Superantigen

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SEB, a typical bacterial superantigen (PDB:3SEB). The b-grasp domain is shown in red, the b-barrel in green, the "disulfide loop" in yellow. Seb.png
SEB, a typical bacterial superantigen (PDB:3SEB). The β-grasp domain is shown in red, the β-barrel in green, the "disulfide loop" in yellow.
SEC3 (yellow) complexed with an MHC class II molecule (green & cyan). The SAgs binds adjacent to the antigen presentation cleft (purple) in the MHC-II. Sec-mhc.png
SEC3 (yellow) complexed with an MHC class II molecule (green & cyan). The SAgs binds adjacent to the antigen presentation cleft (purple) in the MHC-II.
Schematic representation of MHC class II. MHC Class 2.svg
Schematic representation of MHC class II.
The T-cell receptor complex with TCR-a and TCR-b chains, CD3 and z-chain accessory molecules. TCRComplex.png
The T-cell receptor complex with TCR-α and TCR-β chains, CD3 and ζ-chain accessory molecules.

Superantigens (SAgs) are a class of antigens that result in excessive activation of the immune system. Specifically they cause non-specific activation of T-cells resulting in polyclonal T cell activation and massive cytokine release. Superantigens act by binding to the MHC proteins on antigen-presenting cells (APCs) and to the TCRs on their adjacent helper T-cells, bringing the signaling molecules together, and thus leading to the activation of the T-cells, regardless of the peptide displayed on the MHC molecule. [1] SAgs are produced by some pathogenic viruses and bacteria most likely as a defense mechanism against the immune system. [2] Compared to a normal antigen-induced T-cell response where 0.0001-0.001% of the body's T-cells are activated, these SAgs are capable of activating up to 20% of the body's T-cells. [3] Furthermore, Anti-CD3 and Anti-CD28 antibodies (CD28-SuperMAB) have also shown to be highly potent superantigens (and can activate up to 100% of T cells).

Contents

The large number of activated T-cells generates a massive immune response which is not specific to any particular epitope on the SAg thus undermining one of the fundamental strengths of the adaptive immune system, that is, its ability to target antigens with high specificity. More importantly, the large number of activated T-cells secrete large amounts of cytokines, the most important of which is Interferon gamma. This excess amount of IFN-gamma in turn activates the macrophages. The activated macrophages, in turn, over-produce proinflammatory cytokines such as IL-1, IL-6 and TNF-alpha. TNF-alpha is particularly important as a part of the body's inflammatory response. In normal circumstances it is released locally in low levels and helps the immune system defeat pathogens. However, when it is systemically released in the blood and in high levels (due to mass T-cell activation resulting from the SAg binding), it can cause severe and life-threatening symptoms, including shock and multiple organ failure.

Structure

SAgs are produced intracellularly by bacteria and are released upon infection as extracellular mature toxins. [4]

The sequences of these bacterial toxins are relatively conserved among the different subgroups. More important than sequence homology, the 3D structure is very similar among different SAgs resulting in similar functional effects among different groups. [5] [6] There are at least 5 groups of superantigens with different binding preferences. [7]

Crystal structures of the enterotoxins reveals that they are compact, ellipsoidal proteins sharing a characteristic two-domain folding pattern comprising an NH2-terminal β barrel globular domain known as the oligosaccharide / oligonucleotide fold, a long α-helix that diagonally spans the center of the molecule, and a COOH terminal globular domain. [5]

The domains have binding regions for the major histocompatibility complex class II (MHC class II) and the T-cell receptor (TCR), respectively. By bridging these two together, the SAg causes nonspecific activation. [8]

Binding

Superantigens bind first to the MHC class II and then coordinate to the variable alpha- or beta chain of T-cell Receptors (TCR) [6] [9] [10]

MHC Class II

SAgs show preference for the HLA-DQ form of the molecule. [10] Binding to the α-chain puts the SAg in the appropriate position to coordinate to the TCR.

