Persistent allopeptide reactivity and epitope spreading in chronic rejection of organ allografts

Soluble HLA proteins with bound peptides are released from the cell surface by the membrane metalloproteinase

The metalloproteinase (MPase)-mediated pathway of MHC class I processing is a distinct cellular mechanism that generates soluble HLA proteins. It has been implicated in modulation of immune responses induced during transplantation events. It is, therefore, important to define the characteristics of soluble HLA species produced by the MPase pathway. We have previously shown that some mutant peptide-conformed beta(2)-microglobulin (beta(2)m) free heavy chains (HC) with lower affinity for beta(2)m can be released into supernatants by the MPase. These soluble conformed beta(2)m-free HC intermediates can re-associate with beta(2)m in solution giving rise to beta(2)m-associated HC. We now demonstrate that also nonmutant soluble conformed beta(2)m-free HC can be detected in supernatants of activated cells. These soluble HC intermediates appear to have bound peptides and readily re-associate with exogenous beta(2)m producing beta(2)m-associated HC that are stable at physiologic temperature. Thus, generation of peptide-conformed beta(2)m-free HC intermediates is an important step, which precedes generation of both soluble beta(2)m-free and beta(2)m-associated HC by the MPase pathway operating in activated cells.

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.

Mechanisms of chronic rejection

Chronic rejection remains the major obstacle to long-term allograft survival. Detailed understanding of putative etiologic risk factors, both antigen-dependent and -independent, is important for designing effective therapeutic strategies to ameliorate this process. Cell senescence may be an important factor in chronic rejection.

Coronary arteries from human cardiac allografts with chronic rejection contain oligoclonal T cells: persistence of identical clonally expanded TCR transcripts from the early post-transplantation period (endomyocardial biopsies) to chronic rejection (coronary arteries)

Chronic cardiac allograft rejection presents pathologically as graft arteriosclerosis (GA) characterized by recipient T cell and monocyte infiltration. To determine whether oligoclonal T cells are present in coronary arteries of cardiac allografts from patients with GA, we conducted sequencing analysis of beta-chain TCR transcripts from these explanted coronary arteries using the nonpalindromic adaptor-PCR. Substantial proportions of identical beta-chain TCR transcripts in three of five patients were observed, clearly demonstrating the presence of oligoclonal T cells. TCR transcripts from the arteries of two other patients were relative heterogeneous. High proportions of identical CDR3 beta-chain TCR motifs were found in each patient. GENEBANK/EMBL/SWISS PROT database comparison of all sequences revealed that these beta-chain TCR transcripts were novel. Using Vbeta-specific PCR (independent amplification), we found in patient GA03 that the TCR transcript that was clonally expanded in the left anterior descending artery after nonpalindromic adaptor-PCR was also clonally expanded in the right coronary artery of the same allograft. These results demonstrate that this TCR transcript was clonally expanded at different anatomic sides of the cardiac allograft in a systemic manner. In two patients identical beta-chain TCR transcripts that were found to be clonally expanded in the coronary arteries of their explanted cardiac allografts were also found to be clonally explanted in endomyocardial biopsies collected 17 and 21 mo earlier from each patient. The presence of oligoclonal populations of T cells in the rejected graft suggest that these T cells have undergone specific Ag-driven proliferation and clonal expansion early on within the graft and persist throughout the post-transplantation period.

Initial T-cell activation required for transplant vasculopathy in retransplanted rat cardiac allografts

BACKGROUND

A precise understanding of immunological mechanisms is needed to prevent transplant vasculopathy.

METHODS

The developing process of transplant vasculopathy was investigated by retransplanting rat cardiac allografts and measuring the expressions of 21 different genes inside the retransplanted allografts under nonimmunosuppressive conditions.

RESULTS

Significant transplant vasculopathy developed if WKY hearts were grafted to LEW and retransplanted to WKY 5 days after the initial grafting, but it did not in allografts retransplanted 3 days after the initial grafting. The disease did not progress in retransplanted isografts or if nude rats were used as the initial recipients. However, the development of transplant vasculopathy was not affected by changing the second recipients to the F1 progeny of donor x recipient or to nude animals. Among the expressions of 21 different genes observed in allografts at 1, 14, 30, or 60 days after retransplantation, those of T-cell activation-related genes, such as interferon-y and Fas ligand, showed the earliest and the most dramatic difference between 3- and 5-day-retransplanted allografts whereas macrophage/monocyte activation-related genes showed little difference. Furthermore, reverse transcription-polymerase chain reaction analyses of allografts retransplanted to nude animal indicated that T cells of the initial recipient origin survive and remain activated even 60 days after retransplantation.

CONCLUSIONS

The T-cell response occurring between 3 and 5 days after grafting was identified as the critical parameter to the disease progression. Once alloreactive T cells enter a graft, they may be able to survive a long period and promote chronic rejection.

