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It has been suggested that Proteus infection may be involved in the pathogenesis of rheumatoid arthritis (RA). Bacterial and peptide immune responses in patients with RA and other control subjects were investigated in two geographically different populations. Serum samples from Finnish patients with early ( n=72) and advanced ( n=27) RA and 30 Finnish healthy controls, as well as from Japanese RA patients from two different locations: Tokyo ( n=30) and Otsu ( n=30), 18 patients with systemic lupus erythematosus (SLE) and 23 Japanese healthy controls were all screened for the total, and class-specific (IgG, IgA and IgM) antibodies against Proteus mirabilis, Escherichia coli and Serratia marcescens by indirect immunofluorescence assay. These samples were also tested for the determination of levels of isotypic antibodies against the shared epitope involving 16-mer synthetic peptides containing the EQRRAA or ESSRAL sequences and compared to scrambled control peptide by using an enzyme-labeled immunosorbent assay method. Significantly elevated levels of IgG and IgM antibodies to P. mirabilis and antibodies against both EQRRAA and ESSRAL peptides were detected in sera of Finnish patients with early and advanced RA, and in Japanese patients from Otsu or Tokyo compared to their corresponding control groups. In contrast, no difference either in the total or in any of the isotypic antibodies were observed between these groups when serum samples were screened against each of E. coli and S. marcescens or against the control peptide. Furthermore, there was a significant correlation between the antibody levels against Proteus bacteria only and both EQRRAA and ESRRAL peptides. Our findings support the possibility for specific involvement of P. mirabilis in the etiopathogenesis of RA even in early cases.