Tear Proteases and Protease Inhibitors: Potential Biomarkers and Disease Drivers in Ocular Surface Disease

Abstract

Tears are highly concentrated in proteins relative to other biofluids, and a notable fraction of tear proteins are proteases and protease inhibitors. These components are present in a delicate equilibrium that maintains ocular surface homeostasis in response to physiological and temporal cues. Dysregulation of the activity of protease and protease inhibitors in tears occurs in ocular surface diseases including dry eye and infection, and ocular surface conditions including wound healing after refractive surgery and contact lens (CL) wear. Measurement of these changes can provide general information regarding ocular surface health and, increasingly, has the potential to give specific clues regarding disease diagnosis and guidance for treatment. Here, we review three major categories of tear proteases (matrix metalloproteinases, cathepsins, and plasminogen activators [PAs]) and their endogenous inhibitors (tissue inhibitors of metalloproteinases, cystatins, and PA inhibitors), and the changes in these factors associated with dry eye, infection and allergy, refractive surgery, and CLs. We highlight suggestions for development of these and other protease/protease inhibitor biomarkers in this promising field.

Figure 1.. Dry eyes disease classification.

Dry eyes diseases (DED) are categorized into evaporative dry eye (EDE) and aqueous deficient dry eye (ADDE). EDE is mainly caused by meibomian gland dysfunction (MGD)/blepharitis. ADDE is primarily based on defective secretion by the lacrimal gland (LG). ADDE is further subdivided into Sjögren’s syndrome (SS) related and non-Sjögren’s syndrome related (non-SS). Image created with BioRender.

Figure 2.. Proteases and protease inhibitors in the tear film and the cycle of homeostasis/pathogenesis.

Proteases and protease inhibitors maintain ocular surface and tear film homeostasis. A. Dry Eye Disease (DED). Various conditions induce elevated tear osmolarity, production of proinflammatory cytokines and proteases in tears. High tear osmolarity and cathepsin S may increase the expression of proinflammatory cytokines and MMP-9 in corneal epithelium. Elevated tear cytokines may further upregulate proinflammatory cytokines, cathepsin S and MMP-9 which (1) cleave epithelial junctional complexes, accelerate apical corneal epithelial detachment, expose subapical epithelium and; (2) activate sensing nociceptors. Inflammatory cytokines and MMP-9 facilitate the activation of dendritic cells and their migration to the lymph nodes (afferent arm). In lymph nodes, dendritic cells present antigen to CD4⁺ T cells, which can be recruited to the ocular surface (efferent arm). Cytokines produced by CD4⁺ T cells feedback to affect epithelial cell survival and differentiation. B. Corneal Wound Healing. Corneal conditions such as wounding through refractive surgery give rise to alterations in tear composition. MMP-2, MMP-9 and uPA are upregulated in corneal epithelium and contribute to epithelial migration by (1) cleaving epithelial junctional complexes and (2) degradation of ECM. Cytokines in the epithelium also facilitate wound healing by inducing apoptosis in epithelial cells. IL-1α and TGF-β secreted by the epithelium activate fibroblasts (keratocytes) and trans-differentiation of myofibroblasts. (3) Myofibroblasts secret ECM and MMPs, including MMP-1 and MMP-3, which further facilitate stromal healing and ECM remodeling. (4) Plasmin and PAs modulate normal wound healing processes through ECM remodeling, fibrin degradation and pro-MMP activation. Corneal wounding is accompanied by inflammation, upregulation of proinflammatory cytokines and immune cell recruitment leading to increased tear uPA, MMP-8 and MMP-9. Image created with BioRender.