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 identifiedas a glycosylphosphatidylinositol-anchored ectoenzyme expressedon bone marrow stromal or synovial cell lines and having theability to facilitate pre-B cell line growth. The analysis ofthe expression of mouse BST-1/BP-3 on the surface of lymphoidcells in the bone marrow and thymus revealed that it was verytransiently expressed on both B and T cell progenitors undergoinggene rearrangement of the antigen receptor. Among CD45R⁺CD43+B cell progenitors in the bone marrow, BST-1 expression appearedon 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 CD44¹⁰ cells expressed BST-1.In fetuses, BST-1+ cells appeared In the thymus and liver atday 14 and 16 of gestation respectively. The expression levelof BST-1 by fetal thymus was maximal and >60% of thymocyteswere positive for BST-1 at day 15 or 16 and the proportion thengradually decreased during development. Among day 15 fetal thymocytes,BST-1 was negative on the CD44⁺CD25^- fraction, very slightlypositive on the CD44+CD25+ fraction, and strongly positive onthe CD44^10-CD25+ and CD44^-CD25^- fractions. These results showedthat murine BST-1 is a useful marker for lymphoid progenitorcells initiating gene rearrangement of their antigen receptors.