Addition of Cyclophosphamide to T-cell Depletion–Based Nonmyeloablative Conditioning Allows Donor T-cell Engraftment and Clonal Deletion of Alloreactive Host T-cells After Bone Marrow Transplantation

Tocilizumab increases regulatory T cells, reduces natural killer cells and delays graft-versus-host disease development in humanized mice treated with post-transplant cyclophosphamide

Graft-versus-host disease (GVHD) is a life-threatening complication following donor hematopoietic stem cell transplantation, where donor T cells damage host tissues. This study investigated the effect of tocilizumab (TOC) combined with post-transplant cyclophosphamide (PTCy) on immune cell engraftment and GVHD development in a humanized mouse model. NOD-scid-IL2Rγ^null (NSG) mice were injected intraperitoneally with 2 × 10⁷ human (h) peripheral blood mononuclear cells and cyclophosphamide (33 mg kg^-1 ) or saline on days 3 and 4, then TOC or control antibody (0.5 mg mouse^-1 ) twice weekly for 28 days. Mice were monitored for clinical signs of GVHD for either 28 or 70 days. Spleens and livers were assessed for human leukocyte subsets, and serum cytokines and tissue histology were analyzed. In the short-term model (day 28), liver and lung damage were reduced in PTCy + TOC compared with control mice. All groups showed similar splenic hCD45⁺ leukocyte engraftment (55-60%); however, PTCy + TOC mice demonstrated significantly increased (1.5-2-fold) splenic regulatory T cells. Serum human interferon gamma was significantly reduced in PTCy + TOC compared with control mice. Long-term (day 70), prolonged survival was similar in PTCy + TOC (median survival time, > 70 days) and PTCy mice (median survival time, 56 days). GVHD onset was significantly delayed in PTCy + TOC, compared with TOC or control mice. Notably, natural killer cells were reduced (77.5%) in TOC and PTCy + TOC mice. Overall, combining PTCy with TOC increases regulatory T cells and reduces clinical signs of early GVHD, but does not improve long-term survival compared with PTCy alone.

HSCT-Based Approaches for Tolerance Induction in Renal Transplant

Renal transplantation has become the preferred treatment for end stage kidney failure. Although short-term graft survival has significantly improved as advances in immunosuppression have occurred, long-term patient and graft survival have not. Approximately only 50% of renal transplant recipients are alive at 10 years due to the toxicities of immunosuppression and alloimmunity. Emerging research on cell-based therapies is opening a new door for patients to receive the organs they need without sacrificing quality of life and longevity because of drug-based immunosuppression. Research has focused on inducing tolerance, a state in which the body accepts the transplant and graft function is stable. Cell-based therapies to facilitate chimerism and achieve tolerance in major histocompatibility disparate recipients have been developed in mouse, swine, canine, and nonhuman primate models. These findings are now being translated into the clinic in several trials currently underway. Protocols that use a combination of traditional therapeutic agents paired with cell populations including hematopoietic stem cells, regulatory T cells, and facilitating cells are being conducted with the objective to harness the donor immune system to protect the transplanted tissue. The benefits and feasibility of the clinical application of cell-based therapy has been demonstrated, and promising results have been achieved. Here we discuss the preclinical work that has led to the clinical application of the various approaches and a summary of the most current clinical data from groups throughout the world.

A Critical Role for TGF-β/Fc and Nonlytic IL-2/Fc Fusion Proteins in Promoting Chimerism and Donor-Specific Tolerance

BACKGROUND

Immunoglobulin-cytokine fusion molecules have been shown to be the new generation of immunomodulating agents in transplantation tolerance induction. In the present study, we tested whether immunoregulatory cytokine fusion proteins of IL-10/Fc, TGF-β/Fc, or IL-2/Fc would enhance allogeneic bone marrow cell (BMC) engraftment and promote tolerance induction.

METHODS

B6 (H2) mice were conditioned with anti-CD154 (MR1) and rapamycin (Rapa) plus 100 cGy total body irradiation (MR1/Rapa/100 cGy) and transplanted with allogeneic B10.D2 (H2) BMC. Recipients were treated with lytic IL-2/Fc, nonlytic IL-2/Fc, TGF-β/Fc, or IL-10/Fc fusion proteins to promote chimerism to induce tolerance.

