IL9 and oral lichen planus: Moreover, MMP-9 mRNA positively correlated with IL-9 in OLP lesions and both were significantly elevated in the epithelial and lamina propria of erosive OLP compared to healthy controls; in addition, Th9/IL-9 could induce MMP-9 to aggravate OLP disease severity and increase IL-17 and Th17 cells [100].