Virus-specific CD8+ T cells upregulate programmed death-1 expression during acute friend retrovirus infection but are highly cytotoxic and control virus replication

PD-1 knockout on cytotoxic primary murine CD8+ T cells improves their motility in retrovirus infected mice

Cytotoxic T lymphocyte (CTL) motility is an important feature of effective CTL responses and is impaired when CTLs become exhausted, e.g. during chronic retroviral infections. A prominent T cell exhaustion marker is programmed cell death protein 1 (PD-1) and antibodies against the interaction of PD-1 and PD-ligand 1 (PD-L1) are known to improve CTL functions. However, antibody blockade affects all PD-1/PD-L1-expressing cell types, thus, the observed effects cannot be attributed selectively to CTLs. To overcome this problem, we performed CRISPR/Cas9 based knockout of the PD-1 coding gene PDCD1 in naïve Friend Retrovirus (FV)-specific CTLs. We transferred 1,000 of these cells into mice where they proliferated upon FV-infection. Using intravital two-photon microscopy we visualized CTL motility in the bone marrow and evaluated cytotoxic molecule expression by flow cytometry. Knockout of PDCD1 improved the CTL motility at 14 days post infection and enhanced the expression of cytotoxicity markers. Our data show the potential of genetic tuning of naive antiviral CTLs and might be relevant for future designs of improved T cell-mediated therapies.

Cytokine Storm Related to CD4+ T Cells in Influenza Virus-Associated Acute Necrotizing Encephalopathy

Acute necrotizing encephalopathy (ANE) is a rare but deadly complication with an unclear pathogenesis. We aimed to elucidate the immune characteristics of H1N1 influenza virus-associated ANE (IANE) and provide a potential therapeutic approach for IANE. Seven pediatric cases from a concentrated outbreak of H1N1 influenza were included in this study. The patients' CD4+ T cells from peripheral blood decreased sharply in number but highly expressed Eomesodermin (Eomes), CD69 and PD-1, companied with extremely high levels of IL-6, IL-8 in the cerebrospinal fluid and plasma. Patient 2, who showed high fever and seizures and was admitted to the hospital very early in the disease course, received intravenous tocilizumab and subsequently showed a reduction in temperature and a stable conscious state 24 h later. In conclusion, a proinflammatory cytokine storm associated with activated CD4+ T cells may cause severe brain pathology in IANE. Tocilizumab may be helpful in treating IANE.

Characterization of CD4+ and CD8+ T cells responses in the mixed lymphocyte reaction by flow cytometry and single cell RNA sequencing

Background

The Mixed Lymphocyte Reaction (MLR) consists in the allogeneic co-culture of monocytes derived dendritic cells (MoDCs) with T cells from another donor. This in vitro assay is largely used for the assessment of immunotherapy compounds. Nevertheless, the phenotypic changes associated with lymphocyte responsiveness under MLR have never been thoroughly evaluated.

Methods

Here, we used multiplex cytokine and chemokine assays, multiparametric flow cytometry and single cell RNA sequencing to deeply characterize T cells activation and function in the context of CD4+- and CD8+-specific MLR kinetics.

Results

We showed that CD4+ and CD8+ T cells in MLR share common classical markers of response such as polyfunctionality, increased proliferation and CD25 expression but differ in their kinetics and amplitude of activation as well as their patterns of cytokines secretion and immune checkpoints expression. The analysis of immunoreactive Ki-67+CD25+ T cells identified PBK, LRR1 and MYO1G as new potential markers of MLR response. Using cell-cell communication network inference and pathway analysis on single cell RNA sequencing data, we also highlighted key components of the immunological synapse occurring between T cells and the stimulatory MoDCs together with downstream signaling pathways involved in CD4+ and CD8+ T cells activation.

Conclusion

These results provide a deep understanding of the kinetics of the MLR assay for CD4+ or CD8+ T cells and may allow to better characterize compounds impacting MLR and eventually identify new strategies for immunotherapy in cancer.

Hierarchical transcriptional network governing heterogeneous T cell exhaustion and its implications for immune checkpoint blockade

The fundamental principle of immune checkpoint blockade (ICB) is to protect tumor-infiltrating T cells from being exhausted. Despite the remarkable success achieved by ICB treatment, only a small group of patients benefit from it. Characterized by a hypofunctional state with the expression of multiple inhibitory receptors, exhausted T (Tex) cells are a major obstacle in improving ICB. T cell exhaustion is a progressive process which adapts to persistent antigen stimulation in chronic infections and cancers. In this review, we elucidate the heterogeneity of Tex cells and offer new insights into the hierarchical transcriptional regulation of T cell exhaustion. Factors and signaling pathways that induce and promote exhaustion are also summarized. Moreover, we review the epigenetic and metabolic alterations of Tex cells and discuss how PD-1 signaling affects the balance between T cell activation and exhaustion, aiming to provide more therapeutic targets for applications of combinational immunotherapies.

Activated CD8+CD38+ Cells Are Associated With Worse Clinical Outcome in Hospitalized COVID-19 Patients

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), that spread around the world during the past 2 years, has infected more than 260 million people worldwide and has imposed an important burden on the healthcare system. Several risk factors associated with unfavorable outcome were identified, including elderly age, selected comorbidities, immune suppression as well as laboratory markers. The role of immune system in the pathophysiology of SARS-CoV-2 infection is indisputable: while an appropriate function of the immune system is important for a rapid clearance of the virus, progression to the severe and critical phases of the disease is related to an exaggerated immune response associated with a cytokine storm. We analyzed differences and longitudinal changes in selected immune parameters in 823 adult COVID-19 patients hospitalized in the Martin University Hospital, Martin, Slovakia. Examined parameters included the differential blood cell counts, various parameters of cellular and humoral immunity (serum concentration of immunoglobulins, C4 and C3), lymphocyte subsets (CD3⁺, CD4⁺, CD8⁺, CD19⁺, NK cells, CD4⁺CD45RO⁺), expression of activation (HLA-DR, CD38) and inhibition markers (CD159/NKG2A). Besides already known changes in the differential blood cell counts and basic lymphocyte subsets, we found significantly higher proportion of CD8⁺CD38⁺ cells and significantly lower proportion of CD8⁺NKG2A⁺ and NK NKG2A⁺ cells on admission in non-survivors, compared to survivors; recovery in survivors was associated with a significant increase in the expression of HLA-DR and with a significant decrease of the proportion of CD8⁺CD38⁺cells. Furthermore, patients with fatal outcome had significantly lower concentrations of C3 and IgM on admission. However, none of the examined parameters had sufficient sensitivity or specificity to be considered a biomarker of fatal outcome. Understanding the dynamic changes in immune profile of COVID-19 patients may help us to better understand the pathophysiology of the disease, potentially improve management of hospitalized patients and enable proper timing and selection of immunomodulator drugs.

Reversing T-cell exhaustion in immunotherapy: a review on current approaches and limitations

Introduction:T cell functions are altered during chronic viral infections and tumor development. This is mainly manifested by significant changes in T cells' epigenetic and metabolic landscapes, pushing them into an 'exhausted' state. Reversing this T cell exhaustion has been emerging as a 'game-changing' therapeutic approach against cancer and chronic viral infection.Areas covered:This review discusses the cellular pathways related to T cell exhaustion, and the clinical development and possible cellular targets that can be exploited therapeutically to reverse this exhaustion. We searched various databases (e.g. Google Scholar, PubMed, Elsevier, and other scientific database sites) using the keywords T cell exhaustion, T cell activation, co-inhibitory receptors, and reversing T cell exhaustion.Expert opinion:The discovery of the immune checkpoints pathways represents a significant milestone toward understanding and reversing T cell exhaustion. Antibodies that target these pathways have already demonstrated promising activities in reversing T cell exhaustion. Nevertheless, there are still many associated limitations. In this context, next-generation alternatives are on the horizon. This includes the use of small molecules to block the immune checkpoints' receptors, combining them with other treatments, and identifying novel, safer and more effective immunotherapeutic targets.

Nanocarrier vaccines for SARS-CoV-2

The SARS-CoV-2 global pandemic has seen rapid spread, disease morbidities and death associated with substantive social, economic and societal impacts. Treatments rely on re-purposed antivirals and immune modulatory agents focusing on attenuating the acute respiratory distress syndrome. No curative therapies exist. Vaccines remain the best hope for disease control and the principal global effort to end the pandemic. Herein, we summarize those developments with a focus on the role played by nanocarrier delivery.

