Killer instincts: natural killer cells as multifactorial cancer immunotherapy

Abstract

Natural killer (NK) cells integrate heterogeneous signals for activation and inhibition using germline-encoded receptors. These receptors are stochastically co-expressed, and their concurrent engagement and signaling can adjust the sensitivity of individual cells to putative targets. Against cancers, which mutate and evolve under therapeutic and immunologic pressure, the diversity for recognition provided by NK cells may be key to comprehensive cancer control. NK cells are already being trialled as adoptive cell therapy and targets for immunotherapeutic agents. However, strategies to leverage their naturally occurring diversity and agility have not yet been developed. In this review, we discuss the receptors and signaling pathways through which signals for activation or inhibition are generated in NK cells, focusing on their roles in cancer and potential as targets for immunotherapies. Finally, we consider the impacts of receptor co-expression and the potential to engage multiple pathways of NK cell reactivity to maximize the scope and strength of antitumor activities.

Keywords: cancer immunotherapy; innate lymphoid cells; killer immunoglobulin-like receptors (KIR); natural killer cells; signal integration.

Figure 1

Intracellular signaling and integration downstream of the major NK cell receptors. Most NK cell receptors signal via transmembrane domains, including immunotyrosine-based inhibitory motifs (ITIM), immunoreceptor tyrosine-based switch motifs (ITSM), tyrosine-based signaling motif (YINM), immunotyrosine-based activation motifs (ITAM) and immunoglobulin tail tyrosine (ITT). (Left) Inhibitory signals received by NK cells are facilitated by the recruitment and activation of inhibitory SHP-1, SHP-2, and SHIP through ITIM and ITSM. (Right) Activating receptor clustering at the immunologic synapse facilitates the activation of intracellular domains by src-family kinases (src, lyn, fyn), or other kinases, including SAP and EAT2. Signals compete to activate or inhibit downstream intermediaries that can lead to transcription factor-mediated cytotoxicity and cytokine production.

Figure 2

NK cell receptors engaging HLA I and conveying activating or inhibitory signals. KIRs engage conserved epitopes on groups of HLA I molecules. (A) KIR receptors with long (L) cytoplasmic tails generally convey signals for inhibition via an ITIM, except for KIR2DL4. (B) KIR receptors with short (S) cytoplasmic tails and KIR2DL4 convey signals for activation by engaging DAP12, or FcϵRIγ, respectively, which contain ITAMs. Beyond KIR, ILT-2 and 4, and NKG2A (heterodimerized with CD94) bind with MHC I and convey inhibitory signals via ITIMs. NKG2C (heterodimerized with CD94) and NKG2D engage with HLA-E or HLA orthologs which include ULBPs, MIC-A and MIC-B. (C) Finally, NKG2C associates with DAP12 and NKG2D signals via DAP10, a YNIM-containing receptor, each to signal for activation. Green, pink, and purple symbols indicate CAR-NK targets, mAb targets, and small molecule targets, respectively.

Figure 3

TNF receptor superfamily receptor family member receptor and ligand partnerships and NK cells. (A) Death receptors expressed by target cells are induced for apoptosis when their ligand is provided by NK and other cellular sources. (B) Costimulatory TNFRSF members, 4-1BB and CD40L potentiate NK cell response against target cells.

Figure 4

Germline encoded NK cell receptors and ligands that contribute to cancer cell killing. (A) Natural cytotoxicity receptors bind an array of ligands; shown are the most important to tumor cell recognition. (B) CD16a, binds the constant region/fragment crystallizable (Fc), portion of antibodies that, when bound to target cells, enable cross linking of receptors. (C) SLAM family members can signal for activation or inhibition via an immunotyrosine switch motif. Green, pink, and purple symbols indicate CAR-NK targets, mAb targets, and small molecule targets, respectively.

Figure 5

DNAM-1, TIGIT, CD96 and other immunologic checkpoints that contribute to NK cell regulation within a tumor. The adhesion molecule CD112 is recognized by both DNAM-1 and TIGIT. Engagement with DNAM-1 results in activation via ITT signaling, conversely binding to TIGIT results in net inhibition signed through an ITIM. CD155, another adhesion molecule, binds to DNAM-1, TIGIT and CD96; signaling through CD96 through the signaling motifs tyrosine-based sorting motif (YXXM) and ITIM. TIM-3 can bind a variety of ligands including Gal-9, HMGB1 and CEACM1 to signal for inhibition. LAG-3 is known to bind HLA II and Gal-3. PD-1 binds PD-L1 and CTLA4 binds to CD80/CD86 to signal for inhibition. Pink symbols indicate mAb targets.