Induction of hyporesponsiveness to fully allogeneic cardiac grafts by intratracheal delivery of alloantigen

Administration of donor splenocytes via the respiratory tract generates CD8α+ regulatory dendritic cells and induces hyporesponsiveness to fully allogeneic cardiac grafts

BACKGROUND

We previously showed that pretreatment with intratracheal delivery (ITD) of alloantigen induced prolonged cardiac allograft survival and generated regulatory T cells (Tregs) in mice. In this study, we examined the role of splenic dendritic cells (DCs) in the ITD model.

METHODS

CBA mice were treated with ITD from C57BL/10 splenocytes and 7 days later received transplantation of C57BL/10 hearts. In adoptive transfer studies, splenic DCs from ITD-treated mice were transferred into naïve CBA recipients that received C57BL/10 hearts immediately after the transfer. In addition, to determine the role of splenic DCs isolated from ITD-treated mice, the cells were incubated under stimulation with lipopolysaccharide (LPS).

RESULTS

ITD-treated CBA recipients had markedly prolonged allograft survival (median survival time [MST], 67 days) while naïve recipients rejected allografts acutely (MST, 8 days). In adoptive transfer studies, CBA recipients of the transfer of splenic DCs from ITD-treated mice had prolonged allograft survival (MST, 85 days), while CBA recipients of the transfer of splenic DCs from naïve mice did not have prolonged allograft survival (MST, 8 days). In another transfer study, CBA recipients of the transfer of splenic CD8α⁺ DCs from ITD-treated mice had prolonged allograft survival (MST, 79 days), while those receiving splenic CD8α^- DCs from ITD-treated mice did not have prolonged allograft survival (MST, 8 days). In vitro studies showed that ITD-treated splenic DCs produced more IL-10 and less IL-12 than naïve splenic DCs under stimulation with LPS.

CONCLUSIONS

ITD pretreatment induces regulatory DCs, which produce high amounts of IL-10 resulting in the prolongation of graft survival in our model.

Pathways of major histocompatibility complex allorecognition

PURPOSE OF REVIEW

Here, we review the pathways of allorecognition and their potential relevance to the balance between regulatory and effector responses following transplantation.

RECENT FINDINGS

Transplantation between nonidentical members of the same species elicits an immune response that manifests as graft rejection or persistence. Presentation of foreign antigen to recipient T cells can occur via three nonmutually exclusive routes, the direct, indirect and semi-direct pathways. Allospecific T cells can have effector or regulatory functions, and the relative proportions of the two populations activated following alloantigen presentation are two of the factors that determine the clinical outcome. Regulatory T cells have been the subject of significant research, and there is now greater understanding of their recruitment and function in the context of allorecognition.

SUMMARY

A greater understanding of the mechanisms underlying allorecognition may be fundamental to appreciating how these different populations are recruited and could in turn inform novel strategies for immunomodulation.

Pathways of major histocompatibility complex allorecognition

PURPOSE OF REVIEW

Here, we review the pathways of allorecognition and their potential relevance to the balance between regulatory and effector responses following transplantation.

RECENT FINDINGS

Transplantation between nonidentical members of the same species elicits an immune response that manifests as graft rejection or persistence. Presentation of foreign antigen to recipient T cells can occur via three nonmutually exclusive routes, the direct, indirect and semi-direct pathways. Allospecific T cells can have effector or regulatory functions, and the relative proportions of the two populations activated following alloantigen presentation are two of the factors that determine the clinical outcome. Regulatory T cells have been the subject of significant research, and there is now greater understanding of their recruitment and function in the context of allorecognition.

SUMMARY

A greater understanding of the mechanisms underlying allorecognition may be fundamental to appreciating how these different populations are recruited and could in turn inform novel strategies for immunomodulation.

Depletion of alveolar macrophages abrogates prolongation of cardiac allograft survival induced by intratracheal delivery of alloantigen

BACKGROUND

We previously showed that pretreatment with intratracheal delivery (ITD) of alloantigen induced prolonged cardiac allograft survival and generated regulatory cells in mice. In this study, we examined the role of alveolar macrophages (AM) in our ITD model.

