Abstract 629: IFN-Gamma-Dependent Repression of TET2 in Allograft Vasculopathy Exacerbates VSMC Activation

Abstract

Coronary allograft vasculopathy (CAV) occurs in 50% of heart transplant recipients at 10 years after surgery. Allograft failure secondary to CAV accounts for 30% of deaths in heart transplant recipients. CAV is characterized by concentric intimal hyperplasia (IH) in the graft vasculature that causes ischemic injury to the myocardium. T cell-derived INF-γ is a well-established driver of IH in CAV. The molecular mechanisms of IFN-γ-dependent phenotypic modulation of vascular smooth muscle cells (VSMCs) in CAV are not fully elucidated. Our group recently showed that TET methylcytosine dioxygenase 2 (TET2) is a master regulator of VSMC phenotype. We hypothesized that IFN-γ modulates TET2 expression and/or function in VSMCs in CAV. We found, using a minor histocompatibility mismatch aorta graft model of allograft vasculopathy, that TET2 expression was decreased in the graft neointima relative to aortic VSMCs prior to grafting. Lineage tracing studies showed that TET2 activity was decreased in donor VSMC-derived cells present in the neointima. Nuclear localization of phospho-STAT1 correlated with TET2 repression in these cells suggesting that IFN-γ signaling may mediate repression of TET2. Further, we found that IFN-y was sufficient to repress TET2 expression in human and mouse VSMCs in vitro. TET2 repression by IFN-γ was not repressed by rapamycin, an mTOR inhibitor known to be effective against CAV but which has significant side effects. We hypothesized that IFN-γ signaling directly represses TET2. IFN-γ rapidly induced activation of STAT1 in VSMCs in vitro. Transcription factor binding site prediction suggested potential STAT1 binding sites on the TET2 promoter in both human and mouse. ChIP-PCR confirmed that STAT1 occupancy occurs at the TET2 promoter in response to IFN-γ. Studies are ongoing to determine if STAT1 directly negatively regulates TET2 transcription in response to IFN-γ, and if TET2 overexpression can rescue the effect of IFN-γ on VSMC activation and IH in in vivo models of allograft vasculopathy. Since IFN-y-dependent repression of TET2 is not reversed by rapamycin, elucidating this pathway represents an opportunity for developing complementary therapies to mTOR inhibition that may result in improved outcomes and fewer side effects.