Coexpression of PD-1, 2B4, CD160 and KLRG1 on exhausted HCV-specific CD8+ T cells is linked to antigen recognition and T cell differentiation

Enrichment of LAG-3, but not PD-1, on double negative T cells at the female genital tract

PROBLEM

The expression of inhibitory markers such as LAG-3 and PD-1 on T lymphocytes regulates immune function. Their expression at the genital mucosa is poorly understood, but regulation of immune activation at the female genital tract likely controls susceptibility to sexually transmitted infections.

METHOD OF STUDY

Cervical mononuclear cells were phenotyped by flow cytometry. Concentrations of cytokines were determined in cervical-vaginal lavage samples by bead array.

RESULTS

LAG-3 expression was significantly elevated at the genital mucosa and was associated with expression of CCR5 and CD69. Double negative (DN) T cells expressed the highest levels of LAG-3, but not PD-1, and were more activated than other T lymphocytes.

CONCLUSION

The elevated expression of LAG-3 at the genital tract suggests it may regulate T-cell activation, and identify cells susceptible to HIV infection. The enrichment of LAG-3 on DN T cells suggests LAG-3 may contribute to the immunoregulatory activity of these cells.

Inhibitory phenotype of HBV-specific CD4+ T-cells is characterized by high PD-1 expression but absent coregulation of multiple inhibitory molecules

Background

T-cell exhaustion seems to play a critical role in CD8⁺ T-cell dysfunction during chronic viral infections. However, up to now little is known about the mechanisms underlying CD4⁺ T-cell dysfunction during chronic hepatitis B virus (CHB) infection and the role of inhibitory molecules such as programmed death 1 (PD-1) for CD4⁺ T-cell failure.

Methods

The expression of multiple inhibitory molecules such as PD-1, CTLA-4, TIM-3, CD244, KLRG1 and markers defining the grade of T-cell differentiation as CCR7, CD45RA, CD57 and CD127 were analyzed on virus-specific CD4⁺ T-cells from peripheral blood using a newly established DRB1*01-restricted MHC class II Tetramer. Effects of in vitro PD-L1/2 blockade were defined by investigating changes in CD4⁺ T-cell proliferation and cytokine production.

Results

CD4⁺ T-cell responses during chronic HBV infection was characterized by reduced Tetramer⁺CD4⁺ T-cell frequencies, effector memory phenotype, sustained PD-1 but low levels of CTLA-4, TIM-3, KLRG1 and CD244 expression. PD-1 blockade revealed individualized patterns of in vitro responsiveness with partly increased IFN-γ, IL-2 and TNF-α secretion as well as enhanced CD4⁺ T-cell expansion almost in treated patients with viral control.

Conclusion

HBV-specific CD4⁺ T-cells are reliably detectable during different courses of HBV infection by MHC class II Tetramer technology. CD4⁺ T-cell dysfunction during chronic HBV is basically linked to strong PD-1 upregulation but absent coregulation of multiple inhibitory receptors. PD-L1/2 neutralization partly leads to enhanced CD4⁺ T-cell functionality with heterogeneous patterns of CD4⁺ T-cell rejunivation.

Impaired selectin-dependent leukocyte recruitment induces T-cell exhaustion and prevents chronic allograft vasculopathy and rejection

Selectin-selectin ligand interactions mediate the initial steps in leukocyte migration, an integral part of immune responses. Fucosyltransferase-VII (FucT-VII), encoded by Fut7, is essential for biosynthesis of selectin ligands. In an established model of cardiac allograft vasculopathy and chronic rejection, Fut7(-/-) recipients exhibited long-term graft survival with minimal vasculopathy compared with WT controls. Graft survival was associated with CD4 T-cell exhaustion in the periphery, characterized by impaired effector cytokine production, defective proliferation, increased expression of inhibitory receptors programmed death-1 (PD-1) and T cell Ig- and mucin-domain-containing molecule-3 (Tim-3), low levels of IL-7Rα on CD4 T cells, and reduced migration of polyfunctional CD4 memory T cells to the allograft. Blocking PD-1 triggered rejection only in Fut7(-/-) recipients, whereas depleting regulatory T cells had no effect in either Fut7(-/-) or WT recipients. Adoptive transfer experiments confirmed that this CD4 T cell-exhausted phenotype is seen primarily in Fut7(-/-) CD4 T cells. These data suggest that impaired leukocyte recruitment is a novel mechanism leading to CD4 T-cell exhaustion. Our experimental system serves as an excellent model to study CD4 T-cell exhaustion as a dominant mechanism of transplant tolerance. Further, targeting FucT-VII may serve as a promising strategy to prevent chronic allograft rejection and promote tolerance.

Gut immune response in the presence of hepatitis C virus infection

Hepatitis C virus (HCV) is an important etiologic agent of hepatitis and a major cause of chronic liver infection that often leads to cirrhosis, fibrosis and hepatocellular carcinoma. Although, HCV is a hepatotropic virus, there is strong evidence that HCV could replicate extra-hepatic in the gastrointestinal tissue which could serve as a reservoir for HCV. The outcome of HCV infection depends mainly on the host innate and adaptive immune responses. Innate immunity against HCV includes mainly nuclear factor cells and activation of IFN-related genes. There is an immunologic link between the gut and the liver through a population of T-cells that are capable of homing to both the liver and gut via the portal circulation. However, little is known on the role of Gut immune response in HCV. In this review we discussed the immune regulation of Gut immune cells and its association with HCV pathogenesis, various outcomes of anti-HCV therapy, viral persistence and degree of liver inflammation. Additionally, we investigated the relationship between Gut immune responses to HCV and IL28B genotypes, which were identified as a strong predictor for HCV pathogenesis and treatment outcome after acute infection.

