Background: To investigate the effect of highly active antiretroviral therapy (HAART) on HIV- and EBV-specific CD8+ T cells, total number and function of these cells was determined using tetrameric HLA-peptide complexes and IFN-g ELISPOT assays after peptide-stimulation, respectively.

Methods: 16 HIV-infected individuals on HAART, selected for the presence of HLA-type A2 and/or B8, were studied.

Results: HAART, which led to a decrease in HIV load and a significant increase in CD4+ T cells, induced a significant decrease in HIV-specific tetramer+ T cells, whereas EBV-specific tetramer+ T cells did not change. In addition, individuals who temporarily failed on therapy showed a temporary increase in the number of HIV-specific T cells. Interestingly, there was an increase in the ratio of  IFN-g-producing T cells/ total number of both HIV- and EBV-specific T cells in the majority of individuals, suggesting that the function of virus-specific T cells is improved in individuals successfully treated with HAART. Despite this relative functional improvement of EBV-specific T cells, no significant changes were observed in EBV load. In 4 subjects who temporarily failed on HAART later on, numbers of HIV-specific T cells increased with increases in HIV load. The ratio of IFNg-producing T cells, both for HIV and EBV, paralleled CD4⁺ T cell kinetics. These data suggest that differences in the pool size of antigen-specific T cells seem to be determined solely by the presence of antigen, whereas function seems to be related to differences in CD4+ T-cell numbers. In 5 individuals we were also able to determine the phenotype of virus-specific CD8+ T cells based on CD27-expression. No change was observed after HAART, suggesting that antigen stimulation during a longer time period is necessary to induce higher levels of CD27-negative effector T cells.

Conclusions: Overall, these data indicate that HAART improves the antigen responsiveness of both HIV- and EBV-specific T cells, which is associated with an increase in CD4+ T cells.