Direct Pathway Of Allorecognition Research Articles

OR2 Recipient NK cell receptor repertoire comprising activating KIR2DS5 and licensed KIR3DL1 is associated with improved chronic lung allograft dysfunction (CLAD) free survival when donor misses HLA-BW4 ligand

Aim Alloimmune responses manifesting as chronic lung allograft dysfunction (CLAD) limit long-term survival of lung transplant. Natural killer (NK) cells expressing an inhibitory KIR (iKIR) recognizing self-HLA can be activated when confronted with allograft lacking a ligand for that iKIR. Here we investigated if allograft missing ligand for recipient iKIR that had a cognate HLA ligand would enhance CLAD-free survival. We also analyzed the role of recipient’s activating KIRs (aKIR) on CLAD-free survival. Methods Using luminex rSSO methods, KIR and HLA genes were typed for 263 lung transplant recipients and HLA genes were typed for all donors. CLAD was determined by date of FEV1 or FVC drop to KIR-HLA genotypes. Results Bx KIR genotype recipients (i.e., carrying more a KIR s) had higher median CLAD-free survival than those carrying AA genotypes (Fig. A). Recipients with all 5 B haplotype-associated a KIR s ( 2DS1, S2, S3, S5, 3DS1 ) had enhanced CLAD-free survival relative to those missing one or more of these a KIR s. Particularly, recipients carrying 2DS5 showed enhanced CLAD-free survivals than those missing 2DS5 (Fig. B). Further, CLAD-free survival was higher for recipients carrying KIR3DL1 + Bw4 when donors missing Bw4 ligand (Fig. C). Allografts missing relevant ligands for recipients carrying 2DL1 + C2, 2DL2/L3 + C1, and/or 3DL2 + A3/A11 pairs were not associated with differential survival. Recipients with 2DS5, 3DL1, and Bw4 receiving donors missing Bw4 ligand showed the longest CLAD-free survival (p = 0.01, 9.2 median survival years; Fig. D). Conclusions Recipient NK cells expressing aKIRs and/or licensed KIR3DL1 may respond and deplete donor-derived antigen presenting cells that do not express Bw4 ligand, and thus impair direct allorecognition pathways which in turn delay CLAD. Functional studies will be required to understand the mechanism of KIR-HLA interactions in protecting from CLAD.

Nanoparticle delivery of donor antigens for transplant tolerance in allogeneic islet transplantation

Human islet cell transplantation is a promising treatment for type 1 diabetes; however, long-term donor-specific tolerance to islet allografts remains a clinically unmet goal. We have previously shown that recipient infusions of apoptotic donor splenocytes chemically treated with 1-ethyl-3-(3′-dimethylaminopropyl)-carbodiimide (donor ECDI-SP) can mediate long-term acceptance of full major histocompatibility complex (MHC)-mismatched murine islet allografts without the use of immunosuppression. In this report, we investigated the use of poly(lactide-co-glycolide) (PLG) particles in lieu of donor ECDI-SP as a synthetic, cell-free carrier for delivery of donor antigens for the induction of transplant tolerance in full MHC-mismatched murine allogeneic islet transplantation. Infusions of donor antigen-coupled PLG particles (PLG-dAg) mediated tolerance in ∼20% of recipient mice, and the distribution of cellular uptake of PLG-dAg within the spleen was similar to that of donor ECDI-SP. PLG-dAg mediated the contraction of indirectly activated T cells but did not modulate the direct pathway of allorecognition. Combination of PLG-dAg with a short course of low dose immunosuppressant rapamycin at the time of transplant significantly improved the tolerance efficacy to ∼60%. Furthermore, altering the timing of PLG-dAg administration to a schedule that is more feasible for clinical transplantation resulted in equal tolerance efficacy. Thus, the combination therapy of PLG-dAg infusions with peritransplant rapamycin represents a clinically attractive, biomaterials-based and cell-free method for inducing long-term donor-specific tolerance for allogeneic cell transplantation, such as for allogeneic islet transplantation.

