A novel pathway of chronic allograft rejection mediated by NK cells and alloantibody

Chronic allograft vasculopathy (CAV) in murine heart allografts can be elicited by adoptive transfer of donor specific antibody (DSA) to class I MHC antigens and is independent of complement. Here we address the mechanism by which DSA causes CAV. B6.RAG1^?/? or B6.RAG1^?/?C3^?/? (H‐2^b) mice received B10.BR (H‐2^k) heart allografts and repeated doses of IgG2a, IgG1 or F(ab’)₂ fragments of IgG2a DSA (anti‐H‐2^k). Intact DSA regularly elicited markedly stenotic CAV in recipients over 28 days. In contrast, depletion of NK cells with anti‐NK1.1 reduced significantly DSA‐induced CAV, as judged morphometrically. Recipients genetically deficient in mature NK cells (γ‐chain knock out) also showed decreased severity of DSA‐induced CAV. Direct NK reactivity to the graft was not necessary. F(ab’)₂ DSA fragments, even at doses twofold higher than intact DSA, were inactive. Graft microvascular endothelial cells responded to DSA in vivo by increased expression of phospho‐extracellular signal‐regulated kinase (pERK), a response not elicited by F(ab’)₂ DSA. We conclude that antibody mediates CAV through NK cells, by an Fc dependent manner. This new pathway adds to the possible mechanisms of chronic rejection and may relate to the recently described C4d‐negative chronic antibody‐mediated rejection in humans.