The diversity of costimulatory and inhibitory receptor pathways and the regulation of antiviral T cell responses

T-Cell Immunoglobulin and ITIM Domain (TIGIT) Associates with CD8+ T-Cell Exhaustion and Poor Clinical Outcome in AML Patients

PURPOSE

T-cell immunoglobulin and immunoreceptor tyrosine-based inhibitory motif (ITIM) domain (TIGIT) is a recently identified T-cell coinhibitory receptor. In this study, we aimed to determine the clinical impact of TIGIT in patients with acute myelogenous leukemia (AML) and dissect the role of TIGIT in the pathogenesis of leukemia progression.

EXPERIMENTAL DESIGN

TIGIT expression on T cells from peripheral blood collected from patients with AML was examined by flow cytometry. The correlation of TIGIT expression to clinical outcomes, including rate of complete remission and relapse post-allogeneic stem cell transplantation (alloSCT) in AML patients, was analyzed. Phenotypic and functional study (cytokine release, proliferation, killing, and apoptosis) of TIGIT-expressing T cells were performed. Using siRNA to silence TIGIT, we further elucidated the regulatory role of TIGIT in the T-cell immune response by dissecting the effect of TIGIT knockdown on cytokine release and apoptosis of T cells from AML patients.

RESULTS

TIGIT expression on CD8(+) T cells is elevated in AML patients and high-TIGIT correlates with primary refractory disease and leukemia relapse post-alloSCT. TIGIT(+) CD8(+) T cells display phenotypic features of exhaustion and exhibit functional impairment manifested by low production of cytokines and high susceptibility to apoptosis. Importantly, their functional defects are reversed by TIGIT knockdown.

CONCLUSIONS

TIGIT contributes to functional T-cell impairment and associates with poor clinical outcome in AML. Our study suggests that blockade of TIGIT to restore T-cell function and antitumor immunity may represent a novel effective leukemia therapeutic. Clin Cancer Res; 22(12); 3057-66. ©2016 AACR.

T cells Exhibit Reduced Signal Transducer and Activator of Transcription 5 Phosphorylation and Upregulated Coinhibitory Molecule Expression After Kidney Transplantation

BACKGROUND

T-cell depletion therapy is associated with diminished interleukin (IL)-7/IL-15-dependent homeostatic proliferation resulting in incomplete T-cell repopulation. Furthermore, it is associated with impaired T-cell functions. We hypothesized that this is the result of impaired cytokine responsiveness of T cells, through affected signal transducer and activator of transcription (STAT)5 phosphorylation and upregulation of coinhibitory molecules.

MATERIALS AND METHODS

Patients were treated with T cell-depleting rabbit antithymocyte globulin (rATG) (6 mg/kg, n = 17) or nondepleting, anti-CD25 antibody (basiliximab, 2 × 40 mg, n = 25) induction therapy, in combination with tacrolimus, mycophenolate mofetil, and steroids. Before and the first year after transplantation, IL-7 and IL-2 induced STAT5 phosphorylation, and the expression of the coinhibitory molecules programmed cell death protein 1 (PD-1), T cell immunoglobulin mucin-3 (TIM-3), lymphocyte activation gene-3 (LAG-3), cytotoxic T-lymphocyte-associated protein 4 (CTLA-4), cluster of differentiation (CD) 160, and CD244 was measured by flow cytometry.

RESULTS

The first year after rATG, CD4+, and CD8+ T cells were affected in their IL-7-dependent phosphorylation of STAT5 (pSTAT5) which was most outspoken in the CD8+ memory population. The capacity of CD4+ and CD8+ T cells to pSTAT5 in response to IL-2 decreased after both rATG and basiliximab therapy. After kidney transplantation, the percentage of TIM-3+, PD-1+, and CD160+CD4+ T cells and the percentage of CD160+ and CD244+CD8+ T cells increased, with no differences in expression between rATG- and basiliximab-treated patients. The decrease in pSTAT5 capacity CD8+ T cells and the increase in coinhibitory molecules were correlated.

CONCLUSIONS

We show that memory T cells in kidney transplant patients, in particular after rATG treatment, have decreased cytokine responsiveness by impaired phosphorylation of STAT5 and have increased expression of coinhibitory molecules, processes which were correlated in CD8+ T cells.

A Jekyll and Hyde Profile: Type 1 Interferon Signaling Plays a Prominent Role in the Initiation and Maintenance of a Persistent Virus Infection

The hallmarks of persistent viral infections are exhaustion of virus-specific T cells, elevated production of interleukin 10 (IL-10) and programmed death-1 (PD-1) the dominant negative regulators of the immune system and disruption of secondary lymphoid tissues. Within the first 12-24 hours after mice are infected with lymphocytic choriomeningitis virus (LCMV) clone 13, which is used as a model of persistent virus infection, we note generation of high titers of type 1 interferon. Blockade of type 1 interferon significantly lessens IL-10 and PD-1/PD-L1, allows normal secondary lymphoid architecture and re-establishes antiviral T-cell function, thus eradicating the virus and clearing the infection. Hence, type 1 interferon is a master reostat for establishing persistent viral infection.

Inhibitory Receptor Expression on CD8+ T Cells Is Linked to Functional Responses against Trypanosoma cruzi Antigens in Chronic Chagasic Patients

In mammals, chronic diseases resulting from infectious agents have been associated with functional T cell response deficiency, a high frequency of terminally differentiated T cells, the presence of monofunctional Ag-specific T cells, and increased expression of inhibitory receptors. Similar to other chronic diseases, the progressive loss of certain functional activities during Trypanosoma cruzi infection might result in the inability to control replication of this parasite. To examine this hypothesis, we evaluated the differentiation and cell effector function of CD8(+) T cells and characterized the expression of inhibitory receptors and the presence of the parasite in the bloodstream of chagasic patients. The results showed that patients at an advanced severe disease stage had a higher frequency of terminally differentiated CD8(+) T cells than patients at an early stage of the disease. A monofunctional CD8(+) T cell response was observed in patients at an advanced stage, whereas the coexpression of markers that perform three and four functions in response to parasite Ags was observed in patients at a less severe disease stage. The frequency of CD8(+) T cells producing granzyme B and perforin and those expressing inhibitory receptors was higher in symptomatic patients than in asymptomatic patients. Taken together, these findings suggest that during the course of Chagas disease, CD8(+) T cells undergo a gradual loss of function characterized by impaired cytokine production, the presence of advanced differentiation, and increased inhibitory receptor coexpression.

Programmed death-1 & its ligands: promising targets for cancer immunotherapy

Novel strategies for cancer treatment involving blockade of immune inhibitors have shown significant progress toward understanding the molecular mechanism of tumor immune evasion. The preclinical findings and clinical responses associated with programmed death-1 (PD-1) and PD-ligand pathway blockade seem promising, making these targets highly sought for cancer immunotherapy. In fact, the anti-PD-1 antibodies, pembrolizumab and nivolumab, were recently approved by the US FDA for the treatment of unresectable and metastatic melanoma resistant to anticytotoxic T-lymphocyte antigen-4 antibody (ipilimumab) and BRAF inhibitor. Here, we discuss strategies of combining PD-1/PD-ligand interaction inhibitors with other immune checkpoint modulators and standard-of-care therapy to break immune tolerance and induce a potent antitumor activity, which is currently a research area of key scientific pursuit.

