Autoantibodies in human anti-Ro sera specifically recognize deproteinized hY5 Ro RNA

We report the existence of a novel autoantibody specificity linked to anti-Ro antibodies. Sera from two patients with anti-Ro ribonucleoprotein (RNP) antibodies also contained antibodies that immunoprecipitated specifically either the deproteinized RNA component of the RohY5 RNP particle, or intact in vitro transcribed hY5 RNA. No serum recognized specifically the other hY RNAs. A mutant hY5 RNA with additional nucleotides (nt) at both extremities was not immunoprecipitated, possibly because of altered secondary structure. Following digestion of hY5 RNA with ribonuclease T1, the smallest immunoprecipitable RNA fragments were 27 and 31 nt long, and respectively mapped to the 5' and 3' ends of hY5 RNA, excluding the La-binding region. Base pairing between the 27 and 31 nt long fragments was required for recognition by antibodies. Our data indicate that the epitope bound by anti-hY5 RNA antibodies is conformational. We have previously reported that most anti-Ro sera contain a population of antibodies specific for the RohY5 RNP. Since antibodies to the deproteinized hY RNAs within anti-Ro sera are also restricted to anti-hY5 RNA, a direct role for the human-specific RohY5 particles in the immunization process leading to the production of anti-Ro antibodies is suggested.