Control of virus-specific CD8+ T-cell exhaustion and immune-mediated pathology by E3 ubiquitin ligase Cbl-b during chronic viral infection

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.

Low-dose IL-2 therapy invigorates CD8+ T cells for viral control in systemic lupus erythematosus

Autoimmune diseases are often treated by glucocorticoids and immunosuppressive drugs that could increase the risk for infection, which in turn deteriorate disease and cause mortality. Low-dose IL-2 (Ld-IL2) therapy emerges as a new treatment for a wide range of autoimmune diseases. To examine its influence on infection, we retrospectively studied 665 patients with systemic lupus erythematosus (SLE) including about one third receiving Ld-IL2 therapy, where Ld-IL2 therapy was found beneficial in reducing the incidence of infections. In line with this clinical observation, IL-2 treatment accelerated viral clearance in mice infected with influenza A virus or lymphocytic choriomeningitis virus (LCMV). Noticeably, despite enhancing anti-viral immunity in LCMV infection, IL-2 treatment exacerbated CD8+ T cell-mediated immunopathology. In summary, Ld-IL2 therapy reduced the risk of infections in SLE patients and enhanced the control of viral infection, but caution should be taken to avoid potential CD8+ T cell-mediated immunopathology.

Severely ill COVID-19 patients display impaired exhaustion features in SARS-CoV-2-reactive CD8+ T cells

The molecular properties of CD8+ T cells that respond to SARS-CoV-2 infection are not fully known. Here, we report on the single-cell transcriptomes of >80,000 virus-reactive CD8+ T cells, obtained using a modified Antigen-Reactive T cell Enrichment (ARTE) assay, from 39 COVID-19 patients and 10 healthy subjects. COVID-19 patients segregated into two groups based on whether the dominant CD8+ T cell response to SARS-CoV-2 was 'exhausted' or not. SARS-CoV-2-reactive cells in the exhausted subset were increased in frequency and displayed lesser cytotoxicity and inflammatory features in COVID-19 patients with mild compared to severe illness. In contrast, SARS-CoV-2-reactive cells in the dominant non-exhausted subset from patients with severe disease showed enrichment of transcripts linked to co-stimulation, pro-survival NF-κB signaling, and anti-apoptotic pathways, suggesting the generation of robust CD8+ T cell memory responses in patients with severe COVID-19 illness. CD8+ T cells reactive to influenza and respiratory syncytial virus from healthy subjects displayed polyfunctional features and enhanced glycolysis. Cells with such features were largely absent in SARS-CoV-2-reactive cells from both COVID-19 patients and healthy controls non-exposed to SARS-CoV-2. Overall, our single-cell analysis revealed substantial diversity in the nature of CD8+ T cells responding to SARS-CoV-2.

E3 ubiquitin ligase Casitas B lineage lymphoma-b and its potential therapeutic implications for immunotherapy

The distinction of self from non-self is crucial to prevent autoreactivity and ensure protection from infectious agents and tumors. Maintaining the balance between immunity and tolerance of immune cells is strongly controlled by several sophisticated regulatory mechanisms of the immune system. Among these, the E3 ligase ubiquitin Casitas B cell lymphoma-b (Cbl-b) is a newly identified component in the ubiquitin-dependent protein degradation system, which is thought to be an important negative regulator of immune cells. An update on the current knowledge and new concepts of the relevant immune homeostasis program co-ordinated by Cbl-b in different cell populations could pave the way for future immunomodulatory therapies of various diseases, such as autoimmune and allergic diseases, infections, cancers and other immunopathological conditions. In the present review, the latest findings are comprehensively summarized on the molecular structural basis of Cbl-b and the suppressive signaling mechanisms of Cbl-b in physiological and pathological immune responses, as well as its emerging potential therapeutic implications for immunotherapy in animal models and human diseases.

Cellular and Molecular Mechanisms of CD8+ T Cell Differentiation, Dysfunction and Exhaustion

T cells follow a triphasic distinct pathway of activation, proliferation and differentiation before becoming functionally and phenotypically “exhausted” in settings of chronic infection, autoimmunity and in cancer. Exhausted T cells progressively lose canonical effector functions, exhibit altered transcriptional networks and epigenetic signatures and gain constitutive expression of a broad coinhibitory receptor suite. This review outlines recent advances in our understanding of exhausted T cell biology and examines cellular and molecular mechanisms by which a state of dysfunction or exhaustion is established, and mechanisms by which exhausted T cells may still contribute to pathogen or tumour control. Further, this review describes our understanding of exhausted T cell heterogeneity and outlines the mechanisms by which checkpoint blockade differentially engages exhausted T cell subsets to overcome exhaustion and recover T cell function.