Two immunogenic passenger dendritic cell subsets in the rat liver have distinct trafficking patterns and radiosensitivities

The aim of this study was to investigate the trafficking patterns, radiation sensitivities, and functions of conventional dendritic cell (DC) subsets in the rat liver in an allotransplantation setting. We examined DCs in the liver, hepatic lymph, and graft tissues and recipient secondary lymphoid organs after liver transplantation from rats treated or untreated by sublethal irradiation. We identified two distinct immunogenic DC subsets. One was a previously reported population that underwent blood-borne migration to the recipient's secondary lymphoid organs, inducing systemic CD8(+) T-cell responses; these DCs are a radiosensitive class II major histocompatibility complex (MHCII)(+) CD103(+) CD172a(+) CD11b(-) CD86(+) subset. Another was a relatively radioresistant MHCII(+) CD103(+) CD172a(+) CD11b(+) CD86(+) subset that steadily appeared in the hepatic lymph. After transplantation, the second subset migrated to the parathymic lymph nodes (LNs), regional peritoneal cavity nodes, or persisted in the graft. Irradiation completely eliminated the migration and immunogenicity of the first subset, but only partly suppressed the migration of the second subset and the CD8(+) T-cell response in the parathymic LNs. The grafts were acutely rejected, and intragraft CD8(+) T-cell and FoxP3(+) regulatory T-cell responses were unchanged. The radioresistant second subset up-regulated CD25 and had high allostimulating activity in the mixed leukocyte reaction, suggesting that this subset induced CD8(+) T-cell responses in the parathymic LNs and in the graft by the direct allorecognition pathway, leading to the rejection. Conventional rat liver DCs contain at least two distinct immunogenic passenger subsets: a radiosensitive blood-borne migrant and a relatively radioresistant lymph-borne migrant. LNs draining the peritoneal cavity should be recognized as a major site of the intrahost T-cell response by the lymph-borne migrant. This study provides key insights into liver graft rejection and highlights the clinical implications of immunogenic DC subsets.

Inhibition of T-Cell Expansion Caused by Inducible Costimulator/B7h Costimulation Blockade in Direct Allorecognition Pathway

Objective Inducible costimulator (ICOS)/B7h costimulation plays a crucial role in acute and chronic allograft rejection. To test the role of the ICOS signal in T-cell activation and expansion, we used ICOS-Fc–targeted B cells as donor antigen presenting cells to challenge the allogeneic response in vitro. Methods In vitro, the binding of ICOS-Fc with B7h on splenic B cells was confirmed by flow cytometry analysis. To evaluate the capacity of ICOS-Fc–targeted B cells to elicit an allogeneic response in vitro, we performed mixed lymphocyte reactions. Results The binding of B7h on splenic B cells by ICOS-Fc was confirmed at a saturating concentration of 100 μg/mL. Blockade of ICOS/B7h in direct allorecognition depressed proliferation of alloreactive T cells in vitro. Conclusions ICOS/B7h signal plays an important role in direct allorecognition, eliciting allogeneic responses in vitro.

Direct Pathway T-Cell Alloactivation Is More Rapid Than Indirect Pathway Alloactivation

Direct Pathway T-Cell Alloactivation Is More Rapid Than Indirect Pathway Alloactivation

Indirect CD4+ TH1 Response, Antidonor Antibodies and Diffuse C4d Graft Deposits in Long-Term Recipients Conditioned by Donor Antigens Priming

Priming of recipients by DST induces long-term survival of mismatched allografts in adult rats. Despite these recipients developing inducible T regulatory cells able to transfer long-term graft survival to a secondary host, a state of chronic rejection is also observed. We revisited the molecular donor MHC targets of the cellular response in acute rejection and analyzed the cellular and humoral responses in recipients with long-term graft survival following transplantation. We found three immunodominant peptides, all derived from LEW.1W RT1.D(u) molecules to be involved in acute rejection of grafts from unmodified LEW.1A recipients. Although the direct pathway of allorecognition was reduced in DST-treated recipients, the early CD4+ indirect pathway response to dominant peptides was almost unimpaired. We also detected early and sustained antidonor class I and II antibody subtypes with diffuse C4d deposits on graft vessels. Finally, long-term accepted grafts displayed leukocyte infiltration, endarteritis and fibrosis, which evolved toward vascular narrowing at day 100. Altogether, these data suggest that the chronic graft lesions developed in long-term graft recipients are the result of progressive humoral injury associated with a persisting indirect T helper response. These features may represent a useful model for understanding and manipulating chronic active antibody-mediated rejection in human.

