Heavy-chain antibody

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A heavy-chain antibody is an antibody which consists only of two heavy chains and lacks the two light chains usually found in antibodies.

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In common antibodies, the antigen binding region consists of the variable domains of the heavy and light chains (VH and VL). Heavy-chain antibodies can bind antigens despite having only VH domains. This observation has led to the development of a new type of antibody fragments with potential use as drugs, so-called single-domain antibodies. [1]

Discovery

In 1989 a group of biologists led by Raymond Hamers at the Free University Brussels investigated the immune system of dromedaries. In addition to the expected four-chain antibodies, they identified simpler antibodies consisting only of two heavy chains. This discovery was published in Nature in 1993. [2] In 1995 a research team at the University of Miami found a different type of heavy-chain antibodies in sharks. [3]

In cartilaginous fishes

A heavy-chain shark antibody (left) and a heavy-chain camelid antibody (middle) in comparison to a common antibody (right). Heavy chains are shown in a darker shade, light chains in a lighter shade. Heavy chain and common antibody.svg
A heavy-chain shark antibody (left) and a heavy-chain camelid antibody (middle) in comparison to a common antibody (right). Heavy chains are shown in a darker shade, light chains in a lighter shade.

The immunoglobulin new antigen receptor (IgNAR) of cartilaginous fishes (for example sharks) is a heavy-chain antibody. IgNAR shows significant structural differences to other antibodies. It has five constant domains (CH) per chain instead of the usual three, several disulfide bonds in unusual positions, and the complementarity-determining region 3 (CDR3) forms an extended loop covering the site which binds to a light chain in other antibodies. These differences, in combination with the phylogenetic age of the cartilaginous fishes, have led to the hypothesis that IgNAR could be more closely related to a primordial antigen-binding protein than the mammalian immunoglobulins. To test this hypothesis, it would be necessary to discover IgNAR or similar antibodies in vertebrates that are phylogenetically still older, like the jawless fish lamprey and hagfish. [4] Non-vertebrates do not have antibodies at all.

Sharks, and possibly other cartilaginous fishes, have immunoglobulin M (IgM) and immunoglobulin W (IgW) as well, both types with two heavy and two light chains. [5]

In camelids

The only mammals with heavy-chain (IgG-like) antibodies are camelids such as dromedaries, camels, llamas and alpacas. [6] This is a secondary development: The heavy chains of these antibodies have lost one of their constant domains (CH1) and undergone modifications in the variable domain (VH), both structural elements necessary for the binding of light chains. In one subgroup, the missing CH1 seems to be replaced by an extended hinge region, as shown in the image. [1] [2] Despite their different overall structure, camelid heavy-chain antibodies share several properties with IgNAR, for example the extended CDR3 loop and the conformation of the CDR1. It has been reasoned that these similarities are caused by functional requirements, or convergent evolution, rather than a genuine relationship. [4]

About 50% of the antibodies in camelids are of the ordinary mammalian heavy/light-chain type. [7] It is not known whether any type of animal has only heavy-chain antibodies and completely lacks the common type with two heavy and two light chains.

Heavy-chain camelid antibodies have been found to be just as specific as a regular antibody and in some cases they are more robust. As well, they are easily isolated using the same phage panning procedure used for traditional antibodies, allowing them to be cultured ex vivo in large concentrations.[ citation needed ] Phage-displayed dromedary camel VHH libraries have been made for isolating single-domain antibodies against SARS-CoV-2 and other virus infections.[ citation needed ] The smaller size and single domain make these antibodies easier to transform into bacterial cells for bulk production, making them ideal for research purposes. [8]

Related Research Articles

<span class="mw-page-title-main">Antibody</span> Protein(s) forming a major part of an organisms immune system

An antibody (Ab) is the secreted form of a B cell receptor; the term immunoglobulin (Ig) can refer to either the membrane-bound form or the secreted form of the B cell receptor, but they are, broadly speaking, the same protein, and so the terms are often treated as synonymous. Antibodies are large, Y-shaped proteins belonging to the immunoglobulin superfamily which are used by the immune system to identify and neutralize antigens such as bacteria and viruses, including those that cause disease. Antibodies can recognize virtually any size antigen with diverse chemical compositions from molecules. Each antibody recognizes one or more specific antigens. Antigen literally means "antibody generator", as it is the presence of an antigen that drives the formation of an antigen-specific antibody. Each tip of the "Y" of an antibody contains a paratope that specifically binds to one particular epitope on an antigen, allowing the two molecules to bind together with precision. Using this mechanism, antibodies can effectively "tag" a microbe or an infected cell for attack by other parts of the immune system, or can neutralize it directly.

