Molecular mechanisms of T cell co-stimulation and co-inhibition

B cells in teleost fish act as pivotal initiating APCs in priming adaptive immunity: an evolutionary perspective on the origin of the B-1 cell subset and B7 molecules

The long-held paradigm that B cells cannot uptake nonspecific particulate Ags for the initiation of primary adaptive immunity has been challenged by the recent discovery that teleost B cells have potent phagocytic and microbicidal abilities. This discovery provides preliminary clues that primitive B cells might act as initiating APCs in priming adaptive immunity. In this study, zebrafish B cells clearly showed a potent Ag-presenting ability to both soluble Ags and bacterial particles to prime naive CD4(+) T cell activation. This finding demonstrates the innate-like nature of teleost B cells in the interface of innate and adaptive immunity, indicating that they might consist of a major population of initiating APCs whose performance is similar to that of dendritic cells. Given the functional similarities between teleost B cells and the mammalian B-1 subset, we hypothesize that B-1 lineage and teleost B cells might originate from a common ancestor with potent phagocytic and initiating APC capacities. In addition, CD80/86 and CD83 costimulatory signals were identified as being essential for B cell-initiated adaptive immunity. This result suggests that the costimulatory mechanism originated as early as the origin of adaptive immunity and is conserved throughout vertebrate evolution. In fish, only a single CD80/86 copy exists, which is similar to mammalian CD86 rather than to CD80. Thus, CD86 might be a more primordial B7 family member that originated from fish. This study provides valuable insights into the evolutionary history of professional APCs, B cell lineages, and the costimulatory mechanism underlying adaptive immunity as a whole.

Prospects for therapeutic tolerance in humans

PURPOSE OF REVIEW

To provide an overview of recent advances and future possibilities for therapeutic tolerance.

RECENT FINDINGS

Allograft survival despite complete immunosuppressant withdrawal has been demonstrated in selected renal-transplant recipients with haematopoietic chimerism. Early clinical trials of mesenchymal stromal cell therapy have shown promising results in several autoimmune diseases. Regulatory T cells show potential benefit in graft versus host disease, although challenges to ex-vivo expansion remain. Targeted modulation of T-cell function in vivo with monoclonal antibodies has shown beneficial effects in phase II/III trials of multiple sclerosis (alemtuzumab) and type I diabetes mellitus (teplizumab, otelixizumab). Emerging data from animal models suggest an important role for the commensal microbiome in the maintenance and disruption of immune tolerance with parallels in human studies.

SUMMARY

After years of slow progress, recent research has reduced the translational gap between animal models and clinical therapeutic tolerance. Early detection of autoimmunity, potentially at preclinical stages, offers a window of opportunity for tolerogenic therapy. Reliable biomarkers of tolerance are urgently needed to provide objective measurements of the effectiveness of tolerogenic therapies, and to allow intelligent immunosuppressant withdrawal in patients whose autoimmune disease is stable.

VIDEO ABSTRACT AVAILABLE

See the Video Supplementary Digital Content 1 (http://links.lww.com/COR/A8).

Tim-3 directly enhances CD8 T cell responses to acute Listeria monocytogenes infection

T cell Ig and mucin domain (Tim) 3 is a surface molecule expressed throughout the immune system that can mediate both stimulatory and inhibitory effects. Previous studies have provided evidence that Tim-3 functions to enforce CD8 T cell exhaustion, a dysfunctional state associated with chronic stimulation. In contrast, the role of Tim-3 in the regulation of CD8 T cell responses to acute and transient stimulation remains undefined. To address this knowledge gap, we examined how Tim-3 affects CD8 T cell responses to acute Listeria monocytogenes infection. Analysis of wild-type (WT) mice infected with L. monocytogenes revealed that Tim-3 was transiently expressed by activated CD8 T cells and was associated primarily with acquisition of an effector phenotype. Comparison of responses to L. monocytogenes by WT and Tim-3 knockout (KO) mice showed that the absence of Tim-3 significantly reduced the magnitudes of both primary and secondary CD8 T cell responses, which correlated with decreased IFN-γ production and degranulation by Tim-3 KO cells stimulated with peptide Ag ex vivo. To address the T cell-intrinsic role of Tim-3, we analyzed responses to L. monocytogenes infection by WT and Tim-3 KO TCR-transgenic CD8 T cells following adoptive transfer into a shared WT host. In this setting, the accumulation of CD8 T cells and the generation of cytokine-producing cells were significantly reduced by the lack of Tim-3, demonstrating that this molecule has a direct effect on CD8 T cell function. Combined, our results suggest that Tim-3 can mediate a stimulatory effect on CD8 T cell responses to an acute infection.

