Allogeneic chimerism with low-dose irradiation, antigen presensitization, and costimulator blockade in H-2 mismatched mice

To condition or not to condition-That is the question: The evolution of nonmyeloablative conditions for transplantation

In 1985, Eugene Cronkite and his colleagues published, in Experimental Hematology, data indicating that five consecutive "transfusions" of large numbers of marrow cells significantly increase the number of donor-derived cells detected by day 10 of a spleen colony-forming assay, the most primitive hematopoietic cells detectable at that time, present in the host for as long as 2 months posttransfusion (Cronkite EP, Bullis JE, Brecher G. Marrow transfusions increase pluripotent stem cells in normal hosts. Exp Hematol 1985;13:802-805). These data provided the first evidence that donor hematopoietic stem cells (HSCs) may persist in vivo for some time in recipients when transfused and not transplanted, that is, not subjected to treatments that deplete their marrow niches of endogenous HSCs. The limited technology available at the time prevented Dr. Cronkite from pursuing this observation into the development of nonmyeloablated transplantation procedures, and his experiment, as well as the term bone marrow transfusion, has since been long forgotten. In recent years, the scientific need to clarify HSC functions in nonstressed hosts and the clinical need to develop transplantation procedures with levels of morbidity/mortality acceptable for curing inherited hematologic disorders have inspired the search for nonmyeloablative transplantation procedures, including methods that "outcompete" endogenous host HSCs such as those pioneered by Dr. Cronkite's experiments using high transfusion doses. This review describes the technical progress made since Dr. Cronkite's insightful work, which has finally found its path to the clinic.

Establishment of transplantation tolerance via minimal conditioning in aged recipients

Mixed hematopoietic chimerism is a powerful means of generating donor-specific tolerance, allowing long-term graft acceptance without lifelong dependence on immunosuppressive drugs. To avoid the need for whole body irradiation and associated side effects, we utilized a radiation-free minimal conditioning regime to induce long-term tolerance across major histocompatibility barriers. We found that low-dose busulfan, in combination with host T cell depletion and short-term sirolimus-based immunosuppression, facilitated efficient donor engraftment. Tolerance was achieved when mice were transplanted with whole or T cell-depleted bone marrow, or purified progenitor cells. Tolerance induction was associated with an expansion in regulatory T cells and was not abrogated in the absence of a thymus, suggesting a dominant or compensatory peripheral mode of tolerance. Importantly, we were able to generate durable chimerism and tolerance to donor skin grafts in both young and aged mice, despite age-related thymic atrophy and immune senescence. Clinically, this is especially relevant as the majority of transplant recipients are older patients whose immune recovery might be dangerously slow and would benefit from radiation-free minimal conditioning regimes that allow efficient donor engraftment without fully ablating the recipient immune system.

The peripheral chimerism of bone marrow-derived stem cells after transplantation: regeneration of gastrointestinal tissues in lethally irradiated mice

Bone marrow–derived cells represent a heterogeneous cell population containing haematopoietic stem and progenitor cells. These cells have been identified as potential candidates for use in cell therapy for the regeneration of damaged tissues caused by trauma, degenerative diseases, ischaemia and inflammation or cancer treatment. In our study, we examined a model using whole-body irradiation and the transplantation of bone marrow (BM) or haematopoietic stem cells (HSCs) to study the repair of haematopoiesis, extramedullary haematopoiesis and the migration of green fluorescent protein (GFP⁺) transplanted cells into non-haematopoietic tissues. We investigated the repair of damage to the BM, peripheral blood, spleen and thymus and assessed the ability of this treatment to induce the entry of BM cells or GFP⁺lin⁻Sca-1⁺ cells into non-haematopoietic tissues. The transplantation of BM cells or GFP⁺lin⁻Sca-1⁺ cells from GFP transgenic mice successfully repopulated haematopoiesis and the haematopoietic niche in haematopoietic tissues, specifically the BM, spleen and thymus. The transplanted GFP⁺ cells also entered the gastrointestinal tract (GIT) following whole-body irradiation. Our results demonstrate that whole-body irradiation does not significantly alter the integrity of tissues such as those in the small intestine and liver. Whole-body irradiation also induced myeloablation and chimerism in tissues, and induced the entry of transplanted cells into the small intestine and liver. This result demonstrates that grafted BM cells or GFP⁺lin⁻Sca-1⁺ cells are not transient in the GIT. Thus, these transplanted cells could be used for the long-term treatment of various pathologies or as a one-time treatment option if myeloablation-induced chimerism alone is not sufficient to induce the entry of transplanted cells into non-haematopoietic tissues.

Signal one and two blockade are both critical for non-myeloablative murine HSCT across a major histocompatibility complex barrier

Non-myeloablative allogeneic haematopoietic stem cell transplantation (HSCT) is rarely achievable clinically, except where donor cells have selective advantages. Murine non-myeloablative conditioning regimens have limited clinical success, partly through use of clinically unachievable cell doses or strain combinations permitting allograft acceptance using immunosuppression alone. We found that reducing busulfan conditioning in murine syngeneic HSCT, increases bone marrow (BM):blood SDF-1 ratio and total donor cells homing to BM, but reduces the proportion of donor cells engrafting. Despite this, syngeneic engraftment is achievable with non-myeloablative busulfan (25 mg/kg) and higher cell doses induce increased chimerism. Therefore we investigated regimens promoting initial donor cell engraftment in the major histocompatibility complex barrier mismatched CBA to C57BL/6 allo-transplant model. This requires full myeloablation and immunosuppression with non-depleting anti-CD4/CD8 blocking antibodies to achieve engraftment of low cell doses, and rejects with reduced intensity conditioning (≤75 mg/kg busulfan). We compared increased antibody treatment, G-CSF, niche disruption and high cell dose, using reduced intensity busulfan and CD4/8 blockade in this model. Most treatments increased initial donor engraftment, but only addition of co-stimulatory blockade permitted long-term engraftment with reduced intensity or non-myeloablative conditioning, suggesting that signal 1 and 2 T-cell blockade is more important than early BM niche engraftment for transplant success.

