Targeting natural killer cells in cancer immunotherapy

Journal name:
Nature Immunology
Volume:
17,
Pages:
1025–1036
Year published:
DOI:
doi:10.1038/ni.3518
Received
Accepted
Published online

Abstract

Alteration in the expression of cell-surface proteins is a common consequence of malignant transformation. Natural killer (NK) cells use an array of germline-encoded activating and inhibitory receptors that scan for altered protein-expression patterns, but tumor evasion of detection by the immune system is now recognized as one of the hallmarks of cancer. NK cells display rapid and potent immunity to metastasis or hematological cancers, and major efforts are now being undertaken to fully exploit NK cell anti-tumor properties in the clinic. Diverse approaches encompass the development of large-scale NK cell–expansion protocols for adoptive transfer, the establishment of a microenvironment favorable to NK cell activity, the redirection of NK cell activity against tumor cells and the release of inhibitory signals that limit NK cell function. In this Review we detail recent advances in NK cell–based immunotherapies and discuss the advantages and limitations of these strategies.

At a glance

Figures

  1. Various approaches to therapy with the adoptive transfer of NK cells.
    Figure 1: Various approaches to therapy with the adoptive transfer of NK cells.

    NK cells can be obtained from either the patient or from a donor. (a) In autologous transfer, NK cells from the patient are activated and expanded in vitro in the presence of cytokines. Historically, IL-2 has been used for this purpose, but findings now suggest that the combination of IL-12, IL-15 and IL-18 might generate NK cells that are more functional and have memory properties. Feeder cells can be added to the culture. Irradiated human lymphoblastic K562 cells are often used as feeder cells and can be engineered to express cytokines (such as IL-15 and IL-21) and/or co-stimulatory molecules. The expanded and activated NK cells are then transferred back into the patient, who generally receives cytokine administration (IL-2, in most cases) to sustain the expansion and function of the infused NK cells. Although autologous NK cells might recognize activating signals such as stress molecules on cancer cells, their anti-tumor activity is limited by the inhibitory signal transmitted by self HLA molecules. (b) In allogeneic transfer, NK cells can be obtained from HLA-matched or haploidentical (partially matched) donors. NK cells are expanded through processes similar to those used for autologous transfer, but T cells should be removed to avoid GVHD. In this setting, the best responses are obtained when haploidentical donors do not express KIRs that recognize the patient's HLA molecules, because donor NK cells do not receive an inhibitory signal from the patient's cancer cells. (c) CARs can be engineered in autologous or allogeneic NK cells or in NK cell lines such as NK-92. CARs are designed by the fusion an antigen-binding domain (derived from a mAb scFv of known specificity) with a hinge region, a transmembrane domain and one or more stimulatory molecules. Each CAR has the CD3ζ chain (or sometimes the FcRγ chain) as its main signaling domain. Additionally, one or two co-stimulatory domain(s), usually from CD28 or CD137, can be added to the CAR construct; this leads to increased persistence and superior functionality. CARs from the first generation have no stimulatory domain, whereas CARs from the second generation and third generation have one co-stimulatory domain or two co-stimulatory domains, respectively. CAR engineering endows NK cells with antigen specificity. The binding of a CAR to the tumor antigen delivers a potent activating signal that triggers NK cell cytotoxicity, which results in elimination of the cancer cell.

  2. Targeting the tumor microenvironment to improve NK cell responses.
    Figure 2: Targeting the tumor microenvironment to improve NK cell responses.

    Various strategies can be adopted to create a microenvironment favorable to NK cells to improve the efficacy of NK cell–based therapies. Cytokines are generally administered to expand adoptively transferred NK cells, with IL-2 (1) being the most widely used in the clinic. However, IL-2 also activates Treg cells that hamper NK cell activity partly through production of the immunosuppressive cytokine TGF-β. In contrast, IL-15 (2) does not activate Treg cells and provides the additional advantage of stimulating both NK cells and cytotoxic CD8+ T cells. Other cytokines such as IL-12 (3), IL-18 (4) and IL-21 (5) can potentiate NK cell responses. Moreover, some anti-tumor agents have NK cell–modulating properties beyond their direct toxicity toward cancer cells. Genotoxic drugs (6), proteasome inhibitors (7) or immunomodulatory drug (IMiDs) (8) sensitize tumor cells to NK cell–mediated killing by altering their expression of surface molecules (downregulation of HLA molecules and upregulation of stress-induced ligands of activating receptors on NK cells). Moreover, immunomodulatory drugs (8) and the tyrosine-kinase inhibitor imatinib (9) stimulate NK cell function either directly or indirectly through the activation of other immune-cell subsets such as DCs. CD39 and CD73 are two enzymes expressed in the tumor microenvironment that contribute to production of the immunosuppressive metabolite adenosine. Blockade of these two enzymes via mAbs (10 and 11) or small-molecule inhibitors (14 and 15) restores NK cell function. Alternatively, blockade of the high-affinity adenosine receptor A2A (13) might prevent the direct suppressive effect of adenosine on NK cells. Finally, the immunosuppressive cytokine TGF-β substantially represses NK cell activity, and this pathway can be blocked by neutralizing this cytokine (12) or its receptor (16). MDSC, myeloid-derived suppressor cell.

