Perfect partnership

The function of CD4⁺CD25⁺ regulatory T (T_Reg) cells is mediated by cooperation between a transcription factor that functions as a lineage-specification factor for T_Reg cells and a transcription factor that regulates T-cell activation and anergy, report Anjana Rao and colleagues in Cell.

The nuclear factor of activated T cells (NFAT) family of proteins regulates T-cell activation but is also involved in the control of thymocyte development, T-cell differentiation and self-tolerance. Following its activation, NFAT is dephosphorylated and translocates to the nucleus, where it forms strong, cooperative complexes with activator protein 1 (AP1) to induce the expression of a large number of genes that are central to eliciting an immune response. Another transcription factor, forkhead box P3 (FOXP3), is expressed by T_Reg cells and has been identified as a major marker and regulator of the development and function of T_Reg cells.

NFAT is a common regulator in the immune system, where it can switch transcriptional partners

Many of the genes that are targeted by NFAT are also regulated by FOXP3: for example, NFAT activates the interleukin-2 (IL2) and IL4 genes, whereas FOXP3 represses them, and both NFAT and FOXP3 upregulate expression of the T_Reg-cell markers cytotoxic T-lymphocyte antigen 4 (CTLA4) and CD25. So by what mechanism does FOXP3 influence the expression of NFAT-dependent genes? Rao and colleagues investigated the molecular interactions between NFAT and FOXP3. They found that FOXP3 carried out its repressive activity by targeting the cooperative NFAT–AP1 complexes specifically, rather than other NFAT configurations. FOXP3 formed a cooperative complex with NFAT on DNA, excluding AP1 without displacing NFAT from the DNA, as had been proposed previously.

Because the sequences of the forkhead domains of FOXP proteins are highly conserved, the authors could predict the structure of the NFAT–FOXP3 complex based on the crystal structure of an NFAT–FOXP2–DNA complex that they had solved. They then introduced graded, structure-guided mutations into the NFAT-interacting residues of FOXP3 and predicted that these mutations would progressively disrupt the interaction between FOXP3 and NFAT.

To compare the activities of wild-type and mutant FOXP3 proteins, Rao and colleagues expressed both proteins in primary mouse CD4⁺ T cells. The graded mutations at the NFAT–FOXP3 interface did indeed cause a progressive loss of FOXP3 function, and interfered with the ability of FOXP3 to repress expression of IL-2, to upregulate expression of CTLA4 and CD25, and to confer suppressor function in an in vivo mouse model of autoimmune diabetes.

The authors suggest that NFAT is a common regulator in the immune system, where it can switch transcriptional partners from AP1 to FOXP3, thereby converting the effector T-cell activation programme into the T_Reg-cell suppressor programme. Because the selective blocking of the interaction of NFAT with AP1, without interfering with the NFAT–FOXP3 interaction, could therefore induce tolerance, these findings might have important therapeutic implications.