Engineering Natural Killer Cells for Cancer Immunotherapy

The past several years have seen tremendous advances in the engineering of immune effector cells as therapy for cancer. While chimeric antigen receptors (CARs) have been used extensively to redirect the specificity of autologous T cells against hematological malignancies with striking clinical results, studies of CAR-modified natural killer (NK) cells have been largely preclinical. In this review, we focus on recent advances in NK cell engineering, particularly on preclinical evidence suggesting that NK cells may be as effective as T cells in recognizing and killing targets after genetic modification. We will discuss strategies to introduce CARs into both primary NK cells and NK cell lines in an effort to provide antigen specificity, the challenges of manufacturing engineered NK cells, and evidence supporting the effectiveness of this approach from preclinical and early-phase clinical studies using CAR-engineered NK cells. CAR-NK cells hold great promise as a novel cellular immunotherapy against refractory malignancies. Notably, NK cells can provide an "off-the-shelf" product, eliminating the need for a personalized and patient-specific product that plagues current CAR-T cell therapies. The ability to more potently direct NK cell-mediated cytotoxicity against refractory tumors through the expression of CAR is likely to contribute to the recent paradigm shift in cancer treatment. The past several years have seen tremendous advances in the engineering of immune effector cells as therapy for cancer. While chimeric antigen receptors (CARs) have been used extensively to redirect the specificity of autologous T cells against hematological malignancies with striking clinical results, studies of CAR-modified natural killer (NK) cells have been largely preclinical. In this review, we focus on recent advances in NK cell engineering, particularly on preclinical evidence suggesting that NK cells may be as effective as T cells in recognizing and killing targets after genetic modification. We will discuss strategies to introduce CARs into both primary NK cells and NK cell lines in an effort to provide antigen specificity, the challenges of manufacturing engineered NK cells, and evidence supporting the effectiveness of this approach from preclinical and early-phase clinical studies using CAR-engineered NK cells. CAR-NK cells hold great promise as a novel cellular immunotherapy against refractory malignancies. Notably, NK cells can provide an "off-the-shelf" product, eliminating the need for a personalized and patient-specific product that plagues current CAR-T cell therapies. The ability to more potently direct NK cell-mediated cytotoxicity against refractory tumors through the expression of CAR is likely to contribute to the recent paradigm shift in cancer treatment. The past several years have seen tremendous advances in the engineering of immune effector cells as therapy for cancer. Chimeric antigen receptors (CARs) have been used extensively to redirect the specificity of autologous T cells against lymphoid leukemia and lymphoma with striking clinical results. The most success has been reported in acute lymphoblastic leukemia (ALL), with several groups reporting complete remission (CR) rates as high as 90% after administration of a single dose of CD19-CAR-T cells following lymphodepleting chemotherapy, although the responses have been short lived in some cases. The favorable responses in early-phase trials of CD19-CAR-T cells (even in heavily pretreated children and adults)1Maude S.L. Frey N. Shaw P.A. Aplenc R. Barrett D.M. Bunin N.J. Chew A. Gonzalez V.E. Zheng Z. Lacey S.F. et al.Chimeric antigen receptor T cells for sustained remissions in leukemia.N. Engl. J. Med. 2014; 371: 1507-1517Crossref PubMed Scopus (1214) Google Scholar, 2Davila M.L. Riviere I. Wang X. Bartido S. Park J. Curran K. Chung S.S. Stefanski J. Borquez-Ojeda O. Olszewska M. et al.Efficacy and toxicity management of 19-28z CAR T cell therapy in B cell acute lymphoblastic leukemia.Sci. Transl. Med. 2014; 6: 224ra25Crossref PubMed Scopus (690) Google Scholar, 3Lee D.W. Kochenderfer J.N. Stetler-Stevenson M. Cui Y.K. Delbrook C. Feldman S.A. Fry T.J. Orentas R. Sabatino M. Shah N.N. et al.T cells expressing CD19 chimeric antigen receptors for acute lymphoblastic leukaemia in children and young adults: a phase 1 dose-escalation trial.Lancet. 2015; 385: 517-528Abstract Full Text Full Text PDF PubMed Scopus (695) Google Scholar have resulted in ongoing commercialization efforts in an attempt to make this therapy more widely available.4June C.H. Riddell S.R. Schumacher T.N. Adoptive cellular therapy: a race to the finish line.Sci. Transl. Med. 2015; 7: 280ps7Crossref PubMed Scopus (129) Google Scholar, 5Sadelain M. CAR therapy: the CD19 paradigm.J. Clin. Invest. 2015; 125: 3392-3400Crossref PubMed Scopus (79) Google Scholar However, CAR-modified T cells have a number of practical limitations. The generation of an autologous CAR-T cell product for each individual patient is logistically cumbersome and too restrictive for general clinical use. The manufacturing of CAR-T cells often takes a number of weeks, making it impractical for the treatment of a patient with rapidly advancing disease. Furthermore, it is not always possible to collect enough lymphocytes from heavily pretreated (and often lymphopenic) patients to allow for the successful generation of clinically relevant doses of CAR-T cells. An allogeneic "off-the-shelf" product could overcome these logistic challenges; however, allogeneic T cells (even if human leukocyte antigen [HLA] matched) carry a significant risk of graft-versus-host disease (GVHD) mediated through their native α-β T cell receptor (TCR). Natural killer (NK) cells are highly cytotoxic immune effectors, killing their targets in a non-specific manner.6Locatelli F. Moretta F. Brescia L. Merli P. Natural killer cells in the treatment of high-risk leukemia.Semin. Immunol. 2014; 26: 173-179Crossref PubMed Scopus (0) Google Scholar, 7Farag S.S. Caligiuri M.A. Human natural killer cell development and biology.Blood Rev. 2006; 20: 123-137Abstract Full Text Full Text PDF PubMed Scopus (284) Google Scholar NK cells lack the potential to cause GVHD8Moretta L. Locatelli F. Pende D. Marcenaro E. Mingari M.C. Moretta A. 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In this review, we focus on recent advances in NK cell engineering, particularly on preclinical evidence suggesting that NK cells may be as equally effective as T cells in recognizing and killing targets after genetic modification. We will discuss strategies to introduce CARs into both primary NK cells and NK cell lines in an effort to provide antigen specificity, the challenges of manufacturing engineered NK cells, and evidence supporting the effectiveness of this approach from preclinical and early-phase clinical studies using CAR-engineered NK cells. NK cells are innate immune effectors with the ability to exert rapid cytotoxicity against cancer and virus-infected cells without prior sensitization, hence the designation "natural" killers. 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