Kinetics of CD8+ effector T cell responses and induced CD4+ regulatory T cell responses during Friend retrovirus infection

Regulatory T cells suppress the motility of cytotoxic T cells in Friend retrovirus-infected mice

Antiviral immunity often requires CD8+ cytotoxic T lymphocytes (CTLs) that actively migrate and search for virus-infected targets. Regulatory T cells (Tregs) have been shown to suppress CTL responses, but it is not known whether this is also mediated by effects on CTL motility. Here, we used intravital 2-photon microscopy in the Friend retrovirus (FV) mouse model to define the impact of Tregs on CTL motility throughout the course of acute infection. Virus-specific CTLs were very motile and had frequent short contacts with target cells at their peak cytotoxic activity. However, when Tregs were activated and expanded in late-acute FV infection, CTLs became significantly less motile and contacts with target cells were prolonged. This phenotype was associated with development of functional CTL exhaustion. Tregs had direct contacts with CTLs in vivo and, importantly, their experimental depletion restored CTL motility. Our findings identify an effect of Tregs on CTL motility as part of their mechanism of functional impairment in chronic viral infections. Future studies must address the underlying molecular mechanisms.

Superior antiviral activity of IFNβ in genital HSV-1 infection

Type I interferons (IFNs) present the first line of defense against viral infections, providing antiviral, immunomodulatory and antiproliferative effects. The type I IFN family contains 12 IFNα subtypes and IFNβ, and although they share the same receptor, they are classified as non-redundant, capable to induce a variety of different IFN-stimulated genes. However, the biological impact of individual subtypes remains controversial. Recent data propose a subtype-specificity of type I IFNs revealing unique effector functions for different viruses and thus expanding the implications for IFNα-based antiviral immunotherapies. Despite extensive research, drug-resistant infections with herpes simplex virus type 1 (HSV-1), which is the common agent of recurrent orogenital lesions, are still lacking a protective or curing therapeutic. However, due to the risk of generalized infections in immunocompromised hosts as well as the increasing incidence of resistance to conventional antiherpetic agents, HSV infections raise major health concerns. Based on their pleiotropic effector functions, the application of type I IFNs represents a promising approach to inhibit HSV-1 replication, to improve host immunity and to further elucidate their qualitative differences. Here, selective IFNα subtypes and IFNβ were evaluated for their therapeutic potential in genital HSV-1 infections. Respective in vivo studies in mice revealed subtype-specific differences in the reduction of local viral loads. IFNβ had the strongest antiviral efficacy against genital HSV-1 infection in mice, whereas IFNα1, IFNα4, and IFNα11 had no impact on viral loads. Based on flow cytometric analyses of underlying immune responses at local and peripheral sites, these differences could be further assigned to specific modulations of the antiviral immunity early during HSV-1 infection. IFNβ led to enhanced systemic cytokine secretion and elevated cytotoxic responses, which negatively correlated with viral loads in the vaginal tract. These data provide further insights into the diversity of type I IFN effector functions and their impact on the immunological control of HSV-1 infections.

IFNα subtype-specific susceptibility of HBV in the course of chronic infection

Chronic hepatitis B virus (HBV) infection continues to be a major health problem worldwide and remains hard to be cured. Therapy with interferon (IFN) α is an important method for the clinical treatment of chronic hepatitis B. IFNα exhibits direct antiviral effects as well as immunomodulatory activities, which can induce sustained antiviral responses in part of the treated chronic hepatitis B patients. Numerous IFNα subtypes with high sequence identity between 76-96% exist which are characterized by diverse, non-redundant biological activities. Our previous studies have demonstrated that the clinically approved IFNα2 is not the most effective subtype for the anti-HBV treatment among all IFNα subtypes. So far very little is known about the IFNα subtype expression pattern during early HBV infection and the IFNα subtype-specific susceptibility during persistent HBV infection as well as its related cellular mechanism. Here we determined the Ifna subtype mRNA expression during acute and chronic HBV infection by using the well-established hydrodynamic injection (HDI) mouse model and we revealed a transient but strong expression of a panel of Ifna subtypes in the spleen of HBV persistent replication mice compared to HDI controls. Immunotherapy with distinct IFNα subtypes controlled chronic HBV infection. IFNα subtype-mediated antiviral response and immune activation were comprehensively analyzed in an AAV-HBV persistent infection murine model and murine IFNα2 was identified as the most effective subtype in suppression of HBV replication. Further analysis of the immune response revealed a strong immunomodulatory activity of murine IFNα2 on splenic and intrahepatic NK and T cell activation during persistent HBV infection. Taken together, our data provide IFNα subtype-specific differences in the antiviral and immunomodulatory effector responses and a strong expression of all IFNα subtypes in the spleen during persistent HBV infection in mice. This knowledge will support the development of novel immunotherapeutic strategies for chronic hepatitis B infection.

Immunotherapy With Interferon α11, But Not Interferon Beta, Controls Persistent Retroviral Infection

Type I Interferons (IFNs), including numerous IFNα subtypes and IFNβ, are key molecules during innate and adaptive immune responses against viral infections. These cytokines exert various non-redundant biological activities, although binding to the same receptor. Persistent viral infections are often characterized by increased IFN signatures implicating a potential role of type I IFNs in disease pathogenesis. Using the well-established Friend retrovirus (FV) mouse model, we compared the therapeutic efficacy of IFNα11 and IFNβ in acute and chronic retroviral infection. We observed a strong antiviral activity of both IFNs during acute FV infection, whereas only IFNα11 and not IFNβ could also control persistent FV infection. The therapeutic treatment with IFNα11 induced the expression of antiviral IFN-stimulated genes (ISG) and improved cytotoxic T cell responses. Finally, dysfunctional CD8+ T cells solely regained cytotoxicity after IFNα11 treatment. Our data provide evidence for opposing activities of type I IFNs during chronic retroviral infections. IFNβ was shown to be involved in immune dysfunction in chronic infections, whereas IFNα11 had a strong antiviral potential and reactivated exhausted T cells during persistent retroviral infection. In contrast, during acute infection, both type I IFNs were able to efficiently suppress FV replication.

