Early host CD8 T-cell recovery and sensitized anti-donor interleukin-2-producing and cytotoxic T-cell responses associated with marrow graft rejection following nonmyeloablative allogeneic bone marrow transplantation

Reduced positive selection of a human TCR in a swine thymus using a humanized mouse model for xenotolerance induction

BACKGROUND

Tolerance-inducing approaches to xenotransplantation would be optimal and may be necessary for long-term survival of transplanted pig organs in human patients. The ideal approach would generate donor-specific unresponsiveness to the pig organ without suppressing the patient's normal immune function. Porcine thymus transplantation has shown efficacy in promoting xenotolerance in humanized mice and large animal models. However, murine studies demonstrate that T cells selected in a swine thymus are positively selected only by swine thymic epithelial cells, and therefore, cells expressing human HLA-restricted TCRs may not be selected efficiently in a transplanted pig thymus. This may lead to suboptimal patient immune function.

METHODS

To assess human thymocyte selection in a pig thymus, we used a TCR transgenic humanized mouse model to study positive selection of cells expressing the MART1 TCR, a well-characterized human HLA-A2-restricted TCR, in a grafted pig thymus.

RESULTS

Positive selection of T cells expressing the MART1 TCR was inefficient in both a non-selecting human HLA-A2- or swine thymus compared with an HLA-A2+ thymus. Additionally, CD8 MART1 TCRbright T cells were detected in the spleens of mice transplanted with HLA-A2+ thymi but were significantly reduced in the spleens of mice transplanted with swine or HLA-A2- thymi. [Correction added on October 15, 2019, after first online publication: The missing superscript values +, -, and bright have been included in the Results section.] CONCLUSIONS: Positive selection of cells expressing a human-restricted TCR in a transplanted pig thymus is inefficient, suggesting that modifications to improve positive selection of cells expressing human-restricted TCRs in a pig thymus may be necessary to support development of a protective human T-cell pool in future patients.

Immune monitoring of transplant patients in transient mixed chimerism tolerance trials

This review focuses on mechanistic studies performed in recipients of non-myeloablative bone marrow transplant regimens developed at Massachusetts General Hospital in HLA-identical and HLA-mismatched haploidentical combinations, initially as a platform for treatment of hematologic malignancies with immunotherapy in the form of donor leukocyte infusions, and later in combination with donor kidney transplantation for the induction of allograft tolerance. In patients with permanent mixed chimerism, central deletion may be a major mechanism of long-term tolerance. In patients in whom donor chimerism is only transient, the kidney itself plays a significant role in maintaining long-term tolerance. A high throughput sequencing approach to identifying and tracking a significant portion of the alloreactive T cell receptor repertoire has demonstrated biological significance in transplant patients and has been useful in pointing to clonal deletion as a long-term tolerance mechanism in recipients of HLA-mismatched combined kidney and bone marrow transplants with only transient chimerism.

Cooperation of CD4+ T cells and CD8+ T cells and release of IFN-γ are critical for antileukemia responses of recipient mice treated by microtransplantation

Previous studies have demonstrated that infusion of allogeneic matched and haploidentical peripheral blood stem cells with minimal conditioning (microtransplantation) achieved durable responses in patients with refractory leukemia/lymphoma in the absence of engraftment. The mechanisms underlying this response have not been thoroughly elucidated, while host-versus-graft reactions are likely to have an important role. The present study established a mismatched microtransplantation mouse model of leukemia to study the roles of CD4⁺ T cells and CD8⁺ T cells in changes of interferon (IFN)-γ and interleukin (IL)-4 release to explore the mechanisms of the effects of microtransplantation. It was demonstrated that IFN-γ is critical to the antileukemia response in a mouse model of microtransplantation. The therapeutic efficacy was associated with the number of CD4⁺ T cells (Pearson's r=0.722). In addition, CD8⁺ T cells increased the release of IFN-γ with assistance from CD4⁺ T cells. IL-2 augmented IFN-γ release, partly by increasing CD4⁺ T cells (42.8 vs. 35.6%; P<0.05). The present study suggested that the release of IFN-γ via cooperation of CD4⁺ T cells and CD8⁺ T cells represents a crucial mechanism in the antileukemia responses of recipient leukemic mice treated by microtransplantation. During this process, the cooperation of CD4⁺ T cells and CD8⁺ T cells was demonstrated to have a major role in the antileukemia effect. IL-2 may be developed into an agent used for improving the efficacy of microtransplantation by increasing CD4⁺ T cells.

A novel approach to measuring cell-mediated lympholysis using quantitative flow and imaging cytometry

In this study, we established a novel isotope-free approach for the detection of cell-mediated lympholysis (CML) in MHC defined peripheral blood mononuclear cells (PBMCs) using multiparameter flow and imaging cytometry. CML is an established in vitro assay to detect the presence of cytotoxic effector T-lymphocytes precursors (CTLp). Current methods employed in the identification of CTLp in the context of transplantation are based upon the quantification of chromium ((51)Cr) released from target cells. In order to adapt the assay to flow cytometry, primary porcine PBMC targets were labeled with eFluor670 and incubated with major histocompatibility complex (MHC) mismatched effector cytotoxic lymphocytes (CTLs). With this method, we were able to detect target-specific lysis that was comparable to that observed with the (51)Cr-based assay. In addition, the use of quantitative cell imaging demonstrates the presence of accessory cells involved in the cytotoxic pathway. This innovative technique improves upon the standard (51)Cr release assay by eliminating the need for radioisotopes and provides enhanced characterization of the interactions between effector and target cells. This technique has wide applicability to numerous experimental and clinical models involved with effector-cell interactions.

