Mechanism of liver allograft rejection: the indirect recognition pathway

Repetitive gastric aspiration leads to augmented indirect allorecognition after lung transplantation in miniature swine

INTRODUCTION

Lung transplant recipients with documented gastroesophageal reflux disease (GERD) are at increased risk for graft dysfunction. Here, we present the first large-animal model of gastric aspiration after allogeneic lung transplantation and some preliminary data demonstrating the effect of chronic aspiration on the direct and indirect pathways of allorecognition.

METHODS

Left orthotopic lung transplants (n=3) were performed in miniature swine across a major histocompatibility complex class I disparity, followed by 12 days of high-dose cyclosporine A. At the time of transplantation, a transtracheal catheter was placed at the carina, above the bronchial anastomosis. A gastrostomy tube was placed for daily aspiration of gastric contents. Subsequently, graft lungs were instilled with gastric aspirate daily (3 mL/hrX8 hr/day) for 50 days. Recipients were followed up with daily complete blood count, scheduled chest radiographs, and biopsies. In vitro studies, including cell-mediated lympholysis, mixed lymphocyte reactions, and peptide proliferation assays, were performed. Results from these three recipients were compared with those of historical controls (n=6) who were treated identically, except for the tracheal cannulation and simulated gastric aspiration.

RESULTS

Two of the experimental animals were euthanized with nonviable lungs soon after the postoperative day 50 biopsy. In both cases the native lung was normal. The third animal survived over 180 days without the evidence of chronic rejection. After immunosuppressive treatment, all animals demonstrated donor-specific hyporesponsiveness by assays of direct alloresponse (cell-mediated lympholysis, mixed lymphocyte reaction). A significant response to synthetic donor-derived class I peptide, however, was seen in all animals. A more pronounced and diffuse response was seen in the animals rejecting their grafts. The historical controls showed medium-term graft survival with evidence of chronic rejection in the majority of animals, as previously reported.

CONCLUSION

In a model of GERD after lung transplantation, a spectrum of clinical outcomes was observed. The in vitro data suggest that acid reflux enhances the indirect alloresponse to processed donor class I antigen, giving mechanistic insight into the manner in which GERD may be deleterious to the transplanted lung.

Monitoring of indirect allorecognition: wishful thinking or solid data?

Monitoring of T cells involved in the alloimmune response after transplantation requires the availability of reliable in vitro assays for the detection of T cells with both direct and indirect allospecificity. While generally accepted assays exist to measure helper and cytotoxic T cells involved in direct allorecognition, consensus about an assay for monitoring indirect T-cell allorecognition in clinical transplantation is lacking. Many studies claim a relationship between the reactivity of T cells with indirect allospecificity and graft rejection, but different protocols are used and essential controls are often lacking. In this review, the disadvantages and pitfalls of the current approaches are discussed, in some cases supported by the results of our own in vitro experiments. We conclude that an international workshop is necessary to establish and validate a uniform, robust and reliable assay for the monitoring of transplant recipients and to study the actual role of indirect allorecognition in acute and chronic rejection.

FasL expression in hepatic antigen-presenting cells and phagocytosis of apoptotic T cells by FasL+ Kupffer cells are indicators of rejection activity in human liver allografts

Fas-Fas ligand (FasL) interaction and apoptosis are important in the mechanism of allograft rejection. However, the interaction between donor and recipient cells, specifically focusing on antigen-presenting cells (APCs), under various conditions is poorly understood in human liver allografts. FasL expression on APCs, its association with apoptosis, and the origin of apoptotic lymphocytes in human liver allografts were assessed by immunohistochemistry and in situ hybridization. We found increased expression of FasL on Kupffer cells (KCs) and endothelium in acute cellular rejection (n = 20) and to lesser extent in chronic rejection (n = 6) and septic cholangitis (n = 5) compared with stable grafts and normal controls. In addition, the graft specificity of infiltrating T cells was confirmed by polymerase chain reaction examination of T-cell receptor-gamma loci. T-cell apoptosis occurred at a higher rate in acute cellular rejection than in chronic rejection or septic cholangitis. The number of apoptotic bodies derived from recipient lymphocytes correlated with the severity of rejection and was reversed by treatment. FasL(+) KCs phagocytosed CD4(+) interferon-gamma(+) T cells, rather than CD4(+) interleukin-4(+) T cells, suggesting a role of KCs in regulating CD4(+) T-cell subset differentiation. In conclusion, our data suggest that FasL expression on APCs and phagocytosis of apoptotic T cells by FasL(+) KCs are indicators of rejection activity in human liver allografts.

