Monoclonal Antibodies Specific for Immunorecessive Epitopes of Glucuronoxylomannan, the Major Capsular Polysaccharide of Cryptococcus neoformans, Reduce Serotype Bias in an Immunoassay for Cryptococcal Antigen

Immunoassay for detection of glucuronoxylomannan (GXM), the major capsular polysaccharide of Cryptococcus neoformans, is an important tool for diagnosis of cryptococcosis. However, immunoassays that are based solely or in part on detection with polyclonal antibodies may show serotype bias in detection of GXM, particularly limited sensitivity for serotype C. In this study, we describe detection of GXM in an antigen capture sandwich enzyme-linked immunosorbent assay (ELISA) that used a cocktail of two monoclonal antibodies (MAbs). MAb F12D2 was previously produced by immunization with GXM that had been treated to remove O-acetyl groups, a major source of serotype specificity. MAb F12D2 has a high degree of reactivity with GXM of serotypes A, B, C, and D, but the reactivity with serotype D was less than was found with other MAbs. MAb 339 is highly reactive with GXM of serotypes A and D. Use of a combination of the two MAbs produced an immunoassay that had the best properties of both MAbs, including good reactivity with serotype C, which is an emerging threat in sub-Saharan Africa. These results suggest that next-generation immunoassays for diagnosis of cryptococcosis may be formulated by (i) use of immunization and hybridoma screening strategies that are designed to prospectively meet the needs of immunoassay performance and (ii) careful selection of MAbs that span the expected polysaccharide serotypes in the subject patient population.