Ординатура / Офтальмология / Английские материалы / Eye, Retina, and Visual System of the Mouse_Chalupa, Williams_2008

particularly those requiring a genetically altered host or specific immunologic reagents. It was reported that different strains of mice had disparate cytokine responses correlated with disease severity (Hume et al., 2005). Aged mice also were more susceptible to disease than young mice (Girgis et al., 2004), and bacterial toxins, especially α-toxin, could mediate corneal disease in mice (Girgis et al., 2005). Differences in severity of the keratitis in aged versus young mice correlated with their susceptibility to α-toxin. In the mouse model of S. aureus-induced endophthalmitis (a serious and potentially blinding ocular infection usually associated with recent intraocular surgery), it was shown that Fas ligand, but not complement, is critical for control of the disease (Engelbert and Gilmore, 2005).

The mouse has also been used as a model to study conjunctival infection. Immune responses in mice after conjunctival exposure to Chlamydia trachomatis have been examined (Barsoum et al., 1988). Chlamydia was consistently isolated from the conjunctiva and draining lymph node at 1 and 7 days after conjunctival exposure, but by about 5 weeks, blastogenic responsiveness was very low, perhaps reflecting a state of immunosuppression (Barsoum et al., 1988).

The introduction of Listeria monocytogenes into the anterior chamber of mice was used to test whether immune privilege (anterior chamber–associated immune deviation, ACAID) of the anterior chamber, resulting in reduced DTH, extended to this bacterial pathogen (Li and Niederkorn, 1997). The results showed that immune privilege is not extended to all foreign antigens that enter the anterior chamber, and as a result, some intraocular antigens can provoke a strong systemic DTH response.

Mouse eye infection models have also been used to study the role of Toll receptors in disease. The Toll family of receptors (TLR), conserved throughout evolution from flies to humans, is central in initiating innate immune responses. This family of receptors, composed of transmembrane molecules, links the extracellular compartment where contact and recognition of microbial pathogens occur, and the intracellular compartment, where signaling cascades leading to cellular responses are initiated. Gene array data showed that the expression of TLRs and related molecules, including CD14, soluble IL-1ra, TLR6, and IL-18R-accessory protein, were significantly elevated in susceptible (C57BL/6) versus resistant (BALB/c) mice following challenge with P. aeruginosa (Huang and Hazlett, 2003). In a sterile keratitis model (Khatri et al., 2002), when C3H/HeJ (TLR4 point mutation) and control mice were treated with lipopolysaccharide from P. aeruginosa, a significant increase in stromal thickness and haze was seen in the cornea of control mice but not in TLR4 mutant mice, and the severity of the disease coincided with PMN stromal infiltration. Another study showed that the corneal epithelium has functional TLR2 and TLR9, and that TLR2, TLR4, and TLR9 signal

through myeloid differentiation factor 88 (MyD88) (Johnson et al., 2005). Recent evidence showed that a single Ig IL-1R related molecule (SIGIRR) is differentially expressed in BALB/c (resistant) compared to B6 (susceptible) mice and that this Toll receptor is critical in resistance to P. aeruginosa infection in BALB/c animals, functioning to downregulate type 1 immunity and negatively regulating IL-1 and TLR4 signaling (Huang et al., 2006). siRNA treatment to knockdown TLR9 was also found to influence bacterial keratitis, leading to reduced inflammation but also decreased bacterial killing (Huang et al., 2005). Modulation of bacterial factors has also been shown to reduce ocular virulence (Zolfaghar et al., 2006; Parks and Hobden, 2005).

Mouse models of fungal infection

Compared with bacterial infections, there are relatively few studies on the pathogenesis of oculomycoses, and a complete understanding of appropriate therapy is lacking. A murine model of keratomycosis has been established, despite mice being innately resistant to the fungal pathogen, Candida albicans. Moderate to severe keratomycosis was established using immunocompromised mice and a route of corneal scarification (Wu et al., 2003). A mouse model of corneal fusariosis by infection with the fungal pathogen Fusarium solani has also been established and permits evaluation of fungal infection and pathogenesis. Immunosuppression and surface scarification were also needed for this model to establish infection (Wu et al., 2004).

