Cell Death Pathways and Heart Transplant Rejection

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The long-term survival of allografts and patients receiving deceased donor organs remain poor. There is evidence to suggest that cold ischemia (CI) and the subsequent reperfusion during transplants causes cellular injury, which leads to an induction of T cell alloimmunity against donor tissue and ultimately graft rejection. The mechanism by which ischemia reperfusion (IR) initiates cellular injury, the manner of the injury, and how this injury impacts alloresponse are poorly understood. To better mimic human deceased donor transplants, the Fairchild laboratory developed an allograft heart transplant model whereby the donor organ is subjected to extended CI prior to transplantation followed by administration of an CTLA4Ig costimulatory blockade. Allograft hearts subjected to CI were rejected whereas those not subjected to CI survived. Preliminary studies using this model indicated that complement deficiency in the recipients prolonged the survival of CI-treated allografts, correlating with reduced circulating TNFα. As TNFα is a known inducer of necroptosis, an inflammatory cell death, additional studies showed that organs from CYLD-deficient donors, (which are defective in necroptosis) survived longer than wild type organs whereas organs from SHARPIN- deficient donors (which have accelerated necroptosis) were rapidly rejected. Based on these studies, we have proposed and will test the unifying hypothesis that complement activation following CI/IR during transplants leads to the induction of TNFα, which then induces necroptosis of cells in donor tissues. In turn, this inflammatory cellular injury activates T cell alloimmunity to cause rejection. This collaborative study brings together in a highly synergistic manner the unique strengths of three laboratories to address the above hypothesis. (1) In Aim 1, we will examine the role of mannose-binding lectin (MBL)-initiated complement activation in triggering TNF-dependent necroptosis in our allograft transplant model using MBL, TNF and TNFRs knockouts/knockins. (2) In Aim 2, the role of the TNF cell death pathway in CI-initiated rejection will be further dissected using donor organs derived from various knockouts/knockins of CYLD, RIPK1, RIPK3, Caspase-8 and SHARPIN. (3) Necroptosis releases damage-associated molecular patterns (DAMPs) and thus in Aim 3, we will examine how DAMPs are sensed by host antigen-presenting cells and how this leads to induction of the T cell alloresponse. All three aims are highly significant, as they will provide insights into which molecules during (1) initiation, (2) cellular injury and (3) effector phases of graft rejection may be targeted for therapeutic blockade in transplants. The approach has strong potential to directly and positively impact outcomes of transplant patients.