Induction of tolerance by intrathymic inoculation of alloantigen

Immune modulation permits tolerance and engraftment in a murine model of late-gestation transplantation

ABSTRACT

In utero hematopoietic cell transplantation is an experimental nonmyeloablative therapy with potential applications in hematologic disorders, including sickle cell disease (SCD). Its clinical utility has been limited due to the early acquisition of T-cell immunity beginning at ∼14 weeks gestation, posing significant technical challenges and excluding treatment fetuses evaluated after the first trimester. Using murine neonatal transplantation at 20 days postcoitum (DPC) as a model for late-gestation transplantation (LGT) in humans, we investigated whether immune modulation with anti-CD3 monoclonal antibody (mAb) could achieve donor-specific tolerance and sustained allogeneic engraftment comparable with that of the early-gestation fetal recipient at 14 DPC. In allogeneic wild-type strain combinations, administration of anti-CD3 mAb with transplantation resulted in transient T-cell depletion followed by central tolerance induction confirmed by donor-specific clonal deletion and skin graft tolerance. Normal immune responses to third-party major histocompatibility complex and viral pathogens were preserved, and graft-versus-host disease did not occur. We further demonstrated the successful application of this approach in the Townes mouse model of SCD. These findings confirm the developing fetal T-cell response as a barrier to LGT and support transient T-cell depletion as a safe and effective immunomodulatory strategy to overcome it.

Regulatory T cells promote alloengraftment in a model of late-gestation in utero hematopoietic cell transplantation

In utero hematopoietic cell transplantation (IUHCT) has the potential to cure congenital hematologic disorders including sickle cell disease. However, the window of opportunity for IUHCT closes with the acquisition of T-cell immunity, beginning at approximately 14 weeks gestation, posing significant technical challenges and excluding from treatment fetuses evaluated after the first trimester. Here we report that regulatory T cells can promote alloengraftment and preserve allograft tolerance after the acquisition of T-cell immunity in a mouse model of late-gestation IUHCT. We show that allografts enriched with regulatory T cells harvested from either IUHCT-tolerant or naive mice engraft at 20 days post coitum (DPC) with equal frequency to unenriched allografts transplanted at 14 DPC. Long-term, multilineage donor cell chimerism was achieved in the absence of graft-versus-host disease or mortality. Decreased alloreactivity among recipient T cells was observed consistent with donor-specific tolerance. These findings suggest that donor graft enrichment with regulatory T cells could be used to successfully perform IUHCT later in gestation.

Effect of storage period of red blood cell suspensions on helper T-cell subpopulations

BACKGROUND

The aim of this study was to investigate the immunological alterations that occur during the storage of erythrocyte suspensions which may lead to transfusion-related immunomodulation following allogeneic blood transfusion.

MATERIALS AND METHODS

One part of the erythrocyte suspensions obtained from donors was leucoreduced while the other part was not. The leucoreduced (LR) and non-leucoreduced (NL) erythrocyte suspensions were then further divided into three equal amounts which were stored for 0, 21 or 42 days prior to measurements, by enzyme-linked immunosorbent assays, of cytokine levels in their supernatants. T-helper (Th) lymphocyte subgroups and gene expression were analysed in the NL erythrocyte suspensions by flow cytometry and real-time polymerase chain reaction, respectively. Results were compared to those of storage day 0.

RESULTS

By day 21, the number of Th2 cells had increased significantly and the numbers of Th1, Th22 and Treg cells had decreased significantly in the NL erythrocyte suspensions. On day 42 the numbers of Th2 and Treg cells in the NL suspensions were significantly increased while the number of Th1 cells was significantly decreased. The levels of transcription factors (TBX21, GATA3, and SPI.1) were significantly decreased on days 21 and 42, and AHR, FOXP3 and RORC2 levels were significantly increased on day 42 in NL erythrocyte suspensions. The decrease in interleukin-22 and increase in transforming growth factor-β levels found in NL erythrocyte suspensions on day 21 were statistically significant. Elevated levels of interleukin-17A were found in both LR and NL erythrocyte suspensions on day 42.

DISCUSSION

Our results suggest that allogeneic leucocytes and cytokines may play significant roles in the development of transfusion-related immunomodulation.

Intrathymic injection of lentiviral vector curtails the immune response in the periphery of normal mice

BACKGROUND

Gene transfer in the thymus, based on HIV-derived lentiviral vectors, is a promising avenue for modulation of T cell selection and autoimmunity. However, the impact of intrathymic (IT) injections on an antigen-specific immune response elicited in the periphery of normal mice has not been investigated yet.

METHODS

Highly concentrated stocks of lentiviral vectors expressing the soluble form of hemaglutinin of the influenza virus (LvHA) were injected in the thymus of normal BALB/c mice. The CD4 and CD8-mediated immune responses to HA after peripheral immunization were measured by various parameters.

RESULTS

We first show that a lentiviral vector expressing the luciferase was detected for at least 2 months after IT-injections. We then show that the LvHA vector could elicit a functional CD4- and CD8-T cell-mediated immune responses in the peripheral lymphoid organs of BALB/c mice. IT-injection of the LvHA vector significantly curbed this response: lower numbers of transferred HA-specific CD4(+) T cells were found in LvHA-injected compared to control animals. Furthermore, lower frequencies of HA-specific CD8(+) T cells, interferon γ-producing cells and cytotoxic cells were detected from 3 weeks to 3 months in LvHA-injected mice compared to controls. However, these reduced CD8-mediated responses were not increased after depletion of CD25(+) cells in vitro or in vivo.

CONCLUSIONS

The results obtained in the present study show that injection of the LvHA lentiviral vector significantly curtailed the immune response to the same antigen in the periphery. Increased selection of HA-specific regulatory T cells and negative selection of HA-specific CD8(+) T cell precursors may explain the results. Our work establish the feasibility of IT-injections of lentiviral vectors to manipulate T cell tolerance in the thymus of normal mice, for basic and pre-clinical research.

Immunomodulatory strategies prevent the development of autoimmune emphysema

Background

The presence of anti-endothelial cell antibodies and pathogenic T cells may reflect an autoimmune component in the pathogenesis of emphysema. Whether immune modulatory strategies can protect against the development of emphysema is not known.

Methods

Sprague Dawley rats were immunized with human umbilical vein endothelial cells (HUVEC) to induce autoimmune emphysema and treated with intrathymic HUVEC-injection and pristane. Measurements of alveolar airspace enlargement, cytokine levels, immuno histochemical, western blot analysis, and T cell repertoire of the lung tissue were performed.

Results

The immunomodulatory strategies protected lungs against cell death as demonstrated by reduced numbers of TUNEL and active caspase-3 positive cells and reduced levels of active caspase-3, when compared with lungs from HUVEC-immunized rats. Immunomodulatory strategies also suppressed anti-endothelial antibody production and preserved CNTF, IL-1alpha and VEGF levels. The immune deviation effects of the intrathymic HUVEC-injection were associated with an expansion of CD4+CD25+Foxp3+ regulatory T cells. Pristane treatment decreased the proportion of T cells expressing receptor beta-chain, Vβ16.1 in the lung tissue.

