Differential sensitivity to transforming growth factor (TGF)-{beta} of CBA and of CBA/N B cells demonstrates that the IgG2b inducing factor in synovial fluid from rheumatoid arthritis patients is not identical to TGF-{beta}

Synovial fluid from patients with rheumatoid arthritis (RA-SF)contains in vivo produced cytokines and inflammatory mediators,including a factor that induces IgG2b production of lipopolysaccharide(LPS) preactivated murine B lymphocytes. In order to determinethe mechanism by which RA-SF acts on LPS activated mouse B cells,CBA/N mice were used as an experimental model. The X-linkedimmunodeficiency of these mice is caused by a point mutationin the Bruton's tyrosine kinase (btk) gene. We have earliershown that RA-SF can reconstitute the CBA/N B cell deficiencyin vitro and in vivo, with regard to IgG2b production afterLPS stimulation. Since transforming growth factor (TGF)-ßhas been suggested to be a switch factor for IgG2b, we aimedat investigating the role of TGF-ß in our experimentalsystem. We found that TGF-ß could not mimic the effectof RA-SF on CBA spleen cells. A small increase of IgG2b secretionwas observed with spleen cells from normal CBA mice, whereasIg secretion of all isotypes was suppressed in CBA/N spleencells treated with TGF-ß at any concentration. Neutralizingantibodies against TGF-ß suppressed the response ofCBA B cells, whereas the response by CBA/N B cells was enhancedby the same antibody preparation. Here we also show that theabnormal B cell responsiveness to TGF-ß, typical ofCBA/N, co-segregates with the btk mutation in male (CBA x CBA/N)F₂spleen cells. This was determined by allele specific PCR recognizingthe identified base substitutions of the btk gene, typical ofthe two strains. We propose that RA-SF contains a factor, separatefrom TGF-ß, that is involved in the differentiationof IgG2b expressing cells.