CHARACTERIZATION OF A B-CELL PROGENITOR PRESENT IN NEONATAL BONE-MARROW AND SPLEEN BUT NOT IN ADULT BONE-MARROW AND SPLEEN

The neonatal period marks an important time in mammalian immunologic d evelopment, yet it is often ignored in studies of lymphocyte developme nt. We identified a cell population with the phenotype heat stable Ag (HSA)(low) lin- CD43(low) that contained B cell progenitors at a high frequency in the neonatal bone marrow and spleen. Although cells with a similar phenotype can be identified in the bone marrow and spleen of adult animals, these populations showed a greatly reduced frequency o f B cell progenitors. B Lineage cells were detected after 7 days in cu lture at a frequency of 1:15 when HSA(low) lin(-) CD43(low) cells from neonatal bone marrow were cultured on stromal cells and IL-7 under li miting dilution conditions. Under similar conditions, the equivalent p opulation in adult bone marrow had a frequency of B cell progenitors t hat was less than 1:2000, The expression of terminal deoxynucleotidyl transferase in freshly sorted neonatal HSA(low) lin(-) CD43(low) cells suggested that cells committed to the lymphocyte lineage were present in this population, These data suggested that the HSA(low) lin(-) CD4 3(low) population of cells represents a pool of B lineage precursors t hat may be responsible for filling the immune compartment early in neo natal life.