Toll-like Receptor 2 is crucial for commensal-specific immune responses on the ocular surface

Abstract

Abstract Conjunctiva of the eye is a mucosal immune tissue and harbors a variety of immune cells. It is exposed to diverse microorganisms that come in contact with the ocular surface. Previously, we demonstrated that colonization of the ocular mucosa with the commensal bacterium, Corynebacterium mastitidis (C. mast), results in increased resistance of the ocular surface to infectious fungal and bacterial pathogens, and that this effect is mediated by IL-17A produced by γδ T cells that respond to C. mast. In this study, we show that Toll-like receptor 2 (TLR2) is important for activation and proliferation of γδ T cells and for their IL-17A production in response to C. mast. TLR2−/− mice failed to recruit γδ T cells and neutrophils to the conjunctiva upon C. mast inoculation, and IL-17A-producing γδ T cells were reduced in eye-draining lymph nodes of these mice. In vitro experiments revealed that IL-17A production to C. mast by gd T cells was dependent on APC and that TLR2 expression was needed on both γδ T cells and APCs. Exogenous IL-1 could only partly compensate for the deficiency in IL-17A production by TLR2−/− gdT cells, indicating a need for another TLR2-dependent signal. Flow cytometry and RNA-Seq analyses suggested that dependence on TLR2 for IL-17A production and IL-17A related gene expression in response to C. mast is higher in the Vγ6 than in the Vγ4 subset. Notably, the transcriptomic analysis also implicated dysregulation of mitochondrial pathways in TLR2−/− cells, which was functionally confirmed by the Seahorse assay. We conclude that TLR2 is required for sensing the commensal C. mast in the conjunctiva and eye-draining lymph nodes, to drive the local IL-17 response and maintain immune homeostasis at the ocular surface.