addition, the lipid layer prevents skin lipids on the lid margins from entering and disrupting the tear Þlm [35, 39, 52, 54].

The lipid layer varies in composition. Polar lipids such as phospholipids, sphingomyelin, ceramides, and cerebrosides are found adjacent to the aqueous phase of the tear Þlm [35, 39, 52, 55], while non-polar lipids, including wax and cholesterol esters, triglycerides, and free fatty acids, associate with the polar lipids and form the lipidÐ air interface [35, 39, 52, 56]. Even spreading of the lipid layer is important because accumulation of lipid in thick patches, especially the non-polar oils, may contaminate the mucin layer, rendering it unwettable [57, 58]. Blinking helps spread the lipid layer evenly over the tear Þlm surface [59]. Uniform tear Þlm spreading is also facilitated by the low surface tension of the lipidÐair interface, about half that of an aqueousÐair interface [60].

13.9.3 Aqueous Components

The aqueous layer of the tear Þlm, or the more aqueous portion of the mucin gel, contains dissolved oxygen, electrolytes, and numerous proteins, including growth factors that help maintain a trophic and protective environment for the ocular surface epithelium. The health of ocular surface epithelial tissues depends on growth factors, such as EGF [61, 62], HGF, and KGF [63, 64]. Immunoglobulins and other proteins such as lactoferrin [65], lysozyme [66], defensins [67], and immunoglobulin A [68] protect the ocular surface from infection by bacteria and viruses. Still other proteins, such as TGF-β(beta) and interleukin-1 receptor antagonist, help minimize ocular surface inßammation [69Ð71].

Tear Þlm electrolytes, such as Na+, K+, Cl−, Ca2+, and others, present in concentrations similar to those found in serum, result in a normal tear osmolarity of about 300 mOsm/L [72, 73], which helps maintain normal epithelial cell volume. Ions help solubilize proteins, and in some cases, are essential for enzymatic activity. Proper osmolarity is also required for maintenance of normal corneal nerve membrane potential and for cellular homeostasis and secretory function.

The roles of the main and accessory lacrimal glands in tear secretion and the pathogenesis of dry eye disease remain unresolved. Most of the normal daily tear ßow comes from the accessory lacrimal glands (the glands of Wolfring and Krause) located in the superior conjunctival fornix and in the upper lid just superior to the meibomian glands. The main lacrimal gland is thought to function primarily as a reservoir supplying the ocular surface with copious amounts of ßuid to wash out infectious or irritating particles that threaten epithelial integrity [74]. Removal of the main lacrimal glands from squirrel monkeys resulted in no damage to the cornea, conjunctiva, or eyelid, indicating that accessory lacrimal gland function was sufÞcient to maintain a healthy ocular surface [75]. The same procedure performed in cats and humans led to signs of KCS [76], suggesting an important role for the main lacrimal glands in maintaining ocular surface health in these species [77].

Most tear secretion by the LFU is driven by stimulation of the afferent sensory nerves on the ocular surface. These signals are integrated in the central nervous system and may be modiÞed or altered by cortical functions such as emotion. Tear production decreases by as much as 66% following topical anesthesia of the ocular surface [4] or to below detectable levels under general anesthesia when all emotional and afferent sensory stimulus for tear secretion is removed [78]. Anesthesia of the nasal mucosa decreased tear secretion in the ipsilateral eye by about 34% [79]. The effects of anesthesia underscore the importance of neuronal control for normal tear production.

13.10The Pathophysiology of Dry Eye

13.10.1 Loss of Hormonal Support

Both the lacrimal and meibomian glands are androgen responsive tissues. Androgens have been found to suppress inßammation in the lacrimal glands [18Ð20]. Relative androgen deÞciency might explain the greater prevalence of

dry eye in women. Consistent with this, SjšgrenÕs syndrome KCS occurs almost exclusively in women [80]. Androgen levels decrease with age in both sexes and may be responsible in part for the age-related deterioration in tear secretion. Hormone replacement therapy in postmenopausal women may also be associated with dry eye. Women taking estrogen replacement therapy are at a greater risk for developing dry eye than women taking a combination of estrogen and progesterone [81]. It has not been established whether oral contraceptives alter aqueous tear production [82].

much greater osmotic stress [86]. This is supported by a study that found a doubling of tear osmolarity in an experimental murine model of dry eye using sodium ion concentration in tear washings collected from the entire ocular surface to measure tear osmolarity [87].