RESULTS

Donor chimerism was achieved in 20% of recipients conditioned with MR1/Rapa/100 cGy. The addition of TGF-β/Fc (5- or 10-day treatment) or nonlytic IL-2/Fc (10-day treatment) fusion proteins to the conditioning resulted in engraftment in nearly 100% of recipients. In contrast, lytic IL-2/Fc or IL-10/Fc had no effect. The combination of nonlytic IL-2/Fc and TGF-β/Fc had a synergistic effect to promote engraftment and resulted in significantly higher donor chimerism compared with recipients conditioned with TGF-β/MR1/Rapa/100 cGy. Engraftment was durable in the majority of chimeras and increased over time. The chimeras accepted donor skin grafts and promptly rejected third-party skin grafts. Moreover, specific T cell receptor-Vβ5.½ and TCR-Vβ11 clonal deletion was detected in host T cells in chimeras, suggesting central tolerance to donor alloantigens.

CONCLUSIONS

Allogeneic BMC engraftment is enhanced with TGF-β/Fc fusion protein treatment. TGF-β/Fc and nonlytic IL-2/Fc exert a synergistic effect in promotion of alloengraftment and donor-specific transplant tolerance, significantly decreasing the minimum total body irradiation dose required.

Transplantation tolerance

Although transplantation has been a standard medical practice for decades, marked morbidity from the use of immunosuppressive drugs and poor long-term graft survival remain important limitations in the field. Since the first solid organ transplant between the Herrick twins in 1954, transplantation immunology has sought to move away from harmful, broad-spectrum immunosuppressive regimens that carry with them the long-term risk of potentially life-threatening opportunistic infections, cardiovascular disease, and malignancy, as well as graft toxicity and loss, towards tolerogenic strategies that promote long-term graft survival. Reports of "transplant tolerance" in kidney and liver allograft recipients whose immunosuppressive drugs were discontinued for medical or non-compliant reasons, together with results from experimental models of transplantation, provide the proof-of-principle that achieving tolerance in organ transplantation is fundamentally possible. However, translating the reconstitution of immune tolerance into the clinical setting is a daunting challenge fraught with the complexities of multiple interacting mechanisms overlaid on a background of variation in disease. In this article, we explore the basic science underlying mechanisms of tolerance and review the latest clinical advances in the quest for transplantation tolerance.

Mixed chimerism and split tolerance: mechanisms and clinical correlations

Establishing hematopoietic mixed chimerism can lead to donor-specific tolerance to transplanted organs and may eliminate the need for long-term immunosuppressive therapy, while also preventing chronic rejection. In this review, we discuss central and peripheral mechanisms of chimerism induced tolerance. However, even in the long-lasting presence of a donor organ or donor hematopoietic cells, some allogeneic tissues from the same donor can be rejected; a phenomenon known as split tolerance. With the current goal of creating mixed chimeras using clinically feasible amounts of donor bone marrow and with minimal conditioning, split tolerance may become more prevalent and its mechanisms need to be explored. Some predisposing factors that may increase the likelihood of split tolerance are immunogenicity of the graft, certain donor-recipient combinations, prior sensitization, location and type of graft and minimal conditioning chimerism induction protocols. Additionally, split tolerance may occur due to a differential susceptibility of various types of tissues to rejection. The mechanisms involved in a tissue's differential susceptibility to rejection include the presence of polymorphic tissue-specific antigens and variable sensitivity to indirect pathway effector mechanisms. Finally, we review the clinical attempts at allograft tolerance through the induction of chimerism; studies that are revealing the complex relationship between chimerism and tolerance. This relationship often displays split tolerance, and further research into its mechanisms is warranted.

Toward minimal conditioning protocols for allogeneic chimerism in tolerance resistant recipients

Mixed chimerism is a promising approach toward generating donor-specific immunological tolerance. However, chimerism induction can be toxic; therefore, there is an effort to develop non-myeloablative, minimal intensity protocols that can generate chimerism without the toxic side effects. Recently, with the goal of creating a minimalistic chimerism induction protocol in the tolerance resistant non-obese diabetic (NOD) mouse model, we identified pre-existing T cells as cells that resist fully allogeneic chimerism. With monoclonals targeting NOD T cells, we showed that long-term chimerism and tolerance toward donor islets could be established. However, this promising new protocol relied on the administration of a single dose of anti-CD40 ligand, which is not clinically applicable. In refining protocols to move even closer to clinical utility, we report here initial success at generating fully allogeneic mixed chimerism in NOD mice by adding cyclophosphamide to the conditioning regimen in place of anti-CD40 ligand antibodies.

