OX40 facilitates control of a persistent virus infection

TNF/TNFR Superfamily Members in Costimulation of T Cell Responses-Revisited

Prosurvival tumor necrosis factor receptor (TNFR) superfamily (TNFRSF) members on T cells, including 4-1BB, CD27, GITR, and OX40, support T cell accumulation during clonal expansion, contributing to T cell memory. During viral infection, tumor necrosis factor superfamily (TNFSF) members on inflammatory monocyte-derived antigen-presenting cells (APCs) provide a postpriming signal (signal 4) for T cell accumulation, particularly in the tissues. Patients with loss-of-function mutations in TNFR/TNFSF members reveal a critical role for 4-1BB and CD27 in CD8 T cell control of Epstein-Barr virus and other childhood infections and of OX40 in CD4 T cell responses. Here, on the 20th anniversary of a previous Annual Review of Immunology article about TNFRSF signaling in T cells, we discuss the effects of endogenous TNFRSF signals in T cells upon recognition of TNFSF members on APCs; the role of TNFRSF members, including TNFR2, on regulatory T cells; and recent advances in the incorporation of TNFRSF signaling in T cells into immunotherapeutic strategies for cancer.

Activation of immune checkpoint OX40 inhibits HBV replication in a mouse model

BACKGROUND & AIMS

The costimulatory molecules OX40 and OX40L, members of the tumor necrosis factor (receptor) superfamily, play an important role in viral control through the activation of T cells. We speculate that activating the immune checkpoint OX40 may promote the inhibition of hepatitis B virus (HBV) replication.

METHODS

To test this hypothesis, we investigated the expression dynamics of OX40/OX40L and studied the effects of activation of OX40 on HBV replication, and further explored the possible mechanism.

RESULTS

We found that the percentage of T cells expressing OX40 was lower in adult patients with chronic hepatitis B (CHB) than in healthy adults and was negatively correlated with serum viral load. In contrast, the percentage of B cells and monocytes expressing OX40L was increased in adult patients with CHB and positively correlated with liver inflammatory indicators. The expression of OX40 in T cells and OX40L in monocytes was positively correlated with age in healthy donors. In addition, the levels of serum HBsAg and intrahepatic HBV DNA decreased in an HBV mouse model with an agonistic antibody that activates OX40. This viral inhibition process coincides with changes in liver inflammation, the ratio of T cell subsets, and T cell-related cytokines. Finally, we found that the OX40 activation-mediated inhibition of HBV replication was more dependent on CD8+ T cells than on CD4+ T cells.

CONCLUSIONS

The expression levels of the immune checkpoints OX40/OX40L in adult patients with CHB are closely related to virus clearance. The activation of OX40 can suppress HBV replication through a mechanism that is more dependent on CD8+ T cells than on CD4+ T cells. Thus, OX40 is a promising therapeutic target for the treatment of CHB.

HIV-1 adapts to HLA class II-associated selection pressure exerted by CD4+ and CD8+ T cells

Developing an effective HIV-1 vaccine is a global health priority, but HIV-1 mutational escape from T cells poses a challenge. While escape from human leukocyte antigen class I (HLA-I)-restricted CD8+ T cells is well characterized, less is known about HLA-II-restricted T cell escape. We used computational methods to identify 149 sites across the HIV-1 clade B genome under HLA-II-associated selection. Functional assays, including activation-induced intracellular cytokine staining and enzyme-linked immunospot for interferon-γ, revealed diverse mechanisms of HIV-1 adaptation to HLA-II-associated immune pressure, ranging from loss to sustained antigen recognition. T cell receptor and RNA sequencing demonstrated variable clonotype overlap of T cell clones to recognize adapted versus non-adapted peptides, with cells targeting adapted peptides exhibiting a dysfunctional transcriptomic state. Moreover, incorporating HLA-II-associated adaptation strengthened the correlation between Gag-specific viral adaptation and poor disease outcomes. Last, we mapped viral regions prone to HLA-II-associated adaptation and found that these adaptations can increase in frequency within populations.

Role of the Crosstalk B:Neoplastic T Follicular Helper Cells in the Pathobiology of Nodal T Follicular Helper Cell Lymphomas

Angioimmunoblastic T-cell lymphoma (AITL), the most common form of peripheral T-cell lymphoma, originates from follicular helper T (Tfh) cells and is notably resistant to current treatments. The disease progression and maintenance, at least in early stages, are driven by a complex interplay between neoplastic Tfh and clusters of B-cells within the tumor microenvironment, mirroring the functional crosstalk observed inside germinal centers. This interaction is further complicated by recurrent mutations, such as TET2 and DNMT3A, which are present in both Tfh cells and B-cells. These findings suggest that the symbiotic relationship between these 2 cell types could represent a therapeutic vulnerability. This review examines the key components and signaling mechanisms involved in the synapses between B-cells and Tfh cells, emphasizing their significant role in the pathobiology of AITL and potential as therapeutic targets.

OX40/OX40 ligand and its role in precision immune oncology

Immune checkpoint inhibitors have changed the treatment landscape for various malignancies; however, their benefit is limited to a subset of patients. The immune machinery includes both mediators of suppression/immune evasion, such as PD-1, PD-L1, CTLA-4, and LAG-3, all of which can be inhibited by specific antibodies, and immune-stimulatory molecules, such as T-cell co-stimulatory receptors that belong to the tumor necrosis factor receptor superfamily (TNFRSF), including OX40 receptor (CD134; TNFRSF4), 4-1BB (CD137; TNFRSF9), and glucocorticoid-induced TNFR-related (GITR) protein (CD357; TNFRSF18). In particular, OX40 and its binding ligand OX40L (CD134L; TNFSF4; CD252) are critical for immunoregulation. When OX40 on activated T cells binds OX40L on antigen-presenting cells, T-cell activation and immune stimulation are initiated via enhanced T-cell survival, proliferation and cytotoxicity, memory T-cell formation, and abrogation of regulatory T cell (Treg) immunosuppressive functions. OX40 agonists are in clinical trials both as monotherapy and in combination with other immunotherapy agents, in particular specific checkpoint inhibitors, for cancer treatment. To date, however, only a minority of patients respond. Transcriptomic profiling reveals that OX40 and OX40L expression vary between and within tumor types, and that only ~ 17% of cancer patients have high OX40 and low OX40L, one of the expression patterns that might be theoretically amenable to OX40 agonist enhancement. Taken together, the data suggest that the OX40/OX40L machinery is a critical part of the immune stimulatory system and that understanding endogenous expression patterns of these molecules and co-existing checkpoints merits further investigation in the context of a precision immunotherapy strategy for cancer therapy.

Impact of Chronic HIV Infection on Acute Immune Responses to SARS-CoV-2

ABSTRACT

There is mounting evidence that HIV infection is a risk factor for severe presentations of COVID-19. We hypothesized that the persistent immune activation associated with chronic HIV infection contributes to worsened outcomes during acute COVID-19. The goals of this study were to provide an in-depth analysis of immune response to acute COVID-19 and investigate relationships between immune responses and clinical outcomes in an unvaccinated, sex- and race-matched cohort of people with HIV (PWH, n = 20) and people without HIV (PWOH, n = 41). We performed flow cytometric analyses on peripheral blood mononuclear cells from PWH and PWOH experiencing acute COVID-19 (≤21-day postsymptom onset). PWH were younger (median 52 vs 65 years) and had milder COVID-19 (40% vs 88% hospitalized) compared with PWOH. Flow cytometry panels included surface markers for immune cell populations, activation and exhaustion surface markers (with and without SARS-CoV-2-specific antigen stimulation), and intracellular cytokine staining. We observed that PWH had increased expression of activation (eg, CD137 and OX40) and exhaustion (eg, PD1 and TIGIT) markers as compared to PWOH during acute COVID-19. When analyzing the impact of COVID-19 severity, we found that hospitalized PWH had lower nonclassical (CD16 + ) monocyte frequencies, decreased expression of TIM3 on CD4 + T cells, and increased expression of PDL1 and CD69 on CD8 + T cells. Our findings demonstrate that PWH have increased immune activation and exhaustion as compared to a cohort of predominately older, hospitalized PWOH and raises questions on how chronic immune activation affects acute disease and the development of postacute sequelae.

