Anatomy
The ocular surface and its adnexa comprise the cornea, the conjunctiva with bulbar, fornical and palpebral parts, the main lacrimal gland, the glands of the eyelids, i.e. meibomian, Moll's, and accessory lacrimal glands and the nasolacrimal system with the upper and lower puncta, the paired lacrimal canaliculi, the lacrimal sac and nasolacrimal duct. The nasolacrimal ducts collect the tear fluid from the ocular surface and convey it into the nasal cavity whereas all other structures contribute to formation of the preocular tear film. The tear film serves to protect and lubricate the ocular surface, as well as to provide the major refractive surface for the visual system.
The preocular tear film (see chapter 1 by J.M. Tiffany) contains water, protective antimicrobials, cytokines, lipids, and mucins and can be divided in three components: a lipid component, an aqueous component, and a mucus component. The lipid component is secreted by the meibomian glands in the eyelid and forms the superficial layer of the tear film. The aqueous component contains electrolytes, water, and a large variety of proteins, peptides and glycopeptides and is primarily secreted by the lacrimal gland as well as the accessory lacrimal glands (glands of Krause; glands of Wolfring) of the lids. The mucus component is the product of conjunctival goblet and epithelial cells, corneal epithelial cells and acinar as well as excretory duct cells of the lacrimal gland, which have recently been shown to produce mucins.
Ocular Surface
The apical surface of the ocular surface epithelia, both corneal and conjunctival, provide a specialized interface between the tear fluid and the epithelium that stabilizes the fluid layer. That interface includes the undulating membrane ridges on the apical cell's apical membrane, termed microplicae, and emanating from their apices, a layer termed the glycocalyx. Membrane-bound mucins of corneal and conjunctival epithelial cells are present in the glycocalyx layer; soluble mucins (MUC5AC) from conjunctival goblet cells (figs lb, 2b) as well as MUCs 5B and 7 from lacrimal glands are in solution in the tear film. Both MUC5B and MUC7 have been shown to bind bacteria and contribute to innate immunity of the tear film. Beside MUC5AC, conjunctival goblet cells secrete the trefoil factor family (TFF) peptides TFF1 and TFF3. TFF peptides are, together with mucins, typical constituents of mucus gels that influence the rheological properties of the tear film, promote migration of corneal epithelial cells, have antiapoptotic properties, and induce cell scattering. Conjunctival and corneal epithelial cells are able to react against pathogens by the production of inducible antimicrobial peptides. Moreover, in certain disease states the corneal cells are able to produce TFF3.
Lacrimal Gland
The lacrimal gland is anterior in the superolateral region of the orbit, and is divided into two parts by the levator palpebrae superioris muscle. The lacrimal gland consists of acini that are built of a luminar lining of columnar epithelial cells that are surrounded by a basal layer of myoepithelial cells and an enclosing basement membrane. The human lacrimal gland is a tubuloalveolar gland of serous type. Intercalated and 6-12 interlobular ducts drain the secretions into the conjunctival fornix beneath the temporal bone. The tubules discharge without any characteristic excretory duct system (histologic distinction from serous salivary glands) into the interlobular ducts. The connective tissue between the acini contains accumulations of lymphocytes as well as many plasma cells mainly secreting IgA and being part of the eye-associated lymphoid tissue (EALT). As already mentioned, the lacrimal gland produces electrolytes, water, and a large variety of proteins, peptides and glycopeptides. Of these, recent research regarding tear film rheology and innate immunity focus on production of different constitutively and inducible antimicrobial peptides, such as 3-defensins , surfactant proteins A and D as well as MUCs 4, 5AC, 5B and 7 that are secreted into the tear film.
Eyelid
The skeleton of the eyelid is a collagen plate called the tarsus. It contains a row of branched alveolar sebaceous glands, unrelated to the eyelashes. These tarsal or Meibomian glands have punctate openings along the free edge of the eyelid close to its posterior margin. They produce a lipid material whose synthesis is dependent on neuronal, hormonal, and vascular factors. This lipid material is fluid, spreads easily, is a surfactant as well as an aqueous barrier and must remain functional after a blink. To satisfy these requirements, the Meibomian lipids have a specific composition. Even after delivery it may be modified by lipases produced by ocular bacteria, and modifications in the lipid components can lead to unique disease states. Sexual hormones, especially androgens, seem to play a decisive role in Meibomian physiology.
