372 Mechanism of Cytosolic Nucleic Acid-Induced CX3CL1 Expression of Human Keratinocytes

Psoriasis is a multifactorial, chronic inflammatory skin disease, the development of which is affected by both genetic and environmental factors. Cytosolic nucleic acid fragments, recognized as pathogen- and danger-associated molecular patterns, are highly abundant in psoriatic skin. It is known that psoriatic skin exhibits increased levels of CX3CL1 (fractalkine) compared to healthy skin. Recently we carried out a qPCR array containing 84 psoriasis relevant genes to identify genes showing altered expression upon nucleic acid analogue treatment. One of the genes showing the largest transcriptional changes was CX3CL1. Our aim was to investigate the mechanism of how psoriasis-associated high free nucleic acid levels influence CX3CL1 production in keratinocytes. To induce inflammatory processes, normal human epidermal keratinocytes (NHEK) were transiently transfected with synthetic dsDNA analogue poly(dA:dT) and dsRNA analogue poly(I:C) or were treated with psoriasis-related cytokines (TNFα, IL-17A, IL-12). Nucleic acid analogues caused elevated CX3CL1 mRNA expression levels, but to a different degree, however, cytokine treatments had no effect on CX3CL1 mRNA transcription. To clarify which receptors and pathways play a role in nucleic acid-induced CX3CL1 mRNA expression of keratinocytes, we silenced several nucleic acid receptors and used specific inhibitors to decrease the activity of the signalling pathways. Detection of CX3CL1 mRNA levels by qPCR showed that silencing of IFH1, RIG1, cGAS receptors and inhibition of p38, and STAT pathways resulted in a decrease in the nucleic acid-induced CX3CL1 mRNA expression in keratinocytes. Our results suggest that free nucleic acids induce epithelial CX3CL1 production through IFIH1, RIG1 and cGAS receptors and specific signalling pathways and contribute to the elevated level of this cytokine that is also seen in psoriasis.