The detection of raised levels of IgM to Proteus mirabilis in sera from patients with rheumatoid arthritis.

An analysis by ELISA of 100 rheumatoid factor (RF)-positive sera selected at random from a collection of sera from patients with various auto-immune diseases and joint pains, and 100 RF-negative sera from the same collection matched by patient age and gender, showed that the RF-positive sera had highly significantly (p < 0.0001) raised levels of IgM antibody, but not IgG antibody, to Proteus mirabilis over those of the RF-negative sera. This response was subsequently found to be associated with sera from patients who clinically had rheumatoid arthritis (RA). Sera from the RA patients had significantly greater amounts (p = 0.026) of IgM antibody to P. mirabilis than to the other organisms tested and these values were also highly significantly different (p < 0.0001) from P. mirabilis IgM antibody levels in matched RF-negative sera. Sera from RA patients also had significantly greater amounts of IgA to P. mirabilis (p < 0.0001) and greater amounts of IgM to Escherichia coli (p < 0.0001) and Klebsiella pneumoniae (p < 0.0001) than those in matched RF-negative sera. Other classes of antibody to these organisms and all classes of antibody to Pseudomonas aeruginosa were not raised in the sera of RA patients over those of RF-negative controls. The IgM response in RA patients was not specific for only one O serotype of P. mirabilis but was associated with all 11 different O serotypes of P. mirabilis tested and those of other Proteus spp. Moreover, the IgM antibodies to Proteus spp. appeared to be independent from C-reactive protein and RF.(ABSTRACT TRUNCATED AT 250 WORDS)