The clinical relevance of antibodies to ribosomal-P common epitope in two targeted systemic lupus erythematosus populations: a large cohort of consecutive patients and patients with active central nervous system disease

Objectives-To develop an enzyme linked immunosorbent assay (ELISA) using as substrate a synthetic 22-aminoacid peptide, corresponding to the ribosomal P0, P1 and P2 common epitope. To study the specificity and sensitivity of the method and evaluate the frequency and clinical associations of anti-P a ntibodies in two groups of systemic lupus erythematosus (SLE) patients: (a) unselected SLE patients and (b) SLE patients with central nervous system ( CNS) involvement. Patients and methods-The C-terminal 22 aminoacid peptide of the ribosomal P proteins (Lys-Lys-Glu-Glu-Lys-Lys-Glu-Glu-Lys-Ser-Glu-Glu-Glu-Asp-Glu-Asp- Met-Gly-Phe-Gly-Leu-Phe-Asp) was synthesised according to Merrifield's soli d phase procedure. Purification of the peptide was performed by preparative high performance liquid chromatography and confirmed by amino acid analysi s. Using this peptide, in a concentration 5 mu g/ml, an ELISA was developed . The presence of anti-P antibodies was evaluated by western blot using pur ified ribosomal proteins from rat liver. Sera from 178 consecutive patients with SLE and 28 patients with SLE and CNS manifestations were tested. Sera from 58 patients with rheumatoid arthritis and 57 patients with primary Sj ogren's syndrome were used as controls. The cut off point of the assay was defined using 124 normal sera. Results-The specificity of the assay was evaluated by homologous inhibition . Pretreatment of positive sera with soluble 22mer peptide of the ribosomal P proteins resulted in 88% inhibition. The concordance between the peptide assay and western blot was found to be 83%. Thirty three of 178 (18.6%) of the unselected SLE patients had antibodies to P-protein common epitope. Th eir presence was associated with more active disease (European Consensus Lu pus Activity Measurement, ECLAM scoring system) (p<0.001), higher levels of anti-ds DNA antibodies (p<0.05) and lower levels of the C4 component of co mplement (p<0.01). Eleven of 28 (39.3%) patients with SLE and active CNS in volvement had antibodies to P-protein. The overall prevalence of anti-P ant ibodies in active CNS disease patients was statistically significantly high er, as compared with unselected SLE patients (chi(2)=6.04, p<0.05). These a ntibodies were found in a high proportion of patients without anticardiolip in antibodies (52.4%) and they were associated with diffuse CNS involvement (psychiatric disorders (71%) and epilepsy (75%)). Conclusions-A synthetic analogue of the common epitope of ribosomal P-prote ins can be use as an antigen for the detection of anti-P antibodies. These antibodies are associated with active SLE and CNS involvement particularly in patients without anticardiolipin antibodies.