HAVCR2
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Hepatitis A virus cellular receptor 2 (HAVCR2), also known as T-cell immunoglobulin and mucin-domain containing-3 (TIM-3), is a
protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residue (biochemistry), residues. Proteins perform a vast array of functions within organisms, including Enzyme catalysis, catalysing metab ...
that in humans is encoded by the ''HAVCR2'' (''TIM-3)''
gene In biology, the word gene has two meanings. The Mendelian gene is a basic unit of heredity. The molecular gene is a sequence of nucleotides in DNA that is transcribed to produce a functional RNA. There are two types of molecular genes: protei ...
. HAVCR2 was first described in 2002 as a cell surface molecule expressed on
IFNγ Interferon gamma (IFNG or IFN-γ) is a dimerized soluble cytokine that is the only member of the type II class of interferons. The existence of this interferon, which early in its history was known as immune interferon, was described by E. F. ...
producing CD4+ Th1 and CD8+ Tc1 cells. Later, the expression was detected in Th17 cells, regulatory T-cells, and innate immune cells (
dendritic cell A dendritic cell (DC) is an antigen-presenting cell (also known as an ''accessory cell'') of the mammalian immune system. A DC's main function is to process antigen material and present it on the cell surface to the T cells of the immune system ...
s,
NK cells Natural killer cells, also known as NK cells, are a type of cytotoxic lymphocyte critical to the innate immune system. They are a kind of large granular lymphocytes (LGL), and belong to the rapidly expanding family of known innate lymphoid cell ...
,
monocyte Monocytes are a type of leukocyte or white blood cell. They are the largest type of leukocyte in blood and can differentiate into macrophages and monocyte-derived dendritic cells. As a part of the vertebrate innate immune system monocytes also ...
s, macrophages). HAVCR2 receptor is a regulator of the immune response.


Discovery

In a screen to identify differentially expressed molecules between Th1 and Th2 cells, Vijay Kuchroo and colleagues first described HAVCR2/TIM-3 in 2002. Kuchroo was the first to characterize the inhibitory function of TIM-3 and its role in inhibiting T cell responses in both autoimmunity and cancer. Similar to other checkpoint inhibitors such as PD-1 and CTLA-4, TIM-3 has been successfully targeted to treat several solid and hematogenous malignancies, including melanoma, AML, and MDS.


Classification

HAVCR2 /TIM-3 is member of TIM immunoregulatory proteins family which is encoded by gene on mouse chromosome 11B1.1 and on human chromosome 5q33.2. This chromosomal region has been repeatedly linked with
asthma Asthma is a common long-term inflammatory disease of the airways of the lungs. It is characterized by variable and recurring symptoms, reversible airflow obstruction, and easily triggered bronchospasms. Symptoms include episodes of wh ...
,
allergy Allergies, also known as allergic diseases, are various conditions caused by hypersensitivity of the immune system to typically harmless substances in the environment. These diseases include Allergic rhinitis, hay fever, Food allergy, food al ...
and
autoimmunity In immunology, autoimmunity is the system of immune responses of an organism against its own healthy cells, tissues and other normal body constituents. Any disease resulting from this type of immune response is termed an " autoimmune disease ...
. The ''TIM'' gene family include another eight members (''TIM-1–8'') on mouse chromosome and three members (''TIM-1'', ''TIM-3'' and '' TIM-4'') on human chromosome.


