Ultrastructural localization of the secretory aspartyl proteinase in Candida albicans cell wall in vitro and in experimentally infected rat vagina

Detection and ultrastructural localization of aspartyl proteinase (Sap) in Candida albicans experimentally infecting rat vagina were studied. Two Sap-positive (Sap(+)) and one Sap-negative (Sap(-)) strains of the fungus, endowed with high and low experimental vaginopathic potential, respectively, were used. Both Sap(+) strains produced consistent Sap levels in the rat vagina, while the Sap-strain did not produce any measurable Sap. Electron microscopy of thin sections of chemically-fixed vaginal scrapings showed clear evidence of hyphae of proteolitic strains of C. albicans invading the keratinized epithelial cell layer of the vagina. The fungal cells exhibited a pronounced fibrillar layer on the cell wall with a marked intermixing of fungal and vaginal materials especially pronunced at the hyphal tip. Post-embedding immunogold techniques with the use of anti-Sap polyclonal and the specifically generated monoclonal antibody GF1 showed that Sap was essentially localized in the cell wall of C. albicans early during infection, in a cytological pattern mirroring Sap localization in C. albicans cells grown in Sap-inductive media in vitro. In summary, the data offer a new biochemical and ultrastructural evidence that Sap is actively secreted during experimental rat vaginitis by C. albicans. Cell wall localization of Sap is probably inherent to this active secretion process.