Psoriasis is a common skin disorder affecting 1 to 3 percent of the world's population.[1, 2] Treatment of the disease results in costs that range between 0.7 to more than 4 billion dollars annually in the US alone . One of the most cost-effective forms of potential treatment for the disease is the use of antithyroid thioureylenes such a propylthiouracil. We previously reported the effectiveness of thioureylene treatment in patients with plaque psoriasis [4–7], a finding since confirmed by others . The mechanism of action of antithyroid thioureylenes in psoriasis remains unclear. The drugs have been studied with relation to their effect in Graves' hyperthyroidism which, like psoriasis, is an autoimmune disorder [9–12] Antithyroid thioureylenes, of which propylthiouracil, methimazole and carbimazole are used in clinical practice, have known immunomodulatory properties [13, 14] and, in the case of psoriasis, also have antiproliferative effects as demonstrated by their ability to produce a decline in markers of cellular proliferation such as proliferative cell nuclear antigen (PCNA).
Skin antigen-presenting cells (Langerhans cells) appear to play an important role in the initiating events that lead to the cytokine cascade that results in keratinocyte proliferation and development of psoriatic lesions [16–18]. Effective therapeutic approaches to psoriasis treatment, such as psoralen UV-A therapy (PUVA), which result in depletion of APCs in the skin [19, 20], and UV-B , lead to one of the most sustained improvement in psoriasis. IL-12, a product of Langerhans cells and skin monocytes plays a pivotal role in T-cell activation that leads to formation of the psoriatic plaque . IL-12 levels are also significantly raised in patients with Graves' hyperthyroidism and are reduced following treatment with propylthiouracil .
CD1a is expressed in differentiated antigen-presenting cells and monocytes and is used as a marker for the presence of these cells in the skin[24–26]. The CD1 molecules are the products of five CD1 genes located on chromosome 1. The genes are closely related to the mixed histocompatibilty complex (MHC) genes indicating origin from a common precursor . CD1 molecules, however, have limited sequence homology to MHC class I and II molecules, and unlike the MHC molecules are also capable of binding non-protein antigens [28, 29].
The present study was performed to examine the effect of propylthiouracil treatment of patients with plaque psoriasis on CD1a expression in their psoriatic lesions in order to determine whether this form of therapy resulted in depletion of APCs from affected skin, that could account for the therapeutic benefit of PTU in this disorder.