Memory T Cells and Their Costimulators in Human Allograft Injury

Participation of blood vessel cells in human adaptive immune responses

Circulating T cells contact blood vessels either when they extravasate across the walls of microvessels into inflamed tissues or when they enter into the walls of larger vessels in inflammatory diseases such as atherosclerosis. The blood vessel wall is largely composed of three cell types: endothelial cells lining the entire vascular tree; pericytes supporting the endothelium of microvessels; and smooth muscle cells forming the bulk of large vessel walls. Each of these cell types interacts with and alters the behavior of infiltrating T cells in different ways, making these cells active participants in the processes of immune-mediated inflammation. In this review, we compare and contrast what is known about the nature of these interactions in humans.

Selective serotonin reuptake inhibitors attenuate the antigen presentation from dendritic cells to effector T lymphocytes

Fluoxetine, one of the selective serotonin reuptake inhibitors (SSRIs), has been found to possess immune modulation effects, in addition to its antidepressant effects. However, it remains unclear whether SSRIs can suppress the antigen-presenting function of dendritic cells (DCs). Therefore, Fluoxetine was applied to a co-culture of Aggregatibacter actinomycetemcomitans (Aa)-reactive T cells (×Aa-T) isolated from Aa-immunized mice and DCs. This resulted in the suppressed proliferation of ×Aa-T stimulated with Aa-antigen presentation by DCs. Specifically, Fluoxetine increased the extracellular 5-hydroxytryptamine (5-HT) in the ×Aa-T/DC co-culture, whereas exogenously applied 5-HT promoted T-cell proliferation in the ×Aa-T/DC co-culture, indicating that Fluoxetine-mediated suppression of ×Aa-T/DC responses cannot be attributed to extracellular 5-HT. Instead, Fluoxetine remarkably suppressed the expression of costimulatory molecule ICOS-L on DCs. Fluoxetine also promoted a greater proportion of CD86(Low) immature DCs than CD86(High) mature DCs, while maintaining the expression levels of CD80, MHC-class-II and PD-L1. These results suggested that Fluoxetine suppressed the ability of DCs to present bacterial antigens to T cells, and the resulting T-cell proliferation, in a SERT/5-HT-independent manner and that diminished expression of ICOS-L on DCs and increase of CD86(Low) immature DCs caused by Fluoxetine might be partially associated with Fluoxetine-mediated suppression of DC/T-cell responses.

Immunological function of the endothelial cell within the setting of organ transplantation

In organ transplantation, development of immunosuppressive treatment and improved diagnosis of allograft rejection has resulted in increased allograft survival in recent years. Nevertheless, rejection remains a major cause of graft loss and a better understanding of the characteristics of the allo-immune response is required to identify new diagnostic and therapeutic tools. The allogeneic immune response depends upon a major family of antigenic targets: the Major Histocompatibility Complex molecules (MHC) which are present on donor cells. These molecules are targets of both the humoral and cellular arms of the graft recipient's immune system: T lymphocytes which are implicated in acute cellular rejection and antibodies which are implicated in antibody-mediated rejection (AMR). Allo-recognition of allograft MHC antigens by either T cells or allo-antibodies is the primary event which can ultimately lead to graft rejection. Although immunosuppressive strategies have mainly focused on the T cell response and acute cellular rejection has therefore become relatively rare, antibody mediated rejection (AMR) remains resistant to conventional immunosuppressive treatment and results in frequent graft loss. Damage to the endothelium is a prominent histological feature of AMR underlining the involvement of endothelial cells in initiating the allo-immune response. Furthermore, endothelial cells express both HLA class I and class II molecules in the context of organ transplantation endowing them with the capacity to present antigen to the recipient T cells. The endothelium should therefore be viewed both as a stimulator of, and as a target for allo-immune responses. In this review, we will summarize current knowledge about the implication of endothelial cells in the allo-immune response in the context of organ transplantation.

CD155 on Human Vascular Endothelial Cells Attenuates the Acquisition of Effector Functions in CD8 T Cells

Objective—

CD155 is a cell surface protein that has recently been described to exert immune regulatory functions. We have characterized the expression of CD155 on human vascular endothelial cells (ECs) and examined its role in the regulation of T-cell activation.

Methods and Results—

CD155 was expressed on resting human vascular ECs and was upregulated in an interferon-γ (IFNγ)–dependent manner. When the function of CD155 in regulating T-cell activation was examined, antibody-mediated neutralization of CD155 did not affect CD8 T-cell proliferation in response to stimulation with ECs. However, neutralization of CD155 activity or small interfering RNA-mediated inhibition of CD155 expression in ECs increased expression of IFNγ and cytotoxic effector function in activated CD8 T cells.

Conclusion—

CD155 is an IFNγ-inducible immune regulatory protein on the surface of human ECs that attenuates the acquisition of effector functions in CD8 T cells.

Cancer immunotherapy using a bispecific NK receptor fusion protein that engages both T cells and tumor cells

T-cell immunotherapy is a promising strategy to treat cancer, but its efficacy, complexity, and costs may pose challenges. In this study, we report the results of an investigation of a new approach to selectively activate a T-cell attack against tumor cells. The immunotherapeutic approach we developed utilizes a bifunctional fusion protein that binds tumor cells through NK (natural killer)-activating receptor NKG2D and that recruits and stimulates T cells through an anti-CD3 single-chain variable fragment (scFv-NKG2D). In vitro, this scFv-NKG2D fusion protein engaged both T cells and tumor cells, stimulating T cells to produce IFN-γ, and cytotoxicity against NKG2D ligand-positive tumor cells. In vivo, expression of scFv-NKG2D by NKG2D ligand-positive tumor cells reduced tumor burden and, in some cases, led to tumor-free survival. Administration of scFv-NKG2D in vivo also promoted survival in a murine lymphoma model. Tumor-free mice were resistant to rechallenge with cognate tumor cells, suggesting that a host-specific immunologic memory response had been generated. Host adaptive immunity including γδ T cells was required for scFv-NKG2D-mediated therapeutic efficacy. ScFv-NKG2D also inhibited the growth of NKG2D ligand-negative B16F10 tumors, reduced the percentage of myeloid-derived suppressor cells and regulatory T cells, and increased the infiltration of T cells, suggesting that scFv-NKG2D may target these immune suppressive cells. Together, these results establish scFv-NKG2D as a promising biological fusion protein to induce effective antitumor immunity.