METHODS

Some CBA mice were given ITD of C57BL/6 splenocytes and underwent transplantation of C57BL/6 hearts 7 days later. In others, AM were depleted with clodronate-loaded liposomes 3 days before ITD. In adoptive transfer studies, whole splenocytes were obtained from ITD-treated CBA mice and administered to naïve CBA secondary recipients, which were given C57BL/6 hearts immediately afterward. Interleukin-10 concentrations in bronchoalveolar lavage fluid were assessed by enzyme-linked immunosorbent assays. Immunohistologic and flow cytometric studies were performed after ITD.

RESULTS

C57BL/6 splenocytes given by ITD were ingested by AM in 2 days and undetectable in paratracheal lymph nodes or spleen tissue. CBA mice given ITD of C57BL/6 splenocytes had markedly prolonged allograft survival (median survival time [MST], 86 days), whereas naïve CBA mice rejected allografts acutely (MST, 8 days). AM-depleted, ITD-treated mice also rejected allografts (MST, 5.5 days). Naïve secondary recipients given adoptive transfer of splenocytes from ITD-treated mice had prolonged allograft survival (MST, >100 days), whereas secondary recipients given adoptive transfer of splenocytes from AM-depleted, ITD-treated mice rejected the grafts (MST, 15.5 days). Interleukin-10 expression in bronchoalveolar lavage fluid was down-regulated in AM-depleted mice compared with naïve mice.

CONCLUSIONS

AM have an important role in the induction of regulatory cells in our model of ITD of alloantigen.

Co-inhibitory molecules: Controlling the effectors or controlling the controllers?

Nearly forty years ago the concept was proposed that lymphocytes are negatively regulated by what are now called co-inhibitory signals. Nevertheless, it is only the more recent identification of numerous co-inhibitors and their critical functions that has brought co-inhibition to the forefront of immunologic research. Although co-inhibitory signals have been considered to directly regulate conventional T cells, more recent data has indicated a convergence between co-inhibitory signals and the other major negative control mechanism in the periphery that is mediated by regulatory T cells. Furthermore, it is now clear that lymphocytes are not the sole domain of co-inhibitory signals, as cells of the innate immune system, themselves controllers of immunity, are regulated by co-inhibitors they express. Thus, in order to better understand negative regulation in the periphery and apply this knowledge to the treatment of disease, a major focus for the future should be the definition of the conditions where co-inhibition controls effector cells intrinsically versus extrinsically (via regulatory or innate cells).

Immunotherapeutic strategies employing RNA interference technology for the control of cancers

The human immune system is comprised of several types of cells that have the potential to eradicate tumors without inflicting damage on normal tissue. Over the past decade, progress in the understanding of tumor biology and immunology has offered the exciting possibility of treating malignant disease with vaccines that exploit the capacity of T cells to effectively and selectively kill tumor cells. However, the immune system frequently fails to mount a successful defense against cancers despite vaccination with tumor-associated antigens. The ability of these vaccines to generate an abundant supply of armed effector T cells is often limited by immunoregulatory signaling pathways that suppress T cell activation. In addition, many tumors create a local microenvironment that inhibits the function of T cells. The attenuation of these pathways, which facilitate the evasion of tumors from immune surveillance, thus represents a potentially effective approach for cancer immunotherapy. Specifically, it may be of interest to modify the properties of dendritic cells, T cells, and tumor cells to downregulate the expression of proteins that diminish the immune response to cancers. RNA interference (RNAi) techniques have developed into a highly effective means of intracellular gene 'knockdown' and may be successfully employed in this way to improve cancer immunotherapies. This strategy has recently been explored both in vitro and in vivo, and has generated significantly enhanced antitumor immunity in numerous studies. Nevertheless, several practical concerns remain to be resolved before RNAi technology can be implemented safely and efficiently in humans. As novel developments and discoveries in molecular biology rapidly continue to unfold, it is likely that this technology may soon translate into a potent form of gene silencing in the clinic with profound applications to cancer immunotherapy.

Antigen-induced suppressor T cells from the skin point of view

The rebirth of interest in suppressor T cells has spawned a vast amount of data that shed light on their biology and role in immune system homeostasis. Since the early studies on the naturally occurring CD4+/CD25+ T regulatory cells, much attention has been focused on ways to induce suppressor T cells in vivo. This review discusses the salient features of the induction of antigen-specific suppressor T cells in a T cell receptor (TCR) transgenic mouse model of experimental allergic encephalomyelitis (EAE) in response to epicutaneous immunization with cognate peptide. We discuss the skin environment as a privileged anatomical site for therapeutic intervention against pro-inflammatory auto-immune disorders using non-invasive approaches for antigen delivery.

