In vitro lysis of tumor cells by antibody and complement: detection of antibody with low cytotoxic reactivity.

The factors affecting the in vitro lysis of PARA-7 tumor cells mediated by antibody and complement were examined with the use of the 51Cr-release assay. The addition of immune syngeneic hamster antisera (HA) to 51Cr-labeled PARA-7 target cells 30 minutes before the addition of guinea pig complement (GPC) failed to produce significant levels of lysis. Similarly, the addition of rabbit anti-PARA-7 serum (RAS) 30 minutes prior to the addition of GPC resulted in low levels of lysis. Longer incubations in the presence of RAS, but not RAS and GPC, increased levels of lysis. Extended incubation in the presence of RAS increased the amount of antibody bound to the target cells. This increase did not appear to be due to the expression of additional antigen, because the increase occurred at 4 degrees C and was not sensitive to inhibitors of protein synthesis. The inability to obtain increased levels of lysis by extended incubation in the presence of both RAS and GPC appeared to be due to the inhibitory effects of GPC on the interaction of antibody and antigen. Similar inhibitory effects could be produced by inactivated GPC and various other substances. When extended incubations in the presence of antiserum were used for the reexamination of the cytolytic activity of syngeneic immune HA, significant lysis was detectable.