Opsonization and phagocytosis of Cryptococcus neoformans

Abstract

Cryptococcus neoformans is unique among the pathogenic fungi because the yeast is surrounded by an antiphagocytic polysaccharide capsule. Host resistance to cryptococcosis is dependent on natural defense mechanisms that cope with this essential fungal virulence factor. Recent studies in several laboratories indicate that, under appropriate circumstances, the antiphagocytic activity of the capsule can be overcome. A reversal of the antiphagocytic properties of the capsule requires (i) activation of the alternative complement pathway by encapsulated cryptococci, leading to deposition of the opsonic ligand iC3b at the capsular surface, (ii) presence of essential cytokines which up-regulate the efficiency of complement-dependent phagocytosis, and (iii) availability of phagocytic cells whose complement receptors are capable of appropriate up-regulation. Deficiencies in one or more components of the opsonization-phagocyte-cytokine triad may account, in part, for several features of the pathogenesis of cryptococcosis as well as the susceptibility of some immunocompromised patients to cryptococcosis. It may be possible to overcome some deficiencies by passive immunization with anticapsular IgG which takes advantage of phagocyte Fc receptors that are constitutively competent for phagocytosis or by therapeutic use of cytokines that up-regulate complement-dependent phagocytosis.