We have previously reported that abundant NETs are present over the ocular surface of patients with several tear deficient DED subtypes (Sjogren's syndrome, ocular GVHD, and Ocular Cicatricial Pemphigoid [OCP]).
3 There are two possible reasons for buildup of NETs on the ocular surface of severe DED subtypes: (1) tear fluid hyperosmolarity in these patients may have enhanced the formation of NETs
9; and/or (2) lack of nuclease due to tear fluid deficiency may have reduced clearance of NETs.
3 Both Sjogren's syndrome and ocular GVHD patients have hyperosmolar tears and tear deficiency,
22 features that favor accumulation of NETs and consequent inflammation.
3,9 Abnormal regulation of NETs (excessive NETosis and deficient nucleases) has been suggested to play a role in other inflammatory conditions as well (e.g., in the pathogenesis of dermatomyositis and polymyositis).
23 Excessive DNA and NETs can cause inflammation; however, eDNA must re-enter a cell and bind its intracellular receptor to stimulate downstream signaling pathways.
24,25 Cathelicidin, an antimicrobial peptide that is a molecular component of NETs, binds eDNA and enhances its intracellular entry.
26 Cathelicidin is localized within neutrophils and NETs on the ocular surface of patients with severe DED, particularly within the mucoid films. Once inside the cell, DNA binds TLR9 to stimulate signaling through MyD88, which initiates a signaling cascade leading to an IFN-type I response.
27,28 Thus, based on our findings and those of others, we have proposed a mechanism for inflammation in severe DED. eDNA and NETs in tear fluid bind cathelicidin and re-enter ocular surface cells to stimulate the TLR9-MyD88 pathway and activate the IFN type I response. The finding in this clinical trial that application of DNase eye drops in DED patients results in a significant reduction in corneal staining builds upon our previous finding that tear fluid eDNA abundance correlates best with corneal staining (
r = 0.55) and weakly with the Schirmer I test (
r = −0.39) and OSDI score (
r = 0.35).
10 Taken together, the implication is that inflammation induced by eDNA may have caused corneal epitheliopathy (which is clinically detected as corneal staining), and degradation of eDNA with DNase eye drops may have reduced eDNA-induced inflammation and consequent corneal epitheliopathy, thus reducing corneal staining. It is also possible that ocular surface disease results from unintended but detrimental bystander damage resulting from epithelial toxicity due to molecular components of NETs. Histones can cause direct cytotoxicity to epithelial cells.
29 Extracellular histones are major mediators of cell death in sepsis.
30 Cathelicidin peptide fragments can cause inflammation, erythema, and telangiectasia, particularly in patients with rosacea.
31 Neutrophil elastase induces epithelial cell apoptosis.
32 Since the molecular components of NETs are decorated on the eDNA backbone, degradation of this eDNA backbone by DNase eye drops may have released these molecules, facilitating their subsequent removal from the ocular surface (e.g., flushing with artificial tears).