Extracellular histones and xenotransplantation

Xenotransplantation may be an alternative source of organs for patients with end-stage organ failure, but problems remain to be overcome. Five factors that are problematic are (a) a sustained systemic inflammatory response in the xenograft recipient, (b) thrombotic microangiopathy and disseminated intravascular coagulation, (c) ischemia-reperfusion injury, (d) complement activation, and (e) vascular endothelial cell injury. In xenotransplantation, histones, which are positively charged proteins, are released into the extracellular space from damaged and activated cells, cause cell and tissue damage, and act as danger/damage-associated molecular patterns (DAMPs) that mediate inflammation, coagulation disorders, an immune response, and cytotoxicity. We have previously demonstrated that serum histones increase after pig-to-baboon organ transplantation and infection. Treatment of the recipient with tocilizumab (interleukin-6 receptor blockade) reduces the level of serum histones and C-reactive protein. In this review, the potential role of extracellular histones in xenotransplantation is discussed, and we briefly summarize the relationship between extracellular histones and the inflammatory response, coagulation dysfunction, ischemia-reperfusion injury, the complement system, and vascular endothelial cell injury.