Entrainment of circadian rhythm to a photoperiod reversal shows retinal dystrophy in RPE65(-/-) mice

Light entrainment of circadian rhythms is mediated by classical "visual" photoreceptors (rods and cones) as well as "nonvisual" photoreceptive elements (light-detecting cells that do not contribute to classical "vision"). This paper aimed to assess whether light entrainment of locomotor circadian rhythms in mice with impaired rods and cones differs from normal controls and whether this technique, alongside existing techniques, could be used to assess visual function. The study was primarily interested in differences between the entrainment of circadian rhythms of normal-sighted C57Bl/6J mouse and the C57Bl/RPE65 knockout mouse (RPE65(-/-)), although C3H/HeJ (rd/rd) mice were included as a preexisting model of retinal degeneration. Circadian rhythms of motor activity before and after a 12-h light reversal were assessed in custom-built cages that continuously monitored movement. The controls showed a significantly higher mesor and amplitude when compared to the RPE65(-/-) and rd/rd mice. Despite the loss of rods and cones, the RPE65(-/-) and rd/rd maintained a 24-h circadian rhythm entrained to light similar to controls and were capable of circadian reentrainment to a 12-h light reversal. Importantly, this light reentrainment of the circadian phase occurred at a significantly slower rate in the retinal degenerate models than in the controls. The RPE65(-/-) model demonstrates a retinal degenerate reentrainment phenotype when compared to the rd/rd model. It is suggested that these retinal degenerate mice retain the ability to detect light for the purposes of circadian rhythm entrainment. However, alterations of specific parameters of the circadian rhythm with loss of rods and cones may provide measures of loss of visual function (sight).

In vivo gene therapy in young and adult RPE65-/- dogs produces long-term visual improvement

Defects in the RPE65 gene, which is selectively expressed in the retinal pigment epithelium (RPE), result in blindness and gradual photoreceptor cell degeneration. Experiments were conducted to assess the efficacy of gene replacement therapy in restoring retinal function in RPE65-/- dogs. Long-term effects of RPE65 gene therapy were assessed using visual behavioral testing and electroretinographic (ERG) recordings at 10-12 weeks and 6-9 months after surgery in five affected dogs. Subretinal injections of similar dosages of two constructs were performed in affected dogs at the ages of 4-30 months: rAAV.RPE65 into one eye and, in four of five dogs, rAAV.GFP contralaterally. Before surgery all RPE65-/- dogs were behaviorally blind with either no or very low-amplitude ERG responses to light stimuli. Marked improvements in visual behavior and ERG responses were observed as early as 4 weeks after surgery in affected animals. Except for light-adapted 50 Hz ERG flicker responses, all ERG parameters tested increased significantly in the eyes treated with the rAAV.RPE65 construct at the early follow-up. Gradual progressive improvements in ERG responses were observed in the RPE65-treated eyes over time. An unexpected finding was that on long-term follow-up, marked improvement of photopic ERG responses were also observed in the contralateral control eye in both young and older dogs. These results are promising for future clinical trials of human patients with retinal degenerative diseases, such as Leber congenital amaurosis, that result from RPE65 gene defects.

Sequence and structure of the mouse gene for RPE65

PURPOSE

To determine the genomic organization of the mouse gene for the retinal pigment epithelium (RPE) specific protein RPE65.

METHODS

A genomic clone containing the entire Rpe65 gene was isolated from a mouse genomic P1 library. Fragments of this clone were subcloned and sequenced by automated fluorescent dideoxy DNA sequencing and analyzed. Direct sequencing of PCR amplification products was used to complete the structure. Primer extension analysis was used to determine the transcription start site.

RESULTS

Southern hybridization of restriction digests of mouse genomic DNA reveals a likely single autosomal gene for Rpe65 with no evidence of pseudogenes. Sequence analysis of the mouse P1 clone for Rpe65 and fragments thereof reveals 14 exons distributed over 27 kbp. The transcription start site is located 57 bp upstream of the initiation codon. The protein encoded by the mouse Rpe65 gene is highly conserved when compared with RPE65s from other species.

CONCLUSIONS

RPE65 is a highly conserved protein and it appears that the genes for the mouse and human RPE65s, at least, are also conserved in overall structure.

Effect of Rpe65 knockout on accumulation of lipofuscin fluorophores in the retinal pigment epithelium

PURPOSE

In all mammalian species examined to date the retinal pigment epithelium (RPE) has been found to accumulate autofluorescent lysosomal storage bodies (lipofuscin) during senescence. Substantial evidence indicates that retinoids in the RPE-retina complex play a major role in RPE lipofuscin formation. Indeed, at least one RPE lipofuscin fluorophore is derived in part from vitamin A aldehyde. However, the precise mechanisms by which retinoids modulate RPE lipofuscin accumulation have not been elucidated. In mice without a functional Rpe65 gene, isomerization of all-trans- to 11-cis-retinol is blocked. Experiments were performed to determine whether this impairment of retinoid metabolism alters RPE lipofuscin accumulation.

METHODS

RPE lipofuscin fluorophore content was compared in 12- to 13-month-old Rpe65(+/+), Rpe65(+/-), and Rpe65(-/-) mice. Lipofuscin fluorophore content was determined using quantitative fluorometric measurements. RPE lipofuscin content was also estimated with quantitative ultrastructural techniques.

RESULTS

In the Rpe65(-/-) mice, RPE lipofuscin fluorophore accumulation was almost abolished. In addition, a significantly reduced accumulation of lipofuscin fluorophores was also observed in the Rpe65(+/-) animals. The inability of the RPE of)Rpe65(-/-) mice to supply 11-cis-retinal from the RPE to the retinal photoreceptors was accompanied by a massive accumulation of lipid droplets in the RPE that appeared to contain substantial amounts of retinoids.

CONCLUSIONS

These findings indicate that formation of RPE lipofuscin fluorophores is almost completely dependent on a normal visual cycle. The absence of retinal (both all-trans and 11-cis) in Rpe65 knockout mice drastically reduced formation of lipofuscin fluorophores in these animals. Even an excessive accumulation of retinyl fatty acid esters in the RPE of Rpe65 knockout mice did not contribute to lipofuscin accumulation.

New views on RPE65 deficiency: the rod system is the source of vision in a mouse model of Leber congenital amaurosis

Leber congenital amaurosis (LCA) is the most serious form of the autosomal recessive childhood-onset retinal dystrophies. Mutations in the gene encoding RPE65, a protein vital for regeneration of the visual pigment rhodopsin in the retinal pigment epithelium, account for 10-15% of LCA cases. Whereas previous studies of RPE65 deficiency in both animal models and patients attributed remaining visual function to cones, we show here that light-evoked retinal responses in fact originate from rods. For this purpose, we selectively impaired either rod or cone function in Rpe65-/- mice by generating double- mutant mice with models of pure cone function (rhodopsin-deficient mice; Rho-/-) and pure rod function (cyclic nucleotide-gated channel alpha3-deficient mice; Cnga3-/-). The electroretinograms (ERGs) of Rpe65-/- and Rpe65-/-Cnga3-/- mice were almost identical, whereas there was no assessable response in Rpe65-/-Rho-/- mice. Thus, we conclude that the rod system is the source of vision in RPE65 deficiency. Furthermore, we found that lack of RPE65 enables rods to mimic cone function by responding under normally cone-isolating lighting conditions. We propose as a mechanism decreased rod sensitivity due to a reduction in rhodopsin content to less than 1%. In general, the dissection of pathophysiological processes in animal models through the introduction of additional, selective mutations is a promising concept in functional genetics.

Expression, purification, and MALDI analysis of RPE65

PURPOSE

RPE65 is preferentially expressed in the retinal pigment epithelium (RPE) and is essential for retinal function. The purpose of the study was to develop methods for the expression of the protein, determine the accurate molecular weight of this expressed protein, and quantitate the amount of RPE65 in the bovine RPE.

METHODS

Human RPE65 was expressed in Sf9 cells using the baculovirus system. The subcellular localization was determined by Western blot analysis and immunocytochemistry. An ELISA was developed for RPE65 and used to measure levels in bovine RPE. Recombinant and native RPE65 were purified by affinity chromatography. Molecular mass was determined by matrix-assisted laser desorption/ionization (MALDI) mass spectrometry.

