Generation of a recombinant 65-kilodalton mannoprotein, a major antigen target of cell-mediated immune response to Candida albicans

A 65-kDa mannoprotein (CaMp65) has long been studied as a major, immunodomi nant antigen of the human opportunistic pathogen Candida albicans. An expre ssion library of C. albicans was screened with serum from mice immunized wi th ScMp65 (ScW10), a Saccharomyces cerevisiae recombinant protein of about 48 kDa. This serum recognized the CaMp65 from a cell wall extract of C. alb icans. After cloning and sequencing of the relevant C. albicans cDNA, an op en reading frame encoding a protein of 379 amino acids was identified. Its deduced amino acid sequence showed regions of identity with all previously characterized tryptic fragments of CaMp65, as well as with the correspondin g regions of ScMp65. A prepeptide of 32 amino acids,vith signal peptidase a nd Kex2 cleavage sites as well as a high number of potential O-glycosylatio n sites but no N-glycosylation sites or GPI anchor were observed in sequenc e studies of CaMp65. A putative adhesin RGD sequence was also present in th e C-terminal region of the molecule. This triplet was absent in the ScMp65. The relevant gene (designated CaMP65) was localized to chromosome R of C. albicans as determined by pulse-held gel electrophoresis. Northern blot ana lysis demonstrated that gene transcription was heat inducible and associate d with germ-tube formation by the fungus. A recombinant, His(6)-tagged prot ein (rCaMp65) was expressed in Escherichia coli under an inducible promoter . After purification by nickel-chelate affinity chromatography, the recombi nant product mas detected as a 47-kDa protein band in immunoblots with the anti-ScMp65 serum, as well as with CaMp65-specific monoclonal antibodies. B oth ScMp65 and CaMp65 were assayed for antigenic stimulation in cultures of peripheral blood mononuclear cells (PBMC) from 10 unselected human donors. While ScMp65 was substantially nonstimulatory, both rCaMp65 and the native CaMp65 were equally able to induce lymphoproliferation of the PBMC from al l the donors. In addition, a number of CD4(+) T-cell clones were generated using a C. albicans mannoprotein fraction as an antigenic stimulant. Severa l of these clones specifically responded to both the native and the recombi nant C. albicans Mp65 but not to ScMp65. Thus, the recombinant Mp65 of C. a lbicans retains antigenicity and, as such, could be a valid, standardized r eagent for serodiagnostic and immunological studies.