Bystander CD8 Research Articles

Chronic viral infections impinge on naive bystander CD8 T cells.

IntroductionEpidemiological data suggest that persistent viral infections impair immune homeostasis and immune responsiveness. Previous studies showed that chronic virus infections negatively impact bystander T‐cell differentiation and memory formation but there is limited knowledge of how chronic virus infections impinge on heterologous naive T‐cell populations.MethodsWe used adoptive transfer of naive CD8 T cells with defined nonviral specificity into hosts, which were subsequently chronically infected with lymphocytic choriomeningitis virus, followed by analyses of numeric, phenotypic, and functional changes provoked in the chronically infected host.ResultsWe demonstrate that chronic virus infections have a profound effect on the number and phenotype of naive bystander CD8 T cells. Moreover, primary expansion upon antigen encounter was severely compromised in chronically infected hosts. However, when naive bystander CD8 T cells were transferred from the chronically infected mice into naive hosts, they regained their expansion potential. Conversely, when chronically infected hosts were supplied with additional antigen‐presenting cells (APCs), primary expansion of the naive CD8 T cells was restored to levels of the uninfected hosts.ConclusionsOur results document numeric, phenotypic, and functional adaptation of bystander naive CD8 T cells during nonrelated chronic viral infection. Their functional impairment was only evident in the chronically infected host, indicating that T‐cell extrinsic factors, in particular the quality of priming APCs, are responsible for the impaired function of naive bystander T cells in the chronically infected hosts.

Hybrid Fc-fused interleukin-7 induces an inflamed tumor microenvironment and improves the efficacy of cancer immunotherapy.

ObjectivesEmerging oncotherapeutic strategies require the induction of an immunostimulatory tumor microenvironment (TME) containing numerous tumor‐reactive CD8+ T cells. Interleukin‐7 (IL‐7), a T‐cell homeostatic cytokine, induces an antitumor response; however, the detailed mechanisms underlying the contributions of the IL‐7 to TME remain unclear. Here, we aimed to investigate the mechanism underlying the induction of antitumor response by hybrid Fc‐fused long‐acting recombinant human IL‐7 (rhIL‐7‐hyFc) through regulation of both adaptive and innate immune cells in the TME.MethodsWe evaluated rhIL‐7‐hyFc‐mediated antitumor responses in murine syngeneic tumor models. We analysed the cellular and molecular features of tumor‐infiltrating lymphocytes (TILs) and changes in the TME after rhIL‐7‐hyFc treatment. Furthermore, we evaluated the antitumor efficacy of rhIL‐7‐hyFc combined with chemotherapy and checkpoint inhibitors (CPIs).ResultsSystemic delivery of rhIL‐7‐hyFc induced significant therapeutic benefits by expanding CD8+ T cells with enhanced tumor tropism. In tumors, rhIL‐7‐hyFc increased both tumor‐reactive and bystander CD8+ TILs, all of which displayed enhanced effector functions but less exhausted phenotypes. Moreover, rhIL‐7‐hyFc suppressed the generation of immunosuppressive myeloid cells in the bone marrow of tumor‐bearing mice, resulting in the immunostimulatory TME. Combination therapy with chemotherapy and CPIs, rhIL‐7‐hyFc elicited a strong antitumor response and even under a T lymphopenic condition by restoring CD8+ T cells. When combined with chemotherapy and CPIs, rhIL‐7‐hyFc administration enhanced antitumor response under intact andlymphopenic conditions by restoring CD8+ T cells.ConclusionTaken together, these data demonstrate that rhIL‐7‐hyFc induces antitumor responses by generating T‐cell‐inflamed TME and provide a preclinical proof of concept of immunotherapy with rhIL‐7‐hyFc to enhance therapeutic responses in the clinic.

