Effector cell stimulation-inhibition of in vitro lymphoma cell DNA synthesis and correlation with in vivo antitumor response.

DNA synthesis by murine lymphoma cells was stimulated up to 20-fold in vitro by syngeneic or allogeneic peritoneal cells (PEC) and peripheral blood lymphocytes, as measured by [3H]thymidine incorporation in a 44-hr assay. The increase in DNA synthesis correlated with an increase in tumor cell number in the cultures. The adherent PEC population was responsible for most of the enhancement. This effect was abrogated by pretreating the adherent cells with the metabolic inhibitors iodoacetamide, KCN, NaF, and dinitrophenol, or with glutaraldehyde, or by heating at 56 degrees. Pretreatment with mitomycin C did not eliminate the stimulatory effect. PEC supernatants did not enhance tumor growth, but effector-to-target cell proximity was shown to be necessary for stimulation. PEC from tumor-immunized mice also stimulated tumor target cell growth, but to a consistently smaller degree than did nonimmune PEC. This immune inhibition correlated with in vivo survival of mice to live tumor challenge and with ability of effector cells to increase life-span in adoptive immunity tests. Differential production of thymidine by normal and immune PEC appeared not to be a factor in these assays. Fractionation of PEC showed that the immune nonadherent cells were inhibitory in vitro and were able to increase survival time in adoptive immunity tests. On the other hand, the adherent PEC from immune mice either inhibited, stimulated, or had no effect on tumor cell DNA synthesis, compared with nonimmune adherent PEC, thus exhibiting no correlation with the immune status of the donor. In one example, administration of the macrophage activator lipopolysaccharide to mice resulted in PEC that were inhibitory in the in vitro assay, although the agent did not produce in vivo immunity. The inhibition of tumor DNA synthesis assay, with unfractionated PEC, exhibited a consistent correlation with the immune status of the host when mice were sensitized to lymphoma cells. However, the variable influence of the adherent PEC population on tumor growth reduced or nullified the immune inhibitory effect in a few cases.