Hemostasis without clot formation: how platelets guard the vasculature in inflammation, infection, and malignancy

• Published in: Blood Vol. 142; no. 17; pp. 1413 - 1425
• Main Authors: Kaiser, Rainer, Escaig, Raphael, Nicolai, Leo
• Format: Journal Article
• Language: English
• Published: United States Elsevier Inc 26.10.2023
• Subjects: Blood Platelets - metabolism; Endothelial Cells; Hemorrhage - metabolism; Hemostasis - physiology; Humans; Inflammation - metabolism; Neoplasms - metabolism; Thrombosis - metabolism; Tumor Microenvironment
• ISSN: 0006-4971; 1528-0020
• DOI: 10.1182/blood.2023020535

[Display omitted] Platelets are key vascular effectors in hemostasis, with activation signals leading to fast recruitment, aggregation, and clot formation. The canonical process of hemostasis is well-characterized and shares many similarities with pathological thrombus formation. However, platelets are also crucially involved in the maintenance of vascular integrity under both steady-state and inflammatory conditions by ensuring blood vessel homeostasis and preventing microbleeds. In these settings, platelets use distinct receptors, signaling pathways, and ensuing effector functions to carry out their deeds. Instead of simply forming clots, they mainly act as individual sentinels that swiftly adapt their behavior to the local microenvironment. In this review, we summarize previously recognized and more recent studies that have elucidated how anucleate, small platelets manage to maintain vascular integrity when faced with challenges of infection, sterile inflammation, and even malignancy. We dissect how platelets are recruited to the vascular wall, how they identify sites of injury, and how they prevent hemorrhage as single cells. Furthermore, we discuss mechanisms and consequences of platelets’ interaction with leukocytes and endothelial cells, the relevance of adhesion as well as signaling receptors, in particular immunoreceptor tyrosine–based activation motif receptors, and cross talk with the coagulation system. Finally, we outline how recent insights into inflammatory hemostasis and vascular integrity may aid in the development of novel therapeutic strategies to prevent hemorrhagic events and vascular dysfunction in patients who are critically ill. Kaiser and colleagues provide a state-of-the-art review of the complex role of platelets in maintaining vascular integrity. The authors describe the processes by which platelets are recruited to the vessel wall, how they prevent hemorrhage, where they facilitate clot formation, and how the processes differ in the setting of inflammatory stresses, where platelets can function as single cells to protect vascular integrity without clot formation.