Near the anterior margin of the eyelids there are two or three rows of stiff hairs - the eyelashes. In the middle of the lid is the cross-striated orbic-ularis oculi muscle, the fiber bundles of its palpebral part overlapping one another like tiles on a roof. The tendon of the cross-striated levator palpebral muscle is inserted into the tarsus; beneath it is the smooth tarsalis muscle. The tone of the latter is determined by autonomic nervous impulses and is supposed to adjust the width of the palpebral opening. The apocrine ciliary glands (Moll's glands) open close to the eyelashes. These apocrine glands are active from birth in producing agents against pathogenic microorganisms in the eyelid shaft and on the ocular surface, i.e. lysozyme, (3-defensin-2, adrenomedullin, lactoferrin, and IgA. In the conjunctival fornix the eyelid also contains small accessory lacrimal glands (Krause's glands, Wolfring's glands). Although much smaller, these glands are histologically comparable to the main lacrimal gland. However, only less is known about the secretions of these small glands and their contribution to tear film physiology.
Nasolacrimal Ducts
A lacrimal system consists of the upper and the lower lacrimal canaliculus, the lacrimal sac and the nasolacrimal duct. The structures are surrounded by a wide ranging cavernous system and are embedded in the osseous canal between the maxilla and the lacrimal bone. The internal wall of each lacrimal canaliculus is lined by a thick non-cornified epithelium resting on a basement membrane. The lacrimal sac and the nasolacrimal duct are lined by a double-layered epithelium with integrated goblet cells sometimes forming characteristic mucous glands. As a draining and secretory system, the nasolacrimal ducts play a role in tear transport by production of MUCs 2, 4, 5AC, 5B, and 7, TFF peptides TFF1 and TFF3 and non-specific immune defense [19]. Moreover, components of tear fluid are absorbed in the nasolacrimal passage and are transported into the surrounding vascular system. This system is similar to a cavernous body that is subject to autonomic control and regulates tear outflow. Tear duct-associated lymphoid tissue (TALT) is present in the efferent tear ducts. Under normal conditions, tear fluid components are constantly absorbed into the blood vessels of the surrounding cavernous body. These vessels are connected to the blood vessels of the outer eye and could act as a feedback signal for tear fluid production, which ceases if these tear components are not absorbed.
Lacrimal Functiona
Unit and Host Defense at the Ocular Surface Lacrimal Functional Unit
The cornea possesses the richest sensory innervation of the body to detect noxious stimuli. The trigeminal sensory neurons (CN V) that innervate the eye vary in their chemical composition and electrophysiological properties, and can be classified according to the stimuli that activate them preferentially: mechanical forces, temperature, or irritant chemicals. Different classes of noxious stimuli (mechanical injuries, heat, extreme cold) activate to a different degree the various populations of sensory fibers of the ocular surface and evoke unpleasant sensations of distinct quality.
It is recognized today that the tear film is secreted reflexively from the 'lacrimal functional unit' that is composed of the ocular surface tissues (cornea and conjunctiva, including goblet cells and Meibomian glands), the lacrimal glands (main and accessory), and their interconnecting sensory (CN V) and autonomic (CN VII) innervation. This reflex secretion is initiated by subconscious stimulation of the highly innervated ocular surface epithelia. The human nasolacrimal ducts are integrated in this reflex arc, as shown by recent investigations.
Host Defense at the Ocular Surface
Some defense mechanisms of the innate immune system have already been mentioned above and it is beyond the scope of this chapter to deal with all of them. However, it should be mentioned that the defense of the ocular surfaces presents a unique challenge in that not only must integrity be maintained against microbial, inflammatory and physical assault, but it must be done while minimizing the risk of loss of corneal transparency. This puts severe limitations on the degree to which scarring or neovascularization can occur in the cornea secondary to any infectious, inflammatory, immunological or wound-healing process. The defense system must be equally effective under two extremes of conditions: those found in the open eye and the closed eye environments. Distinctly different defense strategies are utilized in both open and closed conditions. The extraordinary effectiveness of this system is evidenced by the fact that despite continued exposure to a microbe-rich environment, the external ocular surfaces maintain a very low microbial titer and are highly resistant to breaching by all but a few pathogens.
Eye-Associated Lymphoid Tissue as an Entrance Side for Immunological Events
EALT
The epithelia of the ocular surface, the corneal and conjunctival epithelia, the epithelium of the efferent tear ducts, the Meibomian glands, main and accessory lacrimal glands and lids make up a physiological system that was recently dubbed the lacrimal-ocular surface system. The LOS is organized to maintain the clarity of the cornea - a homeostatic set-point. Like the systems that represent epithelial interfaces between the internal and external environments, i.e., the gastrointestinal, integumentary and respiratory systems, the LOS system collaborates with the innate and adaptive immune system to respond to microbial invasion. The lacrimal glands, conjunctiva and efferent tear ducts constitute one venue of this collaboration area. These tissues are populated by IgA-producing plasma cells and their epithelia actively transport secretory IgA into the nascent tear fluid.
