For this study, by using an Ad5hr-SIVgag
recombinant to prime cellular immune responses in addition to an Ad5hr-SIVenv/rev
recombinant, we have extended earlier findings of partial protection against an SIVmac251
intravaginal challenge using an envelope-based vaccine regimen (6
) for improved viremia control following an intrarectal challenge with the highly pathogenic SIVmac251
strain. In the earlier study, reduction in viremia was observed during acute infection; however, viral loads in immunized macaques quickly rose by 8 weeks postchallenge above acute-phase levels. Here, in contrast, a subset of immunized macaques exhibited falling, rather than increasing, viral burdens at set point. In addition to significantly reduced acute-phase viral burdens, 8 of 12 non-Mamu-A
01 macaques also exhibited statistically significant reductions in viral burden at set point. Further, we have shown that the SIV gag
gene in the Ad recombinant, in addition to the env/rev
genes, is immunogenic and elicits persistent cellular immune responses. Moreover, antibodies in both serum and secretory fluids were elicited by the prime/boost strategy.
Because the vaccine regimen was similar to the one used previously, with the exception of the added Ad5hr-SIVgag
recombinant, we expected that the improved protective efficacy as shown by reductions in set point viremia might be strongly associated with cellular responses to the Gag immunogen. Others have shown that multicomponent vaccine strategies are more efficacious than those relying solely on envelope (10
). In fact, a highly significant correlation of postchallenge Gag response with reduced set point viremia was observed (Fig. and Table ). It is not possible to determine with certainty, however, if this Gag response was solely responsible for the more prolonged viremia control, as the previous experiment used a vaginal exposure and the challenge here was intrarectal. Further experimentation with the same challenge stock administered by the same route will be necessary to resolve this point. The cellular response to Env rather than Gag was a better prechallenge correlate with set point viremia and in macaques with low- and intermediate-viral-burden outcomes was significantly elevated compared to that of macaques with high viral burdens. However, the postchallenge Env response, although statistically significant, was not as strong a correlate as the postchallenge Gag response (Fig. and Table ).
The vaccine regimen elicited strong anti-gp120 binding antibodies in serum and mucosal secretions, and several showed good correlation with protective outcome. Neutralizing antibody titers to the T-cell-line-adapted SIVmac251
isolate were elevated in the macaques with low viral burden compared to those of macaques with high viremia, but statistical significance was not reached. Furthermore, the antibodies did not neutralize primary SIV isolates. However, SIV gp120-specific IgG binding antibodies in nasal and vaginal secretions were significantly elevated in the eight macaques with low- and intermediate-viral-burden outcomes, and paired SIV gp120-specific nasal IgG and IgA antibodies also were significantly higher in these eight macaques. Passive administration of neutralizing IgG antibodies has been shown to protect against vaginal and oral mucosal transmission of SHIV isolates (2
), although the mechanism of protection has not been elucidated. In these studies, vaginal fluids and saliva did not exhibit neutralizing activity against the SHIV isolates. While secretory IgA is thought to be a major defense mechanism at mucosal sites, IgG can clearly exert protective effects by several potential mechanisms (42
). Other than direct neutralization, these include prevention of viral binding or attachment, trapping of virions in mucous, antibody-dependent cell cytotoxicity, and inhibition of transcytosis. One of the advantages of the Ad recombinant vaccine strategy is its ability to target mucosal sites. Here we demonstrate that vaccine-induced antibodies present in secretory fluids are correlated with protective outcome. Whether these antibodies arise from local immune responses or reflect transudation from plasma remains to be determined.
In addition to vaccine-induced immunity, innate immunity can play a role in protection from HIV and SIV infection (15
). Here we investigated whether CD8+
T-cell antiviral activity was correlated with the reduced viral burdens observed. Unfortunately, cells were not available for assay prior to challenge or in the initial weeks postchallenge. However, we examined this response 28 weeks postchallenge at a time when two of the macaques, 773 and 784, still exhibited strong viremia control. We found that both of these macaques exhibited potent CD8+
T-cell antiviral activity, particularly in the acute assay system, which may have contributed to their continued low-level viremia. It has previously been shown that the acute type of CD8+
T-cell suppressor activity develops after viral infection, whereas the endogenous type of activity occurs in both infected and uninfected individuals (3
). We were not able to associate the CD8AA response of the macaques overall with viral burdens, however, and did not observe significant differences in CD8AA levels among the low-, intermediate-, and high-viremia groups of macaques. This is not inconsistent with the fact that at 28 weeks postchallenge, with the exception of macaques 773 and 784, the majority of macaques had begun to escape the level of viremia control observed at set point.
Overall, our study further develops the replication-competent Ad recombinant approach by demonstrating significant protective efficacy against SIVmac251 challenge, with reductions in viremia during both the acute phase and set point of infection. No single immune correlate of protection was discerned. Both cellular immunity to Gag and Env and antibodies in mucosal secretions were associated with development of low to intermediate viral burdens at set point. Furthermore, the contribution of CD8+ T-cell antiviral activity to the continued control of viremia in macaques 773 and 784 cannot be overlooked. We conclude that protection against a pathogenic virus challenge will require broad immunity, activating all components of the immune system.