CD28-independent costimulation of T cells in alloimmune responses

Cyclosporine A, in Contrast to Rapamycin, Affects the Ability of Dendritic Cells to Induce Immune Tolerance Mechanisms

Following organ transplantation, it is essential that immune tolerance is induced in the graft recipient to reduce the risk of rejection and avoid complications associated with the long-term use of immunosuppressive drugs. Immature dendritic cells (DCs) are considered to promote transplant tolerance and may minimize the risk of graft rejection. The aim of the study was to evaluate the effects of immunosuppressive agents: rapamycin (Rapa) and cyclosporine A (CsA) on generation of human tolerogenic DCs (tolDCs) and also to evaluate the ability of these cells to induce mechanisms of immune tolerance. tolDCs were generated in the environment of Rapa or CsA. Next, we evaluated the effects of these agents on surface phenotypes (CD11c, MHC II, CD40, CD80, CD83, CD86, CCR7, TLR2, TLR4), cytokine production (IL-4, IL-6, IL-10, IL-12p70, TGF-β), phagocytic capacity and resistant to lipopolysaccharide activation of these DCs. Moreover, we assessed ability of such tolDCs to induce T cell activation and apoptosis, Treg differentiation and production of Th1- and Th2-characteristic cytokine profile. Data obtained in this study demonstrate that rapamycin is effective at generating maturation-resistant tolDCs, however, does not change the ability of these cells to induce mechanisms of immune tolerance. In contrast, CsA affects the ability of these cells to induce mechanisms of immune tolerance, but is not efficient at generating maturation-resistant tolDCs.

Immune checkpoint inhibitors and cardiotoxicity: possible mechanisms, manifestations, diagnosis and management

INTRODUCTION

Immune checkpoint inhibitors (ICIs) are a new class of anticancer drugs that enhance the immune system function and activate T cells against cancerous cells. Although cardiac complications are not common, they could be accompanied with high morbidity and mortality.

AREAS COVERED

Regarding the importance of cardiac complications and their subsequent burden on individuals and the healthcare system, this review attempts to discuss the mechanism, diagnosis, and management of myocarditis, besides recapitulating the possible mechanism of other cardiac adverse events. Moreover, we briefly discuss the concurrent administration of other chemotherapeutic agents.

EXPERT OPINION

Due to insufficient knowledge concerning the physiopathology of immune-related adverse events (irAEs) and their potential further complications, cardiovascular complications in particular and in the context of this paper's focus, cooperation of oncologists, immunologists, and cardiologists is necessary for the management of patients. Experimental approaches such as using corticosteroids are becoming a part of guidelines for managing cardiac irAEs. However, a unique algorithm for diagnosis and management is necessary, especially in myocarditis cases. Furthermore, more studies are required to resolve current challenges, including prevention of myocarditis, concurrent administration of other chemotherapeutic agents, and re-introducing patients with ICIs.

Development of an intravital imaging system for the synovial tissue reveals the dynamics of CTLA-4 Ig in vivo

There have been many attempts to visualize the inflamed joints using multiphoton microscopy. However, due to the hypervascular and multilayered structure of the inflamed synovium, intravital imaging of the deep synovial tissue has been difficult. Here, we established original intravital imaging systems to visualize synovial tissue and pathological osteoclasts at the pannus–bone interface using multiphoton microscopy. Combined with fluorescence-labeling of CTLA-4 Ig, a biological agent used for the treatment of rheumatoid arthritis, we identified that CTLA-4 Ig was distributed predominantly within the inflamed synovium and bound to CX₃CR1⁺ macrophages and CD140a⁺ fibroblasts 6 h after injection, but not to mature osteoclasts. Intravital imaging of blood and lymphatic vessels in the inflamed synovium further showed that extravasated CTLA-4 Ig was immediately drained through lymphatic vessels under acute arthritic conditions, but the drainage activity was retarded under chronic conditions. These results indicate that this intravital synovial imaging system can serve as a platform for exploring the dynamics of immune cells, osteoclasts, and biological agents within the synovial microenvironment in vivo.

T cell checkpoint regulators in the heart

T lymphocyte-mediated immune responses in the heart are potentially dangerous because they can interfere with the electromechanical function. Furthermore, the myocardium has limited regenerative capacity to repair damage caused by effector T cells. Myocardial T cell responses are normally suppressed by multiple mechanisms of central and peripheral tolerance. T cell inhibitory molecules, so called immune checkpoints, limit the activation and effector function of heart antigen-reactive T cells that escape deletion during development in the thymus. Programmed cell protein death-1 (PD-1) and cytotoxic T-lymphocyte-associated protein-4 (CTLA-4) are checkpoint molecules homologous to the costimulatory receptor CD28, and they work to block activating signals from the T cell antigen receptor and CD28. Nonetheless, PD-1 and CTLA-4 function in different ways and at different steps in a T cell response to antigen. Studies in mice have established that genetic deficiencies of checkpoint molecules, including PD-1, PD-L1, CTLA-4, and lymphocyte activation gene-3, result in enhanced risk of autoimmune T cell-mediated myocarditis and increased pathogenicity of heart antigen-specific effector T cells. The PD-1/PD-L1 pathway appears to be particularly important in cardiac protection from T cells. PD-L1 is markedly up-regulated on myocardial cells by interferon-gamma secreted by T cells and PD-1 or PD-L1 deficiency synergizes with other defects in immune regulation in promoting myocarditis. Consistent with these studies, myocarditis has emerged as a serious adverse reaction of cancer therapies that target checkpoint molecules to enhance anti-tumour T cell responses. Histopathology and immunohistochemical analyses of myocardial tissue from immune checkpoint blockade (ICB)-treated patients echoes findings in checkpoint-deficient mice. Many questions about myocarditis in the setting of cancer immunotherapy still need to be answered, including the nature of the target antigens, genetic risk factors, and variations in the disease with combined therapies. Addressing these questions will require further immunological analyses of blood and heart tissue from patients treated with ICB.

Insights into CD8 T Cell Activation and Exhaustion from a Mouse Gammaherpesvirus Model

(S.R.S.) I was introduced to viral immunology while working in Peter Doherty's laboratory in the early stages of my research career, inspiring a lifelong interest in this area. During those early years under Peter's mentorship, we studied a mouse gammaherpesvirus model (murine gammaherpesvirus-68 [MHV-68]) that provided a useful small animal model for investigating the immunological control of gammaherpesvirus infection. Interestingly, while CD4 T cells were not required for acute control of MHV-68 in the lung, CD8 T cell-mediated control was progressively lost in the absence of CD4 T cell help, leading to viral recrudescence. This was one of several early studies showing that CD8 T cell control of persistent viral infections was lost in the absence of CD4 T cell help, preceding the concept of CD8 T cell exhaustion. Further studies showed that MHV-68 infection of mice offered a unique model for comparing the mechanisms of acute and long-term control of a persistent viral infection and developing strategies for reversing T cell exhaustion. Here, we provide a brief review of the literature on CD8 T cell activation and exhaustion in this model, focusing on the role of CD40 and B7 family members and including some previously unpublished data.

