Phototherapy in psoriasis: a review of mechanisms of action

Abstract

Background: Phototherapy is one of the most efficacious treatment options for psoriasis. New, emerging studies are beginning to define the biologic mechanisms by which phototherapy improves psoriasis.

Methods: To provide an overview of the mechanisms thought to be responsible for the therapeutic effects of phototherapy, a review was performed on all relevant published studies in the Medline database from January 1, 1985, to August 15, 2011.

Findings: Four categories of action were proposed in the literature to describe the effects of phototherapy in psoriasis: (1) alteration of the cytokine profile, (2) induction of apoptosis, (3) promotion of immunosuppression, and (4) all other mechanisms.

Conclusions: Phototherapy acts through a combination of pathways to confer therapeutic benefits in psoriasis, and these different modalities may help explain its particular usefulness in treating this cutaneous disease.

Figure 1

Immunopathogenesis of psoriasis and the impact of phototherapy on altering cytokine profile. In psoriasis, antigen presenting cells (dendritic and macrophage cells) activate naïve helper T (Th) cells and induce their differentiation into Th1 and Th17 cells, leading to the release of Th1 and Th17 cytokines that promote inflammation and epidermal hyperplasia. Phototherapy has been shown to down-regulate the Th1/Th17 pro-inflammatory axis and up-regulate the counter-regulatory Th2 pathway, leading to clinical improvement. Effects of phototherapy shown in red.

Figure 2

Cellular targets of phototherapy-induced apoptosis in the treatment of psoriasis. Phototherapy is believed to induce programmed cell death in T lymphocytes in both the epidermal and dermal psoriatic tissues. Apoptosis of keratinocytes is also thought to occur in lesional and nonlesional epidermis, while Langerhans cell depletion from apoptosis is believed to play a less significant role. Photo courtesy of Beth Ruben, M.D., Departments of Dermatology and Pathology, University of California, San Francisco.