<span class="mw-page-title-main">Immunoglobulin D</span> Antibody isotype

Immunoglobulin D (IgD) is an antibody isotype that makes up about 1% of proteins in the plasma membranes of immature B-lymphocytes where it is usually co-expressed with another cell surface antibody called IgM. IgD is also produced in a secreted form that is found in very small amounts in blood serum, representing 0.25% of immunoglobulins in serum. The relative molecular mass and half-life of secreted IgD is 185 kDa and 2.8 days, respectively. Secreted IgD is produced as a monomeric antibody with two heavy chains of the delta (δ) class, and two Ig light chains.

<span class="mw-page-title-main">Immunoglobulin M</span> One of several isotypes of antibody

Immunoglobulin M (IgM) is the largest of several isotypes of antibodies that are produced by vertebrates. IgM is the first antibody to appear in the response to initial exposure to an antigen; causing it to also be called an acute phase antibody. In humans and other mammals that have been studied, plasmablasts in the spleen are the main source of specific IgM production.

<span class="mw-page-title-main">Single-domain antibody</span> Antibody fragment

A single-domain antibody (sdAb), also known as a Nanobody, is an antibody fragment consisting of a single monomeric variable antibody domain. Like a whole antibody, it is able to bind selectively to a specific antigen. With a molecular weight of only 12–15 kDa, single-domain antibodies are much smaller than common antibodies which are composed of two heavy protein chains and two light chains, and even smaller than Fab fragments and single-chain variable fragments.

<span class="mw-page-title-main">Immunoglobulin heavy chain</span> Large polypeptide subunit of an antibody

The immunoglobulin heavy chain (IgH) is the large polypeptide subunit of an antibody (immunoglobulin). In human genome, the IgH gene loci are on chromosome 14.

V(D)J recombination is the mechanism of somatic recombination that occurs only in developing lymphocytes during the early stages of T and B cell maturation. It results in the highly diverse repertoire of antibodies/immunoglobulins and T cell receptors (TCRs) found in B cells and T cells, respectively. The process is a defining feature of the adaptive immune system.

<span class="mw-page-title-main">B-cell receptor</span> Transmembrane protein on the surface of a B cell

The B-cell receptor (BCR) is a transmembrane protein on the surface of a B cell. A B-cell receptor is composed of a membrane-bound immunoglobulin molecule and a signal transduction moiety. The former forms a type 1 transmembrane receptor protein, and is typically located on the outer surface of these lymphocyte cells. Through biochemical signaling and by physically acquiring antigens from the immune synapses, the BCR controls the activation of the B cell. B cells are able to gather and grab antigens by engaging biochemical modules for receptor clustering, cell spreading, generation of pulling forces, and receptor transport, which eventually culminates in endocytosis and antigen presentation. B cells' mechanical activity adheres to a pattern of negative and positive feedbacks that regulate the quantity of removed antigen by manipulating the dynamic of BCR–antigen bonds directly. Particularly, grouping and spreading increase the relation of antigen with BCR, thereby proving sensitivity and amplification. On the other hand, pulling forces delinks the antigen from the BCR, thus testing the quality of antigen binding.

<span class="mw-page-title-main">Complementarity-determining region</span> Part of the variable chains in immunoglobulins and T cell receptors

Complementarity-determining regions (CDRs) are polypeptide segments of the variable chains in immunoglobulins (antibodies) and T cell receptors, generated by B-cells and T-cells respectively. CDRs are where these molecules bind to their specific antigen and their structure/sequence determines the binding activity of the respective antibody. A set of CDRs constitutes a paratope, or the antigen-binding site. As the most variable parts of the molecules, CDRs are crucial to the diversity of antigen specificities generated by lymphocytes.

<span class="mw-page-title-main">Fragment crystallizable region</span> Tail region of an antibody

The fragment crystallizable region is the tail region of an antibody that interacts with cell surface receptors called Fc receptors and some proteins of the complement system. This region allows antibodies to activate the immune system, for example, through binding to Fc receptors. In IgG, IgA and IgD antibody isotypes, the Fc region is composed of two identical protein fragments, derived from the second and third constant domains of the antibody's two heavy chains; IgM and IgE Fc regions contain three heavy chain constant domains in each polypeptide chain. The Fc regions of IgGs bear a highly conserved N-glycosylation site. Glycosylation of the Fc fragment is essential for Fc receptor-mediated activity. The N-glycans attached to this site are predominantly core-fucosylated diantennary structures of the complex type. In addition, small amounts of these N-glycans also bear bisecting GlcNAc and α-2,6 linked sialic acid residues.