Costimulation Blockade in Transplantation

T cells play a pivotal role in orchestrating immune responses directed against a foreign (allogeneic) graft. For T cells to become fully activated, the T-cell receptor (TCR) must interact with the major histocompatibility complex (MHC) plus peptide complex on antigen-presenting cells (APCs), followed by a second "positive" costimulatory signal. In the absence of this second signal, T cells become anergic or undergo deletion. By blocking positive costimulatory signaling, T-cell allo-responses can be aborted, thus preventing graft rejection and promoting long-term allograft survival and possibly tolerance (Alegre ML, Najafian N, Curr Mol Med 6:843-857, 2006; Li XC, Rothstein DM, Sayegh MH, Immunol Rev 229:271-293, 2009). In addition, costimulatory molecules can provide negative "coinhibitory" signals that inhibit T-cell activation and terminate immune responses; strategies to promote these pathways can also lead to graft tolerance (Boenisch O, Sayegh MH, Najafian N, Curr Opin Organ Transplant 13:373-378, 2008). However, T-cell costimulation involves an incredibly complex array of interactions that may act simultaneously or at different times in the immune response and whose relative importance varies depending on the different T-cell subsets and activation status. In transplantation, the presence of foreign alloantigen incites not only destructive T effector cells but also protective regulatory T cells, the balance of which ultimately determines the fate of the allograft (Lechler RI, Garden OA, Turka LA, Nat Rev Immunol 3:147-158, 2003). Since the processes of alloantigen-specific rejection and regulation both require activation of T cells, costimulatory interactions may have opposing or synergistic roles depending on the cell being targeted. Such complexities present both challenges and opportunities in targeting T-cell costimulatory pathways for therapeutic purposes. In this chapter, we summarize our current knowledge of the various costimulatory pathways in transplantation and review the current state and challenges of harnessing these pathways to promote graft tolerance (summarized in Table 10.1).

Bifidobacteria: A probable missing puzzle piece in the pathogenesis of multiple sclerosis

Multiple sclerosis (MS) is an autoimmune disorder in which the immunopathogenesis is not fully understood. In the recent years, the role of gut microbiome in the pathogenesis of this disorder has been highlighted. Bifidobacteria as a component of gut microbiome might also be involved in MS pathogenesis. Being emerged in early days after birth, bifidobacteria have a prominent role in immune system maturation and function. Some factors like mode of delivery, breast feeding, mother's blood group and her secretory state and also environmental factors could influence its level in the early infancy, which may remain throughout lifetime. In this review, we discussed possible immunopathogenic link between the bifidobacteria and MS.

Immune Checkpoint Molecules in Reproductive Immunology

Immune checkpoint molecules, like CTLA-4, TIM-3, PD-1, are negative regulators of immune responses to avoid immune injury. Checkpoint regulators are thought to actively participate in the immune defense of infections, prevention of autoimmunity, transplantation, and tumor immune evasion. Maternal-fetal immunotolerance represents a real immunological challenge for the immune system of the mother: beside acceptance of the semiallogeneic fetus, the maternal immune system has to be prepared for immune defense mostly against infections. In this particular situation, the role of immune checkpoint molecules could be of special interest. In this review, we describe current knowledge on the role of immune checkpoint molecules in reproductive immunology.

Tumour necrosis factor α augments the inhibitory effects of CTLA-4-Ig on osteoclast generation from human monocytes via induction of CD80 expression

Cytotoxic T lymphocyte antigen-4-immunoglobulin (CTLA-4-Ig) exerts anti-rheumatic action via negative regulation of the co-stimulation process between antigen-presenting cells and T cells. CTLA-4-Ig also binds to CD80/CD86 on monocytes of osteoclast precursors. However, little is known about the effect of CTLA-4-Ig on osteoclastogenesis in rheumatoid arthritis (RA). In this study we evaluated the effects of CTLA-4-Ig on osteoclast generation from human blood monocytes (PBM) and rheumatoid synovial fluid monocytes (RSFM). Highly purified monocytes were cultured with receptor activator of nuclear factor kappa-B ligand (RANKL) and macrophage colony-stimulating factor (M-CSF) in the presence of CTLA-4-Ig. CTLA-4-Ig inhibited RANKL-induced osteoclast generation in PBM and RSFM, as determined by tartrate-resistant acid phosphatase (TRAP) staining and bone resorption assay using osteo assay surface plates. In addition, CTLA-4-Ig reduced the gene and protein expressions of nuclear factor of activated T cells, cytoplasmic 1 (NFATc1) and cathepsin K during osteoclastogenesis. Furthermore, CTLA-4-Ig significantly inhibited cell proliferation during osteoclastogenesis. Interestingly, the gene expression of indoleamine 2,3-dioxygenase-1, an inducer of apoptosis, was enhanced by CTLA-4-Ig. We next examined the effect of tumour necrosis factor (TNF)-α, a major inflammatory cytokine in rheumatoid synovium, on the expression of CD80 and CD86 by flow cytometric analysis. TNF-α potently induced the surface expression of CD80, which is known to have much higher affinity to CTLA-4-Ig than CD86, and this induction was observed at mRNA levels. Interestingly, freshly prepared rheumatoid synovial monocytes also expressed CD80 as much as TNF-α-treated PBM. Furthermore, TNF-α enhanced CTLA-4-Ig-induced inhibition of osteoclastogenesis and cell proliferation. Taken together, the TNF-α-induced CD80 may augment CTLA-4-Ig-induced inhibition of osteoclastogenesis, suggesting that CTLA-4-Ig potently inhibits osteoclast differentiation and protects bone destruction in rheumatoid inflamed joints.

Inhibition of RON kinase potentiates anti-CTLA-4 immunotherapy to shrink breast tumors and prevent metastatic outgrowth

The advent of immune checkpoint blockade as a new strategy for immunotherapy has changed the outlook for many aggressive cancers. Although complete tumor eradication is attainable in some cases, durable clinical responses are observed only in a small fraction of patients, underlining urgent need for improvement. We previously showed that RON, a receptor tyrosine kinase expressed in macrophages, suppresses antitumor immune responses, and facilitates progression and metastasis of breast cancer. Here, we investigated the molecular changes that occur downstream of RON activation in macrophages, and whether inhibition of RON can cooperate with checkpoint immunotherapy to eradicate tumors. Activation of RON by its ligand, MSP, altered the gene expression profile of macrophages drastically and upregulated surface levels of CD80 and PD-L1, ligands for T-cell checkpoint receptors CTLA-4 and PD-1. Genetic deletion or pharmacological inhibition of RON in combination with anti-CTLA-4, but not with anti-PD-1, resulted in improved clinical responses against orthotopically transplanted tumors compared to single-agent treatment groups, resulting in complete tumor eradication in 46% of the animals. Positive responses to therapy were associated with higher levels of T-cell activation markers and tumor-infiltrating lymphocytes. Importantly, co-inhibition of RON and anti-CTLA-4 was also effective in clearing metastatic breast cancer cells in lungs, resulting in clinical responses in nearly 60% of the mice. These findings suggest that RON inhibition can be a novel approach to potentiate responses to checkpoint immunotherapy in breast cancer.

T Cell Cosignaling Molecules in Transplantation

The ultimate outcome of alloreactivity versus tolerance following transplantation is potently influenced by the constellation of cosignaling molecules expressed by immune cells during priming with alloantigen, and the net sum of costimulatory and coinhibitory signals transmitted via ligation of these molecules. Intense investigation over the last two decades has yielded a detailed understanding of the kinetics, cellular distribution, and intracellular signaling networks of cosignaling molecules such as the CD28, TNF, and TIM families of receptors in alloimmunity. More recent work has better defined the cellular and molecular mechanisms by which engagement of cosignaling networks serve to either dampen or augment alloimmunity. These findings will likely aid in the rational development of novel immunomodulatory strategies to prolong graft survival and improve outcomes following transplantation.

