Parameters Regulating Immune-Mediated Hepatitis

CD8 T cells kill their targets in an antigen-specific manner by delivering lethal granzymes to target cells/or by triggering the Fas-mediated apoptotic pathway. Although the mediators of effector T cell liver damage have been well characterised, the intrinsic T cell parameters that regulate CD8 T-cell mediated liver damage have not been previously investigated. This is an important unanswered question, as prolonged and uncontrolled and/or excessive CTL-mediated hepatocyte killing might lead to fatal sequelae of fulminant or chronic hepatitis. Using transgenic (Tg) mouse models we sought to elucidate the mechanisms that regulate immune-mediated hepatitis by investigating the role of T cell survival and function, on the severity of acute hepatitis, and on the onset of chronic hepatitis. We used the well-characterised Met-Kb Tg mouse model in which acute hepatitis is mediated by LN-activated TCR Tg CD8 recognising their cognate antigen in the liver. To promote T cell survival we employed mice that were deficient in the pro-apoptotic molecule Bim. To influence T cell function we used mice that were deficient in Suppressor of Cytokine Signalling -1 (SOCS-1), a negative regulator of cytokines critical for T cell function. We adoptively transferred TCR Tg CD8 T cells deficient in either SOCS-1 or Bim into Met-Kb recipients. Although there was a substantial accumulation of TCR Tg Bimo/o T cells in the livers of Met-Kb hosts, the severity of hepatitis remained unchanged, suggesting that effector T cell survival was not a critical parameter in limiting liver damage. In contrast, TCR Tg SOCS-1o/o T cells induced a heightened severity of acute hepatitis. TCR Tg SOCS-1o/o T cells isolated from the liver displayed upregulated levels of the IL-2 receptor chain, required for the high affinity IL-2 receptor complex, and had higher levels of IFN-, CTL activity, and proliferation rates, consistent with enhanced effector function. These data support a critical role for cytokines in CTL function, and demonstrate that the propensity of CD8 T cells to mediate acute hepatitis is determined by the function rather than the number of CTLs infiltrating the liver. The parameters that allow acute T cell responses to progress into chronic hepatitis remain unclear. To determine if defective T cell function via SOCS-1 deficiency promotes chronic immune-mediated hepatitis we generated mixed bone marrow chimeras. This strategy enabled the endogenous production and continuous input of TCR Tg SOCS-1o/o T cells into Met-Kb hosts. Although TCR Tg SOCS-1o/o T cells accumulated in the liver of Met-Kb chimeras, this was not associated with chronic hepatic inflammation or raised serum ALT levels. Thus, the accumulation of liver-reactive T cells alone was inadequate to cause chronic hepatitis in Met-Kb mice, even in the setting of dysregulated cytokine production by this CD8 T cell population. The final part of this thesis involved investigating the early mechanisms of T-cell deletion in the liver that are responsible for preventing the onset of acute hepatitis. Suicidal emperipolesis is an efficient mechanism of T cell deletion whereby liver-activated CD8 T cells invaded hepatocytes and were degraded by lysosomes inside the host hepatocyte. We have proposed that this mechanism plays a major role in preventing acute immune-mediated hepatitis by depleting the number of circulating liver-specific T cells that are available to induce hepatitis. Autophagy is necessary for the degradation of bulk protein aggregates, and damaged organelles. Such intracellular cargo is sequestered into double membrane vesicles called autophagosomes, which ultimately fuse with lysosomes leading to degradation of the luminal contents. We hypothesized that vesicles containing invading CD8 T cells fused with vesicles generated via the autophagic pathway, leading to degradation of the CD8 T cells and/or CD8 T cells remnants using the same pathways as autophagy. To test this, we adoptively transferred TCR Tg CD8 T cells into mice with constitutive and conditional loss of hepatic autophagy. From our results, we concluded that the deletion of hepatic autophagy did not affect the efficiency of CD8 T cell clearance suggesting that suicidal emperipolesis was not dependent on autophagy. In summary, the data generated from this body of work provides clarification on the factors regulating CD8 T cell-mediated hepatitis; we demonstrated that acute immune-mediated hepatitis is more influenced by cytokine regulation and CD8 T cell function than by the number of liver-infiltrating CTLs. Despite potentiating CTL activity, the continuous input of SOCS-1 deficient T cells into Met-Kb bone marrow chimeras was however insufficient to precipitate chronic hepatitis, suggesting that mitigation of additional parameters are required for the onset of chronic hepatitis in these mice. From a clinical perspective, the results from this study may explain why T cell numbers do not always correspond to elevations in serum ALT levels. This study predicts that immunotherapeutical anti-viral strategies aimed at boosting the priming or effector function of CTLs would be more effective than those that increase CTL survival