COVID-19 therapy with mesenchymal stromal cells (MSC) and convalescent plasma must consider exosome involvement: Do the exosomes in convalescent plasma antagonize the weak immune antibodies?

Exosome extracellular vesicles as biologic therapy for COVID-19 are discussed for two areas. The first involves the growing use of mesenchymal stromal cells (MSCs) for the profound clinical cytokine storm and severe pneumonia in COVID-19 patients. Instead, it is recommended to treat alternatively with their MSC-released exosomes. This is because many reports in the literature and our data have shown that the release of exosomes from the in vivo administered MSC is actually responsible for their beneficial effects. Further, the exosomes are superior, simpler and clinically more convenient compared to their parental MSC. Additionally, in the context of COVID-19, the known tendency of MSC to intravascularly aggregate causing lung dysfunction might synergize with the pneumonia aspects, and the tendency of MSC peripheral vascular micro aggregates might synergize with the vascular clots of the COVID-19 disease process, causing significant central or peripheral vascular insufficiency. The second exosome therapeutic area for severe COVID-19 involves use of convalescent plasma for its content of acquired immune antibodies that must consider the role in this therapy of contained nearly trillions of exosomes. Many of these derive from activated immune modulating cells and likely can function to transfer miRNAs that acting epigenetically to also influence the convalescent plasma recipient response to the virus. There is sufficient evidence, like recovery of patients with antibody deficiencies, to postulate that the antibodies actually have little effect and that immune resistance is principally due to T cell mechanisms. Further, COVID-19 convalescent plasma has remarkably weak beneficial effects if compared to what was expected from many prior studies. This may be due to the dysfunctional immune response to the infection and resulting weak Ab that may be impaired further by antagonistic exosomes in the convalescent plasma. At the least, pre selection of plasma for the best antibodies and relevant exosomes would produce the most optimum therapy for very severely affected COVID-19 patients.

Severely ill COVID-19 patients display augmented functional properties in SARS-CoV-2-reactive CD8 + T cells

The molecular properties of CD8 + T cells that respond to SARS-CoV-2 infection are not fully known. Here, we report on the single-cell transcriptomes of >80,000 virus-reactive CD8 + T cells from 39 COVID-19 patients and 10 healthy subjects. COVID-19 patients segregated into two groups based on whether the dominant CD8 + T cell response to SARS-CoV-2 was 'exhausted' or not. SARS-CoV-2-reactive cells in the exhausted subset were increased in frequency and displayed lesser cytotoxicity and inflammatory features in COVID-19 patients with mild compared to severe illness. In contrast, SARS-CoV-2-reactive cells in the non-exhausted subsets from patients with severe disease showed enrichment of transcripts linked to co-stimulation, pro-survival NF-κB signaling, and anti-apoptotic pathways, suggesting the generation of robust CD8 + T cell memory responses in patients with severe COVID-19 illness. CD8 + T cells reactive to influenza and respiratory syncytial virus from healthy subjects displayed polyfunctional features. Cells with such features were mostly absent in SARS-CoV-2 responsive cells from both COVID-19 patients and healthy controls non-exposed to SARS-CoV-2. Overall, our single-cell analysis revealed substantial diversity in the nature of CD8 + T cells responding to SARS-CoV-2.

E3 Ubiquitin Ligases as Immunotherapeutic Target in Atherosclerotic Cardiovascular Disease

Chronic low-grade inflammation drives atherosclerosis and despite optimal pharmacological treatment of classical cardiovascular risk factors, one third of the patients with atherosclerotic cardiovascular disease has elevated inflammatory biomarkers. Additional anti-inflammatory strategies to target this residual inflammatory cardiovascular risk are therefore required. T-cells are a dominant cell type in human atherosclerotic lesions. Modulation of T-cell activation is therefore a potential strategy to target inflammation in atherosclerosis. Ubiquitination is an important regulatory mechanism of T-cell activation and several E3 ubiquitin ligases, including casitas B-lineage lymphoma proto-oncogene B (Cbl-B), itchy homolog (Itch), and gene related to anergy in lymphocytes (GRAIL), function as a natural brake on T-cell activation. In this review we discuss recent insights on the role of Cbl-B, Itch, and GRAIL in atherosclerosis and explore the therapeutic potential of these E3 ubiquitin ligases in cardiovascular medicine.

