nNIF blocks NETs induced by soluble factors in plasma from COVID-19 patients. (A) We used confocal microscopy to assess NET formation qualitatively. We isolated PMNs from healthy adults (n = 4) and used those for experiments testing 16 different available COVID-19 patient plasma samples for NET inductive capacity. We used plasma from age- and sex-matched healthy donors as a control (n = 10). We incubated PMNs with control plasma or COVID-19 patient plasma and assessed for NET formation after a 2-hour incubation. PMNs were preincubated for 1 hour with nNIF (1 nM) or its inactive scrambled peptide control (SCR; 1 nM). Representative images from each experimental arm show NETs (magenta; white arrows) and nuclear DNA (green). Scale bar, 20 μm. (B) We used a high throughput cell-free DNA fluorescence assay to quantify NET formation in the PMNs treated in (A). The y-axis depicts NETs measured as fold change over baseline relative fluorescence units ± SEM. The PMNs treated with control healthy donor plasma serve as the baseline PMN NET level, arbitrarily set at 1. The P values were determined using 1-way ANOVA with Tukey’s multiple-comparisons post hoc testing.