Tracking donor-reactive T cells: Evidence for clonal deletion in tolerant kidney transplant patients

T cell responses to allogeneic major histocompatibility complex antigens present a formidable barrier to organ transplantation, necessitating long-term immunosuppression to minimize rejection. Chronic rejection and drug-induced morbidities are major limitations that could be overcome by allograft tolerance induction. Tolerance was first intentionally induced in humans via combined kidney and bone marrow transplantation (CKBMT), but the mechanisms of tolerance in these patients are incompletely understood. We now establish an assay to identify donor-reactive T cells and test the role of deletion in tolerance after CKBMT. Using high-throughput sequencing of the T cell receptor B chain CDR3 region, we define a fingerprint of the donor-reactive T cell repertoire before transplantation and track those clones after transplant. We observed posttransplant reductions in donor-reactive T cell clones in three tolerant CKBMT patients; such reductions were not observed in a fourth, nontolerant, CKBMT patient or in two conventional kidney transplant recipients on standard immunosuppressive regimens. T cell repertoire turnover due to lymphocyte-depleting conditioning only partially accounted for the observed reductions in tolerant patients; in fact, conventional transplant recipients showed expansion of circulating donor-reactive clones, despite extensive repertoire turnover. Moreover, loss of donor-reactive T cell clones more closely associated with tolerance induction than in vitro functional assays. Our analysis supports clonal deletion as a mechanism of allograft tolerance in CKBMT patients. The results validate the contribution of donor-reactive T cell clones identified before transplant by our method, supporting further exploration as a potential biomarker of transplant outcomes.

Induction of tolerance through mixed chimerism

"Mixed chimerism" refers to a state in which the lymphohematopoietic system of the recipient of allogeneic hematopoietic stem cells comprises a mixture of host and donor cells. This state is usually attained through either bone marrow or mobilized peripheral blood stem cell transplantation. Although numerous treatment regimens have led to transplantation tolerance in mice, the induction of mixed chimerism is currently the only treatment modality that has been successfully extended to large animals and to the clinic. Here we describe and compare the use of mixed chimerism to establish transplantation tolerance in mice, pigs, monkeys, and in the clinic. We also attempt to correlate the mechanisms involved in achieving tolerance with the nature of the tolerance that has resulted in each case.

Expansion of donor-reactive host T cells in primary graft failure after allogeneic hematopoietic SCT following reduced-intensity conditioning

Graft rejection remains a major obstacle in allogeneic hematopoietic SCT following reduced-intensity conditioning (RIC-SCT), particularly after cord blood transplantation (CBT). In a murine MHC-mismatched model of RIC-SCT, primary graft rejection was associated with activation and expansion of donor-reactive host T cells in peripheral blood and BM early after SCT. Donor-derived dendritic cells are at least partly involved in host T-cell activation. We then evaluated if such an expansion of host T cells could be associated with graft rejection after RIC-CBT. Expansion of residual host lymphocytes was observed in 4/7 patients with graft rejection at 3 weeks after CBT, but in none of the 17 patients who achieved engraftment. These results suggest the crucial role of residual host T cells after RIC-SCT in graft rejection and expansion of host T cells could be a marker of graft rejection. Development of more efficient T cell-suppressive conditioning regimens may be necessary in the context of RIC-SCT.

Tolerance induction in HLA disparate living donor kidney transplantation by donor stem cell infusion: durable chimerism predicts outcome

BACKGROUND

We recently reported that durable chimerism can be safely established in mismatched kidney recipients through nonmyeloablative conditioning followed by infusion of a facilitating cell (FC)-based hematopoietic stem cell transplantation termed FCRx. Here we provide intermediate-term follow-up on this phase II trial.

METHODS

Fifteen human leukocyte antigen-mismatched living donor renal transplant recipients underwent low-intensity conditioning (fludarabine, cyclophosphamide, 200 cGy TBI), received a living donor kidney transplant on day 0, then infusion of cryopreserved FCRx on day +1. Maintenance immunosuppression, consisting of tacrolimus and mycophenolate, was weaned over 1 year.

RESULTS

All but one patient demonstrated peripheral blood macrochimerism after transplantation. Engraftment failure occurred in a highly sensitized (panel reactive antibody [PRA] of 52%) recipient. Chimerism was lost in three patients at 2, 3, and 6 months after transplantation. Two of these subjects had received either a reduced cell dose or incomplete conditioning; the other two had PRA greater than 20%. All demonstrated donor-specific hyporesponsiveness and were weaned from full-dose immunosuppression. Complete immunosuppression withdrawal at 1 year after transplantation was successful in all patients with durable chimerism. There has been no graft-versus-host disease or engraftment syndrome. Renal transplantation loss occurred in one patient who developed sepsis following an atypical viral infection. Two subjects with only transient chimerism demonstrated subclinical rejection on protocol biopsy despite donor-specific hyporesponsiveness.

CONCLUSIONS

Low-intensity conditioning plus FCRx safely achieved durable chimerism in mismatched allograft recipients. Sensitization represents an obstacle to successful induction of chimerism. Sustained T-cell chimerism is a more robust biomarker of tolerance than donor-specific hyporeactivity.

A model for personalized in vivo analysis of human immune responsiveness

Studies of human immune diseases are generally limited to the analysis of peripheral blood lymphocytes of heterogeneous patient populations. Improved models are needed to allow analysis of fundamental immunologic abnormalities predisposing to disease and in which to assess immunotherapies. Immunodeficient mice receiving human fetal thymus grafts and fetal CD34(+) cells intravenously produce robust human immune systems, allowing analysis of human T cell development and function. However, to use humanized mice to study human immune-mediated disorders, immune systems must be generated from adult hematopoietic cells. Here, we demonstrated robust immune reconstitution in mice with hematopoietic stem cells (HSCs) aspirated from bone marrow of adults with type 1 diabetes (T1D) and healthy control volunteers. In these humanized mice, cryopreservation of human leukocyte antigen allele-matched fetal thymic tissue prevented allogeneic adult HSC rejection. Newly generated T cells, which included regulatory T cells (T(regs)), were functional and self-tolerant and had a diverse repertoire. The immune recognition of these mice mimicked that of the adult CD34(+) cell donor, but the T cell phenotypes were more predominantly "naïve" than those of the adult donors. HSCs from T1D and control donors generated similar numbers of natural T(regs) intrathymically; however, peripheral T cells from T1D subjects showed increased proportions of activated or memory cells compared to controls, suggesting possible HSC-intrinsic differences in T cell homeostasis that might underlie immune pathology in T1D. This "personalized immune" mouse provides a new model for individualized analysis of human immune responses that may provide new insights into not only T1D but also other forms of immune function and dysfunction as well.

Emerging concepts in haematopoietic cell transplantation

Haematopoietic cell transplantation (HCT) is the most widely used form of cellular therapy. It is the only known cure for some haematological malignancies and has recently been used in additional clinical settings, such as allograft tolerance induction and treatment of autoimmune diseases. Recent advances have enabled HCT in a wider range of patients with improved outcomes. This Review summarizes the latest developments in this therapy, focusing on issues that will affect future advancement.

