MONOCLONAL-ANTIBODIES REVEAL ADDITIONAL EPITOPES OF SEROTYPE-D CRYPTOCOCCUS-NEOFORMANS CAPSULAR GLUCURONOXYLOMANNAN THAT ELICIT PROTECTIVE ANTIBODIES

Epitope specificity and isotype influence mAb efficacy against Cryptoc occus neoformans; however, the relative contribution of each attribute is poorly understood. To date, only mAbs that recognize two epitopes of capsular glucuronoxylomannan (GXM), defined by the IgG1 mAbs 2H1 an d E1, consistently mediate protection against C. neoformans. The role of epitope specificity was further examined using six additional IgG1 mAbs and serotype D C. neoformans ATCC 24067. mAbs 3C2, 439, and 471 r ecognize the 2H1 epitope, whereas mAbs 339, 1255, and 302 recognize tw o separate epitopes. mAbs 3C2, 439, and 471 competed for GXM with the IgA mAb 18G9, a 2H1 mAb family member, whereas mAbs 302, 339, and 1255 did not. Each mAb bound GXM similarly, as determined by agglutination , direct Ag binding, Ag inhibition, and indirect capsular immunofluore scence assays. mAb apparent affinity constants for GXM ranged from 5 t o 26 x 10(7) M-(1) with mAb 1255 > 3C2 > 339 > 439 > 471 > 302. Each m Ab significantly prolonged survival (p < 0.05); the average survival t imes of control and mice passively immunized with mAbs3C2, 302, 339, 4 39, 471, and 1255 were 10.8, 36.6, 33, 25.5, 24.9, 17, and 22.6 days, respectively. Although each mAb enhanced J774.16 cell fungicidal activ ity, differences were observed in the ability of each mAb to facilitat e attachment and ingestion of cryptococci. These results indicate 1) t wo additional epitope specificities associated with mAb efficacy, 2) d ifferences in opsonic and protective efficacy for IgG1 anti-GXM mAbs, 3) an association between affinity and protective efficacy, and 4) add itional support for association between an annular indirect capsular i mmunofluorescence pattern and mAb efficacy.