- Email: [email protected]
Ocular Surface: Inflammation of the Conjunctiva☆
Ocular Surface: Inflammation of the Conjunctivaq Teruo Nishida, Yamaguchi University Graduate School of Medicine, Ube, Japan Ken Fukuda and Atsuki Fukushima, Kochi Medical School, Nankoku, Japan Ó 2017 Elsevier Inc. All rights reserved.
2 2 3 3 4 5 6 6 6
Inflammation The Conjunctiva and Cornea Tear Fluid: A Reservoir of Inflammatory Cells and Modulators Allergic Reactions in the Conjunctiva Infection of the Conjunctiva or Cornea Dry Eye as an Inflammatory Disease Tear Fluid as a Diagnostic Indicator of Inflammation Connection of the Conjunctiva and Cornea via Tear Fluid Further Reading
Glossary Cell adhesion molecule Cell adhesion molecules are cell surface proteins, usually glycoproteins, that mediate cellto-cell adhesion. They play important roles in the assembly and maintenance of tissues, wound healing, morphogenic cellular movements, cell migration, and metastasis. Intercellular adhesion molecule-1 (ICAM-1) functions in leukocyte adhesion and inflammation. Its expression is induced in various cell types by interferon-g, and it mediates interactions with neutrophils in inflamed tissue. Vascular cell adhesion molecule-1 (VCAM-1) is presented on the surface of various cell types, including endothelial cells, tissue macrophages, fibroblasts, and dendritic cells. Its expression is induced by cytokines, and it plays a key role in the recruitment of eosinophils to sites of inflammation. Collagenase (microbial) Microbial collagenase is a metalloproteinase produced by bacteria that degrades helical regions of native collagen to yield small protein fragments. The preferred cleavage site is immediately upstream of the glycine residue in the sequence –proline– X–glycine–proline–, where X is any amino acid. Six forms (or two classes) of microbial collagenase have been isolated from Clostridium histolyticum; these proteins are immunologically cross-reactive but possess different amino acid sequences and different specificities. Other variants have been isolated from Bacillus cereus, Empedobacter collagenolyticum, Pseudomonas marinoglutinosa, and species of Vibrio and Streptomyces. Helper T cell Helper T cells constitute a specific subpopulation of CD4þ T cells that provide help to other cells of the immune system in mounting an immune response through either direct cell–cell interaction or the
secretion of cytokines. They are also referred to as effector T cells. Several distinct subtypes of helper T cells, designated Th1, Th2, and Th3, have been identified. Matrix metalloproteinase Matrix metalloproteinases (MMPs) constitute an important family of enzymes that regulate composition of the extracellular matrix. They are synthesized as inactive precursor proteins that consist of propeptide, catalytic, and hemopexin domains; proteolytic removal of the propeptide domain results in MMP activation. MMPs are zinc-dependent endopeptidases that cleave one or several constituents of the extracellular matrix as well as nonmatrix proteins, and they play an important role in cleaving fibrillar collagen types I, II, and III into characteristic 3/4 and 1/4 fragments. Some MMPs are associated with the cell membrane, through either a transmembrane domain or glycosylphosphatidylinositol anchor; such MMPs may act within the pericellular environment to influence cell migration. MMP-1, MMP-8, and MMP-13 are also known as collagenase 1, collagenase 2 (neutrophil collagenase), and collagenase 3, respectively. Th1 cytokine Th1 cytokines include interleukin-2, interferon-g, interleukin-12, and tumor necrosis factor-b. They are secreted by Th1 cells and play an important role in cell-mediated immunity and chronic inflammation. In general, Th1 responses are stimulated by intracellular pathogens including viruses as well as certain mycobacteria, yeasts, and parasitic protozoans. Th2 cytokine Th2 cytokines include interleukin (IL)-4, IL5, IL-6, IL-10, and IL-13. They are secreted by Th2 cells and play a key role in the initiation of allergic responses. Th2 responses are also elicited by free-living bacteria and other parasites.
q
Change History: October 2015. Teruo Nishida added co-authors Ken Fukuda and Atsuki Fukushima. Figure 1 was added, Figure 3 was updated, and accordingly the figures were renumbered. Sections “Introduction,” “Tear Fluid,” “Allergic Reactions in the Conjunctiva,” “Dry Eye as an Inflammatory Disease,” and “Further Reading” were updated.
Reference Module in Neuroscience and Biobehavioral Psychology
http://dx.doi.org/10.1016/B978-0-12-809324-5.01395-X
1
2
Ocular Surface: Inflammation of the Conjunctiva
Inflammation Inflammation is a biological response of the living body to injury or other harmful insults including microbial pathogens, allergens, and physical or chemical agents. It serves to protect the body and is the precursor to wound healing. Classical signs and symptoms of inflammation include redness, swelling, heat, pain, and loss of tissue function. Thus, although inflammatory reactions are well regulated to maintain homeostasis of the body and to promote wound repair, they may result in bodily discomfort. In some instances, however, excessive inflammation may result in tissue damage. Classically, inflammation has been considered to begin with a reaction of vascular tissue that renders vessels permeable to blood cells at the site of injury, resulting in the extravasation of such cells. Recent advances in immunology and molecular cell biology have revealed the mechanisms of inflammation at the level of cellular interactions and molecular networks (Fig. 1). Allergens and infection by pathogens are the major pathological triggers for inflammation at the ocular surface. In addition, recent studies of trauma as a cause of inflammation have revealed that dead or dying cells at the site of injury release endogenous danger signalsdknown as alarminsdthat are recognized by receptors expressed on neighboring intact cells and trigger a response to the tissue damage. Necrotic epithelial cells at the ocular surface are thus able to induce or potentiate inflammation and may thereby exacerbate tissue damage.
