Loss of CD4 T cell help correlates with virus persistence during acute hepatitis C virus (HCV) infection, but the underlying mechanism(s) remain unknown. We developed a combined proliferation/intracellular cytokine staining assay to monitor expansion of HCV-specific CD4 T cells and helper cytokines expression patterns during acute infections with different outcomes. We demonstrate that acute resolving HCV is characterized by strong Th1/Th17 responses with specific expansion of IL-21-producing CD4 T cells and increased IL-21 levels in plasma. In contrast, viral persistence was associated with lower frequencies of IL-21-producing CD4 T cells, reduced proliferation and increased expression of the inhibitory receptors T cell immunoglobulin and mucin-domain-containing-molecule-3 (Tim-3), programmed death 1 (PD-1) and cytotoxic T-lymphocyte antigen 4 (CTLA-4) on HCV-specific CD8 T cells. Progression to persistent infection was accompanied by increased plasma levels of the Tim-3 ligand Galectin-9 (Gal-9) and expansion of Gal-9 expressing regulatory T cells (Tregs). In vitro supplementation of Tim-3high HCV-specific CD8 T cells with IL-21 enhanced their proliferation and prevented Gal-9 induced apoptosis. siRNA-mediated knockdown of Gal-9 in Treg cells rescued IL-21 production by HCV-specific CD4 T cells. We propose that failure of CD4 T cell help during acute HCV is partially due to an imbalance between Th17 and Treg cells whereby exhaustion of both CD4 and CD8 T cells through the Tim-3/Gal-9 pathway may be limited by IL-21 producing Th17 cells or enhanced by Gal-9 producing Tregs.
Inflammatory immune cells can modulate activation of hepatic stellate cells (HSCs) and progression of liver fibrosis. Type 3 inflammation characterized by production of interleukin-17A (IL-17) and IL-22 by innate and adaptive immune cells is implicated in many inflammatory conditions of the gut and can be counteracted by regulatory T cells (Tregs), but its contribution to liver fibrosis is still poorly understood. Here, we evaluated the contribution of type 3 inflammation in liver fibrosis using clinical liver biopsies, in vitro stimulation of primary HSCs, and in vivo mouse models. We report dysregulated type 3 responses in fibrotic lesions with increased IL-17+CD4+/FOXP3hiCD4+ratio and increased IL-17 and IL-22 production in advanced liver fibrosis. Neutrophils and mast cells were the main sources of IL-17 in situ in humans. In addition, we demonstrate a new profibrotic function of IL-22 through enhancement of transforming growth factor–β signaling in HSCs in a p38 mitogen-activated protein kinase–dependent manner. In vivo, IL-22RA1 knockout mice exhibited reduced fibrosis in response to thioacetamide and carbon tetrachloride. Blocking either IL-22 or IL-17 production using aryl hydrocarbon receptor or RAR-related orphan receptor gamma-t antagonists resulted in reduced fibrosis. Together, these data have identified a pathogenic role for type 3 immune response mediated by IL-22 in driving liver fibrosis during chronic liver injury.
Activation of hepatic stellate cells (HSCs) is a key event in the initiation of liver fibrosis, characterized by enhanced extracellular matrix (ECM) production and altered degradation. Activation of HSCs can be modulated by cytokines produced by immune cells. Recent reports have implicated the pro-inflammatory cytokine IL-17A in liver fibrosis progression. We hypothesized that IL-17A may enhance activation of HSC and induction of the fibrogenic signals in these cells. The human HSC line LX2 and primary human HSCs were stimulated with increasing doses of IL-17A and compared to TGF-β and PBS-treated cells as positive and negative controls, respectively. IL-17A alone did not induce activation of HSC. However, IL-17A sensitized HSCs to the action of suboptimal doses of TGF-β as confirmed by strong induction of alpha-smooth muscle actin (α-SMA), collagen type I (COL1A1) and tissue inhibitor of matrix metalloproteinase I (TIMP-I) gene expression and protein production. IL-17A specifically upregulated the cell surface expression of TGF-β-RII following stimulation. Pretreatment of HSCs with IL-17A enhanced signaling through the TGF-β-RII as observed by increased phosphorylation of SMAD2/3 in response to stimulation with suboptimal doses of TGF-β. This enhanced TGF-β response of HSCs induced by IL-17A was JNK-dependent. Our results suggest a novel pro-fibrotic function for IL-17A by enhancing the response of HSCs to TGF-β through activation of the JNK pathway. IL-17A acts through upregulation and stabilization of the TGF-β-RII leading to increased SMAD2/3 signaling. These findings represent a novel example of cooperative signaling between an immune cytokine and a fibrogenic receptor.
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