Resolution of a chronic viral infection after interleukin-10 receptor blockade

Direct infection of dendritic cells during chronic viral infection suppresses antiviral T cell proliferation and induces IL-10 expression in CD4 T cells

Elevated levels of systemic IL-10 have been associated with several chronic viral infections, including HCV, EBV, HCMV and LCMV. In the chronic LCMV infection model, both elevated IL-10 and enhanced infection of dendritic cells (DCs) are important for viral persistence. This report highlights the relationship between enhanced viral tropism for DCs and the induction of IL-10 in CD4 T cells, which we identify as the most frequent IL-10-expressing cell type in chronic LCMV infection. Here we report that infected CD8α^neg DCs express elevated IL-10, induce IL-10 expression in LCMV specific CD4 T cells, and suppress LCMV-specific T cell proliferation. DCs exposed in vivo to persistent LCMV retain the capacity to stimulate CD4 T cell proliferation but induce IL-10 production by both polyclonal and LCMV-specific CD4 T cells. Our study delineates the unique effects of direct infection versus viral exposure on DCs. Collectively these data point to enhanced infection of DCs as a key trigger of the IL-10 induction cascade resulting in maintenance of elevated IL-10 expression in CD4 T cells and inhibition of LCMV-specific CD4 and CD8 T cell proliferation.

Fallen angels or risen apes? A tale of the intricate complexities of imbalanced immune responses in the pathogenesis and progression of immune-mediated and viral cancers

Excessive immune responses directed against foreign pathogens, self-antigens, or commensal microflora can cause cancer establishment and progression if the execution of tight immuno-regulatory mechanisms fails. On the other hand, induction of potent tumor antigen-specific immune responses together with stimulation of the innate immune system is a pre-requisite for effective anti-tumor immunity, and if suppressed by the strong immuno-regulatory mechanisms can lead to cancer progression. Therefore, it is crucial that the inevitable co-existence of these fundamental, yet conflicting roles of immune-regulatory cells is carefully streamlined as imbalances can be detrimental to the host. Infection with chronic persistent viruses is characterized by severe immune dysfunction resulting in T cell exhaustion and sometimes deletion of antigen-specific T cells. More often, this is due to increased immuno-regulatory processes, which are triggered to down-regulate immune responses and limit immunopathology. However, such heightened levels of immune disruption cause a concomitant loss of tumor immune-surveillance and create a permissive microenvironment for cancer establishment and progression, as demonstrated by increased incidences of cancer in immunosuppressed hosts. Paradoxically, while some cancers arise as a consequence of increased immuno-regulatory mechanisms that inhibit protective immune responses and impinge on tumor surveillance, other cancers arise due to impaired immuno-regulatory mechanisms and failure to limit pathogenic inflammatory responses. This intricate complexity, where immuno-regulatory cells can be beneficial in certain immune settings but detrimental in other settings underscores the need for carefully formulated interventions to equilibrate the balance between immuno-stimulatory and immuno-regulatory processes.

Molecular and transcriptional basis of CD4⁺ T cell dysfunction during chronic infection

T cell exhaustion is common during chronic infections. Although CD4(+) T cells are critical for controlling viral load during chronic viral infections, less is known about their differentiation and transcriptional program. We defined the phenotypic, functional, and molecular profiles of exhausted CD4(+) T cells. Global transcriptional analysis demonstrated a molecular profile distinct from effector and memory CD4(+) T cells and also from exhausted CD8(+) T cells, though some common features of CD4(+) and CD8(+) T cell exhaustion were revealed. We have demonstrated unappreciated roles for transcription factors (TFs) including Helios, type I interferon (IFN-I) signaling, and a diverse set of coinhibitory and costimulatory molecules during CD4(+) T cell exhaustion. Moreover, the signature of CD4(+) T cell exhaustion was found to be distinct from that of other CD4(+) T cell lineage subsets and was associated with TF heterogeneity. This study provides a framework for therapeutic interventions targeting exhausted CD4(+) T cells.

Murine gammaherpesvirus M2 protein induction of IRF4 via the NFAT pathway leads to IL-10 expression in B cells

Reactivation of the gammaherpesviruses Epstein-Barr virus (EBV), Kaposi's sarcoma-associated herpesvirus (KSHV) and murine gammaherpesvirus 68 (MHV68) from latently infected B cells has been linked to plasma cell differentiation. We have previously shown that the MHV68 M2 protein is important for virus reactivation from B cells and, when expressed alone in primary murine B cells, can drive B cell differentiation towards a pre-plasma cell phenotype. In addition, expression of M2 in primary murine B cells leads to secretion of high levels of IL-10 along with enhanced proliferation and survival. Furthermore, the absence of M2 in vivo leads to a defect in the appearance of MHV68 infected plasma cells in the spleen at the peak of MHV68 latency. Here, employing an inducible B cell expression system, we have determined that M2 activates the NFAT pathway in a Src kinase-dependent manner – leading to induction of the plasma cell-associated transcription factor, Interferon Regulatory Factor-4 (IRF4). Furthermore, we show that expression of IRF4 alone in a B cell line up-regulates IL-10 expression in culture supernatants, revealing a novel role for IRF4 in B cell induced IL-10. Consistent with the latter observation, we show that IRF4 can regulate the IL-10 promoter in B cells. In primary murine B cells, addition of cyclosporine (CsA) resulted in a significant decrease in M2-induced IL-10 levels as well as IRF4 expression, emphasizing the importance of the NFAT pathway in M2- mediated induction of IL-10. Together, these studies argue in favor of a model wherein M2 activation of the NFAT pathway initiates events leading to increased levels of IRF4 – a key player in plasma cell differentiation – which in turn triggers IL-10 expression. In the context of previous findings, the data presented here provides insights into how M2 facilitates plasma cell differentiation and subsequent virus reactivation.

The human viruses Epstein-Barr Virus (EBV) and Kaposi's Sarcoma-associated herpesvirus (KSHV) are members of the gammaherpesvirus family – pathogens that are associated with cancers of lymphoid tissues. Murine gammaherpesvirus 68 (MHV68) infection of laboratory mice provides a small animal model to study how this family of viruses chronically infects their host. The gammaherpesvirus establish a quiescent infection (termed latency) for the lifetime of the individual. However, they are capable of producing progeny virus (termed reactivation) in response to a variety of immune or environmental stimuli. Differentiation of latently infected B cells into plasma cells (the cells producing antibodies) has been associated with reactivation from latency. Notably, the MHV68 M2 protein plays a role in driving differentiation of MHV68 infected B cells to plasma cells. Furthermore, M2 expression results in increased levels of IL-10 (an immune-regulatory cytokine). Here we show that M2 mediated IL-10 production occurs through induction of IRF4 expression, a key player in plasma cell differentiation. This process involves Src kinases and NFAT – both components of B cell receptor signaling. Additionally, mice lacking IRF4 in infected cells show a significant defect in virus reactivation, thereby identifying IRF4 as a crucial component of M2 mediated functions.

Pathogen manipulation of cIL-10 signaling pathways: opportunities for vaccine development?

Interleukin-10 (IL-10) is a tightly regulated, pleiotropic cytokine that has profound effects on all facets of the immune system, eliciting cell-type-specific responses within cells expressing the IL-10 receptor (IL-10R). It is considered a master immune regulator, and imbalances in IL-10 expression, resulting from either inherent or infectious etiologies, have far reaching clinical ramifications. Regarding infectious diseases, there has been accumulating recognition that many pathogens, particularly those that establish lifelong persistence, share a commonality of their natural histories: manipulation of IL-10-mediated signaling pathways. Multiple viral, bacterial, protozoal, and fungal pathogens appear to have evolved mechanisms to co-opt normal immune functions, including those involving IL-10R-mediated signaling, and immune effector pathways away from immune-mediated protection toward environments of immune evasion, suppression, and tolerance. As a result, pathogens can persist for the life of the infected host, many of whom possess otherwise competent immune systems. Because of pathogenic avoidance of immune clearance, persistent infections can exact incalculable physical and financial costs, and represent some of the most vexing challenges for improvements in human health. Enormous benefits could be gained by the development of efficient prevention and/or therapeutic strategies that block primary infection, or clear the infection. There are now precedents that indicate that modalities focusing on pathogen-mediated manipulation of IL-10 signaling may have clinical benefit.

