Resting dendritic cells induce peripheral CD8+ T cell tolerance through PD-1 and CTLA-4

The sound of silence: modulating anergy in T lymphocytes

Understanding the intercellular and intracellular mechanisms that maintain anergy and prevent the induction of full effector function is one avenue that may allow us to manipulate immune responses. Recent studies of T cell receptor (TCR)-proximal signaling events in different models of T cell unresponsiveness have suggested that biochemically distinct forms of anergy may exist in vivo. T cell responsiveness can be altered through the control of the intracellular pool of key second messengers, such as diacylglycerol (DAG) or the lipid modification of signaling molecules, such as the Linker for activated T cells (LAT). Studies on the molecule programmed death-1 (PD-1) and its ligands have revealed that tissue-resident signals are essential in the maintenance of T cell unresponsiveness. Thus, the emerging view is that T cell anergy is a dynamic state whose establishment and maintenance can be influenced by numerous different signaling pathways.

Innate immune control of pulmonary dendritic cell trafficking

Dendritic cells (DC) are potent antigen-presenting cells that are essential for initiating adaptive immune responses. Residing within the airway mucosa, pulmonary DC continually sample the antigenic content of inhaled air and migrate to draining lymph nodes, where they present these antigens to naive T cells. The migratory patterns of pulmonary DC are highly dependent upon inflammatory conditions in the lung. Under steady-state, or non-inflammatory, conditions, pulmonary DC undergo slow but constitutive migration to draining lymph nodes, where they remain for several days and confer antigen-specific tolerance. With the onset of pulmonary inflammation, airway DC trafficking increases dramatically, and these cells rapidly accumulate within draining lymph nodes. However, within a few days, the number of airway-derived DC in lymph nodes stabilizes or declines, even in the face of ongoing pulmonary inflammation. Here, we summarize current understanding of the molecular and cellular mechanisms underlying pulmonary DC trafficking to the lymph node and the recruitment of DC precurors to the lung. It is hoped that an improved understanding of these mechanisms will lead to novel DC-mediated therapeutic strategies to treat immune-related pulmonary disease.

PD-1 upregulation is associated with HBV-specific T cell dysfunction in chronic hepatitis B patients

Programmed death-1 (PD-1) is demonstrated to have an increased expression on antigen-specific T cells during chronic virus infections, and the blockage of PD-1/PD-ligand (PD-L1) pathway could restore the function of exhausted T cells. We measured the PD-1 expression levels on HBV-specific CD8 T cells and investigated the role of PD-1/PD-L1 pathway in T-cell responses of patients with different HBV infection statuses. Compared to the patients with convalescent acute hepatitis B, PD-1 expression on total CD8 T cells from chronic hepatitis B (CHB) patients was significantly upregulated, especially on the HBV pentamer-positive CD8 T cells. And PD-L1, but not PD-L2, was also significantly upregulated on PBMC from CHB patients. In CHB patients, HBV-specific T cells and cellular proliferation could be observed under the recombinant HBV-Ag stimulation in vitro, and blockade of PD-1 pathway significantly enhanced the IFN-gamma production and cellular proliferation of PBMC. Furthermore, PD-1 expression level on HBV-pentamers positive CD8 T cells was positively associated with plasma viral load in CHB patients. Thus, PD-1 upregulation on HBV-specific CD8 T cells is engaged in the dysfunction of T cells and high viraemia in CHB patients, and the antiviral T-cell responses could be improved by the blockade of this inhibitory PD-1/PD-L1 pathway.

Targeting Molecular and Cellular Inhibitory Mechanisms for Improvement of Antitumor Memory Responses Reactivated by Tumor Cell Vaccine

Development of effective vaccination approaches to treat established tumors represents a focus of intensive research because such approaches offer the promise of enhancing immune system priming against tumor Ags via restimulation of pre-existing (memory) antitumoral helper and effector immune cells. However, inhibitory mechanisms, which function to limit the recall responses of tumor-specific immunity, remain poorly understood and interfere with therapies anticipated to induce protective immunity. The mouse renal cell carcinoma (RENCA) tumor model was used to investigate variables affecting vaccination outcomes. We demonstrate that although a whole cell irradiated tumor cell vaccine can trigger a functional antitumor memory response in the bone marrows of mice with established tumors, these responses do not culminate in the regression of established tumors. In addition, a CD103+ regulatory T (Treg) cell subset accumulates within the draining lymph nodes of tumor-bearing mice. We also show that B7-H1 (CD274, PD-L1), a negative costimulatory ligand, and CD4+ Treg cells collaborate to impair the recall responses of tumor-specific memory T cells. Specifically, mice bearing large established RENCA tumors were treated with tumor cell vaccination in combination with B7-H1 blockade and CD4+ T cell depletion (triple therapy treatment) and monitored for tumor growth and survival. Triple treatment therapy induced complete regression of large established RENCA tumors and raised long-lasting protective immunity. These results have implications for developing clinical antitumoral vaccination regimens in the setting in which tumors express elevated levels of B7-H1 in the presence of abundant Treg cells.

Deletional Self-Tolerance to a Melanocyte/Melanoma Antigen Derived from Tyrosinase Is Mediated by a Radio-Resistant Cell in Peripheral and Mesenteric Lymph Nodes

Self-tolerance to melanocyte differentiation Ags limits the ability to generate therapeutic antimelanoma responses. However, the mechanisms responsible for CD8 T cell tolerance to these Ags are unknown. We have used a newly generated TCR-transgenic mouse to establish the basis of tolerance to one such Ag from tyrosinase. Despite expression of tyrosinase transcripts in the thymus, central deletion does not shape the tyrosinase-specific CD8 T cell repertoire. We demonstrate that this endogenously expressed melanocyte Ag is constitutively presented in both peripheral and mesenteric lymph nodes, leading to abortive activation and deletion of tyrosinase-specific CD8 T cells. Importantly, this Ag is not presented by either radio-sensitive dendritic cells, or by radio-resistant Langerhans cells. Thus, for this endogenous Ag, cross-tolerization does not appear to be an operative mechanism. Instead, we find radioresistant tyrosinase mRNA expression in lymphoid compartments where CD8 T cell deletion occurs. This suggests that direct presentation of tyrosinase by radio-resistant lymph node resident cells is entirely responsible for tolerance to this endogenous melanocyte differentiation Ag.

Polymorphisms of Genes for Programmed Cell Death 1 Ligands in Patients with Rheumatoid Arthritis

To investigate the role of ligands for programmed cell death 1 (PD-L) in the pathogenesis of rheumatoid arthritis (RA), 129 patients with RA and 125 unrelated healthy controls were enrolled in this study. The PD-L1 and PD-L2 polymorphisms were determined by the method of polymerase chain reaction (PCR)/direct sequencing or PCR/reaction fragment length polymorphisms. The genotype distributions of PD-L1 6777 C/G were not significantly different between the patients with RA and healthy controls. There was also no significant difference in the allele frequencies of PD-L1 6777 C/G polymorphisms between the patients with RA and controls. Similar findings could also be found in the phenotypes and alleles frequencies of PD-L2 47103 C/T and 47139 T/C polymorphisms between the patients with RA and controls. The patients with PD-L1 6777 G had higher prevalence of rheumatoid nodule in comparison with those without PD-L1 6777 G (p = 0.005, OR = 4.0, 95% CI = 1.5-10.9). In contrast, the PD-L2 47103 C/T and 47139 T/C polymorphisms were not related to the occurrence of rheumatoid nodule. This study demonstrated that the PD-L1 and PD-L2 polymorphisms were not associated with susceptibility to RA in Taiwan. PD-L1 6777 G was associated with the prevalence of rheumatoid nodule.

