Exotoxin

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This figure shows that exotoxins are secreted by bacterial cells, Clostridium botulinum for example, and are toxic to somatic cells. Somatic cells have antibodies on the cell wall to target exotoxins and bind to them, preventing the invasion of somatic cells. The binding of the exotoxin and antibody forms an antigen-antibody interaction and the exotoxins are targeted for destruction by the immune system. If this interaction does not happen, the exotoxins bind to the exotoxin receptors that are on the cell surface and causes death of the host cell by inhibiting protein synthesis. This figure also shows that the application of heat or chemicals to exotoxins can result in the deactivation of exotoxins. The deactivated exotoxins are called toxoids and they are not harmful to somatic cells. Immune Response to Exotoxins.png
This figure shows that exotoxins are secreted by bacterial cells, Clostridium botulinum for example, and are toxic to somatic cells. Somatic cells have antibodies on the cell wall to target exotoxins and bind to them, preventing the invasion of somatic cells. The binding of the exotoxin and antibody forms an antigen-antibody interaction and the exotoxins are targeted for destruction by the immune system. If this interaction does not happen, the exotoxins bind to the exotoxin receptors that are on the cell surface and causes death of the host cell by inhibiting protein synthesis. This figure also shows that the application of heat or chemicals to exotoxins can result in the deactivation of exotoxins. The deactivated exotoxins are called toxoids and they are not harmful to somatic cells.

An exotoxin is a toxin secreted by bacteria. [1] 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.

Contents

They may exert their effect locally or produce systemic effects. Well-known exotoxins include: botulinum toxin produced by Clostridium botulinum ; Corynebacterium diphtheriae toxin, produced during life-threatening symptoms of diphtheria; tetanospasmin produced by Clostridium tetani . The toxic properties of most exotoxins can be inactivated by heat or chemical treatment to produce a toxoid. These retain their antigenic specificity and can be used to produce antitoxins and, in the case of diphtheria and tetanus toxoids, are used as vaccines.

Exotoxins are susceptible to antibodies produced by the immune system, but some exotoxins are so toxic that they may be fatal to the host before the immune system has a chance to mount defenses against them. In such cases, antitoxin, anti-serum containing antibodies, can sometimes be injected to provide passive immunity.

Types

Many exotoxins have been categorized. [2] [3] This classification, while fairly exhaustive, is not the only system used. Other systems for classifying or identifying toxins include:

The same exotoxin may have different names, depending on the field of research.

Type I: cell surface-active

Type I toxins bind to a receptor on the cell surface and stimulate intracellular signaling pathways. Two examples are described below.

Superantigens

Superantigens are produced by several bacteria. The best-characterized superantigens are those produced by the strains of Staphylococcus aureus and Streptococcus pyogenes that cause toxic shock syndrome. Superantigens bridge the MHC class II protein on antigen-presenting cells with the T-cell receptor on the surface of T cells with a particular Vβ chain. As a consequence, up to 50% of all T cells are activated, leading to massive secretion of proinflammatory cytokines, which produce the symptoms of toxic shock.

Heat-stable enterotoxins

Some strains of E. coli produce heat-stable enterotoxins (ST), which are small peptides that are able to withstand heat treatment at 100 °C. Different STs recognize distinct receptors on the cell surface and thereby affect different intracellular signaling pathways. For example, STa enterotoxins bind and activate membrane-bound guanylate cyclase, which leads to the intracellular accumulation of cyclic GMP and downstream effects on several signaling pathways. These events lead to the loss of electrolytes and water from intestinal cells.

Type II: membrane damaging

Membrane-damaging toxins exhibit hemolysin or cytolysin activity in vitro. However, induction of cell lysis may not be the primary function of the toxins during infection. At low concentrations of toxin, more subtle effects such as modulation of host cell signal transduction may be observed in the absence of cell lysis. Membrane-damaging toxins can be divided into two categories, the channel-forming toxins and toxins that function as enzymes that act on the membrane.

Channel-forming toxins

Most channel-forming toxins, which form pores in the target cell membrane, can be classified into two families: the cholesterol-dependent toxins and the RTX toxins.

  • Cholesterol-dependent cytolysins

Formation of pores by cholesterol-dependent cytolysins (CDC) requires the presence of cholesterol in the target cell. The size of the pores formed by members of this family is extremely large: 25-30 nm in diameter. All CDCs are secreted by the type II secretion system; [4] the exception is pneumolysin, which is released from the cytoplasm of Streptococcus pneumoniae when the bacteria lyse.