Immune control and failure in HCV infection--tipping the balance

Despite the development of potent antiviral drugs, HCV remains a global health problem; global eradication is a long way off. In this review, we discuss the immune response to HCV infection and particularly, the interplay between viral strategies that delay the onset of antiviral responses and host strategies that limit or even eradicate infected cells but also contribute to pathogenesis. Although HCV can disable some cellular virus-sensing machinery, IFN-stimulated antiviral genes are induced in the infected liver. Whereas epitope evolution contributes to escape from T cell-mediated immunity, chronic high antigen load may also blunt the T cell response by activating exhaustion or tolerance mechanisms. The evasive maneuvers of HCV limit sterilizing humoral immunity through rapid evolution of decoy epitopes, epitope masking, stimulation of interfering antibodies, lipid shielding, and cell-to-cell spread. Whereas the majority of HCV infections progress to chronic hepatitis with persistent viremia, at least 20% of patients spontaneously clear the infection. Most of these are protected from reinfection, suggesting that protective immunity to HCV exists and that a prophylactic vaccine may be an achievable goal. It is therefore important that we understand the correlates of protective immunity and mechanisms of viral persistence.

CD127 expression, exhaustion status and antigen specific proliferation predict sustained virologic response to IFN in HCV/HIV co-infected individuals

Hepatitis C virus (HCV) infection is a major cause of morbidity and mortality in the HIV co-infected population. Interferon-alpha (IFN-α) remains a major component of anti-HCV therapy despite its deleterious effects on the immune system. Furthermore, IFN-α was recently shown to diminish the size of the latent HIV reservoir. The objectives of this study were to monitor the impact of IFN-α on T cell phenotype and proliferation of HIV and HCV-specific T cells during IFN therapy, and to identify immune markers that can predict the response to IFN in HICV/HIV co-infected patients. We performed longitudinal analyses of T cell numbers, phenotype and function in co-infected patients undergoing IFN-α therapy with different outcomes including IFN-α non-responders (NR) (n = 9) and patients who achieved sustained virologic response (SVR) (n = 19). We examined the expression of activation (CD38, HLA-DR), functional (CD127) and exhaustion markers (PD1, Tim-3, CD160 and CD244) on total CD4 and CD8 T cells before, during and after therapy. In addition, we examined the HIV- and HCV-specific proliferative responses against HIV-p24 and HCV-NS3 proteins. Frequencies of CD127⁺ CD4 T cells were higher in SVR than in NR patients at baseline. An increase in CD127 expression on CD8 T cells was observed after IFN-α therapy in all patients. In addition, CD8 T cells from NR patients expressed a higher exhaustion status at baseline. Finally, SVR patients exhibited higher proliferative response against both HIV and HCV antigens at baseline. Altogether, SVR correlated with higher expression of CD127, lower T cell exhaustion status and better HIV and HCV proliferative responses at baseline. Such factors might be used as non-invasive methods to predict the success of IFN–based therapies in co-infected individuals.

Galectin-9 is rapidly released during acute HIV-1 infection and remains sustained at high levels despite viral suppression even in elite controllers

Galectin-9 (Gal-9) is a β-galactosidase-binding lectin that promotes apoptosis, tissue inflammation, and T cell immune exhaustion, and alters HIV infection in part through engagement with the T cell immunoglobulin mucin domain-3 (Tim-3) receptor and protein disulfide isomerases (PDI). Gal-9 was initially thought to be an eosinophil attractant, but is now known to mediate multiple complex signaling events that affect T cells in both an immunosuppressive and inflammatory manner. To understand the kinetics of circulating Gal-9 levels during HIV infection we measured Gal-9 in plasma during HIV acquisition, in subjects with chronic HIV infection with differing virus control, and in uninfected individuals. During acute HIV infection, circulating Gal-9 was detected as early as 5 days after quantifiable HIV RNA and tracked plasma levels of interleukin (IL)-10, tumor necrosis factor (TNF)-α, and IL-1β. In chronic HIV infection, Gal-9 levels positively correlated with plasma HIV RNA levels (r=0.29; p=0.023), and remained significantly elevated during suppressive antiretroviral therapy (median: 225.3 pg/ml) and in elite controllers (263.3 pg/ml) compared to age-matched HIV-uninfected controls (54 pg/ml). Our findings identify Gal-9 as a novel component of the first wave of the cytokine storm in acute HIV infection that is sustained at elevated levels in virally suppressed subjects and suggest that Gal-9:Tim-3 crosstalk remains active in elite controllers and antiretroviral (ARV)-suppressed subjects, potentially contributing to ongoing inflammation and persistent T cell dysfunction.

Protective immunity against hepatitis C: many shades of gray

The majority of individuals who become acutely infected with hepatitis C virus (HCV) develop chronic infection and suffer from progressive liver damage while approximately 25% are able to eliminate the virus spontaneously. Despite the recent introduction of new direct-acting antivirals, there is still no vaccine for HCV. As a result, new infections and reinfections will remain a problem in developing countries and among high risk populations like injection drug users who have limited access to treatment and who continue to be exposed to the virus. The outcome of acute HCV is determined by the interplay between the host genetics, the virus, and the virus-specific immune response. Studies in humans and chimpanzees have demonstrated the essential role of HCV-specific CD4 and CD8 T cell responses in protection against viral persistence. Recent data suggest that antibody responses play a more important role than what was previously thought. Individuals who spontaneously resolve acute HCV infection develop long-lived memory T cells and are less likely to become persistently infected upon reexposure. New studies examining high risk cohorts are identifying correlates of protection during real life exposures and reinfections. In this review, we discuss correlates of protective immunity during acute HCV and upon reexposure. We draw parallels between HCV and the current knowledge about protective memory in other models of chronic viral infections. Finally, we discuss some of the yet unresolved questions about key correlates of protection and their relevance for vaccine development against HCV.

CD8(+) T-Cell Responses in Acute Hepatitis C Virus Infection

Hepatitis C virus (HCV) infects approximately 170 million people worldwide and is a major cause of life-threatening liver diseases such as liver cirrhosis and hepatocellular carcinoma. Acute HCV infection often progresses to chronic persistent infection, although some patients recover spontaneously. The divergent outcomes of acute HCV infection are known to be determined by differences in virus-specific T-cell responses among patients. Of the two major T-cell subsets, CD8⁺ T-cells are known to be the key effector cells that control viral infections via cytolytic activity and cytokine secretion. Herein, we review various aspects of HCV-specific CD8⁺ T-cell responses in acute HCV infection. In particular, we focus on timing of CD8⁺ T-cell responses, relationship between CD8⁺ T-cell responses and outcomes of acute HCV infection, receptor expression on CD8⁺ T-cells, breadth of CD8⁺ T-cell responses, and viral mutations.

