Anti-Myeloma Effects of γδ T Cells Are Hampered by Bone Marrow Stromal Cells but Resumed by Stromal Cell-Derived Osteoblasts.

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Multiple myeloma (MM) remains incurable by conventional chemotherapies, leading to the idea to develop various forms of immunotherapies. γδ T cells are important effectors in the first-line defense against infections and tumors, and play a critical role in host defense and tumor surveillance. Aminobisphosphonates, a potent anti-resorptive agent, can effectively expand γδ T cells in vitro from peripheral blood mononuclear cells (PBMC) in combination with IL-2 in human; thus expanded γδ T cells have been demonstrated to exert potent anti-MM effects and draws considerable attention as a novel immunotherapeutic maneuver. However, in contrast to their in vitro anti-MM effects, their efficacy against MM cell growth in the bone marrow appears to be limited in patients with MM, although their clinical application is underway in MM. MM cells expands in a manner dependent on bone marrow microenvironment, in which stromal cells with defective osteoblast differentiation along with osteoclasts create a microenvironment suitable for MM cell growth and survival (a MM niche) to protect MM cells from various apoptotic insults. Because the effects of MM bone marrow microenvironment on γδ T cell activity is largely unknown, the present study was undertaken to clarify the roles of microenvironmental cells in MM bone marrow in cytotoxic activity of γδ T cells against MM cells. γδ T cells were substantially expanded (30- to 100-fold increase) when PBMC were stimulated with zoledronic acid and IL-2 for 1–2 weeks. When the γδ T cells were added exogenously to co-cultures of PBMC–derived OCs and MM cell lines (RPMI8226 and U266), γδ T cells adhered to OCs as well as MM cells and almost completely destroyed both of them, suggesting the susceptibility of OCs and MM cells to γδ T cells. Because such γδ T cell-mediated cytolysis is contact-dependent, we next explored the adhesion-mediated mechanisms. We found strong surface expression of DNAX accessory molecule-1 (DNAM-1; CD226) along with LFA-1 on γδ T cells, both of which are known as an adhesion molecule with signal transduction, and act as co-stimulatory molecules in cytotoxic T cells and NK cells. Blockade of either DNAM-1 or LFA-1 substantially reduced cytolysis of OCs as well as MM cells by γδ T cells, demonstrating their critical role as co-stimulatory molecules in γδ T cells. In contrast, the cytotoxic activity of γδ T cells against MM cells was potently attenuated in the presence of bone marrow stromal cells. Pretreatment of γδ T cells with stromal cells down-regulated interferon-γ production along with a decrease in DNAM-1 expression by γδ T cells. These results suggest that bone marrow stromal cells may be responsible for attenuation of anti-MM effects by γδT cells in vivo through directly blunting γδ T cell activity in addition to protection of MM cells from apoptosis. We and others have recently demonstrated that terminally differentiated osteblasts derived from stromal cells induce MM cell apoptosis. In contrast to stromal cells, terminally differentiated osteblasts allowed MM cell eradication by γδ T cells. Therefore, induction of terminally differentiation of osteblasts from stromal cells not only ameliorates bone lesions but also may disrupt a MM niche to confer susceptibility to γδ T cells in MM cells, which is hampered by bone marrow stromal cells.