SARS-CoV-2 infection: The role of PD-1/PD-L1 and CTLA-4 axis

The outbreak of SARS-CoV-2 in Wuhan of China in December 2019 and its worldwide spread has turned into the COVID-19 pandemic. Respiratory disorders, lymphopenia, cytokine cascades, and the immune responses provoked by this virus play a major and fundamental role in the severity of the symptoms and the immunogenicity which it causes. Owing to the decrease in the inflammatory responses' regulation in the immune system and the sudden increase in the secretion of cytokines, it seems that an investigation of inhibitory immune checkpoints can influence theories regarding this disease's treatment methods. Acquired cell-mediated immune defense's T-cells have a key major contribution in clearing viral infections thus reducing the severity of COVID-19's symptoms. The most important diagnostic feature in individuals with COVID-19 is lymphocyte depletion, most importantly, T-cells. Due to the induction of interferon-γ (INF-γ) production by neutrophils and monocytes, which are abundantly present in the peripheral blood of the individuals with COVID-19, the expression of inhibitory immune checkpoints including, PD-1 (programmed death), PD-L1 and CTLA4 on the T-cells' surface is enhanced. The purpose of this review is to discuss the functions of these checkpoints and their effects on the dysfunction and exhaustion of T-cells, making them almost ineffective in individuals with COVID-19, especially in the cases with extreme symptoms.

B-Cell Control of Regulatory T Cells in Friend Virus Infection

B lymphocytes have well-established effector roles during viral infections, including production of antibodies and functioning as antigen-presenting cells for CD4+ and CD8+ T cells. B cells have also been shown to regulate immune responses and induce regulatory T cells (Tregs). In the Friend virus (FV) model, Tregs are known to inhibit effector CD8+ T-cell responses and contribute to virus persistence. Recent work has uncovered a role for B cells in the induction and activation of Tregs during FV infection. In addition to inducing Tregs, B cell antibody production and antigen-presenting cell activity is a target of Treg suppression. This review focuses on the dynamic interactions between B cells and Tregs during FV infection.

Prophylactic and therapeutic HBV vaccination by an HBs-expressing cytomegalovirus vector lacking an interferon antagonist in mice

Cytomegalovirus (CMV)-based vaccines show promising effects against chronic infections in nonhuman primates. Therefore, we examined the potential of hepatitis B virus (HBV) vaccines based on mouse CMV (MCMV) vectors expressing the small HBsAg. Immunological consequences of vaccine virus attenuation were addressed by either replacing the dispensable gene m157 ("MCMV-HBsȍ) or the gene M27 ("ΔM27-HBs"), the latter encodes a potent IFN antagonist targeting the transcription factor STAT2. M27 was chosen, since human CMV encodes an analogous gene product, which also induced proteasomal STAT2 degradation by exploiting Cullin RING ubiquitin ligases. Vaccinated mice were challenged with HBV through hydrodynamic injection. MCMV-HBs and ΔM27-HBs vaccination achieved accelerated HBV clearance in serum and liver as well as robust HBV-specific CD8⁺ T-cell responses. When we explored the therapeutic potential of MCMV-based vaccines, especially the combination of ΔM27-HBs prime and DNA boost vaccination resulted in increased intrahepatic HBs-specific CD8⁺ T-cell responses and HBV clearance in persistently infected mice. Our results demonstrated that vaccines based on a replication competent MCMV attenuated through the deletion of an IFN antagonist targeting STAT2 elicit robust anti-HBV immune responses and mediate HBV clearance in mice in prophylactic and therapeutic immunization regimes.

Tumor invasion in the central airway is a risk factor for early-onset checkpoint inhibitor pneumonitis in patients with non-small cell lung cancer

BACKGROUND

Anti-programmed death-1 (PD-1) immunotherapy can cause immune-related pneumonitis, also known as checkpoint inhibitor pneumonitis (CIP). CIP that develops early after the initiation of anti-PD-1 immunotherapy is important because it is more severe than CIP that develops later. However, only a few studies have examined the risk factors for early-onset CIP. Previous studies have reported several risk factors for CIP, including imaging findings of airway obstruction adjacent to lung tumors. However, the utility of this factor is debatable. Therefore, we investigated potential risk factors for early-onset CIP, including tumor invasion in the central airway (TICA), in patients with non-small cell lung cancer (NSCLC) receiving anti-PD-1 therapy.

METHODS

We retrospectively analyzed the medical records and chest computed tomography scans of patients with NSCLC treated with anti-PD-1 antibodies at the Kanagawa Cancer Center in Japan between 1 January 2016, and 30 June 2018. The clinical characteristics and imaging findings, including TICA, were compared between patients with and without early-onset CIP.

RESULTS

Data from 181 eligible patients (114 receiving nivolumab and 67 receiving pembrolizumab) were analyzed. Early-onset CIP occurred in 13 of 79 patients (16.5%) with TICA and 2 of 102 patients (2.0%) without TICA. In multivariate analysis, the odds ratio of early-onset CIP for patients with TICA was 8.2 (95% confidence interval [CI]: 1.98-34.0, P = 0.0037).

CONCLUSIONS

TICA was strongly associated with early-onset CIP in patients with NSCLC. Clinicians should carefully observe patients with TICA, especially within three months of anti-PD-1 antibody administration because of high CIP risk.

KEY POINTS

Significant study findings Tumor invasion in the central airway (TICA) was a predictor of early-onset checkpoint inhibitor pneumonitis (CIP) TICA had good interobserver variability, indicating its utility in clinical practice Patients with TICA might have a higher immune status than patients without What this study adds This is the first study focusing on risk factors for CIP limited to early-onset CIP.

Impaired Cytotoxic CD8+ T Cell Response in Elderly COVID-19 Patients

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection induces a T cell response that most likely contributes to virus control in COVID-19 patients but may also induce immunopathology. Until now, the cytotoxic T cell response has not been very well characterized in COVID-19 patients. Here, we analyzed the differentiation and cytotoxic profile of T cells in 30 cases of mild COVID-19 during acute infection. SARS-CoV-2 infection induced a cytotoxic response of CD8+ T cells, but not CD4+ T cells, characterized by the simultaneous production of granzyme A and B as well as perforin within different effector CD8+ T cell subsets. PD-1-expressing CD8+ T cells also produced cytotoxic molecules during acute infection, indicating that they were not functionally exhausted. However, in COVID-19 patients over the age of 80 years, the cytotoxic T cell potential was diminished, especially in effector memory and terminally differentiated effector CD8+ cells, showing that elderly patients have impaired cellular immunity against SARS-CoV-2. Our data provide valuable information about T cell responses in COVID-19 patients that may also have important implications for vaccine development.IMPORTANCE Cytotoxic T cells are responsible for the elimination of infected cells and are key players in the control of viruses. CD8+ T cells with an effector phenotype express cytotoxic molecules and are able to perform target cell killing. COVID-19 patients with a mild disease course were analyzed for the differentiation status and cytotoxic profile of CD8+ T cells. SARS-CoV-2 infection induced a vigorous cytotoxic CD8+ T cell response. However, this cytotoxic profile of T cells was not detected in COVID-19 patients over the age of 80 years. Thus, the absence of a cytotoxic response in elderly patients might be a possible reason for the more frequent severity of COVID-19 in this age group than in younger patients.

Combination immunotherapy with anti-PD-L1 antibody and depletion of regulatory T cells during acute viral infections results in improved virus control but lethal immunopathology

Combination immunotherapy (CIT) is currently applied as a treatment for different cancers and is proposed as a cure strategy for chronic viral infections. Whether such therapies are efficient during an acute infection remains elusive. To address this, inhibitory receptors were blocked and regulatory T cells depleted in acutely Friend retrovirus-infected mice. CIT resulted in a dramatic expansion of cytotoxic CD4+ and CD8+ T cells and a subsequent reduction in viral loads. Despite limited viral replication, mice developed fatal immunopathology after CIT. The pathology was most severe in the gastrointestinal tract and was mediated by granzyme B producing CD4+ and CD8+ T cells. A similar post-CIT pathology during acute Influenza virus infection of mice was observed, which could be prevented by vaccination. Melanoma patients who developed immune-related adverse events under immune checkpoint CIT also presented with expanded granzyme-expressing CD4+ and CD8+ T cell populations. Our data suggest that acute infections may induce immunopathology in patients treated with CIT, and that effective measures for infection prevention should be applied.

