Broad Neutralization of Human Immunodeficiency Virus Type 1 (HIV-1) Elicited from Human Rhinoviruses That Display the HIV-1 gp41 ELDKWA Epitope

In efforts to develop AIDS vaccine components, we generated combinatorial libraries of recombinant human rhinoviruses that display the well-conserved ELDKWA epitope of the membrane-proximal external region of human immunodeficiency virus type 1 (HIV-1) gp41. The broadly neutralizing human monoclonal antibody 2F5 was used to select for viruses whose ELDKWA conformations resemble those of HIV. Immunization of guinea pigs with different chimeras, some boosted with ELDKWA-based peptides, elicited antibodies capable of neutralizing HIV-1 pseudoviruses of diverse subtypes and coreceptor usages. These recombinant immunogens are the first reported that elicit broad, albeit modest, neutralization of HIV-1 using an ELDKWA-based epitope and are among the few reported that elicit broad neutralization directed against any recombinant HIV epitope, providing a critical advance in developing effective AIDS vaccine components.The development of an AIDS vaccine is an ongoing and urgent challenge. One of the major hurdles is that the specific correlates of protection against human immunodeficiency virus (HIV) are still largely unknown. Nonetheless, most agree that the full complement of cellular and humoral components of the immune system will be needed to combat this virus. This is especially true given that the virus resides permanently in its host, infects the very cells needed to direct effective immune responses, and evades the immune system, either by changing in appearance or hiding in subcellular compartments.A broadly reactive neutralizing antibody response is likely to be critical as a first line of defense upon initial HIV exposure by aiding in the clearance of cell-free virions, targeting infected cells for destruction, and preventing viral spread through cell-to-cell transmission. The presence of inhibitory antibodies in highly exposed persistently seronegative individuals testifies to the importance of the humoral response (9, 37). Additionally, broadly neutralizing serum has been associated with healthier prognoses for infected individuals (27, 65) and may be vital for protecting offspring from their infected mothers (7, 79) and preventing superinfection by heterologous HIV strains (23, 84). Even if complete protection cannot be achieved by vaccine-derived antibodies, an early, well-poised and effective neutralizing antibody repertoire may be able to lower the set point of the viral load following the initial burst of viremia, an outcome that has been reported to translate into improved disease outcomes and reduced transmission of HIV (66, 74). Further benefits of neutralizing antibodies have been seen with passive immunization studies in macaques, in which administration of broadly neutralizing monoclonal antibodies (MAbs) has demonstrated that it is possible to provide protection from—and even sterilizing immunity against—HIV infection (5, 51, 66). There is also evidence that such antibodies may provide therapeutic benefits for chronically infected individuals, analogous to benefits realized with anti-HIV drug treatment regimens (87).Despite the promising potential of broadly neutralizing MAbs, designing immunogens that can elicit such cross-reactive neutralizing responses against HIV has been a surprisingly difficult task. Since the majority of the host''s B-cell response is directed against the envelope (Env) glycoproteins, gp120 and gp41, vaccine efforts have concentrated on these proteins and derivatives thereof in approaches ranging from the use of Env-based peptide cocktails to recombinant proteins and DNAs made with varied or consensus sequences and diverse, heterologous prime/protein boost regimens (reviewed in references 36, 58, and 70). These iterative studies have shown notable improvements in the potency and breadth of neutralizing responses induced. However, concerns exist regarding immunogens containing extraneous epitopes, as is the case with intact subunits of Env, and the nature of the immune responses they may elicit. A polyclonal burst of antibodies against a multitude of nonfunctional epitopes may include a predominance of antibodies that are (i) low affinity and/or nonfunctional (reviewed in reference 72); (ii) isolate specific (25); (iii) able to interfere with the neutralizing capabilities of otherwise-effective antibodies (via steric hindrance or by inducing various forms of B-cell pathology) (67); or (iv) directed against irrelevant epitopes instead of more conserved (and sometimes concealed) epitopes that might be able to elicit more potent and cross-reactive neutralizing responses (28, 71, 91).We have developed a system that can be used to present essentially any chosen epitope in a stable, well-exposed manner on the surface of the cold-causing human rhinovirus (HRV). HRV is itself a powerful immunogen and is able to elicit T-cell as well as serum and mucosal B-cell responses (reviewed by Couch 22]) and has minimal immunologic similarity to HIV (data not shown). Chimeric viruses displaying optimal epitopes should be able to serve as valuable components in an effective vaccine cocktail or as part of a heterologous prime/boost protocol. We have shown previously that HRV chimeric viruses displaying HIV-1 gp120 V3 loop sequences are able to elicit neutralizing responses against HIV-1 (75, 82, 83).In this study, we focused our attention on presenting part of the membrane-proximal external region (MPER) of the transmembrane glycoprotein gp41, a region of approximately 30 amino acids adjacent to the transmembrane domain (reviewed in references 59 and 97). The MPER plays an important role in the process of HIV fusion to the host cell membrane (60, 78). This region is also involved in binding to galactosylceramide, an important component of cell membranes, thus permitting CD4-independent transcytosis of the virus across epithelial cells at mucosal surfaces (1, 2). These functions likely explain this region''s sequence conservation and the efficacy of antibodies directed against the MPER (97), particularly given that an estimated 80% of HIV-1 infections are sexually transmitted at mucosal membranes. In fact, potent responses against the MPER are associated with stronger and broader neutralizing capabilities in infected individuals (68). A conserved, contiguous sequence of the MPER, the ELDKWA epitope (HIV-1 HxB2 gp41 residues 662 to 668), is recognized by the particularly broadly neutralizing human MAb 2F5 (11, 62, 85) and is highly resistant to escape mutation in the presence of 2F5 (49). 2F5 was also used in the MAb cocktails reported to confer passive, protective immunity in macaques (5, 51). In addition, infected individuals producing neutralizing antibodies directed against the ELDKWA epitope have been seen to exhibit better health (16, 29), including persistent seronegativity (8), and reduced transmission of HIV to offspring (89). While none of the vaccine-induced immune responses generated against this region has been effective thus far (19, 24, 26, 33, 35, 38, 40, 42, 44-48, 50, 53, 54, 56, 57, 61, 63, 69, 93, 96) (see Table S1 in the supplemental material), more appropriate presentations of MPER epitopes should produce valuable immunogens that can contribute to a successful vaccine.In this study, we have grafted the ELDKWA epitope onto a surface loop of HRV connected via linkers of variable lengths and sequences and selected for viruses well recognized and neutralized by MAb 2F5. In so doing, we have been able to create immunogens capable of eliciting antibodies whose activities mimic some of those of 2F5. The combinatorial libraries produced were designed to encode a large set of possible sequences and, hence, structures from which we could search for valuable conformations. This work illustrates that HRV chimeras have the potential to present selected HIV epitopes in a focused and immunogenic manner.