Studies on the specificity of the L929 cell bioassay for the measurement of tumour necrosis factor.

We report a study of the specificity of the L929 cell cytotoxicity assay for tumour necrosis factor (TNF) in clinical samples. Normal human serum caused marked killing and detachment of L929 cells. Despite heating at 56 degrees C for 30 min, followed by dilution, normal human serum still retained significant activity that could not be neutralized by a monoclonal antibody to TNF. In part this latter cytotoxicity was spurious, and related to detachment and clumping of viable cells. Recombinant human interleukin 1-beta (rhIL1-beta) and interferon-gamma (rhIFN-gamma) were not cytocidal, and did not synergize with recombinant human TNF (rhTNF) in the L929 cell bioassay. Although an antibody to TNF may be used to confirm specific activity, it is important to be aware of the limitations imposed by non-specific activity in serum and of the potential for synergy between TNF and other cytokines.