vs. marrow or in host tissues. We confirmed and extended the relevance of IL-6 deficiency on GVHD and GVT by treating BMT recipients with anti-mouse IL-6 receptor (IL-6R), MR16-1. == Results == Deficiency of IL-6 in donor T cells led to prolongation of survival. Total inhibition of IL-6 with MR16-1 caused an greater reduction in GVHD-induced mortality even. The reduction in GVHD PD 150606 was independent of the direct effects on T effector cell expansion or donor regulatory T cells. PD 150606 GVT responses were preserved after treatment with MR16-1. == Conclusion == MR16-1 treatment reduced GVHD and preserved sufficient GVT. Tocilizumab, a humanized anti-IL-6R mAb, is approved in several countries including the PD 150606 United States and European Union for the treatment of rheumatoid arthritis and other inflammatory diseases. Blockade of IL-6 with anti-IL-6R mAb therapy may be testable in clinical trials as an adjunct to prevent GVHD in BMT patients without a significant loss of GVT. Keywords:cytokines, IL-6, BMT, GVHD, GVT == INTRODUCTION == Allogeneic hematopoietic cell transplantation (HSCT) is one of the curative therapeutic options for hematological malignancies and solid tumors(1). The mechanism of tumor eradication in allogeneic HSCT recipients is the immune-mediated graft-versus-tumor (GVT) effect. The host hematopoietic system is reconstituted with donor hematopoietic donor Tmem17 and system lymphocytes, PD 150606 especially T cells that have the potential to exert GVT effects concurrently with graft-versus-host PD 150606 disease (GVHD) (2,3). Clinical data have demonstrated that the severity of GVHD is correlated with chance of relapse (1,4). Acute GVHD is a major side effect of allogeneic HSCT including bone marrow transplantation (BMT)(1,5). A large number of pro-inflammatory cytokines are involved in the pathophysiology of GVHD. The presence of a pro-inflammatory milieu is attributable to the (a) conditioning regimen, (b) the allo-reaction, and (c) the damage induced in the host tissues as a consequence of them both (5,6). Several inflammatory cytokines such as interleukin-1 (IL-1), tumor necrosis factor- (TNF-), and interferon- (IFN-) are elevated after allogeneic BMT and known to perpetuate GVHD through either direct cytotoxic effects on host tissues (6) or by activation and/or priming of immune effector cells (7. Blockade of these cytokines modulated the severity of GVHD in mice and humans while several lines of experimental evidence suggest that appropriate intervention with cytokine blockade GVT {Hill, 1999 #27401, 8). IL-6 is known to have broad effects, including activation and maturation of B cells, T cells, and macrophages (9). IL-6 has been directly implicated in many experimental and human inflammatory diseases (9). Blockade of IL-6 signaling with anti-IL-6R mAb demonstrated salutary effects in mouse models of several inflammatory diseases (9,10). More interestingly, blockade of IL-6 with tocilizumab, a humanized monoclonal antibody (mAb) specific for IL-6 receptor (IL-6R), has been shown to have therapeutic effects in many human diseases, such as rheumatoid arthritis, Castlemans disease, and juvenile idiopathic arthritis (9,1113). Tocilizumab is approved for clinical use in the United States now, the European Union, and other countries (14). Several lines of correlative data and indirect evidence suggest that IL-6 may be relevant in the induction and severity of GVHD. For example, IL-6 polymorphisms in the recipients of HLA-matched sibling transplants have been found to be associated with acute and chronic GVHD (15), while serum levels of IL-6 have been found to be elevated in patients with acute GVHD (16). Infusion of exogenous IL-6 has been shown to exacerbate (17), while against the IL-6 receptor (IL-6R) reduced experimental GVHD (18). However, the source of IL-6, whether it plays a direct pathogenic role in GVHD and the impact of its blockade on the critical GVT effects after allogeneic BMT are not known (1821). In parallel and independent experiments performed in different laboratories across two countries, we utilized a combination of genetic and antibody blockade approaches in multiple strain models to demonstrate that IL-6 plays an important role in the severity of GVHD and that its blockade can ameliorate GVHD without leading to a complete loss of GVT responses. Given the safety and efficacy of IL-6 blockade in the treatment of several immunological diseases in humans(22), our data suggest that targeting IL-6 with tocilizumab might be an immediately testable strategy in humans to mitigate acute GVHD without losing the significant anti-tumor benefits of allogeneic BMT. == MATERIALS and METHODS == == Mice == Female C57BL/6 (B6, H-2b, CD45.2+), BALB/c (H-2d, CD45.2+), B6-Ly5.2/Cr (B6-CD45.1, H-2b, CD45.1+), and B6D2F1 (F1, H-2b/d, CD45.2+) mice were purchased from Charles River Laboratories (Wilmington, MA) and B6.129S2-Il6tm1Kopf/J (IL-6/, H-2b, CD45.2+), C3H.SW (H-2b, CD45.2+), and B6(C)-H2bm12/KhEgJ (B6-H-2bm12, CD45.2+) mice were purchased from Jackson Laboratories (Bar Harbor, ME). B6-background GFP-FoxP3 knock-in (GFP-Foxp3, H-2b, CD45.2+) mice were provided by Dr. Rudensky (University of Washington, Seattle).