SAMHD1 Promotes the Antiretroviral Adaptive Immune Response in Mice Exposed to Lipopolysaccharide

SAMHD1 is a potent HIV-1 restriction factor that blocks reverse transcription in monocytes, dendritic cells and resting CD4⁺ T cells by decreasing intracellular dNTP pools. However, SAMHD1 may diminish innate immune sensing and Ag presentation, resulting in a weaker adaptive immune response. To date, the role of SAMHD1 on antiretroviral immunity remains unclear, as mouse SAMHD1 had no impact on murine retrovirus replication in prior in vivo studies. Here, we show that SAMHD1 significantly inhibits acute Friend retrovirus infection in mice. Pretreatment with LPS, a significant driver of inflammation during HIV-1 infection, further unmasked a role for SAMHD1 in influencing immune responses. LPS treatment in vivo doubled the intracellular dNTP levels in immune compartments of SAMHD1 knockout but not wild-type mice. SAMHD1 knockout mice exhibited higher plasma infectious viremia and proviral DNA loads than wild-type mice at 7 d postinfection (dpi), and proviral loads inversely correlated with a stronger CD8⁺ T cell response. SAMHD1 deficiency was also associated with weaker NK, CD4⁺ T and CD8⁺ T cell responses by 14 dpi and weaker neutralizing Ab responses by 28 dpi. Intriguingly, SAMHD1 influenced these cell-mediated immune (14 dpi) and neutralizing Ab (28 dpi) responses in male but not female mice. Our findings formally demonstrate SAMHD1 as an antiretroviral factor in vivo that could promote adaptive immune responses in a sex-dependent manner. The requirement for LPS to unravel the SAMHD1 immunological phenotype suggests that comorbidities associated with a "leaky" gut barrier may influence the antiviral function of SAMHD1 in vivo.

Crosstalk of Microorganisms and Immune Responses in Autoimmune Neuroinflammation: A Focus on Regulatory T Cells

Regulatory T cells (Tregs) are the major determinant of peripheral immune tolerance. Many Treg subsets have been described, however thymus-derived and peripherally induced Tregs remain the most important subpopulations. In multiple sclerosis, a prototypical autoimmune disorder of the central nervous system, Treg dysfunction is a pathogenic hallmark. In contrast, induction of Treg proliferation and enhancement of their function are central immune evasion mechanisms of infectious pathogens. In accordance, Treg expansion is compartmentalized to tissues with high viral replication and prolonged in chronic infections. In friend retrovirus infection, Treg expansion is mainly based on excessive interleukin-2 production by infected effector T cells. Moreover, pathogens seem also to enhance Treg functions as shown in human immunodeficiency virus infection, where Tregs express higher levels of effector molecules such as cytotoxic T-lymphocyte-associated protein 4, CD39 and cAMP and show increased suppressive capacity. Thus, insights into the molecular mechanisms by which intracellular pathogens alter Treg functions might aid to find new therapeutic approaches to target central nervous system autoimmunity. In this review, we summarize the current knowledge of the role of pathogens for Treg function in the context of autoimmune neuroinflammation. We discuss the mechanistic implications for future therapies and provide an outlook for new research directions.

Metabolic requirements of NK cells during the acute response against retroviral infection

Natural killer (NK) cells are important early responders against viral infections. Changes in metabolism are crucial to fuel NK cell responses, and altered metabolism is linked to NK cell dysfunction in obesity and cancer. However, very little is known about the metabolic requirements of NK cells during acute retroviral infection and their importance for antiviral immunity. Here, using the Friend retrovirus mouse model, we show that following infection NK cells increase nutrient uptake, including amino acids and iron, and reprogram their metabolic machinery by increasing glycolysis and mitochondrial metabolism. Specific deletion of the amino acid transporter Slc7a5 has only discrete effects on NK cells, but iron deficiency profoundly impaires NK cell antiviral functions, leading to increased viral loads. Our study thus shows the requirement of nutrients and metabolism for the antiviral activity of NK cells, and has important implications for viral infections associated with altered iron levels such as HIV and SARS-CoV-2.

Metabolic alterations control the fate and function of immune cells in response to infections, but the function of NK cell metabolism in the context of acute viral infections is unclear. Here the authors show that acute NK cell responses to Friend retrovirus involve increased glycolysis and mitochondrial metabolism and require amino acid transport as well as iron sufficiency.

Viral infections in humans and mice with genetic deficiencies of the type I IFN response pathway

Type I IFNs are so-named because they interfere with viral infection in vertebrate cells. The study of cellular responses to type I IFNs led to the discovery of the JAK-STAT signaling pathway, which also governs the response to other cytokine families. We review here the outcome of viral infections in mice and humans with engineered and inborn deficiencies, respectively, of (i) IFNAR1 or IFNAR2, selectively disrupting responses to type I IFNs, (ii) STAT1, STAT2, and IRF9, also impairing cellular responses to type II (for STAT1) and/or III (for STAT1, STAT2, IRF9) IFNs, and (iii) JAK1 and TYK2, also impairing cellular responses to cytokines other than IFNs. A picture is emerging of greater redundancy of human type I IFNs for protective immunity to viruses in natural conditions than was initially anticipated. Mouse type I IFNs are essential for protection against a broad range of viruses in experimental conditions. These findings suggest that various type I IFN-independent mechanisms of human cell-intrinsic immunity to viruses have yet to be discovered.

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.

CD4+CD25+ Cells Are Essential for Maintaining Immune Tolerance in Chickens Inoculated with Bovine Serum Albumin at the Late Stage of Embryonic Development

In this study, the role of chicken CD4⁺CD25⁺ cells during induced immunotolerance was tested. Properties of chicken CD4⁺CD25⁺ cells sorted by flow cytometry were analyzed. Results showed that chicken CD4⁺CD25⁺ cells express IL-10, TGF-β highly and suppress proliferation of CD4⁺CD25⁻ cells in vitro. To induce immunotolerance, embryos were inoculated with bovine serum albumin (BSA) via an intravascular route on embryo incubation day 20 (EID20), and after hatching chicks experienced BSA immunization four times at 7-day intervals. Serum anti-BSA antibodies and CD4⁺CD25⁺ cell ratio was analyzed. Results showed that humoral tolerance was obtained and the CD4⁺CD25⁺ cell percentage in peripheral blood lymphocytes increased along with this progress. Injection of anti-chicken CD25 antibody via an intravascular route on EID16 is applied to block CD4⁺CD25⁺ cells, and the CD4⁺CD25⁺ cell ratio decreased significantly up to 35 d post-hatch. Based on the above, injections of anti-chicken CD25 antibody on EID16 and BSA on EID20 were carried out sequentially, and tolerance level was contrasted to the BSA-injection group. Data revealed the anti-BSA antibodies increased significantly in the CD4⁺CD25⁺ cell-blocked groups indicating that immune tolerance level was weakened. In conclusion, chicken CD4⁺CD25⁺ cells are essential in maintaining induced immune tolerance.

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.

Diverse Immunomodulatory Effects of Individual IFNα Subtypes on Virus-Specific CD8+ T Cell Responses

Clinical administration of Interferon α (IFNα) resulted in limited therapeutic success against some viral infections. Immune modulation of CD8+ T cell responses during IFNα therapy is believed to play a pivotal role in promoting viral clearance. However, these clinical studies primarily focused on IFNα subtype 2. To date, the immunomodulatory roles of the remaining 10-13 IFNα subtypes remains poorly understood, thereby precluding assessments of their potential for more effective treatments. Here, we report that virus-specific CD8+ T cell responses were influenced to various extents by individual IFNα subtypes. IFNα4, 6, and 9 had the strongest effects on CD8+ T cells, including antiproliferative effects, improved cytokine production and cytotoxicity. Interestingly, augmented cytokine responses were dependent on IFNα subtype stimulation of dendritic cells (DCs), while antiproliferative effects and cytotoxicity were mediated by IFNAR signaling in either CD8+ T cells or DCs. Thus, precise modulation of virus-specific CD8+ T cell responses may be feasible for specific antiviral immunotherapies through careful selection and administration of individual IFNα subtypes.

