Specific immunosuppression by postoperative infusion of allogeneic spleen cells: requirement of donor major histocompatibility complex expression and graft-versus-host reactivity

Immunological Consequences of In Utero Exposure to Foreign Antigens

Immunologic tolerance refers to a state of immune nonreactivity specific to particular antigens as an important issue in the field of transplantation and the management of autoimmune diseases. Tolerance conceptually originated from Owen’s observation of blood cell sharing in twin calves. Owen’s conceptual framework subsequently constituted the backbone of Medawar’s “actively acquired tolerance” as the major tenet of modern immunology. Based upon this knowledge, the delivery of genetically distinct hematopoietic stem cells into pre-immune fetuses represented a novel and unique approach to their engraftment without the requirement of myeloablation or immunosuppression. It might also make fetal recipients commit donor alloantigens to memory of their patterns as “self” so as to create a state of donor-specific tolerance. Over the years, the effort made experimentally or clinically toward in utero marrow transplantation could not reliably yield sufficient hematopoietic chimerism for curing candidate diseases as anticipated, nor did allogeneic graft tolerance universally develop as envisaged by Medawar following in utero exposure to various forms of alloantigens from exosomes, lymphocytes or marrow cells. Enduring graft tolerance was only conditional on a state of significant hematopoietic chimerism conferred by marrow inocula. Notably, fetal exposure to ovalbumin, oncoprotein and microbial antigens did not elicit immune tolerance, but instead triggered an event of sensitization to the antigens inoculated. These fetal immunogenic events might be clinically relevant to prenatal imprinting of atopy, immune surveillance against developmental tumorigenesis, and prenatal immunization against infectious diseases. Briefly, the immunological consequences of fetal exposure to foreign antigens could be tolerogenic or immunogenic, relying upon the type or nature of antigens introduced. Thus, the classical school of “actively acquired tolerance” might oversimplify the interactions between developing fetal immune system and antigens. Such interactions might rely upon fetal macrophages, which showed up earlier than lymphocytes and were competent to phagocytose foreign antigens so as to bridge toward antigen-specific adaptive immunity later on in life. Thus, innate fetal macrophages may be the potential basis for exploring how the immunological outcome of fetal exposure to foreign antigens is determined to improve the likelihood and reliability of manipulating fetal immune system toward tolerization or immunization to antigens.

Liver transplant tolerance and its application to the clinic: can we exploit the high dose effect?

The tolerogenic properties of the liver have long been recognised, especially in regard to transplantation. Spontaneous acceptance of liver grafts occurs in a number of experimental models and also in a proportion of clinical transplant recipients. Liver graft acceptance results from donor antigen-specific tolerance, demonstrated by the extension of tolerance to other grafts of donor origin. A number of factors have been proposed to be involved in liver transplant tolerance induction, including the release of soluble major histocompatibility (MHC) molecules from the liver, its complement of immunosuppressive donor leucocytes, and the ability of hepatocytes to directly interact with and destroy antigen-specific T cells. The large tissue mass of the liver has also been suggested to act as a cytokine sink, with the potential to exhaust the immune response. In this review, we outline the growing body of evidence, from experimental models and clinical transplantation, which supports a role for large tissue mass and high antigen dose in the induction of tolerance. We also discuss a novel gene therapy approach to exploit this dose effect and induce antigen-specific tolerance robust enough to overcome a primed T cell memory response.

Approaching the promise of operational tolerance in clinical transplantation

Long-term acceptance of transplanted organs without requirement for indefinite immunosuppression remains the ultimate goal of transplant clinicians and scientists. This clinical state of allograft acceptance termed "operational tolerance" has been elusive in routine practice. However, there are published reports of recipients where immunosuppression has been discontinued, by intention or patient noncompliance, in which the outcome is a nondestructive immune response and normal function. The question now arises how clinical operational tolerance might be achieved in the majority of recipients. This review provides an overview of current approaches to achieve operational tolerance, including the use of donor bone marrow and depletion of recipient T cells and the resistance of liver transplants to rejection. It also describes the key role of clinical immune monitoring and future approaches to tolerance induction including inhibition of T-cell signaling, manipulation of costimulatory pathways, and expansion of regulatory T cells. The principles of these experimental approaches may ultimately be extended to provide safe and effective control of transplant rejection and induction of clinical operational tolerance.

