Lineage commitment of epithelial Langerhans dendritic cells

Langerhans dendritic cells (LCs) form a dense cellular network in the epidermis, and represent a highly abundant DC subtype characterized by unique phenotype and function. An important unresolved question in the field has been how LCs develop from progenitors, and how their development might differ from other DC subsets. Lineage decisions of hematopoietic cells are controlled by a highly regulated interplay between activating and inhibitory transcription factors. Although it is known that LCs originate from a monocytopoietic pathway, the transcriptional mechanisms underlying LC commitment remain poorly understood. Our group could previously demonstrate that LC differentiation from human hematopoietic progenitor cells is absolutely dependent on the cytokine TGF-ß1, and that TGF-ß1 instructs LC lineage commitment of monocytopoietic progenitors. Similarly, LC precursors in vivo require TGF-ß1 for their differentiation. In preliminary unpublished experiments we identified transcription factors regulated by TGF-ß1 during LC lineage instruction. These factors were identified in highly purified human myeloid committed progenitor cells that undergo LC differentiation in presence of TGF-ß1. Here we aim to provide a functional understanding of the molecular hematopoietic mechanisms underlying LC lineage specification. Our specific aims are to perform: - detailed expression analyses of candidate factors during LC lineage commitment and myelopoiesis using a well- established human hematopoietic differentiation model. - systematic gain and loss of function analyses of above-mentioned transcription factors during LC commitment and myelopoiesis. - an analysis of how these transcriptional regulators might participate in gene regulatory circuitries during LC commitment and myelopoiesis - in vivo validation experiments

Langerhans cells (LC) represent dendritic cells (DC) in stratified epidermal/mucosal tissues. There, they form dense cellular networks and can be identified by specific immunophenotypic characteristics. Despite their high degree of conservation throughout vertebrate evolution, their function is still incompletely understood. On the one hand, LCs seem to play an important role in antiviral innate and adaptive immunity (e.g. against HIV). On the other hand, they may instruct T cells to ignore epithelial and environmental antigens (i.e. induction of immunologic tolerance). Among various immune cell lineages, LCs are developmentally most closely related to monocytes/macrophages. Cell culture methods have been established that allow to generate large numbers of LCs from hematopoietic progenitor cells or blood monocytes in vitro. These methods are of substantial interest for cell therapy studies in the fields of cancer immunotherapy and tolerance induction. The molecular mechanism underlying LC differentiation and function however remained poorly defined. In our project, we performed gene array profiling in LC progenitors to identify molecules involved in LC differentiation and function. We identified several transcription factors involved in LC lineage differentiation. We found that LC precursors that populate the prenatal epidermis differ from monocytes in expression of certain key lineage identity transcription factors. Additionally, we provided a model on how blood monocytes are transcriptionally reprogrammed to adopt LC characteristics. Moreover, we identified cell membrane-associated signaling molecules: TROP2 can be used as marker for LCs; AXL mediates silent phagocytosis of dead cells and inhibits inappropriate immune responses within the epidermis. Furthermore, we identified a specific micro-RNA highly expressed in LCs (miR-146a) that inhibits bacterial activation of LCs and thus might be critical for host tolerance to commensal bacteria at body surfaces.