IL-10: achieving balance during persistent viral infection

The clearance of viral infections is reliant on the coordination and balance of inflammatory factors necessary for viral destruction and immunoregulatory mechanisms necessary to prevent host pathology. In the case of persistent viral infections, immunoregulatory pathways prevent the immune response from clearing the virus, resulting in a long-term equilibrium between host and pathogen. Consequently, negative immune regulators are being considered as a therapeutic target to treat persistent and chronic viral infections. In this review, we will highlight the current understanding of the important negative immune regulator interleukin-10 (IL-10) in persistent viral infection. Though its main role for the host is to limit immune-mediated pathology, IL-10 is a multifunctional cytokine that differentially regulates a number of different hematopoietic cell types. IL-10 has been shown to play a role in a number of infectious diseases and many viral pathogens specifically exploit the IL-10 pathway to help evade host immunity. Recent advances have demonstrated that manipulation of IL-10 signaling during persistent viral infection can alter T cell responses in vivo and that this manipulation can lead to the clearance of persistent viral infection. Furthermore, there have been crucial advances in the understanding of factors that induce IL-10. We summarize lessons learned about IL-10 in model organisms and human persistent infections and conclude with the potential use of IL-10 to treat persistent viral infections.

The molecular basis of IL-10 function: from receptor structure to the onset of signaling

Assembly of the cell surface IL-10 receptor complex is the first step in initiating IL-10 signaling pathways that regulate intestinal inflammation, viral persistence and even tumor surveillance. The discovery of IL-10 homologs in the genomes of herpes viruses suggests IL-10 signaling pathways can be manipulated at the level of the receptor complex. This chapter will describe our current molecular understanding of IL-10 receptor assembly based on crystal structures and biochemical analyses of cellular and viral IL-10 receptor complexes.

Networking at the level of host immunity: immune cell interactions during persistent viral infections

Persistent viral infections are the result of a series of connected events that culminate in diminished immunity and the inability to eliminate infection. By building our understanding of how distinct components of the immune system function both individually and collectively in productive versus abortive responses, new potential therapeutic targets can be developed to overcome immune dysfunction and thus fight persistent infections. Using lymphocytic choriomeningitis virus (LCMV) as a model of a persistent virus infection and drawing parallels to persistent human viral infections such as human immunodeficiency virus (HIV) and hepatitis C virus (HCV), we describe the cellular relationships and interactions that determine the outcome of initial infection and highlight immune targets for therapeutic intervention to prevent or treat persistent infections. Ultimately, these findings will further our understanding of the immunologic basis of persistent viral infection and likely lead to strategies to treat human viral infections.

Plasmacytoid dendritic cells mediate the regulation of inflammatory type T cell response for optimal immunity against respiratory Chlamydia pneumoniae infection

Chlamydia pneumoniae (Cpn) infection is a leading cause for a variety of respiratory diseases and has been implicated in the pathogenesis of chronic inflammatory diseases. The regulatory mechanisms in host defense against Cpn infection are less understood. In this study, we investigated the role of plasmacytoid dendritic cells (pDCs) in immune regulation in Cpn respiratory tract infection. We found that in vivo depletion of pDCs increased the severity of infection and lung pathology. Mice depleted of pDC had greater body weight loss, higher lung bacterial burden and excessive tissue inflammation compared to the control mice. Analysis of specific T cell cytokine production pattern in the lung following Cpn infection revealed that pDC depleted mice produced significantly higher amounts of inflammatory cytokines, especially TNF-α, but lower IL-10 compared to the controls. In particular, pDC depleted mice showed pathogenic T cell responses characterized by inflammatory type-1 (CD8 and CD4) and inflammatory Th2 cell responses. Moreover, pDC depletion dramatically reduced CD4 regulatory T cells (Tregs) in the lungs and draining lymph nodes. Furthermore, pDC-T cell co-culture experiments showed that pDCs isolated from Cpn infected mice were potent in inducing IL-10 producing CD4 Tregs. Together, these findings provide in vivo evidence for a critical role of pDCs in homeostatic regulation of immunity during Cpn infection. Our findings highlight the importance of a ‘balanced’ immune response for host protective immunity and preventing detrimental immunopathology during microbial infections.

PC61 (anti-CD25) treatment inhibits influenza A virus-expanded regulatory T cells and severe lung pathology during a subsequent heterologous lymphocytic choriomeningitis virus infection

Prior immunity to influenza A virus (IAV) in mice changes the outcome to a subsequent lymphocytic choriomeningitis virus (LCMV) infection and can result in severe lung pathology, similar to that observed in patients that died of the 1918 H1N1 pandemic. This pathology is induced by IAV-specific memory CD8(+) T cells cross-reactive with LCMV. Here, we discovered that IAV-immune mice have enhanced CD4(+) Foxp3(+) T-regulatory (Treg) cells in their lungs, leading us to question whether a modulation in the normal balance of Treg and effector T-cell responses also contributes to enhancing lung pathology upon LCMV infection of IAV-immune mice. Treg cell and interleukin-10 (IL-10) levels remained elevated in the lungs and mediastinal lymph nodes (mLNs) throughout the acute LCMV response of IAV-immune mice. PC61 treatment, used to decrease Treg cell levels, did not change LCMV titers but resulted in a surprising decrease in lung pathology upon LCMV infection in IAV-immune but not in naive mice. Associated with this decrease in pathology was a retention of Treg in the mLN and an unexpected partial clonal exhaustion of LCMV-specific CD8(+) T-cell responses only in IAV-immune mice. PC61 treatment did not affect cross-reactive memory CD8(+) T-cell proliferation. These results suggest that in the absence of IAV-expanded Treg cells and in the presence of cross-reactive memory, the LCMV-specific response was overstimulated and became partially exhausted, resulting in a decreased effector response. These studies suggest that Treg cells generated during past infections can influence the characteristics of effector T-cell responses and immunopathology during subsequent heterologous infections. Thus, in humans with complex infection histories, PC61 treatment may lead to unexpected results.

Therapeutic depletion of natural killer cells controls persistent infection

Persistent viral infections are associated with host and viral factors that impair effective antiviral immunity. Natural killer (NK) cells contribute to establishment of persistent lymphocytic choriomeningitis virus (LCMV) infection in mice through suppression of virus-specific T cell responses during the first few days of infection, but NK cell depletion during those early time points can enable severe T cell-mediated immune pathology and death of the host. Here we show that long after their peak in cytolytic activation, NK cells continue to support viral persistence at later times of infection. Delayed depletion of NK cells, 2 to 3 weeks after infection, enhanced virus-specific T cell responses and viral control. This enhancing effect of delayed NK cell depletion on antiviral immunity, in contrast to early NK cell depletion, was not associated with increased morbidity and mortality, and mice quickly regained weight after treatment. The efficacy of the depletion depended in part upon the size of the original virus inoculum, the viral load at the time of depletion, and the presence of CD4 T cells. Each of these factors is an important contributor to the degree of CD8 T cell dysfunction during viral persistence. Thus, NK cells may continuously contribute to exhaustion of virus-specific T cells during chronic infection, possibly by depleting CD4 T cells. Targeting of NK cells could thus be considered in combination with blockade of other immunosuppressive pathways, such as the interleukin-10 (IL-10) and programmed death 1 (PD-1) pathways, as a therapy to cure chronic human infections, including those with HIV or hepatitis C virus. IMPORTANCE Persistent virus infections are a major threat to global human health. The capacity of viruses, including HIV and hepatitis C virus, to overwhelm or subvert host immune responses contributes to a prolonged state of dampened antiviral immune functionality, which in turn facilitates viral persistence. Recent efforts have focused on therapeutics that can restore the effector functions of these functionally exhausted virus-specific T cells in order to expedite viral clearance. Here we establish that natural killer (NK) cells actively contribute to immune dysfunction and viral persistence at later stages of infection. This previously undescribed mechanism of immune suppression during chronic infection provides a vital clue for the design of novel therapeutic strategies targeting NK cell immunosuppressive activity in order to restore immune function and enhance viral control in chronically infected individuals.

