Bacterial products in donor airways prevent the induction of lung transplant tolerance

Although postoperative bacterial infections can trigger rejection of pulmonary allografts, the impact of bacterial colonization of donor grafts on alloimmune responses to transplanted lungs remains unknown. Here, we tested the hypothesis that bacterial products present within donor grafts at the time of implantation promote lung allograft rejection. Administration of the toll-like receptor 2 (TLR2) agonist Pam₃Cys₄ to Balb/c wild-type grafts triggered acute cellular rejection after transplantation into B6 wild-type recipients that received perioperative costimulatory blockade. Pam₃Cys₄-triggered rejection was associated with an expansion of CD8⁺ T lymphocytes and CD11c⁺CD11b^hiMHC (major histocompatibility complex) class II⁺ antigen-presenting cells within the transplanted lungs. Rejection was prevented when lungs were transplanted into TLR2-deficient recipients but not when MyD88-deficient donors were used. Adoptive transfer of B6 wild-type monocytes, but not T cells, following transplantation into B6 TLR2-deficient recipients restored the ability of Pam₃Cys₄ to trigger acute cellular rejection. Thus, we have demonstrated that activation of TLR2 by a bacterial lipopeptide within the donor airways prevents the induction of lung allograft tolerance through a process mediated by recipient-derived monocytes. Our work suggests that donor lungs harboring bacteria may precipitate an inflammatory response that can facilitate allograft rejection.     

Critical role of proinflammatory cytokine IL-6 in allograft rejection and tolerance

The proinflammatory cytokine IL-6 plays an important role in controlling T-cell differentiation, especially the development of Th17 and regulatory T cells. To determine the function of IL-6 in regulating allograft rejection and tolerance, BALB/c cardiac grafts were transplanted into wild-type or IL-6-deficient C57BL/6 mice. We observed that production of IL-6 and IFN-γ was upregulated during allograft rejection in untreated wild-type mice. In IL-6-deficient mice, IFN-γ production was greater than that observed in wild-type controls, suggesting that IL-6 production affects Th1/Th2 balance during allograft rejection. CD28-B7 blockade by CTLA4-Ig inhibited IFN-γ production in C57BL/6 recipients, but had no effect on the production of IL-6. Although wild-type C57BL/6 recipients treated with CTLA4-Ig rejected fully MHC-mismatched BALB/c heart transplants, treatment of IL-6-deficient mice with CTLA4-Ig resulted in graft acceptance. Allograft acceptance appeared to result from the combined effect of costimulatory molecule blockade and IL-6-deficiency, which limited the differentiation of effector cells and promoted the migration of regulatory T cells into the grafts. These data suggest that the blockade of IL-6, or its signaling pathway, when combined with strategies that inhibit Th1 responses, has a synergistic effect on the promotion of allograft acceptance. Thus, targeting the effects of IL-6 production may represent an important part of costimulation blockade-based strategies to promote allograft acceptance and tolerance.     

Chronic rejection of murine cardiac allografts discordant at the H13 minor histocompatibility antigen correlates with the generation of the H13-specific CD8+ cytotoxic T cells

