Xenotransplantation--caution, but no moratorium.
Explore the source record for details and available documents.
Advisory Committees↗
Biomedical subjects
M Sykes
Publications and source records attributed to M Sykes.
Altered expression of Ly-49 receptors on NK cells developing in mixed allogeneic bone marrow chimeras.
Ly-49 molecules are used by NK cells to distinguish 'self' from 'non-self', but the determinants of Ly-49 expression that allow this distinction to be made are not understood. The education of NK cells for self/non-self recognition was studied in murine mixed allogeneic bone marrow chimeras, in which NK cells are of both host and donor origin. Marked alterations in Ly-49 receptor expression were observed on both host and donor NK cells developing in BALB/c --> B6 mixed chimeras. Ly-49A and Ly-49G2 expression was lower on host B6 NK cells of mixed chimeras compared to non-transplanted B6 controls. Among donor BALB/c NK cells, Ly-49C expression levels were reduced, but the proportion of Ly-49C+ cells was increased, whereas Ly-49G2 expression was up-regulated compared to non-transplanted BALB/c controls. Thus, Ly-49 expression on donor and host NK cells developing post-bone marrow transplantation evolves toward the expression pattern of the host and donor strains respectively, due to the presence of the allogeneic MHC. In vitro functional NK cell assays showed that donor NK cells in mixed chimeras were not tolerant to host antigens and that host NK cells were not tolerant to the donor. Our data are consistent with a model in which MHC expression in the environment has a dominant down-regulating effect on the expression of Ly-49 molecules that recognize those MHC molecules, regardless of whether they are self or allogeneic. This down-regulation, combined with the limited repertoire of Ly-49 molecules, may not be sufficient to allow NK cells to be tolerant of MHC antigens of a fully MHC-mismatched allogenic strain.
Animals↗
Outcomes of recipients of both bone marrow and solid organ transplants. A review.
In this review we examine the clinical outcomes of patients who have received both bone marrow transplantation (BMT) and solid organ transplantation (SOT) and discuss the possible immunologic consequences of the dual transplants. We collected cases through a comprehensive literature search (MEDLINE database, English literature only) covering the years 1990 through 1997 and correspondence with the International Bone Marrow Transplant Registry. Our study selected case reports of patients who have undergone both bone marrow and solid organ transplants; cases in which bone marrow transplantation was undertaken as an adjunct ot induce or augment donor-specific tolerance in a recipient to the transplanted organ were excluded. Clinical characteristics included patient's demographic information, underlying disorders for each transplant, source of donor organ or tissue, time between transplants, and immunosuppressive regimens used to prevent graft-versus-host disease (GVHD) or graft rejection. Clinical outcomes included patient survival, complications of transplantation, and donor-specific tolerance that was experienced in many cases. Twenty-one cases of SOT after BMT and 7 cases of BMT after SOT were reviewed. Solid organ transplantations included lung, liver, cardiac, and kidney for a variety of BMT-related complications including GVHD, hepatic veno-occlusive disease, chronic renal failure, end-stage pulmonary disease, and severe cardiomyopathy. Bone marrow transplants were performed following SOT for aplastic anemia and hematologic malignancies. Clinical outcomes for patients who received both BMT and SOT were variable and depended on transplant indication and degree of histocompatibility. Prior bone marrow transplantation may tolerize for a subsequent organ transplant from the same donor. Conversely, severe GVHD may follow BMT from human leukocyte antigen (HLA)-matched donors following SOT. The favorable survival in this high-risk group of patients may represent a literature review bias (that is, an undetermined number of unsuccessful cases may not have been reported). Nonetheless, dual transplantation is clearly feasible in selected cases.
Bone Marrow Transplantation↗
Functionally and phenotypically mature mouse CD8+ T cells develop in porcine thymus grafts in mice.
