Islet rejection in perforin-deficient mice: the role of perforin and Fas

Recognizing Complexity of CD8 T Cells in Transplantation

The major role of CD8+ T cells in clinical and experimental transplantation is well documented and acknowledged. Nevertheless, the precise impact of CD8+ T cells on graft tissue injury is not completely understood, thus impeding the development of specific treatment strategies. The goal of this overview is to consider the biology and functions of CD8+ T cells in the context of experimental and clinical allotransplantation, with special emphasis on how this cell subset is affected by currently available and emerging therapies.

Cytotoxic effector function of CD4-independent, CD8(+) T cells is mediated by TNF-α/TNFR

BACKGROUND

Liver parenchymal cell allografts initiate both CD4-dependent and CD4-independent, CD8(+) T cell-mediated acute rejection pathways. The magnitude of allospecific CD8(+) T cell in vivo cytotoxic effector function is maximal when primed in the presence of CD4(+) T cells. The current studies were conducted to determine if and how CD4(+) T cells might influence cytotoxic effector mechanisms.

METHODS

Mice were transplanted with allogeneic hepatocytes. In vivo cytotoxicity assays and various gene-deficient recipient mice and target cells were used to determine the development of Fas-, TNF-α-, and perforin-dependent cytotoxic effector mechanisms after transplantation.

RESULTS

CD8(+) T cells maturing in CD4-sufficient hepatocyte recipients develop multiple (Fas-, TNF-α-, and perforin-mediated) cytotoxic mechanisms. However, CD8(+) T cells, maturing in the absence of CD4(+) T cells, mediate cytotoxicity and transplant rejection that is exclusively TNF-α/TNFR-dependent. To determine the kinetics of CD4-mediated help, CD4(+) T cells were adoptively transferred into CD4-deficient mice at various times posttransplant. The maximal influence of CD4(+) T cells on the magnitude of CD8-mediated in vivo allocytotoxicityf occurs within 48 hours.

CONCLUSION

The implication of these studies is that interference of CD4(+) T cell function by disease or immunotherapy will have downstream consequences on both the magnitude of allocytotoxicity as well as the cytotoxic effector mechanisms used by allospecific CD8(+) cytolytic T cells.

Donor Fas is not necessary for T-cell-mediated rejection of mouse kidney allografts

It is important to resolve whether T-cell-mediated rejection (TCMR) is mediated by contact-dependent cytotoxicity or by contact-independent inflammatory mechanisms. We recently showed that the cytotoxic molecules perforin and granzymes A and B are not required for TCMR of mouse kidney transplants. Nevertheless, TCMR could still be mediated by cytotoxicity via Fas on donor cells engaging Fas ligand on host T cells. We examined whether the diagnostic TCMR lesions would be abrogated if donor Fas was absent, particularly in hosts deficient in perforin or granzymes A and B. Kidneys from Fas-deficient donors transplanted into major histocompatibility complex (MHC)- mismatched hosts developed tubulitis and diffuse interstitial infiltration indistinguishable from wild-type (WT) allografts, even in hosts deficient in perforin and granzymes A and B. Gene expression analysis revealed similar molecular disturbances in Fas-deficient and WT allografts at day 21 transplanted into WT, perforin and granzyme A/B-deficient hosts, indicating epithelial injury and dedifferentiation. Thus, donor Fas is not necessary for TCMR diagnostic lesions or molecular changes, even in the absence of perforin-granzyme mechanisms. We propose that in TCMR, interstitial effector T cells mediate parenchymal injury by inflammatory mechanisms that require neither the perforin-granzyme nor the Fas-Fas ligand cytotoxic mechanisms.

A blocking anti-CD28-specific antibody induces long-term heart allograft survival by suppression of the PKC theta-JNK signal pathway

This study investigated the effects of a blocking anti-CD28 antibody (Anti-CD28-PV1-IgG3) in vitro and in vivo. Anti-CD28-PV1-IgG3, a hamster-mouse chimeric antibody against murine CD28, which does not provide CD28-positive signaling during TCR-driven T cell activation, enabled long-term survival of heart allografts across a complete mismatch of the MHC in rats. Among the T cell signaling proteins tested in the spleens from recipients, we found that recipients treated with anti-CD28-PV1-IgG3 exhibited suppression of alloantigen-initiated proximal TCR signaling events, including Lck, Zap70, Vav, and PI3K expression, and their PKC theta- and JNK-regulated expression/activation. This leads to attenuation of intragraft T cell infiltration and expression of T cell effector molecules. These results indicate that targeting the CD28 receptor with a blocking antibody leads to long-term allograft survival by reducing activation of alloantigen-mediated key signaling events in T cells that might be crucial for full T cell activation.

Islet allograft rejection by contact-dependent CD8+ T cells: perforin and FasL play alternate but obligatory roles

Though CD8(+) T lymphocytes are important cellular mediators of islet allograft rejection, their molecular mechanism of rejection remains unidentified. Surprisingly, while it is generally assumed that CD8(+) T cells require classic cytotoxic mechanisms to kill grafts in vivo, neither perforin nor FasL (CD95L) are required for acute islet allograft rejection. Thus, it is unclear whether such contact-dependent cytotoxic pathways play an essential role in islet rejection. Moreover, both perforin and CD95L have been implicated in playing roles in peripheral tolerance, further obscuring the role of these effector pathways in rejection. Therefore, we determined whether perforin and/or FasL (CD95L) were required by donor MHC-restricted ('direct') CD8(+) T cells to reject islet allografts in vivo. Islet allograft rejection by primed, alloreactive CD8(+) T cells was examined independently of other lymphocyte subpopulations via adoptive transfer studies. Individual disruption of T-cell-derived perforin or allograft Fas expression had limited impact on graft rejection. However, simultaneous disruption of both pathways prevented allograft rejection in most recipients despite the chronic persistence of transferred T cells at the graft site. Thus, while there are clearly multiple cellular pathways of allograft rejection, perforin and FasL comprise alternate and necessary routes of acute CD8(+) T-cell-mediated islet allograft rejection.

