**4. NK cell modulation for cancer therapy**

The ability of NK cells to kill tumour cells has made them very attractive in immunotherapy. NK cell impairments associated with tumour development and progression have been fre‐ quently reported in cancer patients, including weakened effector functions and an altered phenotype with downregulation of activating NK cell receptors [68]. Different strategies have been employed to repair, replace or enhance the biological functions of autologous or

**Figure 3.** Tumour Evasion from NK cells. Tumour cells use direct and indirect mechanisms to evade NK cell attack. Direct mechanisms include A) upregulation of MHC class I expression B) shedding of soluble ligands for NK cell activation receptors and C) release of inhibitory cytokines. Indirect mechanisms include D) activation of inhibitory regulatory T cells E) killing of immature dendritic cells to prevent NK cell priming F) release of phagocyte‐derived inhibitory cytokines and G) reducing the number of NK progenitor cells to lower NK cell counts. NK: natural killer cell; DC: dendritic cell, IL‐10: interleukin 10; IDO: indoleamine 2,3‐dioxygenase; MHC: major histocompatibility complex; ROS: reactive oxygen species; Th2: T helper cell type 2.

allogeneic NK cells *in vivo* and *ex vivo*. In a clinical setting, the key factors to be considered are the number, purity, proliferative capacity and activation state of NK cells. The most limiting of these factors is obtaining a sufficient number of NK cells, hence the extensive development of *ex vivo* expansion methods for NK cell adoptive immunotherapy applications. The impressive clinical responses seen following administration of chimeric antigen receptor T cells (CAR‐T) has led to trials of CAR‐NK cells at centres in the US and Europe. Reports of pre‐clinical data are encouraging and suggest that the more constrained proliferation of CAR‐NK cells *in vivo* and the lower release of inflammatory cytokines may provide improve the safety profile.

The delivery of IL‐2, IL‐12 and IL‐15 genes to the human NK cell line NK‐92 has also been shown to enhance proliferative and cytotoxic capabilities. These cytokines are known to play important roles in the enhancement of survival and activation of many immune cells including T cells, B cells and NK cells. Strategies to enhance endogenous NK cell function *in vivo* through cytokines were pioneered by Rosenberg et al. who demonstrated great initial potential for IL‐2 administration in advanced cancer patients [69]. *In vitro* stimulation of NK cells by activating cytokines such as IL‐2 is known as the lymphokine‐activated killer (LAK) phenomenon [70]. In early experiments, NK cells were activated *ex vivo* and adoptively transferred to patients with advanced metastatic renal cancer and melanoma along with IL‐2 infusions. However, overall data from clinical trials since then have failed to provide a convincing proof of efficacy [68]. The clinical efficacy of LAK therapy was limited by the toxicity of IL‐2 and the potential expansion of T regulatory cells. Mouse NK cells stimulated *in vitro* with a combination of IL‐12, IL‐15 and IL‐18 were recently shown to have enhanced effector functions and longer survival after adoptive cell transfer [71]. Target cell stimulation of NK cells is an alternative to *in vitro* cytokine stimulation. Recent studies reported a tumour‐priming approach, in which human NK cells are activated by co‐incubation with an NK‐resistant leukaemia cell line in the absence of IL‐2 [72]. The clinical potential of these tumour‐primed NK cells has been explored in acute myeloid leukaemia and multiple myeloma with promising results in autologous and alloge‐ neic settings [73].

The last three decades unravelled different molecular mechanisms governing NK cell‐medi‐ ated anti‐tumour functions. This led to the development of a variety of strategies for NK cell‐based immunotherapy of cancer. However, many challenges still remain as we better our molecular and functional characterization of NK cells and their receptors, and decipher the different signalling pathways involved in NK cell recognition of targets. NK cell responses can differ according to the type, combination and intensity of signals. Thus, a better under‐ standing of tumour‐specific responses at the bench, will lead to novel therapeutic strategies with better efficacy in the clinic.