Genome variations associated with viral susceptibility and calcification in Emiliania huxleyi

Emiliania huxleyi, a key player in the global carbon cycle is one of the best studied coccolithophores with respect to biogeochemical cycles, climatology, and host-virus interactions. Strains of E. huxleyi show phenotypic plasticity regarding growth behaviour, light-response, calcification, acidification, and virus susceptibility. This phenomenon is likely a consequence of genomic differences, or transcriptomic responses, to environmental conditions or threats such as viral infections. We used an E. huxleyi genome microarray based on the sequenced strain CCMP1516 (reference strain) to perform comparative genomic hybridizations (CGH) of 16 E. huxleyi strains of different geographic origin. We investigated the genomic diversity and plasticity and focused on the identification of genes related to virus susceptibility and coccolith production (calcification). Among the tested 31940 gene models a core genome of 14628 genes was identified by hybridization among 16 E. huxleyi strains. 224 probes were characterized as specific for the reference strain CCMP1516. Compared to the sequenced E. huxleyi strain CCMP1516 variation in gene content of up to 30 percent among strains was observed. Comparison of core and non-core transcripts sets in terms of annotated functions reveals a broad, almost equal functional coverage over all KOG-categories of both transcript sets within the whole annotated genome. Within the variable (non-core) genome we identified genes associated with virus susceptibility and calcification. Genes associated with virus susceptibility include a Bax inhibitor-1 protein, three LRR receptor-like protein kinases, and mitogen-activated protein kinase. Our list of transcripts associated with coccolith production will stimulate further research, e.g. by genetic manipulation. In particular, the V-type proton ATPase 16 kDa proteolipid subunit is proposed to be a plausible target gene for further calcification studies.

Macrophage and T cell produced IL-10 promotes viral chronicity

Chronic viral infections lead to CD8⁺ T cell exhaustion, characterized by impaired cytokine secretion. Presence of the immune-regulatory cytokine IL-10 promotes chronicity of Lymphocytic Choriomeningitis Virus (LCMV) Clone 13 infection, while absence of IL-10/IL-10R signaling early during infection results in viral clearance and higher percentages and numbers of antiviral, cytokine producing T cells. IL-10 is produced by several cell types during LCMV infection but it is currently unclear which cellular sources are responsible for induction of viral chronicity. Here, we demonstrate that although dendritic cells produce IL-10 and overall IL-10 mRNA levels decrease significantly in absence of CD11c⁺ cells, absence of IL-10 produced by CD11c⁺ cells failed to improve the LCMV-specific T cell response and control of LCMV infection. Similarly, NK cell specific IL-10 deficiency had no positive impact on the LCMV-specific T cell response or viral control, even though high percentages of NK cells produced IL-10 at early time points after infection. Interestingly, we found markedly improved T cell responses and clearance of normally chronic LCMV Clone 13 infection when either myeloid cells or T cells lacked IL-10 production and mice depleted of monocytes/macrophages or CD4⁺ T cells exhibited reduced overall levels of IL-10 mRNA. These data suggest that the decision whether LCMV infection becomes chronic or can be cleared critically depends on early CD4⁺ T cell and monocyte/macrophage produced IL-10.

Chronic viral infections like Hepatitis B and C Virus (HBV and HCV) and Human Immunodeficiency Virus (HIV) in humans affect more than 500 million people worldwide. While a robust T cell response is a hallmark of many acute infections one hurdle inhibiting the clearance of chronic viral infections is that the immune-suppressive cytokine IL-10 modulates the virus-host balance towards induction of T cell dysfunction. IL-10 is produced by several cell types during chronic Lymphocytic Choriomeningitis Virus (LCMV) infection but it is currently unclear which cellular sources are responsible to promote viral chronicity. Here, we demonstrate that T cell responses improved markedly, and that normally chronic LCMV Clone 13 infection could be cleared when either myeloid cells or T cells lacked IL-10 production. Furthermore, mice depleted of monocytes/macrophages or CD4⁺ T cells exhibited reduced overall levels of IL-10 mRNA. These data suggest that the decision whether LCMV infection becomes chronic or can be cleared critically depends on CD4⁺ T cell and monocyte/macrophage produced IL-10 early during the establishment of viral chronicity.

A targeted mutation within the feline leukemia virus (FeLV) envelope protein immunosuppressive domain to improve a canarypox virus-vectored FeLV vaccine

We previously delineated a highly conserved immunosuppressive (IS) domain within murine and primate retroviral envelope proteins that is critical for virus propagation in vivo. The envelope-mediated immunosuppression was assessed by the ability of the proteins, when expressed by allogeneic tumor cells normally rejected by engrafted mice, to allow these cells to escape, at least transiently, immune rejection. Using this approach, we identified key residues whose mutation (i) specifically abolishes immunosuppressive activity without affecting the "mechanical" function of the envelope protein and (ii) significantly enhances humoral and cellular immune responses elicited against the virus. The objective of this work was to study the immunosuppressive activity of the envelope protein (p15E) of feline leukemia virus (FeLV) and evaluate the effect of its abolition on the efficacy of a vaccine against FeLV. Here we demonstrate that the FeLV envelope protein is immunosuppressive in vivo and that this immunosuppressive activity can be "switched off" by targeted mutation of a specific amino acid. As a result of the introduction of the mutated envelope sequence into a previously well characterized canarypox virus-vectored vaccine (ALVAC-FeLV), the frequency of vaccine-induced FeLV-specific gamma interferon (IFN-γ)-producing cells was increased, whereas conversely, the frequency of vaccine-induced FeLV-specific interleukin-10 (IL-10)-producing cells was reduced. This shift in the IFN-γ/IL-10 response was associated with a higher efficacy of ALVAC-FeLV against FeLV infection. This study demonstrates that FeLV p15E is immunosuppressive in vivo, that the immunosuppressive domain of p15E can modulate the FeLV-specific immune response, and that the efficacy of FeLV vaccines can be enhanced by inhibiting the immunosuppressive activity of the IS domain through an appropriate mutation.

Circulating cytokines and histological liver damage in chronic hepatitis B infection

Each phase of hepatitis B infection stimulates distinct viral kinetics and host immune responses resulting in liver damage and hepatic fibrosis. Our objective has been to correlate host inflammatory immune response including circulating Th1 and Th2 cytokines in patients with chronic hepatitis B infection with liver histopathology. Sixty-four patients with chronic hepatitis B without previous treatment were recruited. The liver histology and histological activity index were assessed for various degrees of necroinflammation and hepatic fibrosis. We determined circulating levels of the Th1 and Th2 cytokines. Forty-six males and 18 females at a median age of 34.5 years were studied. HBeAg was present in 28/64 (43.75%) of the patients. In patients negative for HBeAg, IL-10 and IFN-gamma were significantly correlated with degrees of necroinflammation (r = 0.34, r = 0.38, resp.; P < 0.05). Moreover, TNF-alpha was significantly correlated with degrees of fibrosis (r = 0.35; P < 0.05), and IL-10 and TNF-alpha were significantly correlated with significant fibrosis (r = 0.39, r = 0.35, resp.; P < 0.05). These correlations were found in the HBeAg negative group as opposed to the HBeAg positive group. In HBeAg negative patients, circulating cytokines IL-10 and IFN-gamma were correlated with degrees of necroinflammation, whereas IL-10 and TNF-alpha were correlated with significant fibrosis.

Exogenous OX40 stimulation during lymphocytic choriomeningitis virus infection impairs follicular Th cell differentiation and diverts CD4 T cells into the effector lineage by upregulating Blimp-1

T cell costimulation is a key component of adaptive immunity to viral infection but has also been associated with pathology because of excessive or altered T cell activity. We recently demonstrated that the TNFR family costimulatory molecule OX40 (CD134) is critically required to sustain antiviral T cell and Ab responses that enable control of viral replication in the context of chronic lymphocytic choriomeningitis virus (LCMV) infection. In this study, we investigated whether reinforcing OX40 stimulation through an agonist Ab had the potential to prevent LCMV persistence. We observed that anti-OX40 injection early after LCMV clone 13 infection increased CD8 T cell-mediated immunopathology. More strikingly, OX40 stimulation of virus-specific CD4 T cells promoted expression of the transcriptional repressor Blimp-1 and diverted the majority of cells away from follicular Th cell differentiation. This occurred in both acute and chronic infections, and resulted in dramatic reductions in germinal center and Ab responses to the viral infection. The effect of the OX40 agonist was dependent on IL-2 signaling and the timing of OX40 stimulation. Collectively, our data demonstrate that excessive OX40 signaling can result in deleterious consequences in the setting of LCMV infection.