Contribution of the PD-L1/PD-1 pathway to T-cell exhaustion: an update on implications for chronic infections and tumor evasion

Recent clinical data support ideas of Programmed death receptor-ligand 1 (PD-L1; also called B7-H1, CD274) playing an important role in immune evasion of tumor cells. Expression of PD-L1 on tumors strongly correlates with the survival of cancer patients. PD-L1 on tumors interacts with the co-inhibitory molecule Programmed death receptor-1 (PD-1, CD279) on T cells mediating decreased TCR-mediated proliferation and cytokine production. In animal tumor models, blockade of PD-L1/PD-1 interactions resulted in an improved tumor control. In addition, exhausted T cells during chronic viral infections could be revived by PD-L1 blockade. Thus, targeting PD-L1/PD-1 interactions might improve the efficacy of adoptive cell therapies (ACT) of chronic infections as well as cancers. Obstacles for a general blockade of PD-L1 might be its role in mediating peripheral tolerance. This review discusses the currently available data concerning the role of PD-L1 in tumor immune evasion and envisions possibilities for implementation into ACT for cancer patients.

The function of programmed cell death 1 and its ligands in regulating autoimmunity and infection

The programmed cell death 1 (PD-1) surface receptor binds to two ligands, PD-L1 and PD-L2. Studies have shown that PD-1-PD-L interactions control the induction and maintenance of peripheral T cell tolerance and indicate a previously unknown function for PD-L1 on nonhematopoietic cells in protecting tissues from autoimmune attack. PD-1 and its ligands have also been exploited by a variety of microorganisms to attenuate antimicrobial immunity and facilitate chronic infection. Here we examine the functions of PD-1 and its ligands in regulating antimicrobial and self-reactive T cell responses and discuss the therapeutic potential of manipulating this pathway.

Limited peripheral T cell anergy predisposes to retinal autoimmunity

Autoimmune uveoretinitis accounts for at least 10% of worldwide blindness, yet it is unclear why tolerance to retinal Ags is so fragile and, particularly, to what extent this might be due to defects in peripheral tolerance. To address this issue, we generated double-transgenic mice expressing hen egg lysozyme, under the retinal interphotoreceptor retinoid-binding promoter, and a hen egg lysozyme-specific CD4(+) TCR transgene. In this manner, we have tracked autoreactive CD4(+) T cells from their development in the thymus to their involvement in uveoretinitis and compared tolerogenic mechanisms induced in a variety of organs to the same self-Ag. Our findings show that central tolerance to retinal and pancreatic Ags is qualitatively similar and equally dependent on the transcriptional regulator protein AIRE. However, the lack of Ag presentation in the eye-draining lymph nodes results in a failure to induce high levels of T cell anergy. Under these circumstances, despite considerable central deletion, low levels of retinal-specific autoreactive CD4(+) T cells can induce severe autoimmune disease. The relative lack of anergy induction by retinal Ags, in contrast to the same Ag in other organs, helps to explain the unique susceptibility of the eye to spontaneous and experimentally induced autoimmune disease.

Role of PD-1 and its ligand, B7-H1, in early fate decisions of CD8 T cells

Expression of the PD-1 receptor on T cells has been shown to provide an important inhibitory signal that down-modulates peripheral effector responses in normal tissues and tumors. Furthermore, PD-1 up-regulation on chronically activated T cells can maintain them in a partially reversible inactive state. The function of PD-1 in the very early stages of T-cell response to antigen in vivo has not been fully explored. In this study, we evaluate the role of PD-1 and its 2 B7 family ligands, B7-H1 (PD-L1) and B7-DC (PD-L2), in early fate decisions of CD8 T cells. We show that CD8 T cells specific for influenza hemagglutinin (HA) expressed as a self-antigen become functionally tolerized and express high levels of surface PD-1 by the time of their first cell division. Blockade of PD-1 or B7-H1, but not B7-DC, at the time of self-antigen encounter mitigates tolerance induction and results in CD8 T-cell differentiation into functional cytolytic T lymphocytes (CTLs). These findings demonstrate that, in addition to modulating effector functions in the periphery, B7-H1:PD-1 interactions regulate early T-cell-fate decisions.

Tolerization of tumor-specific T cells despite efficient initial priming in a primary murine model of prostate cancer

In this report, we studied T cell responses to a prostate cancer Ag by adoptively transferring tumor Ag-specific T cells into prostate tumor-bearing mice. Our findings demonstrate that CD8(+) T cells initially encountered tumor Ag in the lymph node and underwent an abortive proliferative response. Upon isolation from the tumor, the residual tumor-specific T cells were functionally tolerant of tumor Ag as measured by their inability to degranulate and secrete IFN-gamma and granzyme B. We next sought to determine whether providing an ex vivo-matured, peptide-pulsed dendritic cell (DC) vaccine could overcome the tolerizing mechanisms of tumor-bearing transgenic adenocarcinoma of the mouse prostate model mice. We demonstrate that tumor Ag-specific T cells were protected from tolerance following provision of the DC vaccine. Concurrently, there was a reduction in prostate tumor size. However, even when activated DCs initially present tumor Ag, T cells persisting within the tolerogenic tumor environment gradually lost Ag reactivity. These results suggest that even though a productive antitumor response can be initiated by a DC vaccine, the tolerizing environment created by the tumor still exerts suppressive effects on the T cells. Furthermore, our results demonstrate that when trying to elicit an effective antitumor immune response, two obstacles must be considered: to maintain tumor Ag responsiveness, T cells must be efficiently primed to overcome tumor Ag presented in a tolerizing manner and protected from the suppressive mechanisms of the tumor microenvironment.

Signalling to suit function: tailoring phosphoinositide 3-kinase during T-cell activation

Members of the CD28 family of co-receptors are crucial determinants of the outcome of T-cell activation. These receptors interact with ligands in the B7 family and either costimulate or co-inhibit signals through antigen-specific receptors. The T-cell-costimulatory molecules CD28 and inducible costimulator recruit and activate class 1A phosphoinositide 3-kinase (PI3K). Interestingly, the co-inhibitory molecules cytotoxic T lymphocyte antigen-4 and B and T lymphocyte attenuator also interact with class 1A PI3K. However, all co-inhibitory receptors share an ability to oppose activation of the key PI3K effector protein kinase B (also known as Akt). Recent evidence suggests that distinct mechanisms exist to limit Akt activation by different co-inhibitory receptors. This article examines how differential positive or negative regulation of the PI3K-Akt signalling pathway by CD28 family receptors enables functional differences between the receptors.

