Allorecognition of isolated, denatured chains of class I and class II major histocompatibility complex molecules. Evidence for an important role for indirect allorecognition in transplantation

Immunological challenges associated with artificial skin grafts: available solutions and stem cells in future design of synthetic skin

The repair or replacement of damaged skins is still an important, challenging public health problem. Immune acceptance and long-term survival of skin grafts represent the major problem to overcome in grafting given that in most situations autografts cannot be used. The emergence of artificial skin substitutes provides alternative treatment with the capacity to reduce the dependency on the increasing demand of cadaver skin grafts. Over the years, considerable research efforts have focused on strategies for skin repair or permanent skin graft transplantations. Available skin substitutes include pre- or post-transplantation treatments of donor cells, stem cell-based therapies, and skin equivalents composed of bio-engineered acellular or cellular skin substitutes. However, skin substitutes are still prone to immunological rejection, and as such, there is currently no skin substitute available to overcome this phenomenon. This review focuses on the mechanisms of skin rejection and tolerance induction and outlines in detail current available strategies and alternatives that may allow achieving full-thickness skin replacement and repair.

Pancreatic Tissue Transplanted in TheraCyte Encapsulation Devices Is Protected and Prevents Hyperglycemia in a Mouse Model of Immune-Mediated Diabetes

Type 1 diabetes (T1D) is characterized by destruction of glucose-responsive insulin-producing pancreatic β-cells and exhibits immune infiltration of pancreatic islets, where CD8 lymphocytes are most prominent. Curative transplantation of pancreatic islets is seriously hampered by the persistence of autoreactive immune cells that require high doses of immunosuppressive drugs. An elegant approach to confer graft protection while obviating the need for immunosuppression is the use of encapsulation devices that allow for the transfer of oxygen and nutrients, yet prevent immune cells from making direct contact with the islet grafts. Here we demonstrate that macroencapsulation devices (TheraCyte) loaded with neonatal pancreatic tissue and transplanted into RIP-LCMV.GP mice prevented disease onset in a model of virus-induced diabetes mellitus. Histological analyses revealed that insulin-producing cells survived within the device in animal models of diabetes. Our results demonstrate that these encapsulation devices can protect from an immune-mediated attack and can contain a sufficient amount of insulin-producing cells to prevent overt hyperglycemia.

Alloreactivity

The alloimmune response between individuals genetically disparate for antigens encoded within the major histocompatibility complex (MHC) remains a substantial barrier to transplantation of solid organs, tissues, and hematopoietic stem cells. Alloreactivity has been an immunological paradox because of its apparent contradiction to the requirement of MHC restriction for the induction of normal T lymphocyte mediated immune responses. Through crystallographic analyses and experimental systems utilizing murine CD8(+) cytolytic T cell clones, major advances have been achieved in understanding the molecular and structural basis of T cell receptor recognition of MHC-peptide complexes and the basis of T cell mediated alloreactivity. These studies have further provided an explanation for the relatively high frequencies of alloreactive T cells compared to the frequencies of T cells for microbial derived antigens.

Pathways of major histocompatibility complex allorecognition

PURPOSE OF REVIEW

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

RECENT FINDINGS

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

SUMMARY

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

Pathways of major histocompatibility complex allorecognition

PURPOSE OF REVIEW

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

RECENT FINDINGS

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

SUMMARY

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

Innate immunity and resistance to tolerogenesis in allotransplantation

The development of immunosuppressive drugs to control adaptive immune responses has led to the success of transplantation as a therapy for end-stage organ failure. However, these agents are largely ineffective in suppressing components of the innate immune system. This distinction has gained in clinical significance as mounting evidence now indicates that innate immune responses play important roles in the acute and chronic rejection of whole organ allografts. For instance, whereas clinical interest in natural killer (NK) cells was once largely confined to the field of bone marrow transplantation, recent findings suggest that these cells can also participate in the acute rejection of cardiac allografts and prevent tolerance induction. Stimulation of Toll-like receptors (TLRs), another important component of innate immunity, by endogenous ligands released in response to ischemia/reperfusion is now known to cause an inflammatory milieu favorable to graft rejection and abrogation of tolerance. Emerging data suggest that activation of complement is linked to acute rejection and interferes with tolerance. In summary, the conventional wisdom that the innate immune system is of little importance in whole organ transplantation is no longer tenable. The addition of strategies that target TLRs, NK cells, complement, and other components of the innate immune system will be necessary to eventually achieve long-term tolerance to human allograft recipients.

Role of T cells in graft rejection and transplantation tolerance

Transplantation is the most effective treatment for end-stage organ failure, but organ survival is limited by immune rejection and the side effects of immunosuppressive regimens. T cells are central to the process of transplant rejection through allorecognition of foreign antigens leading to their activation, and the orchestration of an effector response that results in organ damage. Long-term transplant acceptance in the absence of immunosuppressive therapy remains the ultimate goal in the field of transplantation and many studies are exploring potential therapies. One promising cellular therapy is the use of regulatory T cells to induce a state of donor-specific tolerance to the transplant. This article first discusses the role of T cells in transplant rejection, with a focus on the mechanisms of allorecognition and the alloresponse. This is followed by a detailed review of the current progress in the field of regulatory T-cell therapy in transplantation and the translation of this therapy to the clinical setting.

