Therapeutic application of T regulatory cells in composite tissue allotransplantation

Exploring Costimulatory Blockade-Based Immunologic Strategies in Transplantation: Are They a Promising Immunomodulatory Approach for Organ and Vascularized Composite Allotransplantation?

The field of transplantation, including the specialized area of vascularized composite allotransplantation (VCA), has been transformed since the first hand transplant in 1998. The major challenge in VCA comes from the need for life-long immunosuppressive therapy due to its non-vital nature and a high rate of systemic complications. Ongoing research is focused on immunosuppressive therapeutic strategies to avoid toxicity and promote donor-specific tolerance. This includes studying the balance between tolerance and effector mechanisms in immune modulation, particularly the role of costimulatory signals in T lymphocyte activation. Costimulatory signals during T cell activation can have either stimulatory or inhibitory effects. Interfering with T cell activation through costimulation blockade strategies shows potential in avoiding rejection and prolonging the survival of transplanted organs. This review paper aims to summarize current data on the immunologic role of costimulatory blockade in the field of transplantation. It focuses on strategies that can be applied in vascularized composite allotransplantation, offering insights into novel methods for enhancing the success and safety of these procedures.

Tacrolimus before CTLA4Ig and rapamycin promotes vascularized composite allograft survival in MGH miniature swine

BACKGROUND

We evaluated the outcome of vertical rectus abdominus myocutaneous flap (VRAM) allotransplantation in a mini-pig model, using a combined co-stimulation blockade (Co-SB) and mechanistic target of rapamycin inhibition (mTORi)-based regimen, with or without preceding calcineurin inhibition (CNI).

MATERIALS AND METHODS

VRAM allotransplants were performed between SLA-mismatched MGH miniature swine. Group A (n = 2) was treated continuously with the mTOR inhibitor rapamycin from day -1 in combination with the Co-SB agent cytotoxic T lymphocyte antigen 4-Ig (CTLA4-Ig) from post-operative day (POD) 0. In group B (n = 3), animals received tacrolimus daily from POD 0 to POD 13, followed by rapamycin daily from POD 7 and CTLA4-Ig weekly from POD 7-28. Graft rejection was determined by Banff criteria and host cellular and humoral immunity monitored.

RESULTS

In group A, allografts developed grade-I acute rejection by POD 2 and POD 7, and reached grade-IV by POD 17 and POD 20, respectively. By contrast, in group B, two allografts demonstrated grade-I rejection on POD 30 and grade-IV on POD 74, while the third exhibited grade-I rejection starting on POD 50, though this animal had to be euthanized on POD 58 due to Pneumocystis jirovecii infection. Time-to-event incidence of grade-I rejection was significantly lower in group A compared to group B. During the first 3 weeks post-transplant, no significant differences in anti-donor immunity were observed between the groups.

CONCLUSION

A short course of CNI, followed by combined Co-SB and mTORi significantly delays acute rejection of VRAM allografts in SLA-mismatched miniature swine.

Induction of Endotoxin Tolerance Delays Acute Rejection in a Hindlimb Transplantation Model in Rats

BACKGROUND

Acute rejection is seen in 85 percent of composite vascular allogeneic transplants despite long-term immunosuppression. Recently, it was reported that the induction of endotoxin tolerance prolonged heart allograft survival in mice. However, it produced side effects in all the animals secondary to the inflammatory reaction. Galactomannan has shown endotoxin tolerance without this side effect in vitro. The authors hypothesized that galactomannan-induced endotoxin tolerance delays acute rejection in vascular allogeneic transplantation without the side effects produced by lipopolysaccharide.

METHODS

Twenty-four rat hindlimb transplants were divided into four groups according to the preconditioning received: control, lipopolysaccharide (0.16 ml/kg), galactomannan 72 hours before (galactomannan-72) (8 ml/kg), and galactomannan 24 hours before (galactomannan-24) (8 ml/kg). Median acute rejection time, weight loss, and diarrheal episodes were monitored. Blood samples were collected at 0, 7, 21, 30, 45, and 60 days. Plasma cytokines (i.e., tumor necrosis factor alpha, interferon gamma), peripheral chimerism, and lymphocyte percentages were analyzed.

RESULTS

Median allograft survival was 40 days (range, 40 to 44 days) in the control group, 68 days (range, 61 to 71 days) in the lipopolysaccharide group, and 70 days (range, 69 to 73 days) in both galactomannan groups (p = 0.001). Weight loss was higher in the lipopolysaccharide group (p < 0.001), as was the 83.3 percent rate of diarrheal episodes (control, 0 percent, p = 0.015; galactomannan-72, 0 percent, p = 0.015; and galactomannan-24, 16.7 percent, p = 0.02). Preconditioned rats had higher peripheral blood chimerism (lipopolysaccharide, 2.30 ± 0.13 percent; galactomannan-72, 2.63 ±1.46 percent; and galactomannan-24, 2.47 ± 0.19 percent) compared to the control group (2.06 ± 0.36 percent) (lipopolysaccharide, p = 0.04; galactomannan-72, p = 0.002; and galactomannan-24, p = 0.002).

