Anti-S180 Abs do not enhance in vitro killing of S180 cells. (A) SR/CR mice that had been previously challenged with S180 cells at least twice were anesthetized and the peritoneal cavities washed with 10 ml PBS. The PBS was allowed to equilibrate for at least 2 minutes and was removed by aspiration. Cells were spun down and discarded, and the fluid was analyzed for the presence of anti-S180 Abs. S180 cells were incubated with the peritoneal fluid for 30 min on ice, washed, and stained with a rhodamine-conjugated secondary antibody to identify mouse Abs. WT mice that had never encountered S180 cells served as negative controls. (B) S180 cells were labeled with SR/CR anti-S180 Abs by incubation with SR/CR peritoneal fluid containing anti-S180 Abs. Labeled S180 cells were cultured in standard in vitro killing conditions alone or with purified WT or SR/CR (abbreviated SR) macrophages. The survival of labeled S180 cells was compared to that of unlabeled S180 cells in similar samples to identify the role of Abs in S180 cell killing. Results are reported as the mean and standard deviation of S180 cell survival for at least 2 experiments.