Phenotypic T cell exhaustion in a murine model of bacterial infection in the setting of pre-existing malignancy

While much of cancer immunology research has focused on anti-tumor immunity both systemically and within the tumor microenvironment, little is known about the impact of pre-existing malignancy on pathogen-specific immune responses. Here, we sought to characterize the antigen-specific CD8⁺ T cell response following a bacterial infection in the setting of pre-existing pancreatic adenocarcinoma. Mice with established subcutaneous pancreatic adenocarcinomas were infected with Listeria monocytogenes, and antigen-specific CD8⁺ T cell responses were compared to those in control mice without cancer. While the kinetics and magnitude of antigen-specific CD8⁺ T cell expansion and accumulation was comparable between the cancer and non-cancer groups, bacterial antigen-specific CD8⁺ T cells and total CD4⁺ and CD8⁺ T cells in cancer mice exhibited increased expression of the coinhibitory receptors BTLA, PD-1, and 2B4. Furthermore, increased inhibitory receptor expression was associated with reduced IFN-γ and increased IL-2 production by bacterial antigen-specific CD8⁺ T cells in the cancer group. Taken together, these data suggest that cancer's immune suppressive effects are not limited to the tumor microenvironment, but that pre-existing malignancy induces phenotypic exhaustion in T cells by increasing expression of coinhibitory receptors and may impair pathogen-specific CD8⁺ T cell functionality and differentiation.

Hepatic immune regulation and its involvement in viral hepatitis infection

The liver has unique immune regulatory functions that promote the induction of tolerance rather than responses to antigens encountered locally. These functions are mediated by local expression of coinhibitory receptors and immunosuppressive mediators that help prevent overwhelming tissue damage. Over the years, we have gained more insight into the local regulatory cues that determine the functional complexity of immune responses regulated locally in the liver. Both the unique hepatic microenvironment and the particular liver sinusoidal cell populations, in addition to hepatocytes, actively modulate immune responses locally in the liver and thereby determine the outcome of hepatic immune responses. This is of high biological and clinical relevance in hepatitis B virus and hepatitis C virus infections, which can cause acute and persistent infections associated with chronic inflammation in humans that eventually progress to cirrhosis and hepatocellular carcinoma. Here, we review current knowledge about the balance between immunity and tolerance in the liver and how this may affect our understanding of the determinants of hepatitis B virus and hepatitis C virus clearance, persistence, and virus-induced liver disease.

Regulatory phenotype, PD-1 and TLR3 expression in T cells and monocytes from HCV patients undergoing antiviral therapy: a randomized clinical trial

BACKGROUND & AIMS

The cellular immunity has a profound impact on the status of hepatitis C virus (HCV) infection. However, the response of cellular immunity on the virological response in patients with antiviral treatment remains largely unclear. We aimed to clarify the response of peripheral T cells and monocytes in chronic hepatitis C patients with antiviral treatment.

METHODS

Patients with chronic hepatitis C were treated either with interferon alpha-2b plus ribavirin (n = 37) or with pegylated interferon alpha-2a plus ribavirin (n = 33) for up to 24 weeks. Frequencies of peripheral regulatory T-cells (Tregs), programmed death-1 (PD-1) expressing CD4+ T-cells or CD8+ T-cells and toll-like receptor (TLR) 3 expressing CD14+ monocytes were evaluated by flow cytometry in patients at baseline, 12 and 24 weeks following treatment and in 20 healthy controls.

RESULTS

Frequencies of Tregs, PD-1 and TLR3 expressing cells were higher in patients than those in control subjects (P<0.05). Patients with complete early virological response (cEVR) showed lower Tregs, PD-1 expressing CD4+ or CD8+ T-cells than those without cEVR at 12 weeks (P<0.05). Patients with low TLR3 expressing CD14+ monocytes at baseline had a high rate of cEVR (P<0.05).

CONCLUSIONS

Low peripheral TLR3 expressing CD14+ monocytes at baseline could serve as a predictor for cEVR of antiviral therapy in chronic HCV-infected patients. The cEVR rates were significantly increased in the patients with reduced circulating Tregs, PD-1 expressing CD4+ or CD8+ T-cells.

TRIAL REGISTRATION

Chinese Clinical Trial Registry ChiCTR10001090.

Adaptive immune response during hepatitis C virus infection

Hepatitis C virus (HCV) infection affects about 170 million people worldwide and it is a major cause of liver cirrhosis and hepatocellular carcinoma. HCV is a hepatotropic non-cytopathic virus able to persist in a great percentage of infected hosts due to its ability to escape from the immune control. Liver damage and disease progression during HCV infection are driven by both viral and host factors. Specifically, adaptive immune response carries out an essential task in controlling non-cytopathic viruses because of its ability to recognize infected cells and to destroy them by cytopathic mechanisms and to eliminate the virus by non-cytolytic machinery. HCV is able to impair this response by several means such as developing escape mutations in neutralizing antibodies and in T cell receptor viral epitope recognition sites and inducing HCV-specific cytotoxic T cell anergy and deletion. To impair HCV-specific T cell reactivity, HCV affects effector T cell regulation by modulating T helper and Treg response and by impairing the balance between positive and negative co-stimulatory molecules and between pro- and anti-apoptotic proteins. In this review, the role of adaptive immune response in controlling HCV infection and the HCV mechanisms to evade this response are reviewed.

Immune responses to HCV and other hepatitis viruses

Five human hepatitis viruses cause most of the acute and chronic liver disease worldwide. Over the past 25 years, hepatitis C virus (HCV) in particular has received much interest because of its ability to persist in most immunocompetent adults and because of the lack of a protective vaccine. Here we examine innate and adaptive immune responses to HCV infection. Although HCV activates an innate immune response, it employs an elaborate set of mechanisms to evade interferon (IFN)-based antiviral immunity. By comparing innate and adaptive immune responses to HCV with those to hepatitis A and B viruses, we suggest that prolonged innate immune activation by HCV impairs the development of successful adaptive immune responses. Comparative immunology provides insights into the maintenance of immune protection. We conclude by discussing prospects for an HCV vaccine and future research needs for the hepatitis viruses.

