Introduction

The vertebrate cornea has a unique immune defense to protect against foreign material and a number of microbial pathogens. As the epidermis and dermis protect underlying tissues of the body, the cornea must prevent injury to the delicate structures of the eye. However beyond its protective function, the cornea has evolved to be a highly transparent tissue, which refracts light to enable high-resolution vision. In order for the cornea to maximally transmit light, the extracellular matrix of collagens and proteoglycans have become highly ordered, and all extraneous cells and blood vessels have been excluded. The premium of enhancing visual acuity must have provided a selective advantage for early vertebrates. Improved visual acuity would have increased the fitness of these animals and would have outweighed the disadvantage of having local immune cells and blood vessels at a distance where a time delay in addressing a central corneal infection could lead to blindness.

The first vertebrates were jawless fish that were believed to have evolved some 470 million years ago.1 These creatures had frontal eyes and inhabited the shorelines of ancient oceans. With better vision, these creatures were likely more active and predatory. This advantage along with the later development of jaws enabled bony fish to flourish and establish other habitats. One such habitat was shallow waters where lunged fish made the transition to land several hundred thousand years later.2 To become established in this terrestrial environment, the new vertebrates had to overcome desiccation and novel pathogens such as viruses, bacteria, fungi, protozoa, and helminths.

The exposure of ocular surface to potentially harmful materials including microbes, and toxic substances found in the environment continually places the ocular surface at risk for immunologic events. There are two approaches to understanding immune defense at the corneal surface. One is to study the cellular and molecular elements present in the human cornea for possible roles in mounting an immune defense. Peripheral dendritic cells or Langerhans cells, for example, have been studied for their global antigen presenting properties or keratocytes for their intrinsic microbial defensive peptides. The other method is to investigate immune defensive pathways in a specific animal model of microbial infection. Well-known examples of this approach are studies performed with murine herpes simplex virus (HSV) and onchocerchiasis. Both approaches provide valuable information, but both have limitations. For ethical reasons, studies of how the native human cornea responds to controlled experimental infections cannot be carried out. However, clinical descriptions and investigation of the progression of culture-proven corneal infections and the response to antimicrobials provide some concepts of how the immune system reacts. For example, Neiserria gonorrhea keratitis can proceed rapidly to perforation suggesting that the local immune response is totally inadequate and is overwhelmed. In contrast, helminthic keratitis proceeds slowly with infiltrates and with persistent recurrent disease leading to corneal neovascularization and scarring. In addition, corneas that have been excised for corneal transplantation or biopsy for a particular infection can be valuable for understanding what cells and signaling events are involved, using routine histopathology as well as more sophisticated methods such as immunohistochemistry, polymerase chain reaction, and immunoelectron microscopy. Controlled infections in animal models may provide more detailed analysis of immune pathways, although murine or nonhuman primate experiments may not accurately reflect immune response to human disease. Hence, information from both methods of study is valuable.

Corneal defensive mechanisms

Innate defenses

Innate immunity is the first line of defense against corneal infection.3 Elements of innate immunity are present at birth and provide a nonspecific surveillance system. Physical barriers, such as the bony orbit and the eyelids, guard against traumatic events, which could comprise the corneal surface. Additionally, there are numerous cellular and molecular elements that constantly protect against inoculation of the corneal surface against microorganisms. These elements include tears, corneal nerves, the epithelium, keratocytes, polymorphonuclear cells, and some cytokines.

Tears

The main function of tears is to prevent drying of the cornea. In addition, tears flush foreign particles from the ocular surface, and transport antimicrobial proteins (lactoferrin, lysozyme, lipocalin, and beta-lysin) and immunoglobulins to the ocular surface to prevent infections.4 Chief among the immunoglobulins in tears and at much higher concentrations than serum is immunoglobulin (Ig) A. Secretory IgA binds to bacteria and prevents bacterial adherence to epithelium. Tear IgG as well as IgA can neutralize some viruses and bind bacteria and hence play a role in corneal defense.

