Binding and Internalization of Glucuronoxylomannan, the Major Capsular Polysaccharide of Cryptococcus neoformans , by Murine Peritoneal Macrophages

Abstract

ABSTRACT Glucuronoxylomannan (GXM), the major component of the capsular polysaccharide of Cryptococcus neoformans , is essential to virulence of the yeast. Previous studies found that the interaction between GXM and phagocytic cells has biological consequences that may contribute to the pathogenesis of cryptococcosis. We found that GXM binds to and is taken up by murine peritoneal macrophages. Uptake is dose and time dependent. Examination of the sites of GXM accumulation by immunofluorescence microscopy showed that the pattern was discontinuous and punctate both on the surfaces of macrophages and at intracellular depots. Although resident macrophages showed appreciable accumulation of GXM, uptake was greatest with thioglycolate-elicited macrophages. A modest stimulation of GXM binding followed treatment of resident macrophages with phorbol 12-myristate 13-acetate, but treatment with lipopolysaccharide or gamma interferon alone or in combination had no effect. Accumulation of GXM was critically dependent on cytoskeleton function; a near complete blockade of accumulation followed treatment with inhibitors of actin. GXM accumulation by elicited macrophages was blocked by treatment with inhibitors of tyrosine kinase, protein kinase C, and phospholipase C, but not by inhibitors of phosphatidylinositol 3-kinase, suggesting a critical role for one or more signaling pathways in the macrophage response to GXM. Taken together, the results demonstrate that it is possible to experimentally enhance or suppress binding of GXM to macrophages, raising the possibility for regulation of the interaction between this essential virulence factor and binding sites on cells that are central to host resistance.