Molecular mechanisms of chronic rejection following transplantation

Although significant advances have been made in the field of organ transplantation, chronic rejection remains a major limiting factor for prolonged graft survival. The long-term survival and function of transplanted lungs are limited by the development of bronchiolitis obliterans syndrome (BOS). The 10-yr lung graft survival rate is only 18.6%. Aside from results of several clinical studies that strongly support the concept that BOS results from alloimmune-mediated injury, little is known regarding specific immune effectors or target molecules involved in the pathogenesis of BOS. Studies from our laboratory have provided evidence for the seminal role of CD4+ T-cells in the pathogenesis of obliterative airway disease (OAD) seen in BOS. Prior to any clinically detectable lesions, there is indirect antigen presentation of mismatched major histocompatibility complex (MHC) class I antigen and production of antibodies to these MHC antigens. Both MHC and minor histocompatibility antigen disparities can result in the development of OAD in animal models and preliminary results strongly suggest that peptide vaccination strategies may prevent OAD following heterotopic tracheal transplants. Using a newly developed orthotopic tracheal transplant model, we have obtained evidence for an important and probably exclusive role for airway epithelial cell injury as a primary mechanism for the immunopathogenesis of the development of OAD.

The direct and indirect allogeneic presentation pathway during acute rejection after human cardiac transplantation

Alloreactive T cells may be activated via a direct or an indirect antigen presentation pathway. We questioned whether the frequency of interferon (IFN)-gamma producing cells determined by enzyme-linked immunospot (ELISPOT) assay is an effective tool to monitor the direct and/or indirect presentation pathway. Secondly, we wondered whether early and late acute rejection (AR) are associated with both pathways. Before (n = 15), during (n = 18) and after (n = 16) a period of AR, peripheral blood mononuclear cell (PBMC) samples were tested from 13 heart transplant recipients. The direct presentation pathway was always present. The number of IFN-gamma producing cells reactive to this pathway increased significantly (P = 0.04) during AR and the number decreased (P = 0.005) after AR therapy. In contrast, the indirect allogeneic presentation pathway was present in only eight of 18 AR samples. When the indirect presentation pathway was detectable, it increased significantly during AR. Five of eight of these AR occurred more than 6 months after transplantation. The ELISPOT assay, enumerating alloreactive IFN-gamma producing cells, is a valuable tool to determine the reactivity via both the direct and the indirect presentation pathway. The direct presentation pathway always plays a role in AR, while the indirect pathway contributes especially to late AR.

Indirect recognition of MHC class I allopeptides accelerates lung allograft rejection in miniature swine

The role of indirect allorecognition in graft rejection is examined in two experiments using a swine lung transplantation model. First, two swine received class I mismatched grafts without immunosuppression; another two recipients were treated postoperatively with cyclosporine (CsA). These swine exhibited acute and chronic rejection, respectively. All four recipients developed T-cell reactivity to donor-derived class I major histocompatibility complex (MHC) peptides. Second, six swine were immunized with synthetic donor-derived class I allopeptides prior to transplantation. Control groups consisted of nonimmunized recipients (n = 6) and recipients immunized with an irrelevant peptide (n = 3). These recipients all received a 12-day course of post-operative CsA. Swine immunized with allopeptides exhibited accelerated graft rejection, as compared to both control groups (p < 0.01 and p = 0.03, respectively). Within the experimental group, the dominant histologic finding was acute rejection (AR). Obliterative bronchiolitis (OB) was seen in the graft with the longest survival. Both control groups showed a lesser degree of AR, with four out of six nonimmunized swine ultimately developing OB. These studies suggest that indirect allorecognition is operative during lung allograft rejection, and that pre-transplant sensitization to donor-derived MHC allopeptides can accelerate graft rejection.

Induction of obliterative airway disease in murine tracheal allografts by CD8+ CTLs recognizing a single minor histocompatibility antigen

The role of minor histocompatibility Ag (mHAg)-specific CD8+ CTLs in the pathogenesis of chronic lung allograft rejection (bronchiolitis obliterans syndrome) remains to be elucidated. Thus, the goal of this study was to define the role of a single mHAg mismatch at the polymorphic H13 allele in the development of obliterative airway disease (OAD) after murine heterotopic tracheal transplantation. The H13a and H13b alleles encode for the SSVVGVWYL (SVL9) and SSVIGVWYL (SIL9) mHAgs, respectively, presented in the context of the H2Db MHC class I molecule. Toward this, C56BL/10SnJ (H13a) tracheal allografts were transplanted into congenic B10.CE-H13b Aw(30NX)/Sn (H13b) recipients. The allografts were harvested at 30, 60, and 90 days after transplantation, and OAD lesions (epithelial damage, cellular infiltration, and luminal fibrosis) were confirmed histologically. Selected groups of mice were immunized (s.c.) or tolerized (i.v.) with the SVL9 peptide before transplantation. This single mHAg mismatch induced the development of OAD within 90 days. SVL9 immunization significantly accelerated the kinetics of the OAD lesions. In contrast, SVL9 tolerization completely abrogated the development of OAD. This was correlated with a complete abrogation of H13a-specific CD8+ CTL responses with a significant reduction in the frequency of IFN-gamma-producing CTLs and the activation of TGF-beta-producing CD8+ T cells. In conclusion, a single mHAg mismatch can induce the development of OAD. These data also suggest that mHAg-reactive CD8+ CTLs may play an important role in the pathogenesis of chronic lung allograft rejection in humans.