Deletion of DOCK2, a regulator of the actin cytoskeleton in lymphocytes, suppresses cardiac allograft rejection

Allograft rejection is induced by graft tissue infiltration of alloreactive T cells that are activated mainly in secondary lymphoid organs of the host. DOCK2 plays a critical role in lymphocyte homing and immunological synapse formation by regulating the actin cytoskeleton, yet its role in the in vivo immune response remains unknown. We show here that DOCK2 deficiency enables long-term survival of cardiac allografts across a complete mismatch of the major histocompatibility complex molecules. In DOCK2-deficient mice, alloreactivity and allocytotoxicity were suppressed significantly even after in vivo priming with alloantigens, which resulted in reduced intragraft expression of effector molecules, such as interferon-γ, granzyme B, and perforin. This is mediated, at least in part, by preventing potentially alloreactive T cells from recruiting into secondary lymphoid organs. In addition, we found that DOCK2 is critical for CD28-mediated Rac activation and is required for the full activation of alloreactive T cells. Although DOCK2-deficient, alloreactive T cells were activated in vitro in the presence of exogenous interleukin-2, these T cells, when transferred adoptively, failed to infiltrate into the allografts that were transplanted into RAG1-deficient mice. Thus, DOCK2 deficiency attenuates allograft rejection by simultaneously suppressing multiple and key processes. We propose that DOCK2 could be a novel molecular target for controlling transplant rejection.

Apoptosis-inducing factor of a cytotoxic T cell line: involvement of a secretory phospholipase A2

Natural killer (NK) cells and cytotoxic T lymphocytes (CTLs) kill target cells by the granule-exocytosis pathway and by the engagement of molecules belonging to the tumor necrosis factor family. The involvement of secretory phospholipase A(2) (sPLA(2)) in the cytotoxic process has been proposed in NK cells. However, its molecular identity and intracellular localization remain unknown, and its mechanism of action is poorly understood. Here, we have readdressed this issue by studying the cytotoxic activity of whole cell extracts of a CTL line. We observed that inactivation of the perforin-granzyme pathway at 37 degrees C in the presence of 1 mM Ca(2+) enhanced the ability of CTL extracts to induce apoptosis. This potentiation of cell death was Ca(2+)-dependent, thermo-resistant, and inhibited by 4-bromophenacyl bromide and scalaradial (two inhibitors of sPLA(2)). The involvement of an sPLA(2) was confirmed by blocking the pro-apoptotic activity of the Ca(2+)-treated cell extract with an anti-sPLA(2) polyclonal antibody. By cell fractionation assays, we showed that the pro-apoptotic sPLA(2) was localized in the cytoplasmic fraction but not in perforin-rich granules or plasma membrane fractions. Western blotting analysis revealed the presence of four distinct bands of 56, 29.5, 21, and 15 kDa. The highest molecular weight band was consistent with the expression of a group III sPLA2. Taken together, these data indicate that an apoptosis-inducing sPLA(2) is expressed in the cytosol of a CTL cell line and suggest that it plays an effector role in CTL-mediated cytotoxicity.

A critical role for granzyme B, in addition to perforin and TNFalpha, in alloreactive CTL-induced mouse pancreatic beta cell death

BACKGROUND

The precise effector mechanisms and molecular mediators used by alloreactive cytotoxic T lymphocytes to kill transplanted pancreatic beta cells are poorly defined. We have used mouse (H2b-anti-d) CTLs raised in strains deficient in various key cytotoxic effector molecules to assess the importance of the various signaling pathways mobilized to kill primary mouse pancreatic islet cells, the beta cell line NIT-1, and NIT-1 cells overexpressing dominant-negative FADD and Bcl-2.

METHODS

Death of target cells was assessed using 51Cr release assays.

RESULTS

In short-term assays (<5 hours) beta cell death did not require a functional FasL/Fas pathway, and was not inhibited by Bcl-2. However, the absence of either perforin or granzyme B resulted in cell survival. By contrast, a crucial role for granzyme B was not seen when hematopoietic P815 cells were used as targets, indicating differential regulation of apoptosis. Interestingly, coincubation with CTL for 24 hours revealed an additional but less potent "late phase" of beta cell death that did not require perforin. This delayed death was blocked by dominant-negative FADD, but not by Bcl-2, and was likely to be due to TNFalpha secretion.

CONCLUSIONS

This study suggests that strategies to protect beta cells from allogeneic CTL attack will need to inhibit the perforin/granzyme and probably also the TNFalpha pathway. As there are no known pharmacological approaches to blocking perforin, therapeutic approaches based on overexpressing both dominant negative FADD and an inhibitor of granzyme B may hold promise in prolonging beta cell survival in the allogeneic setting.

