The different binding patterns of two immunoglobulin M monoclonal antibodies to Cryptococcus neoformans serotype A and D strains correlate with serotype classification and differences in functional assays.

Cryptococcus neoformans var. neoformans strains have historically been divided into serotypes A and D on the basis of reactivity with rabbit sera. Previously, we noted that two murine immunoglobulin M monoclonal antibodies (MAbs) to the capsular glucuronoxylomannan produced different indirect immunofluorescence (IF) patterns, described as annular and punctate, when bound to C. neoformans cells from different strains. In this study, we examined the reactivity of these two MAbs, known as 12A1 and 13F1, with 20 C. neoformans var. neoformans strains, of which 13 were serotype A and 7 were serotype D. For all strains, MAb binding was studied by IF and agglutination assays. In addition, we blindly tested the IF patterns of 22 C. neoformans var. neoformans strains. For selected strains, MAb binding was studied by flow cytometry (FACScan) and phagocytosis assays. The epitopes recognized by MAbs 12A1 and 13F1 were found in all of the strains. MAb 12A1 binding produced an annular IF pattern with all of the strains, irrespective of the serotype classification. MAb 13F1 binding produced annular binding with all of the serotype A strains and punctate binding with 19 of 20 serotype D strains. In general, the punctate IF pattern was associated with lower fluorescence intensity, a requirement for higher antibody concentrations to produce yeast cell agglutination, and lower opsonic efficacy. Our results provide strong support for the existing classification of two serological types for strains assigned to variety neoformans and indicate qualitative and quantitative antigenic differences among serotype A and D strains.