Inability to Induce Tolerance Through Direct Antigen Presentation

Both the direct and indirect antigen presentation pathways are important mechanisms for T cell-mediated allograft rejection. Studies using knockout mice and monoclonal antibodies have demonstrated that CD4+ T cells are both necessary and sufficient for the rejection of allogeneic tissues, including skin, heart, and islet. Furthermore, combined blockade of the CD28/B7 and CD154/CD40 costimulatory pathways induces tolerance in multiple CD4+ T-cell dependent allograft models. In this study, we addressed the T-cell requirement for costimulation in direct antigen presentation. We demonstrated that class II-specific alloreactive T-cell receptor transgenic T cells were sufficient to mediate allograft rejection independent of costimulatory blockade. Analysis of the costimulatory capacity of different antigen presenting cell (APC) populations demonstrated that APCs resident within the donor skin, Langerhans cells, are potent stimulators not requiring CD28- or CD154-dependent costimulation for direct major histocompatibility complex (MHC) antigen presentation. These results complement previous work examining the role of costimulation on CD8+ T cells, supporting a model in which the effectiveness of costimulatory blockade in the setting of transplantation may be selective for the indirect pathway of MHC alloantigen presentation.     

CD8 T cell-mediated rejection of intestinal allografts is resistant to inhibition of the CD40/CD154 costimulatory pathway

BACKGROUND: Disruption of the CD40/CD154 pathway inhibits rejection in numerous models. The importance of this pathway on intestinal allograft rejection was examined in this study. METHODS: Intestinal grafts from B6C3F1 mice transplanted into C57BL/6 recipients were assessed histologically for rejection. RESULTS: The monoclonal antibody to CD154, MR1, failed to inhibit rejection in wild-type mice. Similarly, CD154-/- recipient mice rejected intestinal allografts. MR1 did inhibit early rejection in CD8-/- mice, but had no effect in CD4-/- recipients. All MR1-treated CD8-/- recipients eventually developed rejection. No benefit was observed when blockade of the CD40/CD154 pathway by MR1 was combined with blockade of the CD28/B7 pathway by mCTLA4Ig. CONCLUSIONS: These data suggest that CD4+ T cells mediating intestinal allograft rejection may be more dependent upon the CD40/CD154 pathway than CD8+ T cells. This finding highlights the importance of identifying agents that suppress CD8+ T cell-mediated rejection.     

Activation and Maturation of Alloreactive CD4-Independent, CD8+ Cytolytic T Cells

The goal of this study was to determine the in vivo conditions that promote activation of the (CD4-independent) CD8+ T cell-mediated rejection pathway. We have previously noted that hepatocellular but not islet allografts readily activate this rejection pathway. In the current study, we utilized these two cell transplant models to investigate whether differences in host cell recruitment to the graft site, expression of T-cell activation markers by CD8+ graft infiltrating cells (GICs), and/or development of delayed-type hypersensitivity (DTH) and cytotoxic T lymphocyte cell-mediated effector functions could account for the differential transplant outcomes. The collective results demonstrate that recruitment of CD8+ T cells to the site of transplant, CD103 or CD69 expression on CD8+ GICs, and activation of alloreactive DTH responses are insufficient to initiate CD4-independent, CD8-dependent transplant rejection. Instead, rejection by alloreactive (CD4-independent) CD8+ T cells correlated with expression of CD25, CD154 and CD43 by CD8+ GICs, in vitro alloproliferation by recipient CD8+ T cells, and the development of in vivo allospecific cytolytic effector function. These results suggest that tissue-derived factors influence the activation and maturation of (CD4-independent) CD8+ T cells into cytolytic effectors, which correlates with transplant rejection.     

The effect of CD28/B7 blockade on alloreactive T and B cells after liver cell transplantation