Mouse models of parasitic infection

Parasitic infections of the eye are a leading cause of blindness in many parts of the world. Inflammation of the corneal stroma is a serious complication of infection with the nematode parasite, Onchocerca volvulus (Pearlmann et al., 1996), a cause of river blindness in humans. An essential role for PMNs and eosinophils, as well antibody, has been shown for their recruitment (Pearlman et al., 1998; Hall et al., 1999). Further studies showed that disease is exacerbated in BALB/c IL-4 knockout mice and that both IL-4 and Th2 cells were required for disease development (Pearlman et al., 1995). Use of this mouse model of river blindness in which soluble extracts of filarial nematodes were injected into the corneal stroma also made it possible to demonstrate that the predominant inflammatory response in the cornea was due to a species of endosymbiotic Wolbachia bacteria and that the inflammatory response induced was dependent on Tolllike 4 receptors on host cells (Saint Andre et al., 2002), as well as signaling through MyD88 (Gillette-Ferguson et al., 2006).

Critical to infection with another parasite genus, Acanthamoeba, small, free-living protozoa that are widely distrib-

uted in nature, is the ability of the parasite to bind to the corneal surface. Epidemiological studies have shown that contact lens wear is the most significant risk factor, while trauma is the second most significant factor. Poor lens hygiene is frequently encountered in association with tap water rinsing of the lens (O’Brien and Hazlett, 1996). Eleven host species of Acanthamoeba were tested, and parasites failed to bind to or damage intact epithelium in the mouse cornea. Results indicate that A. castellani, the species tested, exercises rigid host specificity at the host cell surface (Niederkorn et al., 1992). Intracorneal inoculation of nude, athymic (lacking T cells) mice with A. hatchetti, however, was capable of inducing keratitis (Paniagua-Crespo et al., 1989).

Mouse models of retinal infection

Herpes simplex may be a cause of viral retinitis or encephalitis in immunologically competent and incompetent adults (O’Brien and Hazlett, 1996). The full-thickness necrotizing retinitis presents as diffuse retinal opacification and edema with perivascular inflammation. In AIDS patients, selected rampant cases of cytomegalovirus retinopathy and encephalitis have also been documented to be coinfected with HSV (O’Brien and Hazlett, 1996). Herpetic retinitis in humans is characterized by a high frequency of bilateral localization. The mouse has been used to examine HSV-1 retinitis after anterior chamber inoculation. Modulation of disease by CD4+ T cells was detected in the uninoculated eye (Azumi et al., 1994); natural killer cells prevented direct anteriorposterior spread of virus (Tanigawa et al., 2000), and spread of HSV-2 to the suprachiasmatic nuclei and retina was detected in T cell–depleted mice (Matsubara and Atherton, 1997). The observation that in the inoculated eye, T cells arrive in the sensory retina at the onset of retinal necrosis and not during acute retinitis and the peak of viral replication, suggests that T cells play a role in the development of retinal necrosis; they may also have a role in resolution of the disease, as they were observed as late as 63 days post infection in the uninoculated eye (Azumi and Atherton, 1998). Injection of HSV-1 into the vitreous of BALB/c mice was also used to examine the occurrence of contralateral virus spread, and the data suggested that spread was mediated by local (nonsynaptic) transfer in the optic chiasm from infected to uninjected axons in the optic nerves (Labetoulle et al., 2000). Nectin-1, a member of the immune globulin superfamily, has been shown to be a receptor for HSV, and this molecule was found widely expressed in murine ocular tissues (Valyi-Nagy et al., 2004).

Murine cytomegalovirus infections (MCMV) (AIDSrelated in human retinitis) were examined in the mouse, and the virus was found to spread to and replicate in the retina after LPS-induced disruption of the blood-retinal barrier in immunosuppressed mice (Zhang et al., 2005a). Apoptosis in

the retina was detected in uninfected retinal cells in this model (Bigger et al., 2000). The mouse has served to allow detection of ocular reactivation of MCMV after immunosuppression of latently infected mice (Zhang et al., 2005b). Therapy for MCMV used a SCID mouse model, and the efficacy of therapy with cidofovir was successfully monitored with electroretinography (Garneau et al., 2003). MCMV retinitis in the mouse was examined in retrovirus-induced immunodeficiency (MAIDS) in mice, and loss of the perforin cytotoxic pathway appeared to predispose mice to retinitis (Dix et al., 2003); susceptibility also correlated with intraocular levels of TNF-α and IFN-γ (Dix and Cousins, 2004).

