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Antibodies, Right-Side-Up?

Updated on January 5, 2013

It Matters!

This post takes a short break from my series on evolution and bioinformatics to report on an exciting finding in the world of bacteriology!

Nearly 700 million annual cases of throat and skin infections are caused by the human bacterial pathogen Streptococcus pyogenes. Many of these are mild and fairly uncomplicated, though there are cases, like the one I’m about to describe, where bacterium invades the blood streams and induces an inflammatory response, often resulting in the death of the patient.

Did you know that this infectious agent also resides in your saliva? It can be a peaceful resident there, causing no symptoms at all. What then, converts this nonviolent colony into an invasive and life-threatening infection?

Researchers at Lund University in Sweden have recently reported that samples from a patient with a serious S. pyogenes infection reveal differences in the orientation of the patient’s antibodies at the surface of the bacteria from different sites in the body.

Whether or not you’re familiar with all of the players in an infection, you’ve probably heard of the Y-shaped proteins called antibodies that are produced by cells of the immune system. They function as part of an alarm system, recognizing and attaching to invaders such as viruses and bacteria, and signaling other cells to destroy the target.

Antibodies are divided into classes, the most common of which is IgG. Each IgG antibody is composed of a fragment crystallizable (Fc) region, and the fragment antigen-binding (Fab) region (the two arms of the Y):

Now, the antibodies must bind in the fragment antigen-binding (Fab) region in order for the immune system to be able to eliminate pathogens. However, many bacteria, including S. pyogenes, have proteins on their surface that interfere with this mechanism, by binding antibodies in the fragment crystallizable (Fc) region.

“By turning the antibodies around so that they are Fc-bound instead the bacteria can avoid detection by immune cells and subsequent killing,” said Pontus Nordenfelt, first author on the paper in which these findings are published.

“For bacteria that are equipped with IgG Fc-binding proteins we show that the orientation of antibodies bound to bacterial surfaces strictly depend on the local concentration of antibodies; we demonstrate this in a human patient as well.”

The study found that in the throat of an infected patient, the IgG antibodies were mostly bound to the bacterial surface via Fc, while in the blood, IgG was mostly bound via Fab.

Further investigation showed that efficient bacterial Fc-binding occurs only in IgG poor environments, such as saliva, where the bacteria are consequently protected from detection by the immune system.

“When there are very high amounts of antibodies, such as in blood, there [are] likely enough antibodies with a higher affinity than the bacterial Fc-binding. This allows the antibodies to bind the correct way in blood,” said Nordenfelt.

Nordenfelt says that the information provided in this study lays out the basic framework for the biological role of these IgG-binding proteins.

“Since these proteins are present in several important bacterial pathogens we hope that this will help future research in bacterial pathogenesis.”

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