IFN-gamma dictates allograft fate via opposing effects on the graft and on recipient CD8 T cell responses

IL-6 inhibition prevents costimulation blockade-resistant allograft rejection in T cell-depleted recipients by promoting intragraft immune regulation in mice

The efficacy of costimulation blockade with CTLA4-Ig (belatacept) in transplantation is limited due to T cell-mediated rejection, which also persists after induction with anti-thymocyte globulin (ATG). Here, we investigate why ATG fails to prevent costimulation blockade-resistant rejection and how this barrier can be overcome. ATG did not prevent graft rejection in a murine heart transplant model of CTLA4-Ig therapy and induced a pro-inflammatory cytokine environment. While ATG improved the balance between regulatory T cells (Treg) and effector T cells in the spleen, it had no such effect within cardiac allografts. Neutralizing IL-6 alleviated graft inflammation, increased intragraft Treg frequencies, and enhanced intragraft IL-10 and Th2-cytokine expression. IL-6 blockade together with ATG allowed CTLA4-Ig therapy to achieve long-term, rejection-free heart allograft survival. This beneficial effect was abolished upon Treg depletion. Combining ATG with IL-6 blockade prevents costimulation blockade-resistant rejection, thereby eliminating a major impediment to clinical use of costimulation blockers in transplantation.

Low dose post-transplant cyclophosphamide and sirolimus induce mixed chimerism with CTLA4-Ig or lymphocyte depletion in an MHC-mismatched murine allotransplantation model

Allogeneic hematopoietic cell transplantation (allo-HCT) offers a curative option for patients with certain non-malignant hematological diseases. High-dose post-transplant cyclophosphamide (PT-Cy) (200 mg/kg) and sirolimus (3 mg/kg), (HiC) synergistically induce stable mixed chimerism. Further, sirolimus and cytotoxic T lymphocyte-associated antigen-4 immunoglobulin (CTLA4-Ig), also known as Abatacept (Aba), promote immune tolerance and allograft survival. Here, in a major histocompatibility complex (MHC)-mismatched allo-HCT murine model, we combined Aba and/or T-cell depleting anti-Thy1.2 (Thy) with a lower dose of PT-Cy (50 mg/kg) and Sirolimus (3 mg/kg), (LoC). While mice in the LoC group showed graft rejection, the addition of Thy to LoC induced similar donor chimerism levels when compared to the HiC group. However, the addition of Aba to LoC led to graft acceptance only in younger mice. When Thy was added to the LoC+Aba setting, graft acceptance was restored in both age groups. Engrafted groups displayed significantly reduced frequencies of recipient-specific interferon-γ-producing T cells as well as an increased frequency in regulatory T cells (Tregs) except in the LoC+Aba group. Splenocytes from engrafted mice showed no proliferation upon restimulation with Balb/c stimulators. Collectively, in combination with Aba or Thy, LoC may be considered to reduce graft rejection in patients who undergo allo-HCT.

IFN-γ Attenuates Spontaneous Lymphocyte Proliferation by Fuelling Regulatory T Cells in HIV-1-Infected Patients

