The nature of the T-cell response to antigen is governed by the activation state of the antigen-presenting dendritic cell (DC). Immature or resting DCs have been shown to induce T-cell responses that may protect against the development of autoimmune disease. Effectively harnessing this "tolerogenic" effect of resting DCs requires that it be disease-specific and that activation of DCs by manipulation ex vivo is avoided. We reasoned that this could be achieved by transferring in vivo partially differentiated myeloid progenitor cells encoding a disease-specific autoantigen. With the aim of preventing autoimmune diabetes, we transferred myeloid progenitor cells encoding proinsulin into NOD mice. Bone marrow (BM) was cultured in granulocyte macrophage colony-stimulating factor (GM-CSF) and transforming growth factor-beta1, a cytokine combination that expands myeloid cells but inhibits terminal DC differentiation, to yield Gr-1(+)/CD11b(+)/CD11c(-) myeloid progenitor cells and a minor population of CD11c(+)/CD11b(+)/CD86(lo) immature DCs. After transfer, Gr-1(+) myeloid cells acquired the characteristics of resting DCs (CD11c(+)/MHC classII(int)/CD86(lo)/CD40(lo)). Gr-1(+) myeloid cells generated from transgenic NOD mice that expressed proinsulin controlled by a major histocompatibility complex (MHC) class II promoter, but not from wild-type NOD mice, transferred into 4-week-old female NOD mice significantly suppressed diabetes development. The transfer of DC progenitors encoding a disease-specific autoantigen is, therefore, an effective immunotherapeutic strategy that could be applied to humans.