<span class="mw-page-title-main">Immunoglobulin class switching</span> Biological mechanism

Immunoglobulin class switching, also known as isotype switching, isotypic commutation or class-switch recombination (CSR), is a biological mechanism that changes a B cell's production of immunoglobulin from one type to another, such as from the isotype IgM to the isotype IgG. During this process, the constant-region portion of the antibody heavy chain is changed, but the variable region of the heavy chain stays the same. Since the variable region does not change, class switching does not affect antigen specificity. Instead, the antibody retains affinity for the same antigens, but can interact with different effector molecules.

<span class="mw-page-title-main">Immunoglobulin light chain</span> Small antibody polypeptide subunit (immunoglobin)

The immunoglobulin light chain is the small polypeptide subunit of an antibody (immunoglobulin).

<span class="mw-page-title-main">Protein L</span>

Protein L was first isolated from the surface of bacterial species Peptostreptococcus magnus and was found to bind immunoglobulins through L chain interaction, from which the name was suggested. It consists of 719 amino acid residues. The molecular weight of protein L purified from the cell walls of Peptostreptoccus magnus was first estimated as 95kD by SDS-PAGE in the presence of reducing agent 2-mercaptoethanol, while the molecular weight was determined to 76kD by gel chromatography in the presence of 6 M guanidine HCl. Protein L does not contain any interchain disulfide loops, nor does it consist of disulfide-linked subunits. It is an acidic molecule with a pI of 4.0. Unlike protein A and protein G, which bind to the Fc region of immunoglobulins (antibodies), protein L binds antibodies through light chain interactions. Since no part of the heavy chain is involved in the binding interaction, Protein L binds a wider range of antibody classes than protein A or G. Protein L binds to representatives of all antibody classes, including IgG, IgM, IgA, IgE and IgD. Single chain variable fragments (scFv) and Fab fragments also bind to protein L.

A bispecific monoclonal antibody is an artificial protein that can simultaneously bind to two different types of antigen or two different epitopes on the same antigen. Naturally occurring antibodies typically only target one antigen. BsAbs can be manufactured in several structural formats. BsAbs can be designed to recruit and activate immune cells, to interfere with receptor signaling and inactivate signaling ligands, and to force association of protein complexes. BsAbs have been explored for cancer immunotherapy, drug delivery, and Alzheimer's disease.

<span class="mw-page-title-main">IGHM</span> Gene in the species Homo sapiens

Ig mu chain C region is a protein that in humans is encoded by the IGHM gene.

Immunoglobulin heavy locus, also known as IGH, is a region on human chromosome 14 that contains a gene for the heavy chains of human antibodies.

<span class="mw-page-title-main">IGHE</span>

Ig epsilon chain C region is a protein that in humans is encoded by the IGHE gene.

<span class="mw-page-title-main">CD79A</span> Mammalian protein found in Homo sapiens

Cluster of differentiation CD79A also known as B-cell antigen receptor complex-associated protein alpha chain and MB-1 membrane glycoprotein, is a protein that in humans is encoded by the CD79A gene.

The basic structure of immunoglobulin (Ig) molecules is a tetramer of two light chains and two heavy chains linked by disulphide bonds. There are two types of light chains: kappa and lambda, each composed of a constant domain (CL) and a variable domain (VL). There are five types of heavy chains: alpha, delta, epsilon, gamma and mu, all consisting of a variable domain (VH) and three or four constant domains. Ig molecules are highly modular proteins, in which the variable and constant domains have clear, conserved sequence patterns. The domains in Ig and Ig-like molecules are grouped into four types: V-set, C1-set, C2-set and I-set. Structural studies have shown that these domains share a common core Greek-key beta-sandwich structure, with the types differing in the number of strands in the beta-sheets as well as in their sequence patterns.

Antibody structure is made up of two heavy-chains and two light-chains. These chains are held together by disulfide bonds. The arrangement or processes that put together different parts of this antibody molecule play important role in antibody diversity and production of different subclasses or classes of antibodies. The organization and processes take place during the development and differentiation of B cells. That is, the controlled gene expression during transcription and translation coupled with the rearrangements of immunoglobulin gene segments result in the generation of antibody repertoire during development and maturation of B cells.

Recombinant antibodies are antibody fragments produced by using recombinant antibody coding genes. They mostly consist of a heavy and light chain of the variable region of immunoglobulin. Recombinant antibodies have many advantages in both medical and research applications, which make them a popular subject of exploration and new production against specific targets. The most commonly used form is the single chain variable fragment (scFv), which has shown the most promising traits exploitable in human medicine and research. In contrast to monoclonal antibodies produced by hybridoma technology, which may lose the capacity to produce the desired antibody over time or the antibody may undergo unwanted changes, which affect its functionality, recombinant antibodies produced in phage display maintain high standard of specificity and low immunogenicity.

References

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