MMP9 and rheumatoid arthritis: Upon assessment of those DEPs up-regulated in RA samples from our studies, we found that many are known to be produced by neutrophils (S100A9, S100A8, RAB7A, DEFA1, HP, LRG1, FGB, CTSG, MPO, LTF, PGLYRP1, SAA2, ORM1, ORM2, MMP9, SERPINA1, TNC, TIMP1, and PSMA6), and several have been classified as neutrophil activation markers, including calprotectin (S100A8 and S100A9 [30] HP [31], LRG1 [32], CTSG [33], LTF [34], PGLYRP1 [35], MPO [36], and MMP9 [37]).