Platelet-erythrocyte complex production is exacerbated by histones and protected by small polyanion histone neutralizing compounds

Platelet-erythrocyte (PE) complexes, composed of single erythrocytes bound to platelets, are a physiological component of the bloodstream and contribute to erythrocyte turnover. Conditions in which PE complex production is increased or their clearance from the circulation is impeded are associated with hypercoagulable states and thrombocytopenia. Sepsis is a serious acute inflammatory condition that is often complicated by major hemostatic derangements, including thrombosis, anemia and thrombocytopenia, and an important driver of sepsis pathology are extracellular histones, which are released into the circulation from activated neutrophils. Histone-neutralizing small polyanions (SPAs) have been developed to block the cytotoxic effects of histones and are beneficial in treating sepsis. This study investigated whether histones may stimulate increased PE complex production as a potential cause of the hemostatic complications in disease, and whether SPAs can prevent complex generation. Treatment with histones in co-incubated human erythrocytes and platelets, or murine blood, increased PE complex production approximately 10-fold compared with control conditions. Mice administered histones by intravenous injection displayed reduced platelet counts but sustained complex levels, suggesting complex production was enhanced within the diminished platelet pool. The platelets in the histone-stimulated complexes were also more activated than cell-free platelets, according to increased surface expression of P-selectin and the fibrin-binding form of integrin αIIbβ3, determined using flow cytometry, and their altered shape and presence of filopodia observed by scanning electron microscopy. Inclusion of SPAs in the histone-treated cells and blood prevented histone-stimulated production of complexes, indicating the importance of the histone-mediated cell injury in their generation and identifying PE complexes as a treatment target.