New research by scientists at Harvard Medical School has found that nerves in the gastrointestinal (GI) tracts of mice can actively protect against infection by pathogenic Salmonella bacteria. The studies, published in Cell, demonstrated that sensory neurons called nociceptors don’t just detect the presence of Salmonella, but also deploy two lines of defense that directly interfere with the bacterium’s ability to infect the intestines. “Our results show the nervous system is not just a simple sensor-and-alert system,” said study lead Isaac Chiu, PhD, a neuro-immunologist and assistant professor of immunology in the Blavatnik Institute at Harvard Medical School. “We have found that nerve cells in the gut go above and beyond. They regulate gut immunity, maintain gut homeostasis, and provide active protection against infection.”
The team’s findings hint that manipulating neurons in the gut, or their mediators, could springboard new therapeutic approaches to inflammatory and infectious diseases. Chiu and colleagues describe their findings in a paper titled, “Gut-Innervating Nociceptor Neurons Regulate Peyer’s Patch Microfold Cells and SFB Levels to Mediate Salmonella Host Defense.”
Nociceptors are specialized pain-sensing neurons that detect harmful stimuli, including heat, noxious chemicals, and inflammatory mediators. “Nociceptors can directly sense bacteria and fungal products to elicit pain and regulate inflammation,” the team wrote. The GI system is particularly rich in sensory nerve cells, and gut-innervating nociceptors are “major sensors of disturbances in the GI tract and initiate protective sensations and neural reflexes, including visceral pain and diarrhea,” the investigators added. What hasn’t been understood is whether these nerve cells also play a role in enteric infections, such as those caused by strains of the potentially fatal food-borne pathogen Salmonella. Pathogenic Salmonella is responsible for a quarter of all bacterial diarrheal diseases worldwide, and causes some 94 million cases of human gastroenteritis and 22 million cases of typhoid fever every year.
The team’s studies were aimed at further investigating the role of nociceptor neurons in defending against the Salmonella enterica serovar Typhimurium (STm), in mice. When STm infects an animal it travels through the GI tract to the small intestine. The body’s healthy gut microbiota can help to limit colonization by pathogens including STm, by directly competing for nutrients, and/or by secreting metabolites or toxins that counteract the invading microorganisms, the authors noted. But if the Salmonella pathogen does take hold it invades the intestinal tissues and then spreads to other parts of the body. It does this primarily through what are known as microfold (M) cells in Peyer’s patches (PPs). Salmonella isn’t the only bacterium that can use M cells as a gateway to infection. “Enteric microbial pathogens including STm, poliovirus, and prions, utilize M cells as entry points to invade and cause disease,” the researchers noted.
Through their experiments in mice, Chui and colleagues discovered that nociceptors embedded in the small intestine and beneath the Peyer’s patches were activated by the presence of STm. Peyer’s patches are clusters of lymphatic and immune tissue found exclusively on the wall of the small intestine. Under normal conditions, they scan the environment, sample substances, and determine what can go into the intestine. To perform this function, Peyer’s patches are studded with M cells, which act as cellular channels that open and close to regulate the influx of substances and microorganisms into the intestine.
To exploit cells as entry points for invading the small intestine, the Salmonella bacterium injects into the gut transcription factors that stimulate intestinal cells to become M cells. The microorganism then latches onto sugars on the surface of the M cells that act as the gates, and effectively holds the gates open to allow its passage into the intestine.
To understand the role of nociceptors in protecting against STm infection, the researchers compared how mice with and without gut nociceptors responded to the bacterium. One group of mice retained intact gut neurons, in another group the nociceptors were genetically disabled or deleted, and yet another cohort had them chemically disabled.
Experiments showed once activated, the nociceptors use two defensive tactics to prevent STm from infecting the intestine and spreading throughout the rest of the body. First, they regulated the M cell gates that allow microorganisms and various substances to go in and out of the small intestine. Second, they boosted the number of protective gut microbes called segmented filamentous bacteria (SFB), which are part of the microbiome in the small intestine.
