Allogeneic stem-cell transplantation (SCT) is effective in part due to the graft-versus-tumor (GvT) potential of the donor graft~ however, successful SCT is often limited by graft-versus-host disease (GvHD). Acute GvHD is a major cause of morbidity and mortality after allogeneic SCT for blood cancers, occurring in 30-70% of transplants. With 28,000 allogeneic transplants performed worldwide each year, 15% of deaths following allogeneic SCT are a direct result of GvHD. Existing modalities for both prevention and treatment of GvHD are immunosuppressive, impair immune recovery and may limit the GvT effect. Our prior work has demonstrated that blockade of CCR5, a chemokine receptor, significantly decreased visceral GvHD in humans. The overarching goal of the current proposal is to exploit pharmacologic contro of lymphocyte trafficking after allogeneic SCT for therapeutic benefit. The central hypothesis is that T-cell trafficking plays a central role in the three major clinical and immunologic outcomes of allogeneic SCT - GvHD, GvT and reconstitution of a healthy immune system. To test this hypothesis, a clinical trial will be conducted in order to simultaneously investigate mechanistic and therapeutic endpoints. The experimental approach revolves around a phase II trial of extended CCR5 blockade in unrelated donor SCT for patients with blood cancers. Aim 1 will determine the clinical and immunologic effects of extended CCR5 blockade after allogeneic SCT and identify predictors of response using appropriate pharmacodynamic and genotypic methods. Aim 2 will determine the effect of extended CCR5 blockade on immune recovery and anti-tumor immunity by conducting a series of validated assays. Aim 3 will identify the role of CCR5 in determining the tissue-tropism of effector T-cells, using a novel sequencing technology to survey the entire repertoire of T-cells and characterize their phenotype. The use of samples from 2 clinical trials together with control samples from a biospecimen bank will add power to the analysis of all 3 aims. This proposal is highly translational, revolves around a human clinical trial, and takes advantage of appropriate immune pharmacodynamic methods, developed specifically for this proposal, and genomic sequencing of T-cell clones to identif the chemokine receptor signature that is involved in T-cell recruitment into specific tissues. Combining patient-oriented research with state of the art immunologic assays will have a dual role - fuel our basic understanding of trafficking mechanisms in transplantation and cancer immunology, and at the same time change the standard of care by introducing a new class of targeted, minimally immunosuppressive medications to the field. If successful, this proposal may lead to a paradigm shift in the prevention of GvHD, setting the stge for a randomized clinical trial that will rely on the same clinical and immunologic endpoints to measure its success.