The life cycle stages of Pneumocystis murina have opposing effects on the immune response to this opportunistic, fungal pathogen.

The cyst cell wall β-glucans of Pneumocystis have been shown to stimulate immune responses in lung epithelial cells, dendritic cells, and alveolar macrophages. Little is known about how the trophic life forms, which do not have a fungal cell wall, interact with these innate immune cells. Here, we report differences in the responses of both neonatal and adult mice to the trophic and cystic life cycle stages of Pneumocystis murina The adult and neonatal immune responses to infection with Pneumocystis murina trophic forms were less robust than the response to infection with a physiologically normal mixture of cysts and trophic forms. Cysts promoted the recruitment of nonresident innate immune cells and T and B cells into the lungs. Cysts, but not trophic forms, stimulated increased IFN-γ cytokine concentrations in the alveolar spaces, and an increase in IFN-γ-producing CD4+ T cells. In vitro, bone marrow-derived dendritic cells (BMDCs) stimulated with cysts produced the proinflammatory cytokines IL-1β and IL-6. In contrast, trophic forms suppressed β-glucan-, LTA-, and LPS-induced IL-1β, IL-6, and TNFα production by BMDCs and antigen presentation to CD4+ T cells. The negative effects of trophic forms were not due to ligation of mannose receptor. Our results indicate that optimal innate and adaptive immune responses to Pneumocystis species are dependent on stimulation with the cyst life cycle stage. Conversely, trophic forms suppress β-glucan-induced proinflammatory responses in vitro, suggesting that the trophic forms dampen cyst-induced inflammation in vivo.