Anti-HLA antibodies in heart transplantation

We have analyzed the relationship between the development of transplant-related coronary artery disease (TRCAD) and the following potential risk factors: (a). number of HLA mismatches between recipient and donor; (b). production of anti-HLA antibodies; (c). growth of lymphocytes infiltrating the graft; and (d). frequency of biopsy proven episodes of acute rejection. The study population consisted of 285 adult heart allograft recipients who were monitored over a period of two years or more. The results demonstrate a significant correlation between TRCAD, generation of anti-HLA class II antibodies and potential of lymphocytes infiltrating the graft to proliferate ex-vivo in medium containing IL-2. Humoral and cellular immune responses to HLA-DR antigens expressed by the graft seem to underlie the development of TRCAD.

Arteriopathy in chronic allograft rejection in liver transplantation

Chronic rejection is an important cause of liver allograft failures. The allograft undergoing chronic rejection shows affected large- and medium-sized muscular arteries with homing of foamy macrophages and enlargement of the intimal area. The objective of this study was to elucidate the pathogenesis of the intimal lesion that causes obliterative arteriopathy by identifying the origin of the foamy macrophages and mesenchymal cells present in the intimal area. Nine allografted livers (6 male and 3 female patients) from sex-mismatched donors undergoing chronic rejection were studied by combined staining of the macrophages or the mesenchymal cells in the intimal area with immunohistochemistry and in situ hybridization using a probe for the human Y chromosome. By using the specimens from female donor allografts transplanted to male recipients, it was found that 62 +/- 11% of CD68+ foamy macrophages and 71 +/- 4% of smooth muscle actin-positive mesenchymal cells in the intimal lesions and a few interstitial myofibroblasts were positive for the Y chromosome probe. This indicated that they were derived from the recipients. In conclusion, the thickening intimal lesion seen in obliterative vasculopathy in liver allografts consists of the foamy macrophages and mesenchymal cells of recipient origin. These circulating recipient cells migrated to the areas in advance of remodeling arteries.

Chronic rejection of murine cardiac allografts discordant at the H13 minor histocompatibility antigen correlates with the generation of the H13-specific CD8+ cytotoxic T cells

BACKGROUND

Minor histocompatibility antigen (mHag) discordances have been shown to play a critical role in graft-versus-host disease after bone marrow transplantation. However, the role of mHag in rejection of solid-organ allografts remains unknown. Therefore, the goal of this study was to define the role of a single mHag discordance derived from the polymorphic H13 allele in the development of cardiac allograft rejection in mice. The H13a and H13b alleles encode for the SSVVGVWYL (SVL9) and SSVIGVWYL (SIL9) mHag bound to the H2Db molecule, respectively.

METHODS

C56BL/10SnJ (H13a) cardiac allografts were transplanted into congenic B10.CE-H13b Aw(30NX)/Sn (H13b) mice. Allograft function was monitored daily and rejection was defined by cessation of heart beat. Rejection was confirmed histologically. The phenotypic and functional characteristics of the graft-infiltrating cells were analyzed by in situ and in vitro staining with H13a-specific tetramers and by chromium-51 (51Cr)-release assay, respectively.

RESULTS

Sixty-five percent of H13-incompatible allografts were rejected in 37.0+/-14.5 days. Sixty-eight percent of the H13a allografts transplanted into H13a-sensitized mice were rejected earlier, in 27.6+/-15.9 days. Rejected allografts showed histopathologic signs of chronic rejection with diffuse mononuclear cell infiltration, concentric intimal hyperplasia, and fibrosis. Both CD8+ (87%) and CD4+ (13%) T cells were observed in rejected allografts. In addition, 60% of the graft-infiltrating CD8+ T cells recognized a H2Db/SVL9 tetramer. Graft-infiltrating CD8+ T cells showed a significant H2Db-restricted, SVL9-specific cytotoxic activity.

CONCLUSIONS

A single mHag discordance, as demonstrated with H13 disparity, results in the pathogenesis of chronic rejection of major histocompatibility complex-matched vascularized solid-organ allograft.

Porcine CD80: cloning, characterization, and evidence for its role in direct human T-cell activation

Previous studies has shown that human anti-pig reactivity in mixed lymphocyte cultures require the indirect presentation of antigens by human antigen presenting cells (APC). Xenoreactivity was inhibited by blockade of human costimulatory molecules. We investigated the role of porcine costimulatory molecules in their ability to activate human T cells directly. Porcine CD80 was cloned from lipopolysaccharide (LPS)-activated porcine lymphocytes. Sequence analysis showed a high degree of conservation in residues involved in CD28/CTLA4. COS cells transfected with porcine CD80 was able to activate human T cells in a cyclosporine independent manner, demonstrating that porcine CD80 can costimulate human T cells. Tumor necrosis factor-alpha (TNF-alpha) activated porcine splenocytes have been shown to up-regulate B7s. In order to test the effect of costimulation blockade in a xeno system, activated splenocytes were cultured with purified CD4+ T cells. The results demonstrated that these cells were capable of activating human T cells and this activation can be blocked by using an antihuman CD80 antibody that demonstrated cross-reactivity to porcine CD80. Non-cross reactive antibodies had no effect, again suggesting direct activation of the human T cells. These data suggest that a reagent that can block both the direct and indirect activation is necessary for a discordant xenotransplant.