The CDCs Streptococcus pneumoniae Pneumolysin, Clostridium perfringens perfringolysin O, and Listeria monocytogenes listeriolysin O cause specific modifications of histones in the host cell nucleus, resulting in down-regulation of several genes that encode proteins involved in the inflammatory response. [5] Histone modification does not involve the pore-forming activity of the CDCs.

  • RTX toxins

RTX toxins can be identified by the presence of a specific tandemly repeated nine-amino acid residue sequence in the protein. The prototype member of the RTX toxin family is haemolysin A (HlyA) of E. coli.[ citation needed ] RTX is also found in Legionella pneumophila . [6]

Enzymatically active toxins

One example is the α toxin of C. perfringens, which causes gas gangrene; α toxin has phospholipase activity.

Type III: intracellular

Type III exotoxins can be classified by their mode of entry into the cell, or by their mechanism once inside.

By mode of entry

Intracellular toxins must be able to gain access to the cytoplasm of the target cell to exert their effects.

  • Some bacteria deliver toxins directly from their cytoplasm to the cytoplasm of the target cell through a needle-like structure. The effector proteins injected by the type III secretion apparatus of Yersinia into target cells are one example.
  • Another group of intracellular toxins is the AB toxins. The 'B'-subunit (binding) attaches to target regions on cell membranes, the 'A'-subunit (active) enters through the membrane and possesses enzymatic function that affects internal cellular bio-mechanisms. A common example of this A-subunit activity is called ADP-ribosylation in which the A-subunit catalyzes the addition of an ADP-ribose group onto specific residues on a protein. The structure of these toxins allows for the development of specific vaccines and treatments. Certain compounds can be attached to the B unit, which is not, in general, harmful, which the body learns to recognize, and which elicits an immune response. This allows the body to detect the harmful toxin if it is encountered later, and to eliminate it before it can cause harm to the host. Toxins of this type include cholera toxin, pertussis toxin, Shiga toxin and heat-labile enterotoxin from E. coli.

By mechanism

Once in the cell, many of the exotoxins act at the eukaryotic ribosomes (especially 60S), as protein synthesis inhibitors. (Ribosome structure is one of the most important differences between eukaryotes and prokaryotes, and, in a sense, these exotoxins are the bacterial equivalent of antibiotics such as clindamycin.)

Other intracellular toxins do not directly inhibit protein synthesis.

  • For example, Cholera toxin ADP-ribosylates, thereby activating tissue adenylate cyclase to increase the concentration of cAMP, which causes the movement of massive amounts of fluid and electrolytes from the lining of the small intestine and results in life-threatening diarrhea.
  • Another example is Pertussis toxin.

Extracellular matrix damage

These "toxins" allow the further spread of bacteria and, as a consequence, deeper tissue infections. Examples are hyaluronidase and collagenase. These molecules, however, are enzymes that are secreted by a variety of organisms and are not usually considered toxins. They are often referred to as virulence factors, since they allow the organisms to move deeper into the hosts tissues. [7]

Medical applications

Vaccinations

Exotoxins have been used to produce vaccines. This process involves inactivating the toxin, creating a toxoid that does not induce toxin-related illness and is well tolerated. [8] A widely used toxoid vaccine is the DPT vaccine, which is usually administered in multiple doses throughout childhood with adjuvants and boosters for long-term immunity. [8] DPT vaccine protects against pertussis, tetanus and diphtheria infections, caused by the exotoxin-producing Bordetella pertussis , Clostridium tetani and Corynebacterium diphtheriae respectively. [9] Vaccination with the toxoids generates antibodies against the exotoxins, forming immunological memory as protection against subsequent infections. [8] [10] The DPT vaccination may cause adverse side effects, such as swelling, redness and fever, and is contraindicated in some populations. [8] Effective vaccination schedules have reduced rates of mortality linked to pertussis, tetanus and diphtheria but formal controlled trials to test the efficacy of the vaccine have not been conducted. [8] Additionally, pertussis persists endemically [9] and is one of the most common causes of vaccine-preventable deaths. [10]

Cancer treatment

As exotoxins are highly potent, there has been development in their application to cancer treatment. Cancer cells can be eliminated without destroying normal cells like in chemotherapy or radiation by attaching an antibody or receptor ligand to the exotoxin, creating a recombinant toxin that is targeted to certain cells. [11] The cancer cell is killed once the toxin is internalized; [11] for example, Pseudomonas exotoxin disrupts protein synthesis after cellular uptake. [12] Multiple versions of recombinant exotoxin A, secreted by Pseudomonas aeruginosa , have entered clinical trials against tumor growth but have yet to be approved by Food and Drug Administration (FDA). [12] A recombinant diphtheria exotoxin has been approved by the FDA for treatment of cutaneous T-cell lymphoma, an immune system cancer. [12] Further testing to improve clinical efficacy of treatment using recombinant exotoxins continues. [11]