Allorecognition and the alloresponse: clinical implications

The artificial transfer of tissues or cells between genetically diverse individuals elicits an immune response that is adaptive and specific. This response is orchestrated by T lymphocytes that are recognizing, amongst others, major histocompatibility complex (MHC) molecules expressed on the surface of the transferred cells. Three pathways of recognition are described: direct, indirect and semi-direct. The sets of antigens that are recognized in this setting are also discussed, namely, MHC protein products, the MHC class I-related chain (MIC) system, minor histocompatibility antigens and natural killer cell receptor ligands. The end product of the effector responses are hyperacute, acute and chronic rejection. Special circumstances surround the situation of pregnancy and bone marrow transplantation because in the latter, the transferred cells are the ones originating the immune response, not the host. As the understanding of these processes improves, the ability to generate clinically viable immunotherapies will increase.

Induction immunosuppression

Induction immunosuppression is intense, prophylactic therapy used at the time of transplantation based on the empiric observation that more powerful immunosuppression is required to prevent acute rejection early. In the past decade, there has been a growing trend towards the use of specialized agents such as antibody therapies for induction. In general, these agents have been shown to reduce the rate of acute rejection. However, their use has not been clearly shown to improve long-term transplant outcomes. This overview will review the biological basis for induction immunosuppression and the mechanisms of action of those specialized induction agents currently in clinical use. Clinical trials investigating induction regimens will be evaluated, and an individualized approach to the use of induction immunosuppressants will be presented.

CMV-infected allogeneic endothelial cells initiate responder and bystander donor HLA class I release via the metalloproteinase cleavage pathway

Although cytomegalovirus (CMV) interferes with major histocompatibility expression in infected cells, both host and donor soluble human leukocyte antigen class I (sHLA-I) are often released into the serum of transplant recipients during CMV infection and may contribute to anti-HLA antibody production and graft rejection. We hypothesized that CMV infection of endothelial cells (EC) induces host T cells to release interferon (IFN)-gamma, which in turn drives the metalloproteinase (MPase)-cleavage pathway of sHLA-I generation in "bystander" uninfected ECs. To test this hypothesis, cultures of peripheral blood mononuclear cells (PBMCs) and either uninfected ECs or CMV-infected ECs (EC/CMV) were established and supernatants were tested in enzyme-linked immunosorbent assay for sHLA-I. Responder PBMC became activated and released sHLA-I via the MPase pathway when stimulated with allogeneic EC/CMV; the sHLA-I release was contact dependent and cytokine independent. In transwell cultures, IFN-gamma released by PBMCs in response to EC/CMV stimulated a release of sHLA-I from uninfected allogeneic ECs across the transwell; this release was also MPase dependent. This implies that CMV infection within the transplanted allograft will not only stimulate the release of self HLA from responding PBMCs, but will also stimulate the release of donor sHLA-I from uninfected bystander ECs, both via the class I MPase-pathway.

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.

HLA-B7 beta-pleated sheet-derived synthetic peptides are immunodominant T-cell epitopes regulating alloresponses

Chronic rejection of transplanted allografts is the major cause of graft loss after clinical solid organ transplantation. Recent data link the indirect presentation of allopeptides to chronic graft loss; thus, identification of immunodominant epitopes on major histocompatibility complex (MHC) antigens could significantly contribute to establishing novel ways for monitoring and managing chronic rejection. Here, we show that synthetic allo-MHC-derived peptides covering the polymorphic region 56 to 120 of HLA-B7 modulate alloresponses. In particular, the 2 beta-pleated sheet-derived peptides covering residues 91 to 105 and 96 to 120, respectively, but not sequences from the alpha1 helix, were presented by autologous peripheral blood lymphocytes to induce T-cell proliferation. In addition, the 2 beta-pleated sheet-derived peptides and the alpha1-derived peptide residues 60 to 75 abrogated lysis of HLA-B7 target cells by anti-HLA-B7 cytotoxic T lymphocytes (CTLs). Although most residues between 91 and 120 are normally not directly accessible to T cells, our results indicate that peptides derived from the lower surface of the peptide-binding groove of HLA-B7 are immunodominant in HLA-B7 alloresponses. To characterize the binding and stability of allopeptides to T cells, the 62-70 peptide-derived from the 60-75 allopeptide that blocked cytotoxicity of anti-HLA-B7 CTL-was synthesized and coupled with fluorescein isothiocyanate. The peptide specifically labeled anti-B7 CTL, but not anti-HLA-A2 CTL as measured by flow cytometry. Peptide binding to CTL was specific at 4 degrees C and remained stable for 12 hours, whereas it remained stable for less than 2 hours at 37 degrees C. These studies allow the identification of HLA-B7 T-cell epitopes and reveal for the first time a novel, previously unrecognized application of synthetic HLA-derived allopeptides to visualize alloreactive T cells.