Nonengraftment haploidentical cellular therapy for hematologic malignancies

Much of the therapeutic benefit of allogeneic transplant is by a graft versus tumor effect. Further data shows that transplant engraftment is not dependant on myeloablation, instead relying on quantitative competition between donor and host cells. In the clinical setting, engraftment by competition alone is not feasible due to the need for large numbers of infused cells. Instead, low-level host irradiation has proven to be an effective engraftment strategy that is stem cell toxic but not myeloablative. The above observations served as the foundation for clinical trials utilizing allogeneic matched and haploidentical peripheral blood stem cell infusions with minimal conditioning in patients with refractory malignancies. Although engraftment was transient or not apparent, there were compelling responses in a heavily pretreated patient population that appear to result from the breaking of tumor immune tolerance by the host through the actions of IFNγ, invariant NK T cells, CD8 T cells, NK cells, or antigen presenting cells.

Murine mobilized peripheral blood stem cells have a lower capacity than bone marrow to induce mixed chimerism and tolerance

Allogeneic bone marrow transplantation (BMT) under costimulation blockade allows induction of mixed chimerism and tolerance without global T-cell depletion (TCD). The mildest such protocols without recipient cytoreduction, however, require clinically impracticable bone marrow (BM) doses. The successful use of mobilized peripheral blood stem cells (PBSC) instead of BM in such regimens would provide a substantial advance, allowing transplantation of higher doses of hematopoietic donor cells. We thus transplanted fully allogeneic murine granulocyte colony-stimulating factor (G-CSF) mobilized PBSC under costimulation blockade (anti-CD40L and CTLA4Ig). Unexpectedly, PBSC did not engraft, even when very high cell doses together with nonmyeloablative total body irradiation (TBI) were used. We show that, paradoxically, T cells contained in the donor PBSC triggered rejection of the transplanted donor cells. Rejection of donor BM was also triggered by the cotransplantation of unmanipulated donor T cells isolated from naïve (nonmobilized) donors. Donor-specific transfusion and transient immunosuppression prevented PBSC-triggered rejection and mixed chimerism and tolerance were achieved, but graft-versus-host disease (GVHD) occurred. The combination of in vivo TCD with costimulation blockade prevented rejection and GVHD. Thus, if allogeneic PBSC are transplanted instead of BM, costimulation blockade alone does not induce chimerism and tolerance without unacceptable GVHD-toxicity, and the addition of TCD is required for success.

Novel vaccination for allergy through gene silencing of CD40 using small interfering RNA

Small interfering RNA (siRNA) is a potent means of inducing gene-specific silencing. Gene silencing strategies using siRNA have demonstrated therapeutic benefits in animal models of various diseases, and are currently in clinical trials. However, the utility of gene silencing as a treatment for allergic diseases has not yet been reported. In this study, we report a novel therapy for allergy through gene silencing of CD40, a critical costimulatory molecule and a key factor in allergic immune responses. Silencing CD40 resulted in generation of immunoregulatory dendritic cells (DCs). Administration of CD40 siRNA remarkably reduced nasal allergic symptoms and local eosinophil accumulation in the OVA-induced allergic mice. The OVA-specific T cell response was inhibited after the CD40 siRNA treatment. Additionally, anti-OVA specific IgE and production of IL-4 and IL-5 of T cells stimulated by OVA were significantly decreased in CD40 siRNA-treated mice. Furthermore, we demonstrated that the therapeutic effects by CD40 siRNA were associated with impaired Ag-presenting functions of DCs and B cells, and generation of regulatory T cells. The present study highlights a therapeutic potential of siRNA-based treatment for allergic diseases.

Development of either split tolerance or robust tolerance along with humoral tolerance to donor and third-party alloantigens in nonmyeloablative mixed chimeras

Hematopoietic chimerism is considered to generate robust allogeneic tolerance; however, tissue rejection by chimeras can occur. This "split tolerance" can result from immunity toward tissue-specific Ags not expressed by hematopoietic cells. Known to occur in chimeric recipients of skin grafts, it has not often been reported for other donor tissues. Because chimerism is viewed as a potential approach to induce islet transplantation tolerance, we generated mixed bone marrow chimerism in the tolerance-resistant NOD mouse and tested for split tolerance. An unusual multilevel split tolerance developed in NOD chimeras, but not chimeric B6 controls. NOD chimeras demonstrated persistent T cell chimerism but rejected other donor hematopoietic cells, including B cells. NOD chimeras also showed partial donor alloreactivity. Furthermore, NOD chimeras were split tolerant to donor skin transplants and even donor islet transplants, unlike control B6 chimeras. Surprisingly, islet rejection was not a result of autoimmunity, since NOD chimeras did not reject syngeneic islets. Split tolerance was linked to non-MHC genes of the NOD genetic background and was manifested recessively in F(1) studies. Also, NOD chimeras but not B6 chimeras could generate serum alloantibodies, although at greatly reduced levels compared with nonchimeric controls. Surprisingly, the alloantibody response was sufficiently cross-reactive that chimerism-induced humoral tolerance extended to third-party cells. These data identify split tolerance, generated by a tolerance-resistant genetic background, as an important new limitation to the chimerism approach. In contrast, the possibility of humoral tolerance to multiple donors is potentially beneficial.

Recent Progress in Tolerance Induction through Mixed Chimerism

Organ transplant recipients require life-long treatment with immunosuppressive drugs. Currently available immunosuppression is associated with substantial morbidity and mortality, and is ineffective in inhibiting chronic rejection and graft loss. Therefore, a permanent state of donor-specific tolerance remains a primary goal for transplantation research. The induction of mixed hematopoietic chimerism is an attractive concept in this regard. Hematopoietic chimerism modulates the immunologic repertoire by extending the mechanisms of self-tolerance to donor-specific allotolerance. Despite recent progress in developing nontoxic bone marrow transplantation protocols for rodents, translation to large animals has remained difficult. Here, we outline the concept of tolerance via mixed chimerism, and review recent progress and remaining challenges in bringing this approach to the clinical setting.

