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Notes: Summary Interactions between keratinocytes and mononuclear cells via cytokines and adhesion molecules are thought to play a crucial part in inflammatory skin diseases. The cytokine-mediated effects of peripheral blood mononuclear cells (PBMC) from patients with atopic eczema (AE) and healthy individuals on keratinocytes (HaCaT) were investigated in vitro. A new coculture model (Transwell system) which consists of a lower and an upper compartment, which are separated by a polycarbonate-treated membrane, was established. 3[H]thymidine incorporation of keratinocytes and lymphocytes, as well as IL-6. IL-8 and IFN-γ synthesis, were measured. Keratinocyte proliferation was significantly enhanced in the presence of PBMC from patients with AE. In contrast, PBMC from normal donors did not enhance HaCaT cell proliferation when they were cocultttred. Lymphocytes from patients with AH showed a significantly enhanced proliferation after coculture with keratinocytes. However, PBMC from normal donors did not proliferate in the presence of HaCaT cells. Keratinocyte supernalants incubated with PBMC from either atopic or normal volunteers induced a suppression of lymphocyte 3[H]thymidine incorporation. In supernatants from cocultures of PBMC from patients with AE and keratinocytes. significantly enhanced amounts of IL-6 and IL-8. compared with normal donor's lymphocytes and HaCaT cells, were measured. No differences in TFN-γ production were observed. When PBMC were cultured without HaCaT cells, supernatants contained equal levels of IL-6, IL-8 and IFN-γ in normal donors and in patients with AE. Interestingly. HaCaT cells spontaneously secrete measurable amounts of IL-6, IL-8 and IFN-γ. Blocking experiments with neutralizing antibodies against these interleukins showed a complete inhibition of keratinocyte proliferation when PBMC from normal donors were used whereas the proliferative potency of PBMC supernalants from patients with AE on keratinocytes remained. Our data indicate that (i) PBMC from patients with AE stimulate keratinocyte proliferation via soluble factor(s) that are different from IL-6. IL-8 and IEN-γ (ii) probably, HaCaT cells spontaneously produce lymphocyte/monocyle inhibitory soluble factors and IL-6, IL-8 as well as IFN-γ and (iii) secretion and/or activity of keratinocytederived inhibitory mediators is regulated via cytokines of PBMC infiltrating inflammatory skin.