Anatomy and Immunology of the Ocular Surface
Erich Knopa, Nadja Knopb
Research Laboratory of the Eye Clinic CVK, Charité-University School of Medicine, Berlin, bDepartment for Cell Biology in Anatomy, Hannover Medical School, Hannover, Germany
The ocularsurface, in a strict sense, consists of the cornea and its major support tissue, the conjunctiva. In a wider anatomical, embryological, and also functional sense, the ocular mucosal adnexa (i.e. the lacrimal gland and the lacrimal drainage system) also belong to the ocular surface. This definition includes the source and the eventual drainage of the tears that are of utmost importance to ocularsurface integrity. The ocular surface is directly exposed to the external environment, and therefore is endangered by a multitude of antigens and pathogenic microorganisms. As a mucosa, it is protected by the mucosal immune system that uses innate and adaptive effector mechanisms present in the tissue and tear film. Immune protection has two partly opposing tasks: the destruction of invading pathogensis counterbalanced by the limitation of inflammatory events that could be deleterious to the subtle structure of the eye. The immune system of the ocular surface forms an eye-associated lymphoid tissue (EALT) that is recognized as a new component of the mucosal immune system. The latter consists of the mucosa-associated lymphoid tissues in different organs of the body. Mucosa- and henceeye-associated lymphoid tissues have certain characteristics that discriminate them from the central immune system. The mechanisms applied are immunological ignorance, tolerance, or an immunosuppressive local microenvironment, all of which prefer non-reactivity and anti-inflammatory immunological responses. The interaction of these mechanisms results in immune privilege of the ocular surface. During eyeclosure, the ocular surface appears to have different requirements that make an innate pro-inflammatory environment more attractive for immune defense. The structural and functional components that contribute to this special immune regulation will be the focus of this chapter.
Copyright © 2007 S. Karger AG, Basel
Anatomy of the Immune System at the Ocular Surface and Adnexa
Cornea The corneaconsists of a transparent connective tissue (stroma) covered by epithelia on both sides. The endothelium that lines the anterior chamber is a monolayer and the outer border of the cornea is a stratified non-keratinized squamous epithelium that is 5–7 cells thick . It seals the stroma from the external environment by luminal junctions and forms a physical barrier against external antigens. Thisis supplemented by a physicochemical barrier of the epithelial-derived mucin layer that protects against the adhesion and entrance of antigens and by mechanical washing effects of the tear fluid and lid wiping combined with the action of protective proteins . In the normal cornea, very few cells can assist in immune defense. Lymphoid cells do not occur under physiological conditions. Thecentral cornea is avascular because blood and lymph vessels end in the limbal zone  and hence prevent an access of the vast majority of immunologically relevant cells. Major histocompatibility complex (MHC) class II-positive dendritic antigenpresenting Langerhans cells are present in the epithelium of the peripheral cornea and their absence from the central cornea was assumed to be a major reasonfor corneal immune privilege. Other dendritic cells (DCs) that are negative for markers of cell activation were recently observed in the central cornea of mice . Further bone marrow-derived DC precursors or macrophage-like cells were reported in the anterior stroma and in the posterior stroma. Conjunctiva Morphology The conjunctiva consists of an epithelium and an underlying loose connective...