Mixed chimerism and tolerance without whole body irradiation in a large animal model

Reprogramming immune responses: enabling cellular therapies and regenerative medicine

Recent advances in cellular therapies have led to the emergence of a multidisciplinary scientific approach to developing therapeutics for a wide variety of diseases and genetic disorders. Although most cell-based therapies currently consist of heterogeneous cell populations, it is anticipated that the standard of care will eventually be well-characterized stem cell lines that can be modified to meet the individual needs of the patient. Many challenges have to be overcome, however, before such "designer cells" can become a clinical reality. One of the major hurdles will be to prevent immune rejection of the therapeutic cells. A patient's immune system may react to genetically modified or allogeneic cells as foreign, leading to their destruction. We propose that specific reprogramming of the immune system to accept cellular therapies can be accomplished by establishing hematopoietic chimerism. Successful engraftment of hematopoietic stem cells (HSCs), which have the same origin as those cells intended for therapeutic use, should lead to a re-education of the immune system so that the donor cells are recognized as self and will not be rejected. Developing safe, nontoxic protocols for reprogramming the immune system is critical to the success of this approach. Two major requirements exist for achieving stable HSC engraftment: (A) depletion or displacement of host stem cells, and (B) adequate immune suppression. Available data indicate that an agent such as busulfan is effective in depleting stem cells and that immune suppression can be accomplished with monoclonal antibodies that specifically target immune-reactive cells in the periphery.

Donor lymphocyte infusion-mediated graft-versus-leukemia effects in mixed chimeras established with a nonmyeloablative conditioning regimen: extinction of graft-versus-leukemia effects after conversion to full donor chimerism

BACKGROUND

We investigated an approach to separating graft-versus-lymphoma (GVL) effects from graft-versus-host disease (GVHD) in mice receiving a nonmyeloablative conditioning regimen allowing establishment of mixed hematopoietic chimerism.

METHODS

We evaluated the ability of donor lymphocyte infusions (DLI) to mediate GVL effects without GVHD in mixed chimeras prepared with cyclophosphamide, anti-T-cell antibodies, and thymic irradiation. To examine the fate of GVH-reactive donor CD8+ T cells, we used the 2C T-cell receptor (TCR) transgenic mouse strain, which carries an Ld-specific transgenic TCR on the B6 background.

RESULTS

Administration of DLI on day 35 post-BMT led to conversion from mixed to full donor chimerism and mediated a powerful GVL effect with complete protection (100% survival) against mortality induced by a host-type lymphoma (EL4) administered 7 days later (100% mortality in non-DLI controls; P<0.001). No GVHD occurred in DLI recipients. Rechallenging the surviving DLI recipients, which had converted to full chimerism, with the same tumor dose 17 weeks later led to rapid tumor mortality. Long-term DLI recipients had anti-host proliferative responses, but not CTL responses in vitro. When given as DLI together with wild-type spleen cells, marked expansion of GVH-reactive 2C CD8+ T cells was observed on day 10, followed by a marked decline in their numbers by week 10 post-DLI.

CONCLUSIONS

Nonmyeloablative induction of mixed chimerism followed by administration of DLI can mediate powerful GVL effects. The late loss of DLI-mediated GVL effects may reflect the eventual loss of donor-derived GVH-reactive CTL, which occurs in association with conversion to full donor chimerism.

Regulatory mechanism of peripheral tolerance: in vitro evidence for dominant suppression of host responses during the maintenance phase of tolerance to renal allografts in miniature swine

BACKGROUND

Studies from our laboratory have demonstrated that a short course of cyclosporine leads to indefinite survival of renal allografts across an MHC class-I barrier in miniature swine. We have recently reported that a peripheral regulatory mechanism appears to be involved in the maintenance of this tolerance since peripheral blood lymphocytes (PBL), exposed to donor antigen in vitro specifically suppressed the generation of anti-donor cytotoxic activity by recipient-matched naive PBL. We have now further investigated the mechanism of this phenomenon to determine the level at which such regulation occurs, and investigated the phenotypes of the cells involved in maintaining dominant suppression.

MATERIALS AND METHODS

PBL from long-term tolerant animals (>6 months after renal transplantation) were pre-stimulated in vitro with donor-type PBL. These cells were then incubated with recipient-matched naive responders and donor-type PBL stimulators in MLR assays. The proliferative activity of the cells was measured by [3H]-thymidine incorporation. Suppression was measured by inhibition of proliferation of naive cells in response to donor PBL when co-cultured with tolerant cells. Flow cytometry was used to study the phenotypes of cells that were present in cell cultures.

RESULTS

Primed PBL from tolerant animals markedly suppressed the proliferative response of recipient-matched naive cells to donor-matched stimulators in vitro. No suppression of proliferation was observed in response to third party stimulators, indicating that the suppression was donor-specific. Primed PBL from naive animals stimulated with donor antigen and co-cultured with unprimed recipient-matched naive cells also demonstrated reduced proliferative responses. However, this decrease in proliferation appeared to be due to a 'burn-out' phenomenon, as assessed by kinetic studies, rather than due to true suppression. Expression of CD25 increased on a sub-population of T cells from tolerant animals following priming with donor antigen. These cell then markedly inhibited further activation of CD25 positive cells in co-cultures with naive responder cells, suggesting a possible mechanism of suppression.

CONCLUSION

These results suggest that the mechanism of tolerance to class-I-mismatched renal allografts, involves a population of regulatory cells that are capable of suppressing proliferative anti-donor responses.

Spleen transplantation in miniature swine: surgical technique and results in major histocompatibility complex-matched donor and recipient pairs

BACKGROUND

Spleen transplantation (Tx) between some strains of rodents can lead to donor-specific tolerance either spontaneously or after a short course of immunosuppression. This study developed a surgical technique for spleen Tx in miniature swine to investigate its immunologic impact in a large animal model.

METHODS

The preferred surgical technique of spleen Tx (n=8) involved excision of the donor spleen with its vascular pedicle to the aorta and portal vein. Carrel patches of donor aorta and portal vein were anastomosed to the abdominal aorta and inferior vena cava, respectively, of the (splenectomized) recipient. The results in four major histocompatibility complex-matched pairs that were mismatched for the porcine allelic antigen are reported. Two recipients were untreated, one received a 12-day course of cyclosporine A (CsA) alone, and one received thymic irradiation (700 cGy) and CsA. Hematopoietic cell chimerism was followed by fluorescence-activated cell sorter, and graft survival was assessed by histology.

