== Donor-specific hyporesponsiveness in the longest surviving animal as measured by serial donor and third-party MLR. == Discussion == Alloantigen exposure prior to the time of organ transplantation has clear immunomodulatory effects. transplantation. Graft survival was assessed without any posttransplant immunosuppression. PVDST alone or in combination 2-Hydroxy atorvastatin calcium salt with tacrolimus was ineffective. However, PVDST in combination with sirolimus significantly prolonged renal allograft survival to a mean of 24 days. Preoperative sirolimus alone had no effect, and peripheral DST with sirolimus prolonged graft survival in 2/4 animals, but resulted in accelerated rejection in 2/4 animals. These data demonstrate that PVDST in combination with sirolimus delays rejection in a modest but measurable way in a rigorous model. It may thus be a preferable method for donor antigen administration. Keywords:Donor-specific transfusion, rhesus monkey, tolerance == Introduction == Alloantigen exposure typically induces a detrimental allospecific immune response. This often results in sensitization, a cellular and/or humoral effector response resulting in the accelerated rejection of transplanted allografts. However, donor-specific transfusion (DST) of allogeneic hematopoietic cells in some settings can induce an allospecific protective or regulatory response that impedes rejection. Various mechanisms have been proposed including activation-induced cell death (AICD) of effector T cells and induction of regulatory T cells (1). Indeed, in several experimental rodent models DST combined with certain preoperative immunosuppressive therapies has been shown to induce a durable state of tolerance (2,3). As such, various DST 2-Hydroxy atorvastatin calcium salt regimens have been proposed as tools to help prevent clinical allograft rejection. These are to be distinguished from mixed chimerism approaches in that DST transfused elements 2-Hydroxy atorvastatin calcium salt are largely eliminated from the circulation rather than establishing durable chimerism; and while microchimerism has been suggested as a beneficial mechanism involved in the DST effect (4) no attempts are made, such as myeloablation, to establish engraftment of the transfused elements or macrochimerism. A clinically relevant antirejection effect of blood transfusion has long been recognized (5), and several investigators have used variable means of DST as adjuvant maneuvers in clinical transplant trials. These have included use of blood transfusion and more recently donor bone marrow infusion with variable and at best subtle results (6-11). Importantly, the clinical use of pretransplant DST must be balanced by the risk of sensitization (12), and given that the antirejection effects are modest and somewhat stochastic, all forms of DST remain investigational. Nevertheless, several trials have been performed using donor bone marrow as a therapeutic adjunct (13). Importantly, the mechanisms facilitating a DST effect have been shown in rodents to be sensitive to concomitant use of immunosuppressive agents, with calcineurin inhibitors (CNIs) generally preventing and mammalian target of rapamycin (mTOR) inhibitors generally promoting a salutary DST effect (14,15). Thus, with improved pharmacologic immunosuppression and most significantly the introduction of CNIs (16), the DST effect has largely dissipated and the practice fallen out of favor (17,18). In addition to the effect of immunosuppressive drugs, the route of DST has been suggested to be significant. The liver is known to play a role in tolerance to environmental antigens, likely an adaptation preventing inappropriate immune responses to gut flora and food antigens (19). Although the mechanisms are not completely elucidated, a growing body of literature implicates liver sinusoidal endothelial cells (LSECs) as playing a critical role in portal tolerance (20-24). These unique antigen-presenting cells (APCs) predispose CD4+T cells to secrete suppressive cytokines (24). Along with Kupffer cells (25), the LSECs may create a protolerant microenvironment within the liver that can potentially alter systemic responses to alloantigens. Expanding on these concepts, investigators have explored multiple rodent models of portal venous (PV) DST and shown it to induce lasting 2-Hydroxy atorvastatin calcium salt tolerance to subsequently placed allografts (25-27). Interestingly, PVDST, unlike peripheral DST, has been shown in mice to be aided by adjuvant CNIs, making it potentially compatible with modern CNI-based regimens (26). These results, though not validated in more clinically relevant models, have prompted limited human trials of PVDST that have suggested that PVDST decreases acute rejection episodes and immunosuppressant requirements (28-30). We have shown that peripheral DST given as part of certain costimulation blockade-based regimens may promote protolerant effects of these therapies in a rigorous nonhuman primate (NHP) renal allograft model (31,32). However, the effects referable to DST administration have not been marked, and to date, a definitive role for DST has not been established, nor has evidence been forthcoming objectively assessing the role of CNIs, mTOR inhibitors, or route. We have therefore preformed studies to specifically evaluate the role of DST and the effect of Cspg2 portal administration and adjuvant immunosuppression in.