Chimeric Hemagglutinin-Based Influenza Virus Vaccines Induce Protective Stalk-Specific Humoral Immunity and Cellular Responses in Mice

The high variation of the influenza virus hemagglutinin (HA), particularly of its immunodominant head epitopes, makes it necessary to reformulate seasonal influenza virus vaccines every year. Novel influenza virus vaccines that redirect the immune response toward conserved epitopes of the HA stalk domain should afford broad and durable protection. Sequential immunization with chimeric HAs (cHAs) that express the same conserved HA stalk and distinct exotic HA heads has been shown to elicit high levels of broadly cross-reactive Abs. In the current mouse immunization studies, we tested this strategy using inactivated split virion cHA influenza virus vaccines (IIV) without adjuvant or adjuvanted with AS01 or AS03 to measure the impact of adjuvant on the Ab response. The vaccines elicited high levels of cross-reactive Abs that showed activity in an Ab-dependent, cell-mediated cytotoxicity reporter assay and were protective in a mouse viral challenge model after serum transfer. In addition, T cell responses to adjuvanted IIV were compared with responses to a cHA-expressing live attenuated influenza virus vaccine (LAIV). A strong but transient induction of Ag-specific T cells was observed in the spleens of mice vaccinated with LAIV. Interestingly, IIV also induced T cells, which were successfully recalled upon viral challenge. Groups that received AS01-adjuvanted IIV or LAIV 4 wk before the challenge showed the lowest level of viral replication (i.e., the highest level of protection). These studies provide evidence that broadly cross-reactive Abs elicited by cHA vaccination demonstrate Fc-mediated activity. In addition, cHA vaccination induced Ag-specific cellular responses that can contribute to protection upon infection.

CBLB Constrains Inactivated Vaccine-Induced CD8+ T Cell Responses and Immunity against Lethal Fungal Pneumonia

Fungal infections in CD4⁺ T cell immunocompromised patients have risen sharply in recent years. Although vaccines offer a rational avenue to prevent infections, there are no licensed fungal vaccines available. Inactivated vaccines are safer but less efficacious and require adjuvants that may undesirably bias toward poor protective immune responses. We hypothesized that reducing the TCR signaling threshold could potentiate antifungal CD8⁺ T cell responses and immunity to inactivated vaccine in the absence of CD4⁺ T cells. In this study, we show that CBLB, a negative regulator of TCR signaling, suppresses CD8⁺ T cells in response to inactivated fungal vaccination in a mouse model of CD4⁺ T cell lymphopenia. Conversely, Cblb deficiency enhanced both the type 1 (e.g., IFN-γ) and type 17 (IL-17A) CD8⁺ T cell responses to inactivated fungal vaccines and augmented vaccine immunity to lethal fungal pneumonia. Furthermore, we show that immunization with live or inactivated vaccine yeast did not cause detectable pathologic condition in Cblb^-/- mice. Augmented CD8⁺ T cell responses in the absence of CBLB also did not lead to terminal differentiation or adversely affect the expression of transcription factors T-bet, Eomes, and RORγt. Additionally, our adoptive transfer experiments showed that CBLB impedes the effector CD8⁺ T cell responses in a cell-intrinsic manner. Finally, we showed that ablation of Cblb overcomes the requirement of HIF-1α for expansion of CD8⁺ T cells upon vaccination. Thus, adjuvants that target CBLB may augment inactivated vaccines and immunity against systemic fungal infections in vulnerable patients.

Diacylglycerol kinase ζ (DGKζ) and Casitas b-lineage proto-oncogene b-deficient mice have similar functional outcomes in T cells but DGKζ-deficient mice have increased T cell activation and tumor clearance

