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

Retreatment with HBV siRNA Results in Additional Reduction in HBV Antigenemia and Immune Stimulation in the AAV-HBV Mouse Model

BACKGROUND AND AIMS

Treatment with siRNAs that target HBV has demonstrated robust declines in HBV antigens. This effect is also observed in the AAV-HBV mouse model, which was used to investigate if two cycles of GalNAc-HBV-siRNA treatment could induce deeper declines in HBsAg levels or prevent rebound, and to provide insights into the liver immune microenvironment.

METHODS

C57Bl/6 mice were transduced with one of two different titers of AAV-HBV for 28 days, resulting in stable levels of HBsAg of about 103 or 105 IU/mL. Mice were treated for 12 weeks (four doses q3wk) per cycle with 3 mg/kg of siRNA-targeting HBV or an irrelevant sequence either once (single treatment) or twice (retreatment) with an 8-week treatment pause in between. Blood was collected to evaluate viral parameters. Nine weeks after the last treatment, liver samples were collected to perform phenotyping, bulk RNA-sequencing, and immunohistochemistry.

RESULTS

Independent of HBsAg baseline levels, treatment with HBV-siRNA induced a rapid decline in HBsAg levels, which then plateaued before gradually rebounding 12 weeks after treatment stopped. A second cycle of HBV-siRNA treatment induced a further decline in HBsAg levels in serum and the liver, reaching undetectable levels and preventing rebound when baseline levels were 103 IU/mL. This was accompanied with a significant increase in inflammatory macrophages in the liver and significant upregulation of regulatory T-cells and T-cells expressing immune checkpoint receptors.

CONCLUSIONS

Retreatment induced an additional decline in HBsAg levels, reaching undetectable levels when baseline HBsAg levels were 3log10 or less. This correlated with T-cell activation and upregulation of Trem2.

Evaluation of RNAi therapeutics VIR-2218 and ALN-HBV for chronic hepatitis B: Results from randomized clinical trials

BACKGROUND & AIMS

Current therapy for chronic hepatitis B virus (cHBV) infection involves lifelong treatment. New treatments that enable HBV functional cure would represent a clinically meaningful advance. ALN-HBV and VIR-2218 are investigational RNA interference therapeutics that target all major HBV transcripts.

METHODS

We report on: i) the safety of single doses of VIR-2218 (modified from ALN-HBV by enhanced stabilization chemistry plus technology to reduce off-target, seed-mediated binding while maintaining on-target antiviral activity) and ALN-HBV in humanized mice; ii) a cross-study comparison of the safety of single doses of VIR-2218 and ALN-HBV in healthy human volunteers (n = 24 and n = 49, respectively); and iii) the antiviral activity of two doses of 20, 50, 100, 200 mg of VIR-2218 (total n = 24) vs. placebo (n = 8), given 4 weeks apart, in participants with cHBV infection.

RESULTS

In humanized mice, alanine aminotransferase (ALT) levels were markedly lower following administration of VIR-2218 compared with ALN-HBV. In healthy volunteers, post-treatment ALT elevations occurred in 28% of participants receiving ALN-HBV compared with none in those receiving VIR-2218. In participants with cHBV infection, VIR-2218 was associated with dose-dependent reductions in hepatitis B surface antigen (HBsAg). The greatest mean reduction of HBsAg at Week 20 in participants receiving 200 mg was 1.65 log IU/ml. The HBsAg reduction was maintained at 0.87 log IU/ml at Week 48. No participants had serum HBsAg loss or hepatitis B surface antibody seroconversion.

CONCLUSIONS

VIR-2218 demonstrated an encouraging hepatic safety profile in preclinical and clinical studies as well as dose-dependent HBsAg reductions in patients with cHBV infection. These data support future studies with VIR-2218 as part of combination regimens with a goal of HBV functional cure.

TRIAL REGISTRATION

ClinicalTrials.gov identifiers: NCT02826018 and NCT03672188.

IMPACT AND IMPLICATIONS

A significant unmet need exists for therapies for chronic HBV (cHBV) infection that achieve functional cure. We report clinical and non-clinical data on two investigational small-interfering RNAs that target HBx, ALN-HBV and VIR-2218, demonstrating that incorporation of enhanced stabilization chemistry plus technology in VIR-2218 reduces its propensity to cause ALT elevations relative to its parent compound, ALN-HBV. We also show that VIR-2218 reduces hepatitis B surface antigen levels in a dose-dependent manner in participants with cHBV infection. These studies support the continued development of VIR-2218 as part of therapeutic regimens for cHBV infection, with the goal of a functional cure, and are important for HBV researchers and physicians.

Clonal lineage tracing reveals mechanisms skewing CD8+ T cell fate decisions in chronic infection

Although recent evidence demonstrates heterogeneity among CD8+ T cells during chronic infection, developmental relationships and mechanisms underlying their fate decisions remain incompletely understood. Using single-cell RNA and TCR sequencing, we traced the clonal expansion and differentiation of CD8+ T cells during chronic LCMV infection. We identified immense clonal and phenotypic diversity, including a subset termed intermediate cells. Trajectory analyses and infection models showed intermediate cells arise from progenitor cells before bifurcating into terminal effector and exhausted subsets. Genetic ablation experiments identified that type I IFN drives exhaustion through an IRF7-dependent mechanism, possibly through an IFN-stimulated subset bridging progenitor and exhausted cells. Conversely, Zeb2 was critical for generating effector cells. Intriguingly, some T cell clones exhibited lineage bias. Mechanistically, we identified that TCR avidity correlates with an exhausted fate, whereas SHP-1 selectively restricts low-avidity effector cell accumulation. Thus, our work elucidates novel mechanisms underlying CD8+ T cell fate determination during persistent infection and suggests two potential pathways leading to exhaustion.

T-cell exhaustion in immune-mediated inflammatory diseases: New implications for immunotherapy

Immune-mediated inflammatory diseases(IMIDs) are referred to as highly disabling chronic diseases affecting different organs and systems. Inappropriate or excessive immune responses with chronic inflammation are typical manifestations. Usually in patients with chronic infection and cancer, due to long-term exposure to persistent antigens and inflammation microenvironment, T-cells are continuously stimulated and gradually differentiate into an exhausted state. Exhausted T-cells gradually lose effector function and characteristics of memory T-cells. However, existing studies have found that exhausted T-cells are not only present in the infection and tumor environment, but also in autoimmunity, and are associated with better prognosis of IMIDs. This suggests new prospects for the application of this reversible process of T-cell exhaustion in the treatment of IMID. This review will focus on the research progress of T-cell exhaustion in several IMIDs and its potential application for diagnosis and treatment in IMIDs.

