The median level of MMP‐9 transcripts appeared to be higher in the COPD‐S group than in the healthy‐S group (Fig. 5B); in contrast, the two groups did not differ significantly in terms of NGAL, MMP‐2, MMP‐12 and IL‐6 expression levels (see Fig. 5B for NGAL and Fig. S4 for MMP‐2, MMP‐12 and IL‐6). Here, MMP12 is linked to chronic obstructive pulmonary disease.