Differential cell surface expression of mannoprotein epitopes in yeast and mycelial forms of Candida albicans

The ultrastructural localization of mannoprotein constituents (MP) of the cell wall of yeast and hyphal forms of Candida albicans was studied by immunoelectron microscopy. To this aim, two monoclonal antibodies (mAbs AF1 and 1D10), recognizing distinct oligomannoside epitopes of MP molecules, and a second antibody coupled to colloidal gold, were employed. Preembedding methods revealed the presence of both AF1- and, albeit to a lesser intensity, 1D10-epitopes within the fibrillar, capsular layer of yeast cells of the fungus, provided this capsule was preserved and stabilized by treatment of whole cells with Concanavalin A. These cell surface-associated MP were absent in hyphal cells, despite the presence in these cells of a capsular layer not different in form and thickness from that present in yeast cells. Postembedding methods showed that both yeast and hyphal forms of growth of C. albicans synthesized the relevant mannoproteins and similarly incorporated them into inner layers of the cell wall. Apparently, however, the "export" of these MP to the outermost, capsular layer occurred in yeast but not in hyphal cells. These ultrastructural data, coupled with previous biochemical ones, emphasize form-associated patterns of MP expression on Candida cell surface. Given the value of MP as main immunogenic components of Candida, this differential expression could be a means by which the fungus evades from, or attenuates host's response.