Escape of leukemia blasts from HLA-specific CTL pressure in a recipient of HLA one locus-mismatched bone marrow transplantation

A case of leukemia escape from an HLA-specific cytotoxic T lymphocyte (CTL) response in a recipient of bone marrow transplantation is presented. Only the expression of HLA-B51, which was a mismatched HLA locus in the graft-versus-host direction, was down-regulated in post-transplant leukemia blasts compared with that in pre-transplant blasts. All CTL clones, that were isolated from the recipient's blood when acute graft-versus-host disease developed, recognized the mismatched B(∗)51:01 molecule in a peptide-dependent manner. The pre-transplant leukemia blasts were lysed by CTL clones, whereas the post-transplant leukemia blasts were not lysed by any CTL clones. The IFN-γ ELISPOT assay revealed that B(∗)51:01-reactive T lymphocytes accounted for the majority of the total alloreactive T lymphocytes in the blood just before leukemia relapse. These data suggest that immune escape of leukemia blasts from CTL pressure toward a certain HLA molecule can lead to clinical relapse after bone marrow transplantation.

Mechanisms of donor-specific tolerance in recipients of haploidentical combined bone marrow/kidney transplantation

We recently reported long-term organ allograft survival without ongoing immunosuppression in four of five patients receiving combined kidney and bone marrow transplantation from haploidentical donors following nonmyeloablative conditioning. In vitro assays up to 18 months revealed donor-specific unresponsiveness. We now demonstrate that T cell recovery is gradual and is characterized by memory-type cell predominance and an increased proportion of CD4⁺ CD25⁺ CD127⁻ FOXP3⁺ Treg during the lymphopenic period. Complete donor-specific unresponsiveness in proliferative and cytotoxic assays, and in limiting dilution analyses of IL-2-producing and cytotoxic cells, developed and persisted for the 3-year follow-up in all patients, and extended to donor renal tubular epithelial cells. Assays in two of four patients were consistent with a role for a suppressive tolerance mechanism at 6 months to 1 year, but later (≥ 18 months) studies on all four patients provided no evidence for a suppressive mechanism. Our studies demonstrate, for the first time, long-term, systemic donor-specific unresponsiveness in patients with HLA-mismatched allograft tolerance. While regulatory cells may play an early role, long-term tolerance appears to be maintained by a deletion or anergy mechanism.

Mixed chimerism, lymphocyte recovery, and evidence for early donor-specific unresponsiveness in patients receiving combined kidney and bone marrow transplantation to induce tolerance

BACKGROUND

We have previously reported operational tolerance in patients receiving human leukocyte antigen-mismatched combined kidney and bone marrow transplantation (CKBMT). We now report on transient multilineage hematopoietic chimerism and lymphocyte recovery in five patients receiving a modified CKBMT protocol and evidence for early donor-specific unresponsiveness in one of these patients.

METHODS

Five patients with end-stage renal disease received CKBMT from human leukocyte antigen-mismatched, haploidentical living-related donors after modified nonmyeloablative conditioning. Polychromatic flow cytometry was used to assess multilineage chimerism and lymphocyte recovery posttransplant. Limiting dilution analysis was used to assess helper T-lymphocyte reactivity to donor antigens.

RESULTS

Transient multilineage mixed chimerism was observed in all patients, but chimerism became undetectable by 2 weeks post-CKBMT. A marked decrease in T- and B-lymphocyte counts immediately after transplant was followed by gradual recovery. Initially, recovering T cells were depleted of CD45RA+/CD45RO(-) "naïve-like" cells, which have shown strong recovery in two patients, and CD4:CD8 ratios increased immediately after transplant but then declined markedly. Natural killer cells were enriched in the peripheral blood of all patients after transplant.For subject 2, a pretransplant limiting dilution assay revealed T helper cells recognizing both donor and third-party peripheral blood mononuclear cells. However, the antidonor response was undetectable by day 24, whereas third-party reactivity persisted.

CONCLUSION

These results characterize the transient multilineage mixed hematopoietic chimerism and recovery of lymphocyte subsets in patients receiving a modified CKBMT protocol. The observations are relevant to the mechanisms of donor-specific tolerance in this patient group.

The combined administration of partially HLA-matched irradiated allogeneic lymphocytes and thalidomide in advanced renal-cell carcinoma: a case report

Renal-cell carcinoma (RCC) is susceptible to immune therapy including the use of the nonmyeloablative allogeneic transplantation (NST). However, NST can produce severe toxicity, might not be appropriate for many patients with metastatic RCC. Other novel allogeneic immunotherapies are designed to induce an autologous immune response directed against the malignancy. In single-arm phase II trials, thalidomide has demonstrated a modest activity in the treatment of advanced RCC. Here we present a case report in which a patient with advanced RCC in the absence of transplant conditioning, that was receiving thalidomide, was infused with partially HLA-matched irradiated allogeneic lymphocytes. In this patient a complete response with weak acute graft-versus-host disease (GVHD) was observed. No evidence of the disease was present over the subsequent 36 months survival of the patient, suggesting that the infusions may have played a major role in the antineoplastic effect. A potential mechanism of this protocol may involve a host-versus-graft reactions-mediated antitumor effect against the malignancy. In addition, the present results suggest that a combination protocol with alternate treatment (e.g., chemotherapy) schedules merit further investigation in the management of various malignancies.

Evidence for anti-tumour effect of allogeneic haematopoietic SCT in cases without sustained donor engraftment

Remissions of haematological malignancies have been reported after allo-SCT, despite donor cell rejection, suggesting that sustained allogeneic engraftment is not mandatory to obtain a lasting anti-tumour effect. To evaluate the potential benefit from transient post-allo-SCT alloreactivity, we took advantage of the Société Française de Greffe de Moëlle et Thérapie Cellulaire (SFGM-TC) registry to colligate 14 patients with an efficient and long-lasting allogeneic (GVL) effect after allo-SCT for haematological malignancies, despite transient or absent engraftment. None received a second allogeneic graft after autologous recovery. The median duration of remission after autologous reconstitution was 118 (12-252) months. Although we cannot exclude the possibility that some patients were cured before allo-SCT, this retrospective analysis does strongly suggest that an efficient GVL effect can be observed without sustained donor engraftment, and that the transient presence of donor T cells might be sufficient to induce a powerful GVL effect.