Human endothelial cells generate Th17 and regulatory T cells under inflammatory conditions

Organ transplantation represents a unique therapeutic option for irreparable organ dysfunction and rejection of transplants results from a breakdown in operational tolerance. Although endothelial cells (ECs) are the first target in graft rejection following kidney transplantation, their capacity to alloactivate and generate particular T lymphocyte subsets that could intervene in this process remains unknown. By using an experimental model of microvascular endothelium, we demonstrate that, under inflammatory conditions, human ECs induced proliferation of memory CD4(+)CD45RA(-) T cells and selectively amplified proinflammatory Th17 and suppressive CD45RA(-)HLA-DR(+)FoxP3(bright) regulatory CD4(+) T lymphocytes (Tregs). Although HLA-DR expression on resting microvascular ECs was sufficient to induce proliferation of memory CD4(+) T cells, Treg amplification was dependent on the interaction with CD54, highly expressed only under inflammatory conditions. Moreover, expansion of Th17 cells was dependent on IL-6 and STAT-3, and inhibition of either specifically impaired Th17, without altering Treg expansion. Collectively these data reveal that the HLA-DR(+) ECs regulate the local inflammatory allogeneic response, promoting either an IL-6/STAT-3-dependent Th17 response or a contact-CD54-dependent regulatory response according to the cytokine environment. Finally, these data open therapeutic perspectives in human organ transplantation based on targeting the IL-6/STAT-3 pathway and/or promoting CD54 dependent Treg proliferation.

Human placental pericytes poorly stimulate and actively regulate allogeneic CD4 T cell responses

OBJECTIVE

Cell-mediated immune responses in peripheral tissues begin with T cell infiltration through endothelial cell (EC) microvessels and accumulation in the perivascular space occupied by pericytes (PC). Here, we investigate how human T cells interact with PC.

METHODS AND RESULTS

We compared human placental PC with autologous umbilical vein EC. Cultured PC express lower levels of major histocompatibility complex (MHC) and positive costimulatory molecules but higher levels of negative costimulatory molecules than do EC. Unlike EC, interferon-γ-treated MHC class II-positive PC (PC(+)) cannot stimulate resting allogeneic CD4 T cell proliferation or cytokine production. Instead, coculture of resting CD4 T cells with PC(+) induces CD25 expression and renders T cells unresponsive to restimulation by EC(+) from the same donor. PC cultured across a semi-permeable membrane decrease alloreactive CD4 T cell proliferation to EC(+), an effect enhanced by pretreatment of PC with interferon-γ and partially reversed by interleukin-10 and transforming growth factor-β neutralization, but do not induce anergy.

CONCLUSIONS

Human placental PC are poorly immunogenic and negatively regulate CD4 T cell responses through contact-dependent and contact-independent mechanisms.

The recall alloresponse following retransplantation is more intense compared with the T cell memory-transfer model

The presence of alloreactive memory T cells in recipient is a critical handicap to achieving transplantation tolerance. To make a mouse model that can as closely as possible mimic the presensitized transplant patient is important for research on this subject. Thus, we developed a novel retransplant model and compared the alloresponse in this model with that in the memory T cells-transfer model (transfer control). Mean survival time of allograft was compared between 3 groups, including blank transplant control, memory transfer control and retransplant groups. Cellular rejection activity in allografts was evaluated via HE staining of cardiac graft section. Proliferation and differentiation of the alloreactive effector T cells were assayed by in vitro mixed lymphocyte reaction and flow cytometry, respectively. Real-time quantitive RT-PCR was used to assess gene expression of cytokines and surum IFN-gamma was measured via ELISA. It showed that the median survival time of allograft in retransplant recipients was significantly shortened compared to that of transfer control, and it was the same in rejection score of graft. Moreover, proliferation and differentiation of the alloreactive effector T cells were more intensive in retransplant recipients than that in transfer control, which was confirmed by in vitro mixed lymphocyte reaction and by flow cytometry of the splenocytes for detecting CD44highCD62L- memory/effector phenotype cells. Furthermore, activation of CD4+ memory T cells is reflected by high level of surum IFN-gamma and the intensive gene expression of IFN-gamma and IL-2 at cardiac allograft in retransplant recipients. Collectively, the recall alloresponse in retransplantation is more intensive than that in a memory-transfer setting, and this retransplant model is closer to the clinic situation than the memory-transfer model in rodents.

Human vascular smooth muscle cells lack essential costimulatory molecules to activate allogeneic memory T cells

OBJECTIVE

The arterial media, populated by vascular smooth muscle cells (VSMC), is an immunoprivileged compartment and, in contrast to the intima or adventitia containing endothelial cells, is generally spared by inflammatory processes, such as arteriosclerosis. To determine mechanisms of medial immunoprivilege, we investigated the ability of human VSMC versus endothelial cells to activate allogeneic T cells in vitro.

METHODS AND RESULTS

Unlike cultured endothelial cells, cultured VSMC do not activate allogeneic memory CD4 or CD8 T cells and fail to effectively support T-cell proliferation to the polyclonal activator, phytohemagglutinin, consistent with a defect in costimulation function. Although many costimulators are comparably expressed on both cell types, endothelial cells but not VSMC basally express OX40 ligand and upregulate inducible costimulator ligand in response to proinflammatory cytokines. OX40 ligand-transduced, but not control- or inducible costimulator ligand-transduced, VSMC acquire the capacity to stimulate allogeneic memory CD4 T cells to produce cytokines and to proliferate in the presence of supplemental l-tryptophan. OX40 ligand overexpression, although not essential, also enhances allogeneic memory CD8 T-cell responses to VSMC after l-tryptophan supplementation.

