Fungal-derived cues promote ocular autoimmunity through a Dectin-2/Card9-mediated mechanism

Uveitis (intraocular inflammation) is a leading cause of vision loss. Although its etiology is largely speculative, it is thought to arise from complex genetic-environmental interactions that break immune tolerance to generate eye-specific autoreactive T cells. Experimental autoimmune uveitis (EAU), induced by immunization with the ocular antigen, interphotoceptor retinoid binding protein (IRBP), in combination with mycobacteria-containing CFA, has many clinical and histopathological features of human posterior uveitis. Studies in EAU have focused on defining pathogenic CD4+ T cell effector responses, such as those of Th17 cells, but the innate receptor pathways precipitating development of autoreactive, eye-specific T cells remain poorly defined. In this study, we found that fungal-derived antigens possess autoimmune uveitis-promoting function akin to CFA in conventional EAU. The capacity of commensal fungi such as C. albicans or S. cerevisae to promote IRBP-triggered EAU was mediated by Card9. Since Card9 is an essential signaling molecule of a subgroup of C-type lectin receptors (CLRs) important in host defense, we further evaluated the proximal Card9-activating CLRs. Using single receptor-deficient mice, we identified Dectin-2, but not Mincle or Dectin-1, as a predominant mediator of fungal-promoted uveitis. Conversely, Dectin-2 activation by α-mannan sufficiently reproduced the uveitic phenotype of EAU, in a process mediated by the Card9- coupled signaling axis and IL-17 production. Taken together, this report relates the potential of the Dectin-2/Card9-coupled pathway in ocular autoimmunity. Not only does it contribute to understanding of how innate immune receptors orchestrate T cell-mediated autoimmunity, it also reveals a previously unappreciated ability of fungal-derived signals to promote autoimmunity.