Neural progenitors derived from human embryonic stem cells are targeted by allogeneic T and natural killer cells

Neural progenitor cells (NPC) of foetal origin or derived from human embryonic stem cells (HESC) have the potential to differentiate into mature neurons after transplantation into the central nervous system, opening the possibility of cell therapy for neurodegenerative disorders. In most cases, the transplanted NPC are genetically unrelated to the recipient, leading to potential rejection of the transplanted cells. Very few data provide reliable information as to the potential immune response of allogeneic neural progenitors derived from HESC. In this study, we analyzed in vitro the allogeneic immune response of T lymphocytes and natural killer (NK) cells to NPC derived from HESC or of foetal origin. We demonstrate that NPC induce T-cell stimulation and a strong NK cytotoxic response. NK-cell activity is unrelated to MHC-I expression but driven by the activating NKG2D receptor. Cyclosporine and dexamethasone previously used in clinical studies with foetal NPC did not only fail to prevent NK alloreactivity but strongly inhibited the terminal maturation from NPC into mature neurons. We conclude that allogenic transplantation of NPC in the central nervous system will most likely require an immunosuppressive regimen targeting allogenic T and NK cells, whereas possible interference with the differentiation of NPC needs to be carefully evaluated.

CD134-allodepletion allows selective elimination of alloreactive human T cells without loss of virus-specific and leukemia-specific effectors

Graft-versus-host disease (GVHD) remains a frequent and severe complication of allogeneic stem cell transplantation (SCT). One approach to reducing alloreactivity is to deplete the graft of alloreactive T cells. Global T cell depletion results in poor immune reconstitution with high mortality from viral infections and disease relapse. Therefore, an approach to selectively deplete alloreactive T cells without compromising other responses would be highly beneficial. We undertook studies to identify an inducible activation marker expressed on alloreactive effector T cells following culture with HLA-mismatched allostimulators. Compared to other markers, CD134 was superior because of its negative baseline expression and rapid upregulation after activation. Depletion of CD134(+) cells from responder populations dramatically reduced specific alloreactivity as determined by reduction of helper T cell precursor frequencies below the threshold predicting development of clinical GVHD while retaining responses to third-party alloantigens. CD134-allodepleted populations retained effectors specific for the Wilms' tumor (WT1) leukemia antigen as determined by WT1 specific pentamers, and CMV-specific effectors as determined by CMV-specific pentamers and CMV-specific ELISpot. Thus, use of CD134-allodepleted grafts may improve allogeneic SCT by reducing GVHD without loss of pathogen-specific and leukemia-specific immunity.

Completely mismatched allogeneic CD3/CD28 cross-linked Th1 memory cells elicit anti-leukemia effects in unconditioned hosts without GVHD toxicity

Fully allogeneic CD3/CD28 cross-linked Th1 cells were found to elicit host-mediated anti-leukemia effects without GVHD toxicity. Mice inoculated with a lethal dose of BCL1 leukemia demonstrated significantly enhanced survival after allogeneic Th1 treatment. Cure rates of 12.5% with a single allogeneic cell infusion and 31.25% with multiple infusions were demonstrated. Cured mice were able to reject rechallenge with a lethal dose of tumor without further treatment. These results suggest that use of intentionally mis-matched, Th1 memory cells infused with cross-linked CD3/CD28 could represent a novel clinical approach to eliciting potent anti-tumor effects in patients without conditioning and without GVHD toxicity.

Manipulating the immune system for anti-tumor responses and transplant tolerance via mixed hematopoietic chimerism

SUMMARY

Stem cells (SCs) with varying potentiality have the capacity to repair injured tissues. While promising animal data have been obtained, allogeneic SCs and their progeny are subject to immune-mediated rejection. Here, we review the potential of hematopoietic stem cells (HSCs) to promote immune tolerance to allogeneic and xenogeneic organs and tissues, to reverse autoimmunity, and to be used optimally to cure hematologic malignancies. We also review the mechanisms by which hematopoietic cell transplantation (HCT) can promote anti-tumor responses and establish donor-specific transplantation tolerance. We discuss the barriers to clinical translation of animal studies and describe some recent studies indicating how they can be overcome. The recent achievements of durable mixed chimerism across human leukocyte antigen barriers without graft-versus-host disease and of organ allograft tolerance through combined kidney and bone marrow transplantation suggest that the potential of this approach for use in the treatment of many human diseases may ultimately be realized.

Chimerism and tolerance in a recipient of a deceased-donor liver transplant

Complete hematopoietic chimerism and tolerance of a liver allograft from a deceased male donor developed in a 9-year-old girl, with no evidence of graft-versus-host disease 17 months after transplantation. The tolerance was preceded by a period of severe hemolysis, reflecting partial chimerism that was refractory to standard therapies. The hemolysis resolved after the gradual withdrawal of all immunosuppressive therapy.

The anti-tumor effect of allogeneic bone marrow/stem cell transplant without graft vs. host disease toxicity and without a matched donor requirement?

The anti-tumor immune response that occurs in allogeneic bone marrow/stem cell transplant (BMT) settings is capable of eradicating tumors that are resistant to chemotherapy/radiation treatment. This anti-tumor immune response, known as the graft vs. tumor (GVT) effect, is the most effective immunotherapy treatment ever discovered. Unfortunately, the clinical application of GVT is severely limited due to the intimate association of GVT with the extremely toxic and often lethal side-effect known as graft vs. host disease (GVHD). It is a major research focus in the field of BMT to develop methods to separate the beneficial GVT effect from the detrimental GVHD toxicity. However, due to the intimate association of these effects, attempts to limit GVHD also have a tendency to limit the GVT effect. We propose a new concept for harnessing the power of the GVT effect without the toxicity of GVHD. Rather than trying to separate GVT from GVHD, we propose that these naturally coupled effects can 'mirrored' onto the host immune system and maintain their intimate association. The 'mirror' of GVHD is a host rejection of a graft (HVG). As rejection of an allograft would not be toxic, an HVG effect coupled to a host vs. tumor (HVT) effect, the 'mirror' of the GVT effect, would provide the anti-tumor effect of BMT without GVHD toxicity. In the 'mirror' setting, the HVT effect must occur against syngeneic tumors, while in the BMT setting the GVT effect occurs in the allogeneic setting. Previous attempts to elicit syngeneic anti-tumor immunity using therapeutic tumor vaccines have had disappointing results in the clinic due to the influence of tumor immunoavoidance mechanisms. We propose that the 'danger' signals that are released as a result of GVHD in the allogeneic BMT setting serve as an adjuvant to the GVT effect disabling tumor immunoavoidance. The chemotherapy/radiation conditioning prior to transplant is a required initiating event to the coupled GVT/GVHD effects. The conditioning releases 'danger' signals that mediate this adjuvant effect. To imitate this immunological event in immunocompetent, non-conditioned patients we propose that infusion of freshly activated, polyclonal CD4+ memory Th1 cells which express CD40L on the cell surface will stimulate a HVT/HVG 'mirror' effect, providing a non-toxic means to elicit the effective immune-mediated anti-tumor effect of BMT without the GVHD toxicity and without the requirement for a matched donor.