Allogeneic T cell responses are regulated by a specific miRNA-mRNA network

Donor T cells that respond to host alloantigens following allogeneic bone marrow transplantation (BMT) induce graft-versus-host (GVH) responses, but their molecular landscape is not well understood. MicroRNAs (miRNAs) regulate gene (mRNA) expression and fine-tune the molecular responses of T cells. We stimulated naive T cells with either allogeneic or nonspecific stimuli and used argonaute cross-linked immunoprecipitation (CLIP) with subsequent ChIP microarray analyses to profile miR responses and their direct mRNA targets. We identified a unique expression pattern of miRs and mRNAs following the allostimulation of T cells and a high correlation between the expression of the identified miRs and a reduction of their mRNA targets. miRs and mRNAs that were predicted to be differentially regulated in allogeneic T cells compared with nonspecifically stimulated T cells were validated in vitro. These analyses identified wings apart-like homolog (Wapal) and synaptojanin 1 (Synj1) as potential regulators of allogeneic T cell responses. The expression of these molecular targets in vivo was confirmed in MHC-mismatched experimental BMT. Targeted silencing of either Wapal or Synj1 prevented the development of GVH response, confirming a role for these regulators in allogeneic T cell responses. Thus, this genome-wide analysis of miRNA-mRNA interactions identifies previously unrecognized molecular regulators of T cell responses.

Directed differentiation of human embryonic stem cells into thymic epithelial progenitor-like cells reconstitutes the thymic microenvironment in vivo

Thymus transplantation has great clinical potential for treating immunological disorders, but the shortage of transplant donors limits the progress of this therapy. Human embryonic stem cells (hESCs) are promising cell sources for generating thymic epithelial cells. Here, we report a stepwise protocol to direct the differentiation of hESCs into thymic epithelial progenitor-like cells (TEPLCs) by mimicking thymus organogenesis with sequential regulation of Activin, retinoic acid, BMP, and WNT signals. The hESC-derived TEPLCs expressed the key thymic marker gene FOXN1 and could further develop in vivo into thymic epithelium expressing the functional thymic markers MHC II and AIRE upon transplantation. Moreover, the TEPLC-derived thymic epithelium could support mouse thymopoiesis in T-cell-deficient mice and promote human T cell generation in NOD/SCID mice engrafted with human hematopoietic stem cells (hHSCs). These findings could facilitate hESC-based replacement therapy and provide a valuable in vitro platform for studying human thymus organogenesis and regeneration.

Blood and beyond: properties of circulating and tissue-resident human virus-specific αβ CD8(+) T cells

CD8(+) αβ T-cell responses form an essential line of defence against viral infections. An important part of the mechanisms that control the generation and maintenance of these responses have been elucidated in experimental mouse models. In recent years it has become clear that CD8(+) T-cell responses in humans not only show similarities, but also display differences to those occurring in mice. Furthermore, while several viral infections occur primarily in specialised organ systems, for obvious reasons, most human CD8(+) T-cell investigations were performed on cells deriving from the circulation. Indeed, several lines of evidence now point to essential functional differences between virus-specific CD8(+) memory T cells found in the circulation and those providing protection in organ systems, such as the lungs. In this review, we will focus on summarising recent insights into human CD8(+) T-cell differentiation in response to several viruses and emphasise that for a complete understanding of anti-viral immunity, it is pivotal to scrutinize such responses in both blood and tissue.

Cutaneous T cell lymphoma expresses immunosuppressive CD80 (B7-1) cell surface protein in a STAT5-dependent manner

In this article, we report that cutaneous T cell lymphoma (CTCL) cells and tissues ubiquitously express the immunosuppressive cell surface protein CD80 (B7-1). CD80 expression in CTCL cells is strictly dependent on the expression of both members of the STAT5 family, STAT5a and STAT5b, as well as their joint ability to transcriptionally activate the CD80 gene. In IL-2-dependent CTCL cells, CD80 expression is induced by the cytokine in a Jak1/3- and STAT5a/b-dependent manner, whereas in the CTCL cells with constitutive STAT5 activation, CD80 expression is also STAT5a/b dependent but is independent of Jak activity. Although depletion of CD80 expression does not affect the proliferation rate and viability of CTCL cells, induced expression of the cell-inhibitory receptor of CD80, CD152 (CTLA-4), impairs growth of the cells. Coculture of CTCL cells with normal T lymphocytes consisting of both CD4(+) and CD8(+) populations or the CD4(+) subset alone, transfected with CD152 mRNA, inhibits proliferation of normal T cells in a CD152- and CD80-dependent manner. These data identify a new mechanism of immune evasion in CTCL and suggest that the CD80-CD152 axis may become a therapeutic target in this type of lymphoma.