HIV infection is characterized by a high degree of immune activation. It has an impact on CD4 cell count and populations' distribution and function. T regulatory cells (Tregs) were found to play a controversial role in the course of infection because of their beneficial effect on the degree of immune activation and unfavorable influence on the antigen-specific responses. The goal of the present work was to study the relationship among interferon-γ (IFN-γ), spontaneous lymphocyte proliferation, and regulatory T cells in HIV patients receiving therapy. Three lymphocyte populations, isolated after a stepwise magnetic separation from 17 individuals, were investigated-peripheral blood lymphocytes, CD4⁺CD45RA^- (CD4⁺TM), and CD4⁺CD45RA^-CD25^- (TMCD25depl.) cells. The spontaneous, phytohemagglutinin (PHA) and HIV-1p24Ag-stimulated IFN-γ production and the spontaneous lymphocyte proliferation were evaluated. The potential of Tregs to establish a productive infection was determined by measurement of free HIV-1p24 antigen. Two types of constellations among subsets were found. In the first one (in 11 subjects), the spontaneous INF-γ inversely correlated with the spontaneous proliferation in all fractions (r = -0.9, p < 0.001). Conversely, in the second group (six subjects), no associations between the selected parameters were observed. The overall increase in p24-stimulated IFN-γ from TMCD25depl. cells was weak. Four samples: one in Tregs and three in TMCD25depl. cells were positive for the free p24 antigen. No association with the CD4⁺ T cell count, percentage of Tregs, and stage of infection was determined. In conclusion, our results demonstrate that IFN-γ could impact the proliferative capacity of non-Treg cells by fuelling Tregs. Furthermore, Tregs may control the spontaneous lymphocyte proliferation, but are less powerful in the suppression of Ag-specific IFN-γ production from non-Treg lymphocytes. The direct viral influence on Treg functions should be also considered.

Effector T Cells and Ischemia-Induced Systemic Angiogenesis in the Lung

Lymphocytes have been shown to modulate angiogenesis. Our previous work showed that T regulatory (Treg) cell depletion prevented angiogenesis. In the present study, we sought to examine T-cell populations during lung angiogenesis and subsequent angiostasis. In a mouse model of ischemia-induced systemic angiogenesis in the lung, we examined the time course (0-35 d) of neovascularization and T-cell phenotypes within the lung after left pulmonary artery ligation (LPAL). T cells increased and reached a maximum by 10 days after LPAL and then progressively decreased, suggestive of a modulatory role during the early phase of new vessel growth. Because others have shown IFN-γ to be angiostatic in tumor models, we focused on this effector T-cell cytokine to control the magnitude of angiogenesis. Results showed that IFN-γ protein is secreted at low levels after LPAL and that mice required Treg depletion to see the full effect of effector T cells. Using Foxp3(DTR) and diphtheria toxin to deplete T regulatory cells, increased numbers of effector T cells (CD8(+)) and/or increased capacity to secrete the prominent angiostatic cytokine IFN-γ (CD4(+)) were seen. In vitro culture of mouse systemic and pulmonary microvascular endothelial cells with IFN-γ showed increased endothelial cell apoptosis. CD8(-/-) mice and IFN-γR(-/-) mice showed enhanced angiogenesis compared with wild-type mice, confirming that, in this model, IFN-γ limits the extent of systemic neovascularization in the lung.

Interferon-γ, interleukin-10 and interferon-inducible protein 10 (CXCL10) as serum biomarkers for the early allograft dysfunction after liver transplantation

Orthotopic liver transplantation (LTP) is nowadays a standard procedure, and provides the chance of survival of patients with end-stage non-treatable chronic liver disease or acute liver failure. Despite long-term survival with a good quality of life in the majority of patients, about 20% develop early allograft dysfunction (EAD), which leads to death or the need for re-transplantation. Therefore, the early diagnosis of EAD and evaluation of its risk factors are very important. Many primary pathological processes leading to EAD are accompanied by the release of different mediators and by a change of biochemical parameters detectable in the peripheral blood. The aim of this study was to investigate cytokines as well as soluble mediators in the serum of patients with and without EAD from our LTP bank, and to evaluate their predictive and prognostic values for EAD. We demonstrated for the first time that the level of IFNγ during the nearest preoperative period may serve as a predictive parameter for EAD. We additionally found that IL-10 and CXCL10 (IP-10) levels in the early postoperative period can be prognostic for EAD. We believe our data expand the spectrum of predictive and prognostic parameters for EAD in LTP.

