Canine adenovirus type 2 attachment and internalization: coxsackievirus-adenovirus receptor, alternative receptors, and an RGD-independent pathway

The best-characterized receptors for adenoviruses (Ads) are the coxsackievirus-Ad receptor (CAR) and integrins alpha(v)beta(5) and alpha(v)beta(3), which facilitate entry. The alpha(v) integrins recognize an Arg-Gly-Asp (RGD) motif found in some extracellular matrix proteins and in the penton base in most human Ads. Using a canine adenovirus type 2 (CAV-2) vector, we found that CHO cells that express CAR but not wild-type CHO cells are susceptible to CAV-2 transduction. Cells expressing alpha(M)beta(2) integrins or major histocompatibility complex class I (MHC-I) molecules but which do not express CAR were not transduced. Binding assays showed that CAV-2 attaches to a recombinant soluble form of CAR and that Ad type 5 (Ad5) fiber, penton base, and an anti-CAR antibody partially blocked attachment. Using fluorescently labeled CAV-2 particles, we found that in some cells nonpermissive for transduction, inhibition was at the point of internalization and not attachment. The transduction efficiency of CAV-2, which lacks an RGD motif, surprisingly mimicked that of Ad5 when tested in cells selectively expressing alpha(v)beta(5) and alpha(v)beta(3) integrins. Our results demonstrate that CAV-2 transduction is augmented by CAR and possibly by alpha(v)beta(5), though transduction can be CAR and alpha(v)beta(3/5) independent but is alpha(M)beta(2), MHC-I, and RGD independent, demonstrating a transduction mechanism which is distinct from that of Ad2/5.     

Crystal structure of species D adenovirus fiber knobs and their sialic acid binding sites

Adenovirus serotype 37 (Ad37) belongs to species D and can cause epidemic keratoconjunctivitis, whereas the closely related Ad19p does not. Primary cell attachment by adenoviruses is mediated through receptor binding of the knob domain of the fiber protein. The knobs of Ad37 and Ad19p differ at only two positions, Lys240Glu and Asn340Asp. We report the high-resolution crystal structures of the Ad37 and Ad19p knobs, both native and in complex with sialic acid, which has been proposed as a receptor for Ad37. Overall, the Ad37 and Ad19p knobs are very similar to previously reported knob structures, especially to that of Ad5, which binds the coxsackievirus-adenovirus receptor (CAR). Ad37 and Ad19p knobs are structurally identical with the exception of the changed side chains and are structurally most similar to CAR-binding knobs (e.g., that of Ad5) rather than non-CAR-binding knobs (e.g., that of Ad3). The two mutations in Ad19p result in a partial loss of the exceptionally high positive surface charge of the Ad37 knob but do not affect sialic acid binding. This site is located on the top of the trimer and binds both alpha(2,3) and alpha(2,6)-linked sialyl-lactose, although only the sialic acid residue makes direct contact. Amino acid alignment suggests that the sialic acid binding site is conserved in several species D serotypes. Our results show that the altered viral tropism and cell binding of Ad19p relative to those of Ad37 are not explained by a different binding ability toward sialyl-lactose.     

Adenovirus Serotype 30 Fiber Does Not Mediate Transduction via the Coxsackie-Adenovirus Receptor

Prior work by members of our laboratory and others demonstrated that adenovirus serotype 30 (Ad30), a group D adenovirus, exhibited novel transduction characteristics compared to those of serotype 5 (Ad5, belonging to group C). While some serotype D adenoviruses bind to the coxsackie-adenovirus receptor (CAR), the ability of Ad30 fiber to bind CAR is unknown. We amplified and purified Ad30 and cloned the Ad30 fiber by overlap PCR. Alignment of Ad30 fiber with Ad3, Ad35, Ad5, Ad9, and Ad17 revealed that Ad30, like Ad9 and Ad17, has a shortened fiber sequence relative to that of Ad5. The knob region of fiber was 45% identical to that of the Ad5 knob regions. We made a chimeric recombinant virus (Ad5GFPf30) in which the Ad5 fiber (amino acids [aa]47 to 582) was replaced with Ad30 fiber sequences (aa 46 to 372), and CAR-mediated viral entry was determined on CAR-expressing Chinese hamster ovary (CHO) cells. While CAR expression significantly increased Ad5GFP-mediated transduction in CHO cells (from 1 to 36%), it did not enhance Ad5GFPf30 gene transfer. Binding of radiolabeled Ad5GFPf30 or Ad30 wild-type virus was also not improved by the expression of CAR. These results suggest that Ad30 fiber is distinct from Ad5, Ad9, and Ad17 fibers in its inability to direct transduction via CAR.     