Recombinant adenoviral vector expressing HCV NS4 induces protective immune responses in a mouse model of Vaccinia-HCV virus infection: a dose and route conundrum

Hepatitis C virus (HCV) leads to chronic infection in the majority of infected patients presumably due to failure or inefficiency of the immune responses generated. Both antibody and cellular immune responses have been suggested to be important in viral clearance. Non-replicative adenoviral vectors expressing antigens of interest are considered as attractive vaccine vectors for a number of pathogens. In this study, we sought to evaluate cellular and humoral immune responses against HCV NS4 protein using recombinant adenovirus as a vaccine vector expressing NS4 antigen. We have also measured the effect of antigen doses and routes of immunization on the quality and extent of the immune responses, especially their role in viral load reduction, in a recombinant Vaccinia-HCV (Vac-HCV) infection mouse model. Our results show that an optimum dose of adenovirus vector (2×10(7)pfu/mouse) administered intramuscularly (i.m.) induces high T cell proliferation, granzyme B-expressing CD8(+) T cells, pro-inflammatory cytokines such as IFN-γ, TNF-α, IL-2 and IL-6, and antibody responses that can significantly reduce the Vac-HCV viral load in the ovaries of female C57BL/6 mice. Our results demonstrate that recombinant adenovirus vector can induce both humoral and cellular protective immunity against HCV-NS4 antigen, and that immunity is intricately controlled by route and dose of immunizing vector.

HTLV-1 specific CD8+ T cell function augmented by blockade of 2B4/CD48 interaction in HTLV-1 infection

CD8+ T cell response is important in the response to viral infections; this response though is regulated by inhibitory receptors. Expression of inhibitory receptors has been positively correlated with CD8+ T cell exhaustion; the consequent effect of simultaneous blockade of these inhibitory receptors on CD8+ T cell response in viral infections have been studied, however, the role of individual blockade of receptor-ligand pair is unclear. 2B4/CD48 interaction is involved in CD8+T cell regulation, its signal transducer SAP (signaling lymphocyte activation molecule (SLAM)-associated protein) is required for stimulatory function of 2B4/CD244 on lymphocytes hence, we analyzed 2B4/CD244 (natural killer cell receptor) and SAP (signaling lymphocyte activation molecule(SLAM)-associated protein) on total CD8+ and HTLV-1 specific CD8+T cells in HTLV-1 infection and the effect of blockade of interaction with ligand CD48 on HTLV-1 specific CD8+ T cell function. We observed a high expression of 2B4/CD244 on CD8+ T cells relative to uninfected and further upregulation on HTLV-1 specific CD8+ T cells. 2B4+ CD8+ T cells exhibited more of an effector and terminally differentiated memory phenotype. Blockade of 2B4/CD48 interaction resulted in improvement in function via perforin expression and degranulation as measured by CD107a surface mobilization on HTLV-1 specific CD8+ T cells. In the light of these findings, we thus propose an inhibitory role for 2B4/CD48 interaction on CD8+T cell function.

2B4 (CD244) induced by selective CD28 blockade functionally regulates allograft-specific CD8+ T cell responses

Blockade of CD28 signals results in the up-regulation of 2B4 on primary CD8⁺ effectors and plays a critical role in controlling antigen-specific CD8⁺ T cell responses.

Mounting evidence in models of both autoimmunity and chronic viral infection suggests that the outcome of T cell activation is critically impacted by the constellation of co-stimulatory and co-inhibitory receptors expressed on the cell surface. Here, we identified a critical role for the co-inhibitory SLAM family member 2B4 (CD244) in attenuating primary antigen-specific CD8⁺ T cell responses in the presence of immune modulation with selective CD28 blockade. Our results reveal a specific up-regulation of 2B4 on antigen-specific CD8⁺ T cells in animals in which CD28 signaling was blocked. However, 2B4 up-regulation was not observed in animals treated with CTLA-4 Ig (abatacept) or CD28 blockade in the presence of anti–CTLA-4 mAb. 2B4 up-regulation after CD28 blockade was functionally significant, as the inhibitory impact of CD28 blockade was diminished when antigen-specific CD8⁺ T cells were deficient in 2B4. In contrast, 2B4 deficiency had no effect on CD8⁺ T cell responses during unmodified rejection or in the presence of CTLA-4 Ig. We conclude that blockade of CD28 signals in the presence of preserved CTLA-4 signals results in the unique up-regulation of 2B4 on primary CD8⁺ effectors, and that this 2B4 expression plays a critical functional role in controlling antigen-specific CD8⁺ T cell responses.

Will there be a vaccine to prevent HCV infection?

Prevention of hepatitis C virus (HCV) infection by vaccination has been a priority since discovery of the virus and the need has not diminished over the past 25 years. Infection rates are increasing in developed countries because of intravenous drug use. Reducing transmission will be difficult without a vaccine to prevent persistence of primary infections, and also secondary infections that may occur after cure of chronic hepatitis C with increasingly effective direct-acting antiviral (DAA) regimens. Vaccine need is also acute in resource poor countries where most new infections occur and DAAs may be unaffordable. Spontaneous resolution of HCV infection confers durable protection, but mechanisms of immunity remain obscure and contested in the context of vaccine design. A vaccine must elicit a CD4+ helper T cell response that does not fail during acute infection. The need for neutralizing antibodies versus cytotoxic CD8+ T cells is unsettled and reflected in the design of two very different vaccines evaluated in humans for safety and immunogenicity. Here we review the status of vaccine development and the scientific and practical challenges that must be met if the burden of liver disease caused by HCV is to be reduced or eliminated.

Liver environment and HCV replication affect human T-cell phenotype and expression of inhibitory receptors

BACKGROUND & AIMS

There is an unclear relationship between inhibitory receptor expression on T cells and their ability to control viral infections. Studies of human immune cells have been mostly limited to T cells from blood, which is often not the site of infection. We investigated the relationship between T-cell location, expression of inhibitory receptors, maturation, and viral control using blood and liver T cells from patients with hepatitis C virus (HCV) and other viral infections.

METHODS

We analyzed 36 liver samples from HCV antibody-positive patients (30 from patients with chronic HCV infection, 5 from patients with sustained virological responses to treatment, and 1 from a patient with spontaneous clearance) with 19 paired blood samples and 51 liver samples from HCV-negative patients with 17 paired blood samples. Intrahepatic and circulating lymphocytes were extracted; T-cell markers and inhibitory receptors were quantified for total and virus-specific T cells by flow cytometry.

