SNOWMASS, COLO. – Mounting circumstantial evidence points to perturbation of bacterial communities in the gut and skin as important environmental triggers for psoriasis and psoriatic arthritis.

A distinctive pattern of alterations in the skin microbiota, termed bacterial "cutaneotypes," has recently been documented in lesional and uninvolved skin of psoriasis patients. Similarly, psoriatic arthritis patients show decreased diversity of their intestinal bacterial community in a pattern similar to patients with inflammatory bowel disease, Dr. Jose U. Scher said at the Winter Rheumatology Symposium sponsored by the American College of Rheumatology.

Dr. Jose U. Scher

The current working hypothesis of disease pathogenesis is that, in individuals genetically predisposed to psoriasis or psoriatic arthritis, this microbial dysbiosis at the cutaneous and gut levels provides an environmental trigger for overt expression of clinical disease.

"This dysbiosis is potentially relevant as a diagnostic and/or therapeutic target in psoriasis and psoriatic arthritis. For example, it may eventually become possible to assess the gut microbiota to predict which psoriasis patients will later develop psoriatic arthritis. And reconstituting the gut flora may turn out to have therapeutic benefit. But much more work is needed," explained Dr. Scher, director of the Microbiome Center for Rheumatology and Autoimmunity at New York University.

About 25%-30% of psoriasis patients develop inflammatory psoriatic arthritis, most often roughly 7 years after onset of their skin disease. Genetics clearly plays a role, as shown in a classic Danish twin registry study with more than 21,000 subjects. Fifty-five percent of the siblings of monozygotic twins with psoriatic arthritis had skin psoriasis, but only 10% of the siblings had psoriatic arthritis, as did 3.8% of siblings of dizygotic twins with psoriatic arthritis. The lesser concordance rate seen for psoriatic arthritis hinted at the importance of environmental factors in disease genesis (Ann. Rheum. Dis. 2008;67:1417-21).

Subclinical gut inflammation is common in psoriatic arthritis. In one early study, histologic evidence of mild or moderate gut mucosal inflammation was detected in 45% of a group of psoriatic arthritis patients, compared with 15% of patients with rheumatoid arthritis and 0% of controls (Scand. J. Rheumatol. 1997;26:92-8).

Also, psoriasis patients are at a roughly 3.5-fold increased risk of developing Crohn’s disease. Among patients with established psoriatic arthritis, this risk climbs to 6.5-fold greater than in nonpsoriatic controls (Ann. Rheum. Dis. 2013;72:1200-5).

In a soon-to-be-published study, Dr. Scher and his coworkers have taken the field a step further, employing high-throughput gene sequencing to analyze the gut bacterial communities of psoriatic arthritis patients and controls. These were all new-onset psoriatic arthritis patients who had never been exposed to systemic corticosteroids, biologic agents, or conventional disease-modifying antirheumatic drugs.

Compared with the stool samples of healthy controls, several major bacterial species were underrepresented or absent in the gut microbiota of psoriatic arthritis patients. These included Akkermansia, the most common mucolytic bacterium in healthy subjects. Intriguingly, Akkermansia counts are decreased 15-fold in Crohn’s disease and 92-fold in ulcerative colitis, according to the rheumatologist.

Other bacterial species markedly less abundant in the psoriatic arthritis patients’ gut flora were Ruminoccocus, Alistipes, and Roseburia. Like Akkermansia, these are mucin-degrading bacteria that promote a healthy gut environment, and they, too, are reduced in inflammatory bowel disease, Dr. Scher said.

He theorized that these disruptions of the bacterial ecosystem might arise from a course of antibiotics, a change in diet, or other insults. The result is activation of dendritic cells to produce interleukin-23, which triggers a proinflammatory cascade including tumor necrosis factor-alpha, interleukin-22, and antimicrobial peptides.

As shown by other investigators (Mucosal Immunol. 2013;6:666-77), these proinflammatory cytokines inhibit RANK ligand, which is the critical factor for differentiation of microfold cells in the gut. These microfold cells, or M cells, are specialized epithelial cells that transport antigens across the gut epithelium and play an important role in maintenance of an efficient immune response. It’s plausible that, when these M cells are defective, the resultant loss of tolerance and chronic inflammatory state can result in psoriatic arthritis, he explained.

Dr. Scher’s colleagues at New York University have used high-throughput gene sequencing to analyze the cutaneous microbiota of lesional and nonlesional skin in psoriasis patients, as well as skin samples from the same sites in healthy controls. The impetus for this study was a hypothesis that psoriasis might represent an inappropriate cutaneous immune response directed against offending bacteria in the skin microbiota.

Sure enough, the bacterial community present in psoriatic lesions displayed a markedly decreased diversity, compared with controls. This decreased diversity also was present, albeit to a lesser extent, in the psoriasis patients’ nonlesional skin. Both the lesional and nonlesional skin of psoriasis patients contained an increased abundance of Corynebacterium, Staphylococcus, and Streptococcus, compared with controls.