Infantile hypophosphatasia: transplantation therapy trial using bone fragments and cultured osteoblasts

BACKGROUND

Hypophosphatasia (HPP) is a rare, heritable, metabolic bone disease due to deficient activity of the tissue-nonspecific isoenzyme of alkaline phosphatase. The infantile form features severe rickets often causing death in the first year of life from respiratory complications. There is no established medical treatment. In 1997, an 8-month-old girl with worsening and life-threatening infantile HPP improved considerably after marrow cell transplantation.

OBJECTIVE

Our aim was to better understand and to advance these encouraging transplantation results.

DESIGN

In 1999, based on emerging mouse transplantation models involving implanted donor bone fragments as well as osteoblast-like cells cultured from bone, we treated a 9-month-old girl suffering a similar course of infantile HPP.

RESULTS

Four months later, radiographs demonstrated improved skeletal mineralization. Twenty months later, PCR analysis of adherent cells cultured from recipient bone suggested the presence of small amounts of paternal (donor) DNA despite the absence of hematopoietic engraftment. This patient, now 8 yr old (7 yr after transplantation), is active and growing, and has the clinical phenotype of the more mild, childhood form of HPP.

CONCLUSIONS

Cumulative experience suggests that, after immune tolerance, donor bone fragments and marrow may provide precursor cells for distribution and engraftment in the skeletal microenvironment in HPP patients to form tissue-nonspecific isoenzyme of alkaline phosphatase-replete osteoblasts that can improve mineralization.

Regulatory T-cell recovery in recipients of haploidentical nonmyeloablative hematopoietic cell transplantation with a humanized anti-CD2 mAb, MEDI-507, with or without fludarabine

OBJECTIVE

We have evaluated T-cell reconstitution and reactivity in patients receiving nonmyeloablative haploidentical hematopoietic cell transplantation (HCT) protocols involving an anti-CD2 monoclonal antibody (MEDI 507) to treat chemorefractory hematopoietic malignancies.

METHODS

Three cohorts of four patients each and one cohort of six patients received one of four Medi-507-based regimens, all of which included cyclophosphamide, thymic irradiation, and a short posttransplantation course of cyclosporine.

RESULTS

Following marked T-cell depletion, initially recovering CD4 and CD8 T cells were mainly memory-type cells. A high percentage of CD4 T cells expressed high levels of CD25 in recipients of all protocols, except the only protocol to include fludarabine, early post-HCT. CD25 expression varied inversely with T-cell concentrations in blood. CD25(high) CD4 T cells expressed Foxp3 and cytotoxic T-lymphocyte-associated protein 4, indicating that they were regulatory T cells (Treg).

CONCLUSIONS

Fludarabine treatment prevents Treg enrichment after haploidentical nonmyeloablative stem cell transplantation, presumably by depleting recipient Tregs. In vitro analyses of allorecognition were consistent with a cytokine-mediated rejection process in one case and in another provided proof of principle that mixed chimerism achieved without graft-vs-host disease induces donor- and recipient-specific tolerance. More reliable achievement of this outcome could provide a promising strategy for organ allograft tolerance induction.

Comparison of outcomes after transplantation of peripheral blood stem cells versus bone marrow following an identical nonmyeloablative conditioning regimen

This is the first study to examine the outcomes in 54 patients with hematologic malignancies who received an HLA-matched related donor bone marrow (BM, n = 42) or GCSF-mobilized peripheral blood stem cells (PBSC, n = 12) following identical nonmyeloablative conditioning with the intention of induction of mixed chimerism (MC) followed by prophylactic donor leukocyte infusion (pDLI) to convert MC to full donor chimerism (FDC) and capture a graft-versus-tumor effect without clinical graft-versus-host disease (GVHD). Neutrophil and platelet recovery were faster and transfusion requirement was less in PBSC recipients (P < 0.05). A total of 48% of BMT recipients achieved FDC with a median conversion time of 84 days, including 13 following pDLI. In contrast, 83% (P = 0.04) in the PBSC group had spontaneous FDC at a median of 14 days, precluding the administration of pDLI. There was no significant difference in the incidences of acute or chronic GVHD, though the rates of chronic GVHD were considerably higher in PBSC group than in the BM group (6/7, 86% vs 10/24, 42%). CD4 and CD8 T-cell recovery was faster in PBSC recipients. In PBSC recipients, a higher number of CD34+ cells was associated with increased rates of severe, grade III-IV acute GVHD.

Preformed antibody, not primed T cells, is the initial and major barrier to bone marrow engraftment in allosensitized recipients

Multiply-transfused individuals are at higher risk for BM rejection. We show that whereas allosensitization resulted in the priming of both cellular and humoral immunity, preformed antibody was the major barrier to engraftment. The generation of cross-reactive alloantibody led to rejection of BM of a different MHC-disparate strain. Imaging studies indicated that antibody-mediated rejection was very rapid (<3 hours) in primed recipients, while T-cell-mediated rejection in nonprimed mice took more than 6 days. Antibody-mediated BM rejection was not due to a defect in BM homing as rejection occurred despite direct intra-BM infusion of donor BM. Rejection was dependent upon host FcR+ cells. BM cells incubated with serum from primed mice were eliminated in nonprimed recipients, indicating that persistent exposure to high-titer antibody was not essential for rejection. High donor engraftment was achieved in a proportion of primed mice by mega-BM cell dose, in vivo T-cell depletion, and high-dose immunoglobulin infusion. The addition of splenectomy to this protocol only modestly added to the efficacy of this combination strategy. These data demonstrate both rapid alloantibody-mediated elimination of BM by host FcR+ cells and priming of host antidonor T cells and suggest a practical strategy to overcome engraftment barriers in primed individuals.