Flow cytometry with anti HLA-antibodies: a simple but highly sensitive method for monitoring chimerism and minimal residual disease after HLA-mismatched stem cell transplantation

Transplantation of HLA-mismatched stem cells may allow determination of chimerism status of single cells by differential expression of HLA molecules. Monoclonal antibodies against HLA antigens can be used to determine the HLA type of sub-populations by standard flow cytometry. Blood samples from 23 patients transplanted from HLA-mismatched family donors were monitored using HLA-specific antibodies. Suitable antibodies could be found for all donor recipient pairs by using differences in HLA Bw4 and Bw6 groups or other serological antigens. Pretransplant controls of donor and recipient were used to correct for variable fluorescence intensities of the antibodies and sub-populations. Owing to the high sensitivity, cell populations with a minimum frequency of 0.1% were detectable. Flow-cytometric analysis was confirmed by chimerism analysis of immunomagnetically isolated T cells by standard PCR technique. In addition to chimerism evaluation, HLA antibodies improved the detection of leukemic cells after transplantation with aberrant phenotype. In conclusion, flow cytometry using antibodies against HLA antigens is an interesting tool for determination of chimerism and minimal residual disease after HLA-mismatched transplantation. Information about the chimerism status is given on a single-cell level and allows fast and convenient analysis of sub-populations.

Resistance to the induction of mixed chimerism in spontaneously diabetic NOD mice depends on the CD40/CD154 pathway and donor MHC disparity

Blockade of CD40/CD154 pathway has proven effective in promoting the induction of allogeneic mixed chimerism. Using NOD mouse model of human type 1 diabetes, we investigated whether allogeneic mixed chimerism can be induced in prediabetic NOD mice and in spontaneously diabetic NOD mice under nonmyeloablative and irradiation-free conditioning therapy and anti-CD154 mAb as a short-term posttransplant treatment. We found that spontaneously diabetic NOD mice are more resistant to the induction of allogeneic mixed chimerism than prediabetic NOD mice under our nonmyeloablative and irradiation-free conditioning therapy. This alloresistance in spontaneously diabetic NOD mice is dependent on the CD40/CD154 pathway and donor MHC disparity.

Apoptotic donor leukocytes limit mixed-chimerism induced by CD40-CD154 blockade in allogeneic bone marrow transplantation

"Mini" allogeneic bone marrow transplants using non-myeloablative conditioning have reduced early treatment-related mortalities, but graft-versus-host disease (GVHD) and graft rejection remain clinical problems. Our preliminary studies indicated that low-dose busulfan conditioning and costimulatory blockade using anti-CD154 monoclonal antibody (mAb) in combination with a pretransplantation "tolerating" dose of bone marrow (BM) cells were sufficient to establish stable mixed-chimerism without GVHD when transplanting moderate doses of T cell depleted (TCD)-BM from major histocompatibility complex (MHC) fully-mismatched donors (Adams AB, Durham MM, Kean L, et al. J Immunol. 2001;167:1103-1111). In this study, donor splenocytes were administered before transplantation as a tolerating cell infusion with a conditioning regimen consisting of low-dose busulfan and anti-CD154 mAb. We compared the ability of viable and apoptotic donor cells of different ex vivo treatments and purified different donor cell populations (CD3(+), CD3(-), CD11b(+), and CD11b(-) splenocytes) to induce tolerance and enhance donor chimerism in a MHC mismatched model of murine bone marrow transplantation. We found that mixed chimerism without GVHD was enhanced by pretransplantation administration of viable allogeneic splenocytes and diminished in mice with prior exposure to apoptotic/necrotic donor splenocytes. CD11b(+)-enriched splenocytes more potently enhanced donor chimerism compared to unfractionated splenocytes or other splenocyte subsets. Mixed lymphocyte cultures demonstrated that apoptotic stimulators overcame the immune-tolerating activity of anti-CD154 mAb and led to increased interferon gamma and tumor necrosis factor alpha synthesis, increased proliferation of responder T cells, and decreased production of interleukin-10. In conclusion, viable donor splenocytes administered before transplantation in combination with costimulatory blockade induced tolerance and enhanced donor chimerism, whereas pretransplantation administration of apoptotic/necrotic donor cells led to host T cell activation and decreased overall donor engraftment.

Inducing mixed chimerism and transplantation tolerance through allogeneic bone marrow transplantation with costimulation blockade

Induction of mixed chimerism (i.e., coexistence of donor and recipient hematopoietic cells) through transplantation of allogeneic donor bone marrow under appropriate host conditioning, is one of the most reliable strategies to induce transplantation tolerance. Robust tolerance is evident in mixed chimeras as they permanently accept donor skin grafts while promptly rejecting third party grafts. Although historically, myeloablative and T-cell depleting regimens have been described, milder protocols involving costimulation blockade have recently been developed. The prototypical murine protocol described in this chapter, involves the use of CTLA4Ig and a monoclonal antibody-specific for CD154 (CD40L) for costimulation blockade, 3 Gy of nonmyeloablative total body irradiation and a conventional number of 20 x 10(6) fully allogeneic bone marrow cells. Flow cytometry is used to determine levels of multilineage hematopoietic chimerism and deletion of donor-reactive CD4+ T cells. Tolerance is assessed in vivo by grafting of donor and third party skin.

Induction of Mixed Chimerism through Transplantation of CD45-Congenic Mobilized Peripheral Blood Stem Cells after Nonmyeloablative Irradiation

Clinical translation of the mixed-chimerism approach for inducing transplantation tolerance would be facilitated if mobilized peripheral blood stem cells (mPBSCs) could be used instead of bone marrow cells (BMCs). Because the use of mPBSCs for this purpose has not been investigated in nonmyeloablative murine protocols, we explored the engraftment potential of mPBSCs in a CD45-congenic model as a first step. After 2, 1.5, or 1 Gy of total body irradiation, CD45.1 B6 hosts received unseparated granulocyte colony-stimulating factor-mobilized CD45.2 B6 PBSCs or unseparated CD45.2 B6 BMCs. The same total cell numbers, or aliquots of mPBSCs and BMCs containing similar numbers of c-kit+ cells, were transplanted both with and without a short course of rapamycin-based immunosuppression (IS). Transplantation of mPBSCs induced long-term multilineage macrochimerism, but chimerism levels were significantly lower than among recipients of the same number of BMCs. Transplanting aliquots containing similar numbers of c-kit+ cells reduced the difference between mPBSCs and BMCs, but lower levels of chimerism were nonetheless observed in mPBSC recipients. Chimerism levels correlated more closely with the number of transplanted progenitor cells as determined by colony-forming unit assays. IS did not affect chimerism levels, indicating that the donor CD45 isoform or other minor disparities do not pose a major barrier to engraftment. Our findings indicate that under nonmyeloablative conditions, progenitor cells contained in mPBSCs have an engraftment capacity similar to progenitor cells from BMCs, allowing induction of lasting mixed chimerism with moderate cell numbers. On a cell-per-cell basis, unseparated BMCs have some advantages that may be minimized if the number of progenitor cells is equalized. These results are expected to facilitate the development of mPBSC-based allogeneic tolerance protocols.

