We investigated the requirement for intimate contact between bone marrow stroma and B lymphoblasts from normal donors and children with leukemia. By scanning electron microscopy, both normal and leukemic cells seeded onto stroma were surrounded by folds of stromal cells or were linked to the stroma by fine tendrils and uropods. Separation of normal B progenitors from stroma by use of microporous membranes led to significantly lower cell recoveries compared with results when contact was unimpeded. For instance, 22.5% +/- 1.8% (mean +/-SEM) of CD19+, CD34+ cells (most immature subset) were recovered after a 7-day culture directly on stroma, compared with 5.2%+/-0.7% after growth on membranes (P < .001 by Student's t test). In 6 of 11 cases of B-lineage acute lymphoblastic leukemia, separation of progenitors from stroma resulted in apoptosis and a greater than 60% reduction in cell recovery. In the remaining 5 cases, however, this effect was much less pronounced, with reductions in cell recoveries ranging from 48.5% to less than 1% (median, 39.0%) of control values. Inhibition of very late antigen-4, a surface molecule critical for adhesion of B lymphoblasts to stroma, was associated with a greater loss of normal CD34+ B progenitors compared with that for equivalent leukemic cells. These results establish direct contact with stroma as a survival requirement of normal B lymphoblasts and show marked heterogeneity in stromal dependency among B-lineage leukemic cells.

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