Specific secretory immunity depends on sophisticated cooperation between the mucosal B cell system and an epithelial glycoprotein called the secretory component. Initial stimulation of Ig-producing B cells is believed to take place mainly in organized mucosa-associated lymphoid tissue (MALT). It has become evident that MALT is characterized by considerable region-alization or compartmentalization, perhaps determined by the different cellular expression profiles of adhesion molecules and/or the local antigenic repertoire. Antigenic stimulation of B cells results in the generation of predominantly IgA-synthesizing blasts that leave the mucosae via efferent lymphatics, pass through the associated lymph nodes into the thoracic duct, and enter the circulation. The cells then return selectively to the lamina propria as plasma cells or memory B cells by means of homing mechanisms.
Organized lymphoid tissue in the conjunctiva (conjunctiva-associated lymphoid tissue - CALT) and efferent tear duct system TALT have recently been termed collectively EALT [41]. However, aggregated follicles that fulfill the criteria for designation as EALT occur only in somewhat less than a third of conjunctivae and nasolacrimal ducts from unselected cadavers with no known history of disease involving the eye, efferent tear ducts, or nose. In most cases, only lymphocytes and other defense cells are amply present subepithelially, i.e. inside the conjunctiva and efferent tear ducts that do not form aggregated follicles. It is as yet unclear whether special types of bacteria, viruses, allergic reactions, or other factors, such as some type of immune deviation, are responsible for the development of EALT in humans. However, when EALT is present, it can provide the basis from which primary low-grade B cell lymphoma of the MALT type may arise.
EALT as an Entrance Side for Immunological Events Some organs of the human body (anterior eye chamber, brain, placenta, testicle) have a special immunological state of reduced activation of the specific and non-specific immune system. This condition of local immune suppression, termed the immune privilege, is expressed in delayed or totally suppressed rejection of allogenic transplantations in these organs; this is illustrated by the maintenance of the immunophenotypic immature placenta in the maternal organism and in the survival of corneal and lens transplants in the anterior eye chamber. The biological functions of the immune privilege are evident: tolerance of a foreign antigen is obviously better in some organs than its rejection, and this can be achieved only at the expense of T-cell-mediated cytolysis of local cells. Such cell loss is not replaceable in poorly regenerative, postmitotic, or highly differentiated tissues. Therefore, some viruses survive in the central nervous system, as their elimination by T-effector cells would doubtlessly lead to neural cell death with subsequent severe neurological deficit or even individual death. A similar situation exists in the anterior eye chamber [44] and the testicle. Such immune suppression is not necessary in regenerative organs, like the liver or the skin, since all the cells needed for this process are able to proliferate and redifferentiate.
The mechanisms that maintain the immune privilege are non-uniform among different organs, and they are not understood in detail. Besides the classic concept of mechanical tissue barriers (i.e. the blood-brain, blood-testis and blood-retina barriers), we must consider the expression of so-called death ligands (CD95, TRAIL, TNF) that induce apoptosis of potentially dangerous T cells, as well as a special form of antigen presentation that produces immune tolerance. Such immune deviation was first described in the anterior eye chamber. There, injection of foreign antigen does not lead to a local T-cell reaction (type IV immune reaction) as it does at other body locations, but rather produces systemic tolerance against the inoculated antigen. In this way, antigens are not attacked in the anterior eye chamber, thus protecting the sensitive visual system against inflammatory damage. In this way, the immune privilege of the anterior eye chamber allows transplantation of allogenic lenses, artificial intraocular lenses, and corneae (although type IV immune reactions are possible after corneal transplantation in rare cases).
Such tolerance is known to be transferable by injection of splenocytes from an animal primed by inoculation of an antigen into a second animal, demonstrating that antigens from the anterior eye chamber receive a signal that produces immune deviation and that regulatory T cells have developed. In contrast to the spleen, the cervical lymph nodes do not play a critical role in the induction of immune deviation, as was shown in rats by Yamagami and Dana. Nevertheless, the drainage routes of the antigens from the anterior eye chamber and the location of their origin, as well as the passage of the belonging antigen-presenting cells, are unclear. In particular, it is not clear what role is played by the conjunctiva and the nasolacrimal ducts, as well as the lymphoid tissues associated with them, in the immune privilege of the anterior chamber of the eye.
