Antibody affinity may influence antigenic modulation of the common acute lymphoblastic leukemia antigen in vitro

We have produced a new monoclonal antibody designated BA-3. An extensive side-by-side serologic comparison of BA-3 with the anti-common acute lymphoblastic leukemia antigen (CALLA) monoclonal antibody J-5 was undertaken. Cells examined included established leukemic cell lines, malignant cells from patients with newly diagnosed leukemia/lymphoma, and normal hematopoietic tissues. In all experiments the cellular distribution of the antigens recognized by BA-3 and J-5 were identical when analyzed by immunofluorescent microscopy and the FACS. Iodination of NALM-6 cells, followed by radioimmunoprecipitation and SDS-PAGE, indicated that BA-3 (like J-5) precipitated a glycoprotein of approximately 100,000 daltons. Competitive binding studies using ¹²⁵I-labeled BA-3 indicated that B-3 and J-5 were binding to closely associated (if not identical) epitopes on CALLA. Calculation of the equilibrium constant (K value) for BA-3 and J-5, and the approximate number of CALLA molecules per cell, was graphically determined using Scatchard plots. BA-3 and J-5 were shown to have K values of approximately 2.7 X 10⁷ M^-1 and 7.2 x 10⁷ M^-1 respectively, with NALM-6 cells expressing 1 x 10⁵ to 2 x 10⁵ CALLA molecules per cell. Additional studies with BA-3 failed to demonstrate significant antigenic modulation of CALLA in vitro using fresh leukemic cells and leukemic cell lines. Thus, we suggest that antibody affinity may be a significant factor influencing antigenic modulation of CALLA in vitro.