Immune mechanisms of dry-eye disease

Austin K Mircheff and Joel E Schechter

subjects, Waterhouse4 found at least slight adenitis in the lacrimal glands of between 8% and 22% of men in different age groups, with no indication of an age-associated increase. In contrast, the frequency of adenitis, which he interpreted as an autoimmune phenomenon, increased in women, from 22% in women younger than 44 years to 65% in women 75 and older. Whaley et al5 determined the frequency of decreased Schirmer test scores and increased rose Bengal staining in inpatients hospitalized for various indications but explicitly excluding autoimmune diseases. Because they found no correlation between the ocular surface findings and various autoantibodies, they concluded that dry-eye disease in their subjects was due to atrophic changes, rather than autoimmune phenomena. Subsequent postmortem and biopsy studies, discussed in detail in the section on pathology below, documented the frequency of age- associated fibrosis and parenchymal atrophy, and they generally also demonstrated associations with increased lymphocytic infiltration.

A development in oral pathology is of interest in this context. Daniels and Whitcher6 described histopathological features of labial salivary glands from patients who could and could not be diagnosed as having Sjögren’s syndrome on the basis of serum autoantibodies and clinical diagnoses of xerostomia and dry eye. Their conclusion might seem paradoxical to those who have been taught that the patho-physiology of Sjögren’s syndrome is autoimmune-mediated destruction of the secretory parenchyma: While parenchyma was replaced by lymphocyte aggregates, Sjögren’s syndrome cases were distinguished by an absence of acinar atrophy or ductal dilatation, even in parenchymal areas immediately adjacent to large aggregates.

EpidemiologyThis topic is reviewed in Chapter 14.

Genetics and risk factorsThere is a significant genetic influence on the incidence of Sjögren’s syndrome, since having a first-degree relative with an autoimmune disease increases the risk sevenfold.7 An association between human leukocyte antigen (HLA) DR

C H A P T E R 15

Clinical background

Key symptoms and signsThe symptoms of dry-eye disease are discussed in detail in Chapter 14. A point that bears emphasis is that patient presentation is extremely variable. Patients may experience troublesome symptoms but present none of the standard clinical signs and even have increased fluid production; may report symptoms and show ocular surface pathology but have normal fluid production; or may show decreased fluid production but deny symptoms.

Historical developmentPapers describing fibrotic and atrophic changes in the lac-rimal glands of aged subjects began appearing in the late nineteenth century. In 1903 Schirmer noted that fluid pro-duction varied widely among normal individuals but tended to decrease with increasing age and to decrease more severely in women. Later investigators1 substantiated Schirmer’s conclusions. In the 1930s, Sjögren introduced the terms “keratoconjunctivitis sicca” (KCS) and “sicca complex,” and reported findings from patients with the sicca complex and arthritis, including documentation of inflammatory changes in glands obtained from several of the affected individuals.

The concept of autoimmune diseases emerged in the sub-sequent decade, along with the discovery that patients’ sera frequently contained antibodies directed against certain tissues or intracellular structures. Bloch and Bunim2 showed that the sicca complex and glandular histopathology occurred in patients with other autoimmune diseases in addition to rheumatoid arthritis. Bloch et al3 reported that the sicca complex and autoantibody titers also occurred in patients with no sign of autoimmune disease affecting other tissues. The distinction between primary Sjögren’s syndrome and secondary Sjögren’s syndrome was established by the mid-1970s.

The nature of lacrimal gland atrophy and dysfunction outside the setting of Sjögren’s syndrome and other inflam-matory diseases has been somewhat controversial. Examin-ing lacrimal gland and salivary gland histology in autopsy

Pathology

115

Pathology

Lacrimal glandMost studies of the histopathology of the human lacrimal gland in normal aging have confirmed the earlier finding that aging is associated with increased fibrosis, ductal pathol-ogy, acinar atrophy, and infiltration of immune cells. Damato et al22 and Pepose et al23 described the presence of lymphoid aggregates or foci, and, occasionally secondary follicles, even in individuals without a history of autoim-mune disease. Roen et al24 and Obata25 noted the frequent occurrence of ductal dilatation, and Obata also documented the frequency of fatty infiltration. In an analysis of postmor-tem lacrimal glands, Obata et al26 found that diffuse fibrosis, diffuse acinar atrophy, and periductal fibrosis were more frequent in the orbital lobes of elderly women, and in the palpebral lobes of aging men. They also noted that the frequency of lymphoid foci increased with age and, as Wieczorek et al27 had found, that most foci were located near intralobular or interlobular ducts, i.e., those within, but at the periphery of, a lobule.

Nasu et al28 compared lacrimal glands from subjects with autoimmune diseases and with no history of autoimmune diseases. They concluded that the entire population shared common histopathological features and differed only by degree. Although the incidence of lymphoid foci was highest in patients with Sjögren’s syndrome and other autoimmune diseases, only 36% of lacrimal glands from subjects without autoimmune diseases appeared to be free of infiltrates. In the remainder, the incidence and severity of infiltration were highest among those older than 40 years and the incidence was nearly identical in males (63.9%) and females (62.8%).

The immunohistopathology of the lacrimal glands in patients with Sjögren’s syndrome presents some diversity. Pflugfelder et al29 found that, of 6 patients, lymphocytic infil-tration was diffuse in 4 and focal in 2. Tsubota et al30 com-pared the histopathological features of the lacrimal glands of subjects with Mikulicz’s disease and Sjögren’s syndrome, which share several features, including massive infiltration by essentially identical proportions of CD4+, CD8+, and CD21+ lymphocytes. Whereas fluid production is severely impaired in patients with Sjögren’s syndrome, patients with Mikulicz’s disease retain exocrine function, and their ocular surfaces appear normal.

