Eye (2003) 17, 949–956 & 2003 Nature Publishing Group All rights reserved 0950-222X/03 $25.00 www.nature.com/eye Immune defense at EK Akpek and JD Gottsch CAMBRIDGE OPHTHALMOLOGICAL SYMPOSIUM the ocular surface Abstract vertebrates. Improved visual acuity would have increased the fitness of these animals and would The ocular surface is constantly exposed to a have outweighed the disadvantage of having wide array of microorganisms. The ability of local immune cells and blood vessels at a the outer ocular system to recognize pathogens distance where a time delay in addressing a as foreign and eliminate them is critical to central corneal infection could lead to blindness. retain corneal transparency, hence The first vertebrates were jawless fish that preservation of sight. Therefore, a were believed to have evolved some 470 million combination of mechanical, anatomical, and years ago.1 These creatures had frontal eyes and immunological defense mechanisms has inhabited the shorelines of ancient oceans. With evolved to protect the outer eye. These host better vision, these creatures were likely more defense mechanisms are classified as either a active and predatory. This advantage along with native, nonspecific defense or a specifically the later development of jaws enabled bony fish acquired immunological defense requiring to flourish and establish other habitats. One previous exposure to an antigen and the such habitat was shallow waters where lunged development of specific immunity. Sight- fish made the transition to land several hundred threatening immunopathology with thousand years later.2 To become established in autologous cell damage also can take place this terrestrial environment, the new vertebrates after these reactions. This article discusses the had to overcome desiccation and novel innate and acquired corneal elements of the pathogens such as viruses, bacteria, fungi, immune defense at the ocular surface. The protozoa, and helminths. relative roles of the various factors The exposure of ocular surface to potentially contributing to prevention of eye infection harmful materials including microbes, and toxic remain to be fully defined. substances found in the environment Eye (2003) 17, 949–956. doi:10.1038/ continually places the ocular surface at risk for sj.eye.6700617 immunologic events. There are two approaches to understanding immune defense at the Keywords: cornea; defensins; calgranulins; corneal surface. One is to study the cellular and neutrophils; herpes; oncocerchiasis molecular elements present in the human cornea for possible roles in mounting an immune defense. Peripheral dendritic cells or The Wilmer Eye Institute Introduction The Johns Hopkins Langerhans cells, for example, have been University School of The vertebrate cornea has a unique immune studied for their global antigen presenting Medicine defense to protect against foreign material and a properties or keratocytes for their intrinsic Baltimore, MA, USA number of microbial pathogens. As the microbial defensive peptides. The other method epidermis and dermis protect underlying is to investigate immune defensive pathways in Correspondence: tissues of the body, the cornea must prevent a specific animal model of microbial infection. JD Gottsch injury to the delicate structures of the eye. Well-known examples of this approach are The Wilmer Eye Institute 600 North Wolfe studies performed with murine herpes simplex However beyond its protective function, the St Maumenee # 321 cornea has evolved to be a highly transparent virus (HSV) and onchocerchiasis. Both Baltimore tissue, which refracts light to enable high- approaches provide valuable information, but MD 21287-9238, USA resolution vision. In order for the cornea to both have limitations. For ethical reasons, Tel: þ 1 410 955 7928 maximally transmit light, the extracellular studies of how the native human cornea Fax: þ 1 410 614 2816 E-mail: jgottsch@ responds to controlled experimental infections matrix of collagens and proteoglycans have jhmi.edu become highly ordered, and all extraneous cells cannot be carried out. However, clinical descriptions and investigation of the and blood vessels have been excluded. The Received: 28 February 2003 premium of enhancing visual acuity must have progression of culture-proven corneal infections Accepted in revised form: provided a selective advantage for early and the response to antimicrobials provide 28 February 2003 Immune defense at the ocular surface EK Akpek and JD Gottsch 950 some concepts of how the immune system reacts. For membrane is ruptured by infectious agents or trauma.5 example, Neiserria gonorrhea keratitis can proceed rapidly This capacity to secrete IL-1a is also shared by stromal to perforation suggesting that the local immune response