Cryptococcus gattii Capsule Blocks Surface Recognition Required for Dendritic Cell Maturation Independent of Internalization and Antigen Processing

Abstract

Cryptococcus gattii is an emerging fungal pathogen on the west coast of Canada and the United States that causes a potentially fatal infection in otherwise healthy individuals. In previous investigations of the mechanisms by which C. gattii might subvert cell-mediated immunity, we found that C. gattii failed to induce dendritic cell (DC) maturation, leading to defective T cell responses. However, the virulence factor and the mechanisms of evasion of DC maturation remain unknown. The cryptococcal polysaccharide capsule is a leading candidate because of its antiphagocytic properties. Consequently, we asked if the capsule of C. gattii was involved in evasion of DC maturation. We constructed an acapsular strain of C. gattii through CAP59 gene deletion by homologous integration. Encapsulated C. gattii failed to induce human monocyte-derived DC maturation and T cell proliferation, whereas the acapsular mutant induced both processes. Surprisingly, encapsulation impaired DC maturation independent of its effect on phagocytosis. Indeed, DC maturation required extracellular receptor signaling that was dependent on TNF-α and p38 MAPK, but not ERK activation, and the cryptococcal capsule blocked this extracellular recognition. Although the capsule impaired phagocytosis that led to pH-dependent serine-, threonine-, and cysteine-sensitive protease-dependent Ag processing, it was insufficient to impair T cell responses. In summary, C. gattii affects two independent processes, leading to DC maturation and Ag processing. The polysaccharide capsule masked extracellular detection and reduced phagocytosis that was required for DC maturation and Ag processing, respectively. However, the T cell response was fully restored by inducing DC maturation.

Conservation of Intracellular Pathogenic Strategy among Distantly Related Cryptococcal Species

“…Both C. gattii and C. neoformans required opsonization by either complement or antibodies, as the capsule of both cryptococcal species complexes is inherently antiphagocytic (12). Despite significant structural differences in the capsular polysaccharide, we observed very little difference in the efficiency of phagocytosis after antibody opsonization but significant differences in complement-dependent opsonization (18,20). IgG is the preferred opsonin because it so efficient, with the caveat that its high efficiency may mask differences in the pathogen surface molecule.…”

“…Despite causing similar disease in humans and animals, there appear to be quantitative differences between the two species complexes with regard to the immune response that they elicit. In general, C. gattii species complex strains induce a less protective immune response in immunocompetent hosts that is associated with structural differences in the capsular polysaccharide and that reduces DC activation (18,20).…”

“…The VGII strains further inhibit neutrophil and CD4 ϩ T helper cell migration to the lung (13,16,17). A possible mechanism for this reduced protective immunity is inhibition of DC maturation, which may be related to C. gattii polysaccharide capsule effects (18).…”

The cause and effect of Cryptococcus interactions with the host

“…Polysaccharide modification may contribute to the increased virulence of some C. gatti strains [25]: decreased O-acetylation of capsule in the C. gatti VGIIc isolate JP02 was associated with lower pro-inflammatory cytokine production, a result recapitulated by deacetylation of C. neoformans H99 capsule. C. gatti capsule prevents dendritic cell maturation independent of internalisation, and this can be overcome by co-stimulation with tumour necrosis alpha [27]. Thus, it may be reduced internalisation of the JP02 strain that causes reduced cytokine release by DCs.…”

Differential In Vitro Cytokine Induction by the Species of Cryptococcus gattii Complex

“…Cryptococcus gattii sensu lato is also able to disturb the inflammatory process, inducing low levels of proinflammatory cytokine tumor necrosis factor alpha (TNF-␣) production in vitro (27). The cryptococcal capsule components glucuronoxylomannan and galactoxylomannan dampen inflammation by suppressing the NF-B pathway and blocking surface antigen recognition (27,28). In consequence, dendritic cell maturation is compromised, leading to release of TNF-␣, interleukin-12 (IL-12), and IL-23 at low levels and to decreased major histocompatibility complex class II molecule expression.…”