Mouse CD4+ T cells efficiently develop in fetal pig thymus (FP THY) grafts and repopulate the periphery of T cell and NK cell-depleted, thymectomized (ATX) mice. However, efficient peripheral repopulation of mouse CD8+ T cells does not occur in these mice. We have therefore evaluated the maturation and function of mouse CD8 single positive (SP) thymocytes in fetal pig thymus and liver fragment (FP THY LIV) grafts. Phenotypic maturity, as measured by upregulated expression of TCR, class I MHC, and Qa-2, and downregulated expression of heat stable antigen (HSA) on CD8 SP cells in FP THY grafts, was similar to that in host thymi of euthymic control mice. Cytolytic T lymphocyte (CTL) activity of thymocytes from FP THY grafts was similar to that of thymocytes from host thymi of euthymic mice, indicating that functional maturation of CD8 SP cells had taken place in the grafts. Furthermore, similarly efficient deletion of V beta 5.1/5.2+ and V beta 11+ CD8 SP cells was observed in FP THY grafts as in host thymi of euthymic control mice. Similar percentages of V beta 6, V beta 7, and V beta 8.1/8.2 expressing cells were also detected among CD8 SP cells in FP THY grafts and host thymi of euthymic controls. Together, our results suggest that normal positive and negative selection occurs, and that mouse CD8+ cells can undergo normal functional and phenotypic maturation in FP THY grafts. Thus, other explanations must be sought for the failure of CD8+ cells to repopulate the peripheral lymphoid tissues of ATX, T cell-depleted, pig THY/LIV-grafted mice.
Animals↗
Separate regulation of peripheral hematopoietic and thymic engraftment.
Long-term multilineage chimerism, indicating pluripotent hematopoietic stem cell engraftment, was achieved in an Ly5-congenic strain combination without irradiation or other host conditioning when a large number of donor marrow cells (1.4-2x10(8)) was administered. However, the initial (2-4 weeks posttransplantation) percentages of T and B lymphocytes of donor origin were markedly lower than those of myeloid lineages. Steady-state levels of donor and host repopulation of all lineages were reached by 7 to 15 weeks posttransplantation and remained relatively constant for at least 41 weeks. B cell chimerism was similar to that seen in myeloid lineages at steady state. In contrast, long-term donor representation in the T cell lineage was much lower than in the B cell or myeloid lineages. Host treatment with depleting anti-T cell monoclonal antibodies increased the donor contribution to early T cell repopulation, but long-term T cell chimerism was still significantly lower in all lymphohematopoietic tissues, including the thymus, than B cell or myeloid cell chimerism. Pretreatment of hosts with 3.5 Gy of local irradiation to the thymic region further increased the donor contribution to initial T cell repopulation, which equaled that of other lineages at 4 to 7 weeks. However, donor representation in the T cell lineage declined by the time steady-state chimerism was attained and was lower than donor representation in the B cell or myeloid lineages. A higher dose of thymic irradiation (7 Gy) led to a reduction in this discrepancy, so that levels of donor thymopoiesis and hematopoiesis in other lineages were similar by 23 to 27 weeks posttransplantation. The differential contribution of adult donor marrow to long-term, steady-state thymopoiesis vs. hematopoiesis in other lineages under certain conditions in this competitive repopulation assay suggests that functionally distinguishable progenitors are responsible for these activities.
Animals↗
Interleukin-12 preserves the graft-versus-leukemia effect of allogeneic CD8 T cells while inhibiting CD4-dependent graft-versus-host disease in mice.
We have recently demonstrated that a single injection of 4,900 IU of interleukin-12 (IL-12) on the day of bone marrow transplantation (BMT) markedly inhibits acute graft-versus-host disease (GVHD) in a fully major histocompatibility complex plus minor antigen-mismatched BMT model (A/J --> B10, H-2(a) --> H-2(b)), in which donor CD4(+) T cells are required for the induction of acute GVHD. We show here that donor CD8-dependent graft-versus-leukemia (GVL) effects against EL4 (H-2(b)) leukemia/lymphoma can be preserved while GVHD is inhibited by IL-12 in this model. In mice in which IL-12 mediated a significant protective effect against GVHD, marked GVL effects of allogeneic T cells against EL4 were observed. GVL effects against EL4 depended on CD8-mediated alloreactivity, protection was not observed in recipients of either syngeneic (B10) or CD8-depleted allogeneic spleen cells. Furthermore, we analyzed IL-12-treated recipients of EL4 and A/J spleen cells which survived for more than 100 days. No EL4 cells were detected in these mice by flow cytometry, tissue culture, adoptive transfer, necropsies, or histologic examination. Both GVL effects and the inhibitory effect of IL-12 on GVHD were diminished by neutralizing anti-interferon-gamma (IFN-gamma) monoclonal antibody. This study demonstrates that IL-12-induced IFN-gamma production plays a role in the protective effect of IL-12 against GVHD. Furthermore, IFN-gamma is involved in the GVL effect against EL4 leukemia, demonstrating that protection from CD4-mediated GVHD and CD8-dependent anti-leukemic activity can be provided by a single cytokine, IFN-gamma. These observations may provide the basis for a new approach to inhibiting GVHD while preserving GVL effects of alloreactivity.
