How does our immune system distinguish between friendly gut bacteria and deadly pathogens?

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3-9-2019 jr gut microbiome inline
Tiny vesicles emerge from the tip and side of a hook used by a bacterium to latch onto the lining of the gut, as seen in this computer reconstruction superimposed onto a microscope image. The bacterium uses these vesicles to carry antigens (small tan dots at center right) for communication with immune cells. Image credit: M. S. Ladinsky/Science

Observations in mice show that certain filamentous microbes use a hooklike appendage to send messages that researchers believe are aimed at preventing immune cells from attacking the microbes.

The finding, reported in the March 8 Science, could help explain how an immune system distinguishes friendly gut bacteria from deadly pathogens.

Immunologist Ivaylo Ivanov at Columbia University and his colleagues examined segmented filamentous bacteria, a group of gut microbes found in the intestines of many animals including mice, fish and humans. These symbiotic bacteria have a hooklike appendage called a holdfast that attaches them to cells on the gut’s wall.

[T]he vesicles [in the holdfast], like delivery packages, contained the bacteria’s antigens — proteins that immune cells use to recognize a foreign body. Usually, antigens stimulate immune cells to attack and kill an invader. But in this case, although T cells were activated, they didn’t go after the bacteria.

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A T cell’s response may depend on how the antigens are delivered, he suggests. For example, the immune cell may read antigens differently if they are packaged within vesicles released in gut cells rather than exposed on the surface of the invading microbe. “This is just our hypothesis,” Ivanov says.

Read full, original post: How helpful gut microbes send signals that they are friends, not foes

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