Inhibitory Receptors Beyond T Cell Exhaustion

Inhibitory receptors (iRs) are frequently associated with "T cell exhaustion". However, the expression of iRs is also dependent on T cell differentiation and activation. Therapeutic blockade of various iRs, also referred to as "checkpoint blockade", is showing -unprecedented results in the treatment of cancer patients. Consequently, the clinical potential in this field is broad, calling for increased research efforts and rapid refinements in the understanding of iR function. In this review, we provide an overview on the significance of iR expression for the interpretation of T cell functionality. We summarize how iRs have been strongly associated with "T cell exhaustion" and illustrate the parallel evidence on the importance of T cell differentiation and activation for the expression of iRs. The differentiation subsets of CD8 T cells (naïve, effector, and memory cells) show broad and inherent differences in iR expression, while activation leads to strong upregulation of iRs. Therefore, changes in iR expression during an immune response are often concomitant with T cell differentiation and activation. Sustained expression of iRs in chronic infection and in the tumor microenvironment likely reflects a specialized T cell differentiation. In these situations of prolonged antigen exposure and chronic inflammation, T cells are "downtuned" in order to limit tissue damage. Furthermore, we review the novel "checkpoint blockade" treatments and the potential of iRs as biomarkers. Finally, we provide recommendations for the immune monitoring of patients to interpret iR expression data combined with parameters of activation and differentiation of T cells.

Expression of inhibitory receptors on intratumoral T cells modulates the activity of a T cell-bispecific antibody targeting folate receptor

T-cell bispecific antibodies (TCBs) are a novel therapeutic tool designed to selectively recruit T-cells to tumor cells and simultaneously activate them. However, it is currently unknown whether the dysfunctional state of T-cells, embedded into the tumor microenvironment, imprints on the therapeutic activity of TCBs. We performed a comprehensive analysis of activation and effector functions of tumor-infiltrating T-cells (TILs) in different tumor types, upon stimulation by a TCB targeting folate receptor 1 and CD3 (FolR1-TCB). We observed a considerable heterogeneity in T-cell activation, cytokine production and tumor cell killing upon exposure to FolR1-TCB among different FolR1-expressing tumors. Of note, tumors presenting with a high frequency of PD-1^hi TILs displayed significantly impaired tumor cell killing and T-cell function. Further characterization of additional T-cell inhibitory receptors revealed that PD-1^hi TILs defined a T-cell subset with particularly high levels of multiple inhibitory receptors compared with PD-1^int and PD-1^neg T-cells. PD-1 blockade could restore cytokine secretion but not cytotoxicity of TILs in a subset of patients with scarce PD-1^hi expressing cells; in contrast, patients with abundance of PD-1^hi expressing T-cells did not benefit from PD-1 blockade. Our data highlight that FolR1-TCB is a promising novel immunotherapeutic treatment option which is capable of activating intratumoral T-cells in different carcinomas. However, its therapeutic efficacy may be substantially hampered by a pre-existing dysfunctional state of T-cells, reflected by abundance of intratumoral PD-1^hi T-cells. These findings present a rationale for combinatorial approaches of TCBs with other therapeutic strategies targeting T-cell dysfunction.

T-cell exhaustion in the tumor microenvironment

T-cell exhaustion was originally identified during chronic infection in mice, and was subsequently observed in humans with cancer. The exhausted T cells in the tumor microenvironment show overexpressed inhibitory receptors, decreased effector cytokine production and cytolytic activity, leading to the failure of cancer elimination. Restoring exhausted T cells represents an inspiring strategy for cancer treatment, which has yielded promising results and become a significant breakthrough in the cancer immunotherapy. In this review, we overview the updated understanding on the exhausted T cells in cancer and their potential regulatory mechanisms and discuss current therapeutic interventions targeting exhausted T cells in clinical trials.

Activation and Exhaustion of Adaptive Immune Cells in Hepatitis B Infection

In hepatitis B virus (HBV) infection, the immune reaction is responsible for viral clearance and preventing their spread within the host. However, the immune system is dysfunctional in patients with chronic HBV infection, leading to an inadequate immune response against the virus. A major factor contributing to inefficient immune function is the phenomenon of immune exhaustion. Hence, understanding immune activation and exhaustion during HBV infection is important, as it would provide insight in developing immunotherapy to control chronic HBV infection. The aim of this review is to highlight the existing information on immune effector functions and immune exhaustion in response to HBV infection.

Lymphocytic alveolitis is associated with the accumulation of functionally impaired HIV-specific T cells in the lung of antiretroviral therapy-naive subjects

RATIONALE

Lymphocytic alveolitis in HIV-1-infected individuals is associated with multiple pulmonary complications and a poor prognosis. Although lymphocytic alveolitis has been associated with viremia and an increased number of CD8(+) T cells in the lung, its exact cause is unknown.

OBJECTIVES

To determine if HIV-1-specific T cells are associated with lymphocytic alveolitis in HIV-1-infected individuals.

METHODS

Using blood and bronchoalveolar lavage (BAL) cells from normal control subjects and untreated HIV-1-infected individuals, we examined the frequency and functional capacity of HIV-1-specific T cells.

MEASUREMENTS AND MAIN RESULTS

We found that HIV-1-specific T cells were significantly elevated in the BAL compared with blood of HIV-1-infected individuals and strongly correlated with T-cell alveolitis. Expression of Ki67, a marker of in vivo proliferation, was significantly reduced on HIV-1-specific T cells in BAL compared with blood, suggesting a diminished proliferative capacity. In addition, HIV-1-specific CD4(+) and CD8(+) T cells in BAL had higher expression of programmed death 1 (PD-1) and lower cytotoxic T-lymphocyte antigen 4 (CTLA-4) expression than those in the blood. A strong correlation between PD-1, but not CTLA-4, and HIV-1-specific T-cell proliferation was seen, and blockade of the PD-1/PD-L1 pathway augmented HIV-1-specific T-cell proliferation, suggesting that the PD-1 pathway was the main cause of reduced proliferation in the lung.

CONCLUSIONS

These findings suggest that alveolitis associated with HIV-1 infection is caused by the recruitment of HIV-1-specific CD4(+) and CD8(+) T cells to the lung. These antigen-specific T cells display an impaired proliferative capacity that is caused by increased expression of PD-1.

Coinhibitory receptors and CD8 T cell exhaustion in chronic infections

PURPOSE OF REVIEW

To describe the recent data on the role of coinhibitory receptors, such as PD-1, Tim-3, CD160, as mediators of the 'exhaustion' of virus-specific CD8 T cells in chronic infections and particularly in HIV.

RECENT FINDINGS

Exhaustion of chronic virus-specific CD8 T cells is a dynamic process characterized by altered differentiation, impaired function, and compromised proliferation/survival profile of these cells. This process is mediated by coinhibitory receptors expressed on the surface of virus-specific CD8 T cells and an orchestrated function of centrally connected pathways. Coexpression of several coinhibitory receptors characterizes severely exhausted virus-specific CD8 T cells. Several studies suggest a synergistic action, instead of a redundant role, of the different receptors. In-vivo manipulation of the coinhibitory network can rejuvenate exhausted virus-specific CD8 T cell responses and constrain replication of chronic viruses, including HIV.

SUMMARY

Revealing the molecular basis of virus-specific CD8 T cell exhaustion in chronic infections is critical for the understanding of the disease pathogenesis and the designing of novel vaccines aiming to enhance the cytolytic arm of the immune system. This is of particular interest for the development of immunotherapies in the context of a functional cure for HIV.

HBV and the immune response

Hepatitis B virus (HBV) infection acquired in adult life is generally self-limited while chronic persistence of the virus is the prevalent outcome when infection is acquired perinatally. Both control of infection and liver cell injury are strictly dependent upon protective immune responses, because hepatocyte damage is the price that the host must pay to get rid of intracellular virus. Resolution of acute hepatitis B is associated with functionally efficient, multispecific antiviral T-cell responses which are preceded by a poor induction of intracellular innate responses at the early stages of infection. Persistent control of infection is provided by long-lasting protective memory, which is probably sustained by continuous stimulation of the immune system by trace amounts of virus which are never totally eliminated, persisting in an occult episomic form in the nucleus of liver cells even after recovery from acute infection. Chronic virus persistence is instead characterized by a lack of protective T-cell memory maturation and by an exhaustion of HBV-specific T-cell responses. Persistent exposure of T cells to high antigen loads is a key determinant of functional T-cell impairment but also other mechanisms can contribute to T-cell inhibition, including the tolerogenic effect of the liver environment. The degree of T-cell impairment is variable and its severity is related to the level of virus replication and antigen load. The antiviral T-cell function is more efficient in patients who can control infection either partially, such as inactive HBsAg carriers with low levels of virus replication, or completely, such as patients who achieve HBsAg loss either spontaneously or after antiviral therapy. Thus, understanding the features of the immune responses associated with control of infection is needed for the successful design of novel immune modulatory therapies based on the reconstitution of efficient antiviral responses in chronic HBV patients.