Cellular immune therapy for refractory cancers: novel therapeutic strategies

OBJECTIVE

Allogeneic stem cell transplantation is curative for certain cancers, but the high doses of chemotherapy and radiotherapy may lead to toxicity. This review summarizes the field of cellular immune therapy using very-low-dose conditioning for refractory cancers.

METHODS

In our initial study, we treated 25 patients with refractory cancers with 100 cGy total body irradiation followed by allogeneic, nonmobilized peripheral blood cells. Eighteen patients received sibling and seven patients received unrelated cord blood stem cells.

RESULTS

None of the 13 patients with solid tumors achieved donor chimerism or had a sustained response. Twelve patients with hematologic malignancies were treated, 1 received a cord blood transplant and 11 received sibling donor cells. Nine of these 11 patients achieved donor chimerism, ranging from 5% to 100%. Four patients had sustained complete remission of their cancers. The patients who received cord blood transplants did not respond. Development of chimerism correlated with total previous myelotoxic chemotherapy (p < 0.001). We review additional studies in this area, including data in the haploidentical and unrelated donor setting. The data presented comprises studies performed at the four institutions represented by the authors, and a review of other pertinent studies in this area.

CONCLUSIONS

Cellular immune therapy is an emerging application of transplantation therapy, which may be appropriate for refractory cancers. New studies in solid tumors, and with alternative donors, will expand the application of this new and promising treatment.

Robust Tolerance to Fully Allogeneic Islet Transplants Achieved by Chimerism with Minimal Conditioning

BACKGROUND

Whether mixed chimeras induced by nonmyeloablative conditioning are tolerant to challenge with donor allogeneic islet grafts is unknown. Here we investigate whether our nonmyeloablative, costimulation blockade-free and sirolimus (SRL)-based protocol could facilitate mixed chimerism via bone marrow transplantation (BMT) and induce islet allograft tolerance.

METHODS

After low dose (1-3 Gy) total body irradiation (TBI, day -1), with or without prior lymphocyte depletion, C57BL/6 mice were transfused with 40 x 10(6) BALB/c bone marrow cells (day 0) and received SRL (3 mg/kg/day) for 4 weeks. Chimerism was monitored by flow cytometry and the recipients were rendered diabetic chemically and challenged with donor islets.

RESULTS

Mixed chimerism was achieved in mice treated with TBI 3 Gy/SRL but it declined over time in 60% (9/15) of them. Long-term stable chimerism was established in 100% of recipients over 50 weeks with either antilymphocyte serum (ALS, 9/9), anti-CD4 (4/4), or anti-CD4 plus anti-CD8 (5/5) prior to BMT. TBI conditioning could be reduced to 1 Gy, with 90% (9/10) maintaining chimerism in the long-term. When TBI was substituted with cyclophosphamide (CTX) or busulfan (BUS), all mice remained chimeric in the long-term. The chimeras showed no proliferative response to donor antigen and accepted both first and second donor-specific islet grafts indefinitely while rejecting third-party grafts.

CONCLUSIONS

This data provides the first evidence that stable fully allogeneic chimeras induced with BMT after nonmyeloablative conditioning with SRL and lymphocyte-depleting antibodies exhibit robust donor-specific tolerance to islet grafts.

The role of non-deletional tolerance mechanisms in a murine model of mixed chimerism with costimulation blockade

Peripheral and central clonal deletion are important tolerance mechanisms in models using bone marrow transplantation (BMT) with costimulation blockade (CB). However, since tolerance can be found before peripheral deletion is complete and since elimination of recipient CD4(+) cells at the time of BMT prevents tolerance induction, we investigated the potential roles of regulation and anergy in such a murine model. We found that transient elimination of CD25(+) cells or neutralization of IL2 immediately after BMT and CB prevented the induction of skin graft tolerance. Cotransfer into SCID mice of CD4(+) cells taken from chimeras early after BMT, together with naive recipient-type CD4(+) cells significantly prolonged donor skin graft survival. In contrast, cotransfer of CD4(+) cells harvested from chimeras late after BMT did not prolong donor skin graft survival. Besides, depletion of CD25(+) cells in established chimeras several months post-BMT did not break tolerance. In vivo administration of recombinant IL2 inhibited chimerism and tolerance neither early nor late post-BMT, arguing against a decisive role for classical anergy. Thus, CD4 cell-mediated regulation contributes significantly to tolerance induction early after BMT, but appears to have no critical role in the maintenance of tolerance.

Molecular cloning and characterization of canine ICOS

Inducible costimulatory receptor (ICOS) is one recently identified member of the CD28 family of costimulatory molecules. Evidence suggests ICOS functions as a critical immune regulator and, to evaluate these effects, we employed the canine model system that has been used to develop strategies currently in clinical use for hematopoietic stem cell transplantation. To investigate the effects of blocking the ICOS pathway in the canine hematopoietic cell transplantation model, we tested existing murine and human reagents and cloned the full length of the open reading frame of canine ICOS cDNA to allow the development of reagents specific for the canine ICOS. Canine ICOS contains a major open reading frame of 624 nucleotides, encoding a protein of 208 amino acids, and localizes to chromosome 37. Canine ICOS shares 79% sequence identity with human ICOS, 70% with mouse, and 69% with rat. Canine ICOS expression is limited to stimulated PBMC.

Perspective: fundamental and clinical concepts on stem cell homing and engraftment: a journey to niches and beyond

In many ways, the homing of hematopoietic stem cells to bone marrow and other tissues defines these cells and their immediate and long-term fates Once homed, an inevitable series of proliferative and differentiative events presumptively follows. These comments, of course, hold for both homing to marrow, or alternatively, to other nonmarrow tissues. In this review, we will specifically focus on homing and engraftment to bone marrow because this is the best-studied and clinically applicable system.

