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Porcine thymus grafts support robust murine and human thymopoiesis, generating a diverse T cell repertoire that is deleted of donor and host-reactive cells, achieving specific xenograft tolerance. Positive selection is mediated exclusively by the xenogeneic thymic MHC. Although thymectomized, T cell-depleted normal mice usually remain healthy following xenogeneic thymic transplantation, thymus-grafted congenitally athymic mice frequently develop multiorgan autoimmunity. We investigated the etiology of this syndrome by adoptively transferring lymphocyte populations from fetal pig thymus-grafted BALB/c nude mice to secondary BALB/c nude recipients. Fetal pig thymus-grafted nude mice generated normal numbers of CD25+Foxp3+CD4 T cells, but these cells lacked the capacity to block autoimmunity. Moreover, thymocytes and peripheral CD4+CD25− cells from fetal pig thymus-grafted nude mice, but not those from normal mice, induced autoimmunity in nude recipients. Injection of thymic epithelial cells from normal BALB/c mice into fetal pig thymus grafts reduced autoimmunity and enhanced regulatory function of splenocytes. Our data implicate abnormalities in postthymic maturation, expansion, and/or survival of T cells positively selected by a xenogeneic MHC, as well as incomplete intrathymic deletion of thymocytes recognizing host tissue-specific Ags, in autoimmune pathogenesis. Regulatory cell function is enhanced and negative selection of host-specific thymocytes may potentially also be improved by coimplantation of recipient thymic epithelial cells in the thymus xenograft.

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.

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.

Anti-CD154 (CD40L) monoclonal antibody (mAb) plus bone marrow transplantation (BMT) in mice receiving CD8 cell-depleting mAb leads to long-term mixed hematopoietic chimerism and systemic donor-specific tolerance through peripheral and central deletional mechanisms. However, CD4+ T-cell tolerance is demonstrable in vitro and in vivo rapidly following BMT, before deletion of donor-reactive CD4 cells is complete, suggesting the involvement of other mechanisms. We examined these mechanisms in more detail. Spot enzyme-linked immunosorbent (ELISPOT) analysis revealed specific tolerization (within 4 to 15 days) of both T helper 1 (Th1) and Th2 cytokine responses to the donor, with no evidence for cytokine deviation. Tolerant lymphocytes did not significantly down-regulate rejection by naive donor-reactive T cells in adoptive transfer experiments. No evidence for linked suppression was obtained when skin expressing donor alloantigens in association with third-party alloantigens was grafted. T-cell receptor (TCR) transgenic mixing studies revealed that specific peripheral deletion of alloreactive CD4 T cells occurs over the first 4 weeks following BMT with anti-CD154. In contrast to models involving anti-CD154 without BMT, BMT with anti-CD154 leads to the rapid induction of anergy, followed by deletion of pre-existing donor-reactive peripheral CD4+ T cells; the rapid deletion of these cells obviates the need for a regulatory cell population to suppress CD4 cell-mediated alloreactivity. (Blood. 2004;103:4336-4343)

Long-term multilineage chimerism is achieved in CD45 congenic mice receiving high bone marrow doses with or without mediastinal irradiation (MI). Increased donor chimerism results in MI-treated compared with nonirradiated animals, suggesting that MI makes “space” for engraftment of donor pluripotent hematopoietic stem cells (PHSCs). We have now examined whether space is systemic or whether increased engraftment of donor marrow in locally irradiated mice is confined to the irradiated bones. While increased donor chimerism was observed in irradiated bones compared with nonirradiated bones of MI-treated animals 4 weeks following bone marrow transplantation (BMT), these differences were minimal by 40 weeks. MI-treated chimeras contained more adoptively transferable donor PHSCs in the marrow of both irradiated and distant bones compared with non-MI–treated chimeras. Similar proportions of donor PHSCs were present in irradiated and nonirradiated bones of locally irradiated mice at both 4 and 40 weeks. Irradiated bones contained more donor short-term repopulating cells than distant bones at 4 weeks, but not 40 weeks, after BMT. Our study suggests that local proliferation of donor PHSCs in mice receiving local irradiation rapidly leads to a systemic increase in donor PHSC engraftment.