Monocyte-derived dendritic cells perform hemophagocytosis to fine-tune excessive immune responses

Because immune responses simultaneously defend and injure the host, the immune system must be finely regulated to ensure the host's survival. Here, we have shown that when injected with high Toll-like receptor ligand doses or infected with lymphocytic choriomeningitis virus (LCMV) clone 13, which has a high viral turnover, inflammatory monocyte-derived dendritic cells (Mo-DCs) engulfed apoptotic erythroid cells. In this process, called hemophagocytosis, phosphatidylserine (PS) served as an "eat-me" signal. Type I interferons were necessary for both PS exposure on erythroid cells and the expression of PS receptors in the Mo-DCs. Importantly, hemophagocytosis was required for interleukin-10 (IL-10) production from Mo-DCs. Blocking hemophagocytosis or Mo-DC-derived IL-10 significantly increased cytotoxic T cell lymphocyte activity, tissue damage, and mortality in virus-infected hosts, suggesting that hemophagocytosis moderates immune responses to ensure the host's survival in vivo. This sheds light on the physiological relevance of hemophagocytosis in severe inflammatory and infectious diseases.

Cell-intrinsic IL-27 and gp130 cytokine receptor signaling regulates virus-specific CD4⁺ T cell responses and viral control during chronic infection

The outcome of chronic viral infections, which affect millions of people worldwide, is greatly dependent on CD4⁺ T cells. Here we showed that T cell-specific ablation of the common interleukin-6 (IL-6) family receptor, gp130, profoundly compromised virus-specific CD4⁺ T cell survival, T follicular helper responses, and IL-21 production at late stages of chronic lymphocytic choriomeningitis virus (LCMV) infection. These effects were cell intrinsic for CD4⁺ T cells and were accompanied by a reduction of CD8⁺ T cells, antibodies, and a severe failure in viral control. We identified IL-27 as a gp130 cytokine that promoted antiviral CD4⁺ T cell accumulation in vivo and that rapidly induced IL-21 ex vivo. Furthermore, IL-27R was critical for control of persistent LCMV in vivo. These results reveal that gp130 cytokines (particularly IL-27) are key regulators of CD4⁺ T cell responses during an established chronic viral infection, empowering both humoral and cytotoxic immunity.

Vaccination against a virus-encoded cytokine significantly restricts viral challenge

Identification of immune correlates of protection for viral vaccines is complicated by multiple factors, but there is general consensus on the importance of antibodies that neutralize viral attachment to susceptible cells. Development of new viral vaccines has mostly followed this neutralizing antibody paradigm, but as a recent clinical trial of human cytomegalovirus (HCMV) vaccination demonstrated, this singular approach can yield limited protective efficacy. Since HCMV devotes >50% of its coding capacity to proteins that modulate host immunity, it is hypothesized that expansion of vaccine targets to include this part of the viral proteome will disrupt viral natural history. HCMV and rhesus cytomegalovirus (RhCMV) each encode an ortholog to the cellular interleukin-10 (cIL-10) cytokine: cmvIL-10 and rhcmvIL10, respectively. Despite extensive sequence divergence from their host's cIL-10, each viral IL-10 retains nearly identical functionality to cIL-10. Uninfected rhesus macaques were immunized with engineered, nonfunctional rhcmvIL-10 variants, which were constructed by site-directed mutagenesis to abolish binding to the cIL-10 receptor. Vaccinees developed antibodies that neutralized rhcmvIL-10 function with no cross-neutralization of cIL-10. Following subcutaneous RhCMV challenge, the vaccinees exhibited both reduced RhCMV replication locally at the inoculation site and systemically and significantly reduced RhCMV shedding in bodily fluids compared to controls. Attenuation of RhCMV infection by rhcmvIL-10 vaccination argues that neutralization of viral immunomodulation may be a new vaccine paradigm for HCMV by expanding potential vaccine targets.

Negative regulation of type I IFN expression by OASL1 permits chronic viral infection and CD8⁺ T-cell exhaustion

The type I interferons (IFN-Is) are critical not only in early viral control but also in prolonged T-cell immune responses. However, chronic viral infections such as those of human immunodeficiency virus (HIV) and hepatitis C virus (HCV) in humans and lymphocytic choriomeningitis virus (LCMV) in mice overcome this early IFN-I barrier and induce viral persistence and exhaustion of T-cell function. Although various T-cell-intrinsic and -extrinsic factors are known to contribute to induction of chronic conditions, the roles of IFN-I negative regulators in chronic viral infections have been largely unexplored. Herein, we explored whether 2'-5' oligoadenylate synthetase-like 1 (OASL1), a recently defined IFN-I negative regulator, plays a key role in the virus-specific T-cell response and viral defense against chronic LCMV. To this end, we infected Oasl1 knockout and wild-type mice with LCMV CL-13 (a chronic virus) and monitored T-cell responses, serum cytokine levels, and viral titers. LCMV CL-13-infected Oasl1 KO mice displayed a sustained level of serum IFN-I, which was primarily produced by splenic plasmacytoid dendritic cells, during the very early phase of infection (2-3 days post-infection). Oasl1 deficiency also led to the accelerated elimination of viremia and induction of a functional antiviral CD8 T-cell response, which critically depended on IFN-I receptor signaling. Together, these results demonstrate that OASL1-mediated negative regulation of IFN-I production at an early phase of infection permits viral persistence and suppresses T-cell function, suggesting that IFN-I negative regulators, including OASL1, could be exciting new targets for preventing chronic viral infection.

Non-neutralizing antibodies protect from chronic LCMV infection independently of activating FcγR or complement

Chronic viral infections lead to CD8(+) T cell exhaustion, characterized by impaired cytokine secretion. The presence of the immune-regulatory cytokine IL-10 promotes chronic lymphocytic choriomeningitis virus (LCMV) Clone 13 infection in mice, whereas the absence of IL-10/IL-10R signaling early during infection results in viral clearance and higher percentages and numbers of antiviral, cytokine-producing T cells. However, it is currently unclear which cell populations and effector molecules are crucial to protect against chronic infection. In this study, we demonstrate that antiviral, LCMV-binding, non-neutralizing antibodies are needed, in addition to CD4(+) and CD8(+) T cells, to clear a high-dose LCMV infection in mice, in the absence of IL-10. The interaction between CD4(+) T cells and B cells in B-cell follicles via CD40/CD40L, in addition to class switch and/or somatic hypermutation, is crucial for viral control in the absence of IL-10. Interestingly, transfer of LCMV-binding non-neutralizing antibodies protected recipients from chronic infection. In addition, viral clearance in the absence of IL-10R signaling was independent of activating Fcγ receptors and complement. These data highlight that non-neutralizing antibodies effectively contribute to the control of LCMV infection when present prior to infection, suggesting that the induction of neutralizing antibodies is not implicitly necessary for the generation of successful vaccines.

Regulatory CD8+ T cells associated with erosion of immune surveillance in persistent virus infection suppress in vitro and have a reversible proliferative defect

CD4(+) T cell help is critical for CD8(+) T cell memory and immune surveillance against persistent virus infections. Our recent data have showed the lack of CD4(+) T cells leads to the generation of an IL-10-producing CD8(+) T cell population during persistent murine γ-herpesvirus-68 (MHV-68) infection. IL-10 from these cells is partly responsible for erosion in immune surveillance, leading to spontaneous virus reactivation in lungs. In this study, we further characterized the generation, phenotype, and function of these IL-10-producing CD8(+) T cells by comparing with a newly identified IL-10-producing CD8(+) T cell population present during the acute stage of the infection. The IL-10-producing CD8(+) populations in acute and chronic stages differed in their requirement for CD4(+) T cell help, the dependence on IL-2/CD25 and CD40-CD40L pathways, and the ability to proliferate in vitro in response to anti-CD3 stimulation. IL-10-producing CD8(+) T cells in the chronic stage showed a distinct immunophenotypic profile, sharing partial overlap with the markers of previously reported regulatory CD8(+) T cells, and suppressed the proliferation of naive CD8(+) T cells. Notably, they retained the ability to produce effector cytokines and cytotoxic activity. In addition, the proliferative defect of the cells could be restored by addition of exogenous IL-2 or blockade of IL-10. These data suggest that the IL-10-producing CD8(+) T cells arising in chronic MHV-68 infection in the absence of CD4(+) T cell help belong to a subset of CD8(+) regulatory T cells.