Targeting negative regulators downstream of the T cell receptor (TCR) represents a novel strategy to improve cancer immunotherapy. Two proteins that serve as critical inhibitory regulators downstream of the TCR are diacylglycerol kinase ζ (DGKζ), a regulator of Ras and PKC-θ signaling, and Casitas b-lineage proto-oncogene b (Cbl-b), an E3 ubiquitin ligase that predominantly regulates PI(3)K signaling. We sought to compare the signaling and functional effects that result from deletion of DGKζ, Cbl-b, or both (double knockout, DKO) in T cells, and to evaluate tumor responses generated in a clinically relevant orthotopic pancreatic tumor model. We found that whereas deletion of Cbl-b primarily served to enhance NF-κB signaling, deletion of DGKζ enhanced TCR-mediated signal transduction downstream of Ras/Erk and NF-κB. Deletion of DGKζ or Cbl-b comparably enhanced CD8⁺ T cell functional responses, such as proliferation, production of IFNγ, and generation of granzyme B when compared with WT T cells. DKO T cells demonstrated enhanced function above that observed with single knockout T cells after weak, but not strong, stimulation. Deletion of DGKζ, but not Cbl-b, however, resulted in significant increases in numbers of activated (CD44^hi) CD8⁺ T cells in both non-treated and tumor-bearing mice. DGKζ-deficient mice also had enhanced control of pancreatic tumor cell growth compared to Cbl-b-deficient mice. This represents the first direct comparison between mice of these genotypes and suggests that T cell immunotherapies may be better improved by targeting TCR signaling molecules that are regulated by DGKζ as opposed to molecules regulated by Cbl-b.

Immune Regulation by Ubiquitin Tagging as Checkpoint Code

The immune system is equipped with effective machinery to mobilize its activation to defend invading microorganisms, and at the same time, to refrain from attacking its own tissues to maintain immune tolerance. The balance of activation and tolerance is tightly controlled by diverse mechanisms, since breakdown of tolerance could result in disastrous consequences such as the development of autoimmune diseases. One of the mechanisms is by the means of protein ubiquitination, which involves the process of tagging a small peptide ubiquitin to protein substrates. E3 ubiquitin ligases are responsible for catalyzing the final step of ubiquitin-substrate conjugation by specifically recognizing substrates to determine their fates of degradation or functional modification. The ubiquitination process is reversible, which is carried out by deubiquitinating enzymes to release the ubiquitin molecule from the conjugated substrates. Protein ubiquitination and deubiquitination serve as checkpoint codes in many key steps of lymphocyte regulation including the development, activation, differentiation, and tolerance induction. In this chapter, we will discuss a few E3 ligases and deubiquitinating enzymes that are important in controlling immune responses, with emphasis on their roles in T cells.

Scramblase TMEM16F terminates T cell receptor signaling to restrict T cell exhaustion

The scramblase TMEM16F is indispensable for the formation of multivesicular bodies in late endosomes that facilitate TCR degradation and signal termination. Hyperactivation of TMEM16F-deficient T cells in chronic LCMV infection leads to severe T cell exhaustion and uncontrolled virus burden.

In chronic infection, T cells become hyporesponsive to antigenic stimulation to prevent immunopathology. Here, we show that TMEM16F is required to curb excessive T cell responses in chronic infection with virus. TMEM16F-deficient T cells are hyperactivated during the early phase of infection, exhibiting increased proliferation and cytokine production. Interestingly, this overactivation ultimately leads to severe T cell exhaustion and the inability of the host to control viral burden. Mechanistically, we identify TMEM16F as the dominant lipid scramblase in T lymphocytes that transports phospholipids across membranes. TMEM16F is located in late endosomes, where it facilitates the generation of multivesicular bodies for TCR degradation and signal termination. Consequently, TMEM16F deficiency results in sustained signaling and augmented T cell activation. Our results demonstrate that scramblase restricts TCR responses to avoid overactivation, ensuring a well-balanced immune response in chronic infectious disease.

Correlation between low tapasin expression and impaired CD8+ T‑cell function in patients with chronic hepatitis B

Recent studies have demonstrated that chronic hepatitis B virus (HBV) infection is associated with reduced antigen‑presenting capacity and insufficient cytotoxic T lymphocyte (CTL) production. The molecular chaperone tapasin mediates binding of the transporter associated with antigen processing (TAP), and has an important role in endogenous antigen processing and presentation, and the induction of specific CTL responses. The present study aimed to determine whether tapasin is associated with chronic HBV (CHB) infection. The mRNA expression levels of tapasin were detected in peripheral blood mononuclear cells from 27 patients with CHB, 20 patients with acute HBV (AHB) and 26 healthy controls by reverse transcription‑quantitative polymerase chain reaction. In addition, CD8+ T immune responses were evaluated in all groups, and the correlation between tapasin expression and CD8+ responses was analyzed. The results demonstrated that the mRNA expression levels of tapasin were significantly downregulated in patients with CHB compared with in healthy controls and patients with AHB. Furthermore, the apoptotic rate of CD8+ T cells was increased in patients with CHB compared with in the other two groups. The percentage of interferon (IFN)‑γ+CD8+ T cells was reduced in patients with CHB compared with in patients with AHB and healthy controls, and serum cytokine levels (IFN‑γ, interleukin‑2 and tumor necrosis factor‑α) were generally low in patients with CHB. Furthermore, the mRNA expression levels of tapasin were positively correlated with IFN‑γ production by CD8+ T cells, and were inversely correlated with the apoptotic ratio of CD8+ T cells. These results indicate that decreased expression of tapasin may be closely associated with CHB, and suggest an important role for tapasin in the pathogenesis of CHB.