Peptide analogues as a strategy to induce tolerance in T cells with indirect allospecificity

BACKGROUND

It has been demonstrated that indirect recognition of allogeneic MHC molecules might play an important role in provoking graft rejection. Although direct recognition of allogeneic molecules on antigen presenting cells of the graft may induce a state of tolerance, the continuous presentation of processed alloantigens by specialized antigen presenting cells does not allow the same phenomenon to occur. Tolerance to interleukin-2 secreting T cells can be achieved in different ways, among these is the exposure to mutants of the wild type allopeptide. We have investigated whether peptide analogues of the allopeptide can induce tolerance in T cells with indirect allospecificity.

METHODS

T cell clones with indirect anti-HLA-A2-specificity generated from a HLA-A2-DRB1*1502+ patient who chronically rejected a HLA-A2-expressing kidney allograft were used for this study. Nine peptide analogues of HLA-A2 (residues: 103-120) were produced with single amino acid substitutions at the putative T cell receptor for antigen contact positions. Their effect on the proliferation of a panel of T cell clones was evaluated.

RESULTS

Peptide analogues and wild type peptide had similar capacity to bind to the restriction molecule HLA-DRB1*1502. Co-presentation of the peptide analogues 111R/A, H, K and 114H/K, with the wild type peptide inhibited T cell responses, indicative of antagonism. In addition, one analogue 112G/S induced unresponsiveness in the T cells to subsequent culture with the wild type peptide.

CONCLUSIONS

The data presented here suggest that using reagents such as altered peptides may represent a strategy to prevent the activation of T cells with indirect alloreactivity and allograft rejection in vivo.

A limited number of HLA epitopes are recognized from HLA class I-specific antibodies detected in the serum of sensitized patients

The goal of this study was to evaluate the epitope specificity of HLA class I-specific antibodies detected in the serum of sensitized patients awaiting retransplantation. The study group consisted of 22 sensitized from previous graft patients, who produced stable IgG HLA class I-specific antibodies. A total of 60 serum samples were screened and analyzed by two techniques in parallel: the antihuman globulin augmented CDC (AHG-CDC) technique and an ELISA technique. All recipients and donors were typed for class I HLA antigens by a standard lymphocytotoxicity technique. The epitope identification was based on class I HLA antigens sequencing, where the multiple immunogenic epitopes are differentially shared among various HLA antigens. The unique epitope configuration on one HLA antigen represents the private epitope of the specific HLA antigen while epitopes shared by more than one HLA antigen represent public determinants. In some HLA antigens (HLA-A1), more than one private epitope has been defined, while in others (HLA-B35, -B51), the private epitopes are not yet known. In a total of 36 antibody reactivity patterns, the majority of the definable IgG HLA class I-specific antibodies corresponded to the A-locus (75%), and only 25% had specificities against the B-locus antigens, although the number of incompatibilities concerning both loci were almost identical (29 for the HLA antigens of the A-locus and 26 for those of B-locus). All patients produced HLA class I-specific antibodies with specificities against the private epitopes of the immunogenic mismatched HLA antigen(s). In 6/21 cases (28.6%), HLA class I alloreactivity spreading to nongraft HLA antigens was detected and 9 public (shared) immunogenic alloepitopes were recognized. In conclusion, appling the epitope analysis of HLA class I-specific antibodies produced by sensitized from previous graft patients, we were able to define the immunogenic alloepitopes. We consider that the immunogenic alloepitopes, during transplantation course, are mainly private epitopes of mismatched HLA antigens and, in certain cases, shared epitopes between the donor alloantigens and other HLA antigens. This knowledge may offer the potential of transplanting sensitized patients through improved donor selection.

Immunobiology of left ventricular assist devices

The increasing use of implanted biomaterial devices has made it evident that no material is biologically inert. As a result of direct contact with elements of the blood circulation, such as during hemodialysis or after left ventricular assist device (LVAD) implantation, significant changes in systemic immunologic and thrombostatic functions occur. The clinical success of LVAD implantation has, nevertheless, been accompanied by complications arising from an aberrant state of monocyte and T-cell activation, leading to heightened susceptibility of circulating CD4 T cells to undergo activation-induced cell death; this results in progressive defects in cellular immunity and an increased risk of serious infection. Because of the increased state of T-cell activation and the selective loss of Th1 cytokine producing CD4 T cells, LVAD recipients also develop B-cell hyperreactivity and dysregulated immunoglobulin syntheses by unopposed production of Th2 cytokines and increased CD40 Ligand-CD40 interactions. LVADs are currently being evaluated as a permanent therapy for end-stage heart failure. Because these immune dysfunctions appear to be related to the effects of excessive biomaterial associated T-cell activation, future efforts will need to be directed at either altering the physical properties of the materials interacting with the host circulation or pharmacological intervention aimed at inhibiting T-cell activation.

