Antigen location contributes to the pathological features of a transplanted heart graft

Apelin directs endothelial cell differentiation and vascular repair following immune-mediated injury

Sustained, indolent immune injury of the vasculature of a heart transplant limits long-term graft and recipient survival. This injury is mitigated by a poorly characterized, maladaptive repair response. Vascular endothelial cells respond to proangiogenic cues in the embryo by differentiation to specialized phenotypes, associated with expression of apelin. In the adult, the role of developmental proangiogenic cues in repair of the established vasculature is largely unknown. We found that human and minor histocompatibility-mismatched donor mouse heart allografts with alloimmune-mediated vasculopathy upregulated expression of apelin in arteries and myocardial microvessels. In vivo, loss of donor heart expression of apelin facilitated graft immune cell infiltration, blunted vascular repair, and worsened occlusive vasculopathy in mice. In vitro, an apelin receptor agonist analog elicited endothelial nitric oxide synthase activation to promote endothelial monolayer wound repair and reduce immune cell adhesion. Thus, apelin acted as an autocrine growth cue to sustain vascular repair and mitigate the effects of immune injury. Treatment with an apelin receptor agonist after vasculopathy was established markedly reduced progression of arterial occlusion in mice. Together, these initial data identify proangiogenic apelin as a key mediator of coronary vascular repair and a pharmacotherapeutic target for immune-mediated injury of the coronary vasculature.

CD4+ T lymphocytes produce adiponectin in response to transplants

Adiponectin is a pleiotropic cytokine with diverse immunomodulatory effects on macrophages and lymphocytes. In the current paradigm, lymphocytes and macrophages respond to adiponectin that is produced by adipocytes and other parenchymal cells. Using a model of chronic arterial inflammation in cardiac transplants, we found that T cells derived from the recipient migrate to the heart and produce adiponectin locally. The evidence that T cells produce significant amounts of adiponectin is based on 3 experimental approaches. First, CD4+ T cells isolated from the blood and spleen after cardiac transplantation express mRNA for adiponectin. Second, reconstitution of T cell-deficient recipients with transgenic CD4+ T cells that express receptors for donor antigens results in arterial infiltrates containing T cells and increased mRNA expression for adiponectin in cardiac transplants. Third, CD4+ T cells isolated from the allograft secrete adiponectin in vitro. Taken together, these data indicate that adiponectin-competent cells originating in the recipient migrate into the transplant. Establishing T cells as a source of adiponectin provides a new dimension, to our knowledge, to the modulatory effects of adiponectin on immune responses.

Both rejection and tolerance of allografts can occur in the absence of secondary lymphoid tissues

In this study, we showed that aly/aly mice, which are devoid of lymph nodes and Peyer's patches, acutely rejected fully allogeneic skin and heart grafts. They mounted potent inflammatory direct alloresponses but failed to develop indirect alloreactivity after transplantation. Remarkably, skin allografts also were rejected acutely by splenectomized aly/aly (aly/aly-spl(-)) mice devoid of all secondary lymphoid organs. In these recipients, the rejection was mediated by alloreactive CD8(+) T cells presumably primed in the bone marrow. In contrast, cardiac transplants were not rejected by aly/aly-spl(-) mice. Actually, aly/aly-spl(-) mice that spontaneously accepted a heart allotransplant and displayed donor-specific tolerance also accepted skin grafts from the same, but not a third-party, donor via a mechanism involving CD4(+) regulatory T cells producing IL-10 cytokine. Therefore, direct priming of alloreactive T cells, as well as rejection and regulatory tolerance of allogeneic transplants, can occur in recipient mice lacking secondary lymphoid organs.

Allorecognition pathways in transplant rejection and tolerance

With the advent of cellular therapies, it has become clear that the success of future therapies in prolonging allograft survival will require an intimate understanding of the allorecognition pathways and effector mechanisms that are responsible for chronic rejection and late graft loss.Here, we consider current understanding of T-cell allorecognition pathways and discuss the most likely mechanisms by which these pathways collaborate with other effector mechanisms to cause allograft rejection. We also consider how this knowledge may inform development of future strategies to prevent allograft rejection.Although both direct and indirect pathway CD4 T cells appear active immediately after transplantation, it has emerged that indirect pathway CD4 T cells are likely to be the dominant alloreactive T-cell population late after transplantation. Their ability to provide help for generating long-lived alloantibody is likely one of the main mechanisms responsible for the progression of allograft vasculopathy and chronic rejection.Recent work has suggested that regulatory T cells may be an effective cellular therapy in transplantation. Given the above, adoptive therapy with CD4 regulatory T cells with indirect allospecificity is a rational first choice in attempting to attenuate the development and progression of chronic rejection; those with additional properties that enable inhibition of germinal center alloantibody responses hold particular appeal.