CONCLUSIONS

The inability of cultured VSMC to activate memory T cells results from a lack of essential costimulators, particularly OX40 ligand, in addition to indoleamine 2,3-dioxygenase-mediated tryptophan depletion.

Rag2-/- gamma-chain-/- mice as hosts for human vessel transplantation and allogeneic human leukocyte reconstitution

BACKGROUND

Rodent models are a very helpful tool to investigate immunological mechanisms in allograft rejection. The aim of this study was to compare two different immunodeficient recipients in a humanized mouse model of arterial xenotransplantation in terms of reconstitution of the human immune system and rejection of the arterial graft.

METHODS

Side branches of human mammary artery were transplanted as infrarenal aortic interposition grafts into C.B-17-SCID beige and C57BL/6-Rag2(-/-)gammac(-/-) recipients. 7days after surgery mice were reconstituted with 5x10(7) human peripheral blood mononuclear cells (hu PBMCs) and 30days after reconstitution mice were sacrificed and histologic analysis was performed. Peripheral blood and splenocytes were investigated by FACS and ELISA analysis to ensure engraftment of human CD45(+) cells.

RESULTS

Transplant arteriosclerosis developed in non-PBMC-reconstituted C.B-17-SCID beige mice (intimal proliferation: 36.31+/-4.37%), but significantly less in C57BL/6-Rag2(-/-) gammac(-/-) recipients (intimal proliferation: 12.26+/-5.21%). After reconstitution with 5x10(7) unfractionated human PBMCs both mouse strains showed intima proliferation 30days after reconstitution (C.B-17-SCID beige: 28.49+/-7.95% and C57BL/6-Rag2(-/-) gammac(-/-): 44.58+/-11.08%). Whereas only very few human CD45(+) cells were found in mouse blood and spleen of C.B-17-SCID beige mice, C57BL/6-Rag2(-/-) gammac(-/-) mice revealed a reliable reconstitution. In addition, levels of human IgG and IgM within the peripheral blood were markedly higher in C57BL/6-Rag2(-/-) gammac(-/-) recipients.

CONCLUSION

In this study we can show, that the use of C57BL/6-Rag2(-/-) gammac(-/-) mice may be advantageous compared to C.B-17-SCID beige recipients in a humanized mouse model of vessel transplantation.

Immunophenotypic shift of memory CD8 T cells identifies the changes of immune status in the patients after liver transplantation

OBJECTIVE

Chronic or acute rejection is a leading cause of allograft loss after solid organ transplantation and the presence of memory T cells is associated with increased propensity for allograft rejection. The purpose of this study was to investigate the correlation between immunophenotypic shift of memory CD8+ T cells and immune status in the patients after liver transplantation.

MATERIAL AND METHODS

Seventy-three blood samples were collected and varied compartments of memory CD8+ T cells were analysed in non-rejected and rejected patients.

RESULTS

The results show that with time elapsed, the immunophenotypes of memory CD8+ cells shifted from naive T cells to central or intermediate memory cells, and then to effector or terminal memory cells in non-rejected patients. This course was correlated with the expression of CD127 on CD8+ T cells. In rejected patients, the main proportion of CD8+ cells were dominated by naive CD8+ cells and then rapidly restored to the immunophenotypes of memory T cells after effective treatment.

CONCLUSION

These results demonstrated that immunophenotypic shift of memory CD8+ T cells was closely related to the change of the immune status in the patients after liver transplantation. Monitoring the immunophenotypic shift of memory CD8+ T cells is of great importance in the prediction for allograft rejection and treatment effectiveness after liver transplantation.

Understanding gene expression patterns in immune-mediated disorders

Gene expression profiling of peripheral blood cells can provide dynamic information regarding the host response to immune-mediated disorders. AlloMap molecular expression testing from XDx monitors the expression of 20 genes in peripheral blood mononuclear cells (PBMC) to discriminate cardiac allograft recipients of 15 years or greater who are at low risk for acute cellular rejection (ACR). The AlloMap test classifier is based on the expression level of 11 genes, encoding proteins with diverse functions, which are differentially expressed in stable patients with moderate to severe ACR compared to patients without ACR. The nine other test genes are used for normalizing gene expression levels and assuring sample quality. In this work we review the development processes leading to the selection of the 11 informative genes and the derivation of the AlloMap test classifier, and discuss the relationship of peripheral blood gene expression with diverse pathways associated with ACR, including T-cell priming, platelet activation, systemic responses to allograft inflammation, and the overall state of immunosuppression.

Memory T-cell predominance following T-cell depletional therapy derives from homeostatic expansion of naive T cells

T-cell depletion reportedly leads to alterations in the T-cell compartment with predominant survival of memory phenotype CD4 T cells. Here, we asked whether the prevalence of memory T cells postdepletion results from their inherent resistance to depletion and/or to the homeostatic expansion of naive T cells and their phenotypic conversion to memory, which is known to occur in lymphopenic conditions. Using a 'mosaic memory' mouse model with trackable populations of alloreactive memory T cells, we found that treatment with murine antithymocyte globulin (mATG) or antilymphocyte serum (ALS) effectively depleted alloreactive memory CD4 T cells, followed by rapid homeostatic proliferation of endogenous CD4 T cells peaking at 4 days postdepletion, with no homeostatic advantage to the antigen-specific memory population. Interestingly, naive (CD44lo) CD4 T cells exhibited the greatest increase in homeostatic proliferation following mATG treatment, divided more extensively compared to memory (CD44hi) CD4 T cells and converted to a memory phenotype. Our results provide novel evidence that memory CD4 T cells are susceptible to lymphodepletion and that the postdepletional T-cell compartment is repopulated to a significant extent by homeostatically expanded naive T cells in a mouse model, with important important implications for immune alterations triggered by induction therapy.