Priming of CD8+ T cell responses by pathogens typically depends on CD70-mediated interactions with dendritic cells

The CD27/CD70-interaction has been shown to provide a costimulatory and survival signal for T cells in vitro and in vivo. Recently, CD70 expression by DC was found to be important for the priming of CD8+ T cells. We show here that blocking CD70 interactions has a significant impact on priming of CD8+ T cell responses by vaccinia virus (VV), Listeria monocytogenes and vesicular stomatitis virus (VSV) in mice. However, the priming of specific CD8+ T cells upon infection with lymphocytic choriomeningitis virus (LCMV) was only marginally reduced by CD70-blockade. Blocking of CD70 prevented CD8+ T cell priming in DIETER mice, a model in which presentation of LCMV-derived epitopes can be induced selectively in dendritic cells (DC). In contrast, CD70-CD27 interactions were not important for the priming of VSV-specific CD4+ T cells or class switch of neutralizing antibodies. As we show that priming of CD8+ T cells by the pathogens used here is dependent on antigen presentation by DC and that infection results in up-regulation of CD70 on DC, we conclude that CD70 expression on DC plays an important role in the priming of CD8+ T cells by pathogens. Moreover, the lack of CD70 cannot be completely compensated for by other costimulatory molecules.

Donor CD8+ T cells mediate graft-versus-leukemia activity without clinical signs of graft-versus-host disease in recipients conditioned with anti-CD3 monoclonal antibody

Donor CD8(+) T cells play a critical role in mediating graft-vs-leukemia (GVL) activity, but also induce graft-vs-host disease (GVHD) in recipients conditioned with total body irradiation (TBI). In this study, we report that injections of donor C57BL/6 (H-2(b)) or FVB/N (H-2(q)) CD8(+) T with bone marrow cells induced chimerism and eliminated BCL1 leukemia/lymphoma cells without clinical signs of GVHD in anti-CD3-conditioned BALB/c (H-2(d)) recipients, but induced lethal GVHD in TBI-conditioned recipients. Using in vivo and ex vivo bioluminescent imaging, we observed that donor CD8(+) T cells expanded rapidly and infiltrated GVHD target tissues in TBI-conditioned recipients, but donor CD8(+) T cell expansion in anti-CD3-conditioned recipients was confined to lymphohematological tissues. This confinement was associated with lack of up-regulated expression of alpha(4)beta(7) integrin and chemokine receptors (i.e., CXCR3) on donor CD8(+) T cells. In addition, donor CD8(+) T cells in anti-CD3-conditioned recipients were rendered unresponsive, anergic, Foxp3(+), or type II cytotoxic T phenotype. Those donor CD8(+) T cells showed strong suppressive activity in vitro and mediated GVL activity without clinical signs of GVHD in TBI-conditioned secondary recipients. These results indicate that anti-CD3 conditioning separates GVL activity from GVHD via confining donor CD8(+) T cell expansion to host lymphohemological tissues as well as tolerizing them in the host.

Interaction between B7-H1 and PD-1 determines initiation and reversal of T-cell anergy

Although self-reactive T-cell precursors can be eliminated upon recognition of self-antigen presented in the thymus, this central tolerance process is often incomplete, and additional mechanisms are required to prevent autoimmunity. Recent studies indicates that the interaction between B7-H1 and its receptor PD-1 on activated T cells plays an important role in the inhibition of T-cell responses in peripheral organs. Here, we show that, before their exit to the periphery, T cells in lymphoid organs rapidly up-regulate PD-1 upon tolerogen recognition. Ablation of the B7-H1 and PD-1 interaction when T cells are still in lymphoid organs prevents anergy. Furthermore, blockade of B7-H1 and PD-1 interaction could render anergic T cells responsive to antigen. Our results thus reveal previously unappreciated roles of B7-H1 and PD-1 interaction in the control of initiation and reversion of T-cell anergy.

Cognate CD4+ Help Elicited by Resting Dendritic Cells Does Not Impair the Induction of Peripheral Tolerance in CD8+ T Cells

Peripheral tolerance is required to prevent autoimmune tissue destruction by self-reactive T cells that escape negative selection in the thymus. One mechanism of peripheral tolerance in CD8(+) T cells is their activation by resting dendritic cells (DC). In contrast, DC can be "licensed" by CD4(+) T cells to induce cytotoxic function in CD8(+) T cells. The question that then arises, whether CD4(+) T cell help could impair peripheral tolerance induction in self-reactive CD8(+) T cells, has not been addressed. In this study we show that CD4(+) T cell activation by resting DC results in helper function that transiently promotes the expansion and differentiation of cognate CD8(+) T cells. However, both the CD4(+) and CD8(+) T cell populations ultimately undergo partial deletion and acquire Ag unresponsiveness, disabling their ability to destroy OVA-expressing pancreatic beta cells and cause diabetes. Thus, effective peripheral tolerance can be induced by resting DC in the presence of CD4(+) and CD8(+) T cells with specificity for the same Ag.

Incomplete differentiation of antigen-specific CD8 T cells in tumor-draining lymph nodes

CD8 T cells lacking effector activity have been recovered from lymphoid organs of mice and patients with progressing tumors. We explored the basis for lack of effector activity in tumor-bearing mice by evaluating Ag presentation and CD8 T cell function in lymphoid organs over the course of tumor outgrowth. Early after tumor injection, cross-presentation by bone marrow-derived APC was necessary for T cell activation, inducing proliferation and differentiation into IFN-gamma-producing, cytolytic effectors. At later stages of outgrowth, tumor metastasized to draining lymph nodes. Both cross- and direct presentation occurred, but T cell differentiation induced by either modality was incomplete (proliferation without cytokine production). T cells within tumor-infiltrated nodes differentiated appropriately if Ag was presented by activated, exogenous dendritic cells. Thus, activated T cells lacking effector function develop through incomplete differentiation in the lymph nodes of late-stage tumor-bearing mice, rather than through suppression of previously differentiated cells.

Tolerogenic dendritic cells: the ins and outs of outcome

Recent studies point to an important role for dendritic cells (DCs) in the induction of peripheral tolerance, revealing that the maturation and/or activation state of DCs might be a control point for the induction of peripheral tolerance. Recent progress in our understanding of the mechanisms mediating immune tolerance indicates them to be far more complex than hitherto anticipated. Factors deciding the outcome of vaccination with autologous DCs to prevent and treat diseases with an autoimmune background include maturation state of DCs, their administration route, long-term effects, antigen loading, and in vivo microenvironment. DC vaccination, although promising, is far from standardized. In this review, we discuss the ins and outs of DC-mediated immune tolerance and the need for careful experimental design to unequivocally prove the efficacy and reach the goal of optimized use of DCs in autoimmune diseases.