Conjunctiva and corneaThe corneal and conjunctival epithelia undergo marked morphological changes in dry-eye disease. The number of goblet cells in the conjunctival epithelia decreases; cells in the superficial conjunctival epithelial layers flatten, such that the epithelium thins even as the number of strata increases; cells in the most superficial layer lose most of their microvilli and separate from their normally close attachment to the penultimate layer; hyaline bodies, suggested to represent the residua of defunct goblet cells, appear in the epithelium; and vacuoles and other inclusions appear within the cyto-plasm.31,32 The lamina propria underlying areas of affected epithelium becomes increasingly populated by lymphocytes and leukocytes.33 Subsequent studies have confirmed that

alleles and autoantibodies is recognized,8 but other reported genetic associations have been controversial.9 Having deliv-ered a baby doubles the risk for developing Sjögren’s syn-drome 2.1-fold.7

There appear to be no reports of genetic factors influenc-ing dry-eye disease not associated with autoimmune dis-eases. Other risk factors are discussed in Chapter 14.

That most dry-eye patients, and the large majority of patients with Sjögren’s syndrome, are women prompted studies of sex steroid actions in animal models (Box 15.1).

Androgens clearly influence immune cell activity in the lacrimal glands and the status of the ocular surface.10–13 Rocha et al14 proposed that androgens exert their influences by controlling expression of immunomodulatory mediators by parenchymal cells; this important concept is discussed at length in the section on pathophysiology, below. However, much remains to be learned about mechanisms underlying the androgens’ influences. Gonadectomizing or hypophysec-tomizing experimental animals causes significant biochemi-cal and functional changes15,16 but does not cause acinar atrophy or fibrosis on the scale of the changes that occur in the aging human lacrimal gland or normally aging rats17 and mice.18

Other hormones also influence lacrimal gland and ocular surface cytophysiology and immunophysiology. Mathers et al19 found that lacrimal fluid production correlates posi-tively with increasing serum testosterone only in premeno-pausal women. In contrast, in all groups studied, i.e., premenopausal, postmenopausal without hormone replace-ment therapy, and postmenopausal with hormone replace-ment therapy, all measures of lacrimal function correlate negatively with increasing levels of serum prolactin (PRL). Notably, all subjects in this study had PRL values within the normal range.

Findings on the actions of estrogens defy explanations based on a single hormone-regulated process. Women with premature ovarian failure present increased signs and symp-toms but produce fluid at normal rates.20 On the other hand, estrogen replacement therapy is associated with exacerbation of dry-eye symptoms in older women.21

Differential diagnosis, treatment, and prognosisThese topics are addressed in Chapter 14. The extent to which dry-eye disease occurs in association with local and systemic inflammatory diseases should be noted.

Box 15.1 Gender-related lacrimal gland dimorphisms

The rodent lacrimal gland is subject to gender-related dimorphisms. Androgens support10:

• Larger population of dimeric immunoglobulin A (dIgA)-producing plasmacytes

• Greater parenchymal expression of the polymeric immunoglobulin receptor (pIgR)

• Greater secretion of secretory IgA (sIgA) and secretory component (SC)

• Lower basal rate of fluid production

Section2 Dry eye chapter15 Immune mechanisms of dry-eye disease

116

The epithelia of the lacrimal glands, ocular surface, and lacrimal drainage system form a topological continuum with the mucosae of the respiratory system and the gastrointesti-nal system. The ocular surface system, like the respiratory and gastrointestinal systems, contains organized inductive sites for adaptive mucosal immunity, and it also performs both innate and adaptive mucosal immune effector func-tions. Even as the lacrimal epithelia perform the exocrine functions associated with production of the ocular surface fluid, they devote much of their cytophysiology to accom-plishing mucosal immune effector functions.

Cytophysiological apparatusLacrimal epithelial cells employ ion pumps, symporters, exchangers, and channels that are common to essentially all nucleated cells. They generate vectorial ion fluxes by using transport vesicles to insert specific ion transporters into the basal, lateral and apical domains of their plasma mem-branes. Figure 15.2 illustrates the disposition of the ion transporters as an apparatus for secreting Cl− ions and K+ ions through the cells and Na+ ions through the paracellular pathway. This apparatus is distinct from the apparatus that secretes proteins.

increased numbers of lymphocytes are present within the conjunctiva of patients with dry-eye disease. Epithelial cells expressing HLA DR (human major histocompatibility complex (MHC) class II) molecules were present in conjunc-tival impression cytology specimens from 50% of patients34 and in brush cytology specimens from 66% of patients35 with idiopathic dry-eye disease.

Etiology

Explicit concepts are emerging for the mechanisms by which environmental stresses, iatrogenic factors, allergy and infec-tion, and endocrine changes can initiate dry-eye disease. Before presenting these concepts, it is appropriate to review physiological principles and cytophysiological mechanisms that influence disease development.

Pathophysiology

Nexus between the visual system and the mucosal immune system

The normal ocular surface fluid provides a microenviron-ment for the living epithelial cells exposed in the interpalpe-bral regions of the cornea and conjunctiva. Figure 15.1 illustrates the general wiring scheme by which perception of irritation or dryness in the cornea or conjunctiva elicits pro-duction of lacrimal fluid.

