Bone marrow transplantation (BMT) has emerged as a major therapeutic for end-stage blood cell diseases. The success of BMT depends on donor-recipient Mhc alloantigen comptibility and on immuno-suppression. Despite these preventive measures, graft-verus-host disease (GvHD) ensues and hence, the therapeutic potential of BMT is not fully realised. The long-term goal of this project is to delineate the mechanisms by which minor histocompatibility (mH) alloantigens initiate GvHD. We have made significant new discoveries underlying the molecular basis of immunity to mH antigens during the last funding period: a) we discovered the elusive H4 mH alloantigen in which a threonine to isoleucine variation causes differential phosphorylation and alloreactivity; Jb) the duration of mH alloantigen presentation and the avidity of antigen/Tcr interaction impacts immunodominance; c) an unusually high H60-specific CTL precursor (pCTL) frequency impacts its immunodominance over the other mH antigens; and d) CD4 T cell help is essential for primary CTL response to both dominant and recessive mH antigens. From these novel findings several important questions emerge: a) what are the cellular and biochemical bases of mH alloantigen cross-presentation in vivo? and b) what determines immunodominant pCTL frequencies in vivo? In this proposal, we will test the central hypothesis that mH antigens are donated as proteasomal products (but not as the epitope itself) to acceptor dendritic cells for cross-presentation to specific CTL whose repertoire andprecursor frequency are shaped by AIRE-dependent medullary thymic epithelial expression of self variants of the minor alloantigens. Our strategy to test this hypothesis will be to: a) determine the cellular and biochemical mechanisms of Mhc class l-restricted mH antigen processing and cross-presentation; and b) delineate the mechanisms by which self peptide(s) regulate the development of the mH antigen-specific pCTL repertoire. Because of our extensive experience with studies of various aspects of T cell antigen processing, presentation and recognition, we are well situated to undertake the proposed research. Upon completion of this project, we expecf to elucidate the mechanisms by which class l-restricted mH antigens are presented by dendritic cells to specific T cells and how mH alloantigen-specific pCTL repertoires are generated. Insights into these mechanisms can be harnessed to develop preventive and therapeutic strategies against GvHD. Additionally, the beneficial effects of graft-versus-leukemia, a by-product of GvHD, can be utilised as a therapy against cancers.