How the strep bacterium hides from the immune system

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A bacterial pathogen that causes strep throat and other illnesses cloaks itself in fragments of red blood cells to evade detection by the host immune system, according to a study publishing December 3 in the journal Cell Reports. The researchers found that Group A Streptococcus (GAS) produces a previously uncharacterized protein, named S protein, which binds to the red blood cell membrane to avoid being engulfed and destroyed by phagocytic immune cells. By arming GAS with this form of immune camouflage, S protein enhances bacterial virulence and decreases survival in infected mice.
The graphical abstract: pathogen Group A Streptococcus camouflaging as red blood cells. Credit: Dorota Wierzbicki



"Our study describes a completely novel mechanism for immune evasion," says corresponding author David Gonzalez of the University of California, San Diego. "We believe the discovery of this previously overlooked virulence factor, S protein, has broad implications for development of countermeasures against GAS."

GAS is a human-specific pathogen that can cause many different infections, from minor illnesses to very serious and deadly diseases. Some of these conditions include strep throat, scarlet fever, a skin infection called impetigo, toxic shock syndrome, and flesh-eating disease. An estimated 700 million infections occur worldwide each year, resulting in more than half a million deaths. Despite active research, a protective vaccine remains elusive.

To date, penicillin remains a primary drug of choice for combatting GAS infections. But the rate of treatment failures with penicillin has increased to nearly 40% in certain regions of the world. "Due to the high prevalence of GAS infection and the decreasing efficacy of the available set of countermeasures, it is critical to investigate alternative approaches against GAS infection," Gonzalez says.

One alternative approach is to develop novel anti-virulence therapeutics. To avoid immune clearance, GAS expresses a wide variety of molecules called virulence factors to facilitate survival during infection. But the function of many of these proteins remains unknown, hindering the development of alternative pharmacological interventions to combat widespread antibiotic resistance.

To address this gap in knowledge, Gonzalez and co-first authors Igor…
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