The presentation of self and allogeneic MHC peptides to T lymphocytes

T cell Allorecognition Pathways in Solid Organ Transplantation

Transplantation is unusual in that T cells can recognize alloantigen by at least two distinct pathways: as intact MHC alloantigen on the surface of donor cells via the direct pathway; and as self-restricted processed alloantigen via the indirect pathway. Direct pathway responses are viewed as strong but short-lived and hence responsible for acute rejection, whereas indirect pathway responses are typically thought to be much longer lasting and mediate the progression of chronic rejection. However, this is based on surprisingly scant experimental evidence, and the recent demonstration that MHC alloantigen can be re-presented intact on recipient dendritic cells-the semi-direct pathway-suggests that the conventional view may be an oversimplification. We review recent advances in our understanding of how the different T cell allorecognition pathways are triggered, consider how this generates effector alloantibody and cytotoxic CD8 T cell alloresponses and assess how these responses contribute to early and late allograft rejection. We further discuss how this knowledge may inform development of cellular and pharmacological therapies that aim to improve transplant outcomes, with focus on the use of induced regulatory T cells with indirect allospecificity and on the development of immunometabolic strategies. KEY POINTS Acute allograft rejection is likely mediated by indirect and direct pathway CD4 T cell alloresponses.Chronic allograft rejection is largely mediated by indirect pathway CD4 T cell responses. Direct pathway recognition of cross-dressed endothelial derived MHC class II alloantigen may also contribute to chronic rejection, but the extent of this contribution is unknown.Late indirect pathway CD4 T cell responses will be composed of heterogeneous populations of allopeptide specific T helper cell subsets that recognize different alloantigens and are at various stages of effector and memory differentiation.Knowledge of the precise indirect pathway CD4 T cell responses active at late time points in a particular individual will likely inform the development of alloantigen-specific cellular therapies and will guide immunometabolic modulation.

Contributions of direct and indirect alloresponses to chronic rejection of kidney allografts in nonhuman primates

The relative contribution of direct and indirect allorecognition pathways to chronic rejection of allogeneic organ transplants in primates remains unclear. In this study, we evaluated T and B cell alloresponses in cynomolgus monkeys that had received combined kidney/bone marrow allografts and myeloablative immunosuppressive treatments. We measured donor-specific direct and indirect T cell responses and alloantibody production in monkeys (n = 5) that did not reject their transplant acutely but developed chronic humoral rejection (CHR) and in tolerant recipients (n = 4) that never displayed signs of CHR. All CHR recipients exhibited high levels of anti-donor Abs and mounted potent direct T cell alloresponses in vitro. Such direct alloreactivity could be detected for more than 1 y after transplantation. In contrast, only two of five monkeys with CHR had a detectable indirect alloresponse. No indirect alloresponse by T cells and no alloantibody responses were found in any of the tolerant monkeys. Only one of four tolerant monkeys displayed a direct T cell alloresponse. These observations indicate that direct T cell alloresponses can be sustained for prolonged periods posttransplantation and result in alloantibody production and chronic rejection of kidney transplants, even in the absence of detectable indirect alloreactivity.

Chronic graft versus host disease is associated with an immune response to autologous human leukocyte antigen-derived peptides

BACKGROUND

Chronic graft-versus-host disease (cGVHD) is a major complication of allogeneic bone marrow transplantation (BMT) the immunopathogenesis of which is not well understood. Humoral and cellular immunity are both implicated, patients express a range of autoantibodies, but the targets of cellular immunity are not well defined. Autologous human leukocyte antigen (HLA)-derived peptides constitute a significant proportion of the repertoire.

METHODS

We have investigated the response to HLA-derived peptides after allogeneic BMT using gamma-interferon enzyme-linked immunospot assay (ELISPOT). We also studied the release of this gamma-interferon by flow cytometry in a subgroup of responsive patients.