PD-L1 Deficiency within Islets Reduces Allograft Survival in Mice

Background

Islet transplantation may potentially cure type 1 diabetes mellitus (T1DM). However, immune rejection, especially that induced by the alloreactive T-cell response, remains a restraining factor for the long-term survival of grafted islets. Programmed death ligand-1 (PD-L1) is a negative costimulatory molecule. PD-L1 deficiency within the donor heart accelerates allograft rejection. Here, we investigate whether PD-L1 deficiency in donor islets reduces allograft survival time.

Methods

Glucose Stimulation Assays were performed to evaluate whether PD-L1 deficiency has detrimental effects on islet function. Islets isolated from PDL1-deficient mice or wild- type (WT) mice (C57BL/6j) were implanted beneath the renal capsule of streptozotocin (STZ)-induced diabetic BALB/c mice. Blood glucose levels and graft survival time after transplantation were monitored. Moreover, we analyzed the residual islets, infiltrating immune cells and alloreactive cells from the recipients.

Results

PD-L1 deficiency within islets does not affect islet function. However, islet PD-L1 deficiency increased allograft rejection and was associated with enhanced inflammatory cell infiltration and recipient T-cell alloreactivity.

Conclusions

This is the first report to demonstrate that PD-L1 deficiency accelerated islet allograft rejection and regulated recipient alloimmune responses.

Targeting CD28 to prevent transplant rejection

INTRODUCTION

The pivotal role of costimulatory pathways in regulating T-cell activation versus tolerance has stimulated tremendous interest in their manipulation for therapeutic purposes. Of these, the CD28-B7 pathway is arguably the most important and best studied. Therapeutic targets of CD28 are currently used in the treatment of melanoma, autoimmune diseases and in transplantation.

AREAS COVERED

In this review, we summarize our current knowledge of CD28 and cytotoxic T-lymphocyte antigen-4 (CTLA-4) signaling, and review the current state and challenges of harnessing them to promote transplant tolerance.

EXPERT OPINION

Despite the success of belatacept, a first-in-class CTLA-4 fusion protein now clinically used in transplantation, it is apparent that we have only scratched the surface in understanding the complexities of how costimulatory pathways modulate the immune system. Our initial assumption that positive costimulators activate effector T cells and prevent tolerance, while negative costimulators inhibit effector T cells and promote tolerance, is clearly an oversimplified view. Indeed, belatacept is not only capable of blocking deleterious CD28-B7 interactions that promote effector T-cell responses but can also have undesired effects on tolerogenic regulatory T-cell populations.

CD28 family and chronic rejection: "to belatacept...And beyond!"

Kidneys are one of the most frequently transplanted human organs. Immunosuppressive agents may prevent or reverse most acute rejection episodes; however, the graft may still succumb to chronic rejection. The immunological response involved in the chronic rejection process depends on both innate and adaptive immune response. T lymphocytes have a pivotal role in chronic rejection in adaptive immune response. Meanwhile, we aim to present a general overview on the state-of-the-art knowledge of the strategies used for manipulating the lymphocyte activation mechanisms involved in allografts, with emphasis on T-lymphocyte costimulatory and coinhibitory molecules of the B7-CD28 superfamily. A deeper understanding of the structure and function of these molecules improves both the knowledge of the immune system itself and their potential action as rejection inducers or tolerance promoters. In this context, the central role played by CD28 family, especially the relationship between CD28 and CTLA-4, becomes an interesting target for the development of immune-based therapies aiming to increase the survival rate of allografts and to decrease autoimmune phenomena. Good results obtained by the recent development of abatacept and belatacept with potential clinical use aroused better expectations concerning the outcome of transplanted patients.

Multiple layers of CD80/86-dependent costimulatory activity regulate primary, memory, and secondary lymphocytic choriomeningitis virus-specific T cell immunity

The lymphocytic choriomeningitis virus (LCMV) system constitutes one of the most widely used models for the study of infectious disease and the regulation of virus-specific T cell immunity. However, with respect to the activity of costimulatory and associated regulatory pathways, LCMV-specific T cell responses have long been regarded as relatively independent and thus distinct from the regulation of T cell immunity directed against many other viral pathogens. Here, we have reevaluated the contribution of CD28-CD80/86 costimulation in the LCMV system by use of CD80/86-deficient mice, and our results demonstrate that a disruption of CD28-CD80/86 signaling compromises the magnitude, phenotype, and/or functionality of LCMV-specific CD8(+) and/or CD4(+) T cell populations in all stages of the T cell response. Notably, a profound inhibition of secondary T cell immunity in LCMV-immune CD80/86-deficient mice emerged as a composite of both defective memory T cell development and a specific requirement for CD80 but not CD86 in the recall response, while a related experimental scenario of CD28-dependent yet CD80/86-independent secondary CD8(+) T cell immunity suggests the existence of a CD28 ligand other than CD80/86. Furthermore, we provide evidence that regulatory T cells (T(REG)s), the homeostasis of which is altered in CD80/86(-/-) mice, contribute to restrained LCMV-specific CD8(+) T cell responses in the presence of CD80/86. Our observations can therefore provide a more coherent perspective on CD28-CD80/86 costimulation in antiviral T cell immunity that positions the LCMV system within a shared context of multiple defects that virus-specific T cells acquire in the absence of CD28-CD80/86 costimulation.

Enhancement of suboptimal CD8 cytotoxic T cell effector function in vivo using antigen-specific CD80 defective T cells

T cell upregulation of B7 molecules CD80 and CD86 limits T cell expansion in immunodeficient hosts; however, the relative roles of CD80 separate from CD86 on CD4 versus CD8 T cells in a normal immune system are not clear. To address this question, we used the parent-into-F1 (P→F1) murine model of graft-versus-host disease and transferred optimal and suboptimal doses of CD80 and/or CD86 knockout (KO) T cells into normal F1 hosts. Enhanced elimination of host B cells by KO T cells was observed only at suboptimal donor cell doses and was greatest for CD80 KO→F1 mice. Wild-type donor cells exhibited peak CD80 upregulation at day 10; CD80 KO donor cells exhibited greater peak (day 10) donor T cell proliferation and CD8 T cell effector CTL numbers versus wild-type→F1 mice. Fas or programmed cell death-1 upregulation was normal as was homeostatic contraction of CD80 KO donor cells from days 12-14. Mixing studies demonstrated that maximal host cell elimination was seen when both CD4 and CD8 T cells were CD80 deficient. These results indicate an important role for CD80 upregulation on Ag-activated CD4 and CD8 T cells in limiting expansion of CD8 CTL effectors as part of a normal immune response. Our results support further studies of therapeutic targeting of CD80 in conditions characterized by suboptimal CD8 effector responses.

Differential requirement of CD27 costimulatory signaling for naïve versus alloantigen-primed effector/memory CD8+ T cells

CD8(+) memory T cells endanger allograft survival by causing acute and chronic rejection and prevent tolerance induction. We explored the role of CD27:CD70 T-cell costimulatory pathway in alloreactive CD8(+)/CD4(+) T-cell activation. CD27-deficient (CD27(-/-)) and wild-type (WT) B6 mice rejected BALB/c cardiac allografts at similar tempo, with or without depletion of CD4(+) or CD8(+) T cells, suggesting that CD27 is not essential during primary T-cell alloimmune responses. To dissect the role of CD27 in primed effector and memory alloreactive T cells, CD27(-/-) or WT mice were challenged with BALB/c hearts either 10 or 40 days after sensitization with donor-type skin grafts. Compared to WT controls, allograft survival was prolonged in day 40- but not day 10-sensitized CD27(-/-) recipients. Improved allograft survival was accompanied by diminished secondary responsiveness of memory CD8(+) T cells, which resulted from deficiency in memory formation rather than their lack of secondary expansion. Chronic allograft vasculopathy and fibrosis were diminished in CD27(-/-) recipients of class I- but not class II-mismatched hearts as compared to WT controls. These data establish a novel role for CD27 as an important costimulatory molecule for alloreactive CD8(+) memory T cells in acute and chronic allograft rejection.