CD8(+)IL-17(+) T Cells Mediate Neutrophilic Airway Obliteration in T-bet-Deficient Mouse Lung Allograft Recipients

Acute cellular rejection is a known risk factor for the development of obliterative bronchiolitis, which limits the long-term survival of lung transplant recipients. However, the T cell effector mechanisms in both of these processes remain incompletely understood. Using the mouse orthotopic lung transplant model, we investigated whether C57BL/6 T-bet(-/-) recipients of major histocompatibility complex (MHC)-mismatched BALB/c lung grafts develop rejection pathology and allospecific cytokine responses that differ from wild-type mice. T-bet(-/-) recipients demonstrated vigorous allograft rejection at 10 days, characterized by neutrophilic inflammation and predominantly CD8(+) T cells producing allospecific IL-17 and/or IFN-γ, in contrast to IFN-γ-dominant responses in WT mice. CD4(+) T cells produced IL-17 but not IFN-γ responses in T-bet(-/-) recipients, in contrast to WT controls. Costimulation blockade using anti-CD154 Ab significantly reduced allospecific CD8(+)IFN-γ(+) responses in both T-bet(-/-) and WT mice but had no attenuating effect on lung rejection pathology in T-bet(-/-) recipients or on the development of obliterative airway inflammation that occurred only in T-bet(-/-) recipients. However, neutralization of IL-17A significantly attenuated costimulation blockade-resistant rejection pathology and airway inflammation in T-bet(-/-) recipients. In addition, CXCL1 (neutrophil chemokine) was increased in T-bet(-/-) allografts, and IL-17 induced CXCL1 from mouse lung epithelial cells in vitro. Taken together, our data show that T-bet-deficient recipients of complete MHC-mismatched lung allografts develop costimulation blockade-resistant rejection characterized by neutrophilia and obliterative airway inflammation that is predominantly mediated by CD8(+)IL-17(+) T cells. Our data support T-bet-deficient mouse recipients of lung allografts as a viable animal model to study the immunopathogenesis of small airway injury in lung transplantation.

Donor CD4 T cells trigger costimulation blockade-resistant donor bone marrow rejection through bystander activation requiring IL-6

Bone marrow (BM) transplantation under costimulation blockade induces chimerism and tolerance. Cotransplantation of donor T cells (contained in substantial numbers in mobilized peripheral blood stem cells and donor lymphocyte infusions) together with donor BM paradoxically triggers rejection of donor BM through undefined mechanisms. Here, nonmyeloablatively irradiated C57BL/6 recipients simultaneously received donor BM (BALB/c) and donor T cells under costimulation blockade (anti-CD154 and CTLA4Ig). Donor CD4, but not CD8 cells, triggered natural killer-independent donor BM rejection which was associated with increased production of IL-6, interferon gamma (IFN-γ) and IL-17A. BM rejection was prevented through neutralization of IL-6, but not of IFN-γ or IL-17A. IL-6 counteracted the antiproliferative effect of anti-CD154 in vitro. Rapamycin and anti-lymphocyte function-associated antigen 1 negated this effect of IL-6 in vitro and prevented BM rejection in vivo. Simultaneous cotransplantation of (BALB/cxB6)F1, recipient or irradiated donor CD4 cells, or late transfer of donor CD4 cells did not lead to BM rejection, whereas cotransplantation of third party CD4 cells did. Transferred donor CD4 cells became activated, rapidly underwent apoptosis and triggered activation and proliferation of recipient T cells. Collectively, these results provide evidence that donor T cells recognizing the recipient as allogeneic lead to the release of IL-6, which abolishes the effect of anti-CD154, triggering donor BM rejection through bystander activation.

2B4 (CD244) induced by selective CD28 blockade functionally regulates allograft-specific CD8+ T cell responses

Blockade of CD28 signals results in the up-regulation of 2B4 on primary CD8⁺ effectors and plays a critical role in controlling antigen-specific CD8⁺ T cell responses.