MiHA reactive CD4 and CD8 T-cells effect resistance to hematopoietic engraftment following reduced intensity conditioning

Reduced intensity conditioning (RIC) prior to allogeneic hematopoietic cell transplantation (HCT) has shown promise in lowering the incidence of post-transplant complications including infection and graft-versus-host disease. T-cell-mediated graft rejection, however, remains a crucial factor in determining how 'mild' a level of immunosuppression can be administered. Understanding the kinetics of resistance responses as well as the role of CD4+ and CD8+ T cells underlies the development of protocols to circumvent resistance and support hematopoietic engraftment. In these studies, a major histocompatibility complex (MHC)-matched/minor histocompatibility antigen (MiHA) disparate RIC HCT model was developed in which resistance against donor hematopoietic progenitors as well as mature peripheral blood cells could be assessed. Interestingly, resistance was diminished in the absence of either host CD4+ or CD8+ T cells. However, its impairment was more severe in CD4-/- mice where resistance was not detected. Host CD4+ T cells were required for optimal expansion of specific (H60) T-cell receptor (TCR) expressing host anti-donor MiHA reactive CD8+ T cells following HCT. These observations demonstrate a critical role for host CD4+ T cells in resistance against MiHA disparate HCT. This RIC HCT resistance model will be useful for the analysis of the barrier to engraftment mediated by host T cells and the development of strategies to support engraftment.

Myeloma responses and tolerance following combined kidney and nonmyeloablative marrow transplantation: in vivo and in vitro analyses

Six patients with renal failure due to multiple myeloma (MM) received simultaneous kidney and bone marrow transplantation (BMT) from HLA-identical sibling donors following nonmyeloablative conditioning, including cyclophosphamide (CP), peritransplant antithymocyte globulin and thymic irradiation. Cyclosporine (CyA) was given for approximately 2 months posttransplant, followed by donor leukocyte infusions. All six patients accepted their kidney grafts long-term. Three patients lost detectable chimerism but accepted their kidney grafts off immunosuppression for 1.3 to >7 years. One such patient had strong antidonor cytotoxic T lymphocyte (CTL) responses in association with marrow rejection. Two patients achieved full donor chimerism, but resumed immunosuppression to treat graft-versus-host disease. Only one patient experienced rejection following CyA withdrawal. He responded to immunosuppression, which was later successfully withdrawn. The rejection episode was associated with antidonor Th reactivity. Patients showed CTL unresponsiveness to cultured donor renal tubular epithelial cells. Initially recovering T cells were memory cells and were enriched for CD4+CD25+ cells. Three patients are in sustained complete remissions of MM, despite loss of chimerism in two. Combined kidney/BMT with nonmyeloablative conditioning can achieve renal allograft tolerance and excellent myeloma responses, even in the presence of donor marrow rejection and antidonor alloresponses in vitro.

Role of indirect allo- and autoreactivity in anti-tumor responses induced by recipient leukocyte infusions (RLI) in mixed chimeras prepared with nonmyeloablative conditioning

In mixed chimeras prepared with nonmyeloablative conditioning, we previously showed that recipient leukocyte infusions (RLI) induced loss of donor chimerism and anti-tumor responses against the A20 BALB/c B cell lymphoma. We also previously showed that RLI-mediated tumor rejection involved IFN-gamma-producing RLI-derived CD8+ cells and non-RLI, recipient-derived CD4 T cells, leading to the generation of anti-tumor cytotoxic cells. However, the mechanisms of such paradoxical anti-tumor responses remained to be clarified. In the present study, we further explored the cellular mechanisms of the anti-tumor effects of RLI in fully MHC-mismatched and haploidentical strain combinations. In both cases, we show that RLI breaks the tolerance of chimeric T cells toward donor antigens, in association with the in vivo expansion of recipient splenic T, B and CD4-CD8- cells and the production of IFN-gamma. RLI leads to the development of two types of tumor-specific responses. The first is mediated by indirect presentation of donor antigens and occurs independently of tumor injection. The second is observed only in recipients of RLI and tumor and may involve responses to self antigens. Anti-tumor cytotoxicity was mediated by CD8+ or CD4-CD8- effector cells. Thus, anti-tumor cytotoxic responses are generated following complex interactions between recipient APCs presenting donor and recipient antigens and host-type CD4+, CD8+ and CD4-CD8- cells.

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.

Mechanisms of the antitumor responses and host-versus-graft reactions induced by recipient leukocyte infusions in mixed chimeras prepared with nonmyeloablative conditioning: a critical role for recipient CD4+ T cells and recipient leukocyte infusion-derived IFN-gamma-producing CD8+ T cells

Surprisingly, antitumor responses can occur in patients who reject donor grafts following nonmyeloablative hemopoietic cell transplantation. In murine mixed chimeras prepared with nonmyeloablative conditioning, we previously showed that recipient leukocyte infusions (RLI) induced loss of donor chimerism, IFN-gamma production, and antitumor responses against host-type tumors. However, the mechanisms behind this phenomenon remain to be determined. We now demonstrate that the effects of RLI are mediated by distinct and complex mechanisms. Donor marrow rejection is induced by RLI-derived alloactivated T cells, which activate non-RLI-derived, recipient IFN-gamma-producing cells. RLI-derived CD8 T cells induce the production of IFN-gamma by both RLI and non-RLI-derived recipient cells. The antitumor responses of RLI involve mainly RLI-derived IFN-gamma-producing CD8 T cells and recipient-derived CD4 T cells and do not involve donor T cells. The pathways of donor marrow and tumor rejection lead to the development of tumor-specific cell-mediated cytotoxic responses that are not due to bystander killing by alloreactive T cells.

Limiting Dilution Analysis of Interleukin-2 Producing Helper T-cell Frequencies as a Tool in Allogeneic Hematopoietic Cell Transplantation

BACKGROUND

A reliable in vitro test that estimates the level of ongoing alloreactivity would be valuable in allogeneic hematopoietic cell transplantation (HCT) as a help to guide clinical interventions such as donor lymphocyte infusions and changes in the immunosuppression. In the present study, the use of limiting dilution analysis of interleukin-2 (IL-2) producing helper T lymphocyte frequencies (HTL assay) as a way to quantify alloreactivity following HCT was investigated.

METHODS

Serial HTL assays were performed following allogeneic HCT with myeloablative or nonmyeloablative conditioning in 26 patients with hematologic malignancies.

