Increased nerve growth factor- and tyrosine kinase A-like immunoreactivities in prurigo nodularis skin -- an exploration of the cause of neurohyperplasia.

Neurotrophins and their receptors play an important role in cutaneous nerve development and reconstruction after injury. Recent developments indicate that this group of molecules not only exert a neurotrophic action, but are also involved in immune responses and inflammation. Prurigo nodularis is a skin disease characterized by neurohyperplasia and intense itch. In the present study, the localization and distribution of nerve growth factor (NGF) and its receptors were explored by immunohistochemical methods, with the aim of detecting the cause of the neurohyperplasia in the disease. In normal healthy volunteers and in uninvolved skin, NGF immunoreactivity was seldom seen in the basal layer of the epidermis or in the dermis. In prurigo nodularis skin, there was also very little NGF immunoreactivity in the epidermis. However, in the dermis, a huge number of cells showed an NGF-like immunoreactivity. In normal skin of healthy volunteers, only a weak staining for tyrosine kinase A (trkA) was seen in the epidermis, whereas in the dermis, there was no trkA staining seen at all. However, in the prurigo nodularis tissue, the hyperplastic nerves clearly showed trkA immunoreactivity, and it seemed that the staining was only present in the axons. By NGF and p75 NGF receptor double-labelling, both immunoreactivities showed weak staining in the epidermis and dermis of normal skin. However, in the dermis of prurigo nodularis, strong staining for both NGF and NGF receptor antibodies was seen. NGF receptor-immunoreactive nerves were more dense in areas where there were more NGF-immunoreactive cells. The results indicate that in prurigo nodularis skin, NGF is overexpressed, locally infiltrated inflammatory cells may be the source of this NGF, and NGF and its receptors may contribute to the neurohyperplasia of the disease.