PAD4^−/−mice are protected from MI/R injury and their protection is not enhanced by DNase I treatment. (A) (i) Representative TTC stains of transverse sections of the LV 24 hours after MI/R of WT (left), PAD4^−/− mice nontreated with DNase I (middle), and after DNase I infusion (right). Infarcted areas appear as white and are outlined with dotted lines. (ii) Myocardial infarction size in the studied groups (measured as % of LV), established at 24 hours after LAD occlusion (**P < .01, ***P < .001). A portion of PAD4^−/− mice were treated with buffer as a control for DNase I infusion, but because we did not see a difference between buffer-treated and nontreated PAD4^−/− mice, the PAD4^−/− data were combined (blue solid circle). (B) No difference was observed in LV ejection function between healthy (control) WT and control PAD4^−/− mice (blue open circle). Echocardiography analysis performed after 24-hour MI/R revealed a significantly higher EF (%) in PAD4^−/− mice nontreated with DNase I (blue solid circle) as compared with WT group (**P < .01). DNase I infusion to PAD4^−/− mice (purple circle) did not ameliorate cardiac function as compared with non-DNase I–treated PAD4^−/− mice. (C) The level of circulating nucleosomes (fold increase over the baseline) remained unchanged both in PAD4^−/− mice with or without DNase I treatment 24 hours after ischemia onset and was significantly lower as compared with WT mice (*P < .05, **P < .01). (D) Representative Gr-1 immunohistochemical staining of ischemic areas in WT and PAD4^−/− mice. PAD4^−/− mice showed significantly fewer Gr-1–positive cells infiltrating the infarcted myocardium than WT (*P < .05). (E) Immunofluorescence analysis revealed H3cit-positive cells in WT mice, whereas none were found in the PAD4^−/− mice. Scale bars, 50 μm (D-E).