RESULTS

Deviations from single-hit kinetics were frequently observed in the HTL assays and a nonlinear model was therefore used for analysis. The results of this analysis suggested the presence of an inhibitory cell population. Inhibition was observed in the majority of patients and was not restricted to a specific transplant regimen. Inhibition occurred more often with high frequencies of IL-2 producing cells, indicating a physiological role of the putative inhibitory cell population in the regulation of an immune response. Higher frequencies of IL-2 producing cells were observed in patients with acute graft-versus-host disease grades II-IV than in patients with grades 0-I (P = 0.046), indicating that the degree of ongoing alloreactivity is indeed quantified by the HTL assay.

CONCLUSIONS

We find that the HTL assay may yield interesting insight into regulation of immune responses following allogeneic HCT, but because of the complexity of the results obtained, its use as a routine procedure to guide immunosuppression cannot be recommended.

Engraftment of Allogeneic Hematopoietic Stem Cells Requires Both Inhibition of Host-Versus-Graft Responses and ‘Space' for Homeostatic Expansion

BACKGROUND

The establishment of host-versus-graft (HvG) tolerance is the primary aim of reduced intensity conditioning (RIC) regimens for allogeneic stem cell transplantation (SCT). It remains to be clarified to what extent recipient myeloablation is fundamental in the establishment of donor chimerism.

METHODS

We have addressed this question in a murine model of RIC SCT in which the donor-recipient combination produces HvG against the male specific minor histocompatibility antigen HY. In this system engraftment can be monitored by RT-PCR and HvG effectors enumerated by tetramer analysis.

RESULTS

We demonstrate that the dose of irradiation influences donor hemopoietic engraftment and affects generation of anti-donor specific T cells. Chimeric recipients do not mount a HvG immune response, becoming selectively tolerant, as demonstrated by the long term acceptance of skin grafts of donor but not third party origin. However, HvG tolerance is not sufficient to secure engraftment since, even in the absence of HvG, partial myeloablation was still required. The "space" produced by myeloablation and the consequent potential for donor cell expansion could also affect HvG tolerance, since its induction is severely impaired when donor hematopoietic cells have reduced proliferative capacity.

CONCLUSIONS

We conclude that both some degree of myeloablation and HvG tolerance are required for successful engraftment, and that the capacity of donor cells to proliferate influences the induction of HvG tolerance.

CD45 congenic bone marrow transplantation: evidence for T cell-mediated immunity

CD45 congenic mice have been used to study stem cell engraftment in the absence of alloreactivity. Recently, impaired engraftment was reported in this model and attributed to weak immune reactivity. We have confirmed that there is indeed low-level reactivity mediated by CD8(+) cells and alpha beta-TCR(+) T cells. B6 (CD45.2) recipients were conditioned with total body irradiation (TBI) and transplanted with increasing doses of B6 (CD45.1) bone marrow cells (BMCs). Although chimerism was present at 1 month in all recipients, durable engraftment did not occur with <150 cGy of TBI, emphasizing the importance of long-term follow-up in evaluating nonmyeloablative conditioning approaches. A single dose of cyclophosphamide on day 2 also significantly enhanced engraftment. When B6 TCR beta(-/-), TCR delta(-/-), or TCR beta(-/-)/delta(-/-) (CD45.2) mice were transplanted with CD45.1 bone marrow, significantly enhanced engraftment occurred in recipients lacking alpha beta-TCR(+) T cells (p < .00005). Similarly, removal of alpha beta-TCR(+) host T cells in wild-type recipients resulted in enhanced engraftment. Engraftment was also significantly increased in CD8(-/-) and CD4(-/-)/8(-/-) recipients (p < .0005). Taken together, these results demonstrate that alpha beta-TCR(+) and CD8(+) T cells play a critical role in regulating engraftment of CD45 congenic marrow and suggest that these cells are the main effector cells in low-level alloreactivity to the CD45 disparity.

Anti-tumour response despite loss of donor chimaerism in patients treated with non-myeloablative conditioning and allogeneic stem cell transplantation

Based on a murine model, we conducted a series of trials of m-myeloablative human leucocyte antigen (HLA)-matched or mismatched related donor stem cell transplantation (SCT) with the intention of inducing mixed chimaerism (MC), then administering prophylactic donor lymphocyte infusions (DLIs), for the treatment of advanced haematologic malignancies. Preparative therapy consisted of cyclophosphamide, equine anti-thymocyte globulin (ATG) or MEDI-507 (an anti-CD2 monoclonal antibody) for in-vivo T-cell depletion, thymic irradiation on day -1 and cyclosporine alone for graft-versus-host disease (GVHD) prophylaxis. DLIs were given as early as 5 weeks post-SCT in patients with MC without evidence of GVHD. Twenty-two patients ultimately lost their graft (<1% donor cells) that could no be rescued by DLIs. Nine of 22 (41%) patients who lost donor chimaerism achieved an objective response, including three patients who showed evidence of disease regression following DLI, despite continued absence of macrochimaerism. Six patients were alive at 2.5-5.5 years following SCT, including four in continuous complete remission. In summary, it is possible to achieve sustained remission in patients with chemorefractory malignancies following non-myeloablative allogeneic SCT, even in the absence of sustained donor macrochimaerism; DLI may contribute to an ongoing anti-tumour effect in these patients. Immunological mechanisms that correlated with rejection of the graft may have a role in anti-tumour responses via a cell or cytokine-mediated pathway.

Monocyte-mediated T-cell suppression and augmented monocyte tryptophan catabolism after human hematopoietic stem-cell transplantation

T-cell dysfunction after human hematopoietic stem-cell transplantation (HSCT) is generally attributed to intrinsic T-cell defects. Here we show that the characteristic impaired proliferative responses to polyclonal stimulation of post-HSCT peripheral blood mononuclear cells (PB-MCs) were markedly (4-fold) improved by T-cell enrichment. Conversely, addback of post-HSCT monocytes to these enriched T cells dampened their proliferative responses, suggesting that post-HSCT monocytes effectively mediate T-cell suppression. As a mechanism possibly contributing to monocyte-mediated T-cell suppression, we investigated monocyte tryptophan catabolism by indoleamine 2,3-dioxygenase into kynurenine, which has been implicated in regulating T-cell responses. Compared with controls, all post-HSCT monocyte-containing cell cultures (total PBMCs, monocytes, and monocyte/T-cell cocultures), but not monocyte-depleted populations, secreted elevated amounts of kynurenine. Blockade of tryptophan catabolism improved the proliferative responses. The slightly increased kynurenine release and substantial release of neopterin by unstimulated post-HSCT monocytes suggests that they were in a state of continuous activation. Superimposed on this state, stimulation of these cells caused a striking, additional increase (10-fold) in kynurenine release, and they triggered marked apoptosis of autologous post-HSCT T cells. We conclude that the amplified kynurenine release by post-HSCT monocytes, particularly induced upon stimulation, may underlie their suppressor activity, which in turn may contribute to the depressed T-cell immune responses after HSCT.

