The principal host cell of H. capsulatum (Hc) is the M phi within which the pathogenic yeast phase of the fungus multiplies during active disease. The initial interaction between Hc yeasts and M phi therefore is a crucial step in the pathogenesis of histoplasmosis. In the present study, we have identified the major receptor mechanism that mediates the attachment of unopsonized Hc yeasts to human monocyte-derived M phi from peripheral blood. Binding of Hc yeasts by M phi is rapid, temperature dependent, and requires both Ca and Mg ions for optimum activity. Recognition of Hc yeasts does not require Fc receptors, mannosyl/fucosyl receptors, beta-glucan receptors, or secretion of C3 by M phi. Studies were performed on the effect of down regulating specific receptors of the CR3/LFA-1/p150,95 adherence-promoting protein family from the apical portion of M phi to determine the effects upon binding of Hc yeasts. Anti-beta chain mAbs that recognize all three of these proteins blocked binding of yeasts. However, removal of individual receptors with antibodies against the alpha polypeptides caused negligible depression of binding, and removal of any pair caused only modest depression. Thus, each of the members of the CR3/LFA-1/p150,95 family is independently capable of binding Hc. The delineation of this new mechanism for nonopsonic recognition by M phi that is exploited by Hc yeasts will aid in future studies to identify the Hc ligand, to elucidate the stoichiometry of CR3/LFA-1/p150,95 binding, and to determine triggering mechanisms for release of toxic oxygen metabolites.

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