Reduction of human-to-pig cellular response by alteration of porcine MHC with human HLA DPW0401 exogenes

BACKGROUND

In pig-to-human discordant xenotransplantation, the xenograft can be rejected by a formidable human xenogenic T-cell response, even if the graft has gone through hyperacute rejection or delayed xenograft rejection (acute vascular rejection). We therefore examined, in this study, whether the human-to-pig cellular response could be attenuated through the generation of a transgenic pig for human HLA II.

METHODS

With the technique of microinjection, we produced the HLA DPw0401 transgenic pig. The expression of the HLA DPw0401 gene on peripheral blood mononuclear cells (PBMCs) of the transgenic pig was examined by reverse transcriptase-polymerase chain reaction and flow cytometry. The antigenicity of the transgenic HLA DPw0401 molecule was tested by the HLA DPw0401-primed lymphocyte test reagent. The cellular response was analyzed by xenogenic mixed lymphocyte culture.

RESULTS

The mRNA and protein of HLA DPw0401 were expressed in the PBMCs of the transgenic pig. The PBMCs of the HLA transgenic pig induced a stronger cellular reaction to HLA DPw0401-primed lymphocyte test reagents than the nontransgenic littermate pig (n=7, P<0.01). In direct xenogenic mixed lymphocyte culture with responders from HLA DPw0401(+) humans, the PBMCs from the HLA DPw0401 transgenic pig, as compared with those from the normal pig, induced a lower degree of xenogenic cellular response to human PBMCs (n=4, P=0.08).

CONCLUSIONS

Our preliminary data demonstrated the possibility that the human HLA DPw0401 phenotype can be transferred onto porcine cells through the generation of HLA transgenic pigs and make the PBMCs of humans more tolerant to porcine cells.

Synthetic HLA-A2 derived peptides are recognized and presented in renal graft recipients

Indirect presentation of allogeneic MHC antigen is an important pathway by which allografts are rejected and tolerance maintained by regulatory CD4(+) T cells. In this study HLA-A2 derived synthetic peptides were used to determine whether T cells of non-HLA-A2 renal graft recipients, which had been HLA-A2 mismatched to their organ donors, recognize some of the HLA-A2-derived peptides. Among the HLA-A2 mismatched patients, 60% recognized residues 56--69, 65--79, and 75--89. Peripheral blood lymphocytes derived from healthy individuals showed low reactivity towards allopeptides, indicating that sensitization towards HLA-A2 induced response towards HLA-A2 derived peptides. The response to the peptides was blocked by antibodies to HLA-DR, -DQ, and CD4. Depletion of antigen presenting cells abrogated response towards the allopeptides, confirming that the observed proliferation was mediated by the indirect pathway. Interestingly, although none of the HLA-A2 mismatched patients had any signs for either acute or chronic rejection, considerable response to allo-derived HLA-A2 was observed.

Hypothesis: is renal allograft rejection initiated by the response to injury sustained during the transplant process?

Allograft rejection can be caused by numerous factors such as damage to the donor kidney during surgical removal or implantation, injury sustained during the transport process between the donor and recipient, and suboptimal allograft perfusion during the intra- and post-operative period. In cadaveric allografts, damage can occur during cold storage, during the transit stage between donor and recipient, and hemodynamic instability due to the initial damage that caused its removal from the donor (such as brain death or trauma). We hypothesize that rejection requires recognition of this injury in addition to recognition of alloantigens. If indeed injury proves to be one factor in acute rejection episodes, then therapeutic efforts can be made to reduce injury during the transplantation process.

T cell response in xenorecognition and xenografts: a review

Xenotransplantation has recently become a subject of interest for the transplantation community due to the current organ shortage, which could be partially or even totally solved by the development of this strategy. The humoral response, which arises as a result of species disparities, is the major obstacle to the success of xenotransplantation. However, if the use of different strategies such as plasmapheresis, immunoadsorption, the utilization of organs from transgenic pigs for complement regulatory molecules and new immunosuppressive drugs, may allow to overcome or reduce the early antibody mediated rejections (hyperacute or acute vascular rejection), delayed responses based on cellular activations will still occur. In this review, despite the fact that different cell populations have been shown to be implicated in these phenomena (NK, granulocytes, macrophages), we will focus on recent published information concerning T cell response only, in xenorecognition.

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.

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.

Role of donor and recipient antigen-presenting cells in priming and maintaining T cells with indirect allospecificity

BACKGROUND

It has been suggested that the sensitization of recipient T lymphocytes against peptides derived from allogeneic major histocompatibility complex (MHC) antigens in the context of self-MHC molecules may contribute to the pathogenesis of chronic allograft rejection. The purpose of this study was to quantitate and characterize the indirect alloresponse in renal transplantation.

METHODS

An HLA-A2-negative patient whose A2-positive kidney transplant failed as a result of chronic rejection was selected for this study. T-cell clones were raised using a cocktail of peptides corresponding to polymorphic regions of the A2 sequence and studied by measuring their proliferation using [3H]thymidine incorporation. The presence in vivo of HLA-A2-specific T cells was assessed using limiting dilution analysis.