RESULTS

Spleen Tx was technically successful. In two untreated pigs, chimerism was detected in the blood (maximum 5% for 17 and 25 days) and lymph nodes (maximum 6% for 28 and 56 days), but both grafts showed histologic rejection by day 28. In two treated pigs, chimerism was present in the blood for 47 and 57 days, and rejection was prevented, with follow-up for 57 and 217 days, respectively.

CONCLUSION

Spleen Tx in major histocompatibility complex-matched pairs treated with CsA+/-thymic irradiation results in prolonged chimerism and is associated with the development of in vivo unresponsiveness to the transplanted spleen.

Composite tissue allotransplantation in chimeric hosts: part I. Prevention of graft-versus-host disease

BACKGROUND

Mixed allogeneic chimerism (MAC) has been shown to induce tolerance to composite tissue allografts (CTA). However, transplantation of unmanipulated donor-specific limbs results in severe graft-versus-host disease (GVHD). This suggests that nontolerant mature donor-derived cells in the CTA may affect the stability of chimerism, potentially resulting in GVHD. The aim of this study was to develop an approach to study and prevent GVHD in a mixed chimeric-rat hind-limb transplantation model.

METHODS

[ACI-->WF] chimeras received a limb from Wistar Furth (WF) (syngeneic), Fisher (third-party), or ACI (irradiated [1,050 cGy] or nonirradiated) rats. In vitro tolerance was assessed using mixed lymphocyte reactivity (MLR) assays at the time the animals were killed.

RESULTS

[ACI-->WF] chimeras with greater than 85% chimerism exhibited rejection-free survival of donor-specific hind limbs. However, 100% of these animals developed lethal GVHD 22.4+/-2.8 days after limb transplantation. [ACI-->WF] chimeras that underwent transplantation with irradiated ACI or syngeneic WF limbs showed no signs of rejection or GVHD at 5 months. Nonchimeric and third-party controls rejected limbs within 10 days.

CONCLUSIONS

Conditioning of the host WF rats with 950 cGy of irradiation (sublethal, myeloablative) led to high levels of MAC without GVHD. The mature T-cell content of nonirradiated donor (ACI) limbs was sufficient to induce lethal GVHD in 100% of tolerant mixed chimeric [ACI-->WF] hosts. Irradiation of donor limbs before transplantation resulted in long-term donor-specific tolerance and prevented GVHD. These data demonstrate that (1) established chimeras could be susceptible to GVHD caused by immunocompetent donor cells transferred with the hind limb, and (2) inactivating these cells with irradiation prevents GVHD and destabilization of chimerism, and permits rejection-free graft acceptance.

Embryonic stem cells and potency to induce transplantation tolerance

In the past 40 years, many protocols have been extensively studied for the induction of sustainable transplantation tolerance which might lead to protection of allografts from immunological injury in the clinical setting. Despite the enormous success in rodents, there is still no established protocol available yet for widespread clinical trials. Whilst clonal deletion, clonal anergy and suppression, now coined regulation, have been elucidated as the key immunological elements of tolerance, a better understanding of these mechanisms has so far done little to improve on the survival of organ transplants in humans. Haematopoietic chimaerism, as previously described by Medawar and colleagues [1], remains the most robust tool for tolerance induction. Unfortunately, bone marrow or haematopoietic stem cell transplantation for patients awaiting solid organ transplantation remains a high risk therapy, due to the dangers of graft-versus-host disease. Most recent data, however, indicate the potential of embryonic stem cells (ESC) to offer a possible solution for low risk induction of multilineage mixed chimaerism and tolerance not involving any immunosuppression, due to their unique property of low immunogenicity and high plasticity. Here, what we know about ESC in various species, and the potency and drawbacks of ESC for widespread clinical use, will be discussed.

Variable relationship between chimerism and tolerance after hematopoietic cell transplantation without myelosuppressive conditioning

BACKGROUND

We have previously described a mixed chimerism protocol that avoids myelosuppressive conditioning and permits hematopoietic cell transplantation across MHC barriers without the need for whole body irradiation in miniature swine. Here, we report our current experience including animals conditioned without thymic irradiation, and we attempt to define the relationship between long-term chimerism and stable tolerance in these animals.

METHODS

Recipient swine received in vivo T-cell depletion, with or without thymic irradiation on day -2. Cyclosporine was administered for 30 to 60 days beginning on day -1. A total of 1 to 2 x 10(10) /kg cytokine-mobilized donor hematopoietic cells were infused during 3 days. Chimerism was determined by flow cytometry. In vitro tolerance assays and donor-matched kidney transplantation were performed after cessation of cyclosporine.

RESULTS

Most recipients maintained stable chimerism (26 of 35) and were specifically tolerant to donor-matched cells in vitro regardless of whether they received thymic irradiation. Donor-matched kidney transplantations performed in chimeric animals without in vitro antidonor immune responses were accepted without immunosuppression. Some animals developed in vitro evidence of antidonor MHC responsiveness despite the persistence of donor cells in the peripheral blood. Donor-matched kidney transplantations performed in the face of these responses were rejected.

CONCLUSIONS

These data indicate that this nonmyelosuppressive protocol can induce stable chimerism and robust tolerance even in animals conditioned without thymic irradiation. However, the data also demonstrate that macrochimerism does not always correlate with tolerance. Lack of in vitro antidonor immune responses in chimeric animals is an important predictor of renal allograft acceptance in this model.

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.

Persistent chimerism despite antidonor MHC in vitro responses in miniature swine following allogeneic hematopoietic cell transplantation

BACKGROUND

T-cell chimerism predominates in miniature swine receiving hematopoietic-cell transplantation without myelosuppressive conditioning. Several chimeric recipients have become hyporesponsive to donor-major histocompatibility complex (MHC) in vitro and accepted donor-matched renal transplants without immunosuppression. However, some retained antidonor in vitro responses and subsequently rejected donor renal allografts despite the persistence of peripheral blood chimerism. In this study, we characterize the donor cells in both "tolerant" and "nontolerant" chimeric miniature swine.