Unmanipulated Reduced-Intensity Stem Cell Transplantation from a Haploidentical Donor Mismatched at 3 HLA Antigens to a Patient with Leukemic Transformation of Myelodysplastic Syndrome: Successful Second Transplantation after Graft Rejection

We present the case of a patient with myelodysplastic syndrome who experienced leukemia transformation and subsequently underwent transplantation of unmanipulated peripheral blood stem cells from a haploidentical sibling mismatched at 3 HLA antigens, along with a reduced-intensity regimen (fludarabine, busulfan, and anti-T-lymphocyte globulin) and tacrolimus-containing graft-versus-host disease (GVHD) prophylaxis. The patient experienced graft rejection but successfully underwent a second transplantation from the same donor with a slightly intensified conditioning regimen. Although the patient developed life-threatening cytomegalovirus (CMV) pneumonia following the second transplantation, he recovered completely from the pneumonia with intensive supportive therapy. He is still in complete remission past day 1000 in the absence of GVHD. As far as we know, this report is the first to describe a successful second transplantation that was performed for graft rejection following HLA-haploidentical nonmyeloablative stem cell transplantation. Furthermore, we emphasize that patients should be carefully monitored for CMV infection when reduced-intensity conditioning is given repeatedly over a short period.

Targeted pretransplant host lymphocyte depletion prior to T-cell depleted reduced-intensity allogeneic stem cell transplantation

Mixed chimaerism and graft rejection are higher after reduced-intensity allogeneic stem cell transplantation (RIST) with T-cell depleted (TCD) allografts. As host immune status before RIST affects engraftment, we hypothesized that targeted depletion of host lymphocytes prior to RIST would abrogate graft rejection and promote donor chimaerism. Lymphocyte-depleting chemotherapy was administered at conventional doses to subjects prior to RIST with the intent of decreasing CD4(+) counts to <0.05 x 10(9)cells/l. Subjects (n = 18) then received reduced-intensity conditioning followed by ex vivo TCD human leucocyte antigen-matched sibling allografts. All evaluable patients (n = 17) were engrafted; there were no late graft failures. At day +28 post-RIST, 12 patients showed complete donor chimaerism. Mixed chimaerism in the remaining five patients was associated with higher numbers of circulating host CD3(+) cells (P = 0.0032) after lymphocyte-depleting chemotherapy and was preferentially observed in T lymphoid rather than myeloid cells. Full donor chimaerism was achieved in all patients after planned donor lymphocyte infusions. These data reflect the importance of host immune status prior to RIST and suggest that targeted host lymphocyte depletion facilitates the engraftment of TCD allografts. Targeted lymphocyte depletion may permit an individualized approach to conditioning based on host immune status prior to RIST.

Production of donor T cells is critical for induction of donor-specific tolerance and maintenance of chimerism

Nonmyeloablative conditioning has significantly reduced the morbidity associated with bone marrow transplantation. The donor hemopoietic cell lineage(s) responsible for the induction and maintenance of tolerance in nonmyeloablatively conditioned recipients is not defined. In the present studies we evaluated which hemopoietic stem cell-derived components are critical to the induction of tolerance in a total body irradiation-based model. Recipient B10 mice were pretreated with mAbs and transplanted with allogeneic B10.BR bone marrow after conditioning with 100-300 cGy total body irradiation. The proportion of recipients engrafting increased in a dose-dependent fashion. All chimeric recipients exhibited multilineage donor cell production. However, induction of tolerance correlated strictly with early production of donor T cells. The chimeras without donor T cells rejected donor skin grafts and demonstrated strong antidonor reactivity in vitro, while possessing high levels of donor chimerism. These animals lost chimerism within 8 mo. Differentiation into T cells was aborted at a prethymic stage in recipients that did not produce donor T cells. Moreover, donor Ag-driven clonal deletion of recipient T cells occurred only in chimeras with donor T cells. These results demonstrate that donor T cell production is critical in the induction of transplantation tolerance and the maintenance of durable chimerism. In addition, donor T cell production directly correlates with the deletion of potentially alloreactive cells.

Antitumor effect of donor marrow graft rejection induced by recipient leukocyte infusions in mixed chimeras prepared with nonmyeloablative conditioning: critical role for recipient-derived IFN-gamma

Some patients lose chimerism following nonmyeloablative hematopoietic cell transplantation (HCT), yet, surprisingly, enjoy sustained tumor remissions. We hypothesized that host-versus-graft (HVG) alloresponses might induce antitumor effects against recipient tumors. We explored this question in mice by administering recipient leukocyte infusions (RLIs) to mixed chimeras established with nonmyeloablative conditioning. Mixed chimeras were prepared in the B10.A (H2a)-->B6 (H2b) strain combination using depleting anti-T-cell monoclonal antibodies (mAbs), cyclophosphamide, and thymic irradiation. B6 myeloid leukemia cells (MMB3.19) were administered 7 days following donor lymphocyte infusion (DLI) or RLI on day 35. Conversion to full donor chimerism occurred without graft-versus-host disease (GVHD) following DLI, whereas RLI led to loss of chimerism. Both RLI and DLI significantly delayed tumor mortality. In another strain combination (B10.BR [H2k]-->BALB/c [H2d]), RLI-induced or spontaneous loss of chimerism was associated with antitumor effects against the host-type B-cell lymphoma A20. HCT was essential for the antitumor effect of RLI. RLI induced elevated serum interferon-gamma (IFN-gamma) levels, and recipient-derived IFN-gamma was critical for their antitumor effects. Thus, HVG reactions (spontaneous or induced by RLI) mediate antitumor effects against hematologic malignancies via a recipient-derived IFN-gamma-mediated mechanism. A novel approach to achieving anti-tumor effects without the risk of GVHD is suggested.