RESULTS

T-cell clones were specific for a single peptide of HLA-A2, residues 92-120, and restricted by HLA-DRB1*1502. The frequency of interleukin-2-secreting T cells specific for this A2 peptide was 1:86,000, only 2-fold lower than that measured against the recall antigen tetanus toxoid. Capitalizing on the similarity of the donor and recipient DR15 alleles (DRB1*1501 and 1502), the question was addressed as to how these T cells had been primed in vivo. Although the large majority of clones responded to A2 synthetic peptide presented by both DR15 alleles, only 3 of 10 clones responded to cells co-expressing DRB1*1501 and A2.

CONCLUSION

These data suggest that antigen presentation by recipient APCs is responsible for maintaining T cells with indirect allospecificity in vivo and that, in the context of partial DR matching, indirect presentation by the parenchymal cells of the graft may serve to induce tolerance in T cells with indirect allospecificity.

Reduction of alloantibody response to class I major histocompatibility complex by targeting synthetic allopeptides for presentation by B cells

BACKGROUND

PVG.RT1(u) rats develop a strong CD4 T cell-dependent alloantibody response to class I major histocompatibility complex (MHC) A(a) antigen, during which CD4 T helper cells recognize and respond to A(a)-derived peptides presented by recipient class II MHC (indirect allorecognition). On the basis of evidence that CD4 T cells that encounter antigen presented by resting B cells become tolerant, we have targeted synthetic A(a)-derived allopeptides for in vivo presentation to class I MHC-disparate CD4 T cells by resting recipient B cells.

METHODS

PVG.RT1(u) rats were treated with two peptides, P1 and P2, corresponding to the alpha-helical regions of A(a) (residues 57-80 and 143-163), which were conjugated via an N-terminal cysteine residue to monovalent Fab fragments of OX60 monoclonal antibody, which labels membrane IgD-positive B cells.

RESULTS

RT1(u) rats primed with free (nonconjugated) P1 or P2 emulsified in complete Freund's adjuvant produced strong peptide-specific antibody responses and a heightened anti-A(a) antibody response to an A(a)-disparate PVG.R8 heart graft, confirming that each peptide encompasses one or more major T cell determinant for B cell help. Pretreatment of PVG.RT1(u) rats with a mixture of OX60-Fab-P1/P2 conjugates markedly reduced their ability to mount an A(a) antibody response when challenged with either A(a)-disparate blood transfusion or an A(a)-disparate heart graft, although PVG.R8 heart graft survival was not prolonged.

CONCLUSIONS

In this report, we show that synthetic A(a)-derived allopeptides are able, when targeted for in vivo presentation to CD4 T cells by resting B cells, to impair the ability of RT1(u) rats to mount an antibody response to A(a) antigen. All subclasses of IgG anti-A(a) alloantibody were profoundly reduced, suggesting that the responsible mechanism is more likely to be CD4 T helper cell unresponsiveness rather than Th1/Th2 T cell polarization.

Attempts to demonstrate indirect T cell allorecognition of donor MHC peptides in transplant patients

Indirect T cell allorecognition has been shown to play an important role in the rejection of allografts in experimental animals. Although there has been much speculation as to its role in clinical transplantation, especially with regard to chronic rejection, indirect T cell allorecognition has been difficult to demonstrate in transplant patients. In this paper, we looked for in vitro T cell proliferation to synthetic peptides corresponding to donor HLA-A and HLA-B incompatible antigens. Twelve 15 amino acid peptides corresponding to the hypervariable regions of six of the most common HLA class I alleles in Caucasian populations (A1, A2, A3, B7, B8 and B44) were studied. Blood was taken from 12 adult patients following one or more episodes of acute kidney graft rejection, and from three pediatric patients undergoing chronic rejection of heart/lung transplants. The donor-recipient combinations were selected such that at least one of the six HLA antigens above was present in the donor and absent in the recipient. Peripheral blood mononuclear cells from these patients responded strongly in proliferation assays to phytohemagglutinin. However, none responded to the incompatible donor HLA peptides. Compartmentalization of responding T cells, the effects of immunosuppression, and assay sensitivity are discussed as possible explanations for the negative results.

Mechanisms of indirect allorecognition in graft rejection: class II MHC allopeptide-specific T cell clones transfer delayed-type hypersensitivity responses in vivo