Parasympathetic

Sympathetic

Lacrimalnucleus

Trigeminalganglion

Sensory

Figure 15.1 Wiring of a physiological servomechanism. A perception of irritation or dryness by sensory nerve endings in the cornea and conjunctiva elicits afferent signals, which travel through the trigeminal ganglion to reach a lacrimal center in the brainstem. (Note that sensory nerve endings are also present in the lacrimal gland.) Like sensory information from the viscera, signals from the ocular surface are processed and lead to the generation of efferent autonomic secretomotor signals, even when there is no conscious awareness that the status of the ocular surface has deviated from its homeostatic setpoint. The secretomotor signals reach the lacrimal glands by way of both sympathetic and parasympathetic nerves, which release their neurotransmitters in the general vicinity of, but do not form synapses with, parenchymal epithelial cells.

Na+

2Cl–

K+

H+

K+

HCO3–

CaCC

Na+

Na+

2K+

K+3Na+

ADP + Pi

H2O + ATP

Cl–

Cl–

NKCC

AE

NHE

NKA

Figure 15.2 The cytophysiological apparatus for exocrine secretion of Cl− ions, Na+ ions, and, in ductal cells, K+ ions. Secretion of the ions creates the osmotic driving force that causes water to move from the interstitial space to the lumen of the acinus−duct system. Recent studies indicate that the Na+/H+ exchanger (NHE) and the Cl−/HCO3

− exchanger (anion exchanger, AE) work in concert36 and in parallel with the Na+K+2Cl− symporters (NKCC)37,38 to drive Cl− ions from the interstitial fluid to the cytosol, against an unfavorable electrochemical potential difference. Secretagogue-mediated opening of apical Cl− channels allows Cl− ions to flow into the lumen. Flux of Na+ ions through the paracellular pathway dissipates the lumen-negative transepithelial voltage difference that results from the transcellular flux of Cl− ions. Apical K+ channels and K+-Cl− symporters are primarily found in ductal epithelial cells.38 (Not illustrated is the fact that the transporters spend 90% of their time in the intracellular compartments depicted in Figure 15.3. Secretagogue stimulation recruits more Na,K-ATPase (NKA) pump units to the basal lateral plasma membrane and activates NHE exchangers in the basal lateral membrane.)

Pathophysiology

117

dynamic apparatus. The regulated exocrine apparatus exocy-toses proteins, including signaling mediators, into the nascent ocular surface fluid. The transcytotic apparatus takes dimeric IgA (dIgA) up from the stromal fluid and exocytoses secretory IgA (sIgA) at the apical membrane; it also functions as a paracrine secretory apparatus, which exocytoses extracel-lular matrix components and signaling mediators into the stromal space (Box 15.2). A novel paracrine apparatus, dis-cussed below, is induced under certain physiological and pathophysiological conditions. The autophagic-lysosomal apparatus mediates the catabolic turnover of glycoproteins and phospholipids; it communicates with the transcytotic paracrine apparatus at the late endosome.

The lacrimal gland’s innate mucosal immune effector function is to secrete bacteriostatic and bactericidal effector molecules. These include lactoferrin, lactoperoxidase, and lytic enzymes, such as lysozyme and other glycohydrolases, proteases, and phospholipases. Its adaptive mucosal immune function is to deliver sIgA into the ocular surface fluid.

The transcytotic paracrine apparatus that delivers IgA to the ocular surface fluid is described in detail in Box 15.3. The authors39–41 have proposed that, because this apparatus communicates with the autophagic lysosomal apparatus at the late endosome, it also mediates the secretion of autoan-tigens to the underlying stromal space. Moreover, the unique strategy lacrimal epithelial cells use to regulate secretion of sIgA and secretory component acutely may give them a pro-pensity to generate and secrete autoantigen fragments that contain otherwise cryptic epitopes.

The task of delivering of sIgA to the ocular surface fluid requires that lacrimal parenchymal cells maintain the stromal space as a niche for dIgA-secreting plasmacytes. They do so by secreting paracrine mediators that induce plasma-blasts to undergo terminal differentiation and additional mediators that support the survival of mature plasmacytes. While both acinar and ductal epithelial cells secrete sIgA and

The organelles that lacrimal epithelial cells use to secrete proteins are also common to most cell types. However, by using transport vesicles to transfer products between specific organelles, they organize the organelles into apparatus to perform an exocrine function. Figure 15.3 illustrates this

Constitutivetranscytotic – paracrine

apparatus

Inducedparacrineapparatus

Golgi

ER

Recyclingendosome

Earlyendosome

Secretoryvesicles

Constitutive traffic Maturation CCh-Dependent exocytosis

TGN

Regulatedexocrine

apparatus

AutophagosomePre-lysosome

Lateendosome

Stromalside

Tearside

Storagelysosome

Autolysosome

Autophagic – lysosomalapparatus

Figure 15.3 Cytophysiological apparatus for exocrine secretion of glycoproteins, transcytotic secretion of secretory component and secretory IgA, paracrine secretion of signaling mediators, and catabolism of cellular proteins, lipids, and carbohydrates. The apparatus are essentially the same in acinar cells and duct cells, but the exocrine and paracrine secretory products differ.

The glycoproteins that each epithelial cell secretes and the proteins it uses in its ion-transporting apparatus are assembled in the common biosynthetic apparatus, which consists of the endoplasmic reticulum (ER) and the Golgi complex. The products then traffic to the trans-Golgi network (TGN), which is the most complex of the cells’ several sorting nexuses. The TGN sorts specific products into distinct microdomains, and in these microdomains the transport vesicles form that will traffic to the other organelles.