Knowns and Unknowns about CAR-T Cell Dysfunction

Simple Summary

The primary issue of adoptive cell therapy is the poor in vivo persistence. In this context, it is necessary to clarify the fundamental mechanisms of T cell dysfunction. Here we review common dysfunctional states, including exhaustion and senescence, and discuss the challenges associated with phenotypical characterization of these T cell subsets. We overview the heterogeneity among exhausted T cells as well as mechanisms by which T cells get reinvigorated by checkpoint inhibitors. We emphasize that some cancers not responding to such treatment may activate distinct T cell dysfunction programs. Finally, we describe the dysfunction-promoting mechanisms specific for CAR-T cells and the ways to mitigate them.

Abstract

Immunotherapy using chimeric antigen receptor (CAR) T cells is a promising option for cancer treatment. However, T cells and CAR-T cells frequently become dysfunctional in cancer, where numerous evasion mechanisms impair antitumor immunity. Cancer frequently exploits intrinsic T cell dysfunction mechanisms that evolved for the purpose of defending against autoimmunity. T cell exhaustion is the most studied type of T cell dysfunction. It is characterized by impaired proliferation and cytokine secretion and is often misdefined solely by the expression of the inhibitory receptors. Another type of dysfunction is T cell senescence, which occurs when T cells permanently arrest their cell cycle and proliferation while retaining cytotoxic capability. The first section of this review provides a broad overview of T cell dysfunctional states, including exhaustion and senescence; the second section is focused on the impact of T cell dysfunction on the CAR-T therapeutic potential. Finally, we discuss the recent efforts to mitigate CAR-T cell exhaustion, with an emphasis on epigenetic and transcriptional modulation.

Schrödinger's T Cells: Molecular Insights Into Stemness and Exhaustion

T cell stemness and exhaustion coexist as two key contrasting phenomena during chronic antigen stimulation, such as infection, transplant, cancer, and autoimmunity. T cell exhaustion refers to the progressive loss of effector function caused by chronic antigen exposure. Exhausted T (T_EX) cells highly express multiple inhibitory receptors and exhibit severe defects in cell proliferation and cytokine production. The term T cell stemness describes the stem cell-like behaviors of T cells, including self-renewal, multipotency, and functional persistence. It is well accepted that naïve and some memory T cell subsets have stem cell-like properties. When investigating the exhaustive differentiation of T cells in chronic infection and cancer, recent studies highlighted the stemness of "precursors of exhausted" T (T_PEX) cells prior to their terminal differentiation to T_EX cells. Clinically successful checkpoint blockades for cancer treatment appear to invigorate antitumor T_PEX cells but not T_EX cells. Here we discuss the transcriptional and epigenetic regulations of T cell stemness and exhaustion, with a focus on how systems immunology was and will be utilized to define the molecular basis underlying the transition of T_PEX to T_EX cells. We suggest a "stepwise model" of T cell stemness and exhaustion, in which loss of stemness and exhaustion progression are gradual multi-step processes. We provide perspectives on the research needed to define T cell stemness and exhaustion in the transplantation setting, in which allogenic T cells are also chronically exposed to alloantigens. A better understanding of T cell stemness and exhaustion will shed light on developing novel strategies for immunotherapies.

Non-neutralizing antibodies protect against chronic LCMV infection by promoting infection of inflammatory monocytes in mice

Antibodies play an important role in host defense against microorganisms. Besides direct microbicidal activities, antibodies can also provide indirect protection via crosstalk to constituents of the adaptive immune system. Similar to many human chronic viral infections, persistence of Lymphocytic choriomeningitis virus (LCMV) is associated with compromised T- and B-cell responses. The administration of virus-specific non-neutralizing antibodies (nnAbs) prior to LCMV infection protects against the establishment of chronic infection. Here, we show that LCMV-specific nnAbs bind preferentially Ly6Chi inflammatory monocytes (IMs), promote their infection in an Fc-receptor independent way, and support acquisition of APC properties. By constituting additional T-cell priming opportunities, IMs promote early activation of virus-specific CD8 T cells, eventually tipping the balance between T-cell exhaustion and effector cell differentiation, preventing establishment of viral persistence without causing lethal immunopathology. These results document a beneficial role of IMs in avoiding T-cell exhaustion and an Fc-receptor independent protective mechanism provided by LCMV-specific nnAbs against the establishment of chronic infection.

Dynamic adoption of anergy by antigen-exhausted CD4+ T cells

Exhausted immune responses to chronic diseases represent a major challenge to global health. We study CD4⁺ T cells in a mouse model with regulatable antigen presentation. When the cells are driven through the effector phase and are then exposed to different levels of persistent antigen, they lose their T helper 1 (Th1) functions, upregulate exhaustion markers, resemble naturally anergic cells, and modulate their MAPK, mTORC1, and Ca²⁺/calcineurin signaling pathways with increasing dose and time. They also become unable to help B cells and, at the highest dose, undergo apoptosis. Transcriptomic analyses show the dynamic adjustment of gene expression and the accumulation of T cell receptor (TCR) signals over a period of weeks. Upon antigen removal, the cells recover their functionality while losing exhaustion and anergy markers. Our data suggest an adjustable response of CD4⁺ T cells to different levels of persisting antigen and contribute to a better understanding of chronic disease.

Immunogenicity and protective efficacy induced by an mRNA vaccine encoding gD antigen against pseudorabies virus infection

Messenger RNA-based vaccines represent new tools with prophylactic and therapeutic potential characterized by high flexibility of application for infectious diseases. Pseudorabies virus (PRV) is one of the major viruses affecting the pig industry. PRV has serious effects in piglets, sows, and growing-fattening pigs and can lead to huge economic losses. In this study, an envelope glycoprotein D (gD) gene-based specific mRNA vaccine was generated, and a mouse model was used to investigate the protective efficacy of the vaccine. The gD mRNA vaccine and the recombinant plasmid pVAX-gD were transfected into BHK21 cells, and the antigenicity of the expressed proteins was detected by Western blot analysis. Groups of mice were vaccinated with the gD mRNA vaccine, pVAX-gD, and PBS. T cell immune responses were measured by flow cytometry or ELISA and serum neutralization tests every two weeks. The challenge with the PRV-XJ strain was performed eight weeks after the primary immunization, and the response was monitored for 15 days. The levels of specific and neutralizing antibodies in the gD mRNA vaccine group were significantly increased in 8 weeks compared to those in the control group, and cytokine levels, including that of IFN-γ/IL-2, were considerably higher than those in the control animal. Additionally, the proportion of CD4+/CD8+ cells in peripheral lymphocytes was remarkably increased. Our data demonstrate that mRNA is a promising and effective tool for the development of vaccines. The PRV-gD-based mRNA vaccine can elicit an efficient neutralizing antibody response and induce effective protection in mice in defense against PRV infection.

