Peripherally derived CD11b(+) myeloid dendritic cells (mDCs), plasmacytoid DCs, CD8alpha(+) DCs and macrophages accumulate in the central nervous system during relapsing experimental autoimmune encephalomyelitis (EAE). During acute relapsing EAE induced by a proteolipid protein peptide of amino acids 178-191, transgenic T cells (139TCR cells) specific for the relapse epitope consisting of proteolipid protein peptide amino acids 139-151 clustered with mDCs in the central nervous system, were activated and differentiated into T helper cells producing interleukin 17 (T(H)-17 cells). CNS mDCs presented endogenously acquired peptide, driving the proliferation of and production of interleukin 17 by naive 139TCR cells in vitro and in vivo. The mDCs uniquely biased T(H)-17 and not T(H)1 differentiation, correlating with their enhanced expression of transforming growth factor-beta1 and interleukins 6 and 23. Plasmacytoid DCs and CD8alpha(+) DCs were superior to macrophages but were much less efficient than mDCs in presenting endogenous peptide to induce T(H)-17 cells. Our findings indicate a critical function for CNS mDCs in driving relapses in relapsing EAE.