Suppression of mitogen‐induced peripheral B cell differentiation by soluble Fcγ receptors released from lymphocytes

Soluble receptors for FcIgG released from unstimulated human peripheral blood lymphocytes were isolated by affinity chromatography on Sepharose 4B‐IgG. This material was shown to interfere with the differentiation of peripheral blood B cells into Ig‐secreting cells in cultures stimulated with pokeweed or Nocardia opaca extracts. Neither cell viability nor [3H]thymidine incorporation were altered, but the number of Ig‐eontaining cells and that of Ig‐secreting cells were decreased. These effects were dose‐related. They were found to be associated with Fc IgG‐binding soluble material, since absorption on Sepharose 4B‐IgG but not on pepsin‐digested F(ab')2 fragments removed the inhibitory activity. This suppressor factor, released by unstimulated lymphocytes, may represent a human analogue of murine immuno‐globulin‐binding factor (IBF) produced by alloactivated T cells.