Mounting evidence in models of both autoimmunity and chronic viral infection suggests that the outcome of T cell activation is critically impacted by the constellation of co-stimulatory and co-inhibitory receptors expressed on the cell surface. Here, we identified a critical role for the co-inhibitory SLAM family member 2B4 (CD244) in attenuating primary antigen-specific CD8⁺ T cell responses in the presence of immune modulation with selective CD28 blockade. Our results reveal a specific up-regulation of 2B4 on antigen-specific CD8⁺ T cells in animals in which CD28 signaling was blocked. However, 2B4 up-regulation was not observed in animals treated with CTLA-4 Ig (abatacept) or CD28 blockade in the presence of anti–CTLA-4 mAb. 2B4 up-regulation after CD28 blockade was functionally significant, as the inhibitory impact of CD28 blockade was diminished when antigen-specific CD8⁺ T cells were deficient in 2B4. In contrast, 2B4 deficiency had no effect on CD8⁺ T cell responses during unmodified rejection or in the presence of CTLA-4 Ig. We conclude that blockade of CD28 signals in the presence of preserved CTLA-4 signals results in the unique up-regulation of 2B4 on primary CD8⁺ effectors, and that this 2B4 expression plays a critical functional role in controlling antigen-specific CD8⁺ T cell responses.

Elevated levels of interferon-γ production by memory T cells do not promote transplant tolerance resistance in aged recipients

Immunosenescence predisposes the elderly to infectious and autoimmune diseases and impairs the response to vaccination. We recently demonstrated that ageing also impedes development of transplantation tolerance. Unlike their young counterparts (8-12 weeks of age) aged male recipients (greater than 12 months of age) transplanted with a full MHC-mismatched heart are resistant to tolerance mediated by anti-CD45RB antibody. Surprisingly, either chemical or surgical castration restored tolerance induction to levels observed using young recipients. Based on the strong impact of endocrine modulation on transplant tolerance, we explored the impact of ageing and castration on the immune system. Here we report a significant increase in the percentage of T cells that produce interferon-γ (IFN-γ) in aged male versus young male animals and that the overall increase in IFN-γ production was due to an expansion of IFN-γ-producing memory T cells in aged animals. In contrast to IFN-γ production, we did not observe differences in IL-10 expression in young versus old male mice. We hypothesized that endocrine modulation would diminish the elevated levels of IFN-γ production in aged recipients, however, we observed no significant reduction in the percentage of IFN-γ+ T cells upon castration. Furthermore, we neutralized interferon-γ by antibody and did not observe an effect on graft survival. We conclude that while elevated levels of interferon-γ serves as a marker of tolerance resistance in aged mice, other as yet to be identified factors are responsible for its cause. Defining these factors may be relevant to design of tolerogenic strategies for aged recipients.

Heterologous immunity triggered by a single, latent virus in Mus musculus: combined costimulation- and adhesion- blockade decrease rejection

The mechanisms underlying latent-virus-mediated heterologous immunity, and subsequent transplant rejection, especially in the setting of T cell costimulation blockade, remain undetermined. To address this, we have utilized MHV68 to develop a rodent model of latent virus-induced heterologous alloimmunity. MHV68 infection was correlated with multimodal immune deviation, which included increased secretion of CXCL9 and CXCL10, and with the expansion of a CD8(dim) T cell population. CD8(dim) T cells exhibited decreased expression of multiple costimulation molecules and increased expression of two adhesion molecules, LFA-1 and VLA-4. In the setting of MHV68 latency, recipients demonstrated accelerated costimulation blockade-resistant rejection of skin allografts compared to non-infected animals (MST 13.5 d in infected animals vs 22 d in non-infected animals, p<.0001). In contrast, the duration of graft acceptance was equivalent between non-infected and infected animals when treated with combined anti-LFA-1/anti-VLA-4 adhesion blockade (MST 24 d for non-infected and 27 d for infected, p = n.s.). The combination of CTLA-4-Ig/anti-CD154-based costimulation blockade+anti-LFA-1/anti-VLA-4-based adhesion blockade led to prolonged graft acceptance in both non-infected and infected cohorts (MST>100 d for both, p<.0001 versus costimulation blockade for either). While in the non-infected cohort, either CTLA-4-Ig or anti-CD154 alone could effectively pair with adhesion blockade to prolong allograft acceptance, in infected animals, the prolonged acceptance of skin grafts could only be recapitulated when anti-LFA-1 and anti-VLA-4 antibodies were combined with anti-CD154 (without CTLA-4-Ig, MST>100 d). Graft acceptance was significantly impaired when CTLA-4-Ig alone (no anti-CD154) was combined with adhesion blockade (MST 41 d). These results suggest that in the setting of MHV68 infection, synergy occurs predominantly between adhesion pathways and CD154-based costimulation, and that combined targeting of both pathways may be required to overcome the increased risk of rejection that occurs in the setting of latent-virus-mediated immune deviation.

