Activated T cells induce interleukin-12 production by monocytes via CD40-CD40 ligand interaction.

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.