Antigen expression level threshold tunes the fate of CD8 T cells during primary hepatic immune responses

Abstract

CD8 T-cell responses to liver-expressed antigens range from deletional tolerance to full effector differentiation resulting in overt hepatotoxicity. The reasons for these heterogeneous outcomes are not well understood. To identify factors that govern the fate of CD8 T cells activated by hepatocyte-expressed antigen, we exploited recombinant adenoassociated viral vectors that enabled us to vary potential parameters determining these outcomes in vivo. Our findings reveal a threshold of antigen expression within the liver as the dominant factor determining T-cell fate, irrespective of T-cell receptor affinity or antigen cross-presentation. Thus, when a low percentage of hepatocytes expressed cognate antigen, high-affinity T cells developed and maintained effector function, whereas, at a high percentage, they became functionally exhausted and silenced. Exhaustion was not irreversibly determined by initial activation, but was maintained by high intrahepatic antigen load during the early phase of the response; cytolytic function was restored when T cells primed under high antigen load conditions were transferred into an environment of low-level antigen expression. Our study reveals a hierarchy of factors dictating the fate of CD8 T cells during hepatic immune responses, and provides an explanation for the different immune outcomes observed in a variety of immune-mediated liver pathologic conditions. rAAV | CTL | TCR | cytotoxicity T he liver is acknowledged to possess unique tolerogenic properties, which have likely evolved to maintain immunological unresponsiveness toward food-derived and microbial antigens that enter the circulation via the gut (1, 2). This tolerogenic capability of the liver is demonstrated in animal models of liver transplantation, in which liver allografts are accepted across complete MHC mismatch barriers and are able to protect other donor tissues from rejection (reviewed in ref.3). In humans, the tolerogenic hepatic environment is likely to contribute to impaired immune clearance of the hepatitis B virus (HBV) and hepatitis C virus (HCV), which result in persistent infection in a significant proportion of exposed individuals and are associated with major morbidity and mortality. In contrast, effective immune responses to hepatotropic pathogens leading to resolution of infection are observed in most hepatitis A and E virus infections, the majority of individuals infected with HBV during adulthood, and a minority of those infected by HCV (reviewed in refs. 4, 5). The liver is also susceptible to a variety of autoimmune-mediated conditions (6). Collectively, these observations indicate that effective immune responses can be initiated and/or sustained in the liver despite its apparent predisposition toward the generation of tolerance. Unfortunately, there is no small animal model in which to study the parameters that determine the balance between intrahepatic immunity and tolerance in viral hepatitis. Thus, the factors that shape immune outcome have not yet been identified.By studying the fate of ...

Target Density, Not Affinity or Avidity of Antigen Recognition, Determines Adoptive T Cell Therapy Outcomes in a Mouse Lymphoma Model

“…An important role for target cell density in the maintenance of CTL activity is not without precedence in the literature. Recently, similar observations were described in settings of hepatic immunity, where the encounter of a high number of Ag-expressing liver cells elicited CTL impairment (32)(33)(34).…”

Intrahepatic Activation of Naive CD4+ T Cells by Liver-Resident Phagocytic Cells

“…The membrane-bound OVA (mOVA) sequence used to generate rAAV encoding mOVA (rAAV.mOVA) was kindly provided by Dr. Frank Carbone (University of Melbourne), and the generation of this construct was described elsewhere (17). To transduce mice from which KCs were isolated for in vitro assays, the rAAV vector rAAV.GFP-KVIT expressing GFP and a control self-peptide recognized by the Tg Des TCR (17) was used.…”

“…(Tissue-Tek) in liquid nitrogen. Eight-to twelvemicron sections were cut on the cryostat at 213˚C, or thick 200-mm sections in 2% agarose were cut at room temperature (1000Plus Vibratome), before staining as described (17 …”

“…Mice were treated with a high dose (1 3 10 11 viral genome copies) of rAAV.GFP-OVA-Ag85 or control rAAV.GFP-KVIT, which does not express OVA or Ag85 epitopes, as previously described (17). After 2 wk, livers were excised, chopped into fine pieces, and digested with 1 mg/ml collagenase IV (Sigma-Aldrich) and 280 U/ml DNase I (Roche) at room temperature for 40 min.…”

Modeling how reversal of immune exhaustion elicits cure of chronic hepatitis C after the end of treatment with direct‐acting antiviral agents

“…Interferon treatment is unable to induce this reversal,26 explaining the absence of EOT + /SVR with PR treatment, barring a few exceptions 27, 28, 29, 30. When the percentage of hepatocytes infected was reduced below a threshold, exhausted CTLs regained their functionality in a mouse model of HCV infection 31. Anti‐PD‐1 therapy, which prevents or reverses CTL exhaustion, has shown some success in HCV‐infected primates and humans 32, 33.…”