BIOCHEMICAL-CHARACTERIZATION OF CANDIDA-ALBICANS EPITOPES THAT CAN ELICIT PROTECTIVE AND NONPROTECTIVE ANTIBODIES

We previously reported that the immunoglobulin M (IgM) monoclonal anti body (MAb) B6.1 protects mice against disseminated candidiasis, wherea s the IgM MAb B6 does not. Both MAbs are specific for an adhesin fract ion isolated from the cell surface of Candida albicans, but their epit ope specificities differ. In the present study, we examined the surfac e locations of both epitopes and obtained structural information regar ding the B6.1 epitope. Immunofluorescence confocal microscopic analysi s of C. albicans yeast forms shelved that epitope B6.1 is displayed ra ther homogeneously over the entire cell surface, whereas epitope B6 ap pears to have a patchy distribution, Both antibodies were essentially nonreactive with the surfaces of mycelial forms of the fungus, indicat ing that neither epitope is expressed on the surfaces of these forms. For isolation of the B6.1 epitope, the adhesin fraction consisting of cell surface phosphomannan was subjected to mildly acidic (10 mM HCl) hydrolysis and was fractionated into acid-labile and acid-stable porti ons by size exclusion chromatography. Antibody blocking experiments sh owed that the B6.1 epitope is an acid-labile moiety of the phospho man nan and that the B6 epitope is located in the acid-stable fraction. Th e B6 epitope appeared to be mannan because it was stable to heat (boil ing) and protease treatments but was destroyed by alpha-mannosidase di gestion. The B6.1 epitope eluted from the size exclusion column in two fractions. Mass spectroscopic analyses showed that one fraction conta ined material with the size of a mannotriose and that the other was a mixture of mannotriose- and mannotetraose-size substances. Dose respon se inhibition tests of the fractions indicated that the B6.1 epitope i s associated with the mannotriose. Nuclear magnetic resonance (NMR) sp ectroscopic analysis of the epitope yielded data consistent with a bet a-(1-->2)-linked mannotriose. The fine structure of the B6 epitope is under investigation. Information derived from these investigations wil l be useful both in understanding protective versus nonprotective anti body responses to C. albicans and in improving anti-Candida vaccine fo rmulations.