2B4 (CD244) induced by selective CD28 blockade functionally regulates allograft-specific CD8+ T cell responses

Blockade of CD28 signals results in the up-regulation of 2B4 on primary CD8⁺ effectors and plays a critical role in controlling antigen-specific CD8⁺ T cell responses.

Mounting evidence in models of both autoimmunity and chronic viral infection suggests that the outcome of T cell activation is critically impacted by the constellation of co-stimulatory and co-inhibitory receptors expressed on the cell surface. Here, we identified a critical role for the co-inhibitory SLAM family member 2B4 (CD244) in attenuating primary antigen-specific CD8⁺ T cell responses in the presence of immune modulation with selective CD28 blockade. Our results reveal a specific up-regulation of 2B4 on antigen-specific CD8⁺ T cells in animals in which CD28 signaling was blocked. However, 2B4 up-regulation was not observed in animals treated with CTLA-4 Ig (abatacept) or CD28 blockade in the presence of anti–CTLA-4 mAb. 2B4 up-regulation after CD28 blockade was functionally significant, as the inhibitory impact of CD28 blockade was diminished when antigen-specific CD8⁺ T cells were deficient in 2B4. In contrast, 2B4 deficiency had no effect on CD8⁺ T cell responses during unmodified rejection or in the presence of CTLA-4 Ig. We conclude that blockade of CD28 signals in the presence of preserved CTLA-4 signals results in the unique up-regulation of 2B4 on primary CD8⁺ effectors, and that this 2B4 expression plays a critical functional role in controlling antigen-specific CD8⁺ T cell responses.

Identification of B7-H1 as a novel mediator of the innate immune/proinflammatory response as well as a possible myeloid cell prognostic biomarker in sepsis

Identifying relevant mediators responsible for the pathogenesis during sepsis may lead to finding novel diagnostic and therapeutic targets. Recent studies indicate programmed cell death receptor (PD)-1 plays a significant role in the development of immune suppression associated with sepsis. In this study, we determine whether B7-H1, the primary ligand of PD-1, contributes to the pathogenesis of sepsis. We report that B7-H1 is upregulated extensively on various immune cells during sepsis and B7-H1 gene deficiency protects mice from the lethality of sepsis. In terms of the histological development of multiple organ damage and inflammatory cytokine levels in circulation or at infectious site, B7-H1-deficient mice showed a remarkable reduction in these indices when compared with wild-type mice. However, B7-H1 gene-deficient mice did not exhibit a lower bacterial burden when compared with wild-type mice, although they recruited more macrophages and neutrophils into infectious site. In addition, we found that, during sepsis, whereas there were no marked differences affecting ex vivo macrophage cytokine productive capacity between PD-1 and B7-H1 gene-deficient mice, preservation of ex vivo macrophage phagocytic function was only seen in septic PD-1 knockout mouse cells. Finally, higher percentage B7-H1(+) neutrophils in peripheral blood correlated not only with higher levels of pro- and anti-inflammatory cytokines/chemokines (CCL2, IL-6, CXCL2, KC, TNF-α, and IL-10), but with lethal outcome as well. Together, these results indicate B7-H1 contributes to septic morbidity in fashion distinct from PD-1 and suggest B7-H1 expression on neutrophils could be used as a biomarker of septic severity.

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.

CD40 signaling to the rescue: A CD8 exhaustion perspective in chronic infectious diseases

Chronic infectious diseases such as HIV, HBV, and HCV, among others, cause severe morbidity and mortality globally. Progressive decline in CD8 functionality, survival, and proliferative potential-a phenomenon referred to as CD8 exhaustion-is believed to be responsible for poor pathogen control in chronic infectious diseases. While the role of negative inhibitory receptors such as PD-1 in augmenting CD8 exhaustion has been extensively studied, the role of positive costimulatory receptors remains poorly understood. In this review, we discuss how one such costimulatory pathway, CD40-CD40L, regulates CD8 dysfunction and rescue. While the significance of this pathway has been extensively investigated in models of autoimmunity, acute infectious diseases, and tumor models, the role played by CD40-CD40L in regulating CD8 exhaustion in chronic infectious diseases is just beginning to be understood. Considering that monotherapy with blocking antibodies targeting inhibitory PD-1-PD-L1 pathway is only partially effective at ameliorating CD8 exhaustion and that humanized CD40 agonist antibodies are currently available, a better understanding of the role of the CD40-CD40L pathway in chronic infectious diseases will pave the way for the development of more robust immunotherapeutic and prophylactic vaccination strategies.

Cutting edge: IL-12 and type I IFN differentially program CD8 T cells for programmed death 1 re-expression levels and tumor control

Naive CD8 T cells proliferate in response to TCR and CD28 signals, but require IL-12 or type I IFN to survive and develop optimal effector functions. Although murine CTL generated in vitro in response to IL-12 or IFN-α had comparable effector functions, IL-12-stimulated cells were significantly more effective in controlling tumor in an adoptive immunotherapy model. They maintained high numbers and function, whereas IFN-α-stimulated cells declined in number and became exhausted. Consistent with this, IFN-α-stimulated cells in the tumor expressed higher levels of programmed death 1 (PD-1) inhibitory receptor than did IL-12-stimulated cells. When blocking Ab specific for the PD-L1 ligand of PD-1 was administered, the efficacy of IFN-α-stimulated CTL became comparable with that of IL-12-stimulated cells. Thus, IL-12 and IFN-α differentially program CD8 T cells to re-express distinct levels of PD-1 upon re-encountering Ag, resulting in IL-12-stimulated cells being less susceptible to exhaustion in the face of sustained tumor Ag.

A randomized, double-blind, placebo-controlled assessment of BMS-936558, a fully human monoclonal antibody to programmed death-1 (PD-1), in patients with chronic hepatitis C virus infection

Expression of the programmed death 1 (PD-1) receptor and its ligands are implicated in the T cell exhaustion phenotype which contributes to the persistence of several chronic viral infections, including human hepatitis C virus (HCV). The antiviral potential of BMS-936558 (MDX-1106) – a fully human anti-PD-1 monoclonal immunoglobulin-G4 that blocks ligand binding – was explored in a proof-of-concept, placebo-controlled single-ascending-dose study in patients (N = 54) with chronic HCV infection. Interferon-alfa treatment-experienced patients (n = 42) were randomized 5∶1 to receive a single infusion of BMS-936558 (0.03, 0.1, 0.3, 1.0, 3.0 mg/kg [n = 5 each] or 10 mg/kg [n = 10]) or of placebo (n = 7). An additional 12 HCV treatment-naïve patients were randomized to receive 10 mg/kg BMS-936558 (n = 10) or placebo (n = 2). Patients were followed for 85 days post-dose. Five patients who received BMS-936558 (0.1 [n = 1] or 10 mg/kg) and one placebo patient achieved the primary study endpoint of a reduction in HCV RNA ≥0.5 log₁₀ IU/mL on at least 2 consecutive visits; 3 (10 mg/kg) achieved a >4 log₁₀ reduction. Two patients (10 mg/kg) achieved HCV RNA below the lower limit of quantitation (25 IU/mL), one of whom (a prior null-responder) remained RNA-undetectable 1 year post-study. Transient reductions in CD4⁺, CD8⁺ and CD19⁺ cells, including both naïve and memory CD4⁺ and CD8⁺ subsets, were observed at Day 2 without evidence of immune deficit. No clinically relevant changes in immunoglobulin subsets or treatment-related trends in circulating cytokines were noted. BMS-936558 exhibited dose-related exposure increases, with a half-life of 20–24 days. BMS-936558 was mostly well tolerated. One patient (10 mg/kg) experienced an asymptomatic grade 4 ALT elevation coincident with the onset of a 4-log viral load reduction. Six patients exhibited immune-related adverse events of mild-to-moderate intensity, including two cases of hyperthyroidism consistent with autoimmune thyroiditis. Further investigation of PD-1 pathway blockade in chronic viral disease is warranted.