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.

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.

The Cytotoxic Activity of Natural Killer Cells Is Suppressed by IL-10+ Regulatory T Cells During Acute Retroviral Infection

Natural killer (NK) cells play a key role in host defense against cancer and viral infections. It was shown that NK cells are important for the control of acute retroviral infections, but their antiviral activity depends on multiple parameters such as viral inoculation dose, interactions with myeloid cell types and the cytokine milieu. In addition, during an ongoing retroviral infection regulatory T cells (Tregs) can suppress NK cell functions. However, the precise role of Tregs on the initial NK cell response and their immediate antiviral activity after an acute retroviral infection is still unknown. Here we show that thymus-derived Tregs suppress the proliferation, effector functions and cytotoxicity of NK cells very early during acute Friend Retrovirus (FV) infection. Tregs exhibited an activated phenotype and increased the production of the immunosuppressive cytokines IL-10 and TGF-β after FV infection of mice. Neutralization of the immunosuppressive cytokine IL-10 resulted in a significant augmentation of NK cell functions. Although the activation of dendritic cells (DCs) and macrophages as well as the IL-15 cytokine levels were increased after Treg depletion, Tregs mainly affect the NK cell activity in an IL-10-regulated pathway. In this study we demonstrate an IL-10-dependent suppression of NK cells by activated Tregs during the first days of a retroviral infection.

LSECs express functional NOD1 receptors: A role for NOD1 in LSEC maturation-induced T cell immunity in vitro

Liver sinusoidal endothelial cells (LSECs) are organ resident APCs capable of antigen presentation and subsequent tolerization of T cells under physiological conditions. In this study, we investigated whether LSEC pretreatment with NOD-like receptor (NLR) agonists can switch the cells from a tolerogenic to an immunogenic state and promote the development of T cell immunity. LSECs constitutively express NOD1, NOD2 and RIPK2. Stimulation of LSECs with DAP induced the activation of NF-κB and MAP kinases and upregulated the expression of chemokines (CXCL2/9, CCL2/7/8) and cytokines (IFN-γ, TNF-α and IL-2). Pretreatment of LSECs with DAP induced significantly increased IFN-γ and IL-2-production by HBV-stimulated CD8⁺ T cells primed by DAP-treated LSECs. Consistently, a significant reduction in the HBV DNA and HBsAg level occurred in mice receiving T cells primed by DAP-treated LSECs. MDP stimulation had no impact on LSECs or HBV-stimulated CD8⁺ T cells primed with MDP-treated LSECs except for the upregulation of PD-L1. DAP stimulation in vitro could promote LSEC maturation and activate HBV-specific T cell responses. These results are of particular relevance for the regulation of the local innate immune response against HBV infections.

Regulatory T-cell therapy in liver transplantation

Modern immunosuppression drug regimens have produced excellent short-term survival after liver transplantation but it is generally accepted that the side effects of these medications remain a significant contributing factor for less satisfactory long term outcomes. The liver has unique tolerogenic properties as evidenced by the higher rates of operational tolerance seen in liver transplant recipients compared to other solid organ transplants, and therefore, liver transplantation offers an attractive setting in which to study tolerizing therapies. CD4⁺ CD25⁺ FOXP3⁺ regulatory T cells (Tregs) are crucial for maintenance of self-tolerance and prevention of autoimmune disease and are therefore an appealing potential candidate for use as a tolerizing cell therapy. In this review, we summarize the evidence from drug withdrawal trials of spontaneous operational tolerance in liver transplantation, the unique immunology of the hepatic microenvironment, the evidence for the use of CD4⁺ CD25⁺ FOXP3⁺ regulatory T cells as a tolerance inducing therapy in liver transplantation and the challenges in producing clinical grade Treg cell products.

Local Stimulation of Liver Sinusoidal Endothelial Cells with a NOD1 Agonist Activates T Cells and Suppresses Hepatitis B Virus Replication in Mice

Functional maturation of liver sinusoidal endothelial cells (LSECs) induced by a NOD1 ligand (diaminopimelic acid [DAP]) during viral infection has not been well defined. Thus, we investigated the role of DAP-stimulated LSEC maturation during hepatitis B virus (HBV) infection and its potential mechanism in a hydrodynamic injection (HI) mouse model. Primary LSECs were isolated from wild-type C57BL/6 mice and stimulated with DAP in vitro and in vivo and assessed for the expression of surface markers as well as for their ability to promote T cell responses via flow cytometry. The effects of LSEC maturation on HBV replication and expression and the role of LSECs in the regulation of other immune cells were also investigated. Pretreatment of LSECs with DAP induced T cell activation in vitro. HI-administered DAP induced LSEC maturation and subsequently enhanced T cell responses, which was accompanied by an increased production of intrahepatic cytokines, chemokines, and T cell markers in the liver. The HI of DAP significantly reduced the HBsAg and HBV DNA levels in the mice. Importantly, the DAP-induced anti-HBV effect was impaired in the LSEC-depleted mice, which indicated that LSEC activation and T cell recruitment into the liver were essential for the antiviral function mediated by DAP application. Taken together, the results showed that the Ag-presenting ability of LSECs was enhanced by DAP application, which resulted in enhanced T cell responses and inhibited HBV replication in a mouse model.

TLR2 Stimulation Strengthens Intrahepatic Myeloid-Derived Cell-Mediated T Cell Tolerance through Inducing Kupffer Cell Expansion and IL-10 Production

Hepatic APCs play a critical role in promoting immune tolerance in the liver. Recently, we have demonstrated that TLR2 stimulation on liver sinusoidal endothelial cells reverted their suppressive properties to induce T cell immunity. However, there is a paucity of information about how TLR2 stimulation modulates the immunological function of other hepatic APCs. In the current study, we investigated whether TLR2 stimulation influences the function of intrahepatic myeloid-derived cells (iMDCs) and elucidated the mechanisms involved in iMDC-induced T cell immunity. We could show that iMDCs from C57BL/6 mice can potently suppress T cell activation in a cell contact-independent manner. Ag presentation by iMDCs leads to naive CD8 T cell tolerance. To our surprise, instead of inducing cell functional maturation, TLR2 ligand palmitoyl-3-cysteine-serine-lysine-4 (P3C) stimulation further strengthens the suppressive and tolerogenic properties of iMDCs. After P3C administration, the population of Kupffer cells (KCs) of iMDCs dramatically increased. Mechanism analysis shows that KCs are essential for the enhanced inhibition of T cell activation by P3C-stimulated iMDCs. The iMDC-mediated CD8 T cell inhibition was mediated by soluble mediators, one of which was IL-10 secreted by KCs after P3C stimulation. IL-10 blockade could partially abolish iMDC-mediated T cell inhibition. Moreover, hepatitis B virus particle stimulation on iMDCs could also induce IL-10 production by the cells in a TLR2-dependent way. Our results have implications for our understanding of liver-specific tolerance and for the development of strategies to overcome T cell tolerance in situations such as chronic viral liver infections.