Do Natural T Regulatory Cells become Activated to Antigen Specific T Regulatory Cells in Transplantation and in Autoimmunity?

Antigen specific T regulatory cells (T_reg) are often CD4⁺CD25⁺FoxP3⁺ T cells, with a phenotype similar to natural T_reg (nT_reg). It is assumed that nT_reg cannot develop into an antigen specific T_reg as repeated culture with IL-2 and a specific antigen does not increase the capacity or potency of nT_reg to promote immune tolerance or suppress in vitro. This has led to an assumption that antigen specific T_reg mainly develop from CD4⁺CD25⁻FoxP3⁻ T cells, by activation with antigen and TGF-β in the absence of inflammatory cytokines such as IL-6 and IL-1β. Our studies on antigen specific CD4⁺CD25⁺ T cells from animals with tolerance to an allograft, identified that the antigen specific and T_reg are dividing, and need continuous stimulation with specific antigen T cell derived cytokines. We identified that a variety of cytokines, especially IL-5 and IFN-γ but not IL-2 or IL-4 promoted survival of antigen specific CD4⁺CD25⁺FoxP3⁺ T_reg. To examine if nT_reg could be activated to antigen specific T_reg, we activated nT_reg in culture with either IL-2 or IL-4. Within 3 days, antigen specific T_reg are activated and there is induction of new cytokine receptors on these cells. Specifically nT_reg activated by IL-2 and antigen express the interferon-γ receptor (IFNGR) and IL-12p70 (IL-12Rβ2) receptor but not the IL-5 receptor (IL-5Rα). These cells were responsive to IFN-γ or IL-12p70. nT_reg activated by IL-4 and alloantigen express IL-5Rα not IFNGR or IL-12p70Rβ2 and become responsive to IL-5. These early activated antigen specific T_reg, were respectively named Ts1 and Ts2 cells, as they depend on Th1 or Th2 responses. Further culture of Ts1 cells with IL-12p70 induced Th1-like T_reg, expressing IFN-γ, and T-bet as well as FoxP3. Our studies suggest that activation of nT_reg with Th1 or Th2 responses induced separate lineages of antigen specific T_reg, that are dependent on late Th1 and Th2 cytokines, not the early cytokines IL-2 and IL-4.

Human mesenchymal stem cells shift CD8+ T cells towards a suppressive phenotype by inducing tolerogenic monocytes

The mechanisms underlying the immunomodulatory effects of mesenchymal stem cells (MSCs) have been investigated under extreme conditions of strong T cell activation, which induces the rapid death of activated lymphocytes. The objective of this study was to investigate these mechanisms in the absence of additional polyclonal activation. In co-cultures of peripheral mononuclear blood cells with human MSCs (hereafter referred to as hMSCs), we observed a striking decrease in the level of CD8 expression on CD8+ cells, together with decreased expression of CD28 and CD44, and impaired production of IFN-gamma and Granzyme B. This effect was specific to hMSCs, because it was not observed with several other cell lines. Downregulation of CD8 expression required CD14+ monocytes to be in direct contact with the CD8+ cells, whereas the effects of hMSCs on the CD14+ cells were essentially mediated by soluble factors. The CD14+ monocytes exhibited a tolerogenic pattern when co-cultured with hMSCs, with a clear decrease in CD80 and CD86 co-stimulatory molecules, and an increase in the inhibitory receptors ILT-3 and ILT-4. CD8+ cells that were preconditioned by MSCs had similar effects on monocytes and were able to inhibit lymphocyte proliferation. Injection of hMSCs in humanized NSG mice showed similar trends, in particular decreased levels of CD44 and CD28 in human immune cells. Our study demonstrates a new immunomodulation mechanism of action of hMSCs through the modulation of CD8+ cells towards a non-cytotoxic and/or suppressive phenotype. This mechanism of action has to be taken into account in clinical trials, where it should be beneficial in grafts and autoimmune diseases, but potentially detrimental in malignant diseases.