IL-21 restricts virus-driven Treg cell expansion in chronic LCMV infection

Foxp3⁺ regulatory T (Treg) cells are essential for the maintenance of immune homeostasis and tolerance. During viral infections, Treg cells can limit the immunopathology resulting from excessive inflammation, yet potentially inhibit effective antiviral T cell responses and promote virus persistence. We report here that the fast-replicating LCMV strain Docile triggers a massive expansion of the Treg population that directly correlates with the size of the virus inoculum and its tendency to establish a chronic, persistent infection. This Treg cell proliferation was greatly enhanced in IL-21R^−/− mice and depletion of Treg cells partially rescued defective CD8⁺ T cell cytokine responses and improved viral clearance in some but not all organs. Notably, IL-21 inhibited Treg cell expansion in a cell intrinsic manner. Moreover, experimental augmentation of Treg cells driven by injection of IL-2/anti-IL-2 immune complexes drastically impaired the functionality of the antiviral T cell response and impeded virus clearance. As a consequence, mice became highly susceptible to chronic infection following exposure to low virus doses. These findings reveal virus-driven Treg cell proliferation as potential evasion strategy that facilitates T cell exhaustion and virus persistence. Furthermore, they suggest that besides its primary function as a direct survival signal for antiviral CD8⁺ T cells during chronic infections, IL-21 may also indirectly promote CD8⁺ T cell poly-functionality by restricting the suppressive activity of infection-induced Treg cells.

T cell exhaustion represents a state of T cell dysfunction associated with clinically relevant diseases, such as persistent viral infections or cancer. Although the molecular signature of exhausted T cells has been characterized in detail at the functional and transcriptional level, the immunological mechanisms that lead to T cell exhaustion during chronic infections remain poorly understood. Our present study reports two major findings that illustrate a pathway that contributes to T cell exhaustion during viral infection, and indicate its modulation by both, the pathogen and the host. First, we show that a persistence-inducing virus triggers the massive proliferation of Foxp3⁺ regulatory T (Treg) cells and demonstrate the potential of Treg cells to promote T cell exhaustion and chronic infection. Second, we identify IL-21 as a crucial host factor that antagonizes this virus-driven expansion of the Treg population in a cell intrinsic manner independent of IL-2. Thus, in addition to its known pre-dominant direct positive effects on antiviral T cells, IL-21 can also alleviate the suppressive activity of Treg cells. Together, these results suggest enhanced Treg cell responses as a mechanism of immune evasion that could be therapeutically targeted with IL-21.

CD4 T cell responses in latent and chronic viral infections

The spectrum of tasks which is fulfilled by CD4 T cells in the setting of viral infections is large, ranging from support of CD8 T cells and humoral immunity to exertion of direct antiviral effector functions. While our knowledge about the differentiation pathways, plasticity, and memory of CD4 T cell responses upon acute infections or immunizations has significantly increased during the past years, much less is still known about CD4 T cell differentiation and their beneficial or pathological functions during persistent viral infections. In this review we summarize current knowledge about the differentiation, direct or indirect antiviral effector functions, and the regulation of virus-specific CD4 T cells in the setting of persistent latent or active chronic viral infections with a particular emphasis on herpes virus infections for the former and chronic lymphocytic choriomeningitis virus infection for the latter.

Viral infection increases glucocorticoid-induced interleukin-10 production through ERK-mediated phosphorylation of the glucocorticoid receptor in dendritic cells: potential clinical implications

The hypothalamic-pituitary-adrenal axis plays a central role in the adaptive response to stress including infection of pathogens through glucocorticoids. Physical and/or mental stress alter susceptibility to viral infection possibly by affecting this regulatory system, thus we explored potential cellular targets and mechanisms that underlie this phenomenon in key immune components dendritic cells (DCs). Dexamethasone (DEX) treatment and subsequent Newcastle disease virus (NDV) infection most significantly and cooperatively stimulated mRNA expression of the interleukin (IL)-10 in murine bone marrow-derived DCs among 89 genes involved in the Toll-like receptor signaling pathways. NDV increased DEX-induced IL-10 mRNA and protein expression by 7- and 3-fold, respectively, which was observed from 3 hours after infection. Conventional DCs (cDCs), but not plasmacytoid DCs (pDCs) were major sources of IL-10 in bone marrow-derived DCs treated with DEX and/or infected with NDV. Murine cytomegalovirus and DEX increased serum IL-10 cooperatively in female mice. Pre-treatment of DCs with the extracellular signal-regulated kinase (ERK) inhibitor U0126 abolished cooperative induction of IL-10 by DEX and NDV. Further, ERK overexpression increased IL-10 promoter activity stimulated by wild-type human GR but not by its mutant defective in serine 203, whereas ERK knockdown abolished NDV/DEX cooperation on IL-10 mRNA and phosphorylation of the mouse GR at serine 213. NDV also increased DEX-induced mRNA expression of three known glucocorticoid-responsive genes unrelated to the Toll-like receptor signaling pathways in DCs. These results indicate that virus and glucocorticoids cooperatively increase production of anti-inflammatory cytokine IL-10 by potentiating the transcriptional activity of GR in DCs, through which virus appears to facilitate its own propagation in infected hosts. The results may further underlie in part known exacerbation of IL-10/T helper-2-related allergic disorders by stress and viral infection.

Blockade of chronic type I interferon signaling to control persistent LCMV infection

Type I interferons (IFN-I) are critical for antiviral immunity; however, chronic IFN-I signaling is associated with hyperimmune activation and disease progression in persistent infections. We demonstrated in mice that blockade of IFN-I signaling diminished chronic immune activation and immune suppression, restored lymphoid tissue architecture, and increased immune parameters associated with control of virus replication, ultimately facilitating clearance of the persistent infection. The accelerated control of persistent infection induced by blocking IFN-I signaling required CD4 T cells and was associated with enhanced IFN-γ production. Thus, we demonstrated that interfering with chronic IFN-I signaling during persistent infection redirects the immune environment to enable control of infection.

Allergic airway disease in mice alters T and B cell responses during an acute respiratory poxvirus infection

Pulmonary viral infections can exacerbate or trigger the development of allergic airway diseases via multiple mechanisms depending upon the infectious agent. Respiratory vaccinia virus transmission is well established, yet the effects of allergic airway disease on the host response to intra-pulmonary vaccinia virus infection remain poorly defined. As shown here BALB/c mice with preexisting airway disease infected with vaccinia virus developed more severe pulmonary inflammation, higher lung virus titers and greater weight loss compared with mice inoculated with virus alone. This enhanced viremia was observed despite increased pulmonary recruitment of CD8⁺ T effectors, greater IFNγ production in the lung, and high serum levels of anti-viral antibodies. Notably, flow cytometric analyses of lung CD8⁺ T cells revealed a shift in the hierarchy of immunodominant viral epitopes in virus inoculated mice with allergic airway disease compared to mice treated with virus only. Pulmonary IL-10 production by T cells and antigen presenting cells was detected following virus inoculation of animals and increased dramatically in allergic mice exposed to virus. IL-10 modulation of host responses to this respiratory virus infection was greatly influenced by the localized pulmonary microenvironment. Thus, blocking IL-10 signaling in virus-infected mice with allergic airway disease enhanced pulmonary CD4⁺ T cell production of IFNγ and increased serum anti-viral IgG1 levels. In contrast, pulmonary IFNγ and virus-specific IgG1 levels were reduced in vaccinia virus-treated mice with IL-10 receptor blockade. These observations demonstrate that pre-existing allergic lung disease alters the quality and magnitude of immune responses to respiratory poxviruses through an IL-10-dependent mechanism.

