Search for Langerhans stem/progenitor cells

Research project P 14243 Search for Langerhans Atem/Progenitor cells Adelheid ELBE-BÜRGER 06.03.2000 Activation of naive T cells occurs exclusively through antigen presentation by a distinct class of leukocytes, termed dendritic cells (DC). Langerhans cells (LQ are skin-specific members of this family and play a pivotal role in the induction of T cell-mediated immunity against various antigens that are present in or penetrate into skin which include reactive chemicals, alloantigens, microorganisms, and tumor-associated antigens. Upon activation LC leave the epidermis and migrate via the lymphatics to the draining lymph nodes where they initiate T cell responses. During migration LC undergo phenotypic and functional changes which enable them to perform this Rinction. While considerable knowledge exists about the mechanisms of antigen-uptake and the emigration of LC, their ontogeny is only poorly characterized. In both humans and mice DC subtypes arise from distinct, bone marrow- derived lineages which are of either myeloid or lymphoid origin. However, the hematopoietic lineage affiliation of LC is less clear. It is therefore the aim of this proposal to trace and characterize mouse LC progenitors/precursors beginning in the bone marrow until their advent in the skin. Our second research goal is to test the hypothesis that the skin itself harbors dormant LC stem./progenitor/precursor cells. The rationale for this comes from work by others showing not yet well defined dendritic CD34+ cells in murine as well as human skin and our own observations that i) fetal murine epidermis harbors NIHC class IF/ADPase-reactive LC and that ii) human skin contains cells which phenotypically correspond to TGF-P 1 -responsive CD14+ LC precursors seen in hematopoietic precursor cell- derived cultures. We plan to perform a variety of in vivo, ex vivo and in vitro experiments to isolate and characterize these cells, test their functional properties and elucidate the contribution of the cutaneous microenvironment on their growth and differentiation. The insights gained from our studies will have fundamental implications for a better -understanding of LC biology which should be translated on a long-term goal into clinically applicable forms (e.g., the characterization of the LC progenitor/precursor will allow i) to devise new strategies to drive these cells in an immunogenic or tolerogenic direction, ii) to use them as drug targets, and iii) their elimination for transplantation purposes). The identification of hematopoietic stem cells in the skin and the finding that they respond to defined cytokines in vitro, should permit the design of novel immunotherapies particularly in the context of hematopoietic stem cell transplantation.

Activation of naive T cells occurs exclusively through antigen presentation by a distinct class of leukocytes, termed dendritic cells. Langerhans cells (LC) are bone marrow-derived, skin-specific members of this family. While it is established that LC play a dual role in initiating as well as in silencing acquired immune responses, their ontogeny is only poorly understood. Initially we observed that the fetal epidermis contains dendritic epidermal leukocytes (DEL) which do not exhibit LC characteristics (e.g., marker expression and function) and that the appearance of LC in the murine epidermis occurs in the neonatal period. In order to investigate whether fetal skin already contains cells which can differentiate into LC, we have transplanted fetal skin onto adult mice. Our finding that donor LC emerge in the developing epidermis of the graft implies that fetal DEL or, alternatively, dermal precursors can mature into LC. To directly test the first hypothesis, highly purified DEL were cultured in the presence of LC differentiation and maturation factors. Furthermore, we have investigated whether factors produced in the epidermis maintain these cells in an immature state. Our results show that purified DEL are not able to differentiate into LC. In line with these data are findings that autocrine Interleukin-10 secretion partly inhibits the differentiation and maturation of DEL into LC in the fetal and neonatal period. This is the first demonstration of a mechanism explaining the poor immune responsiveness of neonatal skin. The insights gained from our studies have fundamental implications for a better understanding of LC biology. In search for dermal hematopoietic stem cells, we have characterized and subsequently isolated and purified cell populations displaying the hematopoietic marker CD45 and several stem cell markers. Moreover, these populations showed in vitro clonogenic capacity. Experiments to prove their stem cell nature in vivo are planned. Given its easy accessibility, our description of the dermis as a potential source of extramedullary hematopoietic stem cell activity makes it an attractive alternative for blood stem cell therapeutics.