CD4+ T cells isolated from spleen and superÞcial cervical lymph nodes of mice subjected to desiccating stress have been shown to induce severe autoimmune lacrimal keratoconjunctivitis when they are adoptively transferred to nude mouse recipients [16]. These Þndings suggest that ocular surface epithelial cells play a key role in the innate immune/inßammatory response to desiccating stress that facilitates the development of an adaptive immune response.

13.10.2.2 Efferent Arm

In the efferent arm of the immune cycle of dry eye activated CD4+ T cells migrate from the lymph nodes to the ocular surface and lacrimal glands, where if conditions are favorable, they are recruited to the epithelium where the putative lacrimal keratoconjunctivitis-inducing autoantigen is located. In normal immunocompetent individuals, experimental evidence indicates that this process is inhibited by T-regulatory cells in the regional lymph nodes and in the conjunctival epithelium [16]. We have observed that dry eye decreases the numbers of CD8+ and CD103+ conjunctival intraepithelial T cells that may serve as a barrier to migration of pathogenic autoreactive CD4+ T cells into the conjunctival epithelium [99, 100]. T-cell recruitment to and retention in the ocular surface tissues may be facilitated by cytokines and chemokines that are produced by activated epithelia that alter the local immune milieu by increasing the expression of adhesion molecules by vascular endothelial cells and ocular surface epithelial cells [89, 101]. Increased production of MMPs by the ocular surface epithelia also facilitates migration of T cells through the epithelial basement membrane into the epithelium [91]. Dry eye also appears to decrease levels of Fas ligand by the ocular surface epithelia, which has been found to play a role in immune privilege by stimulating apoptosis of Fas-expressing T cells [102]. Cytokines released by the inÞltrating CD4+ T cells are capable of altering conjunctival epithelial homeostasis. IL17 alone or in conjunction with interferon (IFN)- γ(gamma) or TNF-α(alpha) has been found to stimulate the production of inßammatory mediators by mucosal epithelial cells [103Ð110].

We have reported that IFN-γ(gamma) decreases goblet cell differentiation and increases expression of corniÞed envelope precursor proteins such as involucrin and small proline-rich protein-2 [111]. The relationship between T-cell inÞltration of the conjunctiva and loss of goblet cells has been observed in other inßammatory models. Chronic activation of NF-κB signaling in IKBζ knockout mice has been observed to induce CD4+ T-cell migration into the conjunctiva and marked goblet cell loss [112].

These Þndings are consistent with the clinical features of human dry eye disease. Decreased conjunctival goblet cell density is recognized as a sine qua non of SjšgrenÕs syndrome-associated conjunctival disease [113]. Furthermore, increased expression of corniÞed envelope precursor proteins and crosslinking transglutaminase-1 enzyme in the conjunctival epithelium has been observed in SjšgrenÕs syndrome [114]. Finally, epithelial stress pathways activated by osmotic stress and T-cell cytokines such as IFN-γ(gamma) and TNF-α(alpha) may contribute to the increased conjunctival epithelial apoptosis that has been observed in human and murine dry eye [115, 116].

13.11Loss of Ocular Surface Homeostasis

13.11.1Alterations of the Mucin, Lipid, and Aqueous Composition

Inßammation on the ocular surface can alter the tear Þlm function resulting in visual disturbance and irritation, pathological inßammation, and infection. As previously discussed, the cornea is the most densely innervated tissue of the body. Constant irritation of the ocular surface stimulates ocular surface sensory nerve endings to release substance P and CGRP that induce neurogenic inßammation. This neurogenic component of lacrimal keratoconjunctivitis is thought to be a critical factor in the disease pathogenesis. If the environmental stimulation of the ocular surface is not inhibited, the surface shear forces will cause more irritation, abnormal sloughing of the ocular

surface epithelium, and hyperemia of conjunctival vessels. Inßammation is instigated by blood proteins and immune cells diapedising from the vessels into the substantia propria of the conjunctiva. Constant irritation and the resulting chronic inßammation of the ocular surface trigger genes responsible for epithelial differentiation, such as corniÞed envelope precursors. CorniÞcation of the corneal and conjunctival epithelia consequently changes the ocular surface into a poorly lubricated and non-wettable surface [117, 118], thereby increasing the circle of inßammation and further diminishing tear production.

The composition of the tears that are produced when the lacrimal functional unit is inßamed is signiÞcantly altered. These include a rise in osmolarity, reduced concentration of protective factors (i.e., mucins, lipids, proteins), and increased levels of pro-inßammatory cytokines and proteases. Destabilization of the tear Þlm can be caused by loss or deterioration of lacrimal gland, conjunctival goblet cell, or meibomian gland secretions.

13.11.2 Mucins

Studies of dry eye patients have demonstrated that there are alterations in the amount and biochemical characteristics of the tear mucins. Consistent with the loss of conjunctival goblet cells and decreased production of the goblet cell mucin MUC5AC, the level of this mucin in the tear Þlm was found to be decreased in dry eye patients as compared to age and gendermatched healthy individuals [119]. In a different study, SjšgrenÕs syndrome patients had signiÞcantly reduced levels of MUC5AC in their tear ßuid and MUC5AC mRNA transcripts in their conjunctival epithelium [120]. Levels of the gelforming mucin MUC19 have also been found to be reduced in the conjunctival epithelium in SjšgrenÕs syndrome [121].

Mucin synthesis in canine KCS has been reported to have altered mucin glycosylation and changes in mucin subunit linkage [122]. These changes in mucin glycosylation may be

due in part to altered expression of polypeptide GalNAc-transferases (GalNAc-Ts) that have been detected in conjunctival squamous metaplasia [123]. Polypeptide GalNAc-transferases add N-acetyl galactosamine (GalNAc) to serine and threonine residues as the Þrst step in O-glycosylation of mucin. Reduced sulfation of mucin due to metalloproteinase-induced disorganization of the basal lamina of secretory cells has also been reported [124].

13.11.3 Lipids

Meibomian gland dysfunction is a common Þnding in SjšgrenÕs syndrome [125]. The lipid composition of meibomian gland secretions in healthy eyes and those with blepharitis has been investigated [126, 127]; however, there is little known regarding the biochemical changes in lipid composition in dry eye conditions that alter the tear Þlm. Minimal levels of the phospholipids, phosphatidylethanolamine and sphingomyelin, in meibomian gland secretions were signiÞcantly linked with the progression of corneal epithelial disease in KCS [128]. There are two possibilities for these abnormalities. The Þrst could be diminished lipid secretion. The second possibility is degradation of lipids by phospholipases. Phospholipase A2 (PLA2) was increased in the tear ßuid of dry eye patients [129]. Proinßammatory cytokines including IL-1 and TNF- α stimulate the production of this phospholipase. The concentration of tear secretory phospholipase A2 was found to increase in the tear Þlm of KCS patients as SchirmerÕs test values decreased.

13.11.4Pro-inflammatory Aqueous Component

Disruption of the LFU in SjšgrenÕs syndrome can result in dysfunction or death of lacrimal gland secretory acini. Lacrimal gland damage decreases ßuid, electrolyte, and protein secretion. Patients with lacrimal gland disease have been reported