Ex vivo expansion of highly purified NK cells for immunotherapy after haploidentical stem cell transplantation in children.

Background: Allogeneic natural killer (NK) cells are known to show medium to high cytotoxic activity against HLA-nonidentical leukemia or tumor cells. For a possible benefit of post transplant treatment with NK cells after haploidentical stem cell transplantation (haplo-SCT) we developed a clinical scale procedure for NK cell processing observing Good Manufacturing Practice (GMP).
Methods: Allogeneic donor NK cells were selected from 15 unstimulated leukaphereses using two rounds of immunomagnetic T cell depletion, followed by an NK cell enrichment step. CD56 (+)CD3 (-) NK cells were stimulated and expanded in vitro according to GMP. Quality control of NK cell purity, residual T cells and cytotoxic activity was done by multi-coloured flow cytometric analyses.
Results: Purification led to an absolute number of 234-1 237 x 10 (6) CD56 (+)CD3 (-) NK cells from leukapheresis harvests with a median purity of 95 % and a 4 to 6(1/2) log depletion of T cells. After two weeks stimulation with IL-2 a five-fold expansion of NK cells with a T cell contamination below 0.1 % was reached. Median cell viability was 95 % after purification and 99 % after expansion. The IL-2 stimulated NK cells showed a highly increased lytic activity against the MHC-I deficient K562 cells compared to freshly isolated NK cells and a medium cytotoxicity against patients' leukemic cells.
Conclusions: Clinical scale enrichment and activation of allogeneic donor NK cells is feasible. High dose NK cell application may be a new treatment option for pediatric patients with leukemia or solid tumors in case of minimal residual disease or unbalanced chimerism post haplo-SCT as we could show for the first three patients .