RESULTS

The peripheral blood mononuclear cell response was assessed by gamma-interferon ELISPOT in 42 BMT recipients (21 with cGVHD) and 30 healthy donors. Thirteen of 21 patients diagnosed with cGVHD responded to at least one HLA-derived peptide compared with 1 of 21 patients without cGVHD (62% vs. 5%, P<10) and 1 of 30 healthy donors. In all but one patient these peptides correspond with the sequences of autologous HLA. The median single peptide-specific response in ELISPOT was 43/10 peripheral blood mononuclear cells. In a subgroup studied by flow cytometry, gamma-interferon production to individual peptides occurred in 0.04% to 0.18% of CD4 T lymphocytes.

CONCLUSION

These observations identify HLA-derived peptides as targets of a cellular immune response in cGVHD.

Dual effects of the alloresponse by Th1 and Th2 cells on acute and chronic rejection of allotransplants

The contribution of direct and indirect alloresponses by CD4(+) Th1 and Th2 cells in acute and chronic rejection of allogeneic transplants remains unclear. In the present study, we addressed this question using a transplant model in a single MHC class I-disparate donor-recipient mouse combination. BALB/c-dm2 (dm2) mutant mice do not express MHC class I L(d) molecules and reject acutely L(d+) skin grafts from BALB/c mice. In contrast, BALB/c hearts placed in dm2 mice are permanently accepted in the absence of chronic allograft vasculopathy. In this model, CD4(+) T cells are activated following recognition of a donor MHC class I determinant, L(d) 61-80, presented by MHC Class II A(d) molecules on donor and recipient APC. Pre-transplantation of recipients with L(d) 61-80 peptide emulsified in complete Freund's adjuvant induced a Th1 response, which accelerated the rejection of skin allografts, but it had no effect on cardiac transplants. In contrast, induction of a Th2 response to the same peptide abrogated the CD8(+) cytotoxic T cells response and markedly delayed the rejection of skin allografts while it induced de novo chronic rejection of heart transplants. This shows that Th2 cells activated via indirect allorecognition can exert dual effects on acute and chronic rejection of allogeneic transplants.

The indirect alloresponse impairs the induction but not maintenance of tolerance to MHC class I-disparate allografts

We studied the effects of indirect allorecognition on the induction and maintenance phases of tolerance in miniature swine cotransplanted with heart and kidney allografts. MHC class I-mismatched heart and kidney grafts were cotransplanted in recipients receiving CyA for 12 days. Recipients were unimmunized or immunized with a set of donor-derived or control third-party MHC class I peptides either 21 days prior to transplantation or over 100 days after transplantation. T-cell proliferation, delayed type hypersensitivity reaction (DTH) and antibody production were assessed. All animals injected with donor MHC class I peptides developed potent indirect alloresponses specific to the immunizing peptides. While untreated recipients developed stable tolerance, all animals preimmunized with donor allopeptides rejected kidney-heart transplants acutely. In contrast, when peptide immunization was delayed until over 100 days after kidney-heart transplantation, no effects were observed on graft function or in vitro measures of alloimmunity. Donor peptide immunization prevented tolerance when administered to recipients pre transplantation but did not abrogate tolerance when administered to long-term survivors post transplantation. This suggests that the presence of T cells activated via indirect allorecognition represent a barrier to the induction but not the maintenance of tolerance.

