A new immunotoxin built by linking a hemolytic toxin to a monoclonal antibody specific for immature T lymphocytes.

Hybrid molecules built by conjugation between monoclonal antibodies (MAbs) and toxins are currently being experimentally tested as potential new anti-cancer agents. These immunotoxins have mainly used the plant toxin ricin as the toxic component, which inhibits protein synthesis at the ribosome level. We present an alternative for toxic components using a hemolytic toxin acting at the membrane level, due to its phospholipase activity. The hemolytic toxin (HT), isolated from the sea anemone Stoichactis helianthus, has been conjugated to an antibody towards an antigen expressed on immature T lymphocytes (IOR-T6), by means of an artificial disulphide bridge. The hybrid IOR-T6-HT exhibits no hemolytic activity unless it is reduced. It is toxic for cells (CEM) expressing the IOR-T6 antigen and non-toxic for cells (K562) not bearing the antigen. An excess of unconjugated antibody reverses the toxicity. Immunotoxins based on membrane-active, hemolytic toxins can be a useful alternative when directed towards antigens which do not mediate internalization, as is the case for most carcinoma antigens.