Interactions between platelets, leukocytes and the vessel wall provide alternative pathological routes of thrombo-inflammatory leukocyte recruitment. We found that when platelets were activated by a range of agonists in whole blood, they shed platelet-derived extracellular vesicles which rapidly and preferentially bound to blood monocytes compared to other leukocytes. Platelet-derived extracellular vesicle binding to monocytes was initiated by P-selectin-dependent adhesion and was stabilised by binding of phosphatidylserine. These interactions resulted in the progressive transfer of the platelet adhesion receptor GPIbα to monocytes. GPIbα +monocytes tethered and rolled on immobilised von Willebrand Factor or were recruited and activated on endothelial cells treated with TGF-b1 to induce the expression of von Willebrand Factor. In both models monocyte adhesion was ablated by a function-blocking antibody against GPIbα. Monocytes could also bind platelet-derived extracellular vesicle in mouse blood in vitro and in vivo. Intratracheal instillations of diesel nanoparticles, to model chronic pulmonary inflammation, induced accumulation of GPIbα on circulating monocytes. In intravital experiments, GPIbα + -monocytes adhered to the microcirculation of the TGF-b1-stimulated cremaster muscle, while in the ApoE -/model of atherosclerosis, GPIbα + -monocytes adhered to the carotid arteries. In trauma patients, monocytes bore platelet markers within 1 hour of injury, the levels of which correlated with severity of trauma and resulted in monocyte clearance from the circulation. Thus, we have defined a novel thrombo-inflammatory pathway in which platelet-derived extracellular vesicles transfer a platelet adhesion receptor to monocytes, allowing their recruitment in large and small blood vessels, and which is likely to be pathogenic.

O-GlcNAcylation is a newly discovered histone modification implicated in transcriptional regulation, but no structural information on the physical effect of GlcNAcylation on chromatin exists. Here, we generate synthetic, pure GlcNAcylated histones and nucleosomes and reveal that GlcNAcylation can modulate structure through direct destabilization of H2A/H2B dimers in the nucleosome, thus promoting an ‘open' chromatin state. The results suggest that a plausible molecular basis for one role of histone O-GlcNAcylation in epigenetic regulation is to lower the barrier for RNA polymerase passage and hence increase transcription.

S100A8/A9, also known as calprotectin or MRP8/14, is an alarmin primarily secreted by activated myeloid cells and platelets with anti-microbial, pro-inflammatory and pro-thrombotic properties. Increased plasma levels of S100A8/A9 in thrombo-inflammatory diseases are associated with thrombotic complications. We assessed the presence of S100A8/A9 in the plasma and lung autopsies from patients with COVID-19 and investigated the molecular mechanism by which S100A8/A9 affects platelet function and thrombosis. S100A8/A9 plasma levels were increased in patients with COVID-19 and sustained high levels during hospitalization correlated with poor outcomes. Heterodimeric S100A8/A9 was mainly detected in neutrophils and deposited on the vessel wall in COVID-19 lung autopsies. Immobilization of S100A8/A9 with collagen accelerated the formation of a fibrin-rich network following perfusion of recalcified blood at venous shear. In vitro, platelets adhered and partially spread on S100A8/A9 leading to the formation of distinct populations of either P-selectin or phosphatidylserine-positive platelets. Using washed platelets, soluble S100A8/A9 induced phosphatidylserine exposure but failed to induce platelet aggregation, despite GPIIb/IIIa activation and alpha-granule secretion. We identified GPIbα as the receptor for S100A8/A9 on platelets inducing the formation of procoagulant platelets with a supporting role for CD36. The effect of S100A8/A9 on platelets was abolished by recombinant GPIbα ectodomain, platelets from Bernard-Soulier Syndrome patient with GPIb-IX-V deficiency and platelets from mice deficient in the extracellular domain of GPIbα. In conclusion, we identified the S100A8/A9-GPIbα interaction as a novel targetable prothrombotic pathway inducing procoagulant platelets and fibrin formation, in particular in diseases associated with high levels of S100A8/A9, such as COVID-19.

Recent studies have indicated that extracellular vesicles (EV) may play a role in the pathogenesis of Acute Respiratory Distress Syndrome (ARDS). EV have been identified as potential biomarkers of disease severity and prognosis in other pulmonary diseases. We sought to characterize the EV phenotype within ARDS patient broncho-alveolar lavage fluid (BAL), and to determine whether BAL EV could be utilized as a potential biomarker in ARDS. BAL was collected from sepsis patients with and without ARDS, and from esophagectomy patients post-operatively (of whom a subset later developed ARDS during hospital admission). BAL EV were characterized with regards to size, number and cell of origin. Sepsis patients with ARDS had significantly higher numbers of CD14+/CD81+ monocyte-derived BAL EV than sepsis patients without ARDS (p=0.015). However, the converse was observed in esophagectomy patients who later developed ARDS (p=0.003). Esophagectomy patients who developed ARDS also had elevated CD31+/CD63+ and CD31+/CD81+ endothelial-derived BAL EV (p≤0.02) compared to esophagectomy patients who did not develop ARDS. Further studies are required to determine whether CD31+ BAL EV may be a predictive biomarker for ARDS in esophagectomy patients. CD14+/CD81+ BAL EV numbers were significantly higher in those patients with sepsis-related ARDS who died during the 30 days following ICU admission (p=0.027). Thus, CD14+/CD81+ BAL EV are a potential biomarker for disease severity and mortality in sepsis-related ARDS. These findings provide the impetus to further elucidate the contribution of these EV to ARDS pathogenesis.