UCLA

Rejection remains a major challenge to successful organ transplantation. Central to the immune response to the allograft are antibodies reactive to donor human leukocyte antigen (HLA) molecules which trigger acute and chronic antibody-mediated rejection (AMR) and contribute to graft loss. HLA class I antibody-induced injury to the allograft endothelium recruits monocytes, which have been implicated as seminal players in the process of allograft rejection. Recent studies suggest that mTOR inhibitors modulate leukocyte recruitment to endothelium in models of inflammation. As such, we investigated the role of mTOR signaling in monocyte recruitment in HLA class I-induced acute AMR. First, we used an in vitro system of HLA I-stimulated human primary endothelial cells pre-treated with mTOR inhibitors and found that mTOR is a key regulator of monocyte binding to the endothelium. This finding was confirmed in an in vivo murine model of acute AMR, in which MHC I antibody-induced endothelial injury and monocyte infiltration in the graft were significantly reduced with administration of the mTOR inhibitor rapamycin. Additionally, rapamycin treatment blocked phosphorylation of mTOR proteins downstream of HLA I signaling in endothelium. To elucidate the mechanisms responsible for this process we studied the effects of mTOR inhibition on the expression and function of adhesion molecules on the cell surface of HLA I-stimulated endothelial cells. Monocytes firmly adhere to the endothelium via ICAM-1 engagement. mTOR inhibition in HLA I-stimulated endothelial cells impaired ICAM-1 function and hampered the capacity of endothelium to support firm adhesion of monocytes. Next, we explored the requirement of mTOR signaling in monocyte activation and adherence to HLA I-stimulated endothelium. mTOR modulates monocyte activation and adhesiveness following PSGL-1 crosslinking. To conclude, we found mTOR inhibition dampens endothelial activation in response to HLA I antibodies as well as monocyte activation following PSGL-1 engagement, and prevents monocytic infiltration into cardiac allografts. These findings can be translated to the clinical setting, providing the rationale for the application of mTOR inhibitors for mitigating the risks of AMR.