In contrast to their murine counterparts, normal human keratinocytes and human epidermoid cell lines A431 and HaCaT fail to express IL-10 mRNA and protein.

In mice, keratinocyte-derived IL-10 is up-regulated by ultraviolet-B (UVB) radiation and plays a major role in UVB-induced immunosuppression. The present study was designed to examine whether a comparable phenomenon can be detected in man. Freshly isolated or cultured normal human keratinocytes (NHK) and keratinocyte cell lines A431 and HaCaT were stimulated with graded doses of UVB (up to 200 J/m2) or with a variety of other stimuli. RNA was extracted at various time points post-stimulation and analysed by reverse transcriptase-polymerase chain reaction (RT-PCR) using four different IL-10-specific primer pairs and RNA from monocytes or T cells as positive controls. We failed to detect IL-10 mRNA in NHK from 40 different donors (breast, abdomen, leg, scalp, foreskin) and in A431 and HaCaT cells, irrespective of the stimulation used and despite successful stimulation. Supernatants of NHK, A431 and HaCaT cultures were negative for IL-10 protein, as tested by four different ELISAs and a bioassay. Murine keratinocytes, stimulated under comparable conditions and tested by the same techniques, displayed a strong expression of IL-10 mRNA and protein. Remarkably, an IL-10 mRNA signal could be detected in NHK after a second round of PCR amplification. Because NHK suspensions are contaminated with Langerhans cells, melanocytes and possibly fibroblasts, we tested pure populations of each individual cell type to determine the origin of this IL-10 mRNA. Our results clearly indicate that NHK, Langerhans cells and fibroblasts fail to express IL-10 and that melanocytes are the principal source of IL-10 mRNA in normal human epidermis.