Egan et al. demonstrated in mice that potent immunologic tolerance can be achieved by exposure of antigen (ovalbumin) via the conjunctival mucosa. They identified the submandibular lymph node as the principal lymph node in which antigen-bearing antigen-presenting cells are located and in which antigen-specific T-cell clonal expansion occurs following conjunctival application of antigen. Clonal expansion was maintained at an elevated level and the T cells were responsive in vitro during a 10-day period of daily ovalbumin application to the conjunctiva. However, despite continuous antigen application, the number of antigen-specific T cells steadily declined over the 10-day period, and by day 14, the remaining ovalbumin-specific T cells were refractory to secondary challenge with ovalbumin, indicating that they had become anergic in vivo. Egan et al. concluded that the fact that antigen-presenting cells presenting ovalbumin were found only in the submandibular lymph node - and not in other lymph nodes, spleen, or nasal associated lymphoid tissue (NALT) -rules out the possibility that tolerance in this system was due to drainage of antigen through the efferent tear ducts and association with NALT or gastrointestinal-associated lymphoid tissue (GALT).
However, one important point is lacking in the suggestions of Egan et al. It has not yet been appreciated that antigens drained by the tear fluid itself, and not applied intraconjunctivally, would be able to induce immune deviation via CALT and/or TALT. With regard to protection of the cornea against inflammatory destruction, this would be plausible and analogous to the process in the nervous system and the anterior eye chamber. In comparison with gastrointestinal tract MALT (GALT), it is not known as yet whether M cells occur in human CALT and TALT, although they probably do, as their presence has been demonstrated in several animal species. M cells are highly specialized epithelial cells that facilitate uptake and transcytosis of macromolecules and microorganisms. Following transcytosis, antigens to cells of the immune system in lymphoid aggregates are released beneath the epithelium, where antigen processing and presentation and stimulation of specific B and T lymphocytes take place.
According to a definition formulated by Isaacson for MALT of the gut wall (i.e., Peyer's patches), MALT comprises four components organized MALT, a lamina propria, intraepithelial lymphocytes, and an associated lymph node. Circulation of the lymphoid cells in these four components enables them to home to their original and other mucosal sites, where they exert the effector function. Such a response may be dominated by slgA release and may include cytotoxic T-lymphocyte action . In this regard, the submandibular lymph node found by Egan et al. might be the 'associated lymph node' of CALT and TALT, but not of NALT.
Activation of T lymphocytes has been observed in dry eye, which leads to the frequent occurrence of abnormal (pathological) apoptosis in terminally differentiated, acinar epithelial cells of the lacrimal gland [53]. Tears secreted to the ocular surface will then contain proinflammatory cytokines and will inflame the tissues of the ocular surface. Abnormal apoptosis has also been detected in the epithelial cells and lymphocytes of the ocular surface [53]. This ocular surface inflammatory response consists of inflammatory cell infiltration, activation of the ocular surface epithelium with increased expression of adhesion molecules, inflammatory cytokines and pro-apoptotic factors, increased concentrations of inflammatory cytokines in the tear fluid and increased activity of matrix-degrading enzymes in the tear fluid. It has been suggested that the reduction of circulating androgens plays a role in these processes. Treatment with locally applied cyclosporin A eye drops interferes with inter-leukin metabolism, especially of interleukin-6, thus creating a new treatment option that leads to remarkable improvement of the irritation symptoms and ocular surface signs in particular in severe cases of keratoconjunctivitis sicca.
Taken together, these findings support the conclusion that CALT and TALT play a role in the pathogenesis of dry eye. One can imagine that misdirected stimulation of EALT could result in a misguided form of immune deviation at the ocular surface. Within the scope of this event, T cells would no longer be hindered in inducing autoimmunity by apoptosis, finally resulting in the clinical picture of dry eye.
It should be mentioned, however, that a recently published article has placed our understanding of MALT in a different light concerning its functional significance. Alpan et al. demonstrated that a systemic immune response to orally administered soluble antigens does not depend on the presence of functional GALT, but more likely on initiation of immune response by gut-conditioned dendritic cells. This finding suggests that MALT is not required for initiation of a primary immune response to antigens that have entered the body. If present, however, it seems to act in two ways: It produces plasma cell precursors that later migrate into adjacent mucosa, mature to plasma cells, and produce slgA for mucosal protection. It allows uptake of antigens by M cells and presentation of these antigens to virgin T and B cells to initiate a primary immune response. Thus, MALT could represent a second pathway (a kind of safeguard of the adaptive immune system) for initiation of a immune response to antigens that have been incorporated into the mucus layer and, in the case of CALT or TALT, have entered the ocular surface and are drained with tear fluid.
It can be concluded that development of EALT is a common feature frequently observed in symptomatically normal nasolacrimal ducts. Whether special types of bacteria, viruses, or other factors, e.g., immune deviation, are responsible for the development of EALT in humans requires future investigation in prospective and experimental studies.