Conclusions

Our data demonstrate that interventions classically employed to induce central T cell tolerance (thymic inoculation of antigen) or to activate innate immune responses (pristane treatment) can prevent the development of autoimmune emphysema.

Transfusion-related immunomodulation (TRIM): an update

Allogeneic blood transfusion (ABT)-related immunomodulation (TRIM) encompasses the laboratory immune aberrations that occur after ABT and their established or purported clinical effects. TRIM is a real biologic phenomenon resulting in at least one established beneficial clinical effect in humans, but the existence of deleterious clinical TRIM effects has not yet been confirmed. Initially, TRIM encompassed effects attributable to ABT by immunomodulatory mechanisms (e.g., cancer recurrence, postoperative infection, or virus activation). More recently, TRIM has also included effects attributable to ABT by pro-inflammatory mechanisms (e.g., multiple-organ failure or mortality). TRIM effects may be mediated by: (1) allogeneic mononuclear cells; (2) white-blood-cell (WBC)-derived soluble mediators; and/or (3) soluble HLA peptides circulating in allogeneic plasma. This review categorizes the available randomized controlled trials based on the inference(s) that they permit about possible mediator(s) of TRIM, and examines the strength of the evidence available for relying on WBC reduction or autologous transfusion to prevent TRIM effects.

Generation of a tissue-engineered thymic organoid

The thymic microenvironment provides essential support for the generation of a functional and diverse population of human T cells. In particular, the three-dimensional (3D) thymic architecture contributes to critical cell-cell interactions. We report that thymic stroma, arrayed on a synthetic 3D matrix, supports the development of functional human T cells from hematopoietic precursor cells. Newly generated T cells contain T-cell receptor excision circles and are both fully mature and functional. The coculture of T-cell progenitors with thymic stroma can thus be used to generate de novo functional and diverse T-cell populations. This novel tissue engineered thymic system has biological applications for the study of T-lymphopoiesis and self-tolerance as well as potential therapeutic applications including the immune reconstitution of immunocompromised patients and the induction of tolerance in individuals receiving tissue or organ transplants.

Induction of antigen-specific tolerance by intrathymic injection of lentiviral vectors

Immune tolerance to self-antigens is established during lymphocyte differentiation in the thymus, but a simple means to induce antigen-specific tolerance in the thymus is still elusive. We show here that intrathymic injection of a lentiviral vector expressing the hemagglutinin antigen (HA) in TCR-HA transgenic mice resulted in negative selection of HA-specific effector T cells and sustained positive selection of HA-specific regulatory T cells (Tregs). This positive selection increased the number of HA-specific Tregs 10-fold and was comparable with the one observed in TCR-HA transgenic mice crossed with transgenic mice expressing HA under the control of the insulin promoter (Ins-HA). HA expression by radioresistant thymic epithelial cells was sufficient to drive Treg generation. Intrathymic injection of the lentiviral vector also resulted in an enrichment of HA-specific Tregs in peripheral lymphoid organs, which prevented diabetes induced in Ins-HA mice by transfer of HA-specific effector T cells. In this model, HA-specific Tregs inhibited effector T-cell division in pancreatic lymph nodes. Finally, we show that intrathymic injection of a lentiviral vector expressing preproinsulin-2 could reduce the occurrence of spontaneous diabetes in nonobese diabetic mice. Intrathymic gene transfer using lentiviral vectors thus offers new means to manipulate antigen-specific tolerance.

Long-Term Acceptance of Fully Allogeneic Cardiac Grafts by Cotransplantation of Vascularized Thymus in Miniature Swine

BACKGROUND

We have previously reported the ability of both thymokidney and vascularized thymic lobe (VTL) allografts to induce transplantation tolerance to renal allografts across a full major histocompatibility complex (MHC) mismatch in thymectomized miniature swine. However, whether vascularized thymus is capable of inducing tolerance to less tolerogeneic organs when it is transplanted simultaneously is not yet known. The present study investigates cardiac allograft survival and the mechanism of long-term acceptance in recipient swine following cotransplantation of VTL and cardiac grafts from fully MHC-mismatched donors.

METHODS

Animals received a heart graft, a heart graft and a VTL, or a heart graft and a donor thymocyte infusion. Immunosuppressive regimens consisted of 12 or 28 days of tacrolimus.

RESULTS

All animals that received a VTL maintained their grafts significantly longer than their counterparts that received only a heart graft, and those receiving 28 days of tacrolimus maintained their heart grafts long-term. Recipients of a donor thymocyte infusion demonstrated slightly prolonged cardiac graft survival but all rejected their grafts, highlighting the importance of thymic stroma. Cytotoxic T-lymphocyte responses against third-party antigens by cells from tolerant animals showed restriction by both self and donor MHC, whereas responses of controls were restricted to self MHC only. The presence of donor dendritic cells in the VTL grafts and results of co-culture assays suggest that both central and regulatory mechanisms were involved in achieving long-term acceptance.

CONCLUSION

This is the first demonstration of the long-term acceptance of fully MHC-mismatched cardiac allografts in large animals.

Infectious tolerance develops after intrathymic alloantigen-induced acceptance of rat heart allografts can be adoptively transferred

BACKGROUND

We have shown that intrathymic (IT) injection of alloantigen with antirat lymphocyte serum (ALS) treatment can induce donor-specific allograft acceptance. The purpose of this study was to investigate whether T-regulatory (T-reg) cells play a role in the maintenance of donor-specific heart graft tolerance that develops after IT injection of Lewis (LEW, RT1(l)) alloantigen into a Dark Agouti (DA, RT1(a)).

METHODS

Naïve DA rats were injected IT with 2.5 x10(7) LEW donor splenocytes and injected intraperitoneally with 1 mL ALS. Twenty-one days after pretreatment, a LEW or Brown Norway (BN, RT1(n)) heart was transplanted into a treated DA recipient. Splenocytes (1 x 10(8) or 5 x 10(7)) from a LEW heart-tolerant long-term survivor (LTS; >60 days) DA recipient were harvested and adoptively transferred (AT) into an irradiated (450 rad) naïve DA rat 24 hours before transplanting a LEW heart.

RESULTS

All LEW heart allografts were rejected by untreated DA rats in a mean survival time (MST) of 7.4 +/- 1.7 days (n=7). In contrast, 66.7% of LEW heart grafts into IT+ALS-pretreated DA recipients were accepted indefinitely (n=24). When either 1 x 10(8) (n=5) or 5 x 10(7) (n=5) splenocytes from a LEW heart graft-tolerant LTS (>60 days) DA recipient were AT into a new naïve DA rat, all new LEW heart grafts were accepted indefinitely.

