LAUSR

Platelets, best known as essential effector cells in coagulation and hemostasis, are increasingly recognized as major inflammatory cells that play a role in the innate and adaptive arms of the immune system. The interaction of platelets with various cell types of the innate immune system, particularly neutrophils, as well as with structural cells of the vascular endothelium, induces the release of platelet-derivedmediators, thus exerting a protective effect in the physiologic response to diseases and control of invading microorganisms (1). However, it is also known that contrary to this protective anti-infective role of a tightly controlled activation of platelets, aberrant platelet activation can lead to systemic inflammation, with organ dysfunction and thrombotic complications in both acute and chronic inflammatory disorders of infective origin. It is, therefore, pleasing that this Research Topic of Frontiers in Immunology contains a series of up-to-date articles on the role of platelet activation in the pathophysiology of three important host infections, namely the human immunodeficiency virus (HIV), Mycobacterium tuberculosis (TB) and Streptococcus pneumoniae (pneumococcus). The article entitled “Platelets and their role in the pathogenesis of cardiovascular events in patients with community-acquired pneumonia”, by Feldman and Anderson is a review article, concentrating primarily on the role of platelets in the host defense against infection, particularly pneumococcal infection, and describes the current evidence in the literature for how platelet activation may contribute to cardiovascular complications in patients with community-acquired pneumonia. This includes a review of work from the authors’ own laboratory, which uniquely showed that the pneumococcal toxin, pneumolysin, considered by many to be the most important virulence factor of the pneumococcus bacterium, activated platelets, activated formation of neutrophil extracellular traps, and mediated heterotypic aggregation of neutrophils and platelets in vitro. It is thought that the cytolytic action of pneumolysin itself, together with its activation of platelets, plays a major role in the occurrence of cardiac complications in pneumococcal pneumonia. The article entitled “Platelet activation and the immune response to tuberculosis” by Kirwan et al. is a review article that describes the emerging evidence that platelets play a significant role in TB immunopathology. Patients with TB disease often have thrombocytosis, which correlates with disease severity (clinically and radiologically advanced disease) and is associated with a hypercoagulable