TIGIT Inhibits NK Cell-Mediated Killing

NK Cell

Natural killer (NK) cells are a type of lymphocyte critical to the innate immunity. The critical role of NK cells involves rapid defense against pathogen infections, tumor cells and damaged cells. Unlike adaptive immune cells, NK cell-mediated killing do not require prior sensitization or antigens presented on targets. Their functions are accomplished through direct cytolysis, the release of cytokines and chemokines including interferon-γ (IFN-γ), tumor-necrosis factor-α (TNF-α), granulocyte-macrophage colony-stimulating factor (GM‑CSF), interleukin‑6 (IL‑6), CC‑chemokine ligand 3 (CCL3), CCL4 and CCL5.

The functions of NK cells are regulated by a wide range of activating receptors and inhibitory receptors, classified on the basis of the signals they transmit. The activating receptors, such as natural cytotoxicity receptors (NCRs), NKG2D or CD226, stimulates NK cell–mediated killing. In contrast, inhibitory receptors inhibits NK cell functions. Examples of inhibitory NK cell receptors include the KIR (killer cell immunoglobulin-like receptors) and LIR (leukocyte immunoglobulin-like receptors) in humans, the Ly49 family in mice, and heterodimers of the C-type lectin CD94 and NKG2 in both species. Those inhibitory receptors mainly recognize MHC-I molecules. T cell immunoreceptor with immunoglobulin and ITIM domains (TIGIT) is a novel NK inhibitory receptors that bind nectin and nectin-like proteins instead of MHC-I molecules.

TIGIT

TIGIT, also known as VSIG9; VSTM3; WUCAM, is a member of inhibitory immune checkpoints. It is expressed mainly on the surface of T cells and NK cells and is composed of three domains: 1) an extracellular immunoglobulin variable (IgV) domain which is responsible for binding PVR, NECTIN2 and NECTIN3, 2) a transmembrane domain, and 3) an intracellular domain which includes the immunoreceptor tail tyrosine (ITT)-like and ITIM motifs. Physical ligands of TIGIT include PVR (PVS; HVED; CD155; NECL5; TAGE4; Necl-5) with higher affinity, NECTIN2 (HVEB; PRR2; CD112; PVRL2; PVRR2) and NECTIN3 (PPR3; PRR3; CD113; PVRL3; PVRR3; CDW113; NECTIN-3) with lower affinity.

Several published data support the suppression role of TIGIT on NK cells in vitro. For example, the killing of TIGIT transfected NK cells (YTS-TIGIT) was strongly inhibited compared with non-transfected NK cells (YTS), while the increased killing was observed after blockage of TIGIT using antibody.

The inhibitory mechanism has been elucidated which in brief is that TIGIT/PVR interaction leads to phosphorylation of TIGIT’s intracellular domain and further triggers intracellular signaling cascades to limit NK cell functions.

Recently, NK cells are an attractive cell population for cancer immunotherapy because their killing is MHC-independent thus they can kill cancer cells escaped from T cell-mediated immune response. TIGIT as an attractive immunomodulator protein, can regulates both NK and T cells. Thus screening of effective inhibitors of TIGIT should provide promising approaches for cancer treatment.

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