Expanded and armed natural killer cells for cancer treatment.

The capacity of natural killer (NK) cells to recognize and kill transformed cells suggests that their infusion could be used to treat cancer. It is difficult to obtain large numbers of NK cells ex vivo by exposure to cytokines alone but the addition of stimulatory cells to the cultures can induce NK cell proliferation and long-term expansion. Some of these methods have been validated for clinical-grade application and support clinical trials testing feasibility and safety of NK cell administration. Early data indicate that ex vivo expansion of NK cells from healthy donors or from patients with cancer is robust, allowing multiple infusions from a single apheresis. NK cells can transiently expand in vivo after infusion. Allogeneic NK cells are not direct effectors of graft-versus-host disease but this may occur if donor NK cells are infused after allogeneic hematopoietic stem cell transplant, which may activate T cell alloreactivity. NK cells can be directed with antibodies, or engineered using either transient modification by electroporation of mRNA or prolonged gene expression by viral transduction. Thus, expanded NK cells can be armed with activating receptors that enhance their natural anti-tumor capacity or with chimeric antigen receptors that can redirect them towards specific tumor targets. They can also be induced to express cytokines that promote their autonomous growth, further supporting their in vivo expansion. With the implementation of these approaches, expanded and armed NK cells should ultimately become a powerful component of immunotherapy of cancer.