  3. Therapeutic approaches that engage activating receptors on NK cells.
    Figure 3: Therapeutic approaches that engage activating receptors on NK cells.

    mAbs that target tumor-specific antigens and have been approved by the US Food and Drug Administration (FDA) (1) are widely used in the clinic. Their anti-tumor activity is attributed in part to their ability to trigger CD16 (the low-affinity receptor for IgG) on NK cells and induce ADCC. Agonistic mAbs to GITR (2), OX40 (3), CD137 (4) and CD27 (5) are currently being tested in clinical trials. These mAbs have been developed with the primary aim of stimulating T cells, but they might also positively influence NK cell functions. Enthusiasm has been growing for bispecific killer-cell engagers (BiKE) or trispecific killer-cell engagers (TriKE) that link activating receptors on NK cells to tumor antigens. Most of the bispecific engagers (5) trigger CD16, but fusion proteins that bridge NKG2D to tumor antigens (6) have also been designed. Trispecific engagers present three binding sites, and this provides the opportunity of targeting two different tumor antigens (7). The incorporation of IL-15 into a trispecific construct (8) further enhances the activation of NK cells. Alternatively, aptamers can also redirect NK cell activity toward tumor antigens (9) or can stimulate co-stimulatory molecules (10 and 11) to amplify the activation of NK cells. Finally, agonists of the activating receptor CD226 (12) have not been developed yet, but several pieces of evidence indicate that they might improve the anti-tumor activity of NK cells.

  4. Checkpoint inhibitors that 'release' NK cell functions.
    Figure 4: Checkpoint inhibitors that 'release' NK cell functions.

    The balance between activating signals and inhibitory signals received by receptors on NK cells determines the activation of NK cells. Inhibitory ligands expressed on the surface of target tumor cells or on antigen-presenting cells (APC) prevent the activation of NK cells, and abolishing these inhibitory signals by the means of blocking mAbs would restore full NK cell activity. Blocking mAbs directed against CTLA-4 (1), PD-1 (2) or PD-L1 (3) constitute one of the major advances of the past decade in terms of cancer immunotherapy. Even if T cells are considered the key mediators of the impressive efficacy of these checkpoint inhibitors, blockade of the CTLA-4 or PD-1 pathway might also enhance the anti-tumor responses of NK cells. Tim-3 is another checkpoint molecule shared by T cells and NK cells, and mAbs to Tim-3 (4) are currently being tested in cancer patients. In addition, mAbs that specifically block NK cell inhibitory receptors for MHC molecules have entered clinical trials. By blocking KIRs (5) or NKG2A (6), these mAbs release a major brake on the activation of NK cells. Finally, signaling via CD96 and TIGIT can restrain NK cell activity, and blocking mAbs to CD96 (7) or TIGIT (8) have great anti-cancer potential.

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Author information

  1. These authors contributed equally to this work.

    • Nicholas D Huntington &
    • Mark J Smyth

Affiliations

  1. Immunology of Cancer and Infection Laboratory, QIMR Berghofer Medical Research Institute, Herston, Australia.

    • Camille Guillerey &
    • Mark J Smyth
  2. School of Medicine, University of Queensland, Herston, Australia.

    • Camille Guillerey &
    • Mark J Smyth
  3. The Walter and Eliza Hall Institute of Medical Research, Parkville, Australia.

    • Nicholas D Huntington
  4. Department of Medical Biology, The University of Melbourne, Melbourne, Australia.

    • Nicholas D Huntington

Competing financial interests

M.J.S. has research agreements with Medimmune and Bristol Myers Squibb.

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