From glioma gloom to immune bloom: unveiling novel immunotherapeutic paradigms-a review

In tumor therapeutics, the transition from conventional cytotoxic drugs to targeted molecular therapies, such as those targeting receptor tyrosine kinases, has been pivotal. Despite this progress, the clinical outcomes have remained modest, with glioblastoma patients' median survival stagnating at less than 15 months. This underscores the urgent need for more specialized treatment strategies. Our review delves into the progression toward immunomodulation in glioma treatment. We dissect critical discoveries in immunotherapy, such as spotlighting the instrumental role of tumor-associated macrophages, which account for approximately half of the immune cells in the glioma microenvironment, and myeloid-derived suppressor cells. The complex interplay between tumor cells and the immune microenvironment has been explored, revealing novel therapeutic targets. The uniqueness of our review is its exhaustive approach, synthesizing current research to elucidate the intricate roles of various molecules and receptors within the glioma microenvironment. This comprehensive synthesis not only maps the current landscape but also provides a blueprint for refining immunotherapy for glioma, signifying a paradigm shift toward leveraging immune mechanisms for improved patient prognosis.

Cellular and molecular waypoints along the path of T cell exhaustion

Thirty years of foundational research investigating molecular and cellular mechanisms promoting T cell exhaustion are now enabling rational design of T cell-based therapies for the treatment of chronic infections and cancer. Once described as a static cell fate, it is now well appreciated that the developmental path toward exhaustion is composed of a heterogeneous pool of cells with varying degrees of effector potential that ultimately converge on a terminally differentiated state. Recent description of the developmental stages along the differentiation trajectory of T cell exhaustion has provided insight into past immunotherapeutic success and future opportunities. Here, we discuss the hallmarks of distinct developmental stages occurring along the path to T cell dysfunction and the impact of these discrete CD8+ T cell fates on cancer immunotherapy.

N-Linked Glycans Shape Skin Immune Responses during Arthritis and Myositis after Intradermal Infection with Ross River Virus

Arthritogenic alphaviruses are mosquito-borne arboviruses that include several re-emerging human pathogens, including the chikungunya (CHIKV), Ross River (RRV), Mayaro (MAYV), and o'nyong-nyong (ONNV) virus. Arboviruses are transmitted via a mosquito bite to the skin. Herein, we describe intradermal RRV infection in a mouse model that replicates the arthritis and myositis seen in humans with Ross River virus disease (RRVD). We show that skin infection with RRV results in the recruitment of inflammatory monocytes and neutrophils, which together with dendritic cells migrate to draining lymph nodes (LN) of the skin. Neutrophils and monocytes are productively infected and traffic virus from the skin to LN. We show that viral envelope N-linked glycosylation is a key determinant of skin immune responses and disease severity. RRV grown in mammalian cells elicited robust early antiviral responses in the skin, while RRV grown in mosquito cells stimulated poorer early antiviral responses. We used glycan mass spectrometry to characterize the glycan profile of mosquito and mammalian cell-derived RRV, showing deglycosylation of the RRV E2 glycoprotein is associated with curtailed skin immune responses and reduced disease following intradermal infection. Altogether, our findings demonstrate skin infection with an arthritogenic alphavirus leads to musculoskeletal disease and envelope glycoprotein glycosylation shapes disease outcome. IMPORTANCE Arthritogenic alphaviruses are transmitted via mosquito bites through the skin, potentially causing debilitating diseases. Our understanding of how viral infection starts in the skin and how virus systemically disseminates to cause disease remains limited. Intradermal arbovirus infection described herein results in musculoskeletal pathology, which is dependent on viral envelope N-linked glycosylation. As such, intradermal infection route provides new insights into how arboviruses cause disease and could be extended to future investigations of skin immune responses following infection with other re-emerging arboviruses.

Providing a Helping Hand: Metabolic Regulation of T Follicular Helper Cells and Their Association With Disease

T follicular helper (Tfh) cells provide support to B cells upon arrival in the germinal center, and thus are critical for the generation of a robust adaptive immune response. Tfh express specific transcription factors and cellular receptors including Bcl6, CXCR5, PD-1, and ICOS, which are critical for homing and overall function. Generally, the induction of an immune response is tightly regulated. However, deviation during this process can result in harmful autoimmunity or the inability to successfully clear pathogens. Recently, it has been shown that Tfh differentiation, activation, and proliferation may be linked with the cellular metabolic state. In this review we will highlight recent discoveries in Tfh differentiation and explore how these cells contribute to functional immunity in disease, including autoimmune-related disorders, cancer, and of particular emphasis, during infection.

Knowns and Unknowns about CAR-T Cell Dysfunction

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.

A Population of CD4+CD8+ Double-Positive T Cells Associated with Risk of Plasma Leakage in Dengue Viral Infection

According to the WHO 2009 classification, dengue with warning signs is at the risk of developing severe form of dengue disease. One of the most important warning signs is plasma leakage, which can be a serious complication associated with higher morbidity and mortality. We report that the frequency of CD4+CD8+ double-positive (DP) T cells is significantly increased in patients at risk of developing plasma leakage. Transcriptomic analysis demonstrated that CD4+CD8+ DP cells were distinct from CD4+ Single Positive (SP) T cells but co-clustered with CD8+ SP cells, indicating a largely similar transcriptional profile. Twenty significant differentially expressed (DE) genes were identified between CD4+CD8+ DP and CD8+ SP cells. These genes encode OX40 and CCR4 proteins as well as other molecules associated with cell signaling on the cell surface (NT5E, MXRA8, and PTPRK). While comparing the profile of gene expression in CD4+CD8+ DP cells from patients with and without warning signs of plasma leakage, similar expression profile was observed, implying a role of CD4+CD8+ DP cells in plasma leakage through a quantitative increase rather than functional alteration. This study provided novel insight into the host immune response during the acute febrile phase of DENV infection and the role of CD4+CD8+ DP T cells in the pathogenesis of plasma leakage.

A Phase I, Open-Label, Dose-Escalation Study of the OX40 Agonist Ivuxolimab in Patients with Locally Advanced or Metastatic Cancers

PURPOSE

Stimulation of effector T cells is an appealing immunotherapeutic approach in oncology. OX40 (CD134) is a costimulatory receptor expressed on activated CD4⁺ and CD8⁺ T cells. Induction of OX40 following antigen recognition results in enhanced T-cell activation, proliferation, and survival, and OX40 targeting shows therapeutic efficacy in preclinical studies. We report the monotherapy dose-escalation portion of a multicenter, phase I trial (NCT02315066) of ivuxolimab (PF-04518600), a fully human immunoglobulin G2 agonistic monoclonal antibody specific for human OX40.

PATIENTS AND METHODS

Adult patients (N = 52) with selected locally advanced or metastatic cancers received ivuxolimab 0.01 to 10 mg/kg. Primary endpoints were safety and tolerability. Secondary/exploratory endpoints included preliminary assessment of antitumor activity and biomarker analyses.

RESULTS

The most common all-causality adverse events were fatigue (46.2%), nausea (28.8%), and decreased appetite (25.0%). Of 31 treatment-related adverse events, 30 (96.8%) were grade ≤2. No dose-limiting toxicities occurred. Ivuxolimab exposure increased in a dose-proportionate manner from 0.3 to 10 mg/kg. Full peripheral blood target engagement occurred at ≥0.3 mg/kg. Three (5.8%) patients achieved a partial response, and disease control was achieved in 56% of patients. Increased CD4⁺ central memory T-cell proliferation and activation, and clonal expansion of CD4⁺ and CD8⁺ T cells in peripheral blood were observed at 0.1 to 3.0 mg/kg. Increased immune cell infiltrate and OX40 expression were evident in on-treatment tumor biopsies.

CONCLUSIONS

Ivuxolimab was generally well tolerated with on-target immune activation at clinically relevant doses, showed preliminary antitumor activity, and may serve as a partner for combination studies.

