997 Nucleic acid induced fractalkine expression in human keratinocytes

Psoriasis is a multifactorial skin disease, caused by the disturbed homeostasis between the innate and adaptive immune cells, consequently leading to hyperproliferation of keratinocytes. 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 fractalkine (CX3CL1), a chemokine recruiting leukocytes. Our recent large-scale study showed that RNA- and DNA-fragments induce significant transcriptional changes in CX3CL1 expression, thus we aimed to identify the molecular background behind its expression. To induce inflammatory processes, normal human epidermal keratinocytes were transiently transfected with synthetic dsDNA analogue poly(dA:dT) and dsRNA analogue poly(I:C) or were treated with psoriasis-related cytokines. The role of pattern recognition receptors was studied by siRNA-based silencing, signaling pathways were studied by specific inhibitors, while CX3CL1 mRNA expression was monitored by RT-qPCR. Nucleic acid analogues caused elevated CX3CL1 mRNA expression, but to different degree, however cytokine treatments had no effect on it. According to our results, RIG-I, IFIH-1 and cGAS receptors play a role in nucleic-acid induced CX3CL1 expression, through activating p38- and STAT-pathways. Based on the NCBI Database CX3CL1 has two splice variants with the presence (full-length form) or absence of its chemokine motive, possibly altering its chemoattractant properties. We studied the expression of both splice-variants by PCR, using several primer pair combinations, but only the expression of the full-length variant could be verified in keratinocytes. Our results suggest that nucleic acids induce epithelial CX3CL1 production, possibly contributing to the recruitment of professional immune cells into the skin. Although databases predict the existence of multiple splice-variants with different functions, the expression of only the full-length CX3CL1 could be confirmed in keratinocytes.