METHODS

Peripheral blood chimerism was determined by flow cytometry. In vitro antidonor responsiveness was determined by mixed lymphocyte reaction (MLR) and cell-mediated lymphocytotoxicity (CML). Donor cells were separated from chimeras by immunomagnetic bead separation and used as stimulators or targets in CML assays. Phenotypic analysis of donor cells in chimeras was performed using flow cytometry.

RESULTS

Peripheral blood chimerism stabilized beyond 100 days and was made up almost entirely of T cells. PBMC from nontolerant chimeras could be stimulated in vitro to kill donor cells isolated from the mixed chimera itself. In contrast, PBMC from tolerant chimeras hyporesponsive to donor-type cells could not be stimulated in vitro to kill their own sorted donor cells.

CONCLUSIONS

The in vivo persistence of donor T cells in mixed chimeric animals with in vitro antidonor responsiveness is not caused by an inability of these cells to be killed but rather by the poor stimulating capacity of these donor T cells. The nature of donor T cells that persist in the face of in vitro antidonor responses, has important implications for the induction of transplant tolerance by way of the generation of mixed chimerism.

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.

High-level allogeneic chimerism achieved by prenatal tolerance induction and postnatal nonmyeloablative bone marrow transplantation

Clinical application of allogeneic bone marrow transplantation (BMT) has been limited by toxicity related to cytoreductive conditioning and immune response. In utero hematopoietic stem cell transplantation (IUHSCT) is a nonablative approach that achieves mixed chimerism and donor-specific tolerance but has been limited by minimal engraftment. We hypothesized that mixed chimerism achieved by IUHSCT could be enhanced after birth by nonmyeloablative total body irradiation (TBI) followed by same-donor BMT. To test this hypothesis, mixed chimerism was created by IUHSCT in a major histocompatibility complex-mismatched strain combination. After birth, chimeric animals received nonmyeloablative TBI followed by transplantation of donor congenic bone marrow cells. Our results show that: (1) low-level chimerism after IUHSCT can be enhanced to high-level chimerism by this strategy; (2) enhancement of chimerism is dependent on dose of TBI; (3) the mechanism of TBI enhancement is via a transient competitive advantage for nonirradiated hematopoietic stem cells; (4) engraftment observed in the tolerant, fully allogeneic IUHSC transplant recipient is equivalent to a congenic recipient; and (5) host-reactive donor lymphocytes are deleted with no evidence of graft-versus-host disease. This study supports the concept of prenatal tolerance induction to facilitate nonmyeloablative postnatal strategies for cellular therapy. If clinically applicable, such an approach could dramatically expand the application of IUHSCT.

Allograft tolerance induced by intact active bone co-transplantation and anti-CD40L monoclonal antibody therapy

BACKGROUND

One of the most promising approaches to achieving allograft tolerance involves the transient inhibition of co-stimulatory signals in T cells. There is, however, increasing evidence that this approach alone cannot universally elicit allograft tolerance and that adjunct therapies capable of synergizing with co-stimulation blockade may be necessary.

METHODS

We developed a novel tolerance strategy involving co-transplantation of intact allogeneic bone fragments containing active bone marrow (intact active bone [IAB]) with heart allograft and transient anti-CD40L monoclonal antibody therapy.

RESULTS

Mice treated with IAB and anti-CD40L were tolerant to major histocompatibility complex and minor antigen-mismatched cardiac and skin allografts. Heart allografts had normal histology up to 270 days posttransplantation, and the production of graft-reactive antibodies was inhibited. Microchimerism, but no macrochimerism, of donor cells was detected in the peripheral blood or lymphoid organs of tolerant mice receiving IAB and anti-CD40L. Lymphocytes from tolerant mice retained normal proliferative responsiveness to donor cells in vitro but demonstrated a donor-specific loss in the priming of interferon-gamma responses. The ability to produce interleukin-2 or -4 when stimulated with donor cells was normal.

CONCLUSIONS

Contrary to previous reports of the ability of bone marrow cells to induce central deletional tolerance, our data suggest that the regimen involving co-transplantation of IAB on the day of heart allograft transplantation and transient anti-CD40L therapy induces a robust donor-specific peripheral tolerance.

Complete allogeneic hematopoietic chimerism achieved by a combined strategy of in utero hematopoietic stem cell transplantation and postnatal donor lymphocyte infusion

In utero hematopoietic stem cell transplantation (IUHSCTx) can achieve mixed hematopoietic chimerism and donor-specific tolerance without cytoreductive conditioning or immunosuppression. The primary limitation to the clinical application of IUHSCTx has been minimal donor cell engraftment, well below therapeutic levels for most target diseases. Donor lymphocyte infusion (DLI) has been used in postnatal circumstances of mixed chimerism as targeted immunotherapy to achieve a graft-versus-hematopoietic effect and to increase levels of donor cell engraftment. In this report we demonstrate in the murine model that a combined approach of IUHSCTx followed by postnatal DLI can convert low-level, mixed hematopoietic chimerism to complete donor chimerism across full major histocompatibility complex barriers with minimal risk for graft-versus-host disease (GVHD). Time-dated embryonic day 14 (E14) to E15 Balb/c (H-2K(d), CD45.2) fetuses underwent intraperitoneal injection of 5 x 10(6) T-cell-depleted B6 (H-2K(b), CD45.2) bone marrow cells. Chimeric recipients then received transplants at either 4 or 8 weeks of age with 1 of 3 doses (5, 15, or 30 x 10(6) cells) of donor congenic splenocytes (B6-Ly5.2/Cr, H-2K(b), CD45.1). The response to DLI was dose dependent, with conversion to complete donor peripheral blood chimerism in 100% of animals that received high-dose (30 x 10(6) cells) DLI. Only 1 of 56 animals receiving this dose succumbed to GVHD. This study directly supports the potential therapeutic strategy of prenatal tolerance induction to facilitate nontoxic postnatal cellular therapy and organ transplantation, and it has broad implications for the potential treatment of prenatally diagnosed genetic disorders.

