Interleukin-1 a Activity in Necrotic Endothelial Cells Is Controlled by Caspase-1 Cleavage of Interleukin-1 Receptor-2 IMPLICATIONS FOR ALLOGRAFT REJECTION

Inflammation is a key instigator of the immune responses that drive atherosclerosis and allograft rejection. IL-1 alpha, a powerful cytokine that activates both innate and adaptive immunity, induces vessel inflammation after release from necrotic vascular smooth muscle cells (VSMCs). Similarly, IL-1 alpha released from endothelial cells (ECs) damaged during transplant drives allograft rejection. However, IL-1 alpha requires cleavage for full cytokine activity, and what controls cleavage in necrotic ECs is currently unknown. We find that ECs have very low levels of IL-1 alpha activity upon necrosis. However, TNF alpha or IL-1 induces significant levels of active IL-1 alpha in EC necrotic lysates without alteration in protein levels. Increased activity requires cleavage of IL-1 alpha by calpain to the more active mature form. Immunofluorescence and proximity ligation assays show that IL-1 alpha associates with interleukin-1 receptor-2, and this association is decreased by TNf alpha or IL-1 and requires caspase activity. Thus, TNFa or IL-1 treatment of ECs leads to caspase proteolytic activity that cleaves interleukin-1 receptor-2, allowing IL-1 alpha dissociation and subsequent processing by calpain. Importantly, ECs could be primed by IL-1 alpha from adjacent damaged VSMCs, and necrotic ECs could activate neighboring normal ECs and VSMCs, causing them to release inflammatory cytokines and up-regulate adhesion molecules, thus amplifying inflammation. These data unravel the molecular mechanisms and interplay between damaged ECs and VSMCs that lead to activation of IL-1 alpha and, thus, initiation of adaptive responses that cause graft rejection.