CD40 and chronic obstructive pulmonary disease: Of all the proteins above 30% FDR, PD-L1 (immune checkpoint marker), CD68 (a marker of functionally active macrophages (29), CD163 (a marker of functionally inactive macrophages (29), CD40 and CD44 (co-stimulatory B and T cell molecules expressed during viral infections), CD45 (pan-leukocyte marker), and Human Leukocyte Antigen–DR isotype (HLA-DR, ligand of T cell receptor) were significantly higher in the GOLD 1–2 COPD group vs. smokers (Fig. 2A) and GOLD 3–4 COPD group (Fig. 2B) across all the ROIs.