Previous investigations have linked particular MHC class I alleles to control of HIV replication (14
), yet understanding how CD8+
T cells restricted by these protective alleles contribute to viral control remains a mystery. Recently, we identified an association between the Indian rhesus macaque MHC class I allele Mamu
and control of SIVmac239 replication (49
). While identifying CD8+
T-cell responses restricted by this allele in EC macaques, we also discovered that the preliminary peptide binding motif of Mamu-B*08 appears to be similar to the peptide binding motif of HLA-B27 (48
), an MHC class I allele associated with control of HIV replication in humans. The high percentage of Mamu
-positive macaques that become ECs (~50%) and the functional similarity of Mamu-B*08 to HLA-B27 make these MHC class I-defined macaques ideal for modeling human ECs. We therefore studied the immunopathogenic events of acute-phase SIV infection in four Mamu
-positive macaques in an attempt to further understand CD8-mediated viral control in ECs.
In this observational study, we show that three of four Mamu-B*08-positive macaques controlled replication of the pathogenic SIVmac239 isolate (Fig. ). Two of these macaques (r00032 and r02019) were ECs with plasma viral concentrations of approximately 1,000 vRNA copies/ml at 20 weeks postinfection. The third macaque (r01027) showed some measure of control of this highly pathogenic virus, with a plasma viral concentration of <20,000 vRNA copies/ml at 20 weeks postinfection. Only macaque r91003 had plasma viremia similar to the viral set points in the majority of animals that progressed to AIDS (~5 × 105 vRNA copies/ml).
We then investigated how the majority of Mamu
-positive macaques controlled viral replication. Recent experiments have demonstrated that the rapid depletion of the CD4+
memory T cells during the acute phase of HIV/SIV infection might be an important factor contributing to disease progression (44
). Interestingly, while the CD4+
memory T cells in the PBMC were depleted during primary SIV infection in two of the four Mamu
-positive macaques, all four of the Mamu
-positive macaques recovered their CD4+
memory T cells by the chronic phase of infection (Fig. ). By contrast, four Mamu
-negative macaques infected with SIVmac239 experienced acute-phase loss of their CD4+
memory T-cell subset and none of the Mamu
-negative macaques recovered this important CD4+
T-cell subset. At 18 weeks postinfection, the progressor macaque (r91003) started to show signs of CD4+
memory T-cell depletion, likely related to the high plasma virus concentrations in this animal.
Mamu-B*08-restricted CD8+ T-cell responses contributed substantially to the acute phase of SIVmac239 infection (Fig. ). However, the pattern of immunodominance varied from animal to animal, and breadth seemed to correlate with successful control of immunodeficiency virus replication. At 3 weeks postinfection, progressor r91003 focused its Mamu-B*08-restricted CD8+ T-cell responses primarily on only one of the eight mapped Mamu-B*08 CD8+ T-cell epitopes, Vif172-179 RL8. Vif172-179 RL8-specific cells accounted for 87% of the Mamu-B*08-restricted CD8+ T cells detected in this animal. By contrast, the two ECs (r00032 and r02019) divided their robust CD8+ T-cell responses among several Mamu-B*08-restricted epitopes (Fig. to ). Controller macaque r01027 also made several Mamu-B*08-restricted CD8+ T-cell responses at 3 weeks postinfection, but these were of a much lower magnitude than those of the ECs. Indeed, both EC macaques targeted at least the two epitopes in Vif (Vif123-131RL9 and Vif172-179RL8) and one epitope in Nef (Nef137-146RL10) with high-frequency CD8+ T-cell responses. Thus, high-frequency CD8+ T-cell responses against epitopes in Vif and Nef appeared to correlate with a successful disease outcome.
The Vif123-131RL9 response may be especially crucial in the control of viral replication. Vif123-131RL9-specific CD8+ T cells were generated to a sizeable frequency only in the three macaques that controlled viral replication (Fig. to ). Interestingly, we have recently shown that two SIVmac239Δnef-vaccinated Mamu-B*08-positive macaques controlled replication of a pathogenic heterologous challenge with SIVsmE660 (M. R. Reynolds et al., unpublished data). No replication of the challenge virus was seen in one of the two Mamu-B*08-positive macaques, whereas the other had a peak of 14,000 vRNA copies/ml at 2 weeks postchallenge. In the macaque that experienced replication of the SIVsmmE660 challenge virus, we detected only two vaccine-induced anamnestic CD8+ T-cell responses. Both of these were restricted by Mamu-B*08 responses (Vif123-131RL9 and Env573-581KL9). During the acute phase after SIVsmE660 challenge, the Env573-581KL9-specific CD8+ T cells expanded to a frequency of ~1%. However, the Vif123-131RL9-specific CD8+ T-cell response was immunodominant, peaking at 4.84% of the level for CD3+ CD8+ lymphocytes in the peripheral blood at 3 weeks postchallenge, implicating this response in the control of viral replication.