Systemic minor histocompatibility antigen expression in blood endothelial cells prevents T cell-mediated vascular immunopathology

Attenuation of T cell-mediated damage of blood endothelial cells (BECs) in transplanted organs is important to prevent transplant vasculopathy (TV) and chronic rejection. Here, we assessed the importance of minor histocompatibility antigen (mHA) distribution and different coinhibitory molecules for T cell-BEC interaction. A transgenic mHA was directed specifically to BECs using the Tie2 promoter and cellular interactions were assessed in graft-versus-host disease-like and heterotopic heart transplantation settings. We found that cognate CD4(+) T-cell help was critical for the activation of BEC-specific CD8(+) T cells. However, systemic mHA expression on BECs efficiently attenuated adoptively transferred, BEC-specific CD4(+) and CD8(+) T cells and hence prevented tissue damage, whereas restriction of mHA expression to heart BECs precipitated the development of TV. Importantly, the lack of the coinhibitory molecules programmed death-1 (PD-1) and B and T lymphocyte attenuator fostered the initial activation of BEC-specific CD4(+) T cells, but did not affect development of TV. In contrast, TV was significantly augmented in the absence of PD-1 on BEC-specific CD8(+) T cells. Taken together, these results indicate that antigen distribution in the vascular bed determines the impact of coinhibition and, as a consequence, critically impinges on T cell-mediated vascular immunopathology.

Mixed chimerism and split tolerance: mechanisms and clinical correlations

Establishing hematopoietic mixed chimerism can lead to donor-specific tolerance to transplanted organs and may eliminate the need for long-term immunosuppressive therapy, while also preventing chronic rejection. In this review, we discuss central and peripheral mechanisms of chimerism induced tolerance. However, even in the long-lasting presence of a donor organ or donor hematopoietic cells, some allogeneic tissues from the same donor can be rejected; a phenomenon known as split tolerance. With the current goal of creating mixed chimeras using clinically feasible amounts of donor bone marrow and with minimal conditioning, split tolerance may become more prevalent and its mechanisms need to be explored. Some predisposing factors that may increase the likelihood of split tolerance are immunogenicity of the graft, certain donor-recipient combinations, prior sensitization, location and type of graft and minimal conditioning chimerism induction protocols. Additionally, split tolerance may occur due to a differential susceptibility of various types of tissues to rejection. The mechanisms involved in a tissue's differential susceptibility to rejection include the presence of polymorphic tissue-specific antigens and variable sensitivity to indirect pathway effector mechanisms. Finally, we review the clinical attempts at allograft tolerance through the induction of chimerism; studies that are revealing the complex relationship between chimerism and tolerance. This relationship often displays split tolerance, and further research into its mechanisms is warranted.

Anti-complement component C5 mAb synergizes with CTLA4Ig to inhibit alloreactive T cells and prolong cardiac allograft survival in mice

While activation of serum complement mediates antibody-initiated vascular allograft injury, increasing evidence indicates that complement also functions as a modulator of alloreactive T cells. We tested whether blockade of complement activation at the C5 convertase step affects T cell-mediated cardiac allograft rejection in mice. The anti-C5 mAb BB5.1, which prevents the formation of C5a and C5b, synergized with subtherapeutic doses of CTLA4Ig to significantly prolong the survival of C57BL/6 heart grafts that were transplanted into naive BALB/c recipients. Anti-C5 mAb treatment limited the induction of donor-specific IFNγ-producing T cell alloimmunity without inducing Th2 or Th17 immunity in vivo and inhibited primed T cells from responding to donor antigens in secondary mixed lymphocyte responses. Additional administration of anti-C5 mAb to the donor prior to graft recovery further prolonged graft survival and concomitantly reduced both the in vivo trafficking of primed T cells into the transplanted allograft and decreased expression of T cell chemoattractant chemokines within the graft. Together these results support the novel concept that C5 blockade can inhibit T cell-mediated allograft rejection through multiple mechanisms, and suggest that C5 blockade may constitute a viable strategy to prevent and/or treat T cell-mediated allograft rejection in humans.