The ocular surface and its adnexa comprise the cornea, the conjunctiva with bulbar, fornical and palpebral parts, the main lacrimal gland, the glands of the eyelids, i.e. meibomian, Moll's, and accessory lacrimal glands and the nasolacrimal system with the upper and lower puncta, the paired lacrimal canaliculi, the lacrimal sac and nasolacrimal duct. The nasolacrimal ducts collect the tear fluid from the ocular surface and convey it into the nasal cavity whereas all other structures contribute to formation of the preocular tear film. The tear film serves to protect and lubricate the ocular surface, as well as to provide the major refractive surface for the visual system.
The preocular tear film (see chapter 1 by J.M. Tiffany) contains water, protective antimicrobials, cytokines, lipids, and mucins and can be divided in three components: a lipid component, an aqueous component, and a mucus component. The lipid component is secreted by the meibomian glands in the eyelid and forms the superficial layer of the tear film. The aqueous component contains electrolytes, water, and a large variety of proteins, peptides and glycopeptides and is primarily secreted by the lacrimal gland as well as the accessory lacrimal glands (glands of Krause; glands of Wolfring) of the lids. The mucus component is the product of conjunctival goblet and epithelial cells, corneal epithelial cells and acinar as well as excretory duct cells of the lacrimal gland, which have recently been shown to produce mucins.
Ocular Surface
The apical surface of the ocular surface epithelia, both corneal and conjunctival, provide a specialized interface between the tear fluid and the epithelium that stabilizes the fluid layer. That interface includes the undulating membrane ridges on the apical cell's apical membrane, termed microplicae, and emanating from their apices, a layer termed the glycocalyx. Membrane-bound mucins of corneal and conjunctival epithelial cells are present in the glycocalyx layer; soluble mucins (MUC5AC) from conjunctival goblet cells (figs lb, 2b) as well as MUCs 5B and 7 from lacrimal glands are in solution in the tear film. Both MUC5B and MUC7 have been shown to bind bacteria and contribute to innate immunity of the tear film. Beside MUC5AC, conjunctival goblet cells secrete the trefoil factor family (TFF) peptides TFF1 and TFF3. TFF peptides are, together with mucins, typical constituents of mucus gels that influence the rheological properties of the tear film, promote migration of corneal epithelial cells, have antiapoptotic properties, and induce cell scattering. Conjunctival and corneal epithelial cells are able to react against pathogens by the production of inducible antimicrobial peptides. Moreover, in certain disease states the corneal cells are able to produce TFF3.
Lacrimal Gland
The lacrimal gland is anterior in the superolateral region of the orbit, and is divided into two parts by the levator palpebrae superioris muscle. The lacrimal gland consists of acini that are built of a luminar lining of columnar epithelial cells that are surrounded by a basal layer of myoepithelial cells and an enclosing basement membrane. The human lacrimal gland is a tubuloalveolar gland of serous type. Intercalated and 6-12 interlobular ducts drain the secretions into the conjunctival fornix beneath the temporal bone. The tubules discharge without any characteristic excretory duct system (histologic distinction from serous salivary glands) into the interlobular ducts. The connective tissue between the acini contains accumulations of lymphocytes as well as many plasma cells mainly secreting IgA and being part of the eye-associated lymphoid tissue (EALT). As already mentioned, the lacrimal gland produces electrolytes, water, and a large variety of proteins, peptides and glycopeptides. Of these, recent research regarding tear film rheology and innate immunity focus on production of different constitutively and inducible antimicrobial peptides, such as 3-defensins , surfactant proteins A and D as well as MUCs 4, 5AC, 5B and 7 that are secreted into the tear film.
Eyelid
The skeleton of the eyelid is a collagen plate called the tarsus. It contains a row of branched alveolar sebaceous glands, unrelated to the eyelashes. These tarsal or Meibomian glands have punctate openings along the free edge of the eyelid close to its posterior margin. They produce a lipid material whose synthesis is dependent on neuronal, hormonal, and vascular factors. This lipid material is fluid, spreads easily, is a surfactant as well as an aqueous barrier and must remain functional after a blink. To satisfy these requirements, the Meibomian lipids have a specific composition. Even after delivery it may be modified by lipases produced by ocular bacteria, and modifications in the lipid components can lead to unique disease states. Sexual hormones, especially androgens, seem to play a decisive role in Meibomian physiology.
