Interleukin-12 regulates the production of Bacille Calmette-Guérin-induced interferon-gamma from human cells in a CD40-dependent manner

Interferon (IFN)-gamma is a cytokine which plays a critical role in the host defence against human tuberculosis infection. There is evidence that interleukin (IL)-12 can exert a potent effect in stimulating the production of IFN-gamma, and it is well known that a costimulatory signal provided by CD40 may enhance IL-12 production by monocytes/macrophages. However, it is unclear whether CD40-CD40L stimulation is able to regulate the production of mycobacterial-induced IFN-gamma through an IL-12-dependent pathway. In this study, we investigated the capacity of the Bacille Calmette-Guérin (BCG) strain of Mycobacterium bovis to induce the production of interferon-gamma through IL-12 and/or CD40 costimulation from human cells. Our data demonstrate that anti-IL-12 antibodies markedly reduced the levels of IFN-gamma produced by the BCG-stimulated human cells, while exogenous recombinant (r)IL-12 up-regulated the production of IFN-gamma. In addition, the stimulatory effect of IL-12 on BCG-induced IFN-gamma secretion was specific, as it was significantly abolished in the presence of anti-IL-12 antibodies. We also observed that the presence of an anti-CD40L monoclonal antibody significantly inhibited the production of IL-12 and IFN-gamma by human cells activated with BCG. In contrast, an isotype control antibody showed no effect on cytokine production. Furthermore, the presence of a trimeric soluble CD40L agonist (CD40T) in cultures increased the production of IL-12 and IFN-gamma. Importantly, the stimulatory capacity of CD40T on BCG-induced IFN-gamma secretion was blocked by a monoclonal antibody against IL-12, indicating that the effect of CD40T on T cells was mediated through IL-12. Together, these studies are the first to demonstrate that BCG-induced IFN-gamma production by human cells appears to be mediated by IL-12 in a CD40-dependent manner and suggest that CD40-CD40L activation may be a critical mediator in regulating the immune response to stimulation with BCG.