Low dose total body irradiation followed by allogeneic lymphocyte infusion for refractory hematologic malignancy--an updated review

Allogeneic stem cell transplantation is curative for certain cancers, but the high doses of chemotherapy and radiotherapy used in conventional myeloablative conditioning regimens may lead to severe toxicity. In our initial study, we treated 25 patients with refractory cancers with 100 cGy total body irradiation (TBI) followed by allogeneic, non mobilized peripheral blood cells. Eighteen patients received sibling and 7 patients received unrelated cord blood stem cells. None of the 13 patients with solid tumors achieved donor chimerism or had a sustained response. Twelve patients with hematologic malignancies were treated, 1 received a cord blood transplant and 11 received sibling donor cells. Nine of these 11 patients achieved donor chimerism, ranging from 5% to 100%. Four patients had sustained complete remission of their cancers, and 2 are long-term survivors. The development of chimerism correlated with total previous myelotoxic chemotherapy (p < 0.001). This technique is now being extended into the haploidentical setting.

The role of haploidentical stem cell transplantation in the management of children with haematological disorders

The broader application of stem cell transplantation (SCT) for paediatric diseases has been limited by a lack of human leucocyte antigen (HLA)-matched donors. Virtually all children, however have at least one haploidentical parent who could serve as a donor. Such a donor is immediately available and the considerable costs of additional HLA typing, registry and banking expenditures that are necessary to procure an unrelated donor, could be reduced. Recent technological advances appear to have overcome the historical problems of graft rejection and severe graft versus host disease in the haploidentical setting, and in the latest studies the overall survival for children undergoing haploidentical SCT for leukaemia is now comparable with that following unrelated donor bone marrow or cord blood transplantation. Post-transplant infectious complications and leukaemia relapse remain the most important barriers yet to overcome, and new directions in the use of adoptive cellular immunity appear to be promising in this respect. Haploidentical SCT is now a viable option for those children who do not have an HLA compatible sibling or fully matched unrelated donor. The relative merits of a haploidentical family donor versus mismatched unrelated bone marrow or cord blood donation needs to be assessed in prospective, randomized clinical trials.

Simultaneous administration of a low-dose mixture of donor bone marrow cells and splenocytes plus adenovirus containing the CTLA4Ig gene result in stable mixed chimerism and long-term survival of cardiac allograft in rats

T-cell costimulatory blockade combined with donor bone marrow transfusion may induce mixed chimerism, rendering robust tolerance in transplanted organs and cells. However, most protocols entail high doses of donor bone marrow cells (BMCs) or repeated administration of costly agents that block costimulatory pathways, thus delaying clinical development. To circumvent these shortcomings, we developed a strategy in which the dosage of donor BMCs was reduced but compensated by donor splenocytes (SPLCs). Furthermore, repeated administration of costly agents was replaced with a single injection of adenovirus expressing a gene of interest. In rat cardiac transplantation models, cardiac allografts from DA (RT-1a) rats were transplanted heterotopically into the abdomen of LEW (RT-11) recipient rats. Immediately after cardiac transplantation, an adenovirus vector (AdCTLA4Ig; 5 x 10(9) plaque-forming units) containing the gene for CTLA4Ig was administered to recipients (n = 6) simultaneously with a low dose of donor BMCs (1 x 10(8)/rat) and SPLCs (5 x 10(7)/rat) via the portal vein. The treated LEW recipient rats developed long-lasting mixed chimerism (>10% at >100 days) and exhibited long-term cardiac allografts (mean survival time of > 200 days) compared with control recipients. Moreover, recipients displaying long-lasting mixed chimerism accepted subsequent donor skin allografts while promptly rejecting third-party skin allografts. These results suggest that blockade of the CD28-B7 pathway, using adenovirus-mediated CTLA4Ig gene transfer, in concert with a low dosage of donor BMCs and SPLCs, may represent a feasible strategy to induce stable mixed chimerism and permit long-term survival of cardiac allografts.

Tolerance induction by costimulator blockade in 100 cGy treated hosts with varying degrees of genetic disparity

Long-term multilineage allochimerism can be obtained in H2-mismatched B6.SJL to BALB/c transplants with host irradiation of 100 cGy, donor spleen cell pre-exposure and costimulator blockade with anti-CD40 ligand (CD40L) antibody. We evaluated this allochimerism approach in murine marrow transplants with different degrees of major histocompatibility complexe (MHC) mismatching; these include: (1) H2-mismatched transplant H2Kk to H2Kb, (2) full haplo-identical transplant H2Kbd to H2Kbk, (3) a partial haplo-identical transplant H2Kd to H2Kbd and (4) an MHC class II mismatch. Levels of chimerism increased up to 12 weeks and then stayed relatively stable up to 1 year after transplant. At 18 weeks post-transplant, the H2-mismatched, haplo-identical, partial haplo-identical and class II-mismatch transplants evidenced 17.9+/-4.4, 40.7+/-0.9, 25.1+/-4.19 and 33.7+/-3.5% donor chimerism, respectively. Dropping the anti-CD40 antibody treatment and spleen cells or changing the schedule of antibody to one injection, in haplo-identical or full-mismatched transplants resulted in no donor-derived chimerism. On the other hand, these still resulted in minor chimerism in class II-mismatched transplants. Lineage analysis of peripheral blood at 6 and 12 months post-transplant demonstrated a significant shift toward increased chimeric lymphocytes and decreased chimeric granulocytes in the full H2 as compared with haplo-identical or class II transplants. Transplantation with anti-CD40L antibody eliminated both graft-versus-leukemia and graft-versus-host disease (GVHD) and delayed lymphocyte infusion did not rescue animals from fatal leukemia. In conclusion, under the conditions of our tolerization regimen, a haplo transplant gives higher engraftment levels than a full H2 mismatch, and despite lower engraftment levels, a class II-mismatched transplant can be successfully accomplished with only 100 cGy and no CD40L blockade.

The marrow stem cell: the continuum

The marrow hematopoietic stem cell is currently being redefined as to all aspects of its phenotype and its total differentiation capacity. This redefinition now includes its plasticity as to production of nonhematopoietic and hematopoietic cell types, the determinants of its in vivo engraftment potential and its expression of stem cell functional characteristics.