Advances in composite tissue allograft transplantation as related to the hand and upper extremity

The clinical transplantation of composite tissue allografts (CTA) such as human hand or larynx is stimulating discussions among surgeons at national and international forums on the indications, ethical aspects, toxic effects of immunosuppression, and functional results of the first reported cases of unilateral and bilateral hand transplantation. This Clinical Perspective article presents the latest advances in clinical and experimental research related to the field of CTAs. The article presents the historic aspects of CTA, a broad view of the current state of composite tissue transplantation, the mechanism of allograft rejection, current experimental and clinical protocols, and, finally, the future prospects of the standard use of CTAs. It is clear that there is a substantial demand for routine use of CTAs but the treatment protocols need to be optimized and the functional outcomes need to be improved.

Tolerance to vascularized kidney grafts in canine mixed hematopoietic chimeras

BACKGROUND

Recent progress in allogeneic hematopoietic stem cell transplantation provides new methods for reliable engraftment with nonlethal conditioning regimens. These techniques have been successfully applied in the treatment of both malignant and nonmalignant diseases, but have not been fully exploited for their potential to tolerize recipients for organ transplantation. These studies were undertaken to test whether the tolerance of host immune cells toward donor hematopoietic cells in mixed hematopoietic chimeras extends to include a vascularized organ, the kidney.

METHODS

Using nonmyeloablative doses of total body irradiation, a short course of immunosuppression, and hematopoietic stem cells from marrow or peripheral blood sources, five dog lymphocyte antigen-identical canines were made to become stable mixed hematopoietic chimeras with no development of graft-versus-host disease or posttransplant lymphoproliferative disorder. Subsequently, renal transplantations were performed between stem cell donor and recipient littermates, and no additional immunosuppressive therapy was given after stem cell transplantation.

RESULTS

All mixed chimeric dogs demonstrate different, but stable, levels of donor peripheral blood lymphocyte and granulocyte chimerism. With follow-up of longer than 1 year, all of the mixed chimeric dogs (five/five) have excellent renal function with normal serum creatinines (<1.5 mg/dl) and no pathological evidence of rejection on biopsies.

CONCLUSIONS

In a major histocompatibility-matched model, minor antigen differences between donor and recipient are not sufficient to induce a host immune response to a vascularized kidney transplant in mixed hematopoietic chimeras.

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.

Activation of human endothelial cells by mobilized porcine leukocytes in vitro: implications for mixed chimerism in xenotransplantation

BACKGROUND

The induction of immunologic tolerance to pig antigens in primates may facilitate the development of successful clinical xenotransplantation protocols. The infusion of mobilized porcine peripheral blood leukocytes (PBPC, consisting of approximately 2% peripheral blood progenitor cells) into preconditioned baboons, intended to induce mixed hematopoietic cell chimerism, however, results in a severe thrombotic microangiopathy (TM) that includes vascular injury, microvascular thrombosis, and pronounced thrombocytopenia. Because the mechanisms responsible for TM are unclear, we have explored the effects of PBPC on human umbilical vein endothelial cell (HUVEC) activation.

METHODS

Confluent HUVEC monolayers were established in 96-well cell culture clusters. PBPC were mobilized from miniature swine with porcine interleukin 3 (pIL-3), porcine stem cell factor (pSCF), and human granulocyte-colony stimulating factor (hG-CSF) and were collected by leukapheresis. PBPC were added to HUVEC (0-1x10(7) PBPC/well) for 3- to 24-hr periods and, with cell-based ELISA techniques, surface levels of E-selectin, vascular cell adhesion molecule 1 (VCAM-1), and intercellular adhesion molecule 1 (ICAM-1) were measured. In some cases, peripheral blood leukocytes (PBL) were collected from pigs that did not receive pIL-3, pSCF, or hG-CSF and were added to HUVEC. PBPC were also sorted into subsets of CD2- cells, CD2+ cells, and cellular debris, each of which were added separately to HUVEC. Transwell permeable membrane inserts were placed over HUVEC to prevent direct cell-cell contact with PBPC in some instances.

RESULTS

PBPC from different pigs (n=6) induced an increase in the expression of E-selectin, VCAM-1, and ICAM-1 to levels 5, 4, and 2 times greater than baseline, respectively. ICAM-1 expression reached maximum levels after the addition of 6x10(5) PBPC/well. Expression of E-selectin and VCAM-1 increased further with the addition of greater numbers of PBPC, reaching maximum levels after the addition of 1x10(7) PBPC/well. PBPC-induced up-regulation of E-selectin, VCAM-1, and ICAM-1 had a maximum effect after approximately 6 hr, 12 hr, and 6 to 9 hr, respectively (n=3). The effects of fresh and frozen PBPC on HUVEC were similar (n=2). Compared to PBPC, PBL induced higher levels of E-selectin, VCAM-1, and ICAM-1 on HUVEC (n=2). The addition of CD2- cells to HUVEC induced an increase in E-selectin and VCAM-1 to levels 4 times greater than baseline, whereas the addition of CD2+ cells or debris did not elicit a substantial effect (n=2). Transwell permeable membranes prevented PBPC-induced up-regulation of E-selectin, VCAM-1, and ICAM-1 on HUVEC (n=2), suggesting that the mechanism of activation requires direct cell-cell contact.

CONCLUSIONS

Porcine PBPC activate HUVEC, as suggested by an increase in surface E-selectin, VCAM-1, and ICAM-1 levels, and have a maximum effect after 9 hr. Freezing of PBPC does not affect PBPC-induced activation of HUVEC. PBL induce greater activation of HUVEC than do PBPC. CD2- cells are primarily responsible for PBPC-induced activation of HUVEC and direct cell-cell contact is required. Removal of CD2- cells before the administration of PBPC or the use of agents that interrupt PBPC-endothelial cell interactions may prevent or treat TM in baboons.

