CARD9 adaptor molecule is indispensable for protection against Cryptococcosis

Abstract Caspase recruitment domain-containing protein 9 (CARD9) is a critical adaptor molecule triggered by the interaction of C-type lectin receptors (CLRs) with carbohydrate motifs found in fungi. Cryptococcus neoformans is an opportunistic fungal pathogen predominantly affecting patients with suppressed T cell-mediated immunity and is responsible for ~15% of AIDS-related deaths globally. Cryptococcus can mask itself with a unique carbohydrate capsule complex which also serves as its predominant virulence factor. Previous studies have shown that immunization with a C. neoformans strain genetically engineered to express murine IFN-γ, H99γ, or a hyphaeforming mutant, LW10, results in protection against an otherwise lethal challenge with wild type (WT) C. neoformans. We therefore analyzed the role of CARD9 during the induction of protective anti-Cryptococcus immunity in WT and CARD9 knockout (KO) mice given an experimental pulmonary infection with Cryptococcus neoformans strain H99, H99γ or LW10. CARD9 KO mice were significantly more susceptible to C. neoformans infection compared to their WT counterparts. We then evaluated the role of pulmonary macrophages, the first line of defense against cryptococcosis. Pulmonary macrophages from CARD9 KO mice were unable to control fungal burden. These results suggest that CARD9-mediated signaling is required for optimal macrophage fungicidal activity against Cryptococcus. The overall goal of these studies is to define PRR/ligand interactions and downstream signal transduction pathways that are critical for the induction of protective immunity against cryptococcosis.