Platelets: Mechanistic and Diagnostic Significance in Transplantation

In addition to their function in coagulation, platelets recently have been recognized as an important component of innate immune responses. This review relates salient immune functions of platelets to transplants. Platelets are critical bridges between vascular endothelium and leukocytes. Real-time imaging of platelets has demonstrated that platelets rapidly adhere to vascular endothelium and form a nidus for attachment of neutrophils and then monocytes. However, the majority of platelets subsequently release from endothelium and return to the circulation in an activated state. These recycled platelets have the potential to transport proteins and RNA from the graft to the recipient. Some of the platelets that return to the circulation are attached to leukocytes. Platelets have the potential to modulate many elements of the graft and the immune response from the time of organ retrieval through ischemia-reperfusion to acute and chronic rejection. Beyond mechanistic considerations, assays that detect changes in platelet protein or RNA expression could be used to monitor early inflammatory responses in transplants.