A novel in vivo regulatory role of P-glycoprotein in alloimmunity

P-glycoprotein (P-gp) is required for adaptive immunity through defined functions in T cell activation and antigen presenting cell (APC) maturation. The potential role of P-gp as an in vivo regulator of alloimmunity is currently unknown. Here we show that P-gp blockade prolongs graft survival in a murine heterotopic cardiac allotransplantation model through in vivo inhibition of the T helper 1 (Th1) cytokine IFN-gamma and the Th2 product IL-4, and via downregulation of the APC-expressed positive costimulatory molecule CD80. In vitro, the P-gp antagonist PSC833, a non-calcineurin-inhibitory cyclosporine A analogue, specifically inhibited cellular efflux of the P-gp substrate rhodamine-123 in wild-type CD3(+) T cells and MHC class II(+) APCs but not their P-gp knockout counterparts that lacked rhodamine-123 efflux capacity. Additionally, P-gp blockade significantly inhibited murine alloimmune T cell activation in a dose-dependent fashion. In vivo, P-gp blockade significantly prolonged graft survival in Balb/c recipients of C57BL/6 cardiac allografts from 8.5+/-0.5 to 11.7+/-0.5 days (P<0.01), similar in magnitude to the effects of monotherapy with cyclosporine A. Moreover, P-gp blockade, compared to controls, attenuated intragraft expression of CD3 and CD80, but not CD86, and inhibited IFN-gamma and IL-4 production (P<0.05). In the setting of systemic CD86 inhibition, P-gp blockade suppressed IFN-gamma and IL-4 production significantly further (to 98% and 89% inhibition, respectively) compared to either P-gp or anti-CD86 blockade alone, and markedly prolonged allograft survival compared to anti-CD86 blockade alone (40.5+/-4.6 versus 22.5+/-2.6 days, respectively, P<0.01). Our findings define a novel in vivo regulatory role of P-gp in alloimmunity and identify P-gp as a potential therapeutic target in allotransplantation.

The role of T helper cell differentiation in promoting nerve allograft survival with costimulation blockade

OBJECT

Peripheral nerve allografts provide a temporary scaffold for host nerve regeneration and allow for the repair of significant segmental nerve injuries. Despite this potential, nerve allograft transplantation requires temporary systemic immunosuppression. Characterization of the immunological mechanisms involved in the induction of immune hyporesponsiveness to prevent nerve allograft rejection will help provide a basis for optimizing immunomodulation regimens or manipulating donor nerve allografts to minimize or eliminate the need for global immunosuppression.

METHODS

The authors used C57Bl/6 mice and STAT4 and STAT6 gene BALB/c knockout mice. A nonvascularized nerve allograft was used to reconstruct a 1-cm sciatic nerve gap in the murine model. A triple costimulatory blockade of the CD40, CD28/B7, and inducible costimulatory (ICOS) pathways was used. Quantitative assessment was performed at 3 weeks with nerve histomorphometry, walking track analysis, and the enzyme-linked immunospot assay.

RESULTS

The STAT6 -/- mice received 3 doses of costimulation-blocking antibodies and had axonal regeneration equivalent to nerve isografts, while treated STAT4 -/- mice demonstrated moderate axonal regeneration but inferior to the T helper cell Type 2-deficient animals. Enzyme-linked immunospot assay analysis demonstrated a minimal immune response in both STAT4 -/- and STAT6 -/- mice treated with a costimulatory blockade.

CONCLUSIONS

The authors' findings suggest that Type 1 T helper cells may play a more significant role in costimulatory blockade-induced immune hyporesponsiveness in the nerve allograft model, and that Type 2 T helper differentation may represent a potential target for directed immunosuppression.

Paradoxical functions of B7: CD28 costimulation in a MHC class II-mismatched cardiac transplant model

Blockade of the B7: CD28 costimulatory pathway has emerged as a promising therapy to prevent allograft rejection. However, this pathway has also been demonstrated to be important for the generation and maintenance of regulatory T cells. In this study, we investigated the role of the B7: CD28 pathway in the 'bm12 into B6' MHC class II-mismatched vascularized cardiac transplant model of chronic rejection. Allograft rejection was remarkably accelerated in B6 background B7DKO and CD28KO recipients compared with B6 wild-type (WT) recipients. Allograft rejection was associated with a significantly enhanced Th1/Th2 alloreactivity and marked reduction in the ratio of regulatory T cells to CD4(+) effector/memory cells. We noted that administration of anti-B7-1 and anti-B7-2 mAb prior to transplantation also accelerated allograft rejection. Furthermore, depleting CD25(+) cells in B6 WT recipients of bm12 hearts prior to transplant also precipitated rejection at a similar rate. Neither B7/CD28 deficiency nor CD25 depletion affected graft survival in single MHC class I-mismatched (bm1 into B6) recipients. This study highlights the paradoxical functions of B7: CD28 costimulation in a MHC class II-mismatched model, in which the B7: CD28 pathway is demonstrated to be important in preventing rejection through the generation and maintenance of Tregs.

Blocking T cell co-stimulation using a CD80 blocking small molecule reduces delayed type hypersensitivity responses in rhesus monkeys

Blockade of co-stimulation signals between T cells and antigen-presenting cells could be an important approach for treatment of autoimmune diseases and transplant rejection. Recently a series of small compound inhibitors which bind human CD80 (B7-1) and inhibit T cell co-stimulation has been described. To investigate their potency for clinical use, one of these compounds, RhuDex, was evaluated for reactivity with rhesus monkey CD80. The in vitro biological effect on rhesus monkey lymphocytes, the potency for suppression of an inflammatory recall response and the protein-induced delayed type hypersensitivity (DTH) response in the skin were studied. In a rhesus monkey T cell co-stimulation assay RhuDex inhibited proinflammatory cytokine release and cellular proliferation with micromolar potency. Systemic administration of RhuDex to rhesus monkeys inhibited the DTH response significantly, indicating that this compound may inhibit autoimmune mediated inflammatory processes where the target, CD80, is up-regulated.

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.

B7-1 induces immunosuppression when expressed in cultured neonatal mice keratinocytes

BACKGROUND

Chimeric (allo-auto or even xeno-auto) cultured keratinocyte grafting did not exhibit obvious acute rejection or chronic rejection. Although cultured murine keratinocytes were recognized by allogenic CD8+ T cells, they were not rejected. The precise mechanisms underlying this process were unclear. To analyze how keratinocytes attenuated the immune response, we investigated the effect of culturing on neonatal murine keratinocytes and their immunomodulatory properties.

METHODS

Keratinocytes isolated and purified from BALB/c and C57BL/6 neonatal mice were cultured for 7 days. The expression of B7-1, B7-2, B7-H1 and MHC-I was examined by semi-quantitative RT polymerase chain reaction (PCR), fluorescence microscopy and flow cytometry. Cytotoxicity and mixed lymphocyte response (MLR) assays were performed to determine the effects of keratinocytes on cytotoxic T-lymphocyte (CTL) mediated cell lysis and lymphocyte proliferation.

