Killer activation receptor

Last updated September 18, 2024

Killer Activation Receptors (KARs) are receptors expressed on the plasma membrane (cell membrane) of Natural Killer cells (NK cells). KARs work together with Killer Inhibitory Receptors (abbreviated as KIRs in the text), which inactivate KARs in order to regulate the NK cells functions on hosted or transformed cells ^[1].These receptors have a broad binding specificity and are able to broadcast opposite signals. It is the balance between these competing signals that determines if the cytotoxic activity of the NK cell and apoptosis of distressed cell occurs.^[2]

Killer Inhibitory Receptor vs. Killer-cell Immunglobulin-like Receptors

There is sometimes confusion regarding the KIR acronym. The KIR term has been started to be being used parallelly both for the Killer-cell immunoglobulin-like receptors (KIRs) and for the Killer Inhibitory Receptors. The Killer-cell immunoglobulin-like receptors involve both activation and inhibitory receptors.^[3] Killer-cell inhibitory receptors involve both immunoglobulin-like receptors and C-type lectin-like receptors.^[4]

Killer Activation Receptors vs. Killer Inhibitory Receptors

KARs and KIRs have some morphological features in common, such as being transmembrane proteins. The similarities are specially found in the extracellular domains.^[1]

The differences between KARs and KIRs tend to be in the intracellular domains. They can have a tyrosine containing activation or inhibitory motifs in the intracellular part of the receptor molecule (they are called ITAMs and ITIMs).

At first, it was thought that there was only one KAR and one KIR receptor present on the NK cell, known as the two-receptor model.^[2] In the last decade, many different KARs and KIRs, such as NKp46 or NKG2D, have been discovered creating the opposing-signals model.^[1]NKG2D is activated by the cell-surface ligands MICA and ULBP2.^[5]

Even though KARs and KIRs are receptors with antagonistic effects on NK cells, they have some structural characteristics in common. Both receptors are usually transmembrane proteins. Also, the extracellular domains of these proteins tend to have similar molecular features and are responsible for ligand recognition.

The opposing functions of these receptors are due to differences in their intracellular domains. KARs proteins possess positively charged transmembrane residues and short cytoplasmic tails that contain few intracellular signaling domains. In contrast, KIRs proteins usually have long cytoplasmic tails.

As the chains from KARs are not able to mediate any signal transduction in isolation, a common feature of such receptors is the presence of noncovalently linked subunits that contain immunoreceptor tyrosine-based activation motifs (ITAMs) in their cytoplasmic tails. ITAMs are composed of a conserved sequence of amino acids, including two Tyr-x-x-Leu/Ile elements (where x is any amino acid) separated by six to eight amino acid residues. When the binding of an activation ligand to an activation receptor complex occurs, the tyrosine residues in the ITAMs in the associated chain are phosphorylated by kinases, and a signal that promotes natural cytotoxicity is conveyed to the interior of the NK cell. Therefore, ITAMs are involved in the facilitation of signal transduction. These subunits are moreover composed of an accessory signaling molecule such as CD3ζ, the γc chain, or one of two adaptor proteins called DAP10 and DAP12. All of these molecules possess negatively charged transmembrane domains.^[6]

A common feature of members of all KIR is the presence of immunoreceptor tyrosine-based inhibition motifs (ITIMs) in their cytoplasmic tails. ITIMs are composed of the sequence Ile/Val/Leu/Ser-x-Tyr-x-x-Leu/Val, where x denotes any amino acid. The latter are essential to the signaling functions of these molecules. When an inhibitory receptor is stimulated by the binding of MHC class I, kinases and phosphatases are recruited to the receptor complex. This is how ITIMs counteract the effect of kinases initiated by activating receptors and manage to inhibit the signal transduction within the NK cell.

