CD40/CD40 ligand interactions are required for T cell-dependent production of interleukin-12 by mouse macrophages

We have previously shown that T cell receptor-activated mouse T helper (Th)l clones induce the production of interleukin (IL)-12 by splenic antigen-presenting cells (APC). Here, we show that the expression of CD40L by activated T cells is critical for T cell-dependent IL-12 production by mouse macrophages. IL-12 was produced in cultures containing alloreactive Thl clones stimulated with allogeneic peritoneal macrophages. or in cultures of splenocytes stimulated with anti-CD3. Anti-CD40L monoclonal antibodies (mAb) inhibited the production of IL-12, but not IL-2, in these cultures by greater than or equal to 90% and had dramatic inhibitory effects on antigen-dependent proliferation of Thl clones. In addition. both activated T cells and a thl clone derived from CD40L knockout mice failed to induce IL-12 production from splenic APC or peritoneal macrophages. Finally, macrophages cultured in the absence of T cells produced IL-12 upon stimulation with soluble recombinant CD40L in combination with either supernatants from activated Thl clones or with interferon-gamma and granulocyte/macrophage colony-stimulating factor. Thus. both CD40L-dependent and cytokine-mediated signals from activated T cells are required to induce the production of IL-12 by macrophages. A blockade at the level of IL-12 production may explain. at least in part. the dramatic ability of anti-CD40L mAb to inhibit disease in animal models that are dependent upon the generation of a cell-mediated immune response. Moreover. a defect in T cell-dependent induction of IL-12 may contribute to the immune status of humans that lack functional CD40L.