Type I interferon signaling in fibroblastic reticular cells prevents exhaustive activation of antiviral CD8+ T cells

Fibroblastic reticular cells (FRCs) are stromal cells that actively promote the induction of immune responses by coordinating the interaction of innate and adaptive immune cells. However, whether and to which extent immune cell activation is determined by lymph node FRC reprogramming during acute viral infection has remained unexplored. Here, we genetically ablated expression of the type I interferon-α receptor (Ifnar) in Ccl19-Cre⁺ cells and found that sensing of type I interferon imprints an antiviral state in FRCs and thereby preserves myeloid cell composition in lymph nodes of naive mice. During localized lymphocytic choriomeningitis virus infection, IFNAR signaling precipitated profound phenotypic adaptation of all FRC subsets enhancing antigen presentation, chemokine-driven immune cell recruitment, and immune regulation. The IFNAR-dependent shift of all FRC subsets toward an immunostimulatory state reduced exhaustive CD8⁺ T cell activation. In sum, these results unveil intricate circuits underlying type I IFN sensing in lymph node FRCs that enable protective antiviral immunity.

Risk Factors for Acute Rejection in the First Year after Lung Transplant. A Multicenter Study

Rationale: Acute rejection, manifesting as lymphocytic inflammation in a perivascular (acute perivascular rejection [AR]) or peribronchiolar (lymphocytic bronchiolitis [LB]) distribution, is common in lung transplant recipients and increases the risk for chronic graft dysfunction.Objectives: To evaluate clinical factors associated with biopsy-proven acute rejection during the first post-transplant year in a present-day, five-center lung transplant cohort.Methods: We analyzed prospective diagnoses of AR and LB from over 2,000 lung biopsies in 400 newly transplanted adult lung recipients. Because LB without simultaneous AR was rare, our analyses focused on risk factors for AR. Multivariable Cox proportional hazards models were used to assess donor and recipient factors associated with the time to the first AR occurrence.Measurements and Main Results: During the first post-transplant year, 53.3% of patients experienced at least one AR episode. Multivariable proportional hazards analyses accounting for enrolling center effects identified four or more HLA mismatches (hazard ratio [HR], 2.06; P ≤ 0.01) as associated with increased AR hazards, whereas bilateral transplantation (HR, 0.57; P ≤ 0.01) was associated with protection from AR. In addition, Wilcoxon rank-sum analyses demonstrated bilateral (vs. single) lung recipients, and those with fewer than four (vs. more than four) HLA mismatches demonstrated reduced AR frequency and/or severity during the first post-transplant year.Conclusions: We found a high incidence of AR in a contemporary multicenter lung transplant cohort undergoing consistent biopsy sampling. Although not previously recognized, the finding of reduced AR in bilateral lung recipients is intriguing, warranting replication and mechanistic exploration.

NK cells contribute to hepatic CD8+ T cell failure in hepatitis B virus-carrier mice after alcohol consumption

Despite the fact that both Hepatitis B virus (HBV) infection and excessive alcohol consumption represent health problems worldwide, the mechanism by which alcohol affected the progression of HBV-associated liver disease are not completely understood. Therefore, we studied how alcohol affects the development of HBV infection and the role of T cells and NK cells in the antiviral response. Mononuclear cells (MNCs) derived from HBV-carrier mice and wild type (WT) mice were characterized for phenotype by flow cytometry, HBV antigen and gene expression were detected by Radio Immunoassay (RIA), immunohistochemistry and quantitative real-time (qRT)-PCR. Metabolomics changes were detected in mice liver tissue based on ultra high performance liquid tandem chromatography quadrupole time of flight mass spectrometry (UHPLC-QTOFMS). The mice after ethanol consumption shows higher levels of HBV surface Ag (HBsAg), HBV core antigen (HBcAg) and HBV 3.5 kb RNA expression, and a lower level of CD8⁺ T cells during HBV persistence, with an increased lymphocyte activation gene-3 (LAG-3) expression on CD8⁺ T cell. In addition, the energy metabolism was downregulated and the oxidative stress was upregulated in the liver tissue. Furthermore, NK cells depletion results in a lower levels of HBV surface Ag (HBsAg) and HBV 3.5 kb RNA expression, and a higher level of CD8⁺ T cells with reduced expression of LAG-3. In conclusion, alcohol abuse induces CD8⁺ T cells failure after acute HBV infection, but depletion of NK cells could retore CD8⁺ T cell activity. Moreover, downregulation of energy metabolism and upregulation of oxidative stress may also contribute to CD8⁺ T cell failure.

Microenvironment-Dependent Gradient of CTL Exhaustion in the AE17sOVA Murine Mesothelioma Tumor Model

The immune system, and in particular, cytotoxic CD8⁺ T cells (CTLs), plays a vital part in the prevention and elimination of tumors. In many patients, however, CTL-mediated tumor killing ultimately fails in the clearance of cancer cells resulting in disease progression, in large part due to the progression of effector CTL into exhausted CTL. While there have been major breakthroughs in the development of CTL-mediated “reinvigoration”-driven immunotherapies such as checkpoint blockade therapy, there remains a need to better understand the drivers behind the development of T cell exhaustion. Our study highlights the unique differences in T cell exhaustion development in tumor-specific CTL which arises over time in a mouse model of mesothelioma. Importantly, we also show that peripheral tumor-specific T cells have a unique expression profile compared to exhausted tumor-infiltrating CTL at a late-stage of tumor progression in mice. Together, these data suggest that greater emphasis should be placed on understanding contributions of individual microenvironments in the development of T cell exhaustion.