An analysis of the adaptive immune response towards an embryonic stem cell grafts

Background: Although clinical transplantation has had enormous impact on the treatment of premature organ failure, shortage of donor organs continues to be a crucial limiting factor. Embryonic stem cells represent an attractive potential source of replacement tissue because of their inherent pluripotentiality and ability to self-renew. However, before any ES cell-based cellular replacement strategies can be considered, many issues must be addressed. Among these is an evaluation of the potential immune response elicited by any ES cell graft. Because ES cells express very low levels of MHC class I and no MHC class II, their immunogenicity has been questioned. Here we utilize a BM3 TCR transgenic model to analyze the adaptive immune response against an ES cell graft in vivo. 
 Methods: BM3 CD8 TCR-tg T cells (H2K background) specific for the MHC class I molecule H2Kb were labelled with CFSE and adoptively transferred into CBA rag recipients. The following day, ES cells derived from a CBA, B6, or CBK background were implanted beneath the kidney capsule of adoptively transferred mice. Response of the CD8 T cells was measured via CSFE division profiling and graft infiltration. 
 Results: CFSE division profile of naïve BM3 CD8 T cells was unaltered by the presence of either a syngeneic or an allogeneic ES cell graft. These naïve cells were also unable to recognize and infiltrate either a syngeneic or allogeneic ES cell graft on days 5 and 10 post-implantation, despite strong expression of the MHC class I molecule H2Kb by engrafted allogeneic ES cells. On the other hand, H2Kb+ islets begun to be infiltrated by day 5, and were obliterated by a vigorous allogeneic response by day 10. When H2Kb+ islets were implanted into the same kidney as allogeneic ES cells (opposite poles), islet grafts were rapidly infiltrated by CD4 and CD8 T cells and destroyed, but ES cell grafts exhibited markedly reduced cellular infiltrate. In contrast to naïve BM3 CD8 T cells, however, activated cells recognized and mounted an aggressive cytotoxic response against an allogeneic ES cell graft which could be detected by day 6 and resulted in complete graft destruction by day 10. 
 Conclusions: Under certain circumstances, an ES cell graft may have reduced immunogenicity as compared with other conventional tissue or solid organ allografts. This may be due to their lack of passenger APC, which may in turn cripple their ability to elicit a robust allogeneic response via the direct pathway of allorecognition. However, because of their strong upregulation of allogeneic MHC class I molecules after transplantation, they are still likely to elicit a significant rejection response when transplanted into recipients replete with both CD4 and CD8 T cells.

Dendritic cells in transplant tolerance.

Dendritic cells (DCs) are a heterogeneous family of professional APCs involved in priming adaptive immune responses. Donor DCs (direct pathway of allorecognition) and recipient DCs presenting processed donor major histocompatibility complex (MHC) as peptides (indirect pathway of allorecognition) participate actively in graft rejection by stimulating recipient T cell responses following organ transplantation. Recent studies have shown that DCs also play a central role in inducing and maintaining tolerance to self antigens (Ags) through deletion, anergy, and regulation mechanisms. It is easy to see how the remarkable functional plasticity of DCs renders them attractive therapeutic targets for immune modulation. Indeed, in the past few years, successful outcomes in rodent models have built the case that DC-based therapy may provide a novel approach to transplant tolerance. Ongoing research into our understanding of the mechanisms whereby DCs promote tolerance in the steady-state, together with development of biologically, pharmacologically and genetically manipulated ex vivo DCs to mimic/enhance their natural tolerogenicity, should warrant the success of these experimental DCs in establishing long-term allograft survival.

The Direct Pathway of Human T‐Cell Allorecognition is not Tolerized by Stimulation with Allogeneic Peripheral Blood Mononuclear Cells Irradiated with High‐Dose Ultraviolet Ba

Transfusions of high-dose (> or =10,000 Joule/m(2)) ultraviolet-B (UVB)-irradiated allogeneic leukocytes in rodent models have been shown to induce immunologic tolerance that is mediated by allospecific regulatory CD4(+) T cells. Whether these regulatory T cells recognize alloantigens through the direct or indirect pathway of allorecognition is controversial. Here, we demonstrate that the proliferative response obtained in standard primary mixed leukocyte reactions (MLRs) with human peripheral blood mononuclear cells (PBMCs) reflected a CD4(+) T-cell-dependent direct pathway of allorecognition and that high-dose UVB irradiation of PBMCs totally inhibited their capacity to induce a proliferative alloresponse. Re-stimulation with gamma-irradiated PBMCs from the same allogeneic donor (secondary MLR) elicited a proliferative and Th1-deviated response that was similar to the response induced in unprimed PBMCs. Finally, high-dose UVB was found to induce a rapid and massive apoptosis of irradiated PBMCs. Collectively, these data indicate that leukocytes irradiated with high-dose UVB are unable to prime for unresponsiveness or immune deviation in T cells directly recognizing allogeneic major histocompatibility complex molecules. Because it is well-established that antigens within transfused apoptotic cells are captured by resident tolerogenic spleen dendritic cells, we propose that tolerance induced by transfusions of high-dose UVB-irradiated leukocytes primarily involve T cells indirectly recognizing alloantigens.