Complement regulates CD4 T-cell help to CD8 T cells required for murine allograft rejection

Although induction of CD8 T-cell responses to transplants requires CD4-cell help, how this help is transmitted remains incompletely characterized. In vitro, cognate interactions between CD4 T cells and dendritic cells (DCs) induce C3a and C5a production. CD8(+) T cells lacking C3a receptor (C3aR) and C5a receptor (C5aR) proliferate weakly to allogeneic DCs despite CD4 help, indicating that CD4-cell help is mediated, in part, through DC-derived C3a/C5a acting on CD8(+) T cell-expressed C3aR/C5aR. In support of this concept, augmenting DC C5a/C3a production bypasses the requirement for CD4- and CD40-dependent help to wild-type CD8(+) T cells. CD4-deficient recipients of allogeneic heart transplants prime weak CD8 responses and do not acutely reject their grafts. In contrast, CD4-deficient chimeric mice possessing decay accelerating factor deficient (Daf1(-/-)) bone marrow, in which DC C3a/C5a production is potentiated, acutely reject transplants through a CD8 cell-dependent mechanism. Furthermore, hearts transplanted into CD40(-/-) mice prime weak CD8-cell responses and survive indefinitely, but hearts transplanted into Daf1(-/-)CD40(-/-) recipients undergo CD8 cell-dependent rejection. Together, the data indicate that heightened production and activation of immune cell-derived complement bypasses the need for CD40/CD154 interactions and implicate antigen-presenting cell-produced C5a and C3a as molecular bridges linking CD4 help to CD8(+) T cells.

Human endogenous retrovirus transcription profiles of the kidney and kidney-derived cell lines

The human genome comprises approximately 8-9 % of human endogenous retroviruses (HERVs) that are transcribed with tissue specificity. However, relatively few organs have been examined in detail for individual differences in HERV transcription pattern, nor have tissue-to-cell culture comparisons been frequently performed. Using an HERV-specific DNA microarray, a core HERV transcription profile was established for the human kidney comparing 10 tissue samples. This core represents HERV groups expressed uniformly or nearly so in non-tumour kidney tissue. The profiles obtained from non-tumour tissues were compared to 10 renal tumour tissues (renal cell carcinoma, RCC) derived from the same individuals and additionally, to 22 RCC cell lines. No RCC cell line or tumour-specific differences were observed, suggesting that HERV transcription is not altered in RCC. However, when comparing tissue transcription to cell line transcription, there were consistent differences. The differences were irrespective of cancer state and included cell lines derived from non-tumour kidney tissue, suggesting that a specific alteration of HERV transcription occurs when establishing cell lines. In contrast to previous publications, all known HERV-derived tumour antigens, including those identified in RCC, were expressed both in multiple RCC cell lines and several non-tumour tissue-derived cell lines, a result that contrasts with findings from patient samples. The results establish the core kidney transcription pattern of HERVs and reveal differences between cell culture lines and tissue samples.

Differential susceptibility of allogeneic targets to indirect CD4 immunity generates split tolerance

CD4 T cells frequently help to activate CD8 T and B cells that effect transplant rejection. However, CD4 T cells alone can reject transplants, either directly or indirectly. The relative effectiveness of indirect CD4 immunity in rejecting different types of allogeneic grafts is unknown. To address this, we used a TCR transgenic mouse model in which indirect CD4 alloimmunity alone can be studied. We challenged transgenic recipients with hematopoietic cells and shortly thereafter skin transplants that could only be rejected indirectly, and observed Ag-specific indirect donor B cell and skin rejection, but not T cell elimination, reflecting a state of split tolerance. Deficiency of indirect CD4 alloimmunity in donor T cell rejection was also apparent when acute indirect rejection of donor islets occurred despite generation and maintenance of mixed T cell chimerism, due to migration of the few passenger T cells into recipient circulation. Although passenger lymphocytes delayed indirect islet rejection, they enhanced rejection by a full repertoire capable of both direct and indirect reactivity. Interestingly, the persistence of chimerism was associated with the eventual development of tolerance, as demonstrated by acceptance of donor skin grafts given late to hematopoietic cell recipients, and hyporesponsiveness of transgenic T cells from islet recipients in vitro. Mechanistically, tolerance was recessive and associated with progressive down-regulation of CD4. Collectively, our data indicate that indirect CD4 immunity is not equally destructive toward different types of allogeneic grafts, the deficiency of which generates split tolerance. The futility of these responses can convert immunity into tolerance.

