A developmentally regulated microsomal protein specific for the pigment epithelium of the vertebrate retina

In the vertebrate retina, the retinal pigment epithelium (RPE) performs specific functions critical to the normal process of vision. Although some of these functions are well documented, molecular data are still scarce. Using the RPE-specific monoclonal antibody RPE9, raised against human RPE cells, we have identified a novel 65 kD protein, conserved in mammals, birds, and frogs. This RPE-specific protein was found to be nonglycosylated. It was most effectively solubilized in the presence of detergent suggesting that it is associated with the RPE cell membranes. Its partitioning in the detergent phase of Triton X-114 and its solubilization in 0.75 M and 1.0 M KCl suggest that it interacts with the membrane either through a polypeptide anchor or charged amino acids. Cell fractionation by differential solubilization and differential centrifugation demonstrated that the protein was preferentially associated with the microsomal membrane fraction, where it is the major protein. Developmental expression of this 65 kD protein was examined in neonatal rats. Morphologically well-differentiated RPE cells did not express the 65 kD protein at birth. However, expression was detectable at postnatal day 4, that is, one to two days before the photoreceptors develop their outer segments, suggesting that the expression of the 65 kD protein may be coordinated with other developmental events in the intact retina. This is further supported by the fact that RPE cells in confluent culture lose the expression of this protein within two weeks, while they maintain their characteristic epithelial morphology. Because of its specificity, its evolutionary conservation, and its timing of expression, it is possible that this protein may be involved in one of the key roles of RPE and as such is an important molecular marker for RPE differentiation.

Coronavirus (JHM) replication within the retina: analysis of cell tropism in mouse retinal cell cultures

The murine coronavirus, mouse hepatitis virus, JHM strain, induces a retinal degenerative disease in adult BALB/c mice. Coronavirus infections are highly species specific with virus exhibiting a strong tissue and cell specificity. In this report we evaluated the cellular basis of JHM virus retinal tropism. Retinal cultures and retinal pigment epithelial (RPE)-retinal mixed cell cultures were prepared from eyes obtained from Balb/c mice. The ability of JHM virus to infect and replicate in these retinal cultures was evaluated by light microscopy, immunofluorescent staining, electron microscopy, and virus isolation. Cytopathology was not observed and virus could not be detected in supernatant fluid in retinal cultures. However, low levels of infectious virus could be detected within the cells for the first 4 days. This observation suggested that cell-to-cell interactions may be critical since virus particles and virus antigens can be seen in vivo within the neural retina and the RPE. In contrast to the retinal cultures, retinal-RPE mixed cultures were supportive to JHM virus replication. Syncytial cytopathology was observed for the first 4 days and virus was isolated from supernatant fluids. By electron microscopy, virus was found intracellularly within vacuoles and extracellularly at the plasma membrane. After Day 4, a persistent virus infection was established in which cells produced virus for 5 weeks without cytopathic effects or cell death. Double-labeling immunofluorescent studies of retinal-RPE mixed cultures showed that the virus antigen was co-expressed with a Muller cell marker, glutamine synthetase. This cell is the most prominent glial element in the retina. These studies demonstrate that JHM virus is capable of establishing a persistent virus infection in mixed retinal (Muller)-RPE cell cultures. Moreover, these data suggest that cell-to-cell interactions influence the establishment of coronavirus infections in the retina.

Redistribution and reduction of interphotoreceptor retinoid-binding protein during ocular coronavirus infection

Inoculation of the neurotropic coronavirus mouse hepatitis virus strain JHM intravitreally or into the anterior chamber causes acute infection of the retinal pigment epithelium (RPE) and neural retina. Weeks later, many retinas have foci of moderate to severe atrophy. The effect of coronavirus infection (after intravitreal inoculation) was examined on interphotoreceptor retinoid-binding protein (IRBP), the glycolipoprotein in the interphotoreceptor matrix (IPM) thought to transport retinoids between the photoreceptors and the RPE. Changes in IRBP distribution accompanied virus-associated retinal pathology, including photoreceptor loss and RPE abnormalities. Immunohistochemistry on days 3 and 6 showed that IRBP had diffused into the neural retina away from the IPM. The IRBP became localized abnormally in the same areas as virus-induced lesions, shown by staining adjacent sections with a monoclonal antibody specific for the viral nucleocapsid protein. Moreover, the level of IRBP in isolated retinas, measured in an immunoslot-blot assay, decreased significantly by day 3 and remained low through day 23. This decrease was confirmed in eyecups isolated on day 6. It may be caused in part by loss of photoreceptors and diffusion of IRBP through the retina into the vitreous. These studies show that a virus may induce an acute, limited infection in the retina that can be cleared by the host. However, the infection initiated a series of events resulting in long-term reduction and redistribution of a critical photoreceptor protein.

