Model of RelB-mediated regulation of human DC subset generation. RelB promotes the differentiation CD14⁺CD11b⁺ monocytic intermediates from CD34⁺ hematopoietic progenitor cells. These cells can give rise to CD1a⁺CD11b⁺ interstitial-type DCs. CD34⁺ progenitors in which RelB is inhibited (by stable ectopic expression of the RelB inhibitory protein p100ΔN) are arrested at a CD11b^- myeloid cell stage capable of differentiating into other myeloid lineages, including LCs. Our data suggest that CD14^loCD11b^- monocytes represent shared monocyte/LC/DC precursors that require RelB for differentiation into monocytic intermediates of interstitial DCs.