The role of the p14-MP1-MAP kinase scaffold complex in Langerhans cell biology

Dendritic cells (DCs) are pivotal regulators of immunity and tolerance. They form a bridge between innate and adaptive immunity. They initiate virtually each and every adaptive immune reaction and are therefore critical for the organism`s capacity to defend itself against pathogens. Subsets of DCs exist in different tissues. One of the best characterized subsets are Langerhans cells, the DCs of the epidermis and other epithelia. An initial and essential early step in DC function is to take up antigens / pathogens via the endosomal pathway. This is facilitated by several receptors, foremost lectin-like cell surface receptors. Endocytosed / phagocytosed pathogen is then routed into the processing pathways, either the MHC class II pathway for presentation to CD4+ helper T cells or a cross- presentation pathway for presentation to CD8+ cytotoxic T cells. `Antigen processing`, i.e, the fragmentation of pathogenic proteins into small peptides that associate with MHC molecules for recognition by effector T lymphocytes, takes place in `lysosome-related organelles`. Recently the basis for a rare genetic human disease in which albinism is combined with a primary immunodeficiency was discovered: it arises from defects in multiple lysosome-related organelles and is due to a mutation in the gene encoding the endosomal adaptor protein p14. This adaptor is a known regulator of the mitogen-activated protein kinase (MAPK) signal transduction cascade. This work pinpointed an important link between signal transduction, organelle biogenesis and endocytosis. For these reasons it can be hypothesized that the endosomal adaptor protein p14 is also involved in the antigen processing capacity of DCs and possibly other DC functions. Indeed, preliminary observations reveal a profound influence of p14 specifically on Langerhans cells. When p14 is missing in DCs, as in conditional knock-out mice under the CD11c promotor, Langerhans cells were greatly reduced in the epidermis. It is therefore the aim of this project to study in detail the hitherto unknown role of p14 in DCs of the skin, i.e. epidermal Langerhans cells and dermal langerin/CD207+ DCs both in murine and human models. In the mouse, p14 will be conditionally deleted in langerin-expressing cells, using the established "Cre-LoxP" technology. In human Langerhans cells and other DCs, p14 will be inactivated by gene silencing methods. Functional consequences of p14 deletions will be studied with emphasis on the immunizing and tumor-protection capacities of Langerhans cells. Homeostasis of Langerhans cells in the epidermis from birth to adulthood will be investigated. If a critical role for p14 in Langerhans cells for the skin immune system could be substantiated, it would provide a mechanism by which the homeostasis of skin DCs as well as maturation, movement and function of antigen- processing organelles in DCs of the skin could be regulated by extracellular stimuli - ultimately with therapeutic intentions, not only for the p14-dependent immunodeficiency syndrome but more generally for diseases where enhanced DC function and improved immunity is desired, e.g., in cancer.

Dendritic cells (DCs) are pivotal regulators of immunity and tolerance. They form a bridge between innate and adaptive immunity. They initiate virtually each and every adaptive immune reaction and are therefore critical for the organism's capacity to defend itself against pathogens. Subsets of DCs exist in different tissues. One of the best characterized subsets are Langerhans cells, the DCs of the epidermis and other epithelia. The clinical relevance of these skin DCs is underscored by the fact that many common vaccinations are administered into the skin and, consequently, skin DCs mediate the protective effect of the vaccination.Clinical starting point for the project was rare genetic human immunodeficiency disease. It arises from defects in multiple lysosome-related organelles and is due to a mutation in the gene encoding the endosomal adaptor protein p14 / LAMTOR2, a known regulator of the mitogen-activated protein kinase (MAPK) signal transduction cascade. This work pinpointed an important link between signal transduction, organelle biogenesis and endocytosis. Therefore, we hypothesized in this project that LAMTOR2 is also involved DC functions.Surprisingly, when LAMTOR2 was missing in DCs (achieved by genetic methods: "Cre-LoxP") Langerhans cells were almost completely absent from the epidermis of mice. In the completed project we were able to elucidate in detail the cellular and molecular mechanisms for the observed lack of Langerhans cells. Shortly after birth, Langerhans cells settle the epidermis normally. Then, however, they cannot divide and multiply properly and, moreover, they also die at an elevated rate due to apoptosis leading to their absence in the epidermis. In molecular terms, we were able to show that the lack of the LAMTOR2 molecule led to the disruption of the entire multimolecular LAMTOR complex. This caused a marked malfunction of two signal transduction pathways, the ERK and the mTOR pathway.The now completed project represents the first study on the role of LAMTOR2 in DCs and, in particular, in Langerhans cells. Our findings emphasize the high biological importance of the two signaling pathways, in that their malfunctioning can lead to the elimination of almost an entire cell population from the skin. This novel, homeostasis-maintaining function of LAMTOR2 may render it a potential target for influencing Langerhans cell numbers in the epidermis. This may, for instance, be of therapeutic interest in bone-marrow transplanted patients, where one would like to eliminate or reduce Langerhans cells because they are thought to initiate the dangerous graft-versus-host disease. However, before this can eventually be translated into clinical practice, much further basic research on this topic needs to be done.