Protective and nonprotective human immunoglobulin M monoclonal antibodies to Cyptococcus neoformans glucuronoxylomannan manifest different specificities and gene use profiles

The features of protective murine antibodies to the Cryptococcus neoformans capsular polysaccharide glucuronoxylomannan (GXM) have been rigorously investigated; however, the characteristics of protective human antibodies to GYM have not been defined. We produced monoclonal antibodies (MAbs) from XenoMouse mice (transgenic mice that express human immunoglobulin M [IgM], IgG2, and κ) which were immunized with a C. neoformans serotype D strain 24067 GXM-diphtheria toxoid conjugate. This study reports the specificity and efficacy of three human IgM MAbs, G14, G15, and G19, generated from these mice. Each MAb was specific for GXM, but G14 and G19 had different specificity based on their binding to serotype A strain H99 and SB4 GXMs, to which G15 did not bind. Nucleic acid sequence analysis revealed that G15 uses V_H3-64 in the germ line configuration. G14 and G19 use V_H6-1, which has somatic mutations. All of the MAbs use Vκ DPK22/A27. Studies of MAb efficacy in BALB/c mice showed that administration of 0.1 mg, but not 1 or 0.01 mg, of G15 prolonged survival against lethal C. neoformans strain 24067 challenge, whereas G14 and G19 were not protective at any dose. This panel of MAbs illustrates that serotype D GXM has epitopes that elicit human antibodies that can be either protective or nonprotective. Our findings suggest that V_H. gene use may influence GXM specificity and efficacy, and they provide insights into the possible contribution that V_H gene use may have in resistance and susceptibility to cryptococcosis.