Trial Registration

ClinicalTrials.gov NCT00703469 NCT00703469

Memory T cells and their exhaustive differentiation in allograft tolerance and rejection

PURPOSE OF REVIEW

Memory T cells have emerged as a major threat to transplant survival; they are well equipped and well positioned to respond to antigens in an accelerated fashion. They participate in transplant rejection and resist interventions that usually contain naïve T cells. Thus, the means to prevent memory T cells from attacking allotransplants are an important issue in transplantation.

RECENT FINDINGS

Recent studies in other models suggest that effector T cells, which include both freshly activated T cells and memory T cells, can acquire 'an exhausted phenotype' in that they progressively lose their effector activities. This response is highly regulated, antigen specific, and driven primarily by antigen persistence. This exhausted phenotype has not been carefully explored in transplant models, and its role in transplant survival remains largely unknown.

SUMMARY

Studies of T-cell exhaustion may reveal additional facets of the fundamental mechanisms of transplant survival. T-cell exhaustion may be an alternative way of preventing memory development. Future studies are needed to further improve our understanding of T-cell exhaustion in transplantation.

Pharmacological inhibition of VIP signaling enhances antiviral immunity and improves survival in murine cytomegalovirus-infected allogeneic bone marrow transplant recipients

Cytomegalovirus (CMV) infection following allogeneic bone marrow transplant (allo-BMT) is controlled by donor-derived cellular immunity. Vasoactive intestinal peptide (VIP) suppresses Th1 immunity. We hypothesized that blocking VIP-signaling would enhance anti-CMV immunity in murine recipients of allo-BMT. Recipients were transplanted with bone marrow (BM) and T-cells from major histocompatibility complex (MHC)-mismatched VIP-knockout (KO) or wild-type donors, and treated with 7 daily subcutaneous injections of VIPhyb (peptidic VIP-antagonist) or phosphate-buffered saline (PBS). Genetic and pharmacological blockade of VIP-signaling protected allo-BMT recipients from lethal murine CMV (mCMV) infection, improving survival without increasing graft-versus-host disease. Mice treated with VIPhyb or transplanted with VIP-KO allografts had significantly lower viral loads, increased numbers of mCMV-M45-peptide-MHC-tetramer(+) CD8(+) T-cells, with lower PD-1 expression, and enhanced primary and secondary cellular immune responses after mCMV infection than did PBS-treated mice. These results demonstrate that administration of a VIP antagonist after allo-BMT is a promising safely therapeutic approach to enhance antiviral cellular immunity.

Identification of an intrahepatic transcriptional signature associated with self-limiting infection in the woodchuck model of hepatitis B

The woodchuck model of hepatitis B virus (HBV) infection displays many characteristics of human infection and has particular value for characterizing the host immune responses during the development of chronic infection. Using the newly developed custom woodchuck microarray platform, we compared the intrahepatic transcriptional profiles of neonatal woodchucks with self-limiting woodchuck hepatitis virus (WHV) infection to those woodchucks progressing to persistent WHV infection. This revealed that WHV does not induce significant intrahepatic gene expression changes during the early-acute stage of infection (8 weeks), suggesting it is a stealth virus. At the mid-acute phase of infection (14 weeks), resolution was associated with induction of a prominent cytotoxic T-cell signature. Strikingly, this was accompanied by high-level expression of PD-1 and various other inhibitory T-cell receptors, which likely act to minimize liver damage by cytotoxic T cells during viral clearance. In contrast to the expression of perforin and other cytotoxic effector genes, the interferon-γ (IFN-γ) signaling response in the mid-acute phase was comparable to that in chronically infected adult animals. The absence of a strong IFN-α/β transcriptional response indicated that type I IFN is not a critical mediator of self-limiting infection. Nevertheless, a number of antiviral genes, including viperin, were differentially expressed during resolving infection, suggesting that a subset of IFN-stimulated genes (ISG) may play a role in the control of WHV replication.

CONCLUSION

We identified new immune pathways associated with the clearance of hepadnavirus infection revealing novel molecular targets with potential for the therapeutic treatment of chronic hepatitis B.

Evolving role of 2B4/CD244 in T and NK cell responses during virus infection

The signaling lymphocyte activation molecule (SLAM) family receptor, 2B4/CD244, was first implicated in anti-viral immunity by the discovery that mutations of the SLAM-associated protein, SAP/SH2D1A, impaired 2B4-dependent stimulation of T and natural killer (NK) cell anti-viral functions in X-linked lymphoproliferative syndrome patients with uncontrolled Epstein-Barr virus infections. Engagement of 2B4 has been variably shown to either activate or inhibit lymphocytes which express this receptor. While SAP expression is required for stimulatory functions of 2B4 on lymphocytes, it remains unclear whether inhibitory signals derived from 2B4 can predominate even in the presence of SAP. Regardless, mounting evidence suggests that 2B4 expression by NK and CD8 T cells is altered by virus infection in mice as well as in humans, and 2B4-mediated signaling may be an important determinant of effective immune control of chronic virus infections. In this review, recent findings regarding the expression and function of 2B4 as well as SAP on T and NK cells during virus infection is discussed, with a focus on the role of 2B4-CD48 interactions in crosstalk between innate and adaptive immunity.

Combined blockade of programmed death-1 and activation of CD137 increase responses of human liver T cells against HBV, but not HCV

BACKGROUND & AIMS

In patients with chronic hepatitis B virus (HBV) and hepatitis C virus (HCV) infections, antiviral functions of T cells are impaired; these might be increased by blocking T-cell co-inhibitory pathways, such as preventing interaction between the receptor programmed death (PD)-1 and its ligand, PD-L1. We attempted to optimize the restoration of T-cell functions in patients with chronic HBV or HCV infection with a combination of reagents that block PD-1 interaction with PD-L1 and stimulate T-cell signaling via CD137, a member of the tumor necrosis factor-receptor family.

METHODS

We assessed the effects of CD137 stimulation (via CD137L), alone or in combination with antibodies that block PD-1 interaction with PD-L1 (anti-PD-L1), on proliferation and production of interferon-γ and interleukin-2 by intrahepatic and peripheral T cells from patients with chronic HBV or HCV infection. We also analyzed expression of different co-stimulatory molecules on virus-specific CD8+ and forkhead box P3+CD4+ cells by flow cytometry.

RESULTS

Incubation of intrahepatic T cells with CD137L and anti-PD-L1 increased their responses to HBV, but not HCV. However, HCV-specific T cells isolated from peripheral blood were sensitive to these reagents. Virus-specific T cells from some, but not all patients, had increased responses to anti-PD-L1 when CD137L was added because in some cases the combination of anti-PD-L1 and CD137L overstimulated T cells, leading to their inhibition. Intrahepatic HBV- and HCV-specific CD8+ T cells had different costimulatory profiles; liver cells from patients with chronic HBV infection had a higher proportion of forkhead box P3+ regulatory T cells, with higher levels of PD-1, compared with liver cells from patients with chronic HCV infection.

