Proliferation of normal human lymphoid progenitors in culture is dependent on interaction with bone marrow-derived fibroblast-like cells (BM-FB). To investigate possible heterogeneity in this lymphoid- supportive microenvironment, we studied the interaction of a human B- precursor cell line (NALM-6) with BM-FB. NALM-6 cells associate with BM- FB by either adhesion or migration underneath the fibroblast. Individual fibroblasts in the BM-FB layer showed significant variation in the number of migrating NALM-6 cells. Migration of NALM-6 cells was primarily VLA-4-dependent, although residual migration observable after blocking with anti-VLA-alpha 4 antibody was inhibited by anti-VLA-alpha 5 antibody. Migration was not inhibited by blocking either of the known VLA-4 counterreceptors (VCAM-1 or fibronectin), although slight inhibition was observed using a combination of blocking antibodies to VCAM-1 and fibronectin. In contrast, NALM-6 adhesion without migration was significantly inhibitable by anti-VCAM-1 antibody. VCAM-1 or fibronectin expression on individual BM-FB did not correlate with NALM- 6 migration. These results indicate that the adhesion and migration of human B-lymphoid precursors in the bone marrow microenvironment are mechanistically separable events and suggest the possibility of novel VLA-4 ligand(s), which may be important in human lymphopoiesis. Subpopulations of cells in the bone marrow microenvironment may preferentially support important aspects of lymphoid progenitor development.

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