CONCLUSIONS

The donor-specific tolerance that develops after IT+ALS-induced LEW heart acceptance by DA recipients can be transferred adoptively to new naïve DA recipients, thus indicating that it is infectious tolerance.

Tolerance to rat heart grafts induced by intrathymic immunomodulation is mediated by indirect recognition primed CD4+CD25+ Treg cells

BACKGROUND

In a rat model (PVG.R8-to-PVG.1U) disparate for one class I antigen, RT.1Aa, we previously demonstrated that intrathymic immunomodulation with donor antigens resulted in prolonged survival of cardiac allografts that underwent chronic rejection. However, long-term survivors developed a regulatory cell population that prevented both acute and chronic rejection when adoptively transferred into secondary graft recipients. The purpose of this study was to characterize these regulatory cells with particular emphasis on CD4+CD25+ Treg cells.

METHODS

Spleens, lymph nodes, and peripheral blood lymphocytes of secondary tolerant recipients were characterized using antibodies to various T cell markers in flow cytometry. In vitro MLR and in vivo adoptive transfer experiments were conducted to investigate the involvement of CD4+CD25+ T cells in the observed tolerance. The presence of various cytokines in the sera of graft recipients and MLR culture supernatants was tested using ELISA.

RESULTS

Tolerant recipients compared with naive rats had substantially higher percentages of CD4+CD25+ T cells in the spleen (28+/-3% vs. 11+/-5%) and blood (23+/-6% vs. 9+/-4%). Tolerant animals also had higher levels of serum IL-10 than naive and rejecting animals. CD4+CD25+ T cells from secondary long-term graft survivors inhibited donor-specific proliferative responses in vitro that was associated with high IL-10 production. Importantly, depletion of CD4+CD25+ T cells from splenocytes of tolerant rats abrogated their ability to transfer tolerance to tertiary graft recipients.

CONCLUSIONS

Our data demonstrate that cardiac allograft tolerance in this model is mediated by CD4+CD25+ Treg cells primed by indirect recognition and is associated with high levels of IL-10.

Suppression extends to major histocompatibility antigens linked to tolerizing minor histocompatibility antigens, but not the other way round

'Active suppression', a mechanism of transplantation tolerance, can spread to newly introduced minor antigens once these antigens are linked to tolerizing antigens. We explored whether this suppression can extend to major histocompatibility (MHC) antigens and whether this phenomenon can be demonstrated once tolerance is induced to a MHC antigen. Mice were tolerized using donor bone marrow plus CD4 and CD8 monoclonal antibodies. The following strain combinations were used: AKR (H-2k) into CBA (H-2k), a multiple minor difference and B6 (H-2b) into B6(bm12) (H-2b), a MHC class II difference. Tolerance was tested by a donorskingraft. CBA mice tolerant to AKR received a second skin carrying either AKR antigens plus additional multiple minor antigens [F1(AKRxBalb.K)] or carrying additional minors and a MHC class I antigen (B10.AKM-H2M). B6(bm12) (H-2b) tolerant to B6 (H-2b) were grafted with skin from a Balb.B donor (Balb minors linked to the tolerizing class II antigen) or from a B10.A(3R) strain (a MHC class I antigen linked to the tolerizing class II antigen). CBA mice tolerant to AKR accepted F1(AKRxBalb.K) skin, whereas F1(CBAxBalb.K) were rejected. Rejection of B10.AKM/H2M skin by tolerant mice was delayed as compared with nontolerant mice. Tolerant and nontolerant B6(bm12) mice rejected Balb.B skin and B10.A(3R) skin within the same time. Thus, in this model, suppression was linked to minors. Alloreactivity against minors and majors could be suppressed. Suppression linked to a class II antigen could not be demonstrated.

In situ transduction of stromal cells and thymocytes upon intrathymic injection of lentiviral vectors

Background

The thymus is the primary site for T-cell development and induction of self-tolerance. Previous approaches towards manipulation of T-cell differentiation have used intrathymic injection of antigens, as proteins, cells or adenoviruses, leading to transient expression of the foreign protein. Lentiviral vectors, due to their unique ability to integrate into the genome of quiescent cells, may be best suited for long-term expression of a transgene in the thymus.

Results

Young adult mice were injected in the thymus with lentiviral vectors expressing eGFP or the hemaglutinin of the Influenza virus under the control of the ubiquitous phospho glycerate kinase promoter. Thymi were examined 5 to 90 days thereafter directly under a UV-light microscope and by flow cytometry. Intrathymic injection of lentiviral vectors predominantly results in infection of stromal cells that could be detected for at least 3 months. Importantly, hemaglutinin expression by thymic stromal cells mediated negative selection of thymocytes expressing the cognate T-cell receptor. In addition and despite the low multiplicity of infection, transduced thymocytes were also detected, even 30 days after injection.

Conclusions

Our results demonstrate that intrathymic delivery of a lentiviral vector is an efficient means for stable expression of a foreign gene in the thymus. This new method of gene delivery may prove useful for induction of tolerance to a specific antigen and for gene therapy of severe combined immunodeficiencies.

CD25+ immunoregulatory CD4 T cells mediate acquired central transplantation tolerance

Transplantation tolerance is induced reliably in experimental animals following intrathymic inoculation with the relevant donor strain Ags; however, the immunological mechanisms responsible for the induction and maintenance of the tolerant state remain unknown. We investigated these mechanisms using TCR transgenic mice (TS1) that carry T cells specific for an immunodominant, MHC class II-restricted peptide (S1) of the influenza PR8 hemagglutinin (HA) molecule. We demonstrated that TS1 mice reject skin grafts that have transgene-encoded HA molecules (HA104) as their sole antigenic disparity and that intrathymic but not i.v. inoculation of TS1 mice with S1 peptide induces tolerance to HA-expressing skin grafts. Intrathymic peptide inoculation was associated with a dose-dependent reduction in T cells bearing high levels of TCR specific for HA. However, this reduction was both incomplete and transient, with a full recovery of S1-specific thymocytes by 4 wk. Peptide inoculation into the thymus also resulted in the generation of immunoregulatory T cells (CD4+CD25+) that migrated to the peripheral lymphoid organs. Adoptive transfer experiments using FACS sorted CD4+CD25- and CD4+CD25+ T cells from tolerant mice revealed that the former but not the latter maintain the capacity to induce rejection of HA bearing skin allografts in syngeneic hosts. Our results suggest that both clonal frequency reduction in the thymus and immunoregulatory T cells exported from the thymus are critical to transplantation tolerance induced by intrathymic Ag inoculation.