The Conjunctiva and Cornea The ocular surface is composed of the cornea, conjunctiva, lacrimal glands, and associated lid structures. Both the conjunctiva and the cornea are derived from the embryonic epidermis, and they are separated from each other by tear fluid. External insults to the ocular surface evoke different types of inflammatory reactions in the conjunctiva and the cornea that are related to the anatomic differences and physiological roles of these two structures as well as to their connection via tear fluid (Fig. 2). The conjunctiva is a semitransparent membrane that covers the surface of the eye from the back surface of the eyelids to the edge of the transparent cornea. It serves as a barrier at the surface of the eyeball and helps to protect against the invasion of biological, chemical, or physical agents without interrupting the free movement of the eyeball. The surface of the conjunctiva is covered by multiple layers of epithelial cells. The conjunctival epithelium is a relatively inefficient barrier, however, with the result that pathogens, allergens, and biologically active substances readily penetrate into the stroma of the conjunctiva. The conjunctival stroma consists of conjunctival fibroblasts, loosely packed collagen fibers, a vascular system, and abundant immune cells. The triggering of an inflammatory reaction by pathogens or allergens results in enlargement of the blood vessels of the conjunctiva and consequent hyperemia. The associated increase in the permeability of the conjunctival vascular system leads to leakage of liquid components and to the development of conjunctival edema. It also allows the infiltration of blood cells into the conjunctival stroma and the consequent activation of conjunctival fibroblasts. Like the conjunctiva, the cornea faces the external environment. However, unlike the conjunctiva, the cornea is transparent, and its surface must be maintained smooth for the proper transmission of light into the eye. The anatomic structure of the cornea is
Figure 1 Cellular interactions and molecular networks during inflammation. ATP, adenosine triphosphate; HMGB1, high-mobility group box-1; HSPs, heat-shock proteins; IFN, interferon; IL, interleukin; TNF-a, tumor necrosis factor alpha; TRAIL, tumor necrosis factor-related apoptosis-inducing ligand.
Ocular Surface: Inflammation of the Conjunctiva
Figure 2
3
Cornea–tear fluid–conjunctiva axis in ocular surface inflammation. MMP, matrix metalloproteinase.
relatively simple compared with that of the conjunctiva. Its surface is also covered by multiple layers of epithelial cells, but the corneal epithelium provides a much tighter barrier than does the conjunctival epithelium. In the absence of any loss or dysfunction of its component cells, the corneal epithelium prevents the entry of pathogens and allergens into the corneal stroma. The cornea does not contain a vascular system. Although a small number of immune cells such as Langerhans cells is present in the cornea, its major cellular components are epithelial cells, stromal keratocytes (corneal fibroblasts), and endothelial cells. Both the conjunctiva and the cornea are innervated by the ophthalmic branch of the trigeminal nerve, but the cornea is the most sensitive tissue at the ocular surface, and indeed maybe in the entire body, as a result of the high density of sensory nerve endings in the corneal epithelium.
Tear Fluid: A Reservoir of Inflammatory Cells and Modulators Tear fluid functions as a lubricant between the tarsal conjunctiva and the surface of the cornea and serves to maintain the ocular surface wet. It is also important for ensuring the generation of a clear image on the retina. Moreover, it contributes to the biological defense system of the ocular surface, containing immunoglobulin, lactoferrin, lysozyme, and other protective proteins. With regard to inflammation at the ocular surface, tear fluid provides a pathway for the movement of inflammatory cellsdsuch as neutrophils, eosinophils, and lymphocytesdbetween the conjunctiva and the cornea. It also serves as a reservoir of various inflammatory cytokines, chemokines, and growth factors as well as of nutrients and oxygen. Collagen-metabolizing enzymes such as matrix metalloproteinases (MMPs) are present in the tear fluid of individuals with certain ocular inflammatory conditions. The level of MMP-9 in tear fluid is a potential biomarker for inflammation in dry eye disease.
Allergic Reactions in the Conjunctiva The conjunctiva is a common site for allergic reactions (Fig. 3). Clinical characteristics of conjunctival allergic disease include hyperemia, edema, the formation of papillae, discharge, the development of corneal epithelial disorders, and, in some patients, corneal ulcer. Hyperemia and edema result from dilation and an increase in the permeability of the vascular system in the conjunctiva. Conjunctival fibroblasts are responsible for the formation of papillae. Mechanical injury caused by papillae as well as the effects of inflammatory cytokinesdsuch as interleukin (IL)-4, IL-13, and tumor necrosis factor-a (TNF-a)dand of proteinases or cytotoxic granules released from immune cells such as mast cells and eosinophils are responsible for discharge and damage to the corneal epithelium. Disruption of corneal epithelial barrier function results in the spread of inflammation to the cornea and the development of various types of corneal epithelial disorder. Corneal fibroblasts contribute to the pathology of corneal ulceration. The primary cells that mediate allergic reactions at the ocular surface include dendritic cells, mast cells, vascular endothelial cells, eosinophils, T helper 2 (Th2) cells, and conjunctival epithelial cells and fibroblasts, with corneal epithelial cells and corneal fibroblasts also contributing in some cases.