keratocytes. Chronic IL-1a secretion would lead to is totally inadequate and is overwhelmed. In contrast, enhanced immune invasion, neovascularization and helminthic keratitis proceeds slowly with infiltrates and hence destruction of cornea. Interestingly, the corneal with persistent recurrent disease leading to corneal epithelium, but not the keratocytes, is also capable of neovascularization and scarring. In addition, corneas that secreting the soluble and membrane-bound forms of the have been excised for corneal transplantation or biopsy IL-1a receptor (IL-1RII),6 a natural IL-1a antagonist. It for a particular infection can be valuable for appears that corneal epithelial cells evolved the ability to understanding what cells and signaling events are modulate the effects of IL-1a secretion by synthesizing involved, using routine histopathology as well as more IL-1RII. IL-1 R antagonist gene therapy, in experimental sophisticated methods such as immunohistochemistry, models, showed a decreased leucocyte infiltration, polymerase chain reaction, and immunoelectron selectively altered the cytokine profile, and suppressed microscopy. Controlled infections in animal models may corneal neovascularization.7 Thus, the capacity of the provide more detailed analysis of immune pathways, epithelium to modulate the effects of IL-1a on the cornea although murine or nonhuman primate experiments may appears to have important vision-preservation not accurately reflect immune response to human consequences. disease. Hence, information from both methods of study is valuable. Keratocytes Keratocytes also have a defensive capacity during microbial invasion. Under the influence of IL-1a Corneal defensive mechanisms and tumour necrosis factor (TNF)- a, keratocytes synthesize IL-6 and -defensins.8,9 IL-6 interacts Innate defenses synergistically with IL-1 and perhaps TNF-a. Innate immunity is the first line of defense against Defensins hold therapeutic potential in ocular corneal infection.3 Elements of innate immunity are infections as they have a broad spectrum of antimicrobial present at birth and provide a nonspecific surveillance activity (against bacteria, fungi, and viruses) and system. Physical barriers, such as the bony orbit and the accelerate epithelial healing.10 Defensins have also been eyelids, guard against traumatic events, which could identified in neutrophils located in conjunctiva.11 comprise the corneal surface. Additionally, there are In HSV keratitis, IL-8 gene expression, a potent numerous cellular and molecular elements that chemoattractant for neutrophils, by keratocytes has also constantly protect against inoculation of the corneal been demonstrated.12 Il-8 was not found to be expressed surface against microorganisms. These elements include in corneal epithelial cells infected with herpes. tears, corneal nerves, the epithelium, keratocytes, polymorphonuclear cells, and some cytokines. Corneal nerves Corneal nerves are important in the innate defense of the cornea by relaying sensory Tears The main function of tears is to prevent drying of information leading to reflexive movements to protect the cornea. In addition, tears flush foreign particles from the eye.13 Sensations of discomfort and pain may also the ocular surface, and transport antimicrobial proteins release neuropeptides that have the capability to induce (lactoferrin, lysozyme, lipocalin, and beta-lysin) and cytokine activity. Two neuropeptides, calcitonin gene- immunoglobulins to the ocular surface to prevent related peptide and substance P, are released from the infections.4 Chief among the immunoglobulins in tears termini of corneal sensory neurons in response to and at much higher concentrations than serum is pain.14,15 Both can bind to human corneal epithelial cells immunoglobulin (Ig) A. Secretory IgA binds to bacteria and induce IL-8 synthesis with a resultant neutrophil and prevents bacterial adherence to epithelium. Tear IgG influx. as well as IgA can neutralize some viruses and bind bacteria and hence play a role in corneal defense. Complement Complement comprises a series of effector and regulatory proteins that activate each other in an Epithelial cells Corneal epithelial cells actively and orderly fashion to generate biologically active molecules, passively participate in protecting the ocular surface. such as enzymes, opsonins, anaphylotoxins, and These cells are capable of secreting cytokines to activate chemotaxins.16 immune defenses to protect against microbial invasion. All seven complement components are more The cytokine interleukin (IL)-1a is stored in epithelial concentrated in the peripheral cornea as compared to the cells where it is passively released when the cell central cornea. This distribution may result from the Eye Immune defense