T cell immunity after a viral infection versus T cell tolerance induced by soluble viral peptides

Subset-specific and temporal control of effector and memory CD4+ T cell survival

Following their proliferative expansion and differentiation into effector cells like Th1, Tfh, and T central memory precursors (Tcmp), most effector CD4+ T cells die, while some survive and become memory cells. Here, we explored how Bcl-2 family members controlled the survival of CD4+ T cells during distinct phases of mouse acute LCMV infection. During expansion, we found that Th1 cells dominated the response, downregulated expression of Bcl-2, and did not require Bcl-2 for survival. Instead, they relied on the anti-apoptotic protein, A1 for survival. Similarly, Th17 cells in an EAE model also depended on A1 for survival. However, after the peak of the response, CD4+ effector T cells required Bcl-2 to counteract Bim to aid their transition into memory. This Bcl-2 dependence persisted in established memory CD4+ T cells. Combined, these data show a temporal switch in Bcl-2 family-mediated survival of CD4+ T cells over the course of an immune response. This knowledge can help improve T cell survival to boost immunity and conversely, target pathogenic T cells.

Differential trafficking of ligands trogocytosed via CD28 versus CTLA4 promotes collective cellular control of co-stimulation

Intercellular communication is crucial for collective regulation of cellular behaviors. While clustering T cells have been shown to mutually control the production of key communication signals, it is unclear whether they also jointly regulate their availability and degradation. Here we use newly developed reporter systems, bioinformatic analyses, protein structure modeling and genetic perturbations to assess this. We find that T cells utilize trogocytosis by competing antagonistic receptors to differentially control the abundance of immunoregulatory ligands. Specifically, ligands trogocytosed via CD28 are shuttled to the T cell surface, enabling them to co-stimulate neighboring T cells. In contrast, CTLA4-mediated trogocytosis targets ligands for degradation. Mechanistically, this fate separation is controlled by different acid-sensitivities of receptor-ligand interactions and by the receptor intracellular domains. The ability of CD28 and CTLA4 to confer different fates to trogocytosed ligands reveals an additional layer of collective regulation of cellular behaviors and promotes the robustness of population dynamics.

Peptide emulsions in incomplete Freund's adjuvant create effective nurseries promoting egress of systemic CD4+ and CD8+ T cells for immunotherapy of cancer

Water-in-oil emulsion incomplete Freund's adjuvant (IFA) has been used as an adjuvant in preventive and therapeutic vaccines since its development. New generation, highly purified modulations of the adjuvant, Montanide incomplete seppic adjuvant (ISA)-51 and Montanide ISA-720, were developed to reduce toxicity. Montanide adjuvants are generally considered to be safe, with adverse events largely consisting of antigen and adjuvant dose-dependent injection site reactions (ISRs). Peptide vaccines in Montanide ISA-51 or ISA-720 are capable of inducing both high antibody titers and durable effector T cell responses. However, an efficient T cell response depends on the affinity of the peptide to the presenting major histocompatibility complex class I molecule, CD4⁺ T cell help and/or the level of co-stimulation. In fact, in the therapeutic cancer vaccine setting, presence of a CD4⁺ T cell epitope seems crucial to elicit a robust and durable systemic T cell response. Additional inclusion of a Toll-like receptor ligand can further increase the magnitude and durability of the response. Use of extended peptides that need a processing step only accomplished effectively by dendritic cells (DCs) can help to avoid antigen presentation by nucleated cells other than DC. Based on recent clinical trial results, therapeutic peptide-based cancer vaccines using emulsions in adjuvant Montanide ISA-51 can elicit robust antitumor immune responses, provided that sufficient tumor-specific CD4⁺ T cell help is given in addition to CD8⁺ T cell epitopes. Co-treatment with PD-1 T cell checkpoint inhibitor, chemotherapy or other immunomodulatory drugs may address local and systemic immunosuppressive mechanisms, and further enhance efficacy of therapeutic cancer peptide vaccines in IFA and its modern variants. Blinded randomized placebo-controlled trials are critical to definitively prove clinical efficacy. Mineral oil-based adjuvants for preventive vaccines, to tackle spread and severity of infectious disease, induce immune responses, but require more studies to reduce toxicity.

Rethinking peripheral T cell tolerance: checkpoints across a T cell's journey

Following their exit from the thymus, T cells are endowed with potent effector functions but must spare host tissue from harm. The fate of these cells is dictated by a series of checkpoints that regulate the quality and magnitude of T cell-mediated immunity, known as tolerance checkpoints. In this Perspective, we discuss the mediators and networks that control the six main peripheral tolerance checkpoints throughout the life of a T cell: quiescence, ignorance, anergy, exhaustion, senescence and death. At the naive T cell stage, two intrinsic checkpoints that actively maintain tolerance are quiescence and ignorance. In the presence of co-stimulation-deficient T cell activation, anergy is a dominant hallmark that mandates T cell unresponsiveness. When T cells are successfully stimulated and reach the effector stage, exhaustion and senescence can limit excessive inflammation and prevent immunopathology. At every stage of the T cell's journey, cell death exists as a checkpoint to limit clonal expansion and to terminate unrestrained responses. Here, we compare and contrast the T cell tolerance checkpoints and discuss their specific roles, with the aim of providing an integrated view of T cell peripheral tolerance and fate regulation.

