Effects of ricin A chain conjugates of monoclonal antibodies to human alpha-fetoprotein and placental alkaline phosphatase on antigen-producing tumor cells in culture.

Two tumor-associated proteins, alpha-fetoprotein (AFP) and placental alkaline phosphatase (PLAP), were investigated as target proteins for antibody:cytotoxin conjugates. An AFP-producing hepatoma cell line (HepG2) and a PLAP-producing cervical carcinoma cell line (SKGIIIa) were used as target cells. Both cell lines were equally susceptible to the toxic effects of intact ricin. Immunofluorescent studies showed that AFP could be detected at the surface of the HepG2 cells in a speckled distribution, while PLAP was uniformly distributed over the surface of the SKGIIIa cells. The anti-AFP ricin A chain conjugate was not cytotoxic to either cell line at low concentrations and killed both types of cells at high concentrations. The anti-PLAP conjugate at low concentrations was 100-fold more toxic than the anti-AFP conjugate to the PLAP-producing SKGIIIa cells. At high concentrations, it also killed both types of cells. The enhanced toxicity of the anti-PLAP conjugate to the SKGIIIa cells was inhibited by an excess of unconjugated anti-PLAP but not anti-AFP, indicating that the uptake of the conjugate depends on specific cell surface binding to the antigen. The indiscriminate toxicity observed at high concentrations of either conjugate was not inhibited by unconjugated antibody, suggesting that this effect depends on conjugate uptake independent of the identity of the antigen. These results emphasize the importance of the properties of the target antigen to the cytotoxic effects of antibody conjugates as well as the need for caution in experiments using high concentrations of conjugates. They suggest that PLAP may be a suitable target for immunotoxin therapy of human cancer.