The Involvement of IL-34 in Inflammatory and Malignant Skin Diseases.

The mononuclear phagocyte system depends upon CSF1R and its 2 ligands, CSF1 and IL-34. IL-34 is expressed constitutively in the skin in mice and humans from a conserved keratinocyte-specific promoter, suggesting a specific function in epidermal homeostasis. IL-34 deficiency leads to loss of Langerhans cells, the major antigen-presenting cell of the epidermis. Decreased IL-34 expression has been reported in inflammatory and malignant epithelial skin diseases, including atopic dermatitis, psoriasis, eczema, UV-damaged skin, response to common skin sensitizers and various types of squamous cell carcinomas. We discuss the possible contribution of the loss of IL-34 to immune dysregulation in the skin.