Circulating Histones Induce Inflammatory Responses in Monocytes: Effects of Heparins as Histone-Neutralizing Agents

Background The formation and release of Extracellular Traps (ETs), characterizing the mechanism of ETosis, represents both a cellular death mechanism and an additional immune system pathway against infections. The uncontrolled occurrence of this process results in an excessive release of these ETs, which can have dangerous effects on the host. The core of these structures is made up of extracellular DNA that is connected to various nuclear, cytosolic, and granular proteins. The biomolecular characterization of ET revealed that histones, in particular, can promote and/or amplify hyperinflammatory response, coagulation cascade, endothelial dysfunction, and eventually multi-organ failure. On these bases, extracellular histones could become potential biomarkers and predictive laboratory tools for numerous human diseases, including cancer, thrombosis, autoimmune disorders, systemic lupus erythematosus, Sepsis, and COVID-19 infection. The discovery of therapeutic approaches aimed at the neutralization and inhibition of extracellular histones activity could improve the clinical course and outcomes of these pathologies. Experimental Objectives and Approaches This project was designed to I) investigate the ability of histones to alter monocyte morphology and functions, promoting the onset of a pro-inflammatory phenotype, and II) to evaluate the ability of heparin variants to limit the histone-induced inflammatory profiles. To this end, we used an ex vivo human whole-blood experimental model. We used histones as inflammatory triggers and tested their ability to promote the release of a wide panel of inflammatory mediators, and how this could be inhibited by using four commercially available anticoagulant drugs (Enoxaparin, Unfractionated Heparin, Sulodexide, and Fondaparinux). Furtherly, we explored the impact of extracellular histones on the cytomorphological features of human monocytes by evaluating the alterations of MDW (Monocyte Distribution Width), a hematological laboratory parameter associated with monocytic heterogeneity in response to massive inflammatory stimulation. Results Our studies demonstrated the direct role of histones as contributing agents in the initiation of cytokine storm, as found in vivo in some severe human disorders such as Sepsis and COVID -19. Besides, we observed that heparin variants (Enoxaparin, Unfractionated Heparin, Sulodexide, and Fondaparinux) have different inhibiting effects on histone-Induced Inflammation, thus representing potential candidates to limit their harmful effects. Finally, the most intriguing and innovative discovery is that histones can act as MDW modifiers, by altering circulating monocyte volume, cytoplasmic granularity and vacuolization, and nuclear structure, all alterations that are generally found in viral and classical septic patients but never referred to histones, to date. Final Considerations Taken together all these findings highlight the dangerous effect of circulating histones, suggesting that a greater understanding of their biomolecular mechanisms of action, as well as their neutralization, might have a positive impact on the outcomes of many life-threatening clinical disorders.