The Microbiome, Systemic Immune Function, and Allotransplantation

Diverse effects of the microbiome on solid organ transplantation are beginning to be recognized. In allograft recipients, microbial networks are disrupted by immunosuppression, nosocomial and community-based infectious exposures, antimicrobial therapies, surgery, and immune processes. Shifting microbial patterns, including acute infectious exposures, have dynamic and reciprocal interactions with local and systemic immune systems. Both individual microbial species and microbial networks have central roles in the induction and control of innate and adaptive immune responses, in graft rejection, and in ischemia-reperfusion injury. Understanding the diverse interactions between the microbiome and the immune system of allograft recipients may facilitate clinical management in the future.

Modulation of Immune Cell Functions by the E3 Ligase Cbl-b

Maintenance of immunological tolerance is a critical hallmark of the immune system. Several signaling checkpoints necessary to balance activating and inhibitory input to immune cells have been described so far, among which the E3 ligase Cbl-b appears to be a central player. Cbl-b is expressed in all leukocyte subsets and regulates several signaling pathways in T cells, NK cells, B cells, and different types of myeloid cells. In most cases, Cbl-b negatively regulates activation signals through antigen or pattern recognition receptors and co-stimulatory molecules. In line with this function, cblb-deficient immune cells display lower activation thresholds and cblb knockout mice spontaneously develop autoimmunity and are highly susceptible to experimental autoimmunity. Interestingly, genetic association studies link CBLB-polymorphisms with autoimmunity also in humans. Vice versa, the increased activation potential of cblb-deficient cells renders them more potent to fight against malignancies or infections. Accordingly, several reports have shown that cblb knockout mice reject tumors, which mainly depends on cytotoxic T and NK cells. Thus, targeting Cbl-b may be an interesting strategy to enhance anti-cancer immunity. In this review, we summarize the findings on the molecular function of Cbl-b in different cell types and illustrate the potential of Cbl-b as target for immunomodulatory therapies.

The Fusion Protein of CTP-HBcAg18-27-Tapasin Mediates the Apoptosis of CD8(+)T Cells and CD8(+) T Cell Response in HLA-A2 Transgenic Mice

BACKGROUND

HBV-specific cytotoxic T lymphocyte (CTL) activity is believed to play a critical role in controlling HBV infection. The phosphatidylinositol 3-kinase (PI3K)/Akt signaling pathway manipulates cell fate decisions in many different cell types by regulating the activity of downstream effectors. We have previously testified that the fusion protein of CTP-HBcAg18-27--Tapasin could enter the cytoplasm of dendritic cells and efficiently induce robust specific CTL response in vitro.

OBJECTIVES

In the present study, we evaluated specific CTL response and the level of apoptosis of CD8+ T cells induced by CTP-HBcAg18-27-Tapasin in HLA-A2 transgenic mice (H-2Kb). Meanwhile, we preliminary investigated PI3K, phosphorylation level of Akt, and mammalian target of rapamycin (mTOR) as positive regulator of the magnitude and effector function of the hepatitis B virus-specific cytotoxic T lymphocytes in HLA-A2 transgenic mice.

MATERIALS AND METHODS

HLA-A2 transgenic mice were immunized by intramuscular injection in the hind legs three times at one-week intervals with PBS, CTP-HBcAg18-27-Tapasin (50 μg), CTP-HBcAg18-27 (50 μg), HBcAg18-27-Tapasin (50 μg), and HBcAg18-27 (50 μg). One week after the last immunization, mice were sacrificed and splenocytes were harvested in strile condition. The specific CTL response was analyzed by flow cytometry and enzyme linked immunosorbent assay (ELISA); the expression of (PI3K)/Akt signaling was detected by RT-PCR and western blot.

