Background: Platelet activation plays a critical role in thrombosis and thrombo-inflammation. Inhibition of platelet activation is a cornerstone in treatment of acute myocardial ischemia, however with an increased risk of bleeding. Previously, we found that the chemokine receptor ACKR3 (CXCR7) inhibits platelet activation.

Methods: We generated a megakaryocyte/platelet-specific knock-out mouse strain PF4-Cre; Ackr3^-/- and characterized platelet function in vitro and in vivo. We performed ischemia/reperfusion experiments (transient LAD-ligation) in PF4-Cre; Ackr3^-/- and wild type siblings to assess the effect of genetic Ackr3 deficiency in platelets on heart injury. Transthoracic echocardiogram (TTE) were performed to access heart function before and after I/R. Tissue inflammation, injury and regeneration was characterized by gross pathology, histology, histochemistry and immunophenotyping of infiltrating inflammatory cells.

Results: Genetic deficiency of platelet Ackr3 promotes platelet activation (e.g. increased a-granula release, aggregation, intracellular Ca²⁺, and platelet-mediated thrombus formation). This genetically-induced platelet hyperreactivity in PF4-Cre; Ackr3^-/- animals enhances tissue injury in ischemic myocardium 24 hours after I/R, e.g. infract size (infarct / % of area at risk) was increased by 26 % (p<0.05), and aggravates tissue inflammation and systemic thrombo-inflammation. The expression of proliferative markers (Ki67) was significantly reduced by 66 % in PF4-Cre; Ackr3^-/- samples compared to wild type controls. After 28 days of I/R fibrotic areas within the ischemic myocardium were significantly enlarged by 124 % (p<0.05) in PF4-Cre; Ackr3^-/- compared to wild type. The observed enhanced tissue injury in PF4-Cre; Ackr3^-/- samples was sustained and infract size (infarct / % of area at risk) increased by 118 %, p<0.0001. Whereas neovascularization was significantly reduced by 35 % (p < 0.001). TTE revealed a sustained effect on heart function, e.g. the fractional shorting was significantly reduced by 36 % in PF4-Cre; Ackr3^-/- animals after 7 days (p<0.05) and 37 % after 28 days (p<0.05) compared to wild type animals.

Conclusions: We demonstrate that the platelet chemokine receptor ACKR3 is a critical regulator of organ injury following ischemia/reperfusion. Therefore, platelet hyperreactivity enhances inflammation and formation of fibrosis following transient I/R. Modulation of ACKR3 may provide a novel strategy to control platelet reactivity following I/R and to prevent myocardial dysfunction.