CONCLUSIONS

A combination of reagents that prevent interaction between PD-1 and its ligand and activate CD137 signaling increase responses of intrahepatic HBV-specific T cells and circulating HCV-specific T cells. This strategy might be developed to increase T-cell responses to these viruses in patients with chronic hepatitis B or C, and tailoring the dose of CD137L administered will help optimize results.

Natural killer cell responses during viral infections: flexibility and conditioning of innate immunity by experience

Natural killer (NK) cells mediate innate defense against viral infections, but the mechanisms in place to access their functions as needed during diverse challenges while limiting collateral damage are poorly understood. Recent molecular characterization of effects mediated through infection-induced inhibitory/activating NK receptor-ligand pairs and cytokines are providing new insights into pathways regulating their responses and revealing unexpected consequences for NK cell subset effects, maintenance, proliferation and function through times overlapping with adaptive and long-lived immunity. The observations define flexible pathways for experience-induced 'conditioning' and challenge narrowly defined roles for NK cells and innate immunity as first responders with prescribed functions. They suggest that individual experiences as well as genes influence the innate immune resources available to fight off an infection.

Transcriptomic analysis of the woodchuck model of chronic hepatitis B

The Eastern woodchuck (Marmota monax) is naturally infected with woodchuck hepatitis virus (WHV), a hepadnavirus closely related to the human hepatitis B virus (HBV). The woodchuck is used as an animal model for studying chronic hepatitis B (CHB) and HBV-associated hepatocellular carcinoma (HCC) in humans, but the lack of sequence information has hitherto precluded functional genomics analysis. To address this major limitation of the model, we report here the sequencing, assembly, and annotation of the woodchuck transcriptome, together with the generation of custom woodchuck microarrays. Using this new platform, we characterized the transcriptional response to persistent WHV infection and WHV-induced HCC. This revealed that chronic WHV infection, like HBV, is associated with (1) a limited intrahepatic type I interferon response; (2) intrahepatic induction of markers associated with T cell exhaustion; (3) elevated levels of suppressor of cytokine signaling 3 (SOCS3) in the liver; and (4) intrahepatic accumulation of neutrophils. Underscoring the translational value of the woodchuck model, this study also determined that WHV-induced HCC shares molecular characteristics with a subtype of human HCC with poor prognosis.

CONCLUSION

Our data establish the translational value of the woodchuck model and provide new insight into immune pathways which may play a role either in the persistence of HBV infection or the sequelae of CHB.

Therapeutic vaccination against chronic hepatitis C virus infection

Approximately 170 million people worldwide are chronic carriers of Hepatitis C virus (HCV). To date, there is no prophylactic vaccine available against HCV. The standard-of-care therapy for HCV infection involves a combination of pegylated interferon-α and ribavirin. This therapy, which is commonly associated with side effects, has a curative rate varying from 43% (HCV genotype 1) to 80% (HCV genotype 2). In 2011, two direct-acting antiviral agents, telaprevir and boceprevir, were approved by the US Food and drug Administration and are now being used in combination with standard-of-care therapy in selected patients infected with HCV genotype 1. Although both drugs are promising, resulting in a shortening of therapy, these drugs also induce additional side effects and have reduced efficacy in patients who did not respond to standard-of-care previously. An alternative approach would be to treat HCV by stimulating the immune system with a therapeutic vaccine ideally aimed at (i) the eradication of HCV-infected cells and (ii) neutralization of infectious HCV particles. The challenge is to develop therapeutic vaccination strategies that are either at least as effective as antiviral drugs but with lower side effects, or vaccines that, when combined with antiviral drugs, can circumvent long-term use of these drugs thereby reducing their side effects. In this review, we summarize and discuss recent preclinical developments in the area of therapeutic vaccination against chronic HCV infection. Although neutralizing antibodies have been described to exert protective immunity, clinical studies on the induction of neutralizing antibodies in therapeutic settings are limited. Therefore, we will primarily discuss therapeutic vaccines which aim to induce effective cellular immune response against HCV.

Viral acute lower respiratory infections impair CD8+ T cells through PD-1

Viruses are leading causes of severe acute lower respiratory infections (LRIs). These infections evoke incomplete immunity, as individuals can be repeatedly reinfected throughout life. We report that acute viral LRI causes rapid pulmonary CD8+ cytotoxic T lymphocyte (TCD8) functional impairment via programmed death-1/programmed death ligand-1 (PD-1/PD-L1) signaling, a pathway previously associated with prolonged antigenic stimulation during chronic infections and cancer. PD-1-mediated TCD8 impairment occurred acutely in mice following infection with human metapneumovirus or influenza virus. Viral antigen was sufficient for PD-1 upregulation, but induction of PD-L1 was required for impairment. During secondary viral infection or epitope-only challenge, memory TCD8 rapidly reexpressed PD-1 and exhibited severe functional impairment. Inhibition of PD-1 signaling using monoclonal antibody blockade prevented TCD8 impairment, reduced viral titers during primary infection, and enhanced protection of immunized mice against challenge infection. Additionally, PD-1 and PD-L1 were upregulated in the lungs of patients with 2009 H1N1 influenza virus, respiratory syncytial virus, or parainfluenza virus infection. These results indicate that PD-1 mediates TCD8 functional impairment during acute viral infection and may contribute to recurrent viral LRIs. Therefore, the PD-1/PD-L1 pathway may represent a therapeutic target in the treatment of respiratory viruses.

Blocking of PDL-1 interaction enhances primary and secondary CD8 T cell response to herpes simplex virus-1 infection

The blocking of programmed death ligand-1 (PDL-1) has been shown to enhance virus-specific CD8 T cell function during chronic viral infections. Though, how PDL-1 blocking at the time of priming affects the quality of CD8 T cell response to acute infections is not well understood and remains controversial. This report demonstrates that the magnitude of the primary and secondary CD8 T cell responses to herpes simplex virus-1 (HSV-1) infection is subject to control by PDL-1. Our results showed that after footpad HSV-1 infection, PD-1 expression increases on immunodominant SSIEFARL peptide specific CD8 T cells. Additionally, post-infection, the level of PDL-1 expression also increases on CD11c+ dendritic cells. Intraperitoneal administration of anti-PDL-1 monoclonal antibody given one day prior to and three days after cutaneous HSV-1 infection, resulted in a marked increase in effector and memory CD8 T cell response to SSIEFARL peptide. This was shown by measuring the quantity and quality of SSIEFARL-specific CD8 T cells by making use of ex-vivo assays that determine antigen specific CD8 T cell function, such as intracellular cytokine assay, degranulation assay to measure cytotoxicity and viral clearance. Our results are discussed in terms of the beneficial effects of blocking PDL-1 interactions, while giving prophylactic vaccines, to generate a more effective CD8 T cell response to viral infection.

Blockade of Tim-3 signaling restores the virus-specific CD8⁺ T-cell response in patients with chronic hepatitis B

Chronic hepatitis B (CHB) is characterized by functionally impaired virus-specific CD8(+) T-cell responses. However, the mechanism underlying this dysfunction has not been fully clarified. We examined the role of a newly identified protein, T-cell immunoglobulin domain and mucin domain-containing molecule-3 (Tim-3), in regulating the antiviral CD8(+) T-cell response in CHB patients. Tim-3 expression on peripheral virus-specific CD8(+) T cells from 20 CHB patients and 20 healthy controls was determined by flow cytometry. The phenotypes and cytokine-producing capacity were compared between Tim-3(+) CD8(+) and Tim-3(-) CD8(+) T cells. The impact of Tim-3 signaling on cellular proliferation and cytokine-producing capacity was also studied. Tim-3 expression on hepatitis B virus (HBV)-specific CD8(+) T cells was higher than expression on cytomegalovirus (CMV)-specific CD8(+) T cells. Tim-3(+) CD8(+) T cells exhibited proliferative senescence phenotypes and decreased cytokine production upon antigen challenge. Finally, blocking the Tim-3 pathway significantly improved proliferation and antiviral cytokine secretion of CD8(+) T cells in response to HBV-specific antigen peptides. Tim-3 negatively regulates antiviral responses of CD8(+) T cells isolated from CHB patients, and this response is reversed by blocking the Tim-3 pathway.

