Modulation of immune responses by surface polysaccharides of Candida albicans

Fungal polysaccharides, especially those from the cell wall of the nonpathogenic yeast Saccharomyces cerevisiae, have been investigated as modulators of immune function for many years. More recently, because surface-associated components are known to be elaborated and circulate during serious episodes of candidiasis, investigators have taken interest in the opportunistic fungus Candida albicans. This review emphasizes the immunomodulatory activities of mannan and cell wall-derived glycoproteins from C. albicans. Mannan extracted with hot citrate buffer has been shown in an antibody-forming cell assay to be a heterogeneous mixture with components capable of enhancing or suppressing antibody responses when administered to mice at or near the time of immunization with type III pneumococcal polysaccharide (SSS-III), a T helper cell-independent antigen, or with sheep erythrocytes (SRBC), a T helper cell-dependent antigen. The components responsible for these opposing effects have been separated on the basis of size or charge by column chromatography. Two similar cell wall glycoproteins, removed from isolated cell walls by different procedures, induced enhancement only of the immune response to SSS-III and SRBC. The mechanism(s) by which these polysaccharides effect modulatory activity has not been elucidated. The enhancing property does not appear related to a direct mitogenic effect on lymphocytes or to stimulation of the production of B cell growth factors or interleukin 2.