BACKGROUND: Hepatocyte cell lines are beginning to be developed as universal donors for isolated liver cell transplantation, which is a less invasive method than orthotopic liver transplantation for treatment of metabolic liver disease. The immune response to isolated liver cell transplantation and its modification by costimulatory blockade are as yet not well delineated. METHODS: Adenovirus expressing CTLA4Ig was used to study blockade of the costimulatory CD28/B7 pathway in murine models of hepatocyte transplantation, and the effects on alloreactive T and B cells were studied. RESULTS: CTLA4Ig delayed rejection of subcutaneously administered C57L-derived murine hepatoma cells in CBA/J recipients for >50 days. Activation and cytokine secretion by allospecific CD4+ and CD8+ T cells were initially blocked by CTLA4Ig; delayed rejection was associated with tumor infiltration by CD8+ T cells that did not secrete interferon-gamma. CTLA4Ig failed to block transplant rejection in primed mice, indicating that memory effector T cells were resistant to its action. In contrast, CTLA4Ig suppressed both naive and memory alloreactive B cells. High levels of CTLA4Ig mediated acceptance of hepatoma cells delivered directly into the spleen. However, isolated primary C57BL/6 mouse hepatocytes delivered into the spleen were rejected with only moderately delayed kinetics. CONCLUSIONS: Transplant antigenicity, transplant site, and CTLA4Ig dose all affected the survival of transplanted liver cells. CD8+ T cells are significant mediators of hepatocyte transplant rejection and are relatively resistant to costimulatory blockade with CTLA4Ig. Strategies to specifically antagonize CD8+ T cells or to modulate MHC class I expression in association with costimulatory blockade by CTLA4Ig may enhance the clinical feasibility of transplanting allogeneic hepatocytes.     

CD8+ T cell subsets TC1 and TC2 cause different histopathologic forms of murine cardiac allograft rejection

BACKGROUND: CD4+ T cell effector function is sufficient to mediate allograft rejection, and it is suggested that CD8+ T cell-mediated effects are dependent on CD4+ T cell help. CD8+ T cells can be classified into at least two functional subsets: Tc1, producing high amounts of interferon (IFN)-gamma and Tc2, producing interleukin (IL)-4, -5, -10, and -13 and low levels of IFN-gamma. Because these subsets express different chemokine receptors, they may have different capabilities of migrating into grafts. Once in the graft, each subset may perform different effector functions dependent on the cytokines it produces. We asked whether allospecific CD8+ T cells, in the absence of CD4+ T cells, are capable of mediating rejection of a primarily vascularized allograft, and if Tcl and Tc2 cells differ in their ability to mediate rejection. METHODS: Hearts from H-2d mice were transplanted into H-2b RAG 1-/- recipients. Without manipulation, these fully mismatched allografts would survive indefinitely due to the absence of mature T and B cells. We adoptively transferred allo-(H-2d)-reactive Tcl or Tc2 cells from H-2b mice into each recipient. Grafts were harvested and analyzed on predefined timepoints, rejection was graded on a modified ISHLT scale. RESULTS: On day 7, grafts from Tc1- or Tc2-injected animals showed grade 1-2 parenchymal rejection with stable phenotype and comparable distribution of graft infiltrating CD8+ T cells. Adoptive transfer of IFN-gammahigh Tc1, but not of IFN-gammalow Tc2 cells was followed by the development of graft vasculitis, as well as graft arteriopathy. Adoptive transfer of IL-4high IL-5high Tc2, but not of IL-4low IL-5low Tc1 cells lead to extensive infiltration of eosinophils and formation of giant cells. CONCLUSIONS: Both Tc1 and Tc2 cells can mediate murine cardiac allograft rejection in the absence of CD4+ T cell help, although each subset elicits a different type of inflammatory response. In this model, cytokine secretion of either functional CD8+ T effector cell subset is an important effector mechanism in the process of allograft rejection: IFN-gammahigh Tc1 cells are important in early graft vasculitis, although IL-4high IL-5high Tc2 cells promote recruitment of secondary effectors like eosinophils.     

Further Analysis of the T-Cell Subsets and Pathways of Murine Cardiac Allograft Rejection

The present study examined the role of CD4+ and CD8+ T cells in cardiac allograft rejection when either the direct or indirect pathway was eliminated for the CD4+ portion of the response. To study the pathways in vivo, we used genetically altered mouse strains that lack class II antigens as either the donors or recipients for cardiac transplantation. In contrast to earlier published studies, which used different strain combinations, we found that either CD4- or CD8-depletion prolonged cardiac allograft survival moderately, but not indefinitely, in an MHC-mismatched, minor-matched combination. When the CD4+ indirect pathway was eliminated, rapid graft rejection occurred when both T-cell subsets were present and when either CD4+ or CD8+ T cells were depleted. When the CD4+ direct pathway was eliminated, rapid graft rejection occurred when both T-cell subsets were present, there was slow rejection when CD4+ T cells were eliminated, and no rejection was seen for more than 100 days when CD8+ T cells were eliminated. However, the long-surviving allografts on the recipients with only CD4+ cells and an indirect pathway did show evidence of chronic vasculopathy. Thus, either CD4+ or CD8+ T cells can mediate acute cardiac allograft rejection in these experiments when both pathways are available. In addition, CD4+ T cells can provide help for acute rejection through either the direct or indirect pathway. Finally, recipients who have only CD4+ cells and an indirect pathway do not demonstrate acute rejection, but do show evidence of chronic rejection.     

