Mechanisms Involved in Injury and Repair of the Murine lacrimal Gland: Role of Programmed Cell Death and Mesenchymal Stem Cells

The non-keratinized epithelia of the ocular surface are constantly challenged by environmental insults, such as smoke, dust, and airborne pathogens. Tears are the sole physical protective barrier for the ocular surface. Production of tears in inadequate quantity or of inadequate quality results in constant irritation of the ocular surface, leading to dry eye disease, also referred to as keratoconjunctivitis sicca (KCS). Inflammation of the lacrimal gland, such as occurs in Sjogren syndrome, sarcoidosis, chronic graft-versus-host disease, and other pathological conditions, results in inadequate secretion of the aqueous layer of the tear film and is a leading cause of dry eye disease. The hallmarks of lacrimal gland inflammation are the presence of immune cell infiltrates, loss of acinar epithelial cells (the secreting cells), and increased production of proinflammatory cytokines. To date, the mechanisms leading to acinar cell loss and the associated decline in lacrimal gland secretion are still poorly understood. It is also not understood why the remaining lacrimal gland cells are unable to proliferate in order to regenerate a functioning lacrimal gland. This article reviews recent advances in exocrine tissue injury and repair, with emphasis on the roles of programmed cell death and stem/progenitor cells.     

Neural regulation of lacrimal gland secretory processes: Relevance in dry eye diseases

The lacrimal gland is the major contributor to the aqueous layer of the tear film which consists of water, electrolytes and proteins. The amount and composition of this layer is critical for the health, maintenance, and protection of the cells of the cornea and conjunctiva (the ocular surface). Small changes in the concentration of tear electrolytes have been correlated with dry eye syndrome. While the mechanisms of secretion of water, electrolytes and proteins from the lacrimal gland differ, all three are under tight neural control. This allows for a rapid response to meet the needs of the cells of the ocular surface in response to environmental conditions. The neural response consists of the activation of the afferent sensory nerves in the cornea and conjunctiva to stimulate efferent parasympathetic and sympathetic nerves that innervate the lacrimal gland. Neurotransmitters are released from the stimulated parasympathetic and sympathetic nerves that cause secretion of water, electrolytes, and proteins from the lacrimal gland and onto the ocular surface. This review focuses on the neural regulation of lacrimal gland secretion under normal and dry eye conditions.     

Dysfunctional neural regulation of lacrimal gland secretion and its role in the pathogenesis of dry eye syndromes

Tears are a complex fluid consisting of three layers, each of which is secreted by a different set of tissues or glands. The aqueous portion of the tear film is produced predominantly by the lacrimal gland. Dry eye syndromes are diseases in which the amount and composition of tears are altered, which can lead to ocular surface damage. There are many causes for dry eye syndromes. One such cause is the alteration in the functions of nerves innervating the lacrimal gland and the ocular surface. The autoimmune disease Sjogren syndrome can deleteriously affect the innervation of the lacrimal gland. Damage to the sensory nerves in the ocular surface, specifically the cornea, as a result of refractive surgery and normal aging, prevents the normal reflex arc to the lacrimal gland. Both defects can result in decreased tear secretion and dry eye syndromes. This review will discuss the current information regarding neurally-stimulated protein, water, and electrolyte secretion from the lacrimal gland and delineate how nerve dysfunction resulting from a variety of causes decreases secretion from this gland.     

Interaction of EGF family growth factors and neurotransmitters in regulating lacrimal gland secretion

The lacrimal gland is the primary source for the aqueous portion of the tear film. This portion contains water, electrolytes and proteins, which are necessary for the health and maintenance of the cells of the ocular surface. Afferent sensory nerves in the cornea and conjunctiva stimulate efferent parasympathetic and sympathetic nerves in the lacrimal gland. Cholinergic agonists, released from parasympathetic nevres, and norepinephrine, released from sympathetic nerves, are major stimuli of lacrimal gland secretion. These neurotransmitters activate distinct, but overlapping signal transduction pathways leading to lacrimal gland secretion. Other stimuli of lacrimal gland secretion are the EGF family of growth factors. In addition to stimulation of secretion, these growth factors can interact with the cells of the lacrimal gland themselves or with the cells of the ocular surface depending upon the location from which these growth factors are released. This review will focus on the effects of the EGF family of growth factors on the lacrimal gland and their interactions with the pathways stimulated by the neurotransmitters released from nerves.     