Regulatory T cells in retroviral infections

Tight regulation of immune responses is not only critical for preventing autoimmune diseases but also for preventing immunopathological damage during infections in which overactive immune responses may be more harmful for the host than the pathogen itself. Regulatory T cells (Tregs) play a critical role in this regulation, which was discovered using the Friend retrovirus (FV) mouse model. Subsequent FV studies revealed basic biological information about Tregs, including their suppressive activity on effector cells as well as the molecular mechanisms of virus-induced Treg expansion. Treg suppression not only limits immunopathology but also prevents complete elimination of pathogens contributing to chronic infections. Therefore, Tregs play a complex role in the pathogenesis of persistent retroviral infections. New therapeutic concepts to reactivate effector T-cell responses in chronic viral infections by manipulating Tregs also came from work with the FV model. This knowledge initiated many studies to characterize the role of Tregs in HIV pathogenesis in humans, where a complex picture is emerging. On one hand, Tregs suppress HIV-specific effector T-cell responses and are themselves targets of infection, but on the other hand, Tregs suppress HIV-induced immune hyperactivation and thus slow the infection of conventional CD4⁺ T cells and limit immunopathology. In this review, the basic findings from the FV mouse model are put into perspective with clinical and basic research from HIV studies. In addition, the few Treg studies performed in the simian immunodeficiency virus (SIV) monkey model will also be discussed. The review provides a comprehensive picture of the diverse role of Tregs in different retroviral infections and possible therapeutic approaches to treat retroviral chronicity and pathogenesis by manipulating Treg responses.

Regulatory T cells (Tregs) play a very complex role in retroviral infections, and the balance of beneficial versus detrimental effects from Tregs can change between the acute and chronic phase of infection. Therefore, the development of therapeutics to treat chronic retroviral infections via modulation of Tregs requires detailed information regarding both the positive and negative contributions of Tregs in a particular phase of a specific infection. Here, we review the molecular mechanisms that initiate and control Treg responses in retroviral infections as well as the target cells that are functionally manipulated by Tregs. Basic findings from the Friend retrovirus mouse model that initiated this area of research are put into perspective with clinical and basic research from HIV studies. The targeted manipulation of Treg responses holds a bright future for enhancing immune responses to infections, vaccine responses, and for cure or functional cure of chronic retroviral infections.

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.

Immunodominance of Adenovirus-Derived CD8+ T Cell Epitopes Interferes with the Induction of Transgene-Specific Immunity in Adenovirus-Based Immunization

Adenovirus (Ad)-based immunization is a popular approach in vaccine development, and Ad-based vectors are renowned for their potential to induce strong CD8⁺ T cell responses to the encoded transgene. Surprisingly, we previously found in the mouse Friend retrovirus (FV) model that Ad-based immunization did not induce CD8⁺ T cell responses to the FV Leader-Gag-derived immunodominant epitope GagL_85-93 We show now that induction of GagL_85-93-specific CD8⁺ T cells was highly effective when leader-Gag was delivered by plasmid DNA immunization, implying a role for Ad-derived epitopes in mediating unresponsiveness. By immunizing with DNA constructs encoding strings of GagL_85-93 and the two Ad-derived epitopes DNA-binding protein_418-426 (DBP_418-426) and hexon_486-494, we confirmed that Ad epitopes prevent induction of GagL_85-93-specific CD8⁺ T cells. Interestingly, while DBP_418-426 did not interfere with GagL_85-93-specific CD8⁺ T cell induction, the H-2D^d-restricted hexon_486-494 suppressed the CD8⁺ T cell response to the H-2D^b-restricted GagL_85-93 strongly in H-2^b/d mice but not in H-2^b/b mice. This finding indicates that competition occurs at the level of responding CD8⁺ T cells, and we could indeed demonstrate that coimmunization with an interleukin 2 (IL-2)-encoding plasmid restored GagL_85-93-specific CD8⁺ T cell responses to epitope strings in the presence of hexon_486-494 IL-2 codelivery did not restore GagL_85-93 responsiveness in Ad-based immunization, however, likely due to the presence of further epitopes in the Ad vector. Our findings show that seemingly immunodominant transgene epitopes can be dominated by Ad-derived epitopes. These findings underline the importance of thorough characterization of vaccine vectors, and modifications of vectors or immunogens may be required to prevent impaired transgene-specific immune responses.IMPORTANCE Ad-based vectors are widely used in experimental preclinical and clinical immunization studies against numerous infectious agents, such as human immunodeficiency virus, Ebola virus, Plasmodium falciparum, or Mycobacterium tuberculosis Preexisting immunity to Ad-based vectors is widely recognized as a hindrance to the widespread use of Ad-based vectors for immunizations in humans; however, our data show that an immune response to Ad-derived T cell epitopes can also result in loss or impairment of transgene-specific immune responses in prenaive vaccinees due to immune competition. Our results highlight that seemingly immunodominant epitopes may be affected by dominance of vector-derived epitopes, and modifications of the vector design or the immunogens employed in immunization may lead to more effective vaccines.

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.

Interference of retroviral envelope with vaccine-induced CD8+ T cell responses is relieved by co-administration of cytokine-encoding vectors

Background

Retroviral envelope (Env) proteins are known to exhibit immunosuppressive properties, which become apparent not only in retroviral infections, but also in gene-based immunizations using retroviral immunogens, where envelope interferes with the induction of CD8⁺ T cell responses towards another, simultaneously or subsequently delivered, immunogen.

Results

In the Friend retrovirus mouse model, immunization with a plasmid encoding the Friend murine leukemia virus (F-MuLV) Leader-Gag protein resulted in induction of a strong GagL_85–93-specific CD8⁺ T cell response, while the response was completely abrogated by co-immunization with an F-MuLV Env-encoding plasmid. In order to overcome this interference of retroviral envelope, we employed plasmids encoding the cytokines interleukin (IL) 1β, IL2, IL12, IL15, IL21, IL28A or granulocyte–macrophage colony-stimulating factor (GM-CSF) as genetic adjuvants. Co-application of plasmids encoding IL2, IL12, IL21, IL28A and especially GM-CSF rescued the induction of GagL_85–93-specific CD8⁺ T cells in mice vaccinated with FV Leader-Gag and Env. Mice that were immunized with plasmids encoding Leader-Gag and Env and the cytokines IL1β, IL12, IL15, IL28A or GM-CSF, but not Leader-Gag and Env without any cytokine, showed significantly reduced viral loads upon a high-dose Friend virus challenge infection.