Objective Natural killer (NK) cells kill allogeneic cells that lack a class I MHC ligand for clonally distributed killer inhibitory receptors (KIR). Following HLA-mismatched hematopoietic cell transplantation (HCT), donor NK cells might mediate graft-vs-host (GVH) reactions that promote donor chimerism and mediate anti-tumor effects. Additionally, recipient NK cells might mediate donor marrow rejection. We have developed a nonmyeloablative approach to haploidentical HCT involving recipient treatment with a T cell–depleting mAb, Medi-507, that can achieve donor engraftment and mixed hematopoietic chimerism without graft-vs-host disease (GVHD). Donor lymphocyte infusions (DLI) are later administered in an effort to achieve graft-vs-leukemia/lymphoma (GVL) effects without GVHD. It is unknown whether NK cell “tolerance” develops in human mixed chimeras. Methods We have addressed these issues in 12 patients receiving Medi-507-based nonmyeloablative haploidentical HCT. Results NK cells recovered relatively early, despite the presence of circulating anti-CD2 mAb, but the majority of initially recovering cells lacked CD2 expression. These NK cells showed a reduced capacity, compared to those from normal donors, to kill class I–deficient targets. No association was detected between KIR mismatches in the host-vs-graft (HVG) or GVH direction and graft or tumor outcomes in this small series. NK cell chimerism did not correlate with chimerism in other lineages in mixed chimeras. NK cell tolerance to the host was not observed in a patient with full donor chimerism. One patient developed NK cell reactivity against donor-derived lymphoblast targets after loss of chimerism, despite the absence of an HVG KIR mismatch. Conclusion Our results do not show an impact of NK cells on the outcome of nonmyeloablative, even T cell–depleted, HCT across haplotype barriers using an anti-CD2 mAb. Our data also raise questions about the applicability of observations made with NK cell clones to the bulk NK cell repertoire in humans.

We developed a nonmyeloablative conditioning regimen for allogeneic bone marrow transplantation (BMT) followed by donor lymphocyte infusions (DLI) for treatment of chemotherapy refractory malignancies. Although the majority of patients who receive this regimen achieve lasting mixed or full allogeneic chimerism, approximately 30% show initial mixed chimerism followed by loss of the donor graft. These patients recover host hematopoiesis without significant cytopenias. To assess the role of immunologic rejection in graft loss, we compared T-cell recovery and in vitro alloresponses in six patients who lost their marrow graft to that in 16 concurrent patients with sustained donor chimerism.Conditioning included pretransplant cyclophosphamide (150-200 mg/kg), thymic irradiation (700 cGy), and pre- and post-transplant equine antithymocyte globulin (ATG; ATGAM). HLA-identical related donor BMT was followed by DLI at approximately day 35 in patients without graft-vs-host disease.The group with transient chimerism showed significantly increased circulating host T-cell (median 416 cells/mm(3) vs 10 cells/mm(3), p0.05) and CD8 T-cell numbers (354 cells/mm(3) vs 71 cells/mm(3), p0.05) compared to the group with stable mixed or full donor chimerism within the first 100 days post-BMT. All DLI recipients who lost chimerism following DLI had greater than 80% recipient T cells at the time of DLI, whereas those with persistent chimerism had60% host T cells. Graft rejection was associated with the development of a sensitized anti-donor bulk cytotoxic T-lymphocyte (CTL) response in 4 of 6 evaluated patients, compared to only 1 of 10 evaluated patients with sustained chimerism (p0.05). Additionally, 3 of 5 evaluated transient chimeras showed high anti-donor CTL precursor frequencies in limiting dilution assays, and 3 of 4 evaluated transient chimeras showed high anti-donor interleukin-2 (IL-2)-producing T-helper (T(H)) cell frequencies. High anti-donor T(H) or cytotoxic T-lymphocyte precursors were not detected in sustained chimeras.These data indicate that loss of chimerism in patients receiving this nonmyeloablative regimen is due to immune-mediated rejection. This rejection appears to bemediated by recovering recipient cytolytic CD8(+) cells as well as IL-2-producing recipient T(H) cells. These data are the first to demonstrate sensitization of recipient anti-donor IL-2-producing cells in association with human marrow allograft rejection.

