Interleukin (IL)-12 plays a key role not only in protective innate and adaptive T helper cell type 1 (Th1) responses but also in chronic inflammatory diseases. We report here that engagement of CD47 by either monoclonal antibody, its natural ligand thrombospondin (TSP), or 4N1K (a peptide of the COOH-terminal domain of TSP selectively binding CD47) inhibits IL-12 release by monocytes. The suppression occurred after T cell-dependent or -independent stimulation of monocytes and was selective for IL-12 inasmuch as the production of tumor necrosis factor (TNF)-alpha, IL-1, IL-6, and granulocyte/macrophage colony-stimulating factor was not inhibited. CD47 ligation did not alter transforming growth factor (TGF)-beta and IL-10 production, and the suppressive effect on IL-12 was not due to autocrine secretion of TGF-beta or IL-10. The IL-12 inhibition was not mediated by Fcgamma receptor ligation, did not require extracellular Ca(2+) influx, but was reversed by two phosphoinositide 3-kinase inhibitors (wortmannin and Ly294002). Thus, engagement of CD47 on monocytes by TSP, which transiently accumulates at the inflammatory site, is a novel and unexplored pathway to selectively downregulate IL-12 response. The pathway may be relevant in limiting the duration and intensity of the inflammatory response, and in developing novel therapeutic strategies for Th1-mediated diseases.