Most patients with relapsed/refractory myeloma have a high-risk gene expression profile. These patients have poor long-term outcomes with salvage strategies, including transplantation-based therapies and the application of novel drugs. We conducted a pilot trial for relapsed/refractory myeloma patients in which we infused killer cell immunoglobulin-like receptor ligand (KIR-L)-mismatched natural killer (NK) cells from haplo-identical family donors in the setting of an autologous transplant. Such cells have demonstrated remarkable antileukemic effects in the allogeneic transplant setting. Our pilot trial established that transfusions of NK cells were safe, did not cause graft-versus-host disease, and did not affect engraftment of autologous CD34+ stem cells. Additional findings lay the foundation for the work proposed in Project 2. First, we observed that inhibitory KIR-Ls on myeloma cells are down-regulated in vivo after administration of bortezomib, further sensitizing myeloma cells to alloreactive (alio-) NK cell lysis. Second, CS1-specific antibody (Ab) has significant activity against primary myeloma cells in our SCID-hu mouse model, and this activity is synergistic with bortezomib. Third, we have expanded and activated NK cells with K562 cells transfected with membrane-bound interieukin-15 (IL-15) and the costimulatory molecule 4-1 BBL, increasing the number of allo-NK cells available for killing myeloma by rendering subpopulations of previously nonalloreactive NK cells cytotoxic. Specific Aim 1 will evaluate in a clinical trial whether the combination of bortezomib and CS1 Ab can enhance the antimyeloma activity of KIR-L-mismatched NK cells by down-regulating KIR-Ls on myeloma cells and invoking Ab-dependent cellular cytotoxicity, respectively. Specific Aim 2 will examine the activity of human allo-NK cells activated/expanded with K562 cell transfectants in the NOD/SCID/IL2rgamma[null]-hu model, which is deficient of murine NK cells. The potential additive/synergistic effects of combinations of bortezomib, CS1 Ab, and activated/expanded allo-NK cells will be investigated in order to design highly effective allo-NK cell therapy. Once proven effective in the relapsed high-risk disease setting, such therapy could be incorporated into upfront therapy of myeloma patients and be a useful adjunct to autotransplantation and/or novel drugs. This research may also be valuable for developing more efficacious therapies for other NK cell-sensitive malignancies.