Distinct epigenetic features of tumor-reactive CD8+ T cells in colorectal cancer patients revealed by genome-wide DNA methylation analysis

BackgroundTumor-reactive CD8+ tumor-infiltrating lymphocytes (TILs) represent a subtype of T cells that can recognize and destroy tumor specifically. Understanding the regulatory mechanism of tumor-reactive CD8+ T cells has important therapeutic implications. Yet the DNA methylation status of this T cell subtype has not been elucidated.ResultsIn this study, we segregate tumor-reactive and bystander CD8+ TILs, as well as naïve and effector memory CD8+ T cell subtypes as controls from colorectal cancer patients, to compare their transcriptome and methylome characteristics. Transcriptome profiling confirms previous conclusions that tumor-reactive TILs have an exhausted tissue-resident memory signature. Whole-genome methylation profiling identifies a distinct methylome pattern of tumor-reactive CD8+ T cells, with tumor-reactive markers CD39 and CD103 being specifically demethylated. In addition, dynamic changes are observed during the transition of naïve T cells into tumor-reactive CD8+ T cells. Transcription factor binding motif enrichment analysis identifies several immune-related transcription factors, including three exhaustion-related genes (NR4A1, BATF, and EGR2) and VDR, which potentially play an important regulatory role in tumor-reactive CD8+ T cells.ConclusionOur study supports the involvement of DNA methylation in shaping tumor-reactive and bystander CD8+ TILs, and provides a valuable resource for the development of novel DNA methylation markers and future therapeutics.

Immune profiling of tumor-infiltrating T cells using mass cytometry.

2607 Background: Immunotherapy recent successes have opened new avenues for the treatment of cancer and the presence of tumor-specific CD8+ T cells in tumor-bearing individuals offer a promising therapeutic target. However, the detection and profiling of such T cells are challenging due to the need to detect rare antigen-specific T cell subpopulations in patient samples that are limited in size thus making it difficult to exploit these parameters for predictive signatures of clinical response. Moreover, the identification and analysis of neoantigen-specific CD8+ T-cells in tumor-bearing individuals is challenging due to the small pool of such cells. Methods: In order to identify therapy-relevant tumor antigens and to facilitate a concurrent in-depth characterization of cells directed towards these targets, immunoSCAPE leverages the high-dimensional immune profiling capabilities of cytometry by time of flight (CyTOF) combined with a unique technology allowing the identification rare antigen-specific T-cell subsets. Results: We applied this technology to patient tumor-infiltrating lymphocytes from human cancer samples and tumor-derived neoantigens recognized by T-cells were identified and characterized. Interestingly, the majority of patient-derived tumor infiltrates consisted of tumor-unrelated T-cells characterized by a diverse phenotype. Strikingly, the expression of CD39 was absent from these bystander cells, suggesting that CD39 could be a useful biomarker for the identification of putative tumor-reactive T cells. Conclusions: Simultaneous immune profiling revealed that tumor-unrelated, bystander CD8+ T-cells are phenotypically different in human tumor infiltrates and identified CD39 as a putative marker of neoantigen-specific T-cells. By providing insights into the nature, frequency and phenotype of antigen-specific T-cells, immunoSCAPE’s unique target discovery and high-dimensional immune profiling platform is a valuable tool for the development of novel diagnostic and therapeutic strategies in immunotherapy.

CD39+CD8+ T cells exhibit a distinct phenotype among tumor-infiltrating tumor-antigenspecific CD8+ T cells

Abstract Recent studies have spotlighted CD39 expression on tumor-infiltrating CD8+ T cells as an exhaustion marker (Cancer Research 2018, 1:78) and tumor antigen reactivity (Nature Communications 2018, 9:1). In the present study, we examined characteristics of CD39+CD8+ T cells among tumor-infiltrating lymphocytes (TILs) obtained from various types of tumors, including ovarian cancer, stomach cancer and hepatocellular carcinoma, to gain better understanding of CD39 expression on CD8+ T cells. To identify tumor antigen-specific CD8+ T cells, we used 11 HLA-A*A201 multimers specific for tumor associated antigens, including MAGE A1, NY-ESO-1, and AFP. CD39 expression of tumor antigen-specific CD8+ T cells was significantly higher in TILs than in peripheral blood lymphocytes. However, all of tumor-infiltrating tumor antigen-specific CD8+ T cells did not express CD39. As known previously (Nature 2018, 557:7706), tumor-infiltrating bystander CD8+ T cells, specific to viral epitopes, lack CD39 expression. Among tumor-infiltrating tumor antigen-specific CD8+ T cells, CD39+CD8+ T cells exhibited more severely exhausted features than CD39−CD8+ T cells. For example, the percentage of multiple immune checkpoint receptors-expressing cells was significantly higher in CD39+CD8+ T cells compared to CD39−CD8+ T cells. Moreover, higher proportion of T-betlow/Eomeshighcells and and lower expression of CD127 were observed in CD39+CD8+ T cells. In addition, CD39+CD8+ T cells expressed higher levels of tissue residency markers, such as CD103 and CD69. In conclusion, CD39+CD8+ T cells exhibit distinct phenotypes that represent severe exhaustion and terminal differentiation among tumor-infiltrating tumor-antigen-specific CD8+ T cells.