CONCLUSIONS

Induction of endotoxin tolerance delays acute rejection in the rat hindlimb transplantation model. Galactomannan preconditioning has no lipopolysaccharide side effects and was equally effective in delaying acute rejection.

CLINICAL RELEVANCE STATEMENT

The contributions of this experimental work are very incipient. Although the use of galactomannan in clinical practice requires more studies to assess its safety, there is no doubt that immunomodulation may be one of the responses that solve the problem of long-term immunosuppression.

Local delivery strategies to restore immune homeostasis in the context of inflammation

Immune homeostasis is maintained by a precise balance between effector immune cells and regulatory immune cells. Chronic deviations from immune homeostasis, driven by a greater ratio of effector to regulatory cues, can promote the development and propagation of inflammatory diseases/conditions (i.e., autoimmune diseases, transplant rejection, etc.). Current methods to treat chronic inflammation rely upon systemic administration of non-specific small molecules, resulting in broad immunosuppression with unwanted side effects. Consequently, recent studies have developed more localized and specific immunomodulatory approaches to treat inflammation through the use of local biomaterial-based delivery systems. In particular, this review focuses on (1) local biomaterial-based delivery systems, (2) common materials used for polymeric-delivery systems and (3) emerging immunomodulatory trends used to treat inflammation with increased specificity.

The Positive Impact of Donor Bone Marrow Cells Transplantation into Immunoprivileged Compartments on the Survival of Vascularized Skin Allografts

Using the vascularized skin allograft (VSA) model, we compared the tolerogenic effects of different allogeneic bone marrow transplantation (BMT) delivery routes into immunoprivileged compartments under a 7-day protocol immunosuppressive therapy. Twenty-eight fully MHC mismatched VSA transplants were performed between ACI (RT1^a) donors and Lewis (RT1¹) recipients in four groups of seven animals each, under a 7-day protocol of alfa/beta TCRmAb/CsA (alpha/beta-TCR monoclonal antibodies/Cyclosporine A therapy). Donor bone marrow cells (BMC) (100 × 106 cells) were injected into three different immunoprivileged compartments: Group 1: Control, without cellular supportive therapy, Group 2: Intracapsular BMT, Group 3: Intragonadal BMT, Group 4: Intrathecal BMT. In Group 2, BMC were transplanted under the kidney capsule. In Group 3, BMC were transplanted into the right testis between tunica albuginea and seminiferous tubules, and in Group 4, cells were injected intrathecally. The assessment included: skin evaluation for signs and grade of rejection and immunohistochemistry for donor cells engraftment into host lymphoid compartments. Donor-specific chimerism for MHC class I (RT1^a) antigens and the presence of CD4⁺/CD25⁺ T cells were assessed in the peripheral blood of recipients. The most extended allograft survival, 50–78 days, was observed in Group 4 after intrathecal BMT. The T cells CD4⁺/CD25⁺ in the peripheral blood were higher after intrathecal BMC injection than other experimental groups at each post-transplant time point. Transplantation of BMC into immunoprivileged compartments delayed rejection of fully mismatched VSA and induction of robust, donor-specific chimerism.

A new heterotropic vascularized model of total urinary bladder transplantation in a rat model

This study developed a new procedure of urinary bladder transplantation on a rat model (n = 40). Heterotopic urinary bladder transplantation (n = 10) in the right groin vessels was performed. Direct urinary bladder examination, microangiography, histological analysis, and India ink injection were performed to evaluate the proposed method's functionality. Observation time was four weeks. One week after the procedure, the graft survival rate was 80%, two urinary bladders were lost due to anastomosis failure. The rest of the grafts survived two weeks without any complications. Lack of transitional epithelium or smooth muscle layer loss and lack of inflammatory process development were observed. This study was performed in order to obtain the necessary knowledge about urinary bladder transplantation. The proposed technique offers a new approach to the existing orthotropic models.

The characteristics of regulatory macrophages and their roles in transplantation

Regulatory macrophages (Mregs) are a subtype of macrophages that are involved in regulating immune responses and inhibiting activated T lymphocyte proliferation. With advances in our basic understanding of Mregs and the revelation of their biological characteristics, Mregs have become a focus of research. In addition to promoting malignant tumor progression, Mregs also play an immunosuppressive role in inflammatory diseases and transplantation. Recent studies have shown that Mregs are closely associated with the induction of transplantation immune tolerance. Immune regulatory cell treatment as an adjunct immunosuppressive therapy offers new insights into the mechanism by which transplantation immune tolerance is established. The application of Mreg-based cellular immunotherapy has shown promise in clinical solid organ transplantation. Here, we provide a comprehensive overview of Mreg morphology, phenotype, induction and negative immunoregulatory function and discuss the role of Mregs in different transplantation models as well as their potential application value in clinical organ transplantation.