Overexpression of CD6 and PD-1 Identifies Dysfunctional CD8+ T-Cells During Chronic SIV Infection of Rhesus Macaques

Effective CD8⁺ T-cell responses play an important role in determining the course of SIV/HIV viral infection. Here we identified a unique population of dysfunctional CD8⁺ T-cells in lymphoid tissues and bronchoalveolar lavage (BAL) of rhesus macaques with chronic SIV infection characterized by co-expression of CD6 and PD-1. The frequency of CD6 and PD-1 co-expressing CD8⁺ T-cells was significantly increased in lymphoid tissues and BAL during chronic SIV infection compared to pre-infection levels. These CD6⁺PD-1⁺CD8⁺ T-cells displayed impaired proliferation, cytokine secretion and cytotoxicity compared to their CD6^-PD-1⁺CD8⁺ T cell counterparts. The frequency of CD8⁺PD-1⁺ and CD8⁺CD6^-PD-1⁺ T-cells in the lymph node and bone marrow did not correlate with SIV viral load, whereas the frequency of CD8⁺CD6⁺PD-1⁺ T-cells positively correlated with SIV viral load in these tissues highlighting the contribution of CD6 to disease progression. CD6⁺PD-1⁺CD8⁺ T-cells expressed elevated levels of SHP2 phosphatase compared to CD6^-PD-1⁺CD8⁺ T-cells providing a potential mechanism by which CD6 may induce T-cell dysfunction during chronic SIV infection. Combined targeting of CD6 and PD-1 effectively revived the CD8⁺ T-cell proliferative response in vitro suggesting a strategy for potential therapeutic benefit.

Friend retrovirus studies reveal complex interactions between intrinsic, innate and adaptive immunity

Approximately 4.4% of the human genome is comprised of endogenous retroviral sequences, a record of an evolutionary battle between man and retroviruses. Much of what we know about viral immunity comes from studies using mouse models. Experiments using the Friend virus (FV) model have been particularly informative in defining highly complex anti-retroviral mechanisms of the intrinsic, innate and adaptive arms of immunity. FV studies have unraveled fundamental principles about how the immune system controls both acute and chronic viral infections. They led to a more complete understanding of retroviral immunity that begins with cellular sensing, production of type I interferons, and the induction of intrinsic restriction factors. Novel mechanisms have been revealed, which demonstrate that these earliest responses affect not only virus replication, but also subsequent innate and adaptive immunity. This review on FV immunity not only surveys the complex host responses to a retroviral infection from acute infection to chronicity, but also highlights the many feedback mechanisms that regulate and counter-regulate the various arms of the immune system. In addition, the discovery of molecular mechanisms of immunity in this model have led to therapeutic interventions with implications for HIV cure and vaccine development.

PD1+CCR2+CD8+ T Cells Infiltrate the Central Nervous System during Acute Japanese Encephalitis Virus Infection

Japanese encephalitis (JE) is a viral encephalitis disease caused by Japanese encephalitis virus (JEV) infection. Uncontrolled inflammatory responses in the central nervous system (CNS) are a hallmark of severe JE. Although the CCR2-CCL2 axis is important for monocytes trafficking during JEV infection, little is known about its role in CNS trafficking of CD8⁺ T cells. Here, we characterized a mouse model of JEV infection, induced via intravenous injection (i.v.) and delineated the chemokines and infiltrating peripheral immune cells in the brains of infected mice. The CNS expression of chemokines, Ccl2, Ccl3, and Ccl5, and their receptors, Ccr2 or Ccr5, was significantly up-regulated after JEV infection and was associated with the degree of JE pathogenesis. Moreover, JEV infection resulted in the migration of a large number of CD8⁺ T cells into the CNS. In the brains of JEV-infected mice, infiltrating CD8⁺ T cells expressed CCR2 and CCR5 and were found to comprise mainly effector T cells (CD44⁺CD62L^-). JEV infection dramatically enhanced the expression of programmed death 1 (PD-1) on infiltrating CD8⁺ T cells in the brain, as compared to that on peripheral CD8⁺ T cells in the spleen. This effect was more pronounced on infiltrating CCR2⁺CD8⁺ T cells than on CCR2^-CD8⁺ T cells. In conclusion, we identified a new subset of CD8⁺ T cells (PD1⁺CCR2⁺CD8⁺ T cells) present in the CNS of mice during acute JEV infection. These CD8⁺ T cells might play a role in JE pathogenesis.

A functional subset of CD8+ T cells during chronic exhaustion is defined by SIRPα expression

Prolonged exposure of CD8+ T cells to antigenic stimulation, as in chronic viral infections, leads to a state of diminished function termed exhaustion. We now demonstrate that even during exhaustion there is a subset of functional CD8+ T cells defined by surface expression of SIRPα, a protein not previously reported on lymphocytes. On SIRPα+ CD8+ T cells, expression of co-inhibitory receptors is counterbalanced by expression of co-stimulatory receptors and it is only SIRPα+ cells that actively proliferate, transcribe IFNγ and show cytolytic activity. Furthermore, target cells that express the ligand for SIRPα, CD47, are more susceptible to CD8+ T cell-killing in vivo. SIRPα+ CD8+ T cells are evident in mice infected with Friend retrovirus, LCMV Clone 13, and in patients with chronic HCV infections. Furthermore, therapeutic blockade of PD-L1 to reinvigorate CD8+ T cells during chronic infection expands the cytotoxic subset of SIRPα+ CD8+ T cells.

The PD-1/PD-L1 Pathway Affects the Expansion and Function of Cytotoxic CD8+ T Cells During an Acute Retroviral Infection

Cytotoxic CD8⁺ T lymphocytes (CTL) efficiently control acute virus infections but can become exhausted when a chronic infection develops. The checkpoint receptor PD-1 suppresses the functionality of virus-specific CD8⁺ T cells during chronic infection. However, the role of the PD-L1/PD-1 pathway during the acute phase of infections has not been well characterized. In the current study the effects of PD-1 or PD-L1 deficiency on the CD8⁺ T cell response against Friend retroviral (FV) infection of knockout mice was analyzed during acute infection. We observed an enhanced proliferation, functional maturation, and reduced apoptosis of effector CD8⁺ T cells in the absence of PD-1 or PD-L1. The knockout of PD-L1 had a stronger effect on the functionality of CD8⁺ T cells than that of PD-1. Augmented CTL responses were associated with an improved control of FV replication. The strong phenotype of FV-infected PD-L1 knockout mice was independent of the interaction with CD80 as an additional receptor for PD-L1. Furthermore, we performed a detailed analysis of the production of different granzymes in virus-specific CD8⁺ T cells and observed that especially the simultaneous production of multiple granzymes in individual T cells (multifunctionality) was under the control of the PD-1/PD-L1 pathway. The findings from this study allow for a better understanding of the development of antiviral cytotoxic immunity during acute viral infections.

Effects of Friend Virus Infection and Regulatory T Cells on the Antigen Presentation Function of B Cells

The primary role of B cells in immunity is considered the production of pathogen-specific antibodies, but another, less-well-studied, function of B cells is to present foreign antigens to T cells to stimulate their activation and proliferation. Dendritic cells (DCs) are considered the most important antigen-presenting cells (APCs) for CD8⁺ T cells, but DCs lose APC function when infected with Friend virus (FV), a model retrovirus of mice. Interestingly, B cells were better able to stimulate CD8⁺ T cell responses when they were infected with FV. We also found that the activation status of B cells under homeostatic conditions was potently modulated by regulatory T cells. This study illustrates an important link between B cell and T cell responses and illustrates an additional mechanism by which regulatory T cells suppress critical T cell responses during viral infections.