The X-linked inhibitor of apoptosis protein enhances survival of murine islet allografts

Allotransplantation of pancreatic islets represents a promising approach to treat type 1 diabetes. Destruction of beta-cells in islet allografts involves multiple immune mechanisms that lead to activation of caspases and apoptotic cell death. The X-linked inhibitor of apoptosis (XIAP) inhibits apoptosis induced by a variety of triggers, primarily by preventing the activation of caspases. To determine whether XIAP would protect beta-cells from apoptosis, we used a recombinant adenovirus to overexpress XIAP in transformed murine beta-cells and in freshly isolated islets. In vitro cytokine-induced beta-cell death was decreased to baseline levels in XIAP-transduced MIN-6 and NIT-1 cell lines compared with controls. To evaluate the potential of XIAP overexpression to prevent in vivo allogeneic graft rejection, we transduced Balb/c islets ex vivo with XIAP before transplantation into CBA mice with streptozotocin-induced diabetes. We observed that almost all mice receiving allografts of XIAP-expressing islets maintained normoglycemia until the experiment was terminated (45-72 days posttransplant), whereas control mice receiving islets transduced with adenovirus expressing LacZ were hyperglycemic by approximately 17 days posttransplantation due to graft rejection. Immunohistochemistry revealed preservation of beta-cells and clearance of infiltrating immune cells in the XIAP-expressing islet grafts. The in vitro allogeneic response of splenocytes isolated from recipients of XIAP-expressing grafts 8 weeks posttransplant was similar to that seen in nonprimed allogeneic mice, suggesting that XIAP overexpression may lead to the acceptance of islet allografts in diabetic recipients. Long-term protection of islet allografts by XIAP overexpression may enhance the survival of islet transplants in diabetes.

Differences in Suppressor of Cytokine Signaling-1 (SOCS-1) Expressing Islet Allograft Destruction in Normal BALB/c and Spontaneously-Diabetic NOD Recipient Mice

BACKGROUND

The ability to block interferon signaling represents an important strategy in designing therapies to prevent beta-cell destruction during islet allograft rejection.

METHODS

The SOCS proteins regulate cytokine signaling by blocking activation of JAK/STAT proteins. Using islets isolated from SOCS-1 transgenic mice (SOCS-1-Tg; these mice express SOCS-1 under the control of the human insulin promoter and are on the C57BL6/J background), we investigated whether SOCS proteins can prevent the destruction pancreatic islet cells transplanted beneath the kidney capsule of major histocompatibility complex mismatched normal BALB/c and spontaneously-diabetic NOD mouse recipients.

RESULTS

Immunohistochemical staining for insulin confirmed the presence of donor SOCS-1-Tg islets in islet allografts harvested at 22 days posttransplant, whereas grafts of control non-Tg islets were destroyed by 14 days. In contrast, SOCS-1-Tg allogeneic islets were not protected from beta-cell destruction in clinically diabetic NOD mice. The islet allografts functioned for 1 week posttransplant; however, hyperglycemia returned after 2 weeks and the grafts were destroyed. Rejection of SOCS-1-Tg and non-Tg islets in autoimmune diabetic NOD mice was associated with an infiltrate of both CD4+ and CD8+ T cells and a T2-type cytokine response (IL-4) rather than the conventional T1-type cytokine response observed during islet allograft rejection. Self-antigen upregulation in response to IFN-gamma stimulation did not appear to be a factor in rejection of the islet allografts.

CONCLUSIONS

These results demonstrate that expression of SOCS-1 in islets delays islet allograft rejection but cannot circumvent destruction of the islets by the recurrence of the tissue-specific autoimmune process of spontaneous diabetes.

Immunologic Pathways in a Quantitative Model of Immunosuppression Based on Rejection of an Allogeneic or Xenogeneic Tumor Graft

BACKGROUND

A quantitative model of immunosuppression was previously developed based on the rejection of the allogeneic A/J murine tumor sarcoma 1 (Sa1) in immunocompetent mice. Here, the model is used to evaluate the immunologic mechanisms of graft rejection and to determine the potential of this model to detect synergistic effects of combined immunosuppressive therapies.

METHODS

Wild-type, genetically-deficient, or drug-treated mice were used. Mice were challenged subcutaneously with the allogeneic murine tumor cell line, Sa1, or with the xenogenic human tumor, MDA435. Tumor growth was monitored with time, with increasing tumor size reflecting greater immunosuppression. In some cases, the mice were presensitized with either Sa1 or with A/J splenocytes.

RESULTS

In naïve recipient mice, studies in major histocompatibility complex (MHC)-I-deficient mice and with depleting anti-CD8 monoclonal antibody (mAb) demonstrate that CD8 T cells are important for Sa1 rejection. A modest role for perforin but not for Fas/Fas ligand could be demonstrated. Blockade of CD4 T cells was more effective with decreasing histocompatibility barriers. In contrast, CD4 T cells were critical in second-set rejections, but CD8 T cells were not. Rejection of xenogeneic tumors was also T-cell dependent as demonstrated by anti-CD3 mAb, dependent on both CD4 and CD8 T cells as demonstrated using MHC-I- and MHC-II-deficient mice, but was more vigorous as demonstrated by the lack of effectiveness of immunosuppressive drugs in this model.

CONCLUSIONS

This model can be used to define dominant and partial effects of immunologic pathways as well as synergistic interactions of agents to develop immunosuppressive strategies.