Conclusions

Our data demonstrate the potency of cytokine-encoding vectors as adjuvants and immune modulators in composite vaccines for anti-retroviral immunization.

Electronic supplementary material

The online version of this article (doi:10.1186/s12977-017-0352-7) contains supplementary material, which is available to authorized users.

Combination of nanoparticle-based therapeutic vaccination and transient ablation of regulatory T cells enhances anti-viral immunity during chronic retroviral infection

Background

Regulatory T cells (Tregs) have been shown to limit anti-viral immunity during chronic retroviral infection and to restrict vaccine-induced T cell responses. The objective of the study was to assess whether a combinational therapy of nanoparticle-based therapeutic vaccination and concomitant transient ablation of Tregs augments anti-viral immunity and improves virus control in chronically retrovirus-infected mice. Therefore, chronically Friend retrovirus (FV)-infected mice were immunized with calcium phosphate (CaP) nanoparticles functionalized with TLR9 ligand CpG and CD8⁺ or CD4⁺ T cell epitope peptides (GagL_85–93 or Env gp70_123–141) of FV. In addition, Tregs were ablated during the immunization process. Reactivation of CD4⁺ and CD8⁺ effector T cells was analysed and the viral loads were determined.

Results

Therapeutic vaccination of chronically FV-infected mice with functionalized CaP nanoparticles transiently reactivated cytotoxic CD8⁺ T cells and significantly reduced the viral loads. Transient ablation of Tregs during nanoparticle-based therapeutic vaccination strongly enhanced anti-viral immunity and further decreased viral burden.

Conclusion

Our data illustrate a crucial role for CD4⁺ Foxp3⁺ Tregs in the suppression of anti-viral T cell responses during therapeutic vaccination against chronic retroviral infection. Thus, the combination of transient Treg ablation and therapeutic nanoparticle-based vaccination confers robust and sustained anti-viral immunity.

Electronic supplementary material

The online version of this article (doi:10.1186/s12977-016-0258-9) contains supplementary material, which is available to authorized users.

Characterization of the Treg Response in the Hepatitis B Virus Hydrodynamic Injection Mouse Model

Regulatory T cells (Tregs) play an important role in counter-regulating effector T cell responses in many infectious diseases. However, they can also contribute to the development of T cell dysfunction and pathogen persistence in chronic infections. Tregs have been reported to suppress virus-specific T cell responses in hepatitis B virus (HBV) infection of human patients as well as in HBV animal models. However, the phenotype and expansion of Tregs has so far only been investigated in other infections, but not in HBV. We therefore performed hydrodynamic injections of HBV plasmids into mice and analyzed the Treg response in the spleen and liver. Absolute Treg numbers significantly increased in the liver but not the spleen after HBV injection. The cells were natural Tregs that surprisingly did not show any activation or proliferation in response to the infection. However, they were able to suppress effector T cell responses, as selective depletion of Tregs significantly increased HBV-specific CD8+ T cell responses and accelerated viral antigen clearance. The data implies that natural Tregs infiltrate the liver in HBV infection without further activation or expansion but are still able to interfere with T cell mediated viral clearance.

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.

Reciprocal relationship of T regulatory cells and monocytic myeloid-derived suppressor cells in LP-BM5 murine retrovirus-induced immunodeficiency

Immunomodulatory cellular subsets, including myeloid-derived suppressor cells (MDSCs) and T regulatory cells (Tregs), contribute to the immunosuppressive tumour microenvironment and are targets of immunotherapy, but their role in retroviral-associated immunosuppression is less well understood. Due to known crosstalk between Tregs and MDSCs in the tumour microenvironment, and also their hypothesized involvement during human immunodeficiency virus/simian immunodeficiency virus infection, studying the interplay between these immune cells during LP-BM5 retrovirus-induced murine AIDS is of interest. IL-10-producing FoxP3+ Tregs expanded after LP-BM5 infection. Following in vivo adoptive transfer of natural Treg (nTreg)-depleted CD4+T-cells, and subsequent LP-BM5 retroviral infection, enriched monocytic MDSCs (M-MDSCs) from these nTreg-depleted mice displayed altered phenotypic subsets. In addition, M-MDSCs from LP-BM5-infected nTreg-depleted mice exhibited increased suppression of T-cell, but not B-cell, responses, compared with M-MDSCs derived from non-depleted LP-BM5-infected controls. Additionally, LP-BM5-induced M-MDSCs modulated the production of IL-10 by FoxP3+ Tregs in vitro. These collective data highlight in vitro and for the first time, to the best of our knowledge, in vivo reciprocal modulation between retroviral-induced M-MDSCs and Tregs, and may provide insight into the immunotherapeutic targeting of such regulatory cells during retroviral infection.

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.

Dissecting CD8+ NKT Cell Responses to Listeria Infection Reveals a Component of Innate Resistance

A small pool of NK1.1(+) CD8(+) T cells is harbored among the conventional CD8(+) T cell compartment. Conclusions drawn from the analysis of immune responses mediated by cytotoxic CD8(+) T cells are often based on the total population, which includes these contaminating NK1.1(+) CD8(+) T cells. An unresolved question is whether NK1.1(+) CD8(+) cells are conventional T cells that acquire NK1.1 expression upon activation or delineation into memory phenotype or whether they are a distinct cell population that induces immune responses in a different manner than conventional T cells. To address this question, we used the Listeria monocytogenes model of infection and followed CD8(+) NK1.1(+) T cells and NK1.1(-) CD8(+) T cells during each phase of the immune response: innate, effector, and memory. Our central finding is that CD8(+) NK1.1(+) cells and conventional NK1.1(-) CD8(+) T cells both contribute to the adaptive immune response to Listeria, but only CD8(+) NK1.1(+) cells were equipped with the ability to provide a rapid innate immune response, as demonstrated by early and Ag-independent IFN-γ production, granzyme B expression, and degranulation. More importantly, purified conventional CD8(+) T cells alone, in the absence of any contaminating CD8(+) NK1.1(+) cells, were not sufficient to provide early protection to lethally infected mice. These results highlight the role of CD8(+) NK1.1(+) T cells in mounting early innate responses that are important for host defense and support the therapeutic potential of this subset to improve the effectiveness of protective immunity.