Concomitant allorecognition of mismatched donor HLA class I- and class II-derived peptides in pediatric lung transplant recipients with bronchiolitis obliterans syndrome

BACKGROUND

The authors' previous studies with 2 different adult patient populations demonstrated a correlation between indirect allorecognition of mismatched donor HLA Class I- and Class II-derived peptides and the development of bronchiolitis obliterans syndrome (BOS) after lung transplantation. The aim of this study was to determine whether a parallel allorecognition of mismatched donor HLA Class I- and Class II-derived peptides occurs after lung transplantation and to determine its correlation with the development of BOS after lung transplantation in a group of pediatric patients.

METHODS

Peripheral blood mononuclear cells from 7 BOS-positive and 6 BOS-negative pediatric lung transplant recipients (age, 11.5 +/- 4.4 years) were cultured in the presence of synthetic peptides corresponding to the alpha-chain hypervariable regions of a mismatched donor HLA Class I molecule and the beta-chain hypervariable region of a mismatched donor HLA-DR molecule. The frequencies of HLA Class I and Class II alloreactive T cells were determined using limiting dilution analysis.

RESULTS

A significant increase (p = 0.025) in HLA Class I-alloreactive T cells was observed in BOS-positive patients (7.1 x 10(-5) +/- 4.3 x 10(-5)) compared with BOS-negative patients (2.1 x 10(-5) +/- 1.8 x 10(-6)). In addition, a significant increase (p = 0.033) in HLA Class II-alloreactive T cells also was observed in BOS-positive patients (9.6 x 10(-5) +/- 7.9 x 10(-5)) compared with BOS-negative patients (1.3 x 10(-5) +/- 2.1 x 10(-6)).

CONCLUSIONS

This study indicates that a parallel CD4+ T-cell alloreactivity to both donor HLA Class I and Class II molecules may play a role in the pathogenesis of BOS both in adult and pediatric lung transplant recipients.

Tailoring of immunosuppression in renal and liver allograft recipients displaying donor specific T-suppressor cells

Although transplantation tolerance cannot be yet reliably achieved in humans, there is evidence that active immunosuppression contributes to the maintenance of quiescence. However, the mechanism that underlies quiescence and the precise identity of regulatory cells are not completely understood. We have demonstrated that allograft recipients who remain rejection-free display allospecific T-suppressor cells (Ts). Ts express the CD8(+) CD28(-) phenotype, recognize major histocompatibility complex (MHC) class I antigens, and suppress the up-regulation of costimulatory molecules induced by CD40 ligation of donor antigen presenting cells. The presence of Ts is inversely correlated with T cell alloreactivity to donor MHC peptides, alloantibody production, and rejection. Monitoring of Ts has been successfully used in our studies for tailoring immunosuppression in kidney and liver allograft recipients.

Dendritic cells crossprime allo-specific self-restricted CD4(+) T cells after coculture with dead allogeneic cells

Indirect alloreactivity, i.e., the recognition of allopeptides on self-MHC molecules, contributes both to acute and chronic rejection of transplants. The antigen presenting cell priming these allo-specific self-restricted T cells is unknown. We demontrate that dendritic cells, which have been matured in the presence of necrotic allogeneic cells, can crossprime allo-specific self-restricted CD4(+) T cells in vitro. We demonstrate dendrtitic cell mediated crosspriming of HLA-DR13 specific, HLA-DR7 restricted and HLA-DR1 specific, HLA-DR11 restricted CD4(+) T cells. The allo-specific self-restricted CD4(+) T cells primed in our culture system secrete predominantly Th1 and not Th2 cytokines. The use of dendritic cells to monitor the indirect pathway of alloreactivity should help to design and understand interventions against acute and chronic transplant rejection.

Soluble proteins and haptens on bone marrow-derived dendritic cells are presented to host CD4 T cells in an MHC-restricted manner

Because of their potent antigen-presenting capacity, dendritic cells (DC) have been used extensively in immunotherapy protocols. Our purpose was to functionally characterize mouse bone marrow-derived DC (BMDC) in vitro (in protein antigen- and hapten-specific assays) and in vivo (injecting soluble protein- and hapten-pulsed DC) to determine their suitability for the generation of T(h) cell responses. Furthermore, we determined whether there is cross-presentation on MHC class II molecules during in vivo protein and hapten sensitization. Co-culture of protein-pulsed [with hen egg lysozyme (HEL) or with pigeon cytochrome c (CYT)] DC with T cells from HEL- or CYT- sensitized mice induced antigen-specific T cell proliferation, but compared to cultured Langerhans cells (LC), BMDC required higher protein antigen-pulsing concentrations (100 microg and 1 mg/ml). In contrast, at low protein concentrations (10 microg/ml), BMDC stimulated an HEL-specific hybridoma very efficiently. Using an in vitro T cell proliferation assay and in vivo delayed-type hypersensitivity and contact sensitivity assays, we found that protein- and hapten-pulsed BMDC were able to sensitize syngeneic but not allogeneic hosts. Furthermore, if we injected BALB/c- and C57BL/6-derived HEL-pulsed BMDC into F1 mice, specific secondary proliferation of primed T cells occurred only when antigen-pulsed stimulator cells syngeneic to the injected BMDC were used. Using this model system we found that soluble proteins and haptens are presented by injected BMDC to host T cells in an MHC-restricted manner in vivo.

