A clinically relevant CTLA4-Ig-based regimen induces chimerism and tolerance to heart grafts

BACKGROUND

We determined whether a nontoxic CTLA4-Ig-based conditioning regimen effected mixed chimerism and donor-specific tolerance when heart and bone marrow were transplanted simultaneously.

METHODS

Fully mismatched rat strain combinations were used. Recipients received total-body irradiation (300 centigrays), bone marrow (10(8) cells), and cardiac transplants from the donor on day 0. Subsequently, recipient animals received CTLA4-Ig (2 mg/kg, every other day, x 5 doses), tacrolimus (1 mg/kg/day; days 0 to 9), and one dose (10 mg) of antilymphocyte serum on day 10.

RESULTS

All bone marrow recipients (n = 7) developed mixed chimerism (mean = 25% +/- 9% at 1 year) and accepted cardiac allografts permanently (> 375 +/- 32 days). Recipients that received conditioning regimen but no bone marrow (n = 5) rejected donor hearts within 51 +/- 13 days (p < 0.01). Recipients that accepted heart grafts also permanently accepted (> 180 days) donor-specific skin grafts, but rapidly rejected (< 10 days) third-party skin grafts.

CONCLUSIONS

A nontoxic CTLA4-Ig-based conditioning regimen effects mixed chimerism and donor-specific tolerance when heart and bone marrow are transplanted simultaneously. This regimen may have clinical application.

Long-term survival of rat cardiac allografts by intrathymic plus portal venous injections of donor bone marrow cells and short-term tacrolimus immunosuppression

Intrathymic (IT) or portal venous (PV) injection of donor antigens has been shown to prolong organ acceptance in low responder rat strain combinations. We determined whether a combination of these strategies would prolong cardiac allograft survival in high responder combinations. Wistar Furth rats received 1 x 10(8) ACI rat bone marrow cells (BMCs) via IT, intravenous (IV), PV, IV + PV, IT + IV or IT + PV route at the time of ACI cardiac transplantation. Without tacrolimus (FK), all grafts were acutely rejected. With FK immunosuppression (1.5 mg/kg per day, I. M., days 0-4), single BMC injection did not increase graft survival beyond 93 days, whereas 70% of grafts survived indefinitely ( > 150 days) when IT and PV BMCs were combined. Animals receiving IT and PV BMCs also had less allograft vasculopathy. Thus, IT and PV injections of donor BMCs under a brief course of FK synergistically improve cardiac allograft survival.

Combined host-conditioning with CTLA4-Ig, tacrolimus, anti-lymphocyte serum, and low-dose radiation leads to stable mixed hematopoietic chimerism

The toxic dose of irradiation required to achieve stable mixed hematopoietic chimerism is the major limitation to its clinical application in transplantation and other nonmalignant conditions such as hemoglobinopathies. This study examines the additive effect of costimulatory blockage, to our previously described tacrolimus-based conditioning regimen, in further reducing the dose of total-body irradiation to achieve stable mixed chimerism in rats. Fully mismatched, 4- to 6-week-old ACI and Wistar Furth rats were used as donors and recipients, respectively. Recipients were administered CTLA4-Ig 2mg/kg/day (alternate days) in combination with tacrolimus 1 mg/kg/day (daily) from day 0 through day +10, anti-lymphocyte serum 10 mg at day +10 (single dose), and total-body irradiation ranging from 100-600 cGy, prior to bone marrow transplantation (day 0) with 100 x 10(6) of T-cell-depleted bone marrow cells. Levels of donor chimerism were determined over a period of 12 months. The short course of CTLA4-Ig, tacrolimus, and ALS led to dramatic engraftments at reduced doses of irradiation: 100% (5/5) and 93% (13/14) of the animals developed mixed chimerism at 400 cGy and 300 cGy, respectively. At 300 cGy, recipients exhibited durable, multilineage mixed chimerism at 365 days with donor cells ranging from 19-42% (mean 23.4%) with no evidence of graft-vs-host disease. These mixed chimeras exhibited in vitro (mixed lymphocyte reaction) and in vivo (skin grafts) donor-specific tolerance. This study suggests that addition of costimulatory blockade to a tacrolimus-based conditioning regimen reduces the dose of irradiation required to achieve stable multilineage chimerism in rats.

