Mechanisms of Murine Lacrimal Gland Repair after Experimentally Induced Inflammation

The nonkeratinized epithelia of the ocular surface are constantly exposed to and challenged by environmental insults such as smoke, dust, and airborne pathogens. Tears are the sole physical protective barrier for the ocular surface.1,2 They form a complex thin film that fulfills several functions, from forming a smooth refractive surface over the corneal surface and lubricating the eyelids to maintaining an optimal extracellular environment for the epithelial cells of the cornea and conjunctiva.1,2 The tear film consists of three interacting layers. The innermost layer coating the cornea and conjunctiva is a mucous layer.3 The large middle layer is an aqueous layer.4 Finally, the outer layer is a lipid layer that floats on the aqueous layer to retard its evaporation.5 Production of tears in inadequate quantity or of inadequate quality results in constant irritation of the ocular surface, leading to dry eye syndrome. There are two major subtypes of dry eye syndrome: evaporative dry eye, caused by perturbations of the lipid layer of the tear film, and aqueous-deficient dry eye, also known as keratoconjunctivitis sicca (KCS), caused by inadequate production of the aqueous layer of the tear film.6 The mechanisms leading to insufficient lacrimal gland secretion are still poorly understood. It is believed that chronic inflammation of the lacrimal gland is a major contributor to insufficient secretion. Chronic inflammation of the lacrimal gland occurs in several pathologic instances (for a review, see Zoukhri7), such as autoimmune diseases (Sjogren syndrome, sarcoidosis, diabetes) or after organ transplantation (chronic graft versus host disease), or simply as a result of aging. The hallmarks of lacrimal gland inflammation are the presence of focal lymphocytic infiltrates, increased production of proinflammatory cytokines, and destruction of the tear-producing parenchymal cells.7 There is a large body of literature on the regenerative capacity of the salivary glands.8–10 Long-term (7–21 days) ligation of the main excretory ducts of salivary glands leads to atrophy of these glands and has been used as an experimental model of human diseases of the salivary glands.8–10 Salivary glands with ligated ducts show signs of inflammation, edema, and death of the acinar cells.8–10 When the duct ligation is released, the salivary glands go through a process of repair through the proliferation of acinar, ductal, or myoepithelial cells and the synthesis and deposition of extracellular matrix.8,9 Similarly, studies on the pancreas11,12 and mammary glands13 reported self-regenerating capabilities of these tissues. Salivary glands go through a phase of inflammation/destruction of parenchymal cells followed by a period of proliferation/tissue repair. Remodeling of tissue after injury or trauma often recapitulates the same cellular events that govern embryonic tissue development. Apoptosis, a form of programmed cell death, is crucial during embryonic development and is equally important in adult organs to maintain normal cellular homeostasis.14 Under physiological conditions, the occurrence of apoptosis in tissues is typically a rare event, but apoptotic cells become readily identifiable under pathophysiological conditions such as inflammation.14 The role of apoptosis in tissue atrophy/repair is well documented.14 Another form of cell death has also been implicated in tissue repair. Autophagic cell death (also called type 2 cell death; apoptosis is type 1 and necrosis is type 3) is characterized by the appearance of double- or multiple-membrane delimited cytoplasmic vesicles (autophagic vesicles or vacuoles) engulfing bulk cytoplasm or cytoplasmic organelles such as mitochondria and endoplasmic reticulum.15,16 Autophagic vesicles are then destroyed by the cellular lysosomal system.15,16 Apoptosis and autophagic cell death are not mutually exclusive phenomena; they may occur simultaneously in tissues or even conjointly in the same cell.15,16 The presence of stem cells in adult tissues is a very active area of research because of the potential clinical benefits. Several studies on the salivary glands and the pancreas have shown that stem/progenitor cells are present in these tissues and are involved in their regeneration.9,17,18 In previous studies, we developed an animal model of acute, experimentally induced inflammation to study the role of IL-1 in impairing lacrimal gland function. We reported that a single injection of IL-1 into the lacrimal gland inhibited neurally induced and agonist-induced protein secretion measured 1 day after injection.19 We also reported that IL-1 (IL-1β or IL-1α) injection induced a severe inflammatory response in the lacrimal gland that led to the destruction of the acinar cells and resulted in reduced aqueous tear production.7,20 Furthermore, we showed that the effects of IL-1 on lacrimal gland functions were transient. By the seventh (BALB/c mice) or the thirteenth (C57BL/6 mice) day after IL-1 injection, inflammation resolved and the lacrimal gland recovered.20 The purpose of the present study was to determine the cellular mechanisms involved in lacrimal gland injury and repair in this model of acute, experimentally induced inflammation.