Differences in gene expression between the skin and synovial tissues of individuals with psoriatic arthritis could explain why treatments targeting proinflammatory mechanisms don’t improve joint symptoms in some patients.
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A paper published in Annals of the Rheumatic Diseases presents the results of an observational, open-label study involving 27 patients with active psoriatic arthritis, 18 of whom were treated with anti–tumor necrosis factor (anti-TNF) therapies and 9 with the monoclonal antibody ustekinumab (Stelara). This drug targets the axis of proinflammatory cytokine interleukin-23 and effector cytokine IL-12, which are believed to play an important role both in skin and nail psoriasis, and psoriatic arthritis.
However, while anti–IL-23 antibodies seem to work well to address skin manifestations of psoriasis, they tend to improve joint symptoms only in selected patients.
“The lack of a clear mechanism to explain such divergent responses prompted this study,” said Dr. Alessandra Nerviani, lead author of the study, from the Barts and The London School of Medicine & Dentistry.
Participants also had biopsies taken from the synovium – in particular, from joints that were clinically and ultrasonographically active – and from lesional and nonlesional skin for gene expression analysis.
In terms of treatment response, the ustekinumab-treated group showed significantly higher scores for erythrocyte sedimentation rate, joint pain, and disease activity, compared with the anti–TNF-treated group. Psoriasis Area and Severity Index scores were similar in both treatment arms, but significantly more patients in the anti-TNF group met the EULAR Disease Activity Score for response (70.6% vs. 22.2%).
The gene expression analysis, which assessed the activity of 80 genes related to inflammation in 14 patient samples from synovial tissue, lesional skin, and nonlesional skin, found that patterns of expression in the synovium clustered away from those from skin.
This was particularly the case when it came to genes related to drug targets. The targets for anti-TNF showed similar levels of expression in both skin and synovial tissue. However, the targets for ustekinumab – namely interleukin (IL)–23A, IL-23R and, IL-12B – showed higher levels of expression in lesional skin than in nonlesional skin and synovial tissue.
“Interestingly, we observed that, while some patients did express IL-23 cytokines/receptor in both skin and joint, others had discordant expression, that is, active IL-23 pathway in the lesional skin but not in the synovium,” the authors wrote.
When researchers then stratified patients according to how much synovial inflammation they had, they found that patients who had higher scores also showed higher expression of genes for IL-12B and IL-23R, but not IL-23A, despite showing no other major clinical differences.
The authors also looked at the protein expression levels for IL-23p40, IL-23p19 and IL-23R, and found that while the percentage of cells positive for these proteins was significantly higher in lesional, compared with nonlesional skin, it was also higher in the synovium among patients with more inflammation.
“Except for the LIKERT patient score, we did not detect other significant correlations between IL-23 axis expression and clinical parameters at baseline, suggesting that patients with comparable disease severity may have, in fact, heterogeneous histopathological features and expression of drug targets within the diseased synovium,” they wrote.