Monoclonal antibodies recognising the cluster 2 antigen associated with human small cell lung cancer mediate the toxic effects of ricin A chain in an indirect assay of immunotoxin cytotoxicity.

Monoclonal antibodies (Mabs) submitted to the Second International Workshop on Small Cell Lung Cancer Antigens were screened for their ability to mediate the toxic effects of ricin A chain against the NCI-H69 cell line in an indirect assay of immunotoxin cytotoxicity. Cluster 1 Mabs, recognising the neural cell adhesion molecule, mediated little or no cytotoxic effect in combination with screening agent, ricin A chain linked to an antibody Fab' fragment recognising either mouse or rat Mabs. In contrast, cluster 2 Mabs, recognising an epithelial tumour-associated antigen, generally mediated potent cytotoxic effects with the screening agent, inhibiting the incorporation of 3H-leucine by NCI-H69 cells by between 90% and 99%. Measurements of Mab binding to the NCI-H69 cell line by indirect immunofluorescence and flow cytometry indicated that the cluster 2 Mabs generally bound in higher amounts than the cluster 1 Mabs suggesting that the cluster 1 Mabs were ineffective in the screen because they did not bind to the cells in sufficient amounts. However, Mabs recognising antigens other than cluster 1 bound to NCI-H69 cells in amounts similar to those of the cluster 2 Mabs yet did not mediate potent cytotoxic effects in the indirect assay suggesting that the cluster 2 antigen may be internalised in a fashion favouring the delivery of ricin A chain to the cytosol.