Structure

HAVCR2 belongs to TIM family cell surface receptor proteins. These proteins share a similar structure, in which the extracellular region consists of membrane distal single variable immunoglobulin domain (IgV), a
glycosylated Glycosylation is the reaction in which a carbohydrate (or ' glycan'), i.e. a glycosyl donor, is attached to a hydroxyl or other functional group of another molecule (a glycosyl acceptor) in order to form a glycoconjugate. In biology (but not ...
mucin domain of variable length located closer to the membrane
transmembrane A transmembrane protein is a type of integral membrane protein that spans the entirety of the cell membrane. Many transmembrane proteins function as gateways to permit the transport of specific substances across the membrane. They frequently u ...
region, and intracellular stem. The IGV domain is form by two antiparallel beta sheets that are linked by
disulfide bridges In chemistry, a disulfide (or disulphide in British English) is a compound containing a functional group or the anion. The linkage is also called an SS-bond or sometimes a disulfide bridge and usually derived from two thiol groups. In inorg ...
between four conserved cysteines. Cysteine bridges create a CC´ loop and an FG loop in the domain which make unique cleft characteristics for TIM-3 proteins. The cleft is stabilized by
disulfide In chemistry, a disulfide (or disulphide in British English) is a compound containing a functional group or the anion. The linkage is also called an SS-bond or sometimes a disulfide bridge and usually derived from two thiol groups. In inorg ...
and
hydrogen bond In chemistry, a hydrogen bond (H-bond) is a specific type of molecular interaction that exhibits partial covalent character and cannot be described as a purely electrostatic force. It occurs when a hydrogen (H) atom, Covalent bond, covalently b ...
s and is a binding site for ligands such as CEACAM-1 and
phosphatidylserine Phosphatidylserine (abbreviated Ptd-L-Ser or PS) is a phospholipid and is a component of the cell membrane. It plays a key role in cell cycle signaling, specifically in relation to apoptosis. It is a key pathway for viruses to enter cells via a ...
. The extracellular portion of the IgV domain may also be glycosylated and this glycan-binding sites is recognizes by
carbohydrate A carbohydrate () is a biomolecule composed of carbon (C), hydrogen (H), and oxygen (O) atoms. The typical hydrogen-to-oxygen atomic ratio is 2:1, analogous to that of water, and is represented by the empirical formula (where ''m'' and ''n'' ...
domain of another ligands galectin-9 (Gal-9). The mucin domain is variable in a member of the TIM family, in TIM3 it is the smallest domain and has regions rich in
serine Serine (symbol Ser or S) is an α-amino acid that is used in the biosynthesis of proteins. It contains an α- amino group (which is in the protonated − form under biological conditions), a carboxyl group (which is in the deprotonated − ...
,
proline Proline (symbol Pro or P) is an organic acid classed as a proteinogenic amino acid (used in the biosynthesis of proteins), although it does not contain the amino group but is rather a secondary amine. The secondary amine nitrogen is in the p ...
and
threonine Threonine (symbol Thr or T) is an amino acid that is used in the biosynthesis of proteins. It contains an α-amino group (which is in the protonated −NH form when dissolved in water), a carboxyl group (which is in the deprotonated −COO− ...
. This region also contains target sites for O- and N-linked glycosylation. The transmembrane domain anchors the HAVCR2 protein in the cytoplasmic membrane of the cell. The intracellular domain of HAVCR2 is called C-terminal cytoplasmic tail. It contains five conserved tyrosine residues that interact with multiple components of
T-cell receptor The T-cell receptor (TCR) is a protein complex, located on the surface of T cells (also called T lymphocytes). They are responsible for recognizing fragments of antigen as peptides bound to major histocompatibility complex (MHC) molecules. ...
(TCR) complex, mediates intercellular
signaling pathways Signal transduction is the process by which a chemical or physical signal is transmitted through a cell as a series of molecular events. Proteins responsible for detecting stimuli are generally termed receptors, although in some cases the term ...
and negatively regulates its function.