Friend virus (FV) is a naturally occurring mouse retrovirus that infects dividing cells of the hematopoietic lineage, including antigen-presenting cells (APCs). The infection of APCs by viruses often induces their dysfunction, and it has been shown that FV infection reduces the ability of dendritic cells (DCs) to prime critical CD8⁺ T cell responses. Nonetheless, mice mount vigorous CD8⁺ T cell responses, so we investigated whether B cells might serve as alternative APCs during FV infection. Direct ex vivo analysis of B cells from FV-infected mice revealed that infected but not uninfected B cells upregulated expression of the costimulatory molecules CD80, CD86, and CD40, as well as major histocompatibility complex class II (MHC-II) molecules. Furthermore, in vitro studies showed that, compared to uninfected B cells from the same mice, the FV-infected B cells had significantly enhanced APC function, as measured by their capacity to prime CD8⁺ T cell activation and proliferation. Thus, in contrast to DCs, infection of B cells with FV enhanced their APC capacity and ability to stimulate the CD8⁺ T cell responses essential for virus control. FV infections also induce the activation and expansion of regulatory T cells (Tregs), so it was of interest to determine the impact of Tregs on B cell activation. The upregulation of costimulatory molecule expression and APC function of B cells was even more strongly enhanced by in vivo depletion of regulatory T cells than infection. Thus, Tregs exert potent homeostatic suppression of B cell activation that is partially overcome by FV infection.

Hantavirus-Driven PD-L1/PD-L2 Upregulation: An Imperfect Viral Immune Evasion Mechanism

Viruses often subvert antiviral immune responses by taking advantage of inhibitory immune signaling. We investigated if hantaviruses use this strategy. Hantaviruses cause viral hemorrhagic fever (VHF) which is associated with strong immune activation resulting in vigorous CD8+ T cell responses. Surprisingly, we observed that hantaviruses strongly upregulate PD-L1 and PD-L2, the ligands of checkpoint inhibitor programmed death-1 (PD-1). We detected high amounts of soluble PD-L1 (sPD-L1) and soluble PD-L2 (sPD-L2) in sera from hantavirus-infected patients. In addition, we observed hantavirus-induced PD-L1 upregulation in mice with a humanized immune system. The two major target cells of hantaviruses, endothelial cells and monocyte-derived dendritic cells, strongly increased PD-L1 and PD-L2 surface expression upon hantavirus infection in vitro. As an underlying mechanism, we found increased transcript levels whereas membrane trafficking of PD-L1 was not affected. Further analysis revealed that hantavirus-associated inflammatory signals and hantaviral nucleocapsid (N) protein enhance PD-L1 and PD-L2 expression. Cell numbers were strongly reduced when hantavirus-infected endothelial cells were mixed with T cells in the presence of an exogenous proliferation signal compared to uninfected cells. This is compatible with the concept that virus-induced PD-L1 and PD-L2 upregulation contributes to viral immune escape. Intriguingly, however, we observed hantavirus-induced CD8+ T cell bystander activation despite strongly upregulated PD-L1 and PD-L2. This result indicates that hantavirus-induced CD8+ T cell bystander activation bypasses checkpoint inhibition allowing an early antiviral immune response upon virus infection.

The Outer Membrane Vesicles of Aeromonas hydrophila ATCC® 7966TM: A Proteomic Analysis and Effect on Host Cells

Gram-negative bacteria release outer membrane vesicles (OMVs) into the extracellular environment. OMVs have been studied extensively in bacterial pathogens, however, information related with the composition of Aeromonas hydrophila OMVs is missing. In this study we analyzed the composition of purified OMVs from A. hydrophila ATCC® 7966TM by proteomics. Also we studied the effect of OMVs on human peripheral blood mononuclear cells (PBMCs). Vesicles were grown in agar plates and then purified through ultracentrifugation steps. Purified vesicles showed an average diameter of 90-170 nm. Moreover, 211 unique proteins were found in OMVs from A. hydrophila; some of them are well-known as virulence factors such as: haemolysin Ahh1, RtxA toxin, extracellular lipase, HcpA protein, among others. OMVs from A. hydrophila ATCC® 7966TM induced lymphocyte activation and apoptosis in monocytes, as well as over-expression of pro-inflammatory cytokines. This work contributed to the knowledge of the composition of the vesicles of A. hydrophila ATCC® 7966TM and their interaction with the host cell.

Tissue-resident memory T cells populate the human brain

Most tissues are populated by tissue-resident memory T cells (T_RM cells), which are adapted to their niche and appear to be indispensable for local protection against pathogens. Here we show that human white matter-derived brain CD8⁺ T cells can be subsetted into CD103^-CD69⁺ and CD103⁺CD69⁺ T cells both with a phenotypic and transcription factor profile consistent with T_RM cells. Specifically, CD103 expression in brain CD8⁺ T cells correlates with reduced expression of differentiation markers, increased expression of tissue-homing chemokine receptors, intermediate and low expression of the transcription factors T-bet and eomes, increased expression of PD-1 and CTLA-4, and low expression of cytolytic enzymes with preserved polyfunctionality upon activation. Brain CD4⁺ T cells also display T_RM cell-associated markers but have low CD103 expression. We conclude that the human brain is surveilled by T_RM cells, providing protection against neurotropic virus reactivation, whilst being under tight control of key immune checkpoint molecules.

Association Between Imaging Findings of Airway Obstruction Adjacent to Lung Tumors and the Onset of Interstitial Lung Disease After Nivolumab

BACKGROUND

Compared to conventional cytotoxic chemotherapy, immune checkpoint inhibitors have shown a significant efficacy in the treatment of lung cancer. Although interstitial lung disease (ILD) is an important adverse event in immunotherapy, risk factors for ILD remain unclear.

PATIENTS AND METHODS

In this multicenter cohort study (UMIN000025908), 201 patients who were treated with nivolumab were retrospectively reviewed. Associations between the incidence of ILD and patient characteristics were evaluated. ILD grade and progression-free survival were analyzed according to the presence or absence of imaging findings of airway obstruction adjacent to lung tumors (IAOT).

RESULTS

In the multivariate analysis, the odds ratio (OR) of ILD for patients with a history of radiation pneumonitis or IAOT was 3.96 (p=0.012) and 6.59 (p=0.004), respectively. ILD occurred in six (37.5%) out of 16 patients with IAOT and 19 (10.3%) out of 185 patients without IAOT. Three out of the six patients with ILD and IAOT had ILD of grade 4 or more. The median progression-free survival of patients with and without IAOT was 0.9 and 3.2 months, respectively (p<0.001).

CONCLUSION

IAOT was strongly associated with the occurrence of ILD after therapy with nivolumab.

Programmed cell death-1 3'-untranslated region polymorphism is associated with spontaneous clearance of hepatitis B virus infection

Hepatitis B virus (HBV)-specific CD8+ T cells play an important role in the clearance of HBV infection. Programmed cell death-1 (PD-1), an immunosuppressive molecule that regulates T-cell activation and peripheral immune tolerance, is increasingly shown to influence the outcome of HBV infection. rs10204525, a single-nucleotide polymorphism in the 3'-untranslated region (3'-UTR) of PD-1, has been associated with susceptibility and disease progression of chronic HBV infection in far-eastern patients. The aim of our study was to assess the impact of rs10204525 variation on HBV infection in Moroccan patients. A total of 236 patients with chronic HBV infection and 134 individuals with spontaneous HBV resolution were genotyped using a Taqman assay. In addition, PD-1 mRNA expression in peripheral blood nuclear cells was determined by quantitative reverse-transcription polymerase chain reaction. We found that the AA genotype is protective (odds ratio, 0.43; 95% confidence interval, 0.19 to 0.97; P = 0.038) against HBV infection. Interestingly, PD-1 messenger RNA (mRNA) expression analysis has revealed that chronic HBV carriers with GG and GA displayed higher levels of PD-1 mRNA compared with corresponding genotypes in resolved subjects (P = 0.031 and 0.014, respectively). Our data suggest that Mediterranean HBV-infected patients carrying PD-1 GG and GA genotypes at rs10204525 have high PD-1 mRNA expression and may be more prone to installation of chronicity.

Quis Custodiet Ipsos Custodes? Regulation of Cell-Mediated Immune Responses Following Viral Lung Infections

Viral lung infections are leading causes of morbidity and mortality. Effective immune responses to these infections require precise immune regulation to preserve lung function after viral clearance. One component of airway pathophysiology and lung injury associated with acute respiratory virus infection is effector T cells, yet these are the primary cells required for viral clearance. Accordingly, multiple immune mechanisms exist to regulate effector T cells, limiting immunopathology while permitting clearance of infection. Much has been learned in recent years about regulation of T cell function during chronic infection and cancer, and it is now clear that many of these mechanisms also control inflammation in acute lung infection. In this review, we focus on regulatory T cells, inhibitory receptors, and other cells and molecules that regulate cell-mediated immunity in the context of acute respiratory virus infection.