Acute lung transplant rejection is associated with localized increase in T-cell IFNgamma and TNFalpha proinflammatory cytokines in the airways

BACKGROUND: Allograft rejection remains a major cause of morbidity and mortality after lung transplantation and is associated with increased gene expression for proinflammatory cytokines. T cells are a major cell type involved in graft rejection. There have been no previous studies of cytokine production by T cells from blood, bronchoalveolar lavage (BAL), and intraepithelial T cells from bronchial brushings (BB) during rejection episodes; we hypothesized that T-cell proinflammatory cytokines would be increased in the airways during rejection episodes despite standard immunosuppression regimens. METHOD: To investigate changes in cytokine profiles during rejection episodes, whole blood, BAL, and BB from stable lung transplant patients and those with acute rejection were stimulated in vitro and intracellular cytokine production by CD8- (CD4+) and CD8+ T-cell subsets determined using multiparameter flow cytometry. RESULTS: Transforming growth factor (TGF)-beta was significantly decreased in blood CD4+ and CD8+ T cells while interferon (IFN)-gamma and tumor necrosis factor (TNF)-alpha were significantly increased in BAL CD4+ and CD8+ T cells in patients with evidence of rejection. There was no change in CD4:CD8, interleukin (IL)-2, or IL-4 between stable and rejecting groups. CONCLUSIONS: Acute lung transplant rejection is associated with decreased intracellular T-cell TGFbeta in blood and increased intracellular IFNgamma and TNFalpha in BAL CD4+ and CD8+ T cells. Drugs that effectively reduce airway T-cell IFNgamma and TNFalpha proinflammatory cytokine production may improve current protocols for reducing acute graft rejection in lung transplant patients.     

RNA干扰技术阻断CD40/CD40L共刺激通路对小鼠移植心存活时间的影响

目的 探讨慢病毒介导RNA干扰(RNAi)技术阻断CD40/CD40L共刺激通路对小鼠移植心存活时间的影响.方法 以针对小鼠CD40基因的RNAi慢病毒载体在体外感染供者骨髓来源的树突状细胞(DC),制备低表达CD40的耐受性DC(Tol-DC).荧光实时定量聚合酶链反应及流式细胞术检测DC感染前后CD40 mRNA及DC表面抗原CD40、CD11c和MHCⅡ的表达.建立小鼠异位腹腔心脏移植模型.在小鼠异位心脏移植前7 d,经静脉给受者输注体外制备的低表达CD40的Tol-EC(慢病毒感染DC注射组),并设置单纯移植对照组和未感染DC注射组作为对照.观察各组移植心的存活时间,评定各组术后第7天移植心排斥反应的病理分级.结果 CD40-RNAi慢病毒载体在体外感染DC 48 h后,CD40 mRNA表达明显受到抑制,抑制率为80.9%;CD40表达明显下降,由(74.37±4.08)%降至(40.07±4.03)%(P＜0.05).单纯移植对照组、未感染DC注射组和慢病毒感染DC注射组移植心存活时间分别为:(8±2)d、(9±1)d和(14±4)d,慢病毒感染DC注射组移植心存活时间明显延长,并且移植心排斥反应病理分级显著降低,差异有统计学意义(P＜0.05).结论 阻断CD40/C40L共刺激通路可抑制异系T淋巴细胞的活化,从而抑制急性排斥反应,延长小鼠移植心的存活时间.     

CD4+ T cells responsive through the indirect pathway can mediate skin graft rejection in the absence of interferon-gamma

BACKGROUND: T cells responding through the indirect pathway can induce allograft rejection, but mechanisms of rejection are not known. Interferon-y (IFN-gamma) may be an important mediator of rejection under these circumstances. METHODS: We transferred CD4+ T cells from IFN-gamma-deficient (IFN-gamma-/-) mice into SCID recipients of MHC II-deficient (MHC II-/-) skin grafts. Under these conditions, rejection can only occur via the indirect pathway and cannot be mediated by T-cell production of IFN-gamma. RESULTS: IFN-gamma-/- CD4+ T cells rejected MHC II -/- skin grafts. Flow cytometry revealed only CD4+ T cells in the recipients. Cytokine enzyme-linked immunosorbent spot assays confirmed only indirect recognition with an associated expansion of an alloreactive population of IL-2-, IL-4-, and IL-5-secreting T cells. CONCLUSION: CD4+ T cells recognizing alloantigens via the indirect pathway can mediate skin graft rejection in the absence of IFN-gamma.     

