RheumaView Insights

Psoriasis From the Inside Out: How the Plaque Forms, and Where Our Drugs Act

By Olga Goodman, MD

A two-minute walk from the psoriatic plaque to the molecules — and to the points where disease-modifying drugs intervene.

Few lesions in medicine are as instantly recognizable as a psoriatic plaque, and few are as deceptively complex beneath the surface. What looks like a single thick, silvery patch is the visible end of a self-amplifying conversation between skin cells and the immune system. The short animation above traces that conversation from the plaque down to the molecules, and then shows where each class of disease-modifying drug steps in. This article follows the same story in a little more detail.

The plaque you can see

A classic plaque is sharply demarcated, raised, and erythematous, capped by adherent silvery-white scale, with a predilection for extensor surfaces. Lift the scale and you may provoke pinpoint bleeding — the Auspitz sign. That small clinical detail is a clue to everything happening underneath: the scale, the thickness, and the bleeding points are each the surface signature of a specific structural change in the skin.

Under the surface

In cross-section, the architecture of a plaque is anything but random. The epidermis is markedly thickened — acanthosis — and its lower border plunges into the dermis as regular, elongated rete ridges. Maturation is rushed and disordered: the granular layer thins, and nuclei are retained all the way into the surface scale, a state called parakeratosis. Neutrophils stream upward and collect in small clusters within the upper layers — Munro microabscesses. And in the slender dermal papillae between the ridges, capillaries become dilated and tortuous, rising close to the surface. Those vessels are why scraping the scale draws blood. The plaque, in other words, is the footprint of an epidermis being driven to proliferate far too fast, against a backdrop of brisk inflammation.

The engine: the IL-23 / Th17 axis

What drives the loop? It begins with stress. Injured or stressed keratinocytes release antimicrobial peptides such as LL-37, which partner with the cell’s own nucleic acids and rouse dendritic cells waiting in the dermis. The activated dendritic cell presents to a T-lymphocyte and releases IL-23 and IL-12, polarizing and sustaining effector T-cells — chiefly Th17, with a Th1 contribution.

These effector cells pour out IL-17A, IL-22, and TNF. The cytokines act back on keratinocytes, pushing them to divide too fast and to produce still more chemokines and antimicrobial peptides — which recruit more neutrophils and re-activate the dendritic cells. The result is a feed-forward circuit: a loop that, once running, sustains itself. That is the mechanistic reason psoriasis behaves as a chronic, relapsing disease rather than a one-time event.

Points of application: the DMARDs

The reward for understanding the cascade is practical: every node in the loop is a place a drug can intervene. It helps to organize the disease-modifying antirheumatic drugs — the DMARDs — not by their names but by where they act.

Inside the cell, on metabolism — conventional synthetic DMARDs. These are the long-standing anchors of rheumatic therapy, and they act broadly rather than on a single cytokine. Methotrexate works largely through adenosine- and folate-dependent pathways, blunting proliferation and inflammation; leflunomide inhibits dihydroorotate dehydrogenase, throttling the pyrimidine supply that activated lymphocytes need; sulfasalazine dampens NF-κB-driven inflammation; and hydroxychloroquine alters lysosomal pH and toll-like-receptor signaling to interfere with antigen processing.

Outside the cell — biologic DMARDs. These antibodies and fusion proteins intercept the cascade in the extracellular space. Several neutralize the very cytokines named above — TNF, IL-17, and IL-23, the axis central to psoriatic disease, along with IL-6 and IL-1 in other rheumatic conditions. Others act on immune cells directly: abatacept blocks the CD80/86–CD28 costimulatory “second signal” that licenses T-cells, and rituximab depletes B-cells by way of CD20.

Inside the cell, on signaling — targeted synthetic DMARDs. These small molecules block the intracellular relay rather than the cytokine itself. JAK inhibitors interrupt the JAK–STAT cascade shared by many cytokines; the TYK2 inhibitor (deucravacitinib) selectively dampens IL-23 and type I interferon signaling and is approved for plaque psoriasis; and the PDE4 inhibitor (apremilast) raises intracellular cAMP to rebalance pro- and anti-inflammatory mediators.

Notice how cleanly the most effective agents for psoriatic disease map onto the loop described above — IL-17 and IL-23 inhibitors, TYK2 inhibition, and TNF blockade all strike the IL-23/Th17 axis directly. That alignment is not a coincidence; it is mechanism guiding therapy.

Why the mental model matters

Seeing the plaque as the surface of a loop, rather than a static patch, quietly changes how we approach treatment. We are not merely clearing scale — we are interrupting a circuit at a chosen point. That is the whole idea behind “points of application,” and it is the reason a clear mental model of the cascade is worth more than any list of drug names. When the mechanism is visible, the choice of where to intervene becomes a question you can actually reason about.