Near the anterior margin of the eyelids there are two or three rows of stiff hairs - the eyelashes. In the middle of the lid is the cross-striated orbic-ularis oculi muscle, the fiber bundles of its palpebral part overlapping one another like tiles on a roof. The tendon of the cross-striated levator palpebral muscle is inserted into the tarsus; beneath it is the smooth tarsalis muscle. The tone of the latter is determined by autonomic nervous impulses and is supposed to adjust the width of the palpebral opening. The apocrine ciliary glands (Moll's glands) open close to the eyelashes. These apocrine glands are active from birth in producing agents against pathogenic microorganisms in the eyelid shaft and on the ocular surface, i.e. lysozyme, (3-defensin-2, adrenomedullin, lactoferrin, and IgA. In the conjunctival fornix the eyelid also contains small accessory lacrimal glands (Krause's glands, Wolfring's glands). Although much smaller, these glands are histologically comparable to the main lacrimal gland. However, only less is known about the secretions of these small glands and their contribution to tear film physiology.
Nasolacrimal Ducts
A lacrimal system consists of the upper and the lower lacrimal canaliculus, the lacrimal sac and the nasolacrimal duct. The structures are surrounded by a wide ranging cavernous system and are embedded in the osseous canal between the maxilla and the lacrimal bone. The internal wall of each lacrimal canaliculus is lined by a thick non-cornified epithelium resting on a basement membrane. The lacrimal sac and the nasolacrimal duct are lined by a double-layered epithelium with integrated goblet cells sometimes forming characteristic mucous glands. As a draining and secretory system, the nasolacrimal ducts play a role in tear transport by production of MUCs 2, 4, 5AC, 5B, and 7, TFF peptides TFF1 and TFF3 and non-specific immune defense [19]. Moreover, components of tear fluid are absorbed in the nasolacrimal passage and are transported into the surrounding vascular system. This system is similar to a cavernous body that is subject to autonomic control and regulates tear outflow. Tear duct-associated lymphoid tissue (TALT) is present in the efferent tear ducts. Under normal conditions, tear fluid components are constantly absorbed into the blood vessels of the surrounding cavernous body. These vessels are connected to the blood vessels of the outer eye and could act as a feedback signal for tear fluid production, which ceases if these tear components are not absorbed.
Lacrimal Functiona
Unit and Host Defense at the Ocular Surface Lacrimal Functional Unit
The cornea possesses the richest sensory innervation of the body to detect noxious stimuli. The trigeminal sensory neurons (CN V) that innervate the eye vary in their chemical composition and electrophysiological properties, and can be classified according to the stimuli that activate them preferentially: mechanical forces, temperature, or irritant chemicals. Different classes of noxious stimuli (mechanical injuries, heat, extreme cold) activate to a different degree the various populations of sensory fibers of the ocular surface and evoke unpleasant sensations of distinct quality.
It is recognized today that the tear film is secreted reflexively from the 'lacrimal functional unit' that is composed of the ocular surface tissues (cornea and conjunctiva, including goblet cells and Meibomian glands), the lacrimal glands (main and accessory), and their interconnecting sensory (CN V) and autonomic (CN VII) innervation. This reflex secretion is initiated by subconscious stimulation of the highly innervated ocular surface epithelia. The human nasolacrimal ducts are integrated in this reflex arc, as shown by recent investigations.
Host Defense at the Ocular Surface
Some defense mechanisms of the innate immune system have already been mentioned above and it is beyond the scope of this chapter to deal with all of them. However, it should be mentioned that the defense of the ocular surfaces presents a unique challenge in that not only must integrity be maintained against microbial, inflammatory and physical assault, but it must be done while minimizing the risk of loss of corneal transparency. This puts severe limitations on the degree to which scarring or neovascularization can occur in the cornea secondary to any infectious, inflammatory, immunological or wound-healing process. The defense system must be equally effective under two extremes of conditions: those found in the open eye and the closed eye environments. Distinctly different defense strategies are utilized in both open and closed conditions. The extraordinary effectiveness of this system is evidenced by the fact that despite continued exposure to a microbe-rich environment, the external ocular surfaces maintain a very low microbial titer and are highly resistant to breaching by all but a few pathogens.
Eye-Associated Lymphoid Tissue as an Entrance Side for Immunological Events
EALT
The epithelia of the ocular surface, the corneal and conjunctival epithelia, the epithelium of the efferent tear ducts, the Meibomian glands, main and accessory lacrimal glands and lids make up a physiological system that was recently dubbed the lacrimal-ocular surface system. The LOS is organized to maintain the clarity of the cornea - a homeostatic set-point. Like the systems that represent epithelial interfaces between the internal and external environments, i.e., the gastrointestinal, integumentary and respiratory systems, the LOS system collaborates with the innate and adaptive immune system to respond to microbial invasion. The lacrimal glands, conjunctiva and efferent tear ducts constitute one venue of this collaboration area. These tissues are populated by IgA-producing plasma cells and their epithelia actively transport secretory IgA into the nascent tear fluid.
