Maturation-dependent adhesion of human B cell precursors to the bone marrow microenvironment

Murine B cell precursors can be induced to proliferate in culture if allowed to bind to bone marrow derived adherent cells prepared under specific conditions. We studied the binding of human B cell precursor subpopulations to various in vitro microenvironments to determine which conditions may potentially be suitable models for human B precursor differentiation. Using the markers CD10, CD34, and CD20, B lineage populations of increasing maturation were quantitated: CD10⁺/CD34⁺, CD10⁺/ CD20^-, CD10⁺/CD20⁺, and CD10^-/CD20⁺ cells in marrow, and CD10^-/CD20⁺ mature B cells in peripheral blood. The adhesion of subpopulations of blood and marrow-derived light density cells to adherent cell layers or matrix was studied following a 2-h incubation in 24-well plates. The absolute number of bound B lineage cells was determined by cell counts and flow cytometry analysis. The adherence of B lineage cells to passaged human marrow fibroblasts (BM-FB) was highest in the most immature CD10⁺/CD34⁺ cells (34.3 ± 4.2%), decreasing steadily with each stage of maturation to the peripheral blood B cells (11.2 ± 2.4%). Increased adhesion of CD10⁺ B cell precursors relative to CD10^-/CD20⁺ marrow B cells was confirmed by adhesion studies using sorted cells. The two most immature B lineage cells (CD10⁺CD34⁺ and CD10⁺/CD20^-) showed more adherence to BM-FB than any other cell type tested, except for monocytes. Only B lineage precursor cells, erythroid precursors and CD10^-/CD34⁺ cells showed significantly greater binding to BM-FB than to plastic. B lineage precursors bound equally well to primary and passaged human marrow fibroblasts, but bound significantly less well to passaged human foreskin fibroblasts, primary human marrow stroma, extracellular matrix of marrow fibroblasts, or fibronectin. These results suggest that specific binding to marrow fibroblasts is part of the differentiation program of early B lineage precursors. This binding activity gradually and predictably decreases during B lineage differentiation, in contrast to expression of other binding receptors, such as LFA-1 and CD44, which increase during B lineage maturation.