Animals↗
Interleukin-12 prevents severe acute graft-versus-host disease (GVHD) and GVHD-associated immune dysfunction in a fully major histocompatibility complex haplotype-mismatched murine bone marrow transplantation model.
BACKGROUND: We have recently reported that interleukin (IL)-12 prevents acute graft-versus-host disease (GVHD)-induced mortality in a full major histocompatibility complex- plus multiple minor antigen-mismatched A/J-->B10 bone marrow transplantation (BMT) model. Because most patients have access to a haploidentical, one haplotype-mismatched donor, we have now investigated the protective effect of IL-12 against GVHD and GVHD-associated immune dysfunction in a haploidentical CBD2F1 (H2kxd) --> B6D2F1 (H2bxd) strain combination. METHODS: GVHD was induced by injecting CBD2F1 marrow and spleen cells into lethally irradiated B6D2F1 mice. RESULTS: In untreated control mice, GVHD resulted in 87% mortality by day 8 after BMT, with no survivors beyond day 17. Treatment with a single injection of IL-12 on the day of BMT led to 87% long-term survival, with no significant weight loss, diarrhea or GVHD skin changes. The majority of T cells recovering in these mice showed the CD62L+, CD44low, CD45RBhigh naive phenotype. These T cells showed specific tolerance to both host and donor histocompatibility antigens, but normal anti-third party (H2s) alloresponses in vitro. B-cell proliferative responses to lipopolysaccharide were also normal in IL-12-protected mice. Moreover, normal negative selection of thymocytes bearing T cell receptors with Vbeta that recognize endogenous superantigens was observed among CD4+CD8- thymocytes, indicating a lack of GVHD-associated thymic selection abnormalities in IL-12-protected allogeneic BMT recipients. CONCLUSIONS: IL-12 provides permanent protection against an otherwise severe, rapidly lethal GVHD, with no clinical manifestations of chronic GVHD, immunosuppression or autoimmune features, in a full major histocompatibilty complex haplotype-mismatched murine BMT model.
Acute Disease↗
Positive and negative selection of functional mouse CD4 cells by porcine MHC in pig thymus grafts.
Specific tolerance to discordant xenogeneic donors can be achieved by grafting of fetal pig thymic and liver tissue (FP THY/LIV) to T cell and NK cell-depleted, thymectomized (ATX) mice. Mouse CD4+ T cells develop in FP THY/LIV grafts, and demonstrate remarkably normal immune function, including host-restricted responses to keyhole limpet hemocyanin. We have therefore studied the role of host MHC class II in the development of mouse T cells in FP THY/LIV grafts by comparing their development in ATX MHC class II-deficient (IIKO) and wild-type (H-2b) mice. Mouse CD4+ T cells repopulated T/NK cell-depleted, ATX IIKO mice after grafting with FP THY/LIV, indicating that pig MHC can positively select mouse CD4 cells. Expression of TCR, MHC class I, Qa-2, heat-stable Ag, and CD45RB among double-positive and CD4 single-positive (SP) graft thymocytes in wild-type recipients was similar to that in normal mouse thymi, whereas CD4 SP thymocytes in grafts of IIKO mice showed increased Qa-2 and decreased heat-stable Ag expression, suggesting an increased level of maturity. Double-positive cells in grafts of IIKO mice also expressed higher than normal levels of Qa-2. Deletion within the grafts of Vbeta3+, Vbeta5.1/5.2+, and Vbeta11+ but not Vbeta6+, Vbeta7+, or Vbeta8.1/8.2+ mouse CD4 SP thymocytes in ATX IIKO mice demonstrated that swine leukocyte Ag participates in negative selection of the T cell repertoire. Therefore, porcine MHC mediates positive and negative selection of mouse thymocytes, but host class II MHC molecules also regulate thymocyte maturation in xenogeneic thymic grafts.
