Iron overload is common in patients undergoing hematopoietic cell transplantation (HCT) and recent reports show increased non-relapse mortality in patients with iron overload as determined by increased levels of ferritin. Contributing pre-transplant causes include enhanced intestinal iron absorption secondary to anemia, and red blood cell transfusions. Peri- and post-transplant events appear to further enhance iron accumulation, which may contribute to the clinical picture of hepatic graft-versus-host disease (GVHD). Iron homeostasis is regulated by factors expressed primarily in liver and intestinal tract, both of which are targets of conditioning-related toxicity and of GVHD. Preliminary results in murine models indicate that transplantation of allogeneic (but not syngeneic) T lymphocytes results in altered expression of hepcidin and ferroportin 1, and dysregulation of iron homeostasis. Activated allogeneic T lymphocytes express Fas-ligand (CD178), which crosslinks Fas receptor (CD95), prominently expressed on hepatocytes, thereby inducing hepatic injury. We hypothesize that Fas-mediated signals, which induce apoptosis in hepatocytes, also interfere with the expression of hepcidin, thereby disrupting physiologic regulation of iron levels. However, iron loading also appears to modulate Fas signals, which would lead to two way interactions. Preliminary data suggest that exogenous transferrin (Tf) provides protection against iron accumulation, Fas-induced apoptosis and manifestations of GVHD in this setting by enhancing expression of anti-apoptotic proteins, possibly via transferrin receptor 2. We propose in Aim 1 to characterize the impact of transplantation of allogeneic T lymphocytes on iron homeostasis in murine models. Specifically, we will a) determine the pattern of dysregulation of iron homeostasis and the effect of total body irradiation (TBI), b) determine signals that lead to alterations of hepcidin expression, and c) determine interactions of Fas signals and iron regulation on apoptotic responses in hepatocytes.
In Aim 2 we will characterize the impact of iron on the interaction of pro-apoptotic and cytoprotective signals in hepatocytes and develop strategies that would prevent hepatocyte injury in mice transplanted with allogeneic T lymphocytes. Specifically, we will a) characterize the effect of iron loading and ApoTf on transplant-related hepatic injury, and b) determine the effects of protective interventions other than Tf on hepatic injury and iron deposition. These studies will provide insights into the role of iron in hepatic injury in transplant recipients and should lead to improved strategies to prevent GVHD and improve transplant outcome.
New insights from these studies should lead to the identification of targets for prophylactic or therapeutic interventions aimed at preventing or reducing iron overload and toxicity, not only in the setting of transplantation, but also in other patients who suffer from iron overload and the associated acute and chronic complications.
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