Epithelial cells

Corneal epithelial cells actively and passively participate in protecting the ocular surface. These cells are capable of secreting cytokines to activate immune defenses to protect against microbial invasion. The cytokine interleukin (IL)-1α is stored in epithelial cells where it is passively released when the cell membrane is ruptured by infectious agents or trauma.5 This capacity to secrete IL-1α is also shared by stromal keratocytes. Chronic IL-1α secretion would lead to enhanced immune invasion, neovascularization and hence destruction of cornea. Interestingly, the corneal epithelium, but not the keratocytes, is also capable of secreting the soluble and membrane-bound forms of the IL-1α receptor (IL-1RII),6 a natural IL-1α antagonist. It appears that corneal epithelial cells evolved the ability to modulate the effects of IL-1α secretion by synthesizing IL-1RII. IL-1 R antagonist gene therapy, in experimental models, showed a decreased leucocyte infiltration, selectively altered the cytokine profile, and suppressed corneal neovascularization.7 Thus, the capacity of the epithelium to modulate the effects of IL-1α on the cornea appears to have important vision-preservation consequences.

Keratocytes

Keratocytes also have a defensive capacity during microbial invasion. Under the influence of IL-1α and tumour necrosis factor (TNF)- α, keratocytes synthesize IL-6 and -defensins.8,9 IL-6 interacts synergistically with IL-1 and perhaps TNF-α.

Defensins hold therapeutic potential in ocular infections as they have a broad spectrum of antimicrobial activity (against bacteria, fungi, and viruses) and accelerate epithelial healing.10 Defensins have also been identified in neutrophils located in conjunctiva.11

In HSV keratitis, IL-8 gene expression, a potent chemoattractant for neutrophils, by keratocytes has also been demonstrated.12 Il-8 was not found to be expressed in corneal epithelial cells infected with herpes.

Corneal nerves

Corneal nerves are important in the innate defense of the cornea by relaying sensory information leading to reflexive movements to protect the eye.13 Sensations of discomfort and pain may also release neuropeptides that have the capability to induce cytokine activity. Two neuropeptides, calcitonin gene-related peptide and substance P, are released from the termini of corneal sensory neurons in response to pain.14,15 Both can bind to human corneal epithelial cells and induce IL-8 synthesis with a resultant neutrophil influx.

Complement

Complement comprises a series of effector and regulatory proteins that activate each other in an orderly fashion to generate biologically active molecules, such as enzymes, opsonins, anaphylotoxins, and chemotaxins.16

All seven complement components are more concentrated in the peripheral cornea as compared to the central cornea. This distribution may result from the diffusion of complement components from limbal vessels into the cornea.17 Activation of complement by either the classical or alternate pathway can be involved in corneal inflammation.

Interferons

Interferons (IFN) are a group of proteins made by cells in response to viral infection, which induce a generalized antiviral state in the surrounding cells.18,19 IFN-α is secreted by leucocytes, IFN-β by fibroblasts, and IFN-γ by T cells and natural killer (NK) cells. INFs also stimulate production of major histocompatibility complex (MHC) class I molecules and proteins that enhance the ability of virally infected cells to present viral proteins to T cells. IFN-α and IFN-β activate NK cells in order that virus-infected host cells can be targeted and eliminated.

Cells of innate immunity

Neutrophils are important in protecting the corneal epithelium from invasion by many microorganisms. Normally found in the cornea, neutrophils move through endothelial cells of the limbal vasculature by adhesion to receptors on vascular endothelial cells, a process called diapedesis. The neutrophil is a critical effector cell in innate immunity and plays vital roles in phagocytosis and microbial killing.20

Eosinophils possess surface receptors for IgE and complement components.21 Activation of eosinophils can occur with IL-3, IL-5, and granulocyte colony-stimulating factor. A number of eosinophil granule proteins, such as major basic protein and cationic protein, have been identified which are presumed to play a role in parasitic toxicity.21

Macrophages are important in the innate immune response against microbial infections as they have phagocytic and antigen presenting capabilities as well as secretion of inflammatory cytokines.22 Macrophages are believed to have important modulating effects on T-cell immune response. Although macrophages have traditionally been thought to reside in the conjunctiva, resident macrophages have been recently found in the murine corneal stroma and may play a part in host immune responses.23

NK cells are large granular lymphocytic cells and have no surface antigen receptors.3 NK cells recognize MHC class I molecules through surface receptors delivering signals that inhibit, rather than activate, NK cells. As a consequence, NK cells lyse target cells that have lost or express insufficient amounts of MHC class I molecules, as frequently occurs in tumours, cells infected by certain viruses, antibody-coated cells, undifferentiated cells, and tumour cells.24 Similar to activated macrophages, NK cells secrete TNF-α and IFN-α.

