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The achievement of keratinocyte gene therapy in clinical practice requires fundamental experiments using human keratinocytes or skin. We have recently demonstrated that the in vivo introduction of the interleukin 6 (IL-6) gene into rat keratinocytes induces epidermal proliferation and lymphocyte infiltration into the skin. In this study, we first amplified the human IL-6 cDNA from oligo-dT-primed keratinocyte cDNA and then detected the fully spliced (FS) form and the alternatively spliced (AS) form of IL-6 cDNA. Sequence analysis showed that the AS form, which was composed of the IL-6 coding region with all of exon II deleted except for the first guanine, was identical to that reported to be present in lymphocytes. We constructed the expression vectors phIL6 of the FS form and phIL6S of the AS form. We transplanted human skin onto nude rats and introduced phIL6 and phIL6S into the human keratinocytes using the naked DNA method. Keratinocytes prepared 24 h after introduction from the areas treated with them were examined by reverse transcriptase (RT)-PCR and enzyme linked immunosorbent assay (ELISA). RT-PCR showed that the amounts of FS IL-6 mRNA and AS IL-6 mRNA were similar, whereas the ELISA showed that the amount of FS IL-6 peptide was four times that of the AS IL-6 peptide. Histological examination 48 h after introduction showed that the FS form had induced epidermal proliferation, whereas the AS form had not. The epidermal thickening without lymphocyte infiltration induce by the FS form indicates that keratinocyte proliferation is caused by a direct effect of overexpressed IL-6, and not by a secondary effect of infiltrating lymphocytes. This is the first report of the introduction of a human gene into human keratinocytes to produce a biologically active transgenic gene product in human skin using the naked DNA method.