Ultrastructural changes during adhesion and migration of pre-B lymphoid leukaemia cells within bone marrow stroma

The ultrastructural changes in leukaemic cells on initial contact with, and during migration into, layers of bone marrow stroma in vitro were examined in a variety of types of acute leukaemia and leukaemic cell lines. Bone marrow fibroblasts (BMF) were grown on polycarbonate microporous membranes, and acute leukaemia cells added to cultures and allowed to adhere to BMF for variable periods of time before fixation. Acute lymphoblastic leukaemia (ALL) blasts showed rapid development of surface membrane microvilli on contact with BMF layers. ALL blasts, and the pre-B ALL cell line NALM-6, showed evidence of movement in to the BMF layer within 15-30 min, with intrusion of extended cytoplasic processes into gaps between BMF cytoplasm. ALL cells were frequently seen within the layers of fibroblasts after 30 min incubation, and had pronounced morphological changes, with pseudopodia and attenuated and elongated microvilli interdigitating with the surface of fibroblasts or with strands of extracellular matrix material. Changes were also noted in the surface membrane of BMF adjacent to ALL cells, with invagination of the cytoplasmic membrane and formation of micropits. In contrast to the migratory behaviour of pre-B ALL cells, migration was not observed with acute myeloid leukaemia cells or other leukaemic cell lines. These cells showed membrane activation, with variable degrees of microvillous formation, and in some cases insertion of pseudopodia into BMF layers, but migration was not observed. Ultrastructural immunogold labelling was carried out to determine the localization of leukaemic adhesion molecules and their ligands on BMF. This demonstrated that beta 1 integrins were largely localized to the contact surfaces of both ALL blasts and fibroblasts, with VCAM-1 expressed only on the surface of BMF. These observations confirm the specificity of migratory behaviour for pre-B leukaemic cells, and indicate that a complex pattern of surface and intracellular events mediate this process, including the expression of beta 1 integrins and VCAM-1 at the sites of insertion of leukaemic cells between fibroblast margins.