Less commonly, SAgs attach to the polymorphic MHC class II β-chain in an interaction mediated by a zinc ion coordination complex between three SAg residues and a highly conserved region of the HLA-DR β chain. [6] The use of a zinc ion in binding leads to a higher affinity interaction. [5] Several staphylococcal SAgs are capable of cross-linking MHC molecules by binding to both the α and β chains. [5] [6] This mechanism stimulates cytokine expression and release in antigen presenting cells as well as inducing the production of costimulatory molecules that allow the cell to bind to and activate T cells more effectively. [6]

T-cell receptor

T-cell binding region of the SAg interacts with the Variable region on the Beta chain (Vβ region) of the T-cell Receptor. A given SAg can activate a large proportion of the T-cell population because the human T-cell repertoire comprises only about 50 types of Vβ elements and some SAgs are capable of binding to multiple types of Vβ regions. This interaction varies slightly among the different groups of SAgs. [8] Variability among different people in the types of T-cell regions that are prevalent explains why some people respond more strongly to certain SAgs. Group I SAgs contact the Vβ at the CDR2 and framework region of the molecule. [11] [12] SAgs of Group II interact with the Vβ region using mechanisms that are conformation-dependent. These interactions are for the most part independent of specific Vβ amino acid side-chains. Group IV SAgs have been shown to engage all three CDR loops of certain Vβ forms. [11] [12] The interaction takes place in a cleft between the small and large domains of the SAg and allows the SAg to act as a wedge between the TCR and MHC. This displaces the antigenic peptide away from the TCR and circumvents the normal mechanism for T-cell activation. [6] [13]

The biological strength of the SAg (its ability to stimulate) is determined by its affinity for the TCR. SAgs with the highest affinity for the TCR elicit the strongest response. [14] SPMEZ-2 is the most potent SAg discovered to date. [14]

T-cell signaling

The SAg cross-links the MHC and the TCR inducing a signaling pathway that results in the proliferation of the cell and production of cytokines. This occurs because a cognate antigen activates a T cell not because of its structure per se, but because its affinity allows it to bind the TCR for a lengthy enough time period, and the SAg mimics this temporal bonding. Low levels of Zap-70 have been found in T-cells activated by SAgs, indicating that the normal signaling pathway of T-cell activation is impaired. [15]

It is hypothesized that Fyn rather than Lck is activated by a tyrosine kinase, leading to the adaptive induction of anergy. [16]

Both the protein kinase C pathway and the protein tyrosine kinase pathways are activated, resulting in upregulating production of proinflammatory cytokines. [17]

This alternative signaling pathway impairs the calcium/calcineurin and Ras/MAPkinase pathways slightly, [16] but allows for a focused inflammatory response.

Effects

Direct effects

SAg stimulation of antigen presenting cells and T-cells elicits a response that is mainly inflammatory, focused on the action of Th1 T-helper cells. Some of the major products are IL-1, IL-2, IL-6, TNF-α, gamma interferon (IFN-γ), macrophage inflammatory protein 1α (MIP-1α), MIP-1β, and monocyte chemoattractant protein 1 (MCP-1). [17]

This excessive uncoordinated release of cytokines, (especially TNF-α), overloads the body and results in rashes, fever, and can lead to multi-organ failure, coma and death. [10] [12]

Deletion or anergy of activated T-cells follows infection. This results from production of IL-4 and IL-10 from prolonged exposure to the toxin. The IL-4 and IL-10 downregulate production of IFN-gamma, MHC Class II, and costimulatory molecules on the surface of APCs. These effects produce memory cells that are unresponsive to antigen stimulation. [18] [19]

One mechanism by which this is possible involves cytokine-mediated suppression of T-cells. MHC crosslinking also activates a signaling pathway that suppresses hematopoiesis and upregulates Fas-mediated apoptosis. [20]

IFN-α is another product of prolonged SAg exposure. This cytokine is closely linked with induction of autoimmunity, [21] and the autoimmune disease Kawasaki disease is known to be caused by SAg infection. [14]

SAg activation in T-cells leads to production of CD40 ligand which activates isotype switching in B cells to IgG and IgM and IgE. [22]

To summarize, the T-cells are stimulated and produce excess amounts of cytokine resulting in cytokine-mediated suppression of T-cells and deletion of the activated cells as the body returns to homeostasis. The toxic effects of the microbe and SAg also damage tissue and organ systems, a condition known as toxic shock syndrome. [22]

If the initial inflammation is survived, the host cells become anergic or are deleted, resulting in a severely compromised immune system.

