MMP9 and systemic sclerosis: DEGs were enriched in pathways related to the negative regulation of cell proliferation (GO: 0001501, 19/276, Log p = −12.62) and skeletal system development (GO:0008285, 25/227, Log p = −12.51), including IL6, CXCL8, PTGDS, PTGS2, LTF, MGP, MMP9, MMP13, CCN3, OGN, TNFRSF11B, SFRP2, TBX15, PI15, FAM180A, and GPR68. Previous studies have shown that parvovirus B19 (B19V) significantly increases MMP9 activity and enhances the expression of TGF-β, NF-κB, IL-6, TNF-α, and IL-1β in skin fibrosis of systemic sclerosis (27).