Construction of an antigenic map for human B-cell precursors

The purpose of this study was to evaluate the binding of a panel of monoclonal antibodies to human pre-B cells present in fetal, pediatric, and adult bone marrow. The antibodies included BA-1, BA-2, BA-3 (anti-CALLA), anti-B1, L243 (anti-HLA-DR), and T101. Binding of the monoclonal antibodies to pre-B cells was evaluated at the single-cell level by double fluorochrome analysis. Percentages of BA-1⁺ and anti-B1⁺ pre-B cells were independent of age group. BA-1 bound to approximately 80% of fetal, pediatric, and adult bone marrow pre-B cells, whereas anti-B1 bound to approximately 50%. BA-2 bound to 55% of fetal pre-B cells, but this percentage decreased to 32% in pediatric and 16% in adult bone marrow. CALLA was expressed on less than 10% of fetal, pediatric, and adult bone marrow pre-B cells, and HLA-DR was expressed on greater than 95% of fetal, pediatric, and adult pre-B cells. Although T101 (an anti-T-cell monoclonal antibody) did not bind to pre-B cells, it did bind to approximately 25% of the sIgM⁺ cells in fetal bone marrow. These results suggest a predominant phenotype of L243 (anti-HLA-DR)⁺, BA-1⁺, BA-3 (anti-CALLA)^-, T101^- for the human pre-B cell while phenotypic heterogeneity exists for anti-B1 and BA-2.