Gut cells sound the alarm when parasites invade

To effectively fight an infection, the body must first feel that it has been invaded, and then the affected tissue must send signals to gather resources to fight the intruder. Knowing more about these early stages of pathogen recognition and response can provide scientists with crucial clues when it comes to preventing infections or treating inflammatory diseases resulting from overactive immunity.

That was the intention behind a new study, led by researchers at the University of Pennsylvania School of Veterinary Medicine, which examined infection by the Cryptosporidium parasite. When the team looked for the first signs of “danger” emitted by a host infected with the parasite, they tracked them not to an immune cell, as expected, but to the epithelial cells that line the intestines, where Cryptosporidium settles during a infection. Known as enterocytes, these cells absorb nutrients from the intestine and here have been shown to alert the body to danger through the molecular receptor NLRP6, which is a component of what is known as inflammasome.

“You can think of inflammasome as an alarm system in a home,” says Boris Striepen, professor in the Department of Pathobiology at Penn Vet and senior author of the article, which is being published in the newspaper Proceedings of the National Academy of Sciences. “It has several components – like a camera that monitors the door and sensors on the windows – and, once triggered, it amplifies the first signals to warn of danger and send a help request. The cells also have these different components, and we provide perhaps the clearest example of how a specific receptor in the gut is acting as a sensor for a major intestinal infection. “

Normally, says Striepen, researchers have focused on immune cells, such as macrophages and dendritic cells, as being the first to detect foreign invaders, but this new finding highlights that cells normally not considered part of the immune system – in this case intestinal epithelial cells – are playing important roles in how an immune response is initiated.

“There is a growing body of literature that is really evaluating what epithelial cells are doing to help the immune system detect pathogens,” says Adam Sateriale, first author of the postdoctoral paper in Striepen’s laboratory and now directing his own laboratory at the Francis Crick Institute in London. “They seem to be the first line of defense against infections.”

Striepen’s laboratory has devoted considerable attention to Cryptosporidium, which is one of the leading causes of diarrheal diseases that can be fatal in young children in low-resource areas worldwide. Cryptosporidium is also a threat to people in well-resourced environments, causing half of all waterborne disease outbreaks in the United States. In veterinary medicine, it is known to infect calves, slowing their growth. These infections have no effective treatment and no vaccine.

In the current work, Striepen, Sateriale and colleagues took advantage of a natural mouse species Cryptosporidium that they recently discovered that mimics human infection in many ways. Although the researchers knew that T cells help control the parasite in later stages of the infection, they started looking for clues as to what happens first.

An important clue is the unfortunate link between malnutrition and infection by Cryptosporidium. Early infection with Cryptosporidium and the inflammation of the intestine that accompanies it predisposes children to malnutrition and stunted growth; at the same time, malnourished children are more susceptible to infection. This can lead to a downward spiral, putting children at greater risk of fatal infections. The mechanisms behind this phenomenon are not well understood.

“This led us to think that perhaps some of the danger-detection mechanisms that can lead to inflammation in the intestine also play a role in the broader context of this infection,” adds Striepen.

Together, these links inspired the research team to take a closer look at the inflammasome and its impact on the course of infection in their mouse model. They did this by removing a key component of the inflammasome, an enzyme called caspase-1. “It turns out that animals that did not receive it had much higher levels of infection,” said Sateriale.

Additional work has shown that mice without caspase-1 only in intestinal epithelial cells suffered infections as high as those without caspase-1, demonstrating the crucial role of the epithelial cell.

Consistent with this idea, the team led by Penn Vet showed that, of a variety of candidate receptors, only the loss of the NLRP6 receptor led to a failure to control the infection. NLRP6 is a receptor restricted to epithelial barriers previously linked to the detection and maintenance of the intestinal microbiome, a bacterium that naturally colonizes the intestine. However, experiments revealed that mice never exposed to bacteria and therefore did not have a microbiome, also activated their inflammasome after infection with Cryptosporidium – a sign that this aspect of danger signaling occurs in direct response to the parasite and independent infection of the bacterial community of the intestine.

To track how the triggering of the intestinal inflammasome led to an effective response, the researchers looked at some of the signaling molecules, or cytokines, normally associated with the activation of the inflammasome. They found that the infection leads to the release of IL-18, with animals that do not have this cytokine or the ability to release it with more severe infection.

“And when you add IL-18, you can rescue these mice,” said Sateriale, almost reversing the effects of the infection.

Striepen, Sateriale and colleagues believe there is much more work to be done to find a vaccine against Cryptosporidium. But they say their findings help to illuminate important aspects of the interaction between the parasite, the immune system and the inflammatory response, relationships that can inform these translational goals.

Moving on, they are looking to the later stages of Cryptosporidium infection to see how the host successfully controls it. “Now that we understand how the infection is detected, we would like to understand the mechanisms by which it is controlled,” said Sateriale. “After the system detects a parasite, what is done to restrict its growth and kill it?”