Specific secretory immunity depends on sophisticated cooperation between the mucosal B cell system and an epithelial glycoprotein called the secretory component. Initial stimulation of Ig-producing B cells is believed to take place mainly in organized mucosa-associated lymphoid tissue (MALT). It has become evident that MALT is characterized by considerable region-alization or compartmentalization, perhaps determined by the different cellular expression profiles of adhesion molecules and/or the local antigenic repertoire. Antigenic stimulation of B cells results in the generation of predominantly IgA-synthesizing blasts that leave the mucosae via efferent lymphatics, pass through the associated lymph nodes into the thoracic duct, and enter the circulation. The cells then return selectively to the lamina propria as plasma cells or memory B cells by means of homing mechanisms.
Organized lymphoid tissue in the conjunctiva (conjunctiva-associated lymphoid tissue - CALT) and efferent tear duct system TALT have recently been termed collectively EALT [41]. However, aggregated follicles that fulfill the criteria for designation as EALT occur only in somewhat less than a third of conjunctivae and nasolacrimal ducts from unselected cadavers with no known history of disease involving the eye, efferent tear ducts, or nose. In most cases, only lymphocytes and other defense cells are amply present subepithelially, i.e. inside the conjunctiva and efferent tear ducts that do not form aggregated follicles. It is as yet unclear whether special types of bacteria, viruses, allergic reactions, or other factors, such as some type of immune deviation, are responsible for the development of EALT in humans. However, when EALT is present, it can provide the basis from which primary low-grade B cell lymphoma of the MALT type may arise.
EALT as an Entrance Side for Immunological Events Some organs of the human body (anterior eye chamber, brain, placenta, testicle) have a special immunological state of reduced activation of the specific and non-specific immune system. This condition of local immune suppression, termed the immune privilege, is expressed in delayed or totally suppressed rejection of allogenic transplantations in these organs; this is illustrated by the maintenance of the immunophenotypic immature placenta in the maternal organism and in the survival of corneal and lens transplants in the anterior eye chamber. The biological functions of the immune privilege are evident: tolerance of a foreign antigen is obviously better in some organs than its rejection, and this can be achieved only at the expense of T-cell-mediated cytolysis of local cells. Such cell loss is not replaceable in poorly regenerative, postmitotic, or highly differentiated tissues. Therefore, some viruses survive in the central nervous system, as their elimination by T-effector cells would doubtlessly lead to neural cell death with subsequent severe neurological deficit or even individual death. A similar situation exists in the anterior eye chamber [44] and the testicle. Such immune suppression is not necessary in regenerative organs, like the liver or the skin, since all the cells needed for this process are able to proliferate and redifferentiate.
The mechanisms that maintain the immune privilege are non-uniform among different organs, and they are not understood in detail. Besides the classic concept of mechanical tissue barriers (i.e. the blood-brain, blood-testis and blood-retina barriers), we must consider the expression of so-called death ligands (CD95, TRAIL, TNF) that induce apoptosis of potentially dangerous T cells, as well as a special form of antigen presentation that produces immune tolerance. Such immune deviation was first described in the anterior eye chamber. There, injection of foreign antigen does not lead to a local T-cell reaction (type IV immune reaction) as it does at other body locations, but rather produces systemic tolerance against the inoculated antigen. In this way, antigens are not attacked in the anterior eye chamber, thus protecting the sensitive visual system against inflammatory damage. In this way, the immune privilege of the anterior eye chamber allows transplantation of allogenic lenses, artificial intraocular lenses, and corneae (although type IV immune reactions are possible after corneal transplantation in rare cases).
Such tolerance is known to be transferable by injection of splenocytes from an animal primed by inoculation of an antigen into a second animal, demonstrating that antigens from the anterior eye chamber receive a signal that produces immune deviation and that regulatory T cells have developed. In contrast to the spleen, the cervical lymph nodes do not play a critical role in the induction of immune deviation, as was shown in rats by Yamagami and Dana. Nevertheless, the drainage routes of the antigens from the anterior eye chamber and the location of their origin, as well as the passage of the belonging antigen-presenting cells, are unclear. In particular, it is not clear what role is played by the conjunctiva and the nasolacrimal ducts, as well as the lymphoid tissues associated with them, in the immune privilege of the anterior chamber of the eye.