Stable mixed chimerism after T cell-depleted allogeneic hematopoietic stem cell transplantation using conditioning with low-dose total body irradiation and fludarabine

Although reduced intensity conditioning (RIC) before allografting is associated with low treatment-related morbidity and mortality, graft-versus-host disease (GVHD) remains a significant complication of hematopoietic stem cell transplantation (HSCT). T cell depletion (TCD) has been successfully used in conventional allotransplantation to reduce the incidence of GVHD, but was associated with an increased rate of engraftment failure. In a small cohort of six patients at high risk of developing GVHD we have determined whether sustained engraftment could be achieved using reduced intensity conditioning and T cell depletion in combination. All patients engrafted and 5/6 developed high levels (i.e. > or =95%) of donor chimerism, even though mismatched related or matched unrelated donors were used. Only one patient developed acute GVHD, as he received donor lymphocyte infusions (DLI) for relapse. In summary, TCD might be a useful prophylactic tool in RIC allogeneic HSCT. Although TCD after RIC might be associated with high relapse rate, as 5/6 patients are not in remission, this combined strategy might be appropriate for patients with less aggressive malignant or non-malignant diseases in which high transplant-related morbidity and mortality is not acceptable.

Porcine hematopoiesis on primate stroma in long-term cultures: enhanced growth with neutralizing tumor necrosis factor-alpha and tumor growth factor-beta antibodies

BACKGROUND

Donor hematopoiesis is at a competitive disadvantage when bone marrow transplantation is across species barriers. This could present major limitations to xenogeneic stem cell transplantation as an approach to tolerance induction. An in vitro model of xenogeneic engraftment was established to identify inhibitors of porcine hematopoiesis in a primate environment.

METHODS

Porcine bone marrow cells (BMC), in the presence or absence of primate CD34+ positive cells, were cultured for 4-6 weeks on primate stroma with porcine cytokines. Cellularity and growth of colony-forming cells were indicators of hematopoietic growth. Effects of soluble factors were determined by using Transwell inserts to separate porcine cells from stroma. Neutralizing antibodies for human transforming growth factor-beta (TGF-beta) and tumor necrosis factor-alpha (TNF-alpha) were added to cultures.

RESULTS

Porcine hematopoiesis can be maintained in long-term cultures on primate stroma with pig cytokines. Adding BMC to the stroma below Transwell-containing porcine cells dramatically inhibited porcine hematopoiesis, showing an inhibitory role for soluble factors. Neutralizing antibodies against TNF-alpha or TGF-beta caused a modest enhancement of porcine hematopoiesis; however, the combination of both led to a substantial increase. Inhibitory effects of these cytokines were confirmed by adding TNF-alpha and TGF-beta to porcine cultures.

CONCLUSIONS

Porcine cells may be more sensitive to inhibitory effects of TNF-alpha and TGF-beta than primate cells and are at a disadvantage when in a primate environment. Potential implications of this observation are discussed in the context of establishing specific immune tolerance via mixed chimerism to facilitate xenotransplantation.

Expression of xenogeneic MHC class II molecules in HLA-DR(+) and -DR(-) cells: influence of retrovirus vector design and cellular context

We recently established that molecular chimeras of major histocompatibility complex (MHC) class II molecules, created via retroviral transfer of allogeneic class II cDNAs into bone marrow cells (BMCs), alleviated complications associated with mixed BMC chimeras while leading to T cell tolerance to renal grafts sharing the transferred class II. Initially demonstrated for allogeneic transplants in miniature swine, this concept was extended to T-dependent antibody (Ab) responses to xenogeneic antigens (Ags) in the pig --> baboon combination. Successful down-regulation of T cell responses appeared, however, to be contingent on a tight lineage-specific expression of transferred class II molecules. The present studies were, therefore, designed to evaluate the influence of construct design and cellular environment on expression of retrovirally transferred xenogeneic class II cDNAs. Proviral genomes for pig class II SLA-DR expression, differing only at the marker neo(r) or enhanced green fluorescent protein (EGFP) gene, showed increased membrane SLA-DR density on HLA-DR(-) fibroblasts as well as HLA-DR(+), TF-1 erythroleukemia cells. More importantly, HLA-DR(+) human B cell lines, although efficiently transduced with pig DR retroviruses, exhibited unstable surface pig DR. Surface pig DR- B cells, nevertheless, stimulated autologous human T cells pre-sensitized to pig Ags, a proliferation likely occurring through presentation of class II-derived peptides. Collectively, these data suggest that surface expression of transferred class II molecules is not related to the ability of recipient cells to synthesize xenogeneic class II molecules but rather to their Ag processing capacities.

CD40-ligand in primate cardiac allograft and viral immunity

Our laboratory has studied the role of CD40 ligand (CD40L, CD154) in the primate immune response to allogenic and infectious challenges. We find that intensive early blockade of CD40L reliably attenuates acute rejection of primate cardiac allografts. Monotherapy fails to prevent late graft loss, which often occurs in association with rising antidonor antibody titers and allograft vasculopathy, despite continuing anti-CD40L therapy. In contrast, the primary humoral response to T helper dependent influenza viral antigen is inhibited during anti-CD40L therapy, and responses to subsequent immunization are blunted after discontinuation of therapy. These results are encouraging with regard to the tolerogenic potential of costimulatory blockade for specific T helper dependent antigens. However, these findings also indicate that pathogenic allograft responses in primates are probably not entirely CD40L-dependent. As such, additional immunomodulatory strategies are needed to facilitate tolerance to a transplanted organ.

Allogeneic transplantation using non-myeloablative transplant regimens

Reduced intensity (non-myeloablative) stem cell transplant (NST) preparative regimens are being increasingly used to exploit the curative potential of allogeneic stem cell transplantation without the morbidity and mortality associated with conventional transplantation. Growing confidence in the power of the allogeneic graft-versus-malignancy (GVM) effect makes such an approach attractive. Lower intensity transplants increase the degree of mixed chimerism, both in T cell and myeloid cell lineages. Currently a variety of NST treatment approaches are being developed and in this chapter their safety profile and the immunological characteristics of the mixed chimeric state are described. Results of NST in specific disease categories are still limited but the NST approach appears to have promise in the treatment of both haematological and non-haematological malignancies because of the benefit of low toxicity coupled with a strong graft-versus-malignancy effect. NST regimens are also being explored in high-risk patients with non-malignant disorders. However, at present, there is insufficient data to determine whether NST should replace standard myeloablative transplants in specific disease groups. With their low toxicity, NST are well placed as platforms for future developments in transplant immunology to avoid GVHD and enhance the allograft effect against malignant diseases.

