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 (ELIS A), was developed in order to investigate whether the synthetic heptap eptide 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. Ser a with different autoantibody specificities [45 anti-Sm, 40 anti-U1RNP , 40 anti-Re (SSA)/La(SSB) positive, 21 Antinuclear antibody positive, but negative for antibodies to extractable nuclear antigens (ANA + /E NA -) and 75 normal human sera, ANA negative] and 75 sera from patient s with rheumatoid arthritis (RA) were tested for anti-(PPGMRPP), (SOC) , reactivity in order to evaluate the specificity and sensitivity of t he method to detect anti-Sm antibodies. RNA immunoprecipitation assays for the detection of anti-Sm and anti-U1RNP antibodies and counter im munoelectrophoresis (CIE) for the detection of anti-Ro(SSA) and anti-L a(SSB) antibodies were used as reference techniques. The sensitivity o f 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 wit h 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 hi gher binding in the anti-(PPGMRPP),-(SOC), ELISA compared with anti-Sm /U1RNP ELISA. Intra- and inter-assay precision was measured on four se ra 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 result s indicate that the PPGMRPP peptide anchored to a pentameric SOC as a carrier is a suitable antigen for detecting anti-Sm antibodies and tha t the above ELISA is a rapid, reproducible and valuable screening meth od to test anti-Sm/U1RNP reactivities. (C) 1998 Elsevier Science B.V. All rights reserved.