CD8+ regulatory T cells generated by neonatal recognition of peripheral self-antigen

In comparison with CD4+ regulatory T cells, the generation and function of immunomodulatory CD8+T cells is less well defined. Here we describe the existence of regulatory anti-Kb-specific CD8+ T cells that are rendered tolerant during neonatal life via antigen contact exclusively on keratinocytes. These regulatory T cells maintain tolerance during adulthood as they prevent Kb-specific graft rejection by naïve CD8+ T cells. Third-party immune responses remain unaffected. Up-regulation of TGF-beta1 and granzyme B in the regulatory CD8+ T cell population suggests the involvement of these molecules in common suppressive pathways shared with CD4+ regulatory T cells. In summary, CD8+ regulatory T cells can be induced extrathymically through antigen contact on neonatally accessible parenchymal cells and maintain tolerance throughout adult life.

Dendritic cells: translating innate to adaptive immunity

The innate immune system provides many ways to quickly resist infection. The two best-studied defenses in dendritic cells (DCs) are the production of protective cytokines-like interleukin (IL)-12 and type I interferons-and the activation and expansion of innate lymphocytes. IL-12 and type I interferons influence distinct steps in the adaptive immune response of lymphocytes, including the polarization of T-helper type 1 (Th1) CD4+ T cells, the development of cytolytic T cells and memory, and the antibody response. DCs have many other innate features that do not by themselves provide innate resistance but are critical for the induction of adaptive immunity. We have emphasized three intricate and innate properties of DCs that account for their sentinel and sensor roles in the immune system: (1) special mechanisms for antigen capture and processing, (2) the capacity to migrate to defined sites in lymphoid organs, especially the T cell areas, to initiate immunity, and (3) their rapid differentiation or maturation in response to a variety of stimuli ranging from Toll-like receptor (TLR) ligands to many other nonmicrobial factors such as cytokines, innate lymphocytes, and immune complexes. The combination of innate defenses and innate physiological properties allows DCs to serve as a major link between innate and adaptive immunity. DCs and their subsets contribute to many subjects that are ripe for study including memory, B cell responses, mucosal immunity, tolerance, and vaccine design. DC biology should continue to be helpful in understanding pathogenesis and protection in the setting of prevalent clinical problems.

Protective Regulatory T Cell Generation in Autoimmune Diabetes by DNA Covaccination with Islet Antigens and a Selective CTLA-4 Ligand

DNA vaccination of autoimmune diabetes-prone NOD mice with unmodified target islet antigens, i.e., preproinsulin (PPIns) or glutamic acid decarboxylase 65 (GAD65), is poorly protective. However, in this study, we demonstrate protection against disease by covaccination with a mutant B7-1 molecule (B7-1wa) that binds the negative T cell regulator CTLA-4 (CD152), but not CD28. Codelivery of plasmids encoding a PPIns-GAD65 fusion construct and B7-1wa protected against both insulitis and diabetes. In vitro, the T cells of covaccinated mice had negative responses to both insulin and GAD65, and this was restored by adding blocking antibodies to transforming growth factor beta1 (TGF-beta1), suggesting a role for this cytokine. Adoptive transfer experiments revealed that DNA vaccination generated protective CD4(+) regulatory T cells (Tr) of either CD25(+) or CD25(-) phenotype. Furthermore, vaccinated mice had increased numbers of T cells with Tr-associated markers, such as CTLA-4, Foxp3, and membrane-bound TGF-beta1. Tr cells inhibited the responses of diabetogenic T cells to islet antigens, and depletion of T cells expressing membrane-bound TGF-beta1 abolished the suppressive effect. Thus, selective engagement of CTLA-4 during islet-antigen DNA vaccination induces Tr cells that protect against this autoimmune disease.

Molecules involved in T-B co-stimulation and B cell homeostasis: possible targets for an immunological intervention in autoimmunity

The deepened knowledge of co-stimulatory mechanisms within the immunological synapse and the emerging biological principles governing B cell homeostasis provide a plethora of new possibilities to selectively block or enhance immune responses. These mechanisms are highly relevant to the development of new treatment modalities for autoimmune diseases. Here we review approaches to antagonise members of the CD28-B7 superfamily as well as the TNF receptor ligand superfamily members, BAFF and APRIL, and their corresponding receptors on B cells (BAFF-R, TACI and BCMA). The proof of principle that such manipulations have indeed profound consequences for the human immune response comes from genetically manipulated mouse models, and, more importantly, from human immunodeficiency syndromes. Thus, the recent discovery of deletions in the ICOS, BAFF-R and TACI genes leading to disturbances in late B cell differentiation and hypogammaglobulinaemia underline the potential impact of targeting these molecules for therapeutic strategies in autoimmune disorders.

Induction of allospecific tolerance by immature dendritic cells genetically modified to express soluble TNF receptor

The ability of dendritic cells (DC) to initiate immune responses or induce immune tolerance is strictly dependent on their maturation state. TNF-alpha plays a pivotal role in the differentiation and maturation of DC. Blockade of TNF-alpha action may arrest DC in an immature state, prolonging their window of tolerogenic opportunity. Immature DC (imDC) were transfected with recombinant adenovirus to express soluble TNF-alpha receptor type I (sTNFRI), a specific inhibitor of TNF-alpha. The capacity of sTNFRI gene-modified imDC (DC-sTNFRI) to induce immune tolerance was analyzed. sTNFRI expression renders imDC resistant to maturation induction and impairs their capacity to migrate or present Ag. This process leads to induction of allogeneic T cell hyporesponsiveness and the generation of IL-10-producing T regulatory-like cells. In vivo pretreatment of transplant recipients with DC-sTNFRI induces long-term survival of cardiac allografts in 50% of cases, and leads to a substantial increase in the generation of microchimerism and T regulatory cell numbers. Thus, blockade of TNF-alpha action by sTNFRI genetic modification can inhibit the maturation of DC and potentiate the in vivo capacity of imDC to induce donor-specific immune tolerance and prolong allograft survival.

Contribution of the PD-1 ligands/PD-1 signaling pathway to dendritic cell-mediated CD4+ T cell activation

Dendritic cells (DC) are extremely proficient inducers of naïve CD4+ T cell activation due to their high expression level of peptide-MHC and an array of accessory molecules involved in cell migration, adhesion and co-signaling, including PD-1 ligand 1 (PD-L1) and PD-1 ligand 2 (PD-L2). Whether PD-L1 and PD-L2 have a stimulatory or inhibitory function is a matter of debate, and could be partially dependent on the model system used. In this study we examined the role of PD-L1 and PD-L2 expressed by DC in naïve CD4+ T cell activation in a more physiologically relevant model system, using OVA-specific T cells in combination with various levels of TCR stimulation. Overexpression of PD-L1 or PD-L2 by DC did not inhibit T cell proliferation, even when B7-1 and B7-2 mediated costimulation was absent, although IL-2 production was consistently decreased. Surprisingly, blocking PD-L1 and PD-L2 with soluble programmed death-1 (sPD-1) also inhibited T cell activation, probably via reverse signaling via PD-L1 and/or PD-L2 into DC, leading to reduced DC maturation. This study suggests a relatively minor contribution of PD-1 ligands in DC-driven CD4+ T cell activation and provides evidence for reverse signaling by PD-L1 and PD-L2 into DC, resulting in a suppressive DC phenotype.