Cell replacement therapy for type 1 diabetes

Replacement of the insulin-producing pancreatic islet beta cells represents the ultimate treatment for type 1 diabetes. Recent advances in islet transplantation underscore the urgent need for developing alternatives to human tissue donors, which are scarce. Two possible approaches are the expansion of differentiated beta cells by reversible immortalization and the generation of insulin-producing cells from embryonic or adult stem cells. It is possible that new insights into endocrine pancreas development will ultimately lead to manipulation of progenitor-cell fate towards the beta-cell phenotype of insulin production, storage and regulated secretion. Both allogeneic and autologous surrogate beta cells are likely to require protection from recurring autoimmunity. This protection might take the form of tolerization, cell encapsulation, or cell engineering with immunoprotective genes. If successful, these approaches could lead to widespread cell replacement therapy for type 1 diabetes.

Thymic dendritic cells traffic to thymi of allogeneic recipients and prolong graft survival

We have demonstrated that murine thymic dendritic cells (DCs) isolated from donor mice have the capability to home to thymi of fully allogeneic recipients after intravenous injections, where they induce T cell deletions and prolong donor-strain airway and skin graft survival. In contrast, infused splenic DCs immigrated poorly to thymi, and did not affect graft survival. These findings suggest that preferential homing may be an important mechanistic difference among subpopulations of DCs that mediate immune functions and illustrate a novel methodology that could have utility for induction of specific immunologic nonreactivity to allografts, or other disease-associated antigens.

Exposure to allogeneic plasma and risk of postoperative pneumonia and/or wound infection in coronary artery bypass graft surgery

BACKGROUND

A relationship may exist between the immunomodulatory effects of allogeneic blood transfusion and the volume of transfused allogeneic plasma and/or the number of transfused plasma-containing units.

STUDY DESIGN AND METHODS

The records of 416 consecutive patients undergoing coronary artery bypass graft operations at the Massachusetts General Hospital were reviewed. Exposure to allogeneic plasma in units of transfused RBCs, platelets, FFP, and cryoprecipitate was recorded, and diagnoses of postoperative pneumonia and wound infection were made by CDC criteria. The association between postoperative infection and exposure to allogeneic plasma was calculated by logistic regression analyses adjusting for the effects of confounding factors that related to severity of illness, difficulty of operation, and risk of postoperative pneumonia.

RESULTS

Two hundred seventy-four patients received (mean +/- SE) 636.2 +/- 64.3 mL of allogeneic plasma, contained in 8.5 +/- 0.7 units of blood components. Sixty-four patients (15.4%) developed pneumonia and/or wound infection, and 54 (13.0%) developed pneumonia. Patients who did (n = 64) or did not develop infection received 956.6 +/- 180.6 and 321.3 +/- 39.6 mL of plasma, respectively (p<0.0001). In multivariate analyses, the volume of transfused allogeneic plasma was not associated with postoperative pneumonia and/or wound infection (p = 0.24), pneumonia (p = 0.21), or wound infection (p = 0.74). Similarly, the number of transfused plasma-containing units was not associated with these outcomes (p = 0.08, p = 0.08, and p = 0.46, respectively).

CONCLUSION

A relationship between the exposure to allogeneic plasma and the risk of postoperative pneumonia and/or wound infection was not detected in this study population.

Immunomodulation by intrathymic injection of donor leukocytes in rhesus monkeys

BACKGROUND

Several studies have demonstrated that intrathymic injection of donor cells into adult rodents can result in long-term allograft survival. The rationale for using the intrathymic route of donor cell administration is that in the thymic environment immature T cells are educated to discriminate between self and non-self antigens. The validity of this approach was tested in non-human primates.

METHODS

The effect of the intrathymic injection of allogeneic donor cells was investigated in rhesus monkeys and compared with IV and intracutaneous administration of donor cells. Intrathymic injections were carried out without and with antithymocyte globulin. All animals received subsequently an allogeneic skin graft of the same donor and no immunosuppression post transplantation.

RESULTS

Skin graft survival was slightly shorter in animals treated with IC donor cell injections (mean survival time [MST]=8.9+/-0.52) than untreated control animals (MST=10.0+/-0.44), indicating that this route caused sensitisation. Intravenous donor cell injection showed prolongation of graft survival times (MST=11.6+/-1.69). Intrathymic donor cell injection resulted in a graft survival of 9.2+/-1.44 days although addition of antithymocyte globulin slightly prolonged graft survival to 10.3+/-2.84 (not significant). Whereas the cellular responses after intrathymic and intravenous donor cell injections increased, antithymocyte globulin treated animals did not show an increased cellular response. Recipients of intrathymic donor cells showed a significantly decreased humoral anti-donor response as compared to other groups.

CONCLUSIONS

Donor cell pretreatment alters the subsequent response to an allogeneic skin graft in monkeys and is dependent on the route of donor cell administration. This is also reflected in the alloantibody response and the in vitro cellular reactivity. Intrathymic administration of donor cells does not lead to prolonged skin graft acceptance.

Acceptance reaction: intragraft events associated with tolerance to renal allografts in miniature swine

Inbred miniature swine that are treated for 12 d with a high dose of cyclosporin A develop tolerance to MHC class II matched, class I-mismatched renal allografts. The aim of this study was to clarify the intrarenal allograft events associated with the development of tolerance in this protocol. Morphologic and immunologic studies were performed in serial biopsies from accepting grafts after 12 d of cyclosporin A treatment (n = 4) and were compared with those from untreated control rejecting grafts (n = 4). In accepting grafts with stable function, a transient interstitial infiltrate developed. The cellular infiltrate had many similarities to that in rejecting grafts; both had T cells and macrophages, similar proportions of T-cell subsets, and a similar frequency of in situ nick end labeling (TUNEL)+ apoptotic infiltrating cells. However, the cellular infiltrate in the acceptance reaction was distinguished by less T-cell activation (interleukin-2 receptor+), less proliferation (proliferating cell nuclear antigen+) of infiltrating cells, and less graft cell apoptosis in arteries, tubules, glomeruli, and peritubular capillaries. Thereafter, the infiltrate in the accepting grafts progressively resolved with decreased cell proliferation, activation, and apoptotic graft parenchymal cell injury, but the high frequency of apoptosis persisted in graft-infiltrating cells. In parallel to the intragraft events, donor-specific unresponsiveness developed as assessed by cell-mediated cytotoxicity by blood mononuclear cells in vitro. In conclusion, the acceptance reaction in transplanted grafts is characterized by progressive resolution of T-cell proliferation and activation and of cell-mediated graft injury, as well as prolonged T-cell apoptosis. These intragraft events suggest that both T-cell anergy and T-cell deletion occur in the graft during the development of tolerance. Some of the described immunopathologic findings (activation, proliferation, apoptosis) may be useful in distinguishing acceptance from rejection, as well as in predicting later graft acceptance in tolerance induction protocols.

RBC transfusion and postoperative length of stay in the hospital or the intensive care unit among patients undergoing coronary artery bypass graft surgery: the effects of confounding factors

BACKGROUND

Data on the independent association between perioperative allogeneic blood transfusion (ABT) and postoperative length of stay at the hospital or in the intensive care unit (ICU) are sparse.

