Upregulation of the IL-33/ST2 pathway in dry eye

Dry eye (DE) is a multifactorial disease of the tears and ocular surface that results in symptoms of discomfort, visual disturbance, and tear film instability with potential damage to the ocular surface. Although the pathogenesis of dry eye has not been fully understood, the role of increased tear osmolarity has been established. There is increasing evidence that dry eye is an inflammatory disease. This article aims to investigate the potential pathogenicity of inflammatory cytokine interleukin (IL)-33 and its receptor ST2 in dry eye.

Human conjunctival epithelial cells (HConECs) were stimulated with hyperosmolality to produce a dry eye cell model. Real-time PCR evaluated the IL-33 mRNA level, and western blotting assessed IL-33 protein expression. Clinical data (sex, age, ocular surface disease index [OSDI] score, tear film breakup time, Schirmer test, and corneal fluorescein staining [CFS]) of patients with DE were collected. Conjunctival impression cytology (CIC) specimens were collected to detect the protein expression of IL-33 and ST2 with western blotting. Tears were collected with Schirmer strips, and analyzed using multiplex assay kits to examine IL-33 and its downstream factors IL-4, IL-5, and IL-13.

The IL-33 mRNA level of the HConECs increased in the hyperosmotic state (relative 4.35-fold upregulation, p<0.001). The IL-33 protein expression of HConECs also showed higher levels in the hyperosmotic state (relative 2.22-fold upregulation, p<0.01). A total of 25 patients with dry eye and 20 healthy subjects were enrolled. There were no statistically significant differences in age and sex between the two groups. The OSDI score, tear film breakup time, Schirmer test, and ocular surface staining of the two groups were statistically significantly different. The IL-33 and ST2 protein levels were increased in patients with DE versus controls (IL-33: relative 9.25-fold upregulation, p<0.001; ST2: relative 4.35-fold upregulation, p<0.05). The concentrations of IL-33, IL-13, and IL-5 in tears increased in patients with DE versus controls (IL-33: 3.00-fold upregulation, p<0.0001; IL-13: 6.65-fold upregulation, p<0.0001; IL-5: 16.54 -fold upregulation, p=0.01). IL-13 and IL-5 were statistically significantly correlated with IL-33. The level of IL-33 was positively correlated with the OSDI score and CFS, but was negatively correlated with the Schirmer I test and the tear film breakup time (TBUT). The level of IL-13 was positively correlated only with the CFS, and was negatively correlated with the Schirmer I test. The level of IL-5 was positively correlated with the OSDI score and CFS. We failed to detect the concentration of IL-4, as most samples were below the detection limit.

The IL-33 mRNA and protein levels of HConECs increased under hyperosmolality. The IL-33 and ST2 protein levels were higher in the CIC of patients with DE, and have correlations with disease severity. Moreover, the concentrations of IL-13 and IL-5 released from activated type 2 helper T (Th2) cells increased in the tears of patients with DE. The IL-33/ST2 pathway might play a priming role in the regulation of inflammation of the ocular surface.