For the first time, we have established a link between SIV survivorship, particular MHC-I alleles, and acute-phase CTL responses. Acute-phase CTL responses are extremely difficult to measure in HIV-infected individuals, since investigators rarely have synthetic peptides autologous to the infecting virus. Moreover, even closely related strains of HIV can differ in residues that are critical for CTL recognition. This problem may be exacerbated if viruses harboring variation in CTL responses are transmitted among individuals (12
). We overcame this issue by infecting all 51 animals in our cohort with the clonal virus SIVmac
239 and by assaying immune responses during the acute phase with synthetic peptides identical to the infecting virus.
It is possible that other genes linked to Mamu-A*01 or Mamu-B*17 and -B*29 influence SIV viral replication during chronic infection. Unidentified gene products could exert their effect by reducing the susceptibility of host cells to HIV infection or by slowing the production of new HIV virions in infected cells. However, during the first 2 weeks of infection, prior to the development of the CTL response, SIVmac239 replicated to high titer in all six Mamu-A*01-, -B*17-, and -B*29-positive animals. Indeed, the levels of peak viremia in the Mamu-A*01-, -B*17-, and -B*29-positive animals were indistinguishable from those of animals that progress to simian AIDS with typical kinetics. If non-MHC host cell genes were contributing to the control of viral replication, we would have expected a reduction in peak viremia.
Alternately, it is possible that acute-phase immune responses that are first directed against epitopes bound by Mamu-A*01 and Mamu-B*17 have long-lasting effects on the chronic-phase immune response to SIVmac
239. In five of the six SIV Mamu-A*01-, -B*17-, and -B*29-positive animals, SIV-specific CD4+
responses were detected during chronic infection (data not shown). HIV-infected individuals with low viral setpoints frequently mount strong HIV-specific CD4+
T-lymphocyte responses (49
), while HIV-infected individuals with higher viral loads and macaques with normal SIV disease progression typically do not. These CD4+
responses may act in concert with the CTL responses by coordinating the immune response, suppressing viral replication, and increasing disease-free survival.
Vigorous acute-phase CTL responses in Mamu-A*01-, -B*17-, and -B*29-positive animals may also reduce the extent of viral escape during acute and chronic infection. If extremely effective CTL rapidly curb viral replication following peak viremia, the virus's capacity to spawn viable escape variants may be diminished. This is consistent with our observation that Mamu-A*01-, -B*17-, and -B*29-positive animals have less chronic-phase variation in the Tat28-35
SL8 epitope than typical Mamu-A*01-positive animals. The reduction in diversification, in turn, may preserve the ability of CTL to combat the infection. In the longest-lived Mamu-A*01-, -B*17-, and -B*29-positive animal, animal 95061, low-level Tat28-35
SL8 responses have been transiently detected throughout chronic infection by tetramer staining, IFN-γ ELISPOT, and IFN-γ intracellular cytokine staining (data not shown). Additionally, the rescue of virus from animal 95061 that differs from SIVmac
239 by only a single nucleotide after 140 weeks of infection suggests that viral diversification has been reduced and that viral resurgence is actively prevented by ongoing immune responses. We hypothesize that containment of low-level SIV replication and reduction of viral diversification during chronic infection does not require the same CTL response magnitude that is required for initial viral containment during acute SIV infection. Results observed in animals challenged with SIVsmE660 and immediately treated with antiretroviral therapy indirectly support this hypothesis (27
). In these animals, undetectable plasma virus loads were associated with modest lymphoproliferative responses. Upon depletion of CD8+
cells by in vivo treatment with monoclonal antibody, viral replication could be detected in cultures of lymph node mononuclear cells.
Interestingly, the two SIV Mamu-A*01-, -B*17-, and -B*29-positive animals with the lowest viral loads were both vaccinated prior to SIV challenge. Animal 95061 was vaccinated with DNA and MVA encoding the nonameric Mamu-A*01-restricted CTPYDINQM peptide, whereas animal 95096 was vaccinated with the same epitope delivered as a lipopeptide. None of the other animals that received these minimal vaccines differed clinically from the vaccine-naive controls. However, our observed Mamu-A*01, -B*17, and -B*29 effect may have been amplified by vaccination. Vaccination alone, however, cannot account for the improved outcome of Mamu-A*01, -B*17, -B*29 animals, as the other four animals that expressed these two alleles were vaccine naive.
Understanding the precise role of Mamu-A*01, -B*17, and -B*29 in SIV control will require studies of additional Mamu-A*01-, -B*17, and -B*29-positive animals infected with SIVmac
239 and other SIV strains. It is presently unclear whether the observed protective effect of Mamu-A*01, -B*17, and -B*29 positivity confers any benefit against closely related viruses such as SIVmac
251 or SIV-HIV chimeras such as SHIV89.6P. Interestingly, we previously studied a Mamu-A*01-negative, Mamu-B*17-, and -B*29-positive macaque infected with a rhesus-passaged derivative of SIVmac
239, SIVppm. Though we lacked the technology to fully evaluate the immune response in this animal at the time of the study, she lived for 127 weeks postinfection and was the longest lived animal in that five-animal cohort (9
If these results are generalizable to other virus strains besides SIVmac
239, they may have important implications for preclinical vaccine testing. Because many CTL epitopes bound by Mamu-A
01 have been previously identified, Mamu-A*01 animals are frequently selected for trials of vaccine efficacy. While the Mamu-A
01 allele by itself offers only modest protection from disease progression, the selection of Mamu-A
01-positive animals into vaccine cohorts will occasionally result in the inclusion of Mamu-A*01-, -B*17-, and -B*29-positive animals. If these animals are assigned to vaccine-naive control groups, a vaccine effect could be masked by uncharacteristically low viral loads in the controls. Perhaps more troubling, the inclusion of Mamu-A*01-, -B*17-, and -B*29-positive animals in groups of vaccinated animals could confuse vaccine-mediated viral control with genotype-mediated viral control.
One of the more interesting findings in this study is the dominance of cellular immune responses against targets bound by the alleles associated with slow progression, namely, Mamu-A*01, -B*17, and -B*29. Our association of dominance with Mamu-A*01 and Mamu-B*17 relied on our precise knowledge of strong responses directed against regions of Gag, Tat, and Nef. The three dominant responses restricted by Mamu-A*01 and Mamu-B*17 remained strong irrespective of the other MHC-I alleles present in these animals. Moreover, additional subdominant responses restricted by Mamu-A*01 and Mamu-B*17 may have further magnified the contribution of these alleles to the cellular immune response during acute infection.
Our results show, for the first time, that MHC-I alleles associated with slow disease progression bind epitopes recognized by dominant acute-phase CTL responses. In the two animals with the most effective containment of viremia, wild-type sequences persist in two CTL epitopes that normally escape CTL recognition during early infection. Thus, our study provides the first link between host immunogenetics, acute-phase CTL responses, reduction in CTL escape, and improved disease outcome following infection with a highly pathogenic immunodeficiency virus.