Dendritic cells (DCs) play an important role in CD4⁺ T helper (Th) cell differentiation and in the initiation of both protective and pathogenic immunity. Granulocyte/macrophage colony-stimulating factor (GM-CSF) is a DC growth factor critical for the induction of experimental autoimmune encephalomyelitis (EAE) and other autoimmune diseases, yet its mechanism of action in vivo is not fully defined. We show that GM-CSF is directly required for the accumulation of radiosensitive dermal-derived langerin⁺CD103⁺ DCs in the skin and peripheral lymph nodes under steady-state and inflammatory conditions. Langerin⁺CD103⁺ DCs stimulated naive myelin-reactive T cells to proliferate and produce IFN-γ and IL-17. They were superior to other DC subsets in inducing expression of T-bet and promoting Th1 cell differentiation. Ablation of this subset in vivo conferred resistance to EAE. The current report reveals a previously unidentified role for GM-CSF in DC ontogeny and identifies langerin⁺CD103⁺ DCs as an important subset in CD4⁺ T cell–mediated autoimmune disease.