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 ch emotherapy are currently not well understood. Previous studies have shown t hat primary lymphoma cells obtained from histologically negative bone marro w of non-Hodgkin's lymphoma (NHL) patients grew in long-term bone marrow cu ltures primarily in association with bone marrow stromal cells. Furthermore , the interaction of NHL patient cells with bone marrow stromal cells inhib ited their spontaneous apoptosis. The current studies were designed to char acterize the components of the heterotypic interaction between lymphoma cel ls and bone marrow stromal cells as well as to probe the consequences of th is interaction as it pertains to the potential survival of minimal numbers of lymphoma cells during chemotherapy. Cellular adhesion assays performed i n 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. Modu lation 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%) o r MS-5 cells (60% and 55%), superceding levels obtained using untreated lym phoma 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 d emonstrated that viable bone marrow stromal cells were necessary. Similarly , lymphoma/stromal cell conditioned medium also railed to provide a surviva l advantage. These data demonstrated that viable bone marrow stromal cells possessed the ability to actively inhibit the apoptotic pathways of intimat ely adherent lymphoma cells and this potentially contributes to their survi val during chemotherapy.