Murine coronavirus, mouse hepatitis virus (MHV), JHM strain, induces disease in mice, and a genetic predisposition to disease induction of the retina has been shown (Wang et al., 1996). Intraocular coronavirus injection results in a biphasic retinal disease in susceptible BALB/c mice characterized by acute inflammation and retinal degeneration associated with autoimmune reactivity. Resistant CD-1 mice develop only the early phase of the disease (Vinores et al., 2001). Retinal apoptosis may be one of the host mechanisms contributing to limiting this retinal infection (Wang et al., 2000).

Toxoplasma gondii, a protozoan parasite, can cause severe, life-threatening disease, especially in newborns and immunosuppressed (e.g., HIV-infected) patients. The active phase of retinal disease is self-limited, but it can recur, and the destructive nature of the infection places patients at risk for blindness if tissues critical for vision, such as the optic nerve or macula, are involved (O’Brien and Hazlett, 1996). Toxoplasmosis, caused by an obligate intracellular parasite, is established in mice by either an intravitreal or an instillation route of infection using T. gondii Me-49 strain. The route of inoculation influences the inflammatory pattern and the instillation route was preferred, as it avoids extensive needle damage in a small animal (Tedesco et al., 2005). The effects of sulfamethoxazole treatment have been assessed using other toxoplasmosis strains in IFN-γ knockout mice. This model allowed the quantitative assessment of treatment effects (Norose et al., 2006).

Summary

The mouse provides a valuable animal model for infectious disease studies in the eye. The ability to manipulate genetically based experimental systems because of the availability of numerous inbred strains, the wealth of gene knockout and transgenic animals, and the easy availability of immunological and molecular reagents are only a few of the reasons that this is the case.

acknowledgments Work was supported by NIH grant nos. R01EY02986, R01EY016058, and P30EY04068 and by a grant from Ciba-Vision.

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Uveitis, or inflammation of the uvea, is a generic term that denotes ocular inflammation. The condition may be of infectious or noninfectious origin. Infectious uveitis is discussed in chapter 41. Noninfectious uveitis, the subject of this chapter, was initially referred to as idiopathic but is now known to be of autoimmune or immune system–mediated etiology in many cases. Several human diseases typically characterized by detectable responses to protein antigens found in the retina and uvea fall in this category (Body et al., 2001; Nussenblatt and Whitcup, 2004). Some of these diseases are part of a generalized systemic syndrome in which the eye is only one of the organs involved. Examples of such diseases are anterior uveitis associated with juvenile rheumatoid arthritis or with ankylosing spondylitis (HLA-B27 related), and uveitides involving the posterior pole, such as Behçet’s disease, Vogt- Koyanagi-Harada disease, and ocular sarcoidosis. Other types of uveitis target primarily the eye, with no signs of disease observed in other organs. Examples are sympathetic ophthalmia and birdshot retinochoroidopathy.

Traditionally, the rat (particularly the Lewis strain) served as a model for these diseases, but in the past two decades, mouse models of anterior and posterior uveitis have been developed and have given a tremendous push to basic studies of disease mechanisms. Models of uveitis can be divided into autoimmune, where a particular self antigen is known to be the target of an immunological attack, as opposed to immunemediated, where inflammation is not driven by a response to a particular antigen. Autoimmune models can be further subdivided into induced models, with disease being precipitated by immunization with a retinal or uveal protein, and “spontaneous,” which occur in genetically or surgically altered animals that have not, however, been actively immunized. A graphical representation of mechanistic relationships in rodent models of uveitis is shown in figure 42.1. The various uveitis models are reviewed by Caspi (2006a). This chapter concentrates on mouse models of experimental autoimmune uveitis (uveoretinitis) (EAU), induced and spontaneous, and also mentions endotoxin-induced uveitis (EIU), an important nonautoimmune model of anterior segment inflammation.