Effects of Immunosuppressants on Induction of Regulatory Cells After Intratracheal Delivery of Alloantigen

BACKGROUND

We previously reported that intratracheal delivery (ITD) of alloantigen generated regulatory cells in mice. Here, we examined the effect of various doses of conventional immunosuppressants (FK506, cyclosporine A, azathioprine, mycophenolate mofetil, and rapamycin) on inducing regulatory cells in our model.

METHODS

CBA mice (primary recipients) were given C57BL/6 splenocytes by ITD and either no additional treatment or various doses of an immunosuppressant. Seven days later, splenocytes from these mice were adoptively transferred into naive secondary CBA recipients that underwent C57BL/6 cardiac grafting the same day.

RESULTS

Adoptive transfer from primary recipients given ITD of splenocytes alone induced prolonged allograft survival in secondary recipients (median survival time [MST], 50 days), suggesting that regulatory cells were generated. When ITD of alloantigen was combined with daily administration of 0.1 mg/kg FK506 or 0.2 mg/kg rapamycin, graft survival was similarly prolonged (MST 55 and 50 days, respectively). When combined with 20 or 40 mg/kg MMF or 0.4 mg/kg rapamycin, the majority of recipients demonstrated indefinite survival (MST, >100 days in all groups). When ITD of alloantigen was combined with 0.3, 0.5, or 1.0 mg/kg FK506; 5, 10, or 25 mg/kg cyclosporine A; or 1.0 or 2.0 mg/kg azathioprine, allografts were rejected acutely (MST 7-13 days).

CONCLUSION

Generation of regulatory cells by ITD of alloantigen was facilitated by mycophenolate mofetil and high doses of rapamycin but abrogated by cyclosporine A, azathioprine, and high doses of FK506. Low doses of rapamycin and of FK506 did not interfere with generation of regulatory cells.

Interleukin-10 but not Transforming Growth Factor-β is Essential for Generation and Suppressor Function of Regulatory Cells Induced by Intratracheal Delivery of Alloantigen

BACKGROUND

We previously reported that intratracheal delivery of alloantigen-induced regulatory cells in mouse heart-transplantation model. Here, we investigated roles of interleukin (IL)-10 and transforming growth factor (TGF)-beta in induction and effector phases of the regulatory cells.

METHODS

CBA mice were pretreated with intratracheal delivery of C57BL/10 splenocytes and administration of neutralizing anti-IL-10 or anti-TGF-beta monoclonal antibody (mAb). Seven days after the pretreatment, naive CBA mice (secondary recipients) were given adoptive transfer of splenocytes from the pretreated mice and underwent heart grafting from C57BL/10 mice. To determine roles of these cytokines in the effector phase of the regulatory cells, anti-IL-10 or anti-TGF-beta mAb was administered weekly into the secondary recipients after the adoptive transfer.

RESULTS

Adoptive transfer of splenocytes from CBA mice that had been pretreated with intratracheal delivery of C57BL/10 splenocytes significantly prolonged the survival of C57BL/10 allograft (median survival time [MST] 68 days) as compared with adoptive transfer from untreated CBA mice (MST 12 days). In the induction phase, anti-IL-10 mAb abrogated development of the regulatory cells that afforded prolonged allograft survival in the secondary recipients (MST 20 days), whereas anti-TGF-beta mAb did not abrogate it (MST 88 days). In the effector phase, anti-IL-10 mAb abrogated prolonged allograft survival afforded by adoptive transfer of the regulatory cells in the secondary recipients (MST 27 days), whereas anti-TGF-beta mAb did not abrogate suppressor function of the regulatory cells (MST 53 days).

CONCLUSION

IL-10 but not TGF-beta was required for generation and suppressor function of the regulatory cells induced by intratracheal delivery of alloantigen.

Programmed death-1-programmed death-L1 interaction is essential for induction of regulatory cells by intratracheal delivery of alloantigen

BACKGROUND

Programmed death (PD)-1 has been implicated in peripheral tolerance. The authors investigated the roles of PD-1 and its ligands, PD-L1 and PD-L2, in the induction of regulatory cells by intratracheal delivery of alloantigen.

