Hematopoietic stem cell transplantation (HSCT) can cure a variety of benign and malignant diseases but graft versus host disease (GvHD), mediated by donor T cells, remains a primary cause of transplant related morbidity and mortality. In spite of the intense research efforts prevention and control of GvHD remains currently incomplete and novel therapeutic approaches are required. The present exploratory study will test the hypothesis that inhibition of cdk2 is a novel strategy to prevent GvHD. Studies in our laboratory have indicated that cdk2 has a central role in the generation of T cell immune responses and inhibition of cdk2 activation during antigen recognition is mandatory for induction of T cell anergy in vitro and tolerance in vivo. Cdk2 regulates various pathways and downstream functions. In conjunction with cyclin E, cdk2 phosphorylates the cell cycle inhibitor p27kip1 resulting in ubiquitin-targeted degradation. Cdk2 promotes phosphorylation of Rb on specific sites thereby reversing its ability to sequester E2F, and allowing for interaction of Rb with histone deacetylases (HDAC) and other chromatin remodeling proteins. Cdk2 directly regulates expression of genes including NF?B, Sp1, p300/CBP, and subunits of the RNA polymerase. Cdk2 also phosphorylates Smad3 and antagonizes its antiproliferative function induced by TGF-?, whereas impaired phosphorylation on the cdk-specific sites renders Smad3 more effective in executing its antiproliferative function. While cdk2 is essential for expansion of activated T cells, it is not critical for survival of resting lymphocytes, hematopoiesis or thymocyte development. These properties make cdk2 an attractive novel target for control of GvHD. Using (R)-roscovitine (CYC202), a potent inhibitor of cdk2-cyclin E, we determined that inhibition of cdk2 suppressed expansion of alloreactive T cells in vitro and in vivo and protected from acute lethal GvHD in a mouse model of allogeneic bone marrow transplantation. Our studies revealed that inhibition of cdk2 prevented expansion of alloreactive T cells while sparing Treg cells, promoted apoptosis of T cells escaping cell cycle arrest, abrogated Smad3 phosphorylation on the cdk2-specific site and prevented activation of NF?B. Taken together with the properties of cdk2, our findings suggest that cdk2 might be a novel treatment target for control of alloreactive T cells that mediate GvHD in patients undergoing HSCT. To validate this hypothesis we will undertake the following studies to: 1) Identify the effects of cdk2 inhibition on cellular mechanisms of GvHD. 2) Determine the effects of cdk2 inhibition on immune reconstitution and hematopoiesis after allogeneic BMT. 3) Determine the effects of cdk2 inhibition on cellular and molecular/signaling responses of human T cells. PUBLIC HEALTH RELEVANCE: GvHD is mediated by donor T cells after HSCT and has detrimental effects on the recipient of HSC. Therapeutic control of GvHD remains currently incomplete and novel therapeutic approaches are warranted. Our present exploratory studies will test the hypothesis that inhibition of cdk2 is a novel strategy to prevent GvHD. If such approach is successful, it will have significant clinical implications because more patients will benefit from allogeneic HSCT and achieve cure without the detrimental major side effect of GvHD.