Platelets induce NET formation, which triggers FXIIa-dependent thrombus propagation. (A) Freshly isolated human neutrophils were incubated with platelet supernatant. Cells were stained with a primary antibody directed against DNA–histone complexes and DAPI and visualized by confocal microscopy. Incubation with DNase1 where indicated. Bars, 50 µm. Arrowheads, cell nucleus; arrows, NET. (B) The total number of NETs (left) and NETs per leukocyte (right) was quantified on cross sections of thrombi 48 h after flow restriction in IL4-R/Iba mice and WT animals (n = 3 per group). Data are shown as mean ± SEM. (C) Analysis of thrombus formation (milligrams) in the IVC of C57BL/6 (n = 9), FXII^−/− deficient (n = 7), and FXI^−/− deficient mice (n = 7) 48 h after flow reduction. Dots represent individual experiments; lines show the mean of each group. (D) Quantification of fibrin density as percentage of fibrin-covered area in the IVC thrombus (n = 4 per group). Data are shown as mean ± SEM. (E) The effects of co-incubation of activated platelets (P) and neutrophils (N) on FXII activation in vitro. NETosis was inhibited by an antibody directed against the H2A–H2B–DNA complex. Data are shown as mean ± SEM. (F) Confocal visualization of FXII on NETs, released from isolated human neutrophils. Arrow, FXII bound to Sytox Green⁺ NETs. Bars, 10 µm.