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CD40 ligand/CD40 stimulation regulates the production of IFN-gamma from human peripheral blood mononuclear cells in an IL-12- and/or CD28-dependent manner.

CD40 ligand (CD40L)/CD40 costimulation is an important regulator of Th1 responses. Two mechanisms by which CD40L/CD40 stimulation may enhance IFN-gamma are via direct induction of IL-12 and augmentation of the expression of costimulatory molecules such as B7 from APCs. We examined the ability of CD40L/CD40 stimulation to regulate the production of IFN-gamma through IL-12 and/or CD28 costimulation from human PBMCs stimulated with T cell-specific stimuli. The roles of exogenous and endogenous CD40L/CD40 stimulation were evaluated using a trimeric soluble CD40L agonist (CD40T) and an anti-CD40L Ab, respectively. The presence of CD40T in cultures increased the production of IL-12 and IFN-gamma from PBMCs stimulated with varying amounts of PHA. The mechanism, however, by which CD40T enhanced IFN-gamma varied according to the level of T cell activation. Under maximal stimulatory conditions (PHA, 1/100), an IL-12-dependent pathway was dominant. At relatively low levels of T cell stimulation (PHA, 1/500 and 1/1000), however, an additional IL-12-independent CD28-dependent pathway was elucidated. We further studied the role of exogenous CD28 stimulation in regulating the production of IFN-gamma. The enhancement of IFN-gamma production induced by direct CD28 stimulation was primarily dependent on endogenous IL-12 or CD40L/CD40 stimulation. Together, these data suggest that the production of IFN-gamma involves a complex interaction between two interdependent, yet distinct, costimulatory pathways and provide evidence that CD40T may be an effective adjuvant for the enhancement of responses.