A neutrophil elastase-generated mature form of IL-33 is a potent regulator of endothelial cell activation and proliferative retinopathy

Human interleukin-33 (IL-33) is a 270 amino acid protein that belongs to the IL-1 cytokine family and plays an important role in various inflammatory disorders. Neutrophil proteases (Cathepsin G and Elastase) and mast cell proteases (tryptase and chymase) regulate the activity of IL-33 by processing full-length IL-33 into its mature form. There is little evidence on the role of these mature forms of IL-33 in retinal endothelial cell signaling and pathological retinal angiogenesis. Here, we cloned, expressed, and purified the various mature forms of human IL-33 and then evaluated the effects of IL-33 95-270 , IL-33 99-270 , IL-33 109-270 , and IL-33 112-270 on angiogenesis in human retinal microvascular endothelial cells (HRMVECs). We observed that IL-33 95-270 , IL-33 99-270 , IL-33 109-270 , and IL-33 112-270 significantly induced HRMVEC migration, tube formation and sprouting angiogenesis. However, only IL-33 99-270 could induce HRMVEC proliferation. We used a murine model of oxygen-induced retinopathy (OIR) to assess the role of these mature forms of IL-33 in pathological retinal neovascularization. Our 3′-mRNA sequencing and signaling studies indicated that IL-33 99-270 and IL-33 109-270 were more potent at inducing endothelial cell activation and angiogenesis than the other mature forms. We found that genetic deletion of IL-33 significantly reduced OIR-induced retinal neovascularization in the mouse retina and that intraperitoneal administration of mature forms of IL-33, mainly IL-33 99–270 and IL-33 109–270 , significantly restored ischemia-induced angiogenic sprouting and tuft formation in the hypoxic retinas of IL-33 –/– mice. Thus, our study results suggest that blockade or inhibition of IL-33 cleavage by neutrophil proteases could help mitigate pathological angiogenesis in proliferative retinopathies.