BACKGROUND: Minor histocompatibility antigen (mHag) discordances have been shown to play a critical role in graft-versus-host disease after bone marrow transplantation. However, the role of mHag in rejection of solid-organ allografts remains unknown. Therefore, the goal of this study was to define the role of a single mHag discordance derived from the polymorphic H13 allele in the development of cardiac allograft rejection in mice. The H13a and H13b alleles encode for the SSVVGVWYL (SVL9) and SSVIGVWYL (SIL9) mHag bound to the H2Db molecule, respectively. METHODS: C56BL/10SnJ (H13a) cardiac allografts were transplanted into congenic B10.CE-H13b Aw(30NX)/Sn (H13b) mice. Allograft function was monitored daily and rejection was defined by cessation of heart beat. Rejection was confirmed histologically. The phenotypic and functional characteristics of the graft-infiltrating cells were analyzed by in situ and in vitro staining with H13a-specific tetramers and by chromium-51 (51Cr)-release assay, respectively. RESULTS: Sixty-five percent of H13-incompatible allografts were rejected in 37.0+/-14.5 days. Sixty-eight percent of the H13a allografts transplanted into H13a-sensitized mice were rejected earlier, in 27.6+/-15.9 days. Rejected allografts showed histopathologic signs of chronic rejection with diffuse mononuclear cell infiltration, concentric intimal hyperplasia, and fibrosis. Both CD8+ (87%) and CD4+ (13%) T cells were observed in rejected allografts. In addition, 60% of the graft-infiltrating CD8+ T cells recognized a H2Db/SVL9 tetramer. Graft-infiltrating CD8+ T cells showed a significant H2Db-restricted, SVL9-specific cytotoxic activity. CONCLUSIONS: A single mHag discordance, as demonstrated with H13 disparity, results in the pathogenesis of chronic rejection of major histocompatibility complex-matched vascularized solid-organ allograft.     

A major role for host MHC class I antigen presentation for promoting islet allograft survival

The purpose of this study was to determine the role for CD8 T cells versus generalized MHC class I-restricted antigen presentation in islet allograft rejection and tolerance. Diabetic C57BI/6 (B6, H-2(b)) controls, C57BI/6 CD8-deficient (CD8 KO), or MHC class I-deficient C57BI/6 (beta 2m KO) recipients were grafted with allogeneic BALB/c (H-2(d)) islets. Islet allografts were acutely rejected in untreated B6, CD8 KO, and in beta 2m KO mice, indicating that neither CD8 T cells nor host MHC class I is required for allograft rejection. We then determined the efficacy of costimulation blockade in these same strains. Costimulation blockade with anti-CD154 therapy facilitated long-term islet allograft survival in both B6 and in CD8 KO recipients. However, anti-CD154 treated beta 2m KO recipients were completely refractory to anti-CD154 therapy; all treated animals acutely rejected islet allografts with or without therapy. Also, anti-NK1.1 treatment of wild-type B6 mice abrogated graft prolongation following anti-CD154 therapy. Taken together, results show a dramatic distinction between two forms of MHC class I-restricted pathways in allograft prolongation. Although anti-CD154-induced allograft survival was CD8 T-cell independent, an intact host MHC class I-restricted (beta 2m-dependent) pathway is nevertheless necessary for allograft survival. This pathway required NK1.1+ cells, implicating NK and/or NKT cells in promoting allograft prolongation in vivo.     

CD4+ T cells are sufficient to elicit allograft rejection and major histocompatibility complex class I molecule is required to induce recurrent autoimmune diabetes after pancreas transplantation in mice

BACKGROUND: We characterized the role of T cell subsets and major histocompatibility complex molecules in allograft rejection and recurrence of autoimmune diabetes. METHODS: Adoptive cell transfer and vascularized segmental pancreas transplantation were performed in mice. RESULTS: In an alloimmune response model, transfer of nondiabetic CD4, but not CD8 T cells, elicited pancreas allograft rejection in streptozotocin (STZ)-induced diabetic NOD/scid mice. Pancreas allografts were acutely rejected in STZ-induced diabetic NOD/beta2m mice (confirmed the absence of major histocompatibility complex [MHC] class I and CD8 T cells) and permanently accepted in NOD/CIIT mice (confirmed the absence of MHC class II and CD4 T cells). The results suggest that rejection of pancreas allograft is CD4-dependent and MHC class I-independent. In the autoimmune diabetes model, whole spleen cells obtained from diabetic NOD mice induced autoimmune diabetes in NOD/scid and NOD/CIIT mice, but the onset of diabetes was delayed in NOD/beta2m mice. However, the purified diabetic T cells failed to elicit autoimmune diabetes in NOD/beta2m mice. NOD/scid and NOD/CIIT pancreas grafts were acutely destroyed whereas four of six NOD/beta2m pancreas grafts were permanently accepted in autoimmune diabetic NOD mice. CONCLUSION: CD4 T cells are sufficient for the induction of allograft rejection, and MHC class I molecule is required to induce recurrent autoimmune diabetes after pancreas transplantation in mice.     

