Introduction of the cell survival gene bcl-xL improves the viability of CTLL-2 cells without affecting their IL-2 proliferative response. Implications for the development of bioassays.

A standard method for the quantitation of cytokines is to perform a bioassay in which aliquots of samples are compared to known concentrations of a cytokine in supporting the proliferation of a cytokine-dependent cell line. In most instances however, these cell lines are dependent on the cytokine not only for proliferation but also for survival. For example, a cell line that is commonly utilized for interleukin-2 (IL-2) bioassays is the IL-2-dependent line, CTLL-2. CTLL-2 cells will die rapidly by apoptosis if withdrawn from IL-2, thus these cells can be difficult to maintain in culture for extended periods. Overexpression of the anti-apoptotic protein Bcl-x(L) can enhance CTLL-2 survival in the absence of IL-2. However, while overexpression of Bcl-x(L) can prevent CTLL-2 cells from dying in the absence of IL-2, overexpression of Bcl-x(L) does not impair the ability of CTLL-2 cells to be used for proliferation-based IL-2 bioassays. Thus the bcl-x(L)-transfected CTLL-2 cells are equivalent to the parental cell line for determination of IL-2 levels in a culture supernatant, yet are easier to maintain in culture. Introduction of Bcl-x(L) or Bcl-2 into other factor-dependent cell lines may also simplify their maintenance without significantly affecting their utility in bioassays.