A major factor contributing to epidermal proliferation in inflammatory skin diseases appears to be interleukin 1 or a related protein.

Human peripheral blood leukocytes can stimulate G1(G0)-arrested mouse skin keratinocytes to enter the cell cycle again and synthesize DNA at the maximum rate 15-20 hr later. This growth-promoting activity is released by the monocyte fraction and is shown to have characteristics that have been reported for interleukin 1 (IL-1). Pure IL-1 is active in stimulating keratinocyte cultures as was shown with recombinant human IL-1. An IL-1-like protein released by monocytes-macrophages could explain the hyperproliferative epidermis found in certain types of inflammatory skin diseases.