The role of the lacrimal functional unit in the pathophysiology of dry eye

The majority of dry eye symptoms are due to a chronic inflammation of the lacrimal functional unit resulting in a loss of tear film integrity and normal function. This leads to a reduction in the ability of the ocular surface to respond to environmental challenges. The underlying cause of tear film dysfunction is the alteration of tear aqueous, mucin, and lipid components. This may result from a systemic autoimmune disease or a local autoimmune event. A lack of systemic androgen support to the lacrimal gland has been shown to be a facilitative factor in the initiation of this type of pathophysiology. Tear secretion is controlled by the lacrimal functional unit consisting of the ocular surface (cornea, conjunctiva, accessory lacrimal glands, and meibomian glands), the main lacrimal gland and the interconnecting innervation. If any portion of this functional unit is compromised, lacrimal gland support to the ocular surface is impeded. Factors such as neurogenic inflammation and T cell involvement in the disease pathogenesis as well as newly developed animal models of ocular surface inflammation are discussed.     

Tear dynamics and dry eye

Tears undergo four processes: production by the lacrimal gland, distribution by blinking, evaporation from the ocular surface and drainage through the nasolacrimal duct. Abnormalities in any of these steps can cause dry eye. There are two kinds of tear production, basic and reflex, which can be distinguished from each other by the Schirmer test with nasal stimulation. Reflex tearing is important because it supplies such essential components as EGF and vitamin A, whose deficiency may cause squamous metaplasia. There is no reflex tearing in Sjogren's syndrome because of destruction of the lacrimal gland. In cases of diminished or absent reflex tearing, topical autologous serum is the treatment of choice. Even when there is adequate tear production, insufficient distribution, such as occurs with the decreased blinking associated with the use of video display terminals (VDT), may cause dry eye. Any process or activity that suppresses blinking interferes with tear distribution. Tear evaporation increases under certain conditions and in some diseases. When the exposed ocular surface area is increased, such as in VDT work, tear evaporation increases. Meibomian gland dysfunction (MGD) also causes increased tear evaporation by altering the quality of the oily layer in tears. Tear evaporation can be suppressed by using a warm compresser or a humidifier, narrowing the palpebral fissure, or wearing protective eyeglasses. The tear clearance rate is measured by fluorescein dye dilution in the conjunctiva. When the tear clearance is low, inflammatory cytokines or preservatives accumulate in the conjunctival sac, resulting in ocular surface diseases. Frequent use of artificial tears without preservative is the key treatment. A differential diagnosis of the abnormalities of tear dynamics can give us a proper understanding of the pathogenesis of dry eye. With this knowledge, we can formulate an efficient therapeutic approach.     

TFOS DEWS II pain and sensation report

Pain associated with mechanical, chemical, and thermal heat stimulation of the ocular surface is mediated by trigeminal ganglion neurons, while cold thermoreceptors detect wetness and reflexly maintain basal tear production and blinking rate. These neurons project into two regions of the trigeminal brain stem nuclear complex: ViVc, activated by changes in the moisture of the ocular surface and VcC1, mediating sensory-discriminative aspects of ocular pain and reflex blinking. ViVc ocular neurons project to brain regions that control lacrimation and spontaneous blinking and to the sensory thalamus. Secretion of the main lacrimal gland is regulated dominantly by autonomic parasympathetic nerves, reflexly activated by eye surface sensory nerves. These also evoke goblet cell secretion through unidentified efferent fibers. Neural pathways involved in the regulation of meibomian gland secretion or mucin release have not been identified.In dry eye disease, reduced tear secretion leads to inflammation and peripheral nerve damage. Inflammation causes sensitization of polymodal and mechano-nociceptor nerve endings and an abnormal increase in cold thermoreceptor activity, altogether evoking dryness sensations and pain. Long-term inflammation and nerve injury alter gene expression of ion channels and receptors at terminals and cell bodies of trigeminal ganglion and brainstem neurons, changing their excitability, connectivity and impulse firing. Perpetuation of molecular, structural and functional disturbances in ocular sensory pathways ultimately leads to dysestesias and neuropathic pain referred to the eye surface. Pain can be assessed with a variety of questionaires while the status of corneal nerves is evaluated with esthesiometry and with in vivo confocal microscopy.     