4
Ocular Surface: Inflammation of the Conjunctiva
Figure 3 Clinical characteristics of allergic reactions in the conjunctiva and the cornea. IL, interleukin; MMP, matrix metalloproteinase; TARC, thymus and activation-regulated chemokine; Th2, T helper 2 cell; TNF-a, tumor necrosis factor alpha; TSLP, thymic stromal lymphopoietin; VCAM, vascular cell-adhesion molecule.
Certain allergens that enter tear fluid from the environment are solubilized by the fluid and penetrate through the loose barrier provided by the conjunctival epithelium into the conjunctival stroma. In the stroma, the allergens trigger the secretion of histamine and inflammatory cytokinesdsuch as IL-4, IL-13, and TNF-adfrom mast cells as well as of IL-3 and IL-5 from Th2 cells. Histamine acts on the vascular endothelium to increase vessel permeability and induce vessel enlargement, resulting in conjunctival hyperemia and edema. IL-4, IL-13, and TNF-a released by mast cells activate conjunctival fibroblasts and trigger their secretion of the chemokines eotaxin and TARC (thymus and activation-regulated chemokine). Eotaxin attracts eosinophils to the interstitial space, and the extravasated eosinophils are then activated by IL-3 and IL-5 released by Th2 cells. TARC attracts Th2 cells into the interstitial space, and these cells then serve as an additional source of IL-4, IL-13, and TNF-a. Exposure of conjunctival fibroblasts to IL-4, IL-13, and TNF-a also stimulates the synthesis of collagen and cell proliferation, effects that give rise to the formation of papillae. The protrusive shape of the papillae results in mechanical injury to both conjunctival and corneal epithelia; such injury, together with the effects of IL-4, IL-13, and TNF-a that enter tear fluid from the conjunctiva, leads to disruption of the barrier function of the corneal epithelium and to discharge. Eosinophils that enter tear fluid from the conjunctiva are then able to penetrate into the corneal stroma. IL-4, IL-13, and TNF-a also enter the corneal stroma from tear fluid and activate corneal fibroblasts to express TARC, eotaxin, and vascular cell adhesion molecule-1 (VCAM-1), a cell adhesion molecule for eosinophils. The activated corneal fibroblasts also produce MMPs, which degrade collagen of the extracellular matrix in the corneal stroma, resulting in corneal ulceration. TARC released from corneal fibroblasts passes through tear fluid into the conjunctival stroma, where it further promotes the secretion of IL-4, IL-13, and TNF-a by Th2 cells in a vicious cycle. Recent studies have also shown that cytokines such as thymic stromal lymphopoietin (TSLP) and IL-25 as well as alarmins such as IL-33 and IL-1 derived from epithelial cells at the ocular surface contribute to conjunctival allergic inflammation and activation of corneal fibroblasts. This scenario thus reveals that, although immune cells such as mast cells, Th2 cells, and eosinophils play a prominent role in allergic disorders at the ocular surface, resident epithelial cells and fibroblasts in both the conjunctiva and cornea also contribute to the inflammatory process.
Infection of the Conjunctiva or Cornea The clinical characteristics of infection at the ocular surface include swelling, hyperemia at the conjunctiva, discharge, and epithelial defects and ulceration in the cornea. As with allergic reactions, the reactions of the conjunctiva and cornea to infection differ (Fig. 4). The vascular system of the conjunctiva ensures a robust immune response to infection in this tissue, with conjunctivitis being a relatively mild clinical condition. However, the cornea is avascular and possesses few immune cells, with the result that corneal infection is more serious and may become sight threatening.
Ocular Surface: Inflammation of the Conjunctiva
5
Figure 4 Clinical characteristics of infection in the cornea and conjunctiva. ICAM, intercellular adhesion molecule; IFN-g, interferon-gamma; IL, interleukin; LPS, lipopolysaccharide; MMP, matrix metalloproteinase; PGN, peptidoglycan; TARC, thymus and activation-regulated chemokine; Th2, T helper 2 cell; TLR, toll-like receptor; TNF-a, tumor necrosis factor alpha.
If a pathogen survives the biological defense system in tear fluid, it readily penetrates the conjunctival epithelium and triggers the dilation and permeabilization of conjunctival blood vessels, resulting in swelling and hyperemia. Neutrophils and Th1 cells enter the conjunctival stroma from the bloodstream and serve as the second line of defense against pathogens. Both neutrophils and Th1 cells secrete IL-1, with Th1 cells also secreting interferon-g (IFN-g). These cells and cytokines may be sufficient to inactivate the pathogen and to limit the inflammatory response to the conjunctiva. However, pathogens also act on conjunctival epithelial cells to trigger the secretion of IL-8, IL-6, and TNF-a. These cytokines together with IL-1 and IFN-g can enter tear fluid and, in the presence of damage to the corneal epithelium, may penetrate into the corneal stroma and activate corneal fibroblasts. The tight barrier provided by the corneal epithelium normally prevents the entry of pathogens into the cornea. However, corneal epithelial injury can result in pathogen penetration into the corneal stroma. Various pathogen-associated factors such as lipopolysaccharide (LPS) of Gram-negative bacteria and peptidoglycan (PGN) of Gram-positive bacteria are recognized by Toll-like receptors (TLRs) on the surface of corneal fibroblasts and trigger the production of IL-8 and the expression of intercellular adhesion molecule-1 (ICAM-1) by these cells. IL-8, IL-6, TNF-a, IFN-g, and IL-1 that enter the corneal stroma via tear fluid also induce IL8 production by corneal fibroblasts. IL-8 then attracts neutrophils extravasated from conjunctival blood vessels to the corneal stroma, and these cells interact with corneal fibroblasts via ICAM-1. IL-1 released from neutrophils further stimulates corneal fibroblasts. Corneal infection is associated with the production of two types of collagen-degrading enzyme: collagenase released from the pathogen and MMPs released from corneal fibroblasts. These enzymes destroy stromal collagen, eventually resulting in the development of corneal ulcer. Collagen destruction by MMPs released from activated corneal fibroblasts may continue even if the pathogen has been killed by antimicrobial treatment. Neutrophils were originally thought to destroy stromal collagen, but these cells were subsequently found to promote the production of MMPs by corneal fibroblasts rather than to degrade the collagen themselves. As with ocular allergy, corneal fibroblasts thus play a key role in the progression of the inflammatory response to corneal infection.