Animals↗
Resistance to monoclonal antibody-induced CD8+ T-cell depletion in thymectomized MHC class II-deficient mice.
BACKGROUND: CD8+ T cells are present at higher than normal levels in MHC class II-deficient (IIKO) mice. METHODS: In this study, we have examined the sensitivity of CD8+ T cells to depletion induced by a single injection or multiple injections of an anti-CD8 monoclonal antibody (mAb) (2.43) in IIKO mice in vivo. RESULTS: Thymectomized (ATX) IIKO mice showed the presence of a greater percentage of memory CD8+ T cells (CD44high, CD45RBlow, and MEL-14(-)) in peripheral blood lymphocytes (PBL) by 1 month after ATX compared with age-matched euthymic mice. Although CD8+ cells were not detectable in the periphery at 5 and 14 days after 2.43 injection, CD8+ T cell receptor alpha/beta+ cells expressing the memory phenotype had recovered markedly by 21 days after mAb injection in these ATX IIKO mice. The expression of CD8 beta-chain and Thy-1 as well as the absence of CD4 and of T cell receptor gamma/delta among most recovering CD8+ cells, and their varied Vbeta usage, suggested that these cells were derived from the thymus rather than from extra-thymic T-cell differentiation or from oligoclonal peripheral expansion. In addition, low numbers of CD8+ cells that were coated with mAb (2.43) were detected in the lymph nodes of ATX IIKO mice 7 days after mAb injection. Most of these nondepleted lymph node CD8+ cells expressed the memory phenotype and low levels of CD8beta. Furthermore, the levels of recovering CD8+ cells in PBL of ATX IIKO mice by 21 days after mAb treatment were markedly higher than those in PBL of simultaneously mAb-treated ATX wild-type (B10) mice. CONCLUSION: Together, these studies indicate that memory CD8+ T cells are relatively resistant to mAb-induced depletion in vivo.
Animals↗
Immune restoration by fetal pig thymus grafts in T cell-depleted, thymectomized mice.
Donor-specific tolerance can be induced across a discordant xenogeneic barrier in T/NK cell-depleted, thymectomized (ATX) B10 mice by grafting of fetal pig thymic and liver tissue (FP THY/LIV) under the kidney capsule. We have now examined the phenotype and function of murine T cells that develop in FP THY/LIV grafts in these mice. Mouse CD4+ T cells reached normal levels in PBL by 14 wk, and were maintained up to 30 wk. Similar proportions of splenic CD4+ cells expressed the naive phenotype (CD45RBhighMEL-14+CD44low) in FP THY/LIV graft recipients and euthymic control mice. These CD4 cells were functional, demonstrating normal proliferative responses and up-regulation of CD25 and CD69 after activation by mitogens or alloantigens. They proliferated in response to the protein Ag KLH presented by host MHC following in vivo immunization. ATX B10 mice grafted with FP THY/LIV also cleared Pneumocystis carinii infections, whereas simultaneously-treated ATX B10 mice not receiving FP THY were unable to do so. Discordant xenogeneic thymus grafting can therefore restore immune competence. Thus, in addition to tolerance induction, xenogeneic thymic replacement might have a potential role in the reconstitution of immunity in patients afflicted with immunodeficiencies affecting the thymus.
Animals↗
Discordant xenogeneic neonatal thymic transplantation can induce donor-specific tolerance.
