The role of micro RNAs in epidermal Langerhans cell differentiation

Langerhans cells (LCs) represent highly abundant and evolutionary conserved dendritic cells (DCs) in epidermal/mucosal tissues. LCs reside for long periods of time in the epidermis and arise from skin-resident precursors prenatally. Additionally, LCs can be replenished from blood monocytes during inflammation. However, the molecular mechanisms governing LC development and differentiation are still poorly defined. Research on LC development is of high relevance because (1) LCs seem to exert unique functions not only during anti-microbial recognition and defence but also for the maintenance of peripheral immune tolerance; (2) in vitro generated LC are highly potent inducers of T cells and are thus of substantial relevance for cell-based immunotherapy studies. Therefore, understanding the mechanisms underlying their development is of great importance for our basic understanding of immune system function and for clinically-oriented research. Several previous/preliminary observations prompted this grant application: (1) We recently observed that LCs differ from interstitial-type DCs (intDCs) in the expression of a number of miRNAs; (2) others recently showed that DC-specific ablation of Dicer (a molecule involved in miRNA biogenesis) leads to a substantial drop in LC numbers without affecting other myeloid DC subsets, demonstrating that LC subset differentiation is regulated by the miRNA pathway; (3) epidermal TGF-?1 is a critical positive regulator of LC subset differentiation but is not similarly required for the development other DC subsets. Thus, it is likely that TGF-ß1 and miRNA pathways might intersect for the control of LC subset differentiation. Specific Aims: 1) Identify miRNAs functionally involved in LC versus intDC subset differentiation from CD34+ hematopoietic progenitor cells. 2) Identify downstream mechanisms underlying miRNA function.

Our adaptive immune system is critical for many vital processes, including anti- microbial defense, elimination of cancerous and virus-infected cells and tissue remodeling. Central to these processes is a key function: the ability to discriminate between the bodies own (self) versus foreign molecules and cells by antigen-specific T and B cells. Dysregulation of these processes may result in life threatening autoimmune/inflammatory diseases and cancer. Cells of the immune system (also termed leukocytes or white blood cells) include several specialized subsets that interact with each other to enable effective immune responses. Among these are tissue-resident dendritic cells (DCs), a class of leukocytes involved in the induction and maintenance of self tolerance. DCs are heterogeneous, with a subset thereof residing in environment exposed epithelial barrier tissues, including skin, lung and mucosae (e.g. urogenital; oral/intestinal mucosae). These sentinels of the immune system fulfill complex tasks: they ignore or tolerate health promoting commensal bacteria critical for host physiology (e.g. for food processing), while they induce protective immune responses to harmful disease-causing pathogens. Epithelia- associated DCs recognize pathogens and molecules encountered in tissues. They migrate to regional draining lymph nodes where they present processed peptides in association with histocompatibility molecules to antigen-specific T cells. They are particularly potent in stimulating regulatory T cells, known to prevent inflammation. Despite evidence that the human DC system comprise functionally specialized cell subsets, the molecular mechanism underlying DC subset specification from human progenitor cells remained poorly understood. This project analyzed complex gene regulatory networks known to instruct cell fate decisions. Particular emphasis is put on a class of molecules known as micro RNAs. By cognate interaction, each specific miRNA species target and degrade several mRNAs including also mRNAs that encode for proteins involved in DC subset specification (e.g. transcription factors). A molecular understanding of these processes in human cells is of relevance for the development of new therapeutic concepts targeting immune-system related disease processes.