METHODS

CBA (H-2k) mice were pretreated with intratracheal delivery of C57BL/10 (H-2b) splenocytes and administration of monoclonal antibody (mAb) specific for PD-1, PD-L1, or PD-L2. Seven days later, C57BL/10 hearts were transplanted into the pretreated CBA mice. Some naive CBA mice underwent adoptive transfer of splenocytes from the pretreated CBA mice and transplantation of C57BL/10 heart.

RESULTS

Untreated CBA mice rejected C57BL/10 cardiac grafts acutely (median survival time [MST], 7 days). Pretreatment with intratracheal delivery of C57BL/10 splenocytes prolonged graft survival significantly (MST, 65 days). Administration of control immunoglobulin (Ig) G or anti-PD-L2 mAb did not significantly affect the prolongation (MST, 72 and 68 days, respectively). In contrast, anti-PD-1 or anti-PD-L1 mAb abrogated the prolongation (MST, 18 and 17 days, respectively). Adoptive transfer from mice pretreated with intratracheal delivery of alloantigen plus control IgG or anti-PD-L2 mAb prolonged survival of C57BL/10 grafts in secondary CBA recipients (MST, 72 and 56 days, respectively). However, concurrent administration of anti-PD-1 or anti-PD-L1 mAb abrogated prolonged survival after the adoptive transfer (MST, 14 and 20 days, respectively).

CONCLUSIONS

PD-1-PD-L1 interaction was essential for induction of regulatory cells by intratracheal delivery of alloantigen.

Induction of operational tolerance and generation of regulatory cells after intratracheal delivery of alloantigen combined with nondepleting anti-CD4 monoclonal antibody

BACKGROUND

We previously showed that intratracheal delivery of alloantigen induced prolonged survival of fully allogeneic cardiac grafts in mice. Here, this treatment protocol was combined with nondepleting anti-CD4 monoclonal antibody (mAb) to induce operational tolerance.

METHODS

CBA (H-2k) mice were pretreated with intratracheal delivery of whole splenocytes from C57BL/10 (H-2b) mice or a 15-mer Kb peptide, with or without intraperitoneal administration of nondepleting anti-CD4 mAb (YTS177). Seven days later, C57BL/10 hearts were transplanted into the pretreated CBA mice. In addition, some naive CBA mice underwent adoptive transfer of splenocytes from pretreated CBA mice and transplantation of a C57BL/10 heart on the same day.

RESULTS

Untreated CBA mice rejected C57BL/10 cardiac grafts acutely (median survival time, 12 days). Mice given intratracheal delivery of whole splenocytes or Kb peptide demonstrated prolonged graft survival (median survival time, 84 and 76 days, respectively). Concurrent administration of YTS177 and intratracheal delivery of splenocytes or Kb peptide resulted in indefinite graft survival. Mice with long-surviving C57BL/10 cardiac grafts showed acceptance of skin grafts from C57BL/10 mice but not BALB/c mice, demonstrating that operational tolerance had been induced. Adoptive transfer of splenocytes from mice pretreated with intratracheal delivery of splenocytes or Kb peptide plus YTS177 induced indefinite survival of cardiac grafts in secondary recipients, indicating that regulatory cells had been generated.

CONCLUSION

In a murine model, intratracheal delivery of donor splenocytes or Kb peptide combined with YTS177 induced operational tolerance and generated regulatory cells.

CD27/CD70, CD134/CD134 ligand, and CD30/CD153 pathways are independently essential for generation of regulatory cells after intratracheal delivery of alloantigen

BACKGROUND

We investigated whether blockade of tumor necrosis factor receptor-ligand pathways could generate regulatory cells induced by intratracheal delivery of alloantigen.

METHODS

CBA (H-2k) mice were pretreated with intratracheal delivery of splenocytes (1x10(7)) from C57BL/10 (H-2b) mice and intraperitoneal administration of monoclonal antibody (mAb) specific for CD70, CD134 ligand (CD134L), CD153, or CD137L. Seven days later, C57BL/10 hearts were transplanted into pretreated CBA mice. Some naive CBA mice underwent adoptive transfer of splenocytes (5x10(7)) from pretreated CBA mice and transplantation of a C57BL/10 heart on the same day.

