Outside‐in HLA class I signaling regulates ICAM ‐1 clustering and endothelial cell‐monocyte interactions via mTOR in transplant antibody‐mediated rejection

Antibody‐mediated rejection ( AMR ) resulting in transplant allograft vasculopathy ( TAV ) is the major obstacle for long‐term survival of solid organ transplants. AMR is caused by donor‐specific antibodies to HLA, which contribute to TAV by initiating outside‐in signaling transduction pathways that elicit monocyte recruitment to activated endothelium. Mechanistic target of rapamycin ( mTOR ) inhibitors can attenuate TAV ; therefore, we sought to understand the mechanistic underpinnings of mTOR signaling in HLA class I Ab–mediated endothelial cell activation and monocyte recruitment. We used an in vitro model to assess monocyte binding to HLA I Ab–activated endothelial cells and found mTOR inhibition reduced ezrin/radixin/moesin ( ERM ) phosphorylation, intercellular adhesion molecule 1 (ICAM ‐1) clustering, and monocyte firm adhesion to HLA I Ab–activated endothelium. Further, in a mouse model of AMR , in which C57BL/6. RAG 1 −/− recipients of BALB /c cardiac allografts were passively transferred with donor‐specific MHC I antibodies, mTOR inhibition significantly reduced vascular injury, ERM phosphorylation, and macrophage infiltration of the allograft. Taken together, these studies indicate mTOR inhibition suppresses ERM phosphorylation in endothelial cells, which impedes ICAM ‐1 clustering in response to HLA class I Ab and prevents macrophage infiltration into cardiac allografts. These findings indicate a novel therapeutic application for mTOR inhibitors to disrupt endothelial cell‐monocyte interactions during AMR.