Mechanism of allorecognition and skin graft rejection in CD28 and CD40 ligand double-deficient mice

BACKGROUND: It has been shown that simultaneous blockade of CD28- and CD40-mediated costimulatory signals significantly prolongs allograft survival. Although these results led to an expectation of the establishment of specific immunotolerant therapy for organ transplantation, it became evident that these treatments rarely resulted in indefinite allograft survival. To uncover the mechanisms underlying these costimulation blockade-resistant allograft rejections, we studied the process of allogenic skin graft rejection in CD28 and CD40 ligand (L) double-deficient (double-knockout [dKO]) mice. METHODS: Skin grafts from BALB/c or BALB.B mice were transplanted to C57BL/6 background dKO mice. The frequency of CD4+ and CD8+ T cells responding to alloantigens presented by direct or indirect pathways were defined by the use of a cytostaining assay. RESULTS: BALB/c skin grafts were rapidly rejected by dKO mice. This CD28 and CD40L independent allograft rejection was inhibited by the depletion of CD8+ T cells. In vitro studies indicated that CD8+ T cells from BALB/c skin-grafted dKO mice responded to donor antigen presented only by the direct pathway. Unlike major histocompatibility complex (MHC)-mismatched donors, allogenic skin grafts from MHC-matched donors were accepted by dKO mice. CONCLUSION: In the absence of CD28 and CD40 costimulatory signals, CD8+ T cells recognize MHC antigens by the direct pathway, resulting in the rejection of skin grafts from MHC-mismatched donors. In contrast, MHC-matched and non-MHC-mismatched donor skin grafts indefinitely survive in dKO mice. These results indicated that donor-host MHC matching may still be critical to costimulation blockade therapy for organ transplantation.     

肝癌及癌旁组织中4-1BB分子的表达及意义

目的研究肝癌及癌旁组织中4-1BB分子的表达及意义。方法应用RT-PCR、直接免疫荧光细胞化学法、激光共聚焦显微镜和流式细胞术检测肝癌和癌旁组织以及外周血淋巴细胞中4-1BB分子的表达情况。结果全部肝癌癌旁1cm组织中可检测到4-1BB mRNA，8例肝癌组织和6例癌旁5cm组织中4-1BB mRNA呈弱表达。激光共聚焦显微镜检测到13例癌旁1cm肝组织中浸润淋巴细胞4-1BB^ ／CD4^ 或CD8^ 荧光。两组外周血CD4^ 和CD8^ T细胞表面几乎均未检测到4-1BB分子的表达。结论肿瘤浸润淋巴细胞可能通过4-1BB共刺激信号途径得以有限活化并发挥局部杀伤效应。     

Prolonged survival in rat liver transplantation with mouse monoclonal antibody against an inducible costimulator (ICOS)

BACKGROUND: An inducible costimulator (ICOS), a recently identified costimulatory receptor with a close structural homology to CD28 and CTLA4, is expressed on activated T cells. Interaction with its ligand on antigen-presenting cells stimulates T-cell proliferation to produce a different spectrum of cytokine. The inhibition of ICOS-mediated signal transduction by an anti-ICOS antibody is considered to be capable of protecting against graft rejection in organ transplantation. METHODS: An anti-rat ICOS antibody was intravenously administered into recipients of dark Agouti-to-Lewis liver transplantations. The recipient lymphocytes from mesenteric lymph nodes were harvested on day 7 after transplantation for fluorescence-activated cell sorting analysis, and tissue specimens from the grafts were removed for histologic evaluation. Antigen-specific T-cell proliferation responses were assessed in vitro with anti-ICOS antibody. RESULTS: Monotherapy with the antibody significantly prolonged the graft survival time by inhibiting T-cell activation and its proliferation response. The graft-infiltrating cells, both CD4 and CD8 T cells, were not completely reduced even when rats were administered the antibody, whereas the expression of ICOS almost completely disappeared in these cells. CONCLUSIONS: T-cell activation through the ICOS costimulatory pathway plays an important role in graft rejection, and manipulating its pathway is an effective method for modulating transplantation immunity.     

