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

IRBP: preparation and characterization of site-specific monoclonal antibodies

Interstitial retinoid binding protein (IRBP) is a 136,000 molecular weight photoreceptor cell protein which is a highly pathogenic autoantigen for the induction of experimental autoimmune uveitis (EAU). In this study we produced a series of monoclonal antibodies (MAbs) which define different epitopes in the native molecule. These MAbs were further subdivided into three distinct groups based on a radioimmunoassay, and by ELISA assay using native IRBP and synthetic peptides corresponding to its entire amino acid sequence. Group I MAbs (MAbD7-B1 and MAbC6-B4) bound to native IRBP but not to any synthetic peptides, suggesting that their antigenic epitopes are strictly conformation dependent. Group II MAbs (MAbC7-D3 and MAbG8-H4) bound weakly to multiple peptides which shared amino acid sequence similarity located within each of four homology domains indicating that these epitopes are also conformation dependent. In group III (MAbH3-B5, MAbH7-A5, and MAbB6-D12) MAb binding was localized to a specific peptide. The MAbH3-B5 binding site was further refined to amino acid positions 361 to 367 in the native molecule. MAbH3-B5 was also useful in localizing IRBP in the mouse retina by immunohistochemical techniques. The application of these MAbs in the study of EAU and interphotoreceptor transport mechanisms is discussed.

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.

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.

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.

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.