Depletion of donor-reactive cells as a new concept for improvement of mismatched bone marrow engraftment using reduced-intensity conditioning

Cytotoxic Pathways in Allogeneic Hematopoietic Cell Transplantation

Allogeneic hematopoietic cell transplantation (allo-HCT) is a potentially curative treatment for hematologic malignancies, and other hematologic and immunologic diseases. Donor-derived immune cells identify and attack cancer cells in the patient producing a unique graft-vs.-tumor (GVT) effect. This beneficial response renders allo-HCT one of the most effective forms of tumor immunotherapy. However, alloreactive donor T cells can damage normal host cells thereby causing graft-vs.-host disease (GVHD), which results in substantial morbidity and mortality. To date, GVHD remains as the major obstacle for more successful application of allo-HCT. Of special significance in this context are a number of cytotoxic pathways that are involved in GVHD and GVT response as well as donor cell engraftment. In this review, we summarize progress in the investigation of these cytotoxic pathways, including Fas/Fas ligand (FasL), perforin/granzyme, and cytokine pathways. Many studies have delineated their distinct operating mechanisms and how they are involved in the complex cellular interactions amongst donor, host, tumor, and infectious pathogens. Driven by progressing elucidation of their contributions in immune reconstitution and regulation, various interventional strategies targeting these pathways have entered translational stages with aims to improve the effectiveness of allo-HCT.

Induction of mixed chimerism using combinatory cell-based immune modulation with mesenchymal stem cells and regulatory T cells for solid-organ transplant tolerance

Establishment of mixed chimerism is an ideal approach to induce donor-specific tolerance while expanding its potential in various clinical settings. Despite the developments in partial conditioning regimens, improvements are still needed in reducing toxicity and bone marrow transplantation-related complications. Recently, cell-based therapies, including mesenchymal stem cells (MSCs), have been incorporated in establishing noncytoreductive mixed chimerism protocols; however, its efficacy is only partial and shows reversed immunosuppressive properties. This study demonstrates a novel approach to induce mixed chimerism and tolerance through combinatory cell-based immune modulation (CCIM) of MSCs and regulatory T cells (Tregs). We hypothesize that the interaction between these cells may lead to greater inhibition of host immune responses. Compared with single cell therapy, CCIM induced a higher engraftment rate and robust donor-specific tolerance to skin allografts across full major histocompatibility complex barriers. These regulatory effects were associated with inhibition of natural killer cell cytotoxic activity, CD4(+)IL-17(+) cells, memory B cells, plasma cells, and immunoglobulin production levels along with increased frequencies of CD4(+)Foxp3(+) cells, IL-10-producing mature B cells, and myeloid-derived suppressor cells. Furthermore, CCIM was able to regulate mortality in a graft-versus-host disease model through reciprocal regulation of Treg/Th17. Taken together, we suggest CCIM as a clinically applicable strategy for facilitating the induction of mixed chimerism and permanent tolerance.

Immunotherapeutic approaches to improve graft-versus-tumor effect and reduce graft-versus-host disease

The therapeutic efficacy of allogeneic stem cell transplantation is mainly based on the alloreactive immune response of the graft against the host. However, the graft-versus-host process can be viewed as a double-edged sword since it is responsible for both the beneficial graft-versus-tumor effect and the deleterious graft-versus-host disease. During the last two decades, intensive research has been focused on the development of novel immunotherapeutic methods aimed to dissociate graft-versus-host disease from graft-versus-tumor effect. A brief description of these efforts is discussed in this review.

Immunoregulation of GVHD by triggering the innate immune system with CpG

Stimulation of Toll-like receptors by oligodeoxynucleotide sequences containing a CpG motif provides signals capable of triggering the innate and adaptive immune systems, thereby leading either to stimulation or suppression of immunoreactivities. Similar immunoregulatory capabilities are necessary for achieving the fine balance between engraftment and graft-versus-host disease required in the setup of allogeneic cell therapy. Ligation of CpG to its Toll-like receptors can be accomplished by treatment of the host or pretransplant treatment of the donor in vivo. These different strategies are presented in this review, which summarizes the attempts to maximize beneficial alloreactivity against malignant or other undesirable host cells, while controlling graft-versus-host disease.

Multidonor bone marrow transplantation improves donor engraftment and increases the graft versus tumor effect while decreasing graft-versus-host disease

In partially matched donor transplantation, mandatory T-cell depletion (TCD) increases the risks of rejection/graft failure, relapse, and post-transplant infections. A multi-donor approach was offered to resolve some of these drawbacks. This hypothesis was previously tested in a TCD fully mismatched murine model. However, the effect of multi-donor transplantation (MDT) on graft-versus-host disease (GVHD) and graft versus tumor (GVT) effect were never tested. To assess the safety and efficacy of MDT, we used it in non-TCD transplantation and murine breast carcinoma model. We found that when transplanting non-TCD MDT composed by C57Bl/6 and C3H cells into BALB/c, a consistent trichimerism is established, dominated by C57Bl/6 cells. Following MDT the study animals experienced reduced GVHD compare with those transplanted from C57Bl/6 alone, while the GVT effect was superior. We conclude that MDT may serve as a technique that suppresses GVHD while maintaining the GVT effect.

