Harnessing natural killer cells for cancer immunotherapy: dispatching the first responders

Abstract

Natural killer (NK) cells have crucial roles in the innate immunosurveillance of cancer and viral infections. They are 'first responders' that can spontaneously recognize abnormal cells in the body, rapidly eliminate them through focused cytotoxicity mechanisms and potently produce pro-inflammatory cytokines and chemokines that recruit and activate other immune cells to initiate an adaptive response. From the initial discovery of the diverse cell surface receptors on NK cells to the characterization of regulatory events that control their function, our understanding of the basic biology of NK cells has improved dramatically in the past three decades. This advanced knowledge has revealed increased mechanistic complexity, which has opened the doors to the development of a plethora of exciting new therapeutics that can effectively manipulate and target NK cell functional responses, particularly in cancer patients. Here, we summarize the basic mechanisms that regulate NK cell biology, review a wide variety of drugs, cytokines and antibodies currently being developed and used to stimulate NK cell responses, and outline evolving NK cell adoptive transfer approaches to treat cancer.

Figure 1.. Killer Cell Ig-like Receptors (KIRs) provide diversity, tolerance, and education to human NK cells.

a. Humans inherit different numbers and combinations (haplotypes) of the 14 available killer cell inhibitory receptor (KIR) genes, with only two examples shown. The KIR genes are inherited in close proximity in a locus on chromosome 19. Haplotype A contains combinations of mostly inhibitory KIRs and only two activating KIRs (usually KIR2DL4 and KIR2DS4), while haplotype B contains more activating KIRs. b. Different inhibitory KIRs recognize subsets of the available alleles of the class I human leukocyte antigen complex (HLA). Relative receptor-ligand affinities are represented by thickness of the interlinking lines. Class I HLA subgroups are indicated with their general frequency in the human population. HLA-C alleles can be divided into C1 and C2 subgroups according to either asparagine or lysine at position 80, respectively, and HLA-B alleles can be divided into Bw4 or Bw6 subgroups. c. KIRs are expressed on NK cells in a variegated manner, forming a repertoire that can be tolerized and educated by interaction of at least one KIR with the endogenous ‘self’ class I HLA. A subset of NK cell clones could recognize a cell that has lost only one class I HLA allele (mutant cell #1), whereas if an abnormal cell loses all class I HLA (mutant cell #2), it has lost ligands for all KIR and is susceptible to all NK cells. d. KIR-mediated education during NK cell development. KIRs are stochastically expressed on individual NK cell clones during development. If a KIR recognizes ‘self’ class I HLA (top), it inhibits activation signaling and promotes maturation to a fully competent NK cell (educated), but if no KIR is expressed that recognizes ‘self’ class I HLA, chronic activation signaling results in an hyporesponsive NK cell (uneducated; bottom). Exposure to IL-15 or IL-2 can stimulate an uneducated NK cell into a competent, educated state. Figure 1B has been adapted with permission from REF. and figure 1D has been adapted with permission from REF..

Figure 2.. Antibodies in NK cell immunotherapy.

Antibodies can be used to enhance NK cell function in several different ways. a. Immune checkpoint blockade can be used to prevent the engagement of checkpoint inhibitory receptors on NK cells with their ligands on tumours. b. Antibodies targeting tumour antigens can be engaged via their Fc domains with CD16 on the surface of NK cells, triggering ADCC. c. Next generation bi- and tri-specific antibody constructs have also been designed that more efficiently engage with CD16 (and/or other activating receptors) than classic antitumour monoclonal antibodies. d. Finally, it is being investigated whether inhibitors of the metalloprotease ADAM17 can enhance ADCC by preventing the cleavage of CD16. The diversity of therapeutic strategies on display means they can often be used in tandem to activate NK cells through the interaction of multiple signaling pathways, although targeting the NK cell to the tumour is critical.

Figure 3.. Different strategies for producing bi- or tri-specific antibody constructs.

Many modern antibody constructs do not retain Fc domains and instead engage with CD16 through Fv domains. a. BiKEs and TriKEs are made from various combinations of two or three (respective) Fv fragments from mAbs or ligand molecules linked in a single polypeptide chain, which target some combination of tumor antigen, CD16 and another NK activating receptor. A common TriKE design includes an IL-15 moiety for better stimulation of NK cell activation, survival, and proliferation. b. Other constructs contain multiple Fv domains that form homodimers or homomultimers. Of this category AFM13, a bispecific construct consisting of two anti-CD30 and two anti-CD16 Fv domains, has shown promising results in clinical trials. c. A new innovative type of construct, the NK cell engager, is a complex consisting of different polypeptides that include both Fv and Fc domains. The Fc region of the construct can be modified for efficient CD16 engagement and optimally spaced Fv that optimally engages a NK cell activating receptor (e.g. NKp46) for further potentiation of NK cell stimulation, as well as one tumour antigen.