Ординатура / Офтальмология / Английские материалы / Ocular Therapeutics Eye on New Discoveries_Yorio, Clark, Wax_2007

●Chronic lid margin inflammation (blepharitis)

●Stem cell insufficiency and non-healing epithelial defects.

The aforementioned conditions can lead to (i) changes in the microbiologic flora of the ocular surface, including an increased bacterial load; or (ii) alterations in host defense mechanisms associated with poor clearance of microbiologic threats. When one or more of these conditions coexist, the risk factors can act synergistically to lead to a setting that is ideal for bacterial colonization of the corneal matrix (Charukamnoetkanok and Pineda, 2005). Common clinical syndromes would be the patient with severe dry eye disease who also has blepharitis with common tear film instability and surface epithelial disease, leading to breakdown of the epithelial barrier, the most fundamental defense mechanisms of the ocular surface. Another common setting would be the soft contact lens wearer who develops surface disease from hypoxia-related epithelial disease associated with prolonged wear periods, and whose risk factors for infection are compounded by poor lens hygiene. Finally, another common setting would be the elderly diabetic patient who develops pseudophakic bullous edema after a complicated cataract surgery, and whose condition is characterized with chronic or recurrent surface breakdown.

Almost regardless of cause, there are several patterns to bacterial keratitis. Notwithstanding variations in host immune status that can have profound influences on the course of the infection, the severity of the disease is often determined by two factors:

(i) the pathogen; and (ii) the location of the corneal involvement. In general, gramnegative infections fair far worse than grampositive infections – Pseudomonas aeruginosa and Staphylococcus epidermidis being prime examples of these two types of infection, respectively.

The term “corneal ulcer” has gained widespread use to denote microbial keratitis

but is an imprecise term since its focus is on epithelial loss and stromal thinning (“ulceration”). A more precise term that captures the process of microbial keratitis, in particular due to bacteria, is “stromal infiltrate”. This is the case since colonization of the corneal matrix (stroma) is the first clinically evident phase of the infectious process, regardless of the state of the epithelium or degree of stromal thinning (which often occur later). In particular for less “aggressive” pathogens, the epithelium may be largely intact, or demonstrate minimal fluorescein staining, and there may be no frank thinning/ulceration of the stroma, in particular in the early phases of the infection.

The course of the infection can vary widely. In most cases, peripheral involvement of the cornea is far less serious than central involvement for several reasons: first, peripheral infections, even when serious, may lead to stromal thinning, but often have limited effect on visual acuity since the central cornea remains clear. The only exception would be when deformities are significant enough that they lead to corneal irregular astigmatism, but this is not very common. Second, peripheral infections and inflammatory responses in the cornea are often cleared faster than central infections due to the proximity of the peripheral cornea to the vascularized limbus, which facilitates delivery of immune effectors (e.g. macrophages) that are critical for host defense (the corollary of this is that in autoimmune diseases, proximity to the vasculature can bring the threat of progressive immune destruction).

Classic examples of the two ends of this “severity spectrum” include the patient with “marginal keratitis” and the patient with central suppurative gram-negative infection, with central and paracentral cases of gram-positive infection having an intermediate prognosis. The typical presentation of the patient with marginal keratitis is corneal involvement where the infiltrates are small, round to oval, and within 2–3mm of the limbus. A “clear zone” between the infiltrate and the limbus is characteristic. The vast

majority of these infiltrates are termed “sterile” since culturing results are often negative, leading to the assumption that they are due to an immune response to microbial antigen. At the other end of the spectrum is the patient who presents with a suppurative or “exudative” infection of the central cornea due to Pseudomonas aeruginosa. Often, these cases present with significant corneal thinning due to rapid enzymatic degradation of the stroma (see below) in severe gramnegative infection. These infections represent true ophthalmic emergencies since late or suboptimal treatment can rapidly proceed to perforation.

2. Pathogenic mechanisms

The pathogenesis of ocular infections with bacteria such as Pseudomonas aeruginosa is related to the bacteria’s capacity to adhere to the ocular surface, invade the corneal epithelium, and elaborate proteases and toxins. The host immune response also contributes to the pathogenesis of P. aeruginosa infections of the cornea. The pili on the surface of P. aeruginosa facilitate bacterial binding to glycoproteins expressed on the corneal epithelium (Rudner et al., 1992). P. aeruginosa also expresses many virulence factors that contribute to the pathogenesis of corneal infections including alkaline protease, elastase, exoenzyme S, exotoxin A, endotoxin, polysaccharides, phospholipase C, and leukocidin (O’Brien, 2003).

