NEGATIVE FEEDBACK REGULATION OF T CELLS: INTERLEUKIN 2 AND FOXP3 RECIPROCITY (89.8)

Abstract

Abstract TCR activation induces T cell FOXP3 expression, thereby calling into question FOXP3 as a distinct marker for Regulatory T cells. We investigated the role of IL2 in the TCR/CD3-induction of FOXP3 expression by normal human and murine T cells in vitro. Freshly isolated human PBMCs were stimulated with soluble anti-CD3, and four-color flow cytometry was used to determine the expression of FOXP3. Anti-CD3-activation increased the % of cells expressing FOXP3 in both T cells subsets with a peak at 24h. Both anti-CD25 and anti-IL2 inhibited anti-CD3-stimulated FOXP3 expression. The addition of rIL2 restored and prolonged FOXP3 expression. When anti-CD3-activated PBMCs were monitored daily for 4 days, FOXP3 and IL2 expression remained mutually exclusive, indicating that FOXP3 restricts but does not suppress IL2 expression, even upon TCR/CD3-restimulation. By comparison, soluble anti-CD3/CD28 stimulation +/- rIL2 of murine splenocytes did not up-regulate FOXP3 expression. However, upon solid-phase anti-CD3 activation, the presence of both exogenous TGFβ and rIL2 induced FOXP3 expression by both CD4+ and CD8+ T cells. Therefore, TCR-activated T cell FOXP3 expression is primarily regulated by IL2/IL2R signaling and occurs only transiently in a subset of both CD4+ and CD8+ T cells. Since FOXP3 functions to restrict TCR-induced IL2 gene expression, these results are consistent with an IL2-dependent FOXP3-mediated negative feedback loop that regulates IL2 production via FOXP3 at the level of transcription. Therefore, FOXP3 cannot be considered solely a phenotypic marker for T-Reg cells. Instead, FOXP3 plays a key role in the feedback regulation of IL2 availability, thereby affecting the proliferation and survival of T cells via a passive suppressive mechanism.