ABSTRACTThe fungal pathogenHistoplasma capsulatumparasitizes host phagocytes. To avoid antimicrobial immune responses,Histoplasmayeasts must minimize their detection by host receptors while simultaneously interacting with the phagocyte. PathogenicHistoplasmayeast cells, but not avirulent mycelial cells, secrete the Eng1 protein, which is a member of the glycosylhydrolase 81 (GH81) family. We show thatHistoplasmaEng1 is a glucanase that hydrolyzes β-(1,3)-glycosyl linkages but is not required forHistoplasmagrowthin vitroor for cell separation. However,Histoplasmayeasts lacking Eng1 function have attenuated virulencein vivo, particularly during the cell-mediated immunity stage.Histoplasmayeasts deficient for Eng1 show increased exposure of cell wall β-glucans, which results in enhanced binding to the Dectin-1 β-glucan receptor. Consistent with this, Eng1-deficient yeasts trigger increased tumor necrosis factor alpha (TNF-α) and interleukin-6 (IL-6) cytokine production from macrophages and dendritic cells. While not responsible for large-scale cell wall structure and function, the secreted Eng1 reduces levels of exposed β-glucans at the yeast cell wall, thereby diminishing potential recognition by Dectin-1 and proinflammatory cytokine production by phagocytes. In α-glucan-producingHistoplasmastrains, Eng1 acts in concert with α-glucan to minimize β-glucan exposure: α-glucan provides a masking function by covering the β-glucan-rich cell wall, while Eng1 removes any remaining exposed β-glucans. Thus,HistoplasmaEng1 has evolved a specialized pathogenesis function to remove exposed β-glucans, thereby enhancing the ability of yeasts to escape detection by host phagocytes.IMPORTANCEThe success ofHistoplasma capsulatumas an intracellular pathogen results, in part, from an ability to minimize its detection by receptors on phagocytic cells of the immune system. In this study, we showed thatHistoplasmapathogenic yeast cells, but not avirulent mycelia, secrete a β-glucanase, Eng1, which reduces recognition of fungal cell wall β-glucans. We demonstrated that the Eng1 β-glucanase promotesHistoplasmavirulence by reducing levels of surface-exposed β-glucans on yeast cells, thereby enablingHistoplasmayeasts to escape detection by the host β-glucan receptor, Dectin-1. As a consequence, phagocyte recognition ofHistoplasmayeasts is reduced, leading to less proinflammatory cytokine production by phagocytes and less control ofHistoplasmainfectionin vivo. Thus,Histoplasmayeasts express two mechanisms to avoid phagocyte detection: masking of cell wall β-glucans by α-glucan and enzymatic removal of exposed β-glucans by the Eng1 β-glucanase.
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