Protective conditioning against GVHD and graft rejection after combined organ and hematopoietic cell transplantation

Transfer of multiple loci of donor's genes to induce recipient tolerance in organ transplantation

Donor organ rejection remains a significant problem. The present study aimed to assess whether transferring a donor's major histocompatibility complex (MHC) genes to the recipient could mitigate rejection in organ transplantation. Seven loci of MHC genes from donor mice were amplified and ligated into vectors; the vectors either contained one K locus, seven loci or were empty (control). The vectors were subsequently injected into the thymus of recipients (in heterotransplants, recipient rats received the vector containing one K locus), following which donor mouse hearts were transplanted. Following the transplantation of allograft and heterograft, electrocardiosignals were viable for a significantly longer duration in recipient mice and rats receiving the donor histocompatibility-2 complex (H-2)^d genes compared with those in controls, and in mice that received seven vectors compared with those receiving one vector. Mixed lymphocyte cultures containing cells from these recipients proliferated significantly less compared with mixed lymphocyte cultures containing controls. Also, hearts from H-2^d genes-treated recipients demonstrated less lymphocyte infiltration and necrosis compared with the control recipient. The present study concluded that allograft and heterograft rejection may be mitigated by introducing the donor's MHC into the recipient; transferring seven loci has been demonstrated to be more effective than transferring one locus.

The hematopoietic system in the context of regenerative medicine

Hematopoietic stem cells (HSC) represent the prototype stem cell within the body. Since their discovery, HSC have been the focus of intensive research, and have proven invaluable clinically to restore hematopoiesis following inadvertent radiation exposure and following radio/chemotherapy to eliminate hematologic tumors. While they were originally discovered in the bone marrow, HSC can also be isolated from umbilical cord blood and can be "mobilized" peripheral blood, making them readily available in relatively large quantities. While their ability to repopulate the entire hematopoietic system would already guarantee HSC a valuable place in regenerative medicine, the finding that hematopoietic chimerism can induce immunological tolerance to solid organs and correct autoimmune diseases has dramatically broadened their clinical utility. The demonstration that these cells, through a variety of mechanisms, can also promote repair/regeneration of non-hematopoietic tissues as diverse as liver, heart, and brain has further increased their clinical value. The goal of this review is to provide the reader with a brief glimpse into the remarkable potential HSC possess, and to highlight their tremendous value as therapeutics in regenerative medicine.

New directions for rabbit antithymocyte globulin (Thymoglobulin(®)) in solid organ transplants, stem cell transplants and autoimmunity

In the 30 years since the rabbit antithymocyte globulin (rATG) Thymoglobulin^® was first licensed, its use in solid organ transplantation and hematology has expanded progressively. Although the evidence base is incomplete, specific roles for rATG in organ transplant recipients using contemporary dosing strategies are now relatively well-identified. The addition of rATG induction to a standard triple or dual regimen reduces acute cellular rejection, and possibly humoral rejection. It is an appropriate first choice in patients with moderate or high immunological risk, and may be used in low-risk patients receiving a calcineurin inhibitor (CNI)-sparing regimen from time of transplant, or if early steroid withdrawal is planned. Kidney transplant patients at risk of delayed graft function may also benefit from the use of rATG to facilitate delayed CNI introduction. In hematopoietic stem cell transplantation, rATG has become an important component of conventional myeloablative conditioning regimens, following demonstration of reduced acute and chronic graft-versus-host disease. More recently, a role for rATG has also been established in reduced-intensity conditioning regimens. In autoimmunity, rATG contributes to the treatment of severe aplastic anemia, and has been incorporated in autograft projects for the management of conditions such as multiple sclerosis, Crohn’s disease, and systemic sclerosis. Finally, research is underway for the induction of tolerance exploiting the ability of rATG to induce immunosuppresive cells such as regulatory T-cells. Despite its long history, rATG remains a key component of the immunosuppressive armamentarium, and its complex immunological properties indicate that its use will expand to a wider range of disease conditions in the future.