Heparan sulfate glycosaminoglycans are receptors sufficient to mediate the initial binding of adenovirus types 2 and 5

Cell infection by adenovirus serotypes 2 and 5 (Ad2/5) initiates with the attachment of Ad fiber to the coxsackievirus and Ad receptor (CAR) followed by alpha(v) integrin-mediated entry. We recently demonstrated that heparan sulfate glycosaminoglycans (HS GAGs) expressed on cell surfaces are involved in the binding and infection of Ad2/5 (M. C. Dechecchi, A. Tamanini, A. Bonizzato, and G. Cabrini, Virology 268:382-390, 2000). The role of HS GAGs was investigated using extracellular soluble domain 1 of CAR (sCAR-D1) and heparin as soluble receptor analogues of CAR and HS GAGs in A549 and recombinant CHO cell lines with differential levels of expression of the two receptors and cultured to various densities. Complete inhibition of binding and infection was obtained by preincubating Ad2/5 with both heparin (10 microg/ml) and sCAR-D1 (200 microg/ml) in A549 cells. Partial inhibition was observed when heparin and sCAR-D1 were preincubated separately with Ad. The level of heparin-sensitive [(3)H]Ad2/5 binding doubled in sparse A549 cells (50 to 70,000 cells/cm(2)) with respect to that of cells grown to confluence (200 to 300,000 cells/cm(2)), in parallel with increased expression of HS GAGs. [(3)H]Ad2 bound to sparse CAR-negative CHO cells expressing HS GAGs (CHO K1). No [(3)H]Ad2 binding was observed in CHO K1 cells upon competitive inhibition with heparin and in HS GAG-defective CHO A745, D677, and E606 clones. HS-sensitive Ad2 infection was obtained in CAR-negative sparse CHO K1 cells but not in CHO A745 cells, which were permissive to infection only upon transfection with CAR. These results demonstrate that HS GAGs are sufficient to mediate the initial binding of Ad2/5.     

Infection of the tracheal epithelium by infectious bronchitis virus is sialic acid dependent

Avian Infectious bronchitis virus (IBV) is a coronavirus that infects chickens via the respiratory epithelium as primary target cells. The binding of coronaviruses to the cell surface is mediated by the viral surface protein S. Recently we demonstrated that alpha2,3-linked sialic acid serves as a receptor determinant for IBV on Vero cells and primary chicken embryo kidney cells. Here we analyze the importance of the sialic acid binding activity for the infection of tracheal organ cultures (TOCs) by different IBV strains. Our results show that alpha2,3-linked sialic acid also serves as a receptor determinant on chicken TOCs. Infection of TOCs by IBV results in ciliostasis. Desialylation induced by neuraminidase treatment of tracheal organ cultures prior to infection by IBV delayed the ciliostatic effect or resulted in partial loss of ciliary activity. This effect was observed with both respiratory and nephropathogenic strains. Inhibition of ciliostasis was also observed when TOCs were pretreated with an alpha2,3-specific neuraminidase. Analysis of the tracheal epithelium for reactivity with lectins revealed that the susceptible cells in the epithelium abundantly express alpha2,3-linked sialic acid. These results indicate that alpha2,3-linked sialic acid plays an important role for infection of the respiratory epithelium by IBV.     

