Hybrid molecules prepared by linking toxins to monoclonal antibodies (MAbs) are cytotoxic to cells bearing the target antigen. The toxin most widely used has been the plant toxin ricin as the toxic component, which inhibits protein synthesis at the ribosome level. 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. 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 a MAb directed against carcinoembryonic antigen (CEA), by means of an artificial disulphide bridge. The hybrid alpha CEA-HT exhibits no hemolytic activity unless it is reduced. It is toxic for cells (MDA-MB-134) expressing CEA and not toxic for cells (MDA-MB-231) not bearing CEA. An excess of free antibody reverses toxicity.

Three different hybridoma clones secreting anti-HBsAg antibody were constructed by fusing cells of mouse myeloma line Ag8-X63 with splenocytes from BALB/c mice immunized with recombinant HBsAg and natural HBV. The monoclonal antibodies obtained were characterized immunologically, and two were used to develop UMELISA for detection of HBsAg. This monoclonal assay enabled the detection of 0.1 UPE/mL with reference to the standard of the Paul Ehrlich Institute (Frankfurt, Germany). The assay compared well with a commercially available kit (UMELISA HBsAg) and was used for detection of HBsAg in blood donors.