Model of TGF-β1-dependent LC lineage commitment. Our serum-free differentiation model revealed that the basic cytokine combination SCF, Flt3L, GM-CSF, plus TNFα favors monopoiesis. If TGF-β1 is added, monocyte differentiation is repressed and LCs are induced.^5,7  This lineage-modulating effect of TGF-β1 is associated with marked PU.1 and Id2 up-regulation by progenitor cells. We show here that neither ectopic PU.1 nor ectopic Id2 is sufficient to re-establish LCs in cultures in the absence of TGF-β1. Nevertheless, our data suggest that both factors critically contribute to LC lineage commitment by acting at 2 distinct intersection points. PU.1 strongly enhances LC induction by TGF-β1. Conversely, ectopic Id2 inhibits acquisition of monocytic characteristics by cells generated in the absence of TGF-β1. PU.1 is generally up-regulated during myeloid DC generation (eg, also by Delta-1 [our study] or by IL-4 costimulation²¹ ). Furthermore, ectopic PU.1 induces DC differentiation in several cell systems.^20,21  However, for LCs (DCs of the epithelial/mucosal type), PU.1 lacks lineage instructive activity. These cells still required exogenous TGF-β1 even when PU.1-transduced progenitor cells were stimulated with all the other DC cytokines. Therefore, our data suggest that TGF-β1 downstream signaling processes (X) have to cooperate with PU.1 for LC lineage commitment.