It is becoming increasingly clear that the immune response plays a major role in the pathogenesis of corneal infections with P. aeruginosa (Hazlett, 2004). A large body of data from Hazlett’s laboratory has shown that elements of both the innate and adaptive immune responses contribute to the pathogenesis of bacterial keratitis (Hazlett, 2004). In mice, corneal infections with P. aeruginosa can follow one of two courses. The first pathway is a progressive, ulcerating disease that culminates in perforation of the cornea. By contrast, the second pathway of corneal infection leaves the corneal architecture intact. The destructive pathway occurs

in C57BL/6 mice and is characterized by a T-cell immune response that involves the production of interferon-γ (IFN-γ) and the persistence of a PMN infiltrate in the affected cornea. The resolving, non-destructive form of Pseudomonas keratitis occurs in BALB/c mice and is characterized by a T- cell immune response that promotes corneal infiltration by PMN, but also involves a prompt resolution of corneal inflammation.

The final outcome of experimental corneal Pseudomonas infections is determined by the host’s capacity to generate a measured immune response that rids the eye of the invading bacteria, while inflicting minimal collateral damage to the cornea. The rapid recruitment of PMN into the infected cornea is crucial for eliminating the invading bacteria. Infiltration of PMN into the cornea is largely dictated by the nature of the cytokines produced during the early stages of the corneal infection. In the mouse two chemokines, MIP-2 and KC, stand out as the key chemoattractants and activators of PMN (Driscoll, 1994). IL-1 also plays a critical role in the pathogenesis of corneal Pseudomonas infections through its capacity to prolong PMN infiltration (Rudner et al., 2000). However, the persistence of PMN in the cornea is deleterious and contributes to corneal perforation. PMN produce a variety of proteases and reactive oxygen species that are important for eliminating bacteria, but also inflict extensive damage to innocent bystander cells in the cornea.

The susceptibility to corneal infections with P. aeruginosa is also influenced by the adaptive immune response. When confronted with antigens, the CD4 T-cells of the adaptive immune system make a decision to differentiate into either CD4 T-helper-1 (Th1) cells that preferentially produce IFN- γ or to become CD4 Th2 cells that produce a battery of cytokines including IL-4, IL-5, and IL-13. Some mouse strains, such as C57BL/6, have a predilection to produce Th1 immune responses, while other mouse strains, such as BALB/c, preferentially mount Th2 immune responses. In the case

of the C57BL/6 mouse, corneal infections with Pseudomonas elicit an IL-12-driven Th1 immune response, which is characterized by the persistence of PMN in the cornea and perforation of the cornea. By contrast, BALB/c mice develop a Th2 immune response to Pseudomonas infections, which leads to bacterial clearance and restoration of the corneal integrity. Severity of disease in C57BL/6 mice (“non-healing, perforating” phenotype) can be dramatically mitigated by administering antibodies that either eliminate CD4 T-cells or neutralize the Th1 cytokine, IFN-γ, prior to corneal infection.

Susceptible C57BL/6 mice and resistant BALB/c mice also differ in the nature of the chemokines that are produced when their corneas are infected with P. aeruginosa. The chemokine MIP-1α seems to be particularly important. MIP-1α induces the directional migration (chemotaxis) of various immune and inflammatory cells, including T-cells. Pseudomonas-infected corneas of susceptible C57BL/6 mice produce greater amounts of MIP-1α than corneas of resistant BALB/c mice. Increasing the levels of MIP-1α in the cornea by the intrastromal injection of recombinant MIP-1α protein results in an increased corneal infiltration by CD4 T-cells and converts resistant BALB/c mice into a susceptible (corneal perforation) phenotype (Kernacki et al., 2001)

The immune response to bacterial infections of the ocular surface is yet one more example of the aforementioned “dangerous compromise” negotiated between the immune apparatus and the eye. If the immune response is too intense or prolonged, it can result in perforation of the cornea, as occurs in Pseudomonas infections of the corneas in Th1-prone C57BL/6 mice. A satisfactory compromise is struck in the BALB/c mouse, which mounts an adaptive immune response that swiftly recruits innate immune cells (PMN), but does not promote their persistence once the bacteria are eliminated. In a sense, the clinician’s prudent use of corticosteroids and antibiotics mimics the ocular immune compromise,

which balances anti-inflammatory and anti-bacterial modalities.