Adenovirus type 11 uses CD46 as a cellular receptor

The 51 human adenovirus serotypes are divided into six species (A to F). Many adenoviruses use the coxsackie-adenovirus receptor (CAR) for attachment to host cells in vitro. Species B adenoviruses do not compete with CAR-binding serotypes for binding to host cells, and it has been suggested that species B adenoviruses use a receptor other than CAR. Species B adenoviruses mainly cause disease in the respiratory tract, the eyes, and in the urinary tract. Here we demonstrate that adenovirus type 11 (Ad11; of species B) binds to Chinese hamster ovary (CHO) cells transfected with CD46 (membrane cofactor protein)-cDNA at least 10 times more strongly than to CHO cells transfected with cDNAs encoding CAR or CD55 (decay accelerating factor). Nonpermissive CHO cells were rendered permissive to Ad11 infection upon transfection with CD46-cDNA. Soluble Ad11 fiber knob but not Ad7 or Ad5 knob inhibited binding of Ad11 virions to CD46-transfected cells, and anti-CD46 antibodies inhibited both binding of and infection by Ad11. From these results we conclude that CD46 is a cellular receptor for Ad11.     

Initial interactions of subgenus D adenoviruses with A549 cellular receptors: sialic acid versus alpha(v) integrins

Selected members of the adenovirus family have been shown to interact with the coxsackie adenovirus receptor, alpha(v) integrins, and sialic acid on target cells. Initial interactions of subgenus D adenoviruses with target cells have until now been poorly characterized. Here, we demonstrate that adenovirus type 8 (Ad8), Ad19a, and Ad37 use sialic acid as a functional cellular receptor, whereas the Ad9 and Ad19 prototypes do not.     

Reovirus sigma1 fiber incorporated into adenovirus serotype 5 enhances infectivity via a CAR-independent pathway

Adenovirus serotype 5 (Ad5) has been used for gene therapy with limited success because of insufficient infectivity in cells with low expression of the primary receptor, the coxsackie and adenovirus receptor (CAR). To enhance infectivity in tissues with low CAR expression, tropism expansion is required via non-CAR pathways. Serotype 3 Dearing reovirus utilizes a fiber-like sigma1 protein to infect cells expressing sialic acid and junction adhesion molecule 1 (JAM1). We hypothesized that replacement of the Ad5 fiber with sigma1 would result in an Ad5 vector with CAR-independent tropism. We therefore constructed a fiber mosaic Ad5 vector, designated as Ad5-sigma1, encoding two fibers: the sigma1 and the wild-type Ad5 fiber. Functionally, Ad5-sigma1 utilized CAR, sialic acid, and JAM1 for cell transduction and achieved maximum infectivity enhancement in cells with or without CAR. Thus, we have developed a new type of Ad5 vector with expanded tropism, possessing fibers from Ad5 and reovirus, that exhibits enhanced infectivity via CAR-independent pathway(s).     

Mouse adenovirus type 1 attachment is not mediated by the coxsackie-adenovirus receptor

Common human adenovirus (Ad) vectors are derived from serotype 2 or 5, which use the coxsackie-adenovirus receptor (CAR) as their primary cell receptor. We investigated the receptor usage of mouse adenovirus type 1 (MAV-1), which in vivo is characterized by a pronounced endothelial cell tropism. Alignment of the fiber knob sequences of MAV-1 and those of CAR-using adenoviruses, revealed that amino acid residues, critical for interaction with CAR, are not conserved in the MAV-1 fiber knob. Attachment of MAV-1 to Chinese hamster ovary (CHO) cells was not increased by stable transfection with mouse CAR, whereas the binding efficiency of Ad2 was 20-fold higher in the mouse CAR-transfectant compared to the wild type cells. Also, purified fiber knob of Ad5, which is interchangeable with the Ad2 fiber knob, did not compete with MAV-1 for receptor binding, indicating that MAV-1 binds to a receptor different from CAR. These results support further exploration of an MAV-1-derived vector as a potential vehicle for gene delivery to cell types which are not efficiently transduced by human adenovirus vectors.     

Adenovirus type 5 fiber knob binds to MHC class I alpha2 domain at the surface of human epithelial and B lymphoblastoid cells.