Types of Killer Activation Receptors

Based on their structure there are three different groups of KARS. The first group of receptors is called Natural Cytotoxicity Receptors (NCR), which only includes activation receptors. The two other classes are: Natural Killer Group 2 (NKG2), which includes activation and inhibition receptors, and some KIRs which do not have an inhibitor role.^[7]

The three receptors that are included in the NCR class are NKp46, NKp44 and NKp30. The crystal structure of NKp46, which is representative for all three NCR, has been determined. It has two C2-set immunoglobulin domains, and it’s probable that the binding site for its ligand is near the interdomain hinge.^[8]

There are two NKG2-class receptors which are NKG2D and CD94/NKG2C. NKG2D, which doesn’t bind to CD94, is a homodimeric lectin-like receptor. CD94/NKG2C consists in a complex formed by the CD94 protein, which is a C-type lectin molecule bound to the NKG2C protein. This molecule can bind to five classes of NKG2 (A, B, C, E and H), but the union can trigger an activation or an inhibition response, depending on the NKG2 molecule (CD94/NKG2A, for example, is an inhibitor complex).^[8]

Most KIRs have an inhibitor function, however, a few KIRs that have an activator role also exist. One of these activating KIRs is KIR2DS1, which has an Ig-like structure, like KIRs in general.

Finally, there is CD16, a low affinity Fc receptor (FcγRIII) which contains N-glycosylation sites; therefore, it is a glycoprotein.

Killer Activation Receptors are associated with signaling intracellular chains. In fact, these intracellular domains determine the opposite functions of activation and inhibitory receptors. Activation receptors are associated with an accessory signaling molecule (for instance, CD3ζ) or with an adaptor protein, which can be either DAP10 or DAP12. All of these signaling molecules contain immunoreceptor tyrosine-based activated motifs (ITAMs), which are phosphorylated and consequently facilitate signal transduction.

Each of these receptors has a specific ligand, although some receptors that belong to the same class, such as NCR, recognize similar molecules.

How do they work?

KARs can detect a specific type of molecules: MICA and MICB. These molecules are in MHC class I of human cells and they are related to cellular stress: this is why MICA and MICB appear in infected or transformed cells but they aren't very common in healthy cells. KARs recognize MICA and MICB when they are in a huge proportion and get engaged. This engagement activates the natural killer cell to attack the transformed or infected cells. This action can be done in different ways. NK can kill directly the hosted cell, it can do it by segregating cytokines, IFN-β and IFN-α, or by doing both things.

There are other less common ligands, like carbohydrate domains, which are recognized by a group of receptors: C-type lectins (so named because they have calcium-dependent carbohydrates recognition domains).

In addition to lectins, there are other molecules implicated in the activation of NK. These additional proteins are: CD2 and CD16. The CD16 works in antibody-mediated recognition.

Finally, there is a group of proteins which are related to the activation in an unknown way. These are NKp30, Nkp44 and Nkp46.^[8]

These ligands activate the NK cell, however, before the activation, Killer Inhibition Receptors (KIRs) recognize certain molecules in the MHC class I of the hosted cell and get engaged with them. These molecules are typical of healthy cells but some of these molecules are repressed in infected or transformed cells. For this reason when the hosted cell is really infected the proportion of KARs engaged with ligands is bigger than the proportion of KIRs engaged with MHC I molecules. When this happens the NK is activated and the hosted cell is destroyed. On the other hand, if there are more KIRs engaged with MHC class I molecules than KARs engaged with ligands, the NK isn't activated and the suspicious hosted cell remains alive.^[2]

KARs and KIRs: their role in cancer

One way by which NK cells are able to distinguish between normal and infected or transformed cells is by monitoring the amount of MHC class I molecules cells have on their surface. When it come to an infected and a tumor cell, the expression of MHC class I decreases.^[2]

In cancers, a Killer Activation Receptor (KAR), located on the surface of the NK cell, binds to certain molecules which only appear on cells that are undergoing stress situations. In humans, this KAR is called NKG2D and the molecules it recognizes MICA and MICB. This binding provides a signal which induces the NK cell to kill the target cell.^[9]

Then, Killer Inhibitory Receptors (KIRs) examine the surface of the tumor cell in order to determine the levels of MHC class I molecules it has. If KIRs bind sufficiently to MHC class I molecules, the “killing signal” is overridden to prevent the killing of the cell. However, if KIRs are not sufficiently engaged to MHC class I molecules, killing of the target cell proceeds.^[2]

Related Research Articles

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 cells (ILC) and represent 5–20% of all circulating lymphocytes in humans. The role of NK cells is analogous to that of cytotoxic T cells in the vertebrate adaptive immune response. NK cells provide rapid responses to virus-infected cells, stressed cells, tumor cells, and other intracellular pathogens based on signals from several activating and inhibitory receptors. Most immune cells detect the antigen presented on major histocompatibility complex I (MHC-I) on infected cell surfaces, but NK cells can recognize and kill stressed cells in the absence of antibodies and MHC, allowing for a much faster immune reaction. They were named "natural killers" because of the notion that they do not require activation to kill cells that are missing "self" markers of MHC class I. This role is especially important because harmful cells that are missing MHC I markers cannot be detected and destroyed by other immune cells, such as T lymphocyte cells.

