A monoclonal antibody (SJ-9A4) to P24 present on common alls, neuroblastomas and platelets - II. Characterization of P24 and shedding in vitro and in vivo.

The release of soluble P24 antigen into culture medium by common acute lymphoblastic leukemia (C-ALL) and neuroblastoma (NB) cell lines was studied. P24 release by C-ALL cells was detected using a solid phase indirect radioimmunometric assay (IRA) which combines the specificity of lectins and monoclonal antibodies (MoAb) and using immunoadsorption of labeled P24 in spent medium from cells incubated with 35S-methionine (met). No P24 was present in the medium of cells pulse labeled at 37 degrees C when they were placed at 4 degrees C, thus this is an active process. P24 release by NB cells could not be detected by IRA, but could be detected by immunoadsorption of spent medium of metabolically-labeled cells. The absence of IRA activity of P24 from NB spent medium was due to decreased glycosylation and thus no binding to the lectins employed in the IRA was observed. This was confirmed by lectin affinity chromatography which showed that P24 in the spent medium from C-ALL cells bound Ricinus communis agglutinin (RCA1), wheat germ agglutinin (WGA), concanavalin A (Con A), and lentil lectin (LcH), but not peanut agglutinin (PNA). P24 from NB cell spent medium did not bind to any of these lectins. The lectin affinity of P24 derived from lymphoblasts is consistent with the presence of N-linked oligosaccharide chains having N-acetyl glucosamine residues, a mannose core, and a terminal D-galactose. P24 from C-ALL cell spent medium was present in the 35-45% fraction of a saturated ammonium sulfate (SAS) partition of spent medium. The P24 antigen was detected in the fractionated plasma of five patients with C-ALL at the time of diagnosis and was undetectable when the patients had achieved a complete remission. Plasma from 2 patients with P24 negative ALL, normal human plasma, and normal human serum had no detectable activity.