Unique gene expression profiles of heart allograft rejection in the interferon regulatory factor-1-deficient mouse

Interferon regulatory factor-1 (IRF1) is a transcription factor for many genes involved in innate and adaptive immune responses. By using DNA array technology, we have previously demonstrated that IRF1 is significantly upregulated during acute rejection in rat heart allografts and is restored to isograft levels when recipients are treated with the immunosuppressants tacrolimus or cyclosporin A (CsA). To understand the precise role of IRF1 in transplant rejection, we investigated the rejection responses of mice completely deficient of IRF1 protein. Heterotopic heart transplantations were performed using C57BL/6J wild-type (WT B6) and IRF1-deficient (IRF1-/-) mice as recipients, and C3H mice as donors. Graft survival was determined by abdominal palpation and rejection was confirmed by histology. On day 6 after transplantation, isografts and allografts were harvested and subjected to gene expression analysis by a commercial nylon array and by real-time RT-PCR. Median survival time of heart allografts was 8 days in the WT B6 mice and 10 days in the IRF1-/- mice. The gene expression profiles of allografts from the WT B6 and IRF1-/- recipients were nearly identical to each other and very different from the profile of the isograft control. Both WT B6 and IRF1-/- profiles showed 13 genes upregulated (IFN-gamma, MCP-2, MIP-1alpha, MIP-1beta, CCR5, MIG, IP-10 and others) and one gene downregulated (SDF2) among the 76 genes detectable on the array. In more detailed analyses, distinct cytokine and chemokine gene expression profiles were identified in the allografts from the WT B6 and IRF1-/- recipients. Whereas IL-4, IL-6, IL-13, MCP-1, MCP-3, and MPIF-2 were upregulated, RANTES, IL-2Rgamma and gp130 were downregulated in allografts from the IRF1-/- recipients when compared to the WT B6 control. Although the inactivation of the IRF1 gene did not sufficiently prevent acute allograft rejection in this model, a unique cytokine and chemokine gene expression profile was found in the absence of IRF1.

Lesions of T-cell-mediated kidney allograft rejection in mice do not require perforin or granzymes A and B

Organ allograft rejection is strongly associated with the presence of alloreactive cytotoxic T cells but the role of cytotoxicity in the pathologic lesions is unclear. Previous studies showed that the principal lesions of kidney rejection - interstitial infiltration, tubulitis, and endothelial arteritis - are T-cell-dependent and antibody-independent. We studied the role of cytotoxic granule components perforin and granzymes A and B in the evolution of the T-cell-mediated lesions of mouse kidney transplant rejection. By real-time RT-PCR, allografts rejecting in wild-type hosts at days 5, 7, 21, and 42 showed massively elevated and persistent expression of perforin and granzymes A and B, but evolution of tubulitis and arteritis did not correlate with increasing granzyme or perforin expression. Allografts transplanted into hosts with disrupted genes for perforin or granzymes A and B showed no change in tubulitis, arteritis, or MHC induction. Thus the development of the histologic lesions diagnostic of T-cell-mediated kidney transplant rejection are associated with but not mediated by perforin or granzyme A or B. Together with previous graft survival studies, these results indicate that the granule-associated cytotoxic mechanisms of T cells are not the effectors of T-cell-mediated allograft rejection.

Cutting edge: perforin down-regulates CD4 and CD8 T cell-mediated immune responses to a transplanted organ

Perforin mediates target cell apoptosis by CTLs and NK cells. Although perforin expression correlates strongly with acute allograft rejection, perforin-deficient mice reject allografts with the same kinetics as wild-type recipients. In this study, we tested the hypothesis that while perforin is dispensable for acute rejection, it is essential for down-regulating the alloimmune response by inducing the apoptosis of host immune cells. Using a skin transplantation model, we found that perforin-deficient mice are resistant to the induction of allograft acceptance by agents that block T cell costimulation. Failure to induce allograft acceptance in these mice was observed irrespective of whether the alloimmune response was CD4 or CD8 T cell-mediated and could be attributed to defective apoptosis of activated CD4 and CD8 T cells. In contrast, perforin did not influence T cell proliferation. Therefore, perforin is an essential immunoregulatory molecule that may be required for the induction of transplantation tolerance.

CD103 expression is required for destruction of pancreatic islet allografts by CD8(+) T cells

The mechanisms by which CD8 effector populations interact with epithelial layers is a poorly defined aspect of adaptive immunity. Recognition that CD8 effectors have the capacity to express CD103, an integrin directed to the epithelial cell-specific ligand E-cadherin, potentially provides insight into such interactions. To assess the role of CD103 in promoting CD8-mediated destruction of epithelial layers, we herein examined the capacity of mice with targeted disruption of CD103 to reject pancreatic islet allografts. Wild-type hosts uniformly rejected islet allografts, concomitant with the appearance of CD8(+)CD103(+) effectors at the graft site. In contrast, the majority of islet allografts transplanted into CD103(-/-) hosts survived indefinitely. Transfer of wild-type CD8 cells into CD103(-/-) hosts elicited prompt rejection of long-surviving islet allografts, whereas CD103(-/-) CD8 cells were completely ineffectual, demonstrating that the defect resides at the level of the CD8 cell. CD8 cells in CD103(-/-) hosts exhibited normal effector responses to donor alloantigens in vitro and trafficked normally to the graft site, but strikingly failed to infiltrate the islet allograft itself. These data establish a causal relationship between CD8(+)CD103(+) effectors and destruction of graft epithelial elements and suggest that CD103 critically functions to promote intragraft migration of CD8 effectors into epithelial compartments.