RESULTS

Recombinant human (rH)RPE65 was expressed as a major protein associated with cell membrane in Sf9 cells. The recombinant protein was purified to apparent homogeneity from both the membrane and nonmembrane fractions. The identity of the purified protein was confirmed by Western blot analysis and by partial peptide sequencing. rHRPE65 from the nonmembrane fraction has a mass of 64,867 +/- 80 which is close to the calculated molecular weight from the amino acid sequence including the His-tag (64,663), whereas the membrane-associated rHRPE65 has a molecular mass of 65,380 +/- 150, which is significantly higher than that of the non-membrane-associated form and the calculated molecular weight, suggesting posttranslational modifications. Similarly, native RPE65 was detected in the cytosolic and microsomal fractions of the bovine RPE, with an average level of 3.8 +/- 1.3 and 7.2 +/- 0.4 microg RPE65 per eye, respectively. The cytosolic form had a molecular mass of 61,161 +/- 60, which is close to the calculated value (60,944), whereas that of the microsomal form was 61,961 +/- 170.

CONCLUSIONS

RPE65 is expressed in two forms, one of which is membrane associated and contains significant posttranslational modifications, similar to the native membrane-associated form.

Identification, expression, and substrate specificity of a mammalian beta-carotene 15,15'-dioxygenase

We have identified from mouse the first mammalian beta-carotene 15,15'-dioxygenase (beta-CD), a crucial enzyme in development and metabolism that governs the de novo entry of vitamin A from plant-derived precursors. beta-CD is related to the retinal pigment epithelium-expressed protein RPE65 and belongs to a diverse family that includes the plant 9-cis-epoxycarotenoid dioxygenase and bacterial lignostilbene dioxygenases. beta-CD expression in Escherichia coli cells engineered to produce beta-carotene led to the accumulation of all-trans-retinal at the expense of beta-carotene, confirming that beta-CD catalyzed the central cleavage of this vitamin A precursor. Purified recombinant beta-CD protein cleaves beta-carotene in vitro with a V(max) of 36 pmol of retinal/mg of enzyme/min and a K(m) of 6 microm. Non-provitamin A carotenoids were also cleaved, although with much lower activity. By Northern analysis, a 2.4-kilobase (kb) message was observed in liver, kidney, small intestine, and testis, tissues important in retinoid/carotenoid metabolism. This message encoded a 63-kDa cytosolic protein expressed in these tissues. A shorter transcript of 1.8 kb was found in testis and skin. Developmentally, the 2.4-kb mRNA was abundant at embryonic day 7, with lower expression at embryonic days 11, 13, and 15, suggesting a critical role for this enzyme in gastrulation. Identification of beta-CD in an accessible model organism will create new opportunities to study vitamin A metabolism.

The upstream region of the Rpe65 gene confers retinal pigment epithelium-specific expression in vivo and in vitro and contains critical octamer and E-box binding sites

RPE65 is essential for all-trans- to 11-cis-retinoid isomerization, the hallmark reaction of the retinal pigment epithelium (RPE). Here, we identify regulatory elements in the Rpe65 gene and demonstrate their functional relevance to Rpe65 gene expression. We show that the 5' flanking region of the mouse Rpe65 gene, like the human gene, lacks a canonical TATA box and consensus GC and CAAT boxes. The mouse and human genes do share several cis-acting elements, including an octamer, a nuclear factor one (NFI) site, and two E-box sites, suggesting a conserved mode of regulation. A mouse Rpe65 promoter/beta-galactosidase transgene containing bases -655 to +52 (TR4) of the mouse 5' flanking region was sufficient to direct high RPE-specific expression in transgenic mice, whereas shorter fragments (-297 to +52 or -188 to +52) generated only background activity. Furthermore, transient transfection of analogous TR4/luciferase constructs also directed high reporter activity in the human RPE cell line D407 but weak activity in the non-RPE cell lines HeLa, HepG2, and HS27. Functional binding of potential transcription factors to the octamer sequence, AP-4, and NFI sites was demonstrated by directed mutagenesis, electrophoretic mobility shift assay, and cross-linking. Mutations of these sites abolished binding and corresponding transcriptional activity and indicated that octamer and E-box transcription factors synergistically regulate the RPE65 promoter function. Thus, we have identified the regulatory region in the Rpe65 gene that accounts for tissue-specific expression in the RPE and found that octamer and E-box transcription factors play a critical role in the transcriptional regulation of the Rpe65 gene.

A QTL on distal chromosome 3 that influences the severity of light-induced damage to mouse photoreceptors

C57BL/6J-c(2J) (c2J) albino mice showed much less damage to their photoreceptors after exposure to prolonged light than BALB/c mice and seven other albino strains tested. There were no gender differences, and preliminary studies suggested that the c2J relative protective effect was a complex trait. A genome-wide scan using dinucleotide repeat markers was carried out for the analysis of 194 progeny of the backcross (c2J x BALB/c)F(1) x c2J and the thickness of the outer nuclear layer (ONL) of the retina was the quantitative trait reflecting retinal damage. Our results revealed a strong and highly significant quantitative trait locus (QTL) on mouse Chromosome (Chr) 3 that contributes almost 50% of the c2J protective effect, and three other very weak but significant QTLs on Chrs 9, 12, and 14. Interestingly, the Chrs 9 and 12 QTLs corresponded to relative susceptibility alleles in c2J (or relative protection alleles in BALB/c), the opposite of the relative protective effect of the QTLs on Chrs 3 and 14. We mapped the Rpe65 gene to the apex of the Chr 3 QTL (LOD score = 19.3). Northern analysis showed no difference in retinal expression of Rpe65 message between c2J and BALB/c mice. However, sequencing of the Rpe65 message revealed a single base change in codon 450, predicting a methionine in c2J and a leucine in BALB/c. When the retinas of aging BALB/c and c2J mice reared in normal cyclic light were compared, the BALB/c retinas showed a small but significant loss of photoreceptor cells, while the c2J retinas did not. Finding light damage-modifying genes in the mouse may open avenues of study for understanding age-related macular degeneration and other retinal degenerations, since light exposures may contribute to the course of these diseases.

Protection of Rpe65-deficient mice identifies rhodopsin as a mediator of light-induced retinal degeneration

Light-induced apoptosis of photoreceptors represents an animal model for retinal degeneration. Major human diseases that affect vision, such as age-related macular degeneration (AMD) and some forms of retinitis pigmentosa (RP), may be promoted by light. The receptor mediating light damage, however, has not yet been conclusively identified; candidate molecules include prostaglandin synthase, cytochrome oxidase, rhodopsin, and opsins of the cones and the retinal pigment epithelium (PE). We exposed to bright light two groups of genetically altered mice that lack the visual pigment rhodopsin (Rpe65-/- and Rho-/-). The gene Rpe65 is specifically expressed in the PE and essential for the re-isomerization of all-trans retinol in the visual cycle and thus for the regeneration of rhodopsin after bleaching. Rho-/- mice do not express the apoprotein opsin in photoreceptors, which, consequently, do not contain rhodopsin. We show that photoreceptors lacking rhodopsin in these mice are completely protected against light-induced apoptosis. The transcription factor AP-1, a central element in the apoptotic response to light, is not activated in the absence of rhodopsin, indicating that rhodopsin is essential for the generation or transduction of the intracellular death signal induced by light.

