Exosome-Mediated Activation of Toll-Like Receptor 3 in Stellate Cells Stimulates Interleukin-17 Production by γδ T Cells in Liver Fibrosis

During liver injury, hepatocytes secrete exosomes that include diverse types of self-RNAs. Recently, self-noncoding RNA has been recognized as an activator of Toll-like receptor 3 (TLR3). However, the roles of hepatic exosomes and TLR3 in liver fibrosis are not yet fully understood. Following acute liver injury and early-stage liver fibrosis induced by a single or 2-week injection of carbon tetrachloride (CCl4), increased interleukin (IL)-17A production was detected primarily in hepatic gamma delta T cells in wild-type (WT) mice. However, liver fibrosis and IL-17A production by gamma delta T cells were both significantly attenuated in TLR3 knockout (KO) mice compared with WT mice. More interestingly, IL-17A-producing gamma delta T cells were in close contact with activated hepatic stellate cells (HSCs), suggesting a role for HSCs in IL-17A production by gamma delta T cells. In vitro treatments with exosomes derived from CCl4-treated hepatocytes significantly increased the expression of IL-17A, IL-1 beta, and IL-23 in WT HSCs but not in TLR3 KO HSCs. Furthermore, IL-17A production by gamma delta T cells was substantially increased upon coculturing with exosome-treated WT HSCs or conditioned medium from TLR3-activated WT HSCs. However, similar increases were not detected when gamma delta T cells were cocultured with exosome-treated HSCs from IL-17A KO or TLR3 KO mice. Using reciprocal bone marrow transplantation between WT and TLR3 KO mice, we found that TLR3 deficiency in HSCs contributed to decreased IL-17A production by gamma delta T cells, as well as liver fibrosis. Conclusion: In liver injury, the exosome-mediated activation of TLR3 in HSCs exacerbates liver fibrosis by enhancing IL-17A production by gamma delta T cells, which might be associated with HSC stimulation by unknown self-TLR3 ligands from damaged hepatocytes. Therefore, TLR3 might be a novel therapeutic target for liver fibrosis.