The SpyCEP who cleaved me.....

Streptococcus pyogenes (I'll call it strep for short) is one of many pathogenic bacteria which are known to colonise the respiratory tract. as a result it causes the disease known as strep throat, and another known as scarlet fever. If you are lucky, it gets confined to the respiratory tract. However, in some cases Strep can spread to other organs, where it can cause a disease known as necrotizing fasciitis.

How and why it suddenly causes this disease is not known, although there is some evidence that it follows from a  bruise or contusion. Current hypotheses state that somehow the damage enables strep to colonise.

However, in order for it to colonise these damaged sites, it has to be able to go systemic first without alerting the immune system.

To do this, bacteria like strep secrete proteins and other compounds which attack immune cells, and the communication systems immune cells use in order to co-ordinate their response against the bacteria.

I am going to talk about one specific protein secreted by strep called "Streptococcus pyogenes Cell Envelope Protease". For short, we'll call it SpyCEP. And I'll show how this one virulence factor can have an extraordinary effect on the immune system.

It is known that this protein sits on the cell envelope, and that it can actually be sheared off through an as of yet unknown mechanism.

 It is also known that it has an active site which cleaves an interleukin. Interleukins are molecules through which immune cells (often referred to as leukocytes) interact with eachother.  During the initial stages of infection, when normal cells undergo damage due to the presence of pathogen, they can secrete interleukins. These interleukins act as "help me" signals which recruit phagocytic cells which can engulf the bacteria.

These signals are essential, as the immune system comprises many different cells that perform different functions. Phagocytic cells can engulf and eat bacteria. They can then recruit other cells, such as T and B cells, and shows them the proteins from the offending bacteria, and so allowing these specialist cells to recognize them in the future. The communication between these cells is very important and quite subtle. 

So when strep enters a host, the best way for it to ensure its own survival is to disrupt this immune communication, especially the initial signals that summon phagocytic cells. This is where SpyCEP comes in.

As described before, it acts to cleave interleukins. More specifically, it is known to cleave Interleukin 8, which is specifically known to affect the recruitment of  some special phagocytic cells known as neutrophils. In essence, shutting down that initial help signal early on in infection.

Whilst these facts seem to indicate what  SpyCEP does during an infection, demonstrating it's effectiveness during an actual infection has been challenging. One of the ways to demonstrate this is to knock out the gene that codes for SpyCEP. In a recent paper, a group has done this for a strain of strep which is known to produce lots of this protein.

They then injected these bacteria into mice, and then observed how the infection progressed. it's important to note that mice themselves don't quite have a similar immune system to humans. They have two proteins which perform the same role as interleukin 8, known as KC and MIP-2. Fortunately, spyCEP is able to cleave both of these, and so it's role in mouse infection can be said to be similar to that of humans.

What they found when they injected the streptococci that expressed spyCEP and streptococci that didn't express spyCEP, the numbers of phagocytic cells coming to the site of infection was reduced.

They also measured the amounts of working KC in the mouse thigh tissue and serum, and found that it was reduced in the strain that expressed spyCEP compared with the the strain that had it knocked out. In short, the strain which produced spyCEP managed to spread to other tissues better than the strain without it, and also produce a more severe infection.

However, strep produces a whole variety of different virulence factors. And it is possible that the expression of SpyCEP could help the expression of other virulence factors which are the actual effectors in infection. 

To counteract this possible criticism, they took the gene encoding spyCEP, and incorporated it into a "friendly" bacterium. Lactococcus Lactis is a very different bacterium to strep. 

Lactococcus doesn't cause any human disease. It is used in industry to produce buttermilk and cheese. By all accounts, its quite helpful to humans, and completely harmless.

So they placed this virulence gene into lactococcus lactis by adding DNA with the spyCEP gene on it. So this would show the effect of spyCEP without all the other interfering virulence factors expressed by strep. 

It ended up turning Lactococcus from a friendly bacterium, into a quite unfriendly bacterium.

It was found that when this virulent strain was infected into mice, it was no longer the benign bacterium it was before. Instead of being engulfed, consumed and cleared by neutrophils in quick sharp time, the bacteria not only persists, but causes damage to the surrounding tissues in the same way that strep does. In fact, the expression of this one protein allows the lactococcus to spread to other tissues such as the spleen and the liver. [It should be noted that Lactococcus never at any stage produced an infection as severe as Strep, and also the gene itself is unstable]

This is a unique study, in that they demonstrated how the expression of just one protein is necessary to cause a non-pathogenic bacterium into a pathogenic one, and more importantly they were able to demonstrate the importance of this protein as a result.

This shows the importance of spyCEP in infection, and if it's action can be counterracted, it may provide a way of treating this disease.The incorporation of this protein in a vaccine will help alert the immune cells early on in infection, and will allow them to recognise bacteria better, and prevent it from using spyCEP to evade the immune system.

Kurupati, P., Turner, C., Tziona, I., Lawrenson, R., Alam, F., Nohadani, M., Stamp, G., Zinkernagel, A., Nizet, V., Edwards, R., & Sriskandan, S. (2010). Chemokine-cleaving Streptococcus pyogenes protease SpyCEP is necessary and sufficient for bacterial dissemination within soft tissues and the respiratory tract Molecular Microbiology, 76 (6), 1387-1397 DOI: 10.1111/j.1365-2958.2010.07065.x