FK409, a spontaneous nitric oxide releaser, attenuates allograft vasculopathy in a rat aortic transplant model

Although systemic administration of NO donors has been shown to attenuate the development of neointimal hyperplasia in the balloon injury model, this strategy has not been tested in a model of allograft vasculopathy. In this study, we investigated the effect of FK409, a spontaneous NO releaser, on the development of allograft vasculopathy, using a rat aortic transplant model. Thoracic aortas from ACI rats were transplanted heterotopically into the abdominal aorta of Wistar-Furth rats. Postoperatively, recipients received FK409 orally every 8 hours from the day of transplantation to the time of euthanization. Morphometric and immunohistochemical analyses were performed on the aortic grafts 8 weeks after transplantation. Control allografts showed severe neointimal hyperplasia, which consists mainly of alpha-actin-containing vascular smooth muscle cells. The FK409-treated allografts showed a dose-dependent reduction (statistically significant compared with the control) in the neointimal thickness as the dose increased from 1 to 10 mg/kg (thrice per day). However, there was no significant difference in the neointimal thickness between groups treated with 10 and with 20 mg/kg. FK409 treatment (10 mg/kg) caused a significant decrease in DNA synthesis (5-bromo-2-deoxyuridine [BrdU] uptake), an increase in DNA fragmentation (terminal deoxynucleotidyltransferase-mediated uridine nick-end labeling [TUNEL]), and upregulation of Fas expression, in the neointimal vascular smooth muscle cells. These data suggest that FK409 attenuates the allograft vasculopathy in a rat aortic transplant model.

Induction of hyperacute rejection of skin allografts by CD8+ lymphocytes

BACKGROUND

Second-set rejection is generally regarded as a phenomenon mainly mediated by humoral cytotoxic antibodies, although a few discordant data have been presented. In the reported experiments, we have taken advantage of the absence of production of specific cytotoxic alloantibodies contrasting with the normal development of transplantation cellular immunity, in two murine models: chimeric mice and RAG mice.

METHODS

Chimeras (BALB/c-->CBA) were obtained by transplantation of 2x10(7) fetal liver cells from BALB/c (H-2d) mice to lethally irradiated CBA (H-2k) mice. After hyperimmunization with third-party C57/ BL6 (B6) (H-2b) skin transplants and with injections of 2x10(7) B6 spleen cells, antibody production, and skin graft survival were analyzed. To identify further the factors or cells responsible for accelerated rejection of B6 skin transplants in hyperimmunized chimeras, transfer experiments were carried out involving the injection of serum, whole spleen cells, spleen T cells, spleen CD8+ T cells or spleen CD4+ T cells from chimeras into BALB/c mice that had received 6 Gy irradiation. The recipient mice were then grafted with B6 skin. Similarly, the immunodeficient RAG mice were used to construct a model of recipient animals with anti-H-2d hyperimmunized B6 T cells in the total absence of antibody.

RESULTS

In chimeras, anti-B6 cytotoxic antibodies were not detectable in any of hyperimmunized chimeric mice, yet accelerated rejection of B6 skin transplant occurred: a graft survival of 8.6+/-0.5 days (d), comparable to 8.9+/-0.8 d survival in CBA control mice subjected to the same hyperimmunization procedure, and significantly shorter than that in nonhyperimmunized (BALB/c-->CBA) chimeras (11.6+/-0.5 d) or in non-hyperimmunized CBA control mice (12.1+/-0.6 d). High titers of anti-B6 cytotoxic antibodies were present in the serum of hyperimmunized CBA control mice. In transfer experiments, the graft survival was over 14 d in mice treated with irradiation alone, with irradiation + serum or with irradiation + CD4+ T cells. It was significantly shorter in mice treated with irradiation + whole spleen cells, with irradiation + T cells or with irradiation + CD8+ T cells (8.9+/-0.8 d). Similarly, in immunodeficient RAG mice, reconstitution of the T cell compartment with T cells from hyperimmunized B6 mice led to accelerated rejection of BALB/c skin allografts (11.4+/-1.1 d vs. 18.8+/-0.8 d when T cells were provided by nonimmunized mice). In a second transfer of cells from these reconstituted RAG mice into naive RAG mice, CD8+ T cells were shown to induce accelerated rejection of skin allografts (12.0+/-0.6 d) whereas CD4+ T cells were much less efficient (16.5+/-0.1 d).

CONCLUSION

These data indicate that T cells, and especially the CD8+ subset, can be responsible for second-set rejection in the absence of anti-donor antibodies in chimeric and RAG mouse models. These sensitized CD8+ T cells are also likely to play an important role in normal mice, in addition to that of cytotoxic antibodies.

Prevention of chronic rejection in mouse aortic allografts by combined treatment with CTLA4-Ig and anti-CD40 ligand monoclonal antibody

BACKGROUND

In this study, using a murine model of aortic allotransplantation, the role of blockade of signaling through CD28/B7 and CD40/CD40 ligand costimulatory pathways in the evolvement of posttransplant vasculopathy was examined.

