Transcription factor Foxp3 is essential for the development and function of regulatory T cells, but it does not act in isolation. Benoist and colleagues identify a quintet of factors that facilitate transcriptional regulation by Foxp3. The transcription factor Foxp3 participates dominantly in the specification and function of Foxp3+CD4+ regulatory T cells (Treg cells) but is neither strictly necessary nor sufficient to determine the characteristic Treg cell signature. Here we used computational network inference and experimental testing to assess the contribution of other transcription factors to this. Enforced expression of Helios or Xbp1 elicited distinct signatures, but Eos, IRF4, Satb1, Lef1 and GATA-1 elicited exactly the same outcome, acting in synergy with Foxp3 to activate expression of most of the Treg cell signature, including key transcription factors, and enhancing occupancy by Foxp3 at its genomic targets. Conversely, the Treg cell signature was robust after inactivation of any single cofactor. A redundant genetic switch thus 'locked in' the Treg cell phenotype, a model that would account for several aspects of Treg cell physiology, differentiation and stability.

In the maternal decidua, natural killer (NK) cells, characterized by lack of CD16, are found in direct contact with the fetal extravillous trophoblasts (EVTs). It is yet unknown which factors contribute to the specific homing of this unique NK subset to the decidua. In this study we analyze the chemokine receptor repertoire on various NK populations derived from the peripheral blood and decidua. We show that CXCR4 and CXCR3 receptors are preferentially expressed on CD16– NK subsets derived either from the peripheral blood or the decidua and that these receptors are involved in migration of all NK subsets to their ligands. We further demonstrate in vivo that invading EVTs that eventually perform endovascular invasion express CXCL12, the ligand for CXCR4, but not ligands for CXCR3. Indeed, specific accumulation of the CD16– NK cells at the expense of CD16+ cells was observed only when in vitro migration was performed with ligands for CXCR4. Finally, incubation of the peripheral blood CD16– NK cells with cytokines present in the decidua, especially interleukin 15 (IL-15), resulted in the expression of chemokine receptor repertoire similar to that observed on decidual NK cells, suggesting an additional important regulatory effect of local decidual cytokines.