Association of anti–ribosomal P protein antibodies with neuropsychiatric systemic lupus erythematosus

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Abstract
Objective. Although it has been reported that anti–ribosomal P protein antibodies (anti-P) are highly specific for lupus psychosis, there have been discrepancies among the studies regarding the clinical correlation of these antibodies with systemic lupus erythematosus (SLE). The present study was therefore carried out to reappraise the association of anti-P and neuropsychiatric SLE. Methods. Highly purified synthetic ribosomal P peptides of the carboxyl-terminal 22-amino acid sequence were conjugated to human serum albumin (HSA) with the use of glutaraldehyde. Anti-P in sera from 75 patients with SLE (26 without central nervous system disease [non-CNS], 28 with lupus psychosis, and 21 with nonpsychotic CNS involvement [nonpsychotic CNS lupus]) were analyzed with an enzyme-linked immunosorbent assay (ELISA), using HSA–ribosomal P peptide conjugates as antigens. Anti-P levels were quantitated by subtracting the nonspecific binding activities to HSA. Results. The ELISA was found to be specific for anti-P, as determined by comparison with the results of Western blotting using extracts of HEp-2 cells. Serum anti-P levels were significantly elevated in patients with lupus psychosis, including organic brain syndrome and nonorganic psychosis, compared with those with non-CNS SLE or those with nonpsychotic CNS lupus. There were no significant differences in serum anti-P levels between patients with organic brain syndrome and those with nonorganic psychosis. Anti–P antibodies were not detected in the cerebrospinal fluid of patients with either lupus psychosis or nonpsychotic CNS lupus. Conclusion. Our results, obtained from the highly specific ELISA using HSA–ribosomal P peptide conjugates, confirm the correlation of serum anti-P with lupus psychosis. The data also suggest that differences in the purity of the ribosomal P peptides used might be a major reason for the conflicting results in the literature regarding the association of anti-P with lupus psychosis.