Spontaneous acceptance of mouse kidney allografts is associated with increased Foxp3 expression and differences in the B and T cell compartments

Spontaneous acceptance of organ allografts can identify novel mechanisms of drug-free transplantation tolerance. Spontaneous acceptance occurs in both mouse kidney transplants and rat liver transplants however the early immune processes of mouse kidney acceptance have not been studied. Acceptance of C57BL/6 strain kidney allografts in fully MHC-incompatible B10.BR recipients was compared with rejection (REJ) of heart allografts in the same strain combination. Graft infiltrate and antibody deposition were examined by immunohistochemical staining. Expression of mRNA was measured by quantitative real-time PCR. Apoptosis was examined by TUNEL staining. The majority of kidney allografts were accepted long-term and induced tolerance (TOL) of donor-strain skin grafts, showing that acceptance was not due to immune ignorance. There was an extensive infiltrate of T cells in the TOL kidney that exceeded the level in REJ hearts but subsequently declined. The main differences were deposition of IgG2a antibody in REJ that was absent in TOL, more B cells infiltrating TOL kidneys and a progressive increase in the ratio of CD8:CD4 cells during rejection. There was also significantly greater Foxp3 mRNA expression in TOL. Kidneys from RAG-/- donors were accepted, showing that donor lymphocytes were not necessary for acceptance. Neutralising antibodies to TGF-β administered from day 0 to day 6 did not prevent TOL. On the basis of cytokine expression and apoptosis there was no evidence for immune deviation or deletion as mechanisms of acceptance. In accord with the findings of spontaneous acceptance of liver allografts in rats, the main difference between mouse kidney TOL and heart REJ was in the B cell compartment. The major difference to rat liver allograft acceptance was that apoptosis of infiltrate did not appear to play a role. Instead, increased Foxp3 expression in TOL kidneys implies that regulatory T cells might be important.

Characterization of tolerance induction through prenatal marrow transplantation: the requirement for a threshold level of chimerism to establish rather than maintain postnatal skin tolerance

Hematopoietic chimerism resulting from prenatal marrow transplantation does not consistently result in allotolerance for unidentified causes. In a C57BL/6-into-FVB/N murine model, we transplanted T-cell-depleted adult marrow on gestational day 14 to elucidate the immunological significance of chimerism towards postnatal tolerance. Postnatally, chimerism was examined by flow cytometry, and tolerance by skin transplantation and mixed lymphocyte reaction. Regulatory T cells were quantified by FoxP3 expression. Peripheral chimerism linearly related to thymic chimerism, and predicted the degree of graft acceptance with levels >3% at skin placement, yielding consistent skin tolerance. Low- and high-level chimeras had lower intrathymic CD3(high) expression than microchimeras or untransplanted mice. Regardless of the skin tolerance status in mixed chimeras, donor-specific alloreactivity by lymphocytes was suppressed but could be partially restored by exogenous interleukin-2. Recipients that lost peripheral chimerism did not accept donor skin unless prior donor skin had engrafted at sufficient chimerism levels, suggesting that complete tolerance can develop as a consequence of chimerism-related immunosuppression of host lymphocytes and the tolerogenic effects of donor skin. Thus, hematopoietic chimerism exerted immunomodulatory effects on the induction phase of allograft tolerance. Once established, skin tolerance did not fade away along with spontaneous regression of peripheral and tissue chimerism, as well as removal of engrafted donor skin. Neither did it break following in vivo depletion of increased regulatory T cells, and subcutaneous interleukin-2 injection beneath the engrafted donor skin. Those observations indicate that the maintenance of skin tolerance is multifaceted, neither solely dependent upon hematopoietic chimerism and engrafted donor skin nor on the effects of regulatory T cells or clonal anergy. We conclude that hematopoietic chimerism generated by in utero hematopoietic stem cell transplantation is critical to establish rather than maintain postnatal skin tolerance. Therefore, the diminution of hematopoietic chimerism below a threshold level does not nullify an existing tolerance state, but lessens the chance of enabling complete tolerance.