Analysis of the CD40 and CD28 pathways on alloimmune responses by CD4+ T cells in vivo

BACKGROUND: Blockade of the CD40 and CD28 pathways is a powerful strategy to inhibit CD4-mediated alloimmune responses. In this study, we examine the relative roles of the CD40 and CD28 pathways on CD4-mediated allograft rejection responses, and further characterize the role of these pathways on CD4+ T-cell activation, priming for cytokine production, and cell proliferation in response to alloantigen in vivo. METHODS: BALB/c skin allografts were transplanted onto C57BL/6 Rag 1-/- recipients reconstituted with CD4 cells from CD28-/- or CD40L-/- donors. The popliteal lymph node assay was used to study the role of these pathways on CD4-cell activation and priming in vivo. To investigate the role of CD40 and CD28 blockade on CD4-cell proliferation, the fluorescein dye carboxyfluorescein diacetate succinimidyl ester was used. We performed heterotopic cardiac transplantation using CD40-/- mice to evaluate the role of CD40 on donor versus recipient cells in CD4-mediated rejection. RESULTS: B6 Rag 1-/- recipients reconstituted with CD28-/- CD4+ T cells acutely rejected allografts (median survival time 15 days), whereas recipients reconstituted with CD40L-/- CD4+ T cells had significantly prolonged survival of BALB/c skin grafts (MST 71 days). CD40L blockade was equivalent to or inferior to CD28 blockade in inhibition of in vivo CD4-cell activation, priming for cytokine production, and proliferation responses to alloantigen. BALB/c recipients depleted of CD8 cells promptly rejected donor B6 CD40-/- cardiac allografts, whereas B6 CD40-/- recipients depleted of CD8 cells had significantly prolonged survival of BALB/c wild-type cardiac allografts. CONCLUSIONS: The CD40/CD40L pathway, but not the CD28/B7 pathway, is critical for CD4-mediated rejection responses, however, the responsible mechanisms remain unclear.     

Down-regulation of the immune response to histocompatibility antigens and prevention of sensitization by skin allografts by orally administered alloantigen.

The effects of oral administration of major histocompatibility antigens on the alloimmune response have not been investigated. Lymphocytes from inbred LEW (RT1u) rats that were pre-fed allogeneic WF (RT1l) splenocytes exhibited significant antigen specific reduction of the mixed lymphocyte response in vitro and delayed-type hypersensitivity response in vivo, when compared with unfed controls. In an accelerated allograft rejection model, LEW rats were presensitized with BN (RT1n) skin allografts 7 days before challenging them with (LEW x BN)F1 or BN vascularized cardiac allografts. While sensitized control animals hyperacutely reject their cardiac allografts within 2 days, animals prefed with BN splenocytes maintained cardiac allograft survival to 7 days, a time similar to that observed in unsensitized control recipients. This phenomenon was antigen-specific, as third-party WF grafts were rejected within 2 days. Immunohistologic examination of cardiac allografts harvested on day 2 from the fed animals had markedly reduced deposition of IgG, IgM, C3, and fibrin. In addition, there were significantly fewer cellular infiltrates of total white blood cells, neutrophils, macrophages, T cells, IL-2 receptor-positive T cells, and mononuclear cells with positive staining for the activation cytokines IL-2 and IFN-g. On day 6 posttransplant, the grafts from fed animals showed immunohistologic changes typical of acute cellular rejection usually seen in unsensitized rejecting controls. Feeding allogeneic splenocytes prevents sensitization by skin grafts and transforms accelerated rejection of vascularized cardiac allografts to an acute form typical of unsensitized recipients. Oral administration of alloantigen provides a novel approach to down-regulate the specific systemic alloimmune response against histocompatibility antigens.     