Superantigenicity-independent (indirect) effects

Apart from their mitogenic activity, SAgs are able to cause symptoms that are characteristic of infection. [2]

One such effect is vomiting. This effect is felt in cases of food poisoning, when SAg-producing bacteria release the toxin, which is highly resistant to heat. There is a distinct region of the molecule that is active in inducing gastrointestinal toxicity. [2] This activity is also highly potent, and quantities as small as 20-35 μg of SAg are able to induce vomiting. [10]

SAgs are able to stimulate recruitment of neutrophils to the site of infection in a way that is independent of T-cell stimulation. This effect is due to the ability of SAgs to activate monocytic cells, stimulating the release of the cytokine TNF-α, leading to increased expression of adhesion molecules that recruit leukocytes to infected regions. This causes inflammation in the lungs, intestinal tissue, and any place that the bacteria have colonized. [23] While small amounts of inflammation are natural and helpful, excessive inflammation can lead to tissue destruction.

One of the more dangerous indirect effects of SAg infection concerns the ability of SAgs to augment the effects of endotoxins in the body. This is accomplished by reducing the threshold for endotoxicity. Schlievert demonstrated that, when administered conjunctively, the effects of SAg and endotoxin are magnified as much as 50,000 times. [9] This could be due to the reduced immune system efficiency induced by SAg infection. Aside from the synergistic relationship between endotoxin and SAg, the “double hit” effect of the activity of the endotoxin and the SAg result in effects more deleterious that those seen in a typical bacterial infection. This also implicates SAgs in the progression of sepsis in patients with bacterial infections. [22]

Diseases associated with superantigen production

Treatment

The primary goals of medical treatment are to hemodynamically stabilize the patient and, if present, to eliminate the microbe that is producing the SAgs. This is accomplished through the use of vasopressors, fluid resuscitation and antibiotics. [2]

The body naturally produces antibodies to some SAgs, and this effect can be augmented by stimulating B-cell production of these antibodies. [26]

Immunoglobulin pools are able to neutralize specific antibodies and prevent T-cell activation. Synthetic antibodies and peptides have been created to mimic SAg-binding regions on the MHC class II, blocking the interaction and preventing T cell activation. [2]

Immunosuppressants are also employed to prevent T-cell activation and the release of cytokines. Corticosteroids are used to reduce inflammatory effects. [22]

Evolution of superantigen production

SAg production effectively corrupts the immune response, allowing the microbe secreting the SAg to be carried and transmitted unchecked. One mechanism by which this is done is through inducing anergy of the T-cells to antigens and SAgs. [15] [18] Lussow and MacDonald demonstrated this by systematically exposing animals to a streptococcal antigen. They found that exposure to other antigens after SAg infection failed to elicit an immune response. [18] In another experiment, Watson and Lee discovered that memory T-cells created by normal antigen stimulation were anergic to SAg stimulation and that memory T-cells created after a SAg infection were anergic to all antigen stimulation. The mechanism by which this occurred was undetermined. [15] The genes that regulate SAg expression also regulate mechanisms of immune evasion such as M protein and Bacterial capsule expression, supporting the hypothesis that SAg production evolved primarily as a mechanism of immune evasion. [27]

When the structure of individual SAg domains has been compared to other immunoglobulin-binding streptococcal proteins (such as those toxins produced by E. coli ) it was found that the domains separately resemble members of these families. This homology suggests that the SAgs evolved through the recombination of two smaller β-strand motifs. [28]

"Staphylococcal Superantigen-Like" (SSL) toxins are a group of secreted proteins structurally similar to SAgs. Instead of binding to MHC and TCR, they target diverse components of innate immunity such as complement, Fc receptors, and myeloid cells. One way SSL targets myeloid cells is by binding the siallylactosamine glycan on surface glycoproteins. [29] In 2017, a superantigen was found to also have a glycan-binding ability. [30]

Endogenous and viral SAgs

Minor lymphocyte stimulating (Mls; P03319 ) exotoxins were originally discovered in the thymic stromal cells of mice. These toxins are encoded by SAg genes that were incorporated into the mouse genome from the mouse mammary tumour virus (MMTV). The presence of these genes in the mouse genome allows the mouse to express the antigen in the thymus as a means of negatively selecting for lymphocytes with a variable Beta region that is susceptible to stimulation by the viral SAg. The result is that these mice are immune to infection by the virus later in life. [2]

Similar endogenous SAg-dependent selection has yet to be identified in the human genome, but endogenous SAgs have been discovered and are suspected of playing an integral role in viral infection. Infection by the Epstein–Barr virus, for example, is known to cause production of a SAg in infected cells, yet no gene for the toxin has been found on the genome of the virus. The virus manipulates the infected cell to express its own SAg genes, and this helps it to evade the host immune system. Similar results have been found with rabies, cytomegalovirus, and HIV. [2] In 2001, it was found that EBV actually transactivates a superantigen encoded by the env gene ( O42043 ) of HERV-K18. In 2006, it was found that EBV does so by docking to CD2. [31]

The two viral superantigens have no homology to aforementioned bacterial superantigens, nor are they homologous to each other.