Chimerism and bone marrow based therapies in transplantation

Composite tissue allotransplantation holds a great potential for providing increased knowledge of anatomy and microsurgical experience for life-enhancing reconstructions. Many transplant cases around the world have made this a clinical reality at the present time. Composite tissue allotransplants contain multiple tissue types, including bone, muscle, vessels, nerves, skin, and immune cells and bear a huge antigenic load. Although immunosuppressive drugs are applied successfully to prevent allograft rejection, their side effects pose a barrier to worldwide use. Bone marrow therapy in many tolerance induction protocols, therefore, provides a guide to reaching the target of permanent immunotolerance. Multiple studies suggest that bone marrow is immunomodulatory and may facilitate allograft acceptance. In this review, bone marrow based therapy protocols of clinical and experimental models are presented in two major categories: solid organ and composite tissue transplantation.

Opposite effects of donor apoptotic versus necrotic splenocytes on splenic allograft tolerance

BACKGROUND

Apoptotic cells have immunosuppressive activity, whereas necrotic cells activate immune response, indicating they might have different effects on immune rejection against splenic allografts. The aim of this study was to determine whether administration of apoptotic or necrotic splenocytes of donor origin could impact the acute rejection of splenic allografts.

MATERIALS AND METHODS

Apoptotic or necrotic splenocytes derived from donor rats were induced by irradiation or freeze thaw, respectively. Heterotopic vascularized spleen transplantation was performed from Wistar-Furth (donor) to Sprague-Dawley (recipient) rats, and splenocytes were intravenously injected into the recipients. At different time points, the recipients were sacrificed and the splenic allografts underwent histological examination. The interferon-gamma (IFN-gamma) and transforming growth factor-beta1 (TGF-beta1) in sera, spleens of recipients, and donor splenocytes before administration were measured. Mixed leukocyte reaction (MLR) was detected with recipient splenocytes as effectors and donor splenocytes as stimulators.

RESULTS

Exposure to gamma-irradiation at dose of 10,000 rad caused over 80% splenocytes to become apoptotic. The levels of TGF-beta1 released by apoptotic splenocytes in vitro were significantly higher than that by untreated splenocytes, whereas there was almost no TGF-beta1 detected in necrotic splenocytes culture medium. Administration of apoptotic splenocytes significantly attenuated acute rejection of splenic allografts, evidenced by less severe splenic histological alteration and reduction of histological scores compared with control; whereas necrotic splenocytes exacerbated the acute rejection. Apoptotic splenocytes inhibited production of IFN-gamma but increased the levels of TGF-beta1, whereas necrotic splenocytes showed opposite activity in production of those cytokines. Administration of apoptotic splenocytes inhibited MLR, and necrotic splenocytes promoted MLR.

CONCLUSIONS

The apoptotic and necrotic splenocytes exhibited opposite effects on acute rejection against splenic allografts, and IFN-gamma and TGF-beta1 have been involved in the effects.

Donor Leukocytes Combined With Delayed Immunosupressive Drug Therapy Prolong Limb Allograft Survival

Donor leukocytes administered at the time of transplantation may prolong organ allograft survival. Delayed administration of calcineurin inhibitors, such as FK506 or cyclosporine, may enhance their efficacy. Herein the effectiveness of this strategy to promote limb transplant survival was investigated in the strong histocompatibility barrier of Brown-Norway donor to Lewis recipients. Donor leukocytes (6 x 10(7) intravenously) were injected on the day of transplantation followed on day 1 to 14 with mycophenolate mofetil (MMF; 15 mg/kg/d) and prednisone, (0.5 mg/kg/d) which were then tapered by 20% each week and stopped at week 7. Administration of of FK506 (2 mg/kg/d) was started on day 4 and continued for 8 weeks, then tapered for 4 weeks to a maintenance dose of 0.8 mg/kg/d, which was continued for 12 weeks (group A; n = 8). A control group (n = 8) underwent identical treatment save for donor leukocyte injection but rather commencement of FK506 on day 1. Rejection was common during FK506 tapering in both groups. However group A showed a significantly later onset, a shorter period for reversal of the first rejection, and a significantly lower dosage of FK506 at the time of rejection. After the completion of immunosuppression, rejection occurred significantly later in group A than the control group with one animal surviving without immunosuppression on day 344. This is the first trial of a donor leukocyte injection combined with delayed FK506 administration in limb transplantation, which suggested that it could produce a modest but significant improvement in outcome.

