Circulating Histone Levels Correlate with the Severity of COVID-19 and the Extent of Coagulation Activation and Inflammation

Background

The COVID-19 pandemic has highlighted the potentially lethal consequences of cross-talk between coagulation, inflammation and innate immune processes. Hospitalised COVID-19 patients die of respiratory and multi-organ failure and these patients have evidence of extensive immune cell death, which would result in the release of nuclear material, such as histones. Extracellular histones have been associated with adverse clinical outcome and our work has shown that they are directly pro-coagulant [Blood. 2020, in press], pro-inflammatory [J Immunology. 2013;191(5):2495-2502] (induce C-reactive protein (CRP) release) and can cause pulmonary thrombosis [AJRCCM. 2013;187(2):160-9]. We therefore hypothesize that circulating histones play a central role in affecting poor outcomes in patients with COVID-19.

Methods

One hundred and three COVID-19 patients were recruited at the Royal Liverpool University Hospital, in accordance with the ISARIC/WHO Clinical Characterisation Protocol for Severe Emerging Infections in the UK (CCP-UK). Inclusion criteria: (1) proven confirmed swab positive or high likelihood of infection OR (2) one or more of the following symptoms: fever of ≥ 38⁰C, new cough, dyspnoea OR tachypnoea AND admitted to a healthcare facility. Exclusion criteria: confirmed diagnosis of a pathogen unrelated to COVID-19 and no indication or likelihood of co-infection with a relevant pathogen. Patients were categorised into three groups based on severity of infection: mild (minimal symptoms and/or incidental finding), severe (dyspnoea or hypoxia) and critical (respiratory failure, shock or multi-organ failure). Circulating histones were quantified in patient plasma and associated with severity of infection, coagulation parameters, inflammatory and organ injury markers.

Results

Admission histone levels were significantly (P<0.001) elevated in patients with increasing severity of COVID-19 infection (Mild; 2.00µg/ml [0.68-6.62], Severe; 9.75µg/ml [3.61-21.88], Critical; 23.37µg/ml [11.35-30.02]). Histones were associated with a pro-coagulant (histones vs d-dimer; R=0.596, P<0.001) and pro-inflammatory phenotype (histones vs CRP; R=0.730, P<0.001, histones vs fibrinogen; R=0.677, P<0.001). Increased circulating histones were associated with organ dysfunction including hypoxia (oxygen saturations ≤93%; P=0.008), raised bilirubin (R=0.568, P=0.002) and elevated serum creatinine (R=0.508, P=0.009). Patients with elevated histones required critical care admission (P<0.001), increased duration of mechanical ventilation (R=0.778, P=0.022) and overall length of hospital stay (R=0.618, P<0.001).

Conclusions

Admission histone levels are associated with disease severity, coagulation activation, inflammation and organ dysfunction in COVID-19 patients. This study indicates that elevated circulating histones might play a key role in the immuno-thrombotic pathogenesis of COVID-19.