Transgenic expression of an immunologically privileged retinal antigen extraocularly enhances self tolerance and abrogates susceptibility to autoimmune uveitis

Interphotoreceptor retinoid-binding protein (IRBP) is an immunologically privileged retinal antigen that can elicit experimental autoimmune uveitis (EAU). The nature and extent of tolerance to immunologically privileged self antigens is poorly understood. To investigate whether transgenic expression of IRBP extraocularly enhances tolerance and protects from EAU we prepared mice that express half of the mouse IRBP gene, containing a potent uveitogenic epitope (residues 161 - 180), under control of MHC class II promoter. Transgene mRNA was detectable in many tissues. Transgenic protein was undetectable by conventional assays, but was detected in thymic tissue by lymphocyte proliferation assay after induction of the promoter. Transgenic mice challenged with p161 - 180 did not develop EAU and had reduced immunological responses, but remained susceptible to EAU induced by whole IRBP, that contains additional uveitogenic epitopes. Disease was also induced by wild type T cells specific to p161 - 180. Thus, extraocular expression of a privileged retinal antigen enhances self tolerance, supporting the notion that sequestration contributes to immune privilege. Exceedingly low levels of transgene expression result in tolerance that is both profound and epitope specific, implying anergy or deletion of the endogenous uveitogenic repertoire. The same level of expression is, however, insufficient to tolerize wild-type effector T cells in the periphery.

What can we learn about age-related macular degeneration from other retinal diseases?

Age-related macular degeneration (AMD) is increasingly recognized as a complex genetic disorder in which one or more genes contribute to an individual's susceptibility for developing the condition. Twin and family studies as well as population-based genetic epidemiologic methods have convincingly demonstrated the importance of genetics in AMD, though the extent of heritability, the number of genes involved, and the phenotypic and genetic heterogeneity of the condition remain unresolved. The extent to which other hereditary macular dystrophies such as Stargardts disease, familial radial drusen (malattia leventinese), Best's disease, and peripherin/RDS-related dystrophy are related to AMD remains unclear. Alzheimer's disease, another late onset, heterogeneous degenerative disorder of the central nervous system, offers a valuable model for identifying the issues that confront AMD genetics.

Identification of RPE65 in transformed kidney cells

The protein RPE65 has an important role in retinoid processing and/or retinoid transport in the eye. Retinoids are involved in cell differentiation, embryogenesis and carcinogenesis. Since the kidney is known as an important site for retinoid metabolism, the expression of RPE65 in normal kidney and transformed kidney cells has been examined. The RPE65 mRNA was detected in transformed kidney cell lines including the human embryonic kidney cell line HEK293 and the African green monkey kidney cell lines COS-1 and COS-7 by reverse transcription PCR. In contrast, it was not detected in human primary kidney cells or monkey kidney tissues under the same PCR conditions. The RPE65 protein was also identified in COS-7 and HEK293 cells by Western blot analysis using a monoclonal antibody to RPE65, but not in the primary kidney cells or kidney tissues. The RPE65 cDNA containing the full-length encoding region was amplified from HEK293 and COS-7 cells. DNA sequencing showed that the RPE65 cDNA from HEK293 cells is identical to the RPE65 cDNA from the human retinal pigment epithelium. The RPE65 from COS-7 cells shares 98 and 99% sequence identity with human RPE65 at the nucleotide and amino acid levels, respectively. Moreover, the RPE65 mRNA was detected in three out of four renal tumor cultures analyzed including congenital mesoblastic nephroma and clear cell sarcoma of the kidney. These results demonstrated that transformed kidney cells express this retinoid processing protein, suggesting that these transformed cells may have an alternative retinoid metabolism not present in normal kidney cells.

Rpe65 is necessary for production of 11-cis-vitamin A in the retinal visual cycle

Mutation of RPE65 can cause severe blindness from birth or early childhood, and RPE65 protein is associated with retinal pigment epithelium (RPE) vitamin A metabolism. Here, we show that Rpe65-deficient mice exhibit changes in retinal physiology and biochemistry. Outer segment discs of rod photoreceptors in Rpe65-/- mice are disorganized compared with those of Rpe65+/+ and Rpe65+/- mice. Rod function, as measured by electroretinography, is abolished in Rpe65-/- mice, although cone function remains. Rpe65-/- mice lack rhodopsin, but not opsin apoprotein. Furthermore, all-trans-retinyl esters over-accumulate in the RPE of Rpe65-/- mice, whereas 11-cis-retinyl esters are absent. Disruption of the RPE-based metabolism of all-trans-retinyl esters to 11-cis-retinal thus appears to underlie the Rpe65-/- phenotype, although cone pigment regeneration may be dependent on a separate pathway.

Cloning and localization of RPE65 mRNA in salamander cone photoreceptor cells1

RPE65 is a potential retinoid-processing protein expressed in the retinal pigment epithelium. Mutations in the RPE65 gene have been shown to cause certain inherited retinal dystrophies. Previous studies have shown that salamander cone photoreceptor cells have a unique retinoid processing mechanism which is distinct from that of rods. To determine whether RPE65 is expressed in photoreceptors, the RPE65 cDNA was cloned from a salamander retinal cDNA library. The deduced protein consists of 533 amino acids and is 85% identical to human and bovine RPE65. The RPE65 mRNA was detected in all of the single cone cells isolated from the salamander retina, as well as in the retinal pigment epithelium by RT-PCR, but not in the isolated rods. The RT-PCR products have been confirmed to be RPE65 by DNA sequencing. The results indicate that this potential retinoid processing protein is expressed in the cone photoreceptor cells but not in rods. Therefore, this protein may contribute to the unique retinoid processing capabilities in salamander cones.

Role of the 3'-untranslated region of RPE65 mRNA in the translational regulation of the RPE65 gene: identification of a specific translation inhibitory element

Previously, we demonstrated that explanted bovine retinal pigment epithelium (RPE) cells lose RPE65 protein, a major microsomal protein specific to RPE, while the RPE65 mRNA remains, suggesting posttranscriptional regulation of RPE65 expression in vitro. Accordingly, we analyze here the effect of the 5'- and 3'-untranslated regions (UTRs) of RPE65 mRNA on translational efficiency using in vitro translation systems. We compared the levels of translation products and mRNA stability among RPE65 transcripts containing deletions of the 5'- and 3'-UTRs. First, the 5'-UTR does not affect translational efficiency. However, the 3'-UTR does influence translation efficiency. A putative translation inhibitory element (TIE) is contained within the 170-nucleotide (nt) sequence downstream of the stop codon. There is also a weak destabilizing effect that is associated with the region 3' to the putative TIE. But the effect of this is much less than that of the TIE. This TIE, however, does not inhibit translation of the heterologous chloramphenicol acetyltransferase gene, suggesting that a specific interaction with the upstream RPE65 coding sequence, or its product, may be required. Thus, the posttranscriptional regulation of RPE65 mRNA expression observed in cultured RPE may be via a mechanism of translational inhibition.

RPE65, the major retinal pigment epithelium microsomal membrane protein, associates with phospholipid liposomes

The retinal pigment epithelium (RPE)-specific protein RPE65 is the major protein of the RPE microsomal membrane fraction. Though RPE65 lacks transmembrane domains or signal peptide, detergents are required for its maximally effective solubilization in isotonic buffers. However, in 0.75-1.0 M KCl, RPE65 is as soluble without detergent, indicating a peripheral membrane association. We wished to understand why this non-membrane-inserted protein was so closely associated with RPE microsomal membranes. To explore the possible involvement of interactions with phospholipids, an isotonic salt-soluble extract of RPE was incubated with phosphatidylcholine (PC)/phosphatidylserine (PS)/phosphatidylinositol liposomes and centrifuged to sediment the liposomes. RPE65 cosedimented with the liposome pellet. RPE65 also cosedimented with synthetic dipalmitoyl-, 1-palmitoyl, 2-docosahexaenoyl-PC or dipalmitoyl-PS liposomes. Incubation with 1 mM Ca2+ or 1 mM EGTA had no effect, indicating a Ca2+-independent association. A spectrophotometric assay showed that this interaction of RPE65 with phospholipid vesicles resulted in increased light scattering, consistent with phospholipid vesicle aggregation. Resonance energy transfer experiments showed that any putative aggregation occurred without subsequent vesicle fusion. This PC affinity was further confirmed by incubation of RPE extract with dimyristoyl-PC-immobilized artificial membrane (IAM.PC) matrix. The RPE65 selectively bound and was elutable with 2% detergent. This RPE65-phospholipid liposome association may explain the solubilization characteristics of RPE65 and may be related to the function of RPE65 and to its physical association with the RPE smooth endoplasmic reticulum.