Type 1 diabetes pathogenesis and the role of inhibitory receptors in islet tolerance

Type 1 diabetes (T1D) affects over a million Americans, and disease incidence is on the rise. Despite decades of research, there is still no cure for this disease. Exciting beta cell replacement strategies are being developed, but in order for such approaches to work, targeted immunotherapies must be designed. To selectively halt the autoimmune response, researchers must first understand how this response is regulated and which tolerance checkpoints fail during T1D development. Herein, we discuss the current understanding of T1D pathogenesis in humans, genetic and environmental risk factors, presumed roles of CD4+ and CD8+ T cells as well as B cells, and implicated autoantigens. We also highlight studies in non-obese diabetic mice that have demonstrated the requirement for CD4+ and CD8+ T cells and B cells in driving T1D pathology. We present an overview of central and peripheral tolerance mechanisms and comment on existing controversies in the field regarding central tolerance. Finally, we discuss T cell- and B cell-intrinsic tolerance mechanisms, with an emphasis on the roles of inhibitory receptors in maintaining islet tolerance in humans and in diabetes-prone mice, and strategies employed to date to harness inhibitory receptor signaling to prevent or reverse T1D.

Enhancing immunity prevents virus-induced T-cell-mediated immunopathology in B cell-deficient mice

Hyper-activated or deviated immune responses can result in immunopathological diseases. Paradoxically, immunodeficiency represents a frequent cause of such immune-mediated pathologies. Immunopathological manifestations are commonly treated by immunosuppression, but in situations in which immunodeficiency is the basis of disease development, enhancing immunity may represent an alternative treatment option. Here, we tested this counterintuitive concept in a preclinical model using infection of mice with lymphocytic choriomeningitis virus (LCMV). Firstly, we demonstrate that infection of B-cell-deficient (B^-/- ) but not of wild-type (WT) mice with the LCMV strain Docile induced a rapid and fatal CD8⁺ T-cell-mediated immunopathological disease. Similar to WT mice, LCMV-infected B^-/- mice generated a potent, functional LCMV-specific CD8⁺ T-cell response but exhibited prolonged viral antigen presentation and increased vascular leakage in liver and lungs. Secondly, we were able to prevent this virus-induced immunopathology in B^-/- mice by active or passive T-cell immunizations or by treatment with LCMV-specific virus neutralizing or non-neutralizing monoclonal antibodies (mAb). Thus, boosting antiviral immunity did not aggravate immunopathology in this model, but prevented it by decreasing the formation of target structures for damage-causing CD8⁺ T cells.

K48-linked KLF4 ubiquitination by E3 ligase Mule controls T-cell proliferation and cell cycle progression

T-cell proliferation is regulated by ubiquitination but the underlying molecular mechanism remains obscure. Here we report that Lys-48-linked ubiquitination of the transcription factor KLF4 mediated by the E3 ligase Mule promotes T-cell entry into S phase. Mule is elevated in T cells upon TCR engagement, and Mule deficiency in T cells blocks proliferation because KLF4 accumulates and drives upregulation of its transcriptional targets E2F2 and the cyclin-dependent kinase inhibitors p21 and p27. T-cell-specific Mule knockout (TMKO) mice develop exacerbated experimental autoimmune encephalomyelitis (EAE), show impaired generation of antigen-specific CD8⁺ T cells with reduced cytokine production, and fail to clear LCMV infections. Thus, Mule-mediated ubiquitination of the novel substrate KLF4 regulates T-cell proliferation, autoimmunity and antiviral immune responses in vivo.

The E3 ligase Mule has been previously reported to be essential for B cell development and function by modulating p53 ubiquitination and degradation. Here Hao et al. identify KLF4 as a novel ubiquitination target of Mule and show it controls T cell proliferation and autoimmunity.

Topological Small-World Organization of the Fibroblastic Reticular Cell Network Determines Lymph Node Functionality

Fibroblastic reticular cells (FRCs) form the cellular scaffold of lymph nodes (LNs) and establish distinct microenvironmental niches to provide key molecules that drive innate and adaptive immune responses and control immune regulatory processes. Here, we have used a graph theory-based systems biology approach to determine topological properties and robustness of the LN FRC network in mice. We found that the FRC network exhibits an imprinted small-world topology that is fully regenerated within 4 wk after complete FRC ablation. Moreover, in silico perturbation analysis and in vivo validation revealed that LNs can tolerate a loss of approximately 50% of their FRCs without substantial impairment of immune cell recruitment, intranodal T cell migration, and dendritic cell-mediated activation of antiviral CD8+ T cells. Overall, our study reveals the high topological robustness of the FRC network and the critical role of the network integrity for the activation of adaptive immune responses.

Efficiency of dendritic cell vaccination against B16 melanoma depends on the immunization route

Dendritic cells (DC) presenting tumor antigens are crucial to induce potent T cell-mediated anti-tumor immune responses. Therefore DC-based cancer vaccines have been established for therapy, however clinical outcomes are often poor and need improvement. Using a mouse model of B16 melanoma, we found that the route of preventive DC vaccination critically determined tumor control. While repeated DC vaccination did not show an impact of the route of DC application on the prevention of tumor growth, a single DC vaccination revealed that both the imprinting of skin homing receptors and an enhanced proliferation state of effector T cells was seen only upon intracutaneous but not intravenous or intraperitoneal immunization. Tumor growth was prevented only by intracutaneous DC vaccination. Our results indicate that under suboptimal conditions the route of DC vaccination crucially determines the efficiency of tumor defense. DC-based strategies for immunotherapy of cancer should take into account the immunization route in order to optimize tissue targeting of tumor antigen specific T cells.