RESULTS

Untreated CBA mice rejected C57BL/10 cardiac grafts acutely (median survival time [MST] 12 days). Pretreatment with intratracheal delivery of C57BL/10 donor splenocytes prolonged graft survival significantly (MST 84 days). Mice given intratracheal delivery of alloantigen plus anti-CD70, anti-CD134L, or anti-CD153 mAb, but not those given intratracheal delivery of alloantigen plus anti-CD137L mAb, rejected their graft acutely (MST 16, 14, 10, and 65 days, respectively). Adoptive transfer of splenocytes from mice pretreated with intratracheal delivery of alloantigen plus anti-CD70, CD134L, or CD153 mAb did not prolong survival of C57BL/10 cardiac grafts in naive secondary CBA recipients (MST 14, 11, and 11 days, respectively), whereas adoptive transfer of splenocytes from mice given intratracheal delivery of alloantigen plus anti-CD137L mAb did (MST 75 days).

CONCLUSION

The CD27/CD70, CD134/CD134L, and CD30/CD153 pathways are independently required for generation of regulatory cells in our model.

Role of the CTLA4 pathway in hyporesponsiveness induced by intratracheal delivery of alloantigen

BACKGROUND

The authors previously reported that intratracheal delivery (ITD) of donor alloantigen induced donor-specific hyporesponsiveness to C57BL/10 cardiac allografts in CBA recipients and that blockade of the B7 pathways abrogated that hyporesponsiveness. In this study, the authors used a CD28-deficient model to evaluate which signal, either through CD28 or cytotoxic T-lymphocyte-associated antigen (CTLA4), is involved in the induction of hyporesponsiveness.

METHODS

Seven days before transplantation of hearts from C3H/HeJ (H2k) mice into C57BL/6 (H2b) or CD28-deficient (C57BL/6 background) mice, the transplant recipients were given ITD of donor splenocytes (1 x 10(7)), alone or in combination with human CTLA4-immunoglobulin (Ig) (200 microg).

RESULTS

ITD of C3H splenocytes induced donor-specific hyporesponsiveness to C3H cardiac grafts in C57BL/6 recipients (graft median survival time [MST], 40 days). Administration of CTLA4-Ig concurrently with ITD abrogated the prolonged allograft survival (MST, 12 days). Interestingly, ITD of C3H splenocytes induced prolonged survival of C3H allografts in CD28-deficient recipients (MST, 55 days). Furthermore, administration of CTLA4-Ig combined with ITD of C3H splenocytes abrogated the prolonged survival of C3H allografts in CD28-deficient recipients (MST, 7 days), whereas recipients given isotype-control antibody in combination with ITD of splenocytes had prolonged survival of C3H allografts (MST, 58 days).

CONCLUSIONS

Taken together, the authors' findings indicate that a signal through CTLA4, rather than through CD28, plays an important role in the induction of hyporesponsiveness by ITD of alloantigen in this model.

Induction of indefinite survival of fully allogeneic cardiac grafts and generation of regulatory cells by intratracheal delivery of alloantigens under blockade of the CD40 pathway

BACKGROUND

The authors previously showed that intratracheal delivery (ITD) of donor splenocytes induced prolonged survival of fully allogeneic cardiac grafts in mice. In this study, this treatment protocol was combined with blockade of the CD40 pathway in an attempt to induce operational tolerance.

METHODS

CBA mice were given donor splenocytes (1x107) or Kb peptide (100 microg) by ITD with or without antibody specific for mouse CD40 ligand (MR1, 200 microg) 7 days before transplantation of a C57BL/10 heart. Also, splenocyte (5 x 107) from primary recipient CBA mice given ITD of donor splenocytes or Kb peptide plus MR1 were adoptively transferred into naive CBA secondary recipients 7 days after the pretreatment and C57BL/10 hearts were transplanted into those recipients the same day.

RESULTS

ITD of donor splenocytes and Kb peptide induced prolonged survival of cardiac grafts (median survival time [MST], 74 and 56 days, respectively), whereas naive control mice and mice pretreated with syngeneic splenocytes had acute graft rejection (MST in both groups, 7 days). When MR1 was included, all grafts survived indefinitely (>200 days), but mice pretreated with MR1 alone had graft rejection (MST, 54 days). Mice bearing cardiac grafts had acceptance of skin grafts from C57BL/10 but not BALB/c mice, demonstrating that operational tolerance was induced. Secondary recipients given adoptive transfer of splenocytes from primary recipients of the combined treatment had acceptance of C57BL/10 grafts, suggesting that regulatory cells were generated within 7 days of pretreatment.

CONCLUSIONS

ITD of donor splenocytes or Kb peptide under blockade of the CD40 pathway induced operational tolerance and generated regulatory cells.

