Characterization and partial purification of a novel cytotoxic lymphokine (factor 2) produced by a human B cell line (Karpas 160).
A subclone (160b) of the human B cell (Karpas 160) was shown to produce a novel cytotoxic lymphokine [Factor 2 (F2)] in addition to tumour necrosis factor alpha (TNF-alpha) and beta (TNF-beta). F2 was found to have selective toxicity to numerous human tumour cell lines, particularly the erythroleukaemic cell line K562, whereas TNF-alpha/beta were not cytotoxic to these cells, even at relatively high concentrations. Our studies have shown that F2 activity in crude preparations is heterogeneous both in its molecular weight, isoelectric point (pI) and hydrophobicity, which depends not only on the source of F2 cytotoxicity but also on the conditions of methods for its production. Our studies also showed that F2 was separable from TNF by DE52, S-300 gel filtration and Rotofor isoelectric focusing. F2 was partially purified up to 1000-fold by two procedures. The major active form, as assessed by gel filtration was of mol. wt of 45-67 kDa. On SDS-PAGE, F2 activity was recovered mainly from two regions of the gels corresponding to 10-14 kDa and 60-70 kDa. Antibodies of human TNF-alpha, TNF-beta, IFN-alpha, IFN-gamma, and TGF-beta failed to prevent F2-mediated cytotoxicity to K562. F2 activity was not inhibited by mannose-6-PO4 or mouse mAb to rat granule content, both of which have been reported to block human natural killer cytotoxic factor. Our studies indicated that F2 is likely to be a distinct human cytokine with selective cytotoxic activity against tumour cells.