Working on a commitment

Credit: PhotoDisc

Forkhead box O (FOXO) family transcription factors have a well-identified role in T cell commitment to a regulatory phenotype, in part by inducing the expression of the master regulator of regulatory T (T_Reg) cells — FOXP3. New data published in Nature show that FOXO1 also has a crucial role in maintaining the function of T_Reg cells in a manner independent of FOXP3.

FOXO1 also has a crucial role in maintaining the function of T_Reg cells in a manner independent of FOXP3

These studies were carried out using a series of mutant mice labelled for expression of FOXO1 and/or FOXP3, some of which had a conditional deletion of Foxo1 in FOXP3⁺ T_Reg cells or expressed a mutant form of FOXO1 that cannot be exported from the nucleus. In live-imaging experiments, the authors showed that low-dose CD3-specific antibody treatment triggered the translocation of FOXO1 from the nucleus to the cytosol in conventional T cells, but this did not occur in T_Reg cells owing to markedly decreased AKT-mediated phosphorylation of FOXO1 in T_Reg cells. Therefore, T_Reg cells seem to be relatively resistant to the T cell receptor-induced clearance of FOXO1 from the nucleus compared with conventional T cells.

Conditional deletion of Foxo1 in T_Reg cells did not affect the number of T_Reg cells but led to severe pathology — including splenomegaly and lymphadenopathy — comparable to that of T_Reg cell-deficient FOXP3-mutant mice. So the lymphoproliferative disease resulting from the loss of FOXO1 is a consequence of the loss of T_Reg cell function. Expression of a permanently nuclear FOXO1 mutant in FOXO1-deficient T_Reg cells completely rescued the pathology, showing that nuclear FOXO1 activity is required for T_Reg cell function.

By cross-referencing putative FOXO1-binding sites in the nuclei of T_Reg cells against gene expression in wild-type, FOXO1-deficient and nuclear-FOXO1-mutant T_Reg cells, the authors identified 310 direct FOXO1 target genes. A comparison of the direct target genes of FOXO1 and FOXP3 showed that 90–99% of the targets were specifically regulated by only one of the transcription factors. One such FOXO1-specific target gene in T_Reg cells is the gene encoding interferon-γ (IFNγ).

A prominent feature of wild-type T_Reg cells is the inability to produce pro-inflammatory cytokines. By contrast, FOXO1-deficient T_Reg cells had high levels of IFNγ mRNA and protein. In a T cell-transfer model of colitis, disease could be attenuated by wild-type or Foxo1^−/−Ifng^−/− T_Reg cells, but not by Foxo1^−/− T_Reg cells. Together, the data indicate that Ifng is a crucial target gene repressed by FOXO1 for the control of T_Reg cell function.