Stochastic modelling of T-cell activation

We investigate a specific part of the human immune system, namely the activation of T-cells, using stochastic tools, especially sharp large deviation results. T-cells have to distinguish reliably between foreign and self peptides which are both presented to them by antigen presenting cells. Our work is based on a model studied by Zint et al. (J Math Bio 57(6):841-861, 2008). We are able to dispense with some restrictive distribution assumptions that were used previously, i.e., we establish a higher robustness of the model. A central issue is the analysis of two new perspectives to the scenario (two different quenched systems) in detail. This means that we do not only analyse the total probability of a T-cell activation (the annealed case) but also consider the probability of an activation of one certain clonotype and the probability of a T-cell activation by a certain antigen presentation profile (the quenched cases). Finally, we see analytically that the probability of T-cell activation increases with the number of presented foreign peptides in all three cases.

The new normal: immunomodulatory agents against sepsis immune suppression

Sepsis is the leading cause of death among critically ill patients in intensive care units, and treatment options are limited. Therapies developed against the proinflammatory stage have failed clinically; therefore, new approaches that target the host immune response in sepsis are necessary. Increasing evidence suggests that a major pathophysiological event in sepsis is immune suppression, often resulting in secondary fungal, bacterial, or viral infections. Recent studies from animal sepsis models and patient samples suggest that cytokines such as interleukin-7 (IL-7), IL-15, granulocyte macrophage colony-stimulating factor (GM-CSF), as well as co-inhibitory molecule blockade, such as anti-programmed cell death receptor-1 (anti-PD-1) and anti-B and T lymphocyte attenuator (anti-BTLA), may have utility in alleviating the clinical morbidity associated with sustained sepsis. This review discusses some of these novel immunomodulatory agents and evaluates their potential use as therapeutics.

Hunting complex differential gene interaction patterns across molecular contexts

Heterogeneity in genetic networks across different signaling molecular contexts can suggest molecular regulatory mechanisms. Here we describe a comparative chi-square analysis (CPχ²) method, considerably more flexible and effective than other alternatives, to screen large gene expression data sets for conserved and differential interactions. CPχ² decomposes interactions across conditions to assess homogeneity and heterogeneity. Theoretically, we prove an asymptotic chi-square null distribution for the interaction heterogeneity statistic. Empirically, on synthetic yeast cell cycle data, CPχ² achieved much higher statistical power in detecting differential networks than alternative approaches. We applied CPχ² to Drosophila melanogaster wing gene expression arrays collected under normal conditions, and conditions with overexpressed E2F and Cabut, two transcription factor complexes that promote ectopic cell cycling. The resulting differential networks suggest a mechanism by which E2F and Cabut regulate distinct gene interactions, while still sharing a small core network. Thus, CPχ² is sensitive in detecting network rewiring, useful in comparing related biological systems.

Downregulation of inhibitory SRC homology 2 domain-containing phosphatase-1 (SHP-1) leads to recovery of T cell responses in elderly

BACKGROUND

Immune responses are generally impaired in aged mammals. T cells have been extensively studied in this context due to the initial discovery of their reduced proliferative capacity with aging. The decreased responses involve altered signaling events associated with the early steps of T cell activation. The underlying causes of these changes are not fully understood but point to alterations in assembly of the machinery for T cell activation. Here, we have tested the hypothesis that the T cell pool in elderly subjects displayed reduced functional capacities due to altered negative feedback mechanisms that participate in the regulation of the early steps of T cell activation. Such conditions tip the immune balance in favor of altered T cell activation and a related decreased response in aging.

RESULTS

We present evidence that the tyrosine phosphatase SHP-1, a key regulator of T cell signal transduction machinery is, at least in part, responsible for the impaired T cell activation in aging. We used tyrosine-specific mAbs and Western blot analysis to show that a deregulation of the Csk/PAG loop in activated T cells from elderly individuals favored the inactive form of tyrosine-phosphorylated Lck (Y505). Confocal microscopy analysis revealed that the dynamic movements of these regulatory proteins in lipid raft microdomains was altered in T cells of aged individuals. Enzymic assays showed that SHP-1 activity was upregulated in T cells of aged donors, in contrast to young subjects. Pharmacological inhibition of SHP-1 resulted in recovery of TCR/CD28-dependent lymphocyte proliferation and IL-2 production of aged individuals to levels approaching those of young donors. Significant differences in the active (Y394) and inactive (Y505) phosphorylation sites of Lck in response to T cell activation were observed in elderly donors as compared to young subjects, independently of CD45 isoform expression.