Efficacy of single-agent immunosuppressive regimens in a murine model of vascularized composite allotransplantation

We herein investigate the safety and efficacy of single-agent anti-rejection regimens in a mouse vascularized composite allotransplantation (VCA) model. Orthotopic hind-limb transplantations (Balb/c → C57BL/6) were performed using 6- to 8-week-old mice. A thirty-day regimen of either rapamycin, tacrolimus (both 1, 3, 5 mg/kg/day) or cyclosporine (25, 35, 50 mg/kg/day) was used. Primary endpoints were animal and graft survival, and secondary chimerism and regulatory T-cell levels. For rapamycin and tacrolimus given at 1, 3, and 5 mg/kg/day, median graft survival time (MST) was 23 days (18-28 days), 30 days (23-30 days), and 30 d (30-30 days) and 14 days (13-18 days), 30 days (16-30 days), and 30 days (30-30 days), respectively. For cyclosporine dosed at 25 and 35 mg/kg/day, MST was 15 days (12-18 days) and 21 days (14-27 days). Toxicity from CsA 50 mg/kg led to 100% mortality. Mixed chimerism levels were higher in rapamycin-treated animals than in tacrolimus-treated recipients (P = 0.029). Tacrolimus was superior in preventing leukocyte recruitment to the allograft. In murine VCA, no standardized immunosuppressive regimen exists, limiting comparability of outcomes and survival. Our data demonstrate that rapamycin and tacrolimus maintenance treatment at 5 mg/kg/day both yielded allograft survival (<grade 3 rejection) in all animals. Rapamycin displayed less toxicity and maintained mixed chimerism but was not as potent in controlling leukocyte recruitment compared with tacrolimus.

Role of heterogeneous regulatory T cells in the tumor microenvironment

Regulatory T cells (Tregs) modulate ongoing immune responses to prevent autoimmunity in healthy bodies and inhibit effective anti-tumor immunity responses in tumor patients, leading to tumor progression. The function of Tregs in tumor immunity suggests that elimination of Tregs in the host may enhance the anti-tumor immune response. Despite the success of strategies for depleting Tregs in tumor-bearing patients, the overall clinical efficacy is limited and accompanied by undesirable side effects. The present review describes the diverse anti-tumor roles and differentiation mechanisms of heterogeneous Tregs and proposes methods for modulating them in the tumor microenvironment. This information is critical for improving clinical outcomes and preventing adverse effects in cancer patients receiving immunotherapy targeting Tregs.

Treg-inducing microparticles promote donor-specific tolerance in experimental vascularized composite allotransplantation

For individuals who sustain devastating composite tissue loss, vascularized composite allotransplantation (VCA; e.g., hand and face transplantation) has the potential to restore appearance and function of the damaged tissues. As with solid organ transplantation, however, rejection must be controlled by multidrug systemic immunosuppression with substantial side effects. As an alternative therapeutic approach inspired by natural mechanisms the body uses to control inflammation, we developed a system to enrich regulatory T cells (Tregs) in an allograft. Microparticles were engineered to sustainably release TGF-β1, IL-2, and rapamycin, to induce Treg differentiation from naïve T cells. In a rat hindlimb VCA model, local administration of this Treg-inducing system, referred to as TRI-MP, prolonged allograft survival indefinitely without long-term systemic immunosuppression. TRI-MP treatment reduced expression of inflammatory mediators and enhanced expression of Treg-associated cytokines in allograft tissue. TRI-MP also enriched Treg and reduced inflammatory Th1 populations in allograft draining lymph nodes. This local immunotherapy imparted systemic donor-specific tolerance in otherwise immunocompetent rats, as evidenced by acceptance of secondary skin grafts from the hindlimb donor strain and rejection of skin grafts from a third-party donor strain. Ultimately, this therapeutic approach may reduce, or even eliminate, the need for systemic immunosuppression in VCA or solid organ transplantation.

Composite tissue allotransplantation: opportunities and challenges

Vascularized composite allotransplants (VCAs) have unique properties because of diverse tissue components transplanted en mass as a single unit. In addition to surgery, this type of transplant also faces enormous immunological challenges that demand a detailed analysis of all aspects of alloimmune responses, organ preservation, and injury, as well as the immunogenicity of various tissues within the VCA grafts to further improve graft and patient outcomes. Moreover, the side effects of long-term immunosuppression for VCA patients need to be carefully balanced with the potential benefit of a non-life-saving procedure. In this review article, we provide a comprehensive update on limb and face transplantation, with a specific emphasis on the alloimmune responses to VCA, established and novel immunosuppressive treatments, and patient outcomes.