EREG and chronic obstructive pulmonary disease: A Circos illustration of ligand-receptor pairs responsible for elevated CCIs between monocytes and alveolar epithelial cells as a result of significant expression changes highlighted several pro-inflammatory cytokines up-regulated in COPD-associated monocytes, including IL-1β, TNF, and EREG (Fig. 4C and see Additional file 1: Supplementary Methods).