RESULTS

The results showed that CTP-HBcAg18-27-Tapasin significantly increased the percentages of IFN-γ(+) CD8α(+) T cells, the numbers of these polyfunctional triple-cytokine-producing (IFN-γ, TNF-α, and IL-2) CD8(+)T cells, the secretion of cytokine IFN-γ, IL-2, and TNF-α, while in comparison to control group, it significantly decreased the percentage of apoptotic CD8(+) T cells in HLA-A2 transgenic mice. Moreover, the expression of PI3K, P-Akt, and P-mTOR was significantly upregulated in CTP-HBcAg18-27-Tapasin group compared with control groups.

CONCLUSIONS

In conclusion, CTP-HBcAg18-27-Tapasin could reduce apoptosis of CD8(+) T cells, increase the percentages of IFN-γ(+) CD8α(+) T cells, and elicit cell-mediated immunity in HLA-A2 transgenic mice; these processes were associated with activation of the PI3K/Akt signaling pathway.

Clonal exhaustion as a mechanism to protect against severe immunopathology and death from an overwhelming CD8 T cell response

The balance between protective immunity and immunopathology often determines the fate of the virus-infected host. How rapidly virus is cleared is a function of initial viral load, viral replication rate, and efficiency of the immune response. Here, we demonstrate, with three different inocula of lymphocytic choriomeningitis virus (LCMV), how the race between virus replication and T cell responses can result in different disease outcomes. A low dose of LCMV generated efficient CD8 T effector cells, which cleared the virus with minimal lung and liver pathology. A high dose of LCMV resulted in clonal exhaustion of T cell responses, viral persistence, and little immunopathology. An intermediate dose only partially exhausted the T cell responses and resulted in significant mortality, and the surviving mice developed viral persistence and massive immunopathology, including necrosis of the lungs and liver. This suggests that for non-cytopathic viruses like LCMV, hepatitis C virus, and hepatitis B virus, clonal exhaustion may be a protective mechanism preventing severe immunopathology and death.

FOXO3 regulates the CD8 T cell response to a chronic viral infection

Chronic infections with viruses such as hepatitis B virus, hepatitis C virus, and HIV constitute a major global public health problem. Studies of chronic viral infections in humans and mice show that persistent antigenic stimulation induces dysregulation of T cell responses; virus-specific T cells either undergo clonal deletion or lose their ability to display the full spectrum of effector functions, a condition termed functional exhaustion. The ability to generate and retain sufficient numbers of functionally competent T cells, therefore, becomes vitally important in controlling chronic viral infections. Our understanding of the mechanisms governing T cell homeostasis during chronic viral infections, however, is poor. The phosphatidylinositol 3-kinase (PI3K)/Akt signaling pathway controls cell fate decisions in many cell types by modulating the activity of downstream effectors, including the FOXO family of transcription factors. We have observed dynamic, in vivo alterations in the phosphorylation levels of three key proteins (Akt, FOXO1/FOXO3 [FOXO1/3], and mammalian target of rapamycin [mTOR]) involved in this signaling cascade and have identified the transcription factor FOXO3 as a negative regulator of the magnitude and effector function of CD8 T cells during chronic lymphocytic choriomeningitis virus (LCMV) infection in mice. We report that ablation of FOXO3 in T cells reduced apoptosis, increased the abundance of polyfunctional virus-specific CD8 T cells, and improved viral control. Thus, FOXO3 is a promising candidate target for immunotherapies of chronic viral infection.

Genomic analysis and pathogenic characteristics of lymphocytic choriomeningitis virus strains isolated in Japan

Lymphocytic choriomeningitis virus (LCMV) is a zoonotic pathogen of which mice are the natural reservoir. Different strains and clones of LCMV show different pathogenicity in mice. Here we determined the complete genomic sequences of 3 LCMV strains (OQ28 and BRC which were isolated from mice in Japan and WE(ngs) which was derived from strain WE). Strains OQ28 and BRC showed high sequence homology with other LCMV strains. Although phylogenetic analyses placed these 2 Japanese strains in different subclusters, they belonged to same cluster of LCMV isolates. WE(ngs) and WE had many sequence substitutions between them but fell into same subcluster. The pathogenicity of the 3 new LCMV isolates was examined by inoculating ICR mice with 10² and 10⁴ TCID₅₀ of virus. ICR mice infected with OQ28 or WE(ngs) exhibited severe clinical signs, and some of the infected mice died. In contrast, all ICR mice infected with BRC showed no clinical signs and survived infection. Virus was detected in the blood, organs, or both of most of the surviving ICR mice inoculated with either OQ28 or WE(ngs). However, virus was below the level of detection in all ICR mice surviving infection with strain BRC. Therefore, LCMV strains OQ28 and BRC were genetically classified in the same cluster of LCMV strains but exhibited very different pathogenicity.

