Why protecting your brain against infection takes guts

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The brain is uniquely protected against invading bacteria and viruses, but its defense mechanism has long remained a mystery.

In a new study, researchers found that the brain has a surprising ally in its protection: the gut.

The research was conducted by scientists at the University of Cambridge, UK, and the National Institute of Health, U.S.

The brain is arguably the most important organ in the body, as it controls most other body systems and enables reasoning, intelligence, and emotion.

Humans have evolved a variety of protective measures to prevent physical damage to the brain: it sits in a solid, bony case—the skull—and is wrapped in three layers of watertight tissue known as the meninges.

What has been less clear is how the body defends the brain from infection.

Elsewhere in the body, if bacteria or viruses enter the bloodstream, our immune system kicks in, with immune cells and antibodies that target and eliminate the invader.

However, the meninges form an impermeable barrier preventing these immune cells from entering the brain.

In the study, the team found that the meninges are home to immune cells known as plasma cells, which secrete antibodies.

These cells are specifically positioned next to large blood vessels running within the meninges allowing them to secrete their antibody ‘guards’ to defend the perimeter of the brain.

When the researchers looked at the specific type of antibody produced by these cells, they got a surprise—the antibody they found is normally the type found in the intestine.

The team was able to sequence the antibody genes and plasma cells in the gut and meninges and show that they were related.

In other words, the cells that end up in the meninges are those that have been selectively expanded in the gut, where they have recognized particular pathogens.

The team says even a minor breach of the intestinal barrier will allow bugs to enter the bloodstream, with devastating consequences if they’re able to spread into the brain.

Seeding the meninges with antibody-producing cells that are selected to recognize gut microbes ensures defense against the most likely invaders.

The team also found that this defense system is likely to play an important role in defending humans from infections of the central nervous system—meningitis and encephalitis.

One author of the study is Professor Menna Clatworthy from the Department of Medicine.

The study is published in Nature.

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