Gamma‐Irradiated peripheral blood mononuclear cells can express LAK activity

Peripheral blood mononuclear cells (PBMC) irradiated with high dose 7‐radiation (1000‐5000 rad) are commonly used as feeder cells during the cloning of T lymphocytes, natural killer (NK) and lymphokine activated killer (LAK) cells. We report here that such γ‐irradiated PBMC can be stimulated with interleukin 2 (IL‐2) to express the ability to lyse a variety of tumor cell targets. The non‐major histocompatibility complex (MHC) restricted cytotoxicity demonstrated by irradiated PBMC is, however, lower than that expressed by their non‐irradiated counterparts. The numbers of viable, γ‐irradiated LAK cells are significantly increased by the addition of the mitogen, phytohemagglutinin (PHA). Purification of the γ‐irradiated cells expressing cytotoxic activity by flow cytometry determined that the effector cells were predominantly CD3^‐ cells, although some CD3⁺ cells also expressed moderate LAK activity. The ability of γ‐irradiated cells to proliferate in the presence of PHA alone, or with IL‐2 + PHA, was maximal at day 4‐5; but proliferation, as detected by ³H‐thymidine uptake, was not detectable beyond 12‐15 days of in vitro culture. Because many of the LAK, T cell and NK cell cloning procedures require the presence of feeder layers, growth factors (usually IL‐2) and mitogens, the presence of residual feeder cells expressing cytotoxic activity may affect the specificity of such clones. Thus, efforts should be made to ensure that such γ‐radiation‐resistant cells capable of expressing cytotoxic activity are completely eliminated before the cloned cells are used for further experiments.