Critical influence of natural regulatory CD25+ T cells on the fate of allografts in the absence of immunosuppression

BACKGROUND: Allografts are occasionally accepted in the absence of immunosuppression. Because naturally occurring CD4(+)CD25(+) regulatory T cells (natural CD25(+) Treg cells) have been shown to inhibit allograft rejection, we investigated their influence on the outcome of allografts in nonimmunosuppressed mouse recipients. METHODS: We compared survival times of male CBA/Ca skin grafts in female CBA/Ca recipients expressing a transgenic anti-HY T-cell receptor on a RAG-1(+/+) (A1[M]RAG+) or a RAG-1(-/-) (A1[M]RAG-) background. Depletion of natural CD25(+) Treg cells in A1[M]RAG+ mice was achieved by in vivo administration of the PC61 monoclonal antibody. The influence of natural CD25(+) Treg cells on the fate of major histocompatibility complex class II-mismatched (C57BL/6X bm12)F1 skin or bm12 heart transplants in C57BL/6 recipients was also assessed. Finally, we investigated the impact of natural CD25(+) Treg cells on the production of T-helper (Th)1 and Th2 cytokines in mixed lymphocyte cultures between C57BL/6 CD4(+) CD25(-) T cells as responders and bm12 or (C57BL/6X bm12)F1 antigen-presenting cells as stimulators. RESULTS: Male allografts were spontaneously accepted by female A1(M)RAG+ mice but readily rejected by female A1(M)RAG+ mice depleted of natural CD25(+) Treg cells by pretreatment with the PC61 monoclonal antibody. Depletion of CD25(+) Treg cells also enhanced eosinophil-determined rejection of (C57BL/6X bm12)F1 skin grafts or bm12 cardiac grafts in C57BL/6 recipients. Finally, natural CD25(+) Treg cells inhibited the production of interleukin (IL)-2, interferon-gamma, IL-5, and IL-13 in mixed lymphocyte culture in a dose-dependent manner. CONCLUSION: Natural CD25(+) Treg cells control Th1- and Th2-type allohelper T-cell responses and thereby influence the fate of allografts in nonimmunosuppressed recipients.     

Effects of noninherited maternal antigens on allotransplant rejection in a transgenic mouse model

We studied the influence of noninherited maternal antigen (NIMA) on allotransplant rejection using a mouse transgenic model. CBK transgenic (CBA [H-2k] expressing K(b) MHC class I transgene) mice were used as donors in heart transplantation experiments. Offspring of BM3.3 (CBA anti-K(b) TCR transgenic) male mice and (CBA x CBK)F1 females were used as NIMA (offspring that did not inherit K(b)) and IMA (offspring that inherited K(b) maternal antigen) recipient mice. Survival of allografts was monitored and the alloimmune response evaluated using an ELISPOT assay. IMA mice accepted CBK heart allografts and displayed no alloresponse to K(b+) cells. In contrast, mice never exposed to K(b) (offspring of BM3.3 males and CBA females) acutely rejected their grafts within 18 days posttransplantation and exhibited potent inflammatory alloresponses to K(b+) cells. NIMA mice displayed prolonged survival of allotransplants (MST >60 days). Although no deletion of anti-K(b) TCR transgenic cells was detected in these mice, they had a marked reduction in the frequency of activated alloreactive T cells producing type 1 (IFN-gamma and IL-2) cytokines and concomitant expansion of type 2 (IL-4) cytokine-secreting cells. Finally, depletion of CD4+ T cells from NIMA mice restored acute rejection of CBK hearts. This study is the first demonstration of the tolerogenic effects of NIMA on alloimmunity and allotransplant rejection in a transgenic model. It is shown that, although the NIMA tolerogenic effect is not due to deletion of alloreactive T cells, it is mediated by CD4+ T cells producing type 2 cytokines.     