T lymphocyte-targeted immune checkpoint modulation in glioma

Immunomodulatory therapies targeting inhibitory checkpoint molecules have revolutionized the treatment of solid tumor malignancies. Concerns about whether systemic administration of an immune checkpoint inhibitor could impact primary brain tumors were answered with the observation of definitive responses in pediatric patients harboring hypermutated gliomas. Although initial clinical results in patients with glioblastoma (GBM) were disappointing, recently published results have demonstrated a potential survival benefit in patients with recurrent GBM treated with neoadjuvant programmed cell death protein 1 blockade. While these findings necessitate verification in subsequent studies, they support the possibility of achieving clinical meaningful immune responses in malignant primary brain tumors including GBM, a disease in dire need of additional therapeutic options. There are several challenges involved in treating glioma with immune checkpoint modulators including the immunosuppressive nature of GBM itself with high inhibitory checkpoint expression, the immunoselective blood brain barrier impairing the ability for peripheral lymphocytes to traffic to the tumor microenvironment and the high prevalence of corticosteroid use which suppress lymphocyte activation. However, by simultaneously targeting multiple costimulatory and inhibitory pathways, it may be possible to achieve an effective antitumoral immune response. To this end, there are now several novel agents targeting more recently uncovered “second generation” checkpoint molecules. Given the multiplicity of drugs being considered for combination regimens, an increased understanding of the mechanisms of action and resistance combined with more robust preclinical and early clinical testing will be needed to be able to adequately test these agents. This review summarizes our current understanding of T lymphocyte-modulating checkpoint molecules as it pertains to glioma with the hope for a renewed focus on the most promising therapeutic strategies.

Sindbis Virus with Anti-OX40 Overcomes the Immunosuppressive Tumor Microenvironment of Low-Immunogenic Tumors

Despite remarkable responses to cancer immunotherapy in a subset of patients, many patients remain resistant to therapies. It is now clear that elevated levels of tumor-infiltrating T cells as well as a systemic anti-tumor immune response are requirements for successful immunotherapies. However, the tumor microenvironment imposes an additional resistance mechanism to immunotherapy. We have developed a practical and improved strategy for cancer immunotherapy using an oncolytic virus and anti-OX40. This strategy takes advantage of a preexisting T cell immune repertoire in vivo, removing the need to know about present tumor antigens. We have shown in this study that the replication-deficient oncolytic Sindbis virus vector expressing interleukin-12 (IL-12) (SV.IL12) activates immune-mediated tumor killing by inducing OX40 expression on CD4 T cells, allowing the full potential of the agonistic anti-OX40 antibody. The combination of SV.IL12 with anti-OX40 markedly changes the transcriptome signature and metabolic program of T cells, driving the development of highly activated terminally differentiated effector T cells. These metabolically reprogrammed T cells demonstrate enhanced tumor infiltration capacity as well as anti-tumor activity capable of overcoming the repressive tumor microenvironment. Our findings identify SV.IL12 in combination with anti-OX40 to be a novel and potent therapeutic strategy that can cure multiple types of low-immunogenic solid tumors.

Co-Stimulatory Receptors in Cancers and Their Implications for Cancer Immunotherapy

Immune checkpoint inhibitors (ICIs), including anti-PD-1 and anti-CTLA-4 therapeutic agents, are now approved by the Food and Drug Administration for treatment of various types of cancer. However, the therapeutic efficacy of ICIs varies among patients and cancer types. Moreover, most patients do not develop durable antitumor responses after ICI therapy due to an ephemeral reversal of T-cell dysfunction. As co-stimulatory receptors play key roles in regulating the effector functions of T cells, activating co-stimulatory pathways may improve checkpoint inhibition efficacy, and lead to durable antitumor responses. Here, we review recent advances in our understating of co-stimulatory receptors in cancers, providing the necessary groundwork for the rational design of cancer immunotherapy.

An OX40/OX40L interaction directs successful immunity to hepatitis B virus

Depending on age of acquisition, hepatitis B virus (HBV) can induce a cell-mediated immune response that results in either cure or progressive liver injury. In adult-acquired infection, HBV antigens are usually cleared, whereas in infancy-acquired infection, they persist. Individuals infected during infancy therefore represent the majority of patients chronically infected with HBV (CHB). A therapy that can promote viral antigen clearance in most CHB patients has not been developed and would represent a major health care advance and cost mitigator. Using an age-dependent mouse model of HBV clearance and persistence in conjunction with human blood and liver tissue, we studied mechanisms of viral clearance to identify new therapeutic targets. We demonstrate that age-dependent expression of the costimulatory molecule OX40 ligand (OX40L) by hepatic innate immune cells is pivotal in determining HBV immunity, and that treatment with OX40 agonists leads to improved HBV antigen clearance in young mice, as well as increased strength of T cell responses in young mice and adult mice that were exposed to HBV when they were young and developed a CHB serological profile. Similarly, in humans, we show that hepatic OX40L transcript expression is age-dependent and that increased OX40 expression on peripheral CD4⁺ T cells in adults is associated with HBV clearance. These findings provide new mechanistic understanding of the immune pathways and cells necessary for HBV immunity and identify potential therapeutic targets for resolving CHB.

OX40 (CD134) and OX40 ligand, important immune checkpoints in cancer

Immunotherapy has shown promising results in cancer treatment. Research shows that most patients might be resistant to these therapies. So, new immune therapies are needed. OX40 (CD134) and OX40 ligand (OX40L), costimulatory molecules, express on different types of immune cells. The interaction between OX40 and OX40L (OX40/OX40L) induces the expansion and proliferation of T cells and decreases the immunosuppression of regulatory T (Treg) cells to enhance the immune response to the specific antigen. For the important role OX40 takes in the process of immunity, many clinical trials are focusing on OX40 to find out whether it may have active effects in clinical cancer treatment. The results of clinical trials are still not enough. So, we reviewed the OX40 and its ligand (OX40L) function in cancer, clinical trials with OX40/OX40L and the correlation between OX40/OX40L and other immune checkpoints to add more ideas to tumor feasible treatment.

HCV-specific CD4+ T cells of patients with acute and chronic HCV infection display high expression of TIGIT and other co-inhibitory molecules

The combined regulation of a network of inhibitory and activating T cell receptors may be a critical step in the development of chronic HCV infection. Ex vivo HCV MHC class I + II tetramer staining and bead-enrichment was performed with baseline and longitudinal PBMC samples of a cohort of patients with acute, chronic and spontaneously resolved HCV infection to assess the expression pattern of the co-inhibitory molecule TIGIT together with PD-1, BTLA, Tim-3, as well as OX40 and CD226 (DNAM-1) of HCV-specific CD4+ T cells, and in a subset of patients of HCV-specific CD8+ T cells. As the main result, we found a higher expression level of TIGIT+ PD-1+ on HCV-specific CD4+ T cells during acute and chronic HCV infection compared to patients with spontaneously resolved HCV infection (p < 0,0001). Conversely, expression of the complementary co-stimulatory receptor of TIGIT, CD226 (DNAM-1) was significantly decreased on HCV-specific CD4+ T cells during chronic infection. The predominant phenotype of HCV-specific CD4+ T cells during acute and chronic infection was TIGIT+, PD-1+, BTLA+, Tim-3−. This comprehensive phenotypic study confirms TIGIT together with PD-1 as a discriminatory marker of dysfunctional HCV-specific CD4+ T cells.

OX40 stimulation and PD-L1 blockade synergistically augment HBV-specific CD4 T cells in patients with HBeAg-negative infection

BACKGROUND & AIMS

Current antiviral therapies lack the potential to eliminate persistent hepatitis B virus (HBV) infection. HBV-specific T cells are crucial for HBV control and have recently been shown to be protective in patients following discontinuation of antiviral therapy. Thus, T cell-based approaches may greatly improve the therapeutic landscape of HBV infection. We aimed to augment HBV-specific CD4 T cells from chronically infected patients by targeting different immunological pathways.

METHODS

Expression of various co-stimulatory and inhibitory receptors on HBV- and influenza-specific CD4 T cells was analyzed directly ex vivo by MHC class II-tetramers. Patients infected with HBV genotype D were screened for CD4 T cell responses by IFN-γ ELISpot and intracellular cytokine staining following stimulation with overlapping peptides (OLPs) spanning the HBV-polyprotein. Stimulation with recombinant IL-7, an agonistic OX40-antibody or blockade of PD-L1 was performed in antigen-specific in vitro cultures. Cytokine secretion and expression of transcription factors were analyzed by flow cytometry. Responses targeting influenza, Epstein-Barr virus and tetanus toxoid served as controls.

RESULTS

Tetramer-staining revealed that the IL-7 receptor-alpha (CD127), OX40 and PD-1 constitute possible therapeutic targets as they were all strongly expressed on HBV-specific CD4 T cells ex vivo. The HBV-specific CD4 T cell responses identified by OLP screening targeted predominantly the HBV-polymerase and core proteins. Combined OX40 stimulation and PD-L1 blockade significantly augmented IFN-γ and IL-21 producing HBV-specific CD4 T cells in vitro, suggesting active T helper type 1 cell and follicular T helper cell programs. Indeed, transcription factors T-bet and Bcl6 were strongly expressed in cytokine-producing cells.