See also

Related Research Articles

<span class="mw-page-title-main">Tetanus</span> Bacterial infection characterized by muscle spasms

Tetanus, also known as lockjaw, is a bacterial infection caused by Clostridium tetani and characterized by muscle spasms. In the most common type, the spasms begin in the jaw, and then progress to the rest of the body. Each spasm usually lasts for a few minutes. Spasms occur frequently for three to four weeks. Some spasms may be severe enough to fracture bones. Other symptoms of tetanus may include fever, sweating, headache, trouble swallowing, high blood pressure, and a fast heart rate. Onset of symptoms is typically 3 to 21 days following infection. Recovery may take months, but about 10% of cases prove to be fatal.

<span class="mw-page-title-main">DPT vaccine</span> Combination vaccine

The DPT vaccine or DTP vaccine is a class of combination vaccines against three infectious diseases in humans: diphtheria, pertussis, and tetanus. The vaccine components include diphtheria and tetanus toxoids and either killed whole cells of the bacterium that causes pertussis or pertussis antigens. The term toxoid refers to vaccines which use an inactivated toxin produced by the pathogen which they are targeted against to generate an immune response. In this way, the toxoid vaccine generates an immune response which is targeted against the toxin which is produced by the pathogen and causes disease, rather than a vaccine which is targeted against the pathogen itself. The whole cells or antigens will be depicted as either "DTwP" or "DTaP", where the lower-case "w" indicates whole-cell inactivated pertussis and the lower-case "a" stands for "acellular". In comparison to alternative vaccine types, such as live attenuated vaccines, the DTP vaccine does not contain any live pathogen, but rather uses inactivated toxoid to generate an immune response; therefore, there is not a risk of use in populations that are immune compromised since there is not any known risk of causing the disease itself. As a result, the DTP vaccine is considered a safe vaccine to use in anyone and it generates a much more targeted immune response specific for the pathogen of interest.

<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">Toxoid</span>

A toxoid is an inactivated toxin whose toxicity has been suppressed either by chemical (formalin) or heat treatment, while other properties, typically immunogenicity, are maintained. Toxins are secreted by bacteria, whereas toxoids are altered form of toxins; toxoids are not secreted by bacteria. Thus, when used during vaccination, an immune response is mounted and immunological memory is formed against the molecular markers of the toxoid without resulting in toxin-induced illness. Such a preparation is also known as an anatoxin. There are toxoids for prevention of diphtheria, tetanus and botulism.

Virulence factors are cellular structures, molecules and regulatory systems that enable microbial pathogens to achieve the following:

Adenylate cyclase toxin is a virulence factor produced by some members of the genus Bordetella. Together with the pertussis toxin it is the most important virulence factor of the causative agent of whooping cough, Bordetella pertussis. Bordetella bronchiseptica and Bordetella parapertussis, also able to cause pertussis-like symptoms, also produce adenylate cyclase toxin. It is a toxin secreted by the bacteria to influence the host immune system.

Tetanolysin is a toxin produced by Clostridium tetani bacteria. Its function is unknown, but it is believed to contribute to the pathogenesis of tetanus. The other C. tetani toxin, tetanospasmin, is more definitively linked to tetanus. It is sensitive to oxygen.

Cytolysin refers to the substance secreted by microorganisms, plants or animals that is specifically toxic to individual cells, in many cases causing their dissolution through lysis. Cytolysins that have a specific action for certain cells are named accordingly. For instance, the cytolysins responsible for the destruction of red blood cells, thereby liberating hemoglobins, are named hemolysins, and so on. Cytolysins may be involved in immunity as well as in venoms.

<span class="mw-page-title-main">Pore-forming toxin</span> Protein-produced toxins that create pores in cell membrane

Pore-forming proteins are usually produced by bacteria, and include a number of protein exotoxins but may also be produced by other organisms such as apple snails that produce perivitellin-2 or earthworms, who produce lysenin. They are frequently cytotoxic, as they create unregulated pores in the membrane of targeted cells.