The Role of ERO1α in Modulating Cancer Progression and Immune Escape

Endoplasmic reticulum oxidoreductin-1 alpha (ERO1α) was originally shown to be an endoplasmic reticulum (ER) resident protein undergoing oxidative cycles in concert with protein disulfide isomerase (PDI) to promote proper protein folding and to maintain homeostasis within the ER. ERO1α belongs to the flavoprotein family containing a flavin adenine dinucleotide utilized in transferring of electrons during oxidation-reduction cycles. This family is used to maintain redox potentials and protein homeostasis within the ER. ERO1α's location and function has since been shown to exist beyond the ER. Originally thought to exist solely in the ER, it has since been found to exist in the golgi apparatus, as well as in exosomes purified from patient samples. Besides aiding in protein folding of transmembrane and secretory proteins in conjunction with PDI, ERO1α is also known for formation of de novo disulfide bridges. Public databases, such as the Cancer Genome Atlas (TCGA) and The Protein Atlas, reveal ERO1α as a poor prognostic marker in multiple disease settings. Recent evidence indicates that ERO1α expression in tumor cells is a critical determinant of metastasis. However, the impact of increased ERO1α expression in tumor cells extends into the tumor microenvironment. Secretory proteins requiring ERO1α expression for proper folding have been implicated as being involved in immune escape through promotion of upregulation of programmed death ligand-1 (PD-L1) and stimulation of polymorphonuclear myeloid derived suppressor cells (PMN-MDSC's) via secretion of granulocytic colony stimulating factor (G-CSF). Hereby, ERO1α plays a pivotal role in cancer progression and potentially immune escape; making ERO1α an emerging attractive putative target for the treatment of cancer.

Prediction of PD-L1 inhibition effects for HIV-infected individuals

The novel therapies with immune checkpoint inhibitors hold great promises for patients with chronic virus infections and cancers. This is based mainly on the partial reversal of the exhausted phenotype of antigen-specific cytotoxic CD8 T cells (CTL). Recently, we have shown that the restoration of HIV-specific T cell function depends on the HIV infection stage of an infected individual. Here we aimed to answer two fundamental questions: (i) Can one estimate growth parameters for the HIV-specific proliferative responsiveness upon PD-L1 blockade ex vivo? (ii) Can one use these parameter estimates to predict clinical benefit for HIV-infected individuals displaying diverse infection phenotypes? To answer these questions, we first analyzed HIV-1 Gag-specific CD8 T cell proliferation by time-resolved CFSE assays and estimated the effect of PD-L1 blockade on division and death rates, and specific precursor frequencies. These values were then incorporated into a model for CTL-mediated HIV control and the effects on CTL frequencies, viral loads and CD4 T cell counts were predicted for different infection phenotypes. The biggest absolute increase in CD4 T cell counts was in the group of slow progressors while the strongest reduction in virus loads was observed in progressor patients. These results suggest a significant clinical benefit only for a subgroup of HIV-infected individuals. However, as PD1 is a marker of lymphocyte activation and expressed on several lymphocyte subsets including also CD4 T cells and B cells, we subsequently examined the multiple effects of anti-PD-L1 blockade beyond those on CD8 T cells. This extended model then predicts that the net effect on HIV load and CD4 T cell number depends on the interplay between positive and negative effects of lymphocyte subset activation. For a physiologically relevant range of affected model parameters, PD-L1 blockade is likely to be overall beneficial for HIV-infected individuals.

CD4+ T cell exhaustion revealed by high PD-1 and LAG-3 expression and the loss of helper T cell function in chronic hepatitis B

Background

Immune inhibitory receptors play an important role in chronic infections. However, little is known about their role in hepatitis B virus (HBV) infection. Here, we analyzed the relationship between programmed death-1 (PD-1) and lymphocyte activation gene-3 (LAG-3) expression on CD4⁺ T cells and HBV disease progression.

Results

PD-1 and LAG-3 expression was significantly higher on CD4⁺ T cells from HBV patients than on those from the HCs. In addition, a significant positive correlation was found between the PD-1 and LAG-3 expression levels and the ALT(alanine aminotransferase) level. CD4⁺ T cell function was inhibited by high PD-1 and LAG-3 levels, and CD4⁺ T cells with high PD-1 and LAG-3 expression lost the ability to secrete IFN-γ, IL-2 and TNF-α. Furthermore, blockade of the PD-1 and LAG-3 pathways reversed the damage to CD4⁺ T cell proliferation and cytokine secretion.

Conclusions

CD4⁺ T cell exhaustion during chronic HBV had high PD-1 and LAG-3 expression and the absence of helper T cell cytokines, including IFN-γ, IL-2 and TNF-α. After blocking PD-L1 and LAG-3, CD4⁺ T cell function in chronic hepatitis B patients was partially restored.

Enforcing the checkpoints: harnessing T-cell exhaustion for therapy of T1D

PURPOSE OF REVIEW

Although checkpoint inhibitor blockade is now widely used clinically for cancer immunotherapy, the reverse process, (i.e. induction of checkpoints to slow autoimmunity) has not been extensively explored. CD8 T-cell exhaustion is a state of immune hyporesponsiveness that may be harnessed to treat autoimmunity.

RECENT FINDINGS

We focus on the potential role of CD8 T-cell exhaustion as a mechanism of peripheral tolerance in T1D and its therapeutic implications.

SUMMARY

CD8 T-cell exhaustion is a continuum in which cells change from precursor to terminally exhausted cells. Current thinking based on studies in cancer and chronic viral infection invokes a three-signal model for development of T-cell exhaustion, with persistent antigen, negative costimulatory signals and chronic inflammation comprising signals 1-3, respectively. Transcriptional signatures of CD8 T-cell exhaustion were associated with better prognosis across several autoimmune diseases, most profoundly in systemic diseases. In T1D, CD8 exhaustion was promoted by treatment with anti-CD3 therapy (teplizumab) and was more evident in islet-specific CD8 T cells of slow progressors, suggesting a beneficial role in T1D also. Thus, we apply this three-step process of exhaustion to discuss potential treatments to augment CD8 T-cell exhaustion in T1D.