Immunostimulatory combinations: designing the next generation of vaccine adjuvants

Agents that activate dendritic cells are essential components for vaccines and can be conceptualized as molecular adjuvants. Other molecular adjuvants affect downstream factors that shape the resulting immune response. This review provides a compendium of recently studied molecular adjuvants, focusing on CD8+ T cell responses, which have important roles in HIV vaccines. Reference is also made to CD8+ T cell antitumor responses, where parallel studies of molecular adjuvants are being pursued. Molecular adjuvants can be considered in the following groups: TNF superfamily molecules such as CD40 ligand; agonists for TLRs; agonists for NAIP, CIITA, HET-E, TP-1-leucine-rich repeat pathway receptors, such as nucleotide-binding and oligomerization domain (NOD)1, NOD2, and cryopyrin; chemokines; ILs; CSFs; IFNs; alarmins; and purinergic P2X7 receptor agonists. Complementing these positively acting agents are strategies to reduce the immunosuppressive effects of CD4+CD25+ regulatory T cells and negatively acting factors such as TGF-beta, IL-10, suppressor of cytokine signaling 1, and programmed cell death-1 using neutralizing antibodies, antisense, and small interfering RNA. Especially effective are combinations of molecular adjuvants, which can elicit a massive expansion of antigen-specific CD8+ T cells and show unprecedented efficacy in vaccine and tumor models. Taken together, these new approaches provide significant incremental progress in the development of vaccines to elicit cell-mediated immunity against HIV and other pathogens.

Programmed death-1 (PD-1) is a marker of germinal center-associated T cells and angioimmunoblastic T-cell lymphoma

Programmed death-1 (PD-1), a member of the CD28 costimulatory receptor family, is expressed by germinal center-associated T cells in reactive lymphoid tissue. In a study of a wide range of lymphoproliferative disorders, neoplastic T cells in 23 cases of angioimmunoblastic lymphoma were immunoreactive for PD-1, but other subtypes of T cell and B cell non-Hodgkin lymphoma, as well as classic Hodgkin lymphoma, did not express PD-1. The pattern of PD-1 immunostaining of neoplastic cells in angioimmunoblastic lymphoma was similar to that reported for CD10, a recently described marker of neoplastic T cells in angioimmunoblastic lymphoma. Tumor-associated follicular dendritic cells in cases of angioimmunoblastic lymphoma were found to express PD-L1, the PD-1 ligand. In addition, PD-1-positive reactive T cells formed rosettes around neoplastic L&H cells in 14 cases of nodular lymphocyte predominant Hodgkin lymphoma studied. These findings, along with data from previous studies, suggest that angioimmunoblastic lymphoma is a neoplasm of germinal center-associated T cells and that there is an association of germinal center-associated T cells and neoplastic cells in nodular lymphocyte predominant Hodgkin lymphoma. PD-1 is a useful new marker for angioimmunoblastic lymphoma and lends further support to a model of T-cell lymphomagenesis in which specific subtypes of T cells may undergo neoplastic transformation and result in specific, distinct histologic, immunophenotypic, and clinical subtypes of T-cell neoplasia.

A protective role for programmed death 1 in progression of murine adriamycin nephropathy

Programmed death 1 (PD-1) is a novel member of the CD28/cytotoxic T-lymphocyte-associated protein-4 superfamily, which plays an important role in the regulation of activated T cells. However, it is not clear how PD-1 is expressed in normal and diseased kidney, nor if it has a role in progression of chronic renal disease. PD-1 expression and the effect of monoclonal anti-PD-1 antibody (Ab) were examined in murine adriamycin nephropathy (AN). BALB/c mice were divided into three groups: (a) normal mice, (b) adriamycin (ADR) with control immunoglobulin (Ig)G (ADR-IgG), and (c) ADR with anti-PD-1 Ab (ADR-Ab). AN was induced by a single intravenous injection of ADR. Anti-PD-1 Ab was given by intraperitoneal injection on alternate days from day 0 to day 10, or to day 18. Animals were killed at week 4. Renal function, histological change, and cytokine expression were examined. PD-1 mRNA was detected in kidney tissue of mice with AN in a dose- and time-dependent manner. PD-1 was mainly expressed on injured tubule cells and some interstitial cells, which co-stained with alpha-smooth muscle actin in AN, but not in normal kidney. Anti-PD-1 treatment up to day 18, but not to day 10, worsened glomerular and tubulointerstitial injury. The ratio of urinary protein/creatinine was significantly higher in ADR-Ab mice than ADR-IgG mice. The number of macrophages was significantly increased in ADR-Ab mice compared with ADR-IgG mice. Blockade of PD-1 worsened progressive renal histopathological and functional injury in murine AN. This suggests a possible protective role for PD-1 in chronic renal disease, and its potential as a treatment to slow disease progression.

CTLA-4 dysregulation of self/tumor-reactive CD8+ T-cell function is CD4+ T-cell dependent

Cytotoxic T lymphocyte-associated antigen 4 (CTLA-4) maintains peripheral tolerance by suppressing T-cell activation and proliferation but its precise role in vivo remains unclear. We sought to elucidate the impact of CTLA-4 expression on self/tumor-reactive CD8(+) T cells by using the glycoprotein (gp) 100-specific T-cell receptor (TCR) transgenic mouse, pmel-1. pmel-1 CLTA-4(-/-) mice developed profound, accelerated autoimmune vitiligo. This enhanced autoimmunity was associated with a small but highly activated CD8(+) T-cell population and large numbers of CD4(+) T cells not expressing the transgenic TCR. Adoptive transfer of pmel-1 CLTA-4(-/-) CD8(+) T cells did not mediate superior antitumor immunity in the settings of either large established tumors or tumor challenge, suggesting that the mere absence of CTLA-4-mediated inhibition on CD8(+) T cells did not directly promote enhancement of their effector functions. Removal of CD4(+) T cells by crossing the pmel-1 CLTA-4(-/-) mouse onto a Rag-1(-/-) background resulted in the complete abrogation of CD8(+) T-cell activation and autoimmune manifestations. The effects of CD4(+) CLTA-4(-/-) T cells were dependent on the absence of CTLA-4 on CD8(+) T cells. These results indicated that CD8(+) CLTA-4(-/-) T-cell-mediated autoimmunity and tumor immunity required CD4(+) T cells in which the function was dysregulated by the absence of CTLA-4-mediated negative costimulation.

The dynamics of dendritic cell–T cell interactions in priming and tolerance

Induction of T cell priming and tolerance require direct encounters between naïve T cells and dendritic cells (DCs). T cell activation and proliferation occur in both situations, but the concomitant dynamics of DC-T cell contacts differ: the contacts between naïve T cells and DCs are more stable during the induction of priming than during the induction of tolerance. However, the extent of the differences between the dynamics of the contacts, and the contribution of contact duration and frequency to the functional T cell outcome, remain a matter of debate. Recently, experimental models have been developed to address the role of contact stability in T cell function in vivo, which use real-time imaging of DC-T cell interactions.

