Immunocytology of cultured IgM-forming cells of mouse

Abstract A factor required for the proliferation of IgM-forming tumor cells of mouse was purified approximately 1500-fold from culture supernatant of phagocytic cells through conventional protein fractionation procedures. The isoelectric point of the factor was pH 6.1 ± 0.1 as measured by isoelectric focusing. Its molecular weight was estimated to be approximately 5 × 10 4 daltons. These characteristics of the factor were not identical with those of the stimulating factor for granulocyte and macrophage colony formation. Antiserum against the purified factor inhibited the growth-promoting activity of the factor. The effect of the antiserum on the normal antibody response of mouse to sheep red blood cells was examined in the tissue culture system. The antiserum inhibited only the late stage in the formation of direct plaque-forming cells.