Thymic microchimerism correlates with the outcome of tolerance-inducing protocols for solid organ transplantation

This study found that pretransplant infusion of donor peripheral blood leukocytes, either total leukocytes (peripheral blood leukocytes) or peripheral blood mononuclear cells (PBMC), under appropriate immunomodulating conditions was more effective than donor bone marrow (BM) in prolonging the survival of rats that received kidney grafts. A higher percentage of MHCII(+) cells was found in donor PBMC than in BM cells, and depletion of MHCII(+) cells from donor PBMC abolished their tolerogenic potential. By the analysis of microchimerism in rats infused with donor cells and killed at different time points thereafter, the better tolerogenic potential of leukocyte infusion related to a higher capability of these cells to engraft the recipient thymus. PCR analysis on OX6-immunopurified cells revealed the presence of donor MHCII(+) cells in the thymus of these animals. The role of intrathymic microchimerism was reinforced by findings that thymectomy at the time of transplant prevented tolerance induction by donor leukocytes. Donor DNA was found in the thymus of most long-term graft animals that survived, but in none of those that rejected their grafts. The presence of intrathymic microchimerism correlated with graft survival, and microchimerism in other tissues was irrelevant. PCR analysis of DNA from thymic cell subpopulations revealed the presence of donor MHCII(+) cells in the thymus of long-term surviving animals. Thus, in rats, donor leukocyte infusion is better than donor BM for inducing graft tolerance, defined by long-term graft survival, donor-specific T cell hyporesponsiveness, and reduced interferon gamma production. This effect appears to occur through migration of donor MHCII(+) cells in the host thymus.

Skin allograft maintenance in a new synchimeric model system of tolerance

Treatment of mice with a single donor-specific transfusion plus a brief course of anti-CD154 mAb uniformly induces donor-specific transplantation tolerance characterized by the deletion of alloreactive CD8+ T cells. Survival of islet allografts in treated mice is permanent, but skin grafts eventually fail unless recipients are thymectomized. To analyze the mechanisms underlying tolerance induction, maintenance, and failure in euthymic mice we created a new analytical system based on allo-TCR-transgenic hemopoietic chimeric graft recipients. Chimeras were CBA (H-2(k)) mice engrafted with small numbers of syngeneic TCR-transgenic KB5 bone marrow cells. These mice subsequently circulated a self-renewing trace population of anti-H-2(b)-alloreactive CD8+ T cells maturing in a normal microenvironment. With this system, we studied the maintenance of H-2(b) allografts in tolerized mice. We documented that alloreactive CD8+ T cells deleted during tolerance induction slowly returned toward pretreatment levels. Skin allograft rejection in this system occurred in the context of 1) increasing numbers of alloreactive CD8+ cells; 2) a decline in anti-CD154 mAb concentration to levels too low to inhibit costimulatory functions; and 3) activation of the alloreactive CD8+ T cells during graft rejection following deliberate depletion of regulatory CD4+ T cells. Rejection of healed-in allografts in tolerized mice appears to be a dynamic process dependent on the level of residual costimulation blockade, CD4+ regulatory cells, and activated alloreactive CD8+ thymic emigrants that have repopulated the periphery after tolerization.

Reducing transplant toxicity

Conventional myeloablative allogeneic hematopoietic cell transplantation produces considerable morbidity and mortality. These generally limit this treatment to patients in good medical condition who are younger than 55 years of age. T-cell-mediated graft-versus-tumor effects play a key role in the elimination of malignancy after allografting. Several investigators have sought to reduce regimen-related toxicities while optimizing graft-versus-tumor effects. Strategies can be broadly classified as (1) reduced-intensity regimens that retain some toxicity, and (2) minimally myelosuppressive regimens that rely on immunosuppression for allogeneic engraftment and resultant graft-versus-tumor effects. Although follow-up has been short, preliminary results are encouraging. Current challenges include defining a regimen that will facilitate full donor engraftment while minimizing toxicities and graft-versus-host disease. If long-term efficacy is demonstrated, such strategies will expand the options for patients who would not qualify for conventional allogeneic transplants.

Therapeutic approaches for transplantation

Developments in marrow and organ transplantation are mutually interactive. There have been several recent advances in stem cell transplantation: to ensure engraftment using larger doses of stem cells; to substantially reduce the incidence of graft-versus-host disease and marrow rejection using monoclonal antibodies; and to reduce toxicity of the preparative regimen through use of so-called nonmyeloablative regimens (mini-transplants). These advances may pave the way for generation of mixed hemopoietic chimerism as an aid to achieving tolerance to organ transplants. The use of short courses of T-cell-depleting antibodies, such as CD3 immunotoxin in primates and CAMPATH-1H in humans, has demonstrated that long-term graft survival may be possible without substantive long-term immunosuppressive treatment of the recipient. The demonstration in rodents that nondepleting antibodies to T cells can give rise to powerful regulatory mechanisms that maintain tolerance to grafts has initiated a major research effort in understanding how these regulatory T cells work, with the prospect of new therapeutic modalities to mimic or enhance their function.

Neuropathy in miniature swine after administration of the mutant diphtheria toxin-based immunotoxin, pCD3-CRM9

BACKGROUND

Effective in vivo T-cell depletion is a critical component of many transplantation tolerance protocols. We have previously demonstrated T-cell depletion in miniature swine using a CRM9-based CD3-immunotoxin, pCD3-CRM9. CRM9 is a mutant form of diphtheria toxin (DT) that binds less efficiently than wild-type DT to the DT receptor (proHB-EGF) of primates. In this report, we describe and characterize the dose-dependent neurotoxicity associated with CRM9-based immunotoxin administration in swine.

METHODS

Miniature swine were treated with varying doses of pCD3-CRM9 followed by daily monitoring for symptoms of neuropathy, including limb weakness, paresis, sluggishness, and/or respiratory distress. Animals demonstrating severe respiratory distress were euthanized and peripheral nerve, spinal cord, and skeletal muscle tissue samples were obtained at autopsy for microscopic examination. Unconjugated CRM9 was administered to one animal to define its toxicity independent of the effects of T-cell depletion.

