Platelet-leukocyte interactions: immunoregulatory role and pathophysiological relevance

Abstract

Blood platelets are the central players in thrombosis and blood coagulation. Moreover, they also exhibit immunoregulatory properties and bridge hemostasis and immunity. Morphological and functional characteristics of the platelets ensure continuous surveillance for the vascular system, recognition of different hazards, development of appropriate response and recruitment of immune cells. Indirect platelet-leukocyte interactions are mediated by immunoregulatory molecules that are released, along with coagulation and thrombosis factors in the course of platelet activation and degranulation. Chemokines, cytokines, growth factors, some of which are synthesized de novo, are released from activated platelets and modulate cellular functions, thus modulating both innate and adaptive immune response. Activated platelets enter contacts with immune cells to form heterotypic aggregates, i.e., platelet-leukocyte complexes that reside in blood circulation along with other blood cells. The aggregate formation and stabilization is mediated by interaction between the molecules expressed on the surface of platelets and leukocytes, in particular, P-selectin (CD62P) and PSGL-1 (CD162). Platelet-monocyte and platelet-neutrophil complexes are most abundant, with platelet-monocyte aggregates being most stable. Moreover, the platelet-derived microvesicles also interact with leukocytes to form heterotypic aggregates, thus, probably, modulating the immune cell functions via transfer of non-coding RNA molecules. Formation of platelet-leukocyte complexes results into mutual activation of platelets and leukocytes. Platelets and platelet-derived microvesicles stimulate phagocytic activity, cytokine secretion, and generation of reactive oxygen species in monocytes and neutrophils, inducing formation of neutrophilic extracellular traps and procoagulant phenotype in monocytes. The blood platelets regulate monocyte differentiation, promote adhesion, as well as transmigration of lymphocytes and NK cells. At the sites of inflammation, platelets enhance extravasation and infiltration of leukocytes into the damaged tissue. Impaired interactions of platelets with endothelial layer and immune cells may underlie pathogenic conditions. Increased level of circulating plateletleukocyte complexes is observed in various disorders including cardiovascular diseases, acute ischemic stroke, respiratory disorders, renal pathologies, liver diseases, diabetes, reproductive disorders, bacterial and viral infections. Further studies of platelet-leukocyte interactions are warranted to unveil pathogenic mechanisms and to develop new therapeutic approaches.