Inflammation programs self-reactive CD8+ T cells to acquire T-box-mediated effector function but does not prevent deletional tolerance

CD8(+) T cells must detect foreign antigens and differentiate into effector cells to eliminate infections. But, when self-antigen is recognized instead, mechanisms of peripheral tolerance prevent acquisition of effector function to avoid autoimmunity. These distinct responses are influenced by inflammatory and regulatory clues from the tissue environment, but the mechanism(s) by which naive T cells interpret these signals to generate the appropriate immune response are unclear. The identification of the molecules operative in these cell-fate decisions is crucial for developing new treatment options for patients with cancer or autoimmunity, where manipulation of T cell activity is desired to alter the course of disease. With the use of an in vivo murine model to examine CD8(+) T cell responses to healthy self-tissue, we correlated self-tolerance with a failure to induce the T-box transcription factors T-bet and Eomes. However, inflammation associated with acute microbial infection induced T-bet and Eomes expression and promoted effector differentiation of self-reactive T cells under conditions that normally favor tolerance. In the context of a Listeria infection, these functional responses relied on elevated T-bet expression, independent of Eomes. Alternatively, infection with LCMV induced higher Eomes expression, which was sufficient in the absence of T-bet to promote effector cytokine production. Our results place T-box transcription factors at a molecular crossroads between CD8(+) T cell anergy and effector function upon recognition of peripheral self-antigen, and suggest that inflammation during T cell priming directs these distinct cellular responses.

Anti-IFN-γ and peptide-tolerization therapies inhibit acute lung injury induced by cross-reactive influenza A-specific memory T cells

Viral infections have variable outcomes, with severe disease occurring in only few individuals. We hypothesized that this variable outcome could correlate with the nature of responses made to previous microbes. To test this, mice were infected initially with influenza A virus (IAV) and in memory phase challenged with lymphocytic choriomeningitis virus (LCMV), which we show in this study to have relatively minor cross-reactivity with IAV. The outcome in genetically identical mice varied from mild pneumonitis to severe acute lung injury with extensive pneumonia and bronchiolization, similar to that observed in patients who died of the 1918 H1N1 pandemic. Lesion expression did not correlate with virus titers. Instead, disease severity directly correlated with and was predicted by the frequency of IAV-PB1703- and IAV-PA224-specific responses, which cross-reacted with LCMV-GP34 and LCMV-GP276, respectively. Eradication or functional ablation of these pathogenic memory T cell populations, using mutant-viral strains, peptide-based tolerization strategies, or short-term anti-IFN-γ treatment, inhibited severe lesions such as bronchiolization from occurring. Heterologous immunity can shape outcome of infections and likely individual responses to vaccination, and can be manipulated to treat or prevent severe pathology.

Repetitive peptide boosting progressively enhances functional memory CTLs

Cytotoxic T lymphocytes (CTLs) play a critical role in controlling intracellular pathogens and cancer cells, and induction of memory CTLs holds promise for developing effective vaccines against critical virus infections. However, generating memory CTLs remains a major challenge for conventional vector-based, prime-boost vaccinations. Thus, it is imperative that we explore nonconventional alternatives, such as boosting without vectors. We show here that repetitive intravenous boosting with peptide and adjuvant generates memory CD8 T cells of sufficient quality and quantity to protect against infection in mice. The resulting memory CTLs possess a unique and long-lasting effector memory phenotype, characterized by decreased interferon-γ but increased granzyme B production. These results are observed in both transgenic and endogenous models. Overall, our findings have important implications for future vaccine development, as they suggest that intravenous peptide boosting with adjuvant following priming can induce long-term functional memory CTLs.

Sphingosine analogue AAL-R increases TLR7-mediated dendritic cell responses via p38 and type I IFN signaling pathways

Sphingosine analogues display immunosuppressive activities and thus have therapeutic potential in the treatment of autoimmune diseases. In this study, we investigated the effects of the sphingosine analogue AAL-R (FTY720 derivative) on dendritic cell (DC) response upon TLR stimulation. Unlike its known immunosuppressive activity, AAL-R increased TLR7-mediated DC responses by elevating the levels of MHC class I and costimulatory molecules and type I IFN expression and by enhancing the capacity of DCs to induce CD8(+) T cell proliferation. Importantly, the stimulatory activity of AAL-R was dependent on type I IFN signaling, as type I IFN receptor-deficient DCs failed to respond to AAL-R. Also, AAL-R activated p38 MAPK to increase type I IFN synthesis and TLR7-mediated DC maturation. These findings enhance our understanding of sphingosine regulation of the host immune system, in particular upon pathogenic infections.

Mechanisms of peptide vaccination in mouse models: tolerance, immunity, and hyperreactivity

The development of synthetic peptide vaccines capable of inducing strong and protective T-cell immunity has taken more than 20 years. Peptide vaccines come in many flavors and although their design is simple, their use is more complicated as the success of a particular peptide vaccine is influenced by many parameters. In fact, peptide vaccination may lead to tolerance, immunity or even hyper-reactivity causing death of the animals. Here we systematically dissect the parameters that influence the final outcome of peptide vaccines as examined in mouse models and this will guide the rational design of new vaccines in the future.

