Graft-versus-leukemia (GVL) response is critical for the curative potential of allogeneic bone marrow transplantation (BMT) in a number of hematological malignancies. However, the application of this effective therapeutic modality has been impeded by the tight link between GVL and the most severe complication of allogeneic BMT, graft-versus-host disease (GVHD). Hence, improved understanding of the immuno-biology of the toxicity (GVHD) and efficacy (GVL) of allogeneic BMT will allow clinicians to better harness this powerful treatment modality. Donor T cells are critical for both GVHD and GVL. While allo-antigen specific nave donor T cells cause GVHD, they also contribute to GVL, explaining the tight linkage between the two processes. However, T cells specific for the tumor specific-antigens (TSA) expressed exclusively by the leukemia contribute to GVL but not to GVHD. Recent observations have brought in a renewed focus on the role of hematopoietic derived antigen presenting cells (APCs) in the presentation of allo-antigens in the context of allogeneic BMT. Because host APCs and the target tissues express allo-antigens, they can present them directly to donor CD8+ T cells. By contrast the TSAs have to be cross-presented on a subset of professional hematopoietic APCs, the CD8+ dendritic cells (DCs), to elicit a robust TSA specific T cell response. To this end we posit a novel approach that does not involve direct manipulation of effector cells (T or NK cells), but one that will focus of antigen presentation by host APCs. Data generated and published during the current funding cycle from our lab, and others, demonstrate that any individual hematopoietic APC subset is dispensable for induction of GVHD, but are collectively, critical for GVL responses. Furthermore, unpublished preliminary data show that a specific subset of DCs, namely the CD8+DC subsets, is required for cross-presentation of TSAs and induction of optimal GVL after allogeneic BMT. However the specific cellular signals and the key molecular pathways that are critical for enhancing cross-presentation of TSAs on host CD8+DCs and thus potentially augment GVL without aggravation of allo-antigen driven GVHD are not known. Therefore, the central premise of our proposal is that it may be possible to separate GVL from GVHD specific responses by targeting antigen presentation pathways in the host CD8+DCs. In this proposal we will build on our exciting preliminary observations and test the following hypotheses. The specific aims of this proposal are: Specific Aim (SA) 1: To determine the cell autonomous role of host CD8+DCs and the signals that promote cross-presentation and enhance GVL after allogeneic BMT. We will test the hypothesis that host CD8+DCs have the unique ability to cross-present TSAs and, that signaling through TLR3-->TRIF and Nod1/2-->Rip2 pathways will enhance their ability to exclusively augment GVL without aggravating GVHD after allogeneic BMT. Specific Aim (SA) 2: To analyze the intracellular mechanisms of cross-presentation in the host APCs that are critical for GVL. We will explore the hypothesis that cross-presentation of TSAs via the phagosome-endoplasmic reticulum (ER)-Golgi intermediate compartment (ERGIC) pathway in CD8+DCs is critical for enhancing GVL responses without exacerbating GVHD. Together, the above studies will illuminate the role of host APC subsets, CD8+DCs, and the critical molecular pathways of TSA cross-presentation in the induction of a robust GVL response without the concomitant exacerbation of GVHD. If successful, it will allow for modulation of a novel mechanism, targeting antigen presentation on APCs to meaningfully separate GVL and GVHD. This information will thus improve our understanding of the immuno-biology of GVH responses and could lead to the development of innovative therapeutic strategies targeting antigen presentation on host APC subsets to enhance TSA specific donor T cell mediated GVL without aggravating GVHD.