MMP9 and keratoconus: Proinflammatory cytokines (IL-1, IL-6, TNF-α, and TGF-β) and enzymes, involved in degradation of collagens such as metalloproteases (MMP-1, MMP-3, and MMP-9), were described to be upregulated in KC when looking at tears and corneal samples [3, 21, 24, 25].