A critical role for the TLR4/TRIF pathway in allogeneic hematopoietic cell rejection by innate immune cells

We show for the first time that signaling through the TLR4/TRIF pathway plays a critical role in allogeneic bone marrow cell (BMC) rejection. This appears to be unique to BMCs as organ allografts are rejected mainly via MyD88 signaling. Using T- or T-/B-cell-deficient mice, we found that BMC allorejection occurred early before T-cell activation and was T- and B-cell independent, suggesting an effector role for innate immune cells in BMC rejection. We further demonstrated the innate immune signaling in BMC allorejection by showing superior engraftment in mice deficient in TRIF or TLR4 but not in MyD88 or TLR3. The restored cytotoxicity in TRIF-deficient recipients transferred with wild-type F4/80(+) or NK1.1(+) cells suggests TRIF signaling dependence on macrophages or NK cells in early BMC rejection. Production of the proinflammatory cytokine IL-6 and TRIF relevant chemokine MCP-1 was significantly increased early after bone marrow transplantation. In vivo specific depletion of macrophages or NK innate immune cells in combination with anti-CD154/rapamycin resulted in additive-enhanced allogeneic engraftment. The requirement for irradiation was completely eliminated when both macrophages and NK cells were depleted in combination with anti-CD154/rapamycin to target T- and B-cells, supporting the hypothesis that two barriers involving innate and adaptive immunity exist in mediating the rejection of allogeneic BMCs. In summary, our results clearly demonstrate a previously unappreciated role for innate immunity in BMC allorejection via signaling through a unique MyD88-independent TLR4/TRIF mechanism. These findings may have direct clinical impact on strategies for conditioning recipients for stem cell transplantation.

The need for inducing tolerance in vascularized composite allotransplantation

Successful hand and face transplantation in the last decade has firmly established the field of vascularized composite allotransplantation (VCA). The experience in VCA has thus far been very similar to solid organ transplantation in terms of the morbidity associated with long-term immunosuppression. The unique immunological features of VCA such as split tolerance and resistance to chronic rejection are being investigated. Simultaneously there has been laboratory work studying tolerogenic protocols in animal VCA models. In order to optimize VCA outcomes, translational studies are needed to develop less toxic immunosuppression and possibly achieve donor-specific tolerance. This article reviews the immunology, animal models, mixed chimerism & tolerance induction in VCA and the direction of future research to enable better understanding and wider application of VCA.

Double negative Treg cells promote nonmyeloablative bone marrow chimerism by inducing T-cell clonal deletion and suppressing NK cell function

The establishment of immune tolerance and prevention of chronic rejection remain major goals in clinical transplantation. In bone marrow (BM) transplantation, T cells and NK cells play important roles for graft rejection. In addition, graft-versus-host-disease (GVHD) remains a major obstacle for BM transplantation. In this study, we aimed to establish mixed chimerism in an irradiation-free condition. Our data indicate that adoptive transfer of donor-derived T-cell receptor (TCR) αβ(+) CD3(+) CD4(-) CD8(-) NK1.1(-) (double negative, DN) Treg cells prior to C57BL/6 to BALB/c BM transplantation, in combination with cyclophosphamide, induced a stable-mixed chimerism and acceptance of C57BL/6 skin allografts but rejection of third-party C3H (H-2k) skin grafts. Adoptive transfer of CD4(+) and CD8(+) T cells, but not DN Treg cells, induced GVHD in this regimen. The recipient T-cell alloreactive responsiveness was reduced in the DN Treg cell-treated group and clonal deletions of TCRVβ2, 7, 8.1/2, and 8.3 were observed in both CD4(+) and CD8(+) T cells. Furthermore, DN Treg-cell treatment suppressed NK cell-mediated BM rejection in a perforin-dependent manner. Taken together, our results suggest that adoptive transfer of DN Treg cells can control both adoptive and innate immunities and promote stable-mixed chimerism and donor-specific tolerance in the irradiation-free regimen.

Innate and adaptive immune responses are tolerized in chimeras prepared with nonmyeloablative conditioning

BACKGROUND

Mixed chimerism is an effective approach for tolerance induction in transplantation. Strategies to achieve mixed chimerism with relatively low toxicity have significantly expanded the clinical use of chimerism.

METHODS

Allogeneic bone marrow transplants were performed between B6 (H2(b)) and BALB/c (H2(d)) mice. Recipient B6 mice were nonmyeloablatively conditioned with anti-αβ-T-cell receptor, anti-CD154, or rapamycin alone or in different combinations. A total of 15 × 10(6) BALB/c bone marrow cells were transplanted after varying doses of cGy of total body irradiation.