Leukoregulin, a novel cytokine enhances the anti-herpesvirus actions of acyclovir

Leukoregulin is a naturally occurring immunologic cytokine which increases membrane permeability and drug uptake in tumor cells but not in normal cells. In this paper we show that leukoregulin also increases membrane permeability of Herpes simplex virus type 1 (HSV-1)-infected cells. More importantly, we demonstrate that leukoregulin significantly enhances the ability of acyclovir (acycloguanosine, ACV) to inhibit the cellular release of infectious HSV-1. The ability of 1-100 microM ACV to inhibit infectious HSV-1 production is increased up to 100-fold when HSV-1-infected human amnion (WISH) cells are treated with 5 units leukoregulin/ml and ACV 3 hr after virus infection. Under these conditions, leukoregulin alone is unable to inhibit HSV-1 infectivity. In addition, three unrelated cytokines, interleukin-1 alpha (IL-1), interferon (IFN)-alpha and IFN-gamma lack the ability to enhance the anti-HSV actions of ACV when their treatment is initiated after HSV-1 infection. These findings demonstrate that a combination of immunotherapy and chemotherapy can produce a substantial inhibition of herpesvirus replication and provide a rationale for the application of this approach to the interventive treatment of virus infection.

Cytokine-induced modulation of cellular proteins in retinoblastoma. Analysis by flow cytometry

Cytokines are a group of specialized, hormone-like proteins that can exert profound influences on cellular development and on a variety of cellular functions. Retinoblastoma cells are an important model for exploring human malignancy and differentiation. These multipotent embryonic cells are capable of differentiating into neuronal, glial-like and retinal pigment epithelium (RPE)-like elements. This report shows that flow cytometric analysis can be used to measure the expression of both cytoplasmic and cell surface proteins in retinoblastoma cells. The authors used this technique to monitor changes in the expression of selected cellular proteins after exposure to specific cytokines and found that MHC class I molecules were augmented by interferon-alpha (IFN-alpha) and interferon-gamma (IFN-gamma), but not by tumor necrosis factor (TNF). However, the MHC class II molecules were augmented by IFN-gamma but not by IFN-alpha or TNF. The neuronal markers, IRBP and PR-6, the glial-like marker, GFAP, and the RPE cell markers, RPE-9 and RPE-15, were not altered by any of the cytokines tested. Furthermore, IFN-gamma induced a striking enhancement of the expression of the photoreceptor cell protein, S-antigen. In contrast, IFN-alpha and TNF did not affect the expression of S-antigen. These studies show that the cytokine, IFN-gamma, can enhance a distinct cellular protein associated with cells committed to a specific cell lineage.

Ocular tropisms of murine coronavirus (strain JHM) after inoculation by various routes

The coronavirus mouse hepatitis virus (MHV, strain JHM) infects tissues in the anterior and posterior segments when injected intravitreally into adult mouse eyes. Infection causes progressive damage to the photoreceptors and retinal pigment epithelium (RPE), resulting in a disease the authors have termed JHM retinopathy. To determine whether this virus is retinotropic independent of route of inoculation, the authors injected mice with virus by several different routes: into the anterior chamber (AC), onto the cornea, intranasally, or intracerebrally. Inoculation into the AC produced effects similar to those after intravitreal inoculation, although slightly slower in onset. Viral antigen was detected in the anterior portion of the iris on day 3, and by day 6, was also located primarily in the inner nuclear layer, photoreceptors, Müller cells, and RPE. However, by day 10, viral antigens were only detected in a few cells in the ganglion cell layer. Infectious virus was isolated from neural retinas on days 3 and 6, but not on day 10. In contrast, infectious virus could not be isolated from contralateral eyes. After 14 weeks, specific regions of some retinas were atrophied, with most of the retinal layers involved. Inoculation by other routes also resulted in virus-induced disease. Scarification of the cornea with virus, but not application of virus droplets alone, caused pathologic changes in the corneal epithelium and stroma and subtle effects on the ganglion cell and inner plexiform layers. Intracerebral inoculation of virus affected mainly the RPE. Pathologic effects and viral antigens were not detected in eyes from four mice inoculated intranasally. These results show that a murine coronavirus is retinotropic when introduced by several direct routes and one indirect route. Moreover, these studies show that long-lasting retinal disorders ranging in intensity from mild to severe can occur after coronavirus infection.

