Abstract
Early secretion of IL-12 by mouse dendritic cells (DCs) instructs T cells to make IFN-γ. However, only activated, but not naive T cells are able to license DCs for IL-12 production. We hypothesized that it might be due to different levels of CD40L expression on the surface of these cells, as CD40 signals are required for IL-12 production. Using quantitative cell-free systems incorporating CD40L in lipid bilayers combined with total internal reflection fluorescence microscopy and flow cytometry, we show that as low as ∼200 CD40L molecules/μm2 in combination with IL-4 is sufficient to induce IL-12 production by DCs. Remarkably, CD40L alone is adequate to induce IL-23 secretion by DCs. Thus, although activated T cells have somewhat higher levels of CD40L, it is the combination of CD40L and the cytokines they secrete that licenses DCs and influences the effector class of the immune response.