Preservation of tear film integrity and inhibition of corneal injury by dexamethasone in a rabbit model of lacrimal gland inflammation-induced dry eye.

PURPOSE: The aim of this study was to establish a clinically relevant short-term animal model of dry eye with utility in identifying compounds with potential therapeutic efficacy. METHODS: Rabbit lacrimal glands were injected with the T-cell mitogen Concanavalin A (Con A) and inflammation, tear function, and corneal epithelial cell integrity were subsequently assessed. The inflammatory response was characterized by quantifying biochemical markers of inflammation ex vivo and by confirming inflammatory cell influx by histology. Matrix metalloproteinase-9 (MMP-9) and proinflammatory cytokines IL-1beta, IL-8, and TGF-beta1 were quantified in tissue extracts. Tear function was monitored by measuring tear fluorescein clearance and tear breakup time (TBUT). Corneal epithelial cell integrity was determined by quantifying the uptake of methylene blue dye following the exposure of rabbits to a low-humidity environment. The anti-inflammatory corticosteroid, dexamethasone, was administered topically as indicated for each study. RESULTS: Histopathologic evaluation of lacrimal glands injected with Con A revealed a pronounced inflammatory process characterized by lymphocytic infiltration, multifocal necrosis, and fibroplasia. Elevated levels of MMP-9 and cytokines IL-1beta, IL-8, and TGF-beta1 were detected in the lacrimal gland and cornea. Inflammation of the rabbit lacrimal gland following an injection of Con A significantly reduced tear clearance and TBUT and increased susceptibility to desiccation-induced corneal damage. Dexamethasone was prophylactically and therapeutically effective in this inflammation model of dry eye, restoring tear function and inhibiting corneal injury following topical ocular application. CONCLUSIONS: Characteristics of this rabbit lacrimal gland inflammation model of dry eye are consistent with the current understanding of dry eye as a local ocular surface inflammatory response to abnormal tear volume and composition. These results suggest that this rabbit model of dry eye may be employed to assess the therapeutic efficacy of mechanistically diverse agents on clinically relevant signs of ocular surface disease. These methods were strategically developed to be applicable for advancing drug discovery for a broad spectrum of dry eye patients.     

Does androgen insufficiency cause lacrimal gland inflammation and aqueous tear deficiency?

PURPOSE: The current investigators have shown that androgen treatment suppresses inflammation and stimulates the function of lacrimal glands in mouse models of Sj?gren's syndrome. Recently, others have hypothesized that androgen insufficiency induces an autoimmune process in lacrimal tissue, leading to inflammation, a Sj?gren's syndrome-like pathology, and aqueous tear deficiency. The purpose of the present study was to test this hypothesis. METHODS: Lacrimal glands were obtained from adult testicular feminized (Tfm) and control mice; castrated rats, guinea pigs, and rabbits; and castrated rats without anterior or whole pituitary glands and were processed for histology and image analysis. Tear volumes were measured in mice, in patients taking antiandrogen medications, and in age-matched human control subjects. RESULTS: Tfm mice, which are completely resistant to classical androgen action, did not have increased lymphocyte infiltration in their lacrimal glands or decreased tear volumes. No inflammation was evident in lacrimal tissues of male or female rats, guinea pigs, or rabbits 12 to 31 days after castration, no inflammation existed in rat lacrimal glands 15 to 31 days after orchiectomy and pituitary removal, and no aqueous tear deficiency was apparent in patients receiving antiandrogen therapy. CONCLUSIONS: Androgen deficiency may promote the progression of Sj?gren's syndrome and its associated lacrimal gland inflammation, meibomian gland dysfunction, and severe dry eye. However, androgen insufficiency alone does not cause lacrimal gland inflammation, a Sj?gren's syndrome-like pathology in lacrimal tissue, or aqueous tear deficiency in nonautoimmune animals and humans.     

