-coupled, mitochondrion-mediated and caspase-dependent apoptosis, accompanied with a gradual cell cycle arrest from the S phase to the G₂/M phase, which is facilitated by the MVC genome.     

gp340 Promotes Transcytosis of Human Immunodeficiency Virus Type 1 in Genital Tract-Derived Cell Lines and Primary Endocervical Tissue

The human scavenger receptor gp340 has been identified as a binding protein for the human immunodeficiency virus type 1 (HIV-1) envelope that is expressed on the cell surface of female genital tract epithelial cells. This interaction allows such epithelial cells to efficiently transmit infective virus to susceptible targets and maintain viral infectivity for several days. Within the context of vaginal transmission, HIV must first traverse a normally protective mucosa containing a cell barrier to reach the underlying T cells and dendritic cells, which propagate and spread the infection. The mechanism by which HIV-1 can bypass an otherwise healthy cellular barrier remains an important area of study. Here, we demonstrate that genital tract-derived cell lines and primary human endocervical tissue can support direct transcytosis of cell-free virus from the apical to basolateral surfaces. Further, this transport of virus can be blocked through the addition of antibodies or peptides that directly block the interaction of gp340 with the HIV-1 envelope, if added prior to viral pulsing on the apical side of the cell or tissue barrier. Our data support a role for the previously described heparan sulfate moieties in mediating this transcytosis but add gp340 as an important facilitator of HIV-1 transcytosis across genital tract tissue. This study demonstrates that HIV-1 actively traverses the protective barriers of the human genital tract and presents a second mechanism whereby gp340 can promote heterosexual transmission.Through correlative studies with macaques challenged with simian immunodeficiency virus (SIV), the initial targets of infection in nontraumatic vaginal exposure to human immunodeficiency virus type 1 (HIV-1) have been identified as subepithelial T cells and dendritic cells (DCs) (18, 23, 31, 36-38). While human transmission may differ from macaque transmission, the existing models of human transmission remain controversial. For the virus to successfully reach its CD4⁺ targets, HIV must first traverse the columnar mucosal epithelial cell barrier of the endocervix or uterus or the stratified squamous barrier of the vagina or ectocervix, whose normal functions include protection of underlying tissue from pathogens. This portion of the human innate immune defense system represents a significant impediment to transmission. Studies have placed the natural transmission rate of HIV per sexual act between 0.005 and 0.3% (17, 45). Breaks in the epithelial barrier caused by secondary infection with other sexual transmitted diseases or the normal physical trauma often associated with vaginal intercourse represent one potential means for viral exposure to submucosal cells and have been shown to significantly increase transmission (reviewed in reference 11). However, studies of nontraumatic exposure to SIV in macaques demonstrate that these disruptions are not necessary for successful transmission to healthy females. This disparity indicates that multiple mechanisms by which HIV-1 can pass through mucosal epithelium might exist in vivo. Identifying these mechanisms represents an important obstacle to understanding and ultimately preventing HIV transmission.Several host cellular receptors, including DC-specific intercellular adhesion molecule-grabbing integrin, galactosyl ceramide, mannose receptor, langerin, heparan sulfate proteoglycans (HSPGs), and chondroitin sulfate proteoglycans, have been identified that facilitate disease progression through binding of HIV virions without being required for fusion and infection (2, 3, 12, 14, 16, 25, 29, 30, 43, 46, 50). These host accessory proteins act predominately through glycosylation-based interactions between HIV envelope (Env) and the host cellular receptors. These different host accessory factors can lead to increased infectivity in cis and trans or can serve to concentrate and expose virus at sites relevant to furthering its spread within the body. The direct transcytosis of cell-free virus through primary genital epithelial cells and the human endometrial carcinoma cell line HEC1A has been described (7, 9); this is, in part, mediated by HSPGs (7). Within the HSPG family, the syndecans have been previously shown to facilitate trans infection of HIV in vitro through binding of a specific region of Env that is moderately conserved (7, 8). This report also demonstrates that while HSPGs mediate a portion of the viral transcytosis that occurs in these two cell types, a significant portion of the observed transport occurs through an HSPG-independent mechanism. Other host cell factors likely provide alternatives to HSPGs for HIV-1 to use in subverting the mucosal epithelial barrier.gp340 is a member of the scavenger receptor cysteine-rich (SRCR) family of innate immune receptors. Its numerous splice variants can be found as a secreted component of human saliva (34, 41, 42) and as a membrane-associated receptor in a large number of epithelial cell lineages (22, 32, 40). Its normal cellular function includes immune surveillance of bacteria (4-6, 44), interaction with influenza A virus (19, 20, 32, 51) and surfactant proteins in the lung (20, 22, 33), and facilitating epithelial cell regeneration at sites of cellular inflammation and damage (27, 32). The secreted form of gp340, salivary agglutinin (SAG), was identified as a component of saliva that inhibits HIV-1 transmission in the oral pharynx through a specific interaction with the viral envelope protein that serves to agglutinate the virus and target it for degradation (34, 35, 41). Interestingly, SAG was demonstrated to form a direct protein-protein interaction with HIV Env (53, 54). Later, a cell surface-associated variant of SAG called gp340 was characterized as a binding partner for HIV-1 in the female genital tract that could facilitate virus transmission to susceptible targets of infection (47) and as a macrophage-expressed enhancer of infection (10).