1990 Sir Henry Wellcome medal and prize winner. Leukoregulin: a new biotherapeutic cytokine in the search for more effective anti-viral pharmacologic agents

This investigation examines whether cytokines, as exemplified by leukoregulin, with their immense potential for biorecognition and target cell modulation as a result of their complex three-dimensional structure, have the potential to provide new directions for biotherapy of infectious disease. Leukoregulin is a naturally occurring immunologic cytokine, secreted by stimulated lymphocytes, which increases membrane permeability and drug uptake in tumor but not in normal cells. This study demonstrates that leukoregulin also increases the plasma membrane permeability of cells acutely infected with herpes simplex type 1 virus and that the increase in membrane permeability is accompanied by a 10- to 100-fold increase in the ability of acyclovir to inhibit the release of infectious virus when the cells are treated with leukoregulin 3 hours after infection with the virus. This is the first demonstration that a cytokine, alone or in combination with anti-viral chemotherapy, can effectively inhibit virus replication in human cells following acute virus infection, which indicates that combination immunotherapy and chemotherapy have the potential to completely inhibit the production of infectious virus by acutely infected human cells.

Cytokine-mediated activation of a neuronal retinal resident cell provokes antigen presentation

The retinal pigment epithelial (RPE) cell has long been considered an important regulatory cell, maintaining physiological and structural balance within the retina. We have previously shown that the RPE cell may also be important in autoimmunity and transplantation. These cells can be induced by cytokines to express MHC class II Ag in ocular inflammatory and autoimmune conditions. In this report we show that isolated rat RPE cells can be induced to express class II Ag following incubation with rat rIFN-gamma. The ability of RPE cells to present Ag was determined by both T cell proliferation assays and IL-2 production. Only the Ia-positive RPE cells can present retinal Ag (S-Ag and interphotoreceptor-binding protein) to specifically sensitized rat Th cells. Moreover, the ability of chloroquine to inhibit this activity suggests that the RPE cell is also capable of processing Ag prior to Ag presentation. These studies indicate that cytokine-mediated activation of RPE cells may be a basic component of ocular immunity and an important aspect of RPE cell transplantation.

Cytokine-mediated activation of a neuronal retinal resident cell provokes antigen presentation

The retinal pigment epithelial (RPE) cell has long been considered an important regulatory cell, maintaining physiological and structural balance within the retina. We have previously shown that the RPE cell may also be important in autoimmunity and transplantation. These cells can be induced by cytokines to express MHC class II Ag in ocular inflammatory and autoimmune conditions. In this report we show that isolated rat RPE cells can be induced to express class II Ag following incubation with rat rIFN-gamma. The ability of RPE cells to present Ag was determined by both T cell proliferation assays and IL-2 production. Only the Ia-positive RPE cells can present retinal Ag (S-Ag and interphotoreceptor-binding protein) to specifically sensitized rat Th cells. Moreover, the ability of chloroquine to inhibit this activity suggests that the RPE cell is also capable of processing Ag prior to Ag presentation. These studies indicate that cytokine-mediated activation of RPE cells may be a basic component of ocular immunity and an important aspect of RPE cell transplantation.

Murine coronavirus induces an acute and long-lasting disease of the retina

The ability of the coronavirus mouse hepatitis virus, strain JHM, to grow in the retinas of BALB/c mice was examined. Inoculation into the vitreous chamber produced significant changes. Immunoperoxidase staining of frozen sections with either monoclonal or polyclonal antiserum revealed coronaviral antigens in the iris, ciliary body, and a few ganglion cells on day 1. The retinal pigment epithelial cells began expressing viral antigen on day 2 and large amounts of antigen were present in these cells on day 3. Viral antigens were detected in all layers of the neural retina by day 6 and were absent after day 7. Infectious virus was recovered from retinas harvested at 5 days. The drop in viral antigen expression was correlated with an elevation in virus-specific antibody; the latter began to rise on day 5 and plateaued after day 8. In hematoxylin and eosin- or periodic acid -Schiff-stained sections of virus-inoculated left eyes, but not mock-inoculated right eyes, lesions spanning all layers of the neural retina were detected by day 3. Subsequently abnormalities in retinal pigment epithelial cells appeared, sometimes around the entire circumference of the retina. Significant retinal abnormalities, notably photoreceptor degeneration, persisted through 6 weeks. These results demonstrate that coronaviruses can cause acute infection of the posterior pole of the eye, resulting in only a mild inflammatory response and long-lasting disease. This murine disease may be considered a model for degenerative diseases of the pigment epithelium and photoreceptors in man.