Recent animal studies suggest that indirect T-cell recognition of alloantigen plays an important role in allograft rejection and tolerance. In this study, we generated T cell clones from Lewis (LEW, RT1(l)) rat lymph node cells that had been primed in vivo by immunization with immunogenic class II MHC allopeptide RT1.D(u)beta2, representing residues 20-44 of the polymorphic beta chain of RT1.Dbeta(u) (Wistar Furth [WF]). Using reverse transcriptase polymerase chain reaction transcript analysis with specific rat T cell receptor Vbeta primers, we show that six out of nine T cell clones specifically proliferated to RT1.D(u)beta2 and expressed Vbeta 9. One of these clones, clone 2F4, which specifically proliferated to RT1.D(u)beta2 in a dose-response fashion and produced interferon-gamma in response to restimulation by RT1.D(u)beta2, was selected for further studies. The beta-chains of RT1.D(l) and RT1.D(u) residues 20-44 differ by two amino acids at positions 30 and 38. We synthesized two peptides, each containing a single polymorphic site: RT1.D(u)beta 20-33 and RT1.D(u)beta 31-44. Both these peptides were immunogenic by LEW responders, since lymph node cells primed by immunization proliferated equally to the peptides in vitro. Interestingly, in vitro dose-response studies with clone 2F4 showed better proliferative response to peptide RT1.D(u)beta 20-33 than to peptide RT1.D(u)beta 31-44, indicating that this T cell clone preferentially recognizes a single amino acid difference on residue 30. Finally, it has been suggested that indirect allorecognition by CD4+ T cells mediate graft rejection by delayed-type hypersensitivity responses, although definitive studies are lacking. Systemic injection of the 2F4 clone to naive LEW rats elicited an antigen-specific delayed-type hypersensitivity response against RT1.D(u)beta2 peptide and WF splenocytes, confirming indirect presentation in vivo. These data demonstrate that Th1 cell clones generated by in vivo priming via the indirect pathway utilize specific T cell receptor Vbeta and recognize single amino acid differences in the allopeptide. More importantly,these Th1 clones are capable of mediating a specific immune response in vivo. These studies with MHC allopeptide-specific T cell clones further delineate the cellular mechanisms of indirect allorecognition and provide a potential strategy to study its role in acute and chronic rejection, and tolerance.

Indirect recognition of donor HLA-DR peptides in organ allograft rejection

To determine whether indirect allorecognition is involved in heart allograft rejection T cells obtained from peripheral blood and graft biopsy tissues were expanded in the presence of IL-2 and tested in limiting dilution analysis (LDA) for reactivity to synthetic peptides corresponding to the hypervariable regions of the mismatched HLA-DR antigen(s) of the donor. Serial studies of 32 patients showed that T cell reactivity to donor allopeptides was strongly associated with episodes of acute rejection. The frequency of allopeptide reactive T cells was 10-50-fold higher in the graft than in the periphery indicating that T cells activated via the indirect allorecognition pathway participate actively in acute allograft rejection. In recipients carrying a graft differing by two HLA-DR alleles the response appeared to target only one of the mismatched antigens of the donor. Indirect allorecognition was restricted by a single HLA-DR antigen of the host and directed against one immunodominant peptide of donor HLA-DR protein. However, intermolecular spreading was demonstrated in patients with multiple rejection episodes by showing that they develop allopeptide reactivity against the second HLA-DR antigen. These data imply that early treatment to suppress T cell responses through the indirect pathway of allorecognition, such as tolerance induction to the dominant donor determinant, may be required to prevent amplification and perpetuation of the rejection process.

T cell receptor biases and clonal proliferations among lung transplant recipients with obliterative bronchiolitis

Obliterative bronchiolitis (OB) is the most serious late complication of lung transplantation, but the pathogenesis of this disorder has not been elucidated. We sought evidence that OB is mediated by a cellular immunologic response by characterizing T cell antigen receptor beta-chain variable gene (TCRBV) repertoires in lung allograft recipients. Expression levels of 27 TCRBV among recipients were determined by multiprobe RNase protection assay after PCR amplification. In comparison to recipients with no evidence of rejection (n = 9), the PBL TCRBV repertoires of OB subjects (n = 16) exhibited more frequent expansions (16 vs. 9% of all measured TCRBV, P < 0.02), and the magnitudes of these abnormalities were greater (8.2 +/- 0.8 vs. 4.5 +/- 0.3 SD from mean normal values, P < 0.01). TCRBV sequencing showed these expansions were composed of clonal or oligoclonal populations. Thus, T cell responses in the recipients are marked by highly selective clonal expansions, presumably driven by indirect recognition of a limited number of immunodominant alloantigens. These processes are exaggerated among allograft recipients with OB, implying that cognate immune mechanisms are important in the pathogenesis of the disorder. Furthermore, the prominence of finite, distinct TCR phenotypes raise possibilities for development of novel diagnostic modalities and targeted immunotherapies for OB and other manifestations of chronic allograft rejection.

The immune response: the afferent arm

The key to understanding afferent immunity is the mechanism of activation of T lymphocytes by specialized antigen presenting cells, which bind antigenic peptide to Class II major histocompatibility molecules, and stimulate T cells via Signal 1 (antigen) and Signal 2 (costimulation). The best studied costimulatory pathway is the interaction of B7-1 or B7-2 ligand molecules on antigen presenting cells with CD28 or CTLA-4 receptors on T cells. T cell signaling occurs through the T cell receptor-CD3 complex and is augmented by cosignaling via CD4, CD8, and CD45. The activation of T cells to alloantigen occurs by either a direct pathway of recognition of allogenic major histocompatibility molecules (with or without an associated endogenous peptide), or by an indirect pathway of recognition of processed donor alloantigens via recipient antigen presenting cells. Afferent immunity on the musculoskeletal system is of special interest because of the absence of viable donor antigen presenting cells in processed grafts that makes them susceptible to the indirect pathway of alloantigen recognition.

