Fig. 4.

Delayed NET clearance due to defective DNase activity in COVID-19. (a) DNase activity measured in plasma samples from COVID-19 patients (n=43) or healthy donors (n=39) using the DNase Alert QC System. Data represent mean ± s.e.m., p-value, two-tailed unpaired Student's t-test. (b) DNase activity measured in plasma samples from COVID-19 patients (n=43) or healthy donors (n=39) using the SRED assay. Representative pictures are shown for COVID-19 (orange box) and healthy donors (grey box). Data represent mean ± s.e.m., p-value, two-tailed unpaired Student's t-test. (c) NET degradation capacity of COVID-19 plasma. DNA fluorescence was quantified from COVID-19 plasma (n=36) and healthy donor plasma (n=24). Data represent mean ± s.e.m., p-value, two-tailed unpaired Student's t-test. Representative immunofluorescence images of in vitro generated NETs from healthy neutrophils are shown upon incubation with healthy donor or COVID-19 plasma. (d) Activation of purified FXII by NETs was measured by conversion of the chromogenic substrate H-D-Pro-Phe-Arg-p-nitroaniline (S-2302) at an absorption of λ=405 nm. S-2302 was added either in the presence or absence of the FXIIa inhibitor rHA-infestin-4 (INH) or DNase I (Pulmozyme). Data represent mean ± s.e.m., p-value, two-tailed unpaired Student's t-test (n=6 replicates per time point from two independent experiments). Arbitrary units (AU), Scale bar: 25 µm (c). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)