Although there are differences in the structure and composition of the retina between human and mouse, in the case

of immune-mediated uveitis, anatomical interspecies differences are thought to be secondary. Of primary importance is the relationship between the eye and the immune system, which is believed to be similar in both species. This permits extrapolation from mouse models to the human in terms of immunological mechanisms involved in the pathogenesis and resolution of ocular autoimmune and inflammatory disease. The following discussion addresses the rapidly expanding uveitis research exploiting mouse models, its contribution to the understanding of fundamental mechanisms driving the disease, and the translational potential of using mouse models in terms of approaches to therapy.

Endotoxin-induced uveitis:

Nonautoimmune anterior uveitis

Endotoxin-induced uveitis (EIU) is a model of anterior uveitis induced by peripheral injection of bacterial endotoxin (lipopolysaccharide, LPS), with primary involvement of the innate immune response elements, such as resident ocular macrophages that secrete cytokines and chemokines (Smith et al., 1998). EIU, initially developed in rats (Rosenbaum et al., 1980), was first reported in endotoxin-sensitive C3H/HeN mice by Kogiso et al. in 1992, and subsequently C3H/SW and FVB/N were also found to be susceptible (Li et al., 1995). EIU typically develops and peaks within 24 hours of a peripheral injection of endotoxin. A second peak of disease has been described 5 days after the first (Kozhich et al., 2000).

Although the EIU model is very robust, its limitation is that it may be “a model in search of a disease.” Namely, there is some debate over which human disease it should represent. There may be parallels to anterior uveitis without systemic disease. It has also been used to represent anterior uveitis associated with rheumatic disease; however, LPS uveitis is of short duration and self-limiting, unlike anterior uveitis associated with rheumatic syndromes. A human equivalent to EIU that is not usually considered may be what is known as toxic anterior chamber syndrome, a sterile inflammation seen occasionally after intraocular surgery (e.g., cataract extraction) that is usually transient and is

Figure 42.1 Inflammatory and autoimmune animal models of uveitis. Except as noted, all models are induced (endotoxin or immunization with the specified antigen). , disease inducible in Lewis rats; , disease inducible in both mice and rats. BSA, bovine serum albumin; EAAU, experimental autoimmune anterior

Figure 42.2 Typical histology of EIU and EAU in the mouse. Healthy compared to diseased eye tissue in EIU (A and B) and in EAU (C and D). A, Healthy anterior chamber. Shown is the angle where the iris and ciliary body connect to the sclera. C, cornea; CB, ciliary body; I, iris; S, sclera. B, EIU in a C3H mouse induced by subcutaneous injection of LPS. Note infiltration of inflammatory cells in the angle and around the iris and ciliary processes. Original magnification ×1200. C, Healthy mouse retina.

attributed to improper composition of surgical solutions (nonphysiologic or containing an irritant) or inadequate sterilization (endotoxin contamination) of solutions and instruments (Mamalis et al., 2006; Holland et al., 2007). It seems that a variety of noxious stimuli, not just LPS, can precipitate this syndrome. The release of pro-inflammatory cytokines could conceivably be triggered by a variety of noxious stimuli that cause tissue damage and constitute endogenous “danger”

uveitis; EAPU, experimental autoimmune posterior uveitis; EIU, endotoxin-induced uveitis; EMIU, experimental melanin-protein induced uveitis; RAU, recurrent anterior uveitis; VKH, Vogt- Koyanagi-Harada syndrome.

C, choroid; G, ganglion cell layer; P, photoreceptor cell layer; R, retinal pigment epithelium; S, sclera; V, vitreous. D, EAU in the B10.RIII mouse induced by immunization with IRBP. Note structural disorganization and loss of nuclei in the ganglion and photoreceptor cell layers, retinal folds, subretinal exudate, vasculitis, damage to the retinal pigment epithelium, and choroid inflammation. Original magnification ×1000. See color plate 39. (Photomicrographs courtesy of Dr. Chi-Chao Chan, NEI, NIH.)

signals. In this scenario, endotoxin would be a model of a spectrum of causes having in common the triggering of local cytokine and chemokine release from innate immune and resident ocular cells. Irrespective of its exact human parallel, EIU continues to provide a useful model for the study of mechanisms of inflammation and inflammatory cell recruitment into the eye. Typical histological appearance of EIU in a C3H mouse can be seen in figure 42.2B.