Association Between Imaging Findings of Airway Obstruction Adjacent to Lung Tumors and the Onset of Interstitial Lung Disease After Nivolumab

BACKGROUND

Compared to conventional cytotoxic chemotherapy, immune checkpoint inhibitors have shown a significant efficacy in the treatment of lung cancer. Although interstitial lung disease (ILD) is an important adverse event in immunotherapy, risk factors for ILD remain unclear.

PATIENTS AND METHODS

In this multicenter cohort study (UMIN000025908), 201 patients who were treated with nivolumab were retrospectively reviewed. Associations between the incidence of ILD and patient characteristics were evaluated. ILD grade and progression-free survival were analyzed according to the presence or absence of imaging findings of airway obstruction adjacent to lung tumors (IAOT).

RESULTS

In the multivariate analysis, the odds ratio (OR) of ILD for patients with a history of radiation pneumonitis or IAOT was 3.96 (p=0.012) and 6.59 (p=0.004), respectively. ILD occurred in six (37.5%) out of 16 patients with IAOT and 19 (10.3%) out of 185 patients without IAOT. Three out of the six patients with ILD and IAOT had ILD of grade 4 or more. The median progression-free survival of patients with and without IAOT was 0.9 and 3.2 months, respectively (p<0.001).

CONCLUSION

IAOT was strongly associated with the occurrence of ILD after therapy with nivolumab.

Regulatory T cells suppress virus-specific antibody responses to Friend retrovirus infection

Recent vaccine studies with experimental antigens have shown that regulatory T cells (Tregs) constrain the magnitude of B cell responses. This homeostatic Treg-mediated suppression is thought to reduce the potential of germinal center (GC) responses to generate autoreactive antibodies. However, essentially opposite results were observed in live influenza infections where Tregs promoted B cell and antibody responses. Thus, it remains unclear whether Tregs dampen or enhance B cell responses, especially during live viral infections. Here, we use mice infected with Friend retrovirus (FV), which induces a robust expansion of Tregs. Depletion of Tregs led to elevated activation, proliferation, and class switching of B cells. In addition, Treg depletion enhanced the production of virus-specific and virus-neutralizing antibodies and reduced FV viremia. Thus, in contrast to influenza infection, Tregs either directly or indirectly suppress B cells during mouse retroviral infection indicating that the ultimate effect of Tregs on B cell responses is specific to the particular infectious agent.

Hepatitis B Virus-Specific CD8+ T Cells Maintain Functional Exhaustion after Antigen Reexposure in an Acute Activation Immune Environment

Chronic hepatitis B virus (HBV) infection is characterized by the presence of functionally exhausted HBV-specific CD8+ T cells. To characterize the possible residual effector ability of these cells, we reexposed CD8+ T cells from chronically HBV replicating mice to HBV antigens in an acute activation immune environment. We found that after transfer into naive mice, exhausted CD8+ T cells reexpanded in a comparable magnitude as naive CD8+ T cells in response to acute HBV infection; however, their proliferation intensity was significantly lower than that of CD8+ T cells from acute-resolving HBV replicating mice (AR mice). The differentiation phenotypes driven by acute HBV replication of donor exhausted and naive CD8+ T cells were similar, but were different from those of their counterparts from AR mice. Nevertheless, exhausted CD8+ T cells maintained less activated phenotype, an absence of effector cytokine production and poor antiviral function after HBV reexposure in an acute activation immune environment. We thus conclude that exhausted CD8+ T cells undergo a stable form of dysfunctional differentiation during chronic HBV replication and switching immune environment alone is not sufficient for the antiviral functional reconstitution of these cells.

[Role of programmed death-1 in viral infectious diseases]

The research on the immunoregulatory effect of programmed death-1 (PD-1) in infectious diseases mainly focuses on chronic viral infection, but there are few studies on acute viral infection. In chronic viral infection, PD-1 is highly expressed on the surface of CD8⁺ T cells, which is a sign of CD8⁺ T cell depletion. Recent studies have shown that in chronic viral infection, PD-1 is also highly expressed on the surface of regulatory T cells and binds to programmed death-ligand 1 (PD-L1) on the surface of exhausted CD8⁺ T cells, resulting in a stronger inhibitory effect on CD8⁺ T cell immunity. Blocking the PD-1/PD-L1 signaling pathway between exhausted CD8⁺ T cells and regulatory T cells can significantly reverse the depletion of CD8⁺ T cells and greatly improve the antiviral effect of CD8⁺ T cells. However, the role of the PD-1/PD-L1 signaling pathway in acute viral infection remains unknown. This article summarizes the latest research on PD-1 in infectious diseases and discusses its role in acute and chronic viral infection.

PD-1/PD-L1 Blockade: Have We Found the Key to Unleash the Antitumor Immune Response?

PD-1–PD-L1 interaction is known to drive T cell dysfunction, which can be blocked by anti-PD-1/PD-L1 antibodies. However, studies have also shown that the function of the PD-1–PD-L1 axis is affected by the complex immunologic regulation network, and some CD8⁺ T cells can enter an irreversible dysfunctional state that cannot be rescued by PD-1/PD-L1 blockade. In most advanced cancers, except Hodgkin lymphoma (which has high PD-L1/L2 expression) and melanoma (which has high tumor mutational burden), the objective response rate with anti-PD-1/PD-L1 monotherapy is only ~20%, and immune-related toxicities and hyperprogression can occur in a small subset of patients during PD-1/PD-L1 blockade therapy. The lack of efficacy in up to 80% of patients was not necessarily associated with negative PD-1 and PD-L1 expression, suggesting that the roles of PD-1/PD-L1 in immune suppression and the mechanisms of action of antibodies remain to be better defined. In addition, important immune regulatory mechanisms within or outside of the PD-1/PD-L1 network need to be discovered and targeted to increase the response rate and to reduce the toxicities of immune checkpoint blockade therapies. This paper reviews the major functional and clinical studies of PD-1/PD-L1, including those with discrepancies in the pathologic and biomarker role of PD-1 and PD-L1 and the effectiveness of PD-1/PD-L1 blockade. The goal is to improve understanding of the efficacy of PD-1/PD-L1 blockade immunotherapy, as well as enhance the development of therapeutic strategies to overcome the resistance mechanisms and unleash the antitumor immune response to combat cancer.

Differential Inhibitory Receptor Expression on T Cells Delineates Functional Capacities in Chronic Viral Infection

Inhibitory receptors have been extensively described for their importance in regulating immune responses in chronic infections and cancers. Blocking the function of inhibitory receptors such as PD-1, CTLA-4, 2B4, Tim-3, and LAG-3 has shown promise for augmenting CD8 T cell activity and boosting pathogen-specific immunity. However, the prevalence of inhibitory receptors on CD4 T cells and their relative influence on CD4 T cell functionality in chronic HIV infection remains poorly described. We therefore determined and compared inhibitory receptor expression patterns of 2B4, CTLA-4, LAG-3, PD-1, and Tim-3 on virus-specific CD4 and CD8 T cells in relation to their functional T cell profile. In chronic HIV infection, inhibitory receptor distribution differed markedly between cytokine-producing T cell subsets with, gamma interferon (IFN-γ)- and tumor necrosis factor alpha (TNF-α)-producing cells displaying the highest and lowest prevalence of inhibitory receptors, respectively. Blockade of inhibitory receptors differentially affected cytokine production by cells in response to staphylococcal enterotoxin B stimulation. CTLA-4 blockade increased IFN-γ and CD40L production, while PD-1 blockade strongly augmented IFN-γ, interleukin-2 (IL-2), and TNF-α production. In a Friend retrovirus infection model, CTLA-4 blockade in particular was able to improve control of viral replication. Together, these results show that inhibitory receptor distribution on HIV-specific CD4 T cells varies markedly with respect to the functional subset of CD4 T cells being analyzed. Furthermore, the differential effects of receptor blockade suggest novel methods of immune response modulation, which could be important in the context of HIV vaccination or therapeutic strategies.IMPORTANCE Inhibitory receptors are important for limiting damage by the immune system during acute infections. In chronic infections, however, their expression limits immune system responsiveness. Studies have shown that blocking inhibitory receptors augments CD8 T cell functionality in HIV infection, but their influence on CD4 T cells remains unclear. We assessed the expression of inhibitory receptors on HIV-specific CD4 T cells and their relationship with T cell functionality. We uncovered differences in inhibitory receptor expression depending on the CD4 T cell function. We also found differences in functionality of CD4 T cells following blocking of different inhibitory receptors, and we confirmed our results in a Friend virus retroviral model of infection in mice. Our results show that inhibitory receptor expression on CD4 T cells is linked to CD4 T cell functionality and could be sculpted by blockade of specific inhibitory receptors. These data reveal exciting possibilities for the development of novel treatments and immunotherapeutics.