Donor deficiency of decay-accelerating factor accelerates murine T cell-mediated cardiac allograft rejection

Decay-accelerating factor (DAF) is a cell surface regulator that accelerates the dissociation of C3/C5 convertases and thereby prevents the amplification of complement activation on self cells. In the context of transplantation, DAF has been thought to primarily regulate antibody-mediated allograft injury, which is in part serum complement-dependent. Based on our previously delineated link between DAF and CD4 T cell responses, we evaluated the effects of donor Daf1 (the murine homolog of human DAF) deficiency on CD8 T cell-mediated cardiac allograft rejection. MHC-disparate Daf1(-/-) allografts were rejected with accelerated kinetics compared with wild-type grafts. The accelerated rejection predominantly tracked with DAF's absence on bone marrow-derived cells in the graft and required allograft production of C3. Transplantation of Daf1(-/-) hearts into wild-type allogeneic hosts augmented the strength of the anti-donor (direct pathway) T cell response, in part through complement-dependent proliferative and pro-survival effects on alloreactive CD8 T cells. The accelerated allograft rejection of Daf1(-/-) hearts occurred in recipients lacking anti-donor Abs. The results reveal that donor DAF expression, by controlling local complement activation on interacting T cell APC partners, regulates the strength of the direct alloreactive CD8(+) T cell response. The findings provide new insights into links between innate and adaptive immunity that could be exploited to limit T cell-mediated injury to an allograft following transplantation.

Decay accelerating factor can control T cell differentiation into IFN-gamma-producing effector cells via regulating local C5a-induced IL-12 production

A newly recognized link between the complement system and adaptive immunity is that decay accelerating factor (DAF), a cell surface C3/C5 convertase regulator, exerts control over T cell responses. Extending these results, we show that cultures of Marilyn TCR-transgenic T cells stimulated with DAF-deficient (Daf1(-/-)) APCs produce significantly more IL-12, C5a, and IFN-gamma compared with cultures containing wild-type APCs. DAF-regulated IL-12 production and subsequent T cell differentiation into IFN-gamma-producing effectors was prevented by the deficiency of either C3 or C5a receptor (C5aR) in the APC, demonstrating a link between DAF, local complement activation, IL-12, and T cell-produced IFN-gamma. Bone marrow chimera experiments verified that bone marrow cell-expressed C5aR is required for optimal differentiation into IFN-gamma-producing effector T cells. Overall, our results indicate that APC-expressed DAF regulates local production/activation of C5a following cognate T cell/APC interactions. Through binding to its receptor on APCs the C5a up-regulates IL-12 production, this in turn, contributes to directing T cell differentiation toward an IFN-gamma-producing phenotype. The findings have implications for design of therapies aimed at altering pathologic T cell immunity.

CD154+ graft antigen-specific CD4+ T cells are sufficient for chronic rejection of minor antigen incompatible heart grafts

We used a defined model system to address the role of minor histocompatibility antigen-specific CD4+ T cells in chronic rejection. The coronary arteries of vascularized heart grafts expressing the model antigen ovalbumin developed intimal hyperplasia in normal recipients and those lacking CD8+ T cells but not in those lacking CD4+ T cells. Furthermore, purified ovalbumin-specific CD4+ T cells from T-cell antigen receptor transgenic mice caused intimal hyperplasia in ovalbumin-expressing heart grafts in lymphocyte-deficient mice. The graft antigen-specific CD4+ T cells only caused intimal hyperplasia when expressing CD154 and were found in the intima of the affected coronary arteries along with CD40+ cells, CD11c+ dendritic cells and CD11b+, Gr-1+ monocytes. These results show that minor histocompatibility antigen-specific CD4+ T cells are required to cause the classical vascular changes of chronic rejection. They are capable of doing so without contributions from other lymphocytes, and may cause intimal hyperplasia by using CD154 to stimulate other non-lymphoid cells in the intima.

The replacement of graft endothelium by recipient-type cells conditions allograft rejection mediated by indirect pathway CD4(+) T cells

BACKGROUND

Whereas the participation of alloreactive T cells sensitized by indirect allorecognition in graft rejection is well documented, the nature of recipient antigen presenting cells recognized by indirect pathway CD4(+) T cells within the graft has yet to be identified. The purpose of this study was to determine the role played by graft endothelium replacement in the immune recognition of cardiac allografts rejected by indirect pathway CD4(+) T cells.