Call for revolution: a new approach to describing allograft deterioration

I propose a set of definable entities in the renal transplant course, eliminating the need for the term 'chronic rejection'. The status of a renal transplant can be defined by the presence and extent of rejection (T-cell-mediated or antibody-mediated); allograft nephropathy (parenchymal atrophy, fibrosis, and fibrous intimal thickening in arteries); transplant glomerulopathy; specific diseases; and factors which could accelerate progression. The level of function and the slope of the loss of function should be separately determined. This approach can be applied both in research and in clinical practice, and can be adapted to other organ transplants.

Predominant IL-10 production in indirect alloreactivity is not associated with rejection

In a prospective study of indirect alloresponse in renal transplantation, we detected proliferation and cytokine production to donor and third-party HLA-DR peptides unrelated to rejection. Twenty of 28 patients (71%) presented proliferation, 29% before and 71% after transplantation. Half of the patients also presented proliferation to third-party peptides. Indirect alloresponse was also detected in 75% of healthy individuals (HI). Variability of response was observed in patients and HI for both proliferation and cytokine production. IL-10 predominance was observed in indirect alloresponses to donor peptides pre- and post-Tx, in contrast with more IFN-gamma and TGF-beta being detected in HI. IL-10 production was frequently detected without proliferation, in contrast with more frequent proliferation being found with IFN-gamma and TGF-beta production. The lack of association of either cytokine or proliferation with rejection, together with the predominance of IL-10 unrelated to proliferation, suggests that regulatory cells may be part of the T cell repertoire involved in indirect alloreactivity.

CD4+ T cell recognition of a single discordant HLA-A2-transgenic molecule through the indirect antigen presentation pathway induces acute rejection of murine cardiac allografts

To further define the role of indirect allorecognition, cardiac allografts from HLA-A2-transgenic (HLA-A2+) C57BL/6 mice were heterotopically transplanted into normal C57BL/6, CD4 T cell-knockout (KO) C57BL/6 mice, CD8 T cell-KO C57BL/6 mice, fully MHC-discordant BALB/c mice (allogeneic control), and HLA-A2+ C57BL/6 mice (syngeneic control). HLA-A2+ grafts were acutely rejected when transplanted into BALB/c mice (mean survival time: 10+/-0.8 days), normal C57BL/6 mice (mean survival time: 16.5+/-2.1 days) as well as CD8-KO mice (mean survival time: 12.8+/-1.3 days). Histopathological analysis revealed classical acute cellular rejection with moderate to severe diffuse interstitial CD4+ and CD8+ cellular infiltrates and significant intra-graft deposition of IgG and complement. In contrast, HLA-A2+ grafts were not rejected when transplanted into CD4-KO mice or HLA-A2+ mice. CD8-KO recipients treated with an anti-CD4 monoclonal antibody, but not with an anti-NK monoclonal antibody, failed to reject their allografts with prolonged administration of antibody (30 days). Spleen cells from mice rejecting HLA-A2+ allografts failed to lyse HLA-A2+ target cells indicating a lack of involvement of CD8+ T cells in the rejection process. In contrast, spleen cells from rejecting animals proliferated significantly to both HLA-A2+ cells and to a peptide derived from the HLA-A2 molecule. Development of anti-HLA-A2 antibodies was observed in all animals rejecting HLA-A2+ allografts. These results suggest that indirect allorecognition of donor MHC class I molecules leads to rejection of cardiac allografts and development of alloantibodies in this unique transplant model in which there is a single MHC discordance between donor and recipient.

Heterogeneity of T cell clones specific for a single indirect alloantigenic epitope (I-Ab/H-2Kd54-68) that mediate transplant rejection

BACKGROUND

One of the complexities of solid organ allograft rejection is the inherent diversity of the specific T cell antigenic epitopes that participate in this response, including the role of direct alloantigen recognition and indirect recognition of donor-derived peptides in recipient antigen-presenting cells. To probe the role of distinct T cell receptor (TCR) avidity differences and the role of cytokine expression patterns of different effector T cells that may participate in allograft rejections, we have identified a dominant allopeptide derived from the H-2Kd molecule, recognized by H-2b CD4 T cells in the context of syngeneic I-Ab.

METHODS

To identify a stimulatory peptide derived from the H-2Kd molecule, a panel of synthetic overlapping peptides was screened for immunogenicity and a panel of T cell clones established. These clones were characterized for TCR Vbeta usage by mAb staining and/or reverse transcribed-polymerase chain reaction analysis, peptide dose sensitivity as a marker of TCR avidity, cytokine expression phenotype in vitro, and their ability to mediate rejection of a vascularized cardiac allograft after adoptive transfer to immunodeficient mice.