Osmotic stress activates signaling pathways in a variety of cell types, including the ocular surface epithelia [88]. We have reported that exposure to increased osmolarity in vivo or in vitro activates mitogen-activated protein kinase (MAPK) pathways, particularly p38 and c-Jun N-terminal kinases, and nuclear factor (NF)-κB in the ocular surface epithelia [89, 90]. These pathways regulate transcription of a wide variety

The immune response on the ocular surface is designed to respond efÞciently to stress and/or microbial insults and, paradoxically, may contribute to autoimmunity. The exact mechanism by which SjšgrenÕs syndrome lacrimal keratoconjunctivitis develops has not been established; however, our research suggests that an autoimmune cycle in the LFU (Fig. 13.1) may be triggered by ocular surface desiccation; perhaps due to inability of the lacrimal glands to adequately respond to signals of ocular surface dryness, which is an early feature of SjšgrenÕs syndrome [83, 84]. This cycle consists of an afferent arm, where desiccating stress on the ocular surface elicits an immune response, and an efferent arm, where activated CD4+ T cells home to the ocular surface and modulate epithelial differentiation.

13.10.2.1 Afferent Arm

The afferent arm of the dry eye immune reaction is initiated by a stress response of the ocular surface epithelial cells to desiccation and/or increased tear osmolarity. Increased tear osmolarity in dry eye has been recognized for decades [85]. Clinical studies have reported increases in mean tear osmolarity of about 10Ð20% in tear samples collected from the inferior tear meniscus [85]; however, ocular surface epithelial cells underlying areas of marked thinning or frank breakup of the tear layer may be subjected to

osmotic stress, by MAPK activation, stimulates production of a variety of inßammatory mediators by the corneal epithelium, including interleukin (IL)-1β(beta), tumor necrosis factor (TNF)-α(alpha), IL-8, and a number of matrix metalloproteinases (MMPs; MMP-1, MMP-3, MMP-9, MMP-10, and MMP-13) [89Ð93].

Under homeostatic conditions, resident corneal dendritic cells are immature and have limited or no MHC class II antigen membrane expression [94]. Following ocular surface desiccating challenge, immune-mediated inßammation is initiated by release of inßammatory cytokines (IL-1, TNF-α(alpha), and IL-6) by the stressed ocular surface epithelial cells. These cytokines activate immature dendritic cells and increase their expression of CC chemokine receptor 7 (CCR7) and major histocompatibility complex class II antigen [94, 95]. Dendritic cells uptake and process antigen, travel to the lymph nodes via the lymphatic system, and activate antigen-speciÞc na•ve T cells. Following activation and differentiation within these secondary lymphoid organs, effector T cells home to the ocular surface.

Dry eye also leads to a decrease in the number of conjunctival goblet cells, which produce and secrete the immunoregulatory molecule transforming growth factor-β2 that has been reported to suppress activation of dendritic cells on the ocular surface [16, 96, 97]. Desiccating stress

Fig. 13.1 Autoimmune cycle in the lacrimal functional unit (LFU) in SjšgrenÕs syndrome. Afferent arm: environmental stimulus initiates an acute immune inßammation on the ocular surface, activating epithelial, dendritic, and endothelial cells. Activated dendritic cells process autoantigen and trafÞc to the regional lymph nodes where

they present antigen to na•ve CD4+ T cells. Efferent arm: primed and targeted CD4+ T cells travel via the circulation to the LFU where they diapedise into the local tissues, releasing pro-inßammatory cytokines which promote chronic inßammatory disease and secretory dysfunction

may also expose autoantigens by the ocular surface and lacrimal gland epithelia. For example, kallikrein 13, an EGF-binding protein, has been implicated as an autoantigen [98]. Our group has found that kallikrein 13 production by the ocular surface epithelia is increased following desiccating stress. Furthermore, decreased EGF production by the lacrimal glands in SjšgrenÕs

syndrome results in less kallikrein 13 binding and greater exposure of the unbound of kallikrein 13 to antigen-presenting cells.

Adoptive transfer models indicate that antigen-loaded antigen-presenting cells migrate from the ocular surface to regional lymph nodes where they present antigen to CD4+ T cells capable of reacting to ocular surface antigens.