We initiated a clinical trial of nonmyeloablative haploidentical stem-cell transplantation (SCT) using MEDI-507, an immunoglobulin-G1 monoclonal anti-CD2 antibody. The trial was based on a preclinical major histocompatibility complex-mismatched bone marrow transplant model in which graft-versus-host disease (GVHD) was prevented and mixed chimerism as a platform for adoptive cellular immunotherapy was reliably induced. Twelve patients (three cohorts of four patients each) received cyclophosphamide, MEDI-507, and haploidentical unmanipulated bone marrow (n=8) or ex vivo T-cell-depleted peripheral blood stem cells (n=4) for chemorefractory hematologic malignancy. A two-dose regimen and schedule modifications of MEDI-507 were undertaken because of graft loss in the first cohort of four patients and GVHD in the second cohort. With ex vivo T-cell-depleted peripheral blood SCT, mixed chimerism occurred in all four patients without GVHD. Two patients, however, subsequently lost their grafts. Nonmyeloablative preparative therapy with MEDI-507 and haploidentical SCT have led to the reliable induction of at least transient mixed chimerism as a potential platform for adoptive cellular immunotherapy.

We have previously shown that high levels of multiline-age mixed hematopoietic chimerism and systemic T-cell tolerance can be achieved in mice without myeloablation through the use of anti-CD40L and costimulatory blockade alone (plus CTLA4Ig) or with recipient CD8 depletion and allogeneic bone marrow transplantation. Chimeric mice permanently accept donor skin grafts (100 days), and rapidly reject third-party grafts. The mechanisms by which costimulatory blockade facilitates the engraftment of allogeneic hematopoietic cells have not been defined. To further understand the in vivo mechanisms by which the administration of anti-CD40L mAb facilitates the engraftment of donor bone marrow and rapidly tolerizes CD4+ T cells, we analyzed the establishment of chimerism and tolerance in CD40L -/- mice. We demonstrate here that anti-CD40L mAb treatment is required only to prevent CD40L/CD40 interactions, and that no signal to the T cell through CD40L is necessary for the induction of CD4+ tolerance. Peripheral deletion of donor-reactive CD4+ T cells occurs rapidly in CD40L -/- mice receiving bone marrow transplantation (BMT), indicating that this deletion in the presence of anti-CD40L is not due to targeting of activated CD4+ cells by the antibody. Complete CD4+ cell tolerance is observed by both skin graft acceptance and in vitro assays before deletion is complete, indicating that additional mechanisms play a role in inducing CD4+ T-cell tolerance as the result of BMT in the presence of CD40/CD40L blockade.

Costimulatory blockade can be used to promote allogeneic marrow engraftment and tolerance induction, but on its own is not 100% reliable. We sought to determine whether one or the other of the CD4 or CD8 T cell subsets of the recipient was primarily responsible for resistance to allogeneic marrow engraftment in mice receiving costimulatory blockade, and to use this information to develop a more reliable, minimal conditioning regimen for induction of mixed chimerism and transplantation tolerance. We demonstrate that a single anti-CD40 ligand mAb treatment is sufficient to completely overcome CD4 cell-mediated resistance to allogeneic marrow engraftment and rapidly induce CD4 cell tolerance, but does not reliably overcome CD8 CTL-mediated alloresistance. The data suggest that costimulation, which activates alloreactive CTL, is insufficient to activate alloreactive CD4 cells when the CD40 pathway is blocked. The addition of host CD8 T cell depletion to anti-CD40 ligand treatment reliably allows the induction of mixed chimerism and donor-specific skin graft tolerance in 3 Gy-irradiated mice receiving fully MHC-mismatched bone marrow grafts. Thus, despite the existence of multiple costimulatory pathways and pathways of APC activation, our studies demonstrate an absolute dependence on CD40-mediated events for CD4 cell-mediated rejection of allogeneic marrow. Exposure to donor bone marrow allows rapid tolerization of alloreactive CD4 cells when the CD40 pathway is blocked, leading to permanent marrow engraftment and intrathymic tolerization of T cells that develop subsequently.

Two major pathways of death of previously activated T cells have been described: activation-induced cell death can be triggered by restimulating activated T cells with high concentrations of Ag, is Fas-dependent, is not influenced by proteins of the Bcl family, and is blocked by cyclosporin A; in contrast, passive cell death is induced by the withdrawal of growth factors and activation stimuli, is Fas-independent, and is blocked by Bcl family proteins. We examined the role of these two forms of cell death in the peripheral deletion of donor-reactive host T cells after allogeneic bone marrow transplantation and costimulatory blockade with anti-CD154 plus CTLA4Ig in two murine models. The substantial decline in donor-reactive CD4 cells seen in wild-type recipients 1 wk after bone marrow transplantation with costimulatory blockade was largely inhibited in Fas-deficient recipients and in Bcl-xL-transgenic recipients. We observed these effects both in a model involving low-dose total body irradiation and a conventional dose of bone marrow, and in a radiation-free regimen using high-dose bone marrow transplantation. Furthermore, cyclosporin A did not completely block the deletion of donor-reactive CD4+ T cells in recipients of bone marrow transplantation with costimulatory blockade. Thus, the deletion of donor-reactive T cells occurring early after bone marrow transplantation with costimulatory blockade has features of both activation-induced cell death and passive cell death. Furthermore, these in vivo data demonstrate for the first time the significance of in vitro results indicating that proteins of the Bcl family can prevent Fas-mediated apoptosis under certain circumstances.