RESULTS

The H-2Kd54-68 peptide was identified as a dominant stimulatory peptide by the ability of T cells from C57BL/6 (H-2b) mice primed by a combination of allogeneic spleen cell injection and mixed peptide immunization to mount an in vitro proliferative response and interferon-gamma production by peptide stimulation. Furthermore, direct immunization with synthetic H-2Kd54-68 peptide of normal C57BL/6 mice resulted in accelerated rejection of both skin and cardiac allografts from B10.D2 (H-2d) mice, but not 3rd party B10.BR (H-2k) grafts. A panel of 15 distinct CD4+ clones specific for H-2Kd54-68 peptide were established and shown to utilize a variety of TCR Vbeta and different apparent TCR avidities to H-2Kd54-68 peptide when stimulated in vitro. To characterize these clones further, two clones were chosen based on the difference of avidity to H-2Kd54-68 peptide. The cytokine expression pattern was determined and indirect alloantigen specificity confirmed by analysis of responses to purified peptide and B10.D2 spleen cells using normal H.2b and I-Abeta chain knockout mice as APC donors. Both of these T cell clones were able to mediate rejection of B10.D2, but not B10.BR hearts, in immunodeficient mice, but the morphological pattern of T cell infiltration was distinct.

CONCLUSIONS

These results demonstrate the potential importance of fine dissection of the alloantigeneic response to solid organ transplants and provide unique insights into the role of TCR avidity and cytokine expression patterns in different morphological patterns of transplant rejection.

CD28/CTLA-4 and CD80/CD86 costimulatory molecules are mainly involved in acceptance or rejection of human liver transplant

CD28/CTLA-4 interactions with their specific B7-ligands (CD80 and CD86) have decisive roles in antigenic and allogenic responses. Recently, experimental transplant studies demonstrated that donor-specific tolerance is achieved by blocking these interactions. The present study analyzes the expression of these co-stimulatory molecules in peripheral blood cells from 74 liver recipients and in 16 liver biopsies, which were classified into acute-rejection (AR, n = 27) and nonacute-rejection (NAR, n = 47) groups, as well as their influence on the in vitro response of in vivo allosensitized cells. The results clearly indicate that in human liver transplant too, B7 and CD28/CTLA-4 expression on B and CD4(+) peripheral lymphocytes respectively, contributes to graft acceptance or rejection, and appears to be of crucial importance in modulating the host alloresponse and specific-CTL generation. In the NAR-group, costimulatory molecule expression remained at basal levels after transplant, whereas in the AR-group these molecules were significantly upregulated on days of AR. CTLA-4 was observed in the infiltrating lymphocytes in most of the biopsies, but CD80 or CD86 were not. Moreover, specific cytotoxicity from the in vivo primed cells was clearly suppressed in the NAR-patients with low co-stimulatory molecule expression, whereas this activity was not modified but rather stimulated in the AR-group. Together, these findings indicate that intervention of CD28/CTLA-4/B7 signaling could be therapeutically useful in clinical transplantation.

Analysis of post-transplant immune status in recipients of liver/bone marrow allografts

The aims of this study were to assess the effect of donor bone marrow infusion on the reactivity of recipient peripheral blood lymphocytes (PBL) to mitogen and to donor and third-party cells after primary liver allotransplantation and to identify any correlation between altered immunoreactivity and HLA mismatches, occurrence of rejection, and immunosuppression. The immunoreactivity of recipient PBL toward frozen donor splenocytes was evaluated in mixed lymphocyte culture (MLC) (n = 29) and cell-mediated lympholysis (CML) (n = 27) assays in time intervals ranging from 0.7 to 27 months after transplant. Overall, the mean anti-donor MLC stimulation index (SI) fell from 25.6 +/- 5.2 preoperatively to 4.8 +/- 1.7 after transplantation (p < 0.002), with 14 out of 29 (48.3%) patients developing donor-specific MLC hyporeactivity. HLA class II mismatches were significantly associated with recipient post-transplant immune profile (p < 0.05): MLC donor specific hyporesponsiveness was observed in 70%, versus 37% of patients who shared a class II antigen, versus those that did not. Of the control group, 61.1% developed donor-specific nonreactivity versus 27.2% in the donor bone marrow cells (DBMC) group (p = 0.02). Donor-specific CML hyporeactivity was observed after transplantation, independent of DBMC infusion, with mean percentage values of pre- and post-transplant donor-specific lysis of 22.4% +/- 4.1% versus 3.1% +/- 1.6%, p = 0.0004, respectively. Our results suggest that DBMC infusion favors development of nonspecific MLC hyporesponsiveness to donor and third-party alloantigen, with maintenance of reactivity to mitogen and no additional effect on T-cell cytotoxicity.