Blockade of inducible costimulator pathway to prevent acute rejection in rat liver transplantation

BACKGROUND

The role of inducible costimulator (ICOS) in transplantation immunity remains unclear.

METHODS

A Lewis-to-Brown-Norway (BN) rat liver transplant model was used to explore the effect of ICOS blockade by small interference RNA. Recipient survival rate, number of CD25/ICOS-positive cells, ICOS mRNA and protein levels, and interferon-gamma and tumor-necrosis factor-alpha levels were determined.

RESULTS

Recipient survival was significantly prolonged in rats treated with RNA interference. On day 7 after transplantation, there was a diminished frequency of CD25/ICOS-positive cells and an increased frequency of apoptotic T cells. Furthermore, we found that ICOS blockade could inhibit mRNA and protein expression of ICOS, decrease plasma levels of interferon-gamma and tumor-necrosis factor-alpha, suppress cell infiltration into grafts, and promote tolerance in the interference group.

CONCLUSIONS

Our data demonstrate that RNA interference is a potent tool to down-modulate ICOS expression and protect allografts from acute rejection.

Primed CD8(+) T-cell responses to allogeneic endothelial cells are controlled by local complement activation

CD8 T cells primed by transplantation recognize allogeneic class I MHC molecules expressed on graft vascular endothelium and contribute to allograft injury. We previously showed that immune cell-derived complement activation fragments are integral to T cell activation/expansion. Herein we tested the impact of local complement production/activation on T cell/endothelial cell (EC) interactions. We found that proinflammatory cytokines upregulated alternative pathway complement production by ECs, yielding C5a. We further found that ECs deficient in the cell surface C3/C5 convertase regulator decay accelerating factor (DAF, CD55) induced greater CD8 T-cell proliferation and more IFNgamma(+) and perforin(+) effector cells than wild-type (WT) ECs. Allogeneic C3(-/-) EC induced little or no CD8 responses. Abrogation of responses following C5a receptor (C5aR) blockade, or augmentation following addition of recombinant C5a demonstrated that the effects were mediated through T-cell-expressed-C5aR interactions. Analyses of in vivo CD8 cell responses to transplanted heart grafts deficient in EC DAF showed similar augmentation. The findings reveal that EC-derived complement triggers secondary CD8 T-cell differentiation and expansion and argue that targeting complement and/or C5aR could limit T-cell-mediated graft injury.

Mechanisms of endothelial dysfunction, injury, and death

Vascular endothelial cells normally perform several key homeostatic functions such as keeping blood fluid, regulating blood flow, regulating macromolecule and fluid exchange with the tissues, preventing leukocyte activation, and aiding in immune surveillance for pathogens. Injury or cell death impairs or prevents conduct of these activities, resulting in dysfunction. Most endothelial cell death is apoptotic, involving activation of caspases, but nonapoptotic death responses also have been described. Stimuli that can cause endothelial injury or death include environmental stresses such as oxidative stress, endoplasmic reticulum stress, metabolic stress, and genotoxic stress, as well as pathways of injury mediated by the innate and adaptive immune systems. Pathways of immune-mediated death include those activated by death receptors as well as those activated by cytolytic granules and reactive oxygen species. The biochemical pathways activated by these injurious stimuli are described herein and will serve as a basis for future development of endothelial protective therapies.

Development of a humanized mouse model to study the role of macrophages in allograft injury

BACKGROUND

Nearly half of all infiltrating leukocytes in rejecting human allografts are macrophages, yet, in comparison with T cells, much less is known about the contribution of this cell type to rejection. Our laboratory has previously described models of rejection of human skin or artery grafts in immunodeficient mouse hosts mediated by adoptively transferred allogeneic T cells. However, mature human monocyte/macrophages have consistently failed to engraft in these animals. Here, we describe the introduction of human CD68+ macrophages into irradiated immunodeficient mice by transplantation of enriched CD34+ hematopoietic stem-cells isolated from peripheral blood of G-colony-stimulating factor pretreated adults.

METHODS

We investigated strains of immunodeficient mice bearing human tissue grafts (skin and artery) inoculated with 1 x 10(6) human CD34+ adult hematopoietic stem cells, peripheral blood monuclear cells autologous to the CD34 donor, or both for human cell engraftment.

RESULTS

In the absence of T cells, CD68+ CD14+ macrophages infiltrate allogeneic human skin but produce little injury or thrombosis. Both responses are enhanced when combined with adoptive transfer of T cells autologous to the hematopoietic stem cells as exemplified by the induction of the macrophage activation marker CD163. CD68+ macrophages also infiltrate allogeneic arterial interposition grafts, producing intimal expansion and calcification in the absence of T cells.

CONCLUSIONS

These new models may be used to study the role of human macrophages in transplant rejection and other pathologies in vivo.

Effector functions of donor-reactive CD8 memory T cells are dependent on ICOS induced during division in cardiac grafts

Alloreactive T-cell memory is present in every transplant recipient and endangers graft survival. Even in the absence of known sensitizing exposures, heterologous immunity and homeostatic T-cell proliferation generate 'endogenous' memory T cells with donor-reactivity. We have recently shown that endogenous donor-reactive CD8 memory T cells infiltrate murine cardiac allografts within hours of reperfusion and amplify early posttransplant inflammation by producing IFN-gamma. Here, we have tested the role of ICOS co-stimulation in eliciting effector function from these memory T cells. ICOS is not expressed on the cell surface of circulating CD8 memory T cells but is rapidly upregulated during cell division within the allograft parenchyma. Donor-reactive CD8 memory T-cell infiltration, proliferation and ICOS expression are regulated by donor class I MHC molecule expression. ICOS blockade significantly reduced IFN-gamma production and other proinflammatory functions of the activated CD8 memory T cells. Our data demonstrate that this induction of ICOS expression within peripheral tissues is an important feature of CD8 memory T-cell activation and identify ICOS as a specific target for neutralizing proinflammatory functions of endogenous CD8 memory T cells.

