Transitional B cell subsets in human bone marrow

Summary B cells originate from precursors in the bone marrow, and the first cells which migrate to the peripheral blood have been classified as ‘transitional B cells’. Transitional B cells have been characterized in human blood with stage 1 ( T 1) and stage 2 ( T 2) subsets being proposed. In the present study, 27 normal human bone marrow samples were analysed for transitional B cell markers by eight‐colour flow cytometry. T 1 transitional B cells ( CD 45 + CD 19 + CD 10 + IgM + IgD lo ) and T 2 transitional B cells ( CD 45 + CD 19 + CD 10 + IgM + IgD + ) were identified in normal bone marrow samples at a mean frequency of 3·2 and 3·1% of total B lineage cells, respectively. A majority of the bone marrow transitional B cells were CD 24 hi CD 38 hi , the phenotype of blood transitional B cells. Consistent with recent peripheral blood data, T 2 B cells had a significantly higher CD 21 expression compared with T 1 B cells (72·4 versus 40·9%) in the bone marrow. These data raise the possibility that transitional B cells are capable of differentiating from T 1 to T 2 B cells within the bone marrow. Furthermore, transitional cells at either stages 1 or 2 might be capable of migrating out of the bone marrow.