The limited supply of human organs for transplantation necessitates the development of methods leading to acceptance of xenografts. To avoid the hazards of the high-dose chronic immunosuppressive pharmacotherapy which would otherwise be required for successful xenografting, it would be desirable to induce permanent tolerance to xenogeneic donors. We have recently demonstrated that xenogeneic donor-specific tolerance can be induced by transplanting fetal pig thymic and hematopoietic tissue into thymectomized, T cell-depleted, and natural killer-cell-depleted mice, or into natural killer cell-depleted nude mice. We have now extended these studies by replacing fetal tissue with neonatal pig thymic and hematopoietic tissue, and by examining the in vivo responses of reconstituted mice to pig skin grafts. Neonatal tissue was studied because it might be more practicable than fetal tissue for the purpose of transplantation to primates. BALB/c nu/nu mice transplanted with neonatal (<24-hr-old) pig thymus and spleen fragments developed circulating mouse CD4+ cells. The pig thymus grafts were necessary for mouse T-cell development, as CD4 recovery did not occur in recipients of neonatal pig splenic tissue alone. The CD4+ cells that developed included Vbeta8.1/2+ T cells in similar proportions as in BALB/c mice, and Vbeta11+ and Vbeta5+ CD4 T cells were deleted almost as completely as in normal BALB/c mice. This deletion was detected among CD4 single-positive graft thymocytes. In 9 of 12 evaluable animals, mixed lymphocyte responses demonstrated tolerance to donor-type pig SLA antigens, with responsiveness to alloantigens and/or third-party pig xenoantigens. Furthermore, grafting of neonatal pig thymus conferred the ability to reject allogeneic mouse skin in 7 of 10 animals. In addition, 7 of 10 animals accepted paternal (donor SLA-matched) skin (median survival time [MST] > 100 days), whereas 4 of 4 animals rejected third-party SLA-mismatched pig skin (MST=40.5 days). We conclude that neonatal pig thymi transplanted to BALB/c nu/nu mice can support the development of mouse CD4+ cells that are functional and specifically tolerant to donor-type pig antigens.
Animals↗
The mechanism of specific prolongation of class I-mismatched skin grafts induced by retroviral gene therapy.
In the present study, we examine the mechanism of specific hyporesponsiveness to major histocompatibility complex (MHC) class I-mismatched skin allografts induced by retrovirus-mediated gene transfer of an allogeneic class I gene into syngeneic bone marrow (BM). Using appropriate congenic recombinant mouse strains, we have mapped MHC determinants that are capable of restoring rapid rejection of Kb-bearing skin grafts. Our results indicate that either a single class I or a single class II alloantigen expressed on skin in association with Kb is able to restore the rapid rejection of Kb-mismatched skin grafts. These data suggest that third-party alloantigens expressed on skin in association with Kb abrogate hyporesponsiveness by providing T cell help. Consistent with this interpretation, spleen cells from mice reconstituted with Kb-transduced BM were unable to elicit a significant anti-Kb cytotoxic T lymphocyte response in vitro unless interleukin-2 was added to the culture medium. Skin graft survival was also analyzed on B10. AKM mice thymectomized 3-4 weeks post-reconstitution with Kb-transduced BM. Thymectomy did not result in significantly prolonged survival of B10. MBR skin grafts compared to euthymic controls, suggesting that even early after reconstitution, intrathymic deletion of Kb-reactive T cells must have been incomplete. Taken together, these data suggest that prolongation of skin allograft survival in this model is controlled at the level of T cell help.
Animals↗
Mixed hematopoietic chimerism and transplantation tolerance.
Durable transplantation tolerance can be reliably achieved by inducing engraftment of hematopoietic cells in recipients initially depleted of T-lymphocytes. Engraftment of donor pluripotent hematopoietic stem cells (PPHSC) produces mixed hematopoietic chimeras in which both host and donor cells coexist and are tolerant of each other. The major mechanism of tolerance in these chimeras is central, intrathymic clonal deletion, which is induced and maintained by immigration of both host and donor marrow-derived cells to the host thymus, ensuring the ongoing central deletion of donor- and host-reactive cells. In this article, approaches developed in our laboratory to induce stable mixed hematopoietic chimerism and specific central deletional allogeneic and xenogeneic tolerance without toxic or myeloablative host conditioning are reviewed.