RESULTS

Levels of the markers PD-1 and 2B4 (but not CD160, TIM-3, or LAG-3) were increased on intrahepatic T cells from healthy and diseased liver tissues compared with T cells from blood. HCV-specific intrahepatic CD8(+) T cells from patients with chronic HCV infection were distinct in that they expressed TIM-3 along with PD-1 and 2B4. In comparison, HCV-specific CD8(+) T cells from patients with sustained virological responses and T cells that recognized cytomegalovirus lacked TIM-3 but expressed higher levels of LAG-3; these cells also had different memory phenotypes and proliferative capacity.

CONCLUSIONS

T cells from liver express different inhibitory receptors than T cells from blood, independent of liver disease. HCV-specific and cytomegalovirus-specific CD8(+) T cells can be differentiated based on their expression of inhibitory receptors; these correlate with their memory phenotype and levels of proliferation and viral control.

CD4+ primary T cells expressing HCV-core protein upregulate Foxp3 and IL-10, suppressing CD4 and CD8 T cells

Adaptive T cell responses are critical for controlling HCV infection. While there is clinical evidence of a relevant role for regulatory T cells in chronic HCV-infected patients, based on their increased number and function; mechanisms underlying such a phenomena are still poorly understood. Accumulating evidence suggests that proteins from Hepatitis C virus can suppress host immune responses. We and others have shown that HCV is present in CD4+ lymphocytes from chronically infected patients and that HCV-core protein induces a state of unresponsiveness in the CD4+ tumor cell line Jurkat. Here we show that CD4+ primary T cells lentivirally transduced with HCV-core, not only acquire an anergic phenotype but also inhibit IL-2 production and proliferation of bystander CD4+ or CD8+ T cells in response to anti-CD3 plus anti-CD28 stimulation. Core-transduced CD4+ T cells show a phenotype characterized by an increased basal secretion of the regulatory cytokine IL-10, a decreased IFN-γ production upon stimulation, as well as expression of regulatory T cell markers, CTLA-4, and Foxp3. A significant induction of CD4+CD25+CD127(low)PD-1(high)TIM-3(high) regulatory T cells with an exhausted phenotype was also observed. Moreover, CCR7 expression decreased in HCV-core expressing CD4+ T cells explaining their sequestration in inflamed tissues such as the infected liver. This work provides a new perspective on de novo generation of regulatory CD4+ T cells in the periphery, induced by the expression of a single viral protein.

Inhibitory Receptor Expression Depends More Dominantly on Differentiation and Activation than "Exhaustion" of Human CD8 T Cells

Under conditions of chronic antigen stimulation, such as persistent viral infection and cancer, CD8 T cells may diminish effector function, which has been termed "exhaustion." Expression of inhibitory Receptors (iRs) is often regarded as a hallmark of "exhaustion." Here we studied the expression of eight different iRs by CD8 T cells of healthy humans, including CTLA-4, PD1, TIM3, LAG3, 2B4, BTLA, CD160, and KLRG1. We show that many iRs are expressed upon activation, and with progressive differentiation to effector cells, even in absence of long-term ("chronic") antigenic stimulation. In particular, we evaluated the direct relationship between iR expression and functionality in CD8 T cells by using anti-CD3 and anti-CD28 stimulation to stimulate all cells and differentiation subsets. We observed a striking up-regulation of certain iRs following the cytokine production wave, in agreement with the notion that iRs function as a negative feedback mechanism. Intriguingly, we found no major impairment of cytokine production in cells positive for a broad array of iRs, as previously shown for PD1 in healthy donors. Rather, the expression of the various iRs strongly correlated with T cell differentiation or activation states, or both. Furthermore, we analyzed CD8 T cells from lymph nodes (LNs) of melanoma patients. Interestingly, we found altered iR expression and lower cytokine production by T cells from metastatic LNs, but also from non-metastatic LNs, likely due to mechanisms which are not related to exhaustion. Together, our data shows that expression of iRs per se does not mark dysfunctional cells, but is rather tightly linked to activation and differentiation. This study highlights the importance of considering the status of activation and differentiation for the study and the clinical monitoring of CD8 T cells.

KLRG1 impairs CD4+ T cell responses via p16ink4a and p27kip1 pathways: role in hepatitis B vaccine failure in individuals with hepatitis C virus infection

Coinfection of hepatitis B virus (HBV) with hepatitis C virus (HCV) is quite common, leading to an increase in morbidity and mortality. As such, HBV vaccination is recommended in HCV-infected individuals. However, HBV vaccine responses in HCV-infected individuals are often blunted compared with uninfected populations. The mechanism for this failure of vaccine response in HCV-infected subjects remains unclear. In this study, we investigated the expression and function of an inhibitory receptor, killer cell lectin-like receptor subfamily G member 1 (KLRG1), in the regulation of CD4(+) T cells and HBV vaccine responses during HCV infection. We demonstrated that KLRG1 was overexpressed on CD4(+) T cells from HCV-infected, HBV vaccine nonresponders compared with HBV vaccine responders. The capacity of CD4(+) T cells to proliferate and secrete IL-2 cytokine was inversely associated with the level of KLRG1 expression. Importantly, blocking KLRG1 signaling resulted in a significant improvement in CD4(+) T cell proliferation and IL-2 production in HCV-infected, HBV vaccine nonresponders in response to TCR stimulation. Moreover, blockade of KLRG1 increased the phosphorylation of Akt (Ser(473)) and decreased the expression of cell cycle inhibitors p16(ink4a) and p27(kip1), which subsequently enhanced the expression of cyclin-dependent kinase 2 and cyclin E. These results suggest that the KLRG1 pathway impairs CD4(+) T cell responses to neoantigen and induces a state of immune senescence in individuals with HCV infection, raising the possibility that blocking this negative-signaling pathway might improve HBV vaccine responses in the setting of chronic viral infection.

