IL-12 UP-REGULATES CD40 LIGAND (CD154) EXPRESSION ON HUMAN T-CELLS

IL-12 is a heterodimeric cytokine produced by APC that promotes the de velopment of CD4(+) Thl cells and their IFN-gamma production after TCR /CD3 triggering. We here investigated the capacity,, of IL-ll to modif y the expression on T cells of CD40 ligand (CD40L or CD154), a molecul e transiently expressed on activated T cells and known to be of utmost importance for cognate interaction with B cells and for activation of dendritic cells and macrophages. Our data demonstrate that IL-12 up-r egulates CD40L expression on anti-CD3-activated human peripheral blood T cells, For optimal induction of CD40L. IL-12 synergizes with IL-2 a s well as with other costimulatory interactions, such as B7/CD28. The effect of IL-12 was observed at both the protein and the mRNA level, T cells costimulated by IL-12 provided more efficient help for IL-4-dep endent B cell proliferation and for Ige; production than when activate d in the absence of IL-12. This helper activity was blocked by an mAb against CD40L, indicating that the effect of IL-12 on B cells is media ted indirectly through CD40L. The data thus suggest that the effects o f IL-12 on cellular and humoral immune responses are partly mediated t hrough CD40L induction.