Emergent autoimmunity in graft-versus-host disease

AbstractDonor T-cell recognition of host alloantigens presented by host antigen-presenting cells (APCs) is necessary for the induction of graft-versus-host disease (GVHD), but whether direct alloreactivity is sufficient for the propagation of GVHD is unknown. In this study, we demonstrate that GVHD cannot be effectively propagated through the direct pathway of allorecognition. Rather, donor T-cell recognition of antigens through the indirect pathway is necessary for the perpetuation of GVHD. Furthermore, GVHD results in the breaking of self tolerance, resulting in the emergence of donor T cells that can cause autoimmune disease in syngeneic recipients. Notably, GVHD-induced autoreactivity is donor APC dependent, transferable into secondary hosts, and involves cells of the innate immune system. These results indicate that donor T-cell–mediated pathologic damage during GVHD becomes donor APC dependent and provide a mechanistic explanation for the long-standing observation that GVHD is associated with autoimmune clinical manifestations. (Blood. 2005;105:4885-4891)

Frontline: Peripheral priming of alloreactive T cells by the direct pathway of allorecognition.

Recent studies, though controversial, have suggested that secondary lymphoid organs may not constitute an essential site for the initiation of immune responses to transplant antigens. However, this issue has never been examined in the context of direct and indirect allorecognition. Here, we characterized immune responses arising in draining lymph nodes and skin allografts, in a murine model based on a single T cell clonotype where these two pathways can be independently studied. In this model, graft rejection by the direct or the indirect pathway occurred with similar kinetics, although initiation of the alloreactive responses was clearly different. During indirect responses, expansion and activation of alloreactive T cells were first observed in draining lymph nodes, at day 7 post-transplant, and graft-infiltrating T cells were observed later, at day 11. In striking contrast, directly activated alloreactive T cells were detected at an early stage inside the graft, and only later in the draining lymph nodes, after skin allograft rejection was almost completed. These results suggest that sensitization of naive T cells through the direct pathway could take place outside secondary lymphoid organs.

TOLEROGENIC ENDOTHELIAL CELLS INHIBIT THE DIRECT RECOGNITION PATHWAY

P928 Aims: Allograft rejection is mediated by T cells with direct and indirect allorecognition capacity. The direct pathway involves T cells which interact directly with professional (dendritic cells, macrophages) or nonprofessional (endothelial cells) Antigen Presenting Cells (APC) of the donor. The indirect pathway involves T cells which recognize dominant allopeptides derived from the processing of necrotic or apoptotic cells of the graft by host APC. Both in the periphery and within the graft, the frequency of T cells with direct allorecognition capacity is 10,000 times higher than that of allopeptide reactive T cells at all times. We have explored the hypothesis that tolerogenic endothelial inhibit the direct recognition pathway. Methods: Human endothelial cells (EC) were conditioned by use of IFN-γ and used for T cell priming. Conditioned EC were incubated with allospecific CD4+CD25+CD45RO+ FOXP3+ T regulatory cells (TR) or CD8+CD28- T suppressor cells. The expression on EC of CD31, CD40, CD54, CD62E CD106 CD58, HLA class I, HLA class II, ILT3, and ILT4, prior and following exposure to TS or TR was measured using flow cytometry and RT-PCR. T suppressor cells were tested in proliferation assays for their capacity to inhibit TH reactivity. Results: The ILT3high, ILT4high phenotype is characteristic of tolerogenic DC which induce regulatory T cells inhibiting the direct allorecognition pathway. However, after the departure of donor APC from the graft, rejection can be triggered by activated endothelial cells (EC). We have explored the hypothesis that EC may be involved in the maintenance of quiescence. Our studies have shown that alloantigen specific CD8+CD28- T cells with suppressor activity are FOXP3 positive and interact with human dendritic cells as well as EC. The interaction of TS with EC results in the induction of the inhibitory receptor ILT4, down-regulation of costimulatory and adhesion molecules and tolerization of EC. In turn, tolerized ILT4high EC elicit the differentiation of CD8+CD28- FOXP3+ T suppressor cells. Similarly alloantigen specific TR elicited the transcriptional activation of ILT3 and ILT4 in EC. Alloantigen specific CD8+CD28- FOXP3+ T cells, which trigger the transcriptional activation of ILT4 in EC, were found in the circulation of 21 heart allograft recipients in quiescence. Patients with a history of early acute rejection episodes (N=7) and/or chronic rejection (N=3) showed no TS or TR. The assay which we developed facilitates the identification of patients who may benefit from partial or complete cessation of immunosuppression, a goal of obvious importance given the morbidity and mortality associated with chronic immunosuppression. Conclusions: Endothelial cells are an important target of antigen specific T effector cells not only in transplantation but also in numerous autoimmune diseases. Agents which inhibit EC activation and/or render them tolerogenic are likely to have a beneficial effect in treatment of rejection, autoimmunity and possibly atherosclerosis.