METHODS

Aortic allografts were transplanted across C57BL/1OJ (H2b)-->C3H (H2k) strain combinations. Transient or more stable blockade of second signaling was achieved by either a single injection or multiple injections of CTLA4-Ig fusion protein (200 microg/dose i.p.) and/or anti-CD40 ligand (CD40L) monoclonal antibody (250 microg i.m.). At day 30 after transplantation, the grafts were harvested for histopathological and immunohistochemical examination.

RESULTS

Similar to allografts of untreated animals, aortic allografts obtained from recipients treated with either CTLA4-Ig or anti-CD40L monoclonal antibody alone exhibited marked narrowing of the lumen primarily due to concentric intimal thickening caused by proliferation of alpha-smooth muscle actin-positive cells. Contemporaneous treatment, however, with either a single injection or multiple injections of CTLA4-Ig and anti-CD40L monoclonal antibody resulted in marked diminution of intimal thickening. Interestingly, concurrent prolonged inhibition of CD28/B7 and CD40/CD40L pathways resulted in complete abrogation of the development of posttransplant arteriopathy.

CONCLUSION

These data suggest that a more stable disruption of signaling through costimulatory pathways may be required to obviate the development of posttransplant vasculopathy.

Evidence for the presence of multilineage chimerism and progenitors of donor dendritic cells in the peripheral blood of bone marrow-augmented organ transplant recipients

We have postulated that the donor leukocyte microchimerism plays a seminal role in the acceptance of allografts by inducing and perpetuating variable degree of donor-specific nonreactivity in long-surviving organ recipients. Limited information is available, however, concerning the phenotype and function of these chimeric cells in humans. The unequivocal presence of donor dendritic cells (DCs), a prominent lineage in the microchimerism observed in rodents and clinical organ recipients, was difficult to demonstrate in bone marrow (BM)-augmented organ transplant recipients. This enigma was resolved by the recent description of a method for propagating circulating human DCs from their progenitors by culture in a medium enriched with granulocyte-macrophage colony-stimulating factor and interleukin 4, a condition known to inhibit outgrowth of monocytes, thus providing a selective growth advantage to committed progenitors of the myeloid lineage. Cells from BM-augmented organ recipients and normal control subjects harvested from 12- to 14-day cultures exhibited dendritic morphology and potent allostimulatory capacity. Using appropriate primers, the presence of donor DNA was verified by polymerase chain reaction within the lineage(null)/class II(bright) sorted DC. Phenotypic analysis of cultured DCs from BM-augmented patients, unlike that of controls, exhibited a marked down-regulation of B7-1 (CD80) while retaining normal levels of expression of B7-2 (CD86) cell surface molecules. The presence of donor DNA was also confirmed by polymerase chain reaction in individually sorted lineage+ (T, B, and NK) cells and macrophages, suggesting that the chimerism in BM-augmented patients is multilineage. The presence of progenitors of donor DCs in the peripheral blood of BM-augmented patients further substantiates the already convincing evidence of stem cell engraftment.

Abrogation of chronic rejection in a murine model of aortic allotransplantation by prior induction of donor-specific tolerance

Aortic allotransplantation in mice has been well established as a model of choice to study the evolvement of chronic rejection, the etiopathology of which is believed to be that of immune origin. This has prompted the postulation that prior induction of donor-specific tolerance would attenuate or abrogate the underlying events that culminate in posttransplant arteriosclerosis. To study the effects of donor-specific tolerance on chronic rejection, we performed orthotopic liver transplantation without immunosuppression in mice 30 days before aortic allotransplantation across C57Bl/ 10J (H2b)-->C3H (H2k) strain combinations (group III). Aortic allografting in syngeneic (group I; C3H-->C3H) and allogeneic (group II, C57Bl/10J-->C3H) animals served as controls. No morphological changes were evidenced in the transplanted aortas in group I animals. Contrarily, aortic allografts in group II animals underwent a self-limiting acute cellular rejection, which resolved completely and was succeeded by day 30 after transplantation by histopathological changes pathognomonic of chronic rejection. There was evidence for diffuse myointimal thickening, progressive concentric luminal narrowing, and patchy destruction of internal elastic membranes resulting in massive vascular obliteration by day 120 after transplantation. It was of interest that no arteriosclerotic changes were observed for the duration of follow-up (up to 120 days after transplantation) in transplanted aortas (liver donor-type) harvested from animals in group III. However, vasculopathy was prominent in third-party aortic grafts transplanted into tolerant recipients. Taken together, these data suggest that prior induction of tolerance abrogates the development of chronic rejection; this protection seems to be donor specific.

