Convergent evolution of MDP-monocyte differentiation in adult skin permits repair of the Langerhans cell network [scRNA-seq]

Langerhans cells (LCs) are a unique population of phagocytes programmed within embryonic skin to maintain tissue and immunological homeostasis at the epidermal barrier site. Unique amongst tissue-resident macrophages, LCs play roles in skin innervation and repair, while also functioning as migrating professional antigen presenting cells, a capability classically assigned to dendritic cells (DC). We current lack insight into the molecular pathways that determine this dichotomy, when they are established, and whether precursors recruited to the adult skin niche access the same instructive signals laid down in utero. Tracking differentiation of monocyte-derived (m)LCs after destruction of embryo-derived cells in murine adult skin, revealed intrinsic intra-epidermal heterogeneity and selection of MDP-monocytes to by-pass gene programmes specifying a macrophage fate. Adopting a process unique to adult skin, subsequent extrinsic specification of EpCAM+ precursors via follicular Notch ligands, and aryl hydrocarbon receptor (Ahr) signalling, combined to determine commitment to resident mLCs. Using a unique human LC dataset, we demonstrated that embryo-derived (e)LCs in newborn human skin retained transcriptional evidence of their macrophage origin, which was superseded by a distinct DC-like immune modules following proliferation in infant skin. Thus, convergent differentiation of monocytes within adult skin exploits novel molecular pathways to replicate conditioning of eLC towards more immunogenic DC-like cells within post-natal skin.