Chronic virus infection compromises memory bystander T cell function in an IL-6/STAT1-dependent manner.

Chronic viral infections are widespread among humans, with ∼8-12 chronic viral infections per individual, and there is epidemiological proof that these impair heterologous immunity. We studied the impact of chronic LCMV infection on the phenotype and function of memory bystander CD8+ T cells. Active chronic LCMV infection had a profound effect on total numbers, phenotype, and function of memory bystander T cells in mice. The phenotypic changes included up-regulation of markers commonly associated with effector and exhausted cells and were induced by IL-6 in a STAT1-dependent manner in the context of chronic virus infection. Furthermore, bystander CD8 T cell functions were reduced with respect to their ability to produce inflammatory cytokines and to undergo secondary expansion upon cognate antigen challenge with major cell-extrinsic contributions responsible for the diminished memory potential of bystander CD8+ T cells. These findings open new perspectives for immunity and vaccination during chronic viral infections.

Helminth-induced IL-4 expands bystander memory CD8+ T cells for early control of viral infection

Infection with parasitic helminths can imprint the immune system to modulate bystander inflammatory processes. Bystander or virtual memory CD8+ T cells (TVM) are non-conventional T cells displaying memory properties that can be generated through responsiveness to interleukin (IL)-4. However, it is not clear if helminth-induced type 2 immunity functionally affects the TVM compartment. Here, we show that helminths expand CD44hiCD62LhiCXCR3hiCD49dlo TVM cells through direct IL-4 signaling in CD8+ T cells. Importantly, helminth-mediated conditioning of TVM cells provided enhanced control of acute respiratory infection with the murid gammaherpesvirus 4 (MuHV-4). This enhanced control of MuHV-4 infection could further be explained by an increase in antigen-specific CD8+ T cell effector responses in the lung and was directly dependent on IL-4 signaling. These results demonstrate that IL-4 during helminth infection can non-specifically condition CD8+ T cells, leading to a subsequently raised antigen-specific CD8+ T cell activation that enhances control of viral infection.

Highlights from Recent Cancer Literature

Highlights from Recent Cancer Literature

IL-12 stimulates CTLs to secrete exosomes capable of activating bystander CD8+ T cells

An effective cytotoxic T lymphocyte (CTL) response against intracellular pathogens is generally accomplished by immense CTL expansion and activation, which can destroy infected cells. Vigorous immune responses can lead to activation of bystander CD8+ T cells, but the contribution from antigen-specific CTLs is not well understood. We found that CTLs secrete extracellular vesicles following antigen stimulation. These CTL-derived vesicles contain CTL proteins and exhibit markers and size profiles consistent with exosomes. Interestingly, further stimulation of CTLs with IL-12 impacts exosome size and leads to selective enrichment of certain exosomal proteins. More important, exosomes from IL-12-stimulated CTLs directly activated bystander naïve CD8+ T cells to produce interferon-γ (IFNγ) and granzyme B (GZB) in the absence of antigens, whereas control exosomes derived from antigen-stimulated CTLs did not. In addition, IL-12 induced exosomes are able to strengthen the effects of weak antigen stimulation on CTLs. Proteomic analysis demonstrates that IL-12 stimulation alters catalytic and binding activities of proteins in CTL exosomes. Our findings indicate that the biological function and morphology of exosomes secreted by CTLs can be influenced by the type of stimulation CTLs receive. Thus, a fully functional, ongoing, antigen-specific CTL response may influence bystander CD8+ T cells through secretion of exosomes.

