NK Cell-Based Immune Checkpoint Inhibition

Abstract

Immunotherapy, with an increasing number of therapeutic dimensions, is becoming an important mode of treatment for cancer patients. The inhibition of immune checkpoints, which are the source of immune escape for various cancers, is one such immunotherapeutic dimension. It has mainly been aimed at T cells in the past, but NK cells are a newly emerging target. Simultaneously, the number of checkpoints identified has been increasing in recent times. In addition to the classical NK cell receptors KIRs, LIRs, and NKG2A, several other immune checkpoints have also been shown to cause dysfunction of NK cells in various cancers and chronic infections. These checkpoints include the revolutionized CTLA-4, PD-1, and recently identified B7-H3, as well as LAG-3, TIGIT & CD96, TIM-3, and the most recently acknowledged checkpoint-members of the Siglecs family (Siglec-7/9), CD200 and CD47. An interesting dimension of immune checkpoints is their candidacy for dual-checkpoint inhibition, resulting in therapeutic synergism. Furthermore, the combination of immune checkpoint inhibition with other NK cell cytotoxicity restoration strategies could also strengthen its efficacy as an antitumor therapy. Here, we have undertaken a comprehensive review of the literature to date regarding NK cell-based immune checkpoints.

Keywords: cancer immunotherapy (CI); immune checkpoint; immune checkpoint inhibitors (ICI); immune therapeutics; natural killer cell (NK).

Figure 1

Types and functions of NK cells. (a) CD56^bright CD16⁻ and CD56^dim CD16⁺ NK cells, termed immature and mature NK cells, respectively, are identified to have functional differences. CD56^bright NK cells produce more cytokines, while CD56^dim CD16⁺ NK cells are more cytotoxic and carry out ADCC (antibody-dependent cell-mediated cytotoxicity). (b) NK cell surface receptors, both activating and inhibitory receptors, carry out NK cell functions through a balance of signals. Inhibitory receptors detect MHC-I ligands on normal cells, and if present, activating signals are terminated, thereby maintaining “self-recognition.” These receptors carry ITIM motifs in their cytoplasmic tail, which recruit SHP1/2 through phosphorylation to carry out its function. Such inhibition is termed “dominant inhibition.” (c) Inhibitory receptors are exploited by cancer through upregulation of ligands, thereby avoiding destruction by NK cells. Hence, antibodies such as lirilumab and monalizumab are developed to block such interaction and enhance NK cell cytotoxicity toward cancer cells. This phenomenon is termed immune checkpoint inhibition. (d) The presence of CD16 receptors on NK cells makes them able to carry out ADCC. Therefore, several antibodies, such as rituximab, elotuzumab, and cetuximab, have been clinically evaluated for synergism with immune checkpoint inhibitors.

Figure 2

Immune checkpoint inhibition observed in Natural Killer cells. (A) Inhibitory receptor-ligand interaction leading to immune escape of cancer cells is termed immune checkpoint inhibition. Inhibitory receptors expressed on the surface of NK cells are illustrated as blue rods, and ligands for these receptors expressed by tumor cells are illustrated as orange rods. (B) Rectangular boxes represent the intracellular domains of the receptors through which inhibition is carried out. Several of these receptors (KIR, ILT2, NKG2A & CD94, TIGIT & CD96, Siglec-7/9, PD-1, and SIRPα) bear 1-3 ITIMs in their cytoplasmic tail and observe ITIM-based inhibition. In addition, TIGIT and PD-1 cytoplasmic tails contain an ITT-like and ITSM motif, respectively. LAG-3, TIM-3, CD200, and CTLA-4 lack an ITIM motif in their cytoplasmic tails. Instead, they have special intracellular tyrosine motifs such as KEELE, Y265, NPXY, and YVKM and YFIP, respectively, which are implicated in carrying out the inhibition process. Antibodies to these receptors are shown within red-outlined boxes. (C) Moreover, several other immune cells, including T cells, B cells, and myeloid cells, express these receptors on their surfaces, as shown on the left panel for each immune checkpoint receptor.

Figure 3

Immunomodulatory effect of lenalidomide on NK cells. Lenalidomide upregulates ligands for NK cell-activating receptors in multiple myeloma and augments NK cell function. Lenalidomide also increases ILT2 expression on CLL and decreases it on NK cells, while its ligand (HLA-E) is reestablished on leukemic cells. These immunomodulatory effects of NK cells were associated with increased NK cell activation and proliferation. ILT2 blockade in this setting further potentiated NK cell functions.

Figure 4

Indirect effects of CTLA-4 blockade on NK cells. (A) Treg increase was correlated with NK cell suppression and decrease IL-2 availability to NK cells, which may be reversed with CTLA-4 blockade. (B) CTLA-4 blockade with antibodies increased CD4⁺ T-cell proliferation and IL-2 production. (C) Ipilimumab blockade of CTLA-4 on tumor cells has been reported to be associated with ADCC, TNF-α release by NK cells, and induction of the IL-2Rα chain on NK cells. (D) Ipilimumab was associated with an increased frequency of CD3⁻ CD56^dim CD16⁺ NK cells with increased TIM-3 expression. CD56^bright CD16⁻ NK cells with increased expression of p46 receptor and TRAIL have also been reported. (E) A combination of IL-2 plus ipilimumab was reported to be associated with increased NK cell infiltration of tumor, as well as a decrease in exhausted and differentiated NK cells.