METHODS

Transgenic RAG2(-/-) mice expressing I-A(b)-restricted male antigen H-Y-specific TcR were studied for their capacity to reject H-2(k) male cardiac allografts. Chronic vascular rejection in this model was due to the indirect recognition of H-Y antigen shed from H-2(k) male allograft and presented by the recipient's own I-A(b) APC to transgenic T cells.

RESULTS

Immunohistochemical analysis of rejected grafts revealed the presence of numerous microvascular endothelial cells (EC) that expressed recipient's I-A MHC class II molecules. This observation suggested that graft endothelium replacement by I-A(b)-positive cells of recipient origin could stimulate the rejection of male H-2(k) graft by I-A(b)--restricted H-Y--specific T cells. To investigate further this possibility, hearts from H-2(b)--into--H-2(k) irradiation bone marrow (BM) chimera were transplanted in transgenic recipients. A direct correlation was observed between the presence of I-A(b)-positive EC within myocardial microvessels and the induction of acute rejection of chimeric H-2(k) male cardiac allografts transplanted in transgenic recipients.

CONCLUSIONS

We conclude that graft endothelium replacement by recipient-type cells is required for the rejection of cardiac allograft mediated by indirect pathway alloreactive CD4(+) T cells.

CD8 T Cells Specific for a Donor-Derived, Self-Restricted Transplant Antigen Are Nonpathogenic Bystanders after Vascularized Heart Transplantation in Mice

CD8 T cell cross-priming, an established mechanism of protective antiviral immunity, was originally discovered during studies involving minor transplantation Ags. It is unclear whether or how cross-primed CD8 T cells, reactive to donor-derived, but recipient class I MHC-restricted epitopes, could injure a fully MHC-disparate, vascularized transplant. To address this question we studied host class I MHC-restricted, male transplantation Ag-reactive T cell responses in female recipients of fully MHC-disparate, male heart transplants. Cross-priming to the immune-dominant determinant HYUtyp occurred at low frequency after heart transplantation. CD8 T cell preactivation through immunization with HYUtyp mixed in CFA did not alter the kinetics of acute rejection. Furthermore, neither HYUtyp immunization nor adoptive transfer of HYUtyp-specific TCR-transgenic T cells affected outcome in 1) a model of chronic rejection in the absence of immunosuppression or 2) a model of allograft acceptance induced by costimulatory blockade. The results support the contention that CD8 T cells reactive to host-restricted, but donor-derived, Ags are highly specific and are nonpathogenic bystanders during rejection of MHC-disparate cardiac allografts.

Lymphoid Sequestration of Alloreactive Memory CD4 T Cells Promotes Cardiac Allograft Survival

Memory T cells specific for donor Ags present a unique challenge in transplantation. In addition to expressing robust immune responses to a transplanted organ, memory T cells may be resistant to the effects of immunosuppressive therapies used to prolong graft survival. In this study, we explore the possibility of controlling deleterious donor-reactive memory CD4 T cells through lymphoid sequestration. We showed that sphingosine 1-phosphate receptor-1 agonist FTY720 induces relocation of circulating memory CD4 T cells into secondary lymphoid organs. Lymphoid sequestration of these donor-reactive memory CD4 T cells prolonged survival of murine heterotopic cardiac allografts and synergizes with conventional costimulatory blockade to further increase graft survival. Despite limited trafficking, memory CD4 T cells remain capable of providing help for the induction of anti-donor CD8 T cell and alloantibody responses. Elimination of antidonor humoral immunity resulted in indefinite allograft survival proving the pathogenicity of alloantibody under these conditions. Overall, this is the first demonstration that FTY720 influences memory CD4 T cell trafficking and attenuates their contribution to allograft rejection. The data have important implications for guiding FTY720 therapy and for designing combinatorial strategies aimed at prolonging allograft survival in sensitized transplant patients with donor-specific memory T cells.