An insight into the diagnosis and pathogenesis of hepatitis C virus infection

This review focuses on research findings in the area of diagnosis and pathogenesis of hepatitis C virus (HCV) infection over the last few decades. The information based on published literature provides an update on these two aspects of HCV. HCV infection, previously called blood transmitted non-A, non-B infection, is prevalent globally and poses a serious public health problem worldwide. The diagnosis of HCV infection has evolved from serodetection of non-specific and low avidity anti-HCV antibodies to detection of viral nucleic acid in serum using the polymerase chain reaction (PCR) technique. Current PCR assays detect viral nucleic acid with high accuracy and the exact copy number of viral particles. Moreover, multiplex assays using real-time PCR are available for identification of HCV-genotypes and their isotypes. In contrast to previous methods, the newly developed assays are not only fast and economic, but also resolve the problem of the window period as well as differentiate present from past infection. HCV is a non-cytopathic virus, thus, its pathogenesis is regulated by host immunity and metabolic changes including oxidative stress, insulin resistance and hepatic steatosis. Both innate and adaptive immunity play an important role in HCV pathogenesis. Cytotoxic lymphocytes demonstrate crucial activity during viral eradication or viral persistence and are influenced by viral proteins, HCV-quasispecies and several metabolic factors regulating liver metabolism. HCV pathogenesis is a very complex phenomenon and requires further study to determine the other factors involved.

Between Scylla and Charybdis: The role of the human immune system in the pathogenesis of hepatitis C

Hepatitis C virus (HCV) frequently elicits only mild immune responses so that it can often establish chronic infection. In this case HCV antigens persist and continue to stimulate the immune system. Antigen persistence then leads to profound changes in the infected host’s immune responsiveness, and eventually contributes to the pathology of chronic hepatitis. This topic highlight summarizes changes associated with chronic hepatitis C concerning innate immunity (interferons, natural killer cells), adaptive immune responses (immunoglobulins, T cells, and mechanisms of immune regulation (regulatory T cells). Our overview clarifies that a strong anti-HCV immune response is frequently associated with acute severe tissue damage. In chronic hepatitis C, however, the effector arms of the immune system either become refractory to activation or take over regulatory functions. Taken together these changes in immunity may lead to persistent liver damage and cirrhosis. Consequently, effector arms of the immune system will not only be considered with respect to antiviral defence but also as pivotal mechanisms of inflammation, necrosis and progression to cirrhosis. Thus, avoiding Scylla - a strong, sustained antiviral immune response with inital tissue damage - takes the infected host to virus-triggered immunopathology, which ultimately leads to cirrhosis and liver cancer - the realm of Charybdis.

Liver immunology

The liver is the largest organ in the body and is generally regarded by nonimmunologists as having little or no lymphoid function. However, such is far from accurate. This review highlights the importance of the liver as a lymphoid organ. Firstly, we discuss experimental data surrounding the role of liver as a lymphoid organ. The liver facilitates tolerance rather than immunoreactivity, which protects the host from antigenic overload of dietary components and drugs derived from the gut and it is instrumental to fetal immune tolerance. Loss of liver tolerance leads to autoaggressive phenomena, which if not controlled by regulatory lymphoid populations, may lead to the induction of autoimmune liver diseases. Liver-related lymphoid subpopulations also act as critical antigen-presenting cells. The study of the immunological properties of liver and delineation of the microenvironment of the intrahepatic milieu in normal and diseased livers provides a platform to understand the hierarchy of a series of detrimental events that lead to immune-mediated destruction of the liver and the rejection of liver allografts. The majority of emphasis within this review will be on the normal mononuclear cell composition of the liver. However, within this context, we will discuss selected, but not all, immune-mediated liver disease and attempt to place these data in the context of human autoimmunity.

T lymphocytes from chronic HCV-infected patients are primed for activation-induced apoptosis and express unique pro-apoptotic gene signature

Although extensive studies have demonstrated the functional impairment of antigen-specific CD4(+) and CD8(+) T-cells during chronic hepatitis C virus (HCV) infection, the functional status of global CD4(+) and CD8(+) T-cells remains unclear. In this report, we recruited 42 long-term (~20 years) treatment-naïve chronic HCV (CHC) patients and 15 healthy donors (HDs) to investigate differences in global CD4(+) and CD8(+) T-cells function. We show that CD4(+) and CD8(+) T-cells from CHC patients underwent increased apoptosis after TCR stimulation. Furthermore, IFN-γ, IL-9 and IP-10 were elevated in CHC patients' plasma and promoted activation-induced T-cells death. Global CD4(+) and CD8(+) T-cells also showed unique transcriptional profiles in the expression of apoptosis-related genes. We identified BCL2, PMAIP1, and CASP1 in CD4(+) T-cells and IER3 and BCL2A1 in CD8(+) T-cells from CHC patients as HCV-specific gene signatures. Importantly, the gene expression patterns of CD4(+) and CD8(+) T-cells from CHC patients differ from those in CD4(+) and CD8(+) T-cells from human immunodeficiency virus type 1 (HIV-1) or hepatitis B virus (HBV) infected individuals. Our results indicate that chronic HCV infection causes a systemic change in cytokine levels that primes T-cells for activation-induced apoptosis. Furthermore, HCV infection programs unique apoptosis-related gene expression profiles in CD4(+) and CD8(+) T-cells, leading to their enhanced activation-induced apoptosis. These results provide novel insights to the pathogenesis of chronic HCV infection.

Emerging concepts in immunity to hepatitis C virus infection

Since the discovery of hepatitis C virus (HCV) by molecular cloning almost a quarter of a century ago, unprecedented at the time because the virus had never been grown in cell culture or detected serologically, there have been impressive strides in many facets of our understanding of the natural history of the disease, the viral life cycle, the pathogenesis, and antiviral therapy. It is apparent that the virus has developed multiple strategies to evade immune surveillance and eradication. This Review covers what we currently understand of the temporal and spatial immunological changes within the human innate and adaptive host immune responses that ultimately determine the outcomes of HCV infection.

Pathogenesis of chronic viral hepatitis: differential roles of T cells and NK cells

Chronic hepatitis B virus (HBV) and hepatitis C virus (HCV) infections account for 57% of cases of liver cirrhosis and 78% of cases of primary liver cancer worldwide and cause a million deaths per year. Although HBV and HCV differ in their genome structures, replication strategies and life cycles, they have common features, including their noncytopathic nature and their capacity to induce chronic liver disease, which is thought to be immune mediated. However, the rate of disease progression from chronic hepatitis to cirrhosis varies greatly among infected individuals, and the factors that regulate it are largely unknown. This review summarizes our current understanding of the roles of antigen-specific and nonspecific immune cells in the pathogenesis of chronic hepatitis B and C and discusses recent findings that identify natural killer cells as regulators of T cell function and liver inflammation.