CONCLUSIONS

Our data suggest that the role of SHP-1 in T cell activation extends to its increased effect in negative feedback in aging. Modulation of SHP-1 activity could be a target to restore altered T cell functions in aging. These observations could have far reaching consequences for improvement of immunosenescence and its clinical consequences such as infections, altered response to vaccination.

Expanding role of T cells in acute kidney injury

PURPOSE OF REVIEW

Recent advances in T cell biology have shed light on the role of T cell subsets in the pathogenesis of acute kidney injury (AKI). The purpose of this review is to harness our understanding of recent advances in T cell biology in tissue injury and repair and provide a mechanistic insight into the role of T cells in the inflammation of AKI.

RECENT FINDINGS

New specific reagents and genetic animal models have led to advances in our understanding of the role of T cell subsets involved in renal injury. Whereas some T cells promote innate renal inflammation and injury, other T cells promote protection and repair. Recent studies illuminated the pathogenic mechanisms of invariant natural killer T (NKT) cells and T helper1-type responses, and the beneficial functions of regulatory T cells and NKT cells are just beginning to be explored. Pharmacologic and cell-based therapies that influence T cell responses to experimental AKI suggest that this is a promising approach to preserve renal function.

SUMMARY

The recent insights gained into how T cells modulate renal injury suggest that strategies targeting specific types of T cells, to either inhibit or enhance their activity, may ameliorate renal injury in patients.

Inhibitory Receptor Expression Depends More Dominantly on Differentiation and Activation than "Exhaustion" of Human CD8 T Cells

Under conditions of chronic antigen stimulation, such as persistent viral infection and cancer, CD8 T cells may diminish effector function, which has been termed "exhaustion." Expression of inhibitory Receptors (iRs) is often regarded as a hallmark of "exhaustion." Here we studied the expression of eight different iRs by CD8 T cells of healthy humans, including CTLA-4, PD1, TIM3, LAG3, 2B4, BTLA, CD160, and KLRG1. We show that many iRs are expressed upon activation, and with progressive differentiation to effector cells, even in absence of long-term ("chronic") antigenic stimulation. In particular, we evaluated the direct relationship between iR expression and functionality in CD8 T cells by using anti-CD3 and anti-CD28 stimulation to stimulate all cells and differentiation subsets. We observed a striking up-regulation of certain iRs following the cytokine production wave, in agreement with the notion that iRs function as a negative feedback mechanism. Intriguingly, we found no major impairment of cytokine production in cells positive for a broad array of iRs, as previously shown for PD1 in healthy donors. Rather, the expression of the various iRs strongly correlated with T cell differentiation or activation states, or both. Furthermore, we analyzed CD8 T cells from lymph nodes (LNs) of melanoma patients. Interestingly, we found altered iR expression and lower cytokine production by T cells from metastatic LNs, but also from non-metastatic LNs, likely due to mechanisms which are not related to exhaustion. Together, our data shows that expression of iRs per se does not mark dysfunctional cells, but is rather tightly linked to activation and differentiation. This study highlights the importance of considering the status of activation and differentiation for the study and the clinical monitoring of CD8 T cells.

T cell Ig and mucin domain-containing protein 3 is recruited to the immune synapse, disrupts stable synapse formation, and associates with receptor phosphatases

CD8(+) CTLs are adept at killing virally infected cells and cancer cells and releasing cytokines (e.g., IFN-γ) to aid this response. However, during cancer and chronic viral infections, such as with HIV, this CTL response is progressively impaired due to a process called T cell exhaustion. Previous work has shown that the glycoprotein T cell Ig and mucin domain-containing protein 3 (Tim-3) plays a functional role in establishing T cell exhaustion. Tim-3 is highly upregulated on virus and tumor Ag-specific CD8(+) T cells, and antagonizing Tim-3 helps restore function of CD8(+) T cells. However, very little is known of how Tim-3 signals in CTLs. In this study, we assessed the role of Tim-3 at the immunological synapse as well as its interaction with proximal TCR signaling molecules in primary human CD8(+) T cells. Tim-3 was found within CD8(+) T cell lipid rafts at the immunological synapse. Blocking Tim-3 resulted in a significantly greater number of stable synapses being formed between Tim-3(hi)CD8(+) T cells and target cells, suggesting that Tim-3 plays a functional role in synapse formation. Further, we confirmed that Tim-3 interacts with Lck, but not the phospho-active form of Lck. Finally, Tim-3 colocalizes with receptor phosphatases CD45 and CD148, an interaction that is enhanced in the presence of the Tim-3 ligand, galectin-9. Thus, Tim-3 interacts with multiple signaling molecules at the immunological synapse, and characterizing these interactions could aid in the development of therapeutics to restore Tim-3-mediated immune dysfunction.

