Alternative Splicing of FOXP3—Virtue and Vice

Regulatory CD4+ T cells (Tr cells), the development of which is critically dependent on X-linked transcription factor Foxp3 (forkhead box P3), prevent self-destructive immune responses. Despite its important role, molecular and functional features conferred by Foxp3 to Tr precursor cells remain unknown. It has been suggested that Foxp3 expression is required for both survival of Tr precursors as well as their inability to produce interleukin (IL)-2 and independently proliferate after T-cell-receptor engagement, raising the possibility that such 'anergy' and Tr suppressive capacity are intimately linked. Here we show, by dissociating Foxp3-dependent features from those induced by the signals preceding and promoting its expression in mice, that the latter signals include several functional and transcriptional hallmarks of Tr cells. Although its function is required for Tr cell suppressor activity, Foxp3 to a large extent amplifies and fixes pre-established molecular features of Tr cells, including anergy and dependence on paracrine IL-2. Furthermore, Foxp3 solidifies Tr cell lineage stability through modification of cell surface and signalling molecules, resulting in adaptation to the signals required to induce and maintain Tr cells. This adaptation includes Foxp3-dependent repression of cyclic nucleotide phosphodiesterase 3B, affecting genes responsible for Tr cell homeostasis.

Foxp3 plays a key role in CD4+ CD25+ T(reg) cell function in mice and represents a specific marker for these cells. Despite the strong association between FOXP3 expression and regulatory function in fresh human T cells, little is known about the dynamics of endogenous FOXP3 expression and its relation to the suppressive function in activated human T cells. Here, we addressed the dynamics of FOXP3 expression during human CD4+ T cell activation by plate-bound anti-CD3 Ab as well as the relationship between its expression and regulatory function at the single-cell level. Our data show that FOXP3 is expressed in a high percentage of activated T cells after in vitro stimulation of human CD4+ CD25- cells. FOXP3 expression is strongly associated with hyporesponsiveness of activated T cells, but is not directly correlated with their suppressive capabilities, as we demonstrate that it is also expressed in activated nonsuppressive T cells. However, in this nonsuppressive T cell population, FOXP3 expression is transient, while it is stably expressed in activated T cells that do display suppressive function, and in natural CD4+ CD25++ T(reg) cells. These data indicate that expression of endogenous FOXP3, in humans, is not sufficient to induce regulatory T cell activity or to identify T(reg) cells.

Thymectomy in mice on neonatal day 3 leads to the development of multiorgan autoimmune disease due to loss of a CD(+)CD25(+) T cell regulatory population in their peripheral lymphoid tissues. Here, we report the identification of a CD4(+) population of regulatory T cells in the circulation of humans expressing high levels of CD25 that exhibit in vitro characteristics identical with those of the CD4(+)CD25(+) regulatory cells isolated in mice. With TCR cross-linking, CD4(+)CD25(high) cells did not proliferate but instead totally inhibited proliferation and cytokine secretion by activated CD4(+)CD25(-) responder T cells in a contact-dependent manner. The CD4(+)CD25(high) regulatory T cells expressed high levels of CD45RO but not CD45RA, akin to the expression of CD45RB(low) on murine CD4(+)CD25(+) regulatory cells. Increasing the strength of signal by providing either costimulation with CD28 cross-linking or the addition of IL-2 to a maximal anti-CD3 stimulus resulted in a modest induction of proliferation and the loss of observable suppression in cocultures of CD4(+)CD25(high) regulatory cells and CD4(+)CD25(-) responder cells. Whereas higher ratios of CD4(+)CD25(high) T cells are required to suppress proliferation if the PD-L1 receptor is blocked, regulatory cell function is shown to persist in the absence of the PD-1/PD-L1 or CTLA-4/B7 pathway. Thus, regulatory CD4 T cells expressing high levels of the IL-2 receptor are present in humans, providing the opportunity to determine whether alterations of these populations of T cells are involved in the induction of human autoimmune disorders.