An epitope, also known as antigenic determinant, is the part of an antigen that is recognized by the immune system, specifically by antibodies, B cells, or T cells. The part of an antibody that binds to the epitope is called a paratope. Although epitopes are usually non-self proteins, sequences derived from the host that can be recognized (as in the case of autoimmune diseases) are also epitopes. [1]
The epitopes of protein antigens are divided into two categories, conformational epitopes and linear epitopes, based on their structure and interaction with the paratope. [2] Conformational and linear epitopes interact with the paratope based on the 3-D conformation adopted by the epitope, which is determined by the surface features of the involved epitope residues and the shape or tertiary structure of other segments of the antigen. A conformational epitope is formed by the 3-D conformation adopted by the interaction of discontiguous amino acid residues. In contrast, a linear epitope is formed by the 3-D conformation adopted by the interaction of contiguous amino acid residues. A linear epitope is not determined solely by the primary structure of the involved amino acids. Residues that flank such amino acid residues, as well as more distant amino acid residues of the antigen affect the ability of the primary structure residues to adopt the epitope's 3-D conformation. [3] [4] [5] [6] [7] 90% of epitopes are conformational. [8]
T cell epitopes [9] are presented on the surface of an antigen-presenting cell, where they are bound to major histocompatibility complex (MHC) molecules. In humans, professional antigen-presenting cells are specialized to present MHC class II peptides, whereas most nucleated somatic cells present MHC class I peptides. T cell epitopes presented by MHC class I molecules are typically peptides between 8 and 11 amino acids in length, whereas MHC class II molecules present longer peptides, 13–17 amino acids in length, [10] and non-classical MHC molecules also present non-peptidic epitopes such as glycolipids.
The part of the antigen that immunoglobulin or antibodies bind to is called a B-cell epitope. [11] B cell epitopes can be divided into two groups: conformational or linear. [11] B cell epitopes are mainly conformational. [12] [13] There are additional epitope types when the quaternary structure is considered. [13] Epitopes that are masked when protein subunits aggregate are called cryptotopes. [13] Neotopes are epitopes that are only recognized while in a specific quaternary structure and the residues of the epitope can span multiple protein subunits. [13] Neotopes are not recognized once the subunits dissociate. [13]
Epitopes are sometimes cross-reactive. This property is exploited by the immune system in regulation by anti-idiotypic antibodies (originally proposed by Nobel laureate Niels Kaj Jerne). If an antibody binds to an antigen's epitope, the paratope could become the epitope for another antibody that will then bind to it. If this second antibody is of IgM class, its binding can upregulate the immune response; if the second antibody is of IgG class, its binding can downregulate the immune response.[ citation needed ]
MHC class I and II epitopes can be reliably predicted by computational means alone, [14] although not all in-silico T cell epitope prediction algorithms are equivalent in their accuracy. [15] There are two main methods of predicting peptide-MHC binding: data-driven and structure-based. [11] Structure based methods model the peptide-MHC structure and require great computational power. [11] Data-driven methods have higher predictive performance than structure-based methods. [11] Data-driven methods predict peptide-MHC binding based on peptide sequences that bind MHC molecules. [11] By identifying T-cell epitopes, scientists can track, phenotype, and stimulate T-cells. [16] [17] [18] [19]
There are two main methods of epitope mapping: either structural or functional studies. [20] Methods for structurally mapping epitopes include X-ray crystallography, nuclear magnetic resonance, and electron microscopy. [20] X-ray crystallography of Ag-Ab complexes is considered an accurate way to structurally map epitopes. [20] Nuclear magnetic resonance can be used to map epitopes by using data about the Ag-Ab complex. [20] This method does not require crystal formation but can only work on small peptides and proteins. [20] Electron microscopy is a low-resolution method that can localize epitopes on larger antigens like virus particles. [20]
Methods for functionally mapping epitopes often use binding assays such as western blot, dot blot, and/or ELISA to determine antibody binding. [20] Competition methods look to determine if two monoclonal antibodies (mABs) can bind to an antigen at the same time or compete with each other to bind at the same site. [20] Another technique involves high-throughput mutagenesis, an epitope mapping strategy developed to improve rapid mapping of conformational epitopes on structurally complex proteins. [21] Mutagenesis uses randomly/site-directed mutations at individual residues to map epitopes. [20] B-cell epitope mapping can be used for the development of antibody therapeutics, peptide-based vaccines, and immunodiagnostic tools. [20] [22]
