Significant progress has been made in recent years toward realizing the potential of natural killer (NK) cells for cancer immunotherapy. NK cells can respond rapidly to transformed and stressed cells and have the intrinsic potential to extravasate and reach their targets in almost all body tissues. In addition to donor-derived primary NK cells, also the established NK cell line NK-92 is being developed for adoptive immunotherapy, and general safety of infusion of irradiated NK-92 cells has been established in phase I clinical trials with clinical responses observed in some of the cancer patients treated. To enhance their therapeutic utility, NK-92 cells have been modified to express chimeric antigen receptors (CARs) composed of a tumor-specific single chain fragment variable antibody fragment fused via hinge and transmembrane regions to intracellular signaling moieties such as CD3ζ or composite signaling domains containing a costimulatory protein together with CD3ζ. CAR-mediated activation of NK cells then bypasses inhibitory signals and overcomes NK resistance of tumor cells. In contrast to primary NK cells, CAR-engineered NK-92 cell lines suitable for clinical development can be established from molecularly and functionally well-characterized single cell clones following good manufacturing practice-compliant procedures. In preclinical in vitro and in vivo models, potent antitumor activity of NK-92 variants targeted to differentiation antigens expressed by hematologic malignancies, and overexpressed or mutated self-antigens associated with solid tumors has been found, encouraging further development of CAR-engineered NK-92 cells. Importantly, in syngeneic mouse tumor models, induction of endogenous antitumor immunity after treatment with CAR-expressing NK-92 cells has been demonstrated, resulting in cures and long-lasting immunological memory protecting against tumor rechallenge at distant sites. Here, we summarize the current status and future prospects of CAR-engineered NK-92 cells as off-the-shelf cellular therapeutics, with special emphasis on ErbB2 (HER2)-specific NK-92 cells that are approaching clinical application.

Antigen-specific redirection of immune effector cells with chimeric antigen receptors (CARs) demonstrated high therapeutic potential for targeting cancers of different origins. Beside CART cells, natural killer (NK) cells represent promising alternative effectors that can be combined with CAR technology. Unlike T cells, primary NK cells and the NK cell line NK-92 can be applied as allogeneic off-the-shelf products with a reduced risk of toxicities. We previously established a modular universal CAR (UniCAR) platform which consists of UniCAR-expressing immune cells that cannot recognize target antigens directly but are redirected by a tumour-specific target module (TM). The TM contains an antigen-binding moiety fused to a peptide epitope which is recognized by the UniCAR molecule, thereby allowing an on/off switch of CAR activity, and facilitating flexible targeting of various tumour antigens depending on the presence and specificity of the TM. Here, we provide proof of concept that it is feasible to generate a universal off-the-shelf cellular therapeutic based on UniCAR NK-92 cells targeted to tumours expressing the disialoganglioside GD2 by GD2-specific TMs that are either based on an antibody-derived single-chain fragment variable (scFv) or an IgG4 backbone. Redirected UniCAR NK-92 cells induced specific killing of GD2-expressing cells in vitro and in vivo, associated with enhanced production of interferon-γ. Analysis of radiolabelled proteins demonstrated that the IgG4-based format increased the in vivo half-life of the TM markedly in comparison to the scFv-based molecule. In summary, UniCAR NK-92 cells represent a universal off-the-shelf platform that is highly effective and flexible, allowing the use of different TM formats for specific tumour targeting.

Chimeric antigen receptor (CAR)‐engineered natural killer (NK) cells represent a promising effector cell type for adoptive cancer immunotherapy. Both, genetically modified donor‐derived NK cells as well as continuously expanding NK‐92 cells are currently under clinical development. To enhance their therapeutic utility for the treatment of pre‐B‐cell acute lymphoblastic leukemia (B‐ALL), we engineered NK‐92 cells by lentiviral gene transfer to express a FMS‐like tyrosine kinase 3 (FLT3)‐specific CAR that contains a composite CD28‐CD3ζ signaling domain. FLT3 has primarily been described as a therapeutic target for acute myeloid leukemia, but overexpression of FLT3 has also been reported in B‐ALL. Exposure of FLT3‐positive targets to CAR NK‐92 cells resulted in conjugate formation between NK and leukemia cells, NK‐cell degranulation and selective cytotoxicity toward established B‐ALL cell lines and primary blasts that were resistant to parental NK‐92. In a SEM B‐ALL xenograft model in NOD‐SCID IL2R γnull mice, treatment with CAR NK‐92 but not parental NK‐92 cells markedly inhibited disease progression, demonstrating high antileukemic activity in vivo. As FLT3 is known to be also expressed on precursor cells, we assessed the feasibility of incorporating an inducible caspase‐9 (iCasp9) suicide switch to enhance safety of our approach. Upon addition of the chemical dimerizer AP20187 to NK‐92 cells coexpressing the FLT3‐specific CAR and iCasp9, rapid iCasp9 activation was observed, precluding further CAR‐mediated cytotoxicity. Our data demonstrate that B‐ALL can be effectively targeted by FLT3‐specific CAR NK cells which may complement CD19‐directed immunotherapies, particularly in cases of inherent or acquired resistance to the latter.