Adoptive Cell Therapy (ACT) represents a promising approach for the treatment of cancer. It is a process in which immune cells such as T-cells or chimeric antigen receptor-expressing T cells (CAR-T cells) are taken from the patient’s own blood, modified, expanded, and infused back into the patient’s body to fight cancer. This method provides effective responses and has been widely used in clinics. However, it can result in graft versus host disease (GVHD), neurotoxicity, and activates hyper cytokine release, which creates side effects including fever, nausea, bad headache, fatigue, rapid heartbeat, low blood pressure, and difficulty in breathing. Hence, alternative immunotherapy would be an important key in this field to circumvent the limitation of T/CAR-T cell therapy.

         An exploration of an alternative way with lower physical risks and comparable efficacy leads to the application of Natural Killer (NK) cells in ACT for immunotherapy.

The immunotherapy perspective has shifted to a safer way: NK cells

         NK cells are lymphocytes of the innate immune system that play an important role in defending our body from various infections. They are regulated by IL-15 signaling and express many inhibitory and activating receptors. The activating receptors including NKG2D, Nkp46, and CD16 can trigger antibody-dependent cellular cytotoxicity (ADCC) which helps target and kill cancer cells. The inhibitory receptors, NKG2A, and KIRs, are inhibited by HLA-E and HLA class I on cancer cells, allowing the cancer cells to evade NK cells. Therefore, the function of NK cells in killing cancer cells is dependent on the balance stimulating these receptors.

Both NK cells derived from the peripheral blood (PB-NK) or umbilical cord blood (UB-NK) are traditionally used in ACT. Although PB-NK and UB-NK have shown success in preclinical and clinical studies, there are also certain limitations. These include poor bone marrow homing and reduced efficacy when cells are cryopreserved, and the complex manufacturing processes also contribute to high costs in treatment.

Apart from NK cells isolated from blood, clonal NK-cell lines were also investigated for ACT. The immortalized cell line NK-92 has shown good anti-tumor effects. They are easy to culture in vitro, and as a clonal population, it allows for homogeneous engineering and genetic manipulation. However, NK-92 cell lines have poor in vivo expansion, which decreases cytotoxicity efficacy.

iPSCs-derived NK cells: A new effective way to generate NK cells for immunotherapy 

Induced pluripotent stem cells (iPSCs) are derived from somatic cells such as fibroblasts and blood cells and are then reprogrammed back into an embryonic-like pluripotent state. iPSCs have high expansion capacity and with their pluripotency, they can differentiate into any desired cells including NK cells. iPSCs clones can be stored as a master cell bank without losing their properties, which provide a robust and reproducible cell source for clinical uses. iPSCs are easily cultured in laboratories which allow genetic modifications, especially for cancer treatment, and are simpler than primary cell models. These attributes make NK cells derived from iPSCs an attractive avenue for ACT.

            Using iPSCs-derived NK cells in clinical for cancer treatments can maintain consistency and overcome the problems of cell variations, which are varied among donors as found in PB and UB-derived NK cells. iPSC-derived NK cells have also shown great cytotoxicity against in vitro hematologic and solid tumor cell lines including lung cancer, hepatocellular cancer, ovarian cancer, etc. They have also been reported to be able to slow down the progression of tumors in the xenograft ovarian cancer model.

Both NK cells derived from the peripheral blood (PB-NK) or umbilical cord blood (UB-NK) are traditionally used in ACT. Although PB-NK and UB-NK have shown success in preclinical and clinical studies, there are also certain limitations. These include poor bone marrow homing and reduced efficacy when cells are cryopreserved, and the complex manufacturing processes also contribute to high costs in treatment. 

Apart from NK cells isolated from blood, clonal NK-cell lines were also investigated for ACT. The immortalised cell line NK-92 has shown good anti-tumour effects. They are easy to culture in vitro, and as a clonal population, it allows for homogeneous engineering and genetic manipulation. However, NK-92 cell lines have poor in vivo expansion, which decreases cytotoxicity efficacy.

iPSCs-derived NK cells: A new effective way to generate NK cells for immunotherapy  

Induced pluripotent stem cells (iPSCs) are derived from somatic cells such as fibroblasts and blood cells and are then reprogrammed back into an embryonic-like pluripotent state. iPSCs have high expansion capacity and with their pluripotency, they can differentiate into any desired cells including NK cells. iPSCs clones can be stored as a master cell bank without losing their properties, which provide a robust and reproducible cell source for clinical uses. iPSCs are easily cultured in laboratories which allow genetic modifications, especially for cancer treatment, and are simpler than primary cell models. These attributes make NK cells derived from iPSCs an attractive avenue for ACT. 

         Using iPSCs-derived NK cells in clinical for cancer treatments can maintain consistency and overcome the problems of cell variations, which are varied among donors as found in PB and UB-derived NK cells. iPSC-derived NK cells have also shown great cytotoxicity against in vitro hematologic and solid tumor cell lines including lung cancer, hepatocellular cancer, ovarian cancer, etc. They have also been reported to be able to slow down the progression of tumours in xenograft ovarian cancer model.

         Beyond iPSCs-derived NK cells, the derived NK cells or iPSC itself can be modified to express CAR for enhancing its efficiency in terms of tumour recognition and killing effects. 

         CAR-NK has recently been drawing significant attention. CAR-NK cell therapy utilises genetically engineered NK cells to target specific cancer. There are several ways in generating CAR-NK cells from multiple sources for immunotherapy from patients. Recently, iPSCs have become an attractive source of CAR-NK cells for “off-the-shelf” CAR-NK cell products. This is attributed to iPSCs able to provide a clonal population, being an easy system for genetic modification, and unlimited proliferative capacity. iPSCs-derived NK cells can be generated by either differentiation into NK cells prior to being genetically engineered with CAR or be engineered into CAR-iPSCs before differentiation into NK cells. The latter method has a higher chance to produce a homogenous population of CAR-NK cells as only a single genetically modified CAR-iPSCs can  be expanded and differentiated into a large number of CAR-NK cells. 

Although CAR-NK cells exhibit higher potential against tumour cell elimination, there are variations in cytotoxicity effects and cytokine productions among different CAR constructs that are designed for NK cells. Therefore, careful optimization of engineering and manufacturing of CAR-NK cells is necessary in order to develop CAR-NK cells as a safe, potent, and “off-the-shelf” therapy prior to application in clinical trials. 

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References

• Karagiannis P, Kim SI. iPSC-Derived Natural Killer Cells for Cancer Immunotherapy. Mol Cells. 2021;44(8):541-548. doi:10.14348/molcells.2021.0078

• Maddineni S, Silberstein JL, Sunwoo JB. Emerging NK cell therapies for cancer and the promise of next generation engineering of iPSC-derived NK cells [published correction appears in J Immunother Cancer. 2022 Sep;10(9):]. J Immunother Cancer. 2022;10(5):e004693. doi:10.1136/jitc-2022-004693