T-cell responses in hepatitis B and C virus infection: similarities and differences

Chronic hepatitis B virus (HBV) and hepatitis C virus (HCV) infection are global health problems affecting 600 million people worldwide. Indeed, HBV and HCV are hepatotropic viruses that can cause acute and chronic liver disease progressing to liver cirrhosis and even hepatocellular carcinoma. Furthermore, co-infections of HBV and HCV with HIV are emerging worldwide. These co-infections are even more likely to develop persistent infection and are difficult to treat. There is growing evidence that virus-specific CD4⁺ and CD8⁺ T-cell responses play a central role in the outcome and pathogenesis of HBV and HCV infection. While virus-specific T-cell responses are able to successfully clear the virus in a subpopulation of patients, failure of these T-cell responses is associated with the development of viral persistence. In this review article, we will discuss similarities and differences in HBV- and HCV-specific T-cell responses that are central in determining viral clearance, persistence and liver disease.

PD-1 promotes immune exhaustion by inducing antiviral T cell motility paralysis

Immune responses to persistent viral infections and cancer often fail because of intense regulation of antigen-specific T cells-a process referred to as immune exhaustion. The mechanisms that underlie the induction of exhaustion are not completely understood. To gain novel insights into this process, we simultaneously examined the dynamics of virus-specific CD8(+) and CD4(+) T cells in the living spleen by two-photon microscopy (TPM) during the establishment of an acute or persistent viral infection. We demonstrate that immune exhaustion during viral persistence maps anatomically to the splenic marginal zone/red pulp and is defined by prolonged motility paralysis of virus-specific CD8(+) and CD4(+) T cells. Unexpectedly, therapeutic blockade of PD-1-PD-L1 restored CD8(+) T cell motility within 30 min, despite the presence of high viral loads. This result was supported by planar bilayer data showing that PD-L1 localizes to the central supramolecular activation cluster, decreases antiviral CD8(+) T cell motility, and promotes stable immunological synapse formation. Restoration of T cell motility in vivo was followed by recovery of cell signaling and effector functions, which gave rise to a fatal disease mediated by IFN-γ. We conclude that motility paralysis is a manifestation of immune exhaustion induced by PD-1 that prevents antiviral CD8(+) T cells from performing their effector functions and subjects them to prolonged states of negative immune regulation.

CD8+ T cell exhaustion during persistent viral infection is regulated independently of the virus-specific T cell receptor

During chronic viral infections, responses by virus-specific CD8(+) T cells become marginalized by the acquisition of functional defects and reduced cell numbers in a process defined as T cell exhaustion. Similarly, T cell tolerance to self-antigen is also characterized by impaired effector function and eventual deletion of self-reactive T cells. Induction of both tolerance and exhaustion involve many shared inhibitory mechanisms, thus similar therapeutic approaches have proven effective in these distinct environments. We previously demonstrated that tolerant self-reactive CD8(+) T cells expressing dual-T cell receptors (i.e., dual-TCR) could be rescued by immunization through a second TCR specific for a foreign antigen. These data revealed that T cell tolerance was regulated at the level of the self-reactive TCR. Here, dual-TCR CD8(+) T cells were used to examine if exhaustion during persistent viral infection could be rescued by an analogous strategy of immunization through a second TCR not involved in recognition of virus. In direct contrast to the rescue achievable in tolerant CD8(+) T cells, exhausted T cells were equally impaired through both TCR. These findings suggest that exhaustion is maintained by defects downstream of the virus-specific TCR, and establish that exhaustion and tolerance are distinctly regulated states of T cell dysfunction.

Anti-viral CD8 T cells and the cytokines that they love

Viral infections cause an immunological disequilibrium that provokes CD8 T cell responses. These cells play critical roles in purging acute infections, limiting persistent infections, and conferring life-long protective immunity. At every stage of the response anti-viral CD8 T cells are sensitive to signals from cytokines. Initially cytokines operate as immunological warning signs that inform of the presence of an infection, and also influence the developmental choices of the responding cells. Later during the course of the response other sets of cytokines support the survival and maintenance of the differentiated anti-viral CD8 T cells. Although many cytokines promote virus-specific CD8 T cells, other cytokines can suppress their activities and thus favor viral persistence. In this review we discuss how select cytokines act to regulate anti-viral CD8 T cells throughout the response and influence the outcome of viral infections.

Reversal of chronic to resolved infection by IL-10 blockade is LCMV strain dependent

Chronic viral infections lead to CD8(+) T-cell exhaustion, characterized by impaired cytokine secretion. The immune-regulatory cytokine IL-10 promotes chronicity of infection with lymphocytic choriomeningitis virus (LCMV) Clone 13, as absence of IL-10 or blocking of IL-10R during early LCMV Clone 13 infection results in viral clearance. Thus, treatment of humans suffering from chronic viral infections with IL-10 neutralizing or IL-10R blocking antibodies was proposed to boost virus-specific T-cell responses to enhance control or even clear the viral infection. Here we demonstrate that although CD4(+) and CD8(+) T cells can produce elevated levels of cytokines in IL-10(-/-) mice early after infection compared with WT mice, IL-10(-/-) mice cannot clear an infection with the quicker replicating LCMV strain Docile, eventually resulting in T-cell exhaustion. These data suggest that the success of IL-10 blockade to control chronic viral infections may critically depend on the virulence of the infecting strain.

CD8+ clonality is associated with prolonged acute plasma viremia and altered mRNA cytokine profiles during the course of feline immunodeficiency virus infection

Acute lentiviral infection is characterized by early CD8(+) cytotoxic T cell (CTL) activity and a subsequent decline in plasma viremia. However, CD8(+) lymphocytes fail to eliminate the virus and a progressive T cell immune dysfunction develops during the course of chronic lentiviral infection. To further define this CD8(+) immune dysfunction we utilized PARR (PCR for antigen receptor rearrangements), a technique which measures clonally expanded lymphocyte populations by comparison of highly conserved T cell receptor (TCR) regions to identify the prevalence of clonal CD8(+) T cells following FIV infection. We then compared phenotype, mRNA profiles, CD8(+) proliferation and plasma viremia during acute and chronic infection for PARR positive (PARR(+)) and PARR negative (PARR(-)) Feline Immunodeficiency Virus (FIV) infected cats. We demonstrated that approximately forty percent of the FIV(+) cats examined exhibit CD8(+) clonality compared to none of the FIV(-) control cats. There were no phenotypic differences between PARR(+) and PARR(-) CD8(+) lymphocytes from FIV(+) cats but retrospective analysis of plasma viremia over the course of infection revealed a delayed peak in plasma viremia and a decline in lymphocyte counts were observed in the PARR(+) group during acute infection. CD8(+) lymphocytes isolated from chronically infected PARR(-) cats exhibited significantly higher mRNA expression of IFN-γ and IL-2 following mitogenic stimulation when compared to PARR(+) CD8(+) lymphocytes. These data suggest that clonal CD8(+) expansion may be related to impaired control of acute viremia and less effective CD8(+) anti-viral function. Using PARR to assess changes in CD8(+) clonality during the progression from acute to chronic FIV infection may help to better characterize the factors which contribute to CD8(+) anergy and lentiviral persistence.

Vaccination against tuberculosis: how can we better BCG?

Tuberculosis remains one of the most significant human diseases of the developing world, accounting for 3800 worldwide deaths per day. Although we currently have a vaccine for tuberculosis, BCG, this is insufficient at protecting from adult pulmonary tuberculosis in the parts of the world where a good vaccine is most needed. This has prompted the search for new vaccination strategies that can protect better than BCG, or can boost BCG-induced immunity. We discuss these subjects in line with what is known of the immune responses to BCG and Mycobacterium tuberculosis - the etiological agent of the disease, as well as the particular difficulties facing development of new vaccines against tuberculosis. A greater understanding of the factors constituting optimal protection against Mycobacterium tuberculosis infection, as well as which pathogenic factors facilitate active disease, will accelerate the delivery of safe vaccines able to restrict active tuberculosis and thus impede contagion.