Heart, but Not Skin, Allografts from Donors Lacking Flt3 Ligand Exhibit Markedly Prolonged Survival Time

Fms-like tyrosine kinase 3 ligand (Flt3L) administration leads to dramatic increases in dendritic cells (DC) in lymphoid and nonlymphoid tissues. Conversely, mice lacking Flt3L (Flt3L(-)/(-)) show severe reductions in both myeloid (CD11c(+)CD8alpha(-)) and lymphoid-related DC (CD11c(+)CD8alpha(+)) in the thymus and secondary lymphoid organs. In this study marked reductions in CD11c(+) interstitial cardiac DC and in dermal, but not epidermal, DC (Langerhans cells) were also observed. CD11c(+) cells that migrated from Flt3L(-/-) skin explants expressed lower surface MHC class II and costimulatory molecules and naive T cell allostimulatory activity than migratory wild-type (wt) C57BL/6 (B6) CD11c(+) cells. We examined the survival of Flt3L(-)/(-) heart or tail skin grafts (H2(b)) in allogeneic wt (BALB/c; H2(d)) recipients. The outcome of transplantation of BALB/c organs into Flt3L(-)/(-) recipients was also determined. Flt3L(-)/(-) mice rejected BALB/c heart or skin grafts with similar kinetics as B6 wt recipients. Trafficking of donor DC into host spleens or draining lymph nodes was markedly reduced after transplantation of Flt3L(-)/(-) heart, but not skin grafts, respectively. Compared with wt hearts, survival of Flt3L(-)/(-) hearts was markedly prolonged in BALB/c recipients (median survival time, 37 and 15 days, respectively; p < 0.001). Skin graft survival was unaffected. Rejection of Flt3L(-/-) hearts was precipitated by infusion of wt donor DC at the time of transplant. Thus, severe depletion of interstitial heart DC resulting from targeted gene disruption prolongs, but does not indefinitely extend, heart survival. Acute rejection of wt grafts in Flt3L(-/-) recipients reflects presumably an intact role of the direct pathway of allorecognition.

CD4+CD25+ regulatory T cells do not significantly contribute to direct pathway hyporesponsiveness in stable renal transplant patients.

CD4(+)CD25(+) regulatory T cells have been shown to regulate a variety of autoimmune and allogeneic responses in mice and humans. The role of CD4(+)CD25(+) cells in regulating alloresponses in human transplant recipients remains uncertain. Previous research has demonstrated a reduced frequency of direct pathway donor-specific T cells in renal transplant recipients when compared with the frequency of T cells reactive to an HLA-matched third party. A number of mechanisms have been proposed to account for this finding; the purpose of this study was to determine whether CD4(+)CD25(+) cells play a significant role. Twelve stable renal transplant patients were investigated using limiting dilution assay (LDA) and ELISPOT for interferon-gamma to determine the effect of depleting CD4(+)CD25(+) cells on the direct pathway alloresponse. The percentage of CD4(+)CD25(+) cells in the peripheral blood of the study patients was equivalent to that of healthy controls. Furthermore, in no case did depletion of CD4(+)CD25(+) cells significantly increase the frequency of donor-specific T cells detected by LDA. This was also found with ELISPOT in all except one patient, in whom depletion revealed an increased frequency of alloreactive T cell to both donor and third party. Finally, kinetic analysis of the LDA data did not indicate regulation against donor when compared with third party. It is concluded that the action of CD4(+)CD25(+) regulatory cells is not the main mechanism of donor-specific hyporesponsiveness in the direct pathway of allorecognition.

