*Purpose: Microvascular inflammation, vascular permeability, and accumulation of intravascular mononuclear cells are characteristics of acute and chronic antibody-mediated rejection. Vascular endothelial cadherin (VE-Cadherin) is a major architectural protein that regulates the endothelial barrier by controlling endothelial cell (EC) permeability and subsequent transendothelial migration of immune subsets. Despite being a significant clinical concern for allograft and patient survival, the mechanisms underlying how antibodies against HLA class II (HLA II Ab) activate the endothelium and mediate inflammation remain poorly understood.

*Methods: Recombinant Class II Transactivator (CIITA) was sub-cloned with pAd/PL-DEST vector to achieve expression of HLA II on vascular endothelium, and the effects of HLA II Ab on EC permeability including VE-Cadherin phosphorylation, internalization and signaling were determined using immunoblotting and multicolor immunofluorescence confocal microscopy.

*Results: Three-dimensional subcellular reconstruction of multiplex confocal images revealed HLA II Ab increased VE-Cadherin phosphorylation at Tyr685 and internalization into EEA1+ early endosomes (Fig), thereby disassembling intercellular junctions and contributing to EC permeability. HLA II Ab activated both Src and ERK; however, HLA II-Ab-induced VE-Cadherin internalization was dependent only on Src, as both pharmacological and siRNA inhibition of Src, but not of ERK, attenuated HLA II Ab-induced phosphorylation of VE-Cadherin at Tyr685 as well as its internalization into EEA1+ early endosomes.

*Conclusions: HLA II Ab induces rapid endocytosis of VE-Cadherin via a Src-dependent tyrosine phosphorylation of VE-Cadherin, thereby disrupting the endothelial barrier function and contributing to vascular permeability, providing a potential therapeutic target for prevention of transplant antibody-mediated rejection.

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