A limitation to using the mouse as a model of anterior uveitis is that no autoimmune model of anterior uveitis in mice has yet been developed. Experimental autoimmune anterior uveitis (EAAU) or experimental melanin-induced uveitis (EMIU), two closely related models of anterior uveitis induced by melanin-associated antigens, so far are inducible only in rats. EAAU/EMIU is felt to be a more relevant model of human anterior uveitis than EIU because of its autoimmune etiology and recurrent nature (Smith et al., 1998). A robust model of mouse anterior uveitis on an autoimmune basis is needed.

Experimental autoimmune uveoretinitis and related models

EAU is autoimmune in nature and is thought to be a reasonably accurate representation of those (mainly posterior) uveitic diseases in which patients exhibit responses to retinal or uveal antigens. Although no single animal model can reproduce the complexity of human uveitis, there are now a number of EAU variants that may be useful for studying distinct aspects of uveitis or that provide unique insights into basic mechanisms of pathogenesis. The target antigen in EAU is usually a retinal antigen; therefore, it manifests as a uveoretinitis. However, the terms uveitis and uveoretinitis are used here interchangeably.

Experimental Autoimmune Uveoretinitis Induced by Active Immunization with Retinal Antigen Most patients who respond to retinal antigen, respond to a limited number of fragments (epitopes) of the retinal soluble antigen (S-Ag, retinal arrestin), although in advanced disease, additional specificities may be recognized (de Smet et al., 2001). This may suggest that S-Ag has a primary involvement and that responses to other antigens are secondary, resulting from autoimmunization to tissue breakdown products. EAU can be elicited in rats by immunization with S-Ag injected in emulsion with complete Freund’s adjuvant (CFA) and has served for many years as a model of human uveitis. Unfortunately, and not for lack of trying by many laboratories, immunization with S-Ag has failed to reliably induce EAU in the commonly available strains of laboratory mice. A compounding difficulty was that many strains of laboratory mice carry the rd gene and have no photoreceptor cells by weaning time, eliminating them from evaluation. Among these are strains such as SJL/J and PL/J, which are susceptible to related autoimmune diseases, such as experimental autoimmune encephalomyelitis (EAE). In 1987 it was found that another retinal antigen, interphotoreceptor retinoid-binding protein (IRBP), is uveitogenic in rats (Gery et al., 1986), and subsequently we found that it also induces uveitis in several strains of mice (Caspi et al., 1988, 1990). As in rats, EAU in mice is induced by active immunization with IRBP in CFA, and a coinjection of

pertussis toxin (PT) as additional adjuvant is usually needed. An exception is the B10.RIII strain (Jax strain B10.RIII-H2r H2-T18b/(71NS)SnJ; stock no. 000457), the most susceptible mouse strain currently known, which develops severe EAU after immunization with IRBP in CFA without the need for PT (Silver et al., 1999). Antibodies are produced as a result of immunization and have an amplifying role (Pennesi et al., 2003). Typical pathology of EAU in a B10.RIII mouse can be seen in figure 42.2D.

Because it is induced by immunization with a soluble protein, the EAU model as described earlier is MHC class II restricted; that is, the antigenic fragments of IRBP are presented by class II (mouse I-A and/or I-E) molecules and recognized by CD4+ T lymphocytes. The uveitogenic fragments of IRBP that induce disease in mice of the H-2r, H-2b, and H-2a haplotypes have been identified, and disease can be induced by immunization with the synthetic peptides or by transfer of cells from donors immunized with the peptides, obviating the need to purify IRBP from natural sources or to produce it recombinantly. Table 42.1 summarizes the uveitogenic fragments of IRBP defined to date.

