Characterization of a Monoclonal Antibody (A12) that Defines a Human Acute Lymphoblastic Leukemia-Associated Differentiation Antigen

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Abstract
A human leukemia-associated differentiation antigen has been identified by a monoclonal antibody (A12) raised to the lymphoblastoid cell line NALM-1. The A12 antigen was expressed on the surface of leukemic cells from patients with common acute lymphoblastic leukemia (c-ALL) as well as on cells of the hematopoietic cell lines NALM-1, Reh-6, Raji, Daudi, CEM, and 8402 as determined by an antibody-binding radioimmunoassay, as well as by indirect immunofluorescence and FACS analysis. This antigen was not detected on normal blood lymphocytes, normal bone-marrow cells or leukemic cells from patients with acute myeloid leukemia (AML). The A12 antigen had an apparent molecular weight of 100 kD as determined by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) and appeared to be related to if not identical with the acute lymphoblastic leukemia antigen CALLA described by others. Cross-blocking experiments indicated that preincubation of NALM-1 cells with antibody A12 or J5 (anti-CALLA) could block subsequent binding of 125I-labeled A12 and J5 antibody. These results suggest that the two monoclonal antibodies recognize identical or closely located antigenic sites. The surface membrane expression of A12 antigen in NALM-1 cells was modulated when the cells were cultured in the presence of A12 antibody. Under these conditions, the expression of Ia antigens was unaffected. Re-expression of A12 antigen occurred within 24 h after transfer of the modulated cells into medium devoid of monoclonal antibody.