Endogenous IgM antibodies decrease MV-dependent TG. (A) Chemiluminescent enzyme-linked immunosorbent assay of plasma IgM levels after depletion of free IgM from pooled EV-free human plasma with increasing amounts of anti–human IgM/agarose beads. (B) TG curves and peak heights of IgM-depleted MV-free plasma triggered by addition of platelet-derived MVs. Plots depict representative experiment of n = 4. Bars represent mean ± SEM of each group. *P < .05, ****P < .001, two-way analysis of variance with Bonferroni’s multiple comparisons test. (C) Percentages of IgM⁺ MVs within annexin V–positive events isolated from plasma of healthy volunteers (n = 22) measured by using flow cytometry. Groups were divided into MV IgM^low and MV IgM^high based on the mean percentage (horizontal bar). (D) Comparison of the procoagulatory potential of IgM^low and IgM^high MV average TG curves and peak heights of pooled MV-depleted plasma reconstituted with isolated IgM^low (n = 9) and IgM^high (n = 13) MVs. Columns and error bars or solid lines and light-colored areas represent the mean ± SEM of each group. (E) Percentages of IgM⁺ MVs of WT (n = 15) and Siglec-G^−/− (n = 18) mice within annexin V–positive events measured by flow cytometry. Bars represent mean ± SEM. (F) Spontaneous TG of platelet-poor plasma from WT and Siglec-G^−/− mice. Average TG curves and peak heights of pooled platelet-poor plasma of WT vs Siglec-G^−/− mice. Columns and error bars or solid lines and light-colored areas represent the mean ± SEM of each group. (D-F) **P < .01, ***P < .005, Mann-Whitney U test. RLU, relative light units.