Previous studies on the production of interleukin-12 (IL-12) have shown that it is released, together with other proinflammatory cytokines, shortly after exposure of phagocytic cells to a variety of pathogens. We here report that IL-12 is also released during the recall response to soluble antigen (Ag) devoid of intrinsic adjuvant activity. We show that activated T cells induce the production of IL-12 by monocytes via a mechanism involving the interaction of T cell-associated CD40 ligand with CD40 on monocytes. The data suggest that Ag presentation on monocytes favors the persistence of type 1 responses.

Interleukin-12 is a recently discovered lymphokine displaying an array of in vitro activities suggesting a major role in protective immunity against infectious agents like viruses. This study provides evidence that IL-12 may also be implicated in the selection of the immunoglobulin isotypes. We show that picomolar concentrations of rIL-12 markedly inhibit the synthesis of IgE by IL4-stimulated PBMC. The suppression of IgE is observed at the protein and at the mRNA levels, it is isotype specific, and it is abolished by neutralizing anti-IL-12 mAbs. IL-12 may suppress IgE synthesis by: (a) inducing the production of IFN-y, a known inhibitor of IgE synthesis and (b) by a novel mechanism which is IFN-y independent. The best evidence for this is from studies on IgE synthesis by IL-4-plus hydrocortisone-stimulated umbilical cord blood lymphocytes, which do not produce detectable amounts of IFN-'y. In such cultures, rIL-12 inhibits IgE synthesis even in the presence of a large excess of neutralizing anti-IFN-y mAb. (J. Clin. Invest. 1992. 90:262-266.)

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)-α, IL-1, IL-6, and granulocyte/macrophage colony-stimulating factor was not inhibited. CD47 ligation did not alter transforming growth factor (TGF)-β and IL-10 production, and the suppressive effect on IL-12 was not due to autocrine secretion of TGF-β or IL-10. The IL-12 inhibition was not mediated by Fcγ receptor ligation, did not require extracellular Ca2+ 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.

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