Relative contribution of lymphocytes to hepatic ischemia reperfusion injury
Richards, James Alexander
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Background Hepatic ischemia reperfusion injury (IRI) results from the interruption and then reinstatement of the liver’s blood supply. IRI involves both an ischemic and an immune-mediated reperfusion phase of tissue injury; similar inflammatory events are seen in other forms of acute (sterile) liver injury (ALI), including paracetamol toxicity. Hypothesis Irrespective of the primary insult, common pathways exist in the pathophysiology of the lymphocyte-mediated secondary liver injury. Natural mechanisms exist to limit lymphocyte function and these pathways can be targeted therapeutically by immunomodulatory agents. Aims 1. To assess the relative importance of different lymphocyte subsets in IRI. 2. To correlate observations in IRI with other models of ALI. 3. To identify possible pharmacological targets. Materials and Methods Three experimental murine models of acute liver injury were utilised to test this hypothesis: murine model of warm hepatic IRI, concanavalin A (con A) hepatitis and paracetamol-induced liver injury. These models were interrogated with a combination of (transgenic and knockout) mouse lines, in vivo antibody depletion and small molecule inhibition. Injury was evaluated primarily in terms of the biochemical marker of liver injury alanine aminotransferase (ALT). Data were correlated with human tissue where possible. Results T cells (CD3εKO vs WT p=0.010), but not other lymphocyte populations (B cells, NK cells, or other innate lymphoid cells), play a central role in warm hepatic IRI. Programmed Death Receptor-1 (PD-1) is a negative regulator of pro-inflammatory cytokine production by T cells and the absence of PD-1 was associated with significantly worse hepatic IRI (p=0.034), con A hepatitis (p=0.00020) and paracetamol-induced liver injury (p=0.0050). Interferon-γ (IFNγ) and T-box expressed in T cells (T-bet) are important mediators of hepatic IRI (p=0.017) and paracetamol induced liver injury (p=0.0007). The absence of IL-6 was associated with significant protection in paracetamol induced liver injury (p=0.006). The infiltrates within the recipient liver of patients transplanted following paracetamol overdose stain positively for PD-1, IFNγ and T-bet. The Janus family of kinases (JAK) play an important role in the common pathways of cytokine signal transduction. In vivo use of a selective JAK1/JAK2 inhibitor is protective in hepatic IRI (p=0.0014), con A hepatitis (p=0.019) and paracetamol-induced liver injury (p=0.0045). Conclusions Common pathways appear to exist in the immune-mediated secondary phase of injury in ALI. Targeting these pathways will complement existing (cytoprotective) treatment strategies.