3. Current and future therapy

Precise measurement of the location, size, and depth of the infection and degree of stromal inflammation is a requirement and first step in evaluation of the patient with bacterial keratitis. There is considerable debate about the need for culturing patients who present with microbial keratitis, leading to a lack of clear guidelines for which there is consensus. In general, two principles should be emphasized: first, there is little (medical) risk to culturing; indeed, the debate about whether to culture or not has focused more on economic cost/benefit ratios, rather than minimizing risk to the patient. Second, most authorities agree that cases presenting with a small peripheral/ marginal infiltrate can be treated empirically. Conversely, cases with larger central infiltrates are best cultured, since speciation of the offending organism and determination of in vitro antibiotic sensitivities can help guide therapy. This is particularly the case, given the rising incidence of antibioticresistant strains.

For small marginal infiltrates, the typical therapeutic regimen involves a brief course of antibiotics followed by a brief course of corticosteroids to help resolution of the inflammation and promote patient comfort. Alternatively, some clinicians elect to employacombinedantibiotic–corticosteroid regimen from the outset to optimize rapid clearance of the infiltrate. For severe cases, early empiric treatment typically begins with fortified antibiotics specially formulated in the pharmacy, or with broadspectrum fluoroquinolones, every 30–60 minutes round the clock for the first 1–3 days, and then tapered based on response to therapy (Alfonso and Crider, 2005; Charukamnoetkanok and Pineda, 2005; Robertson et al., 2005). Once control over spread of the infection occurs, topical corticosteroids may be added very judiciously to help

resolve the inflammatory response and minimize scarring. The risks of added corticosteroid therapy (delay in collagen secretion, suppression of the immune response, elevation in pressure, etc.) should be weighed carefully against their potential advantages. Even with timely provision of optimal antibiotic therapy, some cases of bacterial keratitis progress, leading to perforation or near-perforation.Applicationofcornealglue with bandage contact lens, corneal transplantation, or even keratoprosthesis surgery are occasionally end results of these severe infections.

Future approaches to the management of bacterial keratitis will likely focus on several distinct areas: first, breakdown of the stromal matrix is an important facet in the pathogenesis of corneal scarring, and the role of matrix metalloproteinases (MMPs) in this process is critical. There is increasing interest in inhibition of these enzymes so as to promote less corneal scarring and neovascularization (Brooks and Ollivier, 2004). Second, there is increasing appreciation that there is wide variability in pathogen-surface epithelial interactions, with some strains being more capable than others in invading and/or killing target cells (Lee et al., 2003; Fleiszig, 2006). This may translate into better diagnostic techniques that can determine more precisely the course of infection for a specific strain and the optimal strategies to inhibit bacterial invasion of the cornea. Third, the past several decades have witnesseddramaticimprovementsintheefficacy and coverage of antibiotics, just enough to stay “ahead” of the increasing threats posed by antibiotic-resistant strains. It is therefore very likely that we will see a new generation of fluoroquinolones and other antibiotic classes to help control ocular infections. Fourth, there is increasing appreciation for the pathogenic role of bacterial toxins for mediating the tissue damage seen clinically (Gilmore and Ferretti, 2003). Currently available antibiotics focus on killing, or arresting, the growth of pathogenic bacteria rather than neutralizing the effect of their toxins. It

is very likely that we will witness significant inroads in this area in the coming decade, with likely applications to ocular infection. Lastly, as outlined above in the “Basic mechanisms section”, both innate and adaptive immunity can not only play a critical role in control of infection, but can also participate in the tissue-destructive processes. Translation of observations made in the rodent models of bacterial keratitis, such as targeted antagonism of specific cytokines and chemokines (e.g. IFN-γ and MIP-1α), may one day be applicable to management of clinical bacterial keratitis.

