Opsonization and phagocytosis of Cryptococcus neoformans
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1Department of Microbiology, School of Medicine, University of Nevada, Reno 89557.
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.
Insights
Host resistance to cryptococcosis depends on overcoming the unique antiphagocytic capsule of Cryptococcus neoformans. Effective defense requires complement activation, cytokines, and phagocyte receptors, which can be therapeutically enhanced.
Area of Science:
- Mycology
- Immunology
- Infectious Diseases
Background:
- Cryptococcus neoformans possesses a unique antiphagocytic polysaccharide capsule, a key virulence factor.
- Host resistance to cryptococcosis relies on immune mechanisms to counteract this capsule.
Purpose of the Study:
- To investigate the conditions required to overcome the capsule's antiphagocytic properties.
- To understand the role of the opsonization-phagocyte-cytokine triad in cryptococcosis pathogenesis.
Main Methods:
- Studies examined the activation of the alternative complement pathway by encapsulated Cryptococcus neoformans.
- Investigated the role of cytokines in enhancing complement-dependent phagocytosis.
- Assessed the importance of phagocytic cell complement receptor expression.
Main Results:
- Overcoming the capsule's antiphagocytic activity requires alternative complement pathway activation, iC3b deposition, essential cytokines, and upregulated phagocyte complement receptors.
- Deficiencies in this triad contribute to cryptococcosis pathogenesis and susceptibility in immunocompromised individuals.
Conclusions:
- The opsonization-phagocyte-cytokine triad is crucial for host defense against Cryptococcus neoformans.
- Therapeutic strategies like passive immunization with anticapsular IgG or cytokine therapy may overcome immune deficiencies and enhance cryptococcosis resistance.