LISTERIOLYSIN-O POTENTIATES IMMUNOTOXIN AND BLEOMYCIN CYTOTOXICITY

Antitumor immunotoxins were formed by covalently attaching the ribosom e-inactivating protein ricin A chain (RA) to the antitumor antibodies BR96 and L6. In vitro cytotoxicity assays established that BR96-RA was cytotoxic to H2987 human lung adenocarcinoma cells (IC50 = 6 nM), whi le L6-RA exhibited very low levels of cytotoxic activity (18% cell kil l at 67 nM). The virulence factor from the intracellular pathogen List eria monocytogenes, listeriolysin O (LLO), was able to potentiate the cytotoxicity of BR96-RA and L6-RA by 120- and >1340-fold, respectively , resulting in IC50 values of approximately 50 pM. LLO also potentiate d the cytotoxicity of the peptide anticancer drug bleomycin by a facto r of >2500 but had no effect on the cytotoxic activities of the antica ncer drugs cytarabine and etoposide phosphate. In addition, LLO did no t potentiate the cytotoxic activity of unconjugated ricin A chain or L 6-RA on H2987 cells that were saturated with L6 prior to conjugate tre atment. These results are attributed to LLO-induced alteration of the intracellular trafficking of molecules that are incorporated into acid ic vesicles.