The role of Syk/CARD9 coupled C-type lectins in antifungal immunity

Abstract

Fungal infections are affecting an increasing number of people, and the failure of current therapies in treating systemic infection has resulted in an unacceptably high mortality rate. It is therefore of importance that we understand immune mechanisms operating during fungal infections, in order to facilitate development of adjunctive immunotherapies for the treatment of these diseases. C-type lectin receptors (CLRs) are pattern recognition receptors (PRRs) that are critical for immune responses to fungi. Many of these receptors are coupled to Syk kinase, which allows these receptors to signal via CARD9 leading to NF-κB activation, which in turn contributes to the induction of both innate and adaptive immunity. Dectin-1, Dectin-2 and Mincle are all CLRs that share this common signalling mechanism and have been shown to play key roles in antifungal immunity. This review aims to update existing paradigms and summarise the most recent findings on these CLRs, their signal transduction mechanisms and the collaborations between these CLRs and other PRRs.

Figure 1

Signal transduction pathways and outcomes of Syk/CARD9-coupled receptor signalling in antifungal immunity. Dectin-1/2 and Mincle all signal via Syk kinase, which associates to either the ITAM-like motif of Dectin-1 (shown by red Y in white boxes) or the ITAM on FcRγ (denoted by dark blue Y in white boxes) that associates with Dectin-2 and Mincle. Syk signalling results in a complex of CARD9, BCL10 and MALT1 that ultimately leads to the activation of transcription factors, including NF-κB. These transcription factors mediate the translation of key cytokines that help to promote Th1/Th17 differentiation, which in turn stimulates antifungal mechanisms from innate cells such as neutrophils and macrophages.