Transplant surgery injury recruits recipient MHC class II-positive leukocytes into the kidney

BACKGROUND

CD4 T cells, which are stimulated by the "indirect pathway" of antigen-presentation, participate in rejection. These T cells are sensitized by recipient major histocompatibility complex (MHC) class II-positive leukocytes that migrate into the transplant. Therefore, an important early step in rejection is the immigration of these recipient MHC class II-positive leukocytes into the renal transplant. The regulation of this early step is not understood. We now test the hypothesis that such leukocytes immigrate into the renal transplant in response to ischemic injury occurring during the transplant procedure.

METHODS

We transplanted Brown Norway (BN) kidneys into F1 Lewis/Brown Norway (L/BN) recipients. The F1 recipients are tolerant to the parental BN antigens, and any infiltration of recipient MHC class II-positive leukocytes results from injury occurring during transplantation surgery. In addition, ischemia/reperfusion injury was also induced by temporarily occluding the native renal arteries for 30 minutes. Transplanted kidneys and native kidneys, which suffered ischemia/reperfusion injury, were studied by immunohistochemistry on days 3, 7, 14, and 28 after surgery. Staining by the new monoclonal antibody (mAb) OX62 and antibodies to MHC class II identified dendritic cells. In addition, the following monoclonal antibodies identified: gamma/delta T cells, V65; B cells, OX33; cells that may be macrophages, dendritic cells, or dendritic cell precursors, ED1 (+) and OX62 (-); and recipient class II MHC, OX3.

RESULTS

After transplantation, the serum creatinine increased to 4 mg/dl and then decreased, which was consistent with reversible injury during transplantation and the absence of rejection. We found that the injury of transplantation itself resulted in the infiltration of recipient MHC class II-positive leukocytes into the transplanted kidney. This infiltrate peaked at days 7 to 14 after surgery. The inflammation was peritubular and patchy and involved cortex and outer medulla. Double staining for OX62 and OX3 identified some of the infiltrating leukocytes as dendritic cells. Other recipient leukocytes were MHC class II positive, ED1 positive, and OX62 negative. We also found that MHC class II leukocytes, including dendritic cells, infiltrated native kidneys injured by ischemia/reperfusion injury.

CONCLUSION

To our knowledge, this is the first demonstration that injury to the kidney during transplantation recruits recipient MHC class II-positive leukocytes into the kidney. Some of these leukocytes are dendritic cells.

Indirect recognition of donor HLA class I peptides in lung transplant recipients with bronchiolitis obliterans syndrome

BACKGROUND

The presentation of donor MHC class II-derived peptides by host antigen-presenting cells in the context of self-MHC class II molecules has been suggested as a mechanism for the chronic rejection of kidney and heart allografts. The aim of this study was to determine whether indirect allorecognition of HLA class I-derived peptides occurred in lung transplant (LTx) recipients and whether it correlated with the development of bronchiolitis obliterans syndrome (BOS).

METHODS

Peripheral blood mononuclear cells from LTx recipients were cultured with synthetic peptides corresponding to the hypervariable regions of the mismatched HLA class I antigens of the donor. Proliferation and precursor frequency (PF) of allopeptide reactive T cells were determined by the incorporation of [3H]thymidine into DNA and limiting dilution analysis.

RESULTS

Peripheral blood leukocytes of LTx recipients with BOS mismatched for HLA class I molecules showed a proliferative response three- to fourfold higher than those observed in mismatched recipients without BOS and in normal control individuals (P<0.001). Similarly, the PF of allopeptide-reactive T cell was 3- to 24-fold higher in recipients with BOS compared with recipients without BOS (P<0.05) as well as normal control individuals (P<0.03). The T cell PF to donor-specific allopeptides, as well as irrelevant allopeptides, was not significantly different in LTx recipients without BOS and normal control individuals.

CONCLUSIONS

These data suggest that T cells from LTx recipients are sensitized to mismatched HLA class I antigens. The sensitization was significantly higher in LTx recipients with BOS compared with LTx recipients without BOS. Strategies to block T-cell responses generated by indirect allorecognition after lung transplantation may provide a means for the prevention or treatment of BOS in LTx recipients.

Efficient presentation of phagocytosed cellular fragments on the major histocompatibility complex class II products of dendritic cells

Cells from the bone marrow can present peptides that are derived from tumors, transplants, and self-tissues. Here we describe how dendritic cells (DCs) process phagocytosed cell fragments onto major histocompatibility complex (MHC) class II products with unusual efficacy. This was monitored with the Y-Ae monoclonal antibody that is specific for complexes of I-Ab MHC class II presenting a peptide derived from I-Ealpha. When immature DCs from I-Ab mice were cultured for 5-20 h with activated I-E+ B blasts, either necrotic or apoptotic, the DCs produced the epitope recognized by the Y-Ae monoclonal antibody and stimulated T cells reactive with the same MHC-peptide complex. Antigen transfer was also observed with human cells, where human histocompatibility leukocyte antigen (HLA)-DRalpha includes the same peptide sequence as mouse I-Ealpha. Antigen transfer was preceded by uptake of B cell fragments into MHC class II-rich compartments. Quantitation of the amount of I-E protein in the B cell fragments revealed that phagocytosed I-E was 1-10 thousand times more efficient in generating MHC-peptide complexes than preprocessed I-E peptide. When we injected different I-E- bearing cells into C57BL/6 mice to look for a similar phenomenon in vivo, we found that short-lived migrating DCs could be processed by most of the recipient DCs in the lymph node. The consequence of antigen transfer from migratory DCs to lymph node DCs is not yet known, but we suggest that in the steady state, i.e., in the absence of stimuli for DC maturation, this transfer leads to peripheral tolerance of the T cell repertoire to self.