<span class="mw-page-title-main">Immunological synapse</span> Interface between lymphocyte and target cell

In immunology, an immunological synapse is the interface between an antigen-presenting cell or target cell and a lymphocyte such as a T cell, B cell, or natural killer cell. The interface was originally named after the neuronal synapse, with which it shares the main structural pattern. An immunological synapse consists of molecules involved in T cell activation, which compose typical patterns—activation clusters. Immunological synapses are the subject of much ongoing research.

Killer-cell immunoglobulin-like receptors (KIRs), are a family of type I transmembrane glycoproteins expressed on the plasma membrane of natural killer (NK) cells and a minority of T cells. In humans, they are encoded in the leukocyte receptor complex (LRC) on chromosome 19q13.4; the KIR region is approximately 150 kilobases and contains 14 loci, including 7 protein-coding genes and 2 pseudogenes.

CD94, also known as killer cell lectin-like receptor subfamily D, member 1 (KLRD1) is a human gene.

An immunoreceptor tyrosine-based inhibitory motif (ITIM), is a conserved sequence of amino acids that is found intracellularly in the cytoplasmic domains of many inhibitory receptors of the non-catalytic tyrosine-phosphorylated receptor family found on immune cells. These immune cells include T cells, B cells, NK cells, dendritic cells, macrophages and mast cells. ITIMs have similar structures of S/I/V/LxYxxI/V/L, where x is any amino acid, Y is a tyrosine residue that can be phosphorylated, S is the amino acid serine, I is the amino acid isoleucine, and V is the amino acid valine. ITIMs recruit SH2 domain-containing phosphatases, which inhibit cellular activation. ITIM-containing receptors often serve to target immunoreceptor tyrosine-based activation motif (ITAM)-containing receptors, resulting in an innate inhibition mechanism within cells. ITIM bearing receptors have important role in regulation of immune system allowing negative regulation at different levels of the immune response.

Ly49 is a family of membrane C-type lectin-like receptors expressed mainly on NK cells but also on other immune cells. Their primary role is to bind MHC-I molecules to distinguish between self healthy cells and infected or altered cells. Ly49 family is coded by Klra gene cluster and include genes for both inhibitory and activating paired receptors, but most of them are inhibitory. Inhibitory Ly49 receptors play a role in the recognition of self cells and thus maintain self-tolerance and prevent autoimmunity by suppressing NK cell activation. On the other hand, activating receptors recognise ligands from cancer or viral infected cells and are used when cells lack or have abnormal expression of MHC-I molecules, which activate cytokine production and cytotoxic activity of NK and immune cells.

NKG2 also known as CD159 is a receptor for natural killer cells. There are 7 NKG2 types: A, B, C, D, E, F and H. NKG2D is an activating receptor on the NK cell surface. NKG2A dimerizes with CD94 to make an inhibitory receptor (CD94/NKG2).

MHC class I polypeptide–related sequence A (MICA) is a highly polymorphic cell surface glycoprotein encoded by the MICA gene located within MHC locus. MICA is related to MHC class I and it has similar domain structure, however, it is not associated with β2-microglobulin nor binds peptides as conventional MHC class I molecules do. MICA rather functions as a stress-induced ligand (as a danger signal) for integral membrane protein receptor NKG2D ("natural-killer group 2, member D"). MICA is broadly recognized by NK cells, γδ T cells, and CD8⁺ αβ T cells which carry NKG2D receptor on their cell surface and which are activated via this interaction.

Killer cell immunoglobulin-like receptor 2DL4 is a protein that in humans is encoded by the KIR2DL4 gene.

NKG2-F type II integral membrane protein is a protein that in humans is encoded by the KLRC4 gene.

NKG2-C type II integral membrane protein or NKG2C is a protein that in humans is encoded by the KLRC2 gene. It is also known as or cluster of differentiation 159c (CD159c).