A three-cell cluster hypothesis for noncognate T-B collaboration via direct T cell recognition of allogeneic dendritic cells

In this article, we propose that T cell help for B cells can occur via an unusual three-cell cluster, with recipient CD4+ T helper cells interacting via direct allorecognition with donor dendritic cell class II MHC antigens, recipient B cells interacting with MHC class I (or any other) antigen on the donor dendritic cell surface, and noncognate (i.e., antigen nonspecific) T-B collaboration. In this noncognate pathway, antigen processing by B cells is not required and T cell help is potent because of the high precursor T cell frequency for direct recognition of allogeneic class II MHC molecules. The data supporting this hypothesis are: 1. LEW rat strain recipients of interstitial dendritic cell-free (DAxLEW)F1 kidney allografts were shown to have no detectable antibody to donor class I MHC antigens at day 7 after grafting. By contrast, LEW recipients of normal (DAxLEW)F1 kidneys had strong antibody responses. 2. Consistent wih important role for donor dendritic cells in the early antibody response to donor class I MHC antigens was the finding that it was dependent on donor class II MHC antigens. PVG recipients, previously immunized with pure DA RT1.B class II MHC antigens, had virtually no antibody response to the class I MHC antigens of DA kidney allografts. 3. We confirmed the low and high responder status of PVG and LEW rats, respectively, to DA class I antigens by studying antibody responses to pure DA class I antigens. However, PVG and LEW recipients of DA kidney allografts did not differ in their antibody response to the donor DA class I MHC antigens. This is consistent with this response not requiring the processing and presentation of DA class I antigen by PVG recipients. 4. LEW recipients of interstitial dendritic cell-free (DAxLEW)F1 kidney allografts did eventually develop a strong antibody response to DA class I antigens, but this was delayed by several weeks. That this delayed antibody response was probably mediated by conventional T-B collaboration and that T help was rate limiting in this situation, was demonstrated by immunizing LEW recipients with a DA class I peptide. This markedly accelerated the kinetics of the antibody response to the dendritic cell-free (DAxLEW)F1 kidneys.

Monitoring of soluble HLA alloantigens and anti-HLA antibodies identifies heart allograft recipients at risk of transplant-associated coronary artery disease

The development of accelerated transplant-related coronary artery disease (T-CAD) is the major obstacle to long-term survival of cardiac allografts. We have investigated the role of various demographic and immunologic parameters as prognostic indicators of T-CAD in a population of 274 heart allograft recipients. Our data demonstrate that patients who experience more than 1 episode of acute rejection per year and/or develop antidonor HLA antibodies are at increased risk of developing T-CAD. Using HLA-A2 as a marker for the release of soluble HLA antigens from the donor, we established that recipients displaying circulating donor alloantigens for more than 26 weeks following transplantation are at increased risk of developing T-CAD (P=0.008). This association suggests that the release of alloantigens from the allograft is indicative of chronic injury and/or that it stimulates chronic rejection via the indirect allorecognition pathway. Our findings indicate that patients at risk of developing T-CAD can be identified by monitoring the release of donor alloantigens and production of antidonor HLA antibodies following transplantation.

Indirect T cell recognition in allograft rejection

Allograft rejection has been attributed predominantly to alloreactive T cells which recognize intact allogeneic MHC on donor antigen presenting cells (APC). Following transplantation, T cells may also recognize donor derived MHC antigens which have been processed and presented as antigenic peptides in the context of self-MHC by recipient APC--so called indirect allorecognition. This article discusses the possible role of indirect T cell recognition in the graft rejection response with particular emphasis on recent experimental data obtained from in vivo transplant models. Although the relative contribution of indirect allorecognition to the rejection process remains unclear the available evidence suggests that the contribution of self-MHC restricted T cells to graft rejection may have been underestimated.

Allograft survival following immunization with membrane-bound or soluble peptide MHC class I donor antigens: factors relevant for the induction of rejection by indirect recognition

T cells recognize foreign antigens in the form of peptide fragments resulting from antigen processing by antigen-presenting cells. In contrast to this indirect recognition, MHC molecules of foreign cells can be directly recognized by T cells. Direct recognition has for a long time been considered the only mechanism responsible for transplant rejection. Recent studies have provided evidence of a role of indirect recognition in rejection. In the current series of experiments, we studied the influence of indirect alloactivation, induced either by donor MHC class I peptides or by membrane-bound MHC I molecules, on heart allograft rejection in rats. Recipients were immunized before transplantation with synthetic donor MHC I peptides. The animals developed antibody and T-cell responses. Depending on the rat strain, peptide pretreatment either had no effect on graft survival (DA-->PVG; untreated controls 8.5 +/- 0.6 days, treated rats 9.5 +/- 0.6 days) or led to accelerated rejection (DA-->LEW; untreated controls 7.5 +/- 0.3 days, treated rats 5.1 +/- 0.2 days; P < 0.0002). Importantly, sensitization by indirect activation induced acute rejection in a donor-recipient combination (LEW.1A-->LEW.1WR2) in which neither direct nor indirect recognition led to rejection (untreated controls > 400 days, pretreated rats 15 +/- 4.2 days). Another group of animals was immunized with allogeneic or congenic erythrocytes carrying the MHC I antigen from which the peptides were derived. Although the immunization elicited a measurable immune response, it did not lead to accelerated rejection. We conclude that sensitization by indirect recognition is able to initiate an acute rejection even in recipients in which neither direct nor indirect recognition is effective, and that this effect is strain-dependent. The form in which the donor antigen is administered is decisive for the induction of rejection by indirect activation.