Role of lymphocytic choriomeningitis virus (LCMV) in understanding viral immunology: past, present and future

Lymphocytic choriomeningitis virus (LCMV) is a common infection of rodents first identified over eighty years ago in St. Louis, MO, U.S.A. It is best known for its application in immunological studies. The history of LCMV closely correlates with the development of modern immunology. With the use of LCMV as a model pathogen several key concepts have emerged: Major Histocompatibility Complex (MHC) restriction, T cell memory, persistent infections, T cell exhaustion and the key role of immune pathology in disease. Given the phenomenal infrastructure within this field (e.g., defined immunodominant and subdominant epitopes to all T cell receptor specificities as well as the cognate tetramers for enumeration in vivo) the study of LCMV remains an active and productive platform for biological research across the globe to this day. Here we present a historical primer that highlights several breakthroughs since the discovery of LCMV. Next, we highlight current research in the field and conclude with our predictions for future directions in the remarkable field of LCMV research.

Persistent loss of IL-27 responsiveness in CD8+ memory T cells abrogates IL-10 expression in a recall response

CD8+ T cells are central to the eradication of intracellular pathogens, but they can also act to limit inflammation and immunopathology. During primary respiratory viral infection CD8+ effector T cells release the immunosuppressive cytokine IL-10, which is essential for host survival. Here we report that CD8+ T-cell-derived IL-10 is absent in a recall response. We show in mice that the lack of IL-10 is due to a persistent loss of IL-27 responsiveness in CD8+ memory T cells, caused by down-regulation of the common cytokine receptor, glycoprotein 130. CD8+ memory T cells secreted less IL-10 when activated in the presence of IL-27 than did naïve controls, and retroviral expression of glycoprotein 130 restored IL-10 and reduced IFN-γ production upon restimulation. We demonstrate that human CD8+ memory cells are also characterized by impaired IL-27 responsiveness. Our data suggest that CD8+ T-cell activation involves a persistent loss of specific cytokine receptors that determines the functional potential of these cells during rechallenge infection.

IL-10-producing regulatory B cells in the pathogenesis of chronic hepatitis B virus infection

A regulatory subset of B cells has been found to modulate immune responses in autoimmunity, infection, and cancer, but it has not been investigated in the setting of human persistent viral infection. IL-10 is elevated in patients with chronic hepatitis B virus infection (CHB), but its cellular sources and impact on antiviral T cells have not been addressed. We investigated the role of IL-10 and regulatory B cells in the pathogenesis of CHB. Serum IL-10 levels were studied longitudinally in patients with CHB undergoing spontaneous disease flares. There was a close temporal correlation between IL-10 levels and fluctuations in viral load or liver inflammation. Blockade of IL-10 in vitro rescued polyfunctional virus-specific CD8 T cell responses. To investigate the potential contribution of regulatory B cells, their frequency was measured directly ex vivo and after exposure to stimuli relevant to hepatitis B virus (HBV) (CpG or HBV Ags). IL-10-producing B cells were enriched in patients, and their frequency correlated temporally with hepatic flares, both after stimulation and directly ex vivo. Phenotypically, these cells were predominantly immature (CD19(+)CD24(hi)CD38(hi)) ex vivo; sorted CD19(+)CD24(hi)CD38(hi) cells suppressed HBV-specific CD8 T cell responses in an IL-10-dependent manner. In summary, these data reveal a novel IL-10-producing subset of B cells able to regulate T cell immunity in CHB.

Blockade of IL-10 signaling during bacillus Calmette-Guérin vaccination enhances and sustains Th1, Th17, and innate lymphoid IFN-γ and IL-17 responses and increases protection to Mycobacterium tuberculosis infection

Vaccination with Mycobacterium bovis bacillus Calmette-Guérin (BCG) remains the only prophylactic vaccine against tuberculosis, caused by Mycobacterium tuberculosis, but gives variable protection against pulmonary disease. The generation of host Th1 responses following BCG vaccination is accepted as the major mechanism of protection against M. tuberculosis infection. Early production of IL-17 in the lungs following M. tuberculosis challenge of mice previously vaccinated with M. tuberculosis peptides in adjuvant has been shown to be required for efficient Th1 cell recruitment. IL-10 regulates various processes involved in generation of Th1 and Th17 responses. Previous studies have shown IL-10 as a negative regulator of the immune response to primary M. tuberculosis infection, with Il10(-/-) mice having reduced lung bacterial loads. In this study we show that inhibition of IL-10 signaling during BCG vaccination enhances host-generated Ag-specific IFN-γ and IL-17A responses, and that this regimen gives significantly greater protection against aerogenic M. tuberculosis challenge in both susceptible and relatively resistant strains of mice. In M. tuberculosis-susceptible CBA/J mice, Ab blockade of IL-10R specifically during BCG vaccination resulted in additional protection against M. tuberculosis challenge of >1-log(10) compared with equivalent isotype-treated controls. The protection observed following BCG vaccination concurrent with anti-IL-10R mAb treatment was sustained through chronic M. tuberculosis infection and correlated with enhanced lung Th1 and Th17 responses and increased IFN-γ and IL-17A production by γδ T cells and an innate-like Thy1.2(+)CD3(-) lymphoid population. We show that IL-10 inhibits optimal BCG-elicited protection, therefore suggesting that antagonists of IL-10 may be of great benefit as adjuvants in preventive vaccination against tuberculosis.

Emergence of distinct multiarmed immunoregulatory antigen-presenting cells during persistent viral infection

During persistent viral infection, adaptive immune responses are suppressed by immunoregulatory factors, contributing to viral persistence. Although this suppression is mediated by inhibitory factors, the mechanisms by which virus-specific T cells encounter and integrate immunoregulatory signals during persistent infection are unclear. We show that a distinct population of IL-10-expressing immunoregulatory antigen-presenting cells (APCs) is amplified during chronic versus acute lymphocytic choriomeningitis virus (LCMV) infection and suppresses T cell responses. Although acute LCMV infection induces the expansion of immunoregulatory APCs, they subsequently decline. However, during persistent LCMV infection, immunoregulatory APCs are amplified and parallel the viral replication kinetics. Further characterization demonstrates that immunoregulatory APCs are molecularly and metabolically distinct, and exhibit increased expression of T cell-interacting molecules and negative regulatory factors that suppress T cell responses. Thus, immunoregulatory APCs are amplified during viral persistence and deliver inhibitory signals that suppress antiviral T cell immunity and likely contribute to persistent infection.

OX40 facilitates control of a persistent virus infection

During acute viral infections, clearance of the pathogen is followed by the contraction of the anti-viral T cell compartment. In contrast, T cell responses need to be maintained over a longer period of time during chronic viral infections in order to control viral replication and to avoid viral spreading. Much is known about inhibitory signals such as through PD-1 that limit T cell activity during chronic viral infection, but little is known about the stimulatory signals that allow maintenance of anti-viral T cells. Here, we show that the co-stimulatory molecule OX40 (CD134) is critically required in the context of persistent LCMV clone 13 infection. Anti-viral T cells express high levels of OX40 in the presence of their cognate antigen and T cells lacking the OX40 receptor fail to accumulate sufficiently. Moreover, the emergence of T cell dependent germinal center responses and LCMV-specific antibodies are severely impaired. Consequently, OX40-deficient mice fail to control LCMV clone 13 infection over time, highlighting the importance of this signaling pathway during persistent viral infection.

A robust T cell response is the hallmark of an effective immune response to a variety of invading viruses. In many acute infections, the clearance of the viral pathogen is associated with a short and vigorous T cell response followed by development of pathogen-specific immune memory. However, some viruses can establish persistent infection in their respective host, during which an ongoing T cell response is required in order to prevent overwhelming viral replication. Little is known about the factors that sustain the T cell response in the persistent phase of a viral infection. In this report, we demonstrate that ligation of the OX40 molecule, which is expressed on T cells targeting the virus, is critically required in order to sustain the anti-viral immune response. We show that virus-specific, OX40-deficient T cells fail to accumulate sufficiently and consequently, mice lacking the OX40 receptor are incapable of controlling viral replication. Collectively our data establish OX40 as a crucial signaling molecule during a persistent viral infection.

