Stage-specific expression of mouse BST-1/BP-3 on the early B and T cell progenitors prior to gene rearrangement of antigen receptor

Human bone marrow stromal cell antigen 1 (BST-1) was identified as a glycosylphosphatidyl-inositol-anchored ectoenzyme expressed on bone marrow stromal or synovial cell lines and having the ability to facilitate pre-a cell line growth, The analysis of the expression of mouse BST-1/BP-3 on the surface of lymphoid cells in the bone marrow and thymus revealed that it was very transiently expressed on both a and T cell progenitors undergoing gene rearrangement of the antigen receptor, Among CD45R(+)CD43(+) a cell progenitors in the bone marrow, BST-1 expression appeared on the CD24 (heat stable antigen)(+), CD19(+) or CD117 (c-kit)(+) population. In the thymus, BST-1 was expressed on CD4(-)CD8(-)CD3(-) [triple negative (TN)] CD90 (Thy-1)(+) cells, In TN thymocytes, the majority of CD25(+) cells and CD44(lo/-) cells expressed BST-1, In fetuses, BST-1(+) cells appeared in the thymus and liver at day 14 and 16 of gestation respectively. The expression level of BST-1 by fetal thymus was maximal and >60% of thymocytes were positive for BST-1 at day 15 or 16 and the proportion then gradually decreased during development, Among day 15 fetal thymocytes, BST-1 was negative on the CD44(+)CD25(-) fraction, very slightly positive on the CD44(+)CD25(+) fraction, and strongly positive on the CD44(lo-/)-CD25(+) and CD44(-)CD25(-) fractions, These results showed that murine BST-1 is a useful marker for lymphoid progenitor cells initiating gene rearrangement of their antigen receptors.