Incidence and Pattern of Graft-versus-Host Disease in Patients Undergoing Allogeneic Transplantation after Nonmyeloablative Conditioning with Total Lymphoid Irradiation and Antithymocyte Globulin

Nonmyeloablative (NMA) conditioning with total lymphoid irradiation and antithymocyte globulin (TLI/ATG) has been shown to protect against acute graft-versus-host disease (GVHD). We report here our institutional experience with allogeneic transplantation following NMA conditioning with TLI/ATG (n = 21). GVHD prophylaxis consisted of a combination of a calcineurin inhibitor and mycophenolate mofetil. Median patient age was 59 years. The median followup of surviving patients is 545 days. One patient experienced primary graft rejection. The median time to neutrophil engraftment was 18 days and platelet engraftment was 9.5 days. The cumulative incidence (CI) of grade II–IV acute GVHD at day +100 was 28.6% and 38.1% at day +180. The CI for grade III-IV acute GVHD was 28.6% at day +180. CI of chronic GVHD was 45.2% at 1 year. The CI of disease relapse was 9.5% at 1 year. The rate of nonrelapse mortality (NRM) was 0% at day +100 and only 9.5% at 1 year. The overall and progression free survival at 1 year was 81% and 80.4%, respectively. Our limited, retrospective data show encouraging relapse and NRM rates with TLI/ATG-based NMA conditioning, but with higher than previously reported rates of acute and chronic GVHD, underscoring the need for novel strategies designed to effectively prevent GVHD.

Allo-HSCT for acute leukemia of ambiguous lineage in adults: the comparison between standard conditioning and intensified conditioning regimens

Knowledge concerning the clinical and biological characteristics of acute leukemia of ambiguous lineage (ALAL) is limited so that there has been a lack of uniformity in treatment. In this report, we retrospectively investigated the effect of intensified conditioning on adult ALAL undergoing allogeneic hematopoietic stem cell transplantation (allo-HSCT). A total of 59 patients with ALAL (male in 37 cases and female in 22 cases) were consecutively enrolled in the data analyses. Twenty-four patients received the standard conditioning (total body irradiation (TBI) + cyclophosphamide (CY) or busulfan + CY protocol) and 35 received the intensified conditioning (TBI + CY + etoposide or fludarabine + cytarabine plus TBI + CY + etoposide protocol). Five-year transplant-related mortality was 17.6 ± 9.6 % and 25.5 ± 8.0 %, the 5-year overall survival (OS) post-transplantation was 23.8 ± 8.9 % and 64.0 ± 8.4 %, disease-free survival was 16.7 ± 7.6 % and 55.8 ± 9.4 %, the 5-year cumulative incidence of relapse was 80.8 ± 8.5 % and 28.8 ± 9.9 %, respectively, in the standard and the intensified group (P = 0.380, P = 0.029, P = 0.005, and P < 0.001). Both univariate and multivariate analysis indicated that the intensified conditioning regimen and acute graft-versus-host disease were favorable factors to reduce the relapse. The younger patients, patients with CR at the time of transplantation, and the intensified conditioning regimen were favorable factors to elevate the survival. In conclusion, intensified conditioning regimens followed by allo-HSCT might improve long-term survival and decrease relapse of leukemia in adult ALAL compared to the standard conditioning regimens.

Skin and kidney histological changes in graft-versus-host disease (GVHD) after kidney transplantation

Kidney transplantation (Ktx) is generally performed during end stage renal disease due to a loss of the kidneys' ability to filter wastes from the circulatory system. Acute graft-versus-host disease (GVHD) after Ktx is a life-threatening complication that progresses to organ failure, systemic complications, and death. The current study evaluated the significance of histologic findings of GVHD as obtained from skin biopsies following Ktx in swine. A swine model of Ktx with tacrolimus-based immunosuppression was used to assess possible correlations between acute-graft-cellular rejection and skin histological findings for prediction of GVHD. Animals were divided into a Ktx treatment group or a control group with no Ktx and skin and kidney biopsies were histologically assessed at postoperative days 0, 15, 30, 45 and 60. Skin samples were analyzed and classified from grade 1 to 4 of skin GVHD and the major histopathological changes of kidney acute cellular rejection were described using Banff's score system. We observed a significant linear correlation between the histological grading values of skin biopsy changes and the histological grading values of kidney biopsies (Kendall's tau_b=0.993) in the Ktx experimental group. No histological changes were observed in controls. Our findings demonstrate the diagnostic value of staging skin GVHD after Ktx and suggest it's future utility for monitoring long term Ktx-induced changes.