Granulocytic myeloid-derived suppressor cells suppress virus-specific CD8+ T cell responses during acute Friend retrovirus infection

BACKGROUND

Myeloid-derived suppressor cells (MDSCs) can suppress T cell responses in several different diseases. Previously these suppressive cells were observed to expand in HIV patients and in a mouse retrovirus model, yet their suppressive effect on virus-specific CD8+ T cells in vitro and in vivo has not been characterized thus far.

RESULTS

We used the Friend retrovirus (FV) model to demonstrate that MDSCs expand and become activated during the late phase of acute FV infection. Only the subpopulation of granulocytic MDSCs (gMDSCs) but not monocytic MDSC suppressed virus-specific CD8+ T cell proliferation and function in vitro. gMDSCs expressed arginase 1, high levels of the inhibitory ligand PD-L1 and the ATP dephosphorylating enzyme CD39 on the cell surface upon infection. All three molecules were involved in the suppressive effect of the gMDSCs in vitro. MDSC depletion experiments in FV-infected mice revealed that they restrict virus-specific CD8+ T cell responses and thus affect the immune control of chronic retroviruses in vivo.

CONCLUSIONS

Our study demonstrates that MDSCs become activated and expand during the acute phase of retrovirus infection. Their suppressive activity on virus-specific CD8+ T cells may contribute to T cell dysfunction and the development of chronic infection.

Severe acute interstitial lung disease after nivolumab in three non-small cell lung cancer patients with imaging findings of airway obstruction adjacent to lung tumors

Nivolumab has been associated with unique adverse events known as immune-related adverse events. Although interstitial lung disease (ILD) is a life-threatening immune-related adverse event, the risk of ILD during nivolumab treatment is unclear. In this report, we encountered three patients with stage IV non-small cell lung cancer with signs of lung obstruction caused by tumor-mediated compression on imaging who developed acute ILD within 10 days of commencing nivolumab treatment. The first case involved a 74-year-old Japanese female never-smoker, the second a 67-year-old Japanese female never-smoker, and the third a 75-year-old Japanese female current-smoker. The first patient was administered nivolumab as third-line chemotherapy, the second was administered nivolumab as fifth-line chemotherapy, and the third was administered nivolumab as second-line chemotherapy. Regardless of aggressive treatments for ILD, 2 of 3 patients died. The findings of these cases suggest that obstructive findings in the lungs, which easily cause infections, may be an important risk factor for nivolumab-induced ILD.

The IL-1R/TLR signaling pathway is essential for efficient CD8+ T-cell responses against hepatitis B virus in the hydrodynamic injection mouse model

The outcome of hepatitis B viral (HBV) infection is determined by the complex interactions between replicating HBV and the immune system. While the role of the adaptive immune system in the resolution of HBV infection has been studied extensively, the contribution of innate immune mechanisms remains to be defined. Here we examined the role of the interleukin-1 receptor/Toll-like receptor (IL-1R/TLR) signaling pathway in adaptive immune responses and viral clearance by exploring the HBV mouse model. Hydrodynamic injection with a replication-competent HBV genome was performed in wild-type mice (WT) and a panel of mouse strains lacking specific innate immunity component expression. We found higher levels of HBV protein production and replication in Tlr2^−/−, Tlr23479^−/−, 3d/Tlr24^−/−, Myd88/Trif^−/− and Irak4^−/− mice, which was associated with reduced HBV-specific CD8⁺ T-cell responses in these mice. Importantly, HBV clearance was delayed for more than 2 weeks in 3d/Tlr24^−/−, Myd88/Trif^−/− and Irak4^−/− mice compared to WT mice. HBV-specific CD8⁺ T-cell responses were functionally impaired for producing the cytokines IFN-γ, TNF-α and IL-2 in TLR signaling-deficient mice compared to WT mice. In conclusion, the IL-1R/TLR signaling pathway might contribute to controlling HBV infection by augmenting HBV-specific CD8⁺ T-cell responses.

CD169+ macrophages regulate PD-L1 expression via type I interferon and thereby prevent severe immunopathology after LCMV infection

Upon infection with persistence-prone virus, type I interferon (IFN-I) mediates antiviral activity and also upregulates the expression of programmed death ligand 1 (PD-L1), and this upregulation can lead to CD8⁺ T-cell exhaustion. How these very diverse functions are regulated remains unknown. This study, using the lymphocytic choriomeningitis virus, showed that a subset of CD169⁺ macrophages in murine spleen and lymph nodes produced high amounts of IFN-I upon infection. Absence of CD169⁺ macrophages led to insufficient production of IFN-I, lower antiviral activity and persistence of virus. Lack of CD169⁺ macrophages also limited the IFN-I-dependent expression of PD-L1. Enhanced viral replication in the absence of PD-L1 led to persistence of virus and prevented CD8⁺ T-cell exhaustion. As a consequence, mice exhibited severe immunopathology and died quickly after infection. Therefore, CD169⁺ macrophages are important contributors to the IFN-I response and thereby influence antiviral activity, CD8⁺ T-cell exhaustion and immunopathology.

Differential Expression of Immune Checkpoint Modulators on In Vitro Primed CD4(+) and CD8(+) T Cells

PD-1, TIM-3, and LAG-3 are molecules shown to have immune modulatory properties, and although initially classified as indicators of T cell hyporesponsiveness, it has become clear that they are also associated with the normal course of T cell activation. Functional studies have focused mainly on CD8⁺ T cells during chronic inflammation due to interest in co-opting the cellular immune response to eliminate viral or cancerous threats; however, there remains a relative lack of data regarding the expression of these molecules on CD4⁺ T cells. Here, we report that expression of the immune checkpoint (IC) molecules PD-1, LAG-3, and TIM-3 are differentially expressed on CD4⁺ and CD8⁺ T cells in the allogeneic response resulting from a mixed lymphocyte reaction. In these studies, PD-1 expression is higher on CD4⁺ T cells compared to CD8⁺ T cells. In contrast, TIM-3 is expressed at higher levels on CD8⁺ T cells compared to CD4⁺ T cells with an apparent reciprocity in that PD-1⁺ CD4⁺ T cells are frequently TIM-3^lo/−, while TIM-3-expressing CD8⁺ T cells are largely PD-1^lo/−. In addition, there is a decrease in the frequency of TIM-3⁺ CD4⁺ cells producing IFN-γ and IL-5 compared to TIM-3⁺ CD8⁺ cells. Lastly, the memory T cell phenotype within each IC-expressing subset differs between CD4⁺ and CD8⁺ T cells. These findings highlight key differences in IC expression patterns between CD4⁺ and CD8⁺ T cells and may allow for more effective therapeutic targeting of these molecules in the future.