Blockade of the CD40/CD154 pathway enhances T-cell-depleted allogeneic bone marrow engraftment under nonmyeloablative and irradiation-free conditioning therapy

BACKGROUND

T-cell-depleted bone marrow transplantation (TDBMT) can prevent graft-versus-host disease (GvHD). However, depleting T cells from allogeneic bone marrow often results in failure of bone marrow engraftment under irradiation conditioning. It is not know whether donor T cells are essential for bone marrow engraftment and whether blocking the CD40/CD154 pathway promotes allogeneic TDBM engraftment under nonmyeloablative and irradiation-free fludarabine phosphate and cyclophosphamide conditioning therapy.

METHODS

Using fully major histocompatibility complex (MHC)-matched mouse models, we investigated whether donor T cells are essential for bone marrow engraftment under fludarabine phosphate and cyclophosphamide conditioning therapy. We also determined whether the barrier of allogeneic TDBM could be overcome by blocking the CD40/CD154 pathway. Donor chimerism was detected by flow cytometric analysis. Donor-specific tolerance through establishing mixed chimerism was tested in vivo by skin transplantation and in vitro by mixed leukocyte reaction and enzyme-linked immunospot (ELISPOT) assay.

RESULTS

Compared with unmodified bone marrow, TDBM resulted in poor engraftment when fully MHC-mismatched donors were used. However, anti-CD154 monoclonal antibody (mAb) treatment significantly enhanced donor TDBM engraftment. TDBM engraftment was also seen in CD154 knockout mice. A stable and high level of multilinage donor chimerism was achieved. Recovery of host CD3 T cells was suppressed, and recovery of donor CD3 T cells was promoted, after TDBMT and anti-CD154 mAb treatment. Donor chimerism was established by TDBMT induced donor-specific tolerance in vivo and in vitro.

CONCLUSION

Donor T cells facilitate bone marrow engraftment under nonmyeloablative and irradiation-free conditioning therapy, and the blocking the CD40/CD154 pathway can replace donor T cells to promote TDBM engraftment.

Mechanisms of tolerance induction through the transplantation of donor hematopoietic stem cells: central versus peripheral tolerance

The transplantation of donor hematopoietic stem cells has been used successfully in numerous experimental settings to induce donor-specific tolerance. After appropriate host conditioning, hematopoietic stem-cell transplantation leads to a lasting state of donor macrochimerism that is associated with a robust form of tolerance. One of the key factors in the success of this approach is its reliance on intrathymic clonal deletion to ensure lifelong tolerization of newly developing T cells. Evidence for ongoing central deletion comes from studies following superantigen-reactive T cells and from experiments using mice transgenic for an alloreactive T-cell receptor. In protocols inducing tolerance through macrochimerism, the preexisting mature T-cell repertoire is controlled by either globally destroying all T cells before the hematopoietic cell transplantation or, in more recent models, by tolerizing it through co-stimulation blockade. The peripheral mechanisms induced by hematopoietic stem-cell transplantation and co-stimulation blockade include both extrathymic clonal deletion and the nondeletional mechanisms anergy, suppression, or both. In addition to these immunologic hurdles, a physiologic engraftment barrier has to be surmounted for the successful induction of mixed chimerism. This can be achieved by cytoreductive host treatment or by the infusion of high numbers of donor hematopoietic cells. A detailed delineation of the mechanisms responsible for tolerance induction after hematopoietic stem-cell transplantation is expected to help in the translation of these experimental protocols to clinical organ transplantation.

A substantial level of donor hematopoietic chimerism is required to protect donor-specific islet grafts in diabetic NOD mice

BACKGROUND

Mixed chimerism can induce tolerance to alloantigens and restore self-tolerance to autoantigens, thereby permitting islet transplantation. However, the minimal level of donor chimerism that is required to prevent islet allograft rejection and recurrence of autoimmune diabetes has not been established.

METHODS

We investigated whether allogeneic Balb/c donor chimerism can be induced in C57BL/6 mice, in prediabetic NOD mice, and in diabetic NOD mice after transplantation of a modest dose of bone marrow by using purine nucleoside analogue, fludarabine phosphate and cyclophosphamide conditioning therapy, followed by short-term anti-CD40 ligand monoclonal antibody and rapamycin posttransplant treatment. We also investigated whether the induced donor chimerism is sufficient to prevent the onset of diabetes in prediabetic NOD mice and protect donor islet grafts in diabetic NOD mice.

RESULTS

Allogeneic donor chimerism could be induced under the authors' approach that is nonmyeloablative and radiation-free. Diabetes onset was prevented in chimeric prediabetic NOD mice. The induction of mixed chimerism protected donor-specific islet grafts in diabetic NOD mice. At 60 days after islet transplantation, all donor Balb/c islet grafts survived in diabetic NOD mice whose level of donor-derived lymphocytes was higher than 30% at the time of islet transplantation (n=8). In contrast, Balb/c islet grafts were rejected in five of seven diabetic NOD mice whose level was lower than 30%.

CONCLUSIONS

Our data demonstrate that a donor lymphocyte chimerism (>30%) at the time of islet transplantation is required to protect donor-specific islet grafts, and indicate that a strictly non-irradiation-based protocol can be used to achieve this goal.

The influence of immunosuppressive drugs on tolerance induction through bone marrow transplantation with costimulation blockade

We recently developed a murine protocol for the induction of allogeneic mixed chimerism and tolerance employing nonmyeloablative total body irradiation (TBI), standard-dose bone marrow transplantation (BMT), and costimulation blockade (cobl) with an anti-CD154 monoclonal antibody (mAb) plus CTLA4Ig. We now evaluated whether a short course (1 month) of immunosuppressive drugs, which would be ethically required in the clinical setting of organ transplantation to prevent graft loss in case tolerance is not achieved, interferes with tolerance induced with this regimen. Our results show that calcineurin inhibitors (cyclosporin A [CyA] or tacrolimus [FK]) inhibit development of long-term chimerism and abrogate tolerance induction in this model. Rapamycin (rapa), methylprednisolone (MP), FTY720, and mycophenolate mofetil (MMF), in contrast, have no negative effect on chimerism or tolerance development. Peripheral deletion of donor-reactive T cells, which usually occurs in the weeks following BMT in this model, is blocked by CyA and FK, but not by the other drugs tested. Furthermore, we found that the additional use of compatible immunosuppressive drugs (rapa plus MMF plus MP) allows the dose of TBI to be reduced, so that mixed chimerism and donor skin-graft acceptance can be achieved with 1 Gy using clinically feasible cell numbers. Thus, this protocol of BMT with costimulation blockade can be safely combined with a clinically tested immunosuppressive regimen to permit success with a lower dose of irradiation. These results should facilitate clinical application of this tolerance strategy.