Immune Exhaustion: Past Lessons and New Insights from Lymphocytic Choriomeningitis Virus

Lymphocytic choriomeningitis virus (LCMV) is a paradigm-forming experimental system with a remarkable track record of contributing to the discovery of many of the fundamental concepts of modern immunology. The ability of LCMV to establish a chronic infection in immunocompetent adult mice was instrumental for identifying T cell exhaustion and this system has been invaluable for uncovering the complexity, regulators, and consequences of this state. These findings have been directly relevant for understanding why ineffective T cell responses commonly arise during many chronic infections including HIV and HCV, as well as during tumor outgrowth. The principal feature of exhausted T cells is the inability to elaborate the array of effector functions necessary to contain the underlying infection or tumor. Using LCMV to determine how to prevent and reverse T cell exhaustion has highlighted the potential of checkpoint blockade therapies, most notably PD-1 inhibition strategies, for improving cellular immunity under conditions of antigen persistence. Here, we discuss the discovery, properties, and regulators of exhausted T cells and highlight how LCMV has been at the forefront of advancing our understanding of these ineffective responses.

Highly competent, non-exhausted CD8+ T cells continue to tightly control pathogen load throughout chronic Trypanosoma cruzi infection

Trypanosoma cruzi infection is characterized by chronic parasitism of non-lymphoid tissues and is rarely eliminated despite potent adaptive immune responses. This failure to cure has frequently been attributed to a loss or impairment of anti-T. cruzi T cell responses over time, analogous to the T cell dysfunction described for other persistent infections. In this study, we have evaluated the role of CD8⁺ T cells during chronic T. cruzi infection (>100 dpi), with a focus on sites of pathogen persistence. Consistent with repetitive antigen exposure during chronic infection, parasite-specific CD8⁺ T cells from multiple organs expressed high levels of KLRG1, but exhibit a preferential accumulation of CD69⁺ cells in skeletal muscle, indicating recent antigen encounter in a niche for T. cruzi persistence. A significant proportion of CD8⁺ T cells in the muscle also produced IFNγ, TNFα and granzyme B in situ, an indication of their detection of and functional response to T. cruzi in vivo. CD8⁺ T cell function was crucial for the control of parasite burden during chronic infection as exacerbation of parasite load was observed upon depletion of this population. Attempts to improve T cell function by blocking PD-1 or IL-10, potential negative regulators of T cells, failed to increase IFNγ and TNFα production or to enhance T. cruzi clearance. These results highlight the capacity of the CD8⁺ T cell population to retain essential in vivo function despite chronic antigen stimulation and support a model in which CD8⁺ T cell dysfunction plays a negligible role in the ability of Trypanosoma cruzi to persist in mice.

The parasite Trypanosoma cruzi establishes lifelong infections in humans and other mammals, leading to severe cardiac and gastrointestinal complications known as Chagas disease. Although the factors that enable T. cruzi persistence remain undefined, in this and many other infection models, pathogen persistence has been attributed to the exhaustion of the immune system, particularly of CD8⁺ T cells. Here, we show that the inability of hosts to fully resolve T. cruzi infection is not a result of immune exhaustion and that in fact the T. cruzi-specific CD8⁺ T cell population remains highly competent and actively suppresses parasite outgrowth throughout the chronic infection. These results demonstrate that compromised immunity is not the eventual outcome of all chronic infections and suggest that T. cruzi, and perhaps other pathogens, may employ alternative strategies to subvert immune clearance in the presence of highly functional pathogen-specific effectors. These findings also suggest that interventions to inhibit immune regulatory pathways or to otherwise boost existing immune responses in such infections, will have limited benefit.

The Role of Interferon in Persistent Viral Infection: Insights from Murine Norovirus

Persistent viral infections result from evasion or avoidance of sterilizing immunity, extend the timeframe of virus transmission, and can trigger disease. Prior studies in mouse models of persistent infection have suggested that ineffective adaptive immune responses are necessary for persistent viral infection. However, recent work in the murine norovirus (MNV) model of persistent infection demonstrates that innate immunity can control both early and persistent viral replication independently of adaptive immune effector functions. Interferons (IFNs) are central to the innate control of persistent MNV, apart from a role in modulating adaptive immunity. Furthermore, subtypes of IFN play distinct tissue-specific roles in innate control of persistent MNV infection. Type I IFN (IFN-α/β) controls systemic replication, and type III IFN (IFN-λ) controls MNV persistence in the intestinal epithelium. In this article, we review recent findings in the MNV model, highlighting the role of IFNs and innate immunity in clearing persistent viral infection, and discussing the broader implications of these findings for control of persistent human infections.

Exhaustion and Inflation at Antipodes of T Cell Responses to Chronic Virus Infection

Viruses that have coevolved with their host establish chronic infections that are well tolerated by the host. Other viruses, that are partly adapted to their host, may induce chronic infections where persistent replication and viral antigen expression occur. The former induce highly functional and resilient CD8T cell responses called memory inflation. The latter induce dysfunctional and exhausted responses. The reasons compelling T cell responses towards inflationary or exhausted responses are only partly understood. In this review we compare the two conditions and describe mechanistic similarities and differences. We also provide a list of potential reasons why exhaustion or inflation occur in different virus infections. We propose that T cell-mediated transcriptional repression of viral gene expression provides a critical feature of inflation that allows peaceful virus and host coexistence. The virus is controlled, but its genome is not eradicated. If this mechanism is not available, as in the case of RNA viruses, the virus and the host are compelled to an arms race. If virus proliferation and spread proceed uncontrolled for too long, T cells are forced to strike a balance between viral control and tissue destruction, losing antiviral potency and facilitating virus persistence.

Spatiotemporal Dynamics of Virus Infection Spreading in Tissues

Virus spreading in tissues is determined by virus transport, virus multiplication in host cells and the virus-induced immune response. Cytotoxic T cells remove infected cells with a rate determined by the infection level. The intensity of the immune response has a bell-shaped dependence on the concentration of virus, i.e., it increases at low and decays at high infection levels. A combination of these effects and a time delay in the immune response determine the development of virus infection in tissues like spleen or lymph nodes. The mathematical model described in this work consists of reaction-diffusion equations with a delay. It shows that the different regimes of infection spreading like the establishment of a low level infection, a high level infection or a transition between both are determined by the initial virus load and by the intensity of the immune response. The dynamics of the model solutions include simple and composed waves, and periodic and aperiodic oscillations. The results of analytical and numerical studies of the model provide a systematic basis for a quantitative understanding and interpretation of the determinants of the infection process in target organs and tissues from the image-derived data as well as of the spatiotemporal mechanisms of viral disease pathogenesis, and have direct implications for a biopsy-based medical testing of the chronic infection processes caused by viruses, e.g. HIV, HCV and HBV.

