The PPGMRPP repetitive epitope of the Sm autoantigen: Antigenic specificity induced by conformational changes. Application of the Sequential Oligopeptide Carriers (SOCs)

Calreticulin binds preferentially with B cell linear epitopes of Ro60 kD autoantigen, enhancing recognition by anti-Ro60 kD autoantibodies

Calreticulin is a molecular chaperone to newly synthesized polypeptides. Previous studies suggested that calreticulin is probably a protein member of the Ro/La RNP complex. The aims of this study were (a) to investigate whether linear B cell epitopes of the Ro/La RNP complex are bound to calreticulin and (b) if the complex peptide-calreticulin is recognized specifically by anti-Ro autoantibodies. Calreticulin was isolated from either human or pig spleen using a multi-step purification method and found to interact preferentially with biotinylated peptides derived from the sequence of the Ro60 kD 175-184aa(10p) and 216-232aa(17p). The interaction of the peptide-calreticulin complex was favoured by the combination of heat treatment, divalent cations and ATP. La/SSB epitopes did not react with calreticulin. Peptides corresponding to La/SSB epitopes as well as the common epitope of Sm did not interact with calreticulin. Thirty-eight anti-Ro60 KD positive and 23 anti-Ro60 kD negative sera of patients with systemic lupus erythematosus (SLE) and primary Sjögren's syndrome (pSS) were tested. All anti-Ro60 kD positive sera bound the complex calreticulin-17p, while 95% of the same sera had activity against the complex calreticulin - 10p. Tested individually, calreticulin, pep10p and pep17p presented very low reactivity (8%, 11% and 29%, respectively) against anti-Ro60 kD positive sera. Anti-Ro60 KD negative sera did not exhibit significant reactivity either with calreticulin, 10rho and 17rho or with the complexes calreticulin - 10p and calreticulin-17p (<5%). These results suggest that calreticulin can induce conformation-dependent recognition of the Ro60 kD epitopes, leading eventually to their recognition by autoantibodies. This is the first time that such a relationship is shown between a chaperone protein and fragments of an intracellular autoantigen. This work also provides insights into the understanding of mechanisms for autoantibody production. Furthermore, this association can be proved useful for the development of new sensitive assays for autoantibody detection.

No evidence of epitope spreading after immunization with the major Sm epitope P-P-G-M-R-P-P anchored to sequential oligopeptide carriers (SOCs)

The sequence Pro-Pro-Gly-Meth-Arg-Pro-Pro (PPGMRPP) is the major B-cell epitope of the Sm autoantigen. The aim of the present study was to evaluate the immune response against the native forms of Sm and U1RNP and immune mediated tissue injury after immunization with the sequence PPGMRPP anchored in five copies to a new type helicoid sequential oligopeptide carrier (SOC) formed by the repetitive Lys-Aib-Gly moiety, [(PPGMRPP)(5)SOC(5)]. Rabbits (n=3) were immunized with 0.5 mg of (PPGMRPP)(5)SOC(5)in complete Freud's adjuvant and boosted at days 26, 53, 99; control rabbits were immunized with the PPGMRPP alone (n=3), phosphate buffered saline (PBS) (n=1), SOC(5)alone (n=1), a peptide at aminoacid (aa) position 158-177 of myelin basic protein (MBP aa 158-147) (n=1) and three La/SSB autoantigen B-cell epitopes (n=3). Antibodies to (PPGMRPP)(5)SOC(5)were determined by enzyme linked immunosorbent assay (ELISA); precipitating anti-Sm and anti-U1RNP antibodies were detected by RNA precipitation and western blot on HeLa total cellular and nuclear extract and 12s sucrose gradient fraction of rat liver extracts. High titres of anti-(PPGMRPP)(5)SOC(5)antibodies not recognizing the native forms of Sm or U1RNP antigens were detected in the (PPGMRPP)(5)SOC(5)immunized but not in the control animals. The sera of two (PPGMRPP)(5)SOC(5)immunized but not of the control rabbits recognized a 67 kDa protein in HeLa nuclear extract, distinct from the 70 kDa U1RNP antigen. Diffuse and segmental increase of mesangeal matrix and cells, crescent formation, segmental glomerular necrosis, rarely massive subendothelial deposits occluding the lumen and C3 complement component in the mesangeal area were seen in the kidneys of one (PPGMRPP)(5)SOC(5)immunized, but not of the remaining animals. In conclusion, the immune response induced by (PPGMRPP)(5)SOC(5)was specific for the immunizing epitope but not for the native forms of Sm and U1RNP antigens, but it was associated with immune mediated kidney injury.

A new helicoid-type sequential oligopeptide carrier (SOC(n)) for developing potent antigens and immunogens

A new class of sequential oligopeptide carriers (SOC(n)) for anchoring antigenic/immunogenic peptides has been constructed. The carrier, formed by the repetitive Lys-Aib-Gly moiety, is designed to display a predetermined 3D structure, so that the attached peptides would obtain a defined spatial orientation. Conformational analysis showed that SOC(n) adopt a distorted 3(10)-helical structure, while the coupled peptides preserve their original 'active' structure. Coupling to the carrier may also result to the enhancement of one conformer of the anchored peptide. It is concluded that the structure of SOC(n) offers an optimal presentation of the attached peptides, so that potent antigens or immunogens are generated.

A major Sm epitope anchored to sequential oligopeptide carriers is a suitable antigenic substrate to detect anti-Sm antibodies

A sensitive, highly reproducible, solid-phase enzyme immunoassay (ELISA), was developed in order to investigate whether the synthetic heptapeptide PPGMRPP-a major epitope of the Sm autoantigen-anchored in five copies to a sequential oligopeptide carrier (SOC), [(PPGMRPP)5-SOC5] is a suitable antigenic substrate to identify anti-Sm/antibodies. Sera with different autoantibody specificities [45 anti-Sm, 40 anti-U1RNP, 40 anti-Ro (SSA)/La(SSB) positive, 21 Antinuclear antibody positive, but negative for antibodies to extractable nuclear antigens (ANA + /ENA - ) and 75 normal human sera, ANA negative] and 75 sera from patients with rheumatoid arthritis (RA) were tested for anti-(PPGMRPP)5-(SOC)5 reactivity in order to evaluate the specificity and sensitivity of the method to detect anti-Sm antibodies. RNA immunoprecipitation assays for the detection of anti-Sm and anti-U1RNP antibodies and counter immunoelectrophoresis (CIE) for the detection of anti-Ro(SSA) and anti-La(SSB) antibodies were used as reference techniques. The sensitivity of the method was 98% and the specificity was 68% for the determination of anti-Sm antibodies, while for the determination of anti-Sm and/or anti-U1RNP reactivity (antibodies to snRNPs) the corresponding values were 82% and 86%, respectively. In a comparison of the above assay with an ELISA, using Sm/U1RNP purified complex as immobilized antigen it was shown that the sensitivity of the anti-Sm/U1RNP ELISA in detecting anti-snRNPs was 74%; in addition sera with anti-Sm antibodies gave higher binding in the anti-(PPGMRPP)5-(SOC)5 ELISA compared with anti-Sm/U1RNP ELISA. Intra- and inter-assay precision was measured on four sera with reactivities extending into a wide range of absorbance values showed that the intra-assay coefficient of variation (CV%) ranged from 2.7 to 6 and the inter-assay CV% ranged from 9 to 14.5. These results indicate that the PPGMRPP peptide anchored to a pentameric SOC as a carrier is a suitable antigen for detecting anti-Sm antibodies and that the above ELISA is a rapid, reproducible and valuable screening method to test anti-Sm/U1RNP reactivities.