Dry Eye as an Inflammatory Disease Dry eye is a multifactorial disorder of tear fluid and the ocular surface in which inflammation plays a key role. The numbers of various inflammatory cells and levels of inflammatory mediators have been found to be increased in the conjunctiva and tear fluid of individuals with dry eye. The numbers of CD4þ T cells and human leukocyte antigen (HLA)–DR–positive cells are also increased in the conjunctival epithelium of dry eye patients. Indeed, HLA-DR may serve as a biomarker for the severity of dry eye disease and
6
Ocular Surface: Inflammation of the Conjunctiva
the extent of associated inflammation. The concentrations of various inflammatory mediators including the cytokines IL-1, TNF-a, IL-6, IFN-g, and IL-8 as well as the proteinase MMP-9 are also increased in tear fluid of dry eye patients.
Tear Fluid as a Diagnostic Indicator of Inflammation The measurement of inflammatory cytokines or chemokines and the cellular components of tear fluid provides clinically important information on inflammation at the ocular surface. The presence of eosinophils in tear fluid thus confirms a diagnosis of allergic inflammation, whereas the presence of neutrophils is indicative of infectious inflammation. In addition to being of diagnostic value, the condition of tear fluid can affect the progression of ocular surface inflammation. In individuals with dry eye, for example, the decrease in tear secretion and small volume of tear fluid may result in concentration of inflammatory cells and proteins. The condition of tear fluid should thus be taken into account in the treatment of patients with ocular surface inflammation.
Connection of the Conjunctiva and Cornea via Tear Fluid The surfaces of both the conjunctiva and the cornea are covered by epithelial cells. However, the biological responses of these two tissues to allergens or to pathogens differ markedly. The conjunctiva has a prominent vascular system and contains abundant immune cells, whereas the cornea is transparent and avascular and contains few immune cells. These anatomic differences between the conjunctiva and cornea are reflected in the types of inflammatory condition that affect them. The conjunctiva is the principal target tissue for allergic reactions at the ocular surface, whereas the cornea is the main target for microbial infection or injury. The vascular system of the conjunctiva serves as a key source of immune cells in each of these conditions. The cornea is also affected by inflammatory reactions that occur in the conjunctiva, with the tear fluid that covers the surface of both the conjunctiva and the cornea serving as a conduit for the exchange of immune cells, cytokines, chemokines, and growth factors. The concept of inflammation was first described by Celsus more than 2000 years ago as redness and swelling with heat and pain. In the 19th century, the concept of loss of tissue function associated with inflammation was recognized. Recent advances in cell and molecular biology have revealed the cytokine and chemokine network that underlies inflammation. However, the availability of effective anti-inflammatory drugs other than steroids remains limited. Nonsteroidal antiallergic drugs have been developed and are effective for the treatment of allergic conjunctivitis. Nonsteroidal anti-inflammatory drugs (NSAIDs) are also effective in ameliorating inflammatory reactions. However, anti-inflammatory agents that halt tissue destruction are needed. Further characterization of the bidirectional regulation of conjunctival and corneal resident cells via cytokines and chemokines as well as immune cells released into tear fluid may provide a basis for the development of new drugs effective for the treatment of inflammation at the ocular surface.
Further Reading Ebihara, N., Matsuda, A., Seto, T., et al., 2010. The epithelium takes center stage in allergic keratoconjunctivitis. Cornea 29 (Suppl. 1), 41–47. Hazlett, L.D., 2005. Role of innate and adaptive immunity in the pathogenesis of keratitis. Ocul. Immunol. Inflamm. 13, 133–138. Kolaczkowska, E., Chadzinska, M., Plytyez, B., 2008. Basic concepts of inflammationdfrom pioneer studies until now. In: Romano, G.T. (Ed.), Inflammation Research Perspectives. Nova Science Publishers, New York, pp. 113–168. Kumagai, N., Fukuda, K., Fujitsu, Y., Yamamoto, K., Nishida, T., 2006. Role of structural cells of the cornea and conjunctiva in the pathogenesis of vernal keratoconjunctivitis. Prog. Retin. Eye Res. 25, 165–187. Kumar, V., Abbas, A.K., Fausto, N., Mitchell, R.N., 2007. Robbins Basic Pathology, eighth ed. Saunders-Elsevier, Philadelphia, pp. 31–58. Ley, K., 2001. History of inflammation research. In: Ley, K. (Ed.), Physiology of Inflammation. Oxford University Press, New York, pp. 1–10. Oppenheim, J.J., Yang, D., 2005. Alarmins: chemotactic activators of immune responses. Curr. Opin. Immunol. 17, 359–365. Pearlman, E., Johnson, A., Adhikary, G., et al., 2008. Toll-like receptors at the ocular surface. Ocul. Surf. 6, 108–116. Tuli, S.S., Schultz, G.S., Downer, D.M., 2007. Science and strategy for preventing and managing corneal ulceration. Ocul. Surf. 5, 23–39.