Adenovirus serotype 5 (Ad5) fiber receptor was investigated using reverse antibody biopanning of a phage-displayed hexapeptide library, and virus-neutralizing monoclonal antibodies (mAbs 1D6.3 and 7A2.7) raised against recombinant Ad5 fiber knob. Both mAbs inhibited attachment of Ad5 to HeLa cells. Mimotopes of 1D6.3 showed homology with the C-terminal segment of the alpha2 domain of the heavy chain of human MHC class I molecules (MHC-I alpha2), and mimotopes of 7A2.7 were consensus to human fibronectin type III (FNIII) modules. In vitro, GST-fused MHC-I alpha2- and FNIII-derived oligopeptides interacted with recombinant fibers in a subgroup-specific manner. In vivo, the MHC-I alpha2 synthetic icosapeptide RAIVGFRVQWLRRYFVNGSR showed a net neutralization effect on Ad5 in HeLa cells, whereas the FNIII icosapeptide RHILWTPANTPAMGYLARVS significantly increased Ad5 binding to HeLa cells. Daudi cells, which lack surface expression of HLA class I molecules, showed a weak capacity for Ad5 binding. In beta2-microglobulin-transfected Daudi cells, Ad5 attachment and permissivity were restored to HeLa cell levels, with 4000 receptors per cell and a binding constant of 1.4x10(10)/M. The results suggested that the conserved region of MHC-I alpha2-domain including Trp167 represents a high affinity receptor for Ad5 fiber knob, whereas ubiquitous FNIII modules would serve as auxiliary receptors.     

Sialic acid on herpes simplex virus type 1 envelope glycoproteins is required for efficient infection of cells

Herpes simplex virus type 1 (HSV-1) envelope proteins are posttranslationally modified by the addition of sialic acids to the termini of the glycan side chains. Although gC, gD, and gH are sialylated, it is not known whether sialic acids on these envelope proteins are functionally important. Digestion of sucrose gradient purified virions for 4 h with neuraminidases that remove both alpha2,3 and alpha2,6 linked sialic acids reduced titers by 1,000-fold. Digestion with a alpha2,3-specific neuraminidase had no effect, suggesting that alpha2,6-linked sialic acids are required for infection. Lectins specific for either alpha2,3 or alpha2,6 linkages blocked attachment and infection to the same extent. In addition, the mobility of gH, gB, and gD in sodium dodecyl sulfate-polyacrylamide gel electrophoresis gels was altered by digestion with either alpha2,3 specific neuraminidase or nonspecific neuraminidases, indicating the presence of both linkages on these proteins. The infectivity of a gC-1-null virus, DeltagC2-3, was reduced to the same extent as wild-type virus after neuraminidase digestion, and attachment was not altered. Neuraminidase digestion of virions resulted in reduced VP16 translocation to the nucleus, suggesting that the block occurred between attachment and entry. These results show for the first time that sialic acids on HSV-1 virions play an important role in infection and suggest that targeting virion sialic acids may be a valid antiviral drug development strategy.     

The Coxsackievirus-Adenovirus Receptor Protein Can Function as a Cellular Attachment Protein for Adenovirus Serotypes from Subgroups A, C, D, E, and F

Attachment of an adenovirus (Ad) to a cell is mediated by the capsid fiber protein. To date, only the cellular fiber receptor for subgroup C serotypes 2 and 5, the so-called coxsackievirus-adenovirus receptor (CAR) protein, has been identified and cloned. Previous data suggested that the fiber of the subgroup D serotype Ad9 also recognizes CAR, since Ad9 and Ad2 fiber knobs cross-blocked each other’s cellular binding. Recombinant fiber knobs and ³H-labeled Ad virions from serotypes representing all six subgroups (A to F) were used to determine whether the knobs cross-blocked the binding of virions from different subgroups. With the exception of subgroup B, all subgroup representatives cross-competed, suggesting that they use CAR as a cellular fiber receptor as well. This result was confirmed by showing that CAR, produced in a soluble recombinant form (sCAR), bound to nitrocellulose-immobilized virions from the different subgroups except subgroup B. Similar results were found for blotted fiber knob proteins. The subgroup F virus Ad41 has both short and long fibers, but only the long fiber bound sCAR. The sCAR protein blocked the attachment of all virus serotypes that bound CAR. Moreover, CHO cells expressing human CAR, in contrast to untransformed CHO cells, all specifically bound the sCAR-binding serotypes. We conclude therefore that Ad serotypes from subgroups A, C, D, E, and F all use CAR as a cellular fiber receptor.     