Dendritic Cells Are Dispensable for T Cell Priming and Control of Acute Lymphocytic Choriomeningitis Virus Infection

Dendritic cells (DCs) are considered to be the major APCs with potent activity for priming of naive CD4 and CD8 T cells. However, T cell priming can also be achieved by other APCs including macrophages, B cells, or even nonhematopoietic cell types. Systemic low-dose infection of mice with lymphocytic choriomeningitis virus (LCMV) results in massive expansion of virus-specific CD4 and CD8 T cells. To determine the role of DCs as APCs and source of type I IFNs in this infection model, we used ΔDC mice in which DCs are constitutively ablated because of expression of the diphtheria toxin α subunit within developing DCs. ΔDC mice showed lower serum concentrations of IFN-β and IL-12p40, but normal IFN-α levels during the first days postinfection. No differences were found for proliferation of transferred TCR-transgenic cells during the early phase of infection, suggesting that T cell priming occurred with the same efficiency in wild-type and ΔDC mice. Expansion and cytokine expression of endogenous LCMV-specific T cells was comparable between wild-type and ΔDC mice during primary infection and upon rechallenge of memory mice. In both strains of infected mice the viral load was reduced below the limit of detection with the same kinetic. Further, germinal center formation and LCMV-specific Ab responses were not impaired in ΔDC mice. This indicates that DCs are dispensable as APCs for protective immunity against LCMV infection.

Type I Interferon Receptor Deficiency in Dendritic Cells Facilitates Systemic Murine Norovirus Persistence Despite Enhanced Adaptive Immunity

In order for a virus to persist, there must be a balance between viral replication and immune clearance. It is commonly believed that adaptive immunity drives clearance of viral infections and, thus, dysfunction or viral evasion of adaptive immunity is required for a virus to persist. Type I interferons (IFNs) play pleiotropic roles in the antiviral response, including through innate control of viral replication. Murine norovirus (MNoV) replicates in dendritic cells (DCs) and type I IFN signaling in DCs is important for early control of MNoV replication. We show here that the non-persistent MNoV strain CW3 persists systemically when CD11c positive DCs are unable to respond to type I IFN. Persistence in this setting is associated with increased early viral titers, maintenance of DC numbers, increased expression of DC activation markers and an increase in CD8 T cell and antibody responses. Furthermore, CD8 T cell function is maintained during the persistent phase of infection and adaptive immune cells from persistently infected mice are functional when transferred to Rag1-/- recipients. Finally, increased early replication and persistence are also observed in mixed bone marrow chimeras where only half of the CD11c positive DCs are unable to respond to type I IFN. These findings demonstrate that increased early viral replication due to a cell-intrinsic innate immune deficiency is sufficient for persistence and a functional adaptive immune response is not sufficient for viral clearance.

Self-adjuvanted mRNA vaccines induce local innate immune responses that lead to a potent and boostable adaptive immunity

mRNA represents a new platform for the development of therapeutic and prophylactic vaccines with high flexibility with respect to production and application. We have previously shown that our two component self-adjuvanted mRNA-based vaccines (termed RNActive® vaccines) induce balanced immune responses comprising both humoral and cellular effector as well as memory responses. Here, we evaluated the early events upon intradermal application to gain more detailed insights into the underlying mode of action of our mRNA-based vaccine. We showed that the vaccine is taken up in the skin by both non-leukocytic and leukocytic cells, the latter being mostly represented by antigen presenting cells (APCs). mRNA was then transported to the draining lymph nodes (dLNs) by migratory dendritic cells. Moreover, the encoded protein was expressed and efficiently presented by APCs within the dLNs as shown by T cell proliferation and immune cell activation, followed by the induction of the adaptive immunity. Importantly, the immunostimulation was limited to the injection site and lymphoid organs as no proinflammatory cytokines were detected in the sera of the immunized mice indicating a favorable safety profile of the mRNA-based vaccines. Notably, a substantial boostability of the immune responses was observed, indicating that mRNA can be used effectively in repetitive immunization schedules. The evaluation of the immunostimulation following prime and boost vaccination revealed no signs of exhaustion as demonstrated by comparable levels of cytokine production at the injection site and immune cell activation within dLNs. In summary, our data provide mechanistic insight into the mode of action and a rational for the use of mRNA-based vaccines as a promising immunization platform.

Blockade of interferon Beta, but not interferon alpha, signaling controls persistent viral infection

Although type I interferon (IFN-I) is thought to be beneficial against microbial infections, persistent viral infections are characterized by high interferon signatures suggesting that IFN-I signaling may promote disease pathogenesis. During persistent lymphocytic choriomeningitis virus (LCMV) infection, IFNα and IFNβ are highly induced early after infection, and blocking IFN-I receptor (IFNAR) signaling promotes virus clearance. We assessed the specific roles of IFNβ versus IFNα in controlling LCMV infection. While blockade of IFNβ alone does not alter early viral dissemination, it is important in determining lymphoid structure, lymphocyte migration, and anti-viral T cell responses that lead to accelerated virus clearance, approximating what occurs during attenuation of IFNAR signaling. Comparatively, blockade of IFNα was not associated with improved viral control, but with early dissemination of virus. Thus, despite their use of the same receptor, IFNβ and IFNα have unique and distinguishable biologic functions, with IFNβ being mainly responsible for promoting viral persistence.

Innate and Adaptive Immune Regulation During Chronic Viral Infections

Chronic viral infections represent a unique challenge to the infected host. Persistently replicating viruses outcompete or subvert the initial antiviral response, allowing the establishment of chronic infections that result in continuous stimulation of both the innate and adaptive immune compartments. This causes a profound reprogramming of the host immune system, including attenuation and persistent low levels of type I interferons, progressive loss (or exhaustion) of CD8(+) T cell functions, and specialization of CD4(+) T cells to produce interleukin-21 and promote antibody-mediated immunity and immune regulation. Epigenetic, transcriptional, posttranscriptional, and metabolic changes underlie this adaptation or recalibration of immune cells to the emerging new environment in order to strike an often imperfect balance between the host and the infectious pathogen. In this review we discuss the common immunological hallmarks observed across a range of different persistently replicating viruses and host species, the underlying molecular mechanisms, and the biological and clinical implications.