Induction of long-term graft acceptance by a combination treatment of donor splenocytes and CTLA4Ig in a high responder rat liver transplantation model

The CTLA4Ig has led to an improved survival rate in various allograft transplantation models. We investigated in a high responder rat model (Dark Agouti to Lewis) of orthotopic liver transplantation (ORLT), whether an additional adoptive cell transfer can enhance the effect of CTLA4Ig. After transplantation, recipients (n = 13/group) were treated with donor or third-party splenocytes alone or in combination with CTLA4Ig. Administration of splenocytes alone had no significant effect on survival (median 13 days, range 9-14) compared with untreated controls (median 10 days, range 8-12). CTLA4Ig monotherapy prolonged survival to a median of 30 days (range 11-150) but resulted in long-term graft rejection. The additional administration of third-party splenocytes showed no significant improvement over CTLA4Ig monotherapy. Only the combination of donor splenocytes with CTLA4Ig led to long-term graft acceptance (>150 days) without clinical and/or histological signs of rejection. A higher rate of apoptosis could be detected in livers at early time-points in long-term survivors receiving CTLA4Ig and donor splenocytes. Analysis of cytokine mRNA expression revealed a decrease of interleukin-2 at early time-points in all groups receiving CTLA4Ig; whereas, interferon-gamma was increased in long-term survivors receiving CTLA4Ig and donor cells or donor cells alone. The combination of CTLA4Ig and donor derived splenocytes is potent to induce long-term survival and graft acceptance. The mechanisms appear to involve the induction of an early inflammatory impulse and apoptosis.

Combined donor leucocyte administration and immunosuppressive drug treatment for survival of rat heart allografts

BACKGROUND

Donor leucocytes (DL) play an important role in rat liver transplant tolerance and their postoperative administration can convert rejection to tolerance. They appear to induce early activation, altered patterns of infiltration and death of recipient alloreactive T cells. The ability of immunosuppressive drugs to combine with DL administration was examined in a rat heart transplant model.

METHODS

Immediately after PVG to DA heterotopic heart transplantation, 6 x 10(7) spleen DL were injected. Cyclosporine A (CsA), 1.5 mg/kg/day, or methotrexate (MTX), 0.1 or 0.2 mg/kg/day, were given from day (d) 0 to d4 (early) or from d3 to d7 (delayed). Castanospermine (CAST) was administered from d0 to d7 at 100 or 300 mg/kg/day. In a separate experiment, transplanted hearts and recipient spleens were collected from treatment groups for analysis of infiltrate and cytokine mRNA expression.

RESULTS

Delayed treatment with CsA or early treatment with MTX but not CAST combined with DL to result in prolonged graft survival. Recipients treated with DL and delayed CsA had a reduced level of intra-graft interleukin (IL)-2, interferon (IFN)-gamma, IL-4, and IL-4R mRNA expression and reduced infiltrate compared to DL alone. Early MTX plus DL led to almost complete inhibition of all markers of inflammation during treatment followed by a rapid increase after cessation. In combination with DL, CsA was more effective than MTX for induction of donor-specific tolerance at the dose and administration regimens tested.

CONCLUSIONS

Delayed CsA or early MTX combine with DL to prolong heart allograft survival. Early and extensive inhibition of rejection by MTX was less effective than delayed and partial inhibition of the response by CsA for induction of transplant tolerance.

