CCR6 and pulmonary arterial hypertension: In contrast, endothelial cells largely express several other chemokine receptors including CCR4, CCR5, CCR6, CXCR2, CXCR4, and CXCR5 [30], and CXCR4, which plays a key role in pulmonary arterial hypertension (PAH) through an autocrine signaling mechanism [31].