Crosstalk between Platelets and the Immune System: Old Systems with New Discoveries

Abstract

Platelets are small anucleate cells circulating in the blood. It has been recognized for more than 100 years that platelet adhesion and aggregation at the site of vascular injury are critical events in hemostasis and thrombosis; however, recent studies demonstrated that, in addition to these classic roles, platelets also have important functions in inflammation and the immune response. Platelets contain many proinflammatory molecules and cytokines (e.g., P-selectin, CD40L, IL-1β, etc.), which support leukocyte trafficking, modulate immunoglobulin class switch, and germinal center formation. Platelets express several functional Toll-like receptors (TLRs), such as TLR-2, TLR-4, and TLR-9, which may potentially link innate immunity with thrombosis. Interestingly, platelets also contain multiple anti-inflammatory molecules and cytokines (e.g., transforming growth factor-β and thrombospondin-1). Emerging evidence also suggests that platelets are involved in lymphatic vessel development by directly interacting with lymphatic endothelial cells through C-type lectin-like receptor 2. Besides the active contributions of platelets to the immune system, platelets are passively targeted in several immune-mediated diseases, such as autoimmune thrombocytopenia, infection-associated thrombocytopenia, and fetal and neonatal alloimmune thrombocytopenia. These data suggest that platelets are important immune cells and may contribute to innate and adaptive immunity under both physiological and pathological conditions.

Platelets: an outlook from biology through evidence-based achievements in critical care

“…This phenomenon validates a remarkable increase in our understanding regarding the roles of platelets in haemostasis, thrombosis and wound repair (13). Correspondingly, current evidence has shown that platelets are entangled in many other physiological and pathological processes beyond haemostasis and thrombosis such immuno-thrombosis, the development of atherosclerosis, lymphatic vessel enlargement, angiogenesis and tumour metastasis (14)(15)(16).…”

Multiple roles of the coagulation protease cascade during virus infection

“…2 They are widely expressed by immune cells and other nonimmune cells, such as endothelial cells, epithelial cells, fibroblasts, cardiomyocytes, and platelets. 1,2,4 Viral infections are mainly detected by endosomal TLRs that recognize nucleic acid ligands from the virus. 2,5 TLR3 recognizes the dsRNA of dsRNA viruses and dsRNA that is made during replication of ssRNA viruses.…”

Mechanisms of platelet-mediated liver regeneration

“…Recent studies show that platelets have many functions beyond physiologic or pathologic thrombus formation. They play a role in inflammation, 5,6 are effectors of injury in a variety of pulmonary disorders and syndromes, 7 facilitate tissue repair, 8 and act in the growth and development of metastases of various cancers. 9 It has been well established that platelets are vital for liver regeneration after a partial liver resection in animals.…”