Nuclear factor-κB1 controls the functional maturation of dendritic cells and prevents the activation of autoreactive T cells

Mature dendritic cells (DCs) are crucial for the induction of adaptive immune responses and perturbed DC homeostasis can result in autoimmune disease. Either uncontrolled expansion or enhanced survival of DCs can result in a variety of autoimmune diseases in mouse models. In addition, increased maturation signals, through overexpression of surface Toll-like receptors (TLRs) or stimulation by type I interferon (IFN), has been associated with systemic autoimmunity. Whereas recent studies have focused on identifying factors required for initiating the maturation process, the possibility that resting DCs also express molecules that 'hold' them in an immature state has generally not been considered. Here we show that nuclear factor-κB1 (NF-κB1) is crucial for maintaining the resting state of DCs. Self-antigen-pulsed unstimulated DCs that do not express NF-κB1 were able to activate CD8(+) T lymphocytes and induce autoimmunity. We further show that NF-κB1 negatively regulates the spontaneous production of tumor necrosis factor-α (TNF-α), which is associated with increased granzyme B expression in cytotoxic T lymphocytes (CTLs). These findings provide a new perspective on functional DC maturation and a potential mechanism that may account for pathologic T cell activation.

Immunological perspective of self versus tumor antigens: insights from the RIP-gp model

Self-reactive T cells in the body are controlled by mechanisms of peripheral tolerance that limit their activation and induction of immune pathology. Our understanding of these mechanisms has been advanced by the use of tissue-specific promoters to express neo-self-antigens. Here, we present findings using the RIP-gp (rat insulin promoter-glycoprotein) transgenic mouse, which expresses the lymphocytic choriomeningitis virus glycoprotein (LCMV-gp) specifically in the pancreatic β islet cells. T cells responsive to this antigen remain ignorant of the LCMV-gp expressed by the islets, and breaking tolerance is dependent upon the maturation status of antigen-presenting cells, the avidity of the T-cell receptor ligation, and the level of major histocompatibility complex expression in the pancreas. Furthermore, decreased activity of Casitas B-lineage lymphoma b, a negative regulator of T-cell receptor signaling, can allow recognition and destruction of the pancreatic islets. This review discusses the roles of these factors in the context of anti-tissue responses, both in the setting of autoimmunity and in anti-tumor immunity.

In vitro and in vivo analysis of pro- and anti-inflammatory effects of weak and strong contact allergens

Inflammation is a crucial step in the development of allergic contact dermatitis. The primary contact with chemical allergens, called sensitization, and the secondary contact, called elicitation, result in an inflammatory response in the skin. The ability of contact allergens to induce allergic contact dermatitis correlates to a great extent with their inflammatory potential. Therefore, the analysis of the sensitizing potential of a putative contact allergen should include the examination of its ability and potency to cause an inflammation. In this study, we examined the inflammatory potential of different weak contact allergens and of the strong sensitizer 2,4,6-trinitrochlorobenzene (TNCB) in vitro and in vivo using the contact hypersensitivity model, the mouse model for allergic contact dermatitis. Cytokine induction was analysed by PCR and ELISA to determine mRNA and protein levels, respectively. Inflammation-dependent recruitment of skin-homing effector T cells was measured in correlation with the other methods. We show that the sensitizing potential of a contact allergen correlates with the strength of the inflammatory response. The different methods used gave similar results. Quantitative cytokine profiling may be used to determine the sensitizing potential of chemicals for hazard identification and risk assessment.

TRAIL-expressing CD8+ T cells mediate tolerance following soluble peptide-induced peripheral T cell deletion

Peripheral tolerance controls the action of self-reactive T cells that escape thymic deletion. We showed previously that deletion of Ag-specific CD4+ T cells induced a CD8+ T(reg) population that maintained tolerance by deleting T cells with the same Ag specificity. The present study explored the mechanism of action of these CD8+ T(reg). Following OT-II T cell deletion by soluble OVA₃₂₃₋₃₃₉, B6 mice were unresponsive to challenge after CFA/OVA immunization, and Trail⁻/⁻ or Dr5⁻/⁻ mice were immune, although all strains displayed similar OT-II peripheral deletion. Interestingly, B6 mice remained tolerant to OVA even after a second infusion of OT-II T cells. Tolerance could be transferred to naïve recipients using CD8+ T cells from B6 or Dr5⁻/⁻ mice that experienced peptide-induced peripheral OT-II deletion but not from Trail⁻/⁻ mice. Subsequent investigation found that the mechanism of action of the CD8+ T(reg) was TRAIL-mediated OT-II T cell deletion in a TCR-specific manner. Furthermore, the tolerance was transient, as it was established by 14 days after peptide injection but lost by Day 56. Together, these data provide evidence to suggest that the mechanism behind transient peripheral tolerance induced following T cell deletion is the cytotoxic activity of TRAIL-expressing CD8+ T(reg).

A quantitative measure for discriminating between self and non-self antigens in immune response

We present a new theory of how lymphocyte-antigen interaction is governed. We present 'chronicity', a quantitative record of previous lymphocyte-antigen interactions, which is used to regulate lymphocyte behavior. When the chronicity of a lymphocyte increases with the interaction and gets beyond the lower threshold, the lymphocyte can proliferate. Non-self antigens cause lymphocyte proliferation which destroys the antigen. However, self antigens are not destroyed. When the chronicity gets beyond the upper threshold, the lymphocytes get in the tolerance state ensuring non-destruction of self antigens. The discrimination between self and non-self results from the difference in the termination process between self and non-self antigens, caused by the difference in the frequency between interaction of lymphocyte with both antigens.