CONCLUSIONS

Combined OX40 stimulation and PD-L1 blockade augmented secretion of the helper T cell signature cytokines IFN-γ and IL-21, suggesting that immunotherapeutic approaches can improve HBV-specific CD4 T cell responses.

LAY SUMMARY

CD4 T cells are important in controlling viral infections but are impaired in the context of chronic hepatitis B virus (HBV) infection. Therapeutic approaches to cure chronic HBV infection are highly likely to require an immune-stimulatory component. This study demonstrates that HBV-specific CD4 T cells can be functionally augmented by combined stimulation of the co-stimulatory molecule OX40 and blockade of the inhibitory PD-1 pathway.

Multifactorial heterogeneity of virus-specific T cells and association with the progression of human chronic hepatitis B infection

Associations between chronic antigen stimulation, T cell dysfunction, and the expression of various inhibitory receptors are well characterized in several mouse and human systems. During chronic hepatitis B virus (HBV) infection (CHB), T cell responses are blunted with low frequencies of virus-specific T cells observed, making these parameters difficult to study. Here, using mass cytometry and a highly multiplexed combinatorial peptide–major histocompatibility complex (pMHC) tetramer strategy that allows for the detection of rare antigen-specific T cells, we simultaneously probed 484 unique HLA-A*1101–restricted epitopes spanning the entire HBV genome on T cells from patients at various stages of CHB. Numerous HBV-specific T cell populations were detected, validated, and profiled. T cells specific for two epitopes (HBVpol387and HBVcore169) displayed differing and complex heterogeneities that were associated with the disease progression, and the expression of inhibitory receptors on these cells was not linearly related with their extent of T cell dysfunction. For HBVcore169-specific CD8+T cells, we found cellular markers associated with long-term memory, polyfunctionality, and the presence of several previously unidentified public TCR clones that correlated with viral control. Using high-dimensional trajectory analysis of these cellular phenotypes, a pseudo-time metric was constructed that fit with the status of viral infection in corresponding patients. This was validated in a longitudinal cohort of patients undergoing antiviral therapy. Our study uncovers complex relationships of inhibitory receptors between the profiles of antigen-specific T cells and the status of CHB with implications for new strategies of therapeutic intervention.

CD8 T Cell Exhaustion During Chronic Viral Infection and Cancer

Exhausted CD8 T (Tex) cells are a distinct cell lineage that arise during chronic infections and cancers in animal models and humans. Tex cells are characterized by progressive loss of effector functions, high and sustained inhibitory receptor expression, metabolic dysregulation, poor memory recall and homeostatic self-renewal, and distinct transcriptional and epigenetic programs. The ability to reinvigorate Tex cells through inhibitory receptor blockade, such as αPD-1, highlights the therapeutic potential of targeting this population. Emerging insights into the mechanisms of exhaustion are informing immunotherapies for cancer and chronic infections. However, like other immune cells, Tex cells are heterogeneous and include progenitor and terminal subsets with unique characteristics and responses to checkpoint blockade. Here, we review our current understanding of Tex cell biology, including the developmental paths, transcriptional and epigenetic features, and cell intrinsic and extrinsic factors contributing to exhaustion and how this knowledge may inform therapeutic targeting of Tex cells in chronic infections, autoimmunity, and cancer.

Molecular Control of Follicular Helper T cell Development and Differentiation

Follicular helper T cells (Tfh) are specialized helper T cells that are predominantly located in germinal centers and provide help to B cells. The development and differentiation of Tfh cells has been shown to be regulated by transcription factors, such as B-cell lymphoma 6 protein (Bcl-6), signal transducer and activator of transcription 3 (STAT3) and B lymphocyte-induced maturation protein-1 (Blimp-1). In addition, cytokines, including IL-21, have been found to be important for Tfh cell development. Moreover, several epigenetic modifications have also been reported to be involved in the determination of Tfh cell fate. The regulatory network is complicated, and the number of novel molecules demonstrated to control the fate of Tfh cells is increasing. Therefore, this review aims to summarize the current knowledge regarding the molecular regulation of Tfh cell development and differentiation at the protein level and at the epigenetic level to elucidate Tfh cell biology and provide potential targets for clinical interventions in the future.

Role of TRAFs in Signaling Pathways Controlling T Follicular Helper Cell Differentiation and T Cell-Dependent Antibody Responses

Follicular helper T (T_FH) cells represent a highly specialized CD4⁺ T cell subpopulation that supports the generation of germinal centers (GC) and provides B cells with critical signals promoting antibody class switching, generation of high affinity antibodies, and memory formation. T_FH cells are characterized by the expression of the chemokine receptor CXCR5, the transcription factor Bcl-6, costimulatory molecules ICOS, and PD-1, and the production of cytokine IL-21. The acquisition of a T_FH phenotype is a complex and multistep process that involves signals received through engagement of the TCR along with a multitude of costimulatory molecules and cytokines receptors. Members of the Tumor necrosis factor Receptor Associated Factors (TRAF) represent one of the major classes of signaling mediators involved in the differentiation and functions of T_FH cells. TRAF molecules are the canonical adaptor molecules that physically interact with members of the Tumor Necrosis Factor Receptor Superfamily (TNFRSF) and actively modulate their downstream signaling cascades through their adaptor function and/or E3 ubiquitin ligase activity. OX-40, GITR, and 4-1BB are the TRAF-dependent TNFRSF members that have been implicated in the differentiation and functions of T_FH cells. On the other hand, emerging data demonstrate that TRAF proteins also participate in signaling from the TCR and CD28, which deliver critical signals leading to the differentiation of T_FH cells. More intriguingly, we recently showed that the cytoplasmic tail of ICOS contains a conserved TANK-binding kinase 1 (TBK1)-binding motif that is shared with TBK1-binding TRAF proteins. The presence of this TRAF-mimicking signaling module downstream of ICOS is required to mediate the maturation step during T_FH differentiation. In addition, JAK-STAT pathways emanating from IL-2, IL-6, IL-21, and IL-27 cytokine receptors affect T_FH development, and crosstalk between TRAF-mediated pathways and the JAK-STAT pathways can contribute to generate integrated signals required to drive and sustain T_FH differentiation. In this review, we will introduce the molecular interactions and the major signaling pathways controlling the differentiation of T_FH cells. In each case, we will highlight the contributions of TRAF proteins to these signaling pathways. Finally, we will discuss the role of individual TRAF proteins in the regulation of T cell-dependent humoral responses.

Follicular T Helper Cell Signatures in Primary Biliary Cholangitis and Primary Sclerosing Cholangitis

Primary sclerosing cholangitis (PSC) and primary biliary cholangitis (PBC) are the most common cholestatic liver diseases. While PBC is generally accepted to be an autoimmune disorder characterized by pathognomonic autoantibodies against mitochondrial antigens, the pathogenesis of PSC is less precisely defined; however, some degree of altered immunity toward autoantigens has been suggested. Follicular T helper (Tfh) cells, a distinct clusters of differentiation (CD)4 T‐cell subset specialized in facilitating antibody responses, have been shown to contribute to humoral autoimmunity in various disorders; yet, there is only limited information on possible alterations of Tfh cells in the context of cholestatic liver diseases. Thus, we addressed this important question by analyzing the frequency, activation status, and function of Tfh cells and frequencies of regulatory follicular T helper (Tfr) cells in well‐defined cohorts of patients with PBC and patients with PSC. Interestingly, we observed a significant increase in circulating chemokine (C‐X‐C motif) receptor 5 (CXCR5)+programmed death 1 (PD‐1) +CD4+ Tfh cells in patients with PBC but not in those with PSC. Although the frequency of potentially pathogenic chemokine (C‐C motif) receptor 7 (CCR7)lowCXCR5+PD‐1+CD4+ Tfh cells was increased in both disorders compared to healthy donors, the increase was significantly more pronounced in PBC. Furthermore, in patients with PBC, Tfh cells displayed stronger expression of the activation markers OX40 and inducible costimulator of T cells, correlated with anti‐anti‐mitochondrial antibody M2 and immunoglobulin M titers, and were most significantly increased in patients with cirrhosis. Tfr cell numbers were similarly increased; however, Tfh/Tfr ratios were unaltered in PSC and PBC. These alterations did not correlate with increased secretion of the Tfh signature cytokine interleukin‐21 in sorted CD4 T cells. Conclusion: Significant alterations occur in the Tfh cell compartment in cholestatic liver diseases, suggesting that Tfh cells influence the pathogenesis of PBC and to a lesser extend PSC.