Experimental Autoimmune Uveoretinitis Induced by Adoptive Transfer of Antigen-Specific T Cells EAU is a cell-mediated response with central involvement of T lymphocytes with specificity for the immunizing retinal antigen. The full-blown disease can be induced by transferring T cells from donors immunized for induction of uveitis to genetically compatible recipients (Tarrant et al., 1998). Antigen-specific T cells can be propagated in vitro as longterm antigen-specific T cell lines (Silver et al., 1995). The adoptively transferred model is useful to represent the efferent (effector) phase of disease without any confounding effects of a continuing induction (priming) phase or effects of CFA.

Experimental Autoimmune Uveoretinitis in Mice Expressing a Transgenic Target Antigen in the Retina

A variation on the theme of traditional EAU is the use of transgenic mice that are genetically engineered to express a foreign protein under control of a retina-specific promoter. Mice have been created that express β-galactosidase (β-gal) under the arrestin promoter, or that express hen egg lysozyme (HEL) under the rhodopsin promoter (Gregerson et al., 1999; Ham et al., 2004). This neo-self antigen can serve as an antigenic target for uveitis induced by active immunization or by adoptive transfer of the specific T cells, similarly to the native target IRBP. The advantages of such neo-antigen- driven EAU models are that they can permit more sophisticated approaches and questions that cannot be addressed in the more traditional EAU models, for example, by combining use of such mice with mice expressing a transgenic T cell receptor (TCR) specific for the retinal

neo-self antigen. A disadvantage of the use of neo-self antigen transgenic models is that expression of transgenic antigens is subject to integration effects, potentially resulting in unexpected expression patterns and artifacts. Transgenic mice expressing IRBP-specific TCRs are under development (Caspi et al., 2008).

Experimental Autoimmune Uveoretinitis Induced by Injection of Antigen-Pulsed Dendritic Cells An alternative model of IRBP-EAU that has recently been developed is elicited by adoptive transfer of in vitro matured dendritic cells (DCs) pulsed with the uveitogenic 161–180 peptide of IRBP (Tang et al., 2007). DCs have been identified as the most important antigen-presenting cells and are believed to be the only antigen-presenting cells that prime naive T cells that respond to their antigen for the first time. Understanding the requirements to prime a T cell for effector function versus regulatory function is of paramount importance. It is of interest that the disease induced by matured and antigenpulsed DCs has a different clinical course and appearance and is also associated with a different type of effector response. The limitation of this model is the requirement for large numbers of DCs that have to be administered twice to elicit good disease scores (Tang et al., 2007). Its special advantage is that it provides an alternative model of EAU with unique characteristics and permits direct manipulation of DCs to study requirements for uveitogenic antigen presentation and possibly for tolerogenic presentation to already primed cells.

“Humanized” Experimental Autoimmune Uveoretinitis Model in Human Lymphocyte Antigen Transgenic

Mice Although animal models can teach us about pathogenesis of disease, they cannot serve as templates for dissecting the antigenic specificity of the response, for the simple reason that they recognize different antigens, and different regions in the same antigens. This necessarily follows from the fact that their MHC molecules, which bind and present antigenic fragments to T lymphocytes, are different from those of humans. To address this issue, a number of laboratories have generated genetically engineered

mice that transgenically express human MHC molecules (human lymphocyte antigen, HLA) in lieu of mouse MHC molecules (known as H-2). We examined susceptibility to EAU of single-transgenic mice expressing HLA-DR3, HLADR2, HLA-DQ6, or HLA-DQ8 (Pennesi et al., 2003). The HLA-Tg mice, similarly to wild-type mice, developed EAU after immunization with IRBP. Interestingly, HLA-DR3 Tg mice, and to a lesser extent other HLA transgenic mice, developed EAU with S-Ag, to which the parental wild-type strains are resistant. As mentioned earlier, S-Ag is the retinal antigen to which human patients most often exhibit lymphocyte responses. Interestingly, double-transgenic mice carrying a DR and a DQ molecule, which more closely mimics the situation in humans, exhibited changes in epitope recognition and enhanced susceptibility compared to the single transgenic counterparts (Mattapallil and Caspi, 2008). Thus, the “humanized” EAU model may help to identify the antigenic molecules and their critical regions that might be involved in driving human uveitis, and can help dissect the epistatic effects of HLA antigens on disease susceptibility and epitope recognition.

