Human leukocyte antigen and T cell receptor transgenic models of xeno- and allogeneic graft rejection and neonatal tolerance induction

Success in xenotransplantation will depend on the development of strategies that induce transplantation tolerance. Classic protocols of neonatal tolerance induction, which are extremely effective in allogeneic model systems, have failed in xenogeneic models. To determine whether failure results from an intrinsic property of the xenogeneic major histocompatibility complex (MHC) molecules or, instead, is caused by some limitation in species-specific molecular interactions distinct from the polymorphic domains of xenogeneic MHC molecules, we have established a panel of Tg mice that express a hybrid human MHC class I molecule in which the polymorphic alpha1 and alpha2 domains are derived from either the human HLA-A2, -B7, or B-27 alleles and the remainder of the molecule is derived from the mouse H-2Kb protein. Initial experiments (Chapter 2) evaluated the function of the Tg HLA class I molecules as transplantation antigens. Hybrid HLA class I molecules induced a strong xeno-CD8+ T cell response as assessed by in vitro cell mediated lympholysis assays and were rapidly rejected by non-Tg recipients when presented as skin grafts. This rejection did not depend on CD8+ or CD4+ T cells or B cells. In vitro and in vivo allorecognition of locus-matched Tg hybrid HLA-B alleles was CD8-dependant.;In chapter 3, we addressed the requirement for clonal deletion of alloreactive CTL in an MHC class I-disparate model of neonatal tolerance. Neonatal T cell receptor Tg recipient mice in which the majority of CD8+ T cells expressed a single alpha/beta TCR alloreactive to H-2Ld were pretreated with semi allogeneic, H-2Ld expressing cells. These neonates were unresponsive to H-2Ld-expressing targets in vitro and were tolerant to H-2Ld skin grafts. Their peripheral lymphoid organs demonstrated a profound reduction of Tg CD8+ T cells and the mice were chimeric.;To evaluate the potential for the induction of neonatal tolerance to xenogeneic MHC in the hybrid HLA Tg model system in which species-specific molecular interactions had been optimized, we injected newborn H-2 matched, non-Tg mice with splenocytes from HLA-B7hyb Tg mice (Chapter 4). HLA-B7hyb injected neonates were unresponsive to B7 hyb-expressing targets in vitro. They specifically accepted B7 hyb-expressing skin grafts and demonstrated peripheral chimerism. These experiments suggest that providing species-specific molecular interactions are conserved, xenogeneic MHC does not pose an insurmountable barrier to xenograft tolerance.