Dendritic cells in a mature age

A common view supposes that dendritic cells (DCs) exist in two basic functional states: immature DCs induce tolerance to self, whereas mature DCs induce immunity to foreign antigens. However, the term 'mature' is often used not only functionally to designate immunogenic DCs but also as a phenotypic description of DCs expressing high levels of MHC, adhesion and co-stimulatory molecules. The recent realization that DCs can express such markers under non-immunogenic conditions raises the question of whether the two connotations of the term 'mature' should continue to be used interchangeably. Here, I discuss the origins of the maturation model and how terminology is evolving to better accommodate our current understanding of the function of DCs.

Dendritic cells: vehicles for tolerance induction and prevention of autoimmune diseases

Adaptive immune responses are orchestrated by specialized professional antigen-presenting cells (APCs), the dendritic cells (DCs), which play crucial roles as initiators and modulators of adaptive immune responses. A main feature of DCs is their phenotypic and functional plasticity. In the absence of any inflammation or pathogenic elements, most DCs in peripheral tissues and lymphoid organs have a resting, immature phenotype characterized by high endocytic capacity and low surface expression of MHC- and costimulatory molecules. However, upon interaction with microbial ligands, pro-inflammatory cytokines or CD40Ligand, DCs rapidly acquire an activated phenotype. These mature DCs have a very efficient T cell-priming ability as a consequence of upregulation of MHC- and costimulatory molecules on their cell surface. For this reason, DC-based vaccines have been used successfully to combat infections and malignancies. Nonetheless, evidence is accumulating that, especially immature, or semi-matured, DCs also have a potent ability to tolerize T cells or prevent undesired immune reactions. Therefore, current and prospective strategies to promote the inherent tolerogenic potential of DCs are a rational approach for the therapy of autoimmune diseases such as rheumatoid arthritis (RA). This review summarizes some aspects of the intriguing ability of DCs to steer the outcome of immunity and their potency to modulate the outcome of various pathological conditions.

CTLA-4 lacking the cytoplasmic domain costimulates IL-2 production in T-cell hybridomas

Optimal T-cell activation depends on the antigen-specific signal mediated by the TCR and engagement of costimulatory receptors such as CD28. CTLA-4, a homologous counterpart of CD28, is considered to be a crucial inhibitory receptor. To test its function separately from CD28 in an antigen-driven and ligand-specific model, we stably transfected the T-cell hybridomas A1.1 and DO11.10, which lack significant endogenous CD28 or CTLA-4 expression, with wild-type CTLA-4 (CTLA-4 WT) and a construct lacking the cytoplasmic tail (tailless [TL]). Functional studies were carried out by co-incubation with APC expressing the B7 ligands for CTLA-4 and appropriate MHC molecules loaded with their cognate antigens. IL-2 production on costimulation of CTLA-4WT and TCR did not differ significantly from untransfected controls. However, coligation of TCR and CTLA-4TL resulted in a vigorous IL-2 response specific for the interaction of CTLA-4 with B7. Thus, lack of the cytoplasmic tail converted CTLA-4 into a costimulatory receptor. This indicates that the CTLA-4 inhibitory function may not be attributable to sequestration of the common B7 ligands when competing with CD28. Rather, ligation of B7 by the CTLA-4 extracellular domain can enhance TCR activation, whereas in the full-length receptor, inhibitory signals mediated by the cytoplasmic domain may override this activation.

Epidermal langerhans cell-deficient mice develop enhanced contact hypersensitivity

Epidermal Langerhans cells (LCs), a distinct skin-resident dendritic cell population, acquire antigen in the skin and migrate to draining lymph nodes where they are thought to initiate adaptive immune responses. To examine the functional requirement of LCs in skin immunity, we generated BAC transgenic mice in which the regulatory elements from human Langerin were used to drive expression of diphtheria toxin. The resulting mice have a constitutive and durable absence of epidermal LCs but are otherwise intact. Unexpectedly, we found that contact hypersensitivity (CHS) was amplified rather than abrogated in the absence of LCs. Moreover, we showed that LCs act during the priming and not the effector phase. Thus, LCs not only were dispensable for CHS, but they served to regulate the response, a previously unappreciated function.

Tolerogenic dendritic cells: cytokine modulation comes of age

Dendritic cells (DCs) include a heterogeneous family of professional APCs involved in initiation of immunity and in immunologic tolerance. Specifically, peripheral tolerance can be achieved and maintained by promoting regulatory T-cell (Treg) responses and/or T-cell anergy or deletion. Until recently, immature developmental stages of DC differentiation were believed to induce T-cell anergy or Treg cells, whereas DCs transformed into mature DCs by activation stimuli were thought to represent immunogenic DCs capable of inciting primary T-cell responses. This paradigm has been challenged by the demonstration of Treg-cell expansion by antigen-bearing, fully mature DCs. Similarly, semimature DCs with a distinctive interleukin 10 (IL-10)+ IL-12- cytokine production profile might be endowed with tolerogenic functions, supporting the concept that DC maturation per se should no longer be considered as a distinguishing feature of immunogenic as opposed to tolerogenic DCs (TDCs). Cytokine-modulated TDCs reflect an incomplete or altered status of monocyte differentiation and promote in vitro induction of Treg cells and/or in vivo protection from autoimmune diseases. Several growth factors, including IL-10, transforming growth factor beta (TGF-beta), granulocyte colony-stimulating factor (G-CSF), hepatocyte growth factor (HGF), and vasoactive intestinal peptide (VIP), modulate DC maturation and favor the differentiation of TDCs. From a therapeutic standpoint, cytokine-modulated TDCs might be beneficial for prevention and/or treatment of posttransplantation graft-versus-host disease (GVHD) and autoimmunity.

Regulated compartmentalization of programmed cell death-1 discriminates CD4+CD25+ resting regulatory T cells from activated T cells

More effective discrimination between CD4+CD25+ regulatory T cells (Treg) and activated T cells would significantly improve the current level of purification of Treg and their therapeutic application. We observed that approximately 90% of Treg (positive for the nuclear transcription factor Forkhead winged helix protein-3 and able to inhibit naive T cell proliferation) isolated from the spleens or lymph nodes of normal mice did not express significant levels of the inhibitory receptor programmed cell death-1 (PD-1) on their surface, but retained PD-1 intracellularly. An identical phenotype was also identified for human CD4+CD25(high) T cells isolated from peripheral blood of healthy volunteers. By contrast, activated T cells expressed high levels of surface PD-1 that paralleled up-regulation of CD25 during effector cell expansion. This distinction allowed us to isolate CD4+CD25+PD-1(-) T cells with suppressive activity from mice immunized with mature allogeneic dendritic cells. Although purification was limited to resting Treg because TCR ligation induced up-regulation of surface PD-1, this strategy nevertheless represents a valuable step toward more definitive characterization of Treg and their improved purification for therapeutic assessment.