RESULTS

Excellent T-cell depletion was obtained using doses of pCD3-CRM9 greater than 0.1 mg/kg. However, neurotoxicity was observed at these doses, as manifested by transient muscle weakness or paresis, which in some cases progressed to respiratory failure and death. Dorsal root ganglia samples revealed pathological changes typical of diphtheritic polyneuropathy. The animal receiving unconjugated CRM9 exhibited the same neurotoxic side effects as those receiving the pCD3-CRM9 conjugate.

CONCLUSIONS

Administration of pCD3-CRM9 immunotoxin provides excellent T-cell depletion in miniature swine but is associated with significant dose-dependent neurotoxicity. A possible reason for CRM9-associated neurotoxicity in swine, but not primates, is suggested on the basis of a known amino acid difference in the exodomain of the DT receptor (proHB-EGF) of swine compared with that of primates.

Mixed chimerism

Induction of mixed chimerism has the potential to overcome the current limitations of transplantation, namely chronic rejection, complications of immunosuppressive therapy and the need for xenografts to overcome the current shortage of allogeneic organs. Successful achievement of mixed chimerism had been shown to tolerize T cells, B cells and possibly natural killer cells, the lymphocyte subsets that pose major barriers to allogeneic and xenogeneic transplants. Current understanding of the mechanisms involved in tolerization of each cell type is reviewed. Considerable advances have been made in reducing the potential toxicity of conditioning regimens required for the induction of mixed chimerism in rodent models, and translation of these strategies to large animal models and in a patient are important advances toward more widespread clinical application of the mixed chimerism approach for tolerance induction.

Graft survival in a rhesus renal transplant model after immunotoxin-mediated T-cell depletion is enhanced by mycophenolate and steroids

BACKGROUND

Anti-CD3 immunotoxin (IT), a T-cell-depleting agent, prolongs survival of renal allografts in a rhesus monkey model without the need for long-term immunosuppression. In this study we sought to further prolong allograft survival by giving short-term conventional immunosuppression simultaneous with IT administration.

METHODS

MHC class II mismatched, juvenile rhesus monkeys were paired as donor and recipient for renal transplantation. Recipients received two to three daily doses of IT starting on the day of transplantation. Additional immunosuppression was given for no more than 60 days. Graft function was monitored by serum creatinine and renal biopsies. Flow cytometry was used to monitor T-cell recovery.

RESULTS

Graft survival time (GST) in animals receiving IT was prolonged compared with controls with 50% of IT-treated monkeys surviving >100 days. Animals treated with IT plus mycophenolate mofetil (MMF) and steroids had significantly enhanced GST (mean GST, 305 days) compared with those treated with IT alone (mean GST, 94 days). In contrast, addition of cyclosporine or 40-O-[2-Hydroxyethyl]rapamycin did not significantly increase graft survival time. A comparison among animals from all treatment groups with short (<100 days) and long (>100 days) GST demonstrated that those with the shorter GST had a higher blood T-cell count 2 weeks after transplantation. Full recovery of CD4+ T cells required longer than 6 months.

CONCLUSIONS

A combination with MMF and steroids given for 4 days after renal allograft transplantation significantly increases GST in IT-treated monkeys. We hypothesize that MMF and steroids suppress the initial T-cell activation mediated by IT.

Peripheral regulation of graft-versus-host alloreactivity in mixed chimeric miniature swine

BACKGROUND

Despite the presence of circulating donor-derived T cells during the induction of mixed chimerism across MHC barriers in miniature swine, severe graft-versus-host disease was avoided in the majority of animals. In this study, we investigated the possible roles of recipient and donor lymphoid populations in the regulation of donor-anti-recipient alloreactivity.

METHODS

Mixed chimerism across a full MHC-mismatch barrier was established in miniature swine using a high-dose allogeneic peripheral blood stem cell protocol. Peripheral blood mononuclear cells from mixed chimeric swine were co-cultured with naïve donor-matched responders and naïve recipient-matched stimulators in mixed lymphocyte reactions.

RESULTS

Peripheral blood mononuclear cells from mixed chimeras inhibited donor-anti-recipient proliferation. This suppression was radioresistant to 25 Gy. Suppression of donor-anti-recipient alloreactivity was not observed in mixed lymphocyte co-cultures when donor-derived cells were added in the absence of recipient-derived cells.

CONCLUSIONS

These results suggest an association between the presence of an active and relatively radioresistant cell population, demonstrable in vitro, and the regulation of graft-versus-host disease across MHC barriers in mixed chimeric miniature swine.

Transplantation tolerance induced by mixed chimerism

Although short- and long-term results after organ transplantation have improved considerably in recent years, morbidity and mortality rates in graft recipients remain high. The induction of lifelong donor-specific tolerance would dramatically improve outcome after organ transplantation. Although many tolerance protocols have been successful in rodent studies, most of these approaches have failed when attempted in large animals or humans. Robust tolerance, in contrast, has been demonstrated with mixed chimerism regimens not only in rodents but also in large animal models, including non-human primates. Furthermore, mixed chimerism protocols have been developed that would be feasible in cadaveric, and thus in thoracic, transplantation. The induction of mixed hematopoietic chimerism is therefore an attractive experimental approach for development of clinical tolerance protocols. One of the obstacles to widespread clinical application of this concept is the remaining toxicity of the host conditioning. Recent advances, however, have led to substantially milder protocols that could become clinically acceptable in the foreseeable future. This article provides a short overview of the basic mechanisms by which immunologic tolerance may be induced, describes the concept of mixed chimerism as a promising approach for clinical tolerance induction, and reviews recent progress in developing clinically feasible mixed chimerism protocols.

Skin-specific alloantigens in miniature swine

BACKGROUND

The acceptance of skin allografts has historically been among the most challenging problems in the field of transplantation, attributed, at least in part, to the existence of antigens expressed by skin but not by other tissues. Many studies have suggested the existence of skin-specific antigens in rodents, but data in large-animal models are more limited.

METHODS

We have recently developed protocols for attaining stable mixed hematopoietic chimerism in miniature swine, using MHC-matched donors and recipients. We have now assessed tolerance to donor-derived skin and cardiac allografts in these chimeric animals.