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.

Five patients with end-stage kidney disease received combined kidney and bone marrow transplants from HLA haploidentical donors following nonmyeloablative conditioning to induce renal allograft tolerance. Immunosuppressive therapy was successfully discontinued in four patients with subsequent follow-up of 3 to more than 6 years. This allograft acceptance was accompanied by specific T-cell unresponsiveness to donor antigens. However, two of these four patients showed evidence of de novo antibodies reactive to donor antigens between 1 and 2 years posttransplant. These humoral responses were characterized by the presence of donor HLA-specific antibodies in the serum with or without the deposition of the complement molecule C4d in the graft. Immunofluorescence staining, ELISA assays and antibody profiling using protein microarrays demonstrated the co-development of auto- and alloantibodies in these two patients. These responses were preceded by elevated serum BAFF levels and coincided with B-cell reconstitution as revealed by a high frequency of transitional B cells in the periphery. To date, these B cell responses have not been associated with evidence of humoral rejection and their clinical significance is still unclear. Overall, our findings showed the development of B-cell allo- and autoimmunity in patients with T-cell tolerance to the donor graft.

Five patients with end-stage renal disease received combined bone marrow and kidney transplants from HLA single-haplotype mismatched living related donors, with the use of a nonmyeloablative preparative regimen. Transient chimerism and reversible capillary leak syndrome developed in all recipients. Irreversible humoral rejection occurred in one patient. In the other four recipients, it was possible to discontinue all immunosuppressive therapy 9 to 14 months after the transplantation, and renal function has remained stable for 2.0 to 5.3 years since transplantation. The T cells from these four recipients, tested in vitro, showed donor-specific unresponsiveness and in specimens from allograft biopsies, obtained after withdrawal of immunosuppressive therapy, there were high levels of P3 (FOXP3) messenger RNA (mRNA) but not granzyme B mRNA.

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.

Background. We have previously demonstrated that xenogeneic bone marrow engraftment and donor-specific tolerance can be induced in mice receiving anti-CD4, -CD8, -Thy-1.2, and -NK1.1 monoclonal antibodies (mAbs) on Days –6 and –1, 3 Gy total body irradiation (TBI), and 7 Gy thymic irradiation on Day 0, followed by injection of T-cell depleted (TCD) rat bone marrow cells. We have recently demonstrated that anti-CD40L mAb treatment is sufficient to completely overcome CD4 cell-mediated resistance to allogeneic marrow engraftment and rapidly induce CD4 cell tolerance in an allogeneic combination. Methods. We investigated the ability of anti-CD40L mAb to promote mixed xenogeneic chimerism and donor-specific tolerance in B6 mice receiving anti-CD8, -Thy1.2 and -NK1.1 mAbs and 3 Gy TBI followed by TCD bone marrow transplantation (BMT) from F344 rats. Results. Administration of anti-CD4 mAb in this model could be completely replaced by one injection of anti-CD40L mAb. Evidence for deletional tolerance was obtained in mixed chimeras prepared with this anti-CD40L-based regimen. However, anti-NK1.1 and anti-Thy1.2 mAb could not be replaced by anti-CD40L mAb. Conclusions. These results demonstrate that anti-CD40L in combination with xenogeneic BMT can tolerize preexisting peripheral and intrathymic CD4 cells to xenoantigens. However, anti-CD40L does not prevent NK cell and/or γΔ cell-mediated rejection of xenogeneic bone marrow.