Effective intrahepatic CD8+ T-cell immune responses are induced by low but not high numbers of antigen-expressing hepatocytes

Liver infections with hepatotropic viruses, such as hepatitis B virus and hepatitis C virus are accompanied by viral persistence and immune failure. CD8+ T cells are crucial mediators of the intrahepatic antiviral immune response. Chronic infections of the liver and other organs correlate with T-cell exhaustion. It was previously suggested that high antigen load could result in T-cell exhaustion. We aimed at elucidating the impact of different intrahepatic antigen loads on the quality of CD8+ T-cell-mediated immunity by employing an infection-free transgenic mouse model expressing ovalbumin (Ova) as the target antigen. Adoptive transfer of OT-I cells induced a transient intrahepatic immune response toward both high and low Ova levels. However, antigen clearance was achieved only in mice expressing low antigen levels. In contrast, T cells exposed to high antigen levels underwent exhaustion and became depleted, causing antigen persistence. Moreover, when functional T cells were exposed to high intrahepatic antigen levels, a complete transition toward exhaustion was observed. Thus, this study shows that the antigen expression level in the liver correlates inversely with T-cell immunity in vivo and governs the efficiency of immune responses upon antigen presentation.

Immune outcomes in the liver: Is CD8 T cell fate determined by the environment?

The liver is known for its tolerogenic properties. This unique characteristic is associated with persistent infection of the liver by the hepatitis B and C viruses. Improper activation of cellular adaptive immune responses within the liver and immune exhaustion over time both contribute to ineffective cytotoxic T cell responses to liver-expressed antigens in animal models, and likely play a role in incomplete clearance of chronic hepatitis virus infections in humans. However, under some conditions, functional immune responses can be elicited against hepatic antigens, resulting in control of hepatotropic infections. In order to develop improved therapeutics in immune-mediated chronic liver diseases, including viral hepatitis, it is essential to understand how intrahepatic immunity is regulated. This review focuses on CD8 T cell immunity directed towards foreign antigens expressed in the liver, and explores how the liver environment dictates the outcome of intrahepatic CD8 T cell responses. Potential strategies to rescue unresponsive CD8 T cells in the liver are also discussed.

NK cells inhibit humoral immunity by reducing the abundance of CD4+ T follicular helper cells during a chronic virus infection

There is a need to understand better how to improve B cell responses and immunity to persisting virus infections, which often cause debilitating illness or death. People with chronic virus infection show evidence of improved virus control when there is a strong neutralizing antibody response, and conversely, B cell dysfunction is associated with higher viral loads. We showed previously that NK cells inhibit CD4(+) and CD8(+) T cell responses to disseminating LCMV infection and that depletion of NK cells attenuates chronic infection. Here, we examined the effect of NK cell depletion on B cell responses to LCMV infection in mice. Whereas mice infected acutely generated a peak level of antibody soon after the infection was resolved, mice infected chronically showed a continued increase in antibody levels that exceeded those after acute infection. We found that early NK cell depletion rapidly increased virus-specific antibody levels to chronic infection, and this effect depended on CD4(+) T cells and was associated with elevated numbers of CXCR5(+)CD4(+) TFH cells. However, the NK cell-depleted mice controlled the infection and by 1 mo pi, had lower TFH cell numbers and antibody levels compared with mice with sustained infection. Finally, we show that NK cell depletion improved antiviral CD8(+) T cell responses only when B cells and virus-specific antibody were present. Our data indicate that NK cells diminish immunity to chronic infection, in part, by suppressing TFH cell and antibody responses.

CD169+ macrophages are sufficient for priming of CTLs with specificities left out by cross-priming dendritic cells

Dendritic cells (DCs) are considered the most potent antigen-presenting cells (APCs), which directly prime or cross-prime MHC I-restricted cytotoxic T cells (CTLs). However, recent evidence suggests the existence of other, as-yet unidentified APCs also able to prime T cells. To identify those APCs, we used adenoviral (rAd) vectors, which do not infect DCs but selectively accumulate in CD169(+) macrophages (MPs). In mice that lack DCs, infection of CD169(+) MPs was sufficient to prime CTLs specific for all epitopes tested. In contrast, CTL responses relying exclusively on cross-presenting DCs were biased to selected strong MHC I-binding peptides only. When both DCs and MPs were absent, no CTL responses could be elicited. Therefore, CD169(+) MPs can be considered APCs that significantly contribute to CTL responses.

Evaluation of non-reciprocal heterologous immunity between unrelated viruses

Heterologous immunity refers to the phenomenon whereby a history of an immune response against one pathogen can provide a level of immunity to a second unrelated pathogen. Previous investigations have shown that heterologous immunity is not necessarily reciprocal, such as in the case of vaccinia virus (VACV). Replication of VACV is reduced in mice immune to a variety of pathogens, while VACV fails to induce immunity to several of the same pathogens, including lymphocytic choriomeningitis virus (LCMV). Here we examine the lack of reciprocity of heterologous immunity between VACV and LCMV and find that they induce qualitatively different memory CD8 T cells. However, depending on the repertoire of an individual host, VACV can provide protection against LCMV simply by experimentally amplifying the quantity of T cells cross-reactive with the two viruses. Thus, one cause for lack of reciprocity is differences in the frequencies of cross-reactive T cells in immune hosts.

Coinhibitory receptors and CD8 T cell exhaustion in chronic infections

PURPOSE OF REVIEW

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

RECENT FINDINGS

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

SUMMARY

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

T cell exhaustion during persistent viral infections

Although robust and highly effective anti-viral T cells contribute to the clearance of many acute infections, viral persistence is associated with the development of functionally inferior, exhausted, T cell responses. Exhaustion develops in a step-wise and progressive manner, ranges in severity, and can culminate in the deletion of the anti-viral T cells. This disarming of the response is consequential as it compromises viral control and potentially serves to dampen immune-mediated damage. Exhausted T cells are unable to elaborate typical anti-viral effector functions. They are characterized by the sustained upregulation of inhibitory receptors and display a gene expression profile that distinguishes them from prototypic effector and memory T cell populations. In this review we discuss the properties of exhausted T cells; the virological and immunological conditions that favor their development; the cellular and molecular signals that sustain the exhausted state; and strategies for preventing and reversing exhaustion to favor viral control.

