Anti-ribosomal P protein antibodies detected by immunoblotting in patientswith connective tissue diseases: their specificity for SLE and associationwith IgG anticardiolipin antibodies

Objective-To assess the prevalence and clinical and serological association s of anti-ribosomal P protein antibodies (anti-P antibodies) in patients wi th connective tissue diseases (CTDs) and investigate the immunobiological n ature of autoantibody clustering in which anti-P antibodies play a part. Methods-IgG anti-P antibodies in the sera of 267 patients with CTDs and 31 healthy subjects were analysed by immunoblotting performed on cytoplasmic e xtract of Rail cells. 60 patients with systemic lupus erythematosus (SLE), 32 systemic sclerosis, 46 primary Sjogren's syndrome, 16 poly/dermatomyosit is, 11 rheumatoid arthritis, 8 undifferentiated CTD, 72 overlap CTD, and 22 primary antiphospholipid syndrome were studied. Anti-P antibodies were aff inity purified by elution from nitrocellulose bound antigen and tested by E LISA for their binding activity to cardiolipin. Results-Anti-P antibodies were detected in 16 (6%) patients and in none of the controls: 12/60 SLE (20%) and 4/80 undifferentiated/overlap patients wi th CTD (5%). A close association of IgG antibodies with P proteins and with cardiolipin was seen in lupus sera (p=0.0009, odds ratio 18.33). Anti-P an tibodies from 9 of 12 anti-P lupus serum samples could be affinity purified and none of the affinity purified fractions cross reacted with ELISA plate coated cardiolipin. Conclusions-Anti-P immunoreactivity is a specific marker of SLE and lupus-l ike disease and its detection is recommended as a powerful diagnostic tool. Anti-P antibodies are strongly clustered with IgG anticardiolipin antibodi es in lupus sera, even if they are independently elicited. This suggests th at their cognate autoantigens play a part in a common pathogenetic pathway in SLE.