The immature secretory vesicle is the entry compartment of the classic exocrine secretory apparatus for glycoproteins. The late endosome and the isolation membrane are the entry compartments of the autophagic-lysosomal apparatus. The recycling endosome and early endosome comprise both a transcytotic secretory apparatus and, simultaneously, a paracrine secretory apparatus. Under certain circumstances, both physiological and pathophysiological, the cell is induced to express a second paracrine secretory apparatus.

Autoantigens that are present in the cytosol, as well as autoantigens that are embedded in the lipid bilayers of membrane-bound organelles, enter the luminal spaces of the membrane traffic apparatus through the process of autophagy and through the formation of multivesicular bodies. Both processes are of interest. Formation of multivesicular bodies in lacrimal gland epithelial cells has not been studied, but it is a fundamental cytophysiological process with potentially important implications for autoantigen exposure. Like proteins that are released to the fluid phase, the proteins in microvesicles that may be released from the cell can be taken up, processed, and presented by antigen-presenting cells.

Box 15.2 Acute and long-term regulation of lacrimal epithelial cells

Secretagogues acutely activate:

• The ion- and water-secreting apparatus• The exocrine apparatus for secreting proteins• Transcytotic apparatus for secreting secretory component and

secretory immunoglobulin A

Hormones, cytokines, and inflammatory mediators likely influence:

• Expression of specific components of each apparatus• Expression of secretagogue receptors• Expression of intracellular signaling cascades activated by

secretagogue receptors

Box 15.3 Secretory immunoglobulin A (sIgA)

• Immobilizes microbes, preventing penetration of the epithelial barrier

• Does not fix complement• Reduces the risk that inflammatory responses will be needed

to fight infections

Section2 Dry eye chapter15 Immune mechanisms of dry-eye disease

118

nular spaces to periacinar spaces. Evidence now suggests that the counterpoise between TGF-β and PRL may be a key factor determining the lacrimal gland’s immunophysiological status.

Constitutive exposure of autoantigensExperiments with several animal models have shown that activated, autoantigen-specific effector T cells can adoptively transfer disease to the lacrimal glands.47–50 This finding indi-cates that potentially pathogenic autoantigen epitopes must be displayed by MHC class II on the surfaces of resident antigen-presenting cells.

Most literature on the subject posits that intracellular autoantigens become available for processing and MHC class II-mediated presentation as the result of apoptosis or necrosis. However, the authors and their colleagues have proposed that live, actively functioning lacrimal epithelial cells constantly secrete autoantigens by way of their transcytotic-paracrine apparatus. As illustrated in Figure 15.3, autoantigens that are present in the cytosol, as well as autoantigens that are embedded in the lipid bilayers of membrane-bound organelles, enter the luminal spaces of the membrane traffic apparatus through the process of autophagy and through the formation of multivesicular bodies, and they may be exocytotically secreted from the early endosome.

As detailed in Box 15.5, the traffic of transport vesicles to and from the basal-lateral plasma membrane is extensive

secretory component, they do not express the same spectra of paracrine mediators. More is known about mediators pro-duced by duct cells, while more is known about cytophysi-ological mechanisms of acinar cells.

In mucosal immune effector sites transforming growth factor-β (TGF-β) conveys the signal for plasmablast differen-tiation. In the rabbit lacrimal gland, TGF-β expression is largely localized to interlobular duct epithelial cells. Nor-mally, duct cells apportion TGF-β into both their exocrine secretory apparatus and their transcytotic paracrine appara-tus. TGF-β is synthesized in a proform; proteolytically proc-essed, mature TGF-β typically remains in a noncovalent complex with its latency-associated peptide. Latent TGF-β entering the stroma is thought to associate with the extracel-lular matrix proteoglycan, decorin, which appears to be produced by both ductal and acinar epithelial cells. Thus, decorin may create a diffusion barrier that keeps latent TGF-β concentrated in the spaces surrounding the interlobu-lar ducts, where it would signal to plasmablasts entering the gland from venules that parallel the ducts.

TGF-β also conveys additional signals: proliferative signals to mesenchymal cells and some epithelia, and antiprolifera-tive or proapoptotic signals to other epithelia and lym-phocytes. The antiproliferative signals contribute to its immunosuppressive actions. Moreover, TGF-β can induce expression of its own mRNA in T lymphocytes and antigen-presenting cells. It appears that mucosal immune effector tissues are able to use TGF-β to induce plasmablast differen-tiation because they also secrete paracrine mediators that abrogate TGF-β’s proapoptotic signals. In the lamina propria of the small intestine, interleukin-6 (IL-6) appears to be a major plasmacyte survival factor. In the lactating mammary gland, plasmacyte survival is supported by PRL. Schechter et al42 demonstrated that PRL, like TGF-β, is concentrated in epithelial cells of the interlobular ducts in the rabbit lacrimal gland. Retention of TGF-β in the stromal space surrounding the interlobular ducts may minimize its deleterious actions. In contrast, PRL is thought free to diffuse from the periductal regions to the periacinar regions, where it would promote survival of plasmacytes and acinar cells.

Azzarolo and coworkers43 found that plasmacytes in the rabbit lacrimal gland undergo apoptosis within hours after animals have been ovariectomized. Apoptosis is prevented if either dihydrotestosterone or estradiol is administered prior to ovariectomy. These findings highlight the impor-tance of survival signals for plasmacytes, and they suggest that the sex steroids either support expression of PRL or interact with PRL to generate the survival signal.

