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.

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It is now recognized that IL-12 plays a predominant role in protective immunity against intracellular pathogens by promoting the development of T helper type 1 (Thl) responses. We here report the unexpected observations that IL-12 exerts differential effects on the maturation of "naive" human CD4 T cells isolated from umbilical cord blood or from the blood of healthy adults.

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