Flexibility of the adenovirus fiber is required for efficient receptor interaction

The adenovirus (Ad) fiber protein mediates Ad binding to the coxsackievirus and Ad receptor (CAR) and is thus a major determinant of viral tropism. The fiber contains three domains: an N-terminal tail that anchors the fiber to the viral capsid, a central shaft region of variable length and flexibility, and a C-terminal knob domain that binds to cell receptors. Ad type 37 (Ad37), a subgroup D virus associated with severe ocular infections, is unable to use CAR efficiently to infect host cells, despite containing a CAR binding site in its fiber knob. We hypothesized that the relatively short, inflexible Ad37 fiber protein restricts interactions with CAR at the cell surface. To test this hypothesis, we analyzed the infectivity and binding of recombinant Ad particles containing modified Ad37 or Ad5 fiber proteins. Ad5 particles equipped with a truncated Ad5 fiber or with a chimeric fiber protein comprised of the Ad5 knob fused to the short, rigid Ad37 shaft domain had significantly reduced infectivity and attachment. In contrast, placing the Ad37 knob onto the long, flexible Ad5 shaft allowed CAR-dependent virus infection and cell attachment, demonstrating the importance of the shaft domain in receptor usage. Increasing fiber rigidity by substituting the predicted flexibility modules in the Ad5 shaft with the corresponding regions of the rigid Ad37 fiber dramatically reduced both virus infection and cell attachment. Cryoelectron microscopy (cryo-EM) single-particle analysis demonstrated the increased rigidity of this chimeric fiber. These studies demonstrate that both length and flexibility of the fiber shaft regulate CAR interaction and provide a molecular explanation for the use of alternative receptors by subgroup D Ad with ocular tropism. We present a molecular model for Ad-CAR interactions at the cell surface that explains the significance of fiber flexibility in cell attachment.     

The sialylated oligosaccharides of recombinant bovine lutropin modulate hormone bioactivity

The Asn-linked oligosaccharides from bovine lutropin (bLH(Pit] are predominantly dibranched complex-type structures with the terminal sequence SO4-4GalNAc beta 1,4GlcNAc beta 1,2Man alpha. Recombinant bLH expressed in Chinese hamster ovary cells (bLH(CHO] bears di- (60%) and tribranched (30%) complex-type oligosaccharides; however, these terminate in the sequence Sia alpha 2,3Gal beta 1,4GlcNAc beta 1,2Man alpha. In contrast to the limited spectrum of oligosaccharide structures present on recombinant bLH(CHO), the endogenous glycoproteins synthesized by CHO cells bear a heterogeneous array of Asn-linked oligosaccharides with 0, 1, 2, 3, or 4 sialic acid moieties. The sialic acid moieties on the Asn-linked oligosaccharides of both endogenous glycoproteins and recombinant bLH(CHO) are exclusively alpha 2,3-linked, suggesting that the alpha 2,6-sialyl-transferase is not active in CHO cells. The bioactivities of bLH(Pit) and bLH(CHO) were compared using MA-10 cells following sequential digestion with neuraminidase and beta-galactosidase. Neither the ED50 (dose producing 50% of the maximum response) for progesterone production (7.2 ng/ml) nor the Pmax (maximum level of progesterone produced) (470 ng/ml) was altered for bLH(Pit) by these treatments, consistent with the absence of either sialic acid or Gal on bLH(Pit). The ED50 for progesterone production by recombinant bLH(CHO) (16.4 ng/ml) was significantly greater than for bLH(Pit) but was reduced to 5.3 ng/ml following removal of terminal sialic acid. Removal of the subterminal Gal was without further effect. The Pmax for bLH(CHO) (180 ng/ml) was not altered by these treatments. The reduction in bLH(CHO) bioactivity caused by the presence of terminal sialic acid suggests that the presence of terminal sulfate on bLH(Pit) oligosaccharides may also reduce its bioactivity and may play a modulatory role in regulating hormone bioactivity.     

The human HLA-A*0201 allele, expressed in hamster cells, is not a high-affinity receptor for adenovirus type 5 fiber

The coxsackie B virus and adenovirus receptor (CAR) and the major histocompatibility complex (MHC) class I alpha2 domain have been identified as high-affinity cell receptors for adenovirus type 5 (Ad5) fiber. In this study we show that CAR but not MHC class I allele HLA-A*0201 binds to Ad5 with high affinity when expressed on hamster cells. When both receptors are coexpressed on the cell surface of hamster cells, Ad5 fiber bind to a single high-affinity receptor, which is CAR.     