Function

HAVCR2/TIM-3 is a transmembrane protein of T lymphocytes (CD4+ and CD8+ T cells), other lymphocytes (like NK cells), myeloid cells (monocytes, macrophages, DC, mast cells), or various cells in different tumor types. The receptor is an
immune checkpoint Immune checkpoints are regulators of the immune system. These pathways are crucial for self-tolerance, which prevents the immune system from attacking cells indiscriminately. However, some cancers can protect themselves from attack by stimulat ...
and together with other inhibitory receptors including
programmed cell death protein 1 Programmed cell death protein 1 (PD-1), (CD279 cluster of differentiation 279). PD-1 is a protein encoded in humans by the ''PDCD1'' gene. PD-1 is a cell surface receptor on T cells and B cells that has a role in regulating the immune system's re ...
(PD-1) and lymphocyte activation gene 3 protein (LAG3) mediate the CD8+
T-cell exhaustion T cells (also known as T lymphocytes) are an important part of the immune system and play a central role in the adaptive immune response. T cells can be distinguished from other lymphocytes by the presence of a T-cell receptor (TCR) on their cell ...
in terms of proliferation and secretion of cytokines such as
TNF-alpha Tumor necrosis factor (TNF), formerly known as TNF-α, is a chemical messenger produced by the immune system that induces inflammation. TNF is produced primarily by activated macrophages, and induces inflammation by binding to its receptors o ...
,
IFN-gamma Interferon gamma (IFNG or IFN-γ) is a dimerized soluble cytokine that is the only member of the type II class of interferons. The existence of this interferon, which early in its history was known as immune interferon, was described by E. F. ...
and
IL-2 The Ilyushin Il-2 (Russian language, Russian: Илью́шин Ил-2) is a Ground attack aircraft, ground-attack plane that was produced by the Soviet Union in large numbers during the World War II, Second World War. The word ''shturmovík'' (C ...
. Combined blockade of HAVCR2 and PD-1 led to improved CD8+ T-cell response during the lymphocytic choriomeningitis virus infection. HAVCR2 and PD-1 may be responsible for NK cell exhaustion as well. Similarly, HAVCR2/TIM-3 and VSIR/VISTA may co-exist on macrophages infiltrating different human and mouse tumours where they can co-regulate immunotherapy resistance. HAVCR2 has also been shown as a CD4+ Th1-specific cell surface protein that regulates macrophage activation, regulates the production of cytokines and enhances the severity of experimental autoimmune encephalomyelitis in mice. Is also known the free form of HAVCR2 outside the cell membrane (soluble form), lacking mucin and the transmembrane domain. However, the function of the soluble protein is unknown.


Ligands


Gal-9

HAVCR2 is primarily activated by soluble galectin-9. The engagement leads to stimulation of an influx of calcium to intracellular space and induction of programmed cell death,
apoptosis Apoptosis (from ) is a form of programmed cell death that occurs in multicellular organisms and in some eukaryotic, single-celled microorganisms such as yeast. Biochemistry, Biochemical events lead to characteristic cell changes (Morphology (biol ...
, cell
necrosis Necrosis () is a form of cell injury which results in the premature death of cells in living tissue by autolysis. The term "necrosis" came about in the mid-19th century and is commonly attributed to German pathologist Rudolf Virchow, who i ...
or T cell
anergy In immunology, anergy characterizes the absence of a response from the body's defense mechanisms when confronted with foreign substances. This phenomenon involves the direct induction of peripheral lymphocyte tolerance. When an individual is i ...
. As a consequence, a suppression of Th1 and Th17 responses and induction of immune tolerance occurs, gal-9/HAVCR2 increases the immunosuppressive activity of Treg cells. In addition to galectin-9, several ligands have been identified, such as
phosphatidylserine Phosphatidylserine (abbreviated Ptd-L-Ser or PS) is a phospholipid and is a component of the cell membrane. It plays a key role in cell cycle signaling, specifically in relation to apoptosis. It is a key pathway for viruses to enter cells via a ...
(PtdSer), High Mobility Group Protein 1 (
HMGB1 High mobility group box 1 protein, also known as high-mobility group protein 1 (HMG-1) and amphoterin, is a protein that in humans is encoded by the ''HMGB1'' gene. HMG-1 belongs to the high mobility group and contains a HMG-box domain. Funct ...
) and Carcinoembryonic Antigen Related Cell Adhesion Molecule 1 (
CEACAM1 Carcinoembryonic antigen-related cell adhesion molecule 1 (biliary glycoprotein) (CEACAM1) also known as CD66a (Cluster of Differentiation 66a), is a human glycoprotein, and a member of the carcinoembryonic antigen (CEA) gene family. Function ...
).


