Is it a Bird? Is it a Plane? No...its Superantigens

The human body is a great niche for bacteria. Nutrients run through our blood vessels, and soak our cells. Within the gut, on the skin, and in the nasal tract, many bacteria make a home. And for the most part, we tolerate their presence. Some bacteria are even useful to us. However, there are some bacteria who are not friendly tenants. These bacteria try to invade the body. However, this in itself is no easy task. The body is defended by the immune system, a complex and organized collection of different cells dedicated to fighting off bodily intruders.

 If bacteria are to successfully invade the body, they need to overcome this formidable defence. Many bacteria do this by producing virulence factors. These attack the cells of the immune system, and more importantly, they can interfere with their lines of communication.  Antigen presentation is one form of communication that is important to immune defence, and is a ripe target for attack by bacteria.

 What is Antigen Presentation ?
An antigen is sort of like a mugshot for the immune system to use to identify intruders. There are a variety of phagocytic cells which often patrol the body. Whenever the body is damaged, these cells eat up the damaged bits, and along with that, any infectious bacteria that have colonised the site of damage.
They then present them to T-cells. T-cells are basically in charge of the immune system, and they ultimately decide whether to go on the attack, and how that attack should proceed. After phagocytes have mopped up the debris, they chop it up into small manageable chunks which are bound to a molecule called MHC, and presented to T-cells. These small chunks are referred to as antigens.

 Usually, they come from normal cells from the body. If however, there were bacteria present, then the phagocytes will have eaten a few whilst it was cleaning up. So some of the antigens it presents will come from these bacteria. If the T-cell correctly identifies the bacteria, it can then co-ordinate the immune response against them.

How do bacteria interfere with this process ?

The bacteria can interfere with this process by secreting compounds known as superantigens. These interfere by making all of the antigens presented to the T-Cells look suspicious. This causes the T-cells to marshal the local immune cells against completely harmless cells, and thus ignore the proliferation of a pathogen.

However, if there is too much superantigen present, the T-cells go into overdrive and cause the whole immune system to go into overkill. In an ordinary setting, chemical messages, known as cytokines, can control the behaviour of the cells of the immune system. In this situation, the activated T-cells are secreting a storm of cytokines.

These cytokines causes the body's thermostat, the hypothalamus, to raise the core body temperature. This leads to a fever. These cytokines also cause an immune reaction to the skin, causing rashes and a condition where layers of skin can flake off. As the cytokines spread inflammation throughout the body the symptoms get more severe, leading to death.
 Superantigens have been intensively studied in pathogens such as Staphylococcus aureus and Streptococcus pyogenes, and cause a terrible disease known as toxic shock syndrome. They have also been discovered in bacteria such as Mycoplasma arthritis and  Yersinia pseudotuberculosis.
They are but one of the weapons bacteria use in their fight with the immune system, and an example of when that fight can get out of control, to the detriment of both bacteria and the host.

Fraser, J., & Proft, T. (2008). The bacterial superantigen and superantigen-like proteins Immunological Reviews, 225 (1), 226-243 DOI: 10.1111/j.1600-065X.2008.00681.x