Why is the fetal allograft not rejected?1

In viviparous species, the conceptus must be protected from a potentially hostile maternal immune system. The major histocompatibility complex (MHC) is a genetic region that encodes MHC class I and class II proteins, which present peptide antigens to T lymphocytes and induce graft rejection. The MHC, class II proteins are only expressed on professional, antigen-presenting cells. However, classical, MHC class I proteins are expressed on all nucleated somatic cells. Protection of the conceptus from immune-mediated rejection involves downregulation of classical MHC class I antigen expression on trophoblast cells, which form the external epithelial layer of the placenta, and maintenance of an immunologically favorable immunosuppressive environment in the uterus. Normally, bovine trophoblast cells do not express MHC class I antigens before d 120 of pregnancy. However, during the last third of gestation, trophoblast cells in the inter-placentomal and arcade regions of the placenta express classical, MHC class I proteins, which could potentially induce fetal rejection, as well as nonclassical, MHC class I proteins. A human, nonclassical, MHC class I antigen, human leukocyte antigen G, is an important immunoregulatory factor required for the maintenance of pregnancy. In cattle, MHC class I expression during the last third of pregnancy has no adverse effects and probably contributes to placental separation at parturition. However, somatic cell nuclear transfer (SCNT) conceptuses, the majority of which are aborted between d 30 and 90 of pregnancy, had trophoblast cell expression of MHC class I antigens before d 34 of pregnancy. In conjunction with increased trophoblast MHC class I expression, SCNT pregnancies exhibited a marked increase in the number of stromal lymphocytes in the uteri of surrogate dams. A retrospective study found that SCNT pregnancies established using MHC class I-homozygous cell lines, in which the immunological barrier is greatly reduced, had significantly improved fetal survival from d 28 to term (51% survival for MHC-homozygous and 5% for MHC-heterozygous SCNT fetuses). Consequently, it appears that the high rate of fetal mortality in SCNT pregnancies is due, at least in part, to inappropriate expression of trophoblast, MHC class I antigens resulting in immune-mediated placental rejection. This suggests that appropriate regulation of MHC class I genes is critical for immunological acceptance of an allogeneic conceptus.

Graft rejection mediated by CD4+ T cells via indirect recognition of alloantigen is associated with a dominant Th2 response

CD4(+) T cells that respond to indirectly presented alloantigen have been shown to mediate chronic rejection, however, the role of the indirect pathway in acute rejection has yet to be completely elucidated. To this end, BALB/c or C57BL/6 mice were depleted of CD8(+) T cells and transplanted with class II transactivator (CIITA)-deficient cardiac allografts, which cannot directly present class II alloantigens to CD4(+) T cells. In this manner, the rejection response by CD4(+) cells was forced to rely upon the indirect recognition pathway. When not depleted of CD8(+) cells, both BALB/c and C57BL/6 mice rejected CIITA-/- allografts and a polarized Th1 response was observed. In contrast, when BALB/c recipients of CIITA-/- allografts were depleted of CD8(+) T cells, the grafts were acutely rejected and a strong Th2 response characterized by eosinophil influx into the graft was observed. Interestingly, CD8-depleted C57BL/6 recipients of CIITA-/- allografts did not acutely reject their transplants and a Th2 response was not mounted. These findings indicate that CD4(+) T cells responding to indirectly presented alloantigens mediate graft rejection in a Th2-dominant manner, and provide further evidence for the role of Th2 responses in acute graft rejection.

Allogeneic core amino acids of an immunodominant allopeptide are important for MHC binding and TCR recognition

The indirect alloimmune response seems to be restricted to a few dominant major histocompatibility complex (MHC)-derived peptides responsible for T-cell activation in allograft rejection. The molecular mechanisms of indirect T-cell activation have been studied using peptide analogues derived from the dominant allopeptide in vitro, whereas the in vivo effects of peptide analogues have not been well characterized yet. In the present study, we generated allochimeric peptide analogues by replacing the three allogeneic amino acids 5L, 9L, and 10T in the sequence of the dominant MHC class I allopeptide P1. These allochimeric peptide analogues were used to define the allogeneic amino acids critical for the MHC binding and TCR recognition. We found that position 5 (5L) of the dominant allopeptide acts as an MHC-binding residue, while the other two allogeneic positions, 9 and 10, are important for the T-cell receptor (TCR) recognition. A peptide containing the MHC-binding residue 5L, as the only different amino acid between donor (RT1.A(u)) and recipient (RT1.A(l)) sequences, did not induce proliferation of lymph node cells primed with the dominant peptide and prevented dominant peptide-induced acceleration of allograft rejection. Identification of MHC and TCR contact residues should facilitate the development of antigen-specific therapies to inhibit or regulate the indirect alloimmune response.