Animals↗
Bone marrow chimerism and transplantation tolerance.
Engraftment of allogeneic or xenogeneic pluripotent hematopoietic stem cells into nonmyeloablated but immunodepleted (preconditioned) recipients can produce a state of immunological tolerance to donor and host. Host and donor hematopoietic cells entering the thymus ensure deletion of both donor- and host-reactive thymocytes. Additional mechanisms are involved in tolerance induced in recipients that are not immunodepleted. Grafting of donor thymic tissue to thymectomized recipients is an alternative approach for inducing central T cell tolerance without the requirement for engraftment of donor hematopoietic stem cells. During the past year, advances have been made in understanding both the requirements for preconditioning and the mechanisms of tolerance induction in the above transplantation models.
Bone Marrow Transplantation↗
Induction of high levels of allogeneic hematopoietic reconstitution and donor-specific tolerance without myelosuppressive conditioning.
Donor-specific tolerance induced by bone marrow transplantation (BMT) would allow organ allografting without chronic immunosuppressive therapy. However, the toxicity of conditioning regimens used to achieve marrow engraftment has precluded the clinical use of BMT for tolerance induction. We have developed a BMT strategy that achieves alloengraftment without toxic or myelosuppressive host conditioning. B6 mice received depleting anti-CD4 and anti-CD8 monoclonal antibodies, local thymic irradiation, and a high-dose (174 x 10(6)) of major histocompatibility (MHC)-mismatched B10.A bone marrow cells (BMCs) divided over days 0 through 4. High levels of donor cells were observed among white blood cells (WBCs) of all lineages. Permanent, multilineage mixed chimerism; donor-specific skin-graft tolerance; and in vitro tolerance were observed in most animals. Large numbers of donor class II(high) cells were detected in thymuses of long-term chimeras, and their presence was associated with intrathymic deletion of donor-reactive host thymocytes. The treatment was not associated with significant myelosuppression, toxicity, or graft-versus-host disease (GVHD). Thus, high levels of allogeneic stem-cell engraftment can be achieved without myelosuppressive host conditioning. As stem-cell mobilization and in vitro culture techniques have increased the feasibility of administering high doses of hematopoietic cells to humans, this approach brings hematopoietic cell transplantation closer to clinical use for the induction of central deletional T-cell tolerance.
Animals↗
Cellular modulation of acute graft-vs.-host disease.
Bone marrow transplantation (BMT) is a potentially curative therapy for a number of life-threatening hematologic malignancies; however, the subsequent development of severe graft-vs.-host disease (GVHD) remains a major obstacle, impeding its widespread application. Prevention and treatment of GVHD may involve modulating the host/donor cellular environment following BMT. Suspected mechanisms by which cells with immunoregulatory properties inhibit alloresponses include natural suppressor and veto activity. Cell phenotypes associated with suppressive activity in mice and humans include null cells, double negative T lymphocytes, and natural killer cells. Cellular-based therapies for the treatment of acute GVHD using autologous peripheral blood mononuclear or bone marrow cells have shown promise in reversing GVHD following allogeneic transplantation. In this review, we examine the considerable evidence supporting an immunoregulatory role for both host and donor cells in modulating acute GVHD.
Animals↗
Thymic dependence of loss of tolerance in mixed allogeneic bone marrow chimeras after depletion of donor antigen. Peripheral mechanisms do not contribute to maintenance of tolerance.