Too much of a good thing? Tim-3 and TCR signaling in T cell exhaustion

T cell exhaustion is thought to be a natural mechanism for limiting immune pathology, although it may be desirable to circumvent this mechanism to help eliminate viral reservoirs or tumors. Although there are no definitive markers, a fingerprint for exhausted T cells has been described that includes the transmembrane proteins PD-1, LAG3, and Tim-3. However, apart from the recruitment of tyrosine phosphatases to PD-1, little is known about the biochemical mechanisms by which these proteins contribute to the development or maintenance of exhaustion. Tim-3 contains no known motifs for the recruitment of inhibitory phosphatases, but it may actually increase signaling downstream of TCR/CD3, at least under acute conditions. Other studies showed that T cell exhaustion results from chronic stimulation that extends the effector phase of T cell activation, at the expense of T cell memory. We suggest that Tim-3 may contribute to T cell exhaustion by enhancing TCR-signaling pathways.

Type I interferon signaling enhances CD8+ T cell effector function and differentiation during murine gammaherpesvirus 68 infection

CD8(+) T cell responses are critical to the control of replication and reactivation associated with gammaherpesvirus infection. Type I interferons (IFNs) have been shown to have direct and indirect roles in supporting CD8(+) T cell development and function during viral infection; however, the role of type I interferons during latent viral infection has not been examined. Mice deficient in type I IFN signaling (IFNAR1(-/-) mice) have high levels of reactivation during infection with murine gammaherpesvirus 68 (MHV68), a murine gammaherpesvirus model for Epstein-Barr virus. We hypothesized that type I IFNs function to enhance the anti-gammaherpesvirus CD8(+) T cell response. To test this, IFNAR1(-/-) mice were infected with MHV68 and the CD8(+) T cell response was analyzed. In the absence of type I IFN signaling, there was a marked increase in short-lived effector CD8(+) T cells, and MHV68-specific CD8(+) T cells had upregulated expression of PD-1 and reduced tumor necrosis factor alpha (TNF-α), gamma IFN (IFN-γ), and interleukin-2 (IL-2) production. Suppressing MHV68 replication early in infection using the antiviral cidofovir rescued CD8(+) T cell cytokine production and reduced PD-1 expression. However, suppressing high levels of reactivation in IFNAR1(-/-) mice failed to improve CD8(+) T cell cytokine production during latency. T cell-specific abrogation of type I IFN signaling showed that the effects of type I IFNs on the CD8(+) T cell response during MHV68 infection are independent of direct type I IFN signaling on T cells. Our findings support a model in which type I IFNs likely suppress MHV68 replication, thus limiting viral antigen and facilitating an effective gammaherpesvirus-directed CD8(+) T cell response.

IMPORTANCE

The murine gammaherpesvirus MHV68 has both genetic and biologic homology to the human gammaherpesvirus Epstein-Barr virus (EBV), which infects over 90% of humans. Latent EBV infection and reactivation are associated with various life-threatening diseases and malignancies. Host suppression of gammaherpesvirus latency and reactivation requires both CD8(+) T cells as well as type I interferon signaling. Type I IFNs have been shown to critically support the antiviral CD8(+) T cell response in other virus models. Here, we identify an indirect role for type I IFN signaling in enhancing gammaherpesvirus-specific CD8(+) T cell cytokine production. Further, this function of type I IFN signaling can be partially rescued by suppressing viral replication during early MHV68 infection. Our data suggest that type I IFN signaling on non-T cells can enhance CD8(+) T cell function during gammaherpesvirus infection, potentially through suppression of MHV68 replication.

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.

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.

Minor antigen distribution predicts site-specific graft-versus-tumor activity of adoptively transferred, minor antigen-specific CD8 T Cells

The clinical success of allogeneic T cell therapy for cancer relies on the selection of antigens that can effectively elicit antitumor responses with minimal toxicity toward nonmalignant tissues. Although minor histocompatibility antigens (MiHA) represent promising targets, broad expression of these antigens has been associated with poor responses and T cell dysfunction that may not be prevented by targeting MiHA with limited expression. In this study, we hypothesized that antitumor activity of MiHA-specific CD8 T cells after allogeneic bone marrow transplantation (BMT) is determined by the distribution of antigen relative to the site of tumor growth. To test this hypothesis, we utilized the clinically relevant male-specific antigen HY and studied the fate of adoptively transferred, HY-CD8(+) T cells (HY-CD8) against a HY-expressing epithelial tumor (MB49) and pre-B cell leukemia (HY-E2APBX ALL) in BMT recipients. Transplants were designed to produce broad HY expression in nonhematopoietic tissues (female → male BMT, [F → M]), restricted HY expression in hematopoietic tissues (male → female BMT, [M → F]) tissues, and no HY tissue expression (female → female BMT, [F → F]). Broad HY expression induced poor responses to MB49 despite sublethal graft-versus-host disease and accumulation of HY-CD8 in secondary lymphoid tissues. Antileukemia responses, however, were preserved. In contrast, restriction of HY expression to hematopoietic tissues restored MB49 responses but resulted in a loss of antileukemia responses. We concluded that target alloantigen expression in the same compartment of tumor growth impairs CD8 responses to both solid and hematologic tumors.

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.

T cells maintain an exhausted phenotype after antigen withdrawal and population reexpansion

During chronic infection, pathogen-specific CD8(+) T cells upregulate expression of molecules such as the inhibitory surface receptor PD-1, have diminished cytokine production and are thought to undergo terminal differentiation into exhausted cells. Here we found that T cells with memory-like properties were generated during chronic infection. After transfer into naive mice, these cells robustly proliferated and controlled a viral infection. The reexpanded T cell populations continued to have the exhausted phenotype they acquired during the chronic infection. Thus, the cells underwent a form of differentiation that was stably transmitted to daughter cells. We therefore propose that during persistent infection, effector T cells stably differentiate into a state that is optimized to limit viral replication without causing overwhelming immunological pathology.

T-cell exhaustion due to persistent antigen: quantity not quality?

T-cell exhaustion is characterized by failure to respond to a persistent antigen and is a hallmark of chronic infections and cancer. Here, we discuss several recent reports on T-cell exhaustion, including one in this issue of the European Journal of Immunology where Richter et al. [Eur. J. Immunol. 42:2290-2304] examine the importance of the amount of persistent antigen versus the cell type presenting it to induce CD8(+) T-cell exhaustion, and the consequences for host survival during chronic viral infection.

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.