Lineage-specific transcription factors in unexpected places.

Foxp3 is a transcription factor closely associated with the Treg lineage in humans and mice. In the immune system, Foxp3 appears highly specific for Treg, and is not known to be expressed by other immune cell types. In this issue of the European Journal of Immunology, an article reports that human DC transfected with ectopic Foxp3 unexpectedly acquire an immunosuppressive phenotype. Foxp3-transfected DC suppressed proliferation of naive T cells, and biased the differentiation of CD4(+) cells into Treg-like cells that themselves expressed Foxp3. The molecular mechanism of these effects required functional activity of the immunoregulatory enzyme IDO. Thus, a transcription factor not native to DC nevertheless conferred elements of a regulatory phenotype following ectopic expression.