Various members of the Toll-like receptor family contribute to the innate immune response of human epidermal keratinocytes

Toll-like receptors (TLRs) are important pattern recognition molecules that activate the nuclear factor (NF)-κB pathway leading to the production of antimicrobial immune mediators. As keratinocytes represent the first barrier against exogenous pathogens in human skin, we investigated their complete functional TLR1-10 expression profile. First, reverse transcription-polymerase chain reaction (PCR) analysis revealed a very similar pattern of TLR mRNA expression when comparing freshly isolated human epidermis and cultured primary human keratinocytes. Thus, further experiments were carried out with primary keratinocytes in comparison with the spontaneously immortalized human keratinocyte cell line HaCaT. The quantitative expression of TLR1-10 mRNA in real-time PCR of primary human keratinocytes and HaCaT cells was analysed. Both cell types constitutively expressed TLR2, TLR3, TLR5, and to a lesser extent TLR10. TLR4 was only found in HaCaT cells, TLR1 to a higher degree in primary keratinocytes. In line with this, LPS induced mRNA expression of CD14 and TLR4 only in HaCaT cells. After stimulation with various TLR ligands, the NF-κB-activated chemokine interleukin-8 (IL-8) was measured. In primary keratinocytes and HaCaT cells the TLR3 ligand poly (I:C) was the most potent stimulator of IL-8 secretion. The TLR ligands peptidoglycan, Pam₃Cys and flagellin which bind to TLR2, TLR1/TLR2 heterodimer, and TLR5, respectively, also induced IL-8 secretion, whereas no IL-8 was induced by LPS, R-848, loxoribine and cytosine guanine dinucleotide-containing oligodeoxynucleotide. A corresponding pattern was found in the RelA NF-κB translocation assay after ligand stimulation of primary keratinocytes. These studies provide substantial evidence for a functional TLR expression and signalling profile of normal human keratinocytes contributing to the antimicrobial defence barrier of human skin.