Fatty acid metabolism is essential to maintain functions of CD8+T cells within a hypoxic and hypoglycemic tumor microenvironment

Abstract The efficacy of immunotherapy for solid tumors is dampened by functional declines of tumor-infiltrating T lymphocytes (TILs), which is generally viewed as the result of their exhaustion due to persistent antigenic stimulation. Disputing this notion, we observed in a mouse melanoma model that bystander CD8+TILs increase expression of co-inhibitors and lose effector functions, suggesting alternative mechanisms. Our data show that metabolic stresses within tumor microenvironment (TME) profoundly affect differentiation and functions of TILs. Hypoxia through HIF-1α drives increases of co-inhibitor LAG-3 while lack of glucose (Glu) enhances PD-1 levels on activated CD8+T cells. Both conditions impair functions of CD8+T cells. When deprived of Glu, activated CD8+T cells enhance fatty acid (FA) catabolism and this is further increased under hypoxia. Using 13C-stable isotope tracing in vivo and liquid chromatography-mass spectrometry (LC-MS) analysis, our study show that CD8+TILs in late stage tumors increasingly depend on catabolism of FAs including ketone bodies fueled by exogenous FA uptake and triacylglycerol (TG) turnover to meet their energy demand. Promoting FA catabolism of CD8+TILs increases their PD-1 expression but preserves some effector functions and improves their anti-tumor efficacy. Our findings show that lack of Glu and oxygen plays a critical role in driving the metabolic reprograming and functional exhaustion of CD8+TILs. They further indicate that metabolic interventions that promote FA catabolism of adoptively transferred or vaccine-induced CD8+T cells may improve the efficacy of cancer immunotherapy.

A dead-end host: is there a way out? A position piece on the ebola virus outbreak by the international union of immunology societies.

The fact that not everyone with Ebola virus disease (EVD) has died during the ongoing outbreak in West Africa, with an estimated case fatality rate of 70.8% by September 2014 (1), suggests that some kind of immunity to this virus is possible. If left unchecked, this scenario will undoubtedly shift to a higher figure, as health-care conditions in many of the countries affected may not always enable infected hosts to recover. Although gender differences in the survival, incidence, and/or severity of infection are unknown, the current Ebola virus outbreak represents an unprecedented disaster for humans (1, 2) and a zoonotic successful strategy (3) for a virus that cunningly and rapidly hijacks innate immunity to devastating effect. Human to human transmission is via contact with bodily fluids from symptomatic patients, which was identified in the first epidemic almost 40 years ago, but unlike prior epidemics, which consisted of sporadic short periods of human transmission, there is now a shift to a prolonged period of viral transmission (over 9 months at the time of writing). This is, in part, a reflection of the movement of people into urban densely populated regions (2). In fact, there are distinct parallels between the first outbreak in the Zaire with today’s outbreak – both being caused by Zaire Ebolavirus (1, 4). Importantly, however, the current and ongoing outbreak is occurring in this region for the first time, and there are fears that without blocking transmission, Ebola may become endemic (2). What then are the medical efforts under way to halt the spread of Ebola with immune therapies and vaccine strategies? Should we temper the usual scientific rigor of clinical trials and roll-out candidate vaccines and therapies directly into people? How should the scientific, and specifically the immunology community, meet the rapidity of EVD spread, which without doubt is the major challenge and crisis of today. Ebola, a filovirus, encodes seven genes: nucleoprotein (NP), VP35 (polymerase co-factor), VP40 (matrix protein), glycoprotein (GP), VP30 (transcription activator), VP24 (secondary matrix protein), and RNA-dependent RNA polymerase (5). The most likely routes of Ebola entry into the body are via mucosal surfaces, the conjunctiva, the oropharynx, or injured skin routes (6). From human and monkey studies, the primary targets are dendritic cells, monocytes, macrophages, and Kupffer cells in the liver and entry mechanisms include the GP interacting with an unidentified receptor and inducing: (i) endocytosis mediated by lipid rafts; (ii) macropinocytosis; and (iii) clathrin-mediated transport (5). Experimental models have shown that virus-like particles (VLPs) composed of GP and VP40 activate endothelial cells that upregulate expression of intercellular adhesion molecule 1 (ICAM-1), vascular cell adhesion molecule 1 (VCAM-1), and E-selectin, which jointly results in an increase in endothelial permeability (7). It is also likely that the increase in pro-inflammatory cytokines, shown experimentally and in plasma samples from patients with Ebola infection (5), contribute to enhancing vasodilation and the resulting cytokine storm (8) causes immune dysfunction and general “shut-down” of all immunity. Additionally, monkey models have shown that Ebola infection causes apoptosis in by-stander CD4 and CD8 lymphocytes and NK cells (9). These devastating effects result in the clinical symptoms of hemorrhagic lesions in the skin, mucous membranes, visceral organs, and large effusions in body cavities. There is also multifocal necrosis most predominantly in the liver, spleen, kidneys, testes, and ovaries (5). As with all pathogens, Ebola has evolved to evade innate immunity by hijacking specific anti-viral pathways, resulting in immunosuppression. VP35 and VP24 inhibit type I interferon activity. VP35 inhibits induction of IFN-beta production by suppressing phosphorylation and dimerization of IFN regulatory factor 3 (IRF-3) and enhancing SUMOylation of IRF-7 (5, 10). Viral P24 inhibits type I and type II IFN signaling by inhibiting nuclear signaling transducer ad activator of transcription 1 (STAT 1) (10). The combined effect of these immune evasion strategies along with eliciting acute immune activation, inflammation, and a cytokine storm is lethal for the host and, as this current epidemic has shown, more than two-thirds of infected people with EVD will die. Are there clues from individuals who have survived EVD or properties of the virus that can be exploited to develop vaccine candidates?