移植抗原特异性转基因CD8+T细胞对白细胞介素2的依赖性

目的研究白细胞介素2（IL-2）在调节移植抗原特异性转基因CD8^＋T细胞介导的免疫排斥反应中的作用。方法将经荧光染科CFSE标记后的C57BL／6小鼠和2CTg小鼠（CD4敲除鼠）淋巴细胞分别植入经致死剂量γ射线照射过的两组DBA／2J小鼠体内，检测CD4^＋与CD8^＋T细胞在体内分裂增殖的时相，并用胞浆内IL-2标志染色方法测定活化后T细胞表达IL-2的能力。以Balb／c小鼠为供者，糖尿病2CTg小鼠和2C Tg-IL-2KO小鼠（IL-2敲除鼠）为受者，进行胰岛细胞移植。观察CD8^＋ T细胞在介导移植排斥中的作用。结果DBA／2J小鼠输注了C57BL／6小鼠的淋巴细胞后，CD4^＋与CD8^＋T细胞分裂增殖均非常明显，前者表达大量IL-2，后者则不表达。DBA／2J小鼠输注了2CTg小鼠的淋巴细胞后。在完全没有CD4^＋T细胞存在的情况下，CD8^＋T细胞仍明显分裂增殖并大量表达IL-2。2CTg和2CTg—IL-2KO小鼠移植胰岛细胞后，前者迅速发生排斥反应，胰岛移植物的平均存活时间仅为8d，而后者胰岛移植物的平均存活时间〉50d。结论CD8^＋T细胞在产生和利用IL-2时有很大的可塑性。CD4^＋T细胞存在时，CD8^＋T细胞能有效利用CD4^＋T细来源的IL-2进行分裂增殖，在缺乏CD4^＋T细胞时，则利用自身来源的IL-2进行分裂增殖；移植抗原特异性CD8^＋T细胞的效应功能完全依赖于IL-2，排斥反应由CD8^＋T细胞介导时，阻断IL-2／IL-2受体通路可诱导移植物长期存活。     

IL-4 deficiency prevents eosinophilic rejection and uncovers a role for neutrophils in the rejection of MHC class II disparate skin grafts

BACKGROUND: Acute rejection of MHC class II-disparate bm12 skin grafts by C57BL/6 recipient mice is characterized by massive graft infiltration by eosinophils, together with increased intragraft amounts of IL-4 and IL-5 mRNA. IL-5 blockade prevents the intragraft eosinophil infiltration and prolongs the survival of skin allografts. As the differentiation of T cell precursors into Th2 cells is largely driven by IL-4, we investigated the role of IL-4 in MHC class II-disparate allograft rejection. METHODS: We performed skin grafts from MHC class II incompatible bm12 mice into wild-type C57BL/6 mice (IL-4) or C57BL/6 IL-4 deficient mice (IL-4). Graft survival, in vitro T cell reactivity, and histology were compared. RESULTS: We observed that 50% of IL-4 mice rapidly rejected their bm12 allograft, whereas the other 50% retained their graft 60 days after transplantation. Histological examination of bm12 allografts retained by IL-4 mice showed a normal appearance with no inflammatory infiltrate and no eosinophils. Among IL-4 mice that acutely rejected their bm12 skin graft, we observed a dense polymorphonuclear infiltrate. The depletion of neutrophils significantly prolonged bm12 graft survival. CONCLUSIONS: Eosinophil infiltrates, typical of MHC class II disparate acute skin graft rejection, are critically dependent on the availability of IL-4. IL-4 mice reject MHC class II disparate skin grafts by a pathway of rejection where neutrophils play a direct causal role.     

