Cells of the human B-cell lymphoblastic leukemia line, BALL-1, were stimulated with Sendai virus. It is shown that the crude culture supernatants contain not only interferon-alpha (IFN-alpha) and tumor necrosis factor-alpha (TNF-alpha) but also other cytokines, here collectively termed 'X' cytokines. The latter did not affect the cytostatic or differentiation-inducing effect of IFN-alpha but synergistically increased the cytotoxicity of TNF-alpha.

HL-60 promyelocytic leukemia cells were cultured continuously in the presence of 22,000 IU/ml of purified interferon-alpha (IFN-alpha) for up to 469 days. Study of the properties of the cells at various times during this period showed some changes in their expression of IgG Fc receptors, and in the maturation of the cells as indicated by their esterase content. The differences from control cells cultured in parallel were most marked after exposure to IFN-alpha for some 40-300 days, but were no longer seen after 400-470 days. The difference in tumorigenicity was significant after 150 days, but no longer after 300 days.