Immunotherapeutic effects of IL-7 during a chronic viral infection in mice

Viral persistence during chronic viral infections is associated with a progressive loss of T-cell effector function called functional exhaustion. There is therefore a need to develop immunotherapies to remediate the functional deficits of T cells during these infections. We investigated the immunotherapeutic effects of IL-7 during chronic lymphocytic choriomeningitis virus infection in mice. Our results showed that the effects of IL-7 on T cells depend on the viral load, timing, and duration of treatment during the course of the infection. We document that the effectiveness of IL-7 was constrained by high viral load early in the infection, but treatment for at least 3 weeks during declining viral titers mitigated the programmed contraction of CD8 T cells, markedly enhanced the number of high-quality polyfunctional virus-specific CD8 T cells with a nonexhausted phenotype, and accelerated viral control. Mechanistically, the enhancement of CD8 T-cell responses by IL-7 was associated with increased proliferation and induction of Bcl-2, but not with altered levels of PD-1 or Cbl-b. In summary, our results strongly suggest that IL-7 therapy is a potential strategy to bolster the quality and quantity of T-cell responses in patients with chronic viral infections.

Visualizing the role of Cbl-b in control of islet-reactive CD4 T cells and susceptibility to type 1 diabetes

The E3 ubiquitin ligase Cbl-b regulates T cell activation thresholds and has been associated with protecting against type 1 diabetes, but its in vivo role in the process of self-tolerance has not been examined at the level of potentially autoaggressive CD4(+) T cells. In this study, we visualize the consequences of Cbl-b deficiency on self-tolerance to lysozyme Ag expressed in transgenic mice under control of the insulin promoter (insHEL). By tracing the fate of pancreatic islet-reactive CD4(+) T cells in prediabetic 3A9-TCR × insHEL double-transgenic mice, we find that Cbl-b deficiency contrasts with AIRE or IL-2 deficiency, because it does not affect thymic negative selection of islet-reactive CD4(+) cells or the numbers of islet-specific CD4(+) or CD4(+)Foxp3(+) T cells in the periphery, although it decreased differentiation of inducible regulatory T cells from TGF-β-treated 3A9-TCR cells in vitro. When removed from regulatory T cells and placed in culture, Cblb-deficient islet-reactive CD4(+) cells reveal a capacity to proliferate to HEL Ag that is repressed in wild-type cells. This latent failure of T cell anergy is, nevertheless, controlled in vivo in prediabetic mice so that islet-reactive CD4(+) cells in the spleen and the pancreatic lymph node of Cblb-deficient mice show no evidence of increased activation or proliferation in situ. Cblb deficiency subsequently precipitated diabetes in most TCR:insHEL animals by 15 wk of age. These results reveal a role for peripheral T cell anergy in organ-specific self-tolerance and illuminate the interplay between Cblb-dependent anergy and other mechanisms for preventing organ-specific autoimmunity.

T-cell exhaustion: characteristics, causes and conversion

T-cell exhaustion is characterized by the stepwise and progressive loss of T-cell functions and can culminate in the physical deletion of the responding cells. Exhaustion is well-defined during chronic lymphocytic choriomeningitis virus infection and commonly develops under conditions of antigen-persistence, which occur following many chronic infections that are of significant public health concern including hepatitis B virus, hepatitis C virus and human immunodeficiency virus infections, as well as during tumour outgrowth. Exhaustion is not a uniformly disabled setting as a gradation of phenotypic and functional defects can manifest, and these cells are distinct from prototypic effector, memory and also anergic T cells. We are gaining insights into the extrinsic and intrinsic factors that determine the severity of exhaustion. These include the duration and magnitude of antigenic activation, availability of CD4 T-cell help, the levels of stimulatory and suppressive cytokines, as well as the expression of activatory and inhibitory receptors. More information is now becoming available regarding the molecular mechanisms that attenuate the responsiveness of exhausted T cells. As the parameters that dictate exhaustion are more thoroughly defined, this is fostering the development of methods that prevent and rejuvenate functionally inferior responses. In this article we discuss our current understanding of the properties of exhausted T cells and the mechanisms that promote and maintain this state.

