What are Superantigens (SAgs)?

Superantigens (SAgs) are a type of antigens that stimulate the immune system excessively. It specifically results in non-specific T-cell activation, polyclonal T-cell activation, and large cytokine release.

• Superantigens are microbial or viral toxins that constitute a class of disease-associated, immunostimulatory substances and function as V-restricted, highly potent polyclonal T cell mitogens.
• On the basis of the epitope described by this receptor, they bind major histocompatibility complex (MHC) class-II molecules without any prior processing and excite a high number of T cells (up to 20% of all T cells).
• Their unique capacity to cross-link MHC class II and the T cell receptor (TCR) to form a trimolecular complex is responsible for these features.
• In 1978, Todd et al. discovered that toxic shock syndrome (TSS) is a presentation of staphylococcal infection as a distinct clinical entity. Schlievert et al. and Bergdoll et al. subsequently isolated pyrogenic exotoxin C (PEC) and staphylococcal enterotoxin F (SEF) in 1981.
• The term “superantigen” was used by Kappler and Marrack in 1989 to describe this fundamentally new class of antigens.
• Some pathogenic viruses and bacteria presumably create SAgs as a defence mechanism against the immune system.
• In a normal antigen-induced T-cell response, 0.0001-0.001% of the body’s T-cells are activated, but these SAgs can activate up to 20% of the body’s T-cells.
• In addition, Anti-CD3 and Anti-CD28 antibodies (CD28-SuperMAB) have been demonstrated to be highly potent superantigens (capable of activating up to one hundred percent of T cells).
• The vast number of activated T-cells produces a massive immunological response that is not specific to any one epitope on the SAg, so undermining one of the key advantages of the adaptive immune system, namely its capacity to target antigens with great specificity.
• Importantly, the vast number of activated T-cells secrete large quantities of cytokines, Interferon gamma being the most critical. In turn, this excess amount of IFN-gamma activates the macrophages.
• In result, the activated macrophages overproduce proinflammatory cytokines including IL-1, IL-6, and TNF-alpha.
• TNF-alpha has a crucial role in the inflammatory response of the body. Under normal conditions, it is secreted locally at low concentrations and aids the immune system in combating infections.
• However, when it is released systemically in the blood and in high concentrations (due to mass T-cell activation caused by SAg binding), it can produce severe and life-threatening symptoms, including shock and multiple organ failure.

Features of Superantigens (SAgs)

• Like conventional antigens/haptens, superantigens stimulate antigen-presenting dendritic cells by increasing the expression of HLA-DR antigen and co-stimulatory molecules (CD54, CD83, and CD86) as well as the generation of tumour necrosis factor (TNF) -α.
• This enhancement by superantigens is suppressible by both corticosteroids and cyclosporine, but typical antigen/hapten-induced enhancement is resistant to cyclosporine suppression.
• Superantigens are active at concentrations as low as 10 mol/L. The bioactivity of a superantigen depends on its capacity to bind both MHC class II and TCR.
• Once secreted, superantigens engage with antigen-presenting cells without further processing.
• In spite of interacting with MHC class II molecules outside the antigen-binding groove, they are nevertheless able to induce a fruitful contact with T cells.
• However, amino acid sequence differences imply that each superantigen has an affinity for particular MHC class-II alleles.
• Large families of T lymphocytes readily recognise the MHC class-II superantigen complex, with recognition restricted only by the TCR Vβ component that each family expresses. The majority of superantigens are identified by at least three to five TCR families.
• After connecting with the T cell receptor, superantigen activates a substantial number of resting T cells, up to 20 percent of the total T cells. This is then followed by T cell growth and activation-induced clonal deletion.
• Both in vivo and in vitro superantigen-induced T cells result in increased production of pro-inflammatory cytokines, including TNF-α and -β, interleukin (IL)-2, and INF-γ.