New approaches to specific immunomodulation in transplantation

T cells can recognize foreign MHC antigens by two distinct routes, either directly as intact molecules, or indirectly as processed peptides. Recent evidence strongly suggests that the indirect pathway of allorecognition plays a key role in initiating and sustaining graft rejection. Theoretically, all mismatched HLA alloantigens could generate immunogenic peptides which may be recognized in the context of any of the two self HLA-DR molecules. However, indirect recognition appears to be limited to a single peptide determinant of an allogeneic HLA-DR molecule and restricted by one self HLA-DR molecule. Furthermore, T cells involved in the self-restricted allopeptide recognition express a limited array of T cell receptor variable genes. These findings suggest that selective immune interventions, such as peptide blockade of the self HLA-DR molecule involved in the presentation of the dominant allopeptide, induction of high-zone tolerance or TCR antagonism, may be devised to prevent graft rejection.

Role of indirect allorecognition in allograft rejection

It is now clear from animal studies that indirect allorecognition occurs during allograft rejection and that this pathway plays a role in mediating the rejection process. Whether this pathway is the dominant pathway responsible for chronic rejection remains to be established, but this is an intriguing hypothesis that may have major implications for development of novel therapies for this disorder. In addition, there are data to indicate that indirect allorecognition may play a role in maintenance of allograft acceptance, and that provision of appropriate MHC peptides via different routes can result in potent degrees of specific allo-tolerance. Expanded experimentation in animals and in humans is therefore in order; first, to dissect more closely the molecular basis of allorecognition and establish the role of the indirect pathway in acute versus chronic rejection, and second to utilize the immunomodulatory properties of MHC peptides or other novel strategies which targets indirect allorecognition in promoting graft acceptance.

T cell receptor (TCR) repertoire in alloimmune responses

A large number of alloantigenic determinants could be generated by both the direct and indirect alloantigen presentation pathways. Hence, a heterogeneous population of T cells expressing a wide variety of receptors would be expected to respond to this diverse array of alloantigenic determinants. However, T cells expressing highly restricted T cell receptor (TCR) variable genes have been reported in a variety of alloimmune responses. A similar phenomenon has been observed in a wide variety of other immune responses, from those induced by superantigens, to very specific responses induced by a single peptide presented by a single MHC molecule. Given this scenario, the limited number of T cell clones which dominate an allograft rejection response, or for that matter an autoimmune response or a tumor specific response, could be therapeutically targeted by virtue of the selected TCR expression.

Suppression of the indirect pathway of T cell reactivity by high doses of allopeptide

T helper cells, which recognize allopeptides processed and presented by self APC, contribute to the generation of both cellular and humoral immune responses against allogeneic transplants. We have explored the hypothesis that the indirect T cell recognition pathway is initiated by soluble MHC antigens and that it can be suppressed by high doses of synthetic peptides corresponding to the dominant alloepitope. T cells from a DR11/7 responder were immunized in vitro with recombinant HLA-DR4 (rDR4). Experiments using partially overlapping synthetic peptides showed that the resulting T cell line (TCL) recognized a single dominant epitope mapping within residues 69-88 of the first domain of the DR4 molecule. In vitro immunization with synthetic allopeptides corresponding to other polymorphic regions, were unable to elicit T cell reactivity against rDR4, although at least one of these peptides (corresponding to residues 13-27) was immunogenic, behaving like a cryptic epitope. The rDR4-specific TCL expressed a limited TCR repertoire and provided help to autologous B cells for the production of specific antibodies. The T cell blastogenic response as well as the transcription and secretion of IL-4 (but not IL-2) was efficiently suppressed by high doses of the dominant allopeptide. These findings support the concept that selective immunointervention of indirect allorecognition can be achieved by use of high doses of antigen or TCR vaccination, as proposed for autoimmune diseases.

Effect of crossmatching on outcome in organ transplantation

The complement-dependent cytotoxicity (CDC) crossmatch and the flow cytometry crossmatch (FCXM) are both used prospectively in renal transplantation, and their use is under evaluation in other types of major organ transplantation. The FCXM is the more sensitive method and better predicts outcome in second and subsequent renal allografts. Improved survival has unmasked the detrimental effect of a positive crossmatch on outcome in liver transplantation. Because of the urgent need of liver transplant candidates, it is unrealistic to defer transplantation until a crossmatch-negative donor is found; however, additional therapeutic measures may be taken to improve outcome for crossmatch-positive liver recipients. Some reports suggest that prospective crossmatching may improve outcome for sensitized heart recipients, and, additionally, recent studies have demonstrated that HLA compatibility between donor and recipient is an independent variable affecting survival after heart transplantation, prompting a reassessment of the current practice of transplanting hearts without consideration of the HLA match.

The induction of tolerance to alloantigens using HLA-based synthetic peptides

The identification of immunomodulatory approaches that allow the induction of antigen-specific unresponsiveness is required for long-term graft survival without the complications of chronic immunosuppression. Recent novel strategies based upon treatment with synthetic peptides corresponding to linear sequences of MHC class I and II molecules reproducibly induce tolerance to alloantigens. Although the mechanisms involved are still not completely understood, the phenomenology reported makes these approaches promising for evaluation in clinical trials.