Studies of lacrimal glands of pregnant rabbits demon-strate that the influences of epithelium-derived paracrine mediator extend beyond plasmablast differentiation and plasmacyte survival.42,44

As described in detail in Box 15.4, expression of both TGF-β and PRL increases during pregnancy. The increase of TGF-β appears to be driven by estradiol (E2) and progester-one (PRG), while the increase for PRL is driven, at least in part, by pituitary PRL. PRL induces expression of the novel paracrine apparatus, amplifying delivery of both mediators to the stroma and decreasing delivery of both to the ocular surface fluid. These changes are associated with significant changes in lacrimal fluid production and with a redistribu-tion of lymphocytes and plasmacytes from periductal perive-

Box 15.4 Functional, cytophysiological, and immunophysiological transformations established late in pregnancy

Functionalchanges

• Resting fluid production decreased44

• Pilocarpine-induced fluid production increased44

• Concentration of protein in pilocarpine-induced fluid decreased44

• Prolactin (PRL) and transforming growth factor-β (TGF-β) contents of lacrimal gland fluid decreased44

• Positive rose Bengal staining occurs more frequently44

cytophysiologicalchanges

• PRL and TGF-β contents in parenchymal cells of interlobular ducts and acini increase

• PRL and TGF-β redistribute from primary localizations in apical cytoplasm to dual localizations in apical and basal cytoplasm42

immunophysiologicalchanges

• Lymphocytes and plasmacytes disperse from aggregates in stromal space surrounding interlobular ducts and become concentrated in periacinar spaces42,44

currentunderstandingofendocrinesignals

• Estradiol and progesterone increase expression of TGF-β• Elevated systemic PRL levels increase expression of PRL• PRL induces expression of novel paracrine secretory

apparatus,45 which captures both newly synthesized PRL and PRL endocytosed from the ambient medium for secretion to the stromal space46

Pathophysiology

119

played on the surfaces of parenchymal cells. Nevertheless, the robustness of MHC class II expression by epithelial cells in infiltrated lacrimal glands is consistent with the notion that the lymphocytic infiltration in the aging human lac-rimal gland is driven, at least in part, by parenchymal cells functioning as surrogate antigen-presenting cells.

Maintaining homeostatic statesAlthough lacrimal epithelial cells secrete autoantigens and can be induced to secrete previously cryptic epitopes and to function as surrogate antigen-presenting cells, none of these events in isolation is likely to be the trigger for autoimmune disease.53 Recent experiments, described in Box 15.8, suggest that the TGF-β secreted by ductal epithelial cells enforces tolerance to the lacrimal autoantigens.

and dynamic, as is traffic to the late endosome. As illustrated in Figure 15.4, chronic stimulation may both increase the secretion of autoantigens to the stroma and cause the secre-tion of autoantigen fragments that contain previously cryptic epitopes.

Induced capacity to present autoantigen epitopes directly to T cellsThe transcytotic paracrine apparatus appears also to allow acinar cells to present autoantigen epitopes directly to CD4+ T cells. Studies with thyroid epithelial cells led Bottazzo and coworkers51 to suggest that parenchymal cells that have been induced to express MHC class II may be able to present autoantigen epitopes directly to CD4+ T cells. Epithelial cells from rat and rabbit lacrimal gland spontaneously express MHC class II when they are isolated and placed in primary culture. As noted above, conjunctival epithelial cells from patients with dry-eye disease express MHC class II. Large proportions of epithelial cells in the majority of cadaver donor lacrimal glands studied by Mircheff et al52 expressed MHC class II. Moreover, the number of positive cells gener-ally correlated with the number of lymphocytes present. As explained in Box 15.7, it appears that acinar cells isolated from rabbit lacrimal gland function as surrogate antigen-presenting cells and stimulate the proliferation of patho-genic CD4+ effector T cells in ex vivo models.

The focal organization of lymphocytes in Sjögren’s syn-drome, and the fact the B cells, CD4+ T cells, and dendritic cells are present in the foci would seem to imply that B-cell activation and autoantibody production are driven by autoantigens and immune complexes that are released into the stromal space, rather than by autoantigen epitopes dis-

Box 15.5 Chronic cholinergic stimulation may lead to the appearance of otherwise cryptic autoantigen epitopes

Consequences of chronic exposure to cholinergic agonists, presumed to model chronic exposure to agonistic autoantibodies to the M3AChR:

• Decreased abundance of Gq and G11, the heterotrimeric GTP-binding proteins that couple to the M3AChR, attenuates M3AChR signaling, preventing elevation of cytosolic Ca2+48

• Activation of apparatus for exocrine protein secretion blocked• Activation of apparatus for secreting Cl− ions, Na+ ions, and

H2O blocked49

• Traffic of vesicles recycling to and from the basal lateral plasma membrane decreases to basal rates

• Traffic of vesicles from early endosome to late endosome blocked

• Lysosomal proteases, prevented from taking normal pathway through late endosome to storage lysosome or pre-lysosome, accumulate within endomembrane compartments that remain accessible, including early endosome, recycling endosome, trans-Golgi network, and secretory vesicles50

• Traffic of identified autoantigens is also altered. Acinar cells release the autoantigens to their ambient medium, both as soluble proteins and as constituents of structures with microsomal sedimentation properties, i.e., structures that might be microvesicles

Early endosome

Intactautoantigen

Prematurelyprocessed

autoantigen

Figure 15.4 Lacrimal epithelial cells are thought to secrete autoantigens by way of transport vesicles that bud from the early endosome. When stimulation (Boxes 15.5 and 15.6) causes catalytically active lysosomal proteases to accumulate in the endosomes, they may hydrolyze peptide bonds in the exposed sequences that normally contain dominant T-cell epitopes. Subdominant, i.e., previously cryptic, epitopes may survive and associate with the peptide binding grooves on antigen-presenting cell major histocompatibility complex class II.

