Do Natural T Regulatory Cells become Activated to Antigen Specific T Regulatory Cells in Transplantation and in Autoimmunity?

Antigen specific T regulatory cells (T(reg)) are often CD4(+)CD25(+)FoxP3(+) T cells, with a phenotype similar to natural T(reg) (nT(reg)). It is assumed that nT(reg) cannot develop into an antigen specific T(reg) as repeated culture with IL-2 and a specific antigen does not increase the capacity or potency of nT(reg) to promote immune tolerance or suppress in vitro. This has led to an assumption that antigen specific T(reg) mainly develop from CD4(+)CD25(−)FoxP3(−) T cells, by activation with antigen and TGF-β in the absence of inflammatory cytokines such as IL-6 and IL-1β. Our studies on antigen specific CD4(+)CD25(+) T cells from animals with tolerance to an allograft, identified that the antigen specific and T(reg) are dividing, and need continuous stimulation with specific antigen T cell derived cytokines. We identified that a variety of cytokines, especially IL-5 and IFN-γ but not IL-2 or IL-4 promoted survival of antigen specific CD4(+)CD25(+)FoxP3(+) T(reg). To examine if nT(reg) could be activated to antigen specific T(reg), we activated nT(reg) in culture with either IL-2 or IL-4. Within 3 days, antigen specific T(reg) are activated and there is induction of new cytokine receptors on these cells. Specifically nT(reg) activated by IL-2 and antigen express the interferon-γ receptor (IFNGR) and IL-12p70 (IL-12Rβ2) receptor but not the IL-5 receptor (IL-5Rα). These cells were responsive to IFN-γ or IL-12p70. nT(reg) activated by IL-4 and alloantigen express IL-5Rα not IFNGR or IL-12p70Rβ2 and become responsive to IL-5. These early activated antigen specific T(reg), were respectively named Ts1 and Ts2 cells, as they depend on Th1 or Th2 responses. Further culture of Ts1 cells with IL-12p70 induced Th1-like T(reg), expressing IFN-γ, and T-bet as well as FoxP3. Our studies suggest that activation of nT(reg) with Th1 or Th2 responses induced separate lineages of antigen specific T(reg), that are dependent on late Th1 and Th2 cytokines, not the early cytokines IL-2 and IL-4.