We then examined whether escape from Mamu-B*08-restricted responses could account for progression or control of SIVmac239 replication. It has been previously shown that escape mutations could lead to loss of viral control in SIV-infected macaques (7
). Also, viral escape from the immunodominant HIV-specific response against the HLA-B27-restricted epitope Gag263-272
KK10 has been associated with loss of control of viral replication (8
). We sequenced replicating plasma virus at 18 weeks postinfection and found that the majority of the amino acid replacements at this time were selected for by Mamu-B*08-restricted CD8+
T-cell responses (Table ). By comparison, viral variation was not detected in any of the Mamu-A*02-restricted epitopes, and amino acid replacements were present in only two Mamu-A*01-restricted epitopes (Tat28-35
SL8 and Env726-735
ST10) (see Fig. S1 in the supplemental material). Four of the eight Mamu-B*08-restricted epitopes (Vif123-131
RL10, and Nef246-254
RL9) exhibited viral variation consistent with mutations identified in a previous study (48
). These results may suggest that Mamu-B*08-restricted CD8+
T cells exert more selective pressure than other MHC class I-restricted responses during the critical early phase of infection. Alternatively, the mutations observed in these epitope sequences could occur in regions of the viral genome that are not under strong evolutionary constraints. Our group and others have previously shown that fitness costs may play a role in determining the rate at which escape mutations accumulate and revert in vivo (21
We also followed the ontogeny of mutations by population sequencing these four Mamu-B*08-restricted epitopes at various time points. We discovered the first evidence of viral variation in circulating plasma virus at 10 weeks postinfection and mutations within several Mamu-B*08-restricted epitopes by 13 weeks postinfection (Fig. ). Interestingly, the patterns of amino acid substitutions in the Nef137-146RL10 and Nef246-254RL9 epitopes differentiated the four Mamu-B*08-positive macaques into two separate groups, the ECs and the non-ECs. It will be intriguing to see if viral control is lost through the accumulation of additional mutations as the three macaques that control SIV replication progress further into the chronic phase of infection.
Similar to those in HLA
-positive individuals (6
), the acute-phase CD8+
T-cell responses in Mamu
-positive macaques were dominated by Mamu-B*08-specific CD8+
T-cell responses (Fig. and Fig. ). Surprisingly, Mamu-B*08 appeared to reduce the dominant influence of Mamu-A*01 on CD8+
T-cell responses. Previously, such immunodomination was described to occur in macaques expressing both Mamu-A*01 and Mamu-A*02 during the acute phase of SIVmac251 infection (58
). Typically, in Mamu
-positive macaques, the Mamu-A*01-restricted Tat28-35
SL8- and Gag181-189
T cells account for >50% of the acute-phase SIV-specific immune responses (56
). However, for both r91003 and r01027, the Mamu-A*01-restricted contribution was below 50%. Responses restricted by Mamu-A*02 in r00032 also appeared to be diminished in this Mamu
-positive macaque compared to those in Mamu
-positive macaques that do not express Mamu
. It should be emphasized, however, that these data are derived from only three animals and that larger cohorts of Mamu
-positive macaques expressing other MHC class I molecules should be studied to clarify this issue.
HIV disease progression in HLA
-positive humans appears to be associated with viral escape from the immunodominant Gag263-272
KK10 response during the chronic phase of infection (8
). However, additional HIV-specific CD8+
T-cell responses may be involved in the initial control of viremia. Because immune responses in HIV-infected individuals are normally characterized using IFN-γ ELISPOT with consensus peptides, often after primary infection, definition of the entire breadth of the HLA-B27-restricted CD8+
T-cell responses may be incomplete. While preliminary, our current findings appear to indicate that the breadth of CD8+
T-cell responses restricted by protective MHC class I alleles, rather than a single immunodominant response, may be important in determining the control of replication of the AIDS virus.
Surprisingly, while Mamu-B*08 appears to bind similar peptides to HLA-B27, an immunodominant CD8+
T-cell response directed against Gag has not been identified for Mamu-B*08. Rather, Mamu-B*08 restricts robust CD8+
T-cell responses largely from Vif and Nef. Moreover, we are currently attempting to identify all of the SIV epitopes restricted by Mamu-B*08. At this time, we have identified only a single Gag-specific CD8+
T-cell response, and it is both low frequency and recognized in only a few Mamu
-positive macaques (J. T. Loffredo et al., unpublished data). Hence, while CD8+
T cells targeting Gag have been shown to be extremely effective in controlling immunodeficiency virus replication (7
), it may be possible for individuals to control viremia by directing responses against other proteins as well. Mamu
-positive, SIV-infected macaques may offer an intriguing model for studying such a mechanism. Interestingly, both HLA-B27 and Mamu-B*08 present many epitopes containing two N-terminal basic amino acids. These peptides are relatively resistant to peptidase activity, and thus, those peptides may be more stable, and this may result in more efficient MHC class I antigen presentation (30
Approximately 50% of Mamu
-positive macaques become ECs, controlling replication of the pathogenic SIVmac239 isolate to <1,000 vRNA copies/ml (49
). Given that the chronic-phase viral set points in 175 macaques that progressed to AIDS were ~500,000 vRNA copies/ml, and approximately two-thirds of macaques die by 1 year postinfection (40
), this level of control is remarkable. However, not all Mamu
-positive macaques become ECs after SIVmac239 infection, and the immune system's role in successful viral containment remains difficult to define. Understanding how some of these macaques become progressors or controllers might give us key insights into how to make an effective HIV vaccine. Here, we provide evidence from a small yet provocative study that the immunodominance and breadth of CD8+
T-cell responses restricted by protective MHC class I alleles may facilitate the development of elite control. Further experiments are needed to explore this intriguing idea, especially given the fundamental implications that this might have for future vaccines.