Induction and stability of the anergic phenotype in T cells

One of the mechanisms that are in place to control the activation of mature T cells that bear self-reactive antigen receptors is anergy, a long-term state of hyporesponsiveness that is established in T cells in response to suboptimal stimulation. T cells receive signals that result not only from antigen recognition and costimulation but also from other sources, including cytokine receptors, inhibitory receptors or metabolic sensors. Integration of those signals will determine T cell fate. Under conditions that induce anergy, T cells activate a program of gene expression that leads to the production of proteins that block T cell receptor signaling and inhibit cytokine gene expression. In this review we will examine those signals that determine functional outcome following antigen encounter, review current knowledge of the factors that ensure signaling inhibition and epigenetic gene silencing in anergic cells and explore the mechanisms that lead to the reversal of anergy and the reacquisition of effector functions.

TCR-Engineered T Cells Meet New Challenges to Treat Solid Tumors: Choice of Antigen, T Cell Fitness, and Sensitization of Tumor Milieu

Adoptive transfer of T cells gene-engineered with antigen-specific T cell receptors (TCRs) has proven its feasibility and therapeutic potential in the treatment of malignant tumors. To ensure further clinical development of TCR gene therapy, it is necessary to target immunogenic epitopes that are related to oncogenesis and selectively expressed by tumor tissue, and implement strategies that result in optimal T cell fitness. In addition, in particular for the treatment of solid tumors, it is equally necessary to include strategies that counteract the immune-suppressive nature of the tumor micro-environment. Here, we will provide an overview of the current status of TCR gene therapy, and redefine the following three challenges of improvement: “choice of target antigen”; “fitness of T cells”; and “sensitization of tumor milieu.” We will categorize and discuss potential strategies to address each of these challenges, and argue that advancement of clinical TCR gene therapy critically depends on developments toward each of the three challenges.

Mobilizing and evaluating anticancer T cells: pitfalls and solutions

Immunotherapy is a promising means to fight cancer, prompting a steady increase in clinical trials and correlative laboratory studies in this field. As antitumor T cells play central roles in immunity against malignant diseases, most immunotherapeutic protocols aim to induce and/or strengthen their function. Various treatment strategies have elicited encouraging clinical responses; however, major challenges have been uncovered that should be addressed in order to fully exploit the potential of immunotherapy. Here, we outline pitfalls for the mobilization of antitumor T cells and offer solutions to improve their therapeutic efficacy. We provide a critical perspective on the main methodologies used to characterize T-cell responses to cancer therapies, with a focus on discrepancies between T-cell attributes measured in vitro and protective responses in vivo. This review altogether provides recommendations to optimize the design of future clinical trials and highlights important considerations for the proficient analysis of clinical specimens available for research.

Transient regulatory T cell ablation deters oncogene-driven breast cancer and enhances radiotherapy

Transient ablation of regulatory T cells in a murine model of breast carcinogenesis inhibits primary tumor and lung metastatic growth and enhances the therapeutic effect of radiotherapy, but not immune checkpoint blockade.

Rational combinatorial therapeutic strategies have proven beneficial for the management of cancer. Recent success of checkpoint blockade in highly immunogenic tumors has renewed interest in immunotherapy. Regulatory T (T reg) cells densely populate solid tumors, which may promote progression through suppressing anti-tumor immune responses. We investigated the role of T reg cells in murine mammary carcinogenesis using an orthotopic, polyoma middle-T antigen-driven model in Foxp3^DTR knockin mice. T reg cell ablation resulted in significant determent of primary and metastatic tumor progression. Importantly, short-term ablation of T reg cells in advanced spontaneous tumors led to extensive apoptotic tumor cell death. This anti-tumor activity was dependent on IFN-γ and CD4⁺ T cells but not on NK or CD8⁺ T cells. Combination of T reg cell ablation with CTLA-4 or PD-1/PD-L1 blockade did not affect tumor growth or improve the therapeutic effect attained by T reg cell ablation alone. However, T reg cell targeting jointly with tumor irradiation significantly reduced tumor burden and improved overall survival. Together, our results demonstrate a major tumor-promoting role of T reg cells in an autochthonous model of tumorigenesis, and they reveal the potential therapeutic value of combining transient T reg cell ablation with radiotherapy for the management of poorly immunogenic, aggressive malignancies.

