The main objective of this study was to reveal antiviral determinants of the HIV-specific CD8 T-cell response and to examine the influence of the host genetic (HLA) background on the HIV-specific CD8 T-cell response. The HISIS-CTL cohort is composed of 56 chronically infected female bar workers highly exposed to a mixed-subtype epidemic where subtype C predominates (14
). Within the HISIS-CTL cohort, HLA-B alleles B5801, B8101, and B0702 were associated with a low viral load. This finding is consistent with a larger study in a cohort of subtype C-infected individuals previously conducted in South Africa (18
). We have expanded upon these observations to show that within this cohort of high HIV-1 exposure, it is T-cell responses restricted by these alleles, particularly those targeting certain regions of the Gag protein, that mediate significantly better control of viral load.
The CD8 T-cell specificities found within an HIV-infected individual are largely dependent on the HLA class I alleles expressed, the sequence of the infecting virus (8
), and, partly, the stage of infection (1
). Additionally, HIV-specific CD4 T cells may also help to maintain CD8 T-cell responses of broad HIV specificity (22
). It was first shown in 1995 that the Gag-specific CD8 T-cell response may play an important role in slowing down progression to AIDS (30
). However, recent reports are contradictory with respect to a role for the Gag-specific CD8 T-cell response in controlling viral load. Some studies supported an association of a strong Gag-specific CD8 T-cell response with the control of HIV replication (10
), whereas others showed no such association (1
). Our results suggest that the breadth of epitope recognition within specific regions of Gag strongly contributes to the antiviral efficiency of the CD8 T-cell response during the chronic phase of infection. The expression of certain HLA-B alleles, namely, B0702, B5801, and B8101, was associated with this particular pattern of CD8 T-cell recognition. This finding, and the inverse linear relationship between the number of recognized Gag (or GagR1R3) epitopes and plasma viral load, further supports the notion that the viral load-HLA associations are mediated by the CD8 T-cell response restricted by these HLA alleles. The hypothesis that the breadth of GagR1R3 recognition is an important correlate of the expression of these HLA B alleles is reinforced by the examination of six individuals that have a high viral load despite expressing the protective allele B5801, B8101, or B0702. The breadth of GagR1R3 recognition was significantly reduced in these individuals compared to the other subjects expressing these B alleles (Fig. ).
Is the antigen specificity of CD8 T cells important for the maintenance of antiviral efficiency? In contrast to the Gag response, there was a trend of a broad Nef-specific response with higher viral loads, resembling observations of previous studies (4
). The observed differences between Gag- and Nef-specific responses support the notion that antigen specificity plays a role in maintaining antiviral efficiency of HIV-specific CD8 T cells during the chronic phase of infection.
The observation that the magnitude of Gag- or GagR1R3-responding CD8 T cells, measured by cumulative IFN-γ ELISPOT responses, did not correlate with a low viral load contrasts with most of the recent reports that have found a protective role of strong Gag-specific T-cell responses. One possible explanation for this difference is that the majority of the HISIS-CTL substudy subjects were infected for extended time periods and had high viral loads. An alternative explanation is that the usage of three different Gag peptide sets permitted a better detection of epitope breadth compared to other studies. We have shown previously that that the Gag and Nef peptide sets used during this study did afford better detection of the breadth of the T-cell response (7
Importantly, “protective” HLA B alleles, B0702, B5801, and B8101, were associated with a median plasma viral load below 50,000 RNA copies/ml, constituting a fourfold reduction in the median viral load observed within the whole HISIS-CTL cohort. This comparatively high median viral load of 198,500 RNA copies per ml most likely reflects the long time since primary HIV infection in the majority of the study subjects, 63% of which (35 of 56) were infected for more than 3.5 years. The remaining 21 subjects seroconverted during the study and had a median viral load of 40,600 RNA copies/ml at 24 months after seroconversion, which is in the range observed in most other studies (10
). Together, these results underline that all study subjects were in the chronic phase of HIV infection: 21 were in the postacute or early chronic phase (9 months to 3.5 years of infection), and 35 were in later stages of the chronic phase of HIV infection (>3.5years of infection).
It is also noteworthy that most previous studies (and the present study) used IFN-γ-based assays to detected HIV-specific CD8 T cells. IFN-γ is a sensitive marker for HIV-specific CD8 T-cell activation; however, virus-specific cytolytic activity and IFN-γ secretion upon restimulation do not necessarily correlate during chronic HIV infection (3
). This dislinkage of cytolytic activity and IFN-γ secretion has been ascribed to a phenomenon known as the “partial functional exhaustion” of T cells. In a mouse model of chronic LCMV infection, CD8 T cells of different epitope specificities can vary in their respective functional exhaustion, which is partly dependent on the antigenic load and the time from infection (32
). A similar phenomenon may explain the differences between the present study and previous studies of HIV-1 infection.
One example of a very strong, yet potentially ineffective, CD8 T-cell response during chronic HIV infection is the targeting of the Gag epitope TL9. This epitope is presented by the class I alleles B0702, B4201, and B8101 and is targeted by 41% of the subjects in this study cohort, and responses are among those with the highest magnitude (14
). This epitope is highly conserved within a particular subtype, and there is no evidence supporting viral escape by an epitope point mutation. Additionally, the high magnitude of responding cells and the lack of mutational changes in the immediate epitope proximity within subtype C argue against abrogated epitope processing. Therefore, we believe that neither the epitope itself nor its presentation is changed in the course of chronic infection. Despite all of these parameters, previously assumed to be favorable, we did not find any evidence supporting a substantial role for this immunodominant response in viral control during the chronic phase of infection. It is noteworthy that the three subjects that possess protective allele B8101, but with high viral loads, also targeted this epitope. The persistence of unaltered yet targeted epitopes during the course of chronic HIV infection has also been demonstrated in two recent studies (9
Why may the breadth of Gag or GagR1R3 but not Nef or Env recognition be important for viral control during the chronic stage of HIV infection? One possible explanation is that a broad Gag or GagR1R3 response is more likely to select for viral escape mutants that are characterized by a reduced replicative capacity. Responding T cells would at the same time poorly recognize the mutant epitope(s), and a reduced T-cell receptor stimulation may in turn slow down the functional exhaustion of virus-specific T cells. Indeed, some of the most “protective” epitopes within GagR1R3 are highly variable, and it has been shown previously that mutational escape from an epitope-specific T-cell response can cripple the replicative capacity of the mutant virus (25
), despite it being less well recognized by virus-specific T cells. Episodic epitope reversion may then contribute to the persistence of wild-type epitope-specific T cells (13
) and at the same time “conserve” the antiviral efficiency of these T cells.
It is commonly accepted that the steady-state viral load is a major parameter affecting the rate of disease progression (26
). Within this context, perhaps the most striking observation during this study was the course of the viral load in 19 B5703-negative individuals after seroconversion. Most subjects targeting multiple epitopes within GagR1R3 had a “normal” course of viral load, whereas subjects with a very narrow GagR1R3 response had an unusually high median viral load steady-state level. Therefore, this pattern of HIV-specific T-cell recognition is likely to be associated with slow/normal disease progression, and to the contrary, its absence is likely to be associated with a more rapid progression. Importantly, the latter finding may affect not only survival but also HIV transmission rates. Taken together, our findings highlight the association between the host genetic (HLA) background and the quality of the HIV-specific CD8 T-cell response during the chronic phase of HIV infection. The results may help to explain differences in disease progression on a population level. During evaluations of candidate HIV-1 vaccines, it may be useful to include vaccine immunogens that are capable of stimulating broad and targeted Gag responses in individuals with certain HLA class I alleles in the population.