TLR ligand induced IL-6 counter-regulates the anti-viral CD8(+) T cell response during an acute retrovirus infection

We have previously shown that Toll-like receptor (TLR) agonists contribute to the control of viral infection by augmenting virus-specific CD8⁺ T-cell responses. It is also well established that signaling by TLRs results in the production of pro-inflammatory cytokines such as interleukin 6 (IL-6). However, how these pro-inflammatory cytokines influence the virus-specific CD8⁺ T-cell response during the TLR agonist stimulation remained largely unknown. Here, we investigated the role of TLR-induced IL-6 in shaping virus-specific CD8⁺ T-cell responses in the Friend retrovirus (FV) mouse model. We show that the TLR agonist induced IL-6 counter-regulates effector CD8⁺ T-cell responses. IL-6 potently inhibited activation and cytokine production of CD8⁺ T cells in vitro. This effect was mediated by a direct stimulation of CD8⁺ T cells by IL-6, which induced upregulation of STAT3 phosphorylation and SOCS3 and downregulated STAT4 phosphorylation and T-bet. Moreover, combining TLR stimulation and IL-6 blockade during an acute FV infection resulted in enhanced virus-specific CD8⁺ T-cell immunity and better control of viral replication. These results have implications for our understanding of the role of TLR induced pro-inflammatory cytokines in regulating effector T cell responses and for the development of therapeutic strategies to overcome T cell dysfunction in chronic viral infections.

Friend retrovirus drives cytotoxic effectors through Toll-like receptor 3

Background

Pathogen recognition drives host defense towards viral infections. Specific groups rather than single members of the protein family of pattern recognition receptors (PRRs) such as membrane spanning Toll-like receptors (TLRs) and cytosolic helicases might mediate sensing of replication intermediates of a specific virus species. TLR7 mediates host sensing of retroviruses and could significantly influence retrovirus-specific antibody responses. However, the origin of efficient cell-mediated immunity towards retroviruses is unknown. Double-stranded RNA intermediates produced during retroviral replication are good candidates for immune stimulatory viral products. Thus, we considered TLR3 as primer of cell-mediated immunity against retroviruses in vivo.

Results

Infection of mice deficient in TLR3 (TLR3^−/−) with Friend retrovirus (FV) complex revealed higher viral loads during acute retroviral infection compared to wild type mice. TLR3^−/− mice exhibited significantly lower expression levels of type I interferons (IFNs) and IFN-stimulated genes like Pkr or Ifi44, as well as reduced numbers of activated myeloid dendritic cells (DCs) (CD86⁺ and MHC-II⁺). DCs generated from FV-infected TLR3^−/− mice were less capable of priming virus-specific CD8⁺ T cell proliferation. Moreover, cytotoxicity of natural killer (NK) cells as well as CD8⁺ T cells were reduced in vitro and in vivo, respectively, in FV-infected TLR3^-/- mice.

Conclusions

TLR3 mediates antiretroviral cytotoxic NK cell and CD8⁺ T cell activity in vivo. Our findings qualify TLR3 as target of immune therapy against retroviral infections.

Expanded regulatory T cells in chronically friend retrovirus-infected mice suppress immunity to a murine cytomegalovirus superinfection

It is still unclear whether expanded and activated regulatory T cells (Tregs) in chronic viral infections can influence primary immune responses against superinfections with unrelated viruses. Expanded Tregs found in the spleens of chronically Friend virus (FV)-infected mice decreased murine cytomegalovirus (mCMV)-specific CD8(+) T cell responses during acute mCMV superinfection. This suppression of mCMV-specific T cell immunity was found only in organs with FV-induced Treg expansion. Surprisingly, acute mCMV infection itself did not expand or activate Tregs.

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.

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.

Infection of adult thymus with murine retrovirus induces virus-specific central tolerance that prevents functional memory CD8+ T cell differentiation

In chronic viral infections, persistent antigen presentation causes progressive exhaustion of virus-specific CD8+ T cells. It has become clear, however, that virus-specific naïve CD8+ T cells newly generated from the thymus can be primed with persisting antigens. In the setting of low antigen density and resolved inflammation, newly primed CD8+ T cells are preferentially recruited into the functional memory pool. Thus, continual recruitment of naïve CD8+ T cells from the thymus is important for preserving the population of functional memory CD8+ T cells in chronically infected animals. Friend virus (FV) is the pathogenic murine retrovirus that establishes chronic infection in adult mice, which is bolstered by the profound exhaustion of virus-specific CD8+ T cells induced during the early phase of infection. Here we show an additional evasion strategy in which FV disseminates efficiently into the thymus, ultimately leading to clonal deletion of thymocytes that are reactive to FV antigens. Owing to the resultant lack of virus-specific recent thymic emigrants, along with the above exhaustion of antigen-experienced peripheral CD8+ T cells, mice chronically infected with FV fail to establish a functional virus-specific CD8+ T cell pool, and are highly susceptible to challenge with tumor cells expressing FV-encoded antigen. However, FV-specific naïve CD8+ T cells generated in uninfected mice can be primed and differentiate into functional memory CD8+ T cells upon their transfer into chronically infected animals. These findings indicate that virus-induced central tolerance that develops during the chronic phase of infection accelerates the accumulation of dysfunctional memory CD8+ T cells.

The phenotype and activation status of regulatory T cells during Friend retrovirus infection

The suppressive capacity of regulatory T cells (Tregs) has been extensively studied and is well established for many diseases. The expansion, accumulation, and activation of Tregs in viral infections are of major interest in order to find ways to alter Treg functions for therapeutic benefit. Tregs are able to dampen effector T cell responses to viral infections and thereby contribute to the establishment of a chronic infection. In the Friend retrovirus (FV) mouse model, Tregs are known to expand in all infected organs. To better understand the characteristics of these Treg populations, their phenotype was analyzed in detail. During acute FV-infection, Tregs became activated in the spleen and bone marrow, as indicated by various T cell activation markers, such as CD43 and CD103. Interestingly, Tregs in the bone marrow, which contains the highest viral loads during acute infection, displayed greater levels of activation than Tregs from the spleen. Treg expansion was driven by proliferation but no FV-specific Tregs could be detected. Activated Tregs in FV-infection did not produce Granzyme B (GzmB) or tumor necrosis factor α (TNFα), which are thought to be a potential mechanism for their suppressive activity. Furthermore, Tregs expressed inhibitory markers, such as TIM3, PD-1 and PD-L1. Blocking TIM3 and PD-L1 with antibodies during chronic FV-infection increased the numbers of activated Tregs. These data may have important implications for the understanding of Treg functions during chronic viral infections.

Combining regulatory T cell depletion and inhibitory receptor blockade improves reactivation of exhausted virus-specific CD8+ T cells and efficiently reduces chronic retroviral loads

Chronic infections with human viruses, such as HIV and HCV, or mouse viruses, such as LCMV or Friend Virus (FV), result in functional exhaustion of CD8⁺ T cells. Two main mechanisms have been described that mediate this exhaustion: expression of inhibitory receptors on CD8⁺ T cells and expansion of regulatory T cells (Tregs) that suppress CD8⁺ T cell activity. Several studies show that blockage of one of these pathways results in reactivation of CD8⁺ T cells and partial reduction in chronic viral loads. Using blocking antibodies against PD-1 ligand and Tim-3 and transgenic mice in which Tregs can be selectively ablated, we compared these two treatment strategies and combined them for the first time in a model of chronic retrovirus infection. Blocking inhibitory receptors was more efficient than transient depletion of Tregs in reactivating exhausted CD8⁺ T cells and reducing viral set points. However, a combination therapy was superior to any single treatment and further augmented CD8⁺ T cell responses and resulted in a sustained reduction in chronic viral loads. These results demonstrate that Tregs and inhibitory receptors are non-overlapping factors in the maintenance of chronic viral infections and that immunotherapies targeting both pathways may be a promising strategy to treat chronic infectious diseases.

