CCL2 and pulmonary arterial hypertension: Additionally, circulating levels of inflammatory mediators, such as interleukin (IL)-6, IL-1β, tumor necrosis factor (TNF)-α, and monocyte chemoattractant protein (MCP)-1, have been shown to be elevated in PAH and correlated to the severity of the disease (Humbert et al., 1995; Dolenc et al., 2014).