Normal B lymphopoiesis is dependent on a close relationship between B- cell precursors and the bone marrow (BM) microenvironment. To further understand the mechanisms regulating the proliferation of the malignant counterpart of B-cell precursors, namely precursor-B acute lymphoblastic leukemia (ALL), we examined the adhesion to BM fibroblasts (BMF) of 19 cases of precursor-B ALL using a chromium labeling assay. Eleven of 19 cases showed greater than 10% binding to BMF (range 2.3% to 54.8%, mean 19.1%). Binding was increased approximately twofold by preincubation of BMF with tumor necrosis factor and interleukin-4, which also resulted in upregulation of expression of vascular cell adhesion molecule-1 (VCAM-1) on BMF. The mechanism of attachment was investigated using murine monoclonal antibodies to leukocyte integrins, principally the beta, integrins VLA- 4 and VLA-5, which were demonstrated to be present on most cases by flow cytometry. Statistically significant inhibition of adhesion was observed with antibodies to the beta 1 common subunit, VLA-4, and VLA- 5, whereas little effect was seen with antibodies to VLA-6 or the beta 2 integrin subunit. Preincubation of fibroblasts with an antibody to VCAM-1 (a ligand of VLA-4) inhibited leukemic cell binding in the majority of cases, which was an effect also observed on cytokine- stimulated BMF. However, a minority of cases, as well as the pre-B lines NALM-6 and KM-3, showed no evidence of inhibition of adhesion with anti-VCAM-1 antibodies. Treatment of BMF with antifibronectin antibody alone had little effect on ALL adhesion and did not enhance the inhibitory effect of anti-VCAM-1. These data indicate that precursor-B ALL cells bind to BM stroma through the beta 1 integrins VLA-4 and VLA-5 and that this effect is partly mediated by VCAM-1 on stromal cells, although other undefined VLA ligands are also likely to be involved. Attachment of ALL cells to stroma is likely to play a key role in regulating the survival and growth of these cells through exposure to stromal cytokines.

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