MHC class Ib-restricted CD8 T cells differ in dependence on CD4 T cell help and CD28 costimulation over the course of mouse polyomavirus infection

We recently identified a protective MHC class Ib-restricted CD8 T cell response to infection with mouse polyomavirus. These CD8 T cells recognize a peptide from aa 139-147 of the VP2 viral capsid protein bound to the nonpolymorphic H-2Q9 molecule, a member of the Qa-2 family of β(2)m-associated MHC class Ib molecules. Q9:VP2.139-specific CD8 T cells exhibit an unusual inflationary response characterized by a gradual expansion over 3 mo followed by a stable maintenance phase. We previously demonstrated that Q9:VP2.139-specific CD8 T cells are dependent on Ag for expansion, but not for long-term maintenance. In this study, we tested the hypothesis that the expansion and maintenance components of the Q9:VP2.139-specific T cell response are differentially dependent on CD4 T cell help and CD28 costimulation. Depletion of CD4(+) cells and CD28/CD40L blockade impaired expansion of Q9:VP2.139-specific CD8 T cells, and intrinsic CD28 signaling was sufficient for expansion. In contrast, CD4 T cell insufficiency, but not CD28/CD40L blockade, resulted in a decline in frequency of Q9:VP2.139-specific CD8 T cells during the maintenance phase. These results indicate that the Q9:VP2.139-specific CD8 T cell response to mouse polyomavirus infection depends on CD4 T cell help and CD28 costimulation for inflationary expansion, but only on CD4 T cell help for maintenance.

A whole blood monokine-based reporter assay provides a sensitive and robust measurement of the antigen-specific T cell response

BACKGROUND

The ability to measure T-cell responses to antigens is proving critical in the field of vaccine development and for understanding immunity to pathogens, allergens and self-antigens. Although a variety of technologies exist for this purpose IFNγ-ELISpot assays are widely used because of their sensitivity and simplicity. However, ELISpots cannot be performed on whole blood, and require relatively large volumes of blood to yield sufficient numbers of peripheral blood mononuclear cells. To address these deficiencies, we describe an assay that measures antigen-specific T cell responses through changes in monokine gene transcription. The biological amplification of the IFNγ signal generated by this assay provides sensitivity comparable to ELISpot, but with the advantage that responses can be quantified using small volumes of whole blood.

METHODS

Whole blood or peripheral blood mononuclear cells (PBMCs) from healthy controls and immunosuppressed recipients of solid organ transplants were incubated with peptide pools covering viral and control antigens or mitogen for 20 hours. Total RNA was extracted and reverse transcribed before amplification in a TaqMan qPCR reaction using primers and probes specific for MIG (CXCL9), IP-10 (CXCL10) and HPRT. The induction of MIG and IP-10 in response to stimuli was analysed and the results were compared with those obtained by ELISpot.

RESULTS

Antigen-specific T cell responses can be measured through the induction of MIG or IP-10 gene expression in PBMCs or whole blood with results comparable to those achieved in ELISpot assays. The biological amplification generated by IFNγ-R signaling allows responses to be detected in as little as 25 uL of whole blood and enables the assay to retain sensitivity despite storage of samples for up to 48 hours prior to processing.