RESULTS

B7-1 was highly expressed in cultured, proliferating murine keratinocytes while no expression of B7-2 and B7-H1 was found. Keratinocytes that expressed B7-1 decreased CTL-mediated cell lysis by an interaction between B7-1 and CTLA-4. In addition, autologous keratinocytes but not allogeneic keratinocytes significantly suppressed auto-specific lymphocyte proliferation in a dose-dependent manner. The modulation was dependent on B7-1 expression and its interaction with CTLA-4.

CONCLUSIONS

Cultured murine keratinocytes expressed B7-1, but not B7-2 or B7-H1. The keratinocytes attenuated CTL-mediated lysis and suppressed lymphocyte proliferation via an interaction with B7-1 and CTLA-4. Therefore, separate expression of B7-1 induced immunosuppression. Non-professional APCs (antigen presenting cells) which separately express B7-1 may possess an ability to induce immunotolerance and thus act as a regulatory APC.

Developments in the scientific and clinical understanding of inflammatory myopathies

The idiopathic inflammatory myopathies are chronic autoimmune disorders sharing the clinical symptom of muscle weakness and, in typical cases, inflammatory cell infiltrates in muscle tissue. During the last decade, novel information has accumulated supporting a role of both the innate and adaptive immune systems in myositis and suggesting that different molecular pathways predominate in different subsets of myositis. The type I interferon activity is one such novel pathway identified in some subsets of myositis. Furthermore, nonimmunological pathways have been identified, suggesting that factors other than direct T cell-mediated muscle fibre necrosis could have a role in the development of muscle weakness.

HLA-G-treated tolerogenic dendritic cells induce tolerogenic potential by increasing expression of B7-1 (CD80) molecules

BACKGROUND

By consensus, HLA-G has a role in the induction of tolerance via interaction with dendritic cells and manipulation of co-stimulatory molecule expression. The purpose of this study was to demonstrate that HLA-G modified dendritic cells exhibit a decreased ability to stimulate T-cells in vitro due to increased expression of B7-1, which provides regulatory signalling to T-cells as a consequence of binding CD28 and CD152 ligands.

METHODS

Bone-marrow cells were cultured from Brown Norway (BN) rat femurs and sorted with anti-rat dendritic cell Ab (OX62) microbeads. Isolated dendritic cells (DCs) were treated with HLA-G tetramer for 3 days. Initially, cells were plated with media, alloantigenic splenocytes, and T-cells and then observed in mixed lymphocyte reaction for thymidine uptake. Also, the cells were analyzed by flow cytometry using antibodies for MHC-II (IA), CD80, and CD86.

RESULTS

This study displays that HLA-G-modified DCs decrease induction of an alloproliferative response. In addition, this study demonstrated that HLA-G tetramer treatment decreases CD86 and permits expression of CD80 without altering MHC-II expression.

DISCUSSION

This study specifically investigated the role of B7-1 (CD80), showing that HLA-G treatment of immature DCs allows expression of B7-1 (CD80) without altering MHC-II expression and simultaneously decreases B7-2 (CD86) expression. Therefore, a low level of B7-2 (CD86) allows attenuation of T-cell response after activation, both of which are beneficial to immune tolerance. The selective expression of B7-1 (CD80) is important and may serve as a guide for future therapies aimed at transplant tolerance.

Further study of anti-ICOS immunotherapy for rat cardiac allograft rejection

PURPOSE

To study the effect of B7-CD28 costimulatory signal blockade by adenovirus-mediated cytotoxic T-lymphocyte-associated antigen 4 immunoglobulin (AdCTLA-4Ig) on cardiac allograft survival in DA (RT1(a)) to LEW (Lewis RT1(l)) rat combinations.

METHODS

We evaluated the effect of combined AdCTLA-4Ig and anti-inducible costimulator (ICOS) antibody immunotherapy on rat cardiac allograft acceptance.

RESULTS

Unlike AdCTLA-4Ig alone, anti-ICOS immunotherapy combined with AdCTLA-4Ig induced stable tolerance without causing chronic rejection. The combined immunotherapy also prevented the accelerated cardiac rejection caused by donor-type test skin grafting. Immunohistochemical analyses revealed remarkable inflammatory mononuclear cell infiltration with typical vasculopathy, especially ICOS-positive cells in the grafts, in recipients treated with AdCTLA-4Ig alone. In contrast, anti-ICOS therapy combined with AdCTLA-4Ig reduced the ICOS-positive inflammatory cell infiltration of the graft significantly. The most important finding is that possible cardiac arrest caused by secondary donor-type skin graft was prevented by combined immunotherapy of AdCTLA-4Ig and anti-ICOS antibody, despite skin graft rejection.

CONCLUSIONS

Our results identified a major role played by the ICOS-ICOSL pathway in chronic and accelerated cardiac allograft rejection, providing a novel approach to preventing the chronic rejection of vascularized organ allografts.

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

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

Blockade of the OX40 ligand prolongs corneal allograft survival

Although corneal transplantation is one of the most common tissue transplantations and is known to have a high graft acceptance rate, occasional corneal graft rejection remains a cause of blindness. OX40, a member of the TNF receptor superfamily, is expressed on activated T cells, and transmits a costimulatory signal by binding to OX40 ligand (OX40L) expressed on several cells with antigen-presenting functions. Using a blocking monoclonal antibody (mAb) against murine OX40L, we investigated the role of OX40 in a murine model of corneal transplantation. C3H/He mouse corneas were transplanted to BALB/c mice orthotopically. Administration of anti-OX40L mAb significantly reduced allograft rejection, and increased graft survival rate to 40% at 8 weeks after transplantation, while all corneas were rejected within 5 weeks in control IgG-treated mice. Similar reduced rejection was observed when wild-type donor corneas were transplanted to OX40L-deficient recipients. In vitro study revealed that the anti-OX40L mAb treatment reduced proliferative response and IFN-gamma production of draining lymph node cells in response to stimulation with donor alloantigen. These results demonstrate that OX40L blockade is effective for prolongation of corneal allograft survival by inhibiting recipient T cell activation.

PDL1 is required for peripheral transplantation tolerance and protection from chronic allograft rejection

The PD-1:PDL pathway plays an important role in regulating alloimmune responses but its role in transplantation tolerance is unknown. We investigated the role of PD-1:PDL costimulatory pathway in peripheral and a well established model of central transplantation tolerance. Early as well as delayed blockade of PDL1 but not PDL2 abrogated tolerance induced by CTLA4Ig in a fully MHC-mismatched cardiac allograft model. Accelerated rejection was associated with a significant increase in the frequency of IFN-gamma-producing alloreactive T cells and expansion of effector CD8(+) T cells in the periphery, and a decline in the percentage of Foxp3(+) graft infiltrating cells. Similarly, studies using PDL1/L2-deficient recipients confirmed the results with Ab blockade. Interestingly, while PDL1-deficient donor allografts were accepted by wild-type recipients treated with CTLA4Ig, the grafts developed severe chronic rejection and vasculopathy when compared with wild-type grafts. Finally, in a model of central tolerance induced by mixed allogeneic chimerism, engraftment was not abrogated by PDL1/L2 blockade. These novel data demonstrate the critical role of PDL1 for induction and maintenance of peripheral transplantation tolerance by its ability to alter the balance between pathogenic and regulatory T cells. Expression of PDL1 in donor tissue is critical for prevention of in situ graft pathology and chronic rejection.

