Innate immune mechanisms in transplant allograft vasculopathy.

Purpose Of Review: Allograft vasculopathy is the leading cause of late allograft loss following solid organ transplantation. Ischemia reperfusion injury and donor-specific antibody-induced complement activation confer heightened risk for allograft vasculopathy via numerous innate immune mechanisms, including MyD88, high-mobility group box 1 (HMGB1), and complement-induced noncanonical nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) signaling.

Recent Findings: The role of MyD88, a signal adaptor downstream of the Toll-like receptors (TLR), has been defined in an experimental heart transplant model, which demonstrated that recipient MyD88 enhanced allograft vasculopathy. Importantly, triggering receptor on myeloid receptor 1, a MyD88 amplifying signal, was present in rejecting human cardiac transplant biopsies and enhanced the development of allograft vasculopathy in mice. HMGB1, a nuclear protein that activates Toll-like receptors, also enhanced the development of allograft vasculopathy. Complement activation elicits assembly of membrane attack complexes on endothelial cells which activate noncanonical NF-κB signaling, a novel complement effector pathway that induces proinflammatory genes and potentiates endothelial cell-mediated alloimmune T-cell activation, processes which enhance allograft vasculopathy.

Summary: Innate immune mediators, including HMGB1, MyD88, and noncanonical NF-κB signaling via complement activation contribute to allograft vasculopathy. These pathways represent potential therapeutic targets to reduce allograft vasculopathy after solid organ transplantation.