Background: Different studies have suggested that an impaired function of HIV-specific CTLs could explain its inefficacy to control HIV replication. We have previously found an impaired ability of CTLs to produce IFN-g as well as an immature phenotype. To further explore the functionality of CTLs, the proliferative potential of anti-HIV Gag and anti-HIV Pol CTLs in response to the specific peptide, was examined in a large group of chronically HIV-infected, untreated patients (pts).

Methods: Sixty-one (61) untreated HLA-A0201⁺ pts were included in the study. PBMCs were cultured in vitro, using an optimized protocol, for 12–14 days in the presence of a Gag (SLYNTVATL), Pol (ILKEPVHGV), or a control peptide (LKHIVWASR). After culture, cells were stained with HLA-A2-Gag and HLA-A2-Pol tetrameric complexes and the monoclonal antibodies CD27, CD45RA and CD8, in order to analyze the phenotype Tet⁺ cells. When enough cells were available, a bulk lysis assay was performed to study the function of expanded cells. An HLA-A0201⁺ B-lymphoblastoid cell line loaded with Gag, Pol, or Control peptides was used as target cell line in this assay.

Results: TetPol⁺ cells were able to expand in all cases, with percentages of TetPol⁺ cells at the end of culture ranging from 0.37%–21%. Surprisingly, expansion of TetPol⁺ cells was noticed in 23% of pts who had undetectable levels of these cells at the beginning of culture. This implies that TetPol⁺ must be present in fresh PBMCs, although at very low levels, and supports their remarkable potential in some individuals. Similar stimulation experiments with the Gag peptide showed that only 30% of pts with TetGag⁺ cells showed expansion of these cells. This suggest that Gag-specific cells may not always propagate in culture, despite being detectable in fresh PBMCs. In all cases, expanded Tet⁺ cells had a memory phenotype. In most cases, significant levels of lysis against target cells pulsed with the cognate peptide were found after expansion of Tet⁺ cells. A significant correlation was found between the percentage of Pol lysis (40:1 E:T ratio) after in vitro culture and expansion of TetPol⁺ cells (Spearman´s rho correlation co-efficient = 0.71, p < 0.001).

Conclusions: CTLs directed against an immunodominant epitope in the HIV Gag protein show a very low proliferative potential when stimulated with the specific peptide. This observation may help to explain the inability of CTLs to control viral replication in chronic HIV infection.