SOCS3 deletion in T lymphocytes suppresses development of chronic ocular inflammation via upregulation of CTLA-4 and expansion of regulatory T cells

Suppressors of cytokine signaling (SOCS) proteins are negative-feedback regulators of the JAK/STAT pathway, and SOCS3 contributes to host immunity by regulating the intensity and duration of cytokine signals and inflammatory responses. Mice with Socs3 deletion in myeloid cells exhibit enhanced STAT3 signaling, expansion of Th1 and Th17 cells, and develop severe experimental autoimmune encephalomyelitis. Interestingly, development of the unique IL-17/IFN-γ double-producing (Th17/IFN-γ and Tc17/IFN-γ) subsets that exhibit strong cytotoxic activities and are associated with pathogenesis of several autoimmune diseases has recently been shown to depend on epigenetic suppression of SOCS3 expression, further suggesting involvement of SOCS3 in autoimmunity and tumor immunity. In this study, we generated mice with Socs3 deletion in the CD4 T cell compartment (CD4-SOCS3 knockout [KO]) to determine in vivo effects of the loss of Socs3 in the T cell-mediated autoimmune disease, experimental autoimmune uveitis (EAU). In contrast to the exacerbation of experimental autoimmune encephalomyelitis in myeloid-specific SOCS3-deleted mice, CD4-SOCS3KO mice were protected from acute and chronic uveitis. Protection from EAU correlated with enhanced expression of CTLA-4 and expansion of IL-10-producing regulatory T cells with augmented suppressive activities. We further show that SOCS3 interacts with CTLA-4 and negatively regulates CTLA-4 levels in T cells, providing a mechanistic explanation for the expansion of regulatory T cells in CD4-SOCS3 during EAU. Contrary to in vitro epigenetic studies, Th17/IFN-γ and Tc17/IFN-γ populations were markedly reduced in CD4-SOCS3KO, suggesting that SOCS3 promotes expansion of the Th17/IFN-γ subset associated with development of severe uveitis. Thus, SOCS3 is a potential therapeutic target in uveitis and other autoinflammatory diseases.

T-cell co-stimulatory blockade in transplantation: two steps forward one step back!

INTRODUCTION

The concern about nephrotoxicity with calcineurin inhibitors led to the search of novel agents for immunosuppression. Based on the requirement of T-cell co-stimulatory signals to fully activated naïve T cells, it became clear that blocking these pathways could be an appealing therapeutic target. However, some unexpected findings were noticed in the recent clinical trials of belatacept, including a higher rate of rejection, which warranted further investigation with some interesting concepts emerging from the bench.

AREAS COVERED

This article aims to review the literature of the B7:CD28 co-stimulatory blockade in transplantation, including the basic immunology behind its development, clinical application and potential limitations.

EXPERT OPINION

Targeting co-stimulatory pathways were found to be much more complex than initially anticipated due to the interplay between not only various co-stimulatory pathways but also various co-inhibitory ones. In addition, co-stimulatory signals have different roles in diverse immune cell types. Therefore, targeting CD28 ligands with cytotoxic T lymphocyte antigen-4 (CTLA4)-Ig may have some deleterious effects, including the inhibition of regulatory T cells, blockade of co-inhibitory signals (CTLA4) and promotion of Th17 cells. Co-stimulatory independence of memory T cells was another unforeseen limitation. Learning how to better integrate co-stimulatory targeting with other immunosuppressive agents will be critical for the improvement of long-term graft survival.

Emerging Co-signaling Networks in T Cell Immune Regulation

Co-signaling molecules are surface glycoproteins that positively or negatively regulate the T cell response to antigen. Co-signaling ligands and receptors crosstalk between the surfaces of antigen-presenting cells (APCs) and T cells, and modulate the ultimate magnitude and quality of T cell receptor (TCR) signaling. In the past 10 years, the field of co-signaling research has been advanced by the understanding of underlying mechanisms of the immune modulation led by newly identified co-signaling molecules and the successful preclinical and clinical trials targeting co-inhibitory molecules called immune checkpoints in the treatment of autoimmune diseases and cancers. In this review, we briefly describe the characteristics of well-known B7 co-signaling family members regarding the expression, functions and therapeutic implications and to introduce newly identified B7 members such as B7-H5, B7-H6, and B7-H7.

