ABSTRACT
The production of antibodies against nonnominal immunoglobulin allotypes in rheumatoid arthritis (RA) patients suggests that the immune system of these patients has been exposed to such foreign allotypes. The presence of nonnominal allotypes is, however, a genetic enigma. We searched for nucleotide sequences specific for nonnominal G3mg and G3mb copies in individuals homozygous for these alleles. Using a sensitive and specific nested polymerase chain reaction (PCR) method with genomic DNA from blood of 18 RA patients and 5 normal controls, we found G3mg sequences in 18 of 18 tested G3mb homozygous persons. The allele specificity of the PCR fragments was confirmed by sequencing and RFLP analysis. The PCR products contained genomic nonspliced parts of the nonnominal sequences. An analysis of cDNA from inflammatory tissue of 5 RA patients detected nonnominal G3mb sequences in 1 of 3 tested G3mg homozygotes and G3mg sequences in 2 of 2 tested G3mb homozygotes. The cDNA-derived PCR products contained sequences from normally spliced nonnominal Ig fragments. The results also showed that the nonnominal Ig sequences were present in very low copy numbers, lower than the Mendelian 1-2 copies per cell. The origin of such a low copy number of Ig gene fragments may be explained by a virus-mediated capture and transfer mechanism of Ig gene fragments generated by the normal Ig switch-associated gene excision process.