Analysis of the relationship between chimerism and the allgeneic humoral response

BACKGROUND

Persistence of antigens has been suggested to play a role in two opposing immunological phenomena: tolerance and memory. Therefore, we studied the impact of chimerism on alloreactive antibody (allo-Ab) production in kidney transplant patients.

METHODS

Thirty-five female renal transplant recipients of male donor organs were classified into the following groups: group 1, 13 sensitized uremic patients on dialysis; group 2, 5 nonsensitized uremic patients on dialysis; group 3, six sensitized patients experiencing graft rejection (3 acute vascular, 1 acute cellular, and 2 chronic); and group 4, 11 nonsensitized with functioning allografts (9 with good function, 1 with acute cellular rejection, and 1 with chronic rejection). Mean duration of dialysis after graft failure was similar in groups 1 (56+/-29.7 months) and 2 (41.8+/-42.4 months), as was dialysis efficiency. Chimerism was measured indirectly in the peripheral blood lymphocytes by polymerase chain reaction amplification of a specific Y chromosome DNA gene sequence with a detection sensitivity limit of 1 male cell per 1 million female cells. Allo-Ab production was measured by the PRA-STAT enzyme-linked immunosorbent assay (Sangstat) method.

RESULTS

Chimerism was observed in 60% of groups 1 and 2, 83% of group 3, and 82% of group 4. Among all groups, graft existence, irrespective of its function, positively predicted chimerism in 92% with a sensitivity of 88% and a specificity of 78%. In group 3, all three patients with acute vascular rejection had chimerism and donor-specific allo-Abs. In group 4, eight of the nine patients with no rejection had chimerism.

CONCLUSION

Chimerism relates to persistence of allogeneic stimulus irrespective of its function. Chimerism did not confer protection against allo-Ab production or vascular rejection, and its existence was not crucial for sustenance of allo-Ab production.

Intramolecular and intermolecular spreading during the course of organ allograft rejection

There are two distinct pathways by which T cells may recognize MHC alloantigens. The direct pathway involves T-cell recognition of intact MHC molecules expressed by donor antigen-presenting cells (APCs). The second, or indirect, pathway describes T-cell recognition of peptides derived from the processing and presentation of allogeneic MHC molecules on self APCs. Recent data demonstrates that indirect recognition plays a central role in both acute and chronic rejection of human organ allografts. Our studies have shown that, at the onset of primary acute rejection, recipient T-cell responses to donor HLA-DR alloantigens are limited to a single dominant determinant present on one of the disparate alloantigens and restricted by one of the responder's HLA-DR molecules. In allograft recipients with recurring episodes of rejection, and/or at the onset of chronic rejection, recipient T-cell reactivity may spread to other epitopes within the allogeneic MHC molecule as well as to other alloantigens expressed by graft tissue. Both quantitative and qualitative alterations in T-cell allopeptide reactivity are associated with increased risk of cellular and/or humoral rejection. These studies provide a basis for the design of new therapeutic strategies and for immunologic monitoring of transplant recipients.

New strategies for early diagnosis of heart allograft rejection

BACKGROUND

Allograft rejection is mediated by T cells that recognize allogeneic major histocompatibility complex (MHC) molecules via the direct and indirect pathway. The direct pathway involves T cells that react against MHC/peptide complexes expressed on the surface of donor antigen-presenting cells (APCs). In contrast, T cells involved in the indirect pathway recognize peptides derived from processing and presentation of allogeneic MHC molecules by self (recipient) APCs. To explore the relative contribution of these two pathways to rejection, we have evaluated the response of peripheral blood T cells from 50 heart transplant recipients against donor APCs (direct recognition) and against self APCs pulsed with synthetic peptides corresponding to the hypervariable region of the mismatched HLA-DR antigens of the donor (indirect recognition).

METHODS

T cell reactivity against donor APCs was quantitated by measuring the expression of CD69 on allostimulated CD3+ LDA1+ cells. Reactivity to synthetic allopeptides was determined in limited dilution assays.

RESULTS

Serial studies of the kinetics of direct and indirect recognition showed that both pathways contribute to early acute rejection episodes. Primary rejection was accompanied invariably by indirect recognition of a dominant allopeptide. Intermolecular spreading of T cell epitopes was observed during recurrent rejections. Enhanced recognition of donor alloantigens via the direct pathway was found predominantly during early rejection episodes. A single form of allorecognition was shown to occur in some rejection episodes.

CONCLUSIONS

Monitoring of the direct and indirect pathway of allorecognition provides a reliable method for prediction and differential diagnosis of acute rejection of heart allografts.