T Cell/B Cell Collaboration and Autoimmunity: An Intimate Relationship

Co-ordinated interaction between distinct cell types is a hallmark of successful immune function. A striking example of this is the carefully orchestrated cooperation between helper T cells and B cells that occurs during the initiation and fine-tuning of T-cell dependent antibody responses. While these processes have evolved to permit rapid immune defense against infection, it is becoming increasingly clear that such interactions can also underpin the development of autoimmunity. Here we discuss a selection of cellular and molecular pathways that mediate T cell/B cell collaboration and highlight how in vivo models and genome wide association studies link them with autoimmune disease. In particular, we emphasize how CTLA-4-mediated regulation of CD28 signaling controls the engagement of secondary costimulatory pathways such as ICOS and OX40, and profoundly influences the capacity of T cells to provide B cell help. While our molecular understanding of the co-operation between T cells and B cells derives from analysis of secondary lymphoid tissues, emerging evidence suggests that subtly different rules may govern the interaction of T and B cells at ectopic sites during autoimmune inflammation. Accordingly, the phenotype of the T cells providing help at these sites includes notable distinctions, despite sharing core features with T cells imparting help in secondary lymphoid tissues. Finally, we highlight the interdependence of T cell and B cell responses and suggest that a significant beneficial impact of B cell depletion in autoimmune settings may be its detrimental effect on T cells engaged in molecular conversation with B cells.

The Role of Dendritic Cells in the Differentiation of T Follicular Helper Cells

T follicular helper cells (T_FH) are a subset of recently discovered CD4⁺ T cells. Their major function is to participate in the formation of germinal centres (GCs) and promote B cell proliferation and differentiation to play important roles in the production of antibodies. Currently, the functions of T_FH cells are clear. However, the early differentiation of these cells is not clear. Dendritic cells (DCs) participate in the differentiation of T_FH cells. Therefore, this article reviewed the research progress regarding the influence of DCs on the differentiation of T_FH cells and their major underlying mechanisms.

Dichotomous Expression of TNF Superfamily Ligands on Antigen-Presenting Cells Controls Post-priming Anti-viral CD4+ T Cell Immunity

T cell antigen-presenting cell (APC) interactions early during chronic viral infection are crucial for determining viral set point and disease outcome, but how and when different APC subtypes contribute to these outcomes is unclear. The TNF receptor superfamily (TNFRSF) member GITR is important for CD4⁺ T cell accumulation and control of chronic lymphocytic choriomeningitis virus (LCMV). We found that type I interferon (IFN-I) induced TNFSF ligands GITRL, 4-1BBL, OX40L, and CD70 predominantly on monocyte-derived APCs and CD80 and CD86 predominantly on classical dendritic cells (cDCs). Mice with hypofunctional GITRL in Lyz2⁺ cells had decreased LCMV-specific CD4⁺ T cell accumulation and increased viral load. GITR signals in CD4⁺ T cells occurred after priming to upregulate OX40, CD25, and chemokine receptor CX3CR1. Thus IFN-I (signal 3) induced a post-priming checkpoint (signal 4) for CD4⁺ T cell accumulation, revealing a division of labor between cDCs and monocyte-derived APCs in regulating T cell expansion.

CD30 Is Dispensable for T-Cell Responses to Influenza Virus and Lymphocytic Choriomeningitis Virus Clone 13 but Contributes to Age-Associated T-Cell Expansion in Mice

CD30 is a tumor necrosis factor receptor (TNFR) family member whose expression is associated with Hodgkin’s disease, anaplastic large cell lymphomas, and other T and B lymphoproliferative disorders in humans. A limited number of studies have assessed the physiological role of CD30/CD30 ligand interactions in control of infection in mice. Here, we assess the role of CD30 in T-cell immunity to acute influenza and chronic lymphocytic choriomeningitis virus (LCMV) clone 13 infection, two viral infections in which other members of the TNFR superfamily are important for T-cell responses. We show that CD30 is expressed on activated but not resting CD4 and CD8 T cells in vitro, as well as on regulatory T cells and marginally on T helper 1 cells in vivo during influenza infection. Despite this, CD4 and CD8 T-cell expansion in response to influenza virus was comparable in CD30^+/+ and CD30^−/− littermates, with no discernable role for the pathway in the outcome of influenza infection. Similarly, during persistent infection with LCMV clone 13, CD30 plays no obvious role in CD4 or CD8 T-cell responses, the level of T-cell exhaustion or viral control. In contrast, in the steady state, we observed increased numbers of total CD4 and CD8 T cells as well as increased numbers of regulatory T cells in unimmunized older (~8 months) CD30^+/+ but not in CD30^−/− age-matched littermates. Naive T-cell numbers were unchanged in the aged CD30^+/+ mice compared to their CD30^−/− littermate controls, rather the T-cell expansions were explained by an increase in CD4⁺ and CD8⁺ CD44^mid-hiCD62L⁻ effector memory cells, with a similar trend in the central memory T-cell compartment. In contrast, CD30 did not impact the numbers of T cells in young mice. These data suggest a role for CD30 in the homeostatic regulation of T cells during aging, contributing to memory T-cell expansions, which may have relevance for CD30 expression in human T-cell lymphoproliferative diseases.

Costimulatory and Coinhibitory Receptor Pathways in Infectious Disease

Costimulatory and inhibitory receptors play a key role in regulating immune responses to infections. Recent translation of knowledge about inhibitory receptors such as CTLA-4 and PD-1 into the cancer clinic highlights the opportunities to manipulate these pathways to treat human disease. Studies in infectious disease have provided key insights into the specific roles of these pathways and the effects of their manipulation. Here, recent studies are discussed that have addressed how major inhibitory and costimulatory pathways play a role in regulating immune responses during acute and chronic infections. Mechanistic insights from studies of infectious disease provide opportunities to further expand our toolkit to treat cancer and chronic infections in the clinic.

The TNF Receptor Superfamily in Co-stimulating and Co-inhibitory Responses

Cytokines related to tumor necrosis factor (TNF) provide a communication network essential for coordinating multiple cell types into an effective host defense system against pathogens and malignant cells. The pathways controlled by the TNF superfamily differentiate both innate and adaptive immune cells and modulate stromal cells into microenvironments conducive to host defenses. Members of the TNF receptor superfamily activate diverse cellular functions from the production of type 1 interferons to the modulation of survival of antigen-activated T cells. Here, we focus attention on the subset of TNF superfamily receptors encoded in the immune response locus in chromosomal region 1p36. Recent studies have revealed that these receptors use diverse mechanisms to either co-stimulate or restrict immune responses. Translation of the fundamental mechanisms of TNF superfamily is leading to the design of therapeutics that can alter pathogenic processes in several autoimmune diseases or promote immunity to tumors.

The Ox40/Ox40 Ligand Pathway Promotes Pathogenic Th Cell Responses, Plasmablast Accumulation, and Lupus Nephritis in NZB/W F1 Mice

Ox40 ligand (Ox40L) locus genetic variants are associated with the risk for systemic lupus erythematosus (SLE); however, it is unclear how Ox40L contributes to SLE pathogenesis. In this study, we evaluated the contribution of Ox40L and its cognate receptor, Ox40, using in vivo agonist and antagonist approaches in the NZB × NZW (NZB/W) F1 mouse model of SLE. Ox40 was highly expressed on several CD4 Th cell subsets in the spleen and kidney of diseased mice, and expression correlated with disease severity. Treatment of aged NZB/W F1 mice with agonist anti-Ox40 mAbs potently exacerbated renal disease, which was accompanied by activation of kidney-infiltrating T cells and cytokine production. The agonist mAbs also induced activation and inflammatory gene expression in splenic CD4 T cells, including IFN-regulated genes, increased the number of follicular helper T cells and plasmablasts in the spleen, and led to elevated levels of serum IgM and enhanced renal glomerular IgM deposition. In a type I IFN-accelerated lupus model, treatment with an antagonist Ox40:Fc fusion protein significantly delayed the onset of severe proteinuria and improved survival. These data support the hypothesis that the Ox40/Ox40L pathway drives cellular and humoral autoimmune responses during lupus nephritis in NZB/W F1 mice and emphasize the potential clinical value of targeting this pathway in human lupus.