Differential effect of CD28 versus B7 blockade on direct pathway of allorecognition and self-restricted responses

AbstractImmunosuppression with B7 antagonists might have 2 opposite effects: reducing T-cell costimulation through CD28 but also preventing CTLA-4 from transmitting its negative regulatory signal. We therefore hypothesized that a selective blockade of CD28 might be qualitatively different from blocking B7. It was previously reported that CD28 modulation prolongs allograft survival in the rat and reverses induction of experimental autoimmune encephalomyelitis in mice. However, whether CD28 or B7 blockade results in similar immunosuppression on alloimmune and self-restricted responses to soluble antigens has not yet been investigated. Here, we addressed this issue in vitro with antagonist anti-CD28 Fab fragments and in vivo using the modulating anti-rat JJ319 monoclonal antibody. As in the inhibition of B7 with CTLA4 immunoglobulin, anti-CD28 Fab fragments inhibited allogenic T-cell proliferation in mixed cultures. In vivo modulation of CD28 blocked the expansion of alloreactive T cells and promoted their apoptosis. In contrast, selective blockade of CD28 did not modify T-cell proliferative responses and antibody production to soluble antigens, whereas blocking B7 with CTLA4 immunoglobulin did. Our data show that blocking CD28, while leaving CTLA4-B7 interactions undisturbed, inhibits alloreactive CD4+ T-cell expansion but does not modify the response to nominal antigens presented in the context of a self-major histocompatibility complex. That B7 engagement is needed for self-restricted responses whereas engagement of CD28 is not essential adds to the suggestion that another unidentified ligand of B7 might deliver a costimulatory signal in the absence of CD28.

Intragraft mrna expression of the novel cytokine IL-21 during acute rejection after clinical heart transplantation

phage (CD11b ) infiltration and significantly elevated intragraft mRNA expression of iNOS. During the development of transplant arteriosclerosis (TxA) PECAM-1-/donor endothelial cells were replaced by recipient PECAM-1 / endothelial cells, a process that occurred only in the allogeneic situation. Endothelial replacement commenced 14 days after transplantation and was complete by day 30. Conclusion: PECAM-1 expression by donor endothelial cells may regulate macrophage infiltration into the graft thereby controlling the development of TxA. Furthermore, these data suggest that the alloantigen specificity of the effector cells that mediate endothelial cell destruction may change as the rejection process progresses. Initially cells responding to donor antigens via the direct pathway of allorecognition will play a key role, but later in the response, indirect pathway T cells will become increasingly important.

Transplant Immunobiology – Mechanisims of Graft Rejection

Graft rejection is a complex process involving the interplay between innate and adaptive immunity. Many factors including the nature of the tissue transplanted, donor factors, the ischaemia time, the genetic disparities between the donor and recipient, the site of transplantation, and the immune status of the recipient as well as others all contribute to determining the character of the rejection response. Donor derived passenger leukocytes, immature dendritic cells, are present within solid organ grafts at the time of transplantation. These cells are triggered to migrate out of the graft as a result of the inflammation caused by removing the organ from the donor and transplanting it into the recipient. When the donor passenger cells migrate from the graft to the recipient lymphoid tissue they change their functional properties, including the acquisition of costimulatory molecules, and become potent antigen presenting cells (APC). As a result T cells recognizing the mismatched donor major histocompatibility antigens will be activated thereby triggering acute rejection via the direct pathway of allorecognition. Donor histocompatibility antigens can also be recognized by T cells after processing and presentation by recipient antigen presenting cells. This pathway of allorecognition, the indirect pathway, can also be responsible for acute rejection but has also been shown to play a key role in the development of transplant arteriosclerosis that results in delayed graft loss, often referred to as chronic allograft rejection. In general, activation of CD4+ T cells leads to the orchestration of the effector mechanisms that lead to graft destruction. CD8+ T cells with cytotoxic activity, macrophages, antibody and complement and cytokines have all been implicated in the graft destruction. The precise combination of effector cells and their relative contributions to the destruction of a particular graft will depend on the circumstances that have lead to the initiation of the response.

The influence of histocompatibility on graft rejection and graft survival within a single center population of heart transplant recipients.

