Activating and inhibitory receptors control natural killer (NK) cell activity. T-cell immunoglobulin and ITIM (immunoreceptor tyrosine-based inhibition motif) domain (TIGIT) was recently identified as a new inhibitory receptor on T and NK cells that suppressed their effector functions. TIGIT harbors the immunoreceptor tail tyrosine (ITT)-like and ITIM motifs in its cytoplasmic tail. However, how its ITT-like motif functions in TIGIT-mediated negative signaling is still unclear. Here, we show that TIGIT/PVR (poliovirus receptor) engagement disrupts granule polarization leading to loss of killing activity of NK cells. The ITT-like motif of TIGIT has a major role in its negative signaling. After TIGIT/PVR ligation, the ITT-like motif is phosphorylated at Tyr225 and binds to cytosolic adapter Grb2, which can recruit SHIP1 to prematurely terminate phosphatidylinositol 3-kinase (PI3K) and MAPK signaling, leading to downregulation of NK cell function. In support of this, Tyr225 or Asn227 mutation leads to restoration of TIGIT/PVR-mediated cytotoxicity, and SHIP1 silencing can dramatically abolish TIGIT/PVR-mediated killing inhibition. Meanwhile, normal cells are kept away from their cytotoxicity. Therefore, the discrimination between 'self' normal cells and 'nonself' abnormal cells has to be precisely recognized by NK cells. [5][6][7] A very large repertoire of receptors, both activating and inhibitory, is proved to be critical in NK cell function. The best-known inhibitory receptors of NK cells are the killer-cell immunoglobulin-like receptor family, whose physical ligands are MHC-I molecules that are expressed on self normal cells to protect them from NK cell lysis. 8 Other non-MHC-I inhibitory receptors, which do not associate with MHC-I molecules, are also expressed on NK cells. 9 However, their physiological and pathological significances have not been defined yet.T-cell immunoglobulin and ITIM domain (TIGIT) was recently identified as an inhibitory receptor that is expressed mainly on NK cells, activated CD4 and CD8 T cells. 10-12 TIGIT harbors one extracellular immunoglobulin domain, a type 1 transmembrane region, and an immunoglobulin tail tyrosine (ITT)-like phosphorylation motif followed by an ITIM (immunoreceptor tyrosine-based inhibition motif) of the cytoplasmic tail. 13 The physical ligands of TIGIT were identified as the poliovirus receptor (PVR, or CD155) and the PVRL2 (Nectin2, or CD112). 10,12 TIGIT can bind to PVR of human dendritic cells to enhance interleukin 10 (IL-10) production, which inhibits T-cell activation. 10 Kuchroo et al. 14 showed that TIGIT harbors a T-cell-intrinsic inhibitory function to suppress T-cell activation.Moreover, TIGIT can inhibit NK cell cytolysis through engagement with PVR or PVRL2. 11 TIGIT-deficient mice are more susceptible to autoimmune diseases. 14,15 However, the inhibitory mechanism mediated by TIGIT has not been elucidated.TIGIT contains a classical ITIM motif, which recruits either Src homology (SH) 2 domain-containing protein tyrosine phosphatases SHP1 and SHP...

The Kruppel-like factor 5 (KLF5) transcription factor is highly expressed in high-grade and basal-like breast cancers. However, the mechanism by which KLF5 promotes cell migration and invasion is still not completely understood. In this study, we demonstrate that TNFAIP2, a tumor necrosis factor-α (TNFα)-induced gene, is a direct KLF5 target gene. The expression of TNFAIP2 is highly correlated with the expression of KLF5 in breast cancers. The manipulation of KLF5 expression positively alters TNFAIP2 expression levels. KLF5 directly binds to the TNFAIP2 gene promoter and activates its transcription. Functionally, KLF5 promotes cancer cell proliferation, migration and invasion in part through TNFAIP2. TNFAIP2 interacts with the two small GTPases Rac1 and Cdc42, thereby increasing their activities to change actin cytoskeleton and cell morphology. These findings collectively suggest that TNFAIP2 is a direct KLF5 target gene, and both KLF5 and TNFAIP2 promote breast cancer cell proliferation, migration and invasion through Rac1 and Cdc42.

Although granzymes (Gzms) A-and B-induced cell death pathways have been defined, little is known about how other orphan Gzms function in CTL-mediated cytotoxicity. GzmK and A are tryptases among all the Gzms of humans and they are closely linked on the same chromosome. In this study, we showed that GzmK can be efficiently delivered into target cells with a cationic lipid protein transfection reagent Pro-Ject. We found human GzmK triggers rapid cell death independently of caspase activation. The features of death are characterized by rapid externalization of phosphatidylserine, nuclear morphological changes and single-stranded DNA nicks. GzmK hydrolyzes the nucleosome assembly protein SET in its recombinant and native forms or in intact cells. Cleavage of SET by GzmK abrogates its nucleosome assembly activity. After GzmK loading, SET and DNase NM23H1 rapidly translocate into the nucleus and SET is cleaved, where the nuclease activity of NM23H1 is activated to nick chromosomal DNA.