Sequential synergistic effect of interleukin 2 and interferon-gamma on the differentiation of a Tac-antigen-positive B cell line

The presence of Tac-antigen (Tac-Ag) on human B lymphocytes and its functional significance with regard to the ability of interleukin 2 (IL 2) to modulate B cell differentiation is currently an area of high interest. An Epstein-Barr virus-transformed B cell line (CB) that secretes IgG was 30 to 40% Tac-Ag+ and was used as a model for examining the role of Tac-Ag and IL 2 in B cell differentiation. Recombinant IL 2 alone was found to have a modest but significant effect on CB in enhancing IgG secretion, increasing the plaque-forming cell response from 637 to 1734 at high concentrations (1000 U/ml IL 2) and to 888 at lower concentrations (100 U/ml). In contrast, recombinant interferon-gamma (IFN-gamma) alone had no effect on the differentiation of CB. However, both factors together showed marked synergy in increasing the number of plaque-forming cells to over 3000 by using only 10 U/ml of IFN-gamma and 100 U/ml of IL 2. These two factors were shown to act sequentially in that IL 2 was needed initially, while IFN-gamma was required for the next differentiation step into IgG-secreting cells. The effect of IL 2 on stimulating differentiation was blocked by anti-Tac, indicating that the action of IL 2 is mediated through its Tac-Ag receptor. CB cells were also sorted into Tac+ and Tac- populations and were cultured separately. In 2 wk, both populations reverted to the pattern of the original cell line. Moreover, cell cycle analysis when using double staining procedures indicated that Tac-Ag on the cell surface of CB appears and disappears according to the stage of the cell cycle, and that Tac is most strongly expressed in the S and G2 + M phases. Thus, the present study suggests that certain B cells are capable of responding to sequential stimulation by IL 2 and IFN-gamma with terminal differentiation into Ig-secreting cells, and that the amount of Tac-Ag expression is cell cycle dependent.