High nonspecific reactivity of normal lymphocytes against mycoplasma-infected target cells in cytotoxicity assays.

Several rat tumor cell cultures were deliberately infected with three species of mycoplasma commonly found as contaminants of cell lines grown in vitro, and the effect of mycoplasma infection on the results of cytotoxicity assays was examined. Lymph node cells and spleen cells from normal animals showed an apparently high spontaneous cytotoxic activity against tumor cells infected with either M. arginini or M. hyorhinis, but the reactivity against cells infected with M. orale was not significantly higher than that against uninfected cells. The high reactivity towards tumor cells infected with M. arginini and M. hyorhinis bore a close resemblence to natural cell-mediated immunity in that spleen cells were much more reactive than lymph node cells, spleen cells from nude mice were as effective as spleen cells from normal mice, and the reaction crossed both strain and species barriers. However, closer examination revealed that the cytotoxic effects were directly caused by depletion of arginine or other essential nutrients from the medium. These findings imply that a cautious approach should be taken when interpreting certain aspects of spontaneous cell-mediated cytotoxicity, and that the greatest care be taken to ensure that the cells used as targets in any cytotoxicity test are mycoplasma-free.