The gene for the retinal pigment epithelium-specific protein RPE65 is localized to human 1p31 and mouse 3

The human and murine chromosomal localization for the gene for the retinal pigment epithelium-specific protein RPE65 was determined. Using interspecific backcross analysis, we mapped Rpe65 to the distal end of mouse chromosome 3. In the human, using a human-hamster hybrid panel, RPE65 was mapped to chromosome 1. By the use of fluorescence in situ hybridization, this localization was refined to 1p31. The mouse and human loci for this potential candidate gene for hereditary retinal disease do not match those of any known disease in mouse or man.

Molecular cloning and expression of RPE65, a novel retinal pigment epithelium-specific microsomal protein that is post-transcriptionally regulated in vitro

Studies reported previously from this laboratory have shown that microsomal membranes of the vertebrate retinal pigment epithelium (RPE) contain an RPE-specific 65-kDa protein, RPE65, which bears the determinant recognized by the strictly tissue-specific monoclonal antibody RPE9, and which is developmentally regulated (Hamel, C. P., Tsilou, E., Harris, E., Pfeffer, B. A., Hooks, J. J., Detrick, B., and Redmond, T. M. (1993) J. Neurosci. Res. 34, 414-425). Microsequencing of 17 tryptic and chymotryptic peptides obtained from the in situ digestion of the RPE65 blotted on nitrocellulose yielded primary sequences that were used to generate oligonucleotide probes. An 84-nucleotide guessmer was used to isolate two clones from a bovine RPE lambda Zap II cDNA library. Rapid amplification of cDNA ends was used to complete the 5' and 3' ends, resulting in a 3,115-base pair composite cDNA. The open reading frame encodes a novel protein of 533 amino acid residues with a computed molecular weight of 60,940. This protein does not match any other sequence in the data bases. The 231 amino acids obtained from peptide sequencing match 43% of the amino acid sequence deduced from the cDNA. The protein has a calculated pI of 6.41 and is not predicted to have any transmembrane segments. The open reading frame expressed in Escherichia coli has an apparent molecular weight identical to that of the native protein and is recognized by the monoclonal antibody RPE9, further corroborating its validity. Northern blot analysis detected a major mRNA species of 3.15 kilobases for RPE65, as well as shorter species, only in RPE and not in other tissues (including other ocular tissues). Cultured RPE cells (7 weeks in primary culture) contained RPE65 mRNA in amounts equivalent to fresh RPE. Such cells, however, contained no immunodetectable RPE65. The possible structure of this RPE-specific protein and hypotheses for the absence of translation in vitro are discussed.

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.

Recognition of peptides that are immunopathogenic but cryptic. Mechanisms that allow lymphocytes sensitized against cryptic peptides to initiate pathogenic autoimmune processes

Interphotoreceptor retinoid binding protein (IRBP) is a glycoprotein that localizes in the retina and induces inflammatory changes in this tissue in immunized animals. Certain IRBP-derived peptide determinants are also immunopathogenic, and we have previously shown that these determinants could be either immunodominant or cryptic. Lymphocytes sensitized against the cryptic peptides do not recognize whole IRBP in vitro, and yet these lymphocytes must recognize the protein in vivo to initiate the autoimmune pathogenic process. We have examined here two hypothetical explanations for this dissociation: 1) It is possible that when IRBP is processed in vitro, immunodominant peptide determinants compete with the cryptic ones and inhibit their interaction with the MHC molecules on the APC. This explanation was ruled out here by the finding that the immunodominant peptide 1179-1191 ("W10") did not inhibit the response to a cryptic one, 1158-1180 ("R4"), when added at equivalent and even moderately higher concentrations. 2) The second hypothesis proposes that the cryptic antigenic sites are not generated from IRBP by the APC in vitro, whereas enzymes in the retina digest the protein to yield fragments that generate these antigenic sites upon processing by the APC. In line with this hypothesis, we have found that cleavage of IRBP by certain endoproteinases (Asp-N, Glu-C, or V-8) produced molecules that were recognized in culture by lymphocytes sensitized to the immunopathogenic but cryptic peptide R4. This study, therefore, describes a putative Ag processing mechanism that results in IRBP recognition and, consequently, the initiation of an autoimmune process by lymphocytes sensitized against a cryptic peptide. Furthermore, experiments with R4 and other cryptic peptides have shown that cleavage fragments of up to 38 residues in length can be presented by APC, to stimulate lymphocytes sensitized against these peptides. No responses were stimulated, however, by fragments of 75 or more residues. The data thus provide new insights into the processing and presentation of cryptic peptide determinants by APC.

Unusual immunologic properties of the uveitogenic interphotoreceptor retinoid-binding protein-derived peptide R23

Peptide R23, consisting of residues 1091-1115 of bovine interphotoreceptor retinoid-binding protein (IRBP), had some unusual immunologic properties in Lewis rats. The peptide induced experimental autoimmune uveoretinitis and pinealitis in these rats, but only at high doses. The minimal immunopathogenic dose was found to be 100 nmol/rat. On the other hand, R23 was highly immunogenic in Lewis rats, producing cellular immunity, as measured by the lymphocyte proliferation assay, with a minimal dose of 1 nmol/rat. This unusual dissociation between the uveitogenic and immunogenic activities of R23 was attributed to different sites on the peptide, stimulating either the lymphocytes which induce disease or those which vigorously proliferate in culture. The potent immunogenicity of R23 was consistent with the peptide being immunodominant, as demonstrated by its capacity to be recognized by lymphocytes sensitized against whole IRBP and to stimulate these cells in culture to proliferate and acquire uveitogenic capacity. Likewise, lymphocytes sensitized against R23 are stimulated in culture by whole IRBP. Unexpectedly, peptide R23 was inferior to whole IRBP in its capacity to stimulate uveitogenicity in R23-sensitized lymphocytes. This finding also was attributed to the preferential stimulation by R23 of the lymphocytes specific for the putative "nonuveitogenic" site on the peptide. Peptide R23 also differs from the other tested bovine IRBP-derived peptides in the specificity of its antibodies. Unlike antibodies to the other peptides, those to R23 showed a strong cross reactivity toward whole IRBP.

Recognition of peptides that are immunopathogenic but cryptic. Mechanisms that allow lymphocytes sensitized against cryptic peptides to initiate pathogenic autoimmune processes

Interphotoreceptor retinoid binding protein (IRBP) is a glycoprotein that localizes in the retina and induces inflammatory changes in this tissue in immunized animals. Certain IRBP-derived peptide determinants are also immunopathogenic, and we have previously shown that these determinants could be either immunodominant or cryptic. Lymphocytes sensitized against the cryptic peptides do not recognize whole IRBP in vitro, and yet these lymphocytes must recognize the protein in vivo to initiate the autoimmune pathogenic process. We have examined here two hypothetical explanations for this dissociation: 1) It is possible that when IRBP is processed in vitro, immunodominant peptide determinants compete with the cryptic ones and inhibit their interaction with the MHC molecules on the APC. This explanation was ruled out here by the finding that the immunodominant peptide 1179-1191 ("W10") did not inhibit the response to a cryptic one, 1158-1180 ("R4"), when added at equivalent and even moderately higher concentrations. 2) The second hypothesis proposes that the cryptic antigenic sites are not generated from IRBP by the APC in vitro, whereas enzymes in the retina digest the protein to yield fragments that generate these antigenic sites upon processing by the APC. In line with this hypothesis, we have found that cleavage of IRBP by certain endoproteinases (Asp-N, Glu-C, or V-8) produced molecules that were recognized in culture by lymphocytes sensitized to the immunopathogenic but cryptic peptide R4. This study, therefore, describes a putative Ag processing mechanism that results in IRBP recognition and, consequently, the initiation of an autoimmune process by lymphocytes sensitized against a cryptic peptide. Furthermore, experiments with R4 and other cryptic peptides have shown that cleavage fragments of up to 38 residues in length can be presented by APC, to stimulate lymphocytes sensitized against these peptides. No responses were stimulated, however, by fragments of 75 or more residues. The data thus provide new insights into the processing and presentation of cryptic peptide determinants by APC.