Listeriolysin O (LLO) is a hemolysin produced by the bacterium Listeria monocytogenes, the pathogen responsible for causing listeriosis. The toxin may be considered a virulence factor, since it is crucial for the virulence of L. monocytogenes.

<i>Clostridium tetani</i> Common soil bacterium and the causative agent of tetanus

Clostridium tetani is a common soil bacterium and the causative agent of tetanus. Vegetative cells of Clostridium tetani are usually rod-shaped and up to 2.5 μm long, but they become enlarged and tennis racket- or drumstick-shaped when forming spores. C. tetani spores are extremely hardy and can be found globally in soil or in the gastrointestinal tract of animals. If inoculated into a wound, C. tetani can grow and produce a potent toxin, tetanospasmin, which interferes with motor neurons, causing tetanus. The toxin's action can be prevented with tetanus toxoid vaccines, which are often administered to children worldwide.

Streptolysins are two hemolytic exotoxins from Streptococcus pyogenes. Types include streptolysin O, which is oxygen-labile, and streptolysin S, which is oxygen-stable.

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

The AB toxins are two-component protein complexes secreted by a number of pathogenic bacteria, though there is a pore-forming AB toxin found in the eggs of a snail. They can be classified as Type III toxins because they interfere with internal cell function. They are named AB toxins due to their components: the "A" component is usually the "active" portion, and the "B" component is usually the "binding" portion. The "A" subunit possesses enzyme activity, and is transferred to the host cell following a conformational change in the membrane-bound transport "B" subunit. These proteins consist of two independent polypeptides, which correspond to the A/B subunit moieties. The enzyme component (A) enters the cell through endosomes produced by the oligomeric binding/translocation protein (B), and prevents actin polymerisation through ADP-ribosylation of monomeric G-actin.

<span class="mw-page-title-main">Diphtheria vaccine</span> Vaccine against diphtheria

Diphtheria vaccine is a toxoid vaccine against diphtheria, an illness caused by Corynebacterium diphtheriae. Its use has resulted in a more than 90% decrease in number of cases globally between 1980 and 2000. The first dose is recommended at six weeks of age with two additional doses four weeks apart, after which it is about 95% effective during childhood. Three further doses are recommended during childhood. It is unclear if further doses later in life are needed.

Microbial toxins are toxins produced by micro-organisms, including bacteria, fungi, protozoa, dinoflagellates, and viruses. Many microbial toxins promote infection and disease by directly damaging host tissues and by disabling the immune system. Endotoxins most commonly refer to the lipopolysaccharide (LPS) or lipooligosaccharide (LOS) that are in the outer plasma membrane of Gram-negative bacteria. The botulinum toxin, which is primarily produced by Clostridium botulinum and less frequently by other Clostridium species, is the most toxic substance known in the world. However, microbial toxins also have important uses in medical science and research. Currently, new methods of detecting bacterial toxins are being developed to better isolate and understand these toxins. Potential applications of toxin research include combating microbial virulence, the development of novel anticancer drugs and other medicines, and the use of toxins as tools in neurobiology and cellular biology.

The RTX toxin superfamily is a group of cytolysins and cytotoxins produced by bacteria. There are over 1000 known members with a variety of functions. The RTX family is defined by two common features: characteristic repeats in the toxin protein sequences, and extracellular secretion by the type I secretion systems (T1SS). The name RTX refers to the glycine and aspartate-rich repeats located at the C-terminus of the toxin proteins, which facilitate export by a dedicated T1SS encoded within the rtx operon.

Adenylate cyclase toxin (CyaA) is released from bacterium Bordetella pertussis by the T1SS and released in the host’s respiratory tract in order to suppress its early innate and subsequent adaptive immune defense.

The thiol-activated Cholesterol-dependent Cytolysin(CDC) family is a member of the MACPF superfamily. Cholesterol dependent cytolysins are a family of β-barrel pore-forming exotoxins that are secreted by gram-positive bacteria. CDCs are secreted as water-soluble monomers of 50-70 kDa, that when bound to the target cell, form a circular homo-oligomeric complex containing as many as 40 monomers. Through multiple conformational changes, the β-barrel transmembrane structure is formed and inserted into the target cell membrane. The presence of cholesterol in the target membrane is required for pore formation, though the presence of cholesterol is not required by all CDCs for binding. For example, intermedilysin secreted by Streptococcus intermedius will bind only to target membranes containing a specific protein receptor, independent of the presence of cholesterol, but cholesterol is required by intermedilysin for pore formation. While the lipid environment of cholesterol in the membrane can affect toxin binding, the exact molecular mechanism that cholesterol regulates the cytolytic activity of the CDC is not fully understood.