CONCLUSIONS

A monokine-based reporter assay provides a sensitive measure of antigen-specific T cell activation. Assays can be performed on small volumes of whole blood and remain accurate despite delays in processing. This assay may be a useful tool for studying T cell responses, particularly when samples are limited in quantity or when storage or transportation are required before processing.

CD28 blockade induces division-dependent downregulation of interleukin-2 receptor alpha

BACKGROUND

Blockade of T cell costimulatory molecules represents a promising new method of attenuating donor-reactive T cell responses to promote graft survival following transplantation. However, recent studies in murine models have shown the presence of an initial high frequency of naïve donor-reactive T cells may render this strategy ineffective.

METHODS

In this report, we examined the phenotypic changes associated with CD28 blockade on T cells stimulated at increasing precursor frequencies in vitro.

RESULTS

We found that treatment with the CD28 blocker CTLA-4 Ig resulted in downregulation of the alpha chain of the IL-2 receptor (CD25) in a division-dependent manner. Significantly, blockade of the CD28 pathway was more effective in down-regulating CD25 when the donor-reactive T cell population was present at low as compared to high precursor frequency.

CONCLUSIONS

These results imply that treatment with CD28 blockers and anti-CD25 mAbs may cooperate in promoting graft survival under conditions of low MHC matching where the donor-reactive T cell precursor frequency is high.

Transplant long-surviving induced by CD40-CD40 ligand costimulation blockade is dependent on IFN-γ through its effect on CD4(+)CD25(+) regulatory T cells

BACKGROUND

IFN-γ was documented to be commonly associated with acute rejection. In the present study, we investigated the role of IFN-γ in the transplant long-surviving induced by blocking CD40-CD40 ligand (CD40-CD40L) costimulation and its mechanisms.

METHODS

IFN-γ expression in cardiac allografts and spleens from syngeneic and allogeneic recipients with or without anti-CD40L monoclonal antibody (MR-1) treatment was examined by real-time RT-PCR. The grafts survival time in Wild type (IFN-γ(+/+)) and IFN-γ deficient (IFN-γ(-/-)) recipients was investigated. Mixed lymphocyte reaction (MLR) of CD4(+) T cells and cytotoxic T lymphocyte (CTL) assay of CD8(+) T cells were also studied. FoxP3 expression in allografts and spleens from IFN-γ(+/+) or IFN-γ(-/-) recipients with MR-1 treatment was examined. Furthermore, FoxP3, IL-10 and CTLA-4 expressions and the suppressive capability of CD4(+)CD25(+) regulatory T cells were examined.

RESULTS

Rejected allografts showed significantly higher IFN-γ expression than long-surviving allografts. Allograft survival was not prolonged in nonimmunosuppressed IFN-γ(-/-) mice. Administration of MR-1 induced long-term survival in 90.1% of IFN-γ(+/+) recipients (98±6.6 days) but failed to do so in IFN-γ(-/-) group (16.2±4.0 days). IFN-γ(-/-) recipients facilitated the proliferation and CTL generation of T cells. The allografts and spleens from IFN-γ(+/+) recipients contained higher FoxP3 expression than IFN-γ(-/-) recipients. Moreover, CD4(+)CD25(+) T cells from IFN-γ(+/+) recipients displayed a higher FoxP3 and IL-10 expression and suppressive capability.

CONCLUSION

IFN-γ plays an important role in the long-surviving induced by blocking CD40-CD40L through inhibiting the function of activated T cells and increasing suppressive capability of CD4(+)CD25(+) regulatory T cells.