Low surface expression of B7-1 (CD80) is an immunoescape mechanism of colon carcinoma

Artificially enforced expression of CD80 (B7-1) and CD86 (B7-2) on tumor cells renders them more immunogenic by triggering the CD28 receptor on T cells. The enigma is that such B7s interact with much higher affinity with CTLA-4 (CD152), an inhibitory receptor expressed by activated T cells. We show that unmutated CD80 is spontaneously expressed at low levels by mouse colon carcinoma cell lines and other transplantable tumor cell lines of various tissue origins. Silencing of CD80 by interfering RNA led to loss of tumorigenicity of CT26 colon carcinoma in immunocompetent mice, but not in immunodeficient Rag-/- mice. CT26 tumor cells bind CTLA-4Ig, but much more faintly with a similar CD28Ig chimeric protein, thus providing an explanation for the dominant inhibitory effects on tumor immunity displayed by CD80 at that expression level. Interestingly, CD80-negative tumor cell lines such as MC38 colon carcinoma and B16 melanoma express CD80 at dim levels during in vivo growth in syngeneic mice. Therefore, low CD80 surface expression seems to give an advantage to cancer cells against the immune system. Our findings are similar with the inhibitory role described for the dim CD80 expression on immature dendritic cells, providing an explanation for the low levels of CD80 expression described in various human malignancies.

Rejuvenating exhausted T cells during chronic viral infection

In a recent paper in Nature, show that the immunoreceptor PD-1 is upregulated by "exhausted" T cells during the chronic phase of viral infection in mice. Remarkably, blocking the interaction between PD-1 and its ligand, PD-L1, reactivates these T cells and reduces viral load.

HIV-1 selectively infects a subset of nonmaturing BDCA1-positive dendritic cells in human blood

The infection of cultured monocyte-derived dendritic cells (DCs) with HIV-1 involves CD4 and CCR5 receptors, while transmission to T cells is enhanced at least in part by the lectin DC-SIGN/CD209. In the present study, we studied BDCA-1+ myeloid DCs isolated directly from human blood. These cells express CD4 and low levels of CCR5 and CXCR4 coreceptors, but not DC-SIGN. The myeloid DCs replicate two R5 viruses, BaL and YU2, and transfer infection to activated T cells. The virus productively infects a small fraction of the blood DCs that fail to mature in culture, as indicated by the maturation markers CD83 and DC-LAMP/CD208, and the expression of high CD86 and MHC class II, in contrast to many noninfected DCs. A greater proportion of BDCA-1+ DCs are infected when the virus is pseudotyped with the vesicular stomatitis envelope VSV-G (5-15%), as compared with the R5 virus (0.3-3.5%), indicating that HIV-1 coreceptors may limit the susceptibility of DCs to become infected, or the endocytic route of viral entry used by HIV/vesicular stomatitis virus enhances infectivity. When infected and noninfected cells are purified by cell sorting, the former uniformly express HIV p24 gag and are virtually inactive as stimulators of the allogeneic MLR, in contrast to potent stimulation by noninfected DCs from the same cultures. These results point to two roles for a small fraction of blood DCs in HIV-1 pathogenesis: to support productive infection and to evade the direct induction of T cell-mediated immunity.

Inclusion of Brefeldin A during dendritic cell isolation allows in vitro detection of cross-presented self-antigens

The mechanism of cross-presentation enables dendritic cells (DC) to induce immunity against intracellular pathogens and to tolerize autoreactive CD8 T cells. The antigen-presenting cells (APCs) responsible for cross-presentation of self-antigens have been identified as CD8alpha(+) CD11c(+) DC. Isolation of these cells has been notoriously difficult, and the resulting responses of T cell hybridomas were too low to permit further studies. Here, we demonstrate that inclusion of Brefeldin A (BfA), an agent reported to block MHC class I-peptide complex turnover on the cell surface, during DC isolation from transgenic RIP-mOVA mice facilitated activation and proliferation of naïve OVA-specific CD8(+) T cells in vitro. CD8alpha(+) DC were more efficient than CD8alpha(-) CD11c(+). BfA also reversibly preserved expression of costimulatory molecules by DC, as evidenced by their expression of costimulatory markers and by an increased stimulatory capacity of DC matured in vivo by LPS. We conclude that the use of BfA notably improves sensitivity of detection of cross-presented self-antigens.

Murine Fc receptors for IgG are redundant in facilitating presentation of immune complex derived antigen to CD8+ T cells in vivo

Antigen(Ag)-immunoglobulin (Ig)G complexes (IC) are more efficiently processed and presented than soluble Ag. IC can bind to various cell types via different types of Fc-Receptors or, upon binding to complement factors, by complement receptors. Murine professional antigen-presenting cells (APC) express four types of FcgammaReceptors (FcgammaR) via which they are able to capture IC; three activating receptors (FcgammaRI, III and IV) and one inhibitory receptor (FcgammaRII). It has been demonstrated that FcgammaR play a pivotal role in facilitating the presentation of Ag derived from IC. Nonetheless, relative little information is available on the relative contribution of the activating or inhibitory FcgammaR or complement to the presentation of immune-complexed Ag to CD8+ T cells. To study the contribution of the different FcgammaR and complement receptors in IC-facilitated Ag-presentation, we analyzed the ovalbumin(OVA)-specific CD8+ T cell proliferation in FcgammaR- and complement component 3 (C3)-deficient mice after subcutaneous injection of OVA-IC. Here we show that the efficient Ag-presentation was FcgammaR-, but not C3-mediated, as it was inhibited in FcgammaRI/II/III-deficient mice but unaffected in the C3-depleted mice. Moreover, FcgammaRIV does not play a role under these conditions. However, no difference was found between wild-type and FcgammaRI/III-deficient or wild-type and FcgammaRII-deficient mice. These results indicate that Ag-presentation via the activating FcgammaR is not enhanced in the absence of FcgammaRII, and point to redundancy of the FcgammaR, including FcgammaRII, in the uptake and presentation of s.c. injected soluble IC to CD8+ T cells.

Intensified and protective CD4+ T cell immunity in mice with anti-dendritic cell HIV gag fusion antibody vaccine

Current human immunodeficiency virus (HIV) vaccine approaches emphasize prime boost strategies comprising multiple doses of DNA vaccine and recombinant viral vectors. We are developing a protein-based approach that directly harnesses principles for generating T cell immunity. Vaccine is delivered to maturing dendritic cells in lymphoid tissue by engineering protein antigen into an antibody to DEC-205, a receptor for antigen presentation. Here we characterize the CD4⁺ T cell immune response to HIV gag and compare efficacy with other vaccine strategies in a single dose. DEC-205–targeted HIV gag p24 or p41 induces stronger CD4⁺ T cell immunity relative to high doses of gag protein, HIV gag plasmid DNA, or recombinant adenovirus-gag. High frequencies of interferon (IFN)-γ– and interleukin 2–producing CD4⁺ T cells are elicited, including double cytokine-producing cells. In addition, the response is broad because the primed mice respond to an array of peptides in different major histocompatibility complex haplotypes. Long-lived T cell memory is observed. After subcutaneous vaccination, CD4⁺ and IFN-γ–dependent protection develops to a challenge with recombinant vaccinia-gag virus at a mucosal surface, the airway. We suggest that a DEC-targeted vaccine, in part because of an unusually strong and protective CD4⁺ T cell response, will improve vaccine efficacy as a stand-alone approach or with other modalities.

