CD4IL-21T cells are correlated with regulatory T cells and IL-21 promotes regulatory T cells survival during HIV infection.

Introduction: IL-21 enhances T and natural killer cells survival and antiviral functions without promoting T cell activation during HIV infection, which makes it a better adjuvant in anti-HIV immunotherapy. Due to the pleiotropy and redundancy of cytokines, it is vital to have a comprehensive knowledge of the role of IL-21 in the regulation of immune responses. Regulatory T cells (Tregs) play an important role in immune regulation and are a determinant of immune therapeutic efficacy in certain circumstances. In this study, we explored the direct effect of IL-21 on Tregs during HIV infection, which has not been addressed before.

Methods: Thirty-four HIV treatment-naïve patients were enrolled and the relationship between CD4IL-21T cells and Tregs were studied. The effects of IL-21 on CD4CD25CD127 Tregs' apoptosis, proliferation, and CTLA-4 and TGF-β expression in HIV-infected patients was investigated and compared with the effect of other common γ-chain cytokines.

Results: We found the percentage and absolute numbers of CD4IL-21T cells were positively related to the frequency or absolute numbers of CD4CD25 or CD4CD25CD127 Tregs. Compared with the media-alone control, IL-21, IL-7, and IL-15 could significantly reduce apoptosis of Tregs (p<0.05). IL-21 did not promote the proliferation of Tregs as compared with media alone, while IL-2, IL-7, and IL-15 could significantly increase the proliferation of Tregs (p<0.05). IL-21 enhanced CTLA-4 expression by Tregs (p<0.05), but could not induce TGF-β secretion of Tregs from HIV infected patients. There were no significant differences of the fold induction of apoptosis, proliferation, or CTLA-4 and TGF-β expression by Tregs from HIV-infected patients and normal controls after IL-21 treatment. In vitro experiment showed that pretreatment with IL-21 significantly enhanced the suppressive effect of Tregs on CD8+ T cells' IFN-γ expression.

Conclusion: We conclude that IL-21 promotes the survival and CTLA-4 expression of Tregs and enhanced the suppressive capacity of Tregs during HIV infection. These results broaden the understanding of HIV pathogenesis and provide critical information for HIV interventions.