UL16 binding protein 2 (ULBP2) is a cell surface glycoprotein encoded by ULBP2 gene located on the chromosome 6. ULBP2 is related to MHC class I molecules, but its gene maps outside the MHC locus. The domain structure of ULBP2 differs significantly from those of conventional MHC class I molecules. It does not contain the α3 domain and the transmembrane segment. ULBP2 is thus composed of only the α1α2 domain which is linked to the cell membrane by the GPI anchor.

UL16 binding protein 1 (ULBP1) is a cell surface glycoprotein encoded by ULBP1 gene located on the chromosome 6. ULBP1 is related to MHC class I molecules, but its gene maps outside the MHC locus. The domain structure of ULBP1 differs significantly from those of conventional MHC class I molecules. It does not contain the α3 domain and the transmembrane segment. ULBP1 is thus composed of only the α1α2 domain which is linked to the cell membrane by the GPI anchor. It functions as a stress-induced ligand for NKG2D receptor. ULBP1 is, for example, upregulated during HCMV infection. Binding of HCMV-encoded UL16 glycoprotein to ULBP1 interferes with cell surface localization of ULBP1; this represents another mechanism by which HCMV-infected cells might escape the immune system.

UL16 binding protein 3 (ULBP3) is a cell surface glycoprotein encoded by ULBP3 gene located on the chromosome 6. ULBP3 is related to MHC class I molecules, but its gene maps outside the MHC locus. The domain structure of ULBP3 differs significantly from those of conventional MHC class I molecules. It does not contain the α3 domain and the transmembrane segment. ULBP3 is thus composed of only the α1α2 domain which is linked to the cell membrane by the GPI anchor. It functions as a stress-induced ligand for NKG2D receptor.

The following outline is provided as an overview of and topical guide to immunology:

NKG2D is an activating receptor (transmembrane protein) belonging to the NKG2 family of C-type lectin-like receptors. NKG2D is encoded by KLRK1 (killer cell lectin like receptor K1) gene which is located in the NK-gene complex (NKC) situated on chromosome 6 in mice and chromosome 12 in humans. In mice, it is expressed by NK cells, NK1.1⁺ T cells, γδ T cells, activated CD8⁺ αβ T cells and activated macrophages. In humans, it is expressed by NK cells, γδ T cells and CD8⁺ αβ T cells. NKG2D recognizes induced-self proteins from MIC and RAET1/ULBP families which appear on the surface of stressed, malignant transformed, and infected cells.

Induced-self antigen is a marker of abnormal self, which can be recognized upon infected and transformed cells. Therefore, the recognition of "induced self" is an important strategy for surveillance of infection or tumor transformation - it results in elimination of the affected cells by activated NK cells or other immunological mechanisms. Similarly γδ T cells can recognize induced-self antigens expressed on cells under stress conditions.

CD94/NKG2 is a family of C-type lectin receptors which are expressed predominantly on the surface of NK cells and a subset of CD8⁺ T-lymphocyte. These receptors stimulate or inhibit cytotoxic activity of NK cells, therefore they are divided into activating and inhibitory receptors according to their function. CD94/NKG2 recognize nonclassical MHC glycoproteins class I (HLA-E in human and Qa-1 molecules in the mouse).

KIR2DL3, Killer cell immunoglobulin-like receptor 2DL3 is a transmembrane glycoprotein expressed by the natural killer cells and the subsets of the T cells. The KIR genes are polymorphic, which means that they have many different alleles. The KIR genes are also extremely homologous, which means that they are similar in position, structure and evolutionary origin, but not necessarily in function.

Paired receptors are pairs or clusters of receptor proteins that bind to extracellular ligands but have opposing activating and inhibitory signaling effects. Traditionally, paired receptors are defined as homologous pairs with similar extracellular domains and different cytoplasmic regions, whose genes are located together in the genome as part of the same gene cluster and which evolved through gene duplication. Homologous paired receptors often, but not always, have a shared ligand in common. More broadly, pairs of receptors have been identified that exhibit paired functional behavior - responding to a shared ligand with opposing intracellular signals - but are not closely homologous or co-located in the genome. Paired receptors are highly expressed in the cells of the immune system, especially natural killer (NK) and myeloid cells, and are involved in immune regulation.

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