Mechanism of T cell-mediated endothelial apoptosis

BACKGROUND

Cytotoxic T lymphocyte (CTL)-mediated destruction of allogeneic vascular endothelium is important in the pathogenesis of both acute and chronic allograft rejection. Despite the importance of this phenomenon, the effector mechanisms responsible for endothelial cell killing are not well defined, and conflicting conclusions have been reached based on variation in experimental methodology.

METHODS

We used a recently described method for isolating mouse vascular endothelium to evaluate endothelial cell lysis by CTLs. Endothelial cell destruction was assessed in vitro both by 51Cr release and DNA fragmentation using wild-type and lpr (Fas deficient) endothelium of C3H/HeJ (H2(k)) mice by MHC alloantigen-specific T cells of wild-type, gld (Fas ligand deficient), and perforin-deficient mice on a C57BL/6 (H2(b)) background.

RESULTS

Although maximal lysis of 56.6+/-0.8% was seen when using wild-type targets and effectors, only a moderate decrease in apoptosis to 37.6+/-4.0% was detected when the Fas/Fas ligand death receptor pathway was eliminated. This decrease in cytotoxicity occurred despite the preserved functional capacity of this pathway. Alternatively, a significant decrease in cytotoxicity to 17.4+/-4.7% was seen when the perforin/granzyme exocytosis pathway was eliminated.

CONCLUSIONS

These data indicate that CTLs destroy vascular endothelium primarily by the perforin/granzyme exocytosis pathway with only a minor contribution to apoptosis by the Fas/Fas ligand death receptor pathway. These data are critical for the proper interpretation of studies evaluating acute and chronic allograft rejection and for the design of rational strategies to ameliorate vascular injury concomitant to the rejection process.

FasL is important in costimulation blockade-resistant skin graft rejection

BACKGROUND

Simultaneous blockade of the CD40 and CD28 costimulatory pathways is effective in prolonging allograft survival in murine and primate models. Recent data suggest that intact apoptotic pathways are crucial for the induction of hyporesponsiveness by costimulation blockade. We have studied the impact of fas/fasL signaling, an important T cell apoptotic pathway, on the effects of costimulation blockade. Methods. Wild type, lpr (fas deficient), and gld (fasL deficient), mice were used as donors and recipients in the murine skin graft model. Allograft survival was compared in untreated and costimulation blockade (500 microg anti-CD40L and 500 microg CTLA4-Ig, days 0, 2, 4, 6) treated recipients. In some recipients, CD4+ T cells were depleted using rat anti-murine CD4 (100 microg day -3, -2, -1, and weekly).

RESULTS

gld mice treated with costimulation blockade enjoy a significantly greater increase in skin allograft survival than do wild-type mice. This effect is not replicated using lpr donors or recipients. Experiments in which CD4+ cells were depleted demonstrate that fasL is not necessary for CD8-mediated allograft rejection, and that depletion of CD4+ cells eliminates some of the survival advantage induced by costimulation blockade.

CONCLUSIONS

FasL is not required for the establishment of costimulation blockade induced hyporesponsiveness, but rather appears to be required for normal costimulation blockade resistant rejection. Fas expression is not critical for costimulation blockade resistant rejection, suggesting that fasL may be interacting with other receptors. Further, it appears that CD4+ cells are important in the maintenance of allograft protection induced by costimulation blockade in this model.

Granulysin expression is a marker for acute rejection and steroid resistance in human renal transplantation

Differentiating etiologies of transplant dysfunction without biopsy and optimizing therapy for acute rejection by predicting steroid resistance will reduce patient morbidity. Granulysin is a cytolytic molecule released by CTL and NK cells and coexpressed with effectors of acute allograft rejection, like perforin and granzymes. Granulysin mRNA and protein expression were studied in peripheral blood lymphocytes (PBL; n = 61 total, n = 10 with intercurrent infections) and biopsy tissue from adult and children renal transplant recipients (n = 97) by competitive quantitative-reverse transcriptase-PCR (QC-RT-PCR) and immunohistochemistry. Differences in cell phenotypes were studied in steroid sensitive and resistant acute rejection biopsies. Granulysin was studied in phytohemagglutinin (PHA) stimulated cell lines (donor PBL and CD45RO(+) T cells) by FACS, Western blotting, and RT-PCR after pretreating with cyclosporine A (CSA), azathioprine, mycophenolic acid, and steroids. Granulysin mRNA was significantly increased in patient PBL and transplant biopsies during acute rejection (p < 0.0001) and infection (p < 0.001). Rejecting biopsies alone (n = 53) had mononuclear cell granulysin staining. Steroid resistant biopsies (n = 25) had denser granulysin staining (>2 cells/high power field) and CD45RO(+) lymphocytes, when compared with steroid sensitive (n = 28) rejecting tissue. Granulysin levels were unchanged after azathioprine and mycophenolic acid treatment, decreased after treating activated PBL with steroids and cyclosporine A (CSA), and paradoxically, increased (p < 0.05) after treating CD45RO(+) CTL with CSA. Elevated PBL granulysin is a peripheral marker for acute rejection and infection and dense granulysin staining a tissue marker for steroid resistance. Memory CTL abound in steroid resistant grafts and may have a markedly different response to CSA immunotherapy, suggesting a possible mechanism for steroid resistance.

