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Secretion of IgM-rheumatoid factor and IgM by blood lymphocytes in rheumatoid arthritis.

A direct plaque forming cell (PFC) assay for the detection of complement fixing IgM-rheumatoid factor (RF) secreting lymphocytes was evaluated. The specificity of the RF-PFC assay was demonstrated by the inhibitory action of exogenous human IgG. The sensitivity of the RF-PFC assay was similar to that of a reverse PFC assay detecting all cells secreting IgM (IgM-PFC). In blood from 75% of seropositive patients with rheumatoid arthritis (RA) RF-PFC were demonstrated, median value 6.8 (range: 0.4-1477) RF-PFC/10(6) mononuclear cells. Practically no RF-PFC were detected in seronegative RA patients and controls. Despite the fact that most IgM-RF undoubtedly is produced outside the blood, a positive correlation was found between the number of RF-PFC and the Waaler-Rose titer in serum. The number of circulating IgM secreting cells did not differ significantly between seropositive RA patients, seronegative RA patients and controls. Comparison of RF-PFC and IgM-PFC in seropositive patients revealed that in mean 7% of IgM-secreting cells in blood secrete IgM-RF.