Th细胞及其相关因子参与干眼发病机制的研究进展

干眼是眼表免疫稳态失衡所致的炎症性疾病.由泪膜、角结膜上皮、睑板腺、泪腺以及联络它们的神经通路共同组成的泪腺功能单位(lacrimal functional unit,LFU)是维持眼表生理稳态的完整系统,任何影响这一系统的因素都可以干扰眼表免疫平衡机制引发炎症反应,并最终导致干眼的发生.研究发现,Th细胞及其相关因子在干眼炎症过程中发挥了重要作用,影响Th细胞分化或Th细胞相关因子的异常表达可以控制眼表炎症反应,有助于恢复眼表免疫稳态,有效抑制干眼病理改变和缓解临床症状.     

New insight into lacrimal gland function: Role of the duct epithelium in tear secretion

Tear secretion is a complex process with the involvement of the main and accessory lacrimal glands, corneal and conjunctival epithelial cells and the Meibomian glands. The lacrimal gland is the main source of fluid, electrolytes and proteins in tear fluid. Deficient ion and water secretion results in aqueous deficient dry eye with serious consequences on the integrity of the ocular surface. Functions of acinar cells are widely studied, whereas less information is available about the duct system of the lacrimal gland. Secretory mechanisms of duct epithelium may play an important role in tear production, but only limited studies have tried to elucidate the role of the duct system in tear secretion. Significant progress has been made in the past few years, resulting in new insight into lacrimal gland duct function. New experimental techniques were introduced, which contributed to the exploration of the role of lacrimal gland ducts in more detail. Therefore, the aim of this review is to summarize our present knowledge about the role of ducts in lacrimal gland function and tear secretion, which appears to be the first review with a focus on this topic. Short outline of pancreatic and salivary gland duct functions is also given for the purposes of comparison.     

Antiinflammatory therapy for dry eye

PURPOSE: To present evidence establishing the relationship between inflammation and dry eye and supporting the use of antiinflammatory therapy for dry eye. DESIGN: Analysis of literature. METHODS: Research studies that evaluated inflammation in dry eye pathogenesis and clinical trials of antiinflammatory therapies for dry eye were reviewed. RESULTS: There is increasing evidence that decreased tear secretion, decreased tear turnover, and desiccation promote inflammation on the ocular surface. An increase in soluble mediators (cytokines and proteases) in the tear fluid, adhesion molecule expression by the conjunctival epithelium, and T-cell infiltration of the conjunctiva have been observed in dry eye patients. This inflammation appears to have a role in the pathogenesis of the ocular surface epithelial disease, termed keratoconjunctivitis sicca (KCS), that develops in dry eye. Clinical improvement of KCS has been observed after therapy with antiinflammatory agents including corticosteroids, cyclosporin and doxycycline. Cyclosporin A emulsion was approved by the Food and Drug Administration as therapy for dry eye. Randomized placebo-controlled FDA clinical trials showed that cyclosporine A was superior to vehicle in stimulating aqueous tear production, decreasing corneal punctuate fluorescein staining, reducing symptoms of blurred vision, and decreasing artificial tear use in patients with KCS. No ocular or systemic toxicity was observed from this medication. CONCLUSIONS: Ocular surface and lacrimal gland inflammation has been identified in dry eye that plays a role in the pathogenesis of KCS. Antiinflammatory therapy has efficacy for treating KCS. Cyclosporin A is the first FDA approved therapy for this indication. It improved signs and symptoms of KCS, and it is safe for long-term use.     