Type I and III interferons enhance IL-10R expression on human monocytes and macrophages, resulting in IL-10-mediated suppression of TLR-induced IL-12

Currently, only about 30-50% of chronic hepatitis C virus (HCV) and hepatitis B virus (HBV) patients respond to IFN-based therapy. It has been suggested that IL-10 is involved in suppressing the activity of type I IFNs on antigen-presenting cells (APCs). However, the interaction between type I IFNs and IL-10 is still not clear. Here we report that IFN-α priming upregulated the expression of IL-10R1 on monocytes, and subsequently IL-10 induced a higher level of STAT3 phosphorylation in IFN-primed cells. This indicates that IFN-α increased the sensitivity of monocytes to IL-10, and as a result, TLR-induced IL-12p70 by IFN-pretreated cells was suppressed. Interestingly, both IFN-β and IL-29, a member of the type III IFN family, comparably sensitized monocytes and macrophages to IL-10 stimulation, indicating a general effect of IFN on the activity of IL-10 in APCs. In summary, we demonstrate that one of the consequences of priming human APCs with IFN is to promote the cells' sensitivity to IL-10, which leads to the inhibition of TLR-induced IL-12p70 production. Therefore, type I and III IFNs induce a suboptimal activation of immune cells. These findings are relevant for the development of strategies to further improve IFN-based therapy for patients with multiple sclerosis or viral hepatitis.

Hepatitis C virus-mediated modulation of cellular immunity

The hepatitis C virus (HCV) is a major cause of chronic liver disease globally. A chronic infection can result in liver fibrosis, liver cirrhosis, hepatocellular carcinoma and liver failure in a significant ratio of the patients. About 170 million people are currently infected with HCV. Since 80 % of the infected patients develop a chronic infection, HCV has evolved sophisticated escape strategies to evade both the innate and the adaptive immune system. Thus, chronic hepatitis C is characterized by perturbations in the number, subset composition and/or functionality of natural killer cells, natural killer T cells, dendritic cells, macrophages and T cells. The balance between HCV-induced immune evasion and the antiviral immune response results in chronic liver inflammation and consequent immune-mediated liver injury. This review summarizes our current understanding of the HCV-mediated interference with cellular immunity and of the factors resulting in HCV persistence. A profound knowledge about the intrinsic properties of HCV and its effects on intrahepatic immunity is essential to be able to design effective immunotherapies against HCV such as therapeutic HCV vaccines.

Infected CD8α- dendritic cells are the predominant source of IL-10 during establishment of persistent viral infection

Interleukin-10 (IL-10) is an important factor involved in T-cell dysfunction during persistent viral infection. Although several factors can negatively regulate T-cell activity, targeting of the IL-10 pathway alone is sufficient to regenerate T-cell activity and increase viral control. How IL-10 mediates these effects is unclear. Here, we investigated the cellular source of IL-10 necessary for establishing T-cell exhaustion and viral persistence, using IL-10 reporter mice (VertX), cell-type-specific IL-10 and IL-10 receptor deletion mice, and bone marrow chimeric mice. During establishment of viral persistence, the cellular subset with the most prevalent expression of IL-10 was CD8α(-)CD4(+) dendritic cells (DCs), which produced IL-10 with increasing kinetics until 9 d postinfection. After this time point, DCs exhibited a modest decline in percentage of IL-10(+) cells whereas B cells and CD4(+) T cells increased minimally. Further analysis of the DC population demonstrated that IL-10 was primarily expressed in infected DCs. These DCs were a notable source of IL-10 as mutant mice with a DC-specific deletion of IL-10 had significantly decreased serum levels. Interestingly, viral infection was not directly causative of IL-10 expression; rather, IL-10 production appeared to be linked to type I IFN signaling. Our findings further illuminate the contribution of DCs to the production of IL-10 and to viral persistence.

Antigen amount dictates CD8+ T-cell exhaustion during chronic viral infection irrespective of the type of antigen presenting cell

Chronic viral infections lead to CD8(+) T-cell exhaustion, characterized by impaired cytokine secretion and loss of proliferative capacity. While viral load and T-cell dysfunction correlate, it is currently unclear whether the quality of a cell type presenting antigen determines the degree of T-cell exhaustion or if the overall amount of antigen recognized by T cells promotes exhaustion. We found that chronic lymphocytic chorio-meningitis virus infection led to decreased CD8(+) T-cell exhaustion in DC-MHC class I (MHCI) mice, in which CD8(+) T cells can only recognize antigen on DCs. However, this increase in CD8(+) T-cell function came at the expense of fatal immunopathology. Additional antigen recognition on nonhematopoietic cells in DC-MHCI mice promoted T-cell exhaustion and avoidance of immunopathology. Likewise, increased numbers of antigen-expressing hematopoietic cells, as well as a selective elevation of the number of DCs as the only cell type presenting antigen in DC-MHCI mice, resulted in compromised T-cell function. These results favor a scenario in which the overall amount of antigen exposure, rather than the type of cell engaging with virus-specific CD8(+) T cells, is responsible for their functional exhaustion. Furthermore, exhaustion of virus-specific CD8(+) T cells leads to avoidance of life-threatening immunopathology.

IL-10 suppression of NK/DC crosstalk leads to poor priming of MCMV-specific CD4 T cells and prolonged MCMV persistence

IL-10 is an anti-inflammatory cytokine that regulates the extent of host immunity to infection by exerting suppressive effects on different cell types. Herpes viruses induce IL-10 to modulate the virus-host balance towards their own benefit, resulting in prolonged virus persistence. To define the cellular and molecular players involved in IL-10 modulation of herpes virus-specific immunity, we studied mouse cytomegalovirus (MCMV) infection. Here we demonstrate that IL-10 specifically curtails the MCMV-specific CD4 T cell response by suppressing the bidirectional crosstalk between NK cells and myeloid dendritic cells (DCs). In absence of IL-10, NK cells licensed DCs to effectively prime MCMV-specific CD4 T cells and we defined the pro-inflammatory cytokines IL-12, IFN-γ and TNF-α as well as NK cell activating receptors NKG2D and NCR-1 to regulate this bidirectional NK/DC interplay. Consequently, markedly enhanced priming of MCMV-specific CD4 T cells in Il10^−/− mice led to faster control of lytic viral replication, but this came at the expense of TNF-α mediated immunopathology. Taken together, our data show that early induction of IL-10 during MCMV infection critically regulates the strength of the innate-adaptive immune cell crosstalk, thereby impacting beneficially on the ensuing virus-host balance for both the virus and the host.

Cytomegalovirus (CMV) infections are very widespread in mammalian hosts. Despite the fact that CMVs are usually well controlled by the immune system, they cause persistent life-long infection and have evolved a number of strategies to effectively modulate or hide from host immunity. Since the establishment of an immunosuppressive environment favors virus persistence, IL-10 is one of the host targets that CMVs actively use to tune the virus-host balance toward their own benefit, resulting in prolonged virus persistence and hence increased chance for horizontal transmission. Here, we delineate the mechanisms of how IL-10 exerts its powerful immune-suppressing function in the context of murine cytomegalovirus (MCMV) infection. We found that IL-10 specifically restrains the priming of MCMV-specific CD4 T cell responses by suppressing dendritic cell (DC) - natural killer cell (NK) crosstalk during acute MCMV infection. Target molecules mediating this bi-directional crosstalk between DCs and NK cells were the pro-inflammatory cytokines IL-12, IFN-γ and TNF-α as well as NK cell activating receptors NKG2D and NCR-1 and all of them were markedly suppressed by IL-10. A consequence resulting from this impeded DC-NK cross-talk by IL-10, leading to poor priming of MCMV-specific CD4 T cell responses was increased lytic CMV persistence and reduced development of host tissue damage. Our study indicates that early induction of IL-10 during MCMV infection critically regulates the strength of the innate-adaptive crosstalk, thereby imparting on the ensuing virus-host balance for the benefit of both the virus and the host.

Herpesvirus exploitation of host immune inhibitory pathways

Herpesviruses employ a plethora of mechanisms to circumvent clearance by host immune responses. A key feature of mammalian immune systems is the employment of regulatory pathways that limit immune responsiveness. The primary functions of these mechanisms are to control autoimmunity and limit exuberant responses to harmless antigen in mucosal surfaces. However, such pathways can be exploited by viral pathogens to enable acute infection, persistence and dissemination. Herein, we outline the current understanding of inhibitory pathways in modulating antiviral immunity during herpesvirus infections in vivo and discuss strategies employed by herpesviruses to exploit these pathways to limit host antiviral immunity.