Stem cell plasticity and blood and marrow transplantation: a clinical strategy

The newly described phenomenon of stem cell plasticity raises interesting biological questions and offers exciting opportunities in clinical application. This review uses the well-established practice of blood and marrow transplantation as a paradigm to explore the clinical consequences of this finding. Recently proposed non-myeloablative conditioning regimens have shown that mixed donor-host hematolymphoid chimerism can be established with relatively low toxicity in both animal studies and human trials. Hematopoietic growth factor treatment of transplanted patients can mobilize a large number of donor stem cells to migrate from marrow to non-hematopoietic organs. We propose that these advances, in conjunction with the developmental plasticity of stem cells, can constitute components of a clinical strategy to use blood and marrow transplantation as a platform to treat systemic diseases involving non-hematopoietic tissues.

Combined effects of calcineurin inhibitors or sirolimus with anti-CD40L mAb on alloengraftment under nonmyeloablative conditions

The immunosuppressive drugs, cyclosporine A (CsA), tacrolimus, or sirolimus, were analyzed as single agents and in combination with anti-CD40L monoclonal antibody (mAb) for their effects on alloengraftment in mice conditioned with minimal total body irradiation (TBI). Whereas anti-CD40L mAb facilitated chimerism, neither sirolimus nor CsA resulted in substantial alloengraftment. However, sirolimus was synergistic with anti-CD40L mAb for inducing donor chimerism. Contrary to expectations, CsA, a T-cell receptor (TCR) signaling inhibitor, did not abrogate anti-CD40L mAb-facilitated engraftment but rather increased engraftment in anti-CD40L mAb-treated mice. Although tacrolimus alone or with anti-CD40L mAb resulted in similar levels of donor chimerism, donor T-cell reconstitution was very low in tacrolimus-treated mice. At 1 week after transplantation, CsA decreased thymic numbers more profoundly than sirolimus or tacrolimus in anti-CD40L mAb-treated recipients. In contrast, only sirolimus resulted in a decrease in host splenic T-cell numbers in anti-CD40L mAb-treated recipients. Importantly, sirolimus and anti-CD40L mAb induced profound donor tolerance with 100% acceptance of donor skin grafts placed early after bone marrow transplantation (BMT). In contrast, anti-CD40L mAb alone or in combination with CsA resulted in 12% or less donor skin graft acceptance early (1 month) and 60% or less later (3 months) after BMT. These data have clinical relevance and indicate that immunosuppressive pharmacologic agents enhance anti-CD40L mAb-facilitated alloengraftment and tolerance induction under nonmyeloablative conditioning.

H2-mismatched transplantation with repetitive cell infusions and CD40 ligand antibody infusions without myeloablation

Graft rejection and graft-versus-host disease are major problems in mismatched marrow transplants along with toxicity from standard myeloablative host treatments. We have established a tolerization model, using 1 Gy irradiation, which reduces stem cell capacity to < 10% of control while causing minimal myelosuppression, donor antigen pre-exposure (spleen cells), CD40-ligand antibody blockade and high levels of marrow (40 x 106 cells), which allows for stable long-term multilineage engraftment in H2-mismatched murine marrow transplants. We now show that the establishment of 'microchimaerism' (0.5-3.8%) sets the stage for macrochimaerism, with subsequent marrow infusions in H2-mismatched mice with CD40-ligand blockade only. Neither irradiation nor spleen cell exposure were necessary. When 40 x 106 bone marrow cells were infused on weeks 0, 12, 14 and 16, blood engraftment was about seven times the single 40 x 106 control. When marrow cells were given on weeks 0, 3, 4, 5 and 6, engraftment at 24 weeks post transplant was 17.9 +/- 1.2%, compared with 2.7 +/- 0.8% for the single 40 x 106 control (P = 0.009). We have shown stable, long-term multilineage chimaerism and established that the schedule of marrow administration, not the total cell dose, is critical for tolerization. This approach indicates that microchimaerism can tolerize for subsequent marrow infusions and produce macrochimaerism. This strategy could be applied in clinical human transplants.

Mechanisms of transplant tolerance induction using costimulatory blockade

The potential use of costimulation-blocking reagents to induce transplantation tolerance has recently created considerable excitement. Recent evidence has begun to delineate the mechanisms by which these powerful effects occur. It has become increasingly clear, firstly, that T cell costimulation is mediated by a delicate network of signaling pathways and, secondly, that interference with these systems can lead to numerous different tolerance mechanisms, including immune regulation, anergy and deletion.

Prevention of chronic rejection in murine cardiac allografts: a comparison of chimerism- and nonchimerism-inducing costimulation blockade-based tolerance induction regimens

We have previously described a nonirradiation-based regimen combining costimulation blockade, busulfan, and donor bone marrow cells that promotes stable, high level chimerism, deletion of donor-reactive T cells, and indefinite survival of skin allografts in mice. The purpose of the current study is to determine the efficacy of this tolerance regimen in preventing acute and chronic rejection in a vascularized heart graft model and to compare this regimen with other putative tolerance protocols. Mice receiving costimulation blockade (CTLA4-Ig and anti-CD40 ligand) alone or in combination with donor cells enjoyed markedly prolonged heart graft survival and initially preserved histological structure. However, tolerance was not achieved, as evidenced by the eventual onset of chronic rejection characterized by obliterative vasculopathy and the rejection of secondary skin grafts. In contrast, following treatment with costimulation blockade, busulfan, and bone marrow, heart grafts survived indefinitely without detectable signs of chronic rejection or structural damage, even 100 days after placement of a secondary donor skin graft. We detected multilineage chimerism in peripheral blood, spleen, lymph nodes, and thymus, and peripheral deletion of donor-reactive cells was complete by day 90. These findings indicate that only the CD40/CD28 blockade chimerism induction regimen prevents both acute and chronic rejection of vascularized organ transplants. Further testing of these strategies in a preclinical large animal model is warranted.

