Complement-dependent cytotoxicity against hepatoma cells mediated by IgM antibodies in serum from tumor-bearing rats

Complement-dependent cytotoxic antibodies in syngeneic serum from rats carrying transplanted aminoazo dye-induced hepatomas (D23 and D33) and from rats immunized with irradiated tumor cell or homogenates were studied by a short-term 51Cr release assay. The tumor-bearer sera (TBS) were subjected to chromatography on unsolubilized protein A and Sepharose 4B. The cytolytic activity of D23 TBS was recovered in the IgM molecular weight region, whereas no such activity was obtained in the IgG fraction. As judged from immunodiffusion experiments, the IgM molecular weight fraction did not contain any aggregated or complexed IgG. Moreover, in immunofluorescence tests against viable hepatoma cells, using a specific anti-rat IgG conjugate, the TBS were negative. Cross-testing of D23 and D33 TBS against the two hepatomas and cross-absorption of the sera with tumor plasma membranes revealed no tumor specificity in these cytotoxicity reactions. Furthermore, neither fetal nor early postnatal liver cells could absorb out the activity. Absorptions with adult liver plasma membranes, however, abrogated the cytotoxic activity, and even more effective in this respect were homogenates from kidney and small intestine, thus indicating that the cytotoxic antibodies are directed against 'normal' adult antigen(s) present also in tissues other than liver.