Interferon gamma licensing of human dendritic cells in T-helper-independent CD8+ alloimmunity

The high frequency of allogeneic reactive CD8(+) T cells in human and their resistance to immunosuppression might be one of the reasons why successful tolerance-inducing strategies in rodents have failed in primates. Studies on the requirement for T-helper cells in priming CD8(+) T-cell responses have led to disparate findings. Recent studies have reported CD8(+)-mediated allograft rejection independently of T-helper cells; however, the mechanisms that govern the activation of these T cells are far from being elucidated. In this study, we demonstrated that lipopolysaccharide-treated dendritic cells (DCs) were able to induce proliferation and cytotoxic activity of allogeneic CD8(+) T cells independently of CD4(+) T cells, while adding mycophenolic acid (MPA) to LPS abolished this capacity and resulted in anergic CD8(+) T cells that secreted high levels of interleukin-4 (IL-4), IL-5, IL-10, and transforming growth factor-β. Interestingly, we demonstrated that MPA inhibited the LPS-induced synthesis of tumor necrosis factor-α, IL-12, and interferon-γ (IFN-γ) in DCs. Importantly, we found that adding exogenous IFN-γ to MPA restored both the synthesis of cytokines and the ability to activate CD8(+) T cells. However, adding IL-12 or tumor necrosis factor-α had no effect. These results suggest that IFN-γ has an important role in licensing DCs to prime CD4-independent CD8 allogeneic T cells via an autocrine loop.

IFN-gamma-receptor signaling ameliorates transplant vasculopathy through attenuation of CD8+ T-cell-mediated injury of vascular endothelial cells

Occlusive transplant vasculopathy (TV) is the major cause for chronic graft rejection. Since endothelial cells (EC) are the first graft cells encountered by activated host lymphocytes, it is important to delineate the molecular mechanisms that coordinate the interaction of EC with activated T cells. Here, the interaction of CD8(+) T cells with Ag-presenting EC in vivo was examined using a transgenic heart transplantation model with beta-galactosidase (beta-gal) expression exclusively in EC (Tie2-LacZ hearts). We found that priming with beta-gal peptide-loaded DC failed to generate a strong systemic IFN-gamma response, but elicited pronounced TV in both IFN-gamma receptor (IFNGR)-competent, and ifngr(-/-) Tie2-LacZ hearts. In contrast, stimulation of EC-specific CD8(+) T cells with beta-gal-recombinant mouse cytomegalovirus (MCMV-LacZ) in recipients of ifngr(+/+) Tie2-LacZ hearts did not precipitate significant TV. However, MCMV-LacZ infection of recipients of ifngr(-/-) Tie2-LacZ hearts led to massive activation of beta-gal-specific CD8 T cells, and led to development of fulminant TV. Further analyses revealed that the strong systemic IFN-gamma "storm" associated with MCMV infection induced upregulation of programmed death-1 ligand 1 (PD-L1) on EC, and subsequent attenuation of programmed death-1 (PD-1)-expressing EC-specific CD8(+) T cells. Thus, IFNGR signaling in ECs activates a potent peripheral negative feedback circuit that protects vascularized grafts from occlusive TV.

Translating costimulation blockade to the clinic: lessons learned from three pathways

SUMMARY

As the recognition that costimulatory signals are critical for optimal T-cell activation, proliferation, and differentiation, there has been an explosion in the study of costimulatory molecules and their roles in enhancing anti-donor T-cell responses following transplantation. Here, we focus on the bench-to-beside translation of blocking agents designed to target three critical costimulatory pathways: the CD28/CD80/CD86 pathway, the CD154/CD40 pathway, and the lymphocyte function associated antigen-1/intercellular adhesion molecule pathway. While blockade of each of these pathways proved promising in inhibiting donor-reactive T-cell responses and promoting long-term graft survival in murine models of transplantation, the progression of development of therapeutic agents to block these pathways has each taken a slightly different course. Both logistical and biological pitfalls have accompanied the translation of blockers of all three pathways into clinically applicable therapies, and the development of costimulatory blockade as a substitute for current standard-of-care calcineurin inhibitors has by no means reached completion. Collaboration between both the basic and clinical arenas will further propel the development of costimulation blockers currently in the pipeline, as well as of novel methods to target these critical pathways during transplantation.