Related Research Articles

<span class="mw-page-title-main">Antigen</span> Molecule triggering an immune response (antibody production) in the host

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.

<span class="mw-page-title-main">Immune system</span> Biological system protecting an organism against disease

The immune system is a network of biological systems that protects an organism from diseases. It detects and responds to a wide variety of pathogens, from viruses to parasitic worms, as well as cancer cells and objects such as wood splinters, distinguishing them from the organism's own healthy tissue. Many species have two major subsystems of the immune system. The innate immune system provides a preconfigured response to broad groups of situations and stimuli. The adaptive immune system provides a tailored response to each stimulus by learning to recognize molecules it has previously encountered. Both use molecules and cells to perform their functions.

<span class="mw-page-title-main">T cell</span> White blood cells of the immune system

T cells are one of the important types of white blood cells 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 surface.

<span class="mw-page-title-main">B cell</span> Type of white blood cell

B cells, also known as B lymphocytes, are a type of white blood cell of the lymphocyte subtype. They function in the humoral immunity component of the adaptive immune system. B cells produce antibody molecules which may be either secreted or inserted into the plasma membrane where they serve as a part of B-cell receptors. When a naïve or memory B cell is activated by an antigen, it proliferates and differentiates into an antibody-secreting effector cell, known as a plasmablast or plasma cell. In addition, B cells present antigens and secrete cytokines. In mammals, B cells mature in the bone marrow, which is at the core of most bones. In birds, B cells mature in the bursa of Fabricius, a lymphoid organ where they were first discovered by Chang and Glick, which is why the B stands for bursa and not bone marrow, as commonly believed.

<span class="mw-page-title-main">Cytotoxic T cell</span> T cell that kills infected, damaged or cancerous cells

A cytotoxic T cell (also known as TC, cytotoxic T lymphocyte, CTL, T-killer cell, cytolytic T cell, CD8+ T-cell or killer T cell) is a T lymphocyte (a type of white blood cell) that kills cancer cells, cells that are infected by intracellular pathogens (such as viruses or bacteria), or cells that are damaged in other ways.

<span class="mw-page-title-main">T helper cell</span> Type of immune cell

The T helper cells (Th cells), also known as CD4+ cells or CD4-positive cells, are a type of T cell that play an important role in the adaptive immune system. They aid the activity of other immune cells by releasing cytokines. They are considered essential in B cell antibody class switching, breaking cross-tolerance in dendritic cells, in the activation and growth of cytotoxic T cells, and in maximizing bactericidal activity of phagocytes such as macrophages and neutrophils. CD4+ cells are mature Th cells that express the surface protein CD4. Genetic variation in regulatory elements expressed by CD4+ cells determines susceptibility to a broad class of autoimmune diseases.

In immunology, anergy is a lack of reaction by the body's defense mechanisms to foreign substances, and consists of a direct induction of peripheral lymphocyte tolerance. An individual in a state of anergy often indicates that the immune system is unable to mount a normal immune response against a specific antigen, usually a self-antigen. Lymphocytes are said to be anergic when they fail to respond to their specific antigen. Anergy is one of three processes that induce tolerance, modifying the immune system to prevent self-destruction.

<span class="mw-page-title-main">Exotoxin</span> Toxin from bacteria that destroys or disrupts cells

An exotoxin is a toxin secreted by bacteria. An exotoxin can cause damage to the host by destroying cells or disrupting normal cellular metabolism. They are highly potent and can cause major damage to the host. Exotoxins may be secreted, or, similar to endotoxins, may be released during lysis of the cell. Gram negative pathogens may secrete outer membrane vesicles containing lipopolysaccharide endotoxin and some virulence proteins in the bounding membrane along with some other toxins as intra-vesicular contents, thus adding a previously unforeseen dimension to the well-known eukaryote process of membrane vesicle trafficking, which is quite active at the host–pathogen interface.