Blockade of LIGHT/HVEM and B7/CD28 Signaling Facilitates Long-Term Islet Graft Survival With Development of Allospecific Tolerance

BACKGROUND

Previous studies have shown that blockade of LIGHT, a T-cell costimulatory molecule belonging to the tumor necrosis factor (TNF) superfamily, by soluble lymphotoxin beta receptor-Ig (LTbetaR-Ig) inhibited the development of graft-versus-host disease. The cardiac allografts were significantly prolonged in LIGHT deficient mice. No data are yet available regarding the role of the LIGHT/HVEM pathway in more stringent fully allogeneic models such as skin and islet transplantation models.

METHODS

Streptozotocin-induced chemical diabetic BALB/C mice underwent transplantation with allogeneic C57BL/6 islets and were treated with LTbetaR-Ig, CTLA4-Ig or a combination of both in the early peritransplant period.

RESULTS

Administration of CTLA4-Ig or LTbeta R-Ig alone only increased graft survival to 55 days and 27 days respectively, whereas simultaneous blockade of both pathways significantly prolonged the islet allograft survival for more than 100 days. Long-term survivors were retransplanted with donor-specific (C57BL/6) islets and the grafted islets remained functional for more than 100 days. All of islet allografts were protected against rejection when the mixtures of 1x10(6) CD4+ T cells from tolerant mice and islet allografts were cotransplanted under the renal capsule of the naïve BALB/c recipients.

CONCLUSIONS

These data indicate that: 1) a synergistic effect for prolonged graft survival can be obtained by simultaneously blocking LIGHT and CD28 signaling in the stringent model of islet allotransplantation; 2) development of donor-specific immunological tolerance is associated with the presence of regulatory T-cell activity; and 3) local cotransplantation of the allografts with the regulatory T cells can effectively prevent allograft rejection and induce donor-specific tolerance in lymphocytes-sufficient recipients.

New insights in CD28-independent allograft rejection

CD28 costimulatory blockade induces tolerance in most murine transplant models but fails to do so in stringent transplant models, such as skin transplantation. The precise immunological mechanisms of CD28-independent rejection remain to be fully defined. Using two novel mouse strains in which both CD28 and either CD4 or CD8 are knocked out (CD4(-/-)CD28(-/-) or CD8(-/-)CD28(-/-) mice), we examined mechanisms of CD28-independent CD4(+) or CD8(+) T-cell-mediated allograft rejection. CD4(-/-)CD28(-/-) and CD8(-/-)CD28(-/) deficient mice rejected fully allogeneic skin allografts at a tempo comparable with that in wild-type mice. Rejection proceeded despite significant reduction in alloreactive T-cell clone sizes suggesting the presence of a subset of T cells harnessing alternate CD28-independent costimulatory pathways. Blockade of CD40-CD154 and CD134-CD134L, but not ICOS-B7h pathways in combination significantly prolonged allograft survival in CD8(-/-)CD28(-/-) recipients and to a lesser extent in CD4(-/-)CD28(-/-) recipients. Prolongation in allograft survival was associated with reduced effector-memory T-cell generation, decreased allospecific Th1 cytokine generation and diminished alloreactive T-cell proliferation in vivo. In aggregate, the data identify these two pathways as critical mediators of CD28-independent rejection by CD4(+) and to a lesser extent CD8(+) T cells, and provide novel mechanistic insights into functions of novel T-cell co-stimulatory pathways in vivo.

The influence of inducible costimulator fusion protein (ICOSIg) gene transfer on corneal allograft survival

BACKGROUND

The purpose of this paper is to analyse the effects of local or systemic administration of adenovirus type 5 encoding the inducible costimulator fusion protein (AdICOSIg) on its influence on prolonging corneal allograft survival.

METHODS

The ICOSIg chimeric molecule was generated by fusing the murine ICOS to a rat FcIgG portion and a recombinant adenovirus (Ad) was made thereof. A major histocompatibility complex (MHC) class I/II mismatched rat corneal transplant model was used. The recipients were randomly assigned to receive ex vivo gene-modified corneas expressing either ICOSIg or a single i.p. injection (1.0 x 10(9) infectious particles) of AdICOSIg two days after transplantation and graft survival was analysed. Moreover, the influence of ICOSIg fusion protein on anti-adenovirus immunity also was investigated.

RESULTS

The ex vivo gene transfer of ICOSIg in cultured corneas resulted in high levels of ICOSIg protein in culture supernatants. However, neither ex vivo nor systemic gene therapy resulted in a significant prolongation of graft survival. Interestingly, the generation of anti-adenovirus antibodies could not be inhibited by systemic ICOSIg fusion protein expression.

CONCLUSIONS

Unlike CTLA4Ig, sole ICOSIg gene therapy is not a successful strategy for the prevention of allogeneic graft rejection in corneal transplantation.

Leukocyte-renal epithelial cell interactions regulate lupus nephritis

Renal disease is the major cause of morbidity in patients with lupus. MRL-Fas(lpr) mice share features with human lupus. The tempo, predictability, and homogeneous expression of disease in MRL-Fas(lpr) mice make them an excellent tool to probe the pathogenesis of lupus nephritis and to identify therapeutic targets. This article focuses on the concepts that renal parenchymal cells are active participants that regulate immune responses in the kidney, and that the interaction between parenchymal cells and leukocytes (macrophages, T cells) determine whether the kidney is protected or destroyed during lupus nephritis. In particular we review the role of macrophages, fueled by the principal macrophage developmental molecule, colony stimulating factor-1, in lupus nephritis, and we review T cells and costimulatory pathways and the interaction of these leukocytes with renal parenchymal cells that regulate lupus nephritis.

Natural killer cell subsets in allograft rejection and tolerance

PURPOSE OF REVIEW

To discuss the role of natural killer cells in regulating the survival of transplanted organs.

RECENT FINDINGS

Natural killer cells have been found to have the dual capacity to promote rejection of transplanted organs and be required for the induction of transplantation tolerance. In murine recipients of bone marrow transplants, or in CD28 recipients of cardiac allografts, different natural killer cell subsets have been shown to promote or delay rejection, depending on their major histocompatibility complex class I specificity. In mouse models of skin and islet allograft acceptance mediated by costimulation-targeting therapies, the presence of natural killer cells was found to be essential for long-term graft acceptance, perhaps due to their ability to eliminate donor or recipient immune cells.

SUMMARY

Natural killer cells can either accelerate or avert rejection in a manner that is influenced by both donor-recipient major histocompatibility complex disparity as well as the milieu created by costimulation-targeting therapies. In clinical settings, alloreactivity by defined natural killer cell subsets may be important in achieving tolerance, and the outcome of natural killer cell activity may be influenced by specific immunosuppressive regimens.

