A short-term Western diet facilitated the development of interleukin (IL)-23-mediated psoriasis-like skin and joint inflammation and caused shifts in the intestinal microbiota in a murine model – , say the investigators and other experts who reviewed the findings.
Dr. Samuel T. Hwang
The mice did not become obese during the short duration of the multilayered study, which suggests that a Western diet (high sugar, moderate fat) can be impactful independent of obesity, Samuel T. Hwang, MD, PhD, professor and chair of dermatology at the University of California, Davis, and senior author of the study, said in an interview. The study was published in the Journal of Investigative Dermatology.
Dr. Renuka R. Nayak
In an accompanying commentary, Renuka R. Nayak, MD, PhD, of the department of rheumatology at the University of California, San Francisco, wrote that the findings “add to the mounting evidence suggesting that diet has a prominent role in the treatment of psoriasis and [psoriatic arthritis] and raise the possibility that the microbiome may contribute to disease severity”.
Mice were fed a Western diet (WD) or conventional chow diet for 6 weeks and then injected with IL-23 minicircle (MC) DNA to induce systemic IL-23 overexpression – or a control minicircle DNA injection – and continued on these diets for another 4 weeks.
The mice in the WD/IL-23 MC DNA group developed erythema and scaling and increased epidermal thickness in the ears; such changes were “remarkably milder” or nonexistent in the other groups. Skin and joint immune cell populations, such as gamma delta T cells, neutrophils, and T helper type 17 cytokines were elevated in WD-fed mice, as were other markers of IL-23-mediated joint inflammation.
Recent research has suggested that the gut microbiota is dysbiotic in patients with psoriasis, and this new study found that WD-fed mice had less microbial diversity than that of mice fed a conventional diet. After IL-23 MC delivery, WD-fed reduced microbial diversity and pronounced dysbiosis.
“When we combined the Western diet and IL-23, we saw some very different microbes in abundance. The whole landscape changed,” Dr. Hwang said in the interview.
The data “suggest that WD and overexpression of IL-23 may contribute to gut microbiota dysbiosis in a synergistic and complex manner,” he and his coinvestigators wrote.
Treatment with broad-spectrum antibiotics suppressed IL-23-mediated skin and joint inflammation in the WD-fed mice – and moderately affected skin inflammation in conventionally-fed mice as well – which affirmed the role of dysbiosis.
And “notably,” in another layer of the study, mice that switched diets from a WD to a conventional diet had reduced skin and joint inflammation and increased diversity of gut microbiota. (Mice that were fed a WD for 6 weeks and given the IL-23 MC DNA were randomized to continue this diet for another 4 weeks or switch to a conventional diet.)
Commenting on the new research, Wilson Liao, MD, professor and vice chair of research in the department of dermatology at the University of California, San Francisco, said it “provides evidence” that diet can affect not only psoriasis, but psoriatic arthritis (PsA) as well, “through altering the ratio of good to bad bacteria in the gut.”
Going forward, better understanding “which specific gut bacteria and bacterial products lead to increased psoriatic inflammation, and the immunologic mechanism by which this occurs” will be important and could lead to novel treatments for psoriasis and PsA, said Dr. Liao, director of the UCSF Psoriasis and Skin Treatment Center.
Next on his research agenda, Dr. Hwang said, is the question of “how microbiota in the gut are actually able to influence inflammation at very distant sites in the joints and the skin.
“We want to understand the metabolic mechanisms,” he said, noting that “we invariably talk about cytokines, but there are other substances, like certain bile acids that are metabolized through the gut microbiome,” which may play a role.
The findings also offer a basis for treatment experiments in humans – of diet, probiotic therapy, or selective antibiotic modulation, for instance, Dr. Hwang said.
And in the meantime, the findings should encourage patients who are interested in making dietary changes, such as reducing sugar intake. “There’s wide interest – patients will ask, is there something I can change to make this better?” Dr. Hwang said. “Before, we could say it might be logical, but now we have some evidence. The message now is [high-sugar, moderate-fat] diets, apart from their ability to stimulate obesity, probably have some effects.”
Dietary change may not replace the need for other psoriasis treatments, he said, “but I think there’s good reason to believe that if you do change your diet, your treatment will be better than it would be without that dietary change,” he said.
In their discussion, Dr. Hwang and coauthors note that WD with IL-23 overexpression also decreased the mRNA expression of barrier-forming tight junction proteins, thus increasing intestinal permeability. This finding may be relevant, they wrote, because “leaky gut has been proposed as a pathogenic link between unhealthy diet, gut dysbiosis, and enhanced immune response,” and has been observed in a number of autoimmune diseases, including psoriasis.
Dr. Hwang, lead author Zhenrui Shi, MD, PhD, and coauthors reported no conflicts of interest. Their study was supported by the National Psoriasis Foundation, as well as the National Institutes of Health/National Institute of Arthritis and Musculoskeletal and Skin Diseases, and the National Cancer Institute.