The fall and rise of pharmacology--(re-)defining the discipline?

Pharmacology is an integrative discipline that originated from activities, now nearly 7000 years old, to identify therapeutics from natural product sources. Research in the 19th Century that focused on the Law of Mass Action (LMA) demonstrated that compound effects were dose-/concentration-dependent eventually leading to the receptor concept, now a century old, that remains the key to understanding disease causality and drug action. As pharmacology evolved in the 20th Century through successive biochemical, molecular and genomic eras, the precision in understanding receptor function at the molecular level increased and while providing important insights, led to an overtly reductionistic emphasis. This resulted in the generation of data lacking physiological context that ignored the LMA and was not integrated at the tissue/whole organism level. As reductionism became a primary focus in biomedical research, it led to the fall of pharmacology. However, concerns regarding the disconnect between basic research efforts and the approval of new drugs to treat 21st Century disease tsunamis, e.g., neurodegeneration, metabolic syndrome, etc. has led to the reemergence of pharmacology, its rise, often in the semantic guise of systems biology. Against a background of limited training in pharmacology, this has resulted in issues in experimental replication with a bioinformatics emphasis that often has a limited relationship to reality. The integration of newer technologies within a pharmacological context where research is driven by testable hypotheses rather than technology, together with renewed efforts in teaching pharmacology, is anticipated to improve the focus and relevance of biomedical research and lead to novel therapeutics that will contain health care costs.

KLRG1 negatively regulates natural killer cell functions through the Akt pathway in individuals with chronic hepatitis C virus infection

In this study, we demonstrate that killer cell lectin-like receptor subfamily G member 1 (KLRG1), a transmembrane protein preferentially expressed on T cells, is highly expressed on CD56(+) NK cells, which are significantly reduced in their numbers and functions in the peripheral blood of patients with chronic hepatitis C virus (HCV) infection compared to subjects without infection. KLRG1 expression is also upregulated on healthy NK cells exposed to Huh-7 hepatocytes infected with HCV in vitro. Importantly, the expression levels of KLRG1 are inversely associated with the capacity of NK cells to proliferate and to produce gamma interferon (IFN-γ) but positively associated with apoptosis of NK cells in response to inflammatory cytokine stimulation. KLRG1(+) NK cells, including CD56(bright) and CD56(dim) subsets, exhibit impaired cell activation and IFN-γ production but increased apoptosis compared to KLRG1(-) NK cells, particularly in HCV-infected individuals. Importantly, blockade of KLRG1 signaling significantly recovered the impaired IFN-γ production by NK cells from HCV-infected subjects. Blockade of KLRG1 also enhanced the impaired phosphorylation of Akt (Ser473) in NK cells from HCV-infected subjects. Taken together, these results indicate that KLRG1 negatively regulates NK cell numbers and functions via the Akt pathway, thus providing a novel marker and therapeutic target for HCV infection.

Simultaneous TCR and CD244 signals induce dynamic downmodulation of CD244 on human antiviral T cells

Various cosignaling molecules on T cells can contribute to activation, inhibition, or exhaustion, depending on context. The surface receptor signaling lymphocytic activation molecule (SLAM) family receptor CD244 (2B4/SLAMf4) has been shown to be capable of either inhibitory or enhancing effects upon engagement of its ligand CD48 (SLAMf2). We examined phenotypes of CD8 T cells from HIV(+) and HIV(neg) human donors, specific for HIV and/or respiratory syncytial virus. Cultured and ex vivo CD8 T cells expressed PD-1, CD244, and TIM-3. We found that ex vivo CD8 T cells downregulated CD244 in response to superantigen. Furthermore, cognate peptide induced rapid downregulation of both CD244 and TIM-3, but not PD-1, on CD8 T cell clones. CD244 downmodulation required simultaneous signaling via both TCR and CD244 itself. Using a pH-sensitive fluorophore conjugated to avidin-Ab tetramers, we found that CD244 crosslinking in the presence of TCR signaling resulted in rapid transport of CD244 to an acidic intracellular compartment. Downregulation was not induced by PMA-ionomycin, or prevented by PI3K inhibition, implicating a TCR-proximal signaling mechanism. CD244 internalization occurred within hours of TCR stimulation and required less peptide than was required to induce IFN-γ production. The degree of CD244 internalization varied among cultured CD8 T cell lines of different specificities, and correlated with the enhancement of IFN-γ production in response to CD48 blockade in HIV(+), but not HIV(neg), subjects. Our results indicate that rapid CD244 internalization is induced by a two-signal mechanism and plays a role in modulation of antiviral CD8 T cell responses by CD48-CD244 signaling.

Molecular pathways: coexpression of immune checkpoint molecules: signaling pathways and implications for cancer immunotherapy

The expression of immune checkpoint molecules on T cells represents an important mechanism that the immune system uses to regulate responses to self-proteins. Checkpoint molecules include cytotoxic T lymphocyte antigen-4, programmed death-1, lymphocyte activation gene-3, T-cell immunoglobulin and mucin protein-3, and several others. Previous studies have identified individual roles for each of these molecules, but more recent data show that coexpression of checkpoint molecules occurs frequently on cancer-specific T cells as well as on pathogen-specific T cells in chronic infections. As the signaling pathways associated with each checkpoint molecule have not been fully elucidated, blocking multiple checkpoints with specific monoclonal antibodies results in improved outcomes in several chronic viral infections as well as in a wide array of preclinical models of cancer. Recent clinical data suggest similar effects in patients with metastatic melanoma. These findings support the concept that individual immune checkpoint molecules may function through nonoverlapping molecular mechanisms. Here, we review current data regarding immune checkpoint molecule signaling and coexpression, both in cancer and infectious disease, as well as the results of preclinical and clinical manipulations of checkpoint proteins.