Interferon-gamma enhances the expression of retinal S-antigen, a specific neuronal cell marker

Interferon-gamma (IFN-gamma) is a potent lymphokine which can modify a variety of cellular processes. One of the mechanisms involved in these processes is the ability of IFN-gamma to alter the regulation and expression of cellular proteins. Using analytical flow cytometry, we show that recombinant human IFN-gamma can enhance the expression of retinal S-antigen in retinoblastoma cells. This enhancement was selective since two other retinal cell proteins, interphotoreceptor binding protein (IRBP) and photo-6, were not affected by IFN-gamma treatment. Retinal S-antigen plays an important role in vision and is one of the retinal proteins capable of inducing an inflammatory eye disease called experimental autoimmune uveitis. These studies therefore demonstrate an important role for this lymphokine, that is, the enhanced expression of a neuronal cell protein. This finding may also identify additional mechanisms by which IFN-gamma may participate in immunopathologic events in nervous tissue.

Evaluation of Ia expression in rat ocular tissues following inoculation with interferon-gamma

It is becoming increasingly clear that IFN-gamma is a potent immunoregulatory protein which influences MHC class II (Ia) antigen expression and cellular functions of B cells, T cells, NK cells and macrophages. During the past 5 yr our laboratory has provided evidence that IFN-gamma modulates class II antigens on retinal cells (retinal pigment epithelial cells, endothelial cells) and is localized within the eye during human inflammatory conditions. In this study we evaluate the direct effect of IFN-gamma on ocular tissue. Lewis rats were inoculated intravitreally or under the retina with either recombinant IFN-gamma (20,000 U) or saline. At 2 hr, 1, 2 and 6 days postinoculation, the eyes were removed and frozen sections were evaluated by immunocytochemical staining with monoclonal anti-Ia antibodies and an irrelevant monoclonal anti-T cell antibody. Saline treated tissue and tissue removed 2 hr after IFN-gamma inoculation showed no significant staining for Ia antigens. However, eyes evaluated 24 hr after IFN-gamma inoculation revealed Ia expression on a variety of ocular cells localized in the conjunctiva and anterior segment, such as conjunctival epithelium, keratocytes, iris epithelium, ciliary epithelium and choroidal cells. In the retina, retinal pigment epithelial (RPE) cells were Ia positive only when IFN-gamma was injected directly under the retina. In conjunction with Ia expression, two striking changes were noted. An iritis was seen and infiltrating cells were detected in the inner retinal layers. Both of these phenomena have been observed in certain inflammatory eye diseases. These studies clearly substantiate the concept that IFN-gamma can regulate class II antigens in the eye and thus may perpetuate immune reactivity in this site.

Development and characterization of monoclonal antibodies directed against the retinal pigment epithelial cell

The retinal pigment epithelium consists of a unicellular layer of neuroepithelial cells that are essential for the maintenance of normal function of the neural retina. In order to evaluate more critically this cell in health and disease, we prepared monoclonal antibodies against human retinal pigment epithelial (RPE) cells. Balb/c mice were immunized with human RPE cells. Spleen cells were fused with myeloma cells and resultant hybridomas were selected for antibody production. Supernatants were assayed by immunoperoxidase on frozen sections of human eye tissues. Two hybrids were cloned and ascites were generated in mice. These IgG antibodies react only with RPE cells and show no cross-reactivity with other cells in the eye or with human brain, kidney, skin, salivary glands, lymphocytes or monocytes. These antibodies recognize cell surface molecules that are highly conserved since they can be found in man, monkey, rat, cow, chicken and frog. SDS gel electrophoresis and immunoblot analysis showed that one of the antibodies reacted with a 42,000 MW polypeptide. Evaluation of the developing rat retina revealed that the epitopes are not detected at birth, are weakly present at day 6 and are highly recognized by day 9. These immunoglobulins will allow us to evaluate RPE cells in disease (proliferation, migration) and to probe the bioregulatory functions (phagocytosis, vitamin A transport) of these cells.

