Aim: Antibody-mediated rejection (AMR) is a major issue affecting solid organ transplants, manifesting with recruitment of monocytes to the endothelium of the allograft. The purpose of this study was to determine the mechanisms by which rapamycin inhibits monocyte recruitment to HLA Class I-stimulated endothelial cells (EC), and to recapitulate the findings in an in vivo model of AMR.

Methods: Static and flow-based adhesion assays were used to assess monocyte (MM6 and primary monocyte) recruitment to HLA-activated primary human EC. mTOR inhibition was characterized using pharmacological inhibitors (rapamycin/RAD001) or using mTOR-pathway-direct siRNA. The involvement of the adhesion molecules P-selectin and ICAM-1 was evaluated by ELISA and flow cytometry. mTOR regulation of ICAM-1 function was determined by flow-based adhesion assay and Western blotting. Rag1 knock-out mice heterotopically transplanted with fully-mismatched donor hearts were treated with rapamycin at 1mg/kg/day and received passive transfer of anti-donor D^d+K^d IgG antibodies (MHC I Ab). Grafts recovered after 30 days were examined for histological features of AMR including microvascular inflammation and mononuclear cell infiltration by immunohistochemistry.

Results: Inhibition of mTOR blocks HLA I-induced monocyte recruitment. Furthermore, both mTORC1 and mTORC2 are involved in HLA I-mediated EC activation. ELISA experiments excluded involvement of P-selectin, while shear-flow-based assays revealed mTOR inhibition impairs the EC's ability to support firm adhesion of monocytes. This was due to a decrease in association of phosphorylated ERM proteins with ICAM-1, disrupting ICAM-1 clustering, a requirement for firm adhesion of monocytes to EC. Western blotting demonstrated that mTOR regulates ERM phosphorylation through the RhoA pathway. In vivo results demonstrated there was a significant reduction in endothelial damage and inhibition of monocyte recruitment to cardiac allograft endothelium when the mTOR inhibitor rapamycin was administered. This data suggest that mTOR inhibition dampens endothelial activation in response to HLA I antibodies and prevents macrophage infiltration into cardiac allografts.

Conclusion: Our findings demonstrate that HLA antibody-mediated activation of EC and consequent recruitment of monocytes intersect in a way that is dependent upon mTOR signaling. These studies yield a more advanced understanding of how mTOR regulates the recruitment of immune cells in the transplant setting in response to HLA class I antibodies.

CITATION INFORMATION: Salehi S, Reed E. Monocyte Recruitment to HLA Class I Antibody-Activated Endothelial Cells Is Dependent Upon mTOR. Am J Transplant. 2016;16 (suppl 3).

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