An essential contribution by IFN-gamma to CD8+ T cell-mediated rejection of pancreatic islet allografts

CD8+ T cells have long been considered to be the prototypical cytotoxic lymphocyte subpopulation. However, whether alloreactive CD8+ T cells require traditional cytolytic pathways such as perforin and Fas ligand (FasL) to mediate graft rejection has been a controversial issue. In the present studies, we examined the role of varied effector pathways in CD8+ T cell-mediated rejection of pancreatic islet allografts. Our goal was to systematically determine the relative requirements, if any, of perforin and FasL as well as the proinflammatory cytokine IFN-gamma in triggering graft destruction. To study CD8+ T cell effector pathways independently of other lymphocyte populations, purified alloreactive CD8+ T cells were adoptively transferred into severe combined immune-deficient (SCID) recipients bearing established islet allografts. Results indicate that to reject established islet allografts, primed CD8+ T cells do not require the individual action of the conventional cytotoxic effectors perforin and Fas ligand. In contrast, the ability to produce IFN-gamma is critical for efficient CD8+ T cell-mediated rejection of established islet allografts. Furthermore, alloreactive CD8+ TCR transgenic T cells (2C) also show IFN-gamma dependence for mediating islet allograft rejection in vivo. We speculate from these results that the production of IFN-gamma by alloreactive CD8+ T cells is a rate-limiting step in the process of islet allograft rejection.

Involvement of Fas-Fas ligand interactions in graft rejection

The Fas/Fas ligand (FasL) pathway has been shown to be important in T lymphocyte-mediated cell death and is a key peripheral immunoregulatory mechanism that limits expansion of antigen-activated lymphocytes. The expression of Fas by commonly transplanted organs such as the heart, lung, kidney, and liver suggests that these tissues may be targets of FasL-expressing allospecific cytotoxic T lymphocytes. In this review the current literature examining the Fas/FasL system as a potential cellular effector pathway in tissue injury is discussed. In addition to a deleterious role in destruction of graft tissue, Fas/FasL interactions may have a beneficial role in transplantation. Recent studies suggest that modulation of FasL in target tissue leads to deletion, via apoptosis, of graft infiltrating lymphoid cells. However, an equally compelling series of reports indicate that overexpression of FasL can lead to a heightened immune response. These data are reviewed in the context of strategies to achieve long term allograft survival.

The significance of apoptosis in the liver

Apoptosis is a form of cell death which occurs in normal as well as in pathological tissues. We provide a description of the morphological changes during apoptosis and an overview of the role of apoptosis dysregulation in the pathogenesis of non-neoplastic liver diseases.

Cytotoxic pathways in the skin allograft rejection by CD4+ T cells

BACKGROUND

Two major mechanisms of T cell-mediated cytotoxicity are known: perforin-dependent and Fas-dependent cytotoxic pathways. Previous studies in vitro demonstrated that CD4+ cytotoxic T lymphocytes use the Fas pathway as a primary cytotoxic mechanism, but the cytotoxic mechanisms used by CD4+ T cells in vivo are unclear.

METHODS

We examined the cytotoxic pathways of CD4+ T cells in vivo using a skin allograft model, in which athymic nu/nu mice were transplanted with skin allografts and reconstituted with purified CD4+T cells. Fas-deficient and perforin-deficient mice and anti-tumor necrosis factor (TNF)-alpha monoclonal antibody were used for inactivating each cytotoxic pathway in vivo.

RESULTS

The skin allografts from Fas-deficient mice were readily rejected by the athymic mice reconstituted with purified CD4+ T cells. Perforin-deficient CD4+ T cells could also reject Fas-deficient skin allografts. Furthermore, in vivo treatment with anti-TNF-alpha monoclonal antibody did not prevent the allograft rejection by CD4+ T cells in the absence of both Fas and perforin pathways.

CONCLUSIONS

These results indicate participation of undefined mechanisms other than Fas, perforin, and TNF-alpha pathways in CD4+ T cell-mediated cytotoxicity in vivo.

Analysis of the Th1/Th2 paradigm in transplantation: interferon-gamma deficiency converts Th1-type proislet allograft rejection to a Th2-type xenograft-like response

The rejection mechanisms for fetal proislet allografts and pig proislet xenografts in mice are characterized by different intragraft cytokine mRNA profiles and cellular responses. Allograft rejection is predominantly CD8 T-cell-dependent and is associated with a Th1-type cytokine pattern (i.e., IFN-gamma, IL-2 but no IL-4 or IL-5 mRNA). In contrast, xenograft rejection is CD4 T-cell-dependent and is accompanied by a strong Th2-type response (i.e., enhanced expression of IL-4 and IL-5 mRNA) and by marked eosinophil accumulation at the graft site. We have now examined and compared the regulatory role of IFN-gamma in both proislet allograft and xenograft rejection processes. The histopathology and intragraft cytokine mRNA profile of BALB/c (H-2d) proislet allografts were examined in IFN-gamma-deficient and wild-type C57BL/6J recipient mice. The survival of pig proislet xenografts was also assessed in IFN-gamma -/- and wild-type hosts. Both proislet allografts and xenografts were acutely rejected in IFN-gamma -/- and wild-type mice. Unlike the conventional allograft reaction, which lacks eosinophil infiltration, the rejection of proislet allografts in IFN-gamma-deficient hosts correlated with intragraft expression of IL-4 and IL-5 mRNA (i.e., a Th2-type response) and eosinophil recruitment. The rejection of proislet allografts and xenografts can therefore occur by IFN-gamma-independent pathways; IFN-gamma, however, regulates the pathology of the allograft reaction but not the xenograft response. The immune destruction of proislet allografts is not prevented by Th2 cytokine gene expression; instead, the latter correlated with the recruitment of unconventional inflammatory cells (eosinophils), which may play an accessory role in effecting graft injury. Significantly, the Th1-to-Th2-like switch resulted in the novel conversion of an allograft rejection reaction into a xenograft-like rejection process.

