Thymocyte selection-associated high mobility group box protein TOX is aprotein that in humans is encoded by theTOXgene.[5][6][7] TOX drives T-cell exhaustion[8][9] and plays a role in innate lymphoid cell development.[10][11]
The TOX gene encodes aprotein that belongs to a large superfamily ofchromatin associated proteins that share an approximately 75 amino acidDNA binding motif, theHMG (high mobility group)-box (named after that found in the canonical member of the family, high mobility group protein 1). Somehigh mobility group (HMG) box proteins (e.g.,LEF1) contain a single HMG box motif and bind DNA in a sequence-specific manner, while other members of this family (e.g.,HMGB1) have multiple HMG boxes and bind DNA in a sequence-independent but structure-dependent manner. While TOX has a single HMG-box motif,[7] it is predicted to bind DNA in a sequence-independent manner.[12]
TOX is a member of a small subfamily of proteins (TOX2,TOX3, andTOX4) that share almost identical HMG-box sequences.[12] TOX2 has been identified to play a role in the differentiation of Tfollicular helper cell.[13] TOX2 is thought to be a downstream signal ofBCL-6.[13] TOX3 has been identified as a breast cancer susceptibility locus.[14][15] TOX is highly expressed in thethymus, the site of development ofT lymphocytes.[10]Knockout mice that lack TOX have a severe defect in development of certain subsets of T lymphocytes.[16]
TOX is necessary for T cell persistence but also drivesT cell exhaustion.[17][18][19] An increase in TOX expression is characterized by a weakening of the effector functions of thecytotoxic T cell and upregulation of inhibitory receptors on the cytotoxic T cells.[20][21] TOX promotes the exhausted T cell phenotype throughepigenetic remodeling.[20][22]PD-1 is an inhibitory marker on T cells that increases when TOX is unregulated.[20][23][22] This allows for cancerous cells to evade the cytotoxic T cells through upregulated expression ofPD-L1.[24]
Markers of effector functions that are decreased when TOX is overexpressed areKLRG1,TNF, andIFN-gamma.[8] IFN-gamma and TNF-alpha production are also increased when theTox andTox2 genes are deleted.[9] Upregulation of effector function in cells lacking TOX is not always seen and it has been proposed that inhibitory receptor function is separated from effector CD8+ cytotoxic T cell function.[8] T-cell exhaustion does not occur when TOX is deleted from CD8+ T cells, but the cells instead adopt the KLRG1+ terminal effector state and undergoapoptosis, or programmed cell death.[9] It was therefore proposed that TOX prevents this terminal differentiation and instead promotes exhaustion so that the T-cell has a slightly more sustained response.[9]
Incancer or during chronicviral infection, T-cell exhaustion occurs when cytotoxic T-cells are constantly stimulated.[8][25] TOX is upregulated in CD8+ T cells from chronic infection when compared to acute infection.[8] Patients with cancer typically have high levels of TOX in theirtumor-infiltrating lymphocytes,[8] and anti-tumor immunity is heightened whenTox andTox2 are deleted.[9] TOX and TOX2-deficient tumor-specificCAR T cells additionally have increased antitumor effector cell function as well as decreased levels of inhibitory receptors.[8]
NFAT transcription factors are essential for activating TOX in CD8+ T-cells,[8] and it has been suggested that TOX is a downstream target of NFAT.[9] The expression and function of NR4a (a target of NFAT) and TOX are strongly linked with reducedNR4a expression inTox double knockout T cells and minimizedTox expression inNR4a triple knockout T cells.[9]
TOX is necessary for positive selection in developingthymocytes.[26] Knock out TOX mice shows a requirement of TOX for the CD4 T cell lineage,[26] however CD8 single positive T-cells were still able to develop.[26]
Notch signaling can aid in the development of all innate lymphoid cells, but in TOX-deficient cells, Notch target genes are expressed at low levels, so it is possible that TOX is required for downstream activation of these Notch target genes.[10] TOX was also found to bindHes1, a Notch target gene, in embryonic kidney cells.[10]
SeveralILC3 populations are reduced in the absence of TOX, implicating TOX's role in their development.[10] In the small intestine, major ILC3 populations are normal in TOX-deficient cells, suggesting that gut ILC3 development may occur independently of TOX.[10] Some ILC3 populations in the gut expand in the absence of TOX.[10]
It has been proposed thatNFIL3 and TOX regulate the transition of common lymphoid progenitor to early innate lymphoid progenitor.[11] In NFIL3-deficient mice, the expression of TOX is downregulated, indicating that NFIL3 is directly affecting the expression of TOX which is then acting downstream in ILC development.[11] TOX-deficient mice and NFIL3-deficient mice both lack mature ILCs and ILC progenitors.[11]
^Wilkinson B, Chen JY, Han P, Rufner KM, Goularte OD, Kaye J (March 2002). "TOX: an HMG box protein implicated in the regulation of thymocyte selection".Nature Immunology.3 (3):272–280.doi:10.1038/ni767.PMID11850626.S2CID19716719.
^Stacey SN, Manolescu A, Sulem P, Rafnar T, Gudmundsson J, Gudjonsson SA, et al. (July 2007). "Common variants on chromosomes 2q35 and 16q12 confer susceptibility to estrogen receptor-positive breast cancer".Nature Genetics.39 (7):865–869.doi:10.1038/ng2064.PMID17529974.S2CID7346190.
^abCheng Y, Shao Z, Chen L, Zheng Q, Zhang Q, Ding W, et al. (January 2021). "Role, function and regulation of the thymocyte selection-associated high mobility group box protein in CD8+ T cell exhaustion".Immunology Letters.229:1–7.doi:10.1016/j.imlet.2020.11.004.PMID33186634.S2CID226948315.