Cbl-b in T-cell activation

Peripheral activation of antigen-specific T cells is stringently controlled to prevent immune responses against self-antigens. Only after a T cell is presented with two signals, an antigen and a co-stimulatory signal, can they be fully activated. In case antigen presentation occurs without co-stimulation, T-cell receptor (TCR) signaling pathways are regulated to prevent T-cell activation and induce T-cell tolerance. Thus, for a productive T-cell response to occur, co-stimulatory receptors need to serve the dual role of amplifying the TCR signaling while concomitantly releasing T cells from suppression. Biochemical and genetic studies during the last 10 years have documented the critical role of the E3 ubiquitin-ligase Cbl-b in this fundamental two-signal modulation of T-cell responses. In this review, we will discuss our current understanding on how Cbl-b controls T-cell activation and tolerance, its in vivo implications, as well as mechanisms for tuning T-cell-mediated immune responses by this essential E3 ligase.

Mammary tumors with diverse immunological phenotypes show differing sensitivity to adoptively transferred CD8+ T cells lacking the Cbl-b gene

We tested the efficacy of CD8+ T cells lacking the Cbl-b gene against a panel of mammary tumor lines with different intrinsic sensitivities to T cells. Mice bearing established tumors expressing an ovalbumin-tagged version of HER-2/neu underwent adoptive transfer with Cbl-b-replete or -null CD8+ T cells from OT-I T cell receptor transgenic donor mice. In general, Cbl-b-null OT-I cells showed enhanced expansion, persistence, and capacity for tumor infiltration. This resulted in markedly enhanced efficacy against two tumor lines that normally demonstrate complete (NOP21) or partial (NOP23) regression. Moreover, a third tumor line (NOP6) that normally demonstrates progressive disease underwent complete regression in response to Cbl-b-null OT-I cells. However, a fourth tumor line (NOP18) was resistant to Cbl-b-null OT-I cells owing to a profound barrier to lymphocyte infiltration. Thus, Cbl-b-null CD8+ T cells are generally more efficacious but are nonetheless unable to mediate curative responses against all tumor phenotypes.

PKC-theta modulates the strength of T cell responses by targeting Cbl-b for ubiquitination and degradation

The E3 ubiquitin ligase Casitas B-lineage lymphoma (Cbl-b) is central to antigen-induced immune tolerance and regulates the CD28 dependence of T cell activation. Cbl-b undergoes ubiquitination and proteasomal degradation after adequate costimulation of T cells; however, the mechanism involved is unknown. Here, we identified protein kinase C-theta (PKC-theta) as the critical intermediary for the inactivation of Cbl-b in response to costimulation of T cells through CD28. PKC-theta associated with Cbl-b on stimulation of the T cell receptor. After costimulation of T cells through CD28, Cbl-b was ubiquitinated and degraded through a mechanism that depended on the kinase activity of PKC-theta. Consistent with this mechanism, the impaired responses of PKCtheta-deficient T cells were at least partially restored by the concomitant genetic loss of cblb. Thus, our data establish a nonredundant antagonism between PKC-theta and Cbl-b that regulates T cell activation responses.

MyD88 intrinsically regulates CD4 T-cell responses

Myeloid differentiation factor 88 (MyD88) is an essential adaptor protein in the Toll-like receptor-mediated innate signaling pathway, as well as in interleukin-1 receptor (IL-1R) and IL-18R signaling. The importance of MyD88 in the regulation of innate immunity to microbial pathogens has been well demonstrated. However, its role in regulating acquired immunity to viral pathogens and neuropathogenesis is not entirely clear. In the present study, we examine the role of MyD88 in the CD4(+) T-cell response following lymphocytic choriomeningitis virus (LCMV) infection. We demonstrate that wild-type (WT) mice developed a CD4(+) T-cell-mediated wasting disease after intracranial infection with LCMV. In contrast, MyD88 knockout (KO) mice did not develop wasting disease in response to the same infection. This effect was not the result of MyD88 regulation of IL-1 or IL-18 responses since IL-1R1 KO and IL-18R KO mice were not protected from weight loss. In the absence of MyD88, naïve CD4(+) T cells failed to differentiate to LCMV-specific CD4 T cells. We demonstrated that MyD88 KO antigen-presenting cells are capable of activating WT CD4(+) T cells. Importantly, when MyD88 KO CD4(+) T cells were reconstituted with an MyD88-expressing lentivirus, the rescued CD4(+) T cells were able to respond to LCMV infection and support IgG2a antibody production. Overall, these studies reveal a previously unknown role of MyD88-dependent signaling in CD4(+) T cells in the regulation of the virus-specific CD4(+) T-cell response and in viral infection-induced immunopathology in the central nervous system.