Gene targeting: applications in transplantation research

Gene targeting, the manipulation of gene in the mouse genome using homologous recombination in embryonic stem cells, is a powerful experimental tool that has been widely utilized in a number of disciplines. The ability to precisely alter genes in this way provides an avenue for investigating the role of a gene product in normal and pathological processes in the intact animal, with a precision and efficacy not possible using pharmacological agents, antibodies or engineered proteins. In transplant research, gene targeting provides a unique tool for discriminating the contributions of gene expression in donor versus recipient tissues. This review focuses on several areas in transplantation research where gene targeting has made useful contributions. These include studies of the role of donor and recipient multiple histocompatibility complex antigens in regulating rejection responses, the role of CD4+ T cell in mediating acute rejection, and the functions of cytokines during rejection and tolerance induction. These studies highlight the unique advantages of gene targeting in studies of complex processes in whole animals and illustrate the contributions of this technique to understanding the pathogenesis of allograft rejection.

Immune mechanisms associated with the rejection of fetal murine proislet allografts and pig proislet xenografts: comparison of intragraft cytokine mRNA profiles

BACKGROUND

Previous in vivo depletion studies of CD4 and CD8 T cells indicated that different rejection mechanisms operate for proislet allografts and xenografts. The cellular and molecular mechanisms of acute proislet allograft and xenograft rejection have therefore been characterized and directly compared.

METHODS

The intragraft cytokine mRNA profile in rejecting BALB/c (H-2d) proislet allografts was analyzed in control, CD4 T cell-depleted, and CD8 T cell-depleted CBA/H (H-2k) recipient mice using semi-quantitative reverse transcriptase-assisted polymerase chain reaction (RT-PCR). The cytokine profiles for proislet allografts and pig proislet xenografts at 3-10 days posttransplant were directly compared and correlated with graft histopathology.

RESULTS

Allograft rejection was protracted (2-3 weeks), characterized by infiltrating CD8 T cells and CD4 T cells (no eosinophils) and was associated with a Th1-type CD4 T cell response (IL-2, IFN-gamma, and IL-3 mRNA) and a CD8 T cell-dependent spectrum of cytokine gene expression (IL-2, IFN-gamma, IL-3, and IL-10 mRNA). Xenograft rejection was rapid (6-8 days), involved predominantly CD4 T cells and eosinophils, and in contrast to allografts, exhibited intragraft mRNA expression for the Th2 cytokines IL-4 and IL-5.

CONCLUSIONS

Proislet allograft and xenograft rejection differ in the tempo of destruction, phenotype of the cellular response and intragraft profile of cytokine mRNA. The recruitment of eosinophils only to the site of xenorejection correlates with IL4 and IL-5 mRNA expression. These findings suggest that different anti-rejection strategies may need to be developed to optimally target the allograft and the xenograft response.

Fas-mediated cytotoxicity is not required for rejection of murine nonvascularized heterotopic cardiac allografts

BACKGROUND

Using mice with loss-of-function mutations in the Fas and Fas ligand (FasL) genes (lpr and gld, respectively) in transplantation experiments has resulted in contradictory findings concerning the role of Fas/FasL-mediated cytotoxicity in allograft rejection. The observation that these mutant mice develop an abnormal lymphocyte phenotype with increasing age that is hyporesponsive in vitro led us to examine the possibility that this characteristic might explain seemingly discordant observations in the literature. Therefore, to distinguish between the effects of Fas/FasL pathway disruption and the effects of immune senescence on in vivo cytotoxicity and allograft rejection, we evaluated the survival of cardiac allografts in gld, lpr, and wild-type mice of varying ages.

METHODS

Six- to 21-week-old C3H, C3H/HeJ-Fasl(gld), C57B1/6, and B6.MRL-Fas(lpr) recipients were transplanted with heterotopic, nonvascularized cardiac allografts from neonatal Balb/c, C3H, C57Bl/6, and B6.MRL-Fas(lpr) donors. Mixed lymphocyte reactions were performed in naive gld, lpr, and wild-type animals, 6 and 12 weeks of age. Rejected allografts in gld, lpr, and wild-type recipients and functioning syngeneic transplants were evaluated for intragraft apoptosis by a DNA fragmentation detection assay.

RESULTS

Graft survival was not significantly different between 6-week-old gld and lpr recipients and their respective wild-type controls. However, allograft rejection was delayed significantly in older (13-week) gld mice compared with age-matched wild-type mice (P=0.02) or young (6-week) gld animals (P=0.04). Similarly, 21-week-old lpr mice exhibited prolonged graft survival compared with 6-week-old lpr animals (P=0.01). Reduced alloreactive proliferative responses in 12-week-old gld and lpr mice were observed when compared with age-matched wild-type strains. Rejecting allografts displayed a similar level of intragraft apoptotic cells regardless of mutant or wild-type phenotype or age of recipient.

CONCLUSIONS

The findings of this study confirm that Fas/FasL-mediated cytotoxicity is not required for murine cardiac allograft rejection. Our findings also demonstrate that the observed delayed graft rejection in lpr and gld mice is a consequence of an age-related alteration of the immune system, specific to gld and lpr mice and associated with an in vivo and in vitro hyporeactivity to alloantigens.

