Considering that CCL5 and CX3CL1 were well studied in PAH and ACKR4 is the only gene downregulated in PAH lungs among the three genes, indicating the role of ACKR4 as a protective modulator and immune checkpoint, we therefore selected ACKR4 as the key gene for further analysis. The gene discussed is CCL5; the disease is pulmonary arterial hypertension.