Prostaglandin E2 (PGE2) acts in synergy with other inflammatory stimuli such as tumor necrosis factor (TNF) to induce the maturation of migratory-type monocyte-derived dendritic cells (MoDCs). However, PGE2 has been reported to inhibit IL-12p70 production by MoDCs and to promote the generation of Th2 T cell responses. We demonstrate here that the addition of PGE2 to TNF for the maturation of MoDCs enhanced CD4 and CD8 T cell proliferative responses to neoantigen and recall antigen, and enhanced Th1-type responses. The increased stimulatory capacity of MoDCs matured with PGE2 was associated with a fully mature, migratory-type MoDC phenotype and more rapid down-regulation of the expression of inflammatory chemokines, with up-regulated expression of the constitutive chemokines TARC and MDC. In addition, although MoDCs matured with TNF and PGE2 selectively produced the inhibitory IL-12p40 subunit at steady state, they were able to produce the bioactive IL-12p70 heterodimer after stimulation with CD40 ligand and/or IFN-gamma. Despite increased IL-6 mRNA expression, MoDCs matured with PGE2 did not overcome the suppressive effects of CD4+ CD25+ T cells in allogeneic mixed lymphocyte reactions. In conclusion, MoDCs matured in the presence of PGE2 display characteristics of more efficient antigen-presenting cells that might be optimal for use in cancer vaccine-based clinical trials.

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 (

Anti-CD40L mAb plus bone marrow transplantation (BMT) and recipient CD8 T-cell depletion permits long-term mixed hematopoietic chimerism and systemic donor-specific tolerance to be achieved across full MHC barriers. Initial tolerance is characterized by peripheral deletion of donor-reactive CD4 cells. In regimens using costimulatory blockade without BMT to achieve allograft survival, cyclosporine inhibited graft survival, suggesting that the combination may not be clinically applicable. We assessed the role of cyclosporine-sensitive mechanisms and the mechanisms of T-cell apoptosis involved in the induction of early peripheral CD4+ T-cell tolerance by BMT with anti-CD40L. Neither a short course of cyclosporine (14 days) nor the absence of FAS-mediated activation-induced cell death (AICD) blocked the induction or maintenance of donor-specific tolerance. IL-2 production was not associated with tolerance induction, consistent with the lack of a role for Fas-mediated AICD. Mice in which passive T-cell death was impaired because of constitutive expression of a Bcl-xL transgene did not develop tolerance with this protocol. These data confirm that deletion of donor-reactive T cells is critical for the induction of mixed chimerism and tolerance. However, the mechanisms involved may differ from those involved in costimulatory blockade regimens that do not include BMT.

Much evidence supports the conclusion that immunological responses to donor-specific incompatibilities are a major factor in producing “chronic” transplant rejection, including the arteriopathy (atherosclerosis) commonly present. Our experiments explored the effects of altered immunological responsiveness to these Ags on the formation of arteriopathy in transplanted mouse hearts. Specific immunological nonreactivity, or tolerance, was induced either by neonatal administration of allogeneic spleen cells (from F1 donors between class I-mismatched donor and recipient strains), resulting in “classical” immunological tolerance, or by bone marrow infusion to suitably prepared adult recipients, either fully MHC mismatched or class I mismatched, yielding “mixed chimerism.” Both approaches obviated systemic graft-versus-host effects. In both groups, donor-specific skin grafts survived perfectly and donor cell chimerism persisted. Specific Abs were undetectable in all recipients. Most transplants to either group of tolerant recipients developed striking vasculopathy in their coronary arteries (12 of 15 in neonatal tolerance and 15 of 23 in mixed chimeras). Neointimal infiltrates included CD4 and CD8 T cells and macrophages. Only 2 of 29 contemporary isotransplants showed any evidence of vasculopathy. Recipients essentially incapable of T and B cell responses (C.B-17/SCID and RAG1−/−) were also used. Transplants into these animals developed vasculopathy in 16 of 31 instances. Accordingly, in this setting, vasculopathy develops in the presence of H-2 gene-determined incompatibility even with minimal conventional immune reactivity. Perhaps innate responsiveness, that could include NK cell activity, can create such arteriopathic lesions. More evidence is being sought regarding this process.