Section2 Dry eye chapter15 Immune mechanisms of dry-eye disease

120

cells42,44 and appears to induce both ductal and acinar cells to express the novel paracrine secretory apparatus.45,46 In contrast, it appears that the increase of TGF-β expression in the ducts is mediated by E2, PRG, or E2 and PRG acting in concert, independently of PRL. The role PRL is thought to play maintaining plasmacyte survival by abrogating proap-optotic signals from TGF-β was discussed above. However, PRL is a proliferation factor for T lymphocytes55; it induces T cells to express interferon-γ (IFN-γ); and it alters the phe-notypic expression of dendritic cells to favor TH1 polariza-tion.56 The authors tested the hypothesis that excessive levels of PRL within the lacrimal gland abrogate regulatory signals and initiate autoimmune activation by using an adenovirus vector to transduce cDNA for rabbit PRL into the lacrimal glands of ovary-intact, nonpregnant female rabbits.57 Tran-sient overexpression of PRL led to an acute increase in lym-phocytic infiltration, which then evolved into a chronic, low-grade infiltration. In different settings, the infiltrates were associated with a predominantly TH1 cytokine profile; a mixed TH1 and TH2 profile; and a profile characterized by increases in the expression of tumor necrosis factor-α (TNF-α), TGF-β, IL-1β, and IL-6, but not the TH1 cytokine, IFN-γ,

Figure 15.5 summarizes the theory that TGF-β mediates the generation of regulatory antigen-presenting cells, which in turn mediate the generation of regulatory CD4+ T cells. That regulatory T cells contribute to maintaining immuno-homeostasis is indicated by observations that it is necessary to deplete regulatory T cells before adoptive transfer of acti-vated effector T cells is able to induce disease in the models described by Niederkorn et al48 and Jiang et al50 and by the recent report that autoimmune disease arises spontaneously in CD25 knockout mice.54

Paracrine mediation of endocrine triggersAs described in detail in Box 15.4, during pregnancy increased systemic PRL, perhaps interacting with increased E2 and PRG, increases expression of PRL in lacrimal duct epithelial

Box 15.6 Transcytotic secretion of sIgA and paracrine secretion of autoantigens

A novel mechanism for acutely regulating transcytotic secretory immunoglobulin A secretion has implications for the autoantigens lacrimal epithelial cells expose. The transcytotic-paracrine apparatus is illustrated in Figure 15.3

• Upon reaching the trans-Golgi network (TGN), newly synthesized pIgR enter microdomains, forming transport vesicles that will be targeted to traffic to the exocrine secretory apparatus, the paracrine secretory apparatus, and perhaps also the autophagic lysosomal apparatus

• In immature secretory vesicles, secretory component (SC) portion is proteolytically severed from the membrane-spanning domain and released into the fluid phase. Immature secretory vesicles mature through return of excess membrane to the TGN. Secretagogues induce mature secretory vesicles to fuse with the apical plasma membrane, exocytosing SC along with lactoferrin, lysozyme, lactoperoxidase, and other products

• pIgR trafficking to the early endosome may return to the TGN, presumably by way of the recycling endosome; may traffic to the basal lateral plasma membrane; or enter tubular extensions that separate from the early endosome and form multivesicular bodies by invaginating small vesicles from their limiting membranes

• Multivesicular bodies traffic to prelysosomes, which fuse with storage lysosomes to gain a full complement of proteases, glycohydrolases, and phospholipases that will hydrolyze pIgR

• pIgR trafficking to the recycling endosome may recycle to the TGN; recycle to the early endosome; or undergo proteolytic processing to release SC. Transport vesicles that bud from the recycling endosome and traffic to the apical plasma membrane fuse with it to exocytose SC into the lumen of the acinus duct system

• Plasmacytes populating the stromal spaces secrete dimeric IgA. Binding of dIgA to pIgR at the basal lateral membrane is thought to direct traffic of the complex away from the late endosome and toward the recycling endosome, so that sIgA is exocytosed at the apical plasma membrane

• Acute stimulation with CCh curtails traffic to the late endosome and prelysosome, increasing pIgR traffic to the recycling endosome, and therefore increasing secretion of SC. Chronic exposure to M3AChR agonists persistently blocks traffic to the degradative compartments, but also suppresses expression of pIgR

Box 15.7 A rabbit model of autoimmune lacrimal gland disease induced by autoadoptive, ex vivo to in vivo, transfer

• Acinar cells are isolated from the inferior lobe of one lacrimal gland, obtained through a transconjunctival incision

• Isolated acinar cells express major histocompatibility complex (MHC) class II, which circulates through the network of endomembrane compartments, where it binds autoantigen epitopes, and traffic to the plasma membrane. This appears to allow isolated acinar cells to function as surrogate autoantigen-presenting cells

• Lymphocytes from peripheral blood proliferate in co-cultures with acinar cells from the same donor animal (autologous mixed-cell reaction). Proliferating cells transfer disease to the donor animal’s remaining lacrimal gland, the corneas, and conjunctivae, whether injected directly into the remaining lacrimal gland or injected subcutaneously46

Box 15.8 An ex vivo rat model of immunoregulation

Acinar cells from rat lacrimal glands express major histocompatibility complex (MHC) class II when placed in primary culture. However, rather than becoming immunoactivating, they become immunoregulatory and immunosuppressive. They:

• Inhibit, rather than stimulate, proliferation of lymphocytes in autologous mixed-cell reactions

• Secrete soluble mediators that inhibit the proliferation of lymphocytes in mixed-cell reactions containing acinar cells and lymphocytes from rabbits

• Secrete soluble mediators that inhibit mixed-cell reactions containing ex vivo matured dendritic cells and lymphocytes from rats

• Secrete soluble mediators, including transforming growth factor-β, that control phenotypic expression of dendritic cells maturing ex vivo, permitting expression of surface MHC class II but suppressing expression of CD8653

Pathophysiology

121

Environmental triggersObservations that adoptively transferring activated effector T cells induces autoimmune lacrimal gland pathophysiology, cited above, suggest that activated effector T cells must be able to abrogate the immunosuppressive actions of both epithelial TGF-β and the resident population of regulatory T cells to proliferate and determine the phenotypic expression of the new generation of maturing antigen-presenting cells. Thus, the phenomenon of adoptive transfer offers a plausible expla-nation for the initiation of lacrimal gland disease in second-ary Sjögren’s syndrome and other inflammatory diseases.

Dursun et al58 and Barabino et al59 found that exposing mice to increased flow of dehumidified air causes dry-eye disease. Dursun and coworkers also treated the mice with scopolamine, with the goal of paralyzing lacrimal gland exo-crine function and thereby exacerbating the desiccation of the ocular surface. In this model, the lacrimal glands become infiltrated with lymphocytes.48 Adoptive transfer provides a possible link between environmental stresses, as well as allergy and infection, at the ocular surface and the initiation of autoimmune disease. The cytophysiology of conjunctival epithelial cells has not been elucidated to the same level of detail as in the lacrimal gland. However, like the lacrimal glands, the conjunctivae in humans and rabbits support resi-dent populations of IgA+ plasmacytes. The apicalmost epithe-lial cell layers express pIgR and use a transcytotic apparatus to deliver sIgA into the ocular surface fluid. Therefore, it is reasonable to infer that, like lacrimal epithelial cells, conjunc-tival epithelial cells also secrete autoantigens to their stroma. As Stern and coworkers60 proposed, factors which decrease lacrimal fluid production, such as age-related atrophy, drug side-effects, and iatrogenic sensory denervation, may lead to desiccation, causing corneal and conjunctival epithelial cells to release inflammatory mediators, and perhaps also causing sensory nerve endings to release inflammatory transmitters and neuropeptides. Mediators that increase in the mouse model of desiccating stress include IL-1, TNF-α, IL-6, IL-2, IL-12, and IFN-γ.61 The likely scenario is that the inflamma-tory mediators abrogate the normal influences of TGF-β and the resident population of regulatory T cells, causing the new generation of antigen-presenting cells to mature with immu-noactivating phenotypes that stimulate epitope-specific effector T cells in the lymph nodes. In such cases, dry-eye disease is not the passive consequence of failed lacrimal exo-crine function, but, rather, the manifestation of pathophysi-ological processes that evolve in tandem in the lacrimal gland and the ocular surface tissues.

Chronic disease processesA number of mechanisms contribute to positive-feedback loops that expand and maintain immunopathophysiologi-cal processes. Desiccation at the ocular surface generates efferent secretomotor signals to the lacrimal gland. As described above, chronic stimulation decreases lacrimal fluid production and may also cause lacrimal epithelial cells to secrete autoantigen fragments containing previously cryptic epitopes. Most patients with Sjögren’s syndrome produce autoantibodies against M3AChR. Whether these are inhibitory or agonistic, they likely decrease fluid production and exacerbate ocular surface desiccation.

or the TH2 cytokines, IL-4 and IL-10. These findings suggest that several different immunopathophysiological processes might arise in response to systemic endocrine changes or local processes that increase expression of PRL without coor-dinately increasing expression of TGF-β. E2 and PRG levels decline abruptly at parturition, whereas the systemic PRL level remains elevated during lactation and declines rather gradually during a nonlactating puerperium. An appealing hypothesis, which has not yet been tested, is that the interval between parturition and the return to normal PRL levels and normal cycling is a period of increased risk for autoimmune activation. The analogous hypothesis is that the loss of steroid hormone support for ductal TGF-β expression during normal aging should be investigated, but the immunopatho-physiological processes of aging are likely to be considerably more complicated.

MediatorsMediators

Autoantigens

IgA+

plasmacyte

IgA+

Plasmablast

IgM+

B CellBone marrow

Lymph nodeCD4+

T Cell

CALT

ImmatureAPC

MatureAPC

Figure 15.5 Paracrine mediators secreted by interlobular duct epithelial cells regulate plasmacytes, dendritic cells, and T cells entering the lacrimal gland. This theory posits that, as immature antigen-presenting cells (APC) enter the periductal stromal space of the lacrimal gland, they are induced by transforming growth factor-β (TGF-β) to become regulatory APC, which will constitutively express TGF-β. They take up autoantigens, then mature and traffic to the draining lymph nodes, where they will present epitopes to CD4+ T cells, inducing them to differentiate as regulatory cells. Induced regulatory cells that subsequently enter the lacrimal gland reinforce the TGF-β-mediated signal, inducing the next generation of immature APC to mature as regulatory APC. In the lymph nodes as well as the lacrimal glands, the induced regulatory cells compete with activated, epitope-specific effector T cells for MHC class II-mediated antigenic stimulation. CALT, conjunctiva-associated lymphoid tissue.

