The ocular surface is comprised primarily of the tear film, cornea, and conjunctiva. The lids serve to protect the eye and spread the tears over the ocular surface, and thus must be considered an integral part of the ocular surface. Diseases that affect the ocular surface will almost certainly affect the lids just as lid diseases may have a profound effect on the ocular surface. The tear film is the primary nonspecific defense system of the ocular surface as well as the first refractive layer of the eye. In addition, it maintains a uniform corneal surface and provides nutrients and oxygen to the cornea. Therefore, understanding and preserving proper tear function is an essential component for optimal ocular health in our patients. The study of keratoconjunctivitis sicca (KCS) and the development of topical cyclosporine for the treatment of that disease have profoundly influenced our understanding of the ocular surface immunity and the diseases that affect the ocular surface.
IMMUNOLOGY OF THE OCULAR SURFACE
Understanding the immunological defense systems of the ocular surface allows the clinician to better understand the pathophysiology of ocular surface disorders and the treatments employed to resolve them. Although the sequences of events in the defense mechanisms are complex, it is possible to classify them into two major categories, the afferent and efferent arcs. The afferent arc is the system by which the host “recognizes” offending antigens, and the efferent arc represents the specific reactions against them.
The cornea is somewhat of an “immunologically privileged site” in that it lacks vascularization and lymphatic drainage. Antigens contacting the ocular surface therefore are primarily processed initially in the conjunctiva where antigen-presenting cells (APC’s) with major histocompatibility complex II (MHC-II) molecules on their surface bind antigen. Conjunctival associated lymphoid tissue (CALT), macrophages, and Langerhans’ cells from the limbal region all act as APC’s. The CALT receives antigen for presentation to circulating T-lymphocytes (T-helper cells), and lymphatic channels drain it to regional lymph nodes. Specialized epithelium overlying the CALT differs from the relatively flat epithelium in adjacent areas in that it has elongated microvillae, which aids in the capture of antigens exposed to the ocular surface. Langerhans’ cells and macrophages bind antigens to their surface either for presentation to circulating T-helper cells or to carry them through the lymphatic channels to the draining lymph nodes. In addition, exposure to gamma interferon produced by activated lymphocytes can induce epithelial cells of the cornea and lacrimal acini to display MHC-II and function as APC’s. Thus, as the disease progresses, more and more host cells become involved in recruitment of the host’s immune responses to antigens. In the regional lymph nodes, B and T lymphocytes are stimulated to proliferate and differentiate. Thereafter, the B-lymphocytes migrate to lacrimal and accessory lacrimal gland epithelium while the T lymphocytes migrate back to the area of sensitization to collect in the submucosa of the conjunctiva.
The B-cells are stimulated to multiply by B-cell growth factor and then to produce antibodies by B-cell differentiation factor. Activated T-helper cells secrete these factors. Antibodies bind specifically with the antigen that stimulated its production so the migration of the B-lymphocytes to the lacrimal tissue provides for the local production of surface when it is breached by injury or inflammatory processes (e.g., corneal vascularization).
T lymphocytes may be divided into 3 subsets: helper T cells, killer T cells, and suppressor T cells. Activated helper T cells transform into lymphoblasts and, after proliferating in the regional lymph nodes migrate to the submucosa of the conjunctiva as already discussed. The activated lymphocytes and macrophages produce lymphokines which are soluble proteins that play many vital roles in the cellular events of the immune response and which allow relatively few activated cells to recruit and control a great number of non-sensitized lymphocytes and macrophages. The lymphokines include Interleukin 1, Interleukin 2, macrophage migration-inhibition factor, macrophage activating factor, chemotactic factor, and immune interferon. Killer T cells are cytotoxic to foreign cells or host cells that have been invaded or damaged by antigens as occurs in infection and neoplasia. Suppressor T cells slow the immune response when the exciting cause has been controlled.
Complement activation and the production of interferon are additional components of the immune response. Complement activation results in a cascade of enzymatic reactions that result in disruption of cell membranes contributing to cellular death via both antibody-mediated and cell-mediated immunity. Interferon can interfere with the replication of certain viruses when infected cells are induced to produce it. By virtue of its low molecular weight, it can diffuse into adjacent cells protecting them. Topical and oral use of alpha interferon has been advocated in the treatment of persistent or severe Herpes felis keratoconjunctivitis in cats, but may be most effective when used early in acute infections.
Ocular Surface Immunity and Disease
While the immune system is responsible for protecting the host, the eye is a very sensitive structure and often fares poorly in the face of the disease response. The following examples are familiar to all veterinarians and will serve to stimulate further study to understand and apply this knowledge to other diseases, ocular and systemic.
Keratoconjunctivitis sicca is classically regarded as a disease of the ocular surface caused by a deficiency of aqueous tear secretion. Multiple etiologies are recognized and include injury to the lacrimal glands or their innervation, drug toxicity (sulfa drugs being most commonly involved), drug or procedure induced (secondary to anesthesia, radiation, or the administration of atropine or antihistamines), surgical excision or chronic prolapse of the gland of the third eyelid, congenital dysfunction, heredity, chronic inflammation (blepharoconjunctivitis or keratoconjunctivitis), and immune mediated disease (either autoimmune or allergic/hyperimmune). The development and use of topical cyclosporine for KCS has focused attention on the last 3 categories, which account for the majority of cases. Indeed, both hereditary factors and chronic inflammation are tied inextricably to immune dysfunction as is clear from the degree to which the pattern of breed predispositionparallels that of allergic skin disease.