E. Viral Keratitis

1. Clinical disease

Viral keratitis is second only to bacterial disease as the most common cause of corneal microbiologic disease in the United States and other industrialized countries. Since several hundred thousand patients are affected by it annually, its economic consequences in the context of eye disease is significant (Green and Pavan-Langston, 2006). Since by far the most common cause of viral keratitis is due to Type I herpes simplex, we will limit our discussion here to herpes simplex keratitis (HSK). Other causes of viral keratitis are also primarily due to herpes family viruses, including varicella (herpes zoster), cytomegalovirus (CMV), and Epstein–Barr virus (EBV).

Five general syndromes of HSV infection of the eye are recognized: (i) corneal epithelial disease; (ii) corneal stromal necrotizing disease; (iii) corneal stromal non-necrotizing (“disciform”) disease; (iv) HSV keratouveitis and uveitis; and (v) HSV. In this section, we will focus only on the (three) syndromes that affect primarily the cornea.

2. Corneal epithelial disease

HSV epithelial disease is due to replicating virus in the corneal epithelium, with the pathology due primarily to the cytopathic effect of the virus on the epithelial cell

(Kaye and Choudhary, 2006). The classic “dendritic” lesion of HSV keratitis affecting the epithelium is due to the effect the virus has had on epithelial cells, which classically occurs in a dendritic or branching fashion. Since the cytopathic effect of the virus on infected cells is considerable, areas of the epithelium thus affected lead typically to a frank epithelial defect best visualized with vital dye (fluorescein) staining. While the disease is limited to the epithelium, the underlying anterior stroma is often affected, such that especially with recurrent disease (which occurs in close to one-third of patients with epithelial disease), the stroma can get hazy or scarred. Since herpes viruses are neurotrophic, corneal sensation can be severely affected. The significant and acute diminishment of corneal sensation is a valuable diagnostic clue to HSV keratitis.

3. Corneal stromal disease

HSV can affect the corneal stroma in two distinctly different patterns (Kaye and Choudhary, 2006). Herpes necrotizing stromal keratitis (HSK) can affect the cornea with or without concomitant epithelial infection. While live virus has been isolated from some cases of HSK, this condition is thought to be primarily a delayed-type hypersensitivity response mediated by CD4 T-cells (see discussion below in the “Basic mechanisms” section). The condition is called “necrotizing” since the inflammatory response is so robust that it leads to frank necrosis of the cells and high expression of collagenolytic enzymes (proteases) that lead to breakdown of the collagen matrix. As a result, the cornea can quickly become opacified with rapid ingrowth of (typically deep) stromal vessels. Initially, the inflammation and infiltration of the stroma leads to a picture of stromal edema; however, with resolution of the inflammation and maturation of the vascularization, the cornea can thin. HSK is a common cause of “interstitial keratitis” (IK), characterized by variable corneal thinning, scarring, and presence of active or “ghost” blood vessels, so called since the

endothelialized channels remain without any active blood flow. Other causes of IK are protean, including syphilis, mycobacterial disease, and autoimmune conditions.

Non-necrotizing HSK is also characterized by disease at the level of the stroma, but is distinct from stromal necrotizing disease in the following: absence of significant vascularization, absence of significant thinning, and involvement of the endothelium (“endothelitis”). Similar to necrotizing disease, the pathogenesis here is primarily immune-based (rather than active infection), but the disease is focused on the endothelium (Streilein et al., 1997; Kaye and Choudhary, 2006). Small diffuse keratic precipitates are common. When significant, endothelial dysfunction leads to stromal edema in the central areas of the cornea, leading to a picture descriptively labeled “disciform keratitis”. The condition can wax and wane; with near-resolution of the edema over a period of weeks as long as the inflammatory state is contained and there is adequate endothelial reserve to retain corneal stromal deturgescence.

