The immune system can learn to discriminate between the self and nonself via a complex set of central and peripheral immune tolerance mechanisms. When considering immunological tolerance to the allograft, the high proportion of MHC alloreactive T cells, which generally range from 5 to 10% [16], has been considered as the main hindrance toward tolerance induction. Regarding T cell tolerance, central tolerance refers to the deletion of reactive clones within the thymus during negative selection. Peripheral T cell tolerance encompasses several mechanisms that take place outside the thymus, including peripheral deletion, anergy/exhaustion, and suppressive function of regulatory T cells (Tregs) [17]. The key lies in determining how to apply this mechanism of immunological tolerance, which is inherent in the body system, to induce donor-specific immune tolerance in transplantation.

During the immune destruction of the islet graft, the initial process of rejection is characterized by a rapid infiltration of innate immune cells to the grafts, which can be followed by an antigen-specific T cell response. The established immunosuppression protocol incorporating T cell depletion or anti-TNFa monoclonal antibodies could achieve T cell activation in allogeneic islet transplantation, which contributes to enhancing short-term graft survival [1]. However, the current protocol has been considered insufficient for controlling the humoral immune response, including antibody-mediated rejection, which is a significant mechanism of chronic allograft failure [18]. Indeed, B cells are known to mainly contribute to humoral immunity and boost cellular immunity. However, several experimental models have shown that B cell subsets ameliorate inflammation and autoimmunity disease, suggesting their capability for regulatory function, namely Bregs. Numerous bodies of evidence have indicated that B cells play essential roles in alloimmunity, including differentiation into antibody-producing plasma cells, sustaining long-term humoral immune memory, serving antigen-presenting cells (APCs), organizing the formation of tertiary lymphoid organs, and secreting pro- and anti-inflammatory cytokines (IL-10) [19]. Thus, therapeutic options targeting T cell–B cell interactions are of interest in the development of immunosuppressive protocols for transplantation [19]. Although a detailed description of B cell function in transplantation immunity is beyond the scope of the current paper, the following section summarizes exciting evidence obtained from islet transplantation animal models, demonstrating the significance of B cell functions in inducing and sustaining immunological tolerance.

TIM-1, which was initially reported to be a T cell costimulatory marker, is a member of the T cell immunoglobulin and mucin domain family of costimulatory molecules. However, an in vivo mice study by Ding et al. showed that TIM-1 was constitutively expressed on B cells rather than T cells and that 6–40% of TIM-1+ B cells express IL-10, including transitional, marginal zone, and follicular B cells [20]. Thus, reports have shown that TIM-1 is an inclusive marker of IL-10 + B cells. A mouse model of islet transplantation found that treatment with low-affinity anti-TIM-1, which has functional properties in the Breg development of recipients, leads to significantly prolonged islet allograft survival (approximately 30% of mice achieved long-term engraftment over 100 days) [20]. Interestingly, treating B cell-depleted recipients with anti-TIM-1 significantly enhanced IFN-g and ultimately prevented the commonly observed increase in Th2 cytokines. Therefore, B cells are required for anti-TIM-1-induced Th2 cytokine expression.

Moreover, a mouse islet allograft model found that anti-CD45RB antibody in combination with anti-TIM-1 antibody had a synergistic effect in inducing tolerance in all recipients. The dual antibody treatment significantly expanded regulatory B and T cells depending on the presence of recipient B cells with IL-10 activity [21]. Using B cell-deficient recipients or depleting B cells with anti-CD20 antibody abrogates the anti-CD45RB-induced tolerance following anti-TIM-1 dual antibody treatment. After exploring the reason why B cell depletion prevented the effects of dual antibody treatment on graft survival, the aforementioned study demonstrated that CD19 + CD5 + CD1d + B10 cells might play an important role only in early-stage transplantation tolerance induction following treatment. Additionally, the study also concluded that CD19 + TIM + B cells might play crucial roles in the whole process of tolerance induction and maintenance. These findings may explain why B cell depletion inhibited the effects of dual antibody treatment.

Although several promising approaches for tolerance induction in a rodent model of transplantation have been reported, as discussed earlier, very few have been translated to human or NHP transplantation models. Unlike laboratory mice, NHPs and humans already have a large collection of memory T and B cells at the time of transplantation. Heterologous immunity or the cross-reactive immune response has the potential to be alloreactive. For instance, up to 45% of anti-CMV T cell clones have been reported to be alloreactive [22]. Existing cross-reactive memory immune cells have been a significant hindrance to immunological tolerance in large animals or humans. Thus, tolerance induction in NHPs or humans would be more challenging than that in rodent models, partly due to cross-reactive memory T or B cells. Nevertheless, a few encouraging approaches have led us to believe that immunological tolerance can eventually be achieved in humans, such as mixed chimerism using hematopoietic cell transplantation [23,24] or ADL exposure [15].

One of the most advanced approaches for sustained tolerance is the use of hematopoietic cell transplantation to achieve durable chimerism, where the donor and recipient hematopoietic cells coexist at levels detectable by flow cytometry [24,25,26]. The central feature of the mixed chimerism-based transplant tolerance involves the intrathymic deletion of donor-reactive T cells by irradiation and/or anti-thymocyte globulin [27]. One study on kidney transplantation in NHPs reported that this protocol successfully promoted operationally sustained mixed chimerism in conditioning recipient and long-term engraftment [23]. Additionally, this approach has been successfully translated to both human leukocyte antigen-matched and -mismatched human renal transplant recipients with immunosuppression-free graft survival [24,28]. These results encourage attempts at extending this regimen to islet transplantation. Oura et al. reported the results of the mixed chimerism-based tolerance induction in islet transplantation in an NHP model, in which recipients were treated with a nonmyeloablative condition regimen that included total body irradiation, horse anti-thymocyte globulin anti-CD154 monoclonal antibody, and cyclosporine or anti-CD8 antibody (calcineurin inhibitor-free regimen) [29]. Accordingly, transient chimerism did not induce tolerance of islet graft survival, with islet function having been lost soon after the disappearance of chimerism [29]. Similar to islet transplantation, the induction of transient chimerism did not promote the tolerance of a heart allograft [30].

Table 1 summarizes the available evidence regarding the effectiveness of ADL in inducing immunological tolerance in islet transplantation. Studies using rodent islet transplantation models have demonstrated the efficacy of the peritransplant infusion of ECDI-fixed donor splenocytes in not only allogeneic islet transplantation [15,45] but also xenogeneic islet transplantation [46,47]. Focusing on evidence that promotes clinical implementation, recent data regarding the NHP model reported by Singh et al. can be considered a breakthrough achievement [15]. Their protocol involves peritransplant infusions of MHC-DRB allele-matched apoptotic donor leukocytes under short-term immune suppressions, including antagonistic anti-CD40 antibody 2C10R4, rapamycin, soluble tumor necrosis factor receptor, and anti-interleukin 6 receptor antibody.