A nonmyeloablative conditioning regimen has recently been developed that allows allogeneic marrow engraftment with induction of permanent mixed chimerism and donor-specific tolerance across fully MHC-mismatched allogeneic barriers. We recently demonstrated that tolerance can be broken in these chimeras by administration of an anti-donor class I-specific monoclonal antibody that eliminates donor hematopoietic cells. We have now investigated the role of the thymus in the loss of tolerance observed when chimerism is eliminated in this manner. Mixed chimeras were prepared in B10 (H2b) recipients by treatment with depleting anti-CD4 and anti-CD8 mAbs, 3-Gy whole body irradiation, and 7-Gy thymic irradiation, followed by B10.A (H2a) bone marrow transplantation. Chimeras were thymectomized 7 weeks later, and were either untreated or were depleted of donor cells with anti-donor class I (Dd-specific) mAb 34-2-12. Control chimeras that were not thymectomized also received anti-donor monoclonal antibodies or no further treatment. Of the four groups, only euthymic animals that were depleted of donor antigen showed a loss of tolerance, as evidenced by rejection of B10.A skin grafts. In contrast to untreated control and thymectomized, anti-Dd-treated chimeras, these euthymic anti-Dd-treated chimeras showed significant recovery of Vbeta11+ T cells, which can recognize Mtv antigens presented by donor I-E molecules. The requirement for a thymus for loss of tolerance in the absence of donor antigen was verified in an adoptive transfer model, in which chimera (B10.A-->B10) spleen cells were depleted of donor-type cells ex vivo, adoptively transferred into B6 nu/nu mice, and then further depleted of donor-type antigen with monoclonal antibody treatment in vivo. These B6 nu/nu mice maintained donor-specific tolerance to B10.A skin grafts. The absence of active suppression as a potent mechanism of tolerance in long-term mixed chimeras was confirmed by the loss of mixed chimerism and of tolerance that was readily induced by injection of naive host-type spleen cells. Together, our results suggest that in mixed allogeneic chimeras, intrathymic clonal deletion, and not peripheral suppression or anergy, is the major mechanism maintaining donor-specific tolerance.
Animals↗
Additional monoclonal antibody (mAB) injections can replace thymic irradiation to allow induction of mixed chimerism and tolerance in mice receiving bone marrow transplantation after conditioning with anti-T cell mABs and 3-Gy whole body irradiation.
While allogeneic bone marrow transplantation (BMT) has long been known to be capable of inducing donor-specific tolerance and hence permitting allograft acceptance without immunosuppressive pharmacotherapy, the toxicity of conditioning regimens required to achieve marrow engraftment has precluded the application of this approach to clinical organ transplantation. A relatively nontoxic method of conditioning mice that allows allogeneic bone marrow engraftment and induction of donor-specific skin allograft tolerance has recently been described. This regimen included anti-CD4 and anti-CD8 mAbs administered on day -5, followed by 3-Gy whole body irradiation (WBI) and 7-Gy thymic irradiation (TI) on day 0. To further reduce the potential toxicity of this regimen, we have now attempted to overcome the requirement for TI by administering additional mAb injections before and after BMT. Mixed chimerism and prolonged donor-specific skin graft acceptance were induced in 90% of B10 mice conditioned with anti-CD4 and -CD8 mAbs on days -6 and -1 and 3-Gy WBI on day 0 without TI. Despite long-term acceptance of donor-specific skin grafts, however, some of these animals showed a gradual decline in donor-type hematopoietic repopulation, and 2 of 10 mice regrafted with a second donor-type skin graft 5-9 months after BMT rejected the second and/or the original graft. This rejection after repeat donor-specific skin grafting correlated with a decline in the percentage of donor-type T cells between 6 and 12 weeks after BMT. In contrast, all animals receiving additional mAb injections 7 and 14 days following BMT after conditioning with mAbs on days -6 and -1 and 3-Gy WBI showed stable chimerism and accepted both primary and secondary donor-specific skin grafts. Animals receiving TI in addition to mAb and 3-Gy WBI also showed stable chimerism and long-term acceptance of initial (at 7 weeks) and later repeat donor-specific grafts. In contrast, the majority of mice receiving mAbs only on day -5 or on day -1 only, followed by 3-Gy WBI on day 0 without TI, did not accept initial donor-specific skin grafts, and showed only transient chimerism. Thus, the requirement for thymic irradiation to allow permanent mixed chimerism and donor-specific tolerance induction can be overcome by the administration of additional T cell-depleting mAb injections. These results establish a less toxic method of inducing donor-specific tolerance, thus increasing the potential clinical applicability of this approach to inducing organ allograft acceptance without chronic immunosuppressive therapy.
Animals↗