Prolonged allograft survival in anti-CD4 antibody transgenic mice: lack of residual helper T cells compared with other CD4-deficient mice

BACKGROUND: Investigations of the role of CD4 T lymphocytes in allograft rejection and tolerance have relied on the use of mouse models with a deficiency in CD4 cells. However, in mice treated with depleting monoclonal antibody (mAb) and in MHC class II knockout (KO) mice, there are residual populations of CD4 cells. CD4 KO mice had increased CD4- CD8-TCRalphabeta+ helper T cells, and both strains of KO mice could reject skin allografts at the normal rate. In this study, transgenic mice with no peripheral CD4 cells were the recipients of skin and heart allografts. Results were compared with allograft survival in CD4 and MHC class II KO mice. METHODS: GK5 (C57BL/6 bml mice transgenic for a chimeric anti-CD4 antibody) had no peripheral CD4 cells. These mice, and CD4 and class II KO mice, received BALB/c or CBA skin or cardiac allografts. Some GK5 mice were treated with anti-CD8 mAb to investigate the role of CD8 cells in rejection. CD4 and CD8 cells were assessed by FACS and immunohistochemistry. RESULTS: BALB/c skin on GK5 mice had a mean survival time +/- SD of 24+/-6 days, compared with 9+/-2 days in wild-type mice. Anti-CD8 mAb prolonged this to 66+/-7 days. BALB/c skin survived 10+/-2 days on class II KO and 14+/-2 days on CD4 KO, both significantly less than the survival seen on GK5 recipients (P<0.001). BALB/c hearts survived >100 days in GK5 recipients and in wild-type recipients treated with anti-CD4 mAb at the time of grafting, in contrast to a mean survival time of 10+/-2 days in untreated wild-type mice. Immunohistochemistry revealed that long-term surviving heart allografts from the GK5 recipients had CD8 but no CD4 cellular infiltrate. These hearts showed evidence of transplant vasculopathy. CONCLUSIONS: The GK5 mice, with a complete absence of peripheral CD4 cells, provide the cleanest available model for investigating the role of CD4 lymphocytes in allograft rejection. Prolonged skin allograft survival in these mice compared with CD4 and MHC class II KO recipients was clearly the result of improved CD4 depletion. Nevertheless, skin allograft rejection, heart allograft infiltration, and vascular disease, mediated by CD8 cells, developed in the absence of peripheral CD4 T cells.     

MHC-identical heart and hepatocyte allografts evoke opposite immune responses within the same host

BACKGROUND: Purified allogeneic hepatocytes are highly antigenic and elicit immune responses that are not easily controlled. However, it is not clear whether hepatocytes are not capable of protective immune mechanisms or whether they are not to protection by immune mechanisms that permit long-term survival of other allografts. The purpose of the current study was to determine whether donor-matched allogeneic hepatocytes are protected from rejection in mice that have been induced to accept heart allografts. METHODS: Transient treatment with anti-CD4 monoclonal antibody (mAb) or gallium nitrate (GN) was used to induce acceptance of heterotopic FVB/N (H-2(q)) heart allografts by C57BL/6 (H-2(b)) mice. Transgenic hA1AT-FVB/N hepatocytes were sequentially transplanted into C57BL/6 mice that had accepted FVB/N heart allografts more than 60 days (heart acceptor mice), CD8 depleted C57BL/6 heart acceptor mice, or B-cell knockout (BCKO, H-2(b)) heart acceptor mice. Hepatocyte survival was determined by the detection of secreted transgenic product hA1AT by enzyme-linked immunosorbent assay (ELISA). RESULTS: FVB/N hepatocytes were rejected by day 10-14 posttransplant, while FVB/N heart allografts continued to function in C57BL/6, BCKO, and CD8 depleted heart acceptor mice. When FVB/N hepatocytes and heart allografts were transplanted into C57BL/6 or BCKO mice under short-term cover of anti-CD4 mAb or GN, hepatocyte rejection occurred by day 10 posttransplant, while most heart allografts survived for more than 60 days. CONCLUSIONS: Hepatocyte rejection does not appear to interfere with the of mechanisms that permit heart allograft acceptance. However, immune responses to allogeneic hepatocytes are not to regulation by mechanisms induced in heart acceptor mice. The simultaneous rejection of FVB/N allogeneic hepatocytes and continued acceptance of FVB/N-matched heart allografts is independent of host CD8+ T cells and humoral immunity.     