Bystander Chronic Infection Negatively Impacts Development of CD8+ T Cell Memory

Epidemiological evidence suggests that chronic infections impair immune responses to unrelated pathogens and vaccines. The underlying mechanisms, however, are unclear and distinguishing effects on priming versus development of immunological memory has been challenging. We investigated whether bystander chronic infections impact differentiation of memory CD8(+) T cells, the hallmark of protective immunity against intracellular pathogens. Chronic bystander infections impaired development of memory CD8(+) T cells in several mouse models and humans. These effects were independent of initial priming and were associated with chronic inflammatory signatures. Chronic inflammation negatively impacted the number of bystander CD8(+) T cells and their memory development. Distinct underlying mechanisms of altered survival and differentiation were revealed with the latter regulated by the transcription factors T-bet and Blimp-1. Thus, exposure to prolonged bystander inflammation impairs the effector to memory transition. These data have relevance for immunity and vaccination during persisting infections and chronic inflammation.

Bystander CD8 T cells produce granzyme B and promote exacerbated disease during Leishmania major infection (P3079)

Abstract CD8 T cells are recruited to inflamed tissue independent of their antigen specificity, but whether CD8 T cells with irrelevant specificities contribute to the immune response is unclear. We found that Lymphocytic choriomeningitis virus (LCMV)-specific CD8 T cells from a previous LCMV infection were recruited in large numbers to leishmanial lesions in mice infected with L. major. Importantly, LCMV immune mice developed larger leishmanial lesions than naïve mice, associated with increased CD8 T cell, neutrophil and monocyte infiltration into the lesions. The recruited CD8 T cells produced granzyme B and increased levels of circulating granzyme B were detected in the serum of LCMV immune mice that had been challenged with L. major, suggesting that granzyme B may be promoting the increased inflammation. This prompted us to determine what cytokines might stimulate the production of granzyme B from previously activated CD8 T cells in the absence of T cell receptor signaling. We found that IL-15 induced granzyme B, but not IFN-γ, expression. In contrast, IL-12 + IL-18 induced IFN-γ but little granzyme B expression. Thus, our findings demonstrate that bystander CD8 T cells can influence the outcome of subsequent infections, and that the cytokine milieu may dictate how the bystander memory CD8 T cells respond within inflamed sites.

Dual regulation of CD8+ T cell and NK cells following systemic cancer immunotherapy (P1087)

Abstract NK-CD8 T cell regulation has been demonstrated in viral infection models. It is believed that NK cells regulate CD8 T cells to prevent exacerbated activation resulting in pathological effects. Current immunotherapy regimens use immunostimulation with blockade of negative regulators to achieve optimum antitumor effects. We observe that NK and CD8 T cells play dual roles in their concurrent regulation following systemic administration of IL2 and αTGFβ as the depletion of either population resulted in a higher expansion of the other. This data indicates a possible cytokine competition between both NK and CD8 T cell compartments. However, antigen nonspecific bystander CD8 T cells were particularly enhanced after NK depletion whereas CD4 T cells were not affected. Therefore, this data suggests a possible elimination of CD8 T cells after NK activation which is FasL-dependent as Fas expression is upregulated on CD8 T cells after NK depletion. This mechanism is not mediated by perforin because the IL2 treatment of perforin-deficient mice resulted in a higher CD8 T cell expansion after NK cell depletion. In conclusion, this study illustrates the existence of an NK-CD8 T cell regulation in an antigen-nonspecific stimulation model which demonstrates a tight regulation between adaptive and innate immune responses. Further, the study demonstrates the potential efficacy of increasing antitumor responses by concurrent application of immunostimulation with blockade of negative regulators.

