The role of CD40L in T cell-dependent nitric oxide production by murine macrophages

Upon activation, T cells express CD40L, a member of the TNF cytokine superfamily, which serves as a ligand for CD40 on antigen presenting cells, including dendritic cells, B cells, and macrophages. While initial studies on the function of CD40 focused its role in the regulation of B cell activation, more recent studies have indicated that CD40 ligation may be critical for the initiation of T cell-dependent macrophage activation, including stimulation of nitric oxide production. However, the relative contribution of the CD40 pathway in macrophage nitric oxide production during T-dependent immune responses remains unclear. We have found that while CD40 ligation of macrophages can stimulate nitric oxide production, disruption of CD40 signaling during a T cell-mediated alloimmune response has no appreciable effect on nitric oxide production. If the T cell alloimmune response is restricted to CD4 cells, CD40L blockade has only a minimal effect on nitric oxide production. Rather, IFNgamma, produced by alloactivated T cells, seems to be a necessary 'first' signal for nitric oxide production, while TNFalpha and CD40L each provide independent 'second' signals. Finally, we demonstrate that CD40L stimulates macrophage NO production independent from autocrine TNFalpha stimulation. These results suggest that macrophage nitric oxide production during a T-dependent immune response requires IFNgamma production by CD4 cells whereas TNFalpha and CD40L can each provide important functionally overlapping 'second' signals to costimulate nitric oxide production, though neither is required.