Determinants of in vitro expansion of different human virus-specific FoxP3⁺ regulatory CD8⁺ T cells in chronic hepatitis C virus infection

It has been shown previously that suppressive virus-specific FoxP3⁺ regulatory CD8⁺ T cells can be expanded from human peripheral blood mononuclear cells after in vitro antigen-specific stimulation. This study extended this finding by analysing the mechanisms of virus-specific FoxP3⁺ regulatory CD8⁺ T-cell generation during peptide-specific expansion in vitro. It was shown that hepatitis C virus (HCV)-, influenza virus (FLU)-, Epstein-Barr virus (EBV)- and cytomegalovirus (HCMV)-specific FoxP3⁺ regulatory CD8⁺ T cells could be expanded differentially from the blood of chronically HCV-infected patients following in vitro peptide-specific stimulation. The different ability of virus-specific CD8⁺ T-cell populations to express FoxP3 after continuous antigen stimulation in vitro correlated significantly with the ex vivo differentiation status. Indeed, CD27⁺ CD28⁺ CD57^- HCV-, FLU- and EBV-specific CD8⁺ T cells displayed a significantly higher ability to give rise to FoxP3⁺ regulatory CD8⁺ T cells compared with CD27^- CD28^- CD57⁺ HCMV-specific CD8⁺ T cells. Similar T-cell receptor expression patterns of FoxP3⁺ versus FoxP3^- CD8⁺ T cells of the same antigen specificity indicated that both cell populations were probably expanded from the same virus-specific CD8⁺ T-cell precursor. In addition, no specific antigen-presenting cell populations were required for the generation of FoxP3⁺ CD8⁺ T cells, as CD8⁺-selected virus-specific FoxP3⁺ CD8⁺ T cells could be expanded by peptide presentation in the absence of antigen-presenting cells. Taken together, these results suggest that the ability to expand FoxP3⁺ regulatory CD8⁺ T cells from virusspecific CD8⁺ T cells differs among distinct virus-specific CD8⁺ T-cell populations depending on the differentiation status.