Strategic exposure to donor antigens prior to transplantation can be an effective way for inducting donor-specific tolerance in allogeneic recipients. We have recently shown that pre-transplant infusion of donor splenocytes treated with the chemical cross-linker ethylcarbodiimide (ECDI-SPs) induces indefinite islet allograft survival in a full MHC-mismatched model without the need for any immunosuppression. Mechanisms of allograft protection by this strategy remain elusive. In this study, we show that the infused donor ECDI-SPs differentially target T cells with indirect versus direct allo-specificities. To target indirect allo-specific T cells, ECDI-SPs induce up-regulation of negative, but not positive, co-stimulatory molecules on recipient splenic CD11c+ DCs phagocytosing the injected ECDI-SPs. Indirect allo-specific T cells activated by such CD11c+ DCs undergo robust initial proliferation followed by rapid clonal depletion. The remaining T cells are sequestered in the spleen without homing to the graft site or the graft draining lymph node. In contrast, direct allo-specific T cells interacting with intact donor ECDI-SPs not yet phagocytosed undergo limited proliferation and are subsequently anergized. Furthermore, CD4+CD25+Foxp3+ T cells are induced in lymphoid organs and at the graft site by ECDI-SPs. We conclude that donor ECDI-SPs infusions target host allogeneic responses via a multitude of mechanisms including clonal depletion, anergy and immunoregulation, which act in a synergistic fashion to induce robust transplant tolerance. This simple form of negative vaccination has significant potential for clinical translation in human transplantation.

We have previously shown that pre- and post-transplant infusions of donor splenocytes treated with 1-ethyl-3-(3′-dimethylaminopropyl)-carbodiimide (ECDI-SPs) provide permanent donor-specific protection of islet allografts. The efficacy of donor ECDI-SPs in protecting vascularized cardiac allografts and mechanism(s) of protection are unknown. In the current study, we show that infusions of ECDI-SPs significantly prolong cardiac allograft survival concomitant with an impressive accumulation of CD11b+IDO+ cells in the cardiac allograft, and that the presence of this population is dependent on Gr1+ cells. Consequently, depletion of Gr1+ cells or inhibition of IDO activity abrogates graft protection by ECDI-SPs infusions. In addition, T cells from ECDI-SPs treated recipients secrete high levels of IL-10 and IL-13 upon in vitro restimulation, which are also dampened in recipients treated with the IDO inhibitor. Furthermore, combination of donor ECDI-SPs with a short course of rapamycin provides indefinite cardiac allograft survival in 100% of the recipients. These findings reveal a novel mechanism of donor ECDI-SPs in inducing cardiac transplant tolerance and provide several targets that are amenable to therapeutic manipulations for tolerance induction for cardiac transplantation.

Allogeneic islet cell transplantation is a promising treatment for human type 1 diabetes. Currently, human islets are transplanted via intra-portal infusions. While successful, it leads to significant early islet attrition from instant blood-mediated inflammatory reaction. An extra-hepatic site was established by transplanting islet-loaded microporous poly(lactide-co-glycolide) (PLG) scaffolds into the epididymal fat pad in syngeneic islet transplant models. This study examined this technology in allogeneic islet transplantation and determined whether transplant tolerance could be effectively induced to protect PLG scaffold transplanted allogeneic islets. The efficacy of an established tolerance induction strategy using donor splenocytes treated with ethylcarbodiimide(ECDI) was tested. ECDI-fixed donor splenocytes were infused 7 days before and 1 day after islet transplantation. Immediate normoglycemia was restored, and treated mice maintained indefinite normoglycemia whereas untreated mice rejected islet grafts within 20 days of transplantation. Interestingly, efficacy of tolerance induction was superior in PLG scaffold compared with intra-portal transplanted islets. Protection of PLG scaffold islet allografts was associated with several mechanisms of immune regulation. In summary, PLG scaffolds can serve as an alternative delivery system for islet transplantation that does not impair tolerance induction. This approach of combining tolerance induction with scaffold islet transplantation has potential therapeutic implications for human islet transplantation.

We have previously shown that preemptive infusion of apoptotic donor splenocytes treated with the chemical cross-linker ethylcarbodiimide (ECDI-SPs) induces long-term allograft survival in full MHC-mismatched models of allogeneic islet and cardiac transplantation. The role of myeloid derived suppressor cells (MDSCs) in the graft protection provided by ECDI-SPs is unclear. In this study, we demonstrate that infusions of ECDI-SPs increase two populations of CD11b+ cells in the spleen that phenotypically resemble monocytic-like (CD11b+Ly6CHI) and granulocytic-like (CD11b+Gr1HI) MDSCs. Both populations suppress T cell proliferation in vitro, and traffic to the cardiac allografts in vivo to mediate their protection via inhibition of local CD8 T cell accumulation and potentially also via induction and homing of regulatory T cells. Importantly, repeated treatments with ECDI-SPs induce the CD11b+Gr1HI cells to produce a high level of IFN-γ and to exhibit an enhanced responsiveness to IFN-γ by expressing higher levels of downstream effector molecules ido and nos2. Consequently, neutralization of IFN-γ completely abolishes the suppressive capacity of this population. We conclude that donor ECDI-SPs induce the expansion of two populations of MDSCs important for allograft protection mediated in part by intrinsic IFN-γ dependent mechanisms. This form of preemptive donor apoptotic cell infusions has significant potential for the therapeutic manipulation of MDSCs for transplant tolerance induction.

Peritransplant infusion of apoptotic donor splenocytes cross-linked with ethylene carbodiimide (ECDI-SPs) has been demonstrated to effectively induce allogeneic donor-specific tolerance. The objective of the current study is to determine the effectiveness and additional requirements for tolerance induction for xenogeneic islet transplantation using donor ECDI-SPs. In a rat-to-mouse xenogeneic islet transplant model, we show that rat ECDI-SPs alone significantly prolonged islet xenograft survival but failed to induce tolerance. In contrast to allogeneic donor ECDI-SPs, xenogeneic donor ECDI-SPs induced production of xenodonor-specific antibodies partially responsible for the eventual islet xenograft rejection. Consequently, depletion of B cells prior to infusions of rat ECDI-SPs effectively prevented such antibody production and led to the indefinite survival of rat islet xenografts. In addition to controlling antibody responses, transient B-cell depletion combined with ECDI-SPs synergistically suppressed xenodonor-specific T-cell priming as well as memory T-cell generation. Reciprocally, after initial depletion, the recovered B cells in long-term tolerized mice exhibited xenodonor-specific hyporesponsiveness. We conclude that transient B-cell depletion combined with donor ECDI-SPs is a robust strategy for induction of xenodonor-specific T- and B-cell tolerance. This combinatorial therapy may be a promising strategy for tolerance induction for clinical xenogeneic islet transplantation.