Alpha2,3 and alpha2,6 N-linked sialic acids facilitate efficient binding and transduction by adeno-associated virus types 1 and 6

Recombinant adeno-associated viruses (AAVs) are promising vectors in the field of gene therapy. Different AAV serotypes display distinct tissue tropism, believed to be related to the distribution of their receptors on target cells. Of the 11 well-characterized AAV serotypes, heparan sulfate proteoglycan and sialic acid have been suggested to be the attachment receptors for AAV type 2 and types 4 and 5, respectively. In this report, we identify the receptor for the two closely related serotypes, AAV1 and AAV6. First, we demonstrate using coinfection experiments and luciferase reporter analysis that AAV1 and AAV6 compete for similar receptors. Unlike heparin sulfate, enzymatic or genetic removal of sialic acid markedly reduced AAV1 and AAV6 binding and transduction. Further analysis using lectin staining and lectin competition assays identified that AAV1 and AAV6 use either alpha2,3-linked or alpha2,6-linked sialic acid when transducing numerous cell types (HepG2, Pro-5, and Cos-7). Treatment of cells with proteinase K but not glycolipid inhibitor reduced AAV1 and AAV6 infection, supporting the hypothesis that the sialic acid that facilitates infection is associated with glycoproteins rather than glycolipids. In addition, we determined by inhibitor (N-benzyl GalNAc)- and cell line-specific (Lec-1) studies that AAV1 and AAV6 require N-linked and not O-linked sialic acid. Furthermore, a resialylation experiment on a deficient Lec-2 cell line confirmed a 2,3 and 2,6 N-linked sialic acid requirement, while studies of mucin with O-linked sialic acid showed no inhibition effect for AAV1 and AAV6 transduction on Cos-7 cells. Finally, using a glycan array binding assay we determined that AAV1 efficiently binds to NeuAcalpha2-3GalNAcbeta1-4GlcNAc, as well as two glycoproteins with alpha2,3 and alpha2,6 N-linked sialic acids. Taken together, competition, genetic, inhibitor, enzymatic reconstitution, and glycan array experiments support alpha2,3 and alpha2,6 sialic acids that are present on N-linked glycoproteins as primary receptors for efficient AAV1 and AAV6 viral infection.     

Structural analysis of a fiber-pseudotyped adenovirus with ocular tropism suggests differential modes of cell receptor interactions

Adenovirus (Ad) entry into cells is initiated by the binding of the fiber knob to a cell surface receptor. The coxsackie- and adenovirus receptor (CAR) functions as the attachment receptor for many, but not all, Ad serotypes. Ad type 37 (Ad37), a subgroup D virus that causes keratoconjunctivitis in humans, does not infect cells via CAR despite demonstrated binding of the Ad37 knob to CAR. We have pseudotyped a fiber deletion Ad5 vector with the Ad37 fiber (Ad37f), and this vector retains the ocular tropism of Ad37. Here we present a cryo-electron microscopy reconstruction of Ad37f that shows the entire Ad37 fiber, including the shaft and knob domains. We have previously proposed that Ad37 may not utilize CAR for cell entry because of the geometric constraints imposed by a rigid fiber (E. Wu, J. Fernandez, S. K. Fleck, D. Von Seggern, S. Huang, and G. R. Nemerow, Virology 279:78-89, 2001). Consistent with this hypothesis, our structural results show that the Ad37 fiber is straight and rigid. Modeling of the interaction between Ad37f and host cell receptors indicates that fiber flexibility or rigidity, as well as length, can affect receptor usage and cellular tropism.     

