Asparaginases isolated from Escherichia coli and Erwinia carotovora are effective in the treatment of acute lymphoblastic leukemia in man. During treatment with either of these enzymes, patients frequently experience pronounced toxicity, including liver and pancreatic dysfunction and immunosuppression. The capabilities of these enzymes to hydrolyze L‐glutamine, an important amino acid in mammalian intermediary metabolism, have led investigators to suggest that the glutaminase activity may be responsible for the observed toxicities. Unlike other asparaginases, an enzyme isolated in our laboratory from Vibrio succinogenes, has been shown to be a potent antilymphoma agent and to be highly specific for L‐asparagine. Our previous work has established that the glutaminase‐free asparaginase from V. succinogenes does not suppress the in vivo humoral or cell‐mediated immune responses of mice to sheep red blood cells, whereas the E. coli enzyme abolishes these responses. In this study, we describe the mechanisms by which E. coli asparaginase suppresses specific antibody‐dependent cell‐mediated cytotoxicity. An analysis of the kinetics of appearance of the specific cell‐mediated cytotoxic response of spleen cells after immunization with sheep red blood cells revealed that it paralleled the IgG plaque‐forming cell response. In addition, 18‐h culture supernatants from immune spleen cells conferred upon non‐immune cells the ability to specifically lyse sheep red blood cells. Pretreatment of immune supernatants with protein A‐Sepharose 4B absorbed out the soluble arming factor. An analysis of E. coli asparaginase‐induced suppression of specific antibody‐dependent cell‐mediated cytotoxicity revealed that a decreased synthesis of specific IgG was responsible for the observed reduction in cytotoxic reactivity. When culture supernatants from spleen cells of immunized animals not treated with asparaginase were added to spleen cells from E. coli asparaginase‐treated mice, however, there was an increased cytotoxic response. This suggested that there was an increase in the number or ratio of effector cells for the specific antibody‐dependent cell‐mediated cytotoxic response in spleens of enzyme‐treated animals. Through the characterization of the immunosuppressive effects of two asparaginases—one having the catalytic capability to hydrolyze L‐glutamine and the other lacking this catalytic activity—we now have evidence that the immunosuppressive effects of E. coli asparaginase on specific antibody‐dependent cell‐mediated cytotoxicity cannot be the result of asparagine depletion alone. Our data demonstrate that the glutaminase‐free asparaginase from V. succinogenes is not immunosuppressive and suggest that an asparaginase with high substrate specificity for L‐asparagine may be a safer treatment for leukemia patients.
ASJC Scopus subject areas
- Cancer Research