CpG-enhanced CD8+ T-cell responses to peptide immunization are severely inhibited by B cells

Synthetic peptides encoding protective pathogen-derived epitopes represent--in principle--an ideal approach to T-cell vaccination. Empirically, however, these strategies have not been successful. In the current study, we profiled the early activation of CD8+ T cells by MHC class I-restricted peptide immunization to better understand the biology of this response. We found that CD8+ T cells proliferated robustly in response to low doses of short synthetic peptides in PBS, but failed to acquire effector function or form memory populations in the absence of the TLR ligand CpG. CpG was unique among TLR ligands in its ability to enhance the response to peptide and its adjuvant effects had strict temporal requirements. Interestingly, CpG treatment modulated T-cell expression of the surface receptors PD-1 and CD25, providing insight into its possible adjuvant mechanism. The effects of CpG on peptide immunization were dramatically enhanced in the absence of B cells, demonstrating a unique system of regulation of T-cell responses by these lymphocytes. The results reported here provide insight into the complex response to a simple vaccination regimen, as well as a framework for a rational peptide-based vaccine design to both exploit and overcome targeted aspects of the immune response.

Mouse Models for Type 1 Diabetes

Our understanding of the genetics, aetiology and pathogenesis of Type 1 Diabetes (T1D) was propelled by the discovery of animal models of T1D in the late 1970s and early 1980s, particularly the non-obese diabetic (NOD) mouse. Since then, transgenic and gene-targeting technologies allowed the generation of many models with reduced genetic and pathogenic complexity. These models allowed researchers to zoom in on specific aspects of this complex disease. In this review, we provide an overview of currently available mouse models for T1D.

Systemic antigen cross-presented by liver sinusoidal endothelial cells induces liver-specific CD8 T-cell retention and tolerization

Peripheral CD8 T-cell tolerance can be generated outside lymphatic tissue in the liver, but the course of events leading to tolerogenic interaction of hepatic cell populations with circulating T-cells remain largely undefined. Here we demonstrate that preferential uptake of systemically circulating antigen by murine liver sinusoidal endothelial cells (LSECs), and not by other antigen-presenting cells in the liver or spleen, leads to cross-presentation on major histocompatibility complex (MHC) I molecules, which causes rapid antigen-specific naïve CD8 T-cell retention in the liver but not in other organs. Using bone-marrow chimeras and a novel transgenic mouse model (Tie2-H-2K(b) mice) with endothelial cell-specific MHC I expression, we provide evidence that cross-presentation by organ-resident and radiation-resistant LSECs in vivo was both essential and sufficient to cause antigen-specific retention of naïve CD8 T-cells under noninflammatory conditions. This was followed by sustained CD8 T-cell proliferation and expansion in vivo, but ultimately led to the development of T-cell tolerance.

CONCLUSION

Our results show that cross-presentation of circulating antigens by LSECs caused antigen-specific retention of naïve CD8 T-cells and identify antigen-specific T-cell adhesion as the first step in the induction of T-cell tolerance.

Newcastle disease virus: a promising vector for viral therapy, immune therapy, and gene therapy of cancer

This review deals with the avian paramyxovirus Newcastle disease virus (NDV) and describes properties that explain its oncolytic activity, its tumor-selective replication behavior, and its immune-stimulatory capacity with human cells. The strong interferon response of normal cells upon contact with NDV appears to be the basis for the good tolerability of the virus in cancer patients and for its immune stimulatory properties, whereas the weak interferon response of tumor cells explains the tumor selectivity of replication and oncolysis. Various concepts for the use of this virus for cancer treatment are pointed out and results from clinical studies are summarized. Reverse genetics technology has made it possible recently to clone the genome and to introduce new foreign genes thus generating new recombinant viruses. These can, in the future, be used to transfer new therapeutic genes into tumors and also to immunize against new emerging pathogens. The modular nature of gene transcription, the undetectable rate of recombination, and the lack of a DNA phase in the replication cycle make NDV a suitable candidate for the rational design of a safe and stable vaccine and gene therapy vector.

Immunologic ignorance of vascular endothelial cells expressing minor histocompatibility antigen

Endothelial cells (ECs) presenting minor histocompatibility antigen (mhAg) are major target cells for alloreactive effector CD8(+) T cells during chronic transplant rejection and graft-versus-host disease (GVHD). The contribution of ECs to T-cell activation, however, is still a controversial issue. In this study, we have assessed the antigen-presenting capacity of ECs in vivo using a transgenic mouse model with beta-galactosidase (beta-gal) expression confined to the vascular endothelium (Tie2-LacZ mice). In a GVHD-like setting with adoptive transfer of beta-gal-specific T-cell receptor-transgenic T cells, beta-gal expression by ECs was not sufficient to either activate or tolerize CD8(+) T cells. Likewise, transplantation of fully vascularized heart or liver grafts from Tie2-LacZ mice into nontransgenic recipients did not suffice to activate beta-gal-specific CD8(+) T cells, indicating that CD8(+) T-cell responses against mhAg cannot be initiated by ECs. Moreover, we could show that spontaneous activation of beta-gal-specific CD8(+) T cells in Tie2-LacZ mice was exclusively dependent on CD11c(+) dendritic cells (DCs), demonstrating that mhAgs presented by ECs remain immunologically ignored unless presentation by DCs is granted.