CD28 family of receptors on T cells in chronic HBV infection: Expression characteristics, clinical significance and correlations with PD-1 blockade

The aim of the present study was to investigate the overall clinical expression characteristics of the cluster of differentiation (CD)28 family receptors [CD28, inducible T-cell co-stimulator, programmed cell death protein 1 (PD-1), cytotoxic T-lymphocyte-associated protein 4 and B- and T-lymphocyte attenuator] on T cells in patients with chronic hepatitis B (CHB), analyze the correlations among these receptors and the clinical parameters, and to investigate the effects of PD-1 blockade on the receptor expression profiles, T-cell function and other biological effects. The expression characteristics of the CD28 family of receptors, the effects of PD-1 blockade on the receptor expression profiles and the levels of interferon (IFN)-γ were investigated in the T cells of patients with CHB. In addition, the transcription factor, T-box 21 (T-bet) and GATA binding protein 3 (GATA-3) mRNA expression levels were investigated in the peripheral blood mononuclear cells (PBMCs) of patients with CHB. The expression levels of the CD28 family receptors in the T cells of patients with CHB demonstrated distinct characteristics, for example levels of PD-1 and CTLA-4 on CD4 T cells and ICOS, PD-1, and BTLA on CD8 T cells were increased in cells from patients with CHB compared with those from the healthy individuals. A significant positive correlation was demonstrated among the serum HBV DNA titers and the levels of PD-1 on CD8⁺ T cells with the highest expression of PD-1 corresponding to viral levels >10⁶ IU/ml. A significant positive correlation was observed between the serum HBV DNA titers and the expression levels of BTLA on CD8⁺ T cells with the highest expression of BTLA corresponding to viral levels >10⁶ IU/ml. PD-1 blockade altered the expression profiles of CD28 family receptors in the T cells of patients with CHB, partly enhanced T cell function and increased the ratio of T-bet/GATA-3 mRNA in PBMCs. Thus, CD28 family receptors are potential clinical indicators for the rapid monitoring of changes in T cell function during CHB treatment. Furthermore, PD-1 blockade has a therapeutic potential that may be enhanced by modulating the expression of co-stimulatory and -inhibitory receptors of the CD28 family.

Tetherin/BST-2 promotes dendritic cell activation and function during acute retrovirus infection

Tetherin/BST-2 is a host restriction factor that inhibits retrovirus release from infected cells in vitro by tethering nascent virions to the plasma membrane. However, contradictory data exists on whether Tetherin inhibits acute retrovirus infection in vivo. Previously, we reported that Tetherin-mediated inhibition of Friend retrovirus (FV) replication at 2 weeks post-infection correlated with stronger natural killer, CD4+ T and CD8+ T cell responses. Here, we further investigated the role of Tetherin in counteracting retrovirus replication in vivo. FV infection levels were similar between wild-type (WT) and Tetherin KO mice at 3 to 7 days post-infection despite removal of a potent restriction factor, Apobec3/Rfv3. However, during this phase of acute infection, Tetherin enhanced myeloid dendritic cell (DC) function. DCs from infected, but not uninfected, WT mice expressed significantly higher MHC class II and the co-stimulatory molecule CD80 compared to Tetherin KO DCs. Tetherin-associated DC activation during acute FV infection correlated with stronger NK cell responses. Furthermore, Tetherin+ DCs from FV-infected mice more strongly stimulated FV-specific CD4+ T cells ex vivo compared to Tetherin KO DCs. The results link the antiretroviral and immunomodulatory activity of Tetherin in vivo to improved DC activation and MHC class II antigen presentation.

PD-L1 Expression on Retrovirus-Infected Cells Mediates Immune Escape from CD8+ T Cell Killing

Cytotoxic CD8+ T Lymphocytes (CTL) efficiently control acute virus infections but can become exhausted when a chronic infection develops. Signaling of the inhibitory receptor PD-1 is an important mechanism for the development of virus-specific CD8+ T cell dysfunction. However, it has recently been shown that during the initial phase of infection virus-specific CD8+ T cells express high levels of PD-1, but are fully competent in producing cytokines and killing virus-infected target cells. To better understand the role of the PD-1 signaling pathway in CD8+ T cell cytotoxicity during acute viral infections we analyzed the expression of the ligand on retrovirus-infected cells targeted by CTLs. We observed increased levels of PD-L1 expression after infection of cells with the murine Friend retrovirus (FV) or with HIV. In FV infected mice, virus-specific CTLs efficiently eliminated infected target cells that expressed low levels of PD-L1 or that were deficient for PD-L1 but the population of PD-L1high cells escaped elimination and formed a reservoir for chronic FV replication. Infected cells with high PD-L1 expression mediated a negative feedback on CD8+ T cells and inhibited their expansion and cytotoxic functions. These findings provide evidence for a novel immune escape mechanism during acute retroviral infection based on PD-L1 expression levels on virus infected target cells.

Virus-specific cytotoxic T cells can eliminate infected cells during acute viral infections, but in chronic infections these cells often become dysfunctional or “exhausted.” The inhibitory receptor PD-1 is involved in the suppression of cytotoxic T cell responses in chronic infections. However, during many acute viral infections cytotoxic T cells up-regulate the PD-1 receptor but initially remain competent in killing virus infected target cells. Here we show that the ligand for PD-1, called PD-L1, can be induced on retrovirus infected cells and that the cells with the highest expression of PD-L1 escaped from cytotoxic T cell killing. Thus, PD-L1high infected target cells accumulated during the course of infection, formed the reservoir of virus persistence, and subsequently mediated a negative feedback on cytotoxic T cells via the PD-1 receptor that ultimately resulted in functional exhaustion of these cells. The current results provide evidence for a novel escape mechanism of viruses from cytotoxic T cell responses and may explain how viral reservoirs are established during chronic infections.

Activated regulatory T cells suppress effector NK cell responses by an IL-2-mediated mechanism during an acute retroviral infection

Background

It is well established that effector T cell responses are crucial for the control of most virus infections, but they are often tightly controlled by regulatory T cells (Treg) to minimize immunopathology. NK cells also contribute to virus control but it is not known if their antiviral effect is influenced by virus-induced Tregs as well. We therefore analyzed whether antiretroviral NK cell functions are inhibited by Tregs during an acute Friend retrovirus infection of mice.

Results

Selective depletion of Tregs by using the transgenic DEREG mouse model resulted in improved NK cell proliferation, maturation and effector cell differentiation. Suppression of NK cell functions depended on IL-2 consumption by Tregs, which could be overcome by specific NK cell stimulation with an IL-2/anti-IL-2 mAb complex.

Conclusions

The current study demonstrates that virus-induced Tregs indeed inhibit antiviral NK cell responses and describes a targeted immunotherapy that can abrogate the suppression of NK cells by Tregs.

Electronic supplementary material

The online version of this article (doi:10.1186/s12977-015-0191-3) contains supplementary material, which is available to authorized users.

Selective Involvement of the Checkpoint Regulator VISTA in Suppression of B-Cell, but Not T-Cell, Responsiveness by Monocytic Myeloid-Derived Suppressor Cells from Mice Infected with an Immunodeficiency-Causing Retrovirus

Inhibition of T-cell responses in tumor microenvironments by myeloid-derived suppressor cells (MDSCs) is widely accepted. We demonstrated augmentation of monocytic MDSCs whose suppression of not only T-cell, but also B-cell, responsiveness paralleled the immunodeficiency during LP-BM5 retrovirus infection. MDSCs inhibited T cells by inducible nitric oxide synthase (iNOS)/nitric oxide (NO), but uniquely, inhibition of B cells was ~50% dependent each on iNOS/NO and the MDSC-expressed negative-checkpoint regulator VISTA. Blockade with a combination of iNOS/NO and VISTA caused additive or synergistic abrogation of MDSC-mediated suppression of B-cell responsiveness.

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.

Programmed death-1 controls T cell survival by regulating oxidative metabolism

The coinhibitory receptor programmed death-1 (PD-1) maintains immune homeostasis by negatively regulating T cell function and survival. Blockade of PD-1 increases the severity of graft-versus-host disease (GVHD), but the interplay between PD-1 inhibition and T cell metabolism is not well studied. We found that both murine and human alloreactive T cells concomitantly upregulated PD-1 expression and increased levels of reactive oxygen species (ROS) following allogeneic bone marrow transplantation. This PD-1(Hi)ROS(Hi) phenotype was specific to alloreactive T cells and was not observed in syngeneic T cells during homeostatic proliferation. Blockade of PD-1 signaling decreased both mitochondrial H2O2 and total cellular ROS levels, and PD-1-driven increases in ROS were dependent upon the oxidation of fatty acids, because treatment with etomoxir nullified changes in ROS levels following PD-1 blockade. Downstream of PD-1, elevated ROS levels impaired T cell survival in a process reversed by antioxidants. Furthermore, PD-1-driven changes in ROS were fundamental to establishing a cell's susceptibility to subsequent metabolic inhibition, because blockade of PD-1 decreased the efficacy of later F1F0-ATP synthase modulation. These data indicate that PD-1 facilitates apoptosis in alloreactive T cells by increasing ROS in a process dependent upon the oxidation of fat. In addition, blockade of PD-1 undermines the potential for subsequent metabolic inhibition, an important consideration given the increasing use of anti-PD-1 therapies in the clinic.