Roles of CD4+ and CD8+ T cells in discordant skin xenograft rejection

An essential role of murine CD4+ T cells in immune reactivity and skin graft rejection in discordant xenogeneic combinations have been reported. Our study was conducted to further clarify the roles of CD4+ and CD8+ T cells in discordant skin xenograft rejection, by using CD4 and CD8 knockout [C57BL/6 Cr Slc (B6; H-2b) background] mice. When human skins were grafted on CD8 knockout mice or B6 mice, both hosts rejected human skin grafts within 12 days after grafting. By contrast, survival of human skin grafts was significantly prolonged in CD4 knockout mice (mean survival times=19.3+/-(SD) 1.6 days; median 19 days). Fully allogeneic C3H/He Slc (H-2k) skin grafts were rejected within 14 days in CD4 knockout mice, suggesting that non-CD4+ T cells in CD4 knockout mice were immunocompetent for allograft rejection. In spleens of these recipient mice, CD8+ T cells seemed to be activated 10 days after human skin grafting. Immunohistological analysis revealed the infiltration of CD8+ T cells at the site of transplanted human skin on CD4 knockout mice. To further examine the role of CD8+ T cells in CD4 knockout mice, human skin grafting was performed on day 0 followed by administration of anti-CD8 monoclonal antibody on days 0, 5, and 14. The administration of anti-CD8 monoclonal antibodies caused the significant prolongation of human skin graft survival. These results indicate the following two conclusions: (1) CD4+ T cells have an essential role in rejecting discordant human skin xenografts rapidly and (2) however, CD8+ T cells also are capable of rejecting discordant human skin xenografts.     

CD28 downregulation on CD4+ T cells is associated with age of kidney transplant recipient.

There is a growing body of evidence showing that the intensity of rejection is weaker in older kidney allograft recipients while chronic complications, but not rejection, are the main causes of graft loss. To investigate whether the age of the recipient is a factor affecting the expressions of the CD28, CTLA-4, and CD40L costimulatory molecules on CD4+ T cells. Their expression levels were determined in 78 kidney transplant recipients aged 17-68 years. The expression was assessed on unstimulated and anti-CD3 antibody + IL-2-stimulated CD4+ T cells. Median time after transplantation was 20 months and median serum creatinine was 1.5 mg/dl. Significant correlations between age and CD28 expression (r = -0.4, P = 0.0004) on CD4+ T cells and between age and CTLA-4 expression after stimulation (r = 0.34, P = 0.008) were found. CD40L expression on CD4+ T cells was not affected by recipient age. The decreased expression of CD28 and enhanced expression of CTLA-4 (after stimulation) associated with age may be helpful in transplant acceptance.     

In vivo blockade of macrophage migration inhibitory factor prevents skin graft destruction after indirect allorecognition.

BACKGROUND: The effector mechanisms that ultimately destroy transplanted tissues are poorly understood. In particular, it is not clear how CD4+ T cells primed to donor-derived determinants expressed on recipient MHC molecules (the indirect pathway) can mediate graft destruction in the absence of cognate recognition of peptide: MHC on the graft cells themselves. Macrophage migration inhibitory factor (MIF) inhibits macrophage movement and is a proinflammatory and regulatory cytokine known to be essential for development of delayed-type hypersensitivity reactions. METHODS: To test whether MIF participates in graft destruction following indirect recognition, we studied rejection of MHC-II-deficient skin grafts placed on allogeneic SCID recipients adoptively transferred with na?ve CD4+ T cells, and the recipients were treated with neutralizing anti-MIF monoclonal antibody or isotype control IgG. In this model graft rejection can only occur indirectly as the graft cells lack MHC II for recognition by the recipient CD4+ T cells. RESULTS: We found that in vivo blockade of MIF inhibited indirect CD4+ cell-mediated skin graft destruction, and markedly reduced detectable macrophages within the grafts. The neutralizing anti-MIF antibody significantly inhibited alloreactive DTH but did not prevent T cell priming or interferon-gamma release by primed T cells. CONCLUSIONS: The results strongly implicate MIF as an active participant in skin graft destruction after indirect recognition and suggest that this effect is mediated through an inhibition of macrophage migration and/or function.     

