Expanded Natural Killer (NK) Cells for Immunotherapy: Fresh and Made to Order

Remarkable increases in the dose and activity of NK cells can be achieved by co-culture with the HLA class I deficient cell line K562 that has been genetically modified to express membrane-bound IL15 and the co-stimulatory molecule 41BB-ligand (K562-mb15-41BBL; Fujisaki et al. Cancer Res. 2009;69:4010–4017). We are conducting a clinical trial utilizing these ex-vivo expanded NK cells (ENK) which are produced at the Center for Cell and Gene Therapy (CAGT) at Baylor and then shipped to the University of Arkansas for Medical Sciences (UAMS) for infusion to high-risk relapsed multiple myeloma (MM) patients using the NHLBI-PACT mechanism. Here we report on the characteristics of the ENK cell products sent fresh versus frozen.

Apheresis products were collected from MM patients or healthy donors (HD), cryopreserved, and then shipped to CAGT for GMP grade production, as described (Lapteva et al. Cytotherapy 2012; in press). Briefly, mononuclear cells from thawed and ficolled apheresis products were cultured in Stem Cell Growth Medium (CellGenix) supplemented with 10% fetal bovine serum and 10 U/mL IL2 with stimulator cells at a ratio of 1 NK cell to 10 irradiated K562-mb15-41BBL cells (developed at St. Jude Children's Research Hospital, Memphis, TN). Cells were harvested on day 8–9; products from HD were CD3-depleted. Clinical-grade products were shipped to UAMS overnight either cryopreserved in a dry shipper (n=7) or fresh in 5% human serum albumin on cold packs at 1–11°C (n=4). Cell purity, expression of activating molecules, and viability by 7AAD exclusion was assessed by flow cytometry. Standard 4h chromium-release assays were used to assess potency against K562 cells at a 20:1 ENK: K562 ratio. Student's t-Test was used to determine significance.

From 0.9–1.5×10⁷ starting NK cells, the total number of ENK cells produced was 5.4×10⁹ (range 1.8–24×10⁹). The fold NK-cell expansion was significantly lower for MM patients (n=5, median 22, 12–70 fold) than for HD (n=6, median 95, 31–160 fold; p<0.05). At harvest, median CD3+/CD56+ NK cell purity was 70% (52–88); CD3 depletion of HD products increased CD3+/CD56+ purity to 93% (86–95) resulting in a median CD3+/CD56- T cell content of 0.02% (0.04–1.02). Overall, median viability was 93% (67–98) and potency (defined as lysis of K562 cells at a 20:1 E:T ratio) was 74% (26–92). One product derived from a patient with 21% CD138+ MM cells in the apheresis collection had low expansion (12-fold), viability (66.7%) and potency (26%). For cryopreserved products, viability immediately after thawing was acceptable (median 94%, 75–99) but recovery of viable cells varied from 61% to 100% and thawed ENK failed to lyse K562 cells unless rested overnight. Further, recovery was extremely poor after overnight incubation (median 16%, 10–21). We therefore validated shipment of fresh ENK products. In contrast with frozen NK cells, the median recovery for fresh clinical products post-shipping was 101% (87–151). We confirmed that NK purity, viability, potency and expression of the key activating molecules NKG2D, NKp30, NKp44 and CD226 were retained up to 48h after transfer. ENK further increased by 34% after 72h in vitro incubation in the presence of IL2. Significant in vivo expansion of ENK was observed after infusion of fresh ENK cell products (n=3) but not after infusion of thawed products (n=3, see separate abstract). An additional advantage was that the fresh cells arrived ready to infuse and changes in release criteria relying on rapid and in process testing significantly reduced the time from apheresis collection to ENK infusion, an important consideration when treating high-risk MM patients who can experience rapid disease progression.

We conclude that large numbers of clinical grade ENK cells can be generated from both MM patient and HD derived apheresis products by co-culture with IL2 and K562-mb15-41BBL although less vigorous expansion was observed with patient-derived cells. Upon thawing, cryopreserved ENK cells exhibited inferior recovery and potency, and survived poorly during further in vitro culture. In contrast, freshly formulated and shipped ENK cells have excellent recovery and retain cytolytic ability. Robust in vivo expansion was only seen after infusion of fresh ENK cells. Production assistance by CAGT allowed for the rapid implementation of a novel therapy utilizing fresh ENK cells for poor prognosis MM patients.

No relevant conflicts of interest to declare.