Thymocyte expansion and maturation: crosstalk of CD44v6 on thymocytes and panCD44 on stroma cells

Re-acquisition of immunocompetence after allogeneic bone marrow cell (BMC) transplantation depends on intrathymic maturation of the allogeneic T progenitor cells. We recently reported that CD44 promotes progenitor homing into the thymus and T-cell maturation and now elucidate the molecular mechanisms of CD44-supported thymocyte maturation. Lethally irradiated, tumor-bearing mice, allogeneically reconstituted with T-cell-depleted BMC and a small number of common lymphoid progenitor 2 cells (CLP2) from transgenic (TG) mice, that express ratCD44v4-v7 under the Thy1 promoter, showed accelerated immunocompetent T-cell recovery compared with mice reconstituted with non-transgenic (NTG) CLP2. In addition, graft-versus-host disease was strongly reduced after tumor vaccination. TG, but not NTG double-negative (DN) thymocytes showed high proliferative potential, accompanied by constitutive association of lck with CD44. Importantly, when thymocyte adhesion was strengthened by anti-CD44, co-cultures of DN thymocytes with thymic stroma supported DN thymocyte maturation. The close contact between DN thymocytes and thymic stroma promoted persisting activation of lck and ERK1/2, particularly in CD44v6(+) DN thymocytes. Thus, intrathymic T-cell maturation in allogeneically reconstituted, leukemia-bearing hosts can be considerably accelerated by high CD44v6 expression in early thymocytes, in which proliferation-supporting signals are initiated by a crosstalk between CD44v6 on thymocytes and panCD44 on the thymic stroma.

Clinical experience with mixed chimerism to induce transplantation tolerance

Lymphohematopoietic chimerism was first shown to be associated with donor-specific allograft tolerance more than 60 years ago. However, early clinical experience with bone marrow transplantation soon revealed that conventional, myeloablative approaches were far too toxic and the risk of graft-versus-host disease too great to justify using this technology for the purpose of organ allograft tolerance induction in the absence of malignant disease. In this review, we discuss a step-wise approach that has been applied by several centers to establish less toxic approaches to using hematopoietic cell transplantation (HCT) for tolerance induction. These steps include (i) feasibility and efficacy data for tolerance induction in large animal models; (ii) safety data in clinical trials for patients with hematologic malignancies; and (iii) pilot trials of combined HCT and kidney transplantation for tolerance induction. Thus far, only one published trial conducted at the Massachusetts General Hospital in Boston has achieved long-term acceptance of human leukocyte antigen-mismatched kidney allografts without chronic immunosuppressive therapy. Alternative protocols have been successful in large animals, but long-term organ allograft tolerance has not been reported in patients. Thus, proof-of-principle that nonmyeloablative induction of mixed chimerism can be used intentionally to induce organ allograft tolerance has now been achieved. Directions for further research to make this approach applicable for a broader patient population are discussed.

Mixed chimerism in SCT: conflict or peaceful coexistence?

Stem cell transplants that follow both myeloablative and non-myeloablative conditioning regimens can result in states of mixed chimerism, which can be stable over time. With widespread availability of Y chromosome FISH in sex-mismatched transplantation and DNA-based methodologies for analysis of chimerism in other donor-recipient pairs, further insights have been gained regarding the implications of the mixed chimeric state. In transplants performed for inherited and acquired marrow failure disorders, disease status can be improved with only 10-20% donor cells, and it appears that stable mixed chimerism at that level is an acceptable outcome often leading to a state of tolerance, but an increasing level of recipient cells often precedes graft rejection. In transplants performed for malignant conditions, increasing levels of mixed chimerism may indicate disease relapse, but some cases with stable levels of mixed chimerism have been compatible with prolonged remission states. Understanding when mixed chimerism is an indication of secondary graft failure or impending graft rejection vs a state of tolerance and ongoing propensity for the establishment of a graft-vs-tumor effect is often difficult with currently available technologies and immunologic assays. The ability to understand the implication of mixed chimerism of multiple cell lineages and of varied lymphocyte subsets will remain important areas for future research to best harness the immunologic and other therapeutic benefits of allogeneic transplantation.