Natural killer cells differentiated in vitro from cord blood CD34+ cells are more advantageous for use as an immunotherapy than peripheral blood and cord blood natural killer cells

Background aims. Natural killer (NK) cells have the potential to become a successful immunotherapy as they can target malignant cells without being direct effectors of graft-versus-host disease. Our group has previously shown that large numbers of functional NK cells can be differentiated in vitro from umbilical cord blood (CB) CD34(+) cells. To produce a clinically relevant and effective immunotherapy, we hypothesized that it is essential that the NK cells are able to proliferate and persist in vivo while maintaining an optimal activation status and killing capacity. Methods. We evaluated the proliferation capacity, telomere length and terminal differentiation markers expressed by NK cells differentiated in vitro. We also determined how their cytotoxicity compared with peripheral blood (PB) NK cells and CBNK cells when targeting patient acute myeloid leukemia (AML) blasts and solid tumor cell lines. Results. We found that the differentiated NK cells could respond to interleukin-2 and proliferate in vitro. Telomere length was significantly increased, whereas CD57 expression was significantly reduced compared with PBNK cells. The cytotoxicity of the differentiated NK cells was equivalent to that of the PBNK and CBNK cell controls, and priming consistently led to higher levels of killing of patient leukemic blasts and solid tumor cell lines in vitro. Interestingly, this activation step was not required to observe killing of patient AML blasts in vivo. Conclusion. We are able to generate NK cells from CBCD34(+) cells in high numbers, allowing for multiple infusions of highly cytotoxic NK cells that have potential to further proliferate in vivo, making them a desirable product for application as an immunotherapy in the clinic.