The results of a study headed by researchers at NYU Grossman School of Medicine may explain how norovirus, a common infection that causes vomiting and diarrhea, can trigger disease onset in patients with Crohn’s disease. The research findings suggest that norovirus infection inhibits production of a protein called apoptosis inhibitor five (API5) by a subset of immune system intraepithelial lymphocytes, γδ IELs. AP15 inhibition leads to the death of intestinal epithelial cells (IECs) known as Paneth cells, lining the gut, which enhances susceptibility to intestinal injury. Experiments both in mice that are genetically susceptible to Crohn’s disease, and in human organoids, showed that administering recombinant AP15 protected the Paneth cells and increased mouse survival.

Gastroenterologist Yu Matsuzawa-Ishimoto, MD, PhD, a postdoctoral research fellow at NYU Langone Health, notes that current Crohn’s disease therapies that work by suppressing the immune system put patients at high risk for infection and often become less effective after a few years of use. A treatment approach targeting API5, he suggests, might avert those issues. “Our findings offer new insight into the key role that apoptosis inhibitor five plays in Crohn’s disease,” Matsuzawa-Ishimoto noted. “This molecule may provide a new target for treating this chronic autoimmune illness, which has proven difficult to manage over the long term.”

Matsuzawa-Ishimoto and colleagues reported on their studies in Nature, in a paper titled “The γδ IEL effector API5 masks genetic susceptibility to Paneth cell death.”

Crohn’s disease is an inflammatory bowel disease (IBD), whereby the body’s immune defenses, which are meant to attack invading microbes, mistakenly target the body’s own digestive tract. Disruption of the intestinal epithelial barrier is a hallmark of Crohn’s disease, which often involves debilitating gastrointestinal symptoms and complications, the authors stated. “Although immunosuppressive therapies such as antibodies targeting TNF can be effective, initial treatment fails in approximately 40% of patients and initial responders frequently experience relapse.”

Norovirus, a common infection that causes vomiting and diarrhea, is one of several viruses and bacteria thought to trigger disease onset in Crohn’s patients, but scientists haven’t understood how this happens.

One clue emerged when past studies found that a certain mutation, ATG16L1, is present in most patients with Crohn’s disease. This mutation makes gut lining cells more vulnerable to damage. The mystery deepened again, however, when it was learned that half of all Americans have this same risk-conferring genetic mutation, but fewer than a half-million develop Crohn’s. There have been clues, however. “Investigation of ATG16L1, an autophagy gene linked to Crohn’s disease susceptibility, has supported a link between Paneth cells and intestinal inflammation,” the team further commented. Paneth cells are a particular type of intestinal epithelial cells.

The newly reported work in mice and in human tissue revealed for the first time that in healthy individuals, the γδ IEL cell subset secretes API5, which signals the immune system to halt the attack on gut lining cells. “Using an ex vivo lymphocyte–epithelium co-culture system, we identified apoptosis inhibitor 5 (API5) as a Paneth cell-protective factor secreted by γδ IELs,” the investigators wrote. This protein adds an extra layer of protection against immune damage, so even those with the ATG16L1 mutation can have a healthy gut. However, the researchers then found that norovirus infection blocks T cell secretion of API5 in mice bred to have a rodent form of Crohn’s disease, killing gut-lining cells in the process. “In the Atg16l1-mutant mouse model, viral infection induced a loss of Paneth cells and enhanced susceptibility to intestinal injury by inhibiting the secretion of API5 from γδ IELs,” they stated. The work supports the theory that API5 protects most people with the mutation against the disease until a second trigger, such as norovirus infection, pushes some across the disease threshold.

In one set of experiments, the investigators created organ-like structures out of tissue collected from humans who tested positive for the mutation. Notably, these structures were made only of gut lining cells. The investigators administered API5 into these “mini guts” and found that the treatment protected the gut-lining cells. Administering API5-producing T cells also protected the gut lining.

In experiments centered on mice genetically modified to have the mutation linked to Crohn’s disease in humans, animals that received an injection of API5 survived, while half of the untreated group died. This confirmed the idea that the AP15 protein protects gut cells, the authors suggested. Human tissue studies then found that individuals with Crohn’s disease had between fivefold and tenfold fewer API5-producing T cells in their gut tissue than those without the illness. “Therapeutic administration of recombinant API5 protected Paneth cells in vivo in mice and ex vivo in human organoids with the ATG16L1 risk allele,” they stated.

Commenting on their collective results, the team noted, “Inhibiting γδ IELs or API5 in Atg16l1-mutant mice caused a reduction in the number of Paneth cells and exacerbated intestinal injury, providing a remarkable example of how a lymphocyte subset can mask genetic susceptibility to inflammatory disease Thus, we identify API5 as a protective γδ IEL effector that masks genetic susceptibility to Paneth cell death.”

“The results of our investigation help explain why the genetic links to Crohn’s are much broader than the actual number of people who have the disease,” said study co-senior author and biochemist Shohei Koide, PhD. Koide is a professor in the Department of Biochemistry and Molecular Pharmacology at NYU Langone and is a member of its Perlmutter Cancer Center.

“Our study suggests that when norovirus infects those with a weakened ability to produce apoptosis inhibitor five, it tips the balance toward a full-blown autoimmune disease,” added study co-senior author and microbiologist Ken Cadwell, PhD, the Recanati Family Professor of Microbiology at NYU Langone.

Cadwell cautions that while the study authors derived API5 protein from human tissue rather than rodents, it remains unclear whether the injection treatment can be safely administered in humans. They next plans to explore the long term effects of API5 injections to better understand whether the prospective treatment can effectively manage Crohn’s disease, which can flare up repeatedly over a long period. The team further noted, “If Paneth cell abnormalities contribute to sustaining inflammation, combining anti-inflammatory agents with therapeutic strategies that restore the protective function of IELs, such as API5 administration, may be particularly effective for reversing the course of disease.”