| Abstract: | A guiding principle for HIV vaccine design has been that cellular and humoral immunity work together to provide the strongest degree of efficacy. However, three efficacy trials of Ad5-vectored HIV vaccines showed no protection. Transmission was increased in two of the trials, suggesting that this vaccine strategy elicited CD4+ T-cell responses that provide more targets for infection, attenuating protection or increasing transmission. The degree to which this problem extends to other HIV vaccine candidates is not known. Here, we show that a gp120-CD4 chimeric subunit protein vaccine (full-length single chain) elicits heterologous protection against simian-human immunodeficiency virus (SHIV) or simian immunodeficiency virus (SIV) acquisition in three independent rhesus macaque repeated low-dose rectal challenge studies with SHIV162P3 or SIVmac251. Protection against acquisition was observed with multiple formulations and challenges. In each study, protection correlated with antibody-dependent cellular cytotoxicity specific for CD4-induced epitopes, provided that the concurrent antivaccine T-cell responses were minimal. Protection was lost in instances when T-cell responses were high or when the requisite antibody titers had declined. Our studies suggest that balance between a protective antibody response and antigen-specific T-cell activation is the critical element to vaccine-mediated protection against HIV. Achieving and sustaining such a balance, while enhancing antibody durability, is the major challenge for HIV vaccine development, regardless of the immunogen or vaccine formulation.There are formidable difficulties for developing a vaccine against a retrovirus such as HIV because of the integration of its genes into the DNA of the host target cells upon infection. For HIV, this problem is compounded by HIV-induced immune suppression and the development of variants that escape immune control. Consequently, an effective preventive vaccine against HIV must work early to block HIV infection and quickly kill HIV-infected cells, or both. To date, only antibodies to the HIV envelope glycoprotein (Env) fit this requirement. Available evidence suggests that such antibodies must recognize highly conserved domains and could inhibit infection by direct neutralization or by Fc receptor-dependent effector mechanisms including antibody-dependent cellular cytotoxicity (ADCC) (1, 2). The ideal result would be sterilizing immunity or, at a minimum, a major restriction of the infection (3). Another challenge stems from evolutionary pressures that abrogate the immunogenicity of conserved, functional epitopes on the envelope spike that are potential targets for cross-reactive antibodies. Large areas are masked by a “glycan shield” of carbohydrate molecules and extensive conformational flexibility (sometimes termed “conformational masking”) that dampen immunogenicity of the conserved functional domains (4, 5). The remaining immunogenic domains (“variable” or “V” loops) tolerate a high degree of sequence variability and generate “type-specific” neutralizing antibodies that are not cross-reactive and that limit the efficacy of vaccines that use conventional gp120 monomeric protein.An emerging concern for HIV vaccine development centers on the quantitative and qualitative aspects of T-cell activation elicited by various immunization regimens (6). Although HIV-specific T cells might potentially combat infection, certain patterns of T-cell activation (e.g., involving CD4+ CCR5+ T cells) have the potential to promote HIV replication. The latter possibility is emphasized by the HIV vaccine-associated increased risk of infection seen in two large human clinical trials that selectively generated HIV-specific T-cell responses (7). Similar associations between increased risk of infection and T-cell responses of various sorts have been reported in the nonhuman primate model (8–10). Thus, the ideal HIV vaccine strategy is likely to be one that generates antiviral humoral responses without incurring T-cell activation profiles that promote infection and/or overcome the protective benefits of antibodies. Insights for such an approach can be gained by comparative analyses of nonhuman primate models of HIV infection.The vaccine concept that we have been testing is designed to overcome some of these challenges by stably expressing a highly conserved transition state structure that is exposed on gp120 during a key step in viral entry, exposure of the coreceptor-binding domain consequent to CD4 binding. The prototype immunogen [full-length single chain (FLSC)] is a chimeric protein composed of gp120 from the HIV-1Ba-L isolate fused to the N terminus of the two outer domains of CD4 by a flexible polypeptide linker (11). For studies of rhesus macaques, the construct is modified to contain “self” rhesus macaque CD4 sequences (rhFLSC) to avoid anti-CD4 responses. The rhFLSC elicits antibody responses to highly conserved epitopes, including the coreceptor-binding domain epitopes (CoRBS) and the C1 regions implicated as a potent ADCC target (12). In an earlier study (12), we showed that rhesus macaques vaccinated with rhFLSC formulated with QS21, a saponin adjuvant derived from the soap-bark tree Q. saponaria, exhibited accelerated clearance of plasma viremia and an absence of long-term tissue viral loads compared with unvaccinated controls after a single high-dose rectal challenge with heterologous SHIV162P3. Postinfection control correlated with stronger responses to CD4i epitopes in the rhFLSC-vaccinated animals (CD4i titers > 1:100), compared with macaques that received control immunogens including gp120, soluble CD4, or chemically cross-linked gp120-CD4. Postinfection control did not correlate with anti-CD4 responses, overall anti-gp120–binding titers, or neutralizing activity measured in conventional assays (12), although it did correlate with neutralizing titers in the soluble CD4-triggered assay using HIV-27312A/V434M that selectively detects responses to highly conserved epitopes in the coreceptor-binding site (13). Taken together, this study showed that rhFLSC elicits antibody responses to highly conserved CD4i epitopes that correlate with postinfection control of viremia after a high-dose rectal challenge with SHIV162P3, but it left open the question of whether rhFLSC can elicit antibodies that block acquisition. Acquisition is typically blocked only in high-dose challenge studies when the vaccine and challenge stock are matched (14), which is not the case for rhFLSC and SHIV162P3. For this reason, we performed three independent studies using different rhFLSC immunization schemes and a repeat low-dose rectal challenge model that is thought to be more reflective of sexual HIV transmission (15). These studies were designed in part as a hypothesis-generating exercise with respect to protective immunity. We consistently found (i) inverse correlations between acquisition of infection and certain aspects of humoral immunity and (ii) direct relationships between acquisition of infection and vaccine-elicited T-cell responses. Importantly, in certain test groups the apparent protective benefit of humoral responses is absent when T-cell responses are comparatively high. These results strongly suggest that a successful HIV vaccine will need to elicit protective antibody responses without eliciting attenuating levels of vaccine-elicited T-cell responses. |