The three main stumbling blocks for anticancer T cells

Memory and effector T cells have the potential to counteract cancer progression, but often fail to control the disease, essentially because of three main stumbling blocks. First, clonal deletion leads to relatively low numbers or low-to-intermediate T cell receptor (TCR) affinity of self/tumor-specific T cells. Second, the poor innate immune stimulation by solid tumors is responsible for inefficient priming and boosting. Third, T cells are suppressed in the tumor microenvironment by inhibitory signals from other immune cells, stroma and tumor cells, which induces T cell exhaustion, as demonstrated in metastases of melanoma patients. State-of-the-art adoptive cell transfer and active immunotherapy can partially overcome the three stumbling blocks. The reversibility of T cell exhaustion and novel molecular insights provide the basis for further improvements of clinical immunotherapy.

The role of coinhibitory signaling pathways in transplantation and tolerance

Negative costimulatory molecules, acting through so-called inhibitory pathways, play a crucial role in the control of T cell responses. This negative “second signal” opposes T cell receptor activation and leads to downregulation of T cell proliferation and promotes antigen specific tolerance. Much interest has focused upon these pathways in recent years as a method to control detrimental alloresponses and promote allograft tolerance. However, recent experimental data highlights the complexity of negative costimulatory pathways in alloimmunity. Varying effects are observed from molecules expressed on donor and recipient tissues and also depending upon the activation status of immune cells involved. There appears to be significant overlap and redundancy within these systems, rendering this a challenging area to understand and exploit therapeutically. In this article, we will review the literature at the current time regarding the major negative costimulation pathways including CTLA-4:B7, PD-1:PD-L1/PD-L2 and PD-L1:B7-1, B7-H3, B7-H4, HVEM:BTLA/CD160, and TIM-3:Galectin-9. We aim to outline the role of these pathways in alloimmunity and discuss their potential applications for tolerance induction in transplantation.

The CTLA-4 and PD-1/PD-L1 inhibitory pathways independently regulate host resistance to Plasmodium-induced acute immune pathology

The balance between pro-inflammatory and regulatory immune responses in determining optimal T cell activation is vital for the successful resolution of microbial infections. This balance is maintained in part by the negative regulators of T cell activation, CTLA-4 and PD-1/PD-L, which dampen effector responses during chronic infections. However, their role in acute infections, such as malaria, remains less clear. In this study, we determined the contribution of CTLA-4 and PD-1/PD-L to the regulation of T cell responses during Plasmodium berghei ANKA (PbA)-induced experimental cerebral malaria (ECM) in susceptible (C57BL/6) and resistant (BALB/c) mice. We found that the expression of CTLA-4 and PD-1 on T cells correlates with the extent of pro-inflammatory responses induced during PbA infection, being higher in C57BL/6 than in BALB/c mice. Thus, ECM develops despite high levels of expression of these inhibitory receptors. However, antibody-mediated blockade of either the CTLA-4 or PD-1/PD-L1, but not the PD-1/PD-L2, pathways during PbA-infection in ECM-resistant BALB/c mice resulted in higher levels of T cell activation, enhanced IFN-γ production, increased intravascular arrest of both parasitised erythrocytes and CD8⁺ T cells to the brain, and augmented incidence of ECM. Thus, in ECM-resistant BALB/c mice, CTLA-4 and PD-1/PD-L1 represent essential, independent and non-redundant pathways for maintaining T cell homeostasis during a virulent malaria infection. Moreover, neutralisation of IFN-γ or depletion of CD8⁺ T cells during PbA infection was shown to reverse the pathologic effects of regulatory pathway blockade, highlighting that the aetiology of ECM in the BALB/c mice is similar to that in C57BL/6 mice. In summary, our results underscore the differential and complex regulation that governs immune responses to malaria parasites.

T cells are part of the body's defense system in response to infection. However, once the infection has been suitably controlled, these T cells must be switched off. Inhibitory pathways, such as CTLA-4 and PD-1, are known to send the ‘turn off’ signal to T cells during chronic infections. However, their roles in acute infections, such as malaria, are unclear. We compared the function of these inhibitory pathways in mice that are either susceptible or resistant to severe malarial disease (cerebral malaria). Strikingly, we found that receptors for CTLA-4 and PD-1 are more highly expressed in T cells from susceptible mice than from resistant mice. Therefore, cerebral malaria develops despite the high expression of these inhibitory receptors. Moreover, we demonstrated that blocking these inhibitory receptors in the resistant mice increased the function of T cells, which in turn led to the characteristic signs of cerebral malaria. Finally, reminiscent of what is known for the susceptible strain, we confirmed that certain T cells (CD8⁺) and molecules (IFN-γ) are crucial to the development of cerebral malaria in the otherwise resistant mice. Thus, the CTLA-4 and PD-1 inhibitory pathways have essential, independent and non-redundant roles in regulating the body's complex response to malaria.

Apoptosis - an Ubiquitous T cell Immunomodulator

Apoptosis is a natural process where cells that are no longer required can be eliminated in a highly regulated, controlled manner. Apoptosis is important in maintaining the mammalian immune system and plays a significant role in immune response, positive and negative T cell selection, and cytotoxic death of target cells. When the apoptotic pathways are impaired or are not tightly regulated, autoimmune diseases, inflammatory diseases, viral and bacterial infections and cancers ensue. An imbalance in the anti-apoptotic and pro-apoptotic factors has been implicated in these diseases. Moreover, current therapies directed towards these diseases focus on the modulation of the apoptotic death pathways to regulate the immune response. In this review, we will focus on the process of T cell activation and apoptosis in autoimmune reactions, in response to tumor progression as well as in response to bacterial and viral infections.

Cutting edge: CD40-CD40 ligand pathway plays a critical CD8-intrinsic and -extrinsic role during rescue of exhausted CD8 T cells

CD8 exhaustion mediated by an inhibitory programmed death-1-programmed death ligand-1 (PD-L1) pathway occurs in several chronic infections, including toxoplasmosis. Although blockade of the programmed death-1-PD-L1 pathway revives this response, the role of costimulatory receptors involved in this rescue has not been ascertained in any model of CD8 exhaustion. This report demonstrates that one such costimulatory pathway, CD40-CD40L, plays a critical role during rescue of exhausted CD8 T cells. Blockade of this pathway abrogates the ameliorative effects of anti-PD-L1 treatment on CD8 T cells. Additionally, we demonstrate in an infectious disease model that CD8-intrinsic CD40 signaling is important for optimal CD8 polyfunctionality, proliferation, T-bet upregulation, and IL-21 signaling, albeit in the context of CD8 rescue. The critical role of CD40 during the rescue of exhausted CD8 T cells may provide a rational basis for designing novel therapeutic vaccination approaches.

Mechanism of T cell exhaustion in a chronic environment

T cell exhaustion develops under conditions of antigen-persistence caused by infection with various chronic pathogens, such as human immunodeficiency virus (HIV) and mycobacterium tuberculosis (TB), or by the development of cancer. T cell exhaustion is characterized by stepwise and progressive loss of T cell function, which is probably the main reason for the failed immunological control of chronic pathogens and cancers. Recent observations have detailed some of the intrinsic and extrinsic factors that influence the severity of T cell exhaustion. Duration and magnitude of antigenic activation of T cells might be associated with up-regulation of inhibitory receptors, which is a major intrinsic factor of T cell exhaustion. Extrinsic factors might include the production of suppressive cytokines, T cell priming by either non-professional antigen-presenting cells (APCs) or tolerogenic dendritic cells (DCs), and alteration of regulatory T (Treg) cells. Further investigation of the cellular and molecular processes behind the development of T cell exhaustion can reveal therapeutic targets and strategies for the treatment of chronic infections and cancers. Here, we report the properties and the mechanisms of T cell exhaustion in a chronic environment.