Allogeneic bone marrow transplantation (in immunocompetent adults) has always required cytoreductive treatment of recipients with irradiation or cytotoxic drugs to achieve lasting engraftment at levels detectable by non-PCR-based techniques ('macrochimerism' or 'mixed chimerism'). Only syngeneic marrow engraftment at such levels has been achieved in unconditioned hosts. This requirement for potentially toxic myelosuppressive host pre-conditioning has precluded the clinical use of allogeneic bone marrow transplantation for many indications other than malignancies, including tolerance induction. We demonstrate here that treatment of naive mice with a high dose of fully major histocompatibility complex-mismatched allogeneic bone marrow, followed by one injection each of monoclonal antibody against CD154 and cytotoxic T-lymphocyte antigen 4 immunoglobulin, resulted in multi-lineage hematopoietic macrochimerism (of about 15%) that persisted for up to 34 weeks. Long-term chimeras developed donor-specific tolerance (donor skin graft survival of more than 145 days) and demonstrated ongoing intrathymic deletion of donor-reactive T cells. A protocol of high-dose bone marrow transplantation and co-stimulatory blockade can thus achieve allogeneic bone marrow engraftment without cytoreduction or T-cell depletion of the host, and eliminates a principal barrier to the more widespread use of allogeneic bone marrow transplantation. Although efforts have been made to minimize host pre-treatment for allogeneic bone marrow transplantation for tolerance induction, so far none have succeeded in eliminating pre-treatment completely. Our demonstration that this can be achieved provides the rationale for a safe approach for inducing robust transplantation tolerance in large animals and humans.

Following an in vivo T cell depleting non-myeloablative conditioning regimen, 5 patients, aged 22–49, received combined kidney and bone marrow transplantation from a haploidentical related donor. Rituximab was included in the conditioning for patients 4 and 5. All patients developed initial mixed chimerism but lost it by day 21; no patient developed GVHD. Four patients discontinued immunosuppression from 240 to 422 days after BMT and have remained off immunosuppression for 9 to 52 months with no evidence of allograft rejection. Flow cytometry was used to assess lymphocyte subsets recovering after transplant. CD3 counts recovered slowly, exceeding 500 cells/μl at days +271, +365, +640 and +450. While memory CD45RO+ cells were most prevalent among CD4+ cells, naïve-type CD4+CD45RA+ cells, presumably arising from the recipient thymus, ranged from 8% to 56% at the time when total CD4 counts recovered to >100 cells/μl (days +165, +21, +352, +240). Notably, a very high proportion of initially recovering T cells were CD3+CD4+ expressing CD25 in all patients as early as day 7 and persisted over 1 year in 2 patients. At approximately day +120 and +365, we further characterized these cells for CD127, FOXP3, CD45RO, CD45RA, HLA-DR and CD62L expression. At Day +120, all 4 patients showed increased frequencies (10.7±4.6%) of CD25+CD127-FOXP3+ regulatory T cells (Treg) within the CD4 population compared to healthy subjects (3.8±0.4%). Expression of CD45RO, CD45RA, CD62L and HLA-DR was variable. By 1 year post-transplant, frequencies of Treg had decreased to levels similar to those in normal subjects. In vitro assays for CD8 and CD4 T cell-mediated alloreactivity (CML/MLR) showed development of long-lasting donor-specific unresponsiveness by 3 months after transplant in Patients 2, 4 and 5, and by 9 months in Patient 1. Responses to 3rd party recovered in all patients after a period of unresponsiveness. In Patient 1, in whom anti-donor CML reactivity declined gradually to become unresponsive by 9 months, depletion of CD4+CD25+ cells revealed a residual anti-donor CML and MLR response at 1year but not at 18 months. In 2 other patients, depletion of CD4+CD25+ cells did not reveal an anti-donor response at time points analyzed from day +122 to 2 years. In patients in whom renal tubular epithelial cells (RTEC) were cultured from the donor kidney, loss of killing activity against donor RTEC was observed post-transplant. The high percentage of Treg recovering early after transplant suggests that they may play a role in initial tolerance induction. This regulatory mechanism may be followed by later deletion of donor-reactive T cells. The variable ability to detect regulation of anti-donor reactivity may reflect the strength of the initial response, as patients with weak pre-transplant anti-donor responses and rapid post-transplant development of donor unresponsiveness did not reveal anti-donor response when Treg were depleted. In addition, infiltration of Treg at the graft site, not revealed by the assays described, might be responsible for tolerance in these patients.