Antigen controls IL-7R alpha expression levels on CD8 T cells during full activation or tolerance induction

The high-affinity chain of the IL-7 receptor, IL-7Ralpha (CD127), is expressed by effector CD8 T cells that have the capacity to become memory cells. IL-7Ralpha expression is uniformly high on naive CD8 T cells, and the majority of these cells down-regulate expression upon antigenic challenge. At the peak of expansion, the fraction of effectors expressing high IL-7Ralpha varies depending on the response examined. The signals that a CD8 T cell receives during a response to Ag that lead to altered expression of IL-7Ralpha have not been fully defined. In vitro experiments demonstrated that Ag alone is sufficient to down-regulate IL-7Ralpha on all cells and most of the cells rapidly re-express the receptor upon removal from Ag. Expression was not altered by the B7.1 costimulatory ligand or when IL-12 was present to provide the signal needed for development of effector functions, indicating that TCR engagement is sufficient to regulate IL-7Ralpha expression. Consistent with this, in vivo priming with peptide Ag resulted in IL-7Ralpha expression that inversely correlated with Ag levels, and expression levels were not changed when IL-12 or adjuvant were administered with Ag. A large fraction of the cells present at the peak of expansion had re-expressed IL-7Ralpha, but most of these cells failed to survive; those that did survive expressed high IL-7Ralpha levels. Thus, Ag-dependent signals regulate IL-7Ralpha levels on responding CD8 T cells, and this occurs whether the responding cells become fully activated or are rendered tolerant by administration of peptide Ag alone.

Antiviral CD8 T cells recognize borna disease virus antigen transgenically expressed in either neurons or astrocytes

Borna disease virus (BDV) can persistently infect the central nervous system (CNS) of mice. The infection remains nonsymptomatic as long as antiviral CD8 T cells do not infiltrate the infected brain. BDV mainly infects neurons which reportedly carry few, if any, major histocompatibility complex class I molecules on the surface. Therefore, it remains unclear whether T cells can recognize replicating virus in these cells or whether cross-presentation of viral antigen by other cell types is important for immune recognition of BDV. To distinguish between these possibilities, we used two lines of transgenic mice that strongly express the N protein of BDV in either neurons (Neuro-N) or astrocytes (Astro-N). Since these animals are tolerant to the neo-self-antigen, we adoptively transferred T cells with specificity for BDV N. In nontransgenic mice persistently infected with BDV, the transferred cells accumulated in the brain parenchyma along with immune cells of host origin and efficiently induced neurological disease. Neurological disease was also observed if antiviral T cells were injected into the brains of Astro-N or Neuro-N but not nontransgenic control mice. Our results demonstrate that CD8 T cells can recognize foreign antigen on neurons and astrocytes even in the absence of infection or inflammation, indicating that these CNS cell types are playing an active role in immune recognition of viruses.

CD8+ CTL priming by exact peptide epitopes in incomplete Freund's adjuvant induces a vanishing CTL response, whereas long peptides induce sustained CTL reactivity

Therapeutic vaccination trials, in which patients with cancer were vaccinated with minimal CTL peptide in oil-in-water formulations, have met with limited success. Many of these studies were based on the promising data of mice studies, showing that vaccination with a short synthetic peptide in IFA results in protective CD8(+) T cell immunity. By use of the highly immunogenic OVA CTL peptide in IFA as a model peptide-based vaccine, we investigated why minimal CTL peptide vaccines in IFA performed so inadequately to allow full optimization of peptide vaccination. Injection of the minimal MHC class I-binding OVA(257-264) peptide in IFA transiently activated CD8(+) effector T cells, which eventually failed to undergo secondary expansion or to kill target cells, as a result of a sustained and systemic presentation of the CTL peptides gradually leaking out of the IFA depot without systemic danger signals. Complementation of this vaccine with the MHC class II-binding Th peptide (OVA(323-339)) restored both secondary expansion and in vivo effector functions of CD8(+) T cells. Simply extending the CTL peptide to a length of 30 aa also preserved these CD8(+) T cell functions, independent of T cell help, because the longer CTL peptide was predominantly presented in the locally inflamed draining lymph node. Importantly, these functional differences were reproduced in two additional model Ag systems. Our data clearly show why priming of CTL with minimal peptide epitopes in IFA is suboptimal, and demonstrate that the use of longer versions of these CTL peptide epitopes ensures the induction of sustained effector CD8(+) T cell reactivity in vivo.

Steady-state dendritic cells expressing cognate antigen terminate memory CD8+ T-cell responses

Antigen stimulation of naive T cells in conjunction with strong costimulatory signals elicits the generation of effector and memory populations. Such terminal differentiation transforms naive T cells capable of differentiating along several terminal pathways in response to pertinent environmental cues into cells that have lost developmental plasticity and exhibit heightened responsiveness. Because these cells exhibit little or no need for the strong costimulatory signals required for full activation of naive T cells, it is generally considered memory and effector T cells are released from the capacity to be inactivated. Here, we show that steady-state dendritic cells constitutively presenting an endogenously expressed antigen inactivate fully differentiated memory and effector CD8(+) T cells in vivo through deletion and inactivation. These findings indicate that fully differentiated effector and memory T cells exhibit a previously unappreciated level of plasticity and provide insight into how memory and effector T-cell populations may be regulated.