Prevention of acute graft-vs-host disease by a single low-dose cyclophosphamide injection following allogeneic bone marrow transplantation

OBJECTIVE

Previously, we documented that conditioning based on donor-specific cell transfusion (DST) and subsequent selective depletion of activated donor-reactive cells by cyclophosphamide (CY) facilitates alloengraftment in a murine transplantation model. Transplantation event represents a strong immunogenic stimulus for host-reactive donor T cells that induce graft-vs-host disease (GVHD). Therefore, in this study, we addressed the question of whether a single posttransplantation CY administration (CY2) can prevent acute GVHD-related mortality without compromising engraftment of allogeneic transplant.

MATERIALS AND METHODS

Splenocyte-enriched C57BL/6 bone marrow was transplanted to BALB/c recipients after mild irradiation, and conditioning with DST and 100 mg/kg CY. Following transplantation, recipients were left untreated or given on a specified day a single CY2 injection (50 mg/kg). All animals were monitored for survival, chimerism, and clinical signs of GVHD. Experimental mice that received BCL1 leukemia cells before transplantation were monitored for leukemia-related mortality as well.

RESULTS

Animals that received no CY2 after transplantation died of acute GVHD. A single low-dose CY2 treatment within the first 5 days after transplantation prevented mortality in most recipients. However, only CY2 administration on days +1 or +5 preserved chimerism. Most chimeras survived GVHD-free for >200 days. Prolonged persistence of host-reactive T cells in mice (CY2 on day +5) permitted a reduction to be made in engraftment-essential irradiation dose and preserved a strong graft-vs-leukemia effect of transplantation.

CONCLUSION

Acute GVHD can be prevented in mice by a single properly timed posttransplantation low-dose CY administration.

Allogeneic hematopoietic stem cell transplantation in the elderly

The development of reduced intensity or non-myeloablative conditioning (NST) in preparation for allogeneic stem cell transplantation (SCT) revolutionized the field and led to reconsideration of the dogma of upper age limit that was set up by the transplant centers as an eligibility parameter. Analysis of the literature data showed that NST regimens are associated with decreased transplant related mortality, and graft-versus-host disease, in comparison with standard myeloablative conditioning, in patients above the age of 50-55 years, or in younger patients with significant comorbidities. However we have to mention, that our considerations are based on the retrospective analysis of the literature data, and that well controlled prospective randomized studies are needed in order to definitely assess the role of NST. Comorbidity indices might be proved as the most important parameters for the choice of the most proper regimen for each patient in need and should be included in future trials.

Deletion of Donor-Reactive Cells as a New Conditioning Regimen for Allogeneic Bone Marrow Transplantation

Progress in the development of less toxic conditioning for bone marrow transplantation (BMT) came with the understanding that acceptance of mismatched BM does not require myeloablation of recipients. Lymphocyte deletion by a cocktail of immunosuppressive drugs is generally sufficient to ensure engraftment of compatible BM cells. However, reduced intensity conditioning (RIC) protocols available today do not provide robust tolerance to mismatched allogeneic BM. Herein we discuss 2 new experimental approaches to RIC protocols with the aim of facilitating allogeneic BM engraftment. Both conditioning regimens are based on selective deletion/inactivation of donor-reactive cells before BMT. Our data show that the first conditioning protocol, comprising priming of recipients by a donor-specific lymphocyte transfusion (DST) on day -2 and a single injection of cyclophosphamide, a drug that is predominantly toxic for proliferating cells, on day -1, consistently improves engraftment of allogeneic BM (day 0) in all experimental models tested. The second engraftment enhancing approach is based on the blockade by antagonistic reagents of the signaling pathways that govern the antigen-induced immune response. Combining the signaling blockade with the deletion of activated donor-reactive cells by cytoreductive agents provides additional benefits for transplantation across major histocompatibility barriers.

Hematopoietic stem cell therapy for malignant diseases

Allogeneic bone marrow or blood stem cell transplantation (SCT) has changed its face in the last two decades. The introduction of nonmyeloablative conditioning regimens has reduced procedure toxicity and allowed the application of SCT in patients and conditions in which SCT was not offered in the past. In this review we will summarize the changes and accomplishments achieved in the past years in the field of stem cell transplantation for malignant disorders.