Mixed chimerism (MC) can be induced in mice after non-myeloablative conditioning and allogeneic HSCT. Full donor hematopoiesis conversion from MC can occur when nontolerant donor lymphocyte infusions (DLI) are given weeks following the transplant as a result of lymphohematopoietic GVH reactions that eradicate leukemia/lymphoma without GVHD. Based on this murine model, we initiated a trial in patients with advanced hematologic malignancies of non-myeloablative therapy with HLA-matched related donor HSCT with intentional induction of MC followed by prophylactic DLI (pDLI) at day +35 post-HSCT in those without clinical GVHD. This analysis examines the influence of the kinetics of sustained donor lymphoid (CD3+ cells) chimerism on transplant outcomes. Fifty-four patients with advanced hematologic malignancies received nonmyeloablative preparative therapy (cyclophosphamide, equine antithymocyte globulin, thymic irradiation and a brief course of cyclosporine) and HLA-matched HSCT (bone marrow, BM, n=42; GCSF-mobilized peripheral blood stem cells, PBSC, n=12). Chimerism analyses were performed weekly until day +100 post-HSCT and then once monthly for a year. Twenty-nine patients with aggressive malignant lymphomas (NHL, n=21; HD, n=8) were selected out of 50 evaluable patients in an effort to analyze a relatively uniform group in terms of their disease and sustained chimerism; patients who lost the allograft were removed from this analysis. Twenty five patients attained stable FDC, defined as > or = 90% donor T cells, and 4 maintained stable MC. Using 42 days post-HSCT as the defining point for chimerism conversion 13 patients achieved early FDC and 16 patients achieved late FDC. Median time to achieve FDC in the early and late groups was 14 days (range, 7–42) and 70 days (range, 49–91) respectively. Separation produced fairly equal distribution by disease characteristics as follows (early to late): NHL, 10 to 11 and HD, 3 to 5. The early group included 10 PBSC recipients who did not receive pDLI because of the spontaneous conversion to FDC; in contrast, the majority of the late group received BMT and 13 of them received DLI (pDLI, n=10, therapeutic DLI, n=3). The incidence of acute GVHD (grades II–IV) was similar between the early and late groups with rates of 77% (n=10 of 13) and 69% (n=11 of 16) respectively; for chronic GVHD, there was a tendency toward an increased rate in the early group of 75% (3/4) compared to the late group rate of 63% (5/8). Overall disease response rates in the early and late groups were 62% and 69%, but there was a trend toward improved complete remission rates in the late group relative to the early group (50% vs 31%). Progression-free survival (PFS) and overall survival (OS) at 1 year were improved in the late compared to the early group [31% (5/16) vs 15% (2/13) and 50% (8/16) vs 38% (5/13), respectively]. At 5 years, there was a statistically significant improved PFS and OS rate in the late group over the early group [31% (5/16) vs 0% (0/13), p =0.037, for both PFS and OS]. In conclusion, these data support the observation from our mouse models that the most potent graft-vs-tumor effect occurs in mixed chimeras with late chimerism conversion, often after delayed pDLI.

Mixed chimerism (MC) can be induced in mice across MHC barriers using nonmyeloablative conditioning and T cell depletion. Delayed donor lymphocyte infusions (DLI) convert this MC state to full donor hematopoiesis and impart a powerful graft versus tumor effect without inducing GVHD. Based on this principle, we developed a clinical trial with a goal of inducing a GVH-free mixed chimeric platform for delayed cellular immunotherapy via DLI following nonmyeloablative haploidentical HSCT. This study analyzed graft outcomes in 13 patients who received in vivo and ex vivo TCD, haploidentical (HLA 1–3 Ag mismatched) HSCT for refractory hematologic malignancies (NHL, n=7; CLL, n=1; HD, n=3; CML, n=1; MDS, n=1). Conditioning consisted of MEDI-507 (siplizumab, an anti-CD2 mAb) for in vivo T cell depletion of both the host and donor, cyclophosphamide, fludarabine, thymic irradiation, and a brief course of cyclosporine. An earlier cohort of 4 patients did not receive fludarabine. All patients received ex vivo TCD of G-CSF-mobilized peripheral blood stem cells using a CD34+ cell-selection device. A median of 7.38 x 106 (3.19 x 106 to 1.49 x 107) CD 34+ cells/kg and a median of 5.24 x 104 (6.31 x 102 to 1.61 x 105) CD3+ cells/kg were given. DLI was given as early as 5 weeks in patients without evidence of GVHD. Inhibitory killer immunoglobulin-like receptor (KIR)-HLA epitope mismatches (missing ligand) in the GVH and HVG (host versus graft) directions were assessed based on HLA and KIR genotyping of patients and their donors. Natural killer (NK) cells recovered relatively early, despite the presence of circulating MEDI-507. Split lineage MC was observed in each case, with a predominance of early donor myeloid chimerism and a lower percentage of donor T cell chimerism. Four of the 13 patients eventually lost their grafts despite DLI, including 2 who were not conditioned with fludarabine. Five patients spontaneously achieved full donor chimerism (FDC) and 4 converted to FDC following DLI. The spontaneous chimerism group had significantly fewer HLA mismatches in the HVG direction. Four out of the 5 had only one mismatch, whereas the patients who had graft loss or required DLI had at least two mismatches (p=0.007 by Fisher’s exact test). This finding remained significant when the subgroup of patients not conditioned with fludarabine was excluded from the analysis (p=0.048). Spontaneous FDC was also associated with the presence of graft HLA-C group 1 (HLA-CAsn80) ligand for host NK cell inhibitory receptor KIR2DL2/3 (KIR-L match/compatible, 4 out of 4 evaluable patients) while this graft HLA ligand tended to be absent in the group that lost the graft or required DLI to achieve chimerism (KIR-L mismatch, 5 out of 8 patients) (p=0.08 by Fisher’s exact test). There was no significant relationship between donor KIR and host HLA types in the GVH direction for predicting engraftment. While these observations are preliminary given the heterogeneity of the conditioning regimens and the small number of patients, the data suggest that the lesser degree of HLA mismatching and the presence of KIR ligand compatibility in the HVG direction may influence donor engraftment following nonmyeloablative TCD haploidentical HSCT and warrant further study.

