R.R. the antiapoptotic regulator cellular caspase-8like inhibitory protein. Donor alloreactive T cells used the cognate proteins FasL and TNF-related apoptosis-inducing ligand (TRAIL) (but not TNF or perforin) to mediate tGVHD, thereby damaging thymic stromal cells, cytoarchitecture, and function. Strategies that interfere with Fas/FasL and TRAIL/DR5 interactions may therefore represent a means to attenuate tGVHD and improve T cell reconstitution in allo-BMT recipients. == Introduction == Allogeneic BM transplantation (allo-BMT) is usually a potentially curative therapy for a number of malignant and nonmalignant disorders. Myeloablative and nonmyeloablative conditioning regimens, which may contain radiation, chemotherapy, and immunosuppressive drugs, enable the engraftment of donor hematopoietic stem cells and prevent rejection by the host. Allo-BMT is frequently followed by a prolonged period of profound immune deficiency, which is associated with a high incidence of contamination (1,2) and malignant relapse (3). Evidence from studies of patients receiving allo-BMT suggests that deficient T cell immunity in the first 12 months after transplantation may be due to insufficient DTP3 T cell numbers and restricted T cell repertoire and function (4). A broad T cell receptor repertoire requires the de novo generation of T cells in the thymus (57), and although thymic function in humans is age dependent and decreases after puberty (810), the adult thymus contributes substantially to immune reconstitution after allo-BMT (11). Factors that inhibit thymic function after allo-BMT include thymic damage by the conditioning regimen (9,12) and graft-versus-host disease (GVHD) mediated by donor alloreactive T cells (13,14). Thymic GVHD (tGVHD) damages the architecture and composition of the thymic microenvironment (13,15,16). Since effective development and normal T cell repertoire selection are critically dependent on a structured thymic microenvironment (17), tGVHD results in extended T lymphopenia, coupled with a restricted donor T cell repertoire and the appearance of clones with anti-host reactivity (18,19). Thymic cellularity is usually reduced primarily because of a decrease in CD4+CD8+(double-positive [DP]) thymocytes, which occurs due to the failure of resident pro- and pre-T cells to enter the cell cycle as well as enhanced apoptosis of CD4+CD8+thymocytes (20,21). Although Hollander and colleagues have previously DTP3 suggested that thymic epithelial cells may be targeted by alloreactive donor T cells and damaged via IFN-, these studies were primarily done in a graft-versus-host reaction (GVHR) model system, without conditioning such as radiation or chemotherapy or with in vitro culture of thymic stromal cell lines (22). By contrast, the cellular and molecular mechanisms by which cytotoxic preparative regimens and acute GVHD mediated by donor alloreactive T cells cause damage, as well as the effects of conditioning around the thymus, have not been well studied, although Blazar and colleagues have previously shown that keratinocyte growth factor (KGF, palifermin) may be cytoprotective against tGVHD (23,24). Therefore, we undertake here an analysis in clinically relevant GVHD models to elucidate the development of tGVHD and the DTP3 mechanisms by which allogeneic T cells can infiltrate and damage the thymus. We first demonstrate the exquisite sensitivity of the thymus to damage by even very small numbers of donor alloreactive T cells and then define the trafficking, coactivating or coinhibitory, cytotoxic molecules and cytokines relevant for these alloreactive T cells to cause tGVHD. Furthermore, we have used a clinically relevant radiation-dependent transplantation model to study the effects of radiation on thymic stroma and its impact on tGVHD. == Results == == Donor alloreactive T cells are required for tGVHD and damage the thymus in a dose-dependent manner. == Mature donor T cells in the BM allograft are the primary initiators of acute GVHD, and disease severity correlates with the dose of donor T cells. We therefore began by characterizing the numbers of donor alloreactive T cells in the allograft and their impact on systemic and tGVHD. We performed experiments in the well-defined MHC-disparate mouse model Rabbit Polyclonal to OR2B3 system C57BL/6 (B6, H-2b) BALB/c (H-2d). BALB/c recipients received 8.5 Gy radiation and an allograft made up of 5 106B6 CD45.1 T DTP3 celldepleted BM (TCD-BM) cells and varying numbers of WT B6 CD45.2 T cells,.