The results indicated that in the presence of STm, the gut nociceptors fired back by releasing a neurochemical called calcitonin gene-related peptide (CGRP), which slowed down M cell differentiation, and so reduced the number of entry points for the Salmonella. The experiments also demonstrated that this release of CGRP boosted the presence of SFB, one of the beneficial functions of which is to guard against Salmonella invasion. Although it is not known how they do this, Chiu and colleagues suggest that one plausible mechanism may be that SFB uses its tiny hooks to attach itself to the intestinal wall and form a repellent coating that shields against the disease-causing bacteria.
“We found that nociceptors shaped the small intestine microbiota by maintaining levels of SFB attached to PP and villi,” they wrote. “SFB uses holdfasts to attach to epithelial cells. It is plausible that nociceptor regulation of M cells changes the FAE [follicle-associated epithelia] composition to create more space for SFB attachment.”
The researchers’ studies showed that these defense mechanisms functioned reliably in mice with intact gut nociceptors, but not in the nociceptor-deficient animals. Analyzing intestinal biopsies indicated that the Peyer’s patches in mice with inactivated nociceptors were more densely infiltrated by Salmonella, at a greater rate than in animals with intact neurons. The nociceptor-deficient animals also had fewer protective SFB microbes in their GI tract, succumbed to Salmonella infection at a greater rate, and had more widespread disease than mice with intact gastric nociceptors. “We demonstrate that nociceptor neurons protect against STm invasion by regulating gut microbial homeostasis and M cell abundance. Nociceptors suppressed the density of M cells in PPs to limit STm entry points,” the investigators wrote. “Nociceptors also maintained levels of segmentous filamentous bacteria (SFB), which were necessary and sufficient to protect against STm infection.”
“It is becoming increasingly clear that the nervous system interacts directly with infectious organisms in various ways to affect immunity,” Chiu said. “Bacteria literally do get on our nerves.” The findings are in line with past research by the group, which demonstrated a three-way interplay between infection, and the nervous and immune systems. However, in contrast with the newly reported findings, the previous work showed that the nervous system can, at times, be exploited by infectious organisms to their advantage. Chiu’s prior research had found that nerves in the lungs can alter immune response in serious lung infections with the bacterium Staphylococcus aureus. In another study, Chiu’s team discovered that the bacterium that can cause flesh-eating disease hijacks nerves as a way to dampen immune defenses and weaken the body’s defenses.
The new findings add to a growing body of knowledge demonstrating that the nervous system has a repertoire far broader than signaling to and from the brain. “Our findings illustrate an important cross talk between the nervous system and the immune system,” said study first author Nicole Lai, PhD, research fellow in immunology in the Chiu lab. “It is clearly a bidirectional highway with both systems sending messages and influencing each other to regulate protective responses during infection.”
The gut is so richly innervated that it has been referred to as the second brain. And given the speed at which the nervous system can alert the body of potential threats, the new findings suggest that evolution has taken advantage of this feature for added protection. “If you think about it, the nervous system’s involvement in immunity is an evolutionarily smart way to protect the gut from infection by repurposing an existing feature,” Chiu said.
The researchers say their findings could also help to explain previous observations that the use of opioid drugs that silence pain-sensing nerve fibers, or taking other nerve-modulating drugs, can make people more prone to infections. “If you dial down nerve signaling in an effort to reduce pain, you may be inadvertently also dampening their protective abilities,” Chiu said. “Our observations support that idea.”
The interaction between gut neurons and gatekeeping M cells represents a key area for future research because M cells are also used as entry points by other pathogenic organisms, and by prions that can cause fatal neurodegenerative conditions. The newly reported results point to a possible therapeutic pathway that involves modulating nerve signaling either for boosting gut immunity or intestinal inflammation. “These findings show that neuronal crosstalk with M cells and microbes mediate mucosal protection and could be a future target to treat infectious and inflammatory diseases,” the authors concluded.
“The idea would be that if we could somehow stimulate these protective gut neurons or mimic their activity with a drug, we could activate the immune response and increase the body’s ability to fend off infection,” Chiu said.