The role of the Histamine H 4 receptor in atopic dermatitis and psoriasis.

Atopic dermatitis (AD) and psoriasis are common skin diseases with a high negative impact on patients´ quality of life. Both diseases are mediated by a pro-inflammatory infiltrate consisting of various cell types like T cells, antigen presenting cells and granulocytes and display disturbed keratinocyte differentiation. Given the fact that histamine levels are also highly elevated in inflamed skin, it is likely that histamine plays a relevant role in disease pathology. However, antagonists blocking histamine H1 receptor (HRH1) and HRH2 are largely ineffective in reducing chronic symptoms in AD and psoriasis. Over the last years a lot of research was undertaken to shed light into the mode of action of the latest discovered HRH4. Thereby it could be shown that HRH4 antagonists display anti-pruritic as well as anti-inflammatory effects not only in mouse models but also in first human clinical trials and therefore the HRH4 might present a novel therapeutic target. In this review we summarize the impact of the HRH4 on different cell types, mouse models as well as clinical studies in regard to AD and psoriasis, respectively.