A Highly Efficient Method to Expand CD3^-CD56⁺ NK Cells from Cord Blood Segments

Background: The infusion of in-vitro expanded cord blood derived CD3⁻CD56⁺ NK cells could potentially be used to enhance graft-vs-tumor effects following cord blood transplantation. In order to infuse NK cells derived from the same cord unit used during allogeneic transplantation, we sought to develop a highly efficient culture method to expand large numbers of NK cells in vitro from < 1 ml of cord blood.

Methods: Immunomagnetic beads were used to deplete CD3⁺ T-cells from thawed cord blood. CD3 depleted mononuclear cells (<0.1% CD56⁺) were co-cultured with either irradiated EBV-LCL feeder cells or 41BB transduced K562 cells in X-VIVO 20, 10% human AB serum, and 500 IU/ml hrIL-2 for up to 47 days.

Results: Day 12 EBV-LCL expanded NK cell cultures contained up to 90% CD3⁻CD56⁺ NK cells with less than 0.5% CD3⁺CD56⁺ cells. Expanded cord blood derived CD56⁺NK cells had similar expression of CD16, NKG2D, LFA-1, perforin, and granzymes A and B and had similar cytotoxic function as NK cells expanded from adult PBMC. Surface expression of NK cell TRAIL increased dramatically with in vitro expansion. By day 12, TRAIL surface expression by FACS was at similar levels observed on expanded adult NK cells, although expression gradually declined with prolonged cell culture; on days 12, 20, and 34, 89%, 57% and 11% of cord derived NK cells expressed TRAIL respectively. NK cell cultures expanded with 41BB-transduced APCs had a similar phenotype and cytotoxic function against K562 cells and renal cell carcinoma (RCC) cells as EBV-LCL expanded cells. Furthermore, NK cell cytotoxicity against RCC tumor targets treated with 10 nM bortezomib for 18 hrs (bortezomib upregulates RCC surface expression of DR5) was higher than untreated RCC cells confirming the functional cytolytic activity of TRAIL expressed on cord blood derived NK cells. We next evaluated the feasibility of expanding CD56⁺ NK cells from thawed segments attached to the umbilical cord blood units. TNC numbers of each segment ranged from 3–9 × 10⁶ cells. Unmanipulated thawed segments were co-cultured with irradiated EBV-LCL feeder cells as described above. On Day 12, 43% of viable cells were CD3⁺, 21.5% were CD3⁺CD56⁺ and 36% were CD3⁻ CD56⁺. CD3⁻CD56⁺ NK cells from thawed segments increased 200 – 300 fold with in vitro expansion when maintained in culture for 2–3 weeks (Table). To enrich for pure NK cell populations, subsequent expansions were performed on CD3 depleted cells pooled from three thawed segments stimulated with EBV-LCL; cultures on day 12 contained a pure population of CD3⁻CD56⁺ NK cells with virtually an identical phenotypic and cytotoxicity profile as NK cells expanded from larger aliquots taken directly from the thawed cord blood unit. By day 33, CD3⁻CD56⁺ NK cells expanded by up to 30,000 fold; in one experiment, 391 ×10⁶ CD3^-CD56⁺ NK cells were expanded from only 13,000 NK cells obtained from a pool of three thawed cord segments.

Conclusions: In vitro-expansion of a pure population of CD3⁻CD56⁺ cells derived from cord blood can be achieved using EBV-LCL or 41BB transduced feeder cells. Expanded cells have increased NKG2D and TRAIL expression and enhanced TRAIL-mediated tumor cytotoxicity. Even with very low starting numbers of TNCs, substantial numbers of CD3⁺, CD3⁺56⁺ and CD3⁻CD56⁺ cells can be expanded in vitro from thawed segments using EBV-LCL feeder cells in advance of thawing of the cord unit. These methods are being optimized to allow for clinical scale expansion of NK cells from the same cord unit used for hematopoietic cell transplantation.