Decreased expression of semaphorin-3A, a neurite-collapsing factor, is associated with itch in psoriatic skin.

Pruritus is a common symptom of psoriasis, which affects quality of life. This symptom accompanies the hyper-innervation of sensory C-fibres in psoriatic lesions. Two extracellular molecules, nerve growth factor (NGF) and semaphorin-3A, regulate C-fibre extension. In this study, the expression levels of these 2 molecules in biopsy specimens from psoriatic and healthy skin were quantified by immunohistochemistry and quantitative reverse-transcription PCR. Semaphorin-3A expression was lower in the psoriatic samples compared with the healthy samples, whereas NGF was higher. C-fibre innervation in the epidermis was also increased in psoriatic skin. Semaphorin-3A mRNA expression was negatively correlated with itch intensity and severity of psoriasis. We propose that decreased semaphorin-3A and increased NGF expression levels may trigger the outgrowth of C-fibres, leading to pruritus.