Endothelial cells in allograft rejection

In organ transplantation, blood borne cells and macromolecules (e.g., antibodies) of the host immune system are brought into direct contact with the endothelial cell lining of graft vessels. In this location, graft endothelial cells play several roles in allograft rejection, including the initiation of rejection responses by presentation of alloantigen to circulating T cells; the development of inflammation and thrombosis; and as targets of injury and agents of repair.

Endothelial cells in allograft rejection

In organ transplantation, blood borne cells and macromolecules (e.g., antibodies) of the host immune system are brought into direct contact with the endothelial cell lining of graft vessels. In this location, graft endothelial cells play several roles in allograft rejection, including the initiation of rejection responses by presentation of alloantigen to circulating T cells; the development of inflammation and thrombosis; and as targets of injury and agents of repair.

Amelioration of human allograft arterial injury by atorvastatin or simvastatin correlates with reduction of interferon-gamma production by infiltrating T cells

BACKGROUND

Graft arteriosclerosis (GA) is an important factor limiting long-term outcomes after organ transplantation. We have used a chimeric humanized mouse system to model this arteriopathy in human vessels, and found that the morphologic and functional changes of experimental GA are interferon (IFN)-gamma dependent. This study evaluated whether 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase inhibitors, described as inhibitors of IFN-gamma production, affect GA in our model.

METHODS

C.B.-17 severe combined immunodeficiency-beige mice were transplanted with human artery segments as aortic interposition grafts and inoculated with allogeneic human peripheral blood mononuclear cells (PBMCs) or replication-deficient adenovirus encoding human IFN-gamma. Transplant arteries were analyzed from recipients treated with vehicle vs. atorvastatin or simvastatin at different doses. The effects of statins on T-cell alloresponses to vascular endothelial cells were also investigated in vitro.

RESULTS

Graft arteriosclerosis-like arteriopathy induced by PBMCs was reduced by atorvastatin at 30 mg/kg/day or simvastatin at 100 mg/kg/day that correlated with decreased graft-infiltrating CD3+ T cells. Circulating IFN-gamma was also reduced, as were graft IFN-gamma and IFN-gamma-inducible chemokine transcripts and graft human leukocyte antigen-DR expression. Graft arteriosclerosis directly induced by human IFN-gamma in the absence of human PBMCs was also reduced by atorvastatin, but only at the highest dose of 100 mg/kg/day. Finally, atorvastatin decreased the clonal expansion and production of interleukin-2, but not IFN-gamma, by human CD4+ T cells in response to allogeneic endothelial cells in coculture.

CONCLUSIONS

Our results suggest that a benefit of statin administration in transplantation may include amelioration of GA primarily by inhibiting alloreactive T-cell accumulation and consequent IFN-gamma production and secondarily through suppression of the arterial response to IFN-gamma.

Antigen presentation by human microvascular endothelial cells triggers ICAM-1-dependent transendothelial protrusion by, and fractalkine-dependent transendothelial migration of, effector memory CD4+ T cells

TCR engagement on adherent human effector memory CD4(+) T cells by TNF-treated HUVECs under flow induces formation of a transendothelial protrusion (TEP) by the T cell but fails to induce transendothelial migration (TEM). In contrast, TCR engagement of the same T cell populations by TNF-treated human dermal microvascular cells (HDMEC) not only induces TEP formation, but triggers TEM at or near the interendothelial cell junctions via a process in which TEP formation appears to be the first step. Transduction of adhesion molecules in unactivated HDMEC and use of blocking Abs as conducted with TNF-activated HDMEC indicate that ICAM-1 plays a nonredundant role in TCR-driven TEP formation and TEM, and that TCR-driven TEM is also dependent upon fractalkine. TEP formation, dependence on ICAM-1, and dependence on fractalkine distinguish TCR-induced TEM from IP-10-induced TEM. These in vitro observations suggest that presentation of Ag by human microvascular endothelial cells to circulating CD4(+) effector memory T cells may function to initiate recall responses in peripheral tissues.

Heterogeneous memory T cells in antiviral immunity and immunopathology

Memory T cells are generated following an initial viral infection, and have the potential for mediating robust protective immunity to viral re-challenge due to their rapid and enhanced functional responses. In recent years, it has become clear that the memory T cell response to most viruses is remarkably diverse in phenotype, function, and tissue distribution, and can undergo dynamic changes during its long-term maintenance in vivo. However, the role of this variegation and compartmentalizationof memory T cells in protective immunity to viruses remains unclear. In this review,we discuss the diverse features of memory T cells that can delineate different subsets, the characteristics of memory T cells thus far identified to promote protective immune responses, and how the heterogeneous nature of memory T cells may also promote immunopathology during antiviral responses. We propose that given the profound heterogeneity of memory T cells, regulation of memory T cells during secondary responses could focus the response to participation of specific subsets,and/or inhibit memory T-cell subsets and functions that can lead to immunopathology.

The role for monocyte chemoattractant protein-1 in the generation and function of memory CD8+ T cells

Memory T cells are resistant to the conventional costimulatory blockade and therefore impede tolerance induction. However, their migratory, survival, and functional requirements for chemokines are not well understood. We herein examine the role for MCP-1 or CCL2 in the generation, migration, and function of memory CD8+ T cells. We found that overall generation of both central memory (TCM) and effector memory (TEM) CD8+ T cells was severely impaired in the absence of MCP-1. Importantly, the survival of TEM, but not TCM, CD8+ cells was reduced without MCP-1, whereas the homeostatic proliferation of TCM, but not TEM, CD8+ cells was weakened in MCP-1-/- mice. However, once they were generated in the absence of MCP-1, in vitro function of both subsets of memory cells remained intact as determined by their proliferation and IFN-gamma production. Interestingly, the migration of TCM, but not TEM, CD8+ cells to inflammatory sites was significantly delayed without MCP-1, whereas both subsets of memory cells underwent comparable expansion and apoptosis with or without MCP-1 during the effector phase. Moreover, the function to eliminate a graft of TCM, but not TEM, CD8+ cells was impaired without MCP-1. Thus, this study demonstrates that MCP-1 plays an important role in not only migration but also generation and survival of memory T cells. This finding provides new insight into the requirement of chemokines for the generation, survival, and function of differential subsets of memory T cells and may have clinic implications for tolerance induction.

