In this study, we examined quantitative and qualitative aspects of the HIV-specific CD8+ T-cell response during and after SIT to define the immunologic associates of virological outcome. The principal findings were (i) that the magnitude and breadth of the HIV-specific CD8+ T-cell response was unable to account for differences in viremic control, (ii) that significant sequence evolution within immunodominant targeted CD8+ T-cell epitopes over time did not occur regardless of virological outcome, and (iii) that the range of effector functions elicited by CD8+ T cells in response to antigen encounter was associated with the control of viral replication after cessation of ART.
Recent advances in immunotechnology have allowed a more comprehensive quantitative assessment of the magnitude and breadth of the HIV-specific immune response (1
). In addition, the use of matrices composed of overlapping peptides derived from HIV consensus or autologous viral sequences has enabled detailed analyses of CD8+
T-cell specificity in conjunction with sensitive quantitative techniques (1
). However, despite this enhanced ability to characterize HIV-specific T-cell responses directly ex vivo, neither pangenomic analyses nor more specific epitope-centered studies have been able to identify reliable correlates of protection in chronic HIV infection. Similar data have been reported for individuals undergoing various SIT regimens (6
). In the setting of SIT, while the overall HIV-specific T-cell response has been observed to increase with repeated exposure to bursts of autologous viral replication, neither the magnitude nor the breadth of the HIV-specific CD8+
T-cell response could be correlated with levels of plateau pVL after SIT. Our data suggest a similar pattern. In two individuals who maintained consistently lower levels of pVL in the absence of ART, one (patient 101) exhibited a high-frequency HIV-specific CD8+
T-cell response directed against a broad array of Gag epitopes, while the other (patient 114) developed a narrowly focused response, predominantly to Nef. A third individual (patient 124) with a more modest reduction in viremia off ART exhibited a low-frequency HIV-specific CD8+
T-cell response that was more narrowly targeted to Gag. Conversely, the individual with the worst virological response in the absence of ART (patient 103) exhibited the broadest antiviral CD8+
T-cell response, which targeted predominantly epitopes derived from the Gag protein. Thus, in agreement with numerous SIT studies that concluded that there is rarely an immunologic or virological benefit to this type of therapy (5
), our data suggest that the frequency and breadth of the HIV-specific CD8+
T-cell response observed in the absence of ART in individuals with chronic infection are not necessarily beneficially altered by prolonged periods of SIT with respect to virological outcome.
Detailed sequence analysis of targeted HIV epitopes in our cohort revealed little variation during the course of SIT and the post-ART period. A higher level of variation suggests possible selection pressure exerted by CD8+
T cells and attempts by the virus to escape recognition. However, this was not observed regardless of virological outcome off ART. Indeed, when we examined CD8+
T-cell recognition of autologous HIV epitopes present during and after SIT in two virological responders, we found that a high percentage of CD8+
T cells recognized multiple conserved epitopes in Gag (patient 101) or Nef (patient 114) using distinct HLA-restricted class I alleles. These observations suggest that separate HLA-restricted populations of HIV-specific CD8+
T cells may prevent viral escape through the focused recognition of immunodominant epitopes (39
). Additionally, the lack of viral evolution in the majority of targeted immunodominant epitopes over time despite the presence of strong CD8+
T-cell responses may be due to a high degree of sequence conservation necessitated by biological constraints. The HLA A3-KK9 and -RK9 epitopes in Gag p17, the HLA B15-GY9 epitope in Gag p24, and the RQDILDLWIY(106-115)
region in Nef have all been identified as being conserved regions in HIV that elicit strong CD8+
T-cell responses (2
). Many of these epitopes are more than 90% conserved among clade B sequences in the Los Alamos Sequence Database, and some are highly conserved across other clades, suggesting constraints on sequence evolution within these regions.
Due to the limited predictive value provided by simple quantitative measures of the HIV-specific CD8+
T-cell immune reactivity, we sought to investigate whether the quality of the response was a better indicator of viral replication kinetics in the absence of ART. It is well established that CD8+
T cells primed to eliminate virus-infected targets are capable of a multitude of functions that include various combinations of cytolytic activity together with cytokine and chemokine expression. Previous reports suggested that individuals with progressive HIV disease produce less TNF-α and IL-2, while long-term nonprogressors retain the ability to make both of these antiviral cytokines within their HIV-specific CD8+
T-cell populations. In addition, the expression of MIP1β, but not IFN-γ, has been found to dominate the HIV-specific CD8+
T-cell response regardless of the targeted HIV antigen and the total frequency of the response (13
). When we tested the functionality of virus-specific CD8+
T-cell populations in individuals with good virological outcomes, we found that despite differences in the magnitudes and breadths of the total HIV-specific response, these individuals maintained a higher level of function overall than did individuals with poorer outcomes. It remains to be determined, however, if HIV-specific CD8+
T-cell functionality is a consequence of low levels of plasma viremia or if these cells actively restrict viral replication. A detailed analysis of the contribution of each individual functional response within polyfunctional HIV-specific CD8+
T-cell populations may elaborate the causal nature of host-virus interactions. However, it is likely that CD8+
T-cell functionality alone cannot account for changes in the replication kinetics of virus in the absence of therapy. Despite the small number of patients and the inherent issues associated with the interpretation of observational studies of humans, these data suggest that functional attributes of the HIV-specific CD8+
T-cell response might be important correlates of virological outcome after exposure to SIT regimens and could represent useful biological parameters to measure in the clinical context.