Chimeric antigen receptor engineered natural killer cells for cancer therapy

Abstract

Natural killer (NK) cells, a unique component of the innate immune system, are inherent killers of stressed and transformed cells. Based on their potent capacity to kill cancer cells and good tolerance of healthy cells, NK cells have been successfully employed in adoptive cell therapy to treat cancer patients. In recent years, the clinical success of chimeric antigen receptor (CAR)-T cells has proven the vast potential of gene-manipulated immune cells as the main force to fight cancer. Following the lessons learned from mature gene-transfer technologies and advanced strategies in CAR-T therapy, NK cells have been rapidly explored as a promising candidate for CAR-based therapy. An exponentially growing number of studies have employed multiple sources of CAR-NK cells to target a wide range of cancer-related antigens, showing remarkable outcomes and encouraging safety profiles. Clinical trials of CAR-NK cells have also shown their impressive therapeutic efficacy in the treatment of hematological tumors, but CAR-NK cell therapy for solid tumors is still in the initial stages. In this review, we present the favorable profile of NK cells as a potential platform for CAR-based engineering and then summarize the outcomes and strategies of CAR-NK therapies in up-to-date preclinical and clinical investigations. Finally, we evaluate the challenges remaining in CAR-NK therapy and describe existing strategies that can assist us in devising future prospective solutions.

Keywords: CAR-NK; Cancer treatment; Clinical trials; Immunotherapy; NK cells; Preclinical studies.

Fig. 1

The mechanism of NK activation and self-tolerance. A In healthy conditions, self-HLA class I molecules of healthy cells bind the inhibitory receptors of NK cells such as KIRs and NKG2A/CD94. Dominant inhibitory signaling suppressed the cytolytic ability of NK cells to make autologous healthy cells “licensed”. B The majority of tumor cells downregulate or lost their MHC-I molecule expression to escape from the immune cells attacking. This results in decreasing tumor ligands combining with inhibitory receptors of NK cells, thus NK cells are activated to secret perforin and granzyme to lyse tumor cells. C Overexpressed activating ligands on stressed cells engage with NK cell receptors, leading to superior activating signaling surpassing inhibitory signaling. As a result, NK cells transform into activation state and initiate cell lysing. D Antibody-dependent cell-mediated cytotoxicity, ADCC. The tumor-specific Fc fragment binds CD16 (FcγRIII) of NK cells, resulting in ADCC development. In addition to ADCC, other killing mechanisms of NK cells include death-receptor-mediated and perforin/granzyme-mediated killing activities

Fig. 2

The research progress, advantages, and limitations of various NK cell sources. NK cells can be obtained from 5 different sources: PB, UCB, iPSC, hESC, and NK cell lines. Most cell sources have remarkable tumor-eliminating ability and provide clinically meaningful benefit, having transitioned into in-human studies of different stages. Each source of NK cells has its own set of strengths and limitations

Fig. 3

The evolution of CAR design and emerging strategies on CAR-NK structure. The main distinction of the three generation CARs lies in the number and composition of intracellular domains. In addition to the T-cell based signaling domains, NK-specific receptor (such as DAP10, DAP12, 2B4) has introduced into NK cells to explore CAR-NK therapy. The novel generation CAR strategies utilize the fundamental principles of CAR signaling and innovative approaches to enhance cytotoxicity persistence, trafficking, and safety performance of CAR-NK cells, endowing them multifunctional attributes

Fig. 4

The challenges of CAR-NK existing in the process from lab production to tumor infiltration. The unsatisfactory CAR transduction efficiency and limited proliferation ability add barriers to CAR-NK production. Multiple approaches including virus-mediated and non-viral-mediated transduction have been utilized to boost CAR expression and stability. The ex-vivo expansion are mainly stimulated by cytokines or feeder cell system with limited potential. Upon infusion into the body, the trafficking and infiltration abilities are impeded by the disruptive chemokines/chemokine receptors axis in the dysregulated tumor vasculature. In tumor bed, suppressive cells (Treg cells, Breg cells and MDSCs) and soluble inhibitory cytokines (TGF-β, IL-10 and IL-6) can disrupt NK cell effector functions. The harsh TME owing to the nutrient deficiency, hypoxia, and acidic conditions can further suppress and dampen NK activities