The induction of type 1 immune responses (interleukin [IL]-12, interferon [IFN]-γ) 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-γ, IL-12, nitric oxide, or tumor necrosis factor α. Moreover, CD40L^−/− mice treated with anti–IFN-γ or anti–IL-12 at the time of infection had accelerated mortality, providing further evidence that IL-12 and IFN-γ 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-γ had accelerated mortality, demonstrating that IFN-γ 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.

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