Major histocompatibility antigen expression on the bovine placenta: its relationship to abnormal pregnancies and retained placenta

In viviparous animals, regulation of expression of major histocompatibility complex (MHC) class I antigens by the trophoblast cells, which constitute the outermost layer of the placenta, seems to be critical for maternal immunological acceptance of an allogeneic fetus. Cattle are unusual in this regard, since the bovine trophoblast cells, in specific regions of the uterine/placental interface, normally express MHC class I antigens during the third trimester of gestation. This expression appears to be biologically relevant as MHC class I compatibility between a cow and her fetus has been associated with an increased incidence of placental retention. We have found significant differences in lymphocyte populations, cytokine production, and trophoblast cell apoptosis in the placentomes of MHC-compatible and -incompatible pregnancies at parturition. This suggests that maternal immunological recognition of fetal MHC class I proteins triggers an immune/inflammatory response that contributes to placental separation at parturition in cattle. Early in pregnancy, a complete shutdown of MHC class I expression by trophoblast cells appears to be critical for normal placental development and fetal survival. In bovine somatic cell nuclear transfer (SCNT) pregnancies, there is an extremely high rate of fetal loss between days 30 and 90 of pregnancy. We have shown that in bovine SCNT pregnancies, between days 34 and 63 of gestation, there is both abnormal expression of MHC class I antigens by trophoblast cells and an abnormal accumulation of lymphocytes within the uterine stroma. Consequently, it is likely that activation of the maternal mucosal immune system, within the uterus at the same time when placentomes are being established, interferes with the process of placentome development and leads to immune-mediated abortion. Our data suggest that bovine MHC-compatible pregnancies provide a unique model for studying regulation of the uterine immune system, as well as immune-mediated placental rejection.

Apoptosis and transplantation tolerance

Self-tolerance is maintained by several mechanisms including deletion (via apoptosis) and regulation. Acquired tolerance to allogeneic tissues and organs exploits similar strategies. One key difference between alloantigens and peptide antigens is the enormous number of T cells that are alloreactive. Accumulating evidence suggests that in the face of this large mass of potentially graft-destructive T cells, tolerance requires an initial wave of deletion. This creates a more level playing field in which a smaller number of regulatory T cells can then act to maintain an established tolerant state. Deletion of alloreactive T cells by apoptosis actively promotes immunoregulation as well, by interfering with proinflammatory maturation of antigen presenting cells. This article reviews the immune response to alloantigens, the development and use of both necrotic and apoptotic means of cell death during the evolution of the immune response, and the likely role and mechanisms by which apoptosis promotes, and may even be required for, transplantation tolerance.

Vaccination with allogeneic dendritic cells fused to carcinoma cells induces antitumor immunity in MUC1 transgenic mice

Fusions of autologous tumor cells with allogeneic dendritic cells (DC) represent an approach for the induction of antitumor immunity. In the present studies, we investigated the antitumor effects of vaccinating MUC1-transgenic (MUC1.Tg) mice with MC38/MUC1 carcinoma cells fused to allogeneic DC from BALB/c mice (allo-DC, H-2(d)) or syngeneic DC from C57BL/6 mice (syn-DC, H-2(b)). Both allo and syn fusion cells (FC/MUC1) expressed MHC class II, costimulatory molecules, and the MUC1 antigen. Allo-FC/MUC1 exhibited dual expression of MHC class I haplotypes (H-2(d)/H-2(b))and MUC1 antigen. By contrast, only H-2(b) and MUC1 antigen were expressed by syn-FC/MUC1. CTLs from MUC1.Tg mice immunized with allo- or syn-FC/MUC1 fusion cells lysed MC38/MUC1 targets. Moreover, immunization with allo- or syn-FC/MUC1 was effective in eliminating established MUC1-positive pulmonary metastases in MUC1.Tg mice. These results indicate that immunization of MUC1.Tg mice with syn- or allo-FC/MUC1 is effective in reversing immunologic unresponsiveness to MUC1 antigen and inducing immunity against MUC1-positive tumors. The findings in the present study have broader clinical implications for fusion cell vaccines.