Paramyxovirus activation and inhibition of innate immune responses

Paramyxoviruses represent a remarkably diverse family of enveloped nonsegmented negative-strand RNA viruses, some of which are the most ubiquitous disease-causing viruses of humans and animals. This review focuses on paramyxovirus activation of innate immune pathways, the mechanisms by which these RNA viruses counteract these pathways, and the innate response to paramyxovirus infection of dendritic cells (DC). Paramyxoviruses are potent activators of extracellular complement pathways, a first line of defense that viruses must face during natural infections. We discuss mechanisms by which these viruses activate and combat complement to delay neutralization. Once cells are infected, virus replication drives type I interferon (IFN) synthesis that has the potential to induce a large number of antiviral genes. Here we describe four approaches by which paramyxoviruses limit IFN induction: by limiting synthesis of IFN-inducing aberrant viral RNAs, through targeted inhibition of RNA sensors, by providing viral decoy substrates for cellular kinase complexes, and through direct blocking of the IFN promoter. In addition, paramyxoviruses have evolved diverse mechanisms to disrupt IFN signaling pathways. We describe three general mechanisms, including targeted proteolysis of signaling factors, sequestering cellular factors, and upregulation of cellular inhibitors. DC are exceptional cells with the capacity to generate adaptive immunity through the coupling of innate immune signals and T cell activation. We discuss the importance of innate responses in DC following paramyxovirus infection and their consequences for the ability to mount and maintain antiviral T cells.

CD4 blockade directly inhibits mouse and human CD4(+) T cell functions independent of Foxp3(+) Tregs

CD4(+) helper T cells orchestrate protective immunity against pathogens, yet can also induce undesired pathologies including allergies, transplant rejection and autoimmunity. Non-depleting CD4-specific antibodies such as clone YTS177.9 were found to promote long-lasting T cell tolerance in animal models. Thus, CD4 blockade could represent a promising therapeutic approach for human autoimmune diseases. However, the mechanisms underlying anti-CD4-induced tolerance are incompletely resolved. Particularly, multiple immune cells express CD4 including Foxp3(+) regulatory T cells (Tregs) and dendritic cells (DCs), both controlling the activation of CD4(+)Foxp3(-) helper T cells. Utilizing mixed leukocyte reactions (MLRs) reflecting physiological interactions between T cells and DCs, we report that anti-CD4 treatment inhibits CD4(+)Foxp3(-) T cell proliferation in an IL-2-independent fashion. Notably, YTS177.9 binding induces a rapid internalization of CD4 on both CD4(+)Foxp3(-) T cells and Foxp3(+) Tregs. However, no expansion or activation of immunosuppressive CD4(+)Foxp3(+) Tregs was observed following anti-CD4 treatment. Additionally, cytokine production, maturation and T cell priming capacity of DCs are not affected by anti-CD4 exposure. In line with these data, the selective ablation of Foxp3(+) Tregs from MLRs by the use of diphtheria toxin (DT)-treated bacterial artificial chromosome (BAC)-transgenic DEREG mice completely fails to abrogate the suppressive activity of multiple anti-CD4 antibodies. Instead, tolerization is associated with the defective expression of various co-stimulatory receptors including OX40 and CD30, suggesting altered signaling through the TCR complex. Consistent with our findings in mice, anti-CD4 treatment renders human CD4(+) T cells tolerant in the absence of Tregs. Thus, our results establish that anti-CD4 antibodies can directly tolerize pathogenic CD4(+)Foxp3(-) helper T cells. This has important implications for the treatment of human inflammatory diseases.

The great balancing act: regulation and fate of antiviral T-cell interactions

The fate of T lymphocytes revolves around a continuous stream of interactions between the T-cell receptor (TCR) and peptide-major histocompatibility complex (MHC) molecules. Beginning in the thymus and continuing into the periphery, these interactions, refined by accessory molecules, direct the expansion, differentiation, and function of T-cell subsets. The cellular context of T-cell engagement with antigen-presenting cells, either in lymphoid or non-lymphoid tissues, plays an important role in determining how these cells respond to antigen encounters. CD8(+) T cells are essential for clearance of a lymphocytic choriomeningitis virus (LCMV) infection, but the virus can present a number of unique challenges that antiviral T cells must overcome. Peripheral LCMV infection can lead to rapid cytolytic clearance or chronic viral persistence; central nervous system infection can result in T-cell-dependent fatal meningitis or an asymptomatic carrier state amenable to immunotherapeutic clearance. These diverse outcomes all depend on interactions that require TCR engagement of cognate peptide-MHC complexes. In this review, we explore the diversity in antiviral T-cell behaviors resulting from TCR engagement, beginning with an overview of the immunological synapse and progressing to regulators of TCR signaling that shape the delicate balance between immunopathology and viral clearance.