Identification of the lymphokines, interferon-gamma and interleukin-2, in inflammatory eye diseases

The exact pathogenic mechanisms involved in autoimmune and inflammatory eye diseases are not known. However, studies during the past few years indicate that a T cell infiltrate, T cell sensitization to retinal antigens and expression of major histocompatibility complex (MHC) class II antigens are associated with this process. In this report we show that the lymphokines, IL-2 and IFN-gamma, are present in the human eye during inflammatory and autoimmune diseases. The presence of these lymphokines is associated with a lymphocyte infiltrate, predominantly of T cell origin, and with the expression of MHC class II antigens on both the infiltrating cells and ocular resident cells, that is, retinal pigment epithelial (rpe) cells and retinal vascular endothelial cells. Furthermore, in vitro studies demonstrate that IFN-gamma can enhance the expression of the HLA-DR determinant on both of these cell types. These observations suggest that lymphokine induced class II antigen expression may serve as a local amplification system in autoimmune and inflammatory eye diseases. A better understanding of the role of lymphokines in the mechanisms involved in the development of autoimmunity and inflammation may be beneficial in the treatment of these diseases.

Anti-Ia antibody diminishes ocular inflammation in experimental autoimmune uveitis

An experimental model of inflammatory eye disease, experimental autoimmune uveitis (EAU), was established by injecting rats in the footpad with S-antigen in complete Freund's adjuvant. This model system was used to evaluate the role of major histocompatibility complex (MHC) class II antigens (Ia) in the pathogenesis of this T cell mediated disease. One day prior to S-antigen priming, rats were injected with either anti-Ia antibodies or with mouse ascites. Clinical and histopathological analysis of eyes from rats treated with anti-Ia antibody showed less ocular inflammation as well as a delay in onset of EAU when compared to controls (p = 0.01). Furthermore, immunocytochemical evaluation demonstrated that tissue obtained from animals receiving anti-Ia therapy also expressed less Ia antigen, as well as a diminution in the number of infiltrating macrophages and lymphocytes. These data show that anti-Ia treatment significantly modifies the course of EAU in the rat. In addition, this study suggests that MHC class II antigen expression may be involved in the initiation and continuation of immune responses that results in ocular inflammatory diseases.

Coexpression of neuronal, glial, and major histocompatibility complex class II antigens on retinoblastoma cells

This study identifies the presence of major histocompatibility complex class II antigens on retinoblastoma cells. In addition, the modulation of HLA-DR by interferon-gamma as well as the preferential expression of this major histocompatibility complex molecule over HLA-DQ is described. Double labeling experiments revealed that HLA-DR antigen is shared concomitantly with cells of glial and neuronal character. Investigations such as these underscore the possibility that expression of major histocompatibility complex class II antigens may function as immunological components in the host or play a role in the cellular differentiation of these tumor cells.

Retinal vascular endothelium expresses fibronectin and class II histocompatibility complex antigens in experimental autoimmune uveitis

To analyze the role of the retinal vascular endothelial cells in the development of experimental autoimmune uveitis (EAU), we studied the presence of Ia antigen and FN in retinal vessels of Lewis rats immunized with retinal S antigen. Immunopathologic studies were performed on frozen tissues obtained during various stages of the disease. Our results show that Ia antigen was not present in the normal rat retina, and there was very little FN present in a few retinal vessels. One to two days prior to the histologic and clinical onset of EAU, FN was found to be increased in the retinal vessels. Ia antigen was found to be present in the retinal vessels coincident with the first signs of cellular infiltration. During the stage of maximal cellular infiltration, FN was present diffusely throughout the retina, as well as in the subretinal space, and Ia antigen was found diffusely in the cellular infiltrate. Therefore, FN and Ia antigen reflect the immunomodulation of vascular endothelial cells in EAU, which may be very important in the pathogenesis of retinal S antigen-induced uveitis. Two possible mechanisms for the role of the activation of the retinal vascular endothelium in the development of retinal inflammation in uveitis are discussed.