Classification of Superantigens (SAgs)

The superantigens can be grouped generally into the subsequent families:

1. Endogenous superantigens: Various viruses with genome integration encode these superantigens. Examples include superantigens produced by mouse mammary tumour virus (MMTV) and superantigen associated with Epstein-Barr virus (EBV).
2. Exogenous superantigens: These include the exotoxins that bacteria release. Staphylococcal enterotoxins (A, B, C1 to C3, etc.), streptococcal pyrogenic exotoxins (A1 to A4, C, etc.), and others are some examples.
3. B-cell superantigens: Those superantigens that preferentially excite B lymphocytes. Staphylococcal protein A and protein Fv are examples.

Structure of Superantigens (SAgs)

• Superantigens are globular proteins produced as precursors of 25-30kDa and released as proteins of 22-29kDa.
• They are resistant to proteases and heat denaturation and can be absorbed immunologically intact by the epithelium.
• Superantigens can be analysed using X-ray crystallography and solution nuclear magnetic resonance spectroscopy to determine their macromolecular structure.
• They share a two-domain structure (amino and carboxy terminal domains) and a lengthy solvent-accessible α-helix (part of the carboxy terminal domain) that spans the molecule’s centre.
• The amino terminal domain is made up of a concave β-barrel with a α-helix at one end. This domain mimics the OB-fold (oligosaccharide/oligonucleotide-binding fold) seen in other protein families.
• In these proteins, the OB-fold is implicated in DNA binding and carbohydrate recognition, but superantigens have not yet been assigned these roles.
• Other characteristics of the amino-terminal domain include the presence of a disulphide bridge and a number of hydrophobic residues in solvent-exposed areas of the molecule.
• The amino acid sequence residues between two cysteines constitute a highly mobile loop region.
• The carboxy-terminal domain is composed of a four-stranded β-sheet bordered by a lengthy central α-helix and possesses structural characteristics of the -grasp motif found in other proteins such as ubiquitin, immunoglobulin-binding domains, etc.
• In addition, the amino-terminal tail is packed against the β-grasp motif and is considered part of the carboxy-terminal domain.
• With the exception of SEB, TSST-1, and SSA, the majority of superantigens have either one or two zinc-binding sites, but their locations vary.
• The zinc-binding sites appear to have a direct effect on MHC-class II molecules’ recognition of superantigens.

Examples of Superantigens

• Staphylococcal toxic shock toxin (TSST-1) and Staphylococcal toxic shock syndrome: Superantigenic stimulation induced by TSST-1 causes the release of cytokines such as IL-1 and TNF-α. A sudden increase in TNF-α is accompanied by alterations in endothelium and vascular smooth muscle, which manifest as hypotension, shock, and septic symptoms.
• Staphylococcal exfoliative toxins and Staphylococcal scalded skin syndrome: The staphylococcal exfoliative toxins A and B (ETA and ETB) act as trypsin-like serine protease or as lipase and activate other proteases due to their superantigenic nature. They also bind directly to the desmosomal cadherin desmoglein I (DsgI), which disrupts desmosomes in the granular layer of the epidermis, causing interadesmosomal splitting, characteristic blistering, and skin depigmentation.
• Staphylococcal enterotoxin (SEs) and Staphylococcal scarlatiniform eruption: It is a milder type of SSSS. The culprits are Staphylococci belonging to phage group II. Strains generating this eruption produce staphylococcal enterotoxin (SEs) comparable to toxins responsible for toxic shock syndrome (TSS).
• Atopic dermatitis: AD is a genetically defined, continuously relapsing, inflammatory skin disease with a complicated immunopathogenesis including both acute hypersensitivity and cellular responses. More than half of AD patients’ cutaneous lesions include S. aureus which produces superantigens including SEA, SEB, and TSST-1.
• Cutaneous T-cell lymphoma: Staphylococcal exfoliating toxin (ExT) and toxic shock syndrome toxin-1 in cutaneous T-cell lymphoma (TSST-1)
• Kawasaki syndrome: Streptococcal pyrogenic exotoxin B (SPEB) and streptococcal pyrogenic exotoxin C cause Kawasaki illness (SPEC)