T-cell recognition of an allogeneic RT1-Dbu class II MHC peptide

The allo-antibody response of several rat strains to an unconjugated synthetic 20 amino acid peptide derived from the alpha helical region of the RT1-Du beta chain was tested. The LEW (RT1l) and WAG (RT1u) strains produced little or no antibody; the PVG (RT1c) and DA (RT1av1) strains produced moderate amounts of antibody; while the BN (RT1n) strain produced strong primary and secondary antibody responses. This suggested that the BN strain was able to process and present the RT1-Dbu peptide on its class II molecules. In vitro proliferation studies demonstrated that LEW T cells did not respond to the peptide, whereas BN T cells responded strongly, and that the response in the BN strain was found only in the CD4+ T-cell subset. However, immunisation of BN rats with the RT1-Dbu peptide failed to cause any acceleration of rejection of WAG skin or kidney grafts. Moreover, BN rats primed with WAG skin and kidney grafts did not produce T cells reactive to the RT1-Dbu synthetic peptide. This suggests that the T-cell response of the BN strain to the synthetic major histocompatibility complex peptide was not relevant to the indirect T-cell allo-recognition response to naturally processed RT1-Du beta chains.

Indirect presentation of MHC antigens in transplantation

T cells can recognise foreign major histocompatibility complex (MHC) antigens by two distinct routes, either directly as intact molecules or indirectly as peptides after antigen processing. Danny Shoskes and Kathryn Wood review the evidence that indirect presentation of allopeptides may play a significant role in the events leading to the rejection or acceptance of allo- and xenografts.

Indirect T cell allorecognition: a cyclosporin A resistant pathway for T cell help for antibody production to donor MHC antigens

LEW (RT1(1)) rats were primed for indirect allorecognition of DA (RT1avl) MHC molecules by immunizing either with synthetic peptides corresponding to the polymorphic regions of the RT1.Aavl classical class I MHC molecule, or with the isolated, denatured chains of the RT1.A class I, RT1.B alpha class II and RT1.B beta class II MHC molecules of the DA strain. These primed LEW rats received DA kidney allografts and were treated after grafting with cyclosporin A. Unprimed LEW controls mount a vigorous rejection response to DA kidney allografts and produce a strong antibody response to DA class I MHC antigens. Both the rejection and the antibody responses are virtually completely suppressed by cyclosporin A treatment in these controls. Priming to indirect allorecognition of donor MHC antigens did not diminish the effectiveness of cyclosporin A in suppressing the acute rejection of DA kidney grafts, but cyclosporin A could not suppress the early antibody response to the grafts in the primed rats. This finding could be of importance in clinical transplantation, where antibody-mediated graft damage might play an important role in both acute vascular rejection and chronic rejection.

Donor heart endothelial cells as targets for graft infiltrating lymphocytes after clinical cardiac transplantation

The in vitro cytotoxic reactivity of allograft infiltrating cells cultured from endomyocardial biopsies was tested against endothelial cells (EC) isolated from the own donor heart. EC derived from pieces of atrium were found to be proper targets for graft infiltrating cytotoxic T cells from four patients. The specificity of this cytotoxicity was further analysed by cold target inhibition studies and blocking with anti-CD3, anti-CD4 or anti-CD8 monoclonal antibodies. The experiments revealed that, besides a clearly HLA directed recognition, a more heterogeneous, multispecific response can be found, which might be partially explained by the activity of EC specific T cell clones. We conclude that this system provides a valuable approach to investigate the reactivity of graft infiltrating cells against EC in relation to the clinical course of the transplantation.

T cell recognition of donor major histocompatibility complex class I peptides during allograft rejection

LEW (RTI1) recipients of DA (RTIav1) skin and kidney allografts were tested for the capacity of their T lymphocytes to proliferate to three 22-24-amino acid peptides from the hypervariable regions of the RTI-Aav1 classical MHC class I molecule. Ten days after rejecting second-set DA kidney allografts, spleen cells (but interestingly not lymph node cells) from LEW recipients showed strong, LEW antigen-presenting cell-dependent, CD4+ T cell proliferation to peptide 1 (from the alpha helical region of the alpha 1 domain). CD8+ T cells showed no response to peptide 1. There was no response by the spleen cells to peptide 2 (from the beta sheet of the alpha 2 domain) or peptide 3 (from the alpha helical region of the alpha 2 domain). Immunization of LEW rats with pure RTI-Aav1 class I H chain in Freund's adjuvant gave responses identical to that seen after grafting, i.e. good CD4+ T cell proliferation to peptide 1, but none to peptides 2 and 3. However, immunization of LEW rats with peptides 1, 2 and 3 in Freund's adjuvant resulted in good CD4+ T cell proliferation responses to each of the peptides. These data demonstrate that indirect allorecognition can be stimulated by allograft rejection, and emphasize that the physiological processing of donor antigens will influence which peptides will be important in indirect allorecognition in transplantation.