Coronary artery disease in the transplanted heart

Coronary artery disease in the transplanted heart limits the long-term success of cardiac transplantation. Intravascular ultrasound studies reveal a dual morphology with donor-transmitted and de novo plaques. Coronary vasomotor dysfunction may occur independently of morphological alterations. The disease is characterized by the interaction of activated T lymphocytes with cytokines and donor epicardial and microvascular endothelium. Various noxious stimuli contribute to the continuing inflammatory response. Consequently, adhesion molecule expression is upregulated, leukocytes migrate into the allograft, thrombocytes accumulate, and growth factors are expressed, finally resulting in functional and morphological chronic allograft lesions. Blocking the activation of T cells, CD4+ cytokines, and adhesion molecules may prevent endothelial injury and subsequent intimal thickening. Strategies to decrease the formation of anti-endothelial and anti-HLA-DR antibodies may also be protective, as may antiproliferative drugs, augmentation of endogenous nitric oxide bioactivity, and new immunosuppressive regimens. Revascularization procedures have a limited role in treating significant focal lesions. Retransplantation, the only definitive treatment, remains ethically controversial.

Blockade of CD28-B7, but not CD40-CD154, prevents costimulation of allogeneic porcine and xenogeneic human anti-porcine T cell responses

Despite increasing use of swine in transplantation research, the ability to block costimulation of allogeneic T cell responses has not been demonstrated in swine, and the effects of costimulatory blockade on xenogeneic human anti-porcine T cell responses are also not clear. We have compared the in vitro effects of anti-human CD154 mAb and human CTLA4IgG4 on allogeneic pig T cell responses and xenogeneic human anti-pig T cell responses. Both anti-CD154 mAb and CTLA4IgG4 cross-reacted on pig cells. While anti-CD154 mAb and CTLA4IgG4 both inhibited the primary allogeneic pig MLRs, CTLA4IgG4 (7.88 microg/ml) was considerably more inhibitory than anti-CD154 mAb (100 microg/ml) at optimal doses. Anti-CD154 mAb inhibited the production of IFN-gamma by 75%, but did not inhibit IL-10 production, while CTLA4IgG4 completely inhibited the production of both IFN-gamma and IL-10. In secondary allogeneic pig MLRs, CTLA4IgG4, but not anti-CD154 mAb, induced Ag-specific T cell anergy. CTLAIgG4 completely blocked the indirect pathway of allorecognition, while anti-CD154 mAb blocked the indirect response by approximately 50%. The generation of porcine CTLs was inhibited by CTLA4IgG4, but not by anti-CD154 mAb. Human anti-porcine xenogeneic MLRs were blocked by CTLA4IgG4, but only minimally by anti-CD154 mAb. Finally, CTLA4IgG4 prevented secondary xenogeneic human anti-porcine T cell responses. These data indicate that blockade of the B7-CD28 pathway was more effective than blockade of the CD40-CD154 pathway in inhibiting allogeneic pig T cell responses and xenogeneic human anti-pig T cell responses in vitro. These findings have implications for inhibiting cell-mediated immune responses in pig-to-human xenotransplantation.

Peptide-conformed beta2m-free class I heavy chains are intermediates in generation of soluble HLA by the membrane-bound metalloproteinase

Molecular mechanisms of soluble HLA-release by a membrane-bound metalloproteinase (MPase) are not defined. We have investigated the possibility that certain beta2-microglobulin (beta2m)-free heavy chains (HC) retain peptide-induced conformations before and after the cleavage by using mutant HLA-A2.242K HC with reduced affinity for beta2m. We show that dissociation of HC/beta2m complexes on the surface of C1R lymphoblastoid cells generates both conformed and non-conformed beta2m-free HC recognized by conformation-dependent antibodies. Conformed HC, having bound the HLA-A2-specific peptide HTLV-1 tax 11-19, can retain their proper conformations after dissociation of beta2m. Further, conformed and non-conformed surface beta2m-free HC are cleaved by the MPase, and some released HC preserve their conformations. Exogenous beta2m binds only to conformed HC, and protects them from cleavage as effectively as the MPase inhibitor BB-2116. We propose that soluble HLA-release requires generation of peptide-conformed beta2m-free HC intermediates on the cell surface, which are then cleaved by the MPase and in solution may reassociate with beta2m. Given the role of soluble HLA in the indirect allorecognition, the activity of this MPase may be important in transplant rejection.

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.

Analysis of the reversibility of chronic liver allograft rejection implications for a staging schema

In contrast to all other vascularized organ allografts, chronic rejection (CR) of the liver is potentially reversible. We therefore studied demographic, perioperative, biochemical, and histologic features associated with reversibility or progression to graft failure. Using very stringent clinical and histological criteria, we identified a subgroup of 23 of 916 patients receiving primary liver allografts with CR from the Liver Transplantation Database. Of these, 13 experienced graft failure as a result of CR, and 10 patients recovered to normal histology or liver injury test results. Male-to-female sex mismatch (p = 0.07), younger recipient age (p = 0.09), younger donor age (p = 0.06), white-to-white race match (p = 0.09), primary diagnosis of biliary atresia (p = 0.02), and cold ischemia time of more than 12 hours (p = 0.02) were associated with graft failure. Patients who eventually recovered from CR were more likely to have acute rejection within the first 2 weeks (70% vs 23%; p = 0.04), had a higher number of acute rejection episodes (p = 0.08), and were more likely to have been treated with OKT3 (90% vs 46%, p = 0.07). Although overlap existed in the histopathologic findings between the patients whose grafts failed and those who recovered, those patients who developed bile duct loss in more than 50% of the portal tracts (p < 0.01), severe (bridging) perivenular fibrosis (p = 0.05), and the presence of foam cell clusters (p = 0.06) were more likely to require retransplantation. In contrast to other solid organ allografts, CR of the liver is not an irreversible process. These findings can be used to understand the evolution of CR and to design a biologically correct and clinically relevant staging system.