Analysis of the pivotal residues of the immunodominant and highly uveitogenic determinant of interphotoreceptor retinoid-binding protein

Interphotoreceptor retinoid-binding protein (IRBP), a retinal-specific Ag, induces experimental autoimmune uveitis in a variety of animals. We have previously shown that sequence 1169-1191 of bovine IRBP is the immunodominant epitope of this protein in Lewis rats and is highly immunogenic and uveitogenic in these rats. The active site of peptide 1169-1191 was determined by testing its truncated forms. The shortest peptide to be immunologically active was found to be 1182-1190 (WEGVGVVPD). To determine the role of individual residues of this sequence, we have tested the immunologic activities of nine analogs of peptide 1181-1191, in which each of residues 1182-1190 was substituted with alanine (A). The tested activities included the capacity to induce experimental autoimmune uveitis and cellular responses in immunized rats, as well as the capability to stimulate lymphocytes sensitized against IRBP or the parent peptide 1181-1191. Analogs that did not stimulate these lymphocytes were also tested for their capacity to competitively inhibit the proliferative response to 1181-1191. Analogs A(1184), A(1186), and A(1187) resembled 1181-1191 in their activities, whereas the other analogs exhibited remarkably reduced activities, with several patterns being noticed. Analog A(1182) was inactive in all tests. Analog A(1190) was very weakly uveitogenic and non-immunogenic, but it stimulated lymphocytes sensitized against IRBP or 1181-1191 when added at exceedingly high concentrations. Analogs A(1183) and A(1185) resembled A(1190) in being weakly uveitogenic and A(1185) was also found to be poorly immunogenic. In addition, relatively high concentrations of A(1183) and A(1185) were needed to stimulate lymphocytes sensitized against IRBP or 1181-1191. However, a different pattern of activities was exhibited by analogs A(1188) and A(1189). These peptides were uveitogenic and immunogenic, but failed to stimulate lymphocytes sensitized to IRBP or 1181-1191. Furthermore, A(1188) and A(1189), but not A(1182), also inhibited the response to 1181-1191 of a cell line specific toward this parent peptide. The data are interpreted to show that residues 1188 and 1189 are involved in the interaction of the peptide with the TCR, whereas residues 1182 and 1190 and, perhaps, 1183 and 1185, are pivotal for the binding of peptide 1181-1190 to the MHC molecules on APC.

Analysis of the pivotal residues of the immunodominant and highly uveitogenic determinant of interphotoreceptor retinoid-binding protein

Interphotoreceptor retinoid-binding protein (IRBP), a retinal-specific Ag, induces experimental autoimmune uveitis in a variety of animals. We have previously shown that sequence 1169-1191 of bovine IRBP is the immunodominant epitope of this protein in Lewis rats and is highly immunogenic and uveitogenic in these rats. The active site of peptide 1169-1191 was determined by testing its truncated forms. The shortest peptide to be immunologically active was found to be 1182-1190 (WEGVGVVPD). To determine the role of individual residues of this sequence, we have tested the immunologic activities of nine analogs of peptide 1181-1191, in which each of residues 1182-1190 was substituted with alanine (A). The tested activities included the capacity to induce experimental autoimmune uveitis and cellular responses in immunized rats, as well as the capability to stimulate lymphocytes sensitized against IRBP or the parent peptide 1181-1191. Analogs that did not stimulate these lymphocytes were also tested for their capacity to competitively inhibit the proliferative response to 1181-1191. Analogs A(1184), A(1186), and A(1187) resembled 1181-1191 in their activities, whereas the other analogs exhibited remarkably reduced activities, with several patterns being noticed. Analog A(1182) was inactive in all tests. Analog A(1190) was very weakly uveitogenic and non-immunogenic, but it stimulated lymphocytes sensitized against IRBP or 1181-1191 when added at exceedingly high concentrations. Analogs A(1183) and A(1185) resembled A(1190) in being weakly uveitogenic and A(1185) was also found to be poorly immunogenic. In addition, relatively high concentrations of A(1183) and A(1185) were needed to stimulate lymphocytes sensitized against IRBP or 1181-1191. However, a different pattern of activities was exhibited by analogs A(1188) and A(1189). These peptides were uveitogenic and immunogenic, but failed to stimulate lymphocytes sensitized to IRBP or 1181-1191. Furthermore, A(1188) and A(1189), but not A(1182), also inhibited the response to 1181-1191 of a cell line specific toward this parent peptide. The data are interpreted to show that residues 1188 and 1189 are involved in the interaction of the peptide with the TCR, whereas residues 1182 and 1190 and, perhaps, 1183 and 1185, are pivotal for the binding of peptide 1181-1190 to the MHC molecules on APC.

Localization of the gene for interphotoreceptor retinoid-binding protein to mouse chromosome 14 near Np-1

Interphotoreceptor retinoid-binding protein (IRBP) is a large glycoprotein known to bind retinoids and found primarily in the interphotoreceptor matrix of the retina between the retinal pigment epithelium and the photoreceptor cells. It is thought to transport retinoids between the retinal pigment epithelium and the photoreceptors, a critical role in the visual process. We have used a 900-bp bovine IRBP cDNA fragment to map the corresponding gene, Rbp-3, to mouse chromosome 14 with somatic cell hybrids and have positioned the gene near Np-1 (nucleoside phosphorylase-1) by analysis of the progeny of an intersubspecific backcross. In the human genome, NP maps to human chromosome 14 and RBP3 to human chromosome 10. Thus, these two genes span the putative site of a chromosomal translocation which contributed to divergent karyotype evolution of man and mouse.

Stimulation in vitro of lymphocytes for induction of uveoretinitis without any significant proliferation

Experimental autoimmune uveoretinitis can be adoptively transferred into naive recipients by lymphocytes from rats immunized with uveitogenic Ag, provided these cells are activated in vitro before being injected. The activation of sensitized lymphocytes, by the immunizing Ag, or by cross-reacting Ag, is usually accompanied by vigorous proliferation. We report, however, on a complete dissociation between the capacity of a peptide to generate uveitogenicity and to stimulate proliferation in cultured lymphocytes. This peptide, which occupies sequence 579-591 of the bovine interphotoreceptor retinoid-binding protein (IRBP), is one of the three "repeats" that exhibit partial sequence homologies with the immunodominant and highly immunogenic peptide, 1179-1191. Peptide 579-591 is nonimmunogenic and nonuveitogenic in Lewis rats and does not stimulate any significant proliferation in lymphocytes sensitized against whole IRBP, peptide 1179-1191, or another "repeat" peptide, 271-283, which is immunogenic and uveitogenic. In contrast, peptide 579-591 effectively generates uveitogenicity in the lymphocytes sensitized against these three Ag. Unlike 579-591, peptides 271-283 and 1179-1191 stimulated both proliferation and uveitogenicity in these sensitized lymphocytes. A different pattern of activities was observed with the other "repeat" peptide, 880-892. This peptide did not have any effect on the lymphocytes sensitized against IRBP, 271-283 or 1179-1191, but did stimulate both proliferation and uveitogenicity in lymphocytes sensitized against itself. The data suggest that 579-591 selectively stimulates a lymphocyte subset that is uveitogenic, but is incapable of mounting proliferative responses.

Stimulation in vitro of lymphocytes for induction of uveoretinitis without any significant proliferation

Experimental autoimmune uveoretinitis can be adoptively transferred into naive recipients by lymphocytes from rats immunized with uveitogenic Ag, provided these cells are activated in vitro before being injected. The activation of sensitized lymphocytes, by the immunizing Ag, or by cross-reacting Ag, is usually accompanied by vigorous proliferation. We report, however, on a complete dissociation between the capacity of a peptide to generate uveitogenicity and to stimulate proliferation in cultured lymphocytes. This peptide, which occupies sequence 579-591 of the bovine interphotoreceptor retinoid-binding protein (IRBP), is one of the three "repeats" that exhibit partial sequence homologies with the immunodominant and highly immunogenic peptide, 1179-1191. Peptide 579-591 is nonimmunogenic and nonuveitogenic in Lewis rats and does not stimulate any significant proliferation in lymphocytes sensitized against whole IRBP, peptide 1179-1191, or another "repeat" peptide, 271-283, which is immunogenic and uveitogenic. In contrast, peptide 579-591 effectively generates uveitogenicity in the lymphocytes sensitized against these three Ag. Unlike 579-591, peptides 271-283 and 1179-1191 stimulated both proliferation and uveitogenicity in these sensitized lymphocytes. A different pattern of activities was observed with the other "repeat" peptide, 880-892. This peptide did not have any effect on the lymphocytes sensitized against IRBP, 271-283 or 1179-1191, but did stimulate both proliferation and uveitogenicity in lymphocytes sensitized against itself. The data suggest that 579-591 selectively stimulates a lymphocyte subset that is uveitogenic, but is incapable of mounting proliferative responses.