STUDY DESIGN AND METHODS

The records of 421 consecutive patients undergoing coronary artery bypass graft (CABG) operations at the Massachusetts General Hospital were reviewed. The effect of perioperative ABT in explaining the variation in the postoperative length of stay (LOS) at the hospital or in the ICU was calculated after adjustment for the effects of 20 confounding factors that pertained to severity of illness, difficulty of operation, and risk of postoperative wound infection or pneumonia.

RESULTS

Postoperative LOS averaged (mean +/- SE) 8.0 +/- 0.3 days in the hospital and 50.0 +/- 4.1 hours in the ICU. After adjustment for the effects of confounding factors, the postoperative length of hospitalization increased by 0.837 percent (95% CI, 0.249-1.425%) per RBC unit transfused (p<0.001), and the postoperative length of stay in the ICU increased by 0.873 percent (95% CI, -0.068-1.814%) per RBC unit transfused (p<0.10).

CONCLUSION

Allogeneic blood transfusion was independently associated with longer postoperative stays in the hospital or the ICU, but the observed independent association is perhaps too small to be clinically relevant. This independent association may be due to a relationship between ABT and a higher incidence of septic complications of surgery, or it may reflect the function of blood transfusion as a surrogate marker for severity of illness.

Thymic dendritic cells express inducible nitric oxide synthase and generate nitric oxide in response to self- and alloantigens

Thymocytes maturing in the thymus undergo clonal deletion/apoptosis when they encounter self- or allo-Ags presented by dendritic cells (DCs). How this occurs is a matter of debate, but NO may play a role given its ability of inducing apoptosis of these cells. APC (a mixed population of macrophages (Mphi) and DCs) from rat thymus expressed high levels of inducible NO synthase (iNOS) and produced large amounts of NO in basal conditions whereas iNOS expression and NO production were very low in thymocytes. Analysis by FACS and by double labeling of cytocentrifuged preparations showed that DCs and MPhi both express iNOS within APC. Analysis of a purified preparation of DCs confirmed that these cells express high levels of iNOS and produce large amounts of NO in basal conditions. The capacity of DCs to generate NO was enhanced by exposure to rat albumin, a self-protein, and required a fully expressed process of Ag internalization, processing, and presentation. Peptides derived from portions of class II MHC molecules up-regulate iNOS expression and NO production by DCs as well, both in self and allogeneic combinations, suggesting a role of NO in both self and acquired tolerance. We also found that NO induced apoptosis of rat double-positive thymocytes, the effect being more evident in anti-CD3-stimulated cells. Altogether, the present findings might suggest that DC-derived NO is at least one of the soluble factors regulating events, in the thymus, that follow recognition of self- and allo-Ags.

Kinetic analysis of microchimerism induced by intrathymic injection of allogeneic splenocytes in mice

Allograft survival facilitated by intrathymic (i.t.) injection of allogeneic cells have shown that modifications of T-cell development induce specific tolerance. One hypothesis is that the resulting microchimerism may play a role in preparing the host immune system for the allograft. To investigate whether the deliberate introduction of allogeneic splenocytes into the thymus of adult mice allows the establishment of a lasting donor/recipient microchimerism, a full allogeneic mouse system (H-2 and Mls) with additional sex mismatch was used. Male cells injected into female mice were detected using an optimized nested-polymerase chain reaction which specifically amplifies the SRY gene with a sensitivity of 1/10(4). After i.t. injection, donor cells were observed early both in the lymph nodes and spleen (75 and 25% of mice, respectively). They were still present on day 6, although preferentially in the thymus (100% of mice) than in the lymph nodes (50% of mice) or in the spleen (22% of mice). After intraperitoneal (i.p.) or subcutaneous (s.c.) injection, donor cells were early (2 h) but transiently detected in the thymus, since on day 6 they were detected in 0 and 17% of mice after i.p. and s.c. injection, respectively. Kinetics of donor-cell detection was similar both in the spleen and lymph nodes with a clear decrease in the percentage of mice with donor-cell detection between day 2 and day 6 (20 and 17% of positive mice for the spleen after i.p. and s.c. injections, respectively--20 and 33% of positive mice for the lymph nodes after i.p. and s.c. injections, respectively). Our results clearly show that i.t. injection of allogeneic splenocytes induces a microchimerism which is both more lasting and detected in a higher percentage of mice than by the i.p. and s.c. routes, both at the central (thymus) and peripheral (spleen) levels.

Mechanism of acquired thymic tolerance induced by a single major histocompatibility complex class I peptide with the dominant epitope: differential analysis of regulatory cytokines in the lymphoid and intragraft compartments

BACKGROUND

We have recently shown that intrathymic injection of a combination of immunogenic WAG-derived or Wistar-Furth (WF) (RT1.Au) major histocompatibility complex class I peptides induces acquired systemic tolerance to cardiac and islet allografts in the WF-to-ACI rat combination and therefore hypothesized that identification of the class I peptide dominance may play an important role in the induction of antigen (Ag)-specific tolerance. This study defined the peptide with the dominant epitope among the seven synthetic RT1.Au peptides and analyzed the immunoregulatory cytokines within the lymphoid and intragraft compartments associated with acquired thymic tolerance.

METHODS

ACI recipients were pretreated with intrathymic (IT) injection of 300 microg of the individual seven RT1.Au peptides 7 days before WF or Lewis cardiac transplantation. Cytokine profile of mixed lymphocyte reaction supernatants of T cells obtained from the thymus, mesenteric lymph nodes, spleen, peripheral blood leukocytes, and graft infiltrating cells after donor (WF) or third-party (Lewis) Ag stimulation were measured by enzyme-linked immunosorbent assay, whereas cytokine gene expression was determined by reverse transcription-polymerase chain reaction.

RESULTS

Only IT injection of peptide 5 (93-109) among the seven RT1.Au peptides induced donor-spe cific tolerance to cardiac allografts in the WF-to-ACI rat combination. In addition, intravenous injection of peptide 5 did not prolong WF graft survival in ACI recipients. Analysis of cytokine production by the tolerant recipients showed significant Ag-specific reduction in the production of interleukin (IL)-2 and interferon-gamma (IFN-gamma) in the thymus, mesenteric lymph nodes, spleen, and peripheral blood leukocytes, which was not associated with a concomitant Ag-specific increase in IL-4 and IL-10 production. Measurement of cytokine mRNA expression confirmed undetectable

Transplantation tolerance: the concept and its applicability

Recent advances have enabled researchers to induce tolerance in animal transplant models. Although it has been relatively easy to do so in rodents, it has been much more difficult to translate such strategies into primates. Understanding the cellular and molecular mechanisms of the alloimmune response has prompted the development of novel strategies that may obviate the need for immunosuppression in humans. Mechanisms of tolerance and promising new therapies, as well as the inherent difficulties in bringing them into clinical practice, are reviewed.

