A schematic DC differentiation model in vitro from Lin⁻c-kit⁺ HPCs and the regulating role of TGF-β1. HPCs develop into mature DCs through four stages: proliferating DC progenitor cells, nonproliferating DC precursors, antigen capturing immature DCs, and mature DCs. The cytokine combination of GM-CSF + SCF + TNF can induce the generation of mature DCs from murine Lin⁻c-kit⁺ HPCs through two unrelated differentiation pathways: CD11b^−/dullCD11c⁺ and CD11b^+hiCD11c⁺ DC precursors that can be clearly identified at day 6 of culture. In response to GM-CSF + TNF, both the DC precursor subsets can independently differentiate at day 10 to 14 into mature DCs with distinct phenotype based on the expression of c-fms mRNA, NSE activity, and E-cadherin. TGF-β1 significantly inhibited the generation of CD11b^−/dullCD11c⁺ and CD11b^+hiCD11c⁺ DC precursors from GM-CSF + SCF + TNF-stimulated Lin⁻c-kit⁺HPCs at day 6 of culture. TGF-β1 could also suppress DC maturation from CD11b^+hiCD11c⁺, but not CD11b^−/dullCD11c⁺ DC precursors at day 12 to 14. In collaboration with GM-CSF + SCF, TGF-β1 induced Lin⁻c-kit⁺ HPCs to differentiate solely into monocytes/macrophages. These cells could further differentiate at day 15 to 17 of culture into LC-like DCs expressing high levels of E-caderin, abundant c-fms, and NSE activity, which obviously differs from CD11b^−/dullCD11c⁺ and CD11b^+hiCD11c⁺ DC precursor-derived mature DC subsets.