Using sera from transplant patients with high-titer panel reactive antibody (PRA), we previously described a model for studying the effects of alloantibody binding and complement activation on human endothelial cells (EC). With this model we found that complement membrane attack complexes (MAC) induced rapid post-translational stabilization of NF-kB-inducing kinase (NIK), the activator of non-canonical NF-kB signaling, which was required for inflammatory changes in cultured human EC and for vasculopathic changes in human coronary artery segments implanted into immunodeficient mouse hosts. To elucidate the mechanism of NIK stabilization, we performed a genome-wide siRNA screen and discovered a novel, TRAF3-independent signaling mechanism in which MAC induces NIK stabilization on endosomes. MAC is internalized by EC in a clathrin-, AP2-, and dynamin-dependent mechanism into Rab5+ endosomes. Activated Akt is recruited in a Rab5-dependent manner to the same endosomes which then bind and stabilize NIK and subsequently recruit and activate IKK-alpha, the catalytic activator of p52/RelB complexes, the non-canonical form of NF-kB. Pharmacological inhibition of MAC endocytosis blocked this pathway both in cultured EC and in human coronary artery xenografts. These data reveal a novel endosome-based signaling cascade for activating non-canonical NF-kB and identify new targets for inhibiting complement-mediated tissue injury such as chronic antibody-mediated rejection and cardiac allograft vasculopathy.

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