Production of human alloreactive antibodies by uremic sensitized lymphocytes engrafted in severe combined immunodeficient mice

BACKGROUND

Numerous studies have documented antibody responses to nominal antigens in human peripheral blood lymphocyte (hu-PBL)-severe combined immunodeficient (SCID) mice engrafted with PBL. We suggest, therefore, that engraftment of SCID mice by allosensitized cells produces alloreactive antibodies (allo-Ab). As such, SCID mice can be used as a model to examine the cellular basis of the sustenance of humoral alloresponses.

METHODS

C.B-17 SCID mice were engrafted intraperitoneally with PBL from sensitized uremic patients for 5 weeks and classified into the following: group 1, 8 mice engrafted with unactivated cells (n = 5; n = sensitized patients); group 2, 6 mice engrafted with unactivated PBL (n = 3) followed 2 days later by in vivo activation with interleukin-2 (IL-2) and lipopolysaccharide (LPS); group 3, 11 mice engrafted with cells preactivated in vitro with IL-2 and LPS (n = 6); group 4, 9 mice engrafted with cells preactivated with irradiated allogeneic transformed B cells, IL-2, and LPS (n = 5); group 5, 10 mice engrafted with cells preactivated with IL-2, LPS, and HLA class II allopeptides and adjuvant (n = 6); group 6, 8 control mice engrafted with control cells preactivated with IL-2 and LPS. Two weeks later, each group was rechallenged by the same stimuli. Allo-Ab production was measured by the panel-reactive antibody PRA-STAT ELISA method. Results. Allo-Ab was produced in 54.5% of group 3, 43% of group 4; 80% of group 5, and 0% of groups 1, 2, and 6. The ratio of alloreactive positive wells in the allo-Ab-producing PBL-hu-SCID mice to that present in the respective patient's sera was 82%, 52%, and 87% in groups 3, 4, and 5, respectively. Conclusions. The study demonstrates the feasibility of using the hu-PBL-SCID model to study alloreactive memory B-cell function in humans. Discrete HLA-class allopeptides (group 5) appeared more potent allo-Ab producers than allogeneic cells (group 4).

T cell receptor repertoire usage in allotransplantation: an overview

Lymphocytes express antigen receptors that allow the immune system to specifically recognize antigens. In transplantation, T cells play a critical role in the rejection process, and different protocols inhibiting T cell-mediated alloreactivity efficiently achieve prolongation of allograft survival. T cells can interact with alloantigens by two ways, either by the "indirect" pathway that correspond to the physiological mechanism of T cell immune recognition, or through the "direct" pathway where they recognize alloantigens directly on the surface of donor cells. If some T cells are specifically activated in allorecognition, one should be able to indirectly detect this "selection" by analyzing the T cell receptor usage that could be biased and reflect the preferential amplification of alloreactive lymphocyte subsets. Nevertheless compared with disease states such as cancer or autoimmunity the T cell receptor repertoire is still largely uncharacterized. We review the current results available on T cell repertoire usage in transplantation studies involving humans or various animal models. The T cell receptor repertoire involved in transplantation (restricted or unrestricted) and the features potentially common to alloimmune responses will be discussed.

MHC class I-restricted epitope spreading in the context of tumor rejection following vaccination with a single immunodominant CTL epitope

Epitope spreading is a process whereby epitopes distinct from and non-cross-reactive with an inducing epitope become targets of an evolving immune response. This phenomenon has been associated most notably with the progression of naturally occurring or experimentally induced chronic autoimmune diseases. We have investigated the potential occurrence of epitope spreading in the context of antitumor cytotoxic T cell (CTL) responses using chicken ovalbumin (OVA) as a model antigen. Our results indicate that following rejection of OVA-expressing EG.7 tumor cells effectuated by a CTL response which is induced against the MHC class I-restricted immunodominant epitope OVA257-264, there occurs intramolecular diversification of the CTL response to two additional OVA-derived epitopes, OVA176-183 and OVA55-62, as well as intermolecular spreading to other endogenous tumor-derived determinants. It seems that CTL-mediated tumor cell destruction in vivo favors cross-presentation of additional epitopes with the consequent activation of additional tumor-reactive lymphocytes. The process of epitope spreading in that context has obvious important implications for the design of antigen-specific antitumor immunotherapies.