Effect of cyclosporin A (CyA) on the immunopathological lesion of the labial minor salivary glands from patients with Sjögren's syndrome

Labial minor salivary gland biopsy specimens from 14 patients with Sjögren's syndrome treated either with cyclosporin A (CyA) or placebo (5 mg/kg body weight day for six months) were studied to determine T lymphocyte subsets and HLA-DR antigen expression using the avidin-biotin-peroxidase technique. In all CyA treated patients we observed a decrease in the number of T lymphocytes and in the number of T helper cells, while the percentage of T suppressor cells and B cells was the same in both treated and untreated groups. It was also shown that the HLA-DR antigen expression on the epithelial cells was eliminated in the CyA treated patients. These findings suggest that the HLA-DR antigen expression on the epithelial cells is the result rather than the triggering factor of this T cell mediated process and is probably related to decreased lymphokine production by activated T lymphocytes.

HLA-DR expression by labial minor salivary gland tissues in Sjögren's syndrome

Minor salivary gland biopsy specimens from patients with Sjögren's syndrome (primary and secondary) and from normal controls were examined with the four step biotin-avidin-immunoperoxidase assay. The composition of the infiltrating cells was similar in patients with both primary and secondary Sjögren's syndrome, consisting primarily of T lymphocytes with predominance of T helper/inducer cells. B lymphocytes (Leu-14) were approximately 20-35% of the infiltrating lymphocytes, while only a few OKM1 (monocytes/macrophages) cells and Leu-7+ (natural killer; NK) cells were observed. The majority of infiltrating lymphocytes expressed HLA-DR antigens. In the biopsy specimens of the controls there were no infiltrates; the scattered lymphocytes, however, were also predominantly T lymphocytes. Finally, the glandular epithelial cells (ducts and acini) were inappropriately expressing HLA-DR antigens, in contrast with controls where minimal HLA-DR expression was found.

Expression of HLA-DR antigen on retinal pigment epithelial cells in retinitis pigmentosa

Class II (HLA-DR) antigens are cell surface molecules that play a major role in the initiation and perpetuation of immune responses. Although most cells do not constitutively express class II antigens, selected cells can be stimulated to do so in some immunologically mediated disorders. When retinal pigment epithelial cells were evaluated by either immunoperoxidase or immunofluorescent staining of frozen eye sections from normal individuals, HLA-DR antigens were not detected. In contrast, retinal pigment epithelial cells from two patients with retinitis pigmentosa did express HLA-DR antigens. These findings demonstrated that at some time during the course of retinitis pigmentosa, the retinal pigment epithelial cell is activated to express HLA-DR.

HLA-DR antigens on retinal pigment epithelial cells from patients with uveitis

The pathogenic mechanisms involved in immunologically associated ocular diseases are not clearly understood. To further evaluate these types of disorders, we examined the expression of HLA-DR antigen on eye sections from normal individuals and five patients (six eyes) with ocular inflammation (sympathetic ophthalmia and chronic uveitis). Using immunoperoxidase staining technique and the complement-mediated cytotoxicity assay, we detected the presence of HLA-DR antigens on retinal pigment epithelial (RPE) cells from uveitic eyes. In contrast, HLA-DR antigen was not detected on RPE cells from normal ocular tissue. Our study shows that during the course of human ocular inflammatory disorders, the RPE cell is activated to express HLA-DR antigens. This antigen expression may be important in the initiation and/or perpetuation of immune reactivity in the eye.

Expression of Ia antigen on retinal pigment epithelium in experimental autoimmune uveoretinitis

Recently, human retinal pigment epithelial (RPE) cells have been demonstrated to express class II, HLA-DR, antigens both in vivo and in vitro. HLA-DR antigens were detected on RPE cells from patients with uveitis and retinitis pigmentosa. In addition, in vitro studies revealed that not only does this cell express HLA-DR antigen but also that this antigen can be modulated by the lymphokine, interferon (IFN)-gamma. In this study we evaluated the development of the murine class II, Ia, antigens on RPE cells in experimental autoimmune uveoretinitis (EAU). Ia antigen was evaluated with the avidin-biotin-peroxidase technique. Ia antigen was not detected on RPE cells from normal rats. However, Ia antigen was detected on the surface of RPE cells from EAU rats four days prior to the development of clinical and histopathological EAU. Moreover, the expression of Ia antigen on RPE cells from EAU rats continued to persist until one and one-half months after immunization. This study demonstrates that during the course of EAU the RPE cell is activated to express Ia antigens. This antigen expression may be important in the initiation and/or perpetuation of immune reactivity in the eye.