Control of memory CD4 T cell recall by the CD28/B7 costimulatory pathway

The CD28/B7 costimulatory pathway is generally considered dispensable for memory T cell responses, largely based on in vitro studies demonstrating memory T cell activation in the absence of CD28 engagement by B7 ligands. However, the susceptibility of memory CD4 T cells, including central (CD62L(high)) and effector memory (T(EM); CD62L(low)) subsets, to inhibition of CD28-derived costimulation has not been closely examined. In this study, we demonstrate that inhibition of CD28/B7 costimulation with the B7-binding fusion molecule CTLA4Ig has profound and specific effects on secondary responses mediated by memory CD4 T cells generated by priming with Ag or infection with influenza virus. In vitro, CTLA4Ig substantially inhibits IL-2, but not IFN-gamma production from heterogeneous memory CD4 T cells specific for influenza hemagglutinin or OVA in response to peptide challenge. Moreover, IL-2 production from polyclonal influenza-specific memory CD4 T cells in response to virus challenge was completely abrogated by CTLA4Ig with IFN-gamma production partially inhibited. When administered in vivo, CTLA4Ig significantly blocks Ag-driven memory CD4 T cell proliferation and expansion, without affecting early recall and activation. Importantly, CTLA4Ig treatment in vivo induced a striking shift in the phenotype of the responding population from predominantly T(EM) in control-treated mice to predominantly central memory T cells in CTLA4Ig-treated mice, suggesting biased effects of CTLA4Ig on T(EM) responses. Our results identify a novel role for CD28/B7 as a regulator of memory T cell responses, and have important clinical implications for using CTLA4Ig to abrogate the pathologic consequences of T(EM) cells in autoimmunity and chronic disease.

Costimulation blockade and its possible future use in clinical transplantation

The nonimmune effects of currently used immunosuppressive drugs result in a high incidence of late graft loss due to nephrotoxicity and death of patients. As an immune-specific alternative to conventional immunosuppressants, new biotechnology tools can be used to block the costimulation signals of T-cell activation. Many experimental studies--particularly preclinical studies in nonhuman primates--have focused on blocking the 'classical' B7/CD28 and CD40/CD40L pathways, which are critical in primary T-cell activation. Here, we review the limitations, the recent advances and the first large-scale clinical application of the CTLA4-Ig fusion protein to block the B7/CD28 costimulation pathway. We also focus on new B7/CD28 and tumor necrosis factor (TNF)/TNF-R family costimulatory molecules that can deliver positive or negative costimulation signals regulating the alloimmune response. Strategies that use single agents to block costimulation have often proved to be insufficient. Given the diversity of the different costimulation molecules, future strategies for human transplantation may involve the simultaneous blockade of several selected pathways or the simultaneous use of conventional immunosuppressants.

CD80 and CD86 control antiviral CD8+ T-cell function and immune surveillance of murine gammaherpesvirus 68

The interactions between CD80 and CD86 on antigen-presenting cells and CD28 on T cells serve as an important costimulatory signal in the activation of T cells. Although the simplistic two-signal hypothesis has been challenged in recent years by the identification of different costimulators, this classical pathway has been shown to significantly impact antiviral humoral and cellular immune responses. How the CD80/CD86-CD28 pathway affects the control of chronic or latent infections has been less well characterized. In this study, we investigated its role in antiviral immune responses against murine gammaherpesvirus 68 (MHV-68) and immune surveillance using CD80/CD86(-/-) mice. In the absence of CD80/CD86, primary antiviral CD8(+) T-cell responses and the induction of neutralizing antibodies were severely impaired. During long-term immune surveillance, the virus-specific CD8(+) T cells were impaired in IFN-gamma production and secondary expansion and exhibited an altered phenotype. Surprisingly, a low level of viral reactivation in the lung was observed, and this effect was independent of CD28 and CTLA-4. Thus, CD80 and CD86, signaling through CD28 and possibly another unidentified receptor, are required for optimal immune surveillance and antiviral immune responses to murine gammaherpesvirus.

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

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

CTLA-4 Engagement and Regulatory CD4+CD25+T Cells Independently Control CD8+-Mediated Responses under Costimulation Blockade

Blockade of costimulatory signals is a promising therapeutic target to prevent allograft rejection. In this study, we sought to characterize to what extent CTLA-4 engagement contributes to the development of transplantation tolerance under the cover of CD40/CD40L and CD28/CD86 blockade. In vitro, we found that inhibition of the primary alloresponse and induction of alloantigen hyporesponsiveness by costimulation blockade was abrogated by anti-CTLA-4 mAb. In addition, regulatory CD4(+)CD25(+) T cells (T(REG)) were confirmed to play a critical role in the induction of hyporesponsiveness by anti-CD40L and anti-CD86 mAb. Our data indicated that CTLA-4 engagement is not required for activation or suppressor function of T(REG). Instead, in the absence of either CTLA-4 signaling or T(REG), CD8(+) T cell division was enhanced, whereas the inhibition of CD4(+) T cell division by costimulation blockade remained largely unaffected. In vivo, the administration of additional anti-CTLA-4 mAb abrogated anti-CD40L- and anti-CD86 mAb-induced cardiac allograft survival. Correspondingly, rejection was accompanied by enhanced allograft infiltration of CD8(+) cells. We conclude that CTLA-4 signaling and T(REG) independently cooperate in the inhibition of CD8(+) T cell expansion under costimulation blockade.

Role of natural killer cell subsets in cardiac allograft rejection

To achieve donor-specific immune tolerance to allogeneic organ transplants, it is imperative to understand the cell types involved in acute allograft rejection. In wild-type mice, CD4(+) T cells are necessary and sufficient for acute rejection of cardiac allografts. However, when T-cell responses are suboptimal, such as in mice treated with costimulation-targeting agents or in CD28-deficient mice, and perhaps in transplanted patients taking immunosuppressive drugs, the participation of other lymphocytes such as CD8(+) T cells and NK1.1(+) cells becomes apparent. We found that host NK but not NKT cells were required for cardiac rejection. Ly49G2(+) NK cells suppressed rejection, whereas a subset of NK cells lacking inhibitory Ly49 receptors for donor MHC class I molecules was sufficient to promote rejection. Notably, rejection was independent of the activating receptors Ly49D and NKG2D. Finally, our experiments supported a mechanism by which NK cells promote expansion and effector function of alloreactive T cells. Thus, therapies aimed at specific subsets of NK cells may facilitate transplantation tolerance in settings of impaired T-cell function.

T-cell costimulatory pathways in allograft rejection and tolerance

A key factor driving the underlying pathyphysiology of "chronic rejection" in organ transplantation is a persistent T cell-mediated alloimmune response. Members of both the B7 family (including CD28 and CTLA4) and the tumor necrosis factor (TNF) family, in which the CD40-CD154 pathway is preeminent, play key roles in the T cell response following alloantigen presentation. "Positive" costimulatory molecules promote full T cell activation, whereas a subgroup of costimulatory molecules delivers "negative" costimulatory signals that function to downregulate alloimmune responses. Emerging experimental data point to key differences between the various positive and negative costimulatory molecules in terms of their temporal and spatial expression profiles, their effects of T and B cell subsets, and on their relative importance within the hierarchy of costimulatory signals delivered to the T cell. In this review, we address the role of costimulatory pathways in allograft rejection and tolerance. We will address in particular the potential of the novel costimulatory pathways as targets for tolerance induction in CD28-independent alloresponses, and we will review emerging data that suggests a key role for parenchymal expression of negative costimulatory molecules in the termination of pathogenic immune responses.

Quantitative mRNA expression analysis of co-stimulatory molecules in sequential biopsies from heart allografts

Allograft-specific T-cell activation requires two signals, "signal one" via the T-cell receptor and "signal two" via costimulatory pathways. Animal models provide promising data that blockade of costimulatory signals can lead to T-cell anergy and tolerance. We analyzed 72 formalin-fixed and paraffin-embedded sequential heart allograft biopsies from 22 patients by quantitative real-time reverse transcriptase-polymerase chain reaction for the mRNA expression of the costimulatory molecules CD28, CD80, CD86, CD40, and CD154. mRNA expression levels were correlated to histological grade and time-point of rejection. mRNA expression of costimulatory molecules predominantly expressed by antigen-presenting cells (CD80, CD86, CD40), correlated with histological grade of cell-mediated acute rejection. Costimulatory molecules were up-regulated not only during rejection episodes early after transplantation, but also at late occurring rejection. The present results suggest a simultaneous and long-term therapeutical blockade of more than one costimulatory pathway for the prevention of repetitive T-cell mediated acute rejection episodes after heart transplantation.