Section2 Dry eye chapter15 Immune mechanisms of dry-eye disease

122

glands, immunopathophysiological processes in the former are not always accompanied by failures of exocrine function in the latter. Two principles seem likely to account for complex clinical presentation of dry-eye disease.

First, immunopathophysiological processes in the lac-rimal glands and conjunctivae are fundamentally local, maintained by interactions between parenchymal cells and immune cells, and they can be diverse, with diverse spectra of cytokines and inflammatory mediators. It may be that in some cases the ocular surface tissues are diseased but lac-rimal glands are not, or that the lacrimal gland’s host immu-nopathophysiological processes do not impair their exocrine functions. This point is illustrated by the persistence of lac-rimal fluid production in patients with Mikulicz’s disease, patients with premature ovarian failure, and some patients with ocular surface abnormalities that destabilize the fluid film.

Second, lacrimal epithelial cells use fundamentally differ-ent cytophysiological apparatus for secreting proteins and for producing fluid. The changes that occur during preg-nancy indicate that the capacity to produce fluid can be unaffected, or even enhanced, at the same time that basal fluid production decreases and the apparatus for exocrine proteins secretion is largely replaced by a paracrine secretory apparatus. One wonders whether some of the aging lacrimal glands with atrophic acini but intact ducts might still be producing fluid.

Rolando et al62 found that water evaporates from the ocular surface fluid more rapidly in patients with dry-eye disease, and Mathers63 found that meibomian gland dys-function is associated with increased evaporation. Thus, mei-bomian gland impairment, whether it is the consequence or the initial trigger of an inflammatory process,64 contributes to a positive-feedback loop.

Zoukhri and coworkers65 have shown that IL-1β acts on parasympathetic nerve endings in the lacrimal gland to block the release of acetylcholine and that both IL-1 and TNF-α impair acinar epithelial cells’ ability to activate their exocrine secretory apparatus in response to acute stimulation with CCh. Other studies indicate that acinar cells undergo signifi-cant cytophysiological changes when they are chronically stimulated with histamine or serotonin, the receptors for which appear to couple to heterotrimeric GTP-binding pro-teins other than Gq and G11.66 Elevated concentrations of both decrease cells’ ability to activate the Cl− secretory apparatus in response to optimal doses of CCh.40

Preliminary experiments indicate that certain mediators induce ex vivo acinar cells to express inflammatory cytokines, suggesting that local feedback loops between parenchymal cells and immune cells may influence the nature of the immunopathophysiological process.

Summary

Despite the multiplicity of signaling and immune cell traffic loops that connect the ocular surface tissues and lacrimal

Key references

A complete list of chapter references is available online at www.expertconsult.com. See inside cover for registration details.

3. Bloch KJ, Buchanan WW, Wohl MJ, et al. Sjögren’s syndrome. A clinical, pathological, and serological study of sixty-two cases. Medicine 1965;44:187–231.

10. Sullivan DA, Hann LE. Hormonal influence on the secretory immune system of the eye: endocrine impact on the lacrimal gland accumulation and secretion of IgA and IgA. J Steroid Biochem 1989;314:253–262.

11. Sullivan DA, Edwards JA. Androgen stimulation of lacrimal function in mouse models of Sjögren’s syndrome. J Steroid Biochem Mol Biol 1997;60:237–245.

19. Mathers WD, Stovall D, Lane ZA, et al. Menopause and tear function: the influence of prolactin and sex hormones on human tear production. Cornea 1998;17:353–358.

20. Schaumberg DA, Buring JE, Sullivan DA, et al. Hormone replacement therapy and dry eye. JAMA 2001;286:2114–2119.

26. Obata H, Yamamoto S, Horiuchi H, et al. Histopathologic study of human lacrimal gland. Ophthalmology 1995;102:678–686.

44. Ding C, Chang N, Fong Y-C, et al. Interacting influences of pregnancy and corneal injury on rabbit lacrimal gland immunoarchitecture and function. Invest Ophthalmol Vis Sci 2006;47:1368– 1375.

45. Wang Y, Chiu CT, Nakamura T, et al. Elevated prolactin redirects secretory vesicle traffic in rabbit lacrimal acinar cells. Am J Physiol Endocrinol Metab 2007;292:E1122–E1134.

48. Niederkorn JY, Stern ME, Pflugfelder SC, et al. Desiccating stress induces T cell-mediated Sjögren’s syndrome-like lacrimal keratoconjunctivitis. J Immunol 2006;176:3950–3957.

49. Thomas PB, Zhu Z, Selvam S, et al. Autoimmune dacryoadenitis and keratoconjunctivitis induced in rabbits by subcutaneous injection of autologous

lymphocytes activated ex vivo against lacrimal antigens. J Autoimmun 2008; 31:116–122.

53. de Saint Jean M, Nakamura T, Wang Y, et al. Suppression of lymphocyte proliferation and regulation of dendritic cell phenotype by soluble mediators from rat lacrimal epithelial cells. Scand J Immunol 2009;70:53–62.

60. Stern ME, Beuerman RW, Fox RI, et al. The pathology of dry eye: the interaction between the ocular surface and lacrimal glands. Cornea 1998;17:584–589.

65. Zoukhri D, Macari E, Choi SH, et al. c-Jun NH2-terminal kinase mediates interleukin-1β-induced inhibition of lacrimal gland secretion. J Neurochem 2006;96:126–135.

66. McDonald ML, Wang Y, Selvam S, et al. Cytopathology and exocrine dysfunction induced in ex vivo rabbit lacrimal gland acinar cell models by chronic exposure to histamine or serotonin. Invest Ophthalmol Vis Sci 2009;50:3164–3175.