The TCF1-Bcl6 axis counteracts type I interferon to repress exhaustion and maintain T cell stemness

During chronic viral infections and in cancer, T cells become dysfunctional, a state known as T cell exhaustion. Although it is well recognized that memory CD8 T cells account for the persistence of CD8 T cell immunity after acute infection, how exhausted T cells persist remains less clear. Using chronic infection with lymphocytic choriomeningitis virus clone 13 and tumor samples, we demonstrate that CD8 T cells differentiate into a less exhausted TCF1^high and a more exhausted TCF1^low population. Virus-specific TCF1^high CD8 T cells, which resemble T follicular helper (T_FH) cells, persist and recall better than do TCF1^low cells and act as progenitor cells to replenish TCF1^low cells. We show that TCF1 is both necessary and sufficient to support this progenitor-like CD8 subset, whereas cell-intrinsic type I interferon signaling suppresses their differentiation. Accordingly, cell-intrinsic TCF1 deficiency led to a loss of these progenitor CD8 T cells, sharp contraction of virus-specific T cells, and uncontrolled viremia. Mechanistically, TCF1 repressed several pro-exhaustion factors and induced Bcl6 in CD8 T cells, which promoted the progenitor fate. We propose that the TCF1-Bcl6 axis counteracts type I interferon to repress T cell exhaustion and maintain T cell stemness, which is critical for persistent antiviral CD8 T cell responses in chronic infection. These findings provide insight into the requirements for persistence of T cell immune responses in the face of exhaustion and suggest mechanisms by which effective T cell-mediated immunity may be enhanced during chronic infections and cancer.

OX40 Cooperates with ICOS To Amplify Follicular Th Cell Development and Germinal Center Reactions during Infection

Cognate interactions between T follicular helper (Tfh) cells and B cells are essential for promoting protective Ab responses. Whereas costimulatory receptors such as ICOS are accepted as being important for the induction of Tfh cell fate decision, other molecules may play key roles in amplifying or maintaining the Tfh phenotype. In this study, with vaccinia virus infection in mice, we show that OX40 was expressed on Tfh cells that accumulated at the T/B borders in the white pulp of the spleen and that OX40-dependent signals directly shaped the magnitude and quality of the their response to viral Ags. OX40 deficiency in Tfh cells profoundly impaired the acquisition of germinal center (GC) B cell phenotype, plasma cell generation, and virus-specific Ab responses. Most significantly, we found that sustained interactions between OX40 and its ligand, OX40L, beyond the time of initial encounter with dendritic cells were required for the persistence of high numbers of Tfh and GC B cells. Interestingly, OX40 was coexpressed with ICOS on Tfh cells in and around the GC, and ICOS-ICOSL interactions were similarly crucial at late times for maintenance of the Tfh and GC B cells. Thus, OX40 and ICOS act in a cooperative, nonredundant manner to maximize and prolong the Tfh response that is generated after acute virus infection.

Regulators of Tfh Cell Differentiation

The follicular helper T (Tfh) cells help is critical for activation of B cells, antibody class switching, and germinal center (GC) formation. The Tfh cells are characterized by the expression of CXC chemokine receptor 5 (CXCR5), ICOS, programed death 1 (PD-1), B cell lymphoma 6 (BCL-6), and IL-21. They are involved in clearing infections and are adversely linked with autoimmune diseases and also have a role in viral replication as well as clearance. On the one hand, Tfh cells are generated from naive CD4⁺ T cells with sequential steps involving cytokine signaling (IL-21, IL-6, IL-12, activin A), migration, and positioning in the GC by CXCR5, surface receptors (ICOS/ICOSL, signaling lymphocyte activation molecule-associated protein/signaling lymphocyte activation molecule) as well as transcription factor (BCL-6, c-Maf, and signal transducer and activator of transcription 3) signaling and repressor miR155. On the other hand, Tfh generation is negatively regulated at specific steps of Tfh generation by specific cytokine (IL-2, IL-7), surface receptor (PD-1, CTLA-4), transcription factors B lymphocyte maturation protein 1, signal transducer and activator of transcription 5, T-bet, KLF-2 signaling, and repressor miR 146a. Interestingly, miR-17-92 and FOXO1 act as a positive as well as a negative regulator of Tfh differentiation depending on the time of expression and disease specificity. Tfh cells are also generated from the conversion of other effector T cells as exemplified by Th1 cells converting into Tfh during viral infection. The mechanistic details of effector T cells conversion into Tfh are yet to be clear. To manipulate Tfh cells for therapeutic implication and or for effective vaccination strategies, it is important to know positive and negative regulators of Tfh generation. Hence, in this review, we have highlighted and interlinked molecular signaling from cytokines, surface receptors, transcription factors, ubiquitin ligase, and microRNA as positive and negative regulators for Tfh differentiation.

C-Myc regulation by costimulatory signals modulates the generation of CD8+ memory T cells during viral infection

The signalling mechanisms of costimulation in the development of memory T cells remain to be clarified. Here, we show that the transcription factor c-Myc in CD8⁺ T cells is controlled by costimulatory molecules, which modulates the development of memory CD8⁺ T cells. C-Myc expression was dramatically reduced in Cd28^−/− or Ox40^−/− memory CD8⁺ T cells, and c-Myc over-expression substantially reversed the defects in the development of T-cell memory following viral infection. C-Myc regulated the expression of survivin, an inhibitor of apoptosis, which promoted the generation of virus-specific memory CD8⁺ T cells. Moreover, over-expression of survivin with bcl-xL, a downstream molecule of NF-κB and intracellular target of costimulation that controls survival, in Cd28^−/− or Ox40^−/− CD8⁺ T cells, reversed the defects in the generation of memory T cells in response to viral infection. These results identify c-Myc as a key controller of memory CD8⁺ T cells from costimulatory signals.

Forsythoside A Inhibits BVDV Replication via TRAF2-Dependent CD28-4-1BB Signaling in Bovine PBMCs

Bovine viral diarrhea virus (BVDV), the causative agent of bovine viral diarrhea/mucosal disease (BVD/MD), is an important pathogen of cattle and other wild animals throughout the world. BVDV infection typically leads to an impaired immune response in cattle. In the present study, we investigated the effect of Forsythoside A (FTA) on BVDV infection of bovine peripheral blood mononuclear cells (PBMCs). We found that Forsythoside A could not only promote proliferation of PBMCs and T cells activation but also inhibit the replication of BVDV as well as apoptosis induced by BVDV. FTA treatment could counteract the BVDV-induced overproduction of IFN-γ to maintain the immune homeostasis in bovine PBMCs. At same time, FTA can enhance the secretion of IL-2. What's more, BVDV promotes the expression of CD28, 4-1BB and TRAF-2, which can be modulated by FTA. Our data suggest that FTA protects PBMCs from BVDV infection possibly via TRAF2-dependent CD28-4-1BB signaling, which may activate PBMCs in response to BVDV infection. Therefore, this aids in the development of an effective adjuvant for vaccines against BVDV and other specific FTA-based therapies for preventing BVDV infection.

Viral Persistence Induces Antibody Inflation without Altering Antibody Avidity

Antibodies are implicated in long-term immunity against numerous pathogens, and because of this property, antibody induction is the basis for many vaccines. Little is known about the influence of viral persistence on the evolving antibody response. Here, we examined the characteristics of antibody responses to persistent infection by employing the prototypic betaherpesvirus family member cytomegalovirus (CMV) in experimental mouse models. During the course of infection, mouse CMV (MCMV)-specific IgM and IgG responses are elicited; however, IgG levels gradually inflate in the persistent phase of infection while IgM levels are stably maintained. Whereas CD27-CD70 interactions are dispensable, the CD28/B7 costimulatory pathway is critical for the class switching of MCMV-specific IgM-to-IgG B cell responses, which corresponds to the CD28/B7-dependent formation of CD4(+)T follicular helper cells (TFH) and germinal center (GC) B cells. Furthermore, the initial viral inoculum dose dictates the height of the antibody levels during IgG antibody inflation and relates to the induction of long-lived plasma cells and memory B cells. Antibody avidity nonetheless is not altered after the establishment of viral persistence and occurs independently of the inoculum doses. However, repetitive challenge with intact viral particles, accompanied by increased GC reactivity, promotes the development of high-avidity IgG responses with neutralizing capacity. These insights can be used for the rational design of CMV-based vaccines aimed at inducing antibody responses.