Cyclosporin A-Induced Autologous Graft-Versus-Host Disease: A Prototypical Model of Autoimmunity and Active (Dominant) Tolerance Coordinately Induced by Recent Thymic Emigrants

Cyclosporin A (CSA)-induced autologous graft-vs-host disease (autoGVHD) is an autoimmune syndrome initiated by autoeffector T cells presumed to be exported from the thymus during CSA treatment. The appearance of noncytotoxic immunoregulatory T cell activity after cessation of CSA treatment is also thymus dependent. In the present study, we have tested the hypothesis that both autoeffector and immunoregulatory T cells in CSA-treated rats are recent thymic emigrants (RTEs). Local syngeneic graft-vs-host reaction (synGVHR) and timed thymectomy (Tx) assays revealed that autoeffector T cells appear initially in the thymus and are promptly exported to lymph nodes (LN) during the first week of CSA treatment. In contrast, immunoregulatory thymocytes are first detectable by local synGVHR inhibition assays during the second week of CSA treatment but are not exported to LN until ∼4 days post-CSA. Both the autoeffector and immunoregulatory T cells in LN express Thy-1, a selective marker for RTEs in the rat. However, the autoeffector RTEs have a CD4+8+ phenotype, whereas the immunoregulatory RTEs have a CD4+8− phenotype. Thus, the coordinate formation in and release from the thymus cortex and medulla of autoeffector and immunoregulatory T cells in CSA-treated rats directly demonstrates that centrally induced, nondeletional tolerance can serve as a fail-safe mechanism by which clones of autoeffector T cells that have escaped intrathymic negative selection for self-MHC class II Ag can be suppressed postthymically.

Role of the thymus in transplantation tolerance in miniature swine. III. Surgical manipulation of the thymus interferes with stable induction of tolerance to class I-mismatched renal allografts

BACKGROUND

Previous studies have demonstrated that long-term tolerance of class I mismatched renal allografts in miniature swine is induced by a short course of cyclosporine (CyA), and that a total thymectomy 21 days before transplantation abrogates the induction of stable tolerance. We have now examined the effects of surgical manipulation of the thymus, with or without a reduction in the thymic volume, on the induction of tolerance.

MATERIALS AND METHODS

Miniature swine receiving a transplant of a class I-mismatched renal allograft and 12 days of CyA underwent either (1) a partial thymectomy 21 days before kidney transplantation (day -21), (2) serial thymic biopsies (to evaluate the effect of surgical trauma and reduction in volume of the thymus) or serial incisions of the thymus thymus (to evaluate the effect of surgical trauma without changes in thymic volume), (3) a sham thymectomy on day -21, or serial sham thymic surgery on the same POD as the thymic biopsies and incisions (control animals).

RESULTS

Control animals had a stable plasma creatinine, had donor-specific unresponsiveness in cell-mediated lympholysis (CML) assays, had absence of rejection in kidney biopsy specimens, and did not develop anti-donor class I immunoglobulin (Ig)G alloantibodies. Animals undergoing a partial thymectomy on day -21 or serial thymic biopsies showed severe renal dysfunction, histological evidence of rejection in kidney biopsy specimens and anti-donor reactivity in CML assays; all but one animal developed anti-donor class I IgG alloantibodies. Serial incisions of the thymus induced an increase in plasma creatinine and histological rejection in 1 of 3 animals and anti-donor cytotoxic T cells in vitro in all 3 animals.

CONCLUSIONS

A partial thymectomy or serial thymic biopsies markedly interfere with the induction of tolerance to renal allografts. Serial thymic incisions also interfere with the induction of tolerance, but to a lesser degree. These studies may have implications for tolerance-inducing protocols that involve thymic manipulation.

Acquired Thymic Tolerance: Role of CTLA4 in the Initiation and Maintenance of Tolerance in a Clinically Relevant Autoimmune Disease Model

Injection of Ag into the thymus of adult animals induces specific systemic tolerance. The mechanisms of acquired thymic tolerance include anergy and the deletion of Ag-specific T cells. Here, we report that anergy to nominal Ag induced via acquired thymic tolerance requires CTL-associated Ag 4 (CTLA4) engagement. The role of CTLA4 in the induction and maintenance of tolerance was then investigated in the murine experimental autoimmune encephalomyelitis model. CTLA4 blockade abrogated the induction but not the maintenance phase of acquired thymic tolerance induced by intrathymic injection of myelin Ags. In addition, CTLA4 blockade had a restricted window of action after priming with Ag, which is consistent with the expression patterns of CTLA4 in vivo. We conclude that: 1) the induction of acquired thymic tolerance requires signaling through CTLA4 and 2) CTLA4 does not appear to be required for the maintenance of acquired thymic tolerance. This is the first report documenting the role of a CTLA4 negative signaling pathway in the induction of tolerance in an autoimmune disease model.

T cell-mediated xenograft rejection: specific tolerance is probably required for long term xenograft survival

T cell-mediated mechanisms of xenograft rejection appear resistant to standard immunosuppression protocols used to prevent allograft rejection and, consequently, higher doses of immunosuppressive drugs are required to promote xenograft compared to allograft survival. Evidence from recent studies suggests that porcine xenografts may be especially immunogenic in humans because of a prominent and vigorous indirect xenoresponse and because of the ability of porcine endothelium to activate human T cells. This has led to an anxiety that systemic immunosuppressives, used as the mainstay of therapy for clinical xenotransplantation, may not allow the long-term survival of porcine organs transplanted into human recipients. This article will review the biology of T cell xenoresponses, present the case for the development of novel graft-specific immunosuppressive regimes in clinical xenotransplantation, and review recent experimental progress in this area.

Comparative studies of specific acquired systemic tolerance induced by intrathymic inoculation of a single synthetic Wistar-Furth (RT1U) allo-MHC class I (RT1.AU) peptide or WAG (RT1U)-derived class I peptide

BACKGROUND

Because T cell receptor-MHC class I/self-peptide interactions regulate T-cell development, the presence of MHC allopeptides in the thymus may influence T-cell tolerance to alloantigens. This hypothesis is supported by our most recent finding that intrathymic (IT) inoculation of nonimmunogenic synthetic peptides derived from "WAG" RT1.A induces tolerance to cardiac allografts in the Wistar-Furth (WF)-to-ACI model. To evaluate whether in vivo immunogenicity of MHC peptides is relevant to tolerance induction and to examine the effect of peptide specificity, we compared the effects on graft survival of well-defined, strain-specific immunogenic WF MHC class I peptides (RT1.AU) with closely related but non-strain-specific class I peptides derived from WAG (RT1U).

METHODS

In vivo immunization of seven MHC class I peptides synthesized from RT1.AU sequences showed that two (u-5 and u-7) were immunogenic, whereas five others were not immunogenic in ACI recipients. We then examined the effects on cardiac allograft survival in the WF-to-ACI model of the two immunogenic RT1.AU peptides (u-5 and u-7) and three immunogenic WAG-derived peptides (peptides 1, 2, and 5).

