Hepatitis C Virus-Specific T cell-Derived Transforming Growth Factor beta is Associated with Slow Hepatic Fibrogenesis

Up to 4 million persons in the USA have chronic hepatitis C (CHC) (1). Despite a decline in overall HCV infections, the number of patients with end stage liver disease due to CHC will increase for the next 2 decades (2). Even with highly effective novel therapies, currently 30–50% of infected individuals fail treatment (3). Therefore, a better understanding of mechanisms involved in CHC-related liver disease progression could permit more efficient therapies. Adaptive effector T cells (frequently assessed by measuring production of prototypic T helper 1 cytokine IFNγ) play an important role in control of HCV infection during the acute phase (4). In CHC, effector HCV-specific T cell immune responses are weak in peripheral blood, although they can be / are present in liver (5–8). It is likely that in chronic infection, persistence of inefficient effector T cell responses causes collateral tissue damage and inflammatory reactions, leading to fibrosis and finally cirrhosis. Regulatory/immunosuppressive T cells (Treg) are apparently involved in HCV pathogenesis, although it remains largely unclear whether they play a detrimental role by suppressing effector T cell responses against HCV or are protective by preventing excessive immunological liver damage. Treg consist of heterogeneous populations that can be natural or induced, antigen-specific or not. Natural Treg, at least in vitro, function via antigen-independent, contact-dependent, and cytokine-independent mechanisms (9), whereas cytokine-mediated suppression (mostly IL-10 and/or TGFβ) has been established for peripheral adaptive Treg in vivo (10). This heterogeneity leads to ambiguous marker(s) for identifying Treg. Current optimal Treg markers are expression of Foxp3, a transcription factor (11), high levels of CD25 (although both of these markers can also be expressed by activated effector T cells), as well as minimal CD127 (IL-7 receptor) expression (12). In HCV infection, increased circulating CD4+CD25+Foxp3+ T cells were associated with viral persistence (13, 14) with suppressive activity independent of cytokines and antigen non-specific (15, 16). Histological co-staining of liver infiltrates showed CD4+Foxp3+ cells at high proportion in livers of CHC patients (17), suggesting their involvement in intrahepatic immune regulation, but possibly also amelioration of fibrosis (18). HCV can prime virus-specific CD4+CD25+Foxp3+ Treg with antigen-specific expansion and suppression of HCV-specific CD8+ T cells (19). Treg also include IL-10-producing CD4+ HCV-specific T cells (20), and IL-10 dampens hepatic inflammation, but also leads to increased viral load (21). Peripheral CD4+CD25+ Treg were shown to secrete TGFβ in response to HCV, which was inversely correlated with liver inflammation (22). Suppressive IL-10 producing HCV-specific CD8+ liver infiltrating lymphocytes were also described (23) and have been associated with protection against apoptosis and fibrosis-related laminin production, as CD8 T cells were located in liver areas with both low hepatocyte apoptosis and fibrosis (24). A limitation of previous studies on Treg is use of phenotypic markers to characterize Treg before functional analysis, as opposed to functionally defining relevant Treg first, so as to not miss subsets. We identified in CHC novel blood HCV-specific CD8+CD25-Foxp3- Treg secreting TGFβ, first functionally then phenotypically (25). TGFβ production by CD4+ T cells was also observed in one patient. Suppression of peripheral HCV-specific IFNγ was predominantly mediated by TGFβ rather than IL-10. Presence of HCV-specific CD4 and CD8 T cells producing TGFβ was recently confirmed in PBMC in acute HCV infection (37). Of note, TGFβ is a multi-functional cytokine with unique ability to direct T cell lineage commitment toward either pro-inflammatory Th17 T cells or anti-inflammatory Treg, depending on presence of additional factors, such as IL-6 (26). Significantly, TGFβ is also a key cytokine driving liver fibrogenesis (27). Disruption of the local balance between opposing effects of TGFβ on liver inflammation and fibrogenesis could underline fibrosis progression in CHC. Here we found that TGFβ produced by HCV specific T cells significantly masks T cell effector response only in those patients that show attenuated fibrosis progression. In addition, TGFβ inversely correlated not only with liver inflammation, but also with liver fibrosis progression and fibrogenic HSC gene expression. It is possible that in chronic HCV infection immunoregulatory and anti-inflammatory functions of TGFβ, produced by certain HCV-specific Treg, ameliorate liver inflammation, whilst limiting the fibrotic process.