Purpose: To evaluate T cell recovery and donor chimerism following haploidentical hematopoietic cell transplantation (HCT) with a non-myeloablative conditioning approach that includes T cell depletion of host and donor and delayed DLI. Methods: Eighteen patients, 3 cohorts of 4 patients each and 1 cohort of 6 evaluable patients/10 transplanted, with chemorefractory hematologic malignancies, received related HLA 1–3 of 6, A, B, or DR antigen mismatched donor HCT after non-myeloablative conditioning with Medi-507 (anti-CD2 humanized mAb; Biotransplant, Inc.), cyclophosphamide, thymic irradiation and peritransplant cyclosporine. The patients in Protocols A received a MEDI-507 test dose of 0.1 mg/kg on Day -2 followed by 0.6 mg/kg on Days −1, 0 and +1 and transplantation of unmanipulated bone marrow. In Protocol B, the timing and dose of Medi-507 was modified. The patients in Protocol C and D received the latter Medi-507 protocol, but were given Isolex ®-selected CD34+ cells from G-CSF mobilized PBSC. Protocol D differs from Protocol C with the addition of fludarabine to more reliably achieve sustained chimerism. Donor leukocyte infusions were administered in an effort to convert mixed to full donor chimerism and to achieve a graft-versus-tumor effect. Chimerism was measured by peripheral blood microsatellite markers or by flow cytometry using HLA-specific mAbs. T cell recovery and phenotype were followed by flow cytometry. Because a high percentage of CD4 T cells post- transplant were CD25high, we performed quantitative RTPCR for Foxp3 and CTLA-4 on sorted PBMC populations. Results: T cell depletion early post-HCT was detected in all patients. There was a marked difference in the percentage of graft acceptance/loss, GVHD prevalence, and T cell phenotype related to each protocol modification. The majority (>90%) of CD4 T cells appearing in the first 100 days post-SCT were CD45RO+/CD45RA- “memory” cells and CD8 T cells were CD45RO+CD45RA-/CD62L-. In addition, a remarkably high percentage (19.5–75%, mean 38.1%) of CD4 T cells expressed high levels of CD25 in recipients of Protocols A, B, and C early post-HCT. CD25 expression decreased as T cell counts increased. By quantitative RTPCR, we found that sorted CD25highCD4 T cells expressed Foxp3 and CTLA-4, consistent with a regulatory phenotype. The addition of fludarabine in Protocol D resulted in an overall reduction in the percentage of peripheral CD4CD25high T cells compared to Protocol C at 4 weeks post-HCT (C 23.43% +/−4.7% versus D 2.1% +/− 0.9%, p Conclusions: These data suggest that the addition of fludarabine may increase the incidence of sustained and full donor chimerism. Fludarabine efficiently depleted host CD4+CD25high Foxp3+ cells, which may have a regulatory role, preventing spontaneous chimerism conversion and associated GVHD.