Platelets and bacterial infections

Abstract

Platelets are circulating blood cells classically known for their key roles in mediating hemostasis and vascular wall repair. Platelets also possess a dynamic repertoire of effector functions in the immune continuum that span from rapid innate immune responses to more delayed adaptive and acquired immune activities. Platelets express a wide array of structural and functional characteristics of host defense effector cells that augment host defenses to bacterial infections. Platelets display a diverse range of surface ligands and receptors that recognize and bind bacteria, including complement receptors, FcγRII, toll-like receptors (TLRs), and integrins conventionally described in the hemostatic response, such as αIIbβ3 and GPIb. Both direct and indirect binding of bacteria, bacterial toxins, and other agonists to platelets via fibrinogen, fibronectin, C1q, or von Willebrand factor (vWF) may result in platelet activation. Platelets may also internalize bacteria, although the function and fate of these host defense mechanisms remain incompletely understood. Once activated, platelets transform from quiescent discoid forms to amoeboid cells that chemotax to and target microbial pathogens or ligands displayed by tissues injured during infectious insults. Upon activation and subsequent degranulation, platelets secrete an array of multifunctional host defense and antimicrobial peptides that act as direct anti-infective agents and coordinate additional molecular and cellular host defenses. Platelets also release soluble immunomod-ulatory factors that play crucial roles in the formation of neutrophil extracellular traps (NETs), resulting in bacterial elimination but also enhancing thrombosis in disease situations. Therefore, platelets are increasingly recognized as key effector cells in immune and inflammatory responses to host infection. The multiplicity of events underscores the complexity of platelet-bacterial interaction and illustrates the emerging view that platelets are important sentinel and effector cells in host defenses against pathogens.