IMPORTANCE

Antibodies provide long-term protection to different pathogens. However, how antibody responses develop during persistent virus infection is not entirely clear. Here, we characterize factors that influence the virus-specific antibody response to persistent CMV. This study describes that during persistent infection, CMV-specific IgM antibody levels are stably maintained while IgG2b and IgG2c levels gradually inflate over time. In contrast, the IgG avidity remains similar after the establishment of viral persistence. The induction of T follicular helper cells and GC B cells requires CD4(+)T cell help and CD28/B7 costimulation signals and is essential for the development of CMV-specific IgG antibody responses. Furthermore, neutralizing CMV-specific antibodies appear to develop late after infection, yet the neutralizing capacity can be improved upon repetitive viral challenge that is associated with increased GC reactivity. The results described here could inform the use of CMV-based vaccines and may help to understand how our immune system copes with this persistent virus.

Virus-Specific CD4+ T Cells Have Functional and Phenotypic Characteristics of Follicular T-Helper Cells in Patients With Acute and Chronic HCV Infections

BACKGROUND & AIMS

Follicular T-helper (Tfh) cells contribute to pathogen-specific antibody responses by providing maturation signals to B cells. In mice with viral infections, virus-specific Tfh cells expand and are required to contain the infection. However, less is known about human virus-specific Tfh cells or their functions during infection. We investigated whether virus-specific CD4+ T cells from patients with hepatitis C virus (HCV) infection had phenotypic or functional features of Tfh cells and contribute to the production of HCV-specific antibodies.

METHODS

We collected blood samples from patients with acute and chronic HCV infection and healthy individuals (controls). We performed MHC class II tetramer analyses, assays to detect intracellular cytokines in response to HCV exposure, and analyses to quantify HCV-specific antibodies. In addition, we collected liver tissues from patients with chronic HCV infection or nonviral liver disease to analyze markers of Tfh cells.

RESULTS

HCV-specific CD4+ T cells from patients with acute HCV infection expressed markers of Tfh cells and secreted interleukin 21 in response to HCV exposure. Longitudinal analyses of HCV-specific T-cell responses and antibody responses showed an association between expression of inducible T-cell co-stimulator and induction of virus-specific antibodies in patients with acute HCV infection. Markers of Tfh cells were barely detectable in the peripheral blood samples from patients with chronic HCV infection, but were detected in liver tissues.

CONCLUSIONS

Virus-specific Tfh cells can be detected in blood samples from patients with acute HCV infection; inducible T-cell co-stimulator expression correlates with production of HCV-specific antibodies. In patients with chronic infection, Tfh cells seem to disappear from the blood but are detectable in the liver.

Analysis of a Genetic Polymorphism in the Costimulatory Molecule TNFSF4 with Hematopoietic Stem Cell Transplant Outcomes

Despite stringent procedures to secure the best HLA matching between donors and recipients, life-threatening complications continue to occur after hematopoietic stem cell transplantation (HSCT). Studying single nucleotide polymorphism (SNP) in genes encoding costimulatory molecules could help identify patients at risk for post-HSCT complications. In a stepwise approach we selected SNPs in key costimulatory molecules including CD274, CD40, CD154, CD28, and TNFSF4 and systematically analyzed their association with post-HSCT outcomes. Our discovery cohort analysis of 1157 HLA-A, -B, -C, -DRB1, and -DQB1 matched cases found that patients with donors homozygous for the C variant of rs10912564 in TNFSF4 (48%) had better disease-free survival (P = .029) and overall survival (P = .009) with less treatment-related mortality (P = .006). Our data demonstrate the TNFSF4C variant had a higher affinity for the nuclear transcription factor Myb and increased percentage of TNFSF4-positive B cells after stimulation compared with CT or TT genotypes. However, these associations were not validated in a more recent cohort, potentially because of changes in standard of practice or absence of a true association. Given the discovery cohort, functional data, and importance of TNFSF4 in infection clearance, TNFSF4C may associate with outcomes and warrants future studies.

TNFRs and Control of Chronic LCMV Infection: Implications for Therapy

The control of persistent viral infections requires the immune system to limit the spread of the virus while avoiding immunopathology. Recent studies have revealed that members of the tumor necrosis factor receptor (TNFR) superfamily play unique and pivotal roles in control of chronic lymphocytic choriomeningitis virus (LCMV) infection and in some settings can tip the balance between immune control and immune pathology. We review these findings and discuss how our understanding of the role of TNFRs in the immune response to chronic LCMV infection may shed light on what happens during HIV infection in humans. We discuss preclinical models of TNF/TNFR family-targeted immunotherapy of chronic LCMV infection and evaluate which TNFRs present the most promising targets for immune intervention.

PD-1 Co-inhibitory and OX40 Co-stimulatory Crosstalk Regulates Helper T Cell Differentiation and Anti-Plasmodium Humoral Immunity

The differentiation and protective capacity of Plasmodium-specific T cells are regulated by both positive and negative signals during malaria, but the molecular and cellular details remain poorly defined. Here we show that malaria patients and Plasmodium-infected rodents exhibit atypical expression of the co-stimulatory receptor OX40 on CD4 T cells and that therapeutic enhancement of OX40 signaling enhances helper CD4 T cell activity, humoral immunity, and parasite clearance in rodents. However, these beneficial effects of OX40 signaling are abrogated following coordinate blockade of PD-1 co-inhibitory pathways, which are also upregulated during malaria and associated with elevated parasitemia. Co-administration of biologics blocking PD-1 and promoting OX40 signaling induces excessive interferon-gamma that directly limits helper T cell-mediated support of humoral immunity and decreases parasite control. Our results show that targeting OX40 can enhance Plasmodium control and that crosstalk between co-inhibitory and co-stimulatory pathways in pathogen-specific CD4 T cells can impact pathogen clearance.

Enhanced CD8 T cell responses through GITR-mediated costimulation resolve chronic viral infection

Chronic infections are characterized by the inability to eliminate the persisting pathogen and often associated with functional impairment of virus-specific T-cell responses. Costimulation through Glucocorticoid-induced TNFR-related protein (GITR) can increase survival and function of effector T cells. Here, we report that constitutive expression of GITR-ligand (GITRL) confers protection against chronic lymphocytic choriomeningitis virus (LCMV) infection, accelerating recovery without increasing pathology. Rapid viral clearance in GITRL transgenic mice coincided with increased numbers of poly-functional, virus-specific effector CD8+ T cells that expressed more T-bet and reduced levels of the rheostat marker PD-1. GITR triggering also boosted the helper function of virus-specific CD4 T cells already early in the infection, as was evidenced by increased IL-2 and IFNγ production, and more expression of CD40L and T-bet. Importantly, CD4-depletion experiments revealed that the expanded pool of virus-specific effector CD8 T cells and the ensuing viral clearance in LCMV-infected GITRL tg mice was entirely dependent on CD4 T cells. We found no major differences for NK cell and regulatory T cell responses, whereas the humoral response to the virus was increased in GITRL tg mice, but only in the late phase of the infection when the virus was almost eradicated. Based on these findings, we conclude that enhanced GITR-triggering mediates its protective, anti-viral effect on the CD8 T cell compartment by boosting CD4 T cell help. As such, increasing costimulation through GITR may be an attractive strategy to increase anti-viral CTL responses without exacerbating pathology, in particular to persistent viruses such as HIV and HCV.

The ability of the immune system to rapidly respond to a viral infection is a prerequisite for the survival of an individual. The immediate reaction of innate immune cells and the subsequent response of antigen-specific lymphocytes is usually effective for rapid neutralization and removal of the invading virus. Yet, such protective immune responses need to be well controlled, as they can cause severe tissue damage that may disable the host more than the infection itself. One way that has evolutionarily been proven effective to deal with this balancing act between protective immunity and prevention of immunopathology is to render virus-specific T cells “exhausted” when the virus cannot be eradicated and the host becomes chronically infected. Exhausted T cells progressively lose their ability to kill other cells and produce different cytokines. The benefit of this exhausted state of anti-viral immunity is that it induces less tissue damage, but the downside is obviously less efficient control over the viral infection. Many immunotherapeutic and vaccination strategies against chronic viral infections are currently dedicated to overcome the exhausted state of the virus-specific T cells and thereby clear the virus. However, the accompanying risk is an exaggerated immune response with overt immunopathology. Here we describe in a mouse model that enhanced triggering through the costimulatory molecule GITR on T cells is able to provide protection upon viral infection and clear an otherwise persistent virus, but importantly without the development of collateral damage due to immunopathology. We show that GITR-mediated costimulation enhances a protective CD8 T cell response, for which CD4 T cell help is required. Our study provides new insights in how a particular costimulatory pathway can be utilized to boost anti-viral immunity, which is highly relevant for the development of safe immunotherapeutic strategies against chronic viral infections in humans.