A loss of function, the so-called ‘exhaustion’ of CD8⁺ T cells, is a hallmark of many chronic infections. The T cell exhaustion is mediated by two main mechanisms, the expression of inhibitory receptors on CD8⁺ T cells and virus-induced expansion of regulatory T cells (Tregs), which suppress CD8⁺ T cell activity. Several mouse studies revealed a reactivation of CD8⁺ T cells and reduction in chronic viral loads after blockage of one of these pathways. These results initiated a number of clinical studies mainly with cancer patients, in which blocking antibodies were used to interfere with inhibitory receptor signaling or drugs that deplete Tregs. For the first time we combined the two therapeutic approaches by using transgenic mice in which Tregs can be selectively ablated and injection of blocking antibodies in a chronic retroviral infection. The results indicate that the combination therapy was superior to any single treatment in further augmenting CD8⁺ T cell responses and reducing chronic viral loads. Our findings demonstrate that Tregs and inhibitory receptors are non-overlapping factors in the maintenance of chronic viral infections and that immunotherapies targeting both pathways may be a promising new strategy to treat chronic infectious diseases.

TLR1/2 ligand-stimulated mouse liver endothelial cells secrete IL-12 and trigger CD8+ T cell immunity in vitro

Liver sinusoidal endothelial cells (LSECs) are unique organ-resident APCs capable of Ag cross-presentation and subsequent tolerization of naive CD8(+) T cells. Under certain conditions, LSECs can switch from a tolerogenic to an immunogenic state and promote the development of T cell immunity. However, little is known about the mechanisms of LSECs to induce T cell immunity. In this study, we investigated whether functional maturation of LSECs can be achieved by TLR ligand stimulation and elucidated the mechanisms involved in LSEC-induced T cell immunity. We demonstrate that pretreatment of LSECs with palmitoyl-3-cysteine-serine-lysine-4 (P3C; TLR1/2 ligand) but not poly(I:C) (TLR3 ligand) or LPS (TLR4 ligand) reverted their suppressive properties to induce T cell immunity. Importantly, P3C stimulation caused functional maturation of Ag-presenting LSECs and enabled them to activate virus-specific CD8(+) T cells. The LSEC-mediated CD8(+) T cell immunity was initiated by soluble mediators, one of which was IL-12 secreted at a low but sustained level after P3C stimulation. P3C stimulation did not induce programmed death ligand 1 expression on LSECs, thereby favoring T cell proliferation and activation instead of suppression. Our data suggest that LSECs undergo maturation exclusively in response to TLR1/2 ligand stimulation and that the immunological status of LSECs was dependent upon the balance between programmed death ligand 1 and IL-12 expression. These results have implications for our understanding of liver-specific tolerance and autoimmunity and for the development of strategies to overcome T cell tolerance in situations such as chronic viral liver infections or liver cancer.

Natural regulatory T cells inhibit production of cytotoxic molecules in CD8⁺ T cells during low-level Friend retrovirus infection

Background

Cytotoxic T cells (CTL) play a central role in the control of viral infections. Their antiviral activity can be mediated by at least two cytotoxic pathways, namely the granule exocytosis pathway, involving perforin and granzymes, and the Fas-FasL pathway. It was shown that the level of Friend retrovirus (FV) replication determines the cytotoxic pathway for the control of viral infection. In low-level infection only the Fas pathway is active, whereas cytotoxic molecules are not produced. In the current study, we elucidate the role of CD4⁺ regulatory T cells (Tregs) in suppressing the exocytosis pathway during an asymptomatic low-level infection.

Findings

We show that even a low-level retrovirus infection induced a strong activation and proliferation of natural Tregs. The expanded Tregs suppressed the proliferation of virus-specific CD8⁺ T cells and the production of cytotoxic molecules by these cells. Not surprisingly, the in vivo killing activity of these CD8⁺ T cells was rather weak. Selective depletion of Foxp3⁺ Tregs resulted in de novo granzyme production and augmented virus-specific in vivo killing, but did not affect the low-level virus replication.

Conclusions

Expanded natural Tregs determined the cytotoxic pathways of virus-specific effector CD8⁺ T cells during the acute phase of retroviral infection.

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.

Targeting the B7 family of co-stimulatory molecules: successes and challenges

As more patient data is cross-referenced with animal models of disease, the primary focus on T(h)1 autoreactive effector cell function in autoimmune diseases, such as rheumatoid arthritis and multiple sclerosis, has shifted towards the role of T(h)17 autoreactive effector cells and the ability of regulatory T cells (T(reg)) to modulate the pro-inflammatory autoimmune response. Therefore, the currently favored hypothesis is that a delicate balance between T(h)1/17 effector cells and T(reg) cell function is critical in the regulation of inflammatory autoimmune disease. An intensive area of research with regard to the T(h)1/17:T(reg) cell balance is the utilization of blockade and/or ligation of various co-stimulatory or co-inhibitory molecules, respectively, during ongoing disease to skew the immune response toward a more tolerogenic/regulatory state. Currently, FDA-approved therapies for multiple sclerosis patients are all aimed at the suppression of immune cell function. The other favored method of treatment is a modulation or deletion of autoreactive immune cells via short-term blockade of activating co-stimulatory receptors via treatment with fusion proteins such as CTLA4-Ig and CTLA4-FasL. Based on the initial success of CTLA4-Ig, there are additional fusion proteins that are currently under development. Examples of the more recently identified B7/CD28 family members are PD-L1, PD-L2, inducible co-stimulatory molecule-ligand (ICOS-L), B7-H3, and B7-H4, all of which may emerge as potential fusion protein therapeutics, each with unique, yet often overlapping functions. The expression of both stimulatory and inhibitory B7 molecules seems to play an essential role in modulating immune cell function through a variety of mechanisms, which is supported by findings that suggest each B7 molecule has developed its own indispensable niche in the immune system. As more data are generated, the diagnostic and therapeutic potential of the above B7 family-member-derived fusion proteins becomes ever more apparent. Besides defining the biology of these B7/CD28 family members in vivo, additional difficulty in the development of these therapies lies in maintaining the normal immune functions of recognition and reaction to non-self-antigens following viral or bacterial infection in the patient. Further complicating the clinical translation of these therapies, the mechanism of action identified for a particular reagent may depend upon the method of immune-cell activation and the subset of immune cells targeted in the study.

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.

