The severity of cryptococcus neoformans infection in Fc gamma receptor IIb deficient mice

Abstract

Cryptococcus neoformans, an encapsulated yeast commonly infecting the central nervous system, refers to as cryptococcal meningitis or cryptococcosis is commonly found in Thailand. C. neoformans is able to replicate, survive inside host macrophages, evade from the cells and use macrophages as a vehicle to disseminate to the target organs, a mechanism known as Trojan horse. Therefore, the containment of C. neoformans in phagocytic cell is important for host protective immunity. Cryptococcosis is common in immunocompromised host such as in patients with low CD4+ T helper cells in HIV-infection and patients taking immunosuppressive drugs. However, the incidence of cryptococcosis in healthy individual (HIV-uninfected) is increasing. Interestingly, it was reported that dysfunctional FcγRIIb polymorphisms are associated with cryptococcosis susceptibility. FcγRIIb is the only inhibitory receptor among the family of FcγRs. FcγRIIb activation leads to suppress of phagocytic activity and reduces cytokine production. Because macrophage is important immune cells against cryptococcosis, we hypothesized that dysfunctional FcγRIIb may influence cryptococcosis susceptibility through the alteration of macrophages functions. We investigated cryptococcosis in the FcγRIIb-/- mouse. Mortality after intravenous C. neoformans-induced cryptococcosis FcγRIIb-/- mice was higher than in age-matched wild-types. Severe cryptococcosis in the FcγRIIb-/- mice was demonstrated by high fungal burdens in the internal organs with histological cryptococcoma-like lesions and high levels of TNF-α and IL-6, but not IL-10. Interestingly, FcγRIIb-/- macrophages demonstrated more prominent phagocytosis but did not differ in killing activity in vitro or the production of TNF-α, IL-6 and IL-10 levels, compared to wild-type cells. Indeed, in vivo macrophage depletion with liposomal clodronate attenuated the fungal burdens in FcγRIIb-/- mice, but not wild-type mice. Upon transferring to wild-type mice, FcγRIIb-/- macrophages with phagocytosed Cryptococcus resulted in higher fungal burdens than FcγRIIb+/+ macrophages with phagocytosed Cryptococcus. Moreover, the T cell subpopulation analysis and IFN-γ intracellular staining suggested that cryptococcosis severity in FcγRIIb-/- mice did not cause by T helper cell defect. These results support, at least in part, a model whereby, in FcγRIIb-/- mice, enhanced C. neoformans transmigration occurs through infected macrophages. In summary, prominent phagocytosis, with limited effective killing activity, and high pro-inflammatory cytokine production by FcγRIIb-/- macrophages were correlated with more severe cryptococcosis in FcγRIIb-/- mice. In clinical translation, we propose FcγRIIb loss-of-function- polymorphisms as a new risk factor for cryptococcosis.