The implications of this study are that a properly designed HIV vaccine can affect both the acquisition of HIV and viral replication. This is in contrast to recent findings from preclinical vaccine studies of macaques with the repeated-low-dose challenge model. These previous studies, using a DNA prime-adenovirus 5 (Ad5) boost regimen or recombinant rhesus cytomegalovirus (RhCMV) to immunize macaques, either protected against infection or controlled acute and chronic viral replication, but not both (18
). Using a DNA prime-Ad5 boost regimen with a vaccine encoding all of the SIV proteins except the viral envelope (Env), we recently achieved long-term control of virus replication in six of eight vaccinated animals (49
). However, this DNA prime-Ad5 boost vaccine regimen had no effect in delaying acquisition of SIVsmE660 after repeated low-dose challenges. This suggests that there are fundamental differences between the SIVmac239Δnef and DNA prime-Ad5 boost vaccine regimens. SIVmac239Δnef contains Env and might have induced antibody responses important for eliminating virus particles at the portal of entry (19
). It is also possible that the continuously replicating live-attenuated SIVmac239Δnef vaccine, like the RhCMV vaccine, may have induced the correct type of cellular immune responses of sufficient magnitude at the portal of entry to rapidly eliminate virally infected cells (27
). However, the RhCMV-based vaccine did not control virus replication in the vaccinated animals that became infected. Seemingly, there are essential components present in live-attenuated SIV vaccines that allow them to be effective against infection and in limiting virus replication.
Recent results emerging from a Thai vaccine trial suggest that a reduction in the acquisition of HIV is possible (42
). Even the modest protection against HIV infection observed in that study is an improvement over previous large-scale HIV vaccine efficacy trials that either failed to protect against HIV entirely or possibly even enhanced the rate of HIV infection (6
). Unfortunately, the Thai vaccine failed to have an effect on virus replication in vaccinated individuals (42
). Therefore, the long-term value of the Thai vaccine trial will likely come from determining the precise correlates of protection. However, dissecting out these mechanisms in humans, given the marginally effective nature of the vaccine, will be daunting. Live-attenuated SIV vaccination in the nonhuman primate model of HIV infection may provide an alternative and useful tool for determining the correlates of protection of an effective anti-AIDS virus vaccine.
The mechanisms underlying the protection afforded by live-attenuated SIV vaccination still need to be elucidated fully, particularly at the portal of entry. We know that virus-specific CD8+
T cells induced by live-attenuated SIV likely play an important role in controlling pathogenic SIV infection (11
). Probably the strongest evidence supporting this view is that transient depletion of peripheral CD8+
cells during chronic infection leads to a recrudescence of plasma virus replication in live-attenuated SIV-vaccinated animals (33
). However, the depleting anti-CD8 antibody used in these studies also removes NK cells, which may play an important supportive role in suppression of virus replication, which complicates this analysis. In the current study, we detected broad but modest (in magnitude) vaccine-induced cellular immune responses prior to the initiation of challenges, consistent with previous studies investigating live-attenuated SIV vaccines (31
). In the vaccinated animals that became infected productively with SIVsmE660, we observed an expansion of anamnestic cellular immune responses during the acute phase of infection. Expansion of anamnestic responses was associated with a decline in plasma virus concentrations in four of the five SIVsmE660-infected vaccinated animals. This provides further evidence that vaccine-induced cellular immune responses contribute to controlling heterologous virus infection.
Recent studies indicated that one to three virus strains typically establish infection after sexual transmission of HIV, and this was recapitulated in the repeated-low-dose SIV challenge model (23
). Therefore, it is possible that vaccine-induced immune responses located at the site of infection can eliminate virally infected cells before they establish persistent infection. In the vaccinated animals protected from SIVsmE660 infection, we did not detect evidence of expansion of anamnestic immune responses in the PBMC that would have indicated that vaccine-induced T cells were actively interacting with virally infected cells. However, virus-specific T cells located in mucosal tissues are often effector memory cells with limited potential to expand (8
). Therefore, if vaccine-induced CD4+
T cells rapidly eliminate virally infected cells at the site of infection, we are unlikely to detect evidence of this interaction by using PBMC in IFN-γ ELISPOT assays. Further study is warranted to determine what role, if any, vaccine-induced mucosal immune responses play in protective immunity.
The role of SIV-specific antibodies in protection is less clear. Passive transfer of serum alone from a live-attenuated SIV-vaccinated animal does not protect the recipient from intravenous challenge with pathogenic SIV (2
). This, along with the increase in virus replication after CD8+
T-cell depletion, suggests that neutralizing antibodies alone are not responsible for live-attenuated SIV-induced protection. However, recent evidence suggests that the threshold for neutralizing antibodies to efficiently prevent infection across mucosal surfaces may be lower than originally anticipated (20
). Additionally, none of these results exclude the possibility that nonneutralizing SIV binding antibodies contribute to delayed acquisition. The few viral particles or infected lymphocytes that cross mucosal surfaces may be removed by immune complexes or by antibody-dependent cell-mediated cytotoxicity.
An unfortunate consequence of mucosal challenge with SIVsmE660 is that we and others have observed a portion of naïve animals controlling virus replication during the chronic phase of infection (23
). We did not have any naïve animals control SIVsmE660 replication after intravenous challenge (43
). Since only a few viruses establish infection after mucosal inoculations, this suggests that there may be variants within the stock of SIVsmE660 that are sensitive to neutralizing antibodies or susceptible to host cellular restriction factors that predispose animals to controlling SIVsmE660 replication. The control of SIVsmE660 replication by three of six naïve controls during the chronic phase of infection, combined with recombination between the vaccine and challenge strains of the virus, made it impossible to assess the effects of SIVmac239Δnef vaccination during the chronic phase of infection. Further investigation is needed to determine why some naïve animals have a propensity for controlling chronic SIVsmE660 replication.
In this study, we demonstrate that SIVmac239Δnef vaccination affects both the acquisition of infection and control of virus replication after repeated low-dose heterologous virus challenge. Mathematical modeling suggests that an HIV vaccine duplicating this feat can have a substantial impact on slowing the spread of HIV (3
). Importantly, this effect was observed in animals that did not express the MHC class I alleles previously associated with control of SIVmac239 replication (29
). This suggests that an HIV vaccine might be effective in a diverse population of people, not just among those fortunate enough to express MHC class I alleles associated with control of HIV. Unfortunately, live-attenuated HIV vaccines themselves are unlikely to be developed because of pathogenicity fears (4
) and the possibility of recombination of the vaccine and infecting strains into a more pathogenic virus (17
). Our study, however, supplies new evidence that live-attenuated SIV vaccines provide an ideal model with which to study effective anti-AIDS virus immunity, this time in the setting of a physiologically relevant, repeated-low-dose heterologous virus challenge. Further experiments are needed to define the precise mechanisms of protection associated with live-attenuated SIV vaccines so they can be applied to designing more effective HIV vaccines.