Lots of deep-sea microbes that are invisible to the mammalian immune system

ÖThe immune system of mammals defends itself against known and novel pathogens by picking up common microbial characteristics such as elements of bacterial cell walls or flagella. But a study published today (March 12th) in Scientific immunology indicates that this detection system has some blind spots. The authors found that the majority of microbes collected from the depths of the Pacific Ocean, despite the presence of the well-known bacterial cell wall component lipopolysaccharide (LPS), are invisible to mammalian immune cells.

This work “implies that a pathogen has much more potential to evade our immune responses than we previously thought,” says Christopher LaRock, an immunologist at Emory University School of Medicine who was not involved in the study. The bacterium Yersinia pestisthat causes the plague can modify its LPS and avoid immune recognition, he adds. “We thought things that weren’t discovered were really clever pathogens that figured it all out. . . But here are some bacteria that have never seen a human before and can still escape. “

According to Jonathan Kagan, an immunologist at Boston Children’s Hospital, a classic tenet in his field is that “the immune system recognizes every single microbe we may ever encounter, just in case it is contagious.” But what about those microbes that humans would not sensibly encounter, such as those deep below the surface of the ocean? Kagan, marine ecologist Randi Rotjan of Boston University, and Anna Gauthier, a PhD student at Harvard University who shares time between the Kagan and Rotjan laboratories, set out to find an answer.

The Phoenix Islands Sanctuary in Kiribati is a World Heritage Site of the United Nations Education, Science and Culture.


In 2017 Rotjan and Gauthier spent almost three weeks on board the research vessel of the Schmidt Ocean Institute Falkor in the Phoenix Islands Sanctuary in Kiribati, a country in the central Pacific. They used a remote-controlled diving robot to collect water, coral, sponge, starfish, and sediment samples from various depths of the sea, and then attempted to grow the bacteria found in these samples under various conditions. In the end, they had 117 species of cultivable bacteria that they brought back to Boston.

The researchers found that all of the bacteria were gram negative, which means they had LPS in their cell walls and most were in the genus Mortitella. When the team exposed 50 of the strains to mouse and human macrophages, 80 percent of these microbes escaped detection by mammalian LPS receptors, indicating that the immune system can only take in a fraction of the new bacterial species.

“It was a complete surprise, but looking back it kind of makes sense,” says Rotjan. “There really isn’t any mammalian tissue that these bacteria can interact with, so there’s no pressure for them to evolve or evolve together or really have an evolutionary interaction with us.”

“If you’ve never seen a microbe before, you may not have developed a pattern-recognition system for it,” agrees Victor Nizet, who is a non-contributor to bacterial pathogenesis at the University of California at San Diego. It indicates that “pattern recognition is more of a local or regional phenomenon related to your own environment. . . and not in all possible environments. ”

To determine whether the outer surface of the bacteria had a component that made it easier to escape the detection, the team only exposed mouse and human cells to the LPS isolated from several bacterial strains. The cells were also blind to the LPS alone, possibly due to longer side chains on the lipid portion of the molecule.

“One wonders why they have these longer chains. The question is, do they have an evolutionary advantage in terms of fitness and survival, ”says Sivapriya Kailasan Vanaja, an immunologist at the University of Connecticut who did not participate in the study. Investigating whether there are organisms in the deep sea that can infect the bacteria and whether these hosts have receptors that can sense this type of LPS is a next step, she says The scientist.

AE Gauthier et al., “Deep-Sea Microbes as Tools to Redefine the Rules for Recognizing Innate Immune Patterns”. Sci Immunol, doi: 10.1126 / sciimmunol.abe0531, 2021.

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