Human liver allograft acceptance and the "tolerance assay": in vitro anti-donor T cell assays show hyporeactivity to donor cells, but unlike DTH, fail to detect linked suppression

Human allograft acceptance is associated with immune regulation, characterized by donor-antigen-linked suppression of delayed-type hypersensitivity (DTH). We wished to determine if "classical" in vitro assays of alloreactivity could also detect linked suppression and thus be useful in the clinical diagnosis of active immune regulation. We analyzed peripheral blood mononuclear cells from a group of eight liver transplant recipients, one of whom had stopped all immunosuppression 4.5 years ago yet continues to have good graft function (graft acceptor). The regulator phenotype was defined as the ability to suppress a DTH response to a recall antigen in the presence of donor antigen. Using the trans vivo DTH test, we identified four regulators, and four nonregulators. When we tested two of the regulators for in vitro mixed lymphocyte culture (MLC) and cytotoxic T lymphocyte (CTL) responses to B-lymphoblastoid cell lines (B-LCL), we found both patients to be specifically hyporesponsive to donor compared with third-party B-LCL stimulators. However, in contrast to the linked suppression of DTH seen when a given B-LCL expressed donor-type HLA-B antigens, there was no evidence of linked suppression in vitro, either in CTL, proliferative, or interferon-gamma cytokine release assays. The primary CTL hyporesponsiveness to donor B-LCL could not be reversed by neutralizing antibodies to transforming growth factor beta or interleukin-10, which could restore a strong DTH response to donor B-LCL. We conclude that DTH analysis can readily detect donor antigen-linked suppression in liver transplant recipients. CTL and MLC tests failed to do so. These findings may be relevant to the development of a tolerance assay suitable for use in clinical trials.

Indirect allorecognition in acquired thymic tolerance: induction of donor-specific permanent acceptance of rat islets by adoptive transfer of allopeptide-pulsed host myeloid and thymic dendritic cells

Pancreatic islet transplantation remains a promising approach to the treatment of type 1 diabetes. Unfortunately, graft failure continues to occur because of immunologic rejection, despite the use of potent immunosuppressive agents. It is therefore reasoned that induction of peripheral tolerance by the use of self-dendritic cells (DCs) as a vehicle to deliver specific target antigens to self-T-cells without ex vivo manipulation of the recipient is an attractive strategy in the treatment of type 1 diabetes. The finding that intrathymic inoculation of an immunodominant WF major histocompatibility complex (MHC) Class I (RT1.A(u)) peptide five (P5) or P5-pulsed host myeloid DCs induces acquired thymic tolerance raises the possibility that adoptive transfer of allopeptide-primed host myeloid or lymphoid DCs might induce transplant tolerance. To address this hypothesis, we studied the effects of intravenous transfer of in vitro P5-pulsed syngeneic myeloid DCs or in vivo P5-primed syngeneic lymphoid (thymic) DCs on islet survival in the WF-to-ACI rat combination. In vivo primed thymic DCs isolated from ACI rats given intrathymic inoculation of P5 for 2 days were capable of in vitro restimulation of in vivo P5-primed T-cells (memory cells). In the first series of studies, we showed that intravenous-like intrathymic-inoculation of in vitro P5-pulsed host myeloid DCs induced donor-specific permanent acceptance of islets in recipients transiently immunosuppressed with antilymphocyte serum (ALS). We next examined whether thymic DCs isolated from animals that had been previously intrathymically inoculated with P5 could induce T-cell tolerance. The results showed that intravenous adoptive transfer of in vivo P5-primed thymic DCs led to donor-specific permanent acceptance of islets in recipients transiently immunosuppressed with ALS. This finding suggested that the thymic DCs take up and present P5 to developing T-cells to induce T-cell tolerance, thus providing evidence of a direct link between indirect allorecognition and acquired thymic tolerance. The second series of studies examined the mechanisms involved in this model by exploring whether in vivo generation of peptide-specific alloreactive peripheral T-cells by intravenous inoculation of P5-pulsed self-DCs was responsible for the induction of T-cell tolerance. Intrathymic inoculation of splenic T-cells obtained from syngeneic ACI rats primed with intravenous injection of P5-pulsed DCs with a high in vitro proliferative response to P5 in the context of self-MHC induced donor-specific permanent acceptance of islets from WF donors. In addition, the clinically relevant model of intravenous injection of P5-activated T-cells combined with transient ALS immunosuppression similarly induced transplant tolerance, which was then abrogated by thymectomy of the recipient before intravenous injection of the activated T-cells. These data raise the possibility that circulation of peptide-activated T-cells to the host thymus plays a role in the induction and possibly the maintenance of T-cell tolerance in this model. Our findings suggest that intravenous administration of genetically engineered host DCs expressing alloMHC peptides might have therapeutic potential in clinical islet transplantation for the treatment of autoimmune diabetes.