Critical role of CD4 help in CD154 blockade-resistant memory CD8 T cell activation and allograft rejection in sensitized recipients

Allograft rejection in sensitized recipients remains the major problem in clinical organ transplantation. We have developed a donor-type skin-sensitized mouse cardiac allograft model (BALB/c-->C57BL/6) in which both rejection (<5 days) and alloreactive CD8 activation are resistant to CD154 blockade. First, we attempted to elucidate why CD154 blockade fails to protect cardiac grafts in sensitized recipients. The gene array analysis has revealed that treatment with anti-CD154 mAb (MR1) had distinctive impact on host immunity in naive vs sensitized animals. Unlike in naive counterparts, host sensitization mitigated the impact of CD154 blockade on critical immune signaling pathways. Indeed, we identified 3234 genes in cardiac grafts that were down-regulated by MR1 in naive (at least 5-fold), but remained unaffected in sensitized hosts. Moreover, MR1 treatment failed to prevent accumulation of CD4 T cells in cardiac allografts of sensitized recipients. Then, to determine the role of CD4 help in CD154 blockade-resistant immune response, we used CD4-depleting and CD4-blocking Ab, in conjunction with MR1 treatment. Our data revealed that CD154 blockade-resistant CD8 activation in sensitized mice was dependent on CD4 T cells. In the absence of CD4 help, CD154 blockade prevented differentiation of alloreactive CD8 T cells into CTL effector/memory cells and abrogated acute rejection (cardiac graft survival for >30 days), paralleled by selective target gene depression at the graft site. These results provide the rationale to probe potential synergy of adjunctive therapy targeting CD4 and CD154 to overcome graft rejection in sensitized recipients.

Bystander central memory but not effector memory CD8+ T cells suppress allograft rejection

Memory T cells respond faster and more vigorously than their naive counterparts and are critical for adaptive immunity. However, it is unknown whether and how memory T cells react in the face of irrelevant Ags. It is generally accepted that bystander memory T cells are neutral in immune responsiveness. In this study, we present the first evidence that bystander central memory (TCM), but not effector memory (TEM), CD8+ T cells suppress allograft rejection as well as T cell proliferation in the draining lymph nodes (DLN) of recipient mice. Both bystander TCM and naive T cells, but fewer TEM cells, migrated to DLN, whereas TCM cells exhibited faster turnover than their naive counterparts, suggesting that bystander TCM cells have an advantage over their naive counterparts in suppression. However, bystander TEM cells migrated to inflammatory graft sites, but not DLN, and yet failed to exert their suppression. These findings indicate that bystander memory T cells need to migrate to lymph nodes to exert their suppression by inhibiting responder T cell activation or homeostatic proliferation. Moreover, the suppression mediated by bystander TCM cells was largely dependent on IL-15, as IL-15 was required for their homeostatic proliferation and TCM-mediated suppression of allograft rejection. This suppression also required the presence of TGFbeta1, as TCM cells expressed TGFbeta1 while neutralizing TGFbeta1 abolished their suppression. Thus, bystander TCM, but not TEM, CD8+ T cells are potent suppressors rather than bystanders. This new finding will have an impact on cellular immunology and may have clinic implications for tolerance induction.

ICOS-Dependent and -independent functions of memory CD4 T cells in allograft rejection

Donor-reactive memory T cells undermine the survival of transplanted organs through multiple pathways. We have previously reported that memory CD4 T cells resist treatment with anti-CD154 antibody and donor-specific transfusion (DST/MR1) and promote cardiac allograft rejection via generation of effector CD4 T cells and alloantibody. We hypothesized that the helper functions of memory CD4 T cells are independent of T-cell costimulation through CD154 but instead are regulated by alternative costimulatory pathways. This study investigated how blocking ICOS/B7RP-1 interactions affects functions of donor-reactive memory CD4 T cells. Treatment with blocking anti-ICOS mAb synergized with DST/MR1 and prolonged mouse cardiac allograft survival despite the presence of donor-reactive memory CD4 T cells. While blocking ICOS did not diminish the expansion of preexisting memory CD4 T cells or the induction of allospecific effector T cells, it did inhibit recruitment of the activated memory and effector T cells into the graft. In addition, anti-ICOS mAb treatment in combination with DST/MR1 prevented help provided by memory CD4 T cells for production of donor-specific IgG antibody. These results demonstrate the potential efficacy of ICOS blockade in sensitized transplant patients and provide the foundation for rational use of ICOS blockade in combination with other graft-prolonging strategies.

Strong induction of 4-1BB, a growth and survival promoting costimulatory receptor, in HTLV-1-infected cultured and patients' T cells by the viral Tax oncoprotein

Human T-cell leukemia virus type 1 (HTLV-1), the cause of adult T-cell leukemia, stimulates the growth of infected T cells in cultures and in nonleukemic patients. In the latter, HTLV-1 is found in long-term persisting T-cell clones. The persistence of normal T cells is controlled by the growth-stimulating and antiapoptotic functions of costimulatory receptors, while the growth-stimulating HTLV-1 functions are mediated by the viral oncoprotein Tax. Here we analyzed the impact of Tax on costimulatory receptors in T cells with repressible Tax and found that among these receptors 4-1BB (TNFRSF9/CD137/ILA) was induced most strongly. Up-regulated 4-1BB expression was a consistent feature of all HTLV-1-infected cell lines, whether patient-derived or in vitro transformed. Tax was sufficient to induce the expression of the endogenous 4-1BB gene in uninfected T cells, and it strongly activated (45-fold) the 4-1BB promoter via a single NF-kappaB site. The ligand of 4-1BB was also found on transformed T-cell lines, opening up the possibility of autostimulation. Moreover, 4-1BB expression in patients' lymphocytes ex vivo correlated with Tax expression, strongly suggesting Tax-mediated 4-1BB activation in vivo. Thus, 4-1BB up-regulation by Tax could contribute to growth, survival, and clonal expansion of the infected cells during persistence and disease.