Serial adoptive transfer of experimental autoimmune uveoretinitis in rats

Experimental autoimmune uveoretinitis (EAU) and pinealitis induced by an interphotoreceptor retinoid-binding protein (IRBP)-derived peptide (R4) was serially transferred into naive recipient rats, using spleen cells from recipients of previous "orders" of transfer. The cells initiating the disease in recipients of the first order were either lymph node cells from rats immunized against peptide R4, or lymphocytes of a cell line specific toward this peptide. The serial transfer was successfully carried out through as many as four orders of sequential recipients.

Repeated determinants within the retinal interphotoreceptor retinoid-binding protein (IRBP): immunological properties of the repeats of an immunodominant determinant

Interphotoreceptor retinoid-binding protein (IRBP), a glycoprotein which localizes in the retina and pineal gland, induces inflammatory changes in these organs (EAU and EAP, respectively) when injected into various mammals. We have previously identified a determinant (residues 1169-1191) in bovine IRBP which is immunodominant and highly immunogenic and immunopathogenic in Lewis rats. IRBP exhibits a fourfold repeat structure and we report here on the comparison between the active sequence 1179-1191 and its three repeat peptides. Only one of the repeats, 271-283, cross-reacted with 1179-1191 and exhibited immunodominance, albeit of a low level. Peptide 271-283 was also immunogenic and immunopathogenic in Lewis rats, but with a minimal dose approximately 100 times higher than that of 1179-1191. Peptide 880-892, a nondominant determinant, resembled 271-283 in its immunogenicity, but was markedly less immunopathogenic. No immunological activity was detected in the fourth repeat peptide, 579-591. Peptide 1179-1191 was superior to the other repeats also in its antigenicity, i.e., the capacity to stimulate presensitized lymphocytes in culture: the minimal stimulatory concentrations of 1179-1191 was greater than 1000 times lower than those of 271-283 or 880-892. Furthermore, 1179-1191 was stimulatory at concentrations lower than those of 271-283 even when tested with lymphocytes sensitized against 271-283. A correlation was also found between the immunological activities of the repeat peptides and their amphipathicity. This study thus identifies two new immunopathogenic determinants of IRBP and provides additional data to show the association between immunodominance of peptides and their various immunological activities.

Uveitis and immune responses in primates immunized with IRBP-derived synthetic peptides

Monkeys immunized with bovine IRBP-derived synthetic peptides R4 (sequence 1158-1180) or R14 (1169-1191) developed EAU which was detected by both clinical and histological examinations. The inflammation localized mainly in the choroid, with only minor changes being noticed in the adjacent retinal tissue. EAU developed in only one of the two monkeys immunized with each of the peptides and the animals with disease also showed higher levels of cellular immunity toward the immunizing peptide than did the monkeys with no disease. The cellular immune responses, measured by the lymphocyte proliferation assay, were specific toward the immunizing peptides, with no cross responsiveness to whole IRBP. This finding suggests that the two uveitogenic peptides were non-immunodominant in the tested monkeys. In contrast, peptide R14 is highly immunodominant in the Lewis rat. Also, the fine specificity of the monkey response to R14 differed from that of the Lewis rat. The possible genetic control of the monkey susceptibility to EAU induction by the peptides is discussed and the unique finding of an autoimmune disease induction by a non-immunodominant peptide is underscored.

Immunological features of synthetic peptides derived from the retinal protein IRBP: differences between immunodominant and non-dominant peptides

Interphotoreceptor retinoid-binding protein (IRBP) is a glycoprotein of 1264 residues (bovine) which localizes specifically in the retina and pineal gland and induces inflammatory changes in these organs (EAU and EAP, respectively) in immunized animals. We report here on differences between the immunological activities in Lewis rats of four IRBP-derived synthetic peptides. Only one of these peptides, designated R14 (residues: 1169-1191) is immunodominant, i.e., it has the capacity to stimulate lymphocytes sensitized against whole IRBP. The remaining peptides, R4 (1158-1180), R8 (1197-1209), and R12 (248-266), are non-dominant and are not recognized by IRBP-sensitized lymphocytes. R14 differed profoundly from the other peptides in its immunogenicity, inducing cellular immunity at the low dose of 0.1 nmol/rat, whereas the non-dominant peptides initiated immune responses at doses approximately 100 times higher. R14 was also superior to the non-dominant peptides in its antigenicity, as determined by the lowest concentration required to induce sensitized lymphocytes to proliferate. Responses were stimulated by R14 at a concentration of 10(-6) microM, while the three non-dominant peptides were stimulatory at the much higher concentration of 10(-1) microM. These data support the concept that immunodominance is linked to a high binding affinity of the peptide determinant to the major histocompatibility complex antigens on antigen-presenting cells.

Immune responses to peptides derived from the retinal protein IRBP: immunopathogenic determinants are not necessarily immunodominant

Interphotoreceptor retinoid-binding protein (IRBP), a glycoprotein specific to the retina and pineal gland, induces in immunized rats inflammatory changes in these organs (EAU and EAP, respectively). We report here on the immunological activities in Lewis rats of 10 IRBP-derived peptides. Only one of these peptides (R3) was found to induce high levels of antibodies in immunized rats, as detected by ELISA. On the other hand, the majority of the tested peptides stimulated substantial cellular immune responses, measured by the lymphocyte proliferation assay. None of the tested peptides were recognized, however, by antibodies or lymphocytes from rats immunized with the whole IRBP molecule, thus indicating that these synthetic peptides are nonimmunodominant in the Lewis rat. Two of these peptides, R4 and R9 (which contains R4), were previously found to be immunopathogenic, producing EAU and EAP in immunized Lewis rats. The immune responses to peptide R4 were further examined and the data show that it induces measurable lymphocyte responses only when injected at remarkably high doses (greater than or equal to 67 microgram/rat). Yet, peptide R4 was highly antigenic when tested for stimulation of specifically sensitized lymphocytes in culture. Furthermore, lymphocytes sensitized against R4 exhibited high capacity to adoptively transfer EAU and EAP to naive recipients. The finding of immunopathogenic but nonimmunodominant peptides is discussed.

Cloning of cDNAs encoding human interphotoreceptor retinoid-binding protein (IRBP) and comparison with bovine IRBP sequences

We have determined the sequence of the human interphotoreceptor retinoid-binding protein mRNA from three separately isolated cDNAs. The sequence is 4.28 kb long and encodes a protein of 1247 amino acids (aa) including a putative signal peptide and propeptide. The sequence is shorter (by about 1.67 kb) than the bovine mRNA with the major difference in the lengths located in the 3'-untranslated region. We suggest that this resulted from an insertion in the bovine gene or a large deletion from the human gene. The insertion/deletion is flanked on either side by sequences that are similar in the bovine and human sequences. Like the bovine polypeptide, the deduced protein sequence from the human cDNA contains a fourfold repeat, with each repeat containing about 300 aa. Among the four repeats, the identity is about 30-40%. The identity between the complete bovine and human polypeptide sequences is 84%. The identity between the nucleotide sequences is 83% (excluding the major insertion/deletion). Comparison with the bovine gene indicates that the human sequence may lack about 5-10 bp at the 5' end of the cDNA; it, however, includes a poly(A) tail at the 3' end. Thus, the human sequence is virtually full length, is similar to the bovine sequence, and contains a striking fourfold repeat.