Epitopes are often used in proteomics and the study of other gene products. Using recombinant DNA techniques genetic sequences coding for epitopes that are recognized by common antibodies can be fused to the gene. Following synthesis, the resulting epitope tag allows the antibody to find the protein or other gene product enabling lab techniques for localisation, purification, and further molecular characterization. Common epitopes used for this purpose are Myc-tag, HA-tag, FLAG-tag, GST-tag, 6xHis, [23] V5-tag and OLLAS. [24] Peptides can also be bound by proteins that form covalent bonds to the peptide, allowing irreversible immobilisation. [25] These strategies have also been successfully applied to the development of "epitope-focused" vaccine design. [26] [27]
The first epitope-based vaccine was developed in 1985 by Jacob et al. [28] Epitope-based vaccines stimulate humoral and cellular immune responses using isolated B-cell or T-cell epitopes. [28] [22] [17] These vaccines can use multiple epitopes to increase their efficacy. [28] To find epitopes to use for the vaccine, in silico mapping is often used. [28] Once candidate epitopes are found, the constructs are engineered and tested for vaccine efficiency. [28] While epitope-based vaccines are generally safe, one possible side effect is cytokine storms. [28]
A neoantigenic determinant is an epitope on a neoantigen, which is a newly formed antigen that has not been previously recognized by the immune system. [29] Neoantigens are often associated with tumor antigens and are found in oncogenic cells. [30] Neoantigens and, by extension, neoantigenic determinants can be formed when a protein undergoes further modification within a biochemical pathway such as glycosylation, phosphorylation or proteolysis. This, by altering the structure of the protein, can produce new epitopes that are called neoantigenic determinants as they give rise to new antigenic determinants. Recognition requires separate, specific antibodies.[ citation needed ]
In immunology, an antigen (Ag) is a molecule, moiety, foreign particulate matter, or an allergen, such as pollen, that can bind to a specific antibody or T-cell receptor. The presence of antigens in the body may trigger an immune response.
An antibody (Ab) or immunoglobulin (Ig) is a large, Y-shaped protein belonging to the immunoglobulin superfamily which is 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.
The human leukocyte antigen (HLA) system or complex of genes on chromosome 6 in humans which encode cell-surface proteins responsible for regulation of the immune system. The HLA system is also known as the human version of the major histocompatibility complex (MHC) found in many animals.
An antigen-presenting cell (APC) or accessory cell is a cell that displays an antigen bound by major histocompatibility complex (MHC) proteins on its surface; this process is known as antigen presentation. T cells may recognize these complexes using their T cell receptors (TCRs). APCs process antigens and present them to T cells.
The T-cell receptor (TCR) is a protein complex found on the surface of T cells, or T lymphocytes, that is responsible for recognizing fragments of antigen as peptides bound to major histocompatibility complex (MHC) molecules. The binding between TCR and antigen peptides is of relatively low affinity and is degenerate: that is, many TCRs recognize the same antigen peptide and many antigen peptides are recognized by the same TCR.
In immunology, epitope mapping is the process of experimentally identifying the binding site, or epitope, of an antibody on its target antigen. Identification and characterization of antibody binding sites aid in the discovery and development of new therapeutics, vaccines, and diagnostics. Epitope characterization can also help elucidate the binding mechanism of an antibody and can strengthen intellectual property (patent) protection. Experimental epitope mapping data can be incorporated into robust algorithms to facilitate in silico prediction of B-cell epitopes based on sequence and/or structural data.
In academia, computational immunology is a field of science that encompasses high-throughput genomic and bioinformatics approaches to immunology. The field's main aim is to convert immunological data into computational problems, solve these problems using mathematical and computational approaches and then convert these results into immunologically meaningful interpretations.
In immunology, a linear epitope is an epitope—a binding site on an antigen—that is recognized by antibodies by its linear sequence of amino acids. In contrast, most antibodies recognize a conformational epitope that has a specific three-dimensional shape.
Bacterial display is a protein engineering technique used for in vitro protein evolution. Libraries of polypeptides displayed on the surface of bacteria can be screened using flow cytometry or iterative selection procedures (biopanning). This protein engineering technique allows us to link the function of a protein with the gene that encodes it. Bacterial display can be used to find target proteins with desired properties and can be used to make affinity ligands which are cell-specific. This system can be used in many applications including the creation of novel vaccines, the identification of enzyme substrates and finding the affinity of a ligand for its target protein.