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

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

Mechanisms of acquired thymic tolerance: induction of transplant tolerance by adoptive transfer of in vivo allomhc peptide activated syngeneic T cells

BACKGROUND

Our most recent observation that i.v. injection of Wistar-Furth (WF) major histocompatibility complex Class I peptide 5 (P5)-pulsed self-myeloid or lymphoid dendritic cells (DC) induces transplantation tolerance suggests that adoptive transfer of in vivo allopeptide-primed host T cells might induce acquired tolerance through their interaction with thymic DC.

METHODS

To examine this hypothesis, host myeloid DC cultured in rat granulocyte/macrophage colony stimulating factor and interleukin 4 were pulsed in vitro with P5 and injected intravenously into syngeneic ACI rats. The T cells primed to P5 via the indirect pathway of allorecognition were harvested 7 days later and administered by either intravenously or intrathymically into syngeneic ACI recipients of WF cardiac allografts.

RESULTS

Syngeneic T cells obtained from the spleen of P5-primed rats had a high mixed lymphocyte reaction proliferative response to P5 presented by self-DC. I.v. administration of 2x107 P5-primed alloreactive purified host splenic T cells alone on day -7 significantly (P<0.001) prolonged cardiac allograft survival from 10.5+/-1.0 days to 18.6+/-1.8 days in the WF-to-ACI rat combination. I.v. injection of P5-activated host T cells combined with 0.5 ml antilymphocyte serum (ALS)-transient immunosuppression on day -7 led to 100% donor-specific permanent graft survival (>200 days). Thymectomy before i.v. injection of P5-activated syngeneic T cells led to acute graft rejection, suggesting that the homing of in vivo activated T cells to the host thymus might play a role in the induction of tolerance. To further define the role of the recipient thymus in this model, we examined the effects of intrathymic (i.t.) injection of P5-primed alloreactive T cells on graft survival and found that i.t. administration of the P5-primed T cells on day -7 alone significantly prolonged graft survival (15.0+/-0.7 days) and when combined with 0.5 ml ALS led to donor-specific permanent graft survival. The long-term unresponsive recipients permanently (>100 days) accepted second-set donor-specific cardiac allografts but not third-party (Lewis) grafts.

CONCLUSIONS

These findings demonstrate that the adoptive transfer of splenic T cells primed to an indirectly presented donor peptide induces transplantation tolerance in a transiently immunosuppressed secondary syngeneic recipient. Our data suggest that the interaction of thymic DC with activated peripheral T cells induces alloantigen (Ag)-specific T-cell tolerance by either inactivation or deletion of alloreactive T cells in the thymus. This observation provides the first formal evidence that the interaction between thymic DC and activated peripheral T cells that continuously circulate through the thymus plays an important role in the induction and maintenance of Ag-specific tolerance.

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.