Background: We have recently shown that HIV-specific CD8⁺ T-cells are highly susceptible to CD95/Fas-induced apoptosis, and that this apoptosis is significantly inhibited by IL-15. In this study, we examined the expression of the anti-apoptotic molecules Bcl-2 and Bcl-x_L in HIV-specific and total CD8⁺ T-cells from HIV-infected individuals and uninfected controls directly ex vivo and after IL-15 treatment.

Methods: Freshly isolated PBMC from HIV-infected individuals and uninfected controls and PBMC cultured ±IL-15 for 16 hrs were stained for surface markers (CD8, HIV-specific tetramers) and intracellularly with anti-Bcl-2, anti-Bcl-x_L and isotype controls and expression levels were determined by flow cytometry.

Results: In freshly isolated PBMC, Bcl-2 expression levels were reduced in CD8⁺ T-cells from HIV-infected individuals with a mean fluorescence intensity (MFI) of 466 ±73 compared to uninfected controls (715 ±59). Strikingly, Bcl-2 expression was even further decreased in HIV-specific CD8⁺ T-cells (269 ±40 MFI). Bcl-x_L levels however did not differ between HIV-specific and total CD8⁺ T-cells from HIV-infected individuals and uninfected controls. IL-15 increased the expression levels of Bcl-2 in 16h cultures from 318 ±25 to 592 ±48 MFI for HIV-specific CD8⁺ T-cells, from 354 ±40 to 625 ±88 MFI for total CD8⁺ T-cells from HIV-infected individuals, and from 606 ±57 to 835 ±54 MFI for CD8⁺ T-cells from uninfected controls. IL-15 also enhanced the expression of Bcl-x_L (from 34 ±3.0 to 46 ±0.9 MFI for HIV-specific CD8⁺ T-cells, from 37 ±1.8 to 49 ±3.5 MFI for total CD8⁺ T-cell from HIV-infected individuals, and from 37 ±5.8 to 51 ±9.4 MFI for total CD8⁺ T-cells from uninfected controls).

Conclusion: Our data demonstrate that the anti-apoptotic molecule Bcl-2 is down-regulated in HIV-specific and total CD8⁺ T-cells from HIV-infected individuals compared to uninfected controls. IL-15 treatment enhances the levels of Bcl-2 and Bcl-x_L in HIV-specific and total CD8⁺ T-cells from HIV-infected individuals and CD8⁺ T-cells from uninfected controls. These findings suggest that the anti-apoptotic effect of IL-15 on HIV-specific CD8⁺ T-cells may be mediated by enhancing the levels of the anti-apoptotic molecules Bcl-2 and Bcl-x_L.