PF4 and pulmonary arterial hypertension: CXCL4 is a chemokine with potent antiangiogenic properties which is secreted by megakaryocytes, activated platelets, and plasmacytoid dendritic cells.26 Previous proteome-wide analysis revealed the role of CXCL4 as a biomarker for development of PAH in SSc.27 In contrast to the latter, we found CXCL4 to be significantly lower in SSc-PH patients.