Adoptively Transferred Expanded Natural Killer Cells Inhibit Myeloma Tumor Growth In Vivo.

Abstract 953

Recently developed culture conditions for expanding and activating natural killer (NK) cells may improve immunotherapeutic options for patients with multiple myeloma. We have previously shown that co-incubation of NK cells from multiple myeloma patients and healthy donors with K562 cells genetically modified to express membrane-bound interleukin 15 and the co-stimulatory molecule 41BBL (K562-mb15-41BBL) leads to a dramatic increase in both NK cell number and anti-myeloma activity in vitro. In this study, we tested the anti-myeloma activity of these expanded NK cells in vivo using a murine model, which supports the growth of myeloma cells in a microenvironment that reproduces that of the human bone marrow. We first implanted human fetal bones subcutaneously into NOD/SCID/IL2Rγ null mice and allowed them to engraft. Luciferase transfected OPM2 myeloma cells were then injected into the human bone fragment. Tumor burden was followed by bio-imaging and ELISA for human Ig. NK cells were expanded from healthy donor PBMC by co-culture with irradiated K562-mb15-41BBL cells in the presence of 300U/ml IL2 for 10-12 days. Myeloma tumor bearing mice were dosed 6-7 days after OPM2 injection (bio-imaging confirmed myeloma engraftment) with expanded NK cells via tail vein injection followed by subcutaneous IL2 to prolong NK cell survival. Flow cytometry was used to track the human NK cells in blood. Histology was performed by H&E staining of formalin-fixed, paraffin embedded tissues harvested at the end of the study. Sufficient NK cells (3×10⁹) for dosing mice were obtained from one unit of healthy donor blood and their ability to kill OPM2 myeloma was confirmed in vitro by chromium release assays. In experiment 1, OPM2-bearing mice received two tail vein injections, 48h apart, comprising PBS, 4 ×10⁷, or 16 ×10⁷ (total dose) expanded NK cells with 100U IL2 given twice per week. We observed significant myeloma growth inhibition in the cohort given 16 ×10⁷ NK cells (p<0.04) and circulating human NK cells could be detected up to day 21 post-administration. Measurement of tumor burden by bio-imaging and ELISA were highly concordant (correlation coefficient = 0.995). Histologic analyses confirmed a dramatic reduction in tumor burden in the mice treated with expanded NK cells and revealed that bone loss was more pronounced in the implanted fetal bones of the control cohort not receiving NK cells. We observed in a subsequent experiment that increasing the IL2 dose to 1000U daily led to in vivo expansion of CSFE-labeled NK cells of up to 10 generations by day 6 post-infusion, which was associated with an enhanced anti-tumor effect in the 16 ×10⁷ dose cohort. Furthermore, we found that multiple injections of NK cells (four injections of 4 ×10⁷ NK cells versus two injections of 8 ×10⁷) were better tolerated in the highest dose cohort (3/3 surviving versus 6/12 surviving). In conclusion, we observed a significant anti-myeloma effect against the aggressive plasma cell leukemia cell line OPM2 with expanded NK cells and low dose IL2. Adjusting the IL2 dose to 1000U daily led to in vivo expansion of NK cells, longer term NK cell persistence, and an increased anti-myeloma effect. Importantly, this IL2 dose is comparable to the 3×10⁶ U daily dose well tolerated in a previous trial with non-expanded NK cells in humans. Single doses of NK cells >8×10⁷ led to high morbidity in this model while multiple doses of 4×10⁷ were well tolerated. Experiments are in progress testing the in vivo effect of expanded NK cells on primary myeloma cell tumors in this model. Our findings support the planned clinical application of ex vivo expanded NK cells.