CD4 and acute graft vs. host disease: IL-34 can be secreted by FOXP3-expressing CD4+ and CD8+ Tregs in humans and CD8+CD45RClow/- Tregs in rats (61) and, at least in vitro, it is more efficient at inducing FOXP3-expressing Tregs than M-CSF-1 and these cells efficiently control acute graft-vs.-host disease in vivo in a model of immune humanized immunodeficient mice (62).