Synthesis of an immunopathogenic fusion protein derived from a bovine interphotoreceptor retinoid-binding protein cDNA clone

We have extended the cDNA sequence of bovine interphotoreceptor retinoid-binding protein (IRBP) and subcloned one of the sequenced cDNA fragments into an expression vector. The nucleotide (nt) sequences of four bovine IRBP cDNA clones have been determined. These sequences when assembled cover the 3' proximal 3629 nt of the IRBP mRNA and encode the C-terminal 551 amino acids (aa) of IRBP. This cDNA sequence validates the intron: exon boundaries predicted from the gene. A 2-kb EcoRI insert from lambda IRBP2, one of the clones sequenced, encoding the C-terminal 136 aa of IRBP was subcloned into the expression vector pWR590-1. Escherichia coli carrying this plasmid construction, pXS590-IRBP, produced a fusion protein containing 583 N-terminal aa of beta-galactosidase, three linker aa residues, 136 C-terminal aa of IRBP and possibly a number of additional C-terminal residues due to suppressed termination. This 86-kDa fusion protein, purified by detergent/chaotrope extraction followed by reverse-phase high-performance liquid chromatography, cross-reacted with anti-bovine IRBP on Western blots. This protein induced an experimental autoimmune uveo-retinitis and experimental autoimmune pinealitis in Lewis rats indistinguishable from that induced by authentic bovine IRBP. Thus, it is evident that biological activity of this region of IRBP, as manifested by immuno-pathogenicity, is retained by the fusion protein.

Rat T-cell lines specific to a nonimmunodominant determinant of a retinal protein (IRBP) produce uveoretinitis and pinealitis

Rat lymphocyte lines were established, with specificity toward two synthetic peptides derived from the interphotoreceptor retinoid-binding protein (IRBP), which specifically localizes in the retina and pineal gland. One of the peptides, R4, is immunopathogenic, producing experimental autoimmune uveoretinitis (EAU) and pinealitis (EAP) in immunized rats, while the other peptide, R3, exhibits no detectable immunopathogenicity in rats. The cell lines carry surface markers specific for the helper/inducer subset of T-lymphocytes. When tested by the proliferation assay, the line cells demonstrated major histocompatibility-restricted vigorous responses against the immunizing (homologous) peptide, but failed to recognize the intact IRBP molecule. This finding is in line with other data indicating that peptides R3 and R4 are nonimmunodominant determinants of IRBP for the Lewis rat. Yet, the cell lines specific for R4 were highly immunopathogenic, producing EAU and EAP in naive rats at numbers as low as 0.25 x 10(6), with histopathological changes similar to those induced by active immunization with this peptide. The immunological capacity of the cell lines was further demonstrated by the finding that spleen cells from recipient rats of these lines responded well against the homologous peptides. The uniqueness of this system, in which lymphocytes specific toward a nondominant determinant are immunopathogenic, is underscored and the possible mechanisms of disease induction are discussed.

mRNA for interphotoreceptor retinoid-binding protein (IRBP): distribution and size diversity in vertebrate species

Northern blots of total retinal RNA from a number of different vertebrate species were probed with a cDNA fragment corresponding to translated portions of bovine IRBP mRNA. A hybridizing band was detected in normal human retina (4.6 kb), Y-79 human retinoblastoma cells (4.4 kb), and in retinas of monkey (4.6 kb), guinea-pig (4.9 kb), mouse (6.1 kb), rat (two bands at 5.4 kb and 6.6 kb), rabbit (6.3 kb), cow (6.5 kb), and hamster (7.6 kb). Thus, the IRBP gene is expressed in the retinas of a wide variety of mammalian species. The mRNAs could be readily detected in about 20 micrograms of total RNA, suggesting that, in these species, IRBP message is relatively abundant. In contrast, only very weak hybridization was detected on northern blots of the three bird species examined (chicken, duck, quail). IRBP mRNA is thus relatively well expressed in mammals, but not in birds.

Identification of an immunodominant and highly immunopathogenic determinant in the retinal interphotoreceptor retinoid-binding protein (IRBP)

Interphotoreceptor retinoid-binding protein (IRBP), a glycoprotein specific for the retina and pineal gland, induces inflammatory changes in these two organs in immunized animals. We report here on the identification of an immunodominant determinant of bovine IRBP that is highly immunogenic and immunopathogenic in the Lewis rat. The peptide, which comprises the sequence 1169-1191 of bovine IRBP, was shown to be immunodominant by its capacity to stimulate lymphocytes sensitized against whole IRBP. A comparison was made between peptide 1169-1191 and another peptide, 1158-1180, which is nondominant but is immunogenic and immunopathogenic in the Lewis rat. Peptide 1169-1191 was found to be superior in its immunological capacities; the minimal dose of 1169-1191 needed to induce cellular immune response or disease in Lewis rats (0.02-0.1 nmol/rat) is congruent to 1,000 times smaller than that of 1158-1180. In addition, unlike the ocular disease induced by 1158-1180, the disease produced by 1169-1191 resembled that induced by whole IRBP in its kinetics and histopathological features. The immunological activity of 1169-1191 in the Lewis rat was localized to the 10 residues at the COOH terminus; no such activity was exhibited by the truncated peptide 1169-1188, which comprises the 20 residues at the NH2 terminus of the full peptide. The usefulness of this unique experimental system in analyzing the role of immunodominance in peptide immunogenicity and immunopathogenicity is underscored.

Interphotoreceptor retinoid-binding protein. Gene characterization, protein repeat structure, and its evolution

The gene for bovine interphotoreceptor retinoid-binding protein (IRBP) has been cloned, and its nucleotide sequence has been determined. The IRBP gene is about 11.6 kilobase pairs (kb) and contains four exons and three introns. It transcribed into a large mRNA of approximately 6.4 kb and translated into a large protein of 145,000 daltons. To prove the identity of the genomic clone, we determined the protein sequence of several tryptic and cyanogen bromide fragments of purified bovine IRBP protein and localized them in the protein predicted from its nucleotide sequence. There is a 4-fold repeat structure in the protein sequence with 30-40% sequence identity and many conservative substitutions between any two of the four protein repeats. The third and fourth repeats are the most similar pair. All three of the introns in the IRBP gene fall in the fourth protein repeat. Two of the exons, the first and the fourth, are large, 3173 and 2447 bases, respectively. The introns are each about 1.5-2.2 kb long. The human IRBP gene has a sequence that is similar to one of the introns from the bovine gene. The unexpected gene structure and protein repeat structure in the bovine gene lead us to propose a model for the evolution of the IRBP gene.

Synthetic peptides derived from IRBP induce EAU and EAP in Lewis rats

In an earlier study we isolated three cyanogen bromide cleavage fragments of bovine IRBP that exhibited high levels of immunopathogenicity, producing inflammatory changes in the eyes (EAU) and pineal gland (EAP) of Lewis rats. These fragments have been localized within the IRBP sequence. In order to identify these putative immunopathogenic epitopes of IRBP, nine selected peptide sequences were synthesized and tested for the induction of disease in Lewis rats. Seven of the peptides were found inactive in producing disease while two closely related peptides, designated R4 (23-mer) and R9 (27-mer) were found to reproducibly induce EAU and EAP in immunized rats. No good correlation was found between the immunopathogenicity of the nine tested peptides and their amphipathicity: peptides R4 and R9 were not predicted to form strong amphipathic helices, while peptides selected for their high predicted helical amphipathicity were not immunopathogenic. EAU induced by peptides R4 and R9 was less severe and had a longer onset time than the disease induced by whole IRBP. In addition, the inflammatory changes induced by R4 and R9 in the posterior segment of the eye were less acute than those induced by whole IRBP and included granuloma formation and perivasculitis, features which are not generally seen in rats immunized with whole IRBP. Thus, the changes induced by R4 and R9 more closely resemble those which are characteristically found in human eyes affected by certain uveitic diseases than do changes produced by the intact protein.

