VLA-4 mediated adhesion to bone marrow stromal cells confers chemoresistance to adherent lymphoma cells

The mechanisms of maintenance of residual lymphoma in bone marrow during chemotherapy are currently not well understood. Previous studies have shown that primary lymphoma cells obtained from histologically negative bone marrow of non-Hodgkin's lymphoma (NHL) patients grew in long-term bone marrow cultures primarily in association with bone marrow stromal cells. Furthermore, the interaction of NHL patient cells with bone marrow stromal cells inhibited their spontaneous apoptosis. The current studies were designed to characterize the components of the heterotypic interaction between lymphoma cells and bone marrow stromal cells as well as to probe the consequences of this interaction as it pertains to the potential survival of minimal numbers of lymphoma cells during chemotherapy. Cellular adhesion assays performed in the presence of either neutralizing antibodies to VCAM- or the alpha and beta subunit of VLA-4 resulted in >95%, 82% and 35% inhibition of lymphoma cell line adhesion to the bone marrow stromal line MS-5, respectively. Modulation of VLA-4 affinity by the 8A2 antibody resulted in enhanced secondary adhesion at 24 and 72 hours to either cellular fibronectin (65% and 65%) or MS-5 cells (60% and 55%), superceding levels obtained using untreated lymphoma cells (<20%). The bone marrow stromal cells induced a chemoprotective effect for adherent lymphoma cells over a 3-log dose range of vincristine, resulting in a 2-log increase in the ED50 at day 6 of culture. The failure of glutaraldehyde fixed stromal cells to induce a chemoprotective effect demonstrated that viable bone marrow stromal cells were necessary. Similarly, lymphoma/stromal cell conditioned medium also railed to provide a survival advantage. These data demonstrated that viable bone marrow stromal cells possessed the ability to actively inhibit the apoptotic pathways of intimately adherent lymphoma cells and this potentially contributes to their survival during chemotherapy.