CCL2 and pulmonary arterial hypertension: CCL2 expression is increased in atherosclerotic plaques (Ylaherttuala et al., 1991; Nelken et al., 1991; Takeya et al., 1993), and circulating CCL2 levels are raised in PAH patients and animal models of PAH (Ikeda et al., 2002; Itoh et al., 2006; Sanchez et al., 2007; Soon et al., 2010).