Developing immunologic tolerance for transplantation at the fetal stage

The origin of DCs and capacity for immunologic tolerance in central and peripheral tissues

Dendritic cells (DCs) are specialized immune sentinels that play key role in maintaining immune homeostasis by efficiently regulating the delicate balance between protective immunity and tolerance to self. Although DCs respond to maturation signals present in the surrounding milieu, multiple layers of suppression also co-exist that reduce the infringement of tolerance against self-antigens. These tolerance inducing properties of DCs are governed by their origin and a range of other factors including distribution, cytokines, growth factors, and transcriptional programing, that collectively impart suppressive functions to these cells. DCs directing tolerance secrete anti-inflammatory cytokines and induce naïve T cells or B cells to differentiate into regulatory T cells (Tregs) or B cells. In this review, we provide a detailed outlook on the molecular mechanisms that induce functional specialization to govern central or peripheral tolerance. The tolerance-inducing nature of DCs can be exploited to overcome autoimmunity and rejection in graft transplantation.

Tolerogenic dendritic cells and their applications in transplantation

In transplantation immunology, the ultimate goal is always to successfully and specifically induce immune tolerance of allografts. Tolerogenic dendritic cells (tol-DCs) with immunoregulatory functions have attracted much attention as they play important roles in inducing and maintaining immune tolerance. Here, we focused on tol-DCs that have the potential to promote immune tolerance after solid-organ transplantation. We focus on their development and interactions with other regulatory cells, and we also explore various tol-DC engineering protocols. Harnessing tol-DCs represents a promising cellular therapy for promoting long-term graft functional survival in transplant recipients that will most likely be achieved in the future.

Adoptive infusion of tolerogenic dendritic cells prolongs the survival of pancreatic islet allografts: a systematic review of 13 mouse and rat studies

Objective

The first Phase I study of autologous tolerogenic dendritic cells (Tol-DCs) in Type 1 diabetes (T1D) patients was recently completed. Pancreatic islet transplantation is an effective therapy for T1D, and infusion of Tol-DCs can control diabetes development while promoting graft survival. In this study, we aim to systematically review islet allograft survival following infusion of Tol-DCs induced by different methods, to better understand the mechanisms that mediate this process.

Methods

We searched PubMed and Embase (from inception to February 29^th, 2012) for relevant publications. Data were extracted and quality was assessed by two independent reviewers. We semiquantitatively analyzed the effects of Tol-DCs on islet allograft survival using mixed leukocyte reaction, Th1/Th2 differentiation, Treg induction, and cytotoxic T lymphocyte activity as mechanisms related-outcomes. We discussed the results with respect to possible mechanisms that promote survival.

Results

Thirteen articles were included. The effects of Tol-DCs induced by five methods on allograft survival were different. Survival by each method was prolonged as follows: allopeptide-pulsed Tol-DCs (42.14±44 days), drug intervention (39 days), mesenchymal stem cell induction (23 days), genetic modification (8.99±4.75 days), and other derivation (2.61±6.98 days). The results indicate that Tol-DC dose and injection influenced graft survival. Single-dose injections of 10⁴ Tol-DCs were the most effective for allograft survival, and multiple injections were not superior. Tol-DCs were also synergistic with immunosuppressive drugs or costimulation inhibitors. Possible mechanisms include donor specific T cell hyporesponsiveness, Th2 differentiation, Treg induction, cytotoxicity against allograft reduction, and chimerism induction.

Conclusions

Tol-DCs induced by five methods prolong MHC mismatched islet allograft survival to different degrees, but allopeptide-pulsed host DCs perform the best. Immunosuppressive or costimulatory blockade are synergistic with Tol-DC on graft survival. Multiple injections are not superior to single injection. Yet more rigorously designed studies with larger sample sizes are still needed in future.

Tolerance induction by exosomes from immature dendritic cells and rapamycin in a mouse cardiac allograft model

Background

Dendritic cells (DCs) release bioactive exosomes that play an important role in immune regulation. Because they express low levels of class I major histocompatibility complex (MHC) and co-stimulatory molecules, exosomes derived from donor immature DCs (imDex) prolong allograft survival by inhibiting T-cell activation. However, this effect is limited and does not induce immunological tolerance when imDex are administered alone. Thus, we tested the effect of combined treatment with donor imDex and low-dose rapamycin on inducing tolerance in a mouse cardiac transplantation model.

Methods

ImDex were obtained from the culture supernatant of immature DCs derived from donor mouse (C57BL/6) bone marrow and were injected with suboptimal doses of rapamycin into recipient mouse (BALB/c) before and after transplantation. The capacity of this treatment to induce immune tolerance was analyzed in vitro and in vivo using the mouse cardiac transplantation model.

Results

Donor imDex expressed moderate levels of MHC class II and low levels of MHC class I and co-stimulatory molecules, but neither imDex nor subtherapeutic rapamycin dose alone induced cardiac allograft tolerance. Combined treatment with imDex and rapamycin, however, led to donor specific cardiac allograft tolerance. This effect was accompanied by decreased anti-donor antigen cellular response and an increased percentage of spleen CD4⁺CD25⁺ T cells in recipients. Furthermore, this donor specific tolerance could be further transferred to naïve allograft recipients through injection of splenocytes, but not serum, from tolerant recipients.

Conclusion

Combined with immunosuppressive treatment, donor imDex can prolong cardiac allograft survival and induce donor specific allograft tolerance.