Dominant ex vivo cross-stimulation of CD8+ T-cells with whole soluble gag protein in HIV-infected subjects.

Background: Soluble HIV proteins are often used to detect HIV-specific CD4+ T-helper cell responses in vitro. However, exogenous antigens can also indirectly stimulate CD8+ T-cells and thus complicate assessment of CD4+ T-cell responses.

Objective: To analyze the extent of in vitro HIV-1 Gag p55 protein cross-stimulation to CD8+ T-cells in therapy-naive and highly active antiretroviral therapy (HAART)-treated HIV patients and to correlate this phenomenon with HIV disease progression.

Methods: Gag protein-stimulated T-cell responses were measured in total and CD8-depleted peripheral blood mononuclear cells (PBMCs) by interferon (IFN)-gamma enzyme-linked immunosorbent spot (ELISPOT) assays in 20 therapy-naive and 60 HAART-treated HIV patients. Numbers of spot forming cells (SFCs) relative to CD4+ and CD8+ T-cell subsets were calculated. Gag protein-stimulated responses were correlated with markers of disease progression.

Results: Stimulation of PBMC with HIV-1 Gag protein induced higher CD8+ T-cell responses than CD4+ T-cell responses in both therapy-naive and HAART-treated HIV patients (P < 0.001). Gag protein cross-stimulation of CD8+ T-cells was higher in therapy-naive than in HAART-treated HIV patients (P < 0.001). In HAART-treated HIV patients, we detected an inverse correlation between Gag protein cross-stimulation of CD8+ T-cells and the CD4 count (R = -0.311; P = 0.016). Depletion of CD14+ cells abrogated the responses, suggesting that Gag protein cross-stimulation of CD8+ T-cells depends on antigen processing and presentation by antigen-presenting cells (APCs).

Conclusions: HIV protein cross-presentation to CD8+ T-cells should be taken into account when detecting HIV-specific T-cell responses by stimulation of PBMCs with whole exogenous antigens.