Project 4. Role of Functionally Distinct Dendritic Cell Subsets in Tolerance and Graft-Versus-Host Disease We recently discovered that effector T cell subsets, including THI, TH2 and Treg cells, can induce monocytes to differentiate into dendritic cells (DCs) that, in turn, selectively induce the formation of THI, TH2 and Treg cells from naive T cells, respectively. We hypothesize that this heretofore unknown immunoregulatory mechanism may play an important role in the control of protective immunity, including proinflammatory responses and tolerance induction, as well as pathologic immunity such as that seen in graft versus host disease (GVHD). We further believe that it may be possible to target and /or manipulate these novel DC subsets to treat or prevent such disorders. The overall goals of this project are to assess the possible role of immunoregulatory DCs (DCrreg) in immune tolerance and GVHD in recipients of hematopoetic stem cell transplants with or without organ transplants, and identify molecules that induce the formation and mediate the functions of these DCs. These goals will be pursued by 1) assessing and comparing the phenotypic, functional and gene expression profiles of T cells and non-T antigen presenting cells in the blood and kidneys of transplant recipients who are immunologically tolerant of their grafts versus those who are not, 2) using neutralizing antibodies and gene expression profiling to identify key factors expressed by Treg cells that are responsible for inducing monocytes to differentiate into DCxreg, as well as the key factors expressed by DCrreg that are responsible for inducing the formation of Treg cells from naive T cells, and 3) evaluating the effects of DCrreg, as well as Treg cells induced by DCrreg, in a murine model of acute GVHD and organ transplant tolerance. This Project is highly dependent on interactions with Projects 1 and 3 as well as Cores B and C, which are the sources of clinical materials, mouse models and immune and gene profiling assays, respectively. Success in this project should ultimately lead to safer and more effective ways to achieve immune tolerance in the setting of allogeneic transplantation.