Blockade of 4-1BB (CD137)/4-1BB ligand interactions increases allograft survival

We investigated the role of 4-1BB, a T cell co-stimulatory molecule, in alloimmune responses. In vivo mixed lymphocyte reactions showed that 4-1BB was preferentially expressed on actively dividing CD4⁺ and CD8⁺ T cells. Furthermore, following alloantigen challenge, the draining lymph nodes contained subpopulations of 4-1BB-expressing CD4⁺ and CD8⁺ T cells. 4-1BB-deficient C57BL/6 mice showed a delayed rejection of cardiac transplants mismatched for the major histocompatibility complex. Longer transplant survival was induced by blockade of 4-1BB/4-1BB ligand (4-1BBL) interactions using an anti-4-1BBL monoclonal antibody. Histological analysis showed that prolonged transplant survival in the 4-1BB-deficient and anti-4-1BBL-treated mice correlated with reduced lymphocytic infiltration and vasculitis in the donor heart tissue. Taken together, our data suggest that blockade of 4-1BB/4-1BBL interactions inhibited the expansion of alloreactive T cells and reduced CTL activity against host alloantigen, which in turn resulted in the prolongation of allograft survival. Blockade of the 4-1BB co-stimulatory pathway may be useful for preventing allograft rejection.H.R. Cho and B. Kwon contributed equally to this work.     

Integrin antagonists prevent costimulatory blockade-resistant transplant rejection by CD8(+) memory T cells

The success of belatacept in late-stage clinical trials inaugurates the arrival of a new class of immunosuppressants based on costimulatory blockade, an immunosuppression strategy that disrupts essential signals required for alloreactive T-cell activation. Despite having improved renal function, kidney transplant recipients treated with belatacept experienced increased rates of acute rejection. This finding has renewed focus on costimulatory blockade-resistant rejection and specifically the role of alloreactive memory T cells in mediating this resistance. To study the mechanisms of costimulatory blockade-resistant rejection and enhance the clinical efficacy of costimulatory blockade, we developed an experimental transplant system that models a donor-specific memory CD8(+) T-cell response. After confirming that graft-specific memory T cells mediate costimulatory blockade-resistant rejection, we characterized the role of integrins in this rejection. The resistance of memory T cells to costimulatory blockade was abrogated when costimulatory blockade was coupled with either anti-VLA-4 or anti-LFA-1. Mechanistic studies revealed that in the presence of costimulatory blockade, anti-VLA-4 impaired T-cell trafficking to the graft but not memory T-cell recall effector function, whereas anti-LFA-1 attenuated both trafficking and memory recall effector function. As antagonists against these integrins are clinically approved, these findings may have significant translational potential for future clinical transplant trials.     

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.     

Regulatory role of host CD8+ T lymphocytes in experimental graft-versus-host disease across a single major histocompatibility complex class II incompatibility

BACKGROUND: CD8+ T cells are known to regulate type 2 helper T cell (Th2) alloreactive immune responses but their mode of activation is unclear. We investigated the role of host CD8+ T cells in experimental Th2-type graft-versus-host disease (GVHD) where donor/recipient disparity is restricted to a single major histocompatibility complex (MHC) class II antigen. METHODS: Immunoglobulin (Ig) E serum levels, eosinophilia and lymphoid tissue hyperplasia were compared after injection of bm12 CD4+ T cells in either wild-type or CD8+ T cell-deficient (CD8-/-) C57BL/6 mice. In vitro, we explored effects of the addition of CD8+ T cells from wild-type or IFN-gamma-/- mice in mixed leukocyte cultures prepared with beta2 microglobulin-deficient (beta2m-/-) CD4+ T cells as responders or beta2m dendritic cells as stimulators. RESULTS: HyperIgE resolved after 3 weeks in wild-type hosts whereas it persisted for 6 weeks in CD8-/- hosts. Eosinophil infiltrates in lymph nodes were significantly enhanced in CD8-/- hosts. Increased serum levels of IL-5 and IL-13 in CD8-/- hosts confirmed the enhancement of Th2-type responses in the context of recipient CD8+ T cell deficiency. Hyperplasia of lymph nodes and spleen were similar in both groups, as well as in vivo proliferation of donor CD4+ T cells. In vitro, CD8+ T cell regulation of the alloreactive Th2 response depended on their production of IFN-gamma and did not require expression of beta2m on CD4+ T cells or antigen-presenting cells. CONCLUSIONS: Host CD8+ T cells regulate alloreactive Th2 responses during graft-versus-host disease through an IFN-gamma dependent pathway, independently of the recognition of beta2m-associated MHC class I molecules.