Therapeutic cancer vaccines: are we there yet?

Enthusiasm for therapeutic cancer vaccines has been rejuvenated with the recent completion of several large, randomized phase III clinical trials that in some cases have reported an improvement in progression free or overall survival. However, an honest appraisal of their efficacy reveals modest clinical benefit and a frequent requirement for patients with relatively indolent cancers and minimal or no measurable disease. Experience with adoptive cell transfer-based immunotherapies unequivocally establishes that T cells can mediate durable complete responses, even in the setting of advanced metastatic disease. Further, these findings reveal that the successful vaccines of the future must confront: (i) a corrupted tumor microenvironment containing regulatory T cells and aberrantly matured myeloid cells, (ii) a tumor-specific T-cell repertoire that is prone to immunologic exhaustion and senescence, and (iii) highly mutable tumor targets capable of antigen loss and immune evasion. Future progress may come from innovations in the development of selective preparative regimens that eliminate or neutralize suppressive cellular populations, more effective immunologic adjuvants, and further refinement of agents capable of antagonizing immune check-point blockade pathways.

T cell exhaustion

T cell exhaustion is a state of T cell dysfunction that arises during many chronic infections and cancer. It is defined by poor effector function, sustained expression of inhibitory receptors and a transcriptional state distinct from that of functional effector or memory T cells. Exhaustion prevents optimal control of infection and tumors. Recently, a clearer picture of the functional and phenotypic profile of exhausted T cells has emerged and T cell exhaustion has been defined in many experimental and clinical settings. Although the pathways involved remain to be fully defined, advances in the molecular delineation of T cell exhaustion are clarifying the underlying causes of this state of differentiation and also suggest promising therapeutic opportunities.

The CD4-like molecule LAG-3, biology and therapeutic applications

IMPORTANCE OF THE FIELD

Promising immunotherapeutic agents targeting co-stimulatory pathways are currently being tested in clinical trials. One player in this array of regulatory pathways is the LAG-3/MHC class II axis. The lymphocyte activation gene-3 (LAG-3) is a negative co-stimulatory receptor that modulates T cell homeostasis, proliferation and activation. A recombinant soluble dimeric form of LAG-3 (sLAG-3-Ig, IMP321) shows adjuvant properties and enhances immunogenicity of tumor vaccines. Recent clinical trials produced encouraging results, especially when the human dimeric soluble form of LAG-3 (hLAG-3-Ig) was used in combination with chemotherapy.

AREAS COVERED IN THIS REVIEW

The biological relevance of LAG-3 in vivo. Pre-clinical data demonstrating adjuvant properties, as well as the improvement of tumor immunity by sLAG-3-Ig. Recent advances in the clinical development of the therapeutic reagent IMP321, hLAG-3-Ig, for cancer treatment.

WHAT THE READER WILL GAIN

This review summarizes preclinical and clinical data on the biological functions of LAG-3.

TAKE HOME MESSAGE

The LAG-3 inhibitory pathway is attracting attention, in the light of recent studies demonstrating its role in T cell unresponsiveness, and Treg function after chronic antigen stimulation. As a soluble recombinant dimer, the sLAG-3-Ig protein acts as an adjuvant for therapeutic induction of T cell responses, and may be beneficial to cancer patients when used in combination therapies.

Tim-3 expression on peripheral T cell subsets correlates with disease progression in hepatitis B infection

Background and objective

T-cell immunoglobulin domain and mucin domain-containing molecule-3 (Tim-3) represents a novel mechanism of T-cell dysfunction in chronic viral diseases. However, the role of Tim-3 in the pathogenesis of chronic hepatitis B (CHB) is not well understood. We investigated Tim-3 expression on peripheral T cell subsets and analyzed the relationship between Tim-3 expression and disease progression in HBV infection.

Methods

peripheral blood samples were obtained from CHB patients (n = 40), including 23 patients with moderate CHB [MCHB] and 17 with severe CHB [SCHB]. Control samples were obtained from nine acute hepatitis B patients (AHB) and 26 age-matched healthy subjects. The expression of Tim-3 on T cells was determined by flow cytometry.

Results

Tim-3 expression was elevated on peripheral CD4⁺ and CD8⁺ T cells from AHB and CHB patients compared to those from healthy controls. The percentage of Tim-3⁺ T cells was further increased in SCHB patients relative to MCHB patients and showed a positive correlation with conventional markers for liver injury (alanine aminotransferase (ALT), aspartate transaminase (AST), total bilirubin (TB) and international normalized ratio (INR) level). The frequency of Tim-3-expressing T cells was negatively correlated with T-bet mRNA expression and plasma interferon-gamma (INF-gamma) levels. Further, Tim-3 expression on CD4⁺ or CD8⁺ T cells was reduced in CHB patients with disease remission after antiviral treatment and in AHB patients during the convalescence phase.

Conclusions

Our results suggest that over-expression of Tim-3 is involved in disease progression of CHB and that Tim-3 may participate in skewing of Th1/Tc1 response, which contributes to persistency of HBV infection.

Tumor-induced disruption of proximal TCR-mediated signal transduction in tumor-infiltrating CD8+ lymphocytes inactivates antitumor effector phase

The presence in cancer tissue of Ag-specific, activated tumor infiltrating CD8(+) T cells proves that tumors express Ags capable of eliciting immune response. Therefore, in general, tumor escape from immune-mediated clearance is not attributable to immunological ignorance. However, tumor-infiltrating lymphocytes are defective in effector phase function, demonstrating tumor-induced immune suppression that likely underlies tumor escape. Since exocytosis of lytic granules is dependent upon TCR-mediated signal transduction, it is a reasonable contention that tumors may induce defective signal transduction in tumor infiltrating T cells. In this review, we consider the biochemical basis for antitumor T cell dysfunction, focusing on the role of inhibitory signaling receptors in restricting TCR-mediated signaling in tumor-infiltrating lymphocytes.

Experimental models to study the immunobiology of hepatitis C virus

Effective host immune responses are essential for the control of hepatitis C virus (HCV) infection and persistence of HCV has indeed been attributed to their failure. In recent years, several in vitro and in vivo experimental models have allowed studies of host immune responses against HCV. Numerous observations derived from these models have improved our understanding of the mechanisms responsible for the host's ability to clear the virus as well as of the mechanisms responsible for the host's failure to control HCV replication. Importantly, several findings obtained with these model systems have been confirmed in studies of acutely or chronically HCV-infected individuals. Collectively, several mechanisms are used by HCV to escape host immune responses, such as poor induction of the innate immune response and escaping/impairing adaptive immunity. In this review, we summarize current findings from experimental models available for studies of the immune response targeting HCV and discuss the relevance of these findings for the in vivo situation in HCV-infected humans.

IL-2 complex treatment can protect naive mice from bacterial and viral infection

IL-2 complexes have substantial effects on the cellular immune system, and this approach is being explored for therapeutic application in infection and cancer. However, the impact of such treatments on subsequent encounter with pathogens has not been investigated. In this study, we report that naive mice treated with a short course of IL-2 complexes show enhanced protection from newly encountered bacterial and viral infections. IL-2 complex treatment expands both the NK and CD8 memory cell pool, including a recently described population of preexisting memory-phenotype T cells responsive to previously unencountered foreign Ags. Surprisingly, prolonged IL-2 complex treatment decreased CD8 T cell function and protective immunity. These data reveal the impact of cytokine complex treatment on the primary response to infection.

Platelet-rich plasma may prevent titanium-mesh exposure in alveolar ridge augmentation with anorganic bovine bone

OBJECTIVE

Bone augmentation with the titanium-mesh (Ti-mesh) technique is susceptible to a large rate of complications such as morbidity of bone graft donor site, and mesh exposure to the oral cavity. The purpose of this study was to evaluate the effectiveness of anorganic bovine bone (ABB) in alveolar bone augmentation with the Ti-mesh technique. In addition, we investigated the effect of platelet-rich plasma (PRP) in preventing mesh exposure by using it to cover the Ti-mesh.

