Psoriasis Pathophysiology Insights Carry Treatment Implications

Bruce Jancin

March 05, 2021

Dr Kenneth Brian Gordon

Several surprising recent insights into the pathophysiology of psoriasis may eventually lead to superior treatment strategies for this common T-cell-mediated disease, Kenneth Brian Gordon, MD, said at MedscapeLive's annual Las Vegas Dermatology Seminar, held virtually.

Among these unexpected developments was the startling finding that skin inflammation in mild psoriasis is at least as great as in severe disease; evidence that psoriasis may actually be an autoimmune disease rather than a nonspecific immune-mediated disease; and the newly appreciated importance of interleukin-19 (IL-19) in keratinocyte proliferation, according to Gordon, professor and chair of the department of dermatology at the Medical College of Wisconsin, Milwaukee.

"Our understanding of the pathophysiology of psoriasis is still a work in progress," the dermatologist observed.

Immunoregulatory Deficits in Mild vs Severe Psoriasis

Conventional wisdom has held that mild psoriasis as defined by limited affected body surface area involves less skin inflammation than more extensive severe psoriasis, so less-potent topical therapies are appropriate. Not so, according to Gordon, who highlighted work by James G. Krueger, MD, PhD, head of the laboratory of investigative dermatology at Rockefeller University, New York, and coinvestigators. They demonstrated that overall skin inflammation expressed as the sum of T-cell activation and IL-19-mediated epidermal responses didn't differ in lesions of mild as compared with severe psoriasis. Indeed, mild skin lesions featured a greater number of T-cells, stronger expression of proinflammatory cytokine IL-17A, and greater expression of the central psoriasis transcriptome. The big difference between skin lesions of mild versus severe psoriasis was that severe psoriasis was characterized by strikingly weaker expression of immunoregulatory genes, including programmed death-ligand 1 (PD-L1) and cytotoxic T-lymphocyte-associated protein 4 (CTLA4), than that of mild lesions.

The implication is that IL-17-targeted therapies may be of benefit in a much larger segment of the psoriatic population: namely, those with mild disease, who comprise the majority of psoriasis patients by a wide margin, according to the investigators.

Gordon concurs.

"The primary problem in psoriasis is not so much the inflammatory activity, but the ability to turn off the activity," he explained. "That implies that if a patient wants to get clear or have significant improvement in disease, you can't use a less effective medication just because they have less amount of disease. You're going to need to treat it just as aggressively because the great majority of our medications block the proinflammatory pathways."

The deficit in immunoregulatory action identified by Krueger and colleagues in patients with severe disease could provide a novel therapeutic target. If the deficient immunoregulation could be boosted, it might achieve disease control without need for continuous anti-inflammatory therapy.

Autoimmunity in Psoriasis

"When I started work in psoriasis, we always thought there would be a common antigen for the immune process in the disease. We never found it. So for that reason, we sort of put it aside and called psoriasis a nonspecific immune-mediated disease," Gordon recalled.

That view is being reexamined. "While we're not completely certain, there is now some evidence that there might be autoimmunity in psoriasis," he said.

He cited work by an international team of investigators who identified the cathelicidin antimicrobial peptide LL37 as being overexpressed in psoriatic skin, where it appears to serve as a T-cell autoantigen. LL37-specific CD4+ and CD8+ T-cells are skin homing: They can infiltrate lesional skin, where they produce interferon-gamma and proinflammatory Th17 cytokines. The investigators reported that levels of circulating LL37-specific T cells correlated with disease activity such that they were found in three-quarters of patients with moderate to severe plaque psoriasis.

"As LL37 is able to activate innate immune cells and break innate tolerance to self-nucleic acids, it represents an even more appealing target to treat psoriasis. Therapeutic targeting of LL37-specific T cells may provide new avenues to prevent or treat psoriasis without inducing indiscriminate immunosuppression," the investigators concluded.

Similarly, German investigators have identified ADAMTS-like protein 5 (ADAMTSL5) as an autoantigen specific for melanocytes in psoriasis patients who possess the central psoriasis risk gene, known as HLA-C*06:02, which is present in two-thirds of patients with psoriasis. They proposed that their newly recognized autoimmune pathway may explain how HLA-C*06.02 predisposes to psoriasis.

Growing Clinical Relevance of IL-19

It's now well-established that IL-17 is the pivotal force driving the changes in keratinocytes that define the visible expressions of psoriasis, including plaque scale and thickness, which are due to abnormal keratinocyte maturation and proliferation, respectively. Less well appreciated is the fact that IL-17-activated keratinocytes produce IL-19, which feeds back and further stimulates keratinocyte proliferation.

In light of mounting evidence that IL-19 plays an important role in the pathogenesis of psoriasis and that naked eye assessment of visible psoriasis may not reflect the true extent of inflammation, Brian J. Nickoloff, MD, PhD, and coworkers at Lilly Research Laboratories have developed a novel serum IL-19 immunoassay that appears to provide a much-needed objective biomarker of disease activity in psoriasis patients. They demonstrated that serum IL-19 levels correlated with Psoriasis Area and Severity Index scores, and that treatment with the anti-IL-17A biologic ixekizumab (Taltz) led to rapid reduction of IL-19 down to a normal level.

Moreover, following withdrawal of ixekizumab, IL-19 levels rose prior to clinical relapse, then dropped again in response to retreatment. The hope is that this assay will serve as an accurate tool for assessment of response to therapy.

Gordon reported receiving research funding and/or honoraria from more than a dozen pharmaceutical companies involved in psoriasis therapy.

This article originally appeared on MDedge.com, part of the Medscape Professional Network.

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