RESULTS

A combination of equal amounts (150 microg or 300 microg) of u-5 or u-7 each with 0.5 ml of antilymphocyte serum (ALS) on day -7 led to 60% and 100% permanent graft survival (>150 days), respectively. IT injection of the individual peptides on day -7 showed that only 300 microg of u-5 significantly prolonged graft survival to a median survival time of 17.3 days from 10.5 days in naive recipients. IT injection of 150, 300, and 600 microg of u-5 combined with 0.5 ml of ALS on day -7 led to permanent graft survival (> 150 days) in four of six, nine of nine, and six of six ACI recipients, respectively, compared with a median survival time of 15.4 days in ALS alone-treated controls. In contrast, similar treatments with peptide u-7 with or without 0.5 ml of ALS did not prolong graft survival, thus demonstrating that peptide u-5 alone mediates the observed effects on graft prolongation. A total of 300 microg of u-5 injected IT combined with ALS led to acute rejection of third-party (Lewis) grafts. Intravenous injection of 300 microg of u-5 with ALS also did not prolong WF graft survival in ACI recipients. The long-term unresponsive ACI recipients accepted permanently donor-type (WF) but not third-party (Lewis) second-set cardiac and islet allografts. Similarly, we showed that although IT injection of 600 and 1200 microg of a mixture of immunogenic WAG-derived peptides 1, 2, and 5 combined with 0.5 ml of ALS on day -7 led to permanent WF graft survival in ACI, only IT injection of 300 microg of peptide 2 combined with ALS led to permanent graft survival (>150 days) in four of five animals. To define the underlying mechanisms of tolerance, we examined in vitro the mixed lymphocyte reaction (MLR), cell-mediated lymphocytotoxicity, and cytokine profile of unresponsive recipients. Although the results showed nonspecific T-cell suppression in the MLR at 25 days after transplantation, which correlated with the persistence of ALS immunosuppression, long-term unresponsive animals showed normal MLR to donor and third-party antigens. In contrast, the donor-specific reactive cytotoxic T lymphocytes remained suppressed in short-term and long-term unresponsive rats.

CONCLUSION

Of interest is our finding that IT injection of a short segment of WAG-derived MHC class I peptide induces active acquired tolerance similar to results obtained with the use of pure WF-derived peptide u-5 in the WF-to-ACI rat combination. It is noteworthy that we could not confirm the T helper (Th)1/Th2 paradigm in this model by initial cytokine analysis. Whether induction of tolerance by IT injection of allo-MHC peptides will have clinical usefulness must await results of similar studies in large animals. However, of major interest is the finding that a short segment of RT1.AU represents the tolerogenic

Modulation of alloimmunity to major histocompatibility complex class I by cotransfer of cytokine genes in vivo

Major histocompatibility complex (MHC) class I antigen is a potent stimulus for alloimmune responses and is the principal immunologic target mediating acute cellular rejection of allografts. Using a method of direct in vivo gene transfer of cDNA encoding donor type MHC class I, we showed in a rat model that recipient muscle could express the transferred MHC class I cDNA, resulting in alloimmunization of the recipient. This was most graphically demonstrated by accelerated rejection of cardiac allografts expressing the same MHC class I as encoded by the immunizing cDNA. We now report the use of the particle-mediated gene transfer via a gene gun (Geneva, Middleton, WI, USA) to transfer MHC class I, as well as cytokine gene expression vectors, into rat skin. Compared to intramuscular injection, gene gun transfer to skin resulted in more efficient immunization. Donor-specific cytotoxic T lymphocyte (CTL) responsiveness and antibody levels increased. Furthermore, coexpression of certain cytokine genes with the MHC class I cDNA modulated the immune response. Specifically, coimmunization with IL-10 cDNA abrogated immunity to allo-MHC class I, while coimmunization with GM-CSF cDNA enhanced it. The influence of expression of these genes in skin was demonstrated by alteration of donor cardiac allograft survival. This model is useful for induction and modulation of alloimmune responses and may be used to develop gene therapy strategies to modify them.

Specific tolerance induction and organ transplantation

Induction of tolerance to histocompatibility antigens of an organ donor would eliminate the need for long-term administration of nonspecific immunosuppressive drugs associated with an increased risk of infection and malignancies. Recently, we established a murine model in which recipient mice were treated with a single dose of anti-CD3, anti-CD4, low dose of total body irradiation (TBI) and allogeneic bone marrow cells. Our results clearly demonstrate that stable multilineage mixed chimerism, immunocompetence and permanent donor-specific skin graft tolerance across full major histocompatibility (MHC) barriers can be successfully achieved in this way. The observations that the preparative regimen and skin transplantation can be performed on the same day, and that a significant reduction in irradiation dose is sufficient in haploidentical donor-recipient combinations (MHC-sharing effect), bring our protocol closer to clinical use.

Peritransplant tolerance induction with anti-CD3-immunotoxin: a matter of proinflammatory cytokine control

BACKGROUND

Tolerance is gaining momentum as an approach to reduce lifelong immunosuppressive therapy while improving transplant longevity. Anti-CD3 immunotoxin (IT), FN18-CRM9, has potential to induce tolerance owing to its exceptional ability to deplete sessile lymph node T cells. However, if initiated at the time of transplantation, alpha-CD3-IT alone elicits a proinflammatory cytokine response, precluding establishment of tolerance.

METHODS

Four groups of rhesus monkeys received kidney allografts and immunosuppression. Three groups received alpha-CD3-IT alone or alpha-CD3-IT supplemented with 15-deoxyspergualin (DSG) and/or methylprednisolone (MP). One group received alpha-CD3-monoclonal antibody with DSG and MP. Cytokines were measured by enzyme-linked immunosorbent assay.

RESULTS

Supplementing peritransplant alpha-CD3-IT treatment with a brief course of DSG and MP promoted rejection-free kidney allograft acceptance in 75% of macaques followed for up to 550 days. Among those given alpha-CD3-IT alone or with MP, none were long-term survivors. Tolerance developed after alpha-CD3-IT, DSG, and MP treatment, but not when the unconjugated a-CD3 monoclonal antibody was substituted for IT. Systemic production of proinflammatory cytokines interferon-gamma (IFN-gamma) and tumor necrosis factor-alpha induced after peritransplant alpha-CD3-IT was prevented only in animals given DSG. Despite high levels of interleukin (IL)-12 in the first month after transplant, tolerant recipients exhibited IL-12 resistance, as evidenced by baseline plasma levels of IFN-gamma but elevated IL-4. DSG was shown to inhibit IL-12-driven IFN-gamma production by a mechanism associated with inhibition of nuclear factor kappa-B.

CONCLUSIONS

In this model, peritransplant induction of tolerance is promoted by efficient elimination of sessile lymph node T cells and control of the proinflammatory IFN-gamma response by a mechanism that appears to involve resistance to IL-12.