The PD-1-PD-L pathway in immunological tolerance

Since the first observation of spontaneous autoimmune diseases in programmed cell death 1 (PD-1) knockout mice, PD-1 has been postulated to have essential roles in the regulation of autoimmunity but the precise mechanism was largely unknown. Recent studies clearly demonstrated that PD-1 has dual roles in immunological tolerance: induction and maintenance of peripheral tolerance. PD-1 ligands (PD-Ls) on antigen-presenting cells have been shown to switch off autoreactive T cells and induce peripheral tolerance, whereas those on parenchymal cells prevent tissue destruction by suppressing effector T cells to maintain tolerance. In addition, PD-1 and other immuno-inhibitory receptors have been shown to collaborate in the regulation of tolerance. Here, we review recent studies on the role of PD-1 in immunological tolerance and discuss possible clinical applications of PD-1 manipulation.

The interactions of dendritic cells with antigen-specific, regulatory T cells that suppress autoimmunity

Dendritic cells (DCs) are important for several aspects of the development and function of CD4(+) CD25(+) regulatory T cells (Tregs), which are critical for maintaining peripheral tolerance and preventing autoimmunity. In cultures from human thymus, dendritic cells (DCs) conditioned with thymic stromal lymphopoietin (TSLP) mediate the production of Tregs from CD4(+) CD25(-) thymocytes. In cultures from mouse lymphoid organs, CD86-rich DCs induce the proliferation and improved suppressive function of antigen-specific Tregs. DC-expanded, antigen-specific Tregs show greatly enhanced efficacy relative to polyclonal populations in blocking experimental autoimmunity. In several animal models including NOD diabetes, Tregs directed to one autoantigen are able to block autoimmunity induced by multiple antigens from the target organ. Distinct states of DC differentiation or maturation are likely to be important for the emerging roles of DCs in the biology of Tregs, particularly the control of autoimmunity in an antigen-dependent manner.

The liver as an immunological organ

The liver is a unique anatomical and immunological site in which antigen-rich blood from the gastrointestinal tract is pressed through a network of sinusoids and scanned by antigen-presenting cells and lymphocytes. The liver's lymphocyte population is selectively enriched in natural killer and natural killer T cells which play critical roles in first line immune defense against invading pathogens, modulation of liver injury and recruitment of circulating lymphocytes. Circulating lymphocytes come in close contact to antigens displayed by endothelial cells, Kupffer cells and liver resident dendritic cells in the sinusoids. Circulating lymphocytes can also contact hepatocytes directly, because the sinusoidal endothelium is fenestrated and lacks a basement membrane. This unique anatomy of the liver may facilitate direct or indirect priming of lymphocytes, modulate the immune response to hepatotrophic pathogens and contribute to some of the unique immunological properties of this organ, particularly its capacity to induce antigen-specific tolerance.

Innate (over)immunity and adaptive autoimmune disease

Autoimmune disease is characterized by clinical symptoms mediated by adaptive (T cell and B cell) immune reactions towards autoantigen-expressing tissue. Here we discuss that autoimmune disease is often preceded by autoreactivity, meaning the priming of autoantigen-specific immune cells without relevant tissue damage. Recent experimental evidence has demonstrated that both the induction of autoreactivity and the conversion into autoimmune disease is controlled by the activation of the nonspecific innate immune system. Also, the "inflammatory status" of the target organ critically influences the onset of overt autoimmune disease.

Restoring function in exhausted CD8 T cells during chronic viral infection

Functional impairment of antigen-specific T cells is a defining characteristic of many chronic infections, but the underlying mechanisms of T-cell dysfunction are not well understood. To address this question, we analysed genes expressed in functionally impaired virus-specific CD8 T cells present in mice chronically infected with lymphocytic choriomeningitis virus (LCMV), and compared these with the gene profile of functional memory CD8 T cells. Here we report that PD-1 (programmed death 1; also known as Pdcd1) was selectively upregulated by the exhausted T cells, and that in vivo administration of antibodies that blocked the interaction of this inhibitory receptor with its ligand, PD-L1 (also known as B7-H1), enhanced T-cell responses. Notably, we found that even in persistently infected mice that were lacking CD4 T-cell help, blockade of the PD-1/PD-L1 inhibitory pathway had a beneficial effect on the 'helpless' CD8 T cells, restoring their ability to undergo proliferation, secrete cytokines, kill infected cells and decrease viral load. Blockade of the CTLA-4 (cytotoxic T-lymphocyte-associated protein 4) inhibitory pathway had no effect on either T-cell function or viral control. These studies identify a specific mechanism of T-cell exhaustion and define a potentially effective immunological strategy for the treatment of chronic viral infections.

CTLA-4 and PD-1 receptors inhibit T-cell activation by distinct mechanisms

CTLA-4 and PD-1 are receptors that negatively regulate T-cell activation. Ligation of both CTLA-4 and PD-1 blocked CD3/CD28-mediated upregulation of glucose metabolism and Akt activity, but each accomplished this regulation using separate mechanisms. CTLA-4-mediated inhibition of Akt phosphorylation is sensitive to okadaic acid, providing direct evidence that PP2A plays a prominent role in mediating CTLA-4 suppression of T-cell activation. In contrast, PD-1 signaling inhibits Akt phosphorylation by preventing CD28-mediated activation of phosphatidylinositol 3-kinase (PI3K). The ability of PD-1 to suppress PI3K/AKT activation was dependent upon the immunoreceptor tyrosine-based switch motif located in its cytoplasmic tail, adding further importance to this domain in mediating PD-1 signal transduction. Lastly, PD-1 ligation is more effective in suppressing CD3/CD28-induced changes in the T-cell transcriptional profile, suggesting that differential regulation of PI3K activation by PD-1 and CTLA-4 ligation results in distinct cellular phenotypes. Together, these data suggest that CTLA-4 and PD-1 inhibit T-cell activation through distinct and potentially synergistic mechanisms.

Hydronephrosis associated with antiurothelial and antinuclear autoantibodies in BALB/c-Fcgr2b-/-Pdcd1-/- mice

Because most autoimmune diseases are polygenic, analysis of the synergistic involvement of various immune regulators is essential for a complete understanding of the molecular pathology of these diseases. We report the regulation of autoimmune diseases by epistatic effects of two immunoinhibitory receptors, low affinity type IIb Fc receptor for IgG (FcγRIIB) and programmed cell death 1 (PD-1). Approximately one third of the BALB/c-Fcgr2b^−/−Pdcd1^−/− mice developed autoimmune hydronephrosis, which is not observed in either BALB/c-Fcgr2b^−/− or BALB/c-Pdcd1^−/− mice. Hydronephrotic mice produced autoantibodies (autoAbs) against urothelial antigens, including uroplakin IIIa, and these antibodies were deposited on the urothelial cells of the urinary bladder. In addition, ∼15% of the BALB/c-Fcgr2b^−/−Pdcd1^−/− mice produced antinuclear autoAbs. In contrast, the frequency of the autoimmune cardiomyopathy and the production of anti–parietal cell autoAb, which were observed in BALB/c-Pdcd1^−/− mice, were not affected by the additional FcγRIIB deficiency. These observations suggest cross talk between two immunoinhibitory receptors, FcγRIIB and PD-1, on the regulation of autoimmune diseases.