CCL4 and chronic obstructive pulmonary disease: However, the Circos illustration of ligand-receptor pairs accounting for enhanced CCIs between monocytes and other immune cells revealed that the most extensively involved ligands and receptors were all from COPD-associated monocytes, including chemokines (CCL20 and CCL4), adhesion molecules (ICAM1), and chemokine receptors (CCRL2 and CXCR4) (Additional file 2: Fig. S4B).