The regulation of T follicular helper responses during infection

Following infection, naïve CD4 T cells can differentiate into various functionally distinct effector and memory subsets, including T follicular helper (TFH) cells that orchestrate germinal center (GC) reactions necessary for high-affinity, pathogen-specific antibody responses. The origins and function of this cell type have been extensively examined in response to subunit immunization with model antigens. More recently, we are beginning to also appreciate the extent to which microbial infections shape the generation, function and maintenance of TFH cells. Here, we review recent advances and highlight additional knowledge gaps in our understanding of how microbial infections influence priming, differentiation, localization and activity of TFH cells following acute and chronic infections.

GITR intrinsically sustains early type 1 and late follicular helper CD4 T cell accumulation to control a chronic viral infection

CD4 T cells are critical for control of persistent infections; however, the key signals that regulate CD4 T help during chronic infection remain incompletely defined. While several studies have addressed the role of inhibitory receptors and soluble factors such as PD-1 and IL-10, significantly less work has addressed the role of T cell co-stimulatory molecules during chronic viral infection. Here we show that during a persistent infection with lymphocytic choriomeningitis virus (LCMV) clone 13, mice lacking the glucocorticoid-induced tumor necrosis factor receptor related protein (GITR) exhibit defective CD8 T cell accumulation, increased T cell exhaustion and impaired viral control. Differences in CD8 T cells and viral control between GITR+/+ and GITR-/- mice were lost when CD4 T cells were depleted. Moreover, mixed bone marrow chimeric mice, as well as transfer of LCMV epitope-specific CD4 or CD8 T cells, demonstrated that these effects of GITR are largely CD4 T cell-intrinsic. GITR is dispensable for initial CD4 T cell proliferation and differentiation, but supports the post-priming accumulation of IFNγ+IL-2+ Th1 cells, facilitating CD8 T cell expansion and early viral control. GITR-dependent phosphorylation of the p65 subunit of NF-κB as well as phosphorylation of the downstream mTORC1 target, S6 ribosomal protein, were detected at day three post-infection (p.i.), and defects in CD4 T cell accumulation in GITR-deficient T cells were apparent starting at day five p.i. Consistently, we pinpoint IL-2-dependent CD4 T cell help for CD8 T cells to between days four and eight p.i. GITR also increases the ratio of T follicular helper to T follicular regulatory cells and thereby enhances LCMV-specific IgG production. Together, these findings identify a CD4 T cell-intrinsic role for GITR in sustaining early CD8 and late humoral responses to collectively promote control of chronic LCMV clone 13 infection.

OX40- and CD27-mediated costimulation synergizes with anti-PD-L1 blockade by forcing exhausted CD8+ T cells to exit quiescence

Exhaustion of chronically stimulated CD8(+) T cells is a significant obstacle to immune control of chronic infections or tumors. Although coinhibitory checkpoint blockade with anti-programmed death ligand 1 (PD-L1) Ab can restore functions to exhausted T cell populations, recovery is often incomplete and dependent upon the pool size of a quiescent T-bet(high) subset that expresses lower levels of PD-1. In a model in which unhelped, HY-specific CD8(+) T cells gradually lose function following transfer to male bone marrow transplantation recipients, we have explored the effect of shifting the balance away from coinhibition and toward costimulation by combining anti-PD-L1 with agonistic Abs to the TNFR superfamily members, OX40 and CD27. Several weeks following T cell transfer, both agonistic Abs, but especially anti-CD27, demonstrated synergy with anti-PD-L1 by enhancing CD8(+) T cell proliferation and effector cytokine generation. Anti-CD27 and anti-PD-L1 synergized by downregulating the expression of multiple quiescence-related genes concomitant with a reduced frequency of T-bet(high) cells within the exhausted population. However, in the presence of persistent Ag, the CD8(+) T cell response was not sustained and the overall size of the effector cytokine-producing pool eventually contracted to levels below that of controls. Thus, CD27-mediated costimulation can synergize with coinhibitory checkpoint blockade to switch off molecular programs for quiescence in exhausted T cell populations, but at the expense of losing precursor cells required to maintain a response.

CD4⁺ and CD8⁺ T cell-dependent antiviral immunity requires STIM1 and STIM2

Calcium signaling is critical for lymphocyte function, and intracellular Ca2+ concentrations are regulated by store-operated Ca2+ entry (SOCE) through Ca2+ release-activated Ca2+ (CRAC) channels. In patients, loss-of-function mutations in CRAC channel components ORAI1 and STIM1 abolish SOCE and are associated with recurrent and chronic viral infections. Here, using mice with conditional deletion of Stim1 and its homolog Stim2 in T cells, we determined that both components are required for the maintenance of virus-specific memory CD8+ T cells and recall responses following secondary infection. In the absence of STIM1 and STIM2, acute viral infections became chronic. Early during infection, STIM1 and STIM2 were required for the differentiation of naive CD8+ T cells into fully functional cytolytic effector cells and mediated the production of cytokines and prevented cellular exhaustion in viral-specific CD8+ effector T cells. Importantly, memory and recall responses by CD8+ T cells required expression of STIM1 and STIM2 in CD4+ T cells. CD4+ T cells lacking STIM1 and STIM2 were unable to provide "help" to CD8+ T cells due to aberrant regulation of CD40L expression. Together, our data indicate that STIM1, STIM2, and CRAC channel function play distinct but synergistic roles in CD4+ and CD8+ T cells during antiviral immunity.

OX40 controls effector CD4+ T-cell expansion, not follicular T helper cell generation in acute Listeria infection

To investigate the importance of OX40 signals for physiological CD4⁺ T-cell responses, an endogenous antigen-specific population of CD4⁺ T cells that recognise the 2W1S peptide was assessed and temporal control of OX40 signals was achieved using blocking or agonistic antibodies (Abs) in vivo. Following infection with Listeria monocytogenes expressing 2W1S peptide, OX40 was briefly expressed by the responding 2W1S-specific CD4⁺ T cells, but only on a subset that co-expressed effector cell markers. This population was specifically expanded by Ab-ligation of OX40 during priming, which also caused skewing of the memory response towards effector memory cells. Strikingly, this greatly enhanced effector response was accompanied by the loss of T follicular helper (TFH) cells and germinal centres. Mice deficient in OX40 and CD30 showed normal generation of TFH cells but impaired numbers of 2W1S-specific effector cells. OX40 was not expressed by 2W1S-specific memory cells, although it was rapidly up-regulated upon challenge whereupon Ab-ligation of OX40 specifically affected the effector subset. In summary, these data indicate that for CD4⁺ T cells, OX40 signals are important for generation of effector T cells rather than TFH cells in this response to acute bacterial infection.

The role of viral persistence in flavivirus biology

In nature, vector borne flaviviruses are persistently cycled between either the tick or mosquito vector and small mammals such as rodents, skunks, and swine. These viruses account for considerable human morbidity and mortality worldwide. Increasing and substantial evidence of viral persistence in humans, which includes the isolation of RNA by RT PCR and infectious virus by culture, continues to be reported. Viral persistence can also be established in vitro in various human, animal, arachnid, and insect cell lines in culture. Although some research has focused on the potential roles of defective virus particles, evasion of the immune response through the manipulation of autophagy and/or apoptosis, the precise mechanism of flavivirus persistence is still not well understood. We propose additional research for further understanding of how viral persistence is established in different systems. Avenues for additional studies include determining whether the multifunctional flavivirus protein NS5 has a role in viral persistence, the development of relevant animal models of viral persistence, and investigating the host responses that allow vector borne flavivirus replication without detrimental effects on infected cells. Such studies might shed more light on the viral–host relationships and could be used to unravel the mechanisms for establishment of persistence.

Persistent infections by vector borne flaviviruses are an important, but inadequately studied topic.