Specific and nonspecific antitumor immunity. I. Description of an in vitro assay based on inhibition of DNA synthesis in tumor cells.

As in vitro assay of cell-mediated antitumor immunity based on the inhibition of tumor cell DNA synthesis (IDS) was devised. It was reasoned that both cytolytic and cytostatic antitumor effects could be measured in a quantitative yet generalized manner with this approach. By the use of microculture techniques and simplified methods for the determination of tritiated-thymidine incorporation by all cells in individual cultures, normal and immune effector (E) cell populations were compared in their ability to inhibit the DNA synthesis of a fixed initial number of various target tumor cells. Doubling dilutions of E cells were used to titrate the antitumor effects of normal and immune cells at many E/T (effector to target) ratios. Under conditions of alloimmunization, significant immunologically specific IDS effects could routinely be detected at an E/T ratio of less than 1 or 0.1:1, and under certain conditions at 1:100 or less. Results were highly reproducible with respect to the individual E cell donors, replicate cultures, and repeat experiments. The effects were proportional to visually determined cell destruction and independent of obvious culture artifacts. The IDS method was compared with the 51Cr release technique under various experimental conditions. The results demonstrated that decreases in E/T ratio and/or the addition of excess nonimmune cells to immune effector populations had a similar effect in both assays, which was to decrease the magnitude of the immune cell activity. Addition of excess normal cells reduced the activity of immune cells to a level below that of an equal number of immune cells tested at the same E/T ratio without added nonimmune cells. Both assays detected primarily a T lymphocyte-mediated lytic event when effectors generated by the described alloimmunizations were used. The IDS assay also detected a weak non-T-cell activity in anti-theta plus complement-treated alloimmune spleen. The possibility that this represented antibody-dependent cell-mediated cytotoxicity was raised by the finding that normal spleen ceels plus antitarget antibody had significant activity in the IDS system. The sensitivities of the two methods were compared and the potential of the IDS method was evaluated.