<span class="mw-page-title-main">Enterotoxin</span> Toxin from a microorganism affecting the intestines

An enterotoxin is a protein exotoxin released by a microorganism that targets the intestines. They can be chromosomally or plasmid encoded. They are heat labile (>60⁰), of low molecular weight and water-soluble. Enterotoxins are frequently cytotoxic and kill cells by altering the apical membrane permeability of the mucosal (epithelial) cells of the intestinal wall. They are mostly pore-forming toxins, secreted by bacteria, that assemble to form pores in cell membranes. This causes the cells to die.

<span class="mw-page-title-main">Adaptive immune system</span> Subsystem of the immune system

The adaptive immune system, also known as the acquired immune system, or specific immune system is a subsystem of the immune system that is composed of specialized, systemic cells and processes that eliminate pathogens or prevent their growth. The acquired immune system is one of the two main immunity strategies found in vertebrates.

<span class="mw-page-title-main">T-cell receptor</span> Protein complex on the surface of T cells that recognises antigens

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.

<span class="mw-page-title-main">Fc receptor</span> Surface protein important to the immune system

In immunology, an Fc receptor is a protein found on the surface of certain cells – including, among others, B lymphocytes, follicular dendritic cells, natural killer cells, macrophages, neutrophils, eosinophils, basophils, human platelets, and mast cells – that contribute to the protective functions of the immune system. Its name is derived from its binding specificity for a part of an antibody known as the Fc region. Fc receptors bind to antibodies that are attached to infected cells or invading pathogens. Their activity stimulates phagocytic or cytotoxic cells to destroy microbes, or infected cells by antibody-mediated phagocytosis or antibody-dependent cell-mediated cytotoxicity. Some viruses such as flaviviruses use Fc receptors to help them infect cells, by a mechanism known as antibody-dependent enhancement of infection.

Co-stimulation is a secondary signal which immune cells rely on to activate an immune response in the presence of an antigen-presenting cell. In the case of T cells, two stimuli are required to fully activate their immune response. During the activation of lymphocytes, co-stimulation is often crucial to the development of an effective immune response. Co-stimulation is required in addition to the antigen-specific signal from their antigen receptors.

Understanding of the antitumor immunity role of CD4+ T cells has grown substantially since the late 1990s. CD4+ T cells (mature T-helper cells) play an important role in modulating immune responses to pathogens and tumor cells, and are important in orchestrating overall immune responses.

<span class="mw-page-title-main">Toxic shock syndrome toxin-1</span>

Toxic shock syndrome toxin-1 (TSST-1) is a superantigen with a size of 22 kDa produced by 5 to 25% of Staphylococcus aureus isolates. It causes toxic shock syndrome (TSS) by stimulating the release of large amounts of interleukin-1, interleukin-2 and tumour necrosis factor. In general, the toxin is not produced by bacteria growing in the blood; rather, it is produced at the local site of an infection, and then enters the blood stream.

Short Course Immune Induction Therapy or SCIIT, is a therapeutic strategy employing rapid, specific, short term-modulation of the immune system using a therapeutic agent to induce T-cell non-responsiveness, also known as operational tolerance. As an alternative strategy to immunosuppression and antigen-specific tolerance inducing therapies, the primary goal of SCIIT is to re-establish or induce peripheral immune tolerance in the context of autoimmune disease and transplant rejection through the use of biological agents. In recent years, SCIIT has received increasing attention in clinical and research settings as an alternative to immunosuppressive drugs currently used in the clinic, drugs which put the patients at risk of developing infection, cancer, and cardiovascular disease.

<span class="mw-page-title-main">Streptococcal pyrogenic exotoxin</span>

Streptococcal pyrogenic exotoxins also known as erythrogenic toxins, are exotoxins secreted by strains of the bacterial species Streptococcus pyogenes. SpeA and speC are superantigens, which induce inflammation by nonspecifically activating T cells and stimulating the production of inflammatory cytokines. SpeB, the most abundant streptococcal extracellular protein, is a cysteine protease. Pyrogenic exotoxins are implicated as the causative agent of scarlet fever and streptococcal toxic shock syndrome. There is no consensus on the exact number of pyrogenic exotoxins. Serotypes A-C are the most extensively studied and recognized by all sources, but others note up to thirteen distinct types, categorizing speF through speM as additional superantigens. Erythrogenic toxins are known to damage the plasma membranes of blood capillaries under the skin and produce a red skin rash. Past studies have shown that multiple variants of erythrogenic toxins may be produced, depending on the strain of S. pyogenes in question. Some strains may not produce a detectable toxin at all. Bacteriophage T12 infection of S. pyogenes enables the production of speA, and increases virulence.