4. Basic mechanisms

HSV keratitis presents as either an epithelial, stromal, or endothelial disease. Since the endothelial disease and resultant corneal edema primarily affects the stroma (“disciform keratitis”; see above discussion), from a pathogenic standpoint, we will discuss it as a facet of “stromal” disease. In general, HSV epithelial disease is milder than stromal disease and is believed to be the result of cytopathic effects produced by the virus. HSV epithelial infections heal within about a week and resolution of the lesions coincides with the elimination of replicating virus from the cornea, which can be expedited considerably by anti-viral therapy, either topically or orally. By contrast, stromal keratitis is more severe, resulting in necrotizing lesions that are characterized by a mixed inflammatory infiltrate, neovascularization, collagen degradation,

stromal melting, corneal scarring, edema, and in some cases blindness. It was originally believed that the pathogenesis of HSV keratitis was simply the consequence of viral replication and the cytopathic effects produced by the virus. However, virus titers decline within 6 to 7 days of corneal infection, while the peak of HSV stromal keratitis is at 2 weeks – a time when HSV replication has dissipated and viral antigens are barely detectable within the cornea. This paradox was resolved in the late 1970s when Metcalf and co-workers observed that HSV infection of T-cell-deficient nude mice did not lead to stromal keratitis (Metcalf et al., 1979). However, stromal keratitis was induced if T-cells from HSV immune animals were adoptively transferred to nude mice that were challenged with corneal infections with HSV. This finding indicated that: (a) HSV infection alone did not produce stromal disease; (b) HSV-specific T-cells were necessary for the development of stromal keratitis; and (c) HSV stromal keratitis is an immune-mediated disease.

Although the seminal findings of Metcalf and co-workers were followed by 25 years of intense research, there is still disagreement as to how T-cells contribute to HSV stromal keratitis. At least three different pathogenic mechanisms for HSV stromal keratitis have been proposed. The first mechanism proposes that HSV infection elicits the generation of virus-specific CD4 Th1 cells that enter the infected cornea where they continue to produce proinflammatory cytokines, even in the absence of HSV antigens. The Th1 cytokine IFN-γ can directly damage corneal cells, as well as recruit second level inflammatory cells such as neutrophils. Although the induction of the CD4 Th1 cell population is HSVspecific, the inflammation and tissue injury are not HSV-specific and persist even though virus replication subsides and HSV antigens are either weakly expressed or absent (Gangappa et al., 1998).

A second proposed mechanism also suggests that HSV-specific CD4 Th1 cells are

responsible for initiating stromal keratitis. However, this model suggests that one of the antigens expressed by HSV-1, called UL6, resembles a protein that is expressed on normal corneal cells. This model of antigen mimicry proposes that the immune response to UL6 on HSV-1 and UL6-like determinants on corneal cells leads to widespread destruction of normal corneal cells as well as HSV-infected corneal cells (Zhao et al., 1998). This mechanism is in keeping with the observation that corneal inflammation persists in the absence of detectable HSV antigens and after viral replication has subsided. Although appealing in its simplicity, this model of molecular mimicry remains contentious. Efforts to reproduce this effect with other strains of HSV have failed to confirm cross-reactivity between the UL6 protein and corneal proteins; also, attempts to demonstrate that UL6-specific CD4 T cells produce HSK in mice have failed (Deshpande et al., 2001).

A third model for explaining the pathogenesis of HSV stromal keratitis states that HSV-specific CD4 Th1 cells play a crucial role in both the induction and progression of HSV keratitis (Hendricks et al., 1992). Three compelling observations lend support to this hypothesis: (a) animal studies have demonstrated the close correlation between the development of DTH responses to HSV antigens and the severity of HSV keratitis (Hendricks et al., 1992); (b) HSVspecific CD4 T-cell clones have been isolated from the corneas of HSV keratitis patients (Verjans et al., 2000); and (c) mice whose CD4 T-cells have been tolerized to HSV-1 antigens, by introducing HSV into the anterior chamber of the eye, are protected from HSV keratitis (Ksander and Hendricks, 1987).

The pathogenesis of HSV keratitis demonstrates that the immune privilege of the eye is not absolute, and reminds us of the penalty that is paid when immune privilege is circumvented. In the case of a potentially life-threatening infection, immune privilege gives way to a robust immune response that

rids the eye of the pathogen, but in so doing jeopardizes the visual axis. The balance between immune privilege and immune reactivity is the dilemma that the ocular immune apparatus must confront on a regular basis. The late J. Wayne Streilein recognized the importance of this condition when he referred to ocular immune privilege as “a dangerous compromise that the eye makes with the immune system” (Streilein, 1987).

