Hematopoietic Stem Cell Transplantation Graft-versus-Host Disease in Allogeneic Flagellin, a TLR5 Agonist, Reduces

Graft-versus-host disease (GVHD) is a major cause of morbidity and mortality in patients treated with allogeneic hematopoietic stem cell transplantation (HSCT). Posttransplant immunosuppressive drugs incompletely control GVHD and increase susceptibility to opportunistic infections. In this study, we used flagellin, a TLR5 agonist protein (∼50 kDa) extracted from bacterial flagella, as a novel experimental treatment strategy to reduce both acute and chronic GVHD in allogeneic HSCT recipients. On the basis of the radioprotective effects of flagellin, we hypothesized that flagellin could ameliorate GVHD in lethally irradiated murine models of allogeneic HSCT. Two doses of highly purified flagellin (administered 3 h before irradiation and 24 h after HSCT) reduced GVHD and led to better survival in both H-2 b → CB6F1 and H-2 K → B6 allogeneic HSCT models while preserving >99% donor T cell chimerism. Flagellin treatment preserved long-term posttransplant immune reconstitution characterized by more donor thymic-derived CD4 + CD25 + Foxp3 + regulatory T cells and significantly enhanced antiviral immunity after murine CMV infection. The proliferation index and activation status of donor spleen-derived T cells and serum concentration of proinflammatory cytokines in flagellin-treated recipients were reduced significantly within 4 d posttransplant compared with those of the PBS-treated control recipients. Allogeneic transplantation of radiation chimeras previously engrafted with TLR5 knockout hemato-poietic cells showed that interactions between flagellin and TLR5 expressed on both donor hematopoietic and host nonhemato-poietic cells were required to reduce GVHD. Thus, the peritransplant administration of flagellin is a novel therapeutic approach to control GVHD while preserving posttransplant donor immunity. T he major causes of mortality in cancer patients treated with allogeneic hematopoietic stem cell transplantation (HSCT) are graft-versus-host disease (GVHD), relapse, and opportunistic infections (1, 2). GVHD is caused by allore-active activated donor T cells, in part, through the elaboration of inflammatory cytokines released by host tissues in response to pretransplant conditioning with radiation and/or high-dose che-motherapy (3). Recent data suggest that, beside the donor T cells, host epithelial cells also have a role in the initial presentation of the alloantigen to donor T cells during the initiation of GVHD (4– 6). Approximately 30–40% and 50–60% of long-term survivors of allogeneic HSCT develop acute GVHD and/or chronic GVHD, respectively, despite prophylactic administration of immunosup-pressive drugs (7). GVHD causes host tissue damage and suppresses the function of lymphoid organs, leading to increased susceptibility to opportunistic infections in human and in animal model systems (8, 9). Moreover, the long-term impairment …