| Abstract: | 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.Pancreatic islet transplantation is a promising treatment option for type 1 diabetes (1). However, a major limitation to its widespread clinical application is the shortage of human donor pancreata (2,3). Xenogeneic sources of islets are an attractive alternative. Currently, porcine islets are considered the best suitable substitute for human transplantation because of the unlimited donor source and their functional compatibility in humans (4). Moreover, they may be resistant to recurrent autoimmunity that is potentially present in recipients of islet transplantation (5,6). Unfortunately, the need for aggressive immunosuppression to control xenogeneic rejection is currently prohibitive for its application as a standard therapy for β-cell replacement in humans (7,8). Therefore, effective tolerance strategies for xenogeneic transplantation are urgently needed.Early studies in xenogeneic transplant models point to a critical role of T-cell–mediated processes in xenograft rejection (7–10). However, B cells are increasingly recognized for their role in xenogeneic immunity (11,12). In addition to mediating humoral responses by differentiating into antibody-producing plasma cells, B cells have also been shown to influence T-cell priming, expansion, and differentiation through a variety of mechanisms, including antigen presentation, costimulation, and cytokine production (13–16). Consequently, B-cell deficiency or depletion ameliorates autoimmune diseases, including type 1 diabetes, multiple sclerosis, and rheumatoid or collagen-induced arthritis (17–19). Likewise, B-cell depletion has been demonstrated to prolong allogeneic and xenogeneic graft survival in nonhuman primates (12,20).We have previously shown that intravenous infusion of donor splenocytes cross-linked with ethylene carbodiimide (ECDI-SPs) induces donor-specific tolerance to allogeneic islet and heart grafts (21–23), and the mechanisms of graft protection in these models involve deletion, anergy, and regulation of T cells of direct and indirect allo-specificities (24).In the current study, we tested donor ECDI-SPs in a concordant (rat-to-mouse) xenogeneic islet transplant model. We show that although ECDI-SPs alone significantly prolong islet xenograft survival, additional transient B-cell depletion is required to promote xenogeneic tolerance and indefinite islet xenograft survival. Furthermore, transient B-cell depletion significantly impairs xenogeneic T-cell priming and memory T-cell generation. Reciprocally, during B-cell reconstitution after transient B-cell depletion, the recovered B cells exhibit xenoantigen-specific unresponsiveness in the long-term tolerized hosts. Collectively, our findings establish a novel and effective tolerance therapy for xenogeneic islet transplantation and underscore the critical role of B-cell depletion in this process. |