Alloimmunity to human endothelial cells derived from cord blood progenitors

There is considerable interest in exploiting circulating endothelial progenitor cells (EPCs) for therapeutic organ repair. Such cells may be differentiated into endothelial cells (ECs) in vitro and then expanded for use in tissue engineering. Vessel-derived ECs are variably immunogenic, depending upon tissue source, and it is unknown whether ECs derived from cord blood EPCs are able to initiate an allogeneic response. In this study, we compare the phenotype and alloantigenicity of human cord blood progenitor cell-derived ECs with HUVECs isolated from the same donors. Human cord blood progenitor cell-derived ECs are very similar to HUVECs in the expression of proteins relevant for alloimmunity, including MHC molecules, costimulators, adhesion molecules, cytokines, chemokines, and IDO, and in their ability to initiate allogeneic CD4(+) and CD8(+) memory T cell responses in vitro and in vivo. These findings have significant implications for the use of cord blood EPCs in regenerative medicine or tissue engineering.

Evolving functions of endothelial cells in inflammation

Inflammation is usually analysed from the perspective of tissue-infiltrating leukocytes. Microvascular endothelial cells at a site of inflammation are both active participants in and regulators of inflammatory processes. The properties of endothelial cells change during the transition from acute to chronic inflammation and during the transition from innate to adaptive immunity. Mediators that act on endothelial cells also act on leukocytes and vice versa. Consequently, many anti-inflammatory therapies influence the behaviour of endothelial cells and vascular therapeutics influence inflammation. This Review describes the functions performed by endothelial cells at each stage of the inflammatory process, emphasizing the principal mediators and signalling pathways involved and the therapeutic implications.

Human effector memory CD4+ T cells directly recognize allogeneic endothelial cells in vitro and in vivo

The frequency of circulating alloreactive human memory T cells correlates with allograft rejection. Memory T cells may be divided into effector memory (T(EM)) and central memory (T(CM)) cell subsets, but their specific roles in allograft rejection are unknown. We report that CD4+ T(EM) (CD45RO+ CCR7- CD62L-) can be adoptively transferred readily into C.B-17 SCID/bg mice and mediate the destruction of human endothelial cells (EC) in vascularized human skin grafts allogeneic to the T cell donor. In contrast, CD4+ T(CM) (CD45RO+ CCR7+ CD62L+) are inefficiently transferred and do not mediate EC injury. In vitro, CD4+ T(EM) secrete more IFN-gamma within 48 h in response to allogeneic ECs than do T(CM). In contrast, T(EM) and T(CM) secrete comparable amounts of IFN-gamma in response to allogeneic monocytes (Mo). In the same cultures, both T(EM) and T(CM) produce IL-2 and proliferate in response to IFN-gamma-treated allogeneic human EC or Mo, but T(CM) respond more vigorously in both assays. Blockade of LFA-3 strongly inhibits both IL-2 and IFN-gamma secretion by CD4+ T(EM) cultured with allogeneic EC but only minimally inhibits responses to allogeneic Mo. Blockade of CD80 and CD86 strongly inhibits IL-2 but not IFN-gamma production by in response to allogeneic EC or Mo. Transduction of EC to express B7-2 enhances allogeneic T(EM) production of IL-2 but not IFN-gamma. We conclude that human CD4+ T(EM) directly recognize and respond to allogeneic EC in vitro by secreting IFN-gamma and that this response depends on CD2 but not CD28. Consistent with EC activation of effector functions, human CD4+ T(EM) can mediate allogeneic EC injury in vivo.

Induction of Indoleamine 2,3-Dioxygenase in Vascular Smooth Muscle Cells by Interferon-γ Contributes to Medial Immunoprivilege

Atherosclerosis and graft arteriosclerosis are characterized by leukocytic infiltration of the vessel wall that spares the media. The mechanism(s) for medial immunoprivilege is unknown. In a chimeric humanized mouse model of allograft rejection, medial immunoprivilege was associated with expression of IDO by vascular smooth muscle cells (VSMCs) of rejecting human coronary artery grafts. Inhibition of IDO by 1-methyl-tryptophan (1-MT) increased medial infiltration by allogeneic T cells and increased VSMC loss. IFN-gamma-induced IDO expression and activity in cultured human VSMCs was considerably greater than in endothelial cells (ECs) or T cells. IFN-gamma-treated VSMCs, but not untreated VSMCs nor ECs with or without IFN-gamma pretreatment, inhibited memory Th cell alloresponses across a semipermeable membrane in vitro. This effect was reversed by 1-MT treatment or tryptophan supplementation and replicated by the absence of tryptophan, but not by addition of tryptophan metabolites. However, IFN-gamma-treated VSMCs did not activate allogeneic memory Th cells, even after addition of 1-MT or tryptophan. Our work extends the concept of medial immunoprivilege to include immune regulation, establishes the compartmentalization of immune responses within the vessel wall due to distinct microenvironments, and demonstrates a duality of stimulatory EC signals versus inhibitory VSMC signals to artery-infiltrating T cells that may contribute to the chronicity of arteriosclerotic diseases.

TCR signaling antagonizes rapid IP-10-mediated transendothelial migration of effector memory CD4+ T cells

Human microvascular endothelial cells (ECs) constitutively express MHC class II in peripheral tissues, the function of which remains unknown. In vitro assays have established that the recognition of EC MHC class II can affect cytokine expression, proliferation, and delayed transendothelial migration of allogeneic memory, but not naive, CD4+ T cells. Previously, we have shown that effector memory CD4+ T cells will rapidly transmigrate in response to the inflammatory chemokine IFN-gamma-inducible protein-10 (IP-10) in a process contingent upon the application of venular levels of shear stress. Using two models that provide polyclonal TCR signaling by ECs in this flow system, we show that TCR engagement antagonizes the rapid chemokine-dependent transmigration of memory CD4+ T cells. Inhibitor studies suggest that TCR signaling downstream of Src family tyrosine kinase(s) but upstream of calcineurin activation causes memory CD4+ T cell arrest on the EC surface, preventing the transendothelial migration response to IP-10.