Acquired immunity

When innate immunity fails and the invading microorganism and/or its antigens persist, cell-mediated immunity can help bring microbial replication under control. In some cases, cell-mediated immunity can be out of proportion to the antigenic threat, leading to irreversible tissue destruction. Scarring and disorganization of the extracellular matrix as a result of HSV dermatitis may be inconsequential, whereas the same response in the cornea may lead to a significant permanent visual loss.

Langerhans cells

Langerhans cells are essential sentinel cells that are part of the corneal immune surveillance system.25,26,27 Langerhans cells are antigen presenting cells of the cornea that are responsible for the recognition, processing, and presentation of antigens.25 Langerhans cells have traditionally been noted to have MHC class II antigens and to remain in the periphery of the cornea. However, Langerhans cells have been noted in the central cornea of human infants27 and, recently, MHC class II-negative Langerhans cells have been demonstrated in the central cornea of BALB/c mice.28 When Langerhans cells are summoned, as elsewhere in the body, they must recognize a foreign antigen and discriminate between self and nonself. After Langerhans cells recognize an antigen as nonself, the antigen is processed and is transported to the surface by MHC molecules, either class I or II.29 T cells are activated by the presentation of the antigen on the MHC molecule by binding to the T-cell receptor. The T cells then mature into effector cells, which are CD4+, if the MHC molecules presenting the antigen are class II, or CD8+, if the MHC is class I. The T cells either directly kill foreign invaders (CD8+ cytotoxic cells) or secrete cytokines (CD4+ T helper cells) which bring in other effector cells, mainly macrophages, which are involved in the destruction of pathogens and can activate other inflammatory cells.

Cytokines

Two subsets of T helper cells have been described with differential cytokine production profiles.30,31,32 Th1 cells secrete IL-2 and IFN-γ whereas Th2 cells secrete IL-4 and IL-5, but not IL-2 or IFN-γ. Furthermore, Th1 cells can be cytolytic and can assist B cells with IgG, IgM, and IgA synthesis but not IgE synthesis. Th2 cells are not cytolytic but can provide help for IgE synthesis as well as IgG, IgM, and IgA syntheses. Selection of CD4 Th1 or CD4 Th2 cells occurs in the case of infection or autoimmune diseases. Th1 cells are preferentially selected as participants in inflammatory reactions associated with delayed-type hypersensitivity reactions and low antibody production. Th2 cells, on the other hand, are involved in inflammatory reactions associated with persistent antibody production, including allergic responses in which IgE production is predominant. Cytokines produced by Th cells are of critical importance for the outcome of many infectious diseases. The Th1/Th2 distinction has proven useful in the analysis of immune responses to infections. For example in schistosomiasis, a Th1 response is associated with elimination of the parasite, whereas a Th2 response results in extensive disease and granuloma formation.33 Indeed, the expression of cytokines can dictate the intensity, chronicity, and type of immune/inflammatory response that is produced. These events may be regulated by accumulation of particular cell populations at a site of immune response that can be regulated by the expression of specific chemokines.

Corneal defenses to infection

Immunologic differences exist between the peripheral and central cornea. The peripheral cornea being proximate to the conjunctiva has all of the immunologic machinery necessary to generate an immune response. The peripheral cornea, possessing Langerhans' cells and IgM, also has more C1, the recognition unit of the classic pathway of complement, than the central cornea. Antigen–antibody complexes, whether formed in the cornea itself or whether derived from the tears, aqueous humour, or limbal vessels, may activate complement more effectively in the peripheral than central cornea. To follow are three examples of immune defense at the ocular surface to microbial invasion.

Each organism provokes a different defensive response, but each has components of innate and acquired immunity that play a defensive role. There are two defensive stages for each, an early and a late corresponding to innate and acquired immune responses, respectively. The first stage is an immediate response within minutes to several hours to microbial invasion. A bacterium can replicate in an hour, and after 24 h, 2 million organisms can exist establishing an infection that may overwhelm immune defenses. The innate immune system is charged with this initial defensive response, which always involves a neutrophilic infiltrate. If the initial defensive effort fails, adaptive immunity is called upon where Langerhans cells identify the foreign antigen in the cornea. An acquired response may occur within 24–48 h. The full adaptive immune response, however, may take days.