Antigen and transforming growth factor Beta receptors contribute to long term functional and phenotypic heterogeneity of memory CD8 T cells

Pathogen-specific CD8 T cells provide a mechanism for selectively eliminating host cells that are harboring intracellular pathogens. The pathogens are killed when lytic molecules are injected into the cytoplasm of the infected cells and begin an apoptotic cascade. Activated CD8 T cells also release large quantities of pro-inflammatory cytokines that stimulate other immune cells in the local vicinity. As the alveoli are extraordinarily sensitive to cytokine induced damage, multiple layers of immune regulation limit the activities of immune cells that enter the lungs. These mechanisms include receptor-mediated signaling pathways in CD8 T cells that respond to peptide antigens and transforming growth factor β. Both pathways influence the functional and phenotypic properties of long-lived CD8 T cells populations in peripheral and lymphoid tissues.

CD70 is downregulated by interaction with CD27

Engagement of the receptor CD27 by CD70 affects the magnitude and quality of T cell responses in a variety of infection models, and exaggerated signaling via this pathway results in enhanced immune responses and autoimmunity. One means by which signaling is regulated is tight control of cell surface CD70, which is expressed on dendritic cells (DCs), T cells, and B cells only upon activation. In this article, we show that a second level of regulation also is present. First, although undetectable on the cell surface by flow cytometry, immature DCs have a small pool of CD70 that continuously recycles from the plasma membrane. In addition, surface levels of CD70 on DCs and T cells were higher in mice deficient in CD27, or on DCs for which the interaction between CD70 and CD27 was precluded by blocking Abs. Binding of CD70 by its receptor resulted in downregulation of CD70 transcription and protein levels, suggesting that CD70-mediated "reverse signals" regulate its own levels. Therefore, the ability of CD70 to trigger costimulation is self-regulated when it binds its complementary receptor.

Molecular insights for optimizing T cell receptor specificity against cancer

Cytotoxic CD8 T cells mediate immunity to pathogens and they are able to eliminate malignant cells. Immunity to viruses and bacteria primarily involves CD8 T cells bearing high affinity T cell receptors (TCRs), which are specific to pathogen-derived (non-self) antigens. Given the thorough elimination of high affinity self/tumor-antigen reactive T cells by central and peripheral tolerance mechanisms, anti-cancer immunity mostly depends on TCRs with intermediate-to-low affinity for self-antigens. Because of this, a promising novel therapeutic approach to increase the efficacy of tumor-reactive T cells is to engineer their TCRs, with the aim to enhance their binding kinetics to pMHC complexes, or to directly manipulate the TCR-signaling cascades. Such manipulations require a detailed knowledge on how pMHC-TCR and co-receptors binding kinetics impact the T cell response. In this review, we present the current knowledge in this field. We discuss future challenges in identifying and targeting the molecular mechanisms to enhance the function of natural or TCR-affinity optimized T cells, and we provide perspectives for the development of protective anti-tumor T cell responses.

Host and microbial factors in regulation of T cells in the intestine

The intestine is divided into specialized tissue areas that provide distinct microenvironments for T cells. Regulation of T-cell responses in the gut has been a major focus of recent research activities in the field. T cells in the intestine are regulated by the interplay between host and microbial factors. In the small intestine, retinoic acid (RA) is a major tissue factor that plays important roles in regulation of immune responses. In the large intestine, the influence of RA diminishes, but that of commensal bacterial products increases. RA, gut microbiota, and inflammatory mediators co-regulate differentiation, distribution, and/or effector functions of T cells. Coordinated regulation of immune responses by these factors promotes well-balanced immunity and immune tolerance. Dysregulation of this process can increase infection and inflammatory diseases.

A co-stimulatory trap set by myeloid leukemia cells

The straightforward notion that tumor cells always exert immunosuppressive functions has been contradicted by the finding that myeloid leukemia cells can express potent co-stimulatory molecules. Indeed, the co-stimulatory support offered by leukemia cells can provoke helper T-cell responses. Unfavorably, this interaction allows leukemia cells to acquire immunosuppressive capacities.