Novel Methods for the Analysis of Serum NET Remnants: Evaluation in Patients with Severe COVID-19

Neutrophil extracellular traps (NETs) are web-like structures composed of chromatin and proteins from neutrophil granules. Several studies highlight the heterogeneity of NETs, underscoring the challenges associated with their detection. In patients with COVID-19, high levels of NET fragments, called NET remnants, are detected in the circulation but also in alveoli and bronchioles. NET remnants are usually measured as complexes of DNA and myeloperoxidase (DNA−MPO). Taking advantage of proteomic data on NET composition, we developed new solid-phase assays to detect NET remnants, measuring complexes of DNA with alpha enolase (DNA−eno) or calprotectin (DNA−cal). The two assays were compared with the DNA−MPO test for the detection of in vitro-generated NET and serum NET remnants; all of them showed similar sensitivity in the detection of in vitro-generated NET. In an analysis of 40 patients with severe COVID-19 and 25 healthy subjects, the results of the three assays were highly correlated, and all detected significantly higher levels of NET remnants in patient sera. Moreover, the level of NET remnants correlated with impaired gas exchange and increased with the progressive decline of pulmonary function. The proposed assays thus represent a novel tool with which to evaluate NETosis; using antibodies to different NET constituents may allow their fingerprinting in different disorders.