Psoriasis is characterized by altered epidermal expression of caspase 14, a novel regulator of keratinocyte terminal differentiation and barrier formation.

Caspases are a family of cysteine proteases involved in the effector arm of physiologic cell death [1]. In 1998, a novel caspase designated ‘‘caspase 14’’ was described in embryonic and adult tissues, especially epidermal keratinocytes [2—4]. Unlike other caspases (such as 3, 6 and 7), caspase 14 is not processed by typical death stimuli or activated during apoptosis induced by ultraviolet irradiation or cytotoxic substances. However, caspase 14 is cleaved under conditions leading to terminal differentiation, suggesting a unique role in keratinocyte ‘‘planned cell death’’ in forming the stratum corneum [2,5]. We postulated that psoriasis, a chronic papulosquamous disease with aberrant epidermal proliferation and terminal differentiation [6], may express altered levels of caspase 14. Here, we used immunohistology to compare caspase 14 expression in paraffin embedded sections of normal and psoriatic skin samples. Skin biopsies were obtained under an approved human use protocol after signed informed consent. Skin samples were fixed in 10% neutral buffered formalin and then paraffin embedded. Normal (n = 3) and psoriatic (n = 6) skin samples from different patients were analyzed for caspase 14 presence using a modified avidin-biotin-peroxidase immunohistochemical technique [7]. The 5 mM paraffin sections placed onto positively charged glass slides were deparaffinized in limonene (Sigma, St. Louis, Missouri, USA) and rehydrated in a descending series of ethanols to water. Endogenous peroxidase activity was blocked by a 5-min incubation in 0.3% H2O2. Non-specific binding of the antibody to tissue sections was