Woodchuck hepatitis virus core antigen-based DNA and protein vaccines induce qualitatively different immune responses that affect T cell recall responses and antiviral effects

T helper type 1 (Th1) immunity was considered to play a dominant role in viral clearance of hepadnaviral infection. However, pre-primed Th2 type responses were able to efficiently control hepadnaviral infection in animal models. We investigated how pre-primed Th1/2 responses control hepadnaviral replication using the newly established mouse models. DNA (pWHcIm, pCTLA-4-C) and protein vaccines based on the nucleocapsid protein (WHcAg) of woodchuck hepatitis virus (WHV) primed specific immune responses with distinct features. The pre-primed responses determined the characteristics of recall responses if challenged with a WHcAg-expressing adenoviral vector. Vaccination with pWHcIm and pCTLA4-C facilitated viral control in the hydrodynamic injection model and reduced WHV loads by about 3 and 2 logs in WHV-transgenic mice, respectively, despite of different kinetics of specific CD8+ T cell responses. Thus, pre-primed Th2-biased responses facilitate the development of CD8+ T cell responses in mice compared with naïve controls and thereby confer better viral control.

T cell differentiation in chronic infection and cancer: functional adaptation or exhaustion?

Chronic viral infections and malignant tumours induce T cells that have a reduced ability to secrete effector cytokines and have upregulated expression of the inhibitory receptor PD1 (programmed cell death protein 1). These features have so far been considered to mark terminally differentiated 'exhausted' T cells. However, several recent clinical and experimental observations indicate that phenotypically exhausted T cells can still mediate a crucial level of pathogen or tumour control. In this Opinion article, we propose that the exhausted phenotype results from a differentiation process in which T cells stably adjust their effector capacity to the needs of chronic infection. We argue that this phenotype is optimized to cause minimal tissue damage while still mediating a critical level of pathogen control. In contrast to the presently held view of functional exhaustion, this new concept better reflects the pathophysiology and clinical manifestations of persisting infections, and it provides a rationale for emerging therapies that enhance T cell activity in chronic infection and cancer by blocking inhibitory receptors.

Bystander activation and anti-tumor effects of CD8+ T cells following Interleukin-2 based immunotherapy is independent of CD4+ T cell help

We have previously demonstrated that immunotherapy combining agonistic anti-CD40 and IL-2 (IT) results in synergistic anti-tumor effects. IT induces expansion of highly cytolytic, antigen-independent “bystander-activated” (CD8⁺CD44^high) T cells displaying a CD25⁻NKG2D⁺ phenotype in a cytokine dependent manner, which were responsible for the anti-tumor effects. While much attention has focused on CD4+ T cell help for antigen-specific CD8+ T cell expansion, little is known regarding the role of CD4+ T cells in antigen-nonspecific bystander-memory CD8+ T cell expansion. Utilizing CD4 deficient mouse models, we observed a significant expansion of bystander-memory T cells following IT which was similar to the non-CD4 depleted mice. Expanded bystander-memory CD8+ T cells upregulated PD-1 in the absence of CD4+ T cells which has been published as a hallmark of exhaustion and dysfunction in helpless CD8+ T cells. Interestingly, compared to CD8+ T cells from CD4 replete hosts, these bystander expanded cells displayed comparable (or enhanced) cytokine production, lytic ability, and in vivo anti-tumor effects suggesting no functional impairment or exhaustion and were enriched in an effector phenotype. There was no acceleration of the post-IT contraction phase of the bystander memory CD8+ response in CD4-depleted mice. The response was independent of IL-21 signaling. These results suggest that, in contrast to antigen-specific CD8+ T cell expansion, CD4+ T cell help is not necessary for expansion and activation of antigen-nonspecific bystander-memory CD8+ T cells following IT, but may play a role in regulating conversion of these cells from a central memory to effector phenotype. Additionally, the expression of PD-1 in this model appears to be a marker of effector function and not exhaustion.

Activated CD8+ T cells induce expansion of Vβ5+ regulatory T cells via TNFR2 signaling

Vβ5(+) regulatory T cells (Tregs), which are specific for a mouse endogenous retroviral superantigen, become activated and proliferate in response to Friend virus (FV) infection. We previously reported that FV-induced expansion of this Treg subset was dependent on CD8(+) T cells and TNF-α, but independent of IL-2. We now show that the inflammatory milieu associated with FV infection is not necessary for induction of Vβ5(+) Treg expansion. Rather, it is the presence of activated CD8(+) T cells that is critical for their expansion. The data indicate that the mechanism involves signaling between the membrane-bound form of TNF-α on activated CD8(+) T cells and TNFR2 on Tregs. CD8(+) T cells expressing membrane-bound TNF-α but no soluble TNF-α remained competent to induce strong Vβ5(+) Treg expansion in vivo. In addition, Vβ5(+) Tregs expressing only TNFR2 but no TNFR1 were still responsive to expansion. Finally, treatment of naive mice with soluble TNF-α did not induce Vβ5(+) Treg expansion, but treatment with a TNFR2-specific agonist did. These results reveal a new mechanism of intercellular communication between activated CD8(+) T cell effectors and Tregs that results in the activation and expansion of a Treg subset that subsequently suppresses CD8(+) T cell functions.

Early skewed distribution of total and HIV-specific CD8+ T-cell memory phenotypes during primary HIV infection is related to reduced antiviral activity and faster disease progression

The important role of the CD8+ T-cells on HIV control is well established. However, correlates of immune protection remain elusive. Although the importance of CD8+ T-cell specificity and functionality in virus control has been underscored, further unraveling the link between CD8+ T-cell differentiation and viral control is needed. Here, an immunophenotypic analysis (in terms of memory markers and Programmed cell death 1 (PD-1) expression) of the CD8+ T-cell subset found in primary HIV infection (PHI) was performed. The aim was to seek for associations with functional properties of the CD8+ T-cell subsets, viral control and subsequent disease progression. Also, results were compared with samples from Chronics and Elite Controllers. It was found that normal maturation of total and HIV-specific CD8+ T-cells into memory subsets is skewed in PHI, but not at the dramatic level observed in Chronics. Within the HIV-specific compartment, this alteration was evidenced by an accumulation of effector memory CD8+ T (TEM) cells over fully differentiated terminal effector CD8+ T (TTE) cells. Furthermore, higher proportions of total and HIV-specific CD8+ TEM cells and higher HIV-specific TEM/(TEM+TTE) ratio correlated with markers of faster progression. Analysis of PD-1 expression on total and HIV-specific CD8+ T-cells from PHI subjects revealed not only an association with disease progression but also with skewed memory CD8+ T-cell differentiation. Most notably, significant direct correlations were obtained between the functional capacity of CD8+ T-cells to inhibit viral replication in vitro with higher proportions of fully-differentiated HIV-specific CD8+ TTE cells, both at baseline and at 12 months post-infection. Thus, a relationship between preservation of CD8+ T-cell differentiation pathway and cell functionality was established. This report presents evidence concerning the link among CD8+ T-cell function, phenotype and virus control, hence supporting the instauration of early interventions to prevent irreversible immune damage.

Tetherin promotes the innate and adaptive cell-mediated immune response against retrovirus infection in vivo

Tetherin/BST-2 is a host restriction factor that could directly inhibit retroviral particle release by tethering nascent virions to the plasma membrane. However, the immunological impact of Tetherin during retrovirus infection remains unknown. We now show that Tetherin influences antiretroviral cell-mediated immune responses. In contrast to the direct antiviral effects of Tetherin, which are dependent on cell surface expression, the immunomodulatory effects are linked to the endocytosis of the molecule. Mice encoding endocytosis-competent C57BL/6 Tetherin exhibited lower viremia and pathology at 7 d postinfection with Friend retrovirus (FV) compared with mice encoding endocytosis-defective NZW/LacJ Tetherin. Notably, antiretroviral protection correlated with stronger NK cell responses. In addition, Friend retrovirus infection levels were significantly lower in wild-type C57BL/6 mice than in Tetherin knockout mice at 2 wk postinfection, and antiretroviral protection correlated with stronger NK cell and virus-specific CD8+ T cell responses. The results demonstrate that Tetherin acts as a modulator of the cell-mediated immune response against retrovirus infection in vivo.