Reversal of chimera donor-to-host tolerance in a tolerogen-free environment. Evidence of a nondeletional mechanism

Clonal deletion of self-antigen-reactive T cells is known to be a dominant mechanism for tolerance induction in animals with a normal immune system. This phenomenon is mediated intrathymically by macrophages and dendritic cells. Some recent data have shown that tolerance to antigens expressed on radio-resistant thymic stromal cells results in clonal anergy. This report considers tolerance to host antigens in murine H-2-incompatible chimeras (H-2d-->H-2k) where thymic stromal cells remained of the host origin while virtually all lymphoid cells were replaced by donor H-2d cells. To assess the mechanism responsible for donor-to-host tolerance induction and the possible role of tolerogens in this process, we transferred (H-2d-->H-2k) chimeric lymphoid cells into lethally irradiated H-2d mice (a murine environment free of host H-2k antigens). Engrafted chimeric cells restored immunocompetence of secondary recipients without inducing a graft-versus-host reaction. H-2k skin test-grafts performed four weeks later were acutely rejected (median survival time = 9 days versus 11 days in controls). These results indicate that (A) donor-type lymphocytes reactive to host antigens in (H-2d-->H-2k) chimeras are not deleted during tolerance induction; (B) the continuous presence of the H-2k tolerogens appears to be necessary for the maintenance of nonreactivity to these tolerogens; (C) the anamnestic-like response to the H-2k skin grafts suggests that, during tolerance induction, anti-host (anti-H-2k) memory cells developed, an interpretation consistent with the concept that tolerance can result from a powerful immune response.

Comparison of fresh, cryopreserved and cultured haematopoietic stem cells from fetal liver

The cryopreservation and long-term culture of fetal liver (FL) cells may offer a ready source of haematopoietic stem cells (HSC). To compare these two techniques, an H-2-incompatible murine model was used, in conditions close to those of stem cell transplantation in humans. After cryopreservation, the recovery of colony-forming unit-culture (CFU-C) and of 14-day colony-forming unit-spleen (CFU-S) was 55.5% and 23%, respectively, compared with fresh cells. The rate of engraftment of donor cells was very high in mice reconstituted with either cryopreserved or fresh cells and the resulting chimerism was virtually complete in both cases. Functionally, both groups showed a significant humoral response to sheep red blood cells. Chimeric mice obtained by injection of cryopreserved cells were able to reject third-party SJL mouse (H-2s) skin grafts (11.4 +/- 1.6 days); at the same time, they specifically tolerated skin grafts from BDF1 (H-2b x H-2d) donor mice, indicating that cryopreserved FL cells could induce both tolerance to donor antigens and restore normal immunological responses to third-party alloantigens. Following 4-week cultures, consistent losses in the total number of CFU-C and CFU-S (2.5% and 8.6% yield, respectively) were observed. Cultured FL failed to protect the animals from the lethal effects of irradiation, due to insufficient reconstitution. These results favour the possible use of cryopreserved FL in clinical settings. At present, however, techniques to improve FL cultures and their efficiency for in vivo reconstitution are required.

Accelerated rejection of H-2-incompatible skin allografts in the absence of specific cytotoxic antibodies

Second-set rejection is generally considered to be mediated by cytotoxic humoral antibodies. A few discordant data have been reported, however. To address this question, we have taken advantage of a model in which specific cytotoxic alloantibodies are not produced although transplantation cellular immunity develops. Following a transplant of allogeneic stem cells from fetal liver, chimeric mice (BALB/c-->CBA--i.e., H-2d-->H-2k) were obtained; virtually all peripheral blood lymphocytes and spleen cells were of BALB/c donor origin. Anti-sheep red blood cell humoral response was present, although significantly lower in chimeras than in controls. These allochimeras were hyperimmunized by skin grafts and injections of spleen cells. The survival of skin grafts and the production of antibodies were then analyzed. When hyperimmunized against a third party, B6 (H-2b), chimeric mice were not able to raise a detectable humoral response involving anti-B6 cytotoxic antibodies, yet they rejected B6 skin allografts in an accelerated fashion (8.44 +/- 0.17 days). Control CBA mice rejecting second-set B6 skin grafts within the same delay developed high-titer, specific cytotoxic antisera (mean titer = 140). These data show that cytotoxic allospecific antibodies are not indispensable in the development of second-set accelerated rejection of skin allografts.

Survey of experimental data on fetal liver transplantation

Fetal liver transplantation has been shown to induce hematological and immunological reconstitution in irradiated rodents, dogs, horses, and sheep. Engraftment and reconstitution without GvHD has been readily obtained using histocompatible donors. When mismatched fetal donors were used, a comparatively larger number of donor cells was required, in addition to pre-treatment of host with higher doses of irradiation or irradiation plus chemotherapy. Stem cell suspensions devoid of any T lymphocyte can be transplanted across major histocompatibility barrier without inducing overt GvHD. The transplanted animals become tolerant to both donor and host grafts.