RESULTS

Skin-graft rejection was seen in four of six animals receiving skin grafts taken from the respective hematopoietic donors. In the other two animals, donor-derived skin grafts survived indefinitely. No cardiac-allograft rejection was observed in mixed-chimeric animals that received heart transplants from their hematopoietic donors, even in animals that had already rejected skin allografts from the same donors. In all animals assessed, in vitro hyporesponsiveness to donor hematopoietic cells persisted.

CONCLUSION

These findings support the concept that skin expresses immunogenic alloantigens that either are not expressed or are not immunogenic in cardiac or hematopoietic tissue.

Mixed chimerism and transplantation tolerance

Achieving transplantation tolerance is an important goal in the effort to reduce long-term morbidity and mortality in organ transplant recipients. Robust, lifelong, donor-specific tolerance can be reliably achieved by induction of mixed chimerism in various animal models. To date, the clinical application of these proto-cols has been impeded partly by the potential toxicity of the required host conditioning regimens and the lack of successful studies in large animals. This article reviews the progress achieved in recent years in developing considerably milder conditioning protocols in rodents, and in extending some of these models to achieve permanent mixed chimerism and tolerance in large animals. Advances in the induction of xenogeneic tolerance through mixed chimerism are also discussed.

Advances in transplant immunobiology

The present review briefly addresses the most recent knowledge acquired in the field of transplant immunology. A particular emphasis is placed on articles published during the past 12-18 months that have focused on allorecognition, dendritic cells and tolerance.

Strategies for tolerance induction to composite tissue allografts

The emerging field of composite tissue transplantation offers the potential to replace lost tissues from cadaveric sources. Two major obstacles currently limit the future of composite tissue allotransplantation. The first is chronic rejection, attributed to both antibody deposition and cell-mediated destruction of transplanted tissue. The second obstacle is complications associated with the chronic use of immunosuppressive agents. Our laboratory has been investigating several strategies to induce tolerance to limb tissue allografts to provide solutions to many of the current limitations in allotransplantation. Three strategies show promise in the ability to induce tolerance to organ allografts. The first involves genetic matching at the HLA loci followed by a short course of immunosuppression. The second is the application of a "mixed chimerism" regimen followed by transplantation. The third is costimulatory blockade using a short course of monoclonal antibodies, such as anti-CD40 ligand and CTLA4-Ig after transplantation. Inducing a state of tolerance to limb allografts would eliminate the need for chronic immunosuppression and may also prevent the onset of chronic rejection. The ability to induce allograft tolerance would greatly expand the indications for composite tissue transplantation.

Mixed allogeneic chimerism and tolerance to composite tissue allografts

The development of effective immunosuppressive drugs has made solid organ allotransplantation the preferred approach for treatment of end-organ failure. The benefits of these immunosuppressants outweigh their risks in preventing rejection of lifesaving solid-organ allografts. On the contrary, composite tissue allotransplants are non-lifesaving and whether the risks of immunosuppressants justify their benefits is a subject of debate. Hence, composite tissue allografts (CTA) have not enjoyed widespread clinical application for reconstruction of large tissue defects. Therefore, a method of preventing rejection that would eliminate the need for toxic immunosuppressants is of particular importance in CTA. Bone marrow transplantation (BMT) to establish mixed chimerism induces tolerance to a variety of allografts in animal models. This article reviews mixed chimerism-based tolerance protocols. Their limitations and their relevance to CTA are discussed, highlighting some unique characteristics (high antigenicity and the presence of active bone marrow) that make CTAs different from solid organ allografts.

Nonmyeloablative hematopoietic cell transplantation

The myeloablative doses of chemotherapy and radiation used with conventional allogeneic hematopoietic cell transplantation produce considerable morbidity and mortality that generally limit this treatment to patients younger than 55 years of age and in good general medical condition. It has become clear that T-cell-mediated graft-versus-tumor effects play an important role in the elimination of malignant disease after allotransplants. Several investigators have sought to reduce regimen-related toxicities while optimizing graft-versus-tumor effects. Strategies can be broadly categorized as reduced-intensity regimens that retain some toxicities and require hospitalization, and minimally myelosuppressive regimens that rely on immunosuppression for allogeneic engraftment and resultant graft-versus-tumor effects. The latter approach can be performed in the ambulatory care setting. Preliminary results are encouraging. If long-term efficacy is demonstrated, such strategies would expand treatment options for patients who would otherwise be excluded from receiving conventional allografts.

Posttransplantation lymphoproliferative disease in miniature swine after allogeneic hematopoietic cell transplantation: similarity to human PTLD and association with a porcine gammaherpesvirus

Posttransplantation lymphoproliferative disease (PTLD) is a major complication of current clinical transplantation regimens. The lack of a reproducible large-animal model of PTLD has limited progress in understanding the pathogenesis of and in developing therapy for this clinically important disease. This study found a high incidence of PTLD in miniature swine undergoing allogeneic hematopoietic stem cell transplantation and characterized this disease in swine. Two days before allogeneic peripheral blood stem cell transplantation, miniature swine were conditioned with thymic irradiation and in vivo T-cell depletion. Animals received cyclosporine daily beginning 1 day before transplantation and continuing for 30 to 60 days. Flow cytometry and histologic examination were performed to determine the cell type involved in lymphoproliferation. Polymerase chain reaction was developed to detect and determine the level of porcine gammaherpesvirus in involved lymph node tissue. PTLD in swine is morphologically and histologically similar to that observed in human allograft recipients. Nine of 21 animals developed a B-cell lymphoproliferation involving peripheral blood (9 of 9), tonsils, and lymph nodes (7 of 9) from 21 to 48 days after transplantation. Six of 9 animals died of PTLD and 3 of 9 recovered after reduction of immunosuppression. A novel porcine gammaherpesvirus was identified in involved tissues. Miniature swine provide a genetically defined large-animal model of PTLD with many characteristics similar to human PTLD. The availability of this reproducible large-animal model of PTLD may facilitate the development and testing of diagnostic and therapeutic approaches for prevention or treatment of PTLD in the clinical setting.