Cryptococcus neoformans Galactoxylomannan contains an epitope(s) that is cross-reactive with Aspergillus Galactomannan

Abstract

We report a case of cryptococcosis in which a serum enzyme-linked immunosorbent assay (ELISA) for Aspergillus galactomannan was positive, with no evidence of aspergillosis. Soluble antigens from 19 Cryptococcus neoformans strains and purified carbohydrates of C. neoformans capsule were thus assayed in the Aspergillus galactomannan ELISA. Antigens from all C. neoformans strains, and purified galactoxylomannan, gave a positive reaction, suggesting that C. neoformans galactoxylomannan contains an epitope(s) that is cross-reactive with Aspergillus galactomannan.

FIG. 1.

Proposed structures of the Aspergillus fumigatus galactomannan (adapted from references and with permission) and the Cryptococcus neoformans galactoxylomannan (adapted from reference with permission). The repeating unit of the galactomannan secreted by Aspergillus fumigatus consists of a linear core of mannan residues with α-1,2- and α-1,6-linked residues. Antigenic side chains composed exclusively of β-1,5 galactofuranosyl units are linked to the C-6 and C-3 positions of the α-1,2-linked mannose units of the core. These side chains have an average degree of polymerization of 4 and are the target of the antigalactomannan monoclonal antibody employed in the Platelia Aspergillus test. The repeating unit of the Cryptococcus neoformans galactoxylomannan consists of a backbone of α-1,6-linked galactose units, with side chains composed of galactose, mannose, xylose, and O-acetyl residues branched on alternate galactose residues. Small quantities of galactofuranose are present in the GalXM in some, but not all, C. neoformans isolates (6). The linkage of these galactofuranose units has not been characterized (14). M, mannan; G, galactose; Gf, galactofuranose; X, xylose.