Human macrophage activation factors. I. Multiple molecular species of MAF produced by a human lymphoid cell line

The lymphokine, macrophage activation factor (MAF), is defined functionally as the factor that activates mouse macrophages to kill syngeneic tumor cells. The source of MAF in this study is the Namalwa cell line, a human Burkitt's lymphoma cell line, that produces MAF as a constitutive product. The initial procedure in MAF characterization has been chromatography on Sepharose 6B-C1 column with the denaturating solvent, 6 M guanidine HCl, pH 8.0, in order to reduce the copurification of MAF activity with serum proteins. Namalwa cells incubated in RPMI 1640 with 2% fetal calf serum (FCS) produce MAF in the molecular weight ranges of 70,000 +/- 7000, 25,000 +/- 25000, 12,500 +/- 1250 and less than 10,000; while Namalwa cells, incubated in 10% FCS, produce activity at 12,500 daltons and below. The MAF active fraction from 10% FCS supernatant was reincubated in 10% FCS for 24 hr, and on rechromatography the MAF activity appears from 70,000 to 12,500. Reincubation in 6 M guanidine moves the MAF activity to the column void volume (greater than 100,000 daltons) and to 70,000 daltons. The increase in molecular size of MAF suggests that MAF consits of a small basic molecular unit (less than 10,000 daltons) which associates in larger molecular forms upon incubation at 37 degrees C.