Requirements for induction and maintenance of peripheral tolerance in stringent allograft models

Peripheral tolerance can be achieved in many but not all murine allograft models. The requirements for controlling more aggressive immune responsiveness and generating peripheral tolerance in stringent allograft models are unknown. Understanding these requirements will provide insight toward ultimately achieving tolerance in humans, which are also resistant. We now demonstrate that the combination of donor-specific transfusion, anti-CD45RB, and anti-CD154 uniformly achieves >90-d survival of BALB/c skin allografts on C57BL/6 recipients. Recipients exhibit marked hyporesponsiveness to alloantigen in vitro. In distinct contrast to less rigorous models, engraftment remains absolutely dependent on cytotoxic T lymphocyte antigen 4 signaling, even after grafts are healed, suggesting that prolonged engraftment cannot simply be attributed to more effective depletion of alloreactive T cells but is actively maintained by regulation. Concordantly, we show that both CD4 and CD8 regulatory cells are required and can transfer donor-specific tolerance to naïve recipients. Nonetheless, most recipients ultimately develop gradual graft loss (median survival time = 140 d), suggesting that alloreactive cells emerging from the thymus eventually overwhelm regulatory capacity. In agreement, adding thymectomy to the regimen results in permanent engraftment (>250 d) and donor-specific tolerance not observed previously in this model. These results highlight the potency of both CD4 and CD8 regulatory cells but also suggest that in stringent settings, regulatory T cell longevity and capacity for infectious tolerance compete with prolonged graft immunogenicity and thymic output. These results provide insight into the mechanisms of tolerance in stringent models and provide a rational basis for innovative tolerogenic strategies in humans.

Analysis of the role of negative T cell costimulatory pathways in CD4 and CD8 T cell-mediated alloimmune responses in vivo

Negative costimulatory signals mediated via cell surface molecules such as CTLA-4 and programmed death 1 (PD-1) play a critical role in down-modulating immune responses and maintaining peripheral tolerance. However, their role in alloimmune responses remains unclear. This study examined the role of these inhibitory pathways in regulating CD28-dependent and CD28-independent CD4 and CD8 alloreactive T cells in vivo. CTLA-4 blockade accelerated graft rejection in C57BL/6 wild-type recipients and in a proportion of CD4(-/-) but not CD8(-/-) recipients of BALB/c hearts. The same treatment led to prompt rejection in CD28(-/-) and a smaller proportion of CD4(-/-)CD28(-/-) mice with no effect in CD8(-/-)CD28(-/-) recipients. These results indicate that the CTLA-4:B7 pathway provides a negative signal to alloreactive CD8(+) T cells, particularly in the presence of CD28 costimulation. In contrast, PD-1 blockade led to accelerated rejection of heart allografts only in CD28(-/-) and CD8(-/-)CD28(-/-) recipients. Interestingly, PD-1 ligand (PD-L1) blockade led to accelerated rejection in wild-type mice and in all recipients lacking CD28 costimulation. This effect was accompanied by expansion of IFN-gamma-producing alloreactive T cells and enhanced generation of effector T cells in rejecting allograft recipients. Thus, the PD-1:PD-L1 pathway down-regulates alloreactive CD4 T cells, particularly in the absence of CD28 costimulation. The differential effects of PD-1 vs PD-L1 blockade support the possible existence of a new receptor other than PD-1 for negative signaling through PD-L1. Furthermore, PD-1:PD-L1 pathway can regulate alloimmune responses independent of an intact CD28/CTLA-4:B7 pathway. Harnessing physiological mechanisms that regulate alloimmunity should lead to development of novel strategies to induce durable and reproducible transplantation tolerance.

Therapeutic peptidomimetic strategies for autoimmune diseases: costimulation blockade*

Cognate interactions between immune effector cells and antigen-presenting cells (APCs) govern immune responses. Specific signals occur between the T-cell receptor peptide and APCs and nonspecific signals between pairs of costimulatory molecules. Costimulation signals are required for full T-cell activation and are assumed to regulate T-cell responses as well as other aspects of the immune system. As new discoveries are made, it is becoming clear how important these costimulation interactions are for immune responses. Costimulation requirements for T-cell regulation have been extensively studied as a way to control many autoimmune diseases and downregulate inflammatory reactions. The CD28:B7 and the CD40:CD40L families of molecules are considered to be critical costimulatory molecules and have been studied extensively. Blocking the interaction between these molecules results in a state of immune unresponsiveness termed 'anergy'. Several different strategies for blockade of these interactions are explored including monoclonal antibodies (mAbs), Fab fragments, chimeric, and/or fusion proteins. We developed novel, immune-specific approaches that interfere with these interactions. Using experimental autoimmune encephalomyelitis (EAE), an animal model for multiple sclerosis mediated by central nervous system (CNS)-specific T-cells, we developed a multi-targeted approach that utilizes peptides for blockade of costimulatory molecules. We designed blocking peptide mimics that retain the functional binding area of the parent protein while reducing the overall size and are thus capable of blocking signal transduction. In this paper, we review the role of costimulatory molecules in autoimmune diseases, two of the most well-studied costimulatory pathways (CD28/CTLA-4:B7 and CD40:CD40L), and the advantages of peptidomimetic approaches. We present data showing the ability of peptide mimics of costimulatory molecules to suppress autoimmune disease and propose a mechanism for disease suppression.

Role of the ICOS-B7h Costimulatory Pathway in the Pathophysiology of Chronic Allograft Rejection

BACKGROUND

Inducible costimulator (ICOS) is the third member of the CD28 superfamily and has a unique role in T cell activation and function. Recent studies indicated that the ICOS-B7h pathway plays an important role in alloimmune responses. We further investigated the role of the ICOS pathway in the pathologic process of chronic rejection in vivo.

METHODS

An established major histocompatibility complex class II disparate cardiac transplantation model was used. We treated mice with a blocking anti-B7h monoclonal antibody (mAb) either in the initiation phase (early blockade) or in the progression phase (delayed blockade) of disease. In addition, some mice received mAb in the entire period (whole blockade). At 6 weeks after transplantation, cardiac grafts were evaluated by histopathologic analysis in terms of vasculopathy, fibrosis, and cellular infiltration. The intragraft expressions of cytokines and chemokines were also examined by quantitative real-time polymerase chain reaction analysis.

RESULTS

Early blockade of the ICOS-B7h pathway did not show any protective effect on chronic allograft rejection compared with untreated controls. In contrast, delayed blockade significantly inhibited the development of vasculopathy, fibrosis, and cellular infiltration (P=0.043, P=0.004, and P=0.03 vs. untreated control, respectively). Interestingly, whole blockade did not prevent the chronic rejection process. Furthermore, the inhibitory effect of delayed ICOS blockade on chronic rejection was associated with down-regulation of local intragraft expression of several cytokines and chemokines.

CONCLUSIONS

These data suggest that the ICOS-B7h pathway is critical in the activation of effector/memory T cells that are necessary for the progression of chronic rejection and provide the rationale to develop novel and specific therapies to prevent this process.