Osmoprotektion als therapeutisches Prinzip

Dry eye syndrome is one of the most common disorders encountered in daily ophthalmological practice. New pathophysiological concepts have been developed over the last few years. Hyperosmolarity of the tear film is one of the key pathogenetic factors in the development of a - commonly subclinical - inflammation of the ocular surface, the lacrimal gland and the tear film in dry eye syndrome. Osmoprotective agents act through compatible solutes to prevent - at least in theory -a hyperosmolar tear film from damaging the ocular surface.     

Enzymes of urea synthesis are expressed at the ocular surface, and decreased urea in the tear fluid is associated with dry-eye syndrome

Purpose

The present study aims at determining whether enzymes of urea synthesis are expressed in the human lacrimal gland and in tissues of ocular surface (conjunctiva, cornea), to give evidence for the hypothesis that urea can be locally formed from ocular tissues and is important for the composition of the tear fluid.

Methods

The presences of enzymes (arginase 1, 2 and agmatinase) that directly contribute to the formation of urea were investigated in the lacrimal gland and tissues of ocular surface by RT-PCR and immunohistochemistry. We collected tear fluid, aqueous humour, and blood samples from a total of 38 subjects, and tear fluid samples from a total of 78 subjects, with and without dry-eye syndrome (DES, keratoconjunctivitis sicca), and determined the urea concentration.

Results

The enzymes arginase 1, 2 and agmatinase were expressed in all tissues examined except for arginase 1, which was not expressed in the cornea. There was no correlation of urea concentration in tear fluid with aqueous humour and blood plasma (r?=?0.13, p?=?0.58 and r?=?0.45, p?=?0.05 respectively). However, correlation of urea concentration between aqueous humour and blood plasma was highly significant (r?=?0.7, p?=?0.0001). The concentration of urea in the tear fluid of patients with DES compared to healthy control group was significantly reduced (p?<?0.0001).

Conclusion

Enzymes that are directly involved in the formation of urea are expressed in ocular tissues. This may imply that in the ocular surface is a well-coordinated system of enzymes that can produce urea which might be independent of external urea supply.     

Detection and localization of the hydrophobic surfactant proteins B and C in human tear fluid and the human lacrimal system

PURPOSE: To evaluate the expression and presence of the surfactant proteins (SP) B and C in the lacrimal apparatus at the ocular surface and in tear fluid. METHODS: Expression of SP-B and SP-C was analyzed by RT-PCR in healthy lacrimal gland, conjunctiva, meibomian gland, accessory lacrimal glands, cornea, and nasolacrimal ducts. The deposition of the hydrophobic proteins SP-B and SP-C was determined by Western blot and immunohistochemistry in healthy tissues, tear fluid, and aqueous humor. RESULTS: The presence of both SP-B and SP-C on mRNA and protein level was evidenced in healthy human lacrimal gland, conjunctiva, cornea, and nasolacrimal ducts. Moreover, both proteins were present in tear fluid but were absent in aqueous humor. Immunohistochemical investigations revealed production of both peptides by acinar epithelial cells of the lacrimal gland and additionally by accessory lacrimal glands of the eyelid as well as epithelial cells of the conjunctiva and nasolacrimal ducts. Immunohistochemically, healthy cornea and goblet cells revealed no reactivity. CONCLUSIONS: Besides the recently detected surfactant-associated proteins SP-A and SP-D, our results show that SP-B and SP-C are also peptides of the tear film, the ocular surface, and the lacrimal apparatus. Based on the current knowledge of lowering surface tension in alveolar lung cells, a similar effect of SP-B and SP-C may be assumed concerning the tear film.     

Peroxisome proliferator-activated receptor agonists inhibit interleukin-1beta-mediated nitric oxide production in cultured lacrimal gland acinar cells.

