Endogenous TGF-β activation by reactive oxygen species is key to Foxp3 induction in TCR-stimulated and HIV-1-infected human CD4(+)CD25(- )T cells

BACKGROUND: CD4(+)CD25(+ )T regulatory cells (Tregs) play an important role in regulating immune responses, and in influencing human immune diseases such as HIV infection. It has been shown that human CD4(+)CD25(+ )Tregs can be induced in vitro by TCR stimulation of CD4(+)CD25(- )T cells. However, the mechanism remains elusive, and intriguingly, similar treatment of murine CD4(+)CD25(- )cells did not induce CD4(+)CD25(+)Foxp3(+ )Tregs unless exogenous TGF-β was added during stimulation. Thus, we investigated the possible role of TGF-β in the induction of human Tregs by TCR engagement. We also explored the effects of TGF-β on HIV-1 infection mediated induction of human Tregs since recent evidence has suggested that HIV-1 infection may also impact the generation of Tregs in infected patients. RESULTS: We show here that endogenous TGF-β is key to TCR induction of Foxp3 in human CD4(+)CD25(- )T cells. These events involve, first, the production of TGF-β by TCR and CD28 stimulation and the activation of latent TGF-β by reactive oxygen species generated from the activated T cells. Biologically active TGF-β then engages in the induction of Foxp3. Neutralization of active TGF-β with anti-TGF-β antibody or elimination of ROS with MnTBAP abrogated Foxp3 expression. HIV-1 infection enhanced Foxp3 expression in activated CD4(+)CD25(- )T cells; which was also abrogated by blockade of endogenous TGF-β. CONCLUSION: Several conclusions can be drawn from this work: (1) TCR and CD28-induced Foxp3 expression is a late event following TCR stimulation; (2) TGF-β serves as a link in Foxp3 induction in human CD4(+)CD25(- )T cells following TCR stimulation, which induces not only latent, but also active TGF-β; (3) the activation of TGF-β requires reactive oxygen species; (4) HIV infection results in an increase in Foxp3 expression in TCR-activated CD25(- )T cells, which is also associated with TGF-β. Taken together, our findings reinforce a definitive role of TGF-β not only in the generation of Tregs with respect to normal immune responses, but also is critical in immune diseases such as HIV-1 infection.