Mechanisms of beta cell death in diabetes: a minor role for CD95

Insulin-dependent diabetes mellitus is an autoimmune disease, under polygenic control, manifested only when >90% of the insulin-producing beta cells are destroyed. Although the disease is T cell mediated, the demise of the beta cell results from a number of different insults from the immune system. It has been proposed that foremost amongst these effector mechanisms is CD95 ligand-induced beta cell death. Using the nonobese diabetic lpr mouse as a model system, we have found, to the contrary, that CD95 plays only a minor role in the death of beta cells. Islet grafts from nonobese diabetic mice that carry the lpr mutation and therefore lack CD95 were protected only marginally from immune attack when grafted into diabetic mice. An explanation to reconcile these differing results is provided.

T cells in islet-like cell cluster xenograft rejection: a study in the pig-to-mouse model

BACKGROUND

The aim of the present study was to evaluate the nature of T cells involved in and, presumably, critical to fetal porcine islet-like cell cluster (ICC) xenograft rejection.

METHODS

Normal mice and T cell receptor (TCR)-beta-, TCR-delta-, or TCR-betaxdelta-deficient mice were transplanted with fetal porcine ICC under the kidney capsule. Perforin- or granzyme B (GraB)-deficient mice were used to further characterize T cell-dependent pathways. For evaluation of the role of T cells in the activation process of macrophages, TCR-betaxdelta mutants were treated with recombinant mouse tumor necrosis factor (TNF)-alpha. In addition, normal mice transplanted with porcine ICC were treated with MDL 201,449A, a novel transcriptional inhibitor of TNF-alpha.

RESULTS

In normal mice, the majority of the infiltrating cells were large, macrophage-like cells expressing the macrophage-specific phenotype marker F4/80. CD3+ T lymphocytes were found to be mainly accumulated in the peripheral parts of the ICC xenograft. TCR-beta mutants and TCR-betaxdelta mutants exhibited no signs of xenograft rejection, whereas TCR-delta mutants and perforin- and GraB-deficient animals rejected the ICC xenograft. Posttransplant high-dose recombinant mouse TNF-alpha-treatment of TCR-betaxdelta mutants did not result in fetal porcine ICC xenograft rejection. However, a somewhat increased amount of F4/80+ and Mac-1+ cells was observed within the xenograft area. Similarly, although graft survival was not found to be prolonged, reduced numbers of CD4+ T cells were observed in mice treated with MDL 201,449A.

CONCLUSIONS

In the pig-to-mouse model, fetal porcine ICC xenograft rejection is exclusively dependent on T cells bearing TCR-alphabeta chains. In addition, the absence of perforin or GraB has no influence on the rejection process, suggesting that xenospecific cytolytic T cells are of minor importance. Even if TNF-alpha is of importance to the developing process of ICC xenograft rejection, other cytokines, i.e., interferon-gamma, might efficiently substitute for the lack of TNF-alpha.

CD8+ Cells Are Not Necessary for Allograft Rejection or the Induction of Apoptosis in an Experimental Model of Small Intestinal Transplantation

Allospecific CTL can function as cellular effectors of solid organ graft rejection; however, the specific mechanisms of cell damage remain undetermined. In this study we examined the role of CD8+ T cells in apoptosis and rejection of small intestinal allografts. ACI rat intestinal grafts transplanted into Lewis rat recipients showed apoptosis of epithelial crypt cells on day 3 posttransplant as determined by terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate nick end labeling staining. By day 7 numerous apoptotic crypt cells were detected in allografts, but were rarely observed in FK506-treated allograft recipients, isografts, or native intestine of allograft recipients. To further investigate the mechanism of rejection, recipient rats were depleted of CD8+ cells by treatment with OX-8 mAbs the day before and the day after transplantation of rat small intestinal allografts. Depletion of CD8+ cells from allograft recipients did not alter the tempo or the histologic features of rejection compared with those in the control (IgG-treated) group. Moreover, there was no difference in the number of apoptotic crypt epithelial cells in the grafts of control and CD8-depleted rats. Reverse transcriptase-PCR analyses determined there were similar levels of transcripts for Fas, Fas ligand, perforin, and granzyme B in control and CD8-depleted allograft recipients. By Western blot it was determined that the levels of Fas ligand protein were increased in the CD8-depleted group compared with those in control and FK506-treated allograft recipients. These data suggest that CD8 cells are not required for tissue injury or apoptotic cell death in small intestine allograft rejection.

Dual mechanisms of apoptosis induction by cytotoxic lymphocytes

Cytotoxic T lymphocytes and natural killer cells together comprise the means by which the immune system detects and rids higher organisms of virus-infected or transformed cells. Although differing considerably in the way they detect foreign or mutated antigens, these cells utilize highly analogous mechanisms for inducing target cell death. Both types of effector lymphocytes utilize two principal contact-dependent cytolytic mechanisms. The first of these, the granule exocytosis mechanism, depends on the synergy of a calcium-dependent pore-forming protein, perforin, and a battery of proteases (granzymes), and it results in penetration by effector molecules into the target cell cytoplasm and nucleus. The second, which requires binding of FasL (CD95L) on the effector cell with trimeric Fas (CD95) molecules on receptive target cells, is calcium independent and functions by generating a death signal at the inner leaflet of the target cell membrane. Exciting recent developments have indicated that both cytolytic mechanisms impinge on an endogenous signaling pathway that is strongly conserved in species as diverse as helminths and humans and dictates the death or survival of all cells.