We report a consecutive single center series of 261 patients who received first orthotopic heart transplants from 1986 to 1997. The 1- and 5-year graft survivals were 78 and 68%. The influence of histocompatibility was investigated by comparing graft survival and numbers of treated rejection episodes with HLA-A, -B, and -DR mismatches over different time periods. Recipients with six mismatches for HLA-A+-B+-DR combined (13.4%) had reduced survival at 7 years (47%) when compared with other recipients (64%). In the first year of transplant, recipients with four HLA-A+-B mismatches had significantly reduced actuarial graft survival (P=0.03) with the greatest influence apparent at 6 months [0-3 mismatches (n=193) 85% versus 4 mismatches (n=68) 69%; P=0.005, OR=2.1]. For 182 recipients with functioning hearts at 1 year, the number of rejection episodes treated within this time was strongly influenced by HLA-DR mismatch [0 DR mismatch (n=15) mean 1.2 rejection episodes versus 1 DR mismatch (n=76) mean 2.7 rejection episodes versus 2 DR mismatches (n=91) mean 3.8 rejection episodes: P=0.0002]. Of these 182 transplants, recipients who had more than four treated rejection episodes during the first year had a significantly reduced 7- year survival [<5 rejection episodes (n=133) 85% versus more than four rejection episodes (n=49) 66%; P=0.02, OR=3.4], as did those with two HLA-DR mismatches [0+1 mismatch (n=91) 87% versus 2 mismatches (n=91) 70%; P<0.05, OR=2.4]. We show that graft loss in the first 6 months of transplant is significantly influenced by four HLA-A+-B mismatches. HLA-DR mismatch significantly increases the number of rejection episodes within the first year, without influencing graft survival. After 12 months both >4 rejection episodes in the first year and two HLA-DR mismatches are markers for late graft loss. We postulate that immunological graft loss in the first 6 months is dominated by the direct allorecognition pathway driven by HLA-DR mismatch. This mechanism is later lost or suppressed. Our data highlight HLA-DR mismatch as a marker for late graft loss and we show an advantage to avoiding transplanting hearts with six HLA-A+-B+-DR mismatches and to minimizing HLA-DR mismatches whenever possible.

Chronic rejection of mouse kidney allografts

Chronic renal allograft rejection is the leading cause of late graft failure. However, its pathogenesis has not been defined. To explore the pathogenesis of chronic rejection, we studied a mouse model of kidney transplantation and examined the effects of altering the expression of donor major histocompatibility complex (MHC) antigens on the development of chronic rejection. We found that long-surviving mouse kidney allografts develop pathological abnormalities that resemble chronic rejection in humans. Furthermore, the absence of MHC class I or class II antigens did not prevent the loss of graft function nor alter the pathological characteristics of chronic rejection. Expression of transforming growth factor-beta (TGF-beta), a pleiotropic cytokine suggested to play a role in chronic rejection, was markedly enhanced in control allografts compared with isografts. However, TGF-beta up-regulation was significantly blunted in MHC-deficient grafts. Nonetheless, these differences in TGF-beta expression did not affect the character of chronic rejection, including intrarenal accumulation of collagens. Reduced expression of either class I or II direct allorecognition pathways is insufficient to prevent the development of chronic rejection, despite a reduction in the levels of TGF-beta expressed in the allograft. This suggests that the severity of chronic rejection is independent of the level of MHC disparity between donor and recipient and the level of TGF-beta expression within the allograft.

T cells reactive to a single immunodominant self-restricted allopeptide induce skin graft rejection in mice.

Alloreactive T lymphocytes can respond to foreign MHC complexed with foreign peptides through the direct pathway of allorecognition and can additionally recognize allopeptides expressed in the context of recipient (self) MHC through the indirect pathway. To better elucidate how indirect pathway-responsive CD4(+) T cells mediate allograft rejection, we isolated and characterized a TH1 T cell line from BALB/c recipients of B10.A skin that responds to a defined immunodominant, self-restricted allopeptide, I-Abetak58-71. When transferred into BALB/c severe combined immunodeficiency recipients of B10.A skin allografts, this cell line specifically induced a form of skin graft rejection characterized by the presence of TH1 cytokines, macrophage infiltration, and extensive fibrosis. Recall immune responses and immunofluorescence of the rejecting skin revealed only the presence of the peptide-specific T cells within the recipient animals, with no evidence of a direct pathway alloresponse. These studies demonstrate that T cells reactive to a single self-restricted allopeptide can mediate a form of allogeneic skin graft rejection that exhibits characteristics of a chronic, fibrosing process.