PtdSer

PtdSer is exposed on the surface of apoptotic cells and binds through the FG loop in the IgV domain. The binding of PtdSer with TIM-3 receptor has been shown to cause an uptake of apoptotic cells and is responsible for the
cross-presentation Cross-presentation is the ability of certain professional antigen-presenting cells (mostly dendritic cells) to take up, process and present ''extracellular'' antigens with MHC class I molecules to CD8 T cells (cytotoxic T cells). Cross-priming, th ...
of dying cell-associated antigens by
dendritic cell A dendritic cell (DC) is an antigen-presenting cell (also known as an ''accessory cell'') of the mammalian immune system. A DC's main function is to process antigen material and present it on the cell surface to the T cells of the immune system ...
s. PtdSer binds to the opposite side of the IgV domain of TIM-3 than Gal-9, and although this interaction of PtdSer to TIM-3 has five times less
affinity Affinity may refer to: Commerce, finance and law * Affinity (law), kinship by marriage * Affinity analysis, a market research and business management technique * Affinity Credit Union, a Saskatchewan-based credit union * Affinity Equity Pa ...
than other members of the TIM family, Tim-3 can also bind some other ligand to phagocytose apoptotic cells.


HMGB1

HMGB1 is alarmin and interacts with DNA released from dying cells or pathogen nucleid acid, facilitating absorption by cell and increasing nucleic acid sensing by endosomal Toll-like receptors (TLRs).  HMGB1 binds to HAVCRS2/TIM3 on dendritic cells but its binding site has not been determined. TIM-3 receptor prevents the entry of the
nucleic acid Nucleic acids are large biomolecules that are crucial in all cells and viruses. They are composed of nucleotides, which are the monomer components: a pentose, 5-carbon sugar, a phosphate group and a nitrogenous base. The two main classes of nuclei ...
s into the cell and suppresses activation of TLR signaling in dendritic cells. So the binding of HMGB1suppresses activation of innate immune response.


CEACAM1

The last known TIM3 receptor ligand is
CEACAM1 Carcinoembryonic antigen-related cell adhesion molecule 1 (biliary glycoprotein) (CEACAM1) also known as CD66a (Cluster of Differentiation 66a), is a human glycoprotein, and a member of the carcinoembryonic antigen (CEA) gene family. Function ...
glycoprotein Glycoproteins are proteins which contain oligosaccharide (sugar) chains covalently attached to amino acid side-chains. The carbohydrate is attached to the protein in a cotranslational or posttranslational modification. This process is known a ...
. It is co-expressed with TIM3 T cells but also
monocyte Monocytes are a type of leukocyte or white blood cell. They are the largest type of leukocyte in blood and can differentiate into macrophages and monocyte-derived dendritic cells. As a part of the vertebrate innate immune system monocytes also ...
s,
macrophage Macrophages (; abbreviated MPhi, φ, MΦ or MP) are a type of white blood cell of the innate immune system that engulf and digest pathogens, such as cancer cells, microbes, cellular debris and foreign substances, which do not have proteins that ...
s,
dendritic cell A dendritic cell (DC) is an antigen-presenting cell (also known as an ''accessory cell'') of the mammalian immune system. A DC's main function is to process antigen material and present it on the cell surface to the T cells of the immune system ...
s. It binds to the CC´ and FG loops of the TIM3 protein. CEACAM1 can also bind to TIM3 intracellularly ('' cis'' presentation) and is likely to be important for TIM-3 maturation on cell surface. The CEACAM1 binding contributes to the development of T cell tolerance, triggers the release of BAT3 from TIM-3 leading to inhibition of TCR signaling, and also inhibits the immune response of
myeloid cells A myelocyte is a young cell of the granulocytic series, occurring normally in bone marrow (can be found in circulating blood when caused by certain diseases). Structure When stained with the usual dyes, the cytoplasm is distinctly basophilic ...
.