OT-1 mice display minimal upper genital tract pathology following primary intravaginalChlamydia muridaruminfection

Chlamydia trachomatis is the most common bacterial sexually transmitted disease worldwide and leads to serious pathological sequelae in the upper genital tract (UGT) including pelvic inflammatory disease, ectopic pregnancy, and infertility. Several components of the host immune responses have been shown to contribute to the UGT pathology following genital chlamydial infection. We have shown recently that CD8 + T cells induce the chlamydial UGT pathology via the production of TNF-α. However, those studies did not determine whether the pathology is mediated by bystander or antigen-specific CD8 + T cells. In this study, we compared chlamydial clearance and UGT pathology in OT-1 transgenic mice and the corresponding C57BL/6J wild-type mice following primary intravaginal Chlamydia muridarum infection. All CD8 + T cells in the OT-1 mice respond only to the Ova 257-264 peptide and are incapable of responding to other antigenic epitopes including those of Chlamydia . OT-1 mice displayed vaginal chlamydial clearance comparable to the wild-type animals. However, both oviduct and uterine horn pathology were minimal in the OT-1 mice compared with the high degree of pathology observed in the wild-type animals. These results strongly suggest that Chlamydia -specific, not bystander, CD8 + T cells mediate the UGT pathological sequelae following genital chlamydial infection.

IL-15-activated bystander CD8+ T cells contribute to host injury by NK-like cytolytic activity in acute viral hepatitis. (105.21)

Abstract In viral infection, activation of bystander CD8+ T cells and their role have been controversial. We investigated bystander activation of CD8+ T cells and their contribution to tissue damage in acute hepatitis A (AHA), which is caused by hepatitis A virus (HAV) and manifested by severe liver injury in adults. We show bystander activation of CD8+ T cells specific to irrelevant viruses such as CMV, EBV and influenza A virus during acute necroinflammatory phase. We also demonstrate an unexpected correlation of the bystander activation to the liver injury, indicating a role of activated bystander CD8+ T cells in the liver injury. In contrast, HAV-specific T cell response was associated with viral clearance rather than the liver injury. Intriguingly, IL-15 is produced by HAV infection, and expression of IL-15 and IL-15Rα is increased in AHA. Furthermore, in vitro IL-15 treatment of healthy donors’ lymphocytes recapitulated the activated phenotype of the bystander CD8+ T cells observed in AHA. CD8+ T cells activated in vitro by IL-15 or obtained from the AHA liver exerted NK-like cytolytic activity triggered by NKG2D and/or NKp30 without TCR engagement. In addition, hepatocytes of the AHA liver over-expressed NKG2D ligands, and therefore could be cytolytic targets. Taken together, we report here a human viral disease in which bystander CD8+ T cells are activated by IL-15 and contribute to tissue damage by NK-like cytolytic activity without TCR engagement.

Bystander chronic infection negatively impacts the development of CD8 T cell memory (76.6)

Abstract Accumulating epidemiological evidence suggests that chronic infections compromise immunity against antigenically unrelated infections, but the underlying mechanisms are unclear. We investigated whether antigenically unrelated “bystander” chronic infections impacted the development of memory CD8 T cells, the hallmarks of protective immunity against intracellular pathogens. We demonstrate that chronic bystander infection and inflammation substantially impair memory CD8 T cell responses in several mouse models with similar changes in non-human primates and in chronically HCV -infected humans. Type I IFN -induced molecular pathways were specifically dysregulated in memory CD8 T cells in inflammatory environments and induction of type I IFN by chronic poly(I:C) administration similarly impaired memory CD8 T cell development and function. Chronic inflammation increased the expression of the proapoptotic protein Bim in bystander CD8 T cells, while inhibiting their IL15-induced rescue from apoptosis ex vivo. Two transcriptional regulators of T cell development, Blimp-1 and T-bet, integrated the effects of inflammatory mediators in impairing optimal memory CD8 T cell development. Thus, exposure to chronic bystander infections deregulates the normal effector to memory CD8 T cell transition. These results have direct relevance concerning current therapeutic approaches and vaccination strategies for patients with persisting infections or chronic inflammatory conditions.

