PROJECT SUMMARY/ABSTRACT Hematopoietic stem cells (HSCs) possess the ability to replenish a new functional hematopoietic system in recipients following transplantation and are the only curative treatment for many hematopoietic malignancies and disorders. However, the availability of donor human leukocyte antigen (HLA)–matched allogeneic HSCs is often too low for successful transplantation. Haplotype mismatch (or haploidentical) transplantation, where donors are matched at half of the HLA loci, is an attractive alternative but is limited by increased rejection, often requiring high-dose immunosuppression to sustain donor grafts. Current clinical transplantation strategies primarily target the adaptive immune response, however innate immune cells have recently emerged as key actors in the allograft response in various transplantation models, contributing to both the success and/or rejection of the transplant. Thus, better understanding of the innate immune response in transplantation and how to target it, is critical. We have recently discovered a novel function for the HSC niche molecule Vascular Cell Adhesion Molecule-1 (VCAM1) on HSCs, in which VCAM1 acts as a “don’t-eat-me” signal for innate immune phagocytes in the context of haplotype mismatch transplantation. Hence, manipulating VCAM1 expression levels may represent a promising immuno-regulatory approach to control the activity of innate immune cells promoting haplotype mismatch engraftment. In our first and second aims, we propose to use mouse models of transplantation to dissect the cellular and molecular mechanisms by which VCAM1 promotes immune evasion from phagocytes in the context of major histocompatibility complex (MHC)-I presentation. In our third aim, we will assess the impact of engineering VCAM1 expression levels on donor stem cells in promoting engraftment across the histocompatibility barrier. This proposal will provide valuable insights in the role of MHC-I engagement in controlling the function of innate immune cells, and illuminate new molecular targets to improve outcomes in immunologically mismatched stem cell transplantation assays.