2 2 3 3 4 5 6 6 6
Inflammation The Conjunctiva and Cornea Tear Fluid: A Reservoir of Inflammatory Cells and Modulators Allergic Reactions in the Conjunctiva Infection of the Conjunctiva or Cornea Dry Eye as an Inflammatory Disease Tear Fluid as a Diagnostic Indicator of Inflammation Connection of the Conjunctiva and Cornea via Tear Fluid Further Reading
Glossary Cell adhesion molecule Cell adhesion molecules are cell surface proteins, usually glycoproteins, that mediate cellto-cell adhesion. They play important roles in the assembly and maintenance of tissues, wound healing, morphogenic cellular movements, cell migration, and metastasis. Intercellular adhesion molecule-1 (ICAM-1) functions in leukocyte adhesion and inflammation. Its expression is induced in various cell types by interferon-g, and it mediates interactions with neutrophils in inflamed tissue. Vascular cell adhesion molecule-1 (VCAM-1) is presented on the surface of various cell types, including endothelial cells, tissue macrophages, fibroblasts, and dendritic cells. Its expression is induced by cytokines, and it plays a key role in the recruitment of eosinophils to sites of inflammation. Collagenase (microbial) Microbial collagenase is a metalloproteinase produced by bacteria that degrades helical regions of native collagen to yield small protein fragments. The preferred cleavage site is immediately upstream of the glycine residue in the sequence –proline– X–glycine–proline–, where X is any amino acid. Six forms (or two classes) of microbial collagenase have been isolated from Clostridium histolyticum; these proteins are immunologically cross-reactive but possess different amino acid sequences and different specificities. Other variants have been isolated from Bacillus cereus, Empedobacter collagenolyticum, Pseudomonas marinoglutinosa, and species of Vibrio and Streptomyces. Helper T cell Helper T cells constitute a specific subpopulation of CD4þ T cells that provide help to other cells of the immune system in mounting an immune response through either direct cell–cell interaction or the
secretion of cytokines. They are also referred to as effector T cells. Several distinct subtypes of helper T cells, designated Th1, Th2, and Th3, have been identified. Matrix metalloproteinase Matrix metalloproteinases (MMPs) constitute an important family of enzymes that regulate composition of the extracellular matrix. They are synthesized as inactive precursor proteins that consist of propeptide, catalytic, and hemopexin domains; proteolytic removal of the propeptide domain results in MMP activation. MMPs are zinc-dependent endopeptidases that cleave one or several constituents of the extracellular matrix as well as nonmatrix proteins, and they play an important role in cleaving fibrillar collagen types I, II, and III into characteristic 3/4 and 1/4 fragments. Some MMPs are associated with the cell membrane, through either a transmembrane domain or glycosylphosphatidylinositol anchor; such MMPs may act within the pericellular environment to influence cell migration. MMP-1, MMP-8, and MMP-13 are also known as collagenase 1, collagenase 2 (neutrophil collagenase), and collagenase 3, respectively. Th1 cytokine Th1 cytokines include interleukin-2, interferon-g, interleukin-12, and tumor necrosis factor-b. They are secreted by Th1 cells and play an important role in cell-mediated immunity and chronic inflammation. In general, Th1 responses are stimulated by intracellular pathogens including viruses as well as certain mycobacteria, yeasts, and parasitic protozoans. Th2 cytokine Th2 cytokines include interleukin (IL)-4, IL5, IL-6, IL-10, and IL-13. They are secreted by Th2 cells and play a key role in the initiation of allergic responses. Th2 responses are also elicited by free-living bacteria and other parasites.
q
Change History: October 2015. Teruo Nishida added co-authors Ken Fukuda and Atsuki Fukushima. Figure 1 was added, Figure 3 was updated, and accordingly the figures were renumbered. Sections “Introduction,” “Tear Fluid,” “Allergic Reactions in the Conjunctiva,” “Dry Eye as an Inflammatory Disease,” and “Further Reading” were updated.
Reference Module in Neuroscience and Biobehavioral Psychology
http://dx.doi.org/10.1016/B978-0-12-809324-5.01395-X
1
2
Ocular Surface: Inflammation of the Conjunctiva
Inflammation Inflammation is a biological response of the living body to injury or other harmful insults including microbial pathogens, allergens, and physical or chemical agents. It serves to protect the body and is the precursor to wound healing. Classical signs and symptoms of inflammation include redness, swelling, heat, pain, and loss of tissue function. Thus, although inflammatory reactions are well regulated to maintain homeostasis of the body and to promote wound repair, they may result in bodily discomfort. In some instances, however, excessive inflammation may result in tissue damage. Classically, inflammation has been considered to begin with a reaction of vascular tissue that renders vessels permeable to blood cells at the site of injury, resulting in the extravasation of such cells. Recent advances in immunology and molecular cell biology have revealed the mechanisms of inflammation at the level of cellular interactions and molecular networks (Fig. 1). Allergens and infection by pathogens are the major pathological triggers for inflammation at the ocular surface. In addition, recent studies of trauma as a cause of inflammation have revealed that dead or dying cells at the site of injury release endogenous danger signalsdknown as alarminsdthat are recognized by receptors expressed on neighboring intact cells and trigger a response to the tissue damage. Necrotic epithelial cells at the ocular surface are thus able to induce or potentiate inflammation and may thereby exacerbate tissue damage.