Mouse endothelial cells cross-present lymphocyte-derived antigen on class I MHC via a TAP1- and proteasome-dependent pathway

In vivo studies suggest that vascular endothelial cells (ECs) can acquire and cross-present exogenous Ag on MHC-I but the cellular mechanisms underlying this observation remain unknown. We tested whether primary female mouse aortic ECs could cross-present exogenous male Ag to the T cell hybridoma, MHH, specific for HYUty plus D(b). MHC-I-deficient male spleen cells provided a source of male Ag that could not directly stimulate the MHH cells. Addition of male but not female MHC-I-deficient spleen cells to wild-type syngeneic female EC induced MHH stimulation, demonstrating EC cross-presentation. Lactacystin treatment of the donor male MHC-I-deficient spleen cells, to inhibit proteasome function, markedly enhanced EC cross-presentation showing that the process is most efficient for intact proteins rather than degraded peptide fragments. Additional experiments revealed that this EC Ag-processing pathway is both proteasome and TAP1 dependent. These studies demonstrate that cultured murine aortic ECs can process and present MHC-I-restricted Ag derived from a separate, live cell, and they offer insight into the molecular requirements involved in this EC Ag presentation process. Through this pathway, ECs expressing cross-presented peptides can participate in the effector phase of T cell-mediated inflammatory responses such as autoimmunity, anti-tumor immunity, and transplant rejection.

Transcatheter placement of a low-profile biodegradable pulmonary valve made of small intestinal submucosa: A long-term study in a swine model

OBJECTIVE

We sought to investigate a placement of a percutaneous low-profile prosthetic valve constructed of small intestinal submucosa in the pulmonary position in a swine model.

METHODS

Twelve female farm pigs were stented at the native pulmonary valve to induce pulmonary insufficiency. Once right ventricular dilation occurred, the small intestinal submucosa valve was implanted. The pigs were followed up with transthoracic echocardiographic Doppler scanning. One animal died of heart failure before valve replacement. Animals were euthanized at 1 day, 1 month, 3 months, 6 months, and 12 months after valve implantation.

RESULTS

The small intestinal submucosa pulmonary valve showed effective reversal of pulmonary regurgitation. There were no misplacements during deployment. There were no embolizations. One-year echocardiographic follow-up showed minimal regurgitation and no stenosis for a valve/vessel ratio of 0.78 or greater. Histologic examination demonstrated intensive remodeling of the small intestinal submucosal valve. Within 1 month, the surface was covered by endothelium, and fibroblasts invaded the interior. Over the following months, the small intestinal submucosal valve remodeled without apparent graft rejection.

CONCLUSION

The small intestinal submucosa valve has the potential for graft longevity without the need for anticoagulation or immunosuppression. Histologic remodeling of the valve tissue provides a replacement capable of resembling a native valve that can be placed percutaneously with low-profile delivery systems.

Reepithelialized orthotopic tracheal allografts expand memory cytotoxic T lymphocytes but show no evidence of chronic rejection

BACKGROUND

Acute rejection of mouse tracheal allografts is characterized by infiltration of the lamina propria with CD4+/CD8+ T cells that leads to the destruction of the epithelium and luminal obliteration. The donor epithelium is progressively replaced by recipient-derived epithelium. Once allograft reepithelialization has occurred, immunosuppression can be withdrawn without inciting acute rejection. We hypothesize that reepithelialization will also prevent chronic rejection of the trachea after withdrawal of immunosuppression.

METHODS

BALB/c tracheal grafts were transplanted orthotopically into allogeneic C57BL/6 recipients. Allografted mice were nonimmunosuppressed for 10 or 100 days or immunosuppressed with cyclosporine A continuously for 50 days and then withdrawn from immunosuppression for an additional 50 days. In addition, grafts from this group were then heterotopically retransplanted into isogenic C57BL/6 or allogeneic BALB/c recipients to assess their immunogenicity.

RESULTS

Cyclosporine A-treated mice showed no signs of chronic rejection or priming of cellular immunity as measured by proliferation and cytokine secretion in a mixed leukocyte reaction. However, there was a notable expansion of memory CD8+ T cells specific for donor major histocompatibility complex. When these tracheal allografts were retransplanted heterotopically into C57BL/6 or BALB/c, they demonstrated reduced responses toward BALB/c and primed responses toward C57BL/6, respectively. These results suggest that the grafts express a chimeric phenotype consisting of both BALB/c and C57BL/6 antigens.

CONCLUSION

These observations suggest that long-term withdrawal of immunosuppression does not lead to chronic tracheal rejection even in the presence of alloantigen specific cytotoxic T-lymphocyte responses and that the reepithelialized grafts may contain donor elements that impact the generation of immunity.