Neutrophil calprotectin identifies severe pulmonary disease in COVID-19

Abstract

Severe cases of coronavirus disease 2019 (COVID-19) are regularly complicated by respiratory failure. Although it has been suggested that elevated levels of blood neutrophils associate with worsening oxygenation in COVID-19, it is unknown whether neutrophils are drivers of the thrombo-inflammatory storm or simple bystanders. To better understand the potential role of neutrophils in COVID-19, we measured levels of the neutrophil activation marker S100A8/A9 (calprotectin) in hospitalized patients and determined its relationship to severity of illness and respiratory status. Patients with COVID-19 (n = 172) had markedly elevated levels of calprotectin in their blood. Calprotectin tracked with other acute phase reactants including C-reactive protein, ferritin, lactate dehydrogenase, and absolute neutrophil count, but was superior in identifying patients requiring mechanical ventilation. In longitudinal samples, calprotectin rose as oxygenation worsened. When tested on day 1 or 2 of hospitalization (n = 94 patients), calprotectin levels were significantly higher in patients who progressed to severe COVID-19 requiring mechanical ventilation (8039 ± 7031 ng/ml, n = 32) as compared to those who remained free of intubation (3365 ± 3146, P < 0.0001). In summary, serum calprotectin levels track closely with current and future COVID-19 severity, implicating neutrophils as potential perpetuators of inflammation and respiratory compromise in COVID-19.

Keywords: COVID-19; SARS-CoV-2; calprotectin; neutrophil extracellular traps; neutrophils.

Graphical abstract

FIGURE 1

Calprotectin in sera of COVID-19 patients and its association with clinical studies. (A) Sera from COVID-19 patients (n = 172) and healthy controls (n = 47) were assessed for calprotectin (note log scale). COVID-19 samples were compared to controls by Mann-Whitney test; ****P < 0.0001. (B) For 36 patients, serum samples from two time points were available. Patients were grouped by whether their oxygenation was worsening, stable, or improving; *P < 0.05 by paired Wilcoxon test. (C–E) Calprotectin levels were compared to clinical laboratory results (when available on the same day as the research sample). Spearman’s correlation coefficients were calculated for C-reactive protein (n = 138), absolute neutrophil count (n = 139), and absolute lymphocyte count (n = 139)

FIGURE 2

Levels of calprotectin track closely with oxygenation status. (A, B) Patients (n = 172) were grouped by clinical status: room air (n = 41), noninvasive supplemental oxygen (n = 71), or mechanical ventilation (n = 60). Levels of calprotectin and C-reactive protein were compared by Kruskal-Wallis test corrected by Dunn’s test for multiple comparisons; *P < 0.05 and ****P < 0.0001. (C) Receiver operating characteristic curves based on requirement for mechanical ventilation. (D, E) Calprotectin (n = 172) and C-reactive protein (n = 137) were compared to SpO₂/FiO₂ ratio for each patient, and correlations were determined by Spearman’s test. (F, G) For 94 patients, a calprotectin level was available from hospital day 1 or 2. For 75 of the 94 patients, a CRP level was also available. Patients were then grouped by whether they at any point required mechanical ventilation (vent ever, n = 32) during their hospitalization. Groups were compared by Mann-Whitney test; **P < 0.01 and ****P < 0.0001