Viral abrogation of stem cell transplantation tolerance causes graft rejection and host death by different mechanisms

Tolerance-based stem cell transplantation using sublethal conditioning is being considered for the treatment of human disease, but safety and efficacy remain to be established. We have shown that mouse bone marrow recipients treated with sublethal irradiation plus transient blockade of the CD40-CD154 costimulatory pathway develop permanent hematopoietic chimerism across allogeneic barriers. We now report that infection with lymphocytic choriomeningitis virus at the time of transplantation prevented engraftment of allogeneic, but not syngeneic, bone marrow in similarly treated mice. Infected allograft recipients also failed to clear the virus and died. Postmortem study revealed hypoplastic bone marrow and spleens. The cause of death was virus-induced IFN-alphabeta. The rejection of allogeneic bone marrow was mediated by a radioresistant CD8(+)TCR-alphabeta(+)NK1.1(-) T cell population. We conclude that a noncytopathic viral infection at the time of transplantation can prevent engraftment of allogeneic bone marrow and result in the death of sublethally irradiated mice treated with costimulation blockade. Clinical application of stem cell transplantation protocols based on costimulation blockade and tolerance induction may require patient isolation to facilitate the procedure and to protect recipients.

Rapamycin and T cell costimulatory blockade as post-transplant treatment promote fully MHC-mismatched allogeneic bone marrow engraftment under irradiation-free conditioning therapy

Hematopoietic macrochimerism, established by bone marrow transplantation, can be used as an approach for treating autoimmune disease and inducing transplant tolerance. In this study, we investigated whether a stable, high level of fully MHC-mismatched hematopoietic macrochimerism can be induced by using irradiation-free protocols, and whether rapamycin and T cell costimulatory blockades (anti-CD40L monoclonal antibody (mAb) and CTLA4Ig) as post-transplant treatment promote bone marrow engraftment. Donor-specific blood transfusion (DST), anti-lymphocyte serum (ALS), busulfan, and cyclophosphamide were given pretransplantation. Balb/c (H-2(d)) bone marrow cells, at a dose of 4 x 10(7), were infused into each C57BL/6 mouse (H-2(b)). Rapamycin, anti-CD40L mAb, and CTLA4Ig were then administered, either alone or in combination. Without ALS or busulfan and cyclophosphamide, macrochimerism can only rarely be induced. Donor-specific transfusion (DST) enhances induction of hematopoietic macrochimerism. Rapamycin, anti-CD40L mAb and CTLA4Ig, alone or in combination, induce a stable and high level of hematopoietic macrochimerism. In the chimeric mice, donor-derived cells were detected in all lymphohematopoietic tissues and donor-specific tolerance was induced in vitro. We conclude that a stable and high level of fully MHC-mismatched hematopoietic macrochimerism can be induced in mice after transplanting a single modest dose of bone marrow cells without irradiation. Rapamycin and T cell costimulatory blockade as post-transplant treatment promote bone marrow engraftment.

Tolerance through bone marrow transplantation with costimulation blockade

The routine induction of tolerance in organ transplant recipients remains an unattained goal. The creation of a state of mixed chimerism through allogeneic bone marrow transplantation leads to robust donor-specific tolerance in several experimental models and this approach has several features making it attractive for clinical development. One of its major drawbacks, however, has been the toxicity of the required host conditioning. The use of costimulation blocking reagents (anti-CD 154 monoclonal antibodies and the fusion protein CTLA4Ig) has led to much less toxic models of mixed chimerism in which global T cell depletion of the host is no longer necessary and which has even allowed the elimination of all cytoreductive treatment when combined with the injection of very high doses of bone marrow cells. In this overview we will briefly discuss general features of tolerance induction through bone marrow transplantation, will then describe recent models using costimulation blockade to induce mixed chimerism and will review the mechanisms of tolerance found with these regimens. Finally we will attempt to identify issues related to the clinical introduction of bone marrow transplantation with costimulation blockade which remain unresolved.

Novel conditioning regimens and nonmyeloablative stem cell transplants

Allogeneic stem cell transplantation has proven an effective means of treatment for a variety of malignant and nonmalignant conditions. Stem cell transplantation has traditionally been preceded by the patient receiving high (myeloablative) doses of chemotherapy and total body irradiation, which have been associated with significant transplant-related morbidity and mortality. This finding has precluded some older or heavily pretreated patients from being offered stem cell transplantation and has made other populations, such as those with nonmalignant conditions, reluctant to consider this therapy. Preclinical studies demonstrated that engraftment of donor stem cells could be achieved with nonmyeloablative, but immunosuppressive, preparative regimens that are significantly less toxic. Preliminary clinical studies have shown that patients previously ineligible for allogeneic stem cell transplantation can frequently be safely transplanted and that an immune-mediated graft-versus-malignancy effect plays an important role in the antitumor efficacy of this treatment. The potential to use this nonmyeloablative approach in nonmalignant disorders in which regimen-related toxicity must be minimized is of particular interest in children.

Stem cell engraftment strategies

The donor stem cell phenotype and host microenvironment determine the outcome of a stem cell transplant. In a series of transplant studies in syngeneic male to female or congenic Ly5.1/Ly5.2 models in which hosts have received no or minimal irradiation (100 cGy), evidence overwhelmingly supports the concept that syngeneic engraftment is determined by stem cell competition. These approaches can be extended to H-2 mismatched allogeneic mouse combination when antigen pre-exposure and CD40-CD40 ligand antibody blockage are employed. A human trial in patients with resistant neoplasia infusing pheresed blood with 10(8) CD3 cells/kg showed that tumor responses and complete chimerism occur with very low levels of CD34+ cells/kg and that the extent of previous treatment is a critical factor in determining chimerism. A major feature of transplants is the phenotype of the donor stem cell. This phenotype shows dramatic reversible plasticity involving differentiation, adhesion protein expression, and engraftment with cytokine-induced cell-cycle transit. Homing is probably also plastic. Marked fluctuations in engraftment capacity are also seen at different points in marrow circadian rhythm.