Partial biochemical characterization of IgG1-inducing factor

IgG1 induction factor elevates the IgG1 and suppresses the IgG3 and IgG2b responses in lipopolysaccharide-stimulated murine spleen cell cultures. By the use of a quantitative assay, it was found that the three activities, induction of IgG1 and reduction of IgG3 and IgG2b synthesis, were found in the same fractions after different chromatographic procedures, suggesting that the same molecule was responsible for the effects. The factor was precipitated by 60-90% saturation of ammonium sulfate and was sensitive to proteolytic cleavage and to treatment with a buffer of pH 10. It had an apparent molecular mass of 20 kDa as judged by gel filtration chromatography and was separated into two peaks after isoelectric focusing, pI 7.4-7.2 and 6.4-6.2, respectively. Finally it was weakly hydrophobic and negatively charged at pH 7.55. These characteristics indicate that the factor is different from many previously characterized lymphokines and similar or identical to the B cell stimulating factor-1 (BSF-p1). The relevance of these findings to the mechanism of the immunoglobulin class switch is discussed.