Chronic allograft rejection. Do the Th2 cells preferentially induced by indirect alloantigen recognition play a dominant role?

Chronic rejection has been the major obstacle to the long-term allograft survival in the clinic. Although the etiology of this rejection reaction is multifactorial, alloantigen-specific immune activation plays the most critical role. We herein hypothesize that CD4+ Th2 cells that are preferentially induced by the indirect recognition of allogeneic histocompatibility antigens late in transplantation may play the most critical role in the initiation and/or maintenance of chronic allograft rejection. Immunosuppression used to prevent acute rejection and the nature of antigen-presenting cells and alloligands in the graft may all contribute to immune deviation to the Th2 response. This response may be further perpetuated by type 2 cytokines conceivably produced by activated macrophages, NK cells, and CD8+ T cells in the graft. Cytokines and growth factors induced by this type 2 response, in turn, allow for activation of B, endothelial, and smooth muscle cells that collectively contribute to the pathogenesis of chronic allograft rejection by producing alloantibodies and growth hormones required for interstitial fibrosis, extracellular matrix deposition, and vascular neointimal hyperplasia.

The effect of thymectomy on tolerance induction and cardiac allograft vasculopathy in a miniature swine heart/kidney transplantation model

BACKGROUND

We have previously demonstrated that MHC class I disparate hearts transplanted into miniature swine treated with a short course of cyclosporine developed florid cardiac allograft vasculopathy (CAV) and were rejected within 55 days. However, when a donor-specific kidney is cotransplanted with the heart allograft, recipients become tolerant to donor antigen and accept both allografts long-term without the development of CAV. In the present study, we have investigated the role of the host thymus in the induction of tolerance and prevention of CAV in heart/kidney recipients.

METHODS

Total thymectomies were performed in six animals (postoperative day [POD]-21), which on day 0 received either an isolated MHC class I disparate heart allograft (n=3) or combined class I disparate heart and kidney allografts (n=3), followed in both cases by a 12-day course of cyclosporine (POD 0-11). Graft survival and the development of CAV in these thymectomized recipients were compared to the same parameters in non-thymectomized, cyclosporine-treated recipients bearing either class I disparate heart allografts (n=5) or heart and kidney allografts (n=4).

RESULTS

In the group of animals bearing isolated class I disparate heart allografts, the thymectomized recipients rejected their allografts earlier (POD 8, 22, 27) than the non-thymectomized recipients (POD 33,35,45,47,55). The donor hearts in both the thymectomized and non-thymectomized animals developed florid CAV. In the group of animals bearing combined class I disparate heart and kidney allografts, the nonthymectomized recipients accepted both donor organs long term with no evidence of CAV. In contrast, none of the thymectomized heart/kidney recipients survived >100 days, and they all developed the intimal proliferation of CAV.

CONCLUSION

Thymic-dependent mechanisms are necessary for the induction of acquired tolerance and prevention of CAV in porcine heart/kidney recipients.

Transplantation tolerance: the concept and its applicability

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

The humoral immune response against an HLA class I allodeterminant correlates with the HLA-DR phenotype of the responder

BACKGROUND

The genetic basis for control of alloantibody responses against foreign HLA histocompatibility antigens has never been delineated. The most likely postulate would be that HLA class II alloantigens of the host regulate the response through their ability to present processed HLA allopeptide fragments for the cognate interaction between CD4+ T lymphocytes and B lymphocytes that leads to IgG antibody synthesis.

METHODS

We have analyzed our allosensitized transplant patient population with regard to humoral responsiveness to a serologically defined public HLA class I epitope, Bw4. Peptides representing the linear sequence of the Bw4 epitope (amino acids 74-86) and the alternative Bw6 epitope were synthesized and assayed for binding to a panel of HLA homozygous lymphoblastoid B cells using a quantitative fluorescence binding assay.

RESULTS

We found that 73% of patients who have produced a HLA-Bw4-specific alloantibody express either the HLA-DRB1*01 or HLA-DRB1*03 alloantigen; 19% of the remaining responders expressed HLA-DRB1*04. Analysis of the United Network for Organ Sharing Transplant Registry indicated that the survival of cadaver renal allografts mismatched for Bw4 was significantly compromised in sensitized DRB1*01+ or DRB1*03+ recipients (P<0.01). In vitro, the Bw4 peptide bound strongly to DRB1*01+ and DRB1*03+ lymphoblastoid B cells; no similar binding was observed with Bw6 peptide. These findings were confirmed using murine fibroblast lines transfected with HLA-DR alpha/beta genes and by solid-phase enzyme-linked immunosorbent assay using purified HLA-DR alloantigen.

CONCLUSIONS

We conclude that there are at least two human Ir genes, HLA-DRB1*01 and HLA-DRB1*03, that confer a high risk for both humoral allosensitization and renal allograft failure in situations of HLA-Bw4 incompatibility. These findings may be of future benefit in devising new antigen matching strategies for reducing the risk of humoral HLA allosensitization and chronic allograft rejection.