Virus-Induced Transient Immune Suppression and the Inhibition of T Cell Proliferation by Type I Interferon

Vaccine-induced memory is necessary for protective immunity to pathogens, but many viruses induce a state of transient immune suppression that might contribute to the inability of a vaccine to elicit immunity. We evaluated here the fate of bystander T cells activated by third party cognate antigens during acute viral infections in vivo, using distinct models to track and specifically activate HY and P14 transgenic bystander CD8 T cells in vivo during acute arenavirus infections of mice. Viral infections acted as stimulatory adjuvants when bystander T cells were exposed to an inflammatory milieu and cognate antigens at the beginning of infections, but bystander CD8 T cell proliferation in response to cognate antigen was inhibited 3 to 9 days after virus infection. Reduced proliferation was not dependent on Fas-FasL- or tumor necrosis factor (TNF)-induced activation-induced cell death or on deficiencies of antigen presentation. Instead, reduced proliferation was associated with a delayed onset of division that was an intrinsic defect of T cells. Inhibition of proliferation could be simulated by exposure of T cells to the Toll-like receptor agonist and type I interferon (IFN) inducer poly(I · C). T cells lacking IFN-α/β receptors resisted both the suppressive effects of preexposure to poly(I · C) and the stimulatory effects of type I IFN, indicating that the timing of exposure to IFN can have negative or positive effects on T cell proliferation. Inhibition of T cell receptor-stimulated bystander CD8 T cell proliferation during acute viral infections may reflect the reduced ability of vaccines to elicit protective immunity when administered during an acute illness.

Antitumor Immunity Produced by the Liver Kupffer Cells, NK Cells, NKT Cells, and CD8+CD122+T Cells

Mouse and human livers contain innate immune leukocytes, NK cells, NKT cells, and macrophage-lineage Kupffer cells. Various bacterial components, including Toll-like receptor (TLR) ligands and an NKT cell ligand (α-galactocylceramide), activate liver Kupffer cells, which produce IL-1, IL-6, IL-12, and TNF. IL-12 activates hepatic NK cells and NKT cells to produce IFN-γ, which further activates hepatic T cells, in turn activating phagocytosis and cytokine production by Kupffer cells in a positive feedback loop. These immunological events are essentially evoked to protect the host from bacterial and viral infections; however, these events also contribute to antitumor and antimetastatic immunity in the liver by activated liver NK cells and NKT cells. Bystander CD8+CD122+ T cells, and tumor-specific memory CD8+T cells, are also induced in the liver by α-galactocylceramide. Furthermore, adoptive transfer experiments have revealed that activated liver lymphocytes may migrate to other organs to inhibit tumor growth, such as the lungs and kidneys. The immunological mechanism underlying the development of hepatocellular carcinoma in cirrhotic livers in hepatitis C patients and liver innate immunity as a double-edged sword (hepatocyte injury/regeneration, septic shock, autoimmune disease, etc.) are also discussed.

IFN-αβ and Self-MHC Divert CD8 T Cells into a Distinct Differentiation Pathway Characterized by Rapid Acquisition of Effector Functions

Nonvirus-specific bystander CD8 T cells bathe in an inflammatory environment during viral infections. To determine whether bystander CD8 T cells are affected by these environments, we examined P14, HY, and OT-I TCR transgenic CD8 T cells sensitized in vivo by IFN-alphabeta-inducing viral infections or by polyinosinic:polycytidylic acid. These sensitized cells rapidly exerted effector functions, such as IFN-gamma production and degranulation, on contact with their high-affinity cognate Ag. Sensitization required self-MHC I and indirect effects of IFN-alphabeta, which together upregulated the T-box transcription factor Eomesodermin, potentially enabling the T cells to rapidly transcribe CTL effector genes and behave like memory cells rather than naive T cells. IL-12, IL-15, IL-18, and IFN-gamma were not individually required for sensitization to produce IFN-gamma, but IL-15 was required for upregulation of granzyme B. These experiments indicate that naive CD8 T cells receive signals from self-MHC and IFN-alphabeta and that, by this process, CD8 T cell responses to viral infection can undergo distinct differentiation pathways, depending on the timing of Ag encounter during the virus-induced IFN response.