Expression profiling of TCR-engineered T cells demonstrates overexpression of multiple inhibitory receptors in persisting lymphocytes

Despite significant progress in the development of adoptive cell-transfer therapies (ACTs) using gene-engineered T cells, little is known about the fate of cells following infusion. To address that, we performed a comparative analysis of gene expression between T-cell receptor-engineered lymphocytes persisting in the circulation 1 month after administration and the product that was infused. We observed that 156 genes related to immune function were differentially expressed, including underexpression of stimulators of lymphocyte function and overexpression of inhibitory genes in postinfusion cells. Of genes overexpressed postinfusion, the product of programmed cell death 1 (PDCD1), coinhibitory receptor PD-1, was expressed at a higher percentage in postinfusion lymphocytes than in the infusion product. This was associated with a higher sensitivity to inhibition of cytokine production by interaction with its ligand PD-L1. Coinhibitory receptor CD160 was also overexpressed in persisting cells, and its expression was associated with decreased reactivity, which surprisingly was found to be ligand-independent. These results contribute to a deeper understanding of the properties of transgenic lymphocytes used to treat human malignancies and may provide a rationale for the development of combination therapies as a method to improve ACT.

Galectin-9 and IL-21 mediate cross-regulation between Th17 and Treg cells during acute hepatitis C

Loss of CD4 T cell help correlates with virus persistence during acute hepatitis C virus (HCV) infection, but the underlying mechanism(s) remain unknown. We developed a combined proliferation/intracellular cytokine staining assay to monitor expansion of HCV-specific CD4 T cells and helper cytokines expression patterns during acute infections with different outcomes. We demonstrate that acute resolving HCV is characterized by strong Th1/Th17 responses with specific expansion of IL-21-producing CD4 T cells and increased IL-21 levels in plasma. In contrast, viral persistence was associated with lower frequencies of IL-21-producing CD4 T cells, reduced proliferation and increased expression of the inhibitory receptors T cell immunoglobulin and mucin-domain-containing-molecule-3 (Tim-3), programmed death 1 (PD-1) and cytotoxic T-lymphocyte antigen 4 (CTLA-4) on HCV-specific CD8 T cells. Progression to persistent infection was accompanied by increased plasma levels of the Tim-3 ligand Galectin-9 (Gal-9) and expansion of Gal-9 expressing regulatory T cells (Tregs). In vitro supplementation of Tim-3(high) HCV-specific CD8 T cells with IL-21 enhanced their proliferation and prevented Gal-9 induced apoptosis. siRNA-mediated knockdown of Gal-9 in Treg cells rescued IL-21 production by HCV-specific CD4 T cells. We propose that failure of CD4 T cell help during acute HCV is partially due to an imbalance between Th17 and Treg cells whereby exhaustion of both CD4 and CD8 T cells through the Tim-3/Gal-9 pathway may be limited by IL-21 producing Th17 cells or enhanced by Gal-9 producing Tregs.

Role of T cell immunity in hepatitis C virus infections

Chronic infections with the hepatitis C virus (HCV) are a major global health issue. Viral replication is restricted to hepatocytes, and occurs for decades at high replication rates. Over the last decade, it became accepted that HCV-specific CD4(+) and CD8(+) T cells are crucial for protective immunity to HCV. However, a characteristic feature of persistent HCV infection is the dysfunctional T cell response, and over recent years enormous progress has been made in understanding the mechanisms that dampen the antiviral T cell responses in blood and liver of chronic HCV patients and also impact disease progression.

Molecular mechanisms of T cell co-stimulation and co-inhibition

Co-stimulatory and co-inhibitory receptors have a pivotal role in T cell biology, as they determine the functional outcome of T cell receptor (TCR) signalling. The classic definition of T cell co-stimulation continues to evolve through the identification of new co-stimulatory and co-inhibitory receptors, the biochemical characterization of their downstream signalling events and the delineation of their immunological functions. Notably, it has been recently appreciated that co-stimulatory and co-inhibitory receptors display great diversity in expression, structure and function, and that their functions are largely context dependent. Here, we focus on some of these emerging concepts and review the mechanisms through which T cell activation, differentiation and function is controlled by co-stimulatory and co-inhibitory receptors.

PD-1 coinhibitory signals: the link between pathogenesis and protection

In the majority of HIV-1 infected individuals, the adaptive immune response drives virus escape resulting in persistent viremia and a lack of immune-mediated control. The expression of negative regulatory molecules such as PD-1 during chronic HIV infection provides a useful marker to differentiate functional memory T cell subsets and the frequency of T cells with an exhausted phenotype. In addition, cell-based measurements of virus persistence equate with activation markers and the frequency of CD4 T cells expressing PD-1. High-level expression of PD-1 and its ligands PD-L1 and PD-L2 are found on hematopoietic and non-hematopoietic cells, and are upregulated by chronic antigen stimulation, Type 1 and Type II interferons (IFNs), and homeostatic cytokines. In HIV infected subjects, PD-1 levels on CD4 and CD8 T cells continue to remain high following combination anti-retroviral therapy (cART). System biology approaches have begun to elucidate signal transduction pathways regulated by PD-1 expression in CD4 and CD8 T cell subsets that become dysfunctional through chronic TCR activation and PD-1 signaling. In this review, we summarize our current understanding of transcriptional signatures and signal transduction pathways associated with immune exhaustion with a focus on recent work in our laboratory characterizing the role of PD-1 in T cell dysfunction and HIV pathogenesis. We also highlight the therapeutic potential of blocking PD-1-PD-L1 and other immune checkpoints for activating potent cellular immune responses against chronic viral infections and cancer.

T-cell responses in hepatitis B and C virus infection: similarities and differences

Chronic hepatitis B virus (HBV) and hepatitis C virus (HCV) infection are global health problems affecting 600 million people worldwide. Indeed, HBV and HCV are hepatotropic viruses that can cause acute and chronic liver disease progressing to liver cirrhosis and even hepatocellular carcinoma. Furthermore, co-infections of HBV and HCV with HIV are emerging worldwide. These co-infections are even more likely to develop persistent infection and are difficult to treat. There is growing evidence that virus-specific CD4⁺ and CD8⁺ T-cell responses play a central role in the outcome and pathogenesis of HBV and HCV infection. While virus-specific T-cell responses are able to successfully clear the virus in a subpopulation of patients, failure of these T-cell responses is associated with the development of viral persistence. In this review article, we will discuss similarities and differences in HBV- and HCV-specific T-cell responses that are central in determining viral clearance, persistence and liver disease.

Molecular signatures of T-cell inhibition in