Spontaneous Experimental Autoimmune Uveoretinitis–

Like Models of Uveitis Several models of spontaneous uveitis have been described. They differ from the induced models by not requiring active immunization or transfer of immune cells to trigger the disease, although they develop only in mice that have been genetically or surgically manipulated.

In humans, a type of uveitis known as birdshot retinochoroidopathy is highly associated with HLA-A29. Relative risks between 50 and 249 have been reported in different ethnic groups in various studies (Nussenblatt and Whitcup, 2004). Szpak et al. (2001) made an HLA-A29 Tg mouse and found that it spontaneously develops a posterior uveitis that closely resembles birdshot retinochoroidopathy. The target antigen has not been identified, but, owing to association with HLA-A29, an MHC class I molecule, it is expected to be an epitope(s) recognized by CD8+ effector T cells.

Two other spontaneous models, although developing in a very different way, probably share an underlying etiology,

namely, spontaneous development of responsiveness to IRBP. (1) Ichikawa and collaborators (1991) reported that nude mice implanted with a neonatal rat thymus, which are known to develop a variety of autoimmune diseases, also develop spontaneous uveitis accompanied by humoral and cellular responses to IRBP. (2) A similar situation exists in mice deficient in the gene encoding the AIRE protein (AutoImmune REgulator). AIRE is a molecule that controls ectopic expression of tissue antigens in the thymus, including several retinal antigens. As they age, AIRE knockout mice develop spontaneous uveitis also associated with responses to IRBP (Anderson et al., 2002; Devoss et al., 2006). The insights into basic mechanisms of uveitic disease gained from these models are discussed later in the chapter.

Uveitis Associated with Antitumor Response to

Melanoma Antigen Gp100 (Si) is an antigen associated with melanin pigment that serves as a vaccination target for immunotherapy of melanoma. The antitumor effector cells induced by the vaccination regimen are CD8+ cytotoxic lymphocytes specific to gp100. Successful eradication of experimental melanoma in mouse models as well as in ongoing clinical trials of human melanoma immunotherapy is typically associated with autoimmune vitiligo (Kawakami and Rosenberg, 1997; Overwijk et al., 2003). It has recently been appreciated that these mice, in addition to generalized vitiligo, also develop a uveitis that can be quite severe (D. Palmer and N. P. Restifo, pers. comm., 2007). Of note, some melanoma patients given cellular immunotherapy targeting

gp100 also were observed to develop uveitis (Robinson et al., 2004). This condition is reminiscent of Vogt-Koyanagi- Harada disease, in which patients develop vitiligo and uveitis and exhibit responses to melanin-related antigens (Boyd et al., 2001).

Basic mechanisms involved in uveitic disease as revealed by mouse models

Over the years, the various EAU models have provided invaluable insights into the fundamental mechanisms involved in acquisition of self tolerance to ocular antigens, its breakdown, the subsequent events that result in pathology, and the processes involved in resolution (figure 42.3). Based on combined evidence from many studies, the following discussion presents a putative scenario of the pathogenesis of uveitis. In a nutshell, retina-specific T cells that escape negative selection in the thymus cannot be efficiently tolerized in the periphery, owing to relative inaccessibility of retinal antigens, thus leaving a circulating pool of potentially uveitogenic T cells. “Natural” thymic-derived T-regulatory cells keep them under control, but upon encountering antigen-specific signals in the context of innate immune stimuli (“danger” signals), they can differentiate to an autoaggressive Th1 or Th17 effector phenotype. When they reach the eye and recognize their specific antigen there, they initiate a cascade of recruitment and amplification that culminates in characteristic EAU. Ocular antigens released from inflamed tissue subsequently induce antigen-specific

Figure 42.3 Critical checkpoints in EAU pathogenesis. Schematic representation of critical events and checkpoints in the pathogenesis of EAU as revealed by studies in mouse models. See

color plate 40. (This figure was previously published in Caspi, 2006. Copyright does not apply.)