Crohn's disease, a type of chronic inflammatory bowel disease (IBD), often causes scar-like tissue to build up in the intestines, termed fibrosis. Over time, increased scar tissue builds up in the intestinal walls, causing them to narrow and preventing waste from leaving the body. Most patients with Crohn's disease will need to undergo surgery to treat or prevent life-threatening bowel obstructions and infections.
There are few effective treatments for intestinal fibrosis because researchers have struggled to understand how fibrosis occurs in the intestines on the cellular level. Now, post-doctoral researcher Ju-Hyun Ahn, PhD, working with Janelle Arthur, PhD, associate professor at the Department of Microbiology & Immunology and member of the Center for Gastrointestinal Biology and Disease at the UNC School of Medicine, have implicated interactions between gut bacteria and host cells in causing Crohn's disease-related intestinal fibrosis. The findings, which were published in Cell Host & Microbe, could reveal new, targeted treatments for managing Crohn's disease.
Ju-Hyun Ahn, PhD
"Our research reveals that a small molecule called yersiniabactin can alter host intracellular zinc levels, leading to the activation of macrophages that contributes to fibrosis," said Ahn. "Overall, our discovery suggests that monitoring macrophage activation and zinc imbalances could offer new ways to identify patients at higher risk of fibrosis and develop targeted treatments for managing this debilitating condition."
Macrophages are white blood cells that specialize in engulfing, or "eating," other cells and breaking them down. These immune cells, which are "powered" by zinc, play critical roles in removing harmful pathogens and limiting microbe colonization in our guts. But their roles don't stop there: macrophages are also in charge of activating specialized tissue-producing cells, called fibroblasts, to patch up wounds in the body.
When macrophages are actively fighting infections or responding to inflammation (as is the case with IBD), they activate a protein called hypoxia-inducible factor 1-alpha, or HIF-1α, to help them perform their duties more effectively. HIF-1α+ macrophages can become dysregulated, however, causing their fibroblast-activating signals to get stuck in the "on" position. The excessive activation of fibroblasts results in abnormal tissue growth at sites of injury or inflammation and creates the perfect conditions for fibrosis to occur.
Researchers in the Arthur lab have been tracing the origins and effects of adherent-invasive E. coli, a microbe that naturally colonizes the gut, but can contribute to Crohn's disease in humans when systems go awry. In 2019, the lab found that many adherent-invasive E. coli strains make a small molecule called yersiniabactin and that production of the molecule induces intestinal fibrosis. It was also known that when yersiniabactin is produced, the molecule can steal both iron and zinc from surrounding cells - the very metals that macrophages rely on to function.
(Left) Tissue from a patient with Crohn's disease. (Right) Immunofluorescence staining shows HIF-1α macrophages (red) lining strictures in the intestinal tissue. Credit: Ahn et al. (2024).
Although the lab had already established a link between yersiniabactin and intestinal fibrosis, they wanted to take things one step further: to identify the yersiniabactin-bound metal and see if there was a link between this metal loss and macrophage disruption. Using the gene editing system CRISPR-Cas9 and other methods, researchers found that adherent-invasive E. coli -produced yersiniabactin does indeed steal zinc from macrophages. This ultimately disrupts the macrophages and activates the HIF-1α pathway. Pro-fibrotic genes are then expressed in both the HIF-1α+ macrophages and nearby fibroblasts, ultimately putting them in an activated state known to drive fibrosis.
Ahn and Arthur wondered if these HIF-1α+ macrophages could be localized to sites of fibrosis in IBD patients. Using a fluorescent dye to mark macrophages expressing HIF-1α, they observed under abundant HIF-1α+ macrophages from bowel tissue surgically removed from IBD patients to treat fibrotic complications and obstruction. Researchers then confirmed their findings in their mouse model. HIF-1α+ macrophages were, in fact, highly abundant in sites of inflammation-associated intestinal fibrosis, but could not be found elsewhere in nearby regions of the intestine.
Janelle Arthur, PhD
"We were able to show that HIF-1α+ macrophages are localized to the sites of disease," said Arthur, who is also a member of and the UNC Lineberger Comprehensive Cancer Center. "This really underscores the critical role of gut bacteria in modulating the availability of nutrient metals to promote fibrosis through macrophage-fibroblast interactions during periods of inflammation."
In light of the recent findings, researchers in the Arthur lab will evaluate if the presence of HIF-1α+ macrophages could be used to screen future Crohn's disease patient biopsies for fibrosis. The lab also plans to examine how certain features of the microbiome, such as its ability to produce yersiniabactin and/or steal zinc, could be used to identify patients at risk of developing fibrosis.
However, more research is needed to better understand how HIF-1α+ macrophages contribute to fibrosis. Ahn will continue to pursue questions about pro-fibrotic macrophages in molecular studies and additional mouse models after receiving postdoctoral fellowship funding by the Crohn's and Colitis Foundation.