Treatment for superantigen-mediated diseases

Drugs used for Treatment of superantigen-mediated diseases

• Glatiramer acetate: Glatiramer acetate is a four-amino-acid synthetic co-polymer based on myelin basic protein. It significantly reduces PBMC proliferation, IFN-, and TNF-α release. In vitro, at 200 microgram/ml, similar alterations were seen. It is a first-line treatment for relapsing-remitting multiple sclerosis after several double-blind, placebo-controlled trials.
• Polyclonal human intravenous immunoglobulins (IVIG): In a multicentric, randomised, double-blind, placebo-controlled experiment, high-dose IVIG neutralised superantigen toxins in streptococcal toxic shock syndrome. IVIG neutralised S. pyogenes superantegenicity in vitro and increased bacterial death in a whole blood experiment in mice. IVIG neutralises superantigens and reduces systemic inflammation in mice infected with S. pyogenes. IVIG did not improve inflammatory response, bacterial clearance, or survival in delayed treatment. IVIG preparations neutralise streptococcal superantigens differently, therefore supplementary therapy for severe streptococcal illnesses should optimise IVIG type and dose.
• Doxycycline: Superantigenic staphylococcal exotoxins poison via pro-inflammatory cytokines (SE). Doxycycline suppresses SE-stimulated T cell proliferation and cytokine/chemokine generation by human peripheral blood mononuclear cells. Doxycycline’s anti-inflammatory properties may help mitigate SE’s pathogenic consequences.
• Anisodamine: Raceanisodamine hydrochloride, a tropane alkaloid-containing Chinese herbal extract, is its active constituent. It inhibits shiga toxin-stimulated peripheral blood monocyte TNF-α, IL-β, and IL-8 production (Stx). Enterohemorrhagic E. coli causes HUS with Shiga toxin. Stx-injected mice live longer with anisodamine. Anisodamine inhibits TSST-1-activated T cells and the generation of pro- and anti-inflammatory cytokines from human peripheral mononuclear cells (PBMC). It also inhibits Vβ+ T cell growth after TSST-1 injection, protects mice from mortality, and fights gram-negative bacterial infections. This vasoactive medication treats acute disseminated intravascular coagulation in bacteremic shock patients.

Experimental treatments

• Pirfenadone: Pirfenidone reduced SEB-induced cytokine levels by 50–80% and T cell proliferation by 95% in human peripheral blood cells. SEB was administered systemically or aerosolically to BALB/c mice, followed by a sublethal dose of lipopolysaccharide. Pirfenidone given 2–4.5 hours after SEB increased survival from 0–10% to 80–100%. It also suppressed endotoxin-induced TNF-α production in macrophages and protected mice from shock.
• Ketamine isomers: Ketamine isomers reduced SEB-induced TNF-α production in human whole blood in vitro at concentrations over 50 micromole. Ketamine isomers reduced SEB-induced IL-6 and IL-8 production at doses over 100 and 500 micromole, respectively. S(+)- and R(-)-ketamine have similar effects.
• Triptolide: Oxygenated diterpene from Chinese medicinal herb Tripterygium wilfordii. Triptolide suppresses SE-stimulated T cell proliferation by 98% and PBMC production of IL-1β, IL-6, TNF-α, INF-, MIP-1 α, MIP-1 β. It dose-dependently mitigated lipopolysaccharides.
• Immunoglobulin Y: Passive transfer of chicken-derived IgY against staphylococcal enterotoxin-B (SEB) decreased cytokine responses and protected mice. All rhesus monkeys treated with SEB-specific IgY for 4 hours after challenge survived lethal SEB aerosol exposure. SEB-specific antibodies may protect non-human primates.

References

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