Simultaneous administration of a low-dose mixture of donor bone marrow cells and splenocytes plus adenovirus containing the CTLA4Ig gene result in stable mixed chimerism and long-term survival of cardiac allograft in rats

T-cell costimulatory blockade combined with donor bone marrow transfusion may induce mixed chimerism, rendering robust tolerance in transplanted organs and cells. However, most protocols entail high doses of donor bone marrow cells (BMCs) or repeated administration of costly agents that block costimulatory pathways, thus delaying clinical development. To circumvent these shortcomings, we developed a strategy in which the dosage of donor BMCs was reduced but compensated by donor splenocytes (SPLCs). Furthermore, repeated administration of costly agents was replaced with a single injection of adenovirus expressing a gene of interest. In rat cardiac transplantation models, cardiac allografts from DA (RT-1a) rats were transplanted heterotopically into the abdomen of LEW (RT-11) recipient rats. Immediately after cardiac transplantation, an adenovirus vector (AdCTLA4Ig; 5 x 10(9) plaque-forming units) containing the gene for CTLA4Ig was administered to recipients (n = 6) simultaneously with a low dose of donor BMCs (1 x 10(8)/rat) and SPLCs (5 x 10(7)/rat) via the portal vein. The treated LEW recipient rats developed long-lasting mixed chimerism (>10% at >100 days) and exhibited long-term cardiac allografts (mean survival time of > 200 days) compared with control recipients. Moreover, recipients displaying long-lasting mixed chimerism accepted subsequent donor skin allografts while promptly rejecting third-party skin allografts. These results suggest that blockade of the CD28-B7 pathway, using adenovirus-mediated CTLA4Ig gene transfer, in concert with a low dosage of donor BMCs and SPLCs, may represent a feasible strategy to induce stable mixed chimerism and permit long-term survival of cardiac allografts.

Efficacy of donor splenocytes mixed with bone marrow cells for induction of tolerance in sublethally irradiated mice

BACKGROUND

High dose of bone marrow cells (BMCs) has been reported to be essential to establish donor-specific tolerance. In clinical settings, a large quantity of BMCs is very difficult to be obtained. Our previous report demonstrated that even a low dose of BMCs could establish donor-specific tolerance if mixed with splenocytes (SPLCs). In the present study, various components of SPLCs were purified or removed and were investigated their contribution for enhancement of bone marrow engraftment leading to donor-specific tolerance in sublethally irradiated mice.

METHODS

Sublethally irradiated C57BL/6 recipient mice were intravenously injected 3 x 10(6) BMCs mixed with various components and various numbers of SPLCs harvested from BALB/c donor mice. One week after injection, skin grafting was performed. The degree of chimerism in peripheral blood lymphocytes (PBLs) and in SPLCs was analyzed by FACS 3 months after transplantation.

RESULTS

Recipients receiving 3 X 106 BMCs mixed with 10 x 10(6) T cell-enriched SPLCs established chimerism. Recipients receiving BMCs mixed with macrophage-depleted SPLCs also showed chimeirism and donor-specific tolerance. B cell-enriched SPLCs did not help small dose of BMCs to establish chimerism. Irradiated SPLCs were not effective to induce tolerance even with additional infusion to recipients.

CONCLUSIONS

Active effects of splenic T cells were more important to help engraftment of small dose of BMCs than B cells, but the interaction between T and B cells might play some roles to enhance BMC engraftment. Splenic macrophages or dendritic cells might have some adverse effects against tolerance induction. Fatal graft-versus-host disease (GVHD) might be avoided by depleting adherent cells from SPLCs, so macrophages or dendritic cells were also considered as key components to induce donor-specific tolerance and prevent GVHD in this model.

Delayed low-level calcineurin inhibition promotes allospecific tolerance induction by posttransplantation donor leukocyte infusion

BACKGROUND

Donor lymphocytes infused after organ transplantation can have strong immunoregulatory effects. Application of such protocols for transplant tolerance induction in a clinical setting will, however, require combination of specific immunomodulatory strategies with nonspecific immunosuppressive medication for safety reasons. The aim of this study was to analyze the effects of immunosuppressive treatment on tolerance induction protocols by posttransplantation donor lymphocyte infusion.

METHODS

The interaction of postoperative donor leukocyte infusion with different types, dosage, and timing of immunosuppressive drugs were studied in a rat model of heart transplantation.