Islet xenograft rejection in absence of CD8+ T cells does not require either interferon-gamma or interleukin-5

We recently demonstrated that interleukin-5 and eosinophils mediate rejection of skin allografts when CD8+ T cell-dependent and Th1-type CD4+ T cell-dependent pathways are not functional. The purpose of this study was to determine whether a similar mechanism might be operative during rejection of rat islet xenografts in mice. First, we observed that eosinophils indeed infiltrate rejected islet grafts together with CD4+ and CD8+ T cells. CD8+ T cell depletion significantly enhanced graft survival and a further prolongation of islet function was obtained in combination with interferon-gamma neutralization. However, islet rejection characterized by prominent eosinophil and macrophage infiltration still occurred in this setting. Although eosinophil infiltrates were dramatically reduced in interleukin-5 deficient mice, the ability of these animals to reject islet xenografts under CD8+ T cell depletion and interferon-gamma neutralization was similar to that of wild-type mice. We conclude that in absence of CD8+ T cells and interferon-gamma, macrophages, but not eosinophils, mediate rejection of rat-to-mouse islet xenografts.     

Differential Roles of CD8+ and CD8− T Lymphocytes in Corneal Allograft Rejection in ''High-Risk'' Hosts

We examined the role of perforin and FasL in corneal allograft rejection mediated by CD8+ and CD8 T cells. BALB/c corneas were transplanted orthotopically into vascularized, 'high-risk' graft beds in C57BL/6 mice, perforin knockout mice and FasL-defective gld/gld mice. CD8+ and CD8 T cells were collected following graft rejection and adoptively transferred to SCID mice, which were then challenged with BALB/c corneal allografts. In every case, CD8 T cells could mediate graft rejection when adoptively transferred to SCID mice that received BALB/c corneal allografts. Although CD8+ T cells also mediated graft rejection, the tempo was slower. Moreover, CD8+ T cells collected FasL-defective donors that had rejected corneal allografts, mediated corneal allograft rejection in only 50% of the SCID mice that received the adoptively transferred cells. In some cases, CD8+ T-cell-mediated rejection occurred in the absence of delayed-type hypersensitivity and cytotoxic T-lymphocyte activity, but was associated with CD8+ T-cell-mediated apoptosis of BALB/c corneal cells in vitro. The results demonstrate the redundancy in immune mechanisms of corneal allograft rejection. Either CD8+ or CD8 T cells can produce corneal allograft rejection, however functional FasL is necessary for optimal rejection, even in a high-risk setting.     

Interleukin-18 Affects Local Cytokine Expression But Does Not Impact on the Development of Kidney Allograft Rejection

Interleukin-18 is predominantly a macrophage-derived cytokine with a key role in inflammation and cell-mediated immunity. Having previously demonstrated IL-18 upregulation in a rat model of kidney rejection, here we examined IL-18 in a fully MHC-mismatched murine model of acute kidney rejection using IL-18-deficient recipients (IL-18-/-) and animals administered neutralizing IL-18 binding protein (IL-18BP). Gene expression of IL-18 and its receptor were significantly upregulated in allografts compared to isografts, as was the cellular infiltrate (T cells and macrophages) (p < 0.001). Allografts developed kidney dysfunction (p < 0.05) and tubulitis (p < 0.01) not observed in controls. There was a significant reduction in gene expression of IL-18 downstream pro-inflammatory molecules (iNOS, TNFalpha and IFNgamma) in IL-18-/- recipients (p < 0.01), and IL-18BP-treated animals. The CD4+ infiltrate and IL-4 mRNA expression was greater in the IL-18-/- recipients than wild-type (WT) allografts and IL-18BP-treated animals (p < 0.05), suggesting a Th2-bias which was supported by IFNgamma and IL-4 ELISPOT data and an increased eosinophil accumulation (p < 0.001). Neither IL-18 deficiency nor neutralization prevented renal dysfunction or tubulitis. This study demonstrates increased production of IL-18 in murine kidney allograft rejection and provides evidence that IL-18-induced pathways of inflammation are active. However, neither IL-18 deficiency nor neutralization was protective against the development of allograft rejection.     