IL-2-independent and TNF-α-dependent expansion of Vβ5+ natural regulatory T cells during retrovirus infection

Friend virus infection of mice induces the expansion and activation of regulatory T cells (Tregs) that dampen acute immune responses and promote the establishment and maintenance of chronic infection. Adoptive transfer experiments and the expression of neuropilin-1 indicate that these cells are predominantly natural Tregs rather than virus-specific conventional CD4(+) T cells that converted into induced Tregs. Analysis of Treg TCR Vβ chain usage revealed a broadly distributed polyclonal response with a high proportionate expansion of the Vβ5(+) Treg subset, which is known to be responsive to endogenous retrovirus-encoded superantigens. In contrast to the major population of Tregs, the Vβ5(+) subset expressed markers of terminally differentiated effector cells, and their expansion was associated with the level of the antiviral CD8(+) T cell response rather than the level of Friend virus infection. Surprisingly, the expansion and accumulation of the Vβ5(+) Tregs was IL-2 independent but dependent on TNF-α. These experiments reveal a subset-specific Treg induction by a new pathway.

Host resistance influences patterns of experimental viral adaptation and virulence evolution

Infectious diseases are major threats to all living systems, so understanding the forces of selection that limit the evolution of more virulent pathogens is of fundamental importance; this includes the practical application of identifying possible mitigation strategies for at-risk host populations. The evolution of more virulent pathogens has been classically understood to be limited by the tradeoff between within-host growth rate and transmissibility. Importantly, heterogeneity among hosts can influence both of these factors. However, despite our substantial understanding of how the immune system operates to control pathogen replication during infection, we have only a limited appreciation of how variability in intrinsic (i.e., genetically determined) levels of host resistance influences patterns of pathogen adaptation and virulence evolution. Here, we describe results from experimental evolution studies using a model host-pathogen (virus-mammal) system; we demonstrate that variability in intrinsic levels of resistance among host genotypes can have significant effects on patterns of pathogen adaptation and virulence evolution during serial passage. Both the magnitude of adaptive response as well as the degree of pathogen specialization was positively correlated with host resistance, while mean overall virulence of post-passage virus was negatively correlated with host resistance. These results are consistent with a model whereby resistant host genotypes impose stronger selection on adapting pathogen populations, which in turn leads to the evolution of more specialized pathogen variants whose overall (i.e., mean) virulence across host genotypes is reduced.

In ovo injection of anti-chicken CD25 monoclonal antibodies depletes CD4+CD25+ T cells in chickens

The CD4(+)CD25(+) cells have T regulatory cell properties in chickens. This study investigated the effect of in ovo injection of anti-chicken CD25 monoclonal antibodies (0.5 mg/egg) on CD4(+)CD25(+) cell depletion and on amounts of interleukin-2 mRNA and interferon-γ mRNA in CD4(+)CD25(-) cells posthatch. Anti-chicken CD25 or PBS (control) was injected into 16-d-old embryos. Chicks hatched from eggs injected with anti-chicken CD25 antibodies had a lower CD4(+)CD25(+) cell percentage in the blood until 25 d posthatch. The anti-chicken CD25 antibody injection nearly depleted CD4(+)CD25(+) cells in the blood until 16 d posthatch. At 30 d posthatch, the CD4(+)CD25(+) cell percentage in the anti-CD25-antibody-injected group was comparable with the percentage in the control group. At 16 d posthatch, the anti-chicken CD25 antibody injection decreased CD4(+)CD25(+) cell percentages in the thymus, spleen, and cecal tonsils. Chickens hatched from anti-CD25-antibody-injected eggs had approximately 25% of CD4(+)CD25(+) cells in the cecal tonsils and thymus compared with those in the cecal tonsils and thymus of the control group. The CD4(+)CD25(-) cells from the spleen and cecal tonsils of chicks hatched from anti-chicken-CD25-injected eggs had higher amounts of interferon-γ and interleukin-2 mRNA than CD4(+)CD25(-) cells from the control group. It could be concluded that injecting anti-chicken CD25 antibodies in ovo at 16 d of incubation nearly depleted the CD4(+)CD25(+) cells until 25 d posthatch.

CD4+ T cells develop antiretroviral cytotoxic activity in the absence of regulatory T cells and CD8+ T cells

Conventional CD4(+) T cells play an important role in viral immunity. In most virus infections, they provide essential help for antiviral B and T cell responses. In chronic infections, including HIV infection, an expansion of regulatory T cells (Tregs) has been demonstrated, which can suppress virus-specific CD4(+) T cell responses in vitro. However, the suppressive activity of Tregs on effector CD4(+) T cells in retroviral infection is less well documented in vivo. We took advantage of a transgenic mouse in which Tregs can be selectively depleted to determine the influence of such cells on retrovirus-specific CD4(+) T cell responses during an ongoing infection. Mice were infected with Friend retrovirus (FV), and Tregs were depleted during the acute phase of the infection. In nondepleted mice, activated CD4(+) T cells produced Th1-type cytokines but did not exhibit any antiviral cytotoxicity as determined in a major histocompatibility complex (MHC) class II-restricted in vivo cytotoxic T lymphocyte (CTL) assay. Depletion of Tregs significantly increased the numbers of virus-specific CD4(+) T cells and improved their cytokine production, whereas it induced only very little CD4(+) T cell cytotoxicity. However, after dual depletion of Tregs and CD8(+) T cells, conventional CD4(+) T cells developed significant cytotoxic activity against FV epitope-labeled target cells in vivo and contributed to the control of virus replication. Thus, both Tregs and CD8(+) T cells influence the cytotoxic activity of conventional CD4(+) T cells during an acute retroviral infection.

Control of regulatory T cells is necessary for vaccine-like effects of antiviral immunotherapy by monoclonal antibodies

Regulatory T cells (Tregs) down-regulate immunity and are associated with chronic viral infections, suggesting that their inhibition might be used to treat life-threatening diseases. Using the FrCasE mouse retroviral model, we have recently shown that short mAb-based immunotherapies can induce life-long protective immunity. This finding has a potentially important therapeutical impact because mAbs are increasingly used to treat severe viral infections. We now report that poor anti-FrCasE immunity in infected mice is due to Treg expansion in secondary lymphoid organs because depletion of Tregs restored humoral and cytotoxic T lymphocyte (CTL) antiviral responses. Kinetic analyses show that Treg expansion is not a consequence of chronicity, but rather is associated with viral spread. Moreover, Treg adoptive transfers indicate that production of the immunosuppressive cytokine IL-10 is essential for preventing a protective immune response. Finally, treatment of infected mice with a virus-neutralizing IgG2a shortly after infection prevents Treg expansion and limits immunosuppressive activity. This effect is rapid, necessary for the development of protective immunity, and depends on mAb effector functions. Therefore, manipulating Tregs may be necessary to confer robust antiviral immunity in the context of mAb-based therapy. This concept likely applies to cancer treatment because vaccine-like effects of mAbs have also been observed in certain cancer immunotherapies.