The concentration of IgE (which is responsible for type I hypersensitivity) can be greatly increased in conjunctival plasma cells in allergic keratoconjunctivitis. Histamine receptors are present on the ocular surface and the degranulation of conjunctival mast cells mediated by IgE releases histamine which can then bind to the ocular surface and exert its local effects, none of which are particularly kind to the delicate tissue. Thus with chronic uncontrolled atopy, the cornea and conjunctiva experience a cumulative and often progressive damage, damage that itself facilitates the recruitment of further immune responses.
The immunological response to herpetic keratoconjunctivitis is largely T-cell mediated, and in some cases, may require suppression in conjunction with antiviral therapy in order to maintain normal corneal function. Many veterinary ophthalmologists believe that there is a correlation between herpetic infections in cats and the occurrence of eosinophilic keratitis, which is usually responsive to corticosteroid therapy.
Chronic keratoconjunctivitis (pannus) occurs primarily in German Shepherds and is characterized by infiltration of the superficial cornea and conjunctiva with lymphocytes, pigment, and fibrovascular tissue. It is widely recognized as an immune mediated problem that requires lifelong control by the daily administration of immune suppressive agents (primarily corticosteroids and cyclosporine).
Mooren’s ulcers are ulcerations involving the peripheral corneal stroma in humans; similar occurrences have been noted in dogs and are believed to be immune mediated in response to antigen-antibody complexes that precipitate in the peripheral cornea. The ensuing host response results in keratomalacia or melting of the involved cornea.
CYCLOSPORINE IN VETERINARY OPHTHALMOLOGY
Cyclosporine (CsA) acts primarily on the afferent arm of the immune response during the recognition of the presence of antigen and the mobilization of cells in response. It exerts its activity on lymphocytes but does not interfere with phagocytes or the hemopoietic stem cells, so it is less often associated with opportunistic infections than other agents. It prevents lymphokine release and the subsequent proliferation and activation of T-cells, B-cells and monocytes. It does not, however, block suppressor T-cells so it down-regulates the immune response in favor of tolerance. The mechanisms of action in countering the inhibitory effect on lacrimal secretion by paracrine neurohormones secreted by lymphocytes in the lacrimal and accessory lacrimal glands remain incompletely understood.
Until recently (Fall of 1995), no ophthalmic preparation of cyclosporine was marketed, and it was compounded in oil as an extra label use of the oral product intended for systemic immune suppression. Initially, it was prepared in a 2% solution in olive oil, but irritation was a problem. Subsequently, corn oil was substituted in the formulation and was well tolerated in most patients. Occasionally irritation by 2% solutions may occur, and in such cases, a 1% formulation in corn oil may be less irritating while still effective in stimulating tear secretion. Such formulations have now been in use for approximately 10 years and have been well tolerated with few side effects. However, there are some significant concerns of which the clinician should be aware.
The topical use of these formulations can result in the systemic absorption of cyclosporine in great enough levels to be measured, and a measurable decrease in lymphocyte activation has been documented (Gilger, et. al., 1995 and 1996). No clinical signs were noted in these patients, but clinicians utilizing these formulations should be aware of the potential for interactions or synergistic effects with concurrently used medications. In addition, the degree of absorption from skin contact in humans is unknown but should be considered, especially in owners who are immunocompromised.
Perhaps the greatest cause for concern revolves around the issue of sterility and product stability. Contaminated of corn oil solutions during use by human patients has been documented. In a study of human cornea transplant patients using topical 2% cyclosporine compounded in corn oil under sterile conditions, the bottles were dispensed to the patients after initial cultures of the bottles revealed no bacterial growth. At the time of their rechecks between 25–30 days, 82% of the bottles were positive for bacterial growth. Klebsiella spp. was present in 46%, Pseudomonas spp in 31%, and Staphylococci spp in 27%. (D’Alessandro, et. al.. Invest. Ophthalmol. Vis. Sci. 35(4), 1994).
Spillage of the medication is an occasional complaint from owners, and some people object to the oiliness of the lids and face. Mallasezia spp may occasionally take advantage of such an oily environment.
Optimmune®, a 0.2% ointment formulation of cyclosporine, was introduced by Schering-Plough in the United States in 1995 and has now been approved for use in animals in 25 countries. It is well tolerated, and its prolonged contact time with eye allows the lower concentration to be effective in stimulating tear production and suppressing some disorders of surface ocular immunity. Although studies on its effects on lymphocyte activation have not yet been performed, it is minimally absorbed into the bloodstream, and no systemic side effects were noted during clinical trials. Because ointments do not support bacterial growth, contamination is not a problem. Some clients may have trouble with application, especially if their dog is aggressive or uncooperative or their manual dexterity is compromised by disability or age related infirmity. In comparison studies, the ointment was equally effective in producing tears when compared with formulations in oil, but some veterinary ophthalmologists report that their clinical experience is that it is less effective. Factors that could contribute to the difference include individual variation, poor client compliance with the ointment, or a premature switch from the ointment to the drop.
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