<span class="mw-page-title-main">Enterotoxin type B</span> Enterotoxin produced by the bacteria Staphylococcus aureus

In the field of molecular biology, enterotoxin type B, also known as Staphylococcal enterotoxin B (SEB), is an enterotoxin produced by the gram-positive bacteria Staphylococcus aureus. It is a common cause of food poisoning, with severe diarrhea, nausea and intestinal cramping often starting within a few hours of ingestion. Being quite stable, the toxin may remain active even after the contaminating bacteria are killed. It can withstand boiling at 100 °C for a few minutes. Gastroenteritis occurs because SEB is a superantigen, causing the immune system to release a large amount of cytokines that lead to significant inflammation.

Mucosal-associated invariant T cells make up a subset of T cells in the immune system that display innate, effector-like qualities. In humans, MAIT cells are found in the blood, liver, lungs, and mucosa, defending against microbial activity and infection. The MHC class I-like protein, MR1, is responsible for presenting bacterially-produced vitamin B2 and B9 metabolites to MAIT cells. After the presentation of foreign antigen by MR1, MAIT cells secrete pro-inflammatory cytokines and are capable of lysing bacterially-infected cells. MAIT cells can also be activated through MR1-independent signaling. In addition to possessing innate-like functions, this T cell subset supports the adaptive immune response and has a memory-like phenotype. Furthermore, MAIT cells are thought to play a role in autoimmune diseases, such as multiple sclerosis, arthritis and inflammatory bowel disease, although definitive evidence is yet to be published.

<span class="mw-page-title-main">CD28 family receptor</span> Group of regulatory cell surface receptors

CD28 family receptors are a group of regulatory cell surface receptors expressed on immune cells. The CD28 family in turn is a subgroup of the immunoglobulin superfamily.

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  26. ↑ Erlandsson E, Andersson K, Cavallin A, Nilsson A, et al. (2003). "Identification of the Antigenic Epitopes in Staphylococcal Enterotoxins A and E and Design of a Superantigen for Human Cancer Therapy". J. Mol. Biol. 333 (5): 893–905. doi:10.1016/j.jmb.2003.09.009. PMID   14583188.
  27. ↑ Cleary PP, McLandsborough L, Ikeda L, Cue D, Krawczak J, Lam H (April 1998). "High-frequency intracellular infection and erythrogenic toxin A expression undergo phase variation in M1 group A streptococci". Mol. Microbiol. 28 (1): 157–67. doi: 10.1046/j.1365-2958.1998.00786.x . PMID   9593304.
  28. ↑ Bachert C, Gevaert P, van Cauwenberge P (June 2002). "Staphylococcus aureus enterotoxins: a key in airway disease?". Allergy. 57 (6): 480–7. doi: 10.1034/j.1398-9995.2002.02156.x . PMID   12028112.
  29. ↑ Hermans, SJ; Baker, HM; Sequeira, RP; Langley, RJ; Baker, EN; Fraser, JD (November 2012). "Structural and functional properties of staphylococcal superantigen-like protein 4". Infection and Immunity. 80 (11): 4004–13. doi: 10.1128/IAI.00764-12 . PMC   3486064 . PMID   22949551.
  30. ↑ Langley, RJ; Ting, YT; Clow, F; Young, PG; Radcliff, FJ; Choi, JM; Sequeira, RP; Holtfreter, S; Baker, H; Fraser, JD (September 2017). "Staphylococcal enterotoxin-like X (SElX) is a unique superantigen with functional features of two major families of staphylococcal virulence factors". PLOS Pathogens. 13 (9): e1006549. doi: 10.1371/journal.ppat.1006549 . PMC   5589262 . PMID   28880913.
  31. ↑ Hsiao, Francis C.; Lin, Miao; Tai, Albert; Chen, Gang; Huber, Brigitte T. (15 August 2006). "Cutting Edge: Epstein-Barr Virus Transactivates the HERV-K18 Superantigen by Docking to the Human Complement Receptor 2 (CD21) on Primary B Cells". The Journal of Immunology. 177 (4): 2056–2060. doi: 10.4049/jimmunol.177.4.2056 . PMID   16887963.

Rasooly, R., Do, P. and Hernlem, B. (2011) Auto-presentation of Staphylococcal enterotoxin A by mouse CD4+ T cells. Open Journal of Immunology, 1, 8-14.

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