PATIENTS AND METHODS

Patients included in the clinical trial were randomly allocated by a blinded assistant into two groups. The 30 patients recruited for this study underwent 43 alveolar bone augmentation with the Ti-mesh technique using ABB as graft material in all of them. In 15 patients, the Ti-meshes were covered with PRP (PRP group) whereas in the other 15 the Ti-meshes were not (control group). After 6 months, patients were called for clinical, radiographic, and histological evaluation, and implant placement surgery. A total of 97 implants were placed in the augmented bone and their evolution was followed up for a period of 24 months.

RESULTS

Significant differences were found between the two study groups in terms of complications and bone formation. In the control group, 28.5% of the cases suffered from mesh exposure, while in the PRP group, no exposures were registered. Radiographic analysis revealed that bone augmentation was higher in the PRP group than in the control group. Overall, 97.3% of implants placed in the control group and 100% of those placed in the PRP group were successful during the monitoring period. We suggest that the positive effect of PRP on the Ti-mesh technique is due to its capacity to improve soft tissue healing, thereby protecting the mesh and graft material secured beneath the gingival tissues.

CONCLUSIONS

Alveolar bone augmentation using ABB alone in the Ti-mesh technique is sufficient for implant rehabilitation. Besides, covering the Ti-meshes with PRP was a determining factor in avoiding mesh exposure. Ti-mesh exposure provoked significant bone loss, but in most cases it did not affect the subsequent placement of implants.

Cooperation of Tim-3 and PD-1 in CD8 T-cell exhaustion during chronic viral infection

Inhibitory receptors play a crucial role in regulating CD8 T-cell function during chronic viral infection. T-cell Ig- and mucin-domain-containing molecule-3 (Tim-3) is well known to negatively regulate T-cell responses, but its role in CD8 T-cell exhaustion during chronic infection in vivo remains unclear. In this study, we document coregulation of CD8 T cell exhaustion by Tim-3 and PD-1 during chronic lymphocytic choriomeningitis virus infection. Whereas Tim-3 was only transiently expressed by CD8 T cells after acute infection, virus-specific CD8 T cells retained high Tim-3 expression throughout chronic infection. The majority (approximately 65% to 80%) of lymphocytic choriomeningitis virus-specific CD8 T cells in lymphoid and nonlymphoid organs coexpressed Tim-3 and PD-1. This coexpression of Tim-3 and PD-1 was associated with more severe CD8 T-cell exhaustion in terms of proliferation and secretion of effector cytokines such as IFN-gamma, TNF-alpha, and IL-2. Interestingly, CD8 T cells expressing both inhibitory receptors also produced the suppressive cytokine IL-10. Most importantly, combined blockade of Tim-3 and PD-1 pathways in vivo synergistically improved CD8 T cell responses and viral control in chronically infected mice. Taken together, our study defines a parameter for determining the severity of CD8 T cell dysfunction and for identifying virus-specific CD8 T cells that produce IL-10, and shows that targeting both PD-1 and Tim-3 is an effective immune strategy for treating chronic viral infections.

Regulation of virus-specific CD4+ T cell function by multiple costimulatory receptors during chronic HIV infection

Elevated expression of inhibitory receptors on virus-specific T cells has been implicated as a mechanism by which viruses evade host immune surveillance. Blockade of these pathways during chronic infection leads to increased T cell function and improved immune control of viral replication. To explore the association between costimulatory receptors and HIV replication, we examined the expression of programmed death 1 (PD-1), CTLA-4, T cell Ig domain and mucin domain 3 (TIM-3), and CD28 on HIV-specific CD4(+) T cells from HIV-infected subjects. Greater than 30% of HIV-specific CD4(+) T cells from untreated subjects coexpressed PD-1, CTLA-4, and TIM-3, whereas <2% of CMV- or varicella-zoster virus-specific CD4(+) T cells expressed all three receptors. Coexpression of all three inhibitory receptors on HIV-specific CD4(+) T cells was more strongly correlated with viral load compared with the expression of each receptor individually. Suppression of HIV replication with antiretroviral therapy was associated with decreased expression of all three inhibitory receptors on HIV-specific CD4(+) T cells. Surprisingly, a high percentage of HIV-specific CD4(+) T cells that expressed inhibitory receptors also coexpressed CD28. In vitro blockade of PD-1 binding concurrent with stimulation through CD28 synergistically increased HIV-specific CD4(+) T cell proliferation to a greater extent than did either alone. These findings indicate that HIV-specific CD4(+) T cell responses during chronic infection are regulated by complex patterns of coexpressed inhibitory receptors and that the synergistic effect of inhibitory receptor blockade and stimulation of costimulatory receptors could be used for therapeutic augmentation of HIV-specific CD4(+) T cell function.

The PD-1 pathway in tolerance and autoimmunity

Regulatory T cells (Tregs) and the PD-1: PD-ligand (PD-L) pathway are both critical to terminating immune responses. Elimination of either can result in the breakdown of tolerance and the development of autoimmunity. The PD-1: PD-L pathway can thwart self-reactive T cells and protect against autoimmunity in many ways. In this review, we highlight how PD-1 and its ligands defend against potentially pathogenic self-reactive effector T cells by simultaneously harnessing two mechanisms of peripheral tolerance: (i) the promotion of Treg development and function and (ii) the direct inhibition of potentially pathogenic self-reactive T cells that have escaped into the periphery. Treg cells induced by the PD-1 pathway may also assist in maintaining immune homeostasis, keeping the threshold for T-cell activation high enough to safeguard against autoimmunity. PD-L1 expression on non-hematopoietic cells as well as hematopoietic cells endows PD-L1 with the capacity to promote Treg development and enhance Treg function in lymphoid organs and tissues that are targets of autoimmune attack. At sites where transforming growth factor-beta is present (e.g. sites of immune privilege or inflammation), PD-L1 may promote the de novo generation of Tregs. When considering the consequences of uncontrolled immunity, it would be therapeutically advantageous to manipulate Treg development and sustain Treg function. Thus, this review also discusses how the PD-1 pathway regulates a number of autoimmune diseases and the therapeutic potential of PD-1: PD-L modulation.

Alloantigen expression on non-hematopoietic cells reduces graft-versus-leukemia effects in mice

Allogeneic hematopoietic stem cell transplantation (HSCT) is used effectively to treat a number of hematological malignancies. Its beneficial effects rely on donor-derived T cell-targeted leukemic cells, the so-called graft-versus-leukemia (GVL) effect. Induction of GVL is usually associated with concomitant development of graft-versus-host disease (GVHD), a major complication of allogeneic HSCT. The T cells that mediate GVL and GVHD are activated by alloantigen presented on host antigen-presenting cells of hematopoietic origin, and it is not well understood how alloantigen expression on non-hematopoietic cells affects GVL activity. Here we show, in mouse models of MHC-matched, minor histocompatibility antigen-mismatched bone marrow transplantation, that alloantigen expression on host epithelium drives donor T cells into apoptosis and dysfunction during GVHD, resulting in a loss of GVL activity. During GVHD, programmed death-1 (PD-1) and PD ligand-1 (PD-L1), molecules implicated in inducing T cell exhaustion, were upregulated on activated T cells and the target tissue, respectively, suggesting that the T cell defects driven by host epithelial alloantigen expression might be mediated by the PD-1/PD-L1 pathway. Consistent with this, blockade of PD-1/PD-L1 interactions partially restored T cell effector functions and improved GVL. These results elucidate a previously unrecognized significance of alloantigen expression on non-hematopoietic cells in GVL and suggest that separation of GVL from GVHD for more effective HSCT may be possible in human patients.