Activated neutrophils suppress T-cell proliferation. Purified T cells (either CD4⁺ or CD8⁺) were cultured in the presence or absence of anti-CD3 antibody or anti-CD28 antibody with unstimulated or fMLF-activated neutrophils (unless otherwise indicated). Cells were harvested after 5 to 6 days and analyzed by flow cytometry for CFSE dilution. (A) Representative fluorescence-activated cell sorting (FACS) plots of CFSE dilution of CD4⁺ T cells. (B) Quantification of CD4⁺ (left) and CD8⁺ (right) T-cell proliferation (n = 17). (C) Titration of the cell ratio with 4000 (5:1 ratio), 20 000 (1:1), 40 000 (1:2), 60 000 (1:3), 100 000 (1:5), or 160 000 (1:8) neutrophils per well of a 96-well plate (n = 3-17). (D) Purified T cells were cultured in the presence or absence of IL-15 with unstimulated or fMLF-activated neutrophils (n = 5). (E) Purified T cells were cultured with anti-CD3 and anti-CD28 antibodies (red bars), and in the presence of neutrophils (blue bars) and/or indicated stimuli. Three to 19 donors were tested in duplicate per stimulus. Error bars indicate standard error of the mean (SEM); ****P < .0001; ***P < .001; **P < .01. CR, cytokine receptor; GPCR, G-protein–coupled receptor; TLR, Toll-like receptor.