The Conjunctiva and Cornea The ocular surface is composed of the cornea, conjunctiva, lacrimal glands, and associated lid structures. Both the conjunctiva and the cornea are derived from the embryonic epidermis, and they are separated from each other by tear fluid. External insults to the ocular surface evoke different types of inflammatory reactions in the conjunctiva and the cornea that are related to the anatomic differences and physiological roles of these two structures as well as to their connection via tear fluid (Fig. 2). The conjunctiva is a semitransparent membrane that covers the surface of the eye from the back surface of the eyelids to the edge of the transparent cornea. It serves as a barrier at the surface of the eyeball and helps to protect against the invasion of biological, chemical, or physical agents without interrupting the free movement of the eyeball. The surface of the conjunctiva is covered by multiple layers of epithelial cells. The conjunctival epithelium is a relatively inefficient barrier, however, with the result that pathogens, allergens, and biologically active substances readily penetrate into the stroma of the conjunctiva. The conjunctival stroma consists of conjunctival fibroblasts, loosely packed collagen fibers, a vascular system, and abundant immune cells. The triggering of an inflammatory reaction by pathogens or allergens results in enlargement of the blood vessels of the conjunctiva and consequent hyperemia. The associated increase in the permeability of the conjunctival vascular system leads to leakage of liquid components and to the development of conjunctival edema. It also allows the infiltration of blood cells into the conjunctival stroma and the consequent activation of conjunctival fibroblasts. Like the conjunctiva, the cornea faces the external environment. However, unlike the conjunctiva, the cornea is transparent, and its surface must be maintained smooth for the proper transmission of light into the eye. The anatomic structure of the cornea is
Figure 1 Cellular interactions and molecular networks during inflammation. ATP, adenosine triphosphate; HMGB1, high-mobility group box-1; HSPs, heat-shock proteins; IFN, interferon; IL, interleukin; TNF-a, tumor necrosis factor alpha; TRAIL, tumor necrosis factor-related apoptosis-inducing ligand.
Ocular Surface: Inflammation of the Conjunctiva
Figure 2
3
Cornea–tear fluid–conjunctiva axis in ocular surface inflammation. MMP, matrix metalloproteinase.
relatively simple compared with that of the conjunctiva. Its surface is also covered by multiple layers of epithelial cells, but the corneal epithelium provides a much tighter barrier than does the conjunctival epithelium. In the absence of any loss or dysfunction of its component cells, the corneal epithelium prevents the entry of pathogens and allergens into the corneal stroma. The cornea does not contain a vascular system. Although a small number of immune cells such as Langerhans cells is present in the cornea, its major cellular components are epithelial cells, stromal keratocytes (corneal fibroblasts), and endothelial cells. Both the conjunctiva and the cornea are innervated by the ophthalmic branch of the trigeminal nerve, but the cornea is the most sensitive tissue at the ocular surface, and indeed maybe in the entire body, as a result of the high density of sensory nerve endings in the corneal epithelium.
Tear Fluid: A Reservoir of Inflammatory Cells and Modulators Tear fluid functions as a lubricant between the tarsal conjunctiva and the surface of the cornea and serves to maintain the ocular surface wet. It is also important for ensuring the generation of a clear image on the retina. Moreover, it contributes to the biological defense system of the ocular surface, containing immunoglobulin, lactoferrin, lysozyme, and other protective proteins. With regard to inflammation at the ocular surface, tear fluid provides a pathway for the movement of inflammatory cellsdsuch as neutrophils, eosinophils, and lymphocytesdbetween the conjunctiva and the cornea. It also serves as a reservoir of various inflammatory cytokines, chemokines, and growth factors as well as of nutrients and oxygen. Collagen-metabolizing enzymes such as matrix metalloproteinases (MMPs) are present in the tear fluid of individuals with certain ocular inflammatory conditions. The level of MMP-9 in tear fluid is a potential biomarker for inflammation in dry eye disease.
Allergic Reactions in the Conjunctiva The conjunctiva is a common site for allergic reactions (Fig. 3). Clinical characteristics of conjunctival allergic disease include hyperemia, edema, the formation of papillae, discharge, the development of corneal epithelial disorders, and, in some patients, corneal ulcer. Hyperemia and edema result from dilation and an increase in the permeability of the vascular system in the conjunctiva. Conjunctival fibroblasts are responsible for the formation of papillae. Mechanical injury caused by papillae as well as the effects of inflammatory cytokinesdsuch as interleukin (IL)-4, IL-13, and tumor necrosis factor-a (TNF-a)dand of proteinases or cytotoxic granules released from immune cells such as mast cells and eosinophils are responsible for discharge and damage to the corneal epithelium. Disruption of corneal epithelial barrier function results in the spread of inflammation to the cornea and the development of various types of corneal epithelial disorder. Corneal fibroblasts contribute to the pathology of corneal ulceration. The primary cells that mediate allergic reactions at the ocular surface include dendritic cells, mast cells, vascular endothelial cells, eosinophils, T helper 2 (Th2) cells, and conjunctival epithelial cells and fibroblasts, with corneal epithelial cells and corneal fibroblasts also contributing in some cases.
4
Ocular Surface: Inflammation of the Conjunctiva
Figure 3 Clinical characteristics of allergic reactions in the conjunctiva and the cornea. IL, interleukin; MMP, matrix metalloproteinase; TARC, thymus and activation-regulated chemokine; Th2, T helper 2 cell; TNF-a, tumor necrosis factor alpha; TSLP, thymic stromal lymphopoietin; VCAM, vascular cell-adhesion molecule.
