Mapping immunodominant patterns and HLA class II restriction characteristics of HIV-specific CD4+ T cell responses in acute and chronic HIV-1 subtype C infection.
Increasing evidence suggests that virus-specific CD4+ T cells contribute to immune-mediated control of HIV-1 infection. However, precise details of CD4+ T cell contribution to immune protection against HIV have not been adequately defined and most of the existing data was predominantly generated in clade B HIV-1 infection. Understanding the contribution of CD4+ T cell responses in clade C infection is important for developing vaccines that would be efficacious in sub-Saharan Africa which carries the highest burden of the HIV epidemic in the world. Therefore this study focused on the role of CD4+ T helper cells in the immune response to clade C HIV-1 infection. We tested the hypothesis that HIV-1-specific CD4+ T cell responses and protective class II HLA alleles are important determinants of effective immunological control of HIV-1 infection. Firstly, CD8 depleted PBMCs were used in an IFN-γ ELISPOT assay to conduct a comprehensive analysis of virus-specific CD4+ T cell responses in acute and chronic HIV-1 clade C infection. Thereafter the host genetic effects of class II HLA-DRB1 alleles on HIV viremia were assessed using the HLA-DRB1 restriction assay, where HLA class II-restriction characteristics of detectable responses were defined. Lastly, functional differences of HIV-specific CD4+ T cells were further characterized using flow cytometric analysis. In our study, Gag and Pol regions of the HIV proteome were found to be the most frequently targeted in acute HIV-1 infection (69% of total responses), with CD4+ T cell targeting across the proteome remaining relatively stable over time. In chronic HIV-1 clade C infection, dominant HIV-1-specific CD4+ T cell responses were detectable against a limited number of epitopes. Epitopes in the Gag region were the most targeted by CD4+ T cells (30/40 peptides), with OLP 41 in the Gag p24 region being the most dominant epitope targeted (15% of responses). There were no significant differences observed between total or Gag-specific CD4+ T cell responses and contemporaneous viral load. Interestingly, responses rarely targeted the envelope region in clade C infection, in contrast to multiple epitopes targeted in this protein in previous clade B studies. Functional analysis demonstrated that IFN-γ, IL-2 and TNFα were the most secreted cytokines by HIV-specific CD4+ T cells in 18/25 individuals, with IFN-γ being the most dominant response in individual subjects. The HLA class II DRB1 restriction in clade C HIV infected individuals showed epitope promiscuity, consistent with previous studies in clade B infection. The HLA-DRB1*13:01 allele variant was associated with the highest frequency of responders (22%) in our cohort and restricted the highest number of HIV-specific peptides (9/15). Together, our data identify immunodominant regions of HIV-specific CD4+ T cell responses and their association with viral control during clade C infection. Furthermore, our findings will inform studies aimed at elucidating the underlying mechanism by which CD4+ T cells modulate effective CD8+ T cell and B cell responses. Additionally, these data suggest that epitope promiscuity among class II HLA molecules should be taken into account for vaccines designed to induce CD4+ T cell responses. This information will be critical to vaccine efforts designed to induce these responses, as well as potential therapeutic manipulation of immunity in persons with acute and chronic HIV-1 infection.