High-dose chemotherapy with melphalan and autologous hematopoietic stem cell transplantation (HSCT) is considered standard treatment for patients with multiple myeloma. While autologous HSCT may induce remission in patients resistant to standard chemotherapy, and has been shown to lead to long-lasting disease control in a subgroup of patients, the procedure is not curative. Given enough time and in the absence of a competing cause of death, all patients eventually relapse after auto-HSCT.

The only potentially curative approach currently available in the treatment of multiple myeloma (MM) is stem cell trans-plantation from an allogeneic donor. Allogeneic HSCT eradicates residual myeloma cells through T-cell mediated graft-versus-tumor effects. Allogeneic HSCT is, however, associated with significant risk of graft-versus-host disease and its use is therefore limited to younger patients with high risk dis-ease. Malignant plasma cells in multiple myeloma are also sensitive to natural killer cell lysis. Natural killer cells do not cause graft-versus-host disease, which has led to interest in their therapeutic use in patients with multiple myeloma.

We have previously shown that immunomagnetic separation of a highly pure NK cell product from a leukapheresis is possible and that these cells can be expanded up to 100-fold in a GMP-compatible setting. The current study aims to test the tolerability and feasibility of infusions of in vitro expanded haploidentical NK cells for patients after melphalan 200mg/m2 high dose chemotherapy and autologous HSCT in 10 patients. If feasible, the data will provide a basis for further prospective studies.

Inclusion Criteria:

• > 18 years, with multiple myeloma and indication for an autologous HSCT
• Available related haploidentical donor
• Written informed consent

Exclusion Criteria:

• Patients scheduled for autologous/allogeneic tandem HSCT