Pseudomonas aeruginosa

Many bacterial and fungal organisms have components of their cell membrane and walls that play a vital role in the maintaining viability and have been retained over millions of years. These conserved molecular signatures have been recognized by host immune systems as a sign of incipient microbial invasion and have evolved to elicit an immediate and intense neutrophilic hypersensitivity response.3 Phylectenular corneal disease is an example of a hypersensitivity response most commonly noted with staphylococcal antigens and has been reproduced in animal models by intrastromal injection of ribitol teichoic acid, a staphylococcal protein.34

P. aeruginosa is a common cause of corneal infection, particularly among users of soft contact lenses. Lipopolysaccharide (LPS) is another conserved molecule, a glycolipid in the outer membrane of Gram-negative bacteria including P. aeruginosa.3 Corneal ulcers can be induced by intrastromal delivery of LPS to rabbit cornea, where histopathological examination demonstrates an intense infiltrate predominantly composed of neutrophils,35 in the absence of any live bacteria.

The recruitment of neutrophils is mediated by an ancient ‘toll receptor’, first found in Drosophila, which was found to regulate the dorsal–ventral orientation in embryologic development.3 The toll receptor also upregulated defensin expression in fungally infected Drosophila. A toll-like receptor, TLR4, has been identified in corneal epithelial cells.36 TLR4 was shown to regulate endotoxin-induced keratitis by increased expression of platelet adhesion molecule (PECAM)-1 and macrophage inflammatory factor (MIP)-2, a homologue of IL-8.37 PECAM is necessary to allow the egress or diapedesis of neutrophils from blood vessels into the extracellular matrix. Thus, the immune recognition of LPS as a conserved signature of Gram-negative bacterial invasion allows for the rapid influx of neutrophils from the perilimbal vessels.

Recruitment of neutrophils by MIP-2 in Pseudomonas-infected corneas has been demonstrated in susceptible mice.38 The mechanism by which MIP-2 is upregulated appears to be related to the increase in IL-1 that is noted to occur in infected mice.39 Thus, a corneal infection with Pseudomonas may have a positive feedback loop with increasing IL-1 secretion leading to a sustained neutrophilic infiltrate that is characteristic for the disease.

Contact lens wear has been shown to be a risk factor for the development of Pseudomonas keratitis. An acquired immune defensive component may be a possible explanation for this risk factor as experimental contact lens wear can cause migration of Langerhans cells into the central cornea.40 With Langerhans cells present in the central cornea, an acquired immune response could proceed more rapidly leading to cytokine release and further influx of inflammatory cells.41 Indeed, CD4+ Th1 cells have been shown to increase in mice corneas susceptible to perforation. Further influx of CD4+ positive cells, responsible for persistence of a neutrophilic infiltrate in Pseudomonas keratitis, may be a result of unregulated MIP-1 which is produced by Langerhans cells, activated T cells, and macrophages.41

Herpes simplex virus

Herpes simplex keratitis (HSK) has many different clinical presentations. Replication of the virus in corneal epithelium produces a classic dendrite, while immune reactions to the virus in the stroma and endothelium can produce infiltration, ulceration, vascularization, or oedema. Peripheral ulcerative keratitis can also seen with the disease. The relation of the immune surface defenses and HSV is complex. An intense and/or prolonged immune response to the virus can lead to corneal blindness. There appears to be a balance of ameliorating and exacerbating immune factors to keep viral replication and release of proinflammatory cytokines in check without incurring sight-threatening corneal damage.

Important in the defense against herpetic and other viral infections are mechanisms involved in innate immunity leading to the nonspecific clearing of viral particles. Neutrophils play a central role in the suppression of HSV viral replication in the murine model.42 IFNα also plays a role in viral suppression as mice treated with neutralizing antibody to interferon have elevated viral titres and corneal disease.43 IL-1α and TNF-α can induce corneal epithelial cells and keratocytes to secrete IL-6, which in turn has been found to induce corneal cells to produce MIP-1α and MIP-2.44,45 MIP is known to be a potent chemoattractant for neutrophils and T cells. NK cells also play a role as they indirectly influence the severity of HSK in a mouse model by affecting neutrophil migration into the cornea.46 Macrophages are required for the development of an acquired immune response, presumably by functioning in antigen processing and presentation.47 IFN-α1, similarly, has a protective role, which is dependent upon the local or distal participation of CD4+ and CD8+ T lymphocytes early in the course of the infection.48

