Immunization with Apoptotic Phagocytes Containing Histoplasma capsulatum Activates Functional CD8 + T Cells To Protect against Histoplasmosis

Abstract

ABSTRACT We have previously revealed the protective role of CD8 + T cells in host defense against Histoplasma capsulatum in animals with CD4 + T cell deficiency and demonstrated that sensitized CD8 + T cells are restimulated in vitro by dendritic cells that have ingested apoptotic macrophage-associated Histoplasma antigen. Here we show that immunization with apoptotic phagocytes containing heat-killed Histoplasma efficiently activated functional CD8 + T cells whose contribution was equal to that of CD4 + T cells in protection against Histoplasma challenge. Inhibition of macrophage apoptosis due to inducible nitric oxide synthase (iNOS) deficiency or by caspase inhibitor treatment dampened the CD8 + T cell but not the CD4 + T cell response to pulmonary Histoplasma infection. In mice subcutaneously immunized with viable Histoplasma yeasts whose CD8 + T cells are protective against Histoplasma challenge, there was heavy granulocyte and macrophage infiltration and the infiltrating cells became apoptotic. In mice subcutaneously immunized with carboxyfluorescein diacetate succinimidyl ester (CFSE)-labeled apoptotic macrophages containing heat-killed Histoplasma , the CFSE-labeled macrophage material was found to localize within dendritic cells in the draining lymph node. Moreover, depleting dendritic cells in immunized CD11c-DTR mice significantly reduced CD8 + T cell activation. Taken together, our results revealed that phagocyte apoptosis in the Histoplasma -infected host is associated with CD8 + T cell activation and that immunization with apoptotic phagocytes containing heat-killed Histoplasma efficiently evokes a protective CD8 + T cell response. These results suggest that employing apoptotic phagocytes as antigen donor cells is a viable approach for the development of efficacious vaccines to elicit strong CD8 + T cell as well as CD4 + T cell responses to Histoplasma infection.