French researchers have found out more about how the foodborne pathogen Listeria monocytogenes gets into the brain.

Listeria monocytogenes is responsible for listeriosis, a severe foodborne illness that can lead to a central nervous system infection called neurolisteriosis. This infection is fatal in 30 percent of cases, said researchers.

Scientists have discovered how cells infected with Listeria monocytogenes escape immune responses. A mechanism provides infected cells circulating in the blood with a higher probability of adhering to and infecting certain cells, enabling bacteria to cross a barrier and infect the brain.

The central nervous system is separated from the bloodstream by what is known as the blood-brain barrier. But some pathogens manage to cross it and are able to infect the central nervous system.

Infections of the central nervous system are among the most serious but how pathogens access the brain is not well understood.

Experts from the Institut Pasteur, Université Paris Cité, Inserm and the Paris Public Hospital Network have shed light on the issue with a study published in the journal Nature.

Identifying and understanding the escape mechanisms of infected cells could help development of new therapeutic strategies to prevent infection.

Tricks used by Listeria
Scientists from the Biology of Infection Unit at Institut Pasteur and the Listeria National Reference Center and the World Health Organization’s Collaborating Center led by Marc Lecuit revealed how Listeria monocytogenes infects the central nervous system.

They then developed an experimental model using mice that reproduces the different stages of human listeriosis, and involved virulent strains of Listeria isolated from patients with neurolisteriosis.

“We discovered a specific, unexpected mechanism by which a pathogen increases the life span of the cells it infects by specifically blocking an immune system function that is crucial for controlling infection,” said Lecuit.

The team observed that inflammatory monocytes, a type of white blood cell, are infected by the bacteria. These infected monocytes circulate in the bloodstream and adhere to the cerebral vessels’ cells, allowing Listeria to infect the brain tissue.

They showed that InIB, a Listeria monocytogenes surface protein, enables the bacteria to evade the immune system and survive in the protective niche provided by the infected monocytes. Interaction between InlB and its cellular receptor c-Met blocks cell death by cytotoxic T lymphocytes, which specifically target Listeria-infected cells.

This extends the life of infected cells, raising the number of infected monocytes in the blood and facilitating bacterial spread to host tissues, including the brain.

Scientists said it is possible that other intracellular pathogens such as Toxoplasma gondii and Mycobacterium tuberculosis use similar mechanisms to infect the brain.

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