Targeting Thrombomodulin to Endothelial ICAM-1 Rather Than PECAM-1 Allows Partnering with the Endothelial Protein C Receptor and Enhances Protection of Endothelial Barrier Function

Abstract 3351

The thrombomodulin-protein C pathway has important antithrombotic and anti-inflammatory roles in a variety of disease models and human illnesses, including severe sepsis, acute lung injury, and focal ischemia & reperfusion. We recently reported that anchoring recombinant thrombomodulin (TM) to PECAM-1 on the luminal surface of the vascular endothelium is protective in a mouse model of inflammatory lung injury. To this point, it has been unclear whether targeted thrombomodulin is able to partner with the endothelial protein C receptor (EPCR) in the same way as its endogenous counterpart. Here we demonstrate that anchoring TM to endothelial ICAM-1, rather than PECAM-1, results in approximately 10-fold greater activation of protein C. Furthermore, blocking protein C binding to EPCR results in marked reduction of protein C activation when TM is targeted to ICAM-1, whereas protein C activation is largely independent of EPCR when PECAM-1 is used as the target ligand. Consistent with this in vitro observation, anti-ICAM/TM fusion protein provides greater in vivo protection in a mouse model of inflammatory lung injury than its PECAM-targeted analogue, more potently blocking expression of pro-inflammatory cytokines and more effectively stabilizing endothelial barrier function. Since ICAM, endogenous TM, and EPCR are all thought to localize to microdomains on the apical surface of endothelial cells, we hypothesize that ICAM-targeting more effectively mimics the natural configuration and brings TM within sufficient proximity to allow access to its membrane co-factor. These observations could have profound implications for the therapeutic efficacy of a whole series of endothelial-targeted proto-drugs and their potential for translational success.