Identification and characterization of adsorbed serum sialoglycans on Leishmania donovani promastigotes

Sialic acids as terminal residues of oligosaccharide chains play a crucial role in several cellular recognition events. The presence of sialic acid on promastigotes of Leishmania donovani, the causative organism of Indian visceral leishmaniasis, was demonstrated by fluorimetric high-performance liquid chromatography showing Neu5Ac and, to a minor extent, Neu5,9Ac2. The presence of Neu5Ac was confirmed by GC/MS analysis. Furthermore, binding with sialic acid-binding lectins Sambucus nigra agglutinin (SNA), Maackia amurensis agglutinin (MAA), and Siglecs showed the presence of both alpha2,3- and alpha2,6-linked sialic acids. No endogenous biosynthetic machinery for Neu5Ac could be demonstrated in the parasite. Concomitant western blotting of parasite membranes and culture medium with SNA demonstrated the presence of common sialoglyconjugates (123, 90, and 70 kDa). Similarly, binding of MAA with parasite membrane and culture medium showed three analogous sialoglycans corresponding to 130, 117, and 70 kDa, indicating that alpha2,3- and alpha2,6-linked sialoglycans are adsorbed from the fetal calf serum present in the culture medium. L. donovani promastigotes also reacted with Achatinin-H, a lectin that preferentially identifies 9-O-acetylated sialic acid in alpha2-->6 GalNAc linkage. This determinant was evidenced on parasite cell surfaces by cell agglutination, ELISA, and flow cytometry, where its binding was abolished by pretreatment of cells with a recombinant 9-O-acetylesterase derived from the HE1 region of the influenza C esterase gene. Additionally, binding of CD60b, a 9-O-acetyl GD3-specific monoclonal antibody, corroborated the presence of terminal 9-O-acetylated disialoglycans. Our results indicate that sialic acids (alpha2-->6 and alpha2-->3 linked) and 9-O-acetyl derivatives constitute components of the parasite cell surface.     

Adenovirus type 37 uses sialic acid as a cellular receptor on Chang C cells

Epidemic keratoconjunctivitis (EKC) is a severe eye infection caused mainly by adenovirus type 8 (Ad8), Ad19, and Ad37. We have shown that the EKC-causing adenoviruses use sialic acid as a cellular receptor on A549 cells instead of the coxsackie-adenovirus receptor, which is used by most adenoviruses. Recently, Wu et al. (Virology 279:78-89, 2001) proposed that Ad37 uses a 50-kDa protein as a receptor on Chang C conjunctival cells and that this interaction is independent of sialic acid. According to the American Type Culture Collection, this cell line carries HeLa cell markers and should be considered to be a genital cell line. This prompted us to investigate the function of sialic acid as a cellular receptor for Ad37 in Chang C cells. In this study, we demonstrate that enzymatic removal or lectin-mediated blocking of cell surface sialic acid inhibits the binding of Ad37 virions to Chang C cells, as does soluble, virion-interacting sialic acid-containing substances. The binding was Ca2+ or Mg2+ ion independent and mediated by the knob domain of the trimeric viral fiber polypeptide. Moreover, Ad37 virions infected Chang C cells and two other genital cell lines (HeLa and SiHa) as well as a corneal cell line in a strictly sialic acid-dependent manner. From these results, we conclude that Ad37 uses sialic acid as a major receptor in cell lines derived from both genital and corneal tissues.     

The Cell Adhesion Molecule “CAR” and Sialic Acid on Human Erythrocytes Influence Adenovirus In Vivo Biodistribution

Although it has been known for 50 years that adenoviruses (Ads) interact with erythrocytes ex vivo, the molecular and structural basis for this interaction, which has been serendipitously exploited for diagnostic tests, is unknown. In this study, we characterized the interaction between erythrocytes and unrelated Ad serotypes, human 5 (HAd5) and 37 (HAd37), and canine 2 (CAV-2). While these serotypes agglutinate human erythrocytes, they use different receptors, have different tropisms and/or infect different species. Using molecular, biochemical, structural and transgenic animal-based analyses, we found that the primary erythrocyte interaction domain for HAd37 is its sialic acid binding site, while CAV-2 binding depends on at least three factors: electrostatic interactions, sialic acid binding and, unexpectedly, binding to the coxsackievirus and adenovirus receptor (CAR) on human erythrocytes. We show that the presence of CAR on erythrocytes leads to prolonged in vivo blood half-life and significantly reduced liver infection when a CAR-tropic Ad is injected intravenously. This study provides i) a molecular and structural rationale for Ad–erythrocyte interactions, ii) a basis to improve vector-mediated gene transfer and iii) a mechanism that may explain the biodistribution and pathogenic inconsistencies found between human and animal models.