In a recent article published in Cell Death & Disease, researchers investigated the involvement of gut microbiota and macrophage-neutrophil interactions in the impact of dietary phenolic acids on intestinal inflammation in the context of inflammatory bowel disease (IBD). 

Specifically, they examined the effects of four phenolic acids, chlorogenic acid (CGA), ferulic acid (FA), caffeic acid (CA), and ellagic acid (EA).

Study: Polyphenol-rich diet mediates interplay between macrophage-neutrophil and gut microbiota to alleviate intestinal inflammation. Image Credit: Hryshchyshen Serhii/Shutterstock.com

Background

Given their remarkable plasticity for further differentiation and ability to regulate inflammation to maintain intestinal homeostasis, macrophages, and neutrophils play a critical role in the pathogenesis of IBD.

Likely, IBD arises from disrupted proinflammatory macrophage activation in response to altered gut microbiota and a shift from M1 to M2 phenotype of macrophages alleviates colitis.

Thus, precise regulation of abnormal macrophage polarization is paramount to clear damaged cells and pathogens and reinstate epithelial cell populations.

Dietary polyphenols also play a crucial role in maintaining gut health and regulating immune homeostasis.

About the study

The present study explored whether dietary phenolic acids impacted intestinal inflammation through macrophages-neutrophils and gut microbiota interactions.

To this end, they developed dextran sulfate sodium (DSS) models of colitis using male wild-type (wt) C57BL/6 mice to elucidate how dietary polyphenol influences macrophage-microbiota interactions that alter colitis severity. 

All test animals were housed in specific pathogen-free (SPF) conditions and had free access to a chow diet and drinking water. Phenolic acids were orally administrated in mice for eight days, and on day 3, 3% DSS was provided in drinking water ad libitum for five days to induce acute experimental colitis.

It helped the researchers study the beneficial effect of phenolic acids. They also monitored colitis severity in mice by observing their body weight (BW), histological scores, disease activity index (DAI), and colon length changes. 

After a week of adaptation, they selected four-week-old mice for antibiotic treatment experiments. These animals received a four-antibiotics (ABX) cocktail in the drinking water for two weeks, which depleted their intestinal microbiota before DSS treatment.

Next, the team subjected antibiotic-treated mice to oral inoculation with fecal microbial suspensions at a frequency of once every other day for four weeks. Then, they investigated whether the protective effect of phenolic acids on disease severity relied on the gut microbiota.

Performing fecal microbiota transplantation (FMT) by transferring feces from mice that had consumed either CA or EA into DSS-treated recipients helped assess whether the altered microbiota resulting from CA or EA consumption contributed to reduced severity in the DSS model.

The team also induced colitis in macrophage-depleted male C57BL/6 mice implanted with bone marrow-derived macrophage (BMDMs) from wild-type (wt) mice. They analyzed the immunological basis of increased colitis in these animals by measuring the colon's cytokine levels.

Since colonic tissue of DSS-treated mice had elevated cytokine levels, especially tumor-necrosis factor-alpha (TNF-α), interleukin-1beta (IL-1β), IL-6, and more inflammatory cells, researchers also examined changes in cellular components in macrophages involved in the colitis. 

Analysis of the expression of M1 macrophage signature genes and the frequency of CD11b⁺/CD11c⁺, i.e., the macrophage phenotype in the colon of mice, helped the researchers verify whether phenolic acids mediated the reduction of colitis.

The team specifically investigated whether CGA exerted any modulatory effects on macrophages through suppression of pyruvate kinase M 2 (Pkm2)-dependent glycolysis and NOD-like receptor protein 3 (Nlrp3) activation in response to lipopolysaccharides (LPS). Furthermore, they evaluated the role of neutrophils in colitis.

Results

Phenolic acids protect against DSS-induced colitis using several different mechanisms. Accordingly, CGA alleviated colitis in a macrophage-dependent manner. 

On the other hand, FA ameliorated colitis in a neutrophil-dependent manner, i.e., partly through blocking the formation of neutrophil extracellular traps (NETs).

CGA promoted the phenotypic conversion of M1 to M2 macrophages to reduce colitis. It also required NLRP3 inflammasome to reprogram M1 macrophage polarization and induce IL-1β production to reduce colitis.

Consistent with a recent study, results also showed that PKM2-driven glycolysis contributed to regulated IL-1β release after activation of the NLRP3 inflammasome.

The effects of caffeic and ellagic acids depended on gut microbiota. The latter produced urolithin A in the presence of gut microbes, which exerted its anti-inflammatory effects via IL-22.

Interestingly, these two polyphenolic acids did not directly modulate colitis severity in microbiota-depleted and DSS-induced mice.

However, fecal microbiotas transferred from caffeic acid- or ellagic acid-administrated mice to colitis recipients attenuated colitis, showing that metabolites produced by gut microbes mediated their protective effects against colitis in mice.

Conclusions

Overall, the study data could inform therapeutic approaches targeting IBD. Further studies should determine other factors involved in exerting the beneficial effects of phenolic acids against intestinal inflammation.

Moreover, future studies should explore the mechanisms governing phenolic acids mediated protective effects against intestinal inflammation (e.g., receptors of downstream signals).

Journal reference:

• Han, D., Wu, Y., Lu, D. et al. (2023) Polyphenol-rich diet mediates interplay between macrophage-neutrophil and gut microbiota to alleviate intestinal inflammation. Cell Death Dis 14, 656. doi: https://doi.org/10.1038/s41419-023-06190-4. https://www.nature.com/articles/s41419-023-06190-4