Fibrinogen binding to histones in circulation protects against adverse cellular and clinical outcomes

Circulating histones are released by extensive tissue injury or cell death and play important pathogenic roles in critical illnesses. Their interaction with circulating plasma components and the potential roles in the clinical setting are not fully understood.

We aimed to characterize the interaction of histones with fibrinogen and explore its roles in vitro, in vivo, and in patient samples.

Histone-fibrinogen binding was assessed by electrophoresis and enzyme-linked immunosorbent assay-based affinity assay. Functional significance was explored using washed platelets and endothelial cells in vitro and histone-infusion mouse models in vivo. To determine clinical translatability, a retrospective single-center cohort study was conducted on patients requiring intensive care admission (n = 199) and validated in a cohort of hospitalized patients with COVID-19 (n = 69).

Fibrinogen binds histones through its D-domain with high affinity (calf thymus histones, K_D = 18.0 ± 5.6 nM; histone 3, K_D = 2.7 ± 0.8 nM; and histone 4, K_D = 2.0 ± 0.7 nM) and significantly reduces histone-induced endothelial damage and platelet aggregation in vitro and in vivo in a histone-infusion mouse model. Physiologic concentrations of fibrinogen can neutralize low levels of circulating histones and increase the cytotoxicity threshold of histones to 50 μg/mL. In a cohort of patients requiring intensive care, a histone:fibrinogen ratio of ≥6 on admission was associated with moderate-severe thrombocytopenia and independently predicted mortality. This finding was validated in a cohort of hospitalized patients with COVID-19.

Fibrinogen buffers the cytotoxic properties of circulating histones. Detection and monitoring of circulating histones and histone:fibrinogen ratios will help identify critically ill patients at highest risk of adverse outcomes who might benefit from antihistone therapy.