5. Current and future therapy

a. Current therapy – An exhaustive review of the various treatment modalities available in the management of HSV keratitis is beyond the scope of this text. The brief discussion here will focus on broad general outlines for the medical management of this condition.

The key step in managing HSV keratitis is determining whether the condition is epithelial (and hence due to active replicating virus infection) or primarily immunemediated stromal disease. The mainstay of active epithelial disease is anti-viral therapy, either topical or oral. Usually, a 1–2 week course of anti-viral therapy can eradicate the active infection. Gentle debridement of the corneal epithelium in the affected areas has also been advocated as a means of ridding the corneal tissue of the viral load; either alone, or in conjunction with antiviral therapy, this can be quite helpful.

Management of stromal disease is focused on suppression of the immune mechanisms that drive the process. Topical corticosteroid therapy is therefore the mainstay of the therapy for both necrotizing and non-necrotizing disease. Since stromal disease may be associated with live virus, either in the stroma or the epithelium, most clinicians opt to maintain patients on concurrent anti-viral therapy.

Several caveats are worth mentioning: first, topical anti-viral therapy is quite toxic to the epithelium, including epithelial stem cells. Meta-herpetic disease refers to

non-healing epithelial conditions seen in association with prolonged anti-viral disease; the best approach therefore is judicious use, and then quick termination, of topical anti-viral therapy for replicating viral surface disease. Second, a significant minority of HSV keratitis patients are at risk of developing raises in intraocular pressure/glaucoma.

Therefore, monitoring of pressure is a critical facet in the management of HSV keratitis patients. Third, the Herpetic Eye Disease Studies (HEDS) showed that an oral maintenance dose of Acyclovir can nearly halve the incidence of recurrent orofacial and ocular HSV disease. Hence, patients at risk of recurrent disease who have no contraindication to systemic antiviral disease may be excellent candidates for this prophylactic therapy (Green and Pavan-Langston, 2006).

b. Future therapy – In the past three decades much has been learned about the microbiology of HSV, including factors that regulate its latency, as well as the immunopathogenesis of HSK. Many potential therapeutic approaches have been attempted using laboratory rodent models, including inhibition of angiogenesis, cell adhesion factors, costimulatory molecules, and T-cell subset (e.g. CD4 cells) depletion, to name a few. From a microbiological perspective, the severity of HSK ultimately depends on both pathogen and host-specific responses. From a pathogen standpoint, the availability of excellent anti-viral agents in the recent past (including Famcyclovir and Valacyclovir) has made the management of these patients, from a systemic standpoint, more effective than ever.

However, it is the management of patients with necrotizing stromal disease that remains very challenging. A multifactorial contribution to disease pathogenesis, coupled with significant overlap between the molecular mechanisms that mediate destructive T-cell-mediated responses

and protective host defense mechanisms, has led to no easy solution. For example, suppression of specific chemokines in rodent models can lead to suppressed corneal inflammation and/or angiogenic responses, but may also lead to poor host defense that can lead to uncontrolled disease with an even worse outcome. The widespread use of immunosuppressive medications in transplant, cancer, and rheumatic disease patients, in addition to experience with HIV/ AIDS patients, has provided ample evidence for how suppression of immunity can indeed be associated with a worse prognosis in HSV disease than in non-immuno compromised patients. Future approaches that can better tease apart the pathogenic from the protective defense mechanisms of HSK will be major contributors toward a safer and more effective management of these patients.

F. Cicatrizing and Autoimmune Diseases

1. Clinical disease and pathogenesis

Cicatrizing and autoimmune disorders of the cornea and ocular surface, though rare, represent a highly heterogeneous group of conditions that can cause significant damage to the eye (Akpek and Klhan-Sarac, 2005). When severe and affecting the epithelium, conjunctival shrinkage and fibrosis in the conjunctival substantia propria (stroma) can ensue, leading to frank cicatricial changes and scarring, as seen commonly in cicatrizing pemphigoid. This section provides a brief overview of several of the more common variants of autoimmune disease – specifically, peripheral ulcerative keratitis (PUK), a serious condition that affects the peripheral cornea, and ocular cicatricial pemphigoid (OCP), the prototype cicatrizing disorder of the conjunctiva. In the context of PUK, we will also discuss Mooren’s ulcer, a progressive, painful, and idiopathic peripheral ulceration of the cornea. Given the significant differences in these conditions, the clinical pictures and pathogenesis

will be discussed in one section for each specific entity.