RESULTS

Tolerance could be achieved if donor cell infusion was combined with delayed, but not immediate, low-dose cyclosporine treatment, and this was associated with activation and apoptosis of host lymphocytes. In contrast, combinations with an antibody against the interleukin 2 receptor led to long-term graft survival but severe chronic rejection, and combinations with high-dose cyclosporine or sirolimus led to acute rejection.

CONCLUSIONS

Postoperative donor leukocyte infusion is a potential way for tolerance induction, but the type, dose, and timing of medication are highly critical for its efficacy.

Postoperative administration of donor B cells induces rat kidney allograft acceptance: lack of association with Th2 cytokine expression in long-term accepted grafts

BACKGROUND

Although donor leukocytes are only thought to prolong survival when administered before transplantation, recent evidence shows that they are effective at transplantation. This study aims to identify the leukocyte subset that is most active in prolonging kidney allograft survival and examine the cytokine expression in long-term acceptance.

METHODS

PVG rat kidneys were transplanted to completely MHC class I and class II-mismatched DA recipients. Donor B cells or T cells, purified by negative selection, were injected i.v. at the time of transplantation. Expression of interleukin (IL)-2, IL-4, IL-10, interferon (IFN)-gamma, and transforming growth factor-beta mRNA was measured by quantitative real-time polymerase chain reaction (PCR). Immunohistochemical analysis and terminal deoxynucleotide transferase-mediated dUTP nick-end labelling (TUNEL) staining was used to identify infiltrating cells and apoptotic cells, respectively, in sections of kidney allografts.

RESULTS

Median kidney graft survival time (MST) of B cell-treated animals (n=5) was >300 days, compared with 7 days in untreated animals (n=7) (P=0.003), whereas animals treated with the same number of T cells (n=6) had a MST of 17 days (P=0.1 vs. untreated, P=0.03 vs. B cell-treated). Examination of the long-term (>300 days) accepted grafts from B cell-treated recipients showed little evidence of kidney damage but a moderate perivascular infiltrate consisting of T and B cells. This infiltrate seemed to be quiescent because there was no detectable expression of IL-2 receptors or of apoptotic cells. It produced little or no cytokine mRNA, because expression in the long-term accepted grafts was similar to levels in normal kidneys or syngeneic transplants. There was a marked increase of cytokine mRNA early after transplantation in both leukocyte-treated and untreated grafts, with more rapid appearance of IFN-gamma and IL-10 in leukocyte-treated grafts.

CONCLUSIONS

Donor B cells efficiently induce long-term acceptance of transplanted kidneys in a fully MHC-mismatched rat model when administered at transplantation, by a mechanism that seems to be independent of Th2 cytokine expression within the long-term accepted graft.

Cell-based therapy approaches using dying cells: from tumour immunotherapy to transplantation tolerance induction

Cell-based therapies are promising approaches to treat uncontrolled pathologies, such as tumours. Apoptotic tumour cells have recently been proposed as a source of tumour-associated antigens to stimulate an efficient immune response. However, a complex relationship exists between apoptosis and the immune system. In this review, the different factors that may influence immune responses against apoptotic cells are detailed and discussed in the light of recent publications. These factors include the nature of the phagocytes and the receptors involved in apoptotic cell uptake, as well as the environment in which cells are dying. A possible distinction between apoptosis and necrosis by immune system sentinels adds a further level of complexity. The potential use of the immunomodulatory properties associated with apoptosis to favour engraftment and induce tolerance in transplantation is then discussed. In conclusion, this review will suggest appropriate conditions to efficiently and safely use apoptotic cells as a new cell therapy product.

