The efficacy of complement‐mediated phagocytosis of Cryptococcus neoformans is dependent on the location of C3 in the polysaccharide capsule and involves both direct and indirectC3‐mediated interactions

Abstract

Complement component 3 (C3) is the major opsonin for the pathogenic fungus Cryptococcus neoformans in the non-immune host. However, the efficiency of complement-mediated opsonization varies, depending on the strain, through mechanisms that are not understood. Analysis of complement-mediated phagocytosis for 12 strains grown in Sabouraud medium revealed that phagocytic indices were inversely correlated with capsule volume. In contrast, there was no correlation between phagocytic index and capsule volume for IgG1-opsonized cells. When capsule size was increased, the efficacy of complement-mediated phagocytosis decreased, whereas that of antibody-mediated phagocytosis increased. C3 localized inside the capsule and at the outer capsule edge for poorly phagocytozed and well-phagocytozed strains, respectively. Blocking experiments revealed that complement-mediated phagocytosis occurred through complement receptor 3 (CR3), without significant involvement of CR1 or CR4. Blocking experiments with antibodies to C3 did not completely abrogate yeast cell uptake, consistent with phagocytosis through glucuronoxylomannan-CR3 interactions. Our data explain how some large encapsulated cells avoid phagocytosis and suggest a novel strategy for immune evasion whereby a microbial capsule interferes with phagocytosis by modifying the location of C3 deposition.

Cryptococcus inositol utilization modulates the host protective immune response during brain infection

“…However, binding of C3, and thus the alternating complement activation cascade, is largely dependent on the nature and density of the capsule. Therefore, any alteration in the capsule content or thickness impacts the host-pathogen interactions during cryptococcal infection [42][43][44]. Since complement activation ultimately culminates in the onset of effective host innate immune responses, we suggest that the differential expression of genes involved in various complement activation pathways elicited by the wild type and the itr1aΔ itr3cΔ mutant likely contributes to a corresponding difference in their recognition by the host phagocytes.…”

“…Importantly, the outer capsule of Cryptococcus sp. has been shown to bind with C3, a component of the alternate complement system and activates the respective pathway in vitro [40][41][42]. However, binding of C3, and thus the alternating complement activation cascade, is largely dependent on the nature and density of the capsule.…”

Effects of Voriconazole on Cryptococcus neoformans

“…Furthermore, complement deposition in cells with large capsules occurs in the internal regions of the capsule where it is a less-effective opsonin (40). Several conditions induce capsule growth in vitro (for a review, see reference 21).…”

Antibodies Produced in Response toCryptococcus neoformansPulmonary Infection in Mice Have Characteristics of Nonprotective Antibodies

“…The presence of the capsule is required for virulence and GXM can mediate a large number of deleterious effects in the host (12,20,28,35). The capsular polysaccharide confers antiphagocytic properties to the organism (1,17,36), and injection of polysaccharide into mice can produce Ab unresponsiveness in the host (16,28). Ab titers in both animal experimental infection and human cryptococcosis are usually low (7,9,10), a finding attributed to poor immunogenicity of the capsular polysaccharide (16,28) and/or sequestration of Ab by tissue polysaccharide (10).…”

“…C3 staining with a FITCconjugated goat Ab to mouse complement (5 g/ml; Cappel) localized deep in the capsule of C. neoformans cells in the lung on May 11, 2018 by guest http://iai.asm.org/ (Fig. 2), a location which has been associated with an inability to interact with the receptor in the macrophage and inefficient opsonization (36). Our findings provide insights about the role of Ab responses to cryptococcal polysaccharide during C. neoformans pulmonary infection.…”