2. PUK and Mooren’s ulcer

There are many different classification schemes for immune-mediated peripheral disorders of the cornea. For the sake of this overview, we will divide them into idiopathic and secondary conditions, using Mooren’s ulcer as a prototype of the former, and rheumatoid arthritis (RA)-associated PUK as an example of the latter.

Mooren’s ulcer is a typically progressive and painful ulceration of the peripheral corneal stroma and epithelium that can lead to extensive corneal vascularization and scarring. The ulcer starts in the periphery of the cornea and spreads both centripetally and towards the sclera. Evidence suggests that it is an autoimmune disease, although the exact mechanism is unknown. Mooren’s ulcer is of unknown cause but autoantibodies generated to a stromal autoantigen have been identified in sera of affected patients (Gottsch and Liu, 1998; Gottsch et al., 1999). It is thought that perhaps environmental insults, such as infection, may lead to the expression of autoantigens in the cornea. A large number of infiltrating macrophages and CD4 T- lymphocytes are reported in the affected cornea and adjacent conjunctiva (Kafkala et al., 2006). These cells, or the attendant inflammation, lead to significant overexpression of class II major histocompatibility complex (HLA-DR) antigens by epithelial and stromal cells of the cornea (Zegans et al., 1999). MHC class II cells may then act as antigen-presenting cells for stimulation of a T-cell response that can, in due course, lead to a destructive attack on autoantigen-expressing cells, in this case in the cornea. Humoral immunity has also been implicated in Mooren’s ulcer. Patients with the disease have been reported to have circulating IgG antibodies to human corneal and conjunctival epithelium, elevated serum IgA levels, circulating

immune complexes, and antibodies and complement bound to conjunctival epithelium. Consequently, complement activation may lead to neutrophil chemotaxis and releaseofmatrixmetalloproteinases(MMPs) that not only degrade the stroma, but also “expose” antigens that may then lead to induction of more autoimmunity (Martin et al., 1987).

PUK can also be seen in conjunction with a wide variety of systemic rheumatic conditions, including RA, systemic lupus erythematosus, and vasculitis syndromes (e.g. Wegener’s granulomatosis), to name but a few. These are generally immune-complex mediated, as characterized by a crescentshaped inflammatory response that begins first adjacent to the vascularized limbus and then spreads centripetally, associated commonly with an epithelial defect, but with varying degrees of stromal degradation. In RA, the most common form of autoimmune rheumatic disease, IgM antibodies are formed against IgG. These “rheumatoid factors” lead to immune-complex formation and deposition in many tissues, including the joints, blood vessels, and ocular tissues, leading to a classic type III hypersensitivity reaction. These complexes can activate the complement cascade that in turn attracts neutrophils and macrophages. Upregulation in gene expression of pro-inflammatory cytokines TNF-α and IL-6 has been reported in corneal keratocytes surrounding the ulcer (Prada et al., 2003), suggesting that corneal keratocytes are also activated in this condition. These cytokines can cause MMP production that can lead to matrix breakdown. In addition to these enzymes, reactive oxygen species may also be generated by innate immune cells, such as macrophages and neutrophils, that can also activate MMPs (Smith et al., 1999, 2001).

pemphigoid (OCP), linear IgA disease, Stevens–Johnsonsyndrome,perennialatopic keratoconjunctivitis, and graft-versus-host disease seen after bone marrow or hematopoietic stem cell transplantation. The prototype and most common form of cicatrizing conjunctivitis is OCP, however.