Role of CD4+ and CD8+ T cells in murine skin and heart allograft rejection across different antigenic desparities

The factors that influence the relative contribution of the T cell subsets to allograft rejection remain unclear. We compared skin and heart rejection in CD4 Knockout (KO), and CD8 KO mice across full-, minor-, and class II histocompatibility antigen (HA) mismatches. Skin allografts were rejected by either CD4+ or CD8+ T cells alone at any degree of antigenic mismatch. However, either the absence of CD4+ cells or a lesser degree of HA mismatch resulted in prolongation of graft survival. In contrast, fully allogeneic heart grafts were accepted in CD4 KO recipients, and minor HA mismatched heart grafts were accepted by both CD4 KO and CD8 KO mice. Thus, the T cell subsets required for allograft rejection are determined by the immunogenicity of the tissue transplanted. In the absence of CD8+ T cells, perforin and Fas ligand (FasL) but not granzyme B mRNA were detected in rejecting grafts. Thus, granzyme B is a CD8+ cytotoxic T lymphocyte (CTL)-specific effector molecule.     

Absence of host B7 expression is sufficient for long-term murine vascularized heart allograft survival

CD28 antagonists have been shown to promote long-term graft survival and induce donor-specific tolerance. In this study, the role of CD28/B7 costimulation and the relative importance of host versus donor B7 expression in allograft rejection was assessed in a murine abdominal vascularized heterotopic heart transplant model. Wild-type, CD28-deficient, or B7-1/B7-2-deficient C57BL/6 (B6) mice were grafted with allogeneic wild type or B7-1/B7-2-deficient hearts. The results demonstrate allogeneic heart grafts survive long-term in mCTLA4Ig-treated B6 and untreated B7-1/B7-2-deficient B6 recipients but not CD28KO B6 mice. B7-1/B7-2KO B6 recipients treated with anti-CD28 (PV-1) or recombinant human IL-2 rejected the heart transplants indicating that these mice are immunologically competent to reject grafts if costimulatory signals are supplied or bypassed. Finally, there was no difference in rejection between normal animals transplanted with wild-type versus B7-1/B7-2-deficient hearts. These results support a critical role for B7-expressing host antigen presenting cells in the rejection of heart allografts in mice and differences among B7KO and CD28KO animals.     

An immunodominant minor histocompatibility alloantigen that initiates corneal allograft rejection

BACKGROUND: Murine orthotopic corneal allografts experience immune privilege and have good survival as compared with skin allografts. However, privilege is not complete, and some grafts are still rejected. Unexpectedly, corneas expressing minor histocompatibility (H) alloantigens are rejected at a higher rate than major histocompatibility complex (MHC) disparate grafts. We hypothesize that certain immunodominant minor H alloantigens are extremely immunogenic when expressed in corneal tissue, terminate ocular immune privilege, and initiate corneal allograft rejection. METHODS AND RESULTS: Corneal allograft survival and the role of CD4+ T cells and CD8+ T cells were examined in corneal transplants that expressed genetically defined minor H3 alloantigens. The H3 locus contains at least two minor H genes. H3a is presented by MHC class I and recognized exclusively by CD8+ T cells. H3b is presented by class II and recognized exclusively by CD4+ T cells. Congenic strains that differ from C57BL/10 at (1) H3a, (2) H3b, or (3) H3a+H3b were used for orthotopic corneal and skin transplants. Donor corneas expressing either H3a or H3a+H3b experienced immune privilege and survived longer than skin allografts. By contrast, donor corneas expressing H3b (recognized by CD4+ T cells) experienced vigorous rejection and were eliminated faster than skin allografts. CONCLUSION: There are minor H alloantigens that terminate ocular immune privilege and initiate corneal allograft rejection. These minor H alloantigens are more immunogenic when expressed in corneal tissue than when they are expressed in skin allografts.