Immunogenicity is the ability of a foreign substance, such as an antigen, to provoke an immune response in the body of a human or other animal. It may be wanted or unwanted:
Molecular mimicry is the theoretical possibility that sequence similarities between foreign and self-peptides are enough to result in the cross-activation of autoreactive T or B cells by pathogen-derived peptides. Despite the prevalence of several peptide sequences which can be both foreign and self in nature, just a few crucial residues can activate a single antibody or TCR. This highlights the importance of structural homology in the theory of molecular mimicry. Upon activation, these "peptide mimic" specific T or B cells can cross-react with self-epitopes, thus leading to tissue pathology (autoimmunity). Molecular mimicry is one of several ways in which autoimmunity can be evoked. A molecular mimicking event is more than an epiphenomenon despite its low probability, and these events have serious implications in the onset of many human autoimmune disorders.
MHC-restricted antigen recognition, or MHC restriction, refers to the fact that a T cell can interact with a self-major histocompatibility complex molecule and a foreign peptide bound to it, but will only respond to the antigen when it is bound to a particular MHC molecule.
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.
Polyclonal B cell response is a natural mode of immune response exhibited by the adaptive immune system of mammals. It ensures that a single antigen is recognized and attacked through its overlapping parts, called epitopes, by multiple clones of B cell.
Human leukocyte histocompatibility complex DO (HLA-DO) is an intracellular, dimeric non-classical Major Histocompatibility Complex (MHC) class II protein composed of α- and β-subunits which interact with HLA-DM in order to fine tune immunodominant epitope selection. As a non-classical MHC class II molecule, HLA-DO is a non-polymorphic accessory protein that aids in antigenic peptide chaperoning and loading, as opposed to its classical counterparts, which are polymorphic and involved in antigen presentation. Though more remains to be elucidated about the function of HLA-DO, its unique distribution in the mammalian body—namely, the exclusive expression of HLA-DO in B cells, thymic medullary epithelial cells, and dendritic cells—indicate that it may be of physiological importance and has inspired further research. Although HLA-DM can be found without HLA-DO, HLA-DO is only found in complex with HLA-DM and exhibits instability in the absence of HLA-DM. The evolutionary conservation of both DM and DO, further denote its biological significance and potential to confer evolutionary benefits to its host.
In immunology, a conformational epitope is a sequence of sub-units composing an antigen that come in direct contact with a receptor of the immune system.
Peptide-based synthetic vaccines are subunit vaccines made from peptides. The peptides mimic the epitopes of the antigen that triggers direct or potent immune responses. Peptide vaccines can not only induce protection against infectious pathogens and non-infectious diseases but also be utilized as therapeutic cancer vaccines, where peptides from tumor-associated antigens are used to induce an effective anti-tumor T-cell response.
A peptide microarray is a collection of peptides displayed on a solid surface, usually a glass or plastic chip. Peptide chips are used by scientists in biology, medicine and pharmacology to study binding properties and functionality and kinetics of protein-protein interactions in general. In basic research, peptide microarrays are often used to profile an enzyme, to map an antibody epitope or to find key residues for protein binding. Practical applications are seromarker discovery, profiling of changing humoral immune responses of individual patients during disease progression, monitoring of therapeutic interventions, patient stratification and development of diagnostic tools and vaccines.
Immunodominance is the immunological phenomenon in which immune responses are mounted against only a few of the antigenic peptides out of the many produced. That is, despite multiple allelic variations of MHC molecules and multiple peptides presented on antigen presenting cells, the immune response is skewed to only specific combinations of the two. Immunodominance is evident for both antibody-mediated immunity and cell-mediated immunity. Epitopes that are not targeted or targeted to a lower degree during an immune response are known as subdominant epitopes. The impact of immunodominance is immunodomination, where immunodominant epitopes will curtail immune responses against non-dominant epitopes. Antigen-presenting cells such as dendritic cells, can have up to six different types of MHC molecules for antigen presentation. There is a potential for generation of hundreds to thousands of different peptides from the proteins of pathogens. Yet, the effector cell population that is reactive against the pathogen is dominated by cells that recognize only a certain class of MHC bound to only certain pathogen-derived peptides presented by that MHC class. Antigens from a particular pathogen can be of variable immunogenicity, with the antigen that stimulates the strongest response being the immunodominant one. The different levels of immunogenicity amongst antigens forms what is known as dominance hierarchy.
Antigen-antibody interaction, or antigen-antibody reaction, is a specific chemical interaction between antibodies produced by B cells of the white blood cells and antigens during immune reaction. The antigens and antibodies combine by a process called agglutination. It is the fundamental reaction in the body by which the body is protected from complex foreign molecules, such as pathogens and their chemical toxins. In the blood, the antigens are specifically and with high affinity bound by antibodies to form an antigen-antibody complex. The immune complex is then transported to cellular systems where it can be destroyed or deactivated.