OCP is a chronic, bilateral, and progressive inflammatory disease that leads to conjunctival fibrosis. Its hallmarks are autoantibodies, most commonly IgG, that bind to the epithelial basement membrane zone (BMZ). Specific genetic alleles are associated with OCP that presumably determine the “at risk” population (Hsu et al., 2000; Akpek and Klhan-Sarac, 2005). Binding of autoantibody to the BMZ autoantigen can lead to a type II hypersensitivity reaction that involves complement activation and inflammatory cell infiltration. CD4 T-cells, in addition to neutrophils, mast cells, macrophages, and dendritic cells, are also seen in increased numbers in affected conjunctivae (Sacks et al., 1989). A clinical hallmark of OCP is fibrosis, and it is thought that fibrogenic and angiogenic cytokines, such as TGF-β, platelet-derived growth factor, and basic fibroblast growth factor, contribute to the pathogenesis of disease in OPC. It is this “hyper-fibrotic” response that leads to the formation of adhesion bands (symblephara) between the palpebral and bulbar conjunctivae. The conjunctival shrinkage can also lead to cicatrizing changes of the eyelid, with secondary inturning of lashes (trichiasis) that can cause further corneal epithelial disease. Advanced OCP is hence accompanied by blinding keratopathy, corneal neovascularization, and potentially progressive thinning and perforation. As such, OCP is an example of how a severe conjunctival inflammatory condition can cause corneal disease.

3. Cicatrizing conjunctivitis

Similar to the PUK syndrome, cicatrizing conjunctivitis can be caused by a long list of entities, including ocular cicatricial

4. Current and future therapy

a. Current therapy – The most immediate therapy in these conditions is directed at healing the epithelium. Prolonged epithelial

defects lead to more stromal breakdown, angiogenesis, and a high risk of infection. Aggressive suppression of inflammation and support for epithelial healing by treating concomitant lid disease and aberrant trichiatic lashes and/or application of a bandage contact lens can be very helpful. However, since the vasculature is the source of the immune effectors, and these conditions are often the result of systemic immune mechanisms, their treatment almost always requires the use of systemic immune suppression. This should only be undertaken by individuals expert in their use, which includes knowledge of how best to monitor patients on these medications and what important side effects to monitor for. This is often beyond the expertise of the treating ophthalmologist and requires integration of care with another expert, typically a rheumatologist or oncologist expert in the use of immunosuppressive medications. For OCP, treatment typically is started with dapsone or methotrexate for mild to moderate disease. Patients who do not respond to this can be tried on Imuran; however, the treatment for severe and/or highly progressive disease is a combination of systemic prednisone and cyclophosphamide (Cytoxan) (Ahmed et al., 2004; Chang and McCluskey, 2005; Kim and Foster, 2006). The treatment of PUK often depends on the underlying condition. Active PUK in the setting of a known rheumatic disease is often a sign that the systemic condition is under poor control. Hence, for example, an RA patient who develops a case of PUK should get maximal control of the systemic disease; this may require methotrexate or anti-TNF treatment. Alternatively, a patient with Wegener-type ANCA vasculitis who develops PUK often requires cytotoxic therapy for adequate management of the systemic, not just ocular, disease. Mooren’s ulcer is a diagnosis of exclusion in a setting of no known underlying disease. More chronic, and less aggressive, cases can sometimes be managed by systemic steroids and resection of the conjunctiva, so as

to inhibit the delivery of immune effectors to the corneal periphery. However, severe cases often require potent immunosuppressives. Finally, it is important to emphasize that whether due to non-healing epithelial defect, or local and systemic immune suppression, eyes affected with PUK are at high risk of infection. Judicious use of topical anti-microbials is often necessary to mitigate this risk.

b. Future therapy – In a sense, the “future” of systemic immune modulation is already with us with the adoption of “biologic” agents that target specific cytokines (e.g. TNF, IL-1) or membrane receptors (e.g. CD25). These novel agents, though not without their own attendant risks, have truly revolutionized the management of patients with rheumatologic diseases. However, randomized prospective and controlled studies for their use in ocular disease are few, though currently an area of investigation by several centers. Intravenous Ig, thought not a standard of care, has been used by some investigators for autoimmune surface diseases, such as OCP, and may hold promise for management of patients with considerably less risk than some of the systemic immunosuppressives currently in use (Ahmed et al., 2004; Kim and Foster, 2006). Targeting the molecular factors that promote matrix breakdown and/or fibrosis is another area of significant interest. Inhibition of matrix breakdown with free radical scavengers or suppression of protease (MMP) activity are also strategies being investigated.

G. Uveitis

1. Clinical disease

Uveitis, by definition, means inflammation of the uveal tract, which is comprised of the choroid posteriorly, and the iris anteriorly. However, this is largely a histopathological definition since most of the uveal tract cannot be observed directly using