Human skin dendritic cell fate is differentially regulated by the monocyte identity factor KLF4 during steady state and inflammation.

Abstract Langerhans cell (LC) networks play key roles in immunity and tolerance at body surfaces. LCs are established prenatally and can be replenished from blood monocytes. Unlike skin-resident dermal/interstitial-type DCs (d/intDCs) and inflammatory dendritic epidermal cells (IDECs) appearing in dermatitis/eczema lesions, LCs lack key monocyte-affiliated markers. Inversely, LCs express various epithelial genes critical for their long-term peripheral tissue residency. DCs are functionally involved in inflammatory diseases; however, mechanisms remained poorly understood. In vitro differentiation models of human DCs, gene profiling, gene transduction and immunohistology were used to identify molecules involved in DC subset specification. We here identified the monocyte/macrophage lineage identity transcription factor Kruppel-like factor 4 (KLF4) to be inhibited during LC differentiation from human blood monocytes. Conversely, KLF4 is maintained or induced during dermal DC and monocyte-derived DC/ IDEC differentiation. We showed that in monocytic cells KLF4 has to be repressed to allow their differentiation into LCs. Moreover, respective KLF4 levels in DC subsets positively correlate with pro-inflammatory characteristics. We identified epithelial Notch signaling to repress KLF4 in monocytes undergoing LC commitment. Loss of KLF4 in monocytes transcriptionally de-represses RUNX3 in response to TGF-β1, thereby allowing LC differentiation marked by a low cytokine expression profile. Monocyte differentiation into Langerhans cells depends on activation of Notch signaling and the concomitant loss of KLF4.