HSV replication in corneal epithelium does not always incite stromal disease. Resistance to HSV stromal keratitis in mice is associated with allotypic variation in immunoglobulin genes.49 Cross-tolerization of T cells specific for corneal tissue autoantigens by the circulating Ig-derived peptides was suggested as the possible mechanism. Further studies demonstrated that HSV stromal keratitis was mediated by CD4+ T-cell clones specific for corneal self-antigens, which also recognize an allotype-bearing peptide derived from IgG2a,50 and UL6 peptide of HSV cross-reacts with the IgG2a(b) isotype. Exposure of susceptible mice to a soluble form of this peptide was thought to confer resistance to keratitis. However, other work has cast doubt on this “molecular mimicry” hypothesis as an explanation for the pathogenesis of herpetic stromal keratitis.51

The influx of neutrophils and T cells producing HSV keratitis is greatly influenced by the cytokine MIP-1α.52 In addition, knockout mice for MIP-1α have decreased inflammatory infiltrates with HSV infections. HSK has been found to have an increased expression of the Th1 cytokines IL-2 and IFN gamma,53 and expression of IL-10, which suppresses the expression of these cytokines, ameliorates the development of HSK.54,55

Helminthic keratitis

Parasitic filarial nematodes infect more than 200 million individuals worldwide, causing debilitating inflammatory diseases such as river blindness and lymphatic filariasis.56 The ocular disease is thought to result from microfilariae that migrate from the conjunctiva into the cornea, become immotile and die. An intense immune response to dead microfilaria ensues which upon repeated infestation results in sclerosing keratitis and loss of vision. The mechanism by which the microfilaria become immobile and die in the cornea is unknown. However, a neutrophilic protein, calgraulin C, has been identified on the surface of worms isolated from onchoceral nodules.57 Experiments have shown that calgranulin C can be synthesized by keratocytes and is filariastatic and filaracidal for the helminth Brugia malayi.58 A parasitic muscle protein, paramyosin, is a binding protein for calgranulin C, resulting in a dose-dependent immobilization and at higher concentration killing of worms.59 The binding complex of paramyosin–calgranulin C is highly antigenic and capable of inducing a stromal keratitis mimicking the human onchocercal human corneal disease.59

In human onchoceral conjunctival biopsies, CD4+ cells are prominent with predominant IL-4 expression suggesting a Th2 immune response.60,61 In animal models of onchocerciasis, the clinical disease could be reproduced by the injection of onchocercal extracts.62 The early inflammatory cellular infiltrate is composed of neutrophils, however; eosinophils are the most prominent inflammatory cells when inflammation peaks.63,64 In addition, recruitment of neutrophils and eosinophils to the cornea is antibody dependent, with PECAM-1 and ICAM-1 playing important roles.65 Also, sensitized CD4+ T cells expressing IL-4 and IL-5 and but not IFN-γ cells have been shown to play a role in the keratitis noted after injection with O. volvulus antigen.66 Modulation of cytokines has been shown to affect the severity of keratitis in animal models. By using an IL-4 gene knockout model, a reduced inflammatory infiltrate with eosinophils was noted. Collectively, these studies demonstrate that a predominantly Th2 immunity contributes to the inflammatory response in onchocercal keratitis.

However, recently, using a murine model for river blindness in which soluble extracts of filarial nematodes were injected into the corneal stroma, the predominant inflammatory response in the cornea was demonstrated to be because of a species of endosymbiotic Wolbachia bacteria.67 In addition, the inflammatory response induced by these bacteria was dependent on expression of functional TLR4 on host cells, providing further evidence of how the innate immune system plays a crucial role in the initial inflammatory response to onchocercal infection.

Summary

The ocular surface has many elements to help defend against microbial pathogens. Innate and acquired immune defenses can be both intensely involved in microbial keratitis. However, the elimination of replicating organisms in the early stages of infection is the most desirable, as the least amount of scarring and the greatest amount of vision results. Prolonged infections that ultimately activate the acquired immune system, in general, have the poorest prognosis.