Corneal fibroblasts: Function and markers
Introduction
Transparency of the mammalian cornea depends largely on the condition of the corneal stroma—and, in particular, on the highly organized arrangement of collagen fibrils in the stroma (Maurice, 1957). Indeed, more than 90% of the volume of the corneal stroma consists of extracellular matrix (ECM) including collagen and proteoglycans, with cellular components occupying only 3% of total stromal volume (Otori, 1967). Keratocytes are the principal cells of the corneal stroma and are responsible for metabolism of stromal ECM, including the slow synthesis and secretion as well as the degradation of ECM proteins that contribute to the maintenance of corneal transparency (Funderburgh et al., 2003; Hassell and Birk, 2010; Yam et al., 2020). Both keratocytes and corneal endothelial cells are derived from the neural crest, which also gives rise to the stroma of the iris and ciliary body as well as the trabecular meshwork during ocular development (Gage et al., 2005; Williams and Bohnsack, 2015). Disturbance of cells of neural crest origin can therefore result in developmental disorders of the anterior segment of the eye including Axenfeld-Rieger syndrome (Shields et al., 1985; Sowden, 2007).
Keratocytes in the healthy cornea are quiescent, only rarely undergoing cell division. The epithelial barrier of the cornea is less permeable than are those of the skin and conjunctiva and protects the corneal stroma from various external stimuli such as mechanical injury, infectious microbes, and inflammatory mediators. Keratocytes are activated and transform into fibroblasts or myofibroblasts in response to disruption of the epithelial barrier and consequent exposure of the stroma to such stimuli (Fini, 1999; Jester et al., 1999c; Wilson, 2012). Corneal fibroblasts (CFs) are thus considered to be activated keratocytes in the damaged or diseased cornea. This review highlights the roles of CFs in various inflammatory conditions associated with corneal injury, infection, or immune reaction.
Section snippets
Functions of keratocytes
Quiescent keratocytes in the healthy cornea maintain basal metabolism of ECM and consequent transparency of the corneal stroma (Yam et al., 2020). Scanning electron microscopy of the cornea after removal of stromal ECM by enzymatic digestion and acid hydrolysis revealed that the processes of neighboring keratocytes make contact with each other and thereby form a continuous network structure (Nishida et al., 1988b). Gap junctions were also found to be present at these contact sites by
Role of CFs in the response to injury, wound healing, and sterile inflammation
Damaged cells release various endogenous molecules that signal danger to surrounding tissue. These molecules are known as DAMPs or alarmins, they include cytokines (IL-1α, IL-33), DNA, ATP, heat shock protein 60, high-mobility group box 1 protein (HMGB1), uric acid, S100 proteins, and β-defensin 2 (Chen and Nunez, 2010), and they induce sterile inflammation, an inflammatory response in the absence of pathogen infection or an immunologic reaction. Corneal epithelial abrasion results in the
Role of CFs in infectious keratitis
Human corneal epithelial cells express Toll-like receptor (TLR) 2 and TLR4 intracellularly but not at the cell surface, and they are thus not activated by the corresponding ligands peptidoglycan (PGN) and lipopolysaccharide (LPS) (Ueta et al., 2004). This unresponsiveness may serve to prevent responses to commensal bacteria and contribute to an immunosilent environment at the ocular surface. In vivo experiments with rabbits have also revealed that the intact corneal epithelium acts as a barrier
Role of CFs in the pathogenesis of corneal lesions associated with severe ocular allergic diseases
Ocular allergy is a type of noninfectious ocular inflammation. Ocular allergic diseases are triggered by the invasion of allergens such as dust mites, pollen, or animal cell debris into the conjunctiva. Such diseases are classified as allergic conjunctivitis, atopic keratoconjunctivitis (AKC), vernal keratoconjunctivitis (VKC), or giant papillary conjunctivitis. Allergic conjunctivitis is characterized by ocular itching, hyperemia of the conjunctiva, and a watery discharge that result from the
CF-myofibroblast transformation and the role of TGF-β signaling
During the contraction phase of corneal wound healing, myofibroblasts characterized by the expression of α–smooth muscle actin appear in the corneal stroma. Although myofibroblasts contribute to wound healing after injury by mediating tissue contraction, these cells may also give rise to tissue scarring or fibrosis by releasing large quantities of ECM including collagen, fibronectin, and proteoglycans (Funderburgh et al., 2003; Garana et al., 1992). Myofibroblasts are thus generated in the
Markers of CFs
Whereas keratocytes are quiescent and manifest a flattened and stellate morphology in the healthy cornea, these cells become mitotic and motile in response to corneal injury. The resulting CFs have a fusiform morphology, possess multiple nucleoli, and lack cytoplasmic granules (Fini, 1999; Fini and Stramer, 2005). At later stages of wound healing, CFs undergo further transformation into myofibroblasts that contribute to tissue fibrosis and contraction (Jester et al., 1999c; Saika et al., 2008;
Therapeutic interventions targeting CFs
Potential therapeutic approaches to corneal diseases such as infectious or noninfectious corneal ulcer include elimination of causative factors such as infecting microbes or allergic reactions, inhibition of released collagen-degrading enzymes such as MMPs, and regulation of effector cells such as CFs. Given that CFs act as sentinel cells, serving as the first line of defense in the avascular corneal stroma against various external insults, prolonged and uncontrolled inflammatory responses
Conclusions
We have shown here that CFs act as sentinel cells and promoters of corneal stromal inflammation associated with tissue injury, infection, or allergy. In the healthy cornea, resting keratocytes are quiescent and interconnected in a network structure. They determine the balance between the synthesis and degradation of ECM in the corneal stroma so as to maintain corneal integrity and transparency. In the diseased cornea, however, keratocytes become activated and adopt a fibroblastic phenotype, and
Funding
This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.
Declaration of competing interest
None.
Acknowledgments
I thank Teruo Nishida, M.D., D.Sc. and Koji Sugioka, M.D., Ph.D. for suggestions in development of the manuscript as well as Waka Ishida, Ph.D. for preparation of the figures.
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