Intratracheal delivery of a single major histocompatibility complex class I peptide induced prolonged survival of fully allogeneic cardiac grafts and generated regulatory cells

We have previously reported that intratracheal delivery of donor splenocytes in mice induces hyporesponsiveness to fully allogeneic cardiac grafts and generates regulatory cells. Here, we examined whether an allopeptide would produce the same results. A 15-mer (54-68) peptide corresponding to a hypervariable region of the K(b) molecule was given intratracheally or intravenously to CBA (H2(k)) mice 7 days before transplantation of a C57BL/10 (H2(b)) or BALB/c (H2(d)) heart and was also used in adoptive transfer experiments. Cardiac grafts in recipients given K(b) peptide intratracheally experienced a median survival time (MST) of 56 days, whereas those in recipients given the peptide intravenously were rejected acutely (MST=7.5 days). Adoptive transfer of splenocytes from mice pretreated intratracheally with K(b) peptide to naïve secondary recipients prolonged survival of cardiac grafts (MST = 35 days). Intratracheal delivery of a single major histocompatibility complex class I peptide induced hyporesponsiveness to allogeneic cardiac grafts and generated regulatory cells.

B7/CTLA4 pathway is essential for generating regulatory cells after intratracheal delivery of alloantigen in mice

BACKGROUND

The mechanism of hyporesponsiveness induced by intratracheal (IT) delivery of alloantigen was examined and its effect on cardiac graft survival was assessed in studies in mice.

METHODS

In CBA (H2 ) mice, donor splenocytes were given by IT delivery 7 days before transplantation of a C57BL/10 (H2 ) heart. To determine whether regulatory cells were involved in hyporesponsiveness, splenocytes from mice given IT delivery of alloantigen and antibodies for B7-1, B7-2, or CTLA4 were adoptively transferred to naïve secondary recipients 7 days after delivery; those recipients underwent heart transplantation the same day. Effects on cell proliferation and cytokine production of splenocytes from mice given IT delivery of alloantigen were examined in mixed leukocyte cultures (MLC).

RESULTS

Cardiac graft survival was significantly prolonged in mice given IT delivery of alloantigen (median survival time [MST], 81 days); those given syngeneic splenocytes rejected grafts acutely (MST, 7 days; P<0.05). Adoptive transfer of splenocytes also significantly prolonged survival of cardiac grafts in secondary recipients (MST, 62 days). When B7-1, B7-2, or CTLA4 antibody was combined with IT delivery of alloantigen in the first recipient, all grafts were rejected within 14 days in second recipients after adoptive transfer. In mixed leukocyte cultures, splenocytes from these mice did not respond to alloantigen and production of interleukin-4 and interleukin-10 was increased.

CONCLUSIONS

Donor splenocytes delivered IT induced hyporesponsiveness and regulatory cells in our animal model, and such induction was dependent on B7-1, B7-2, and CTLA4 signals.

Prevention of chronic rejection in murine cardiac allografts: a comparison of chimerism- and nonchimerism-inducing costimulation blockade-based tolerance induction regimens

We have previously described a nonirradiation-based regimen combining costimulation blockade, busulfan, and donor bone marrow cells that promotes stable, high level chimerism, deletion of donor-reactive T cells, and indefinite survival of skin allografts in mice. The purpose of the current study is to determine the efficacy of this tolerance regimen in preventing acute and chronic rejection in a vascularized heart graft model and to compare this regimen with other putative tolerance protocols. Mice receiving costimulation blockade (CTLA4-Ig and anti-CD40 ligand) alone or in combination with donor cells enjoyed markedly prolonged heart graft survival and initially preserved histological structure. However, tolerance was not achieved, as evidenced by the eventual onset of chronic rejection characterized by obliterative vasculopathy and the rejection of secondary skin grafts. In contrast, following treatment with costimulation blockade, busulfan, and bone marrow, heart grafts survived indefinitely without detectable signs of chronic rejection or structural damage, even 100 days after placement of a secondary donor skin graft. We detected multilineage chimerism in peripheral blood, spleen, lymph nodes, and thymus, and peripheral deletion of donor-reactive cells was complete by day 90. These findings indicate that only the CD40/CD28 blockade chimerism induction regimen prevents both acute and chronic rejection of vascularized organ transplants. Further testing of these strategies in a preclinical large animal model is warranted.