Egan et al. demonstrated in mice that potent immunologic tolerance can be achieved by exposure of antigen (ovalbumin) via the conjunctival mucosa. They identified the submandibular lymph node as the principal lymph node in which antigen-bearing antigen-presenting cells are located and in which antigen-specific T-cell clonal expansion occurs following conjunctival application of antigen. Clonal expansion was maintained at an elevated level and the T cells were responsive in vitro during a 10-day period of daily ovalbumin application to the conjunctiva. However, despite continuous antigen application, the number of antigen-specific T cells steadily declined over the 10-day period, and by day 14, the remaining ovalbumin-specific T cells were refractory to secondary challenge with ovalbumin, indicating that they had become anergic in vivo. Egan et al. concluded that the fact that antigen-presenting cells presenting ovalbumin were found only in the submandibular lymph node - and not in other lymph nodes, spleen, or nasal associated lymphoid tissue (NALT) -rules out the possibility that tolerance in this system was due to drainage of antigen through the efferent tear ducts and association with NALT or gastrointestinal-associated lymphoid tissue (GALT).
However, one important point is lacking in the suggestions of Egan et al. It has not yet been appreciated that antigens drained by the tear fluid itself, and not applied intraconjunctivally, would be able to induce immune deviation via CALT and/or TALT. With regard to protection of the cornea against inflammatory destruction, this would be plausible and analogous to the process in the nervous system and the anterior eye chamber. In comparison with gastrointestinal tract MALT (GALT), it is not known as yet whether M cells occur in human CALT and TALT, although they probably do, as their presence has been demonstrated in several animal species. M cells are highly specialized epithelial cells that facilitate uptake and transcytosis of macromolecules and microorganisms. Following transcytosis, antigens to cells of the immune system in lymphoid aggregates are released beneath the epithelium, where antigen processing and presentation and stimulation of specific B and T lymphocytes take place.
According to a definition formulated by Isaacson for MALT of the gut wall (i.e., Peyer's patches), MALT comprises four components organized MALT, a lamina propria, intraepithelial lymphocytes, and an associated lymph node. Circulation of the lymphoid cells in these four components enables them to home to their original and other mucosal sites, where they exert the effector function. Such a response may be dominated by slgA release and may include cytotoxic T-lymphocyte action . In this regard, the submandibular lymph node found by Egan et al. might be the 'associated lymph node' of CALT and TALT, but not of NALT.
Activation of T lymphocytes has been observed in dry eye, which leads to the frequent occurrence of abnormal (pathological) apoptosis in terminally differentiated, acinar epithelial cells of the lacrimal gland [53]. Tears secreted to the ocular surface will then contain proinflammatory cytokines and will inflame the tissues of the ocular surface. Abnormal apoptosis has also been detected in the epithelial cells and lymphocytes of the ocular surface [53]. This ocular surface inflammatory response consists of inflammatory cell infiltration, activation of the ocular surface epithelium with increased expression of adhesion molecules, inflammatory cytokines and pro-apoptotic factors, increased concentrations of inflammatory cytokines in the tear fluid and increased activity of matrix-degrading enzymes in the tear fluid. It has been suggested that the reduction of circulating androgens plays a role in these processes. Treatment with locally applied cyclosporin A eye drops interferes with inter-leukin metabolism, especially of interleukin-6, thus creating a new treatment option that leads to remarkable improvement of the irritation symptoms and ocular surface signs in particular in severe cases of keratoconjunctivitis sicca.
Taken together, these findings support the conclusion that CALT and TALT play a role in the pathogenesis of dry eye. One can imagine that misdirected stimulation of EALT could result in a misguided form of immune deviation at the ocular surface. Within the scope of this event, T cells would no longer be hindered in inducing autoimmunity by apoptosis, finally resulting in the clinical picture of dry eye.
It should be mentioned, however, that a recently published article has placed our understanding of MALT in a different light concerning its functional significance. Alpan et al. demonstrated that a systemic immune response to orally administered soluble antigens does not depend on the presence of functional GALT, but more likely on initiation of immune response by gut-conditioned dendritic cells. This finding suggests that MALT is not required for initiation of a primary immune response to antigens that have entered the body. If present, however, it seems to act in two ways: It produces plasma cell precursors that later migrate into adjacent mucosa, mature to plasma cells, and produce slgA for mucosal protection. It allows uptake of antigens by M cells and presentation of these antigens to virgin T and B cells to initiate a primary immune response. Thus, MALT could represent a second pathway (a kind of safeguard of the adaptive immune system) for initiation of a immune response to antigens that have been incorporated into the mucus layer and, in the case of CALT or TALT, have entered the ocular surface and are drained with tear fluid.
It can be concluded that development of EALT is a common feature frequently observed in symptomatically normal nasolacrimal ducts. Whether special types of bacteria, viruses, or other factors, e.g., immune deviation, are responsible for the development of EALT in humans requires future investigation in prospective and experimental studies.