Organ transplantation in the twenty-first century

Major advances in the understanding of the immunologic process responsible for organ or cellular transplant rejection, a dramatic improvement in available immunosuppressive drugs, development of more sophisticated surgical techniques, and important progress in posttransplant intensive care over the last 30 years have led to a remarkable improvement in success following organ transplantation. Whereas excellent short-term survival of most transplanted organs is readily achieved, graft loss because of chronic rejection and the worsening problem of organ donor shortage remain major concerns in the field of transplantation. Recent advances in immunosuppressive drugs, induction of immunologic tolerance, and gene therapy strategies may help to prolong organ allograft survival in the future. Revised criteria for organ donation and xenotransplantation may one day solve the problem of organ supply. Today, as we approach the next millennium, we are optimistic that the elusive goal of immunologic tolerance will be achieved and perhaps applied to animal tissue. Such will certainly be the challenge for the next century.

Mechanisms of Acquired Thymic Tolerance In Vivo: Intrathymic Injection of Antigen Induces Apoptosis of Thymocytes and Peripheral T Cell Anergy

Intrathymic injection of Ag induces Ag-specific tolerance in several clinically relevant experimental autoimmune and transplantation models. However, the exact mechanisms of acquired thymic tolerance in vivo remain unclear. We investigated the mechanisms of acquired thymic tolerance in mice that are transgenic for the TCR specific for peptide 323-339 of OVA. Intrathymic injection of OVA leads to apoptosis of thymocytes starting as early as 3 h after injection and persisting up to 7 days. Double positive thymocytes undergo apoptosis earlier than single positive thymocytes, and significantly higher percentages of double positive thymocytes ultimately die as compared with single positive cells. Apoptotic cells show decreased surface expression of CD4. In the periphery, T cells from intrathymically injected animals had suppressed proliferation and IL-2 production to OVA compared with T cells from control Ag-injected mice. We conclude that intrathymic injection of Ag induces apoptosis of immature thymocytes and a subpopulation of mature thymocytes and induces prolonged anergy in peripheral T cells in vivo. Understanding the mechanisms of acquired thymic tolerance may lead to development of novel clinical strategies to prevent autoimmune disease and transplant rejection.

Acquired transplant tolerance

Increasing the acceptance rate of organs is the central goal of transplantation research. Long-term survival of vascularized organs without chronic immunosuppressive therapy has been achieved in experimental animals. In humans, the possibility of achieving immunological tolerance and a drug-free state has been reported occasionally in patients who after withdrawal of immunosuppressants because of major toxicity still carry a functioning graft. It has been proposed that organ transplant implies a migratory flux of donor 'passenger' leukocytes out of the graft into the recipient tissue or organs, to establish a persistent condition of 'microchimerism'. Although there is evidence that the same migratory mechanisms apply to all organ grafts, migration of 'passenger' leukocytes is less in kidney and heart than in liver. To enhance the acceptance of organs less tolerogenic than liver, perioperative infusion of donor bone marrow has been attempted to increase the donor 'passenger' leukocyte load. It has been suggested that the established microchimerism is not only associated with long-term acceptance of the graft, but it also plays an active role in induction and maintenance of donor-specific unresponsiveness. However, the intimate mechanism(s) responsible for prolonged graft survival in this setting remain speculative. Experimental evidence is also available that the thymus plays a major role in the development of self-tolerance and is critical in the induction of acquired tolerance to exogenous antigens. It has been reported that after intrathymic injection of donor cells clonal deletion of maturing thymocytes occurs and is the major mechanism in the induction of donor-specific tolerance, since peripheral T-cell component would be devoid of alloreactive population. Studies are warranted in the near future to explore whether the thymus technique can be employed to prolong survival or induce tolerance to allograft in humans. An interesting novel strategy for transplant tolerance is also the oral administration of alloantigens, which has been recently applied to the cardiac transplant model in rat. All these approaches will have a major impact in the near future on transplant medicine, opening new perspectives to obtain indefinite graft survival.

In vivo mechanisms of acquired thymic tolerance

Injection of antigen into the thymus of adult animals induces specific systemic tolerance, but the mechanisms of acquired thymic tolerance are not well understood. To investigate these mechanisms we used a model of intrathymic injection of ovalbumin (OVA) in BALB/c mice. We show an antigen-specific decrease in proliferative responses to OVA, as well as a significant decrease in antigen-specific IL-2 secretion and IFN-gamma production by splenocytes and lymph node cells of tolerant mice. Addition of recombinant IL-2 in vitro reversed the defect in IFN-gamma production by cells from OVA-tolerized animals, but did not reverse the proliferation or IL-2 production defects. By using an adoptive transfer system, where a small population of OVA peptide-specific CD4+ TCR transgenic T cells are transferred into syngeneic normal recipients, we show an absence of peripheral antigen-dependent clonal expansion of transferred CD4+ TCR transgenic cells in tolerant mice in vivo. There was an increase in clonotype-positive T cells in the thymus after immunization, confirming that activated T cells circulate through the thymus. Furthermore, thymectomy after intrathymic injection abrogates the effect of acquired thymic tolerance and restores antigen-dependent clonal expansion in vivo. We conclude that intrathymic injection of antigen induces Th1 cell unresponsiveness and prevents the peripheral expansion of antigen-specific CD4(+) T cells in vivo. This is the first demonstration that in acquired thymic tolerance antigen-specific T cells circulate to the thymus where they may be anergized or ultimately deleted.

Preclinical studies of allograft tolerance in rhesus monkeys: a novel anti-CD3-immunotoxin given peritransplant with donor bone marrow induces operational tolerance to kidney allografts

A major challenge in clinical transplantation today is to design a practical and effective protocol for tolerance induction compatible with cadaver organ transplantation. A preclinical rhesus monkey kidney allograft model using immediate peritransplant anti-CD3 immunotoxin (anti-CD3-IT) and donor bone marrow (DBM) is shown here to induce operational tolerance with prolonged graft survival in the absence of chronic immunosuppressive drugs. Bone marrow harvested from the kidney donor was depleted of mature alloantigen-presenting cells and T cells by removing DR(bright) cells and CD3(bright) cells, respectively. In outbred, major histocompatibility complex-incompatible donor-recipient pairs with high pretransplant mixed lymphocyte response and cytotoxic T lymphocyte precursor activity, four of six allografts survived for periods of 120 days to >1.5 years. Graft acceptance after peritransplant treatment followed robust elimination of both peripheral blood T cells and lymph node T cells. In most recipients given anti-CD3-IT and DBM infusion, anti-donor immunoglobulin G responses were completely inhibited. Microchimerism was observed in all recipients studied, including those not given DBM, but levels of microchimerism did not correlate with graft survival. Anti-CD3-IT induction in combination with modified DBM protocols such as the depletion of mature T cells and DR(bright) antigen-presenting cells may offer new opportunities to improve clinical tolerance protocols beyond those attempted in the clinic to date. Overall, these results with anti-CD3-IT show promise for development of cadaver transplant tolerance induction.