RESULTS

Pretreatment of recipients with anti-CD154 and rapamycin with or without T-cell lymphodepletion reduced the total body irradiation requirement to 100 cGy for establishing stable mixed chimerism. The mixed chimeras accepted donor islet allografts long term. Lymphocytes from mixed chimeras did not respond to host or donor antigens, yet were reactive to major histocompatibility complex-disparate third-party alloantigens, demonstrating functional donor-specific T-cell tolerance. No antibodies against donor and host were detected in mixed chimeras, suggesting humoral tolerance. Mixed chimeras showed no cytotoxicity to donor cells, but a similar rapid killing rate for major histocompatibility complex disparate third-party B10.BR cells compared with T-cell-deficient and wild-type controls in in vivo cytotoxicity assays, suggesting donor-specific tolerance in the innate immune cells was achieved in mixed chimeras.

CONCLUSIONS

Mixed chimeras prepared with low-intensity nonmyeloablative conditioning exhibit systemic tolerance in innate immunity and tolerance in adaptive T- and B-cell immune responses.

Dissociation between peripheral blood chimerism and tolerance to hindlimb composite tissue transplants: preferential localization of chimerism in donor bone

BACKGROUND

Mixed chimerism induces donor-specific tolerance to composite tissue allotransplants (CTAs). In the present studies, we used a nonmyeloablative conditioning approach to establish chimerism and promote CTA acceptance.

METHODS

Wistar Furth (RT1A(u)) rats were conditioned with 600 to 300 cGy total body irradiation (TBI, day-1), and 100 x 10(6) T-cell-depleted ACI (RT1A(abl)) bone marrow cells were transplanted on day 0, followed by a 11-day course of tacrolimus and one dose of antilymphocyte serum (day 10). Heterotopic osteomyocutaneous flap transplantation was performed 4 to 6 weeks after bone marrow transplantation.

RESULTS

Mixed chimerism was initially achieved in almost all recipients, but long-term acceptance of CTA was only achieved in rats treated with 600 cGy TBI. When anti-alphabeta-T-cell receptor (TCR) monoclonal antibody (mAb) (day-3) was added into the regimens, donor chimerism was similar to recipients preconditioned without anti-alphabeta-TCR mAb. However, the long-term CTA survival was significantly improved in chimeras receiving more than or equal to 300 cGy TBI plus anti-alphabeta-TCR mAb. Higher levels of donor chimerism were associated with CTA acceptance. The majority of flap acceptors lost peripheral blood chimerism within 6 months. However, donor chimerism persisted in the transplanted bone at significantly higher levels compared with other hematopoietic compartments. The compartment donor chimerism may be responsible for the maintenance of tolerance to CTA. Long-term acceptors were tolerant to a donor skin graft challenge even in the absence of peripheral blood chimerism.

CONCLUSIONS

Mixed chimerism established by nonmyeloablative conditioning induces long-term acceptance of CTA, which is associated with persistent chimerism preferentially in the transplanted donor bone.

Strategic nonmyeloablative conditioning: CD154:CD40 costimulatory blockade at primary bone marrow transplantation promotes engraftment for secondary bone marrow transplantation after engraftment failure

There is an increased risk of failure of engraftment following nonmyeloablative conditioning. Sensitization resulting from failed bone marrow transplantation (BMT) remains a major challenge for secondary BMT. Approaches to allow successful retransplantation would have significant benefits for BMT candidates living with chronic diseases. We used a mouse model to investigate the effect of preparative regimens at primary BMT on outcome for secondary BMT. We found that conditioning with TBI or recipient T cell lymphodepletion at primary BMT did not promote successful secondary BMT. In striking contrast, successful secondary BMT could be achieved in mice conditioned with anti-CD154 costimulatory molecule blockade at first BMT. Blockade of CD154 alone or combined with T cell depletion inhibits generation of the humoral immune response after primary BMT, as evidenced by abrogation of production of anti-donor Abs. The humoral barrier is dominant in sensitization resulting from failed BMT, because almost all CFSE-labeled donor cells were killed at 0.5 and 3 h in sensitized recipients in in vivo cytotoxicity assay, reflecting Ab-mediated cytotoxicity. CD154:CD40 costimulatory blockade used at primary BMT promotes allogeneic engraftment in secondary BMT after engraftment failure at first BMT. The prevention of generation of anti-donor Abs at primary BMT is critical for successful secondary BMT.