CD40 ligand is not essential for induction of type 1 cytokine responses or protective immunity after primary or secondary infection with Histoplasma capsulatum

The induction of type 1 immune responses (interleukin [IL]-12, interferon [IFN]-gamma) has been shown to be important in mediating protection against many intracellular infections including Histoplasma capsulatum. Costimulatory molecules such as CD40 ligand (CD40L) have been shown to be a central regulator of type 1 responses in vivo. To study the role of CD40L in mediating protection against infection with H. capsulatum, CD40L-deficient (CD40L(-/-)) and CD40L(+/+) mice were infected with H. capsulatum and assessed for various parameters. After a lethal challenge of H. capsulatum, CD40L(-/-) mice were not substantially different from CD40L(+/+) mice in terms of mortality, fungal burden, or production of IFN-gamma, IL-12, nitric oxide, or tumor necrosis factor alpha. Moreover, CD40L(-/-) mice treated with anti-IFN-gamma or anti-IL-12 at the time of infection had accelerated mortality, providing further evidence that IL-12 and IFN-gamma are produced in vivo in the absence of CD40L. In addition, CD40L(-/-) mice infected with a sublethal dose of H. capsulatum survived infection, whereas all mice infected with the same dose and treated with anti-IFN-gamma had accelerated mortality, demonstrating that IFN-gamma but not CD40L was essential for primary immunity to H, capsulatum infection. Interestingly, depletion of either CD4(+) or CD8(+) T cells resulted in accelerated mortality in CD40L(-/-) mice, suggesting a critical role for these cells in response to infection. Finally, CD40L(-/-) mice initially infected with a sublethal dose of H. capsulatum were protected from secondary infection with a lethal dose of H. capsulatum, demonstrating that CD40L is not required for the maintenance of memory immunity.