<span class="mw-page-title-main">Tetanus vaccine</span> Vaccines used to prevent tetanus

Tetanus vaccine, also known as tetanus toxoid (TT), is a toxoid vaccine used to prevent tetanus. During childhood, five doses are recommended, with a sixth given during adolescence.

CRM197 is a non-toxic mutant of diphtheria toxin, currently used as a carrier protein for polysaccharides and haptens to make them immunogenic. There is some dispute about the toxicity of CRM197, with evidence that it is toxic to yeast cells and some mammalian cell lines.

References

  1. ↑ Ryan, Kenneth J.; Ray, C. George, eds. (2010). Sherris medical microbiology (5th ed.). New York: McGraw Hill Medical. ISBN   978-0-07-160402-4.
  2. ↑ Desk Encyclopedia of Microbiology. Amsterdam: Elsevier Academic Press. 2004. p. 428. ISBN   978-0-12-621361-4.
  3. ↑ "Bacterial Pathogenesis: Bacterial Factors that Damage the Host - Producing Exotoxins". Archived from the original on 2010-07-27. Retrieved 2008-12-13.
  4. ↑ Tweten RK (October 2005). "Cholesterol-dependent cytolysins, a family of versatile pore-forming toxins". Infection and Immunity. 73 (10): 6199–209. doi:10.1128/IAI.73.10.6199-6209.2005. PMC   1230961 . PMID   16177291.
  5. ↑ Hamon MA, Batsché E, Régnault B, Tham TN, Seveau S, Muchardt C, Cossart P (August 2007). "Histone modifications induced by a family of bacterial toxins". Proceedings of the National Academy of Sciences of the United States of America. 104 (33): 13467–72. Bibcode:2007PNAS..10413467H. doi: 10.1073/pnas.0702729104 . PMC   1948930 . PMID   17675409.
  6. ↑ D'Auria G, Jiménez N, Peris-Bondia F, Pelaz C, Latorre A, Moya A (January 2008). "Virulence factor rtx in Legionella pneumophila, evidence suggesting it is a modular multifunctional protein". BMC Genomics. 9: 14. doi: 10.1186/1471-2164-9-14 . PMC   2257941 . PMID   18194518.
  7. ↑ Machunis-Masuoka E, Bauman RW, Tizard IR (2004). Microbiology . San Francisco: Pearson/Benjamin Cummings. ISBN   978-0-8053-7590-9.
  8. 1 2 3 4 5 Scott LJ, McCormack PL (February 2013). "Reduced-antigen, combined diphtheria, tetanus, and acellular pertussis vaccine, adsorbed (boostrix(®)): a guide to its use as a single-dose booster immunization against pertussis". BioDrugs. 27 (1): 75–81. doi:10.1007/s40259-012-0009-y. PMID   23329401. S2CID   18873223.
  9. 1 2 Zarei S, Jeddi-Tehrani M, Akhondi MM, Zeraati H, Pourheidari F, Ostadkarampour M, Tavangar B, Shokri F (June 2009). "Primary immunization with a triple diphtheria-tetanus-whole cell pertussis vaccine in Iranian infants: an analysis of antibody response". Iranian Journal of Allergy, Asthma, and Immunology. 8 (2): 85–93. PMID   19671937.
  10. 1 2 Thierry-Carstensen B, Jordan K, Uhlving HH, Dalby T, Sørensen C, Jensen AM, Heilmann C (August 2012). "A randomised, double-blind, non-inferiority clinical trial on the safety and immunogenicity of a tetanus, diphtheria and monocomponent acellular pertussis (TdaP) vaccine in comparison to a tetanus and diphtheria (Td) vaccine when given as booster vaccinations to healthy adults". Vaccine. 30 (37): 5464–71. doi:10.1016/j.vaccine.2012.06.073. PMID   22776216.
  11. 1 2 3 Kreitman RJ (October 1999). "Immunotoxins in cancer therapy". Current Opinion in Immunology. 11 (5): 570–8. doi:10.1016/s0952-7915(99)00005-9. PMID   10508704.
  12. 1 2 3 Weldon JE, Pastan I (December 2011). "A guide to taming a toxin--recombinant immunotoxins constructed from Pseudomonas exotoxin A for the treatment of cancer". The FEBS Journal. 278 (23): 4683–700. doi:10.1111/j.1742-4658.2011.08182.x. PMC   3179548 . PMID   21585657.
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