Clinical significance

HAVCR2 expression is up regulated in
tumor-infiltrating lymphocytes Tumor-infiltrating lymphocytes (TIL) are white blood cells that have left the bloodstream and cell migration, migrated towards a tumor. They include T cells and B cells and are part of the larger category of ‘tumor-infiltrating immune cells’ wh ...
in
lung The lungs are the primary Organ (biology), organs of the respiratory system in many animals, including humans. In mammals and most other tetrapods, two lungs are located near the Vertebral column, backbone on either side of the heart. Their ...
,
gastric The stomach is a muscular, hollow organ in the upper gastrointestinal tract of humans and many other animals, including several invertebrates. The Ancient Greek name for the stomach is ''gaster'' which is used as ''gastric'' in medical terms re ...
,
head and neck cancer Head and neck cancer is a general term encompassing multiple cancers that can develop in the head and neck region. These include cancers of the mouth, tongue, gums and lips (oral cancer), voice box ( laryngeal), throat ( nasopharyngeal, orophary ...
,
schwannoma A schwannoma (or neurilemmoma) is a usually benign nerve sheath tumor composed of Schwann cells, which normally produce the insulating myelin sheath covering peripheral nerves. Schwannomas are homogeneous tumors, consisting only of Schwann cells ...
,
melanoma Melanoma is the most dangerous type of skin cancer; it develops from the melanin-producing cells known as melanocytes. It typically occurs in the skin, but may rarely occur in the mouth, intestines, or eye (uveal melanoma). In very rare case ...
and follicular B-cell non-Hodgkin lymphoma. It is also up-regulated in tumour-associated macrophages in various malignancies, including melanoma, especially in immunotherapy-resistant context. The HAVCR2 pathway may interact with the
PD-1 Programmed cell death protein 1 (PD-1), (CD279 cluster of differentiation 279). PD-1 is a protein encoded in humans by the ''PDCD1'' gene. PD-1 is a cell surface receptor on T cells and B cells that has a role in regulating the immune system's re ...
pathway in the dysfunctional CD8+ T cells and
Treg The regulatory T cells (Tregs or Treg cells), formerly known as suppressor T cells, are a subpopulation of T cells that modulate the immune system, maintain tolerance to self-antigens, and prevent autoimmune disease. Treg cells are immun ...
s in cancer. HAVCR2 is mainly expressed on activated CD8+ T cells and suppresses macrophage activation following PD-1 inhibition. Upregulation was observed in tumors progressing after anti-PD-1 therapy. This seems to be a form of adaptive resistance to immunotherapy. Multiple phase 1/2 clinical trials with anti-HAVCR2 monoclonal antibodies (LY3321367, Eli Lilly and Company; MBG453, Novartis Pharmaceuticals; TSR-022, Tesaro, Inc.) in combination with anti-PD-1 or anti-PD-L1 therapies are ongoing. HAVCR2 is also an exhaustion marker for NK cells. Blockade of this receptor can improve the NK cells antitumor activity in
esophageal cancer Esophageal cancer (American English) or oesophageal cancer (British English) is cancer arising from the esophagus—the food pipe that runs between the throat and the stomach. Symptoms often include dysphagia, difficulty in swallowing and weigh ...
,
melanoma Melanoma is the most dangerous type of skin cancer; it develops from the melanin-producing cells known as melanocytes. It typically occurs in the skin, but may rarely occur in the mouth, intestines, or eye (uveal melanoma). In very rare case ...
and
lung adenocarcinoma Adenocarcinoma of the lung is the most common type of lung cancer, and like other forms of lung cancer, it is characterized by distinct cellular and molecular features. It is classified as one of several non-small cell lung cancers (NSCLC), to d ...
. The role of HAVCR2 in the T-cell dysfunction has been investigated in chronic viral infections. Together with PD-1, HAVCR2 negatively regulate CD8+ T-cells and thus, in vivo blockade of HAVCR2 and PD-1 led to the restoring of antiviral immunity. A recent genome-wide association study (GWAS) has found that genetic variations in ''HAVCR2'' are associated with late-onset sporadic Alzheimer's disease (LOAD). HARVC2 is capable of interacting with amyloid-β precursor protein.


References

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