Lymphocyte responses to retinal-specific antigens in uveitis patients and healthy subjects

A modified procedure for measuring human lymphocyte responses to retinal antigens was introduced and found to be highly sensitive. The main modifications are the use of round bottom culture wells, high antigen concentrations (up to 100 micrograms/ml) and extended incubation time (up to 9 days). Using this procedure, the majority of patients with uveitis, as well as a large proportion of healthy donors were found to respond positively (S.I. greater than or equal to 2.0) toward S-antigen and interphoto-receptor retinoid-binding protein. The responses to the retinal antigens were further enhanced by enriching the cultured lymphocytes for the helper/inducer subset. The responses to the retinal antigens were inferior to those against tuberculin (PPD), in particular in subjects who had been inoculated with BCG. The "secondary" responses to PPD were always higher in magnitude, were stimulated by markedly lower concentrations of antigen and were detected earlier in culture. The notion that the responses to retinal antigen in healthy donors are "primary" in nature was further supported by the findings that (a) lymphocytes from cord blood samples resembled adult lymphocytes in their response to S-Ag and (b) healthy donor lymphocytes which were prestimulated in vitro with S-Ag reacted to this protein by a specific "secondary" fashion. The possible involvement of lymphocytes with reactivity toward retinal antigens in pathogenic autoimmune processes in the eye is discussed.

Cyanogen bromide fragments of bovine interphotoreceptor retinoid-binding protein induce experimental autoimmune uveoretinitis in Lewis rats

Interphotoreceptor retinoid-binding protein (IRBP), a retinal specific antigen, induces experimental autoimmune uveoretinitis (EAU) when injected into Lewis rats. Here we report that certain cyanogen bromide fragments of IRBP are capable of inducing EAU. Bovine IRBP, reduced and S-carboxymethylated, was subjected to cyanogen bromide cleavage. This CNBr digest was subjected to reversed-phase high performance liquid chromatography. Three fragments were purified to apparent homogeneity. These three fragments were subjected to gas-phase amino-terminal sequencing analysis. All three yielded single sequences, confirming their purity. On the basis of this amino-terminal sequencing and sequencing of cDNAs encoding bovine IRBP, two of these sequences, named CB-58 and CB-71, were localized to the C-terminal one-third of the IRBP molecule, whereas the third, a subfragment thought to result from cleavage at a tryptophan residue and named CB-47, was localized to the N-terminal one third of the protein. CB-71 and CB-47 shared a strong homology, suggesting a putative internal gene duplication event in the evolution of IRBP. All three of these fragments when injected into Lewis rats caused moderately severe EAU with early onset at relatively low doses. The histopathologic changes induced were indistinguishable from those caused by the intact protein. It would seem, therefore, that bovine IRBP contains multiple uveitogenic sites.

Uveitis induced in primates by IRBP: humoral and cellular immune responses

In a previous publication, we have reported that immunization with bovine interphotoreceptor retinoid-binding protein (IRBP) produces severe uveitis in monkeys. The present study further examined the uveitogenicity of IRBP and analysed the immune responses in the immunized monkeys. Uveitis developed in monkeys immunized with bovine IRBP at doses as low as 10 micrograms Kg-1 body wt. In contrast, no disease was detected in monkeys immunized with monkey IRBP at the total dose of 100 micrograms Kg-1. Serum antibodies were measured in the immunized monkeys by the enzyme-linked immunosorbent assay, while cellular immunity was determined by skin hypersensitivity and the lymphocyte mitotic reaction in culture. No humoral or cellular immune responses were detected in monkeys immunized with monkey IRBP. Monkeys immunized with bovine IRBP produced antibodies which cross-reacted well with monkey IRBP. On the other hand, these monkeys did not react with monkey IRBP by skin hypersensitivity and their lymphocytes responded in culture to this protein only when tested by a highly sensitive procedure. In contrast, significant responses of the same cells to bovine IRBP were obtained at all tested conditions. These data are interpreted to suggest that the pathogenic cellular immune responses in vivo detect cross-reactivity with this autologous ocular antigen more efficiently than most conventional laboratory tests.

Dissociation between lymphocyte activation for proliferation and for the capacity to adoptively transfer uveoretinitis

We have shown previously that immunization with bovine interphotoreceptor retinoid-binding protein (IRBP) induces in rats severe eye disease, experimental autoimmune uveoretinitis (EAU). This study examined the uveitogenic capacity of IRBP of another species, the monkey, and tested the cross-antigenicity between these two proteins by a battery of immunological assays. Monkey IRBP was found to be approximately 20 times less uveitogenic in Lewis rats than bovine IRBP. High levels of cross-reactivity between bovine and monkey IRBP were demonstrated by antibodies as measured by the enzyme-linked immunosorbent assay, and by the radiometric ear test of delayed-type hypersensitivity, by using rats immunized with either one of the IRBP. On the other hand, lymphocytes from these rats failed to detect the cross-reactivity between the two IRBP by the proliferation response in culture. Yet, such lymphocytes did recognize the nonimmunizing IRBP when activated in culture for acquiring the capacity to adoptively transfer EAU into naive recipients. The data are discussed with regard to the limited usefulness of the lymphocyte proliferation assay for detection of immunopathogenic processes and the role of cross-reacting antigens in initiation of autoimmune responses.

Dissociation between lymphocyte activation for proliferation and for the capacity to adoptively transfer uveoretinitis

We have shown previously that immunization with bovine interphotoreceptor retinoid-binding protein (IRBP) induces in rats severe eye disease, experimental autoimmune uveoretinitis (EAU). This study examined the uveitogenic capacity of IRBP of another species, the monkey, and tested the cross-antigenicity between these two proteins by a battery of immunological assays. Monkey IRBP was found to be approximately 20 times less uveitogenic in Lewis rats than bovine IRBP. High levels of cross-reactivity between bovine and monkey IRBP were demonstrated by antibodies as measured by the enzyme-linked immunosorbent assay, and by the radiometric ear test of delayed-type hypersensitivity, by using rats immunized with either one of the IRBP. On the other hand, lymphocytes from these rats failed to detect the cross-reactivity between the two IRBP by the proliferation response in culture. Yet, such lymphocytes did recognize the nonimmunizing IRBP when activated in culture for acquiring the capacity to adoptively transfer EAU into naive recipients. The data are discussed with regard to the limited usefulness of the lymphocyte proliferation assay for detection of immunopathogenic processes and the role of cross-reacting antigens in initiation of autoimmune responses.

Experimental autoimmune uveoretinitis (EAU) induced by retinal interphotoreceptor retinoid-binding protein (IRBP): differences between EAU induced by IRBP and by S-antigen

Rats immunized with the retinal interphotoreceptor retinoid-binding protein (IRBP) develop an inflammatory eye disease, "experimental autoimmune uveoretinitis" (EAU). The ocular changes which characterize the EAU induced by IRBP resemble those seen in rats which develop EAU by immunization with another retinal protein, S-antigen (S-Ag). Yet, the two antigens do not cross-react antigenically and the two diseases differ by several features: At low doses (less than or equal to 4 micrograms/rat), IRBP was more uveitogenic in Lewis rats than was S-Ag, inducing disease more reproducibly and with earlier onset time. On the other hand, at higher doses (greater than or equal to 20 micrograms/rat) the disease induced by S-Ag was more severe than that induced by the same doses of IRBP. Rats of various inbred strains differed in their susceptibility to EAU induced by these two antigens. In particular, BN rats were more susceptible to IRBP-induced EAU than to the S-Ag-induced disease, while WF and RCS-rdy+ rats developed severe EAU when immunized with S-Ag but showed minimal or no ocular change when immunized with IRBP.

Immunohistochemical analysis of experimental autoimmune uveoretinitis (EAU) induced by interphotoreceptor retinoid-binding protein (IRBP) in the rat

Experimental autoimmune uveoretinitis (EAU) was induced in rats by immunization with bovine interphotoreceptor retinoid-binding protein (IRBP) and studied by immunohistochemistry. In general, the IRBP induction of inflammatory cellular components and expression of immune-related antigens on various non-lymphoid cells resembled those provoked by S-antigen (S-Ag). However, differences were found between the two diseases, including: 1) The increase in T suppressor/cytotoxic cells occurred in IRBP EAU more rapidly than in S-Ag EAU. 2) Fewer numbers of non-lymphoid cells expressed major histocompatibility complex class II surface antigens in IRBP EAU than in S-Ag EAU. The immunopathogenic mechanism of EAU induced by these two retinal antigens are discussed.