Antigen Presentation by Exosomes Released from Peptide-Pulsed Dendritic Cells Is not Suppressed by the Presence of Active CTL

Despite the potency of dendritic cells (DCs) as a vaccine carrier, they are short-lived and sensitive to CTL-mediated elimination. Thus, it is believed that the longevity of Ag presentation by peptide-pulsed DC is limited in vivo. Surprisingly, however, we found that although the majority of injected DCs disappeared from the draining lymph nodes within 7 days, Ag presentation persisted for at least 14 days following DC immunization. This prolonged Ag presentation was not mediated by the remaining injected DCs or through Ag transfer to endogenous APCs. We provide evidence that exosomes released by DCs might be responsible for the persistence of Ag presentation. Functional exosomes could be recovered from the draining lymph nodes of C57BL/6 mice following DC vaccination and, in contrast to DCs, T cell stimulation by exosomes in vivo was not affected by the presence of CTL. Our findings demonstrate that Ag presentation following delivery of DC vaccines persists for longer than expected and indicate that the exosome may play a previously unrecognized role in Ag presentation following DC vaccination. Furthermore, our study reinforces the application of exosomes as a vaccination platform and suggests that exosome-based vaccines may be advantageous for booster immunizations due to their resistance to CTL.

Design and development of synthetic peptide vaccines: past, present and future

Synthetic peptide vaccines aiming at the induction of a protective CD8(+) T-cell response against infectious or malignant diseases are widely used in the clinic but, despite their success in animal models, they do not yet live up to their promise in humans. This review assesses the development of synthetic peptide vaccines, weighs it against the immunological concepts that have emerged, and identifies the key issues that play a role in the failure or success of a synthetic peptide vaccine. The current state-of-the-art peptide vaccine is a complete synthetic inflammatory product that is ingested by professional antigen-presenting cells and stimulates both CD4(+) and CD8(+) T cells.

Complement-Dependent Enhancement of CD8+T Cell Immunity to Lymphocytic Choriomeningitis Virus Infection in Decay-Accelerating Factor-Deficient Mice

Decay-accelerating factor (DAF, CD55) is a GPI-anchored membrane protein that regulates complement activation on autologous cells. In addition to protecting host tissues from complement attack, DAF has been shown to inhibit CD4+ T cell immunity in the setting of model Ag immunization. However, whether DAF regulates natural T cell immune response during pathogenic infection is not known. We describe in this study a striking regulatory effect of DAF on the CD8+ T cell response to lymphocytic choriomeningitis virus (LCMV) infection. Compared with wild-type mice, DAF knockout (Daf-1(-/-)) mice had markedly increased expansion in the spleen of total and viral Ag-specific CD8+ T cells after acute or chronic LCMV infection. Splenocytes from LCMV-infected Daf-1(-/-) mice also displayed significantly higher killing activity than cells from wild-type mice toward viral Ag-loaded target cells, and Daf-1(-/-) mice cleared LCMV more efficiently. Importantly, deletion of the complement protein C3 or the receptor for the anaphylatoxin C5a (C5aR) from Daf-1(-/-) mice reversed the enhanced CD8+ T cell immunity phenotype. These results demonstrate that DAF is an important regulator of CD8+ T cell immunity in viral infection and that it fulfills this role by acting as a complement inhibitor to prevent virus-triggered complement activation and C5aR signaling. This mode of action of DAF contrasts with that of CD59 in viral infection and suggests that GPI-anchored membrane complement inhibitors can regulate T cell immunity to viral infection via either a complement-dependent or -independent mechanism.

Activation-induced non-responsiveness (anergy) limits CD8 T cell responses to tumors

Naïve CD8 T cells respond to signals provided by Ag, costimulation and cytokines by proliferating and differentiating to develop effector functions. Following initial clonal expansion, however, the cells develop activation-induced non-responsiveness (AINR), a form of anergy characterized by an inability to produce IL-2. Cells in the AINR state can carry out effector functions (cytolysis, IFN-gamma production) but cannot continue to proliferate and expand in the face of persisting Ag. AINR limits the ability of activated CTL to control tumor growth but can be reversed by IL-2, provided either therapeutically or by activated CD4 T helper cells, to allow continued expansion.

Molecular basis for checkpoints in the CD8 T cell response: tolerance versus activation

CD8 T cells specific for self-antigens are present in the peripheral lymphoid system and can contribute to autoimmunity or transplant rejection. Whether recognition of Ag leads to full activation, or to induction of tolerance, depends upon availability of cytokine at critical stages of the response. Signals provided by IL-12 and/or IFN-alpha/beta are required for activation of naïve CD8 T cells, and IL-2 is needed to sustain and further expand the effector cells if Ag persists. These critical signaling requirements provide new insights into the factors that regulate the CD8 T cell contributions to development of autoimmunity or rejection of transplants.

T-cell epitope peptide vaccines

T-cell immunotherapy is a promising treatment option for cancer. The identification of tumor antigens that are recognized by the immune system has allowed for the generation of vaccines for various malignancies. Due to the ease of manufacturing and characterizating peptide-based vaccines they have been used to stimulate antitumor T-cells. This article will review the use of peptide-based vaccines for the treatment of cancer by inducing antitumor T-lymphocyte responses.

Improved peptide vaccine strategies, creating synthetic artificial infections to maximize immune efficacy

Soon after it was realized that T-cells recognize their target antigens as small protein fragments or peptides presented by MHC molecules at the cell surface, these peptide epitopes have been tried as vaccines. Human testing of such vaccines, although protective in mouse models, has produced mixed results. Since these initial trials, there has been an tremendous increase in our understanding of how infectious organisms can induce potent immune responses. In this article we review the key changes in the design, formulation and delivery of synthetic peptide vaccines that are applied to improve peptide vaccine strategies.

