Uveitis induced in primates by interphotoreceptor retinoid-binding protein

The isolation of a gene encoding interphotoreceptor retinoid-binding protein

The gene locus for bovine IRBP, as well as several kilobases of sequence flanking the gene on either end, has been cloned. Two of the several clones seem to contain full-length copies of the protein encoding portion of the gene. Using these clones and cDNA clones, we have determined that there are one or perhaps two copies of the IRBP gene per haploid genome in several species. The gene is compact considering the large size of the protein (145,000 daltons) and its large mRNA (about 6.5 kb). Surprisingly, the gene is no more than 14 kb, being fully contained on lambda clones of maximum packaging size 20 kb. Small parts of the gene were sequenced for the purpose of proving the identity of the genomic clones. DNA sequencing of one of the IRBP gene clones demonstrates the existence of an intron in the gene. The sequence analysis of another fragment identified the N-terminus which has been sequenced at the protein level. The DNA sequence analysis showed the existence of a putative signal sequence and the potential existence of a short five amino acid sequence between the signal sequence and the authentic N-terminus of the secreted extracellular IRBP. This confirms and validates the finding of the extra five amino acid sequence that is present on 40-50% of the polypeptides in monkey and human IRBP which have been isolated from the subretinal space. The presence of the appropriate gene sequence for the pentapeptide but its absence in bovine IRBP indicates differences in processing among the vertebrate IRBPs.

Human IRBP: characterization of uveitopathogenic sites

Human interstitial or interphotoreceptor retinoid binding protein (IRBP) is a 136,000 molecular weight photoreceptor cell protein capable of inducing an experimental autoimmune uveitis (EAU) in susceptible animal strains. In order to determine specific sites in human IRBP responsible for its uveitopathogenicity, we synthesized 60 peptides, corresponding to its entire 1262 amino acid sequence, and tested each peptide for its ability to induce an EAU in Lewis rats. Three peptides with extensive amino acid sequence homology, designated HIRBP 715, HIRBP 730, and HIRBP 745, were uveitopathogenic when used at a 50 micrograms immunizing dose. The most potent peptide for the induction of EAU was HIRBP 715. Histopathologically a severe inflammatory response was present in the anterior and posterior segments of the eye. In these eyes the retina was infiltrated extensively with inflammatory cells. Focally the photoreceptor cell layer of the retina was destroyed. There was an associated subretinal exudate as well as an occasional subretinal granuloma. The clinical and histopathological changes in the eyes of rats immunized with peptides HIRBP 730 and HIRBP 745 were less severe as compared to HIRBP 715. One additional peptide, HIRBP 720, without extensive amino acid sequence homology to other regions of human IRBP, was also uveitopathogenic under our experimental conditions. Our study identifies multiple uveitopathogenic sites in the human IRBP molecule and, based on the primary amino acid sequence three of these sites are interrelated by several gene duplications which occurred some 600-800 million years ago in the native IRBP molecule.

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.

Relationship between sympathetic ophthalmia, phacoanaphylatic endophthalmitis, and Vogt-Koyanagi-Harada disease

The more than coincidental occurrence of phacoanaphylatic endophthalmitis (PE) in sympathetic ophthalmia, and the similarity of the dissimilarity between Vogt-Koyanagi-Harada (VKH) disease and sympathetic ophthalmia have been well described both clinically and histopathologically. The etiology and pathogenesis of these three diseases are still not fully understood. Identifying and distinctive characteristics among them include the history of ocular trauma in sympathetic ophthalmia, rupture of the lens capsule in phacoanaphylatic endophthalmitis, and involvement of the skin, ear, and central nervous system in VKH disease. A T-cell-mediated immune reaction to ocular antigens seems to play a major role in sympathetic ophthalmia and VKH disease. A B-cell-related Arthus reaction to lens antigen seems to be the principle mechanism of PE. Thus, these three diseases may represent a spectrum of uveitis. At one end is the delayed-type hypersensitivity disease of sympathetic ophthalmia, whereas at the opposite end is the immune complex disease of PE, with VKH disease in the middle of this uveitic spectrum.

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.

Uveoretinitis in rabbits following immunization with interphotoreceptor retinoid-binding protein

Interphotoreceptor retinoid-binding protein (IRBP) is a glycoprotein found in the interphotoreceptor matrix between the neurosensory retina and the retinal pigment epithelium and is thought to shuttle retinol among cells that border the interphotoreceptor space. Immunization of rabbits with bovine IRBP caused subsequent photoreceptor degeneration, as documented by light- and electron microscopy. Beginning on post-injection day 18, scattered regions had photoreceptor outer segments that were disorganized and shortened or absent. Macrophages were found between the retinal pigment epithelium and neurosensory retina and within choroidal interstitium and blood vessels. Labeling of these cells with a marker specific for monocytic macrophages (RAM11) and absence of labeling with a marker for retinal pigment epithelium (rabbit anti-bovine cellular retinaldehyde-binding protein) suggest that these macrophages were hematogenous in origin. Staining of retinas with fluorescein isothiocyanate (FITC)-conjugated sheep anti-rabbit IgG revealed leakage of rabbit IgG into the interphotoreceptor matrix on and after day 18 in experimental animals but not in controls, suggesting breakdown of the outer blood-retinal barrier. Indirect immunofluorescence with anti-glial fibrillary acidic protein revealed labeling of Müller cells in experimental retinas on and after day 18, but not in control or shorter survival experimental retinas. There were foci of increased cellularity in the choroid on days 18, 26 and 39. From days 26 through 67, the retinal pathology became more widespread. Varying degrees of outer-segment degeneration were present in all parts of the retina and in many areas there was total loss of outer segments and loss of some photoreceptor-cell bodies. The inner retina appeared unaffected in all experimental and control retinas. These results demonstrate that injection of rabbits with bovine IRBP causes retinal photoreceptor degeneration as anti-IRBP titers increase and breakdown of the outer blood-retinal barrier ensues. Further studies will be required to elucidate factor(s) that control accessibility of the neurosensory retina to circulating antibodies against IRBP and other intrinsic retinal proteins.