Development of dry eye disease often occurs in individuals with autoimmune disorders such as Sj?gren's syndrome. The cause of dry eye in these patients is thought to be due, at least in part, to lymphocytic infiltration of the lacrimal glands, with subsequent loss of secretion of the aqueous component of tear film. How this lymphocytic infiltration leads to loss of secretion is not fully understood. We have previously shown that the proinflammatory cytokine, interleukin-1beta (IL-1beta), can stimulate the production of nitric oxide (NO) in cultured lacrimal gland acinar cells. It is possible that IL-1beta, produced by the infiltrating macrophages, stimulates production of inducible nitric oxide synthase (iNOS), and subsequently excessive production of NO. Peroxynitrate and other radical byproducts associated with excessive synthesis of NO may be detrimental to normal function of the lacrimal gland. Here we show that the peroxisome proliferator-activated receptor (PPAR)alpha and gamma agonists can inhibit NO production in cultured lacrimal gland acinar cells. Further, this is accomplished without loss of iNOS expression or tetrahydrobiopterin. These data suggest that the use of ointments or eye drops containing these PPAR agonists may provide an effective therapeutic intervention for the prevention of dry eye in Sj?gren's syndrome patients.     

糖尿病患者睑板腺功能障碍病理机制的研究进展

睑板腺功能障碍是一种以睑板腺终末导管阻塞和(或)睑酯分泌的质或量异常为主要特征的慢性、弥漫性睑板腺病变,临床上可引起泪膜异常和眼表炎症反应,从而导致眼部刺激症状,严重时可能损伤角膜从而影响视功能。睑板腺功能障碍可分为睑酯低排出型和高排出型,低排出型又进一步分为腺泡萎缩型和阻塞型。随着对糖尿病研究的进展,发现在糖尿病早期,患者的睑板腺组织即受到慢性损伤,导致其结构和功能均发生变化,糖尿病患者睑板腺功能障碍的发病率更高,程度更严重。而引起糖尿病患者睑板腺功能障碍的因素很多而且复杂,目前机制并不是很清楚,文章就国内外学者对糖尿病睑板腺功能障碍病理机制的研究进展作一综述。     

糖尿病性干眼发病机制及治疗进展

糖尿病是一种主要以损害微血管为主的病变。随着生活水平的提高,糖尿病患者的数量也逐渐增加,据最新数据显示,全球的糖尿病患病率高达9.3%(4.63亿人)。糖尿病相关的视网膜病变是导致工作人群视力障碍最主要的原因,近年来人们逐渐认识到高糖也可以引起眼表泪膜的不稳定、角膜神经功能障碍、角膜感知功能下降、泪液渗透压升高。糖尿病引起的眼表功能障碍原因复杂,机制多样,包括高糖导致泪膜各层结构破坏、树突状细胞-神经沟通障碍、炎性因子破坏泪腺分泌泪液等。本文就糖尿病性相关干眼的发生机制、研究进展进行综述,以期眼科医师在诊疗糖尿病干眼患者时更多考虑全身情况。     

干眼病理损害中的炎症机制

干眼是一种泪液和眼表的多因素性疾病,可引起眼部不适、视觉障碍、泪膜不稳定和眼表损害,并伴有泪膜渗透性升高和眼表炎症。目前研究表明,炎症在干眼的发病和病理损害中起着重要作用。本文从炎症与泪液渗透压、眼表黏蛋白表达、角结膜上皮结构及功能、泪腺及睑板腺结构和功能等方面对炎症造成干眼眼表和泪液功能损害的可能机制进行综述。     

A single injection of interleukin-1 induces reversible aqueous-tear deficiency, lacrimal gland inflammation, and acinar and ductal cell proliferation

Emerging studies from our laboratory demonstrate that interleukin-1 (IL-1) family members play a major role in impairing lacrimal gland functions. Here we have extended our investigations to observe the effects of IL-1 on aqueous tear production, lacrimal gland secretion, lacrimal gland histology, and acinar and ductal cell proliferation. We demonstrate that a single injection of IL-1 into the lacrimal glands inhibited neurally- as well as agonist-induced protein secretion resulting in decreased tear output. Meanwhile, IL-1 injection induced a severe, but reversible (7-13 days), inflammatory response that led to destruction of lacrimal gland acinar epithelial cells. Finally, we demonstrate that as the inflammatory response subsided and lacrimal gland secretion and tear production returned to normal levels, there was increased proliferation of acinar and ductal epithelial cells. Our work uncovers novel effects of IL-1 on lacrimal gland functions and the potential regenerative capacity of the mouse lacrimal gland.