VLA-4 is involved in the engraftment of the human pre-B acute lymphoblastic leukaemia cell line NALM-6 in SCID mice

Attachment of human pre-B leukaemic cells to human or murine bone marrow stromal cells in vitro is largely mediated by the β₁ integrin VLA-4 binding to VCAM-1. Cells subsequently migrate within the stroma, a process also involving VLA-4. A variant of the pre-B acute lymphoblastic leukaemia cell line NALM-6, designated 4A1, lacking expression of VLA-4, was generated by radiation-induced mutagenesis followed by several rounds of negative selection with immunomagnetic beads, fluorescence activated cell sorting and clonal expansion. In vitro assays using 4A1 cells showed reduced binding to, and migration under, the murine stromal line M2-10B4. Sublethally irradiated mice (n = 19) with severe combined immunodeficiency were injected intravenously with NALM-6 cells. Animals developed signs of leukaemia with hind-limb paralysis at a median of 30 d (95% confidence interval 28–30). Although there were no gross abnormal findings at autopsy, histological analysis revealed extensive marrow replacement and focal liver infiltration with leukaemic blasts, which were confirmed to be of human origin by flow cytometry. 12 mice were injected with a similar number of cells from the VLA-4-negative variant cell line 4A1. Six mice developed signs of leukaemia after 43–74 d, with the remaining six being free of signs of disease after > 100 d (P < 0.001). Mice in this group with leukaemia had a lower incidence of hind-limb paralysis and less leukaemic infiltration in the marrow, but in some cases had large tumour nodules elsewhere. After a single 500 μg intraperitoneal injection of anti-murine VCAM-1 monoclonal antibody (MK2.7), five additional mice were injected with an identical number of wild-type (VLA-4⁺) NALM-6. All animals developed signs of leukaemia after a similar period to those injected with wild-type NALM-6 only. These results demonstrate that the β₁ integrin VLA-4 is involved in the engraftment of the pre-B-cell leukaemic cell line NALM-6 in SCID mice, although the interaction with VCAM-1 is unlikely to be the sole explanation.