Certain allergens that enter tear fluid from the environment are solubilized by the fluid and penetrate through the loose barrier provided by the conjunctival epithelium into the conjunctival stroma. In the stroma, the allergens trigger the secretion of histamine and inflammatory cytokinesdsuch as IL-4, IL-13, and TNF-adfrom mast cells as well as of IL-3 and IL-5 from Th2 cells. Histamine acts on the vascular endothelium to increase vessel permeability and induce vessel enlargement, resulting in conjunctival hyperemia and edema. IL-4, IL-13, and TNF-a released by mast cells activate conjunctival fibroblasts and trigger their secretion of the chemokines eotaxin and TARC (thymus and activation-regulated chemokine). Eotaxin attracts eosinophils to the interstitial space, and the extravasated eosinophils are then activated by IL-3 and IL-5 released by Th2 cells. TARC attracts Th2 cells into the interstitial space, and these cells then serve as an additional source of IL-4, IL-13, and TNF-a. Exposure of conjunctival fibroblasts to IL-4, IL-13, and TNF-a also stimulates the synthesis of collagen and cell proliferation, effects that give rise to the formation of papillae. The protrusive shape of the papillae results in mechanical injury to both conjunctival and corneal epithelia; such injury, together with the effects of IL-4, IL-13, and TNF-a that enter tear fluid from the conjunctiva, leads to disruption of the barrier function of the corneal epithelium and to discharge. Eosinophils that enter tear fluid from the conjunctiva are then able to penetrate into the corneal stroma. IL-4, IL-13, and TNF-a also enter the corneal stroma from tear fluid and activate corneal fibroblasts to express TARC, eotaxin, and vascular cell adhesion molecule-1 (VCAM-1), a cell adhesion molecule for eosinophils. The activated corneal fibroblasts also produce MMPs, which degrade collagen of the extracellular matrix in the corneal stroma, resulting in corneal ulceration. TARC released from corneal fibroblasts passes through tear fluid into the conjunctival stroma, where it further promotes the secretion of IL-4, IL-13, and TNF-a by Th2 cells in a vicious cycle. Recent studies have also shown that cytokines such as thymic stromal lymphopoietin (TSLP) and IL-25 as well as alarmins such as IL-33 and IL-1 derived from epithelial cells at the ocular surface contribute to conjunctival allergic inflammation and activation of corneal fibroblasts. This scenario thus reveals that, although immune cells such as mast cells, Th2 cells, and eosinophils play a prominent role in allergic disorders at the ocular surface, resident epithelial cells and fibroblasts in both the conjunctiva and cornea also contribute to the inflammatory process.
Infection of the Conjunctiva or Cornea The clinical characteristics of infection at the ocular surface include swelling, hyperemia at the conjunctiva, discharge, and epithelial defects and ulceration in the cornea. As with allergic reactions, the reactions of the conjunctiva and cornea to infection differ (Fig. 4). The vascular system of the conjunctiva ensures a robust immune response to infection in this tissue, with conjunctivitis being a relatively mild clinical condition. However, the cornea is avascular and possesses few immune cells, with the result that corneal infection is more serious and may become sight threatening.
Ocular Surface: Inflammation of the Conjunctiva
5
Figure 4 Clinical characteristics of infection in the cornea and conjunctiva. ICAM, intercellular adhesion molecule; IFN-g, interferon-gamma; IL, interleukin; LPS, lipopolysaccharide; MMP, matrix metalloproteinase; PGN, peptidoglycan; TARC, thymus and activation-regulated chemokine; Th2, T helper 2 cell; TLR, toll-like receptor; TNF-a, tumor necrosis factor alpha.
If a pathogen survives the biological defense system in tear fluid, it readily penetrates the conjunctival epithelium and triggers the dilation and permeabilization of conjunctival blood vessels, resulting in swelling and hyperemia. Neutrophils and Th1 cells enter the conjunctival stroma from the bloodstream and serve as the second line of defense against pathogens. Both neutrophils and Th1 cells secrete IL-1, with Th1 cells also secreting interferon-g (IFN-g). These cells and cytokines may be sufficient to inactivate the pathogen and to limit the inflammatory response to the conjunctiva. However, pathogens also act on conjunctival epithelial cells to trigger the secretion of IL-8, IL-6, and TNF-a. These cytokines together with IL-1 and IFN-g can enter tear fluid and, in the presence of damage to the corneal epithelium, may penetrate into the corneal stroma and activate corneal fibroblasts. The tight barrier provided by the corneal epithelium normally prevents the entry of pathogens into the cornea. However, corneal epithelial injury can result in pathogen penetration into the corneal stroma. Various pathogen-associated factors such as lipopolysaccharide (LPS) of Gram-negative bacteria and peptidoglycan (PGN) of Gram-positive bacteria are recognized by Toll-like receptors (TLRs) on the surface of corneal fibroblasts and trigger the production of IL-8 and the expression of intercellular adhesion molecule-1 (ICAM-1) by these cells. IL-8, IL-6, TNF-a, IFN-g, and IL-1 that enter the corneal stroma via tear fluid also induce IL8 production by corneal fibroblasts. IL-8 then attracts neutrophils extravasated from conjunctival blood vessels to the corneal stroma, and these cells interact with corneal fibroblasts via ICAM-1. IL-1 released from neutrophils further stimulates corneal fibroblasts. Corneal infection is associated with the production of two types of collagen-degrading enzyme: collagenase released from the pathogen and MMPs released from corneal fibroblasts. These enzymes destroy stromal collagen, eventually resulting in the development of corneal ulcer. Collagen destruction by MMPs released from activated corneal fibroblasts may continue even if the pathogen has been killed by antimicrobial treatment. Neutrophils were originally thought to destroy stromal collagen, but these cells were subsequently found to promote the production of MMPs by corneal fibroblasts rather than to degrade the collagen themselves. As with ocular allergy, corneal fibroblasts thus play a key role in the progression of the inflammatory response to corneal infection.