Spontaneous acceptance of liver transplants in rodents: evidence that liver leucocytes induce recipient T-cell death by neglect

In many animal models transplanted livers are not rejected, even when there is a complete MHC mismatch between the donor and recipient and the recipient is not immunosuppressed. This distinguishes liver transplants from other organs, such as kidneys and hearts, which are rapidly rejected in mismatched individuals. Acceptance of transplanted livers in a rat model is not due to the absence of an immune response to the liver and there is a rapid, abortive response that is ultimately exhausted. Donor leucocytes transferred with the liver appear to be responsible for both liver acceptance and the abortive activation of the recipient's T cells. The immune mechanism of liver transplant acceptance appears to be due to 'death by neglect' in which T cells are activated to express IL-2 and IFN-gamma mRNA in the recipient lymphoid tissues, but not at adequate levels within the graft. Subsequently the activated T cells die leading to specific clonal deletion of liver donor-reactive T cells. These findings have important implications for liver transplant patients as immunosuppressive drugs that are given to prevent rejection can also interfere with this form of tolerance. In addition, it might be possible to modify the immunosuppressive drug treatment of transplant patients to promote the process of death by neglect of recipient alloreactive T cells.

Posttransplant administration of donor leukocytes induces long-term acceptance of kidney or liver transplants by an activation-associated immune mechanism

Donor leukocytes play a dual role in rejection and acceptance of transplanted organs. They provide the major stimulus for rejection, and their removal from the transplanted organ prolongs its survival. Paradoxically, administration of donor leukocytes also prolongs allograft survival provided that they are administered 1 wk or more before transplantation. Here we show that administration of donor leukocytes immediately after transplantation induced long-term acceptance of completely MHC-mismatched rat kidney or liver transplants. The majority of long-term recipients of kidney transplants were tolerant of donor-strain skin grafts. Acceptance was associated with early activation of recipient T cells in the spleen, demonstrated by a rapid increase in IL-2 and IFN-gamma at that site followed by an early diffuse infiltrate of activated T cells and apoptosis within the tolerant grafts. In contrast, IL-2 and IFN-gamma mRNA were not increased in the spleens of rejecting animals, and the diffuse infiltrate of activated T cells appeared later but resulted in rapid graft destruction. These results define a mechanism of allograft acceptance induced by donor leukocytes that is associated with activation-induced cell death of recipient T cells. They demonstrate for the first time that posttransplant administration of donor leukocytes leads to organ allograft tolerance across a complete MHC class I plus class II barrier, a finding with direct clinical application.

Bone marrow aplasia induced by passenger leukocytes from heart allografts

OBJECTIVE

Organ allografts contain passenger leukocytes that are transferred to the recipient with the transplantation, but their functional relevance to the recipient's immune system is still controversial.

MATERIALS AND METHODS

To clarify the functional capacity of passenger leukocytes, we attempted to enhance their effect in rat heart allograft recipients by selective depletion of recipient leukocytes using a monoclonal antibody (mAb) against a recipient-specific allotype of CD45 (RT7(a)).

RESULTS

Although antibody treatment of the recipient alone led to profound lymphopenia and reversible myelosuppression, additional transplantation of an major histocompatibility complex-incompatible heart graft from an RT7(b) donor led to lethal aplastic anemia in the recipients. This lethal effect was completely abrogated by postoperative anti-CD3 treatment of the recipient and was partially abrogated or delayed by depletion of passenger leukocytes through additional anti-RT7(b) antibody treatment of the recipient or gamma-irradiation of the graft.

CONCLUSIONS

The results suggest a role for both donor and recipient-type T cells for the induction of aplastic anemia in this model. The study shows that, under defined conditions, allogeneic passenger leukocytes in a heart graft can have a profound effect on the recipient's immune system and bone marrow.

Immune activation is required for the induction of liver allograft tolerance: implications for immunosuppressive therapy

Liver transplants in many animal models are unusual because often they are not rejected even when transplanted across complete major histocompatibility complex barriers without immunosuppression. Their paradoxical behavior is even more obvious when the immune mechanism of acceptance is examined. Instead of acceptance resulting from a lack of immune response to the graft, the opposite occurs, and there is an unusual extensive increase in immune activation in acceptance compared with rejection. This abnormal extensive immune activation is driven by donor leukocytes transferred with the liver and results in death of the recipient cells that would normally reject the transplant. Some forms of immunosuppression inhibit this activation-associated liver transplant tolerance. The significance of these findings and possible means to design future treatment protocols for clinical transplantation that optimize management of liver transplant recipients are discussed.