Immunological Tolerance by Indoleamine 2,3 Dioxygenase

To achieve immunological tolerance specifically towards alloantigens while mainting immunity against pathogens remains the main challenge for improving the final outcome in bone marrow transplantation (BMT). Most recently, the expression of indoleamine-2,3-dioxygenase (IDO), the rate limiting enzyme for tryptophan metabolism, by antigen presenting cells (APC) has been implicated to mediate T-cell anergy in vitro and in vivo. The enzymatic activity of IDO deprives stimulated T cells from the access to tryptophan and exposes them to tryptophan derivates. These alterations of the microenvironment are supposed to allow T cells to undergo initial steps of T cell activation but to prevent completion of the cell cycle thus to induce apoptosis. By such mechanisms of mediating T cell non- responsiveness IDO expressing APC are reasonably conceived to bear the potential to induce antigen-specific tolerance. Experimental evidence suggests that IDO can be induced in monocytes/macrophages (mo/mph) and dendritic cells (DC) by transfection of the human IDO gene as well as by several cytokines, including prolonged exposure to interferon gamma (IFN) and several DC maturation agents. The core of the proposed project is to test the hypothesis that APC induced to express IDO will acquire the capacity to mediate allo-antigen specific tolerance. The hypothesis predicts that APC with highly immune stimulatory potential (i.e. DC matured by lipopolysaccharide [LPS] and IFN) will be converted to a tolerogenic APC once IDO is overexpressed (e.g. by transfection). Further, we will systematically explore conditions to optimally and stably induce IDO expression in mo/mph and DC by cytokines. T cells stimulated by IDO expressing APC, in addition to an apoptotic decline, will be examined for a decrease of their proliferative and cytotoxic capacity and an eventual shift of their polarization (Th1 versus Th2 phenotype). To test for allo-antigen specific tolerance we will examine whether T cell populations initially exposed to IDO expressing allogeneic APC will, in secondary culturers, fail to produce effector responses to the alloantigen but maintain responses to pathogens (e.g. tetaus toxoid, candida). Thirdly, it will be important to examine whether the such tolerized T cells exert regulatory function, i.e. will suppress fresh T cells from responding to polyclonal stimulation. If the hypothesis holds true it may be envisioned that by modulating IDO expression APC can be targeted specifically to direct T cell responses they stimulate along an immune effector or a tolerogenic pathway.

To achieve immunological tolerance specifically towards alloantigens while mainting immunity against pathogens remains the main challenge for improving the final outcome in bone marrow transplantation (BMT). Most recently, the expression of indoleamine-2,3-dioxygenase (IDO), the rate limiting enzyme for tryptophan metabolism, by antigen presenting cells (APC) has been implicated to mediate T-cell anergy in vitro and in vivo. The enzymatic activity of IDO deprives stimulated T cells from the access to tryptophan and exposes them to tryptophan derivates. These alterations of the microenvironment are supposed to allow T cells to undergo initial steps of T cell activation but to prevent completion of the cell cycle thus to induce apoptosis. By such mechanisms of mediating T cell non- responsiveness IDO expressing APC are reasonably conceived to bear the potential to induce antigen-specific tolerance. Experimental evidence suggests that IDO can be induced in monocytes/macrophages (mo/mph) and dendritic cells (DC) by transfection of the human IDO gene as well as by several cytokines, including prolonged exposure to interferon gamma (IFN) and several DC maturation agents. The core of the proposed project is to test the hypothesis that APC induced to express IDO will acquire the capacity to mediate allo-antigen specific tolerance. The hypothesis predicts that APC with highly immune stimulatory potential (i.e. DC matured by lipopolysaccharide [LPS] and IFN) will be converted to a tolerogenic APC once IDO is overexpressed (e.g. by transfection). Further, we will systematically explore conditions to optimally and stably induce IDO expression in mo/mph and DC by cytokines. T cells stimulated by IDO expressing APC, in addition to an apoptotic decline, will be examined for a decrease of their proliferative and cytotoxic capacity and an eventual shift of their polarization (Th1 versus Th2 phenotype). To test for allo-antigen specific tolerance we will examine whether T cell populations initially exposed to IDO expressing allogeneic APC will, in secondary culturers, fail to produce effector responses to the alloantigen but maintain responses to pathogens (e.g. tetaus toxoid, candida). Thirdly, it will be important to examine whether the such tolerized T cells exert regulatory function, i.e. will suppress fresh T cells from responding to polyclonal stimulation. If the hypothesis holds true it may be envisioned that by modulating IDO expression APC can be targeted specifically to direct T cell responses they stimulate along an immune effector or a tolerogenic pathway.