The gelatin zymography results showed that MMP-9 and MMP-2 secretion and activation increased in the co-culture of RA FLS and human monocytes/macrophages from synovial fluid of RA patients compared with those in the co-culture of RA FLS and human monocytes from normal peripheral blood (P < 0.05; Figure 3). Here, MMP9 is linked to rheumatoid arthritis.