Dry Eye as an Inflammatory Disease Dry eye is a multifactorial disorder of tear fluid and the ocular surface in which inflammation plays a key role. The numbers of various inflammatory cells and levels of inflammatory mediators have been found to be increased in the conjunctiva and tear fluid of individuals with dry eye. The numbers of CD4þ T cells and human leukocyte antigen (HLA)–DR–positive cells are also increased in the conjunctival epithelium of dry eye patients. Indeed, HLA-DR may serve as a biomarker for the severity of dry eye disease and
6
Ocular Surface: Inflammation of the Conjunctiva
the extent of associated inflammation. The concentrations of various inflammatory mediators including the cytokines IL-1, TNF-a, IL-6, IFN-g, and IL-8 as well as the proteinase MMP-9 are also increased in tear fluid of dry eye patients.
Tear Fluid as a Diagnostic Indicator of Inflammation The measurement of inflammatory cytokines or chemokines and the cellular components of tear fluid provides clinically important information on inflammation at the ocular surface. The presence of eosinophils in tear fluid thus confirms a diagnosis of allergic inflammation, whereas the presence of neutrophils is indicative of infectious inflammation. In addition to being of diagnostic value, the condition of tear fluid can affect the progression of ocular surface inflammation. In individuals with dry eye, for example, the decrease in tear secretion and small volume of tear fluid may result in concentration of inflammatory cells and proteins. The condition of tear fluid should thus be taken into account in the treatment of patients with ocular surface inflammation.
Connection of the Conjunctiva and Cornea via Tear Fluid The surfaces of both the conjunctiva and the cornea are covered by epithelial cells. However, the biological responses of these two tissues to allergens or to pathogens differ markedly. The conjunctiva has a prominent vascular system and contains abundant immune cells, whereas the cornea is transparent and avascular and contains few immune cells. These anatomic differences between the conjunctiva and cornea are reflected in the types of inflammatory condition that affect them. The conjunctiva is the principal target tissue for allergic reactions at the ocular surface, whereas the cornea is the main target for microbial infection or injury. The vascular system of the conjunctiva serves as a key source of immune cells in each of these conditions. The cornea is also affected by inflammatory reactions that occur in the conjunctiva, with the tear fluid that covers the surface of both the conjunctiva and the cornea serving as a conduit for the exchange of immune cells, cytokines, chemokines, and growth factors. The concept of inflammation was first described by Celsus more than 2000 years ago as redness and swelling with heat and pain. In the 19th century, the concept of loss of tissue function associated with inflammation was recognized. Recent advances in cell and molecular biology have revealed the cytokine and chemokine network that underlies inflammation. However, the availability of effective anti-inflammatory drugs other than steroids remains limited. Nonsteroidal antiallergic drugs have been developed and are effective for the treatment of allergic conjunctivitis. Nonsteroidal anti-inflammatory drugs (NSAIDs) are also effective in ameliorating inflammatory reactions. However, anti-inflammatory agents that halt tissue destruction are needed. Further characterization of the bidirectional regulation of conjunctival and corneal resident cells via cytokines and chemokines as well as immune cells released into tear fluid may provide a basis for the development of new drugs effective for the treatment of inflammation at the ocular surface.
Further Reading Ebihara, N., Matsuda, A., Seto, T., et al., 2010. The epithelium takes center stage in allergic keratoconjunctivitis. Cornea 29 (Suppl. 1), 41–47. Hazlett, L.D., 2005. Role of innate and adaptive immunity in the pathogenesis of keratitis. Ocul. Immunol. Inflamm. 13, 133–138. Kolaczkowska, E., Chadzinska, M., Plytyez, B., 2008. Basic concepts of inflammationdfrom pioneer studies until now. In: Romano, G.T. (Ed.), Inflammation Research Perspectives. Nova Science Publishers, New York, pp. 113–168. Kumagai, N., Fukuda, K., Fujitsu, Y., Yamamoto, K., Nishida, T., 2006. Role of structural cells of the cornea and conjunctiva in the pathogenesis of vernal keratoconjunctivitis. Prog. Retin. Eye Res. 25, 165–187. Kumar, V., Abbas, A.K., Fausto, N., Mitchell, R.N., 2007. Robbins Basic Pathology, eighth ed. Saunders-Elsevier, Philadelphia, pp. 31–58. Ley, K., 2001. History of inflammation research. In: Ley, K. (Ed.), Physiology of Inflammation. Oxford University Press, New York, pp. 1–10. Oppenheim, J.J., Yang, D., 2005. Alarmins: chemotactic activators of immune responses. Curr. Opin. Immunol. 17, 359–365. Pearlman, E., Johnson, A., Adhikary, G., et al., 2008. Toll-like receptors at the ocular surface. Ocul. Surf. 6, 108–116. Tuli, S.S., Schultz, G.S., Downer, D.M., 2007. Science and strategy for preventing and managing corneal ulceration. Ocul. Surf. 5, 23–39.