Immunomodulatory properties of FK734, a humanized anti-CD28 monoclonal antibody with agonistic and antagonistic activities

BACKGROUND

We describe immunomodulatory effects of FK734, a humanized version of a mouse anti-human CD28 mAb (clone TN228), in vitro and in a chimeric human-mouse model of allograft rejection.

METHODS

Cytokine production and proliferation were assessed in a mixed lymphocyte reaction containing FK734, human T cells, and endothelial cells or monocytes. FK734 was also administered to SCID mice engrafted with human skin and adoptively transferred with human peripheral blood mononuclear cells allogeneic to the skin graft.

RESULTS

In vitro, FK734 enhanced secretion of interleukin-2 and interferon-gamma as well as proliferation of CD4+ and CD8+ T cells stimulated by allogeneic human leukocyte antigen (HLA)-DR+ human umbilical vein endothelial cells (which lack B7 molecules and FcgammaRs) or by blood monocytes (which express low levels of B7 molecules and FcgammaRs) compared with control mAb, but these effects were significantly smaller than those provided by mAb 28.2, a stimulatory mouse anti-human CD28 mAb, at comparable concentrations. However, FK734 generally inhibited cytokine secretion and T cell proliferation in cocultures with human umbilical vein endothelial cells transduced to express CD86. In vivo using SCID/beige mice bearing human skin with adoptively transferred peripheral blood mononuclear cells, administration of FK734 protected human endothelial cell-lined microvessels, significantly but incompletely reducing endothelial cell injury and T cell infiltration into the graft one or two weeks later.

CONCLUSIONS

FK734 is a partial agonist of CD28 signaling that can reduce human T cell alloresponses in the presence of strong costimulation by B7 molecules in vitro and can reduce T cell-mediated skin allograft rejection in vivo.

Interferon-γ Axis in Graft Arteriosclerosis

Cardiac transplantation is the most effective treatment for advanced heart failure. Despite improvements in immunosuppression therapy that prevent acute rejection, cardiac allografts fail at rates of 3% to 5% per posttransplant year. The hallmark morphological lesion of chronically failing cardiac allografts, also seen in chronic renal and liver graft failure, is luminal stenosis of blood vessels, especially of conduit arteries. Late graft failure results from widespread secondary ischemic injury to the graft parenchyma rather than direct immune-mediated damage. Although this process affects the entire graft vasculature, graft arteriosclerosis is a suitable term to describe the problem because it applies to different types of failing organs and because it emphasizes the central feature, namely an accelerated form of arterial injury and remodeling. The precise pathogenesis of graft arteriosclerosis is unknown. In this review, we make the case that the signature T-helper type 1 cytokine, interferon (IFN)-γ, is a key effector in graft arteriosclerosis, which, together with the IFN-γ–inducing cytokine interleukin-12 and IFN-γ–inducible chemokines such as CXCR3 ligands, constitute a positive feedback loop for T-cell activation, differentiation, and recruitment that we refer to as the IFN-γ axis. We evaluate the evidence to support this hypothesis in clinical observational and experimental animal studies. Additionally, we examine the regulation of IFN-γ production within the artery wall, the effects of IFN-γ on vessel wall cells, and the outcome of therapeutic agents on IFN-γ production and signaling. These observations lead us to suggest that new therapies for graft arteriosclerosis should be optimized which focus on reducing IFN-γ synthesis or actions.

Mantoux Test as a model for a secondary immune response in humans

The Mantoux Test (MT) is a classical delayed-type hypersensitivity (DTH) response to the intradermal injection of tuberculin purified protein derivative (PPD). It represents a cutaneous T cell mediated memory recall immune response. The test is typically used to determine immunity to tuberculosis in humans and positive reactions develop in individuals previously exposed to Mycobacterium tuberculosis, and those immunised with the Bacillus of Calmette and Guérin (BCG) vaccine. In view of its relative accessibility human skin represents a convenient tissue for the investigation of human immune responses. Using the MT, we have been able to determine that significant cellular proliferation and clonal expansion occur at the site of antigen deposition in the skin. Furthermore, cells undergoing proliferation in the skin also undergo accelerated differentiation. Taken together with other studies, in humans and in mice, these observations shed new light on the importance of the microenvironment at the site of the immune response for the proliferation and differentiation of memory T cells.

Expression and Regulation of Human CD275 on Endothelial Cells in Healthy and Inflamed Mucosal Tissues

The CD275-CD278 costimulatory pathway is a new pathway for CD28-B7 family molecules involved in the effector phase of T-cell-mediated immune responses. Expression of CD275 in oral mucosa at healthy and disease states has not been examined. We generated monoclonal antibodies against human CD275 and investigated its expression and regulation in cultured tissue cell lines and oral mucosal tissues. CD275 on monocytes was efficiently upregulated by interleukin-4, while interferon-gamma abrogated this effect. CD275 on cultured endothelial cells (EC) was rapidly enhanced by tumour necrosis factor-alpha. In healthy oral mucosa, CD275 was not detected on keratinocytes, Langerhans cells or intraepithelial lymphocytes within the epithelium or on interstitial dendritic cells or lymphocytes in the sub-epithelium. Constitutive expression of CD275 on EC in the connective tissues was observed in healthy mucosa, but CD275 expression on EC in oral lichen planus was either upregulated or down regulated. Approximately 20% of the T cells found within infiltrating mononuclear cells in the sub-epithelium expressed high levels of the CD278 receptor. CD275 on lymphoid and nonlymphoid cells is positively or negatively regulated by various cytokines. Our results suggest that CD275 on EC is involved in the recruitment or extravasation of receptor-positive effector T cells into inflamed tissues.