Detection of immunoregulatory rheumatoid factor

Regulatory rheumatoid factor (regRF) represents a separate rheumatoid factor population that might be potentially used for clinical diagnostics, whereas regRF-producing lymphocytes serve as a promising therapeutic target for autoimmune diseases. Hence, it is worth outlining conditions allowing to reliably detect regRF in humans and laboratory animals. The method of agglutination with tanned IgG-loaded red blood cells was used to measure level of regulatory rheumatoid factor, because it allows to identify solely regRF as compared to the latex fixation method detecting both the regulatory rheumatoid factor as well as the rheumatoid factor associated with rheumatic diseases. It was found that fresh or frozen serum should be used to detect regRF in rats. Freshly purified rat blood plasma stabilized with heparin, sodium citrate or EDTA did not allow to detect regRF, potentially being linked to the presence of fibrinogen known to demonstrate IgG binding activity. In addition, heparin itself also complicates regRF detection in rats. In humans, regRF was reliably detected in pre-frozen plasma stabilized with sodium citrate. Finally, false-negative results were obtained in a small number of cases while using blood serum for detecting regRF in humans.