ANTIBODIES TO THE PROTEIN CORE OF THE SMALL-CELL LUNG-CANCER WORKSHOPANTIGEN CLUSTER-W4 AND TO THE LEUKOCYTE WORKSHOP ANTIGEN CD24 RECOGNIZE THE SAME SHORT PROTEIN-SEQUENCE LEUCINE-ALANINE-PROLINE

We recently described the identity of the small cell lung cancer (SCLC ) cluster-w4 antigen and the human B cell differentiation marker CD24, a glycosylphosphatidylinositol (GPI)-anchored, highly glycosylated su rface molecule of only 31-35 amino acids 1 51. The specificities of t hree anti-cluster-w4 and of eleven anti-CD24 MoAbs have been investiga ted with respect to their binding capacity to the protein core of clus ter-w4/CD24 antigen. Four overlapping peptides spanning this protein c ore were synthesized. MoAbs shown to bind to two overlapping peptides by antibody binding inhibition using the cluster-w4/CD24-positive SCLC cell line SW2 and by direct peptide binding detected in an ELISA were investigated in more detail. To determine the exact epitopes recogniz ed by these MoAbs, an epitope mapping assay using peptides synthesized onto polyethylene pins was established. The three anti-cluster-w4 MoA bs SWA 11, SWA21 and SWA22 and the anti-CD24 MoAbs OKB2 and ALB9 recog nized the same short leucine-alanine-proline (LAP) sequence in an area without potential glycosylation sites close to the GPI anchor of the protein core of the cluster-w4/CD24 antigen.