Immediate antigen-specific effector functions by TCR-transgenic CD8+ NKT cells

Only recently have natural antigens for CD1d-dependent, invariant Valpha14+ natural killer T (iNKT) cells been identified. Similar data for CD1d-independent and CD8+ NKT cell populations are still missing. Here, we show that the MHC class I-restricted CD8+ TCR-transgenic mouse lines OT-I, P14 and H-Y contain a significant proportion of transgenic CD8+ NK1.1+ T cells. In liver, most of NK1.1+ T cells express CD8alphaalpha homodimers. Transgenic NKT cells did not bind invariant Valpha14-to-Jalpha18 TCR rearrangement (Valpha14i)-specific CD1d/alpha-galactosylceramide tetramers and the frequency of iNKT cells was severely reduced. The activated cell surface phenotype and the distribution of transgenic NKT cells in vivo were similar to that reported for iNKT cells. The OT-I and P14 CD8+ NKT cells recognized their cognate antigen in the context of H2-Kb and produced cytokines shortly after TCR stimulation. Importantly, transgenic NKT cells exerted immediate antigen-specific cytotoxicity in vitro and in vivo. Our results demonstrate the presence of transgenic CD8+ NKT cells in MHC class I-restricted TCR-transgenic animals, which are endowed with rapid antigen-specific effector functions. These data imply that experiments studying naive T cell function in TCR-transgenic animals should be interpreted with caution, and that such animals could be utilized for studying CD8+ NKT cell function in an antigen-specific manner.

Solid tumors “melt” from the inside after successful CD8 T cell attack

Adoptive transfer of tumor-specific T cells represents a promising approach for cancer immunotherapy. Here, we visualized the anti-tumor response of CD8 T cells from P14 TCR-transgenic mice specific for the model antigen GP33 by immunohistology. P14 T cells, adoptively transferred into tumor-bearing hosts, induced regression of established 3LL-A9(GP33) and MCA102(GP33) tumors that express GP33 as a tumor-associated model antigen. Strikingly, the visible effects of P14 T cell attack, such as the destruction of the tumor vasculature and accumulation of granulocytes, were predominantly detected inside the tumor mass. In regressing tumors, P14 T cells were found in the intact rim zone but not in central areas that were infiltrated with granulocytes and lacked CD31(+) endothelial cells. The rim of P14 T cell-treated tumors showed an increase in vascular density and decrease in hypoxia compared to untreated tumors. Hypoxic areas of tumors are known to exhibit decreased sensitivity to radiation therapy or chemotherapy. Thus, our data also imply that adoptive transfer of tumor-specific CD8 T cells might synergize with radiation therapy or chemotherapy in the elimination of solid tumors in vivo.

Tissue-resident memory CD8+ T cells can be deleted by soluble, but not cross-presented antigen

Under noninflammatory conditions, both naive and central memory CD8 T cells can be eliminated in the periphery with either soluble peptide or cross-presented Ag. Here, we assess the tolerance susceptibility of tissue-resident memory CD8 T cells in mice to these two forms of tolerogen. Soluble peptide specifically eliminated the majority of memory CD8 cells present in both lymphoid and extralymphoid tissues including lung and liver, but was unable to reduce the number present in the CNS. In contrast, systemic cross-presentation of Ag by dendritic cells resulted in successful elimination of memory cells only from the spleen, with no significant reduction in the numbers of tissue-resident memory cells in the lung. The fact that tissue-resident memory cells were unable to access cross-presented Ag suggests that either the memory cells in the lung do not freely circulate out of the tissue, or that they circulate through a region in the spleen devoid of cross-presented Ag. Thus, although tissue-resident memory cells are highly susceptible to tolerance induction, both the form of tolerogen and location of the T cells can determine their accessibility to tolerogen and the degree to which they are successfully deleted from specific tissues.

CD8alpha+ dendritic cells selectively present MHC class I-restricted noncytolytic viral and intracellular bacterial antigens in vivo

CD8alpha(+) dendritic cells (DCs) have been shown to be the principal DC subset involved in priming MHC class I-restricted CTL immunity to a variety of cytolytic viruses, including HSV type 1, influenza, and vaccinia virus. Whether priming of CTLs by CD8alpha(+) DCs is limited to cytolytic viruses, which may provide dead cellular material for this DC subset, or whether these DCs selectively present intracellular Ags, is unknown. To address this question, we examined Ag presentation to a noncytolytic virus, lymphocytic choriomeningitis virus, and to an intracellular bacterium, Listeria monocytogenes. We show that regardless of the type of intracellular infection, CD8alpha(+) DCs are the principal DC subset that initiate CD8(+) T cell immunity.

Induction of Peripheral Tolerance in Dual TCR T Cells: an Evidence for Non-dominant Signaling by One TCR

Recently, the existence of T cells with dual T cell receptor (TCR) in the immune system is generally accepted, while it has been controversial whether signals through one TCR would affect the functions of the other. In this study T cells expressing two different TCR were obtained from cross-hybrids of LCMV and AND TCR transgenic mice specific for the gp33 and peptide fragment of PCC (fPCC), respectively. Peptide stimulation demonstrated that the dual TCR T cells functioned independently in an antigen-specific manner. To examine whether the tolerance targeted for the one TCR affects the responsiveness of the other, the cross-hybrids were treated with gp33. Although T cells from F1 mice were rendered anergenic to gp33, no functional changes to fPCC were observed in terms of cellular proliferation and IL-2 secretion, suggesting that the dual TCR T cells remained reactive to fPCC. We therefore propose that signaling through the TCR is receptor-specific and 'negative dominance' of one TCR by tolerance induction is not applicable in this dual TCR system.