Migration of dendritic cells to initiate primary immune responses: determination of inducing and regulatory signals

Migration of dendritic cells is of crucial importance to their strong immunogenic function. Dendritic cells capture antigens (e.g., microbes) in the tissues, specifically in the skin. They efficiently process these antigens into immunogenic peptide / MHC complexes. Such antigen-laden dendritic cells (including Langerhans cells of the epidermis) need to migrate from the site of antigen uptake to the lymphoid organs where they present the MHC / peptide complexes to antigen-specific T lymphocytes which, in turn, become activated and develop into effector T cells. We have previously defined routes of migration in human and murien skin. This FWF project aimed at defining the cytokines involved in the migration process. Studies in explant culture models of murine and human skin We defined a crucial migration-enhancing role for inflammatory cytokines TNF-alpha and interleukin-1 alpha as well as beta. Data from cross-blocking experiments using neutralizing anti-TNF and anti-IL-1 monoclonal antibodies suggest that either cytokine appears to be able to trigger migration independently. This is underscored by our findings in the TNF-alpha / lymphotoxin knock-out mouse of Dr. B. Ryffel, Basel. In these mice, that cannot make any TNF, emigration of dendritic cells from the skin into the culture medium seems to proceed normally. The same phenomenon was observed with knock-out mice that lack both receptors for TNF (gift from Dr. Bluethmann, Basel) as well as in mice that lack TNF-alpha, lymphotoxin-alpha, and IL-6 ("triple knock-out`s" from Dr. H.-P. Eugster, Zürich). This shows that TNF is not an absolute prerequisite for migration, although it is undoubtedly a crucial regulatory cytokine. TNF-a was shown to have a differential effect on the migration: at low concentrations it promoted emigration of dendritic cells; at high concentrations it inhibited. We attempted to find a mechanism for this novel observation and noted that CD54 / ICAM-1 was upregulated on lymphatic endothelial cells by the high concentrations of TNF-a. These data were published in the Journal of Leukocyte Biology (Stoitzner et al.). Colleagues at the Department of Internal Medicine noted that the cytokine IL-16 had a chemotactic effect on dendritic cells from the blood. We therefore wondered whether this cytokine was also operative in the skin. Indeed, IL-16 was found to be a migration-promoting molecule both in murine and in human skin explant cultures. Not only was this observed when IL-16 was experimentally added to skin explant cultures but also in vivo IL-16 seems to exert such an effect. We found (by immunohistochemistry) increased numbers of IL-16-containing cells under circumstances of enhanced dendritic cell migration. As a corollary, we noted that IL-16 can apparently exert its migration-enhancing effects in the absence of CD4, that serves as its receptor: IL-16 increased Langerhans cell migration in CD4-deficient mice. These data are now published in the Journal of Investigative Dermatology. In another set of experiments we found that matrix metalloproteinases (MMP`s), that have been reported to occur on dendritic cells, are crucial for the migration of skin dendritic cells. Inhibitors of MMP`s reproducibly inhibited the emigration of Langerhans cells from the skin. Judging from the use of more specific inhibitory reagents (Tissue inhibitors of metalloproteinases/TIMP, and neutralizing antibodies) we learnt that not only MMP-9 but also MMP- 2 are crucial for migration. We dissected the effects of MMPs on dendritic cells from both cutaneous compartments, i.e., on epidermal Langerhans cells and dermal dendritic cells. Both cells types appeared to be dependent on these enzymes for their migration. MMP-9 knock-out mice (in collaboration with Dr. R. Senior from Washington University, St. Louis, MO) impressivley confirmed the importance of this particular molecule. A manuscript describing these experiments is currently under review at the Journal of Immunology (July 2001). Finally, we were able to show that migration of dendritic cells at a great scale also occurs in vivo, and not only in the skin explant culture model. After "tape stripping" of skin a mild inflammation ensues and an efflux of Langerhans cells via lymphatic vesesls could be observed. These data were extended in that we used such tape- stripped skin as a model to study the migration of dendritic cell precursors from the bone marrow into the epidermis. Indeed, intradermally injected progenitor cells did immigrate into the overlying epidermis that had been stripped 4 days before. They did not migrate into unstripped epidermis. This model may prove useful for studying Langerhans cell ontogeny. It will be further exploited, and a manuscript describing our observations is being prepared. Relevance of dendritic cell migration Dendritic cells are being used in clinical trials with the aim of generating effective anti-tumor responses. First results are published, and they are encouraging. In most studies ex vivo generated, antigen-laden dendritic cells are administered intracutaneously, be it subcutaneously or intradermally. The intravenous route has also been taken. The assumption is that dendritic cells placed into the skin migrate to the draining lymph nodes. This assumption has been proven many times. Yet, even though injected dendritic cells can be recovered from the draining lymph nodes, this process is far from being efficient in quantitative terms. Very few studies have investigated the fate of radioactively labeled, injected dendritic cells in human patients undergoing immunotherapy. Interestingly, most of the injected radioactively labeled cells remained at the injection site; only a minor percentage made its way to the node. This is basically the same as previously found in the mouse. These data emphasize the need for a better understanding of the regulation of dendritic cell migration in order to be able to improve it. Some of the data corroborated in this FWF project may give a handle for further experimentation along these lines. For instance, it may prove helpful to (pre)treat injected cells with inflammatory cytokines. Additionally or alternatively it may be advantageous to help dendritic cells migrate by providing them with extra matrix metalloproteinases. These ideas will be tested in experimental models in animals and, eventually, in clinical study settings.