Antigen presenting cells integrate opposing signals from CD4+ and CD8+ regulatory T lymphocytes to arbitrate the outcomes of immune responses

An individual's set of polymorphic HLA class II and I molecules is known to select the T cell repertoire in the thymus and to present processed antigenic peptides (pAg) to mature peripheral CD4+ T helper (Th) and CD8+ T cytotoxic (Th) cells in the periphery. This review highlights new studies which address how antigen presenting cells (APC) integrate the responses of cognate Th and T suppressor (Ts) cells to determine the outcome of immune responses. Together with other findings, these studies emphasize that understanding the mechanism of immune processes requires consideration of HLA molecules in the context of the peptides they bind, the antigen presenting cells (APC) that express them and the T lymphocytes that recognize them. The activities of lymphocyte and APC surface structures are becoming integrated into a physiological understanding of the cellular interactions between regulatory and effector T cells with APC.

Cellular and humoral mechanisms of vascularized allograft rejection induced by indirect recognition of donor MHC allopeptides

INTRODUCTION

To investigate the role and mechanisms of indirect allorecognition in allograft rejection, we studied whether priming T cells with donor-derived MHC allopeptides could accelerate rejection in a vascularized allograft model.

METHODS

Lewis recipients of fully mismatched Wistar Furth cardiac allografts were immunized before transplantation with donor MHC allopeptides.

RESULTS

Animals immunized with immunogenic class II MHC allopeptides rejected their grafts in a significantly accelerated fashion compared with controls. Additional studies demonstrated that a single immunodominant RT1.D (HLA-DR like) allopeptide was responsible for accelerating the rejection process. Histological analysis of rejected allografts revealed marked vascular rejection in the accelerated, although not the control, group as well as severe cellular rejection. Peak production of IgM and IgG donor-specific alloantibodies was detected by flow cytometry 1 week earlier in the sera of the accelerated group compared with the control group. Immunohistological analysis of grafts from the accelerated compared with the control group showed increased endothelial deposition of IgG2b, C3, and fibrin, and up-regulation of class II MHC molecule expression. Increased intragraft expression of interferon-y and the interferon-gamma-induced chemokines, inducible protein-10 and Mig, and infiltration by activated mononuclear cells expressing CXCR3, the receptor for inducible protein-10 and Mig, was also seen.

CONCLUSION

These novel data provide evidence of a definitive link between indirect allorecognition of donor-derived MHC class II peptides and the cellular and humoral mechanisms of vascularized allograft rejection.

HLA-derived peptides as novel immunomodulatory therapeutics

A growing body of experimental evidence demonstrates that synthetic peptides corresponding to linear sequences of MHC (HLA in humans) proteins have immunomodulatory effects in vitro and in vivo in animal models and in humans. Although the original concept was that these peptides inhibited antigen recognition at the MHC-T cell receptor interface via physical blockade, it is now clear that the mechanisms responsible for the myriad of functional effects are more complex. Recent findings show that some peptides affect signal transduction and cell cycle progression. Fragments of MHC molecules can dampen or downregulate immune responses via a variety of mechanisms. Some soluble MHC molecules or synthetic peptides are capable of inducing and maintaining immunologic tolerance in animals. This information suggests that synthetic peptides themselves or drugs mimicking their effects may represent a new class of immunotherapeutics.

T cell costimulatory blockade: new therapies for transplant rejection

Optimal T cell responses occur when T cells receive both antigen-specific signals through the T cell receptor and non-antigen-specific costimulatory signals through accessory cell surface molecules. The best understood costimulatory receptor is CD28. Signals through the T cell receptor and CD28 cooperatively induce cytokine gene expression and promote T cell proliferation and survival. Negative signals delivered through a related cell surface receptor, cytotoxic T lymphocyte antigen (CTLA-4), act to terminate immune responses and are required for normal immune homeostasis. This article reviews T cell costimulation, including the CD28/CTLA-4 system and other potential costimulatory pathways (such as CD40/CD154), the role of these pathways in normal immune responses, and the potential for the inhibition of these pathways to induce transplantation tolerance.

Sequencing of two HLA-A blank alleles: implications in unrelated bone marrow donor matching

BACKGROUND

DNA-based methods can type for HLA antigens with no or very low cell surface expression. The role of such serological blank antigens in bone marrow donor selection is unknown.

METHODS

HLA-A serological blank antigens detected in two leukemic patients were cloned and sequenced from genomic DNA. mRNA expression and exon 3 splicing were determined by reverse transcriptase-polymerase chain reaction (PCR).

RESULTS

Two patients typed A2/x and A3/x by serology were revealed to be A*01/A*0201 and A*03/A*24 by DNA typing. The HLA-A*O1 blank antigen was identified as the A*0104N allele with a C insertion in exon 4, which results in a stop codon. The HLA-A*24 blank antigen was identified as the A*2402102L allele characterized by a mutation in intron 2 leading to impaired splicing of mainly exon 3. As a consequence, functional A24 mRNA was reduced to less than 5% compared with the other HLA-A allele. For both patients, a search for an unrelated donor was initiated on the basis of the functional absence of the serologically blank allele. The second patient with the A*24 blank antigen could undergo transplantation with marrow from a "fully A/B/C/DRB1/DRB5/DQB1-compatible" donor, homozygous for the A antigen. Donor T cells did not react against the patient in a pretransplantation cytotoxic T lymphocyte precursor frequency test known to detect all class I incompatibilities. However, despite this functional pretransplantation compatibility, the patient died 44 days after bone marrow transplantation, suffering from graft-versus-host disease grade IV. The presence of potentially functional A*2402 mRNA could be demonstrated by reverse transcriptase-PCR and sequencing: 50% of the A24 mRNA molecules were estimated to contain exon 3 with a correct splice site.

CONCLUSIONS

These findings show that serological blank antigens with low mRNA expression might still be recognized after bone marrow transplantation. The determination of such alleles by molecular methods and the assessment of mRNA expression should therefore be included in the unrelated bone marrow donor search protocol.

Evidence of indirect allorecognition in long-term human renal transplantation

The purpose of this study was to investigate indirect alloreactivity in the peripheral blood of long-term renal transplanted patients. We evaluated the T cell proliferative response to a whole pool of donor cell-derived allopeptides, processed and presented by host antigen-presenting cells (APC), rather than to synthetic peptides. For the indirect pathway, proliferation assays were performed using APC-depleted donor cells. Indirect alloreactivity was detected in 57% (8/14) of the patients, 6 of whom presented no evidence of rejection, but 2 patients had a diagnosis of chronic rejection. In 4 of 8 positive cases (50%), proliferation was detected with 5 days of culture, and sometimes indirect alloresponse was the dominant route. We present evidence that the indirect alloproliferative response to a pool of naturally processed donor peptides is present in the peripheral blood of long-term renal transplanted patients irrespective of rejection.

Predominant expression of T helper 2 cytokines and altered expression of T helper 1 cytokines in long-term allograft survival induced by intrathymic immune modulation with donor class I major histocompatibility complex peptides

BACKGROUND

We have recently demonstrated that three synthetic peptides corresponding to the alpha-helices of the alpha1 and alpha2 domains of the donor class I RT1.Aa molecule served as efficient CD4+ T-cell epitopes for indirect recognition of this molecule during cardiac allograft rejection in the PVG.R8-toPVG.1U rat strain combination. These peptides induce long-term graft survival when injected into the thymus 7 days before transplantation under the cover of transient immunosuppression with anti-rat lymphocyte serum. In this study, we analyzed intragraft cytokine gene expression to test whether immune deviation to the T helper (Th) 2 response is associated with long-term allograft survival in this model.

METHODS

Intragraft cytokine gene expression was analyzed using a competitive reverse transcription polymerase chain reaction method we developed for this study. Cytokine gene expression was quantified in control allografts (n=5) with acute rejection and allografts from intrathymically manipulated recipients with acute rejection (n=5), delayed rejection (n=7), or no rejection (n=8).

RESULTS

Long-surviving allografts expressed high levels of interleukin (IL)-4, IL-10, transforming growth factor (TGF)-beta, interferon (IFN)-gamma, and undetectable levels of IL-2. Allografts that were rejected in a delayed fashion expressed mostly IL-2, IFN-gamma, and TGF-beta with low or undetectable levels of IL-4 and IL-10. Acutely rejected allografts from unmanipulated controls or peptide-manipulated recipients expressed high levels of IL-2, IFN-gamma, TGF-beta and undetectable levels of IL-4 or IL-10. All allografts also expressed T-cell receptor Cbeta gene, providing evidence for the presence of T-cell infiltrates in the grafts.

CONCLUSIONS

These observations demonstrate that acute graft rejection in this model is associated with the expression of Th1 cytokines, IL-2, and IFN-gamma, whereas long-term survival is associated with predominant expression of Th2 cytokines, IL-4, and IL-10. The expression of IFN-gamma in long-surviving allografts in the absence of IL-2 provides evidence for altered activation of the Th1 response in this intrathymic immune modulation model.

EBV-B cells as antigen presenting cells in characterization of the self/donor context of allorecognition by T lymphocytes

Alloreactivity is caused by T cell recognition of foreign histocompatibility antigens according to two models: (i) indirect recognition, in which processed allogeneic antigens are presented by self-major histocompatibility complexes like any other foreign antigen, and (ii) direct recognition, where the foreign MHC itself is recognized breaking the T cell recognition rule of self-restriction. This paper uses these two cases of alloantigen presentation as illustrative examples to investigate (i) the capacity of Epstein-Barr virus-transformed B cells (EBV-B cells) to process alloantigens, and (ii) in vitro assays with EBV-B cell lysate as a source of alloantigen, in order to characterize alloreactive T cell populations. A microculture system was established using donor EBV-B cell lysate as a source of the allogeneic antigen and donor or recipient EBV-B cells as antigen presenting cells to investigate whether alloantigen is recognized by effector T cells from the recipient. T lymphocytes produced after expansion in the presence of interleukin-2 from four samples of liver biopsies (three patients) and four samples of bronchoalveolar lavages (four patients) were used as effector cells. Upon human leucocyte antigen class II typing, these expressed the patient phenotype. When the T lymphocytes were from liver grafts, the recognition involved donor antigens presented by donor EBV-B cells (direct recognition). On the other hand, when the T lymphocytes were cultured from lung grafts, they mainly recognized antigens of donor EBV-B cell lysates in a self-restricted context (indirect recognition). These data suggest that EBV-B cells can provide allogeneic determinants recognized by T cells in donor or self-contexts, i.e. through either direct or indirect recognition.

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

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

The presentation of self and allogeneic MHC peptides to T lymphocytes

The presentation of donor-derived MHC peptides by recipient APCs to T cells is an essential component of the rejection of allografts (indirect allorecognition). Initial alloreactive T cell response is confined to a few well processed and presented dominant determinants on donor MHC. However, during long-term graft rejection, T cell response spreads to formerly poorly presented cryptic allogeneic MHC peptides. This phenomenon is likely to play an important role in the amplification and the perpetuation of the rejection process. Additionally, we present evidence that T cell repertoire selection to allogeneic MHC peptides is acquired via recognition of self-MHC peptides presented in the thymus during ontogeny. Supporting this view, we have shown that indirect alloresponses can lead to self-T cell tolerance breakdown to cross-reactive determinants on self-MHC molecules or alternatively that sensitization of recipients to self-MHC peptides can lead to accelerated graft rejection. It is therefore essential to determine the factors which govern the processing and presentation of self and allogeneic MHC molecules and to elucidate the mechanisms regulating subsequent T cell responses in order to design antigen-specific based immune therapies in transplantation.

Preformed IgG antibodies against major histocompatibility complex class II antigens are major risk factors for high-grade cellular rejection in recipients of heart transplantation

BACKGROUND

Preformed anti-HLA antibodies reacting specifically with donor lymphocytes have been associated with acute vascular rejection and early cardiac allograft failure. However, the effect of preformed anti-HLA antibodies directed against allogeneic major histocompatibility complex (MHC) class I or II antigens of a donor panel on heart transplantation outcome has not been extensively studied.

METHODS AND RESULTS

The study group consisted of 68 patients who received cardiac transplants between 1989 and 1996 and who were at high risk for developing anti-HLA antibodies before transplantation. The effect of preformed antibodies against allogeneic MHC class I or class II antigens on the development of early high-grade cellular rejection and on cumulative annual rejection frequency was determined. Both patients with left ventricular assist devices and retransplantation candidates had a similar increase in the frequency of IgG anti-MHC class II antibodies (IgG anti-II) compared with control subjects (P<0.0001), whereas the frequency of IgG anti-MHC class I antibodies (IgG anti-I) was elevated only in patients with left ventricular assist devices. Pretransplantation IgG anti-II predicted early development of high-grade cellular rejection (P=0.006) and higher cumulative annual rejection frequency (P<0.001) in both of these sensitized patient groups. Among retransplantation recipients, a match between donors 1 and 2 at HLA-A additionally predicted an earlier time to a high-grade cellular rejection.

CONCLUSIONS

These results emphasize the importance of specifically screening heart transplantation candidates for the presence of IgG antibodies directed against MHC class II molecules and suggest that strategies aimed at their reduction may have an impact on the onset and frequency of high-grade cellular rejections after transplantation.

Determinant spreading: lessons from animal models and human disease

Spreading of the immune response is a common theme in organ-specific and systemic autoimmune diseases. We evaluated whether some of the mixed antinuclear antibody patterns characteristic of systemic autoimmunity might be the result of determinant spreading from a single initiating event. Immunisation of healthy mice with individual protein components of the La/Ro ribonucleoprotein (RNP) targeted in systemic lupus erythematosus and primary Sjögren's syndrome induced autoantibodies recognising Ro60 (SS-A), Ro52 (SS-A) and La (SS-B) and in some cases the molecular chaperones calreticulin and Grp78. The endogenous antigen(s) driving determinant spreading might be derived from physiological apoptosis which could explain the involvement of some chaperone proteins in the autoimmune response. Diversified anti-La/Ro antibody responses were initiated by challenge with a single subdominant T epitope of La even though some self epitopes of La were efficiently tolerised. The pattern of autoantibody responses in primary Sjögren's syndrome was strongly influenced by HLA class II phenotype which we speculate controls activation of T cells recognising defined peptides from the La/Ro RNP. In this way, HLA class II alleles may be critical in influencing initiation and spreading of systemic autoimmune reactions. Molecular mimicry of such determinants by exogenous agents might readily initiate spreading of an autoimmune response in genetically susceptible hosts.

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.