Functional cooperation of Toll-like receptor signaling and the high-affinity receptor for IgE, FcεRI, on human Langerhans cells

Abstract

Atopic dermatitis (AD) is a multifactorial severe skin disease with increasing incidence in western countries. In the disease, Langerhans cells (LC) play a pivotal role in bridging innate and adaptive immunity through sensing invading pathogens by pattern recognition receptors (PRR) like Toll-like receptors (TLR) and presenting them to naïve T cells. A hallmark of skin LC of atopic individuals is the expression of the high-affinity receptor for IgE, FcεRI. <br /> The aim of this study was to shed light on the regulatory mechanisms involved in FcεRI expression on activated human LC. Therefore, CD34 positive hematopoietic stem cell-derived LC (CD34LC) were used for stimulation assays with TLR ligands as well as with heat-killed <em>Staphylococcus aureus</em> (<em>S.a.</em>) and <em>Streptococcus pyogenes</em> (<em>S.p.</em>) to mimic the pathogens and the commensals colonizing human skin. Stimulation of CD34LC with all ligands resulted in a significant downregulation of FcεRI. Analysis of the factors promoting the expression of FcεRI revealed that the main transcription factor PU.1 and the less expressed transcription factors Yy1, Elf-1, Hmgb1 and Hmgb2 were significantly downregulated upon activation of the cells with the ligands used here. <br /> Since PU.1 was the strongest expressed transcription factor in CD34LC, its regulation was further investigated. PU.1 is<em> inter alia</em> controlled by microRNA-155 (miRNA-155), a miRNA that is induced in macrophages and DC upon activation. The expression of miRNA-155 was examined and, additionally, CD34LC were transfected with pre-miRNA-155 molecules to confirm the effect of those miRNA on the expression of FcεRI. In summary, a high induction of miRNA-155 was observed under all stimulation conditions followed by a decrease of PU.1, which was further confirmed by ectopic miRNA-155. In line with PU.1 downregulation, FcεRI expression also decreased with all stimuli and with ectopic miRNA-155, too.<br /> Another important immunological miRNA is miRNA-146a, which is expressed constitutively on LC and facilitates endotoxin-induced tolerance. miRNA-146a analysis was included in this study to check a putative involvement of this miRNA in the regulation of FcεRI expression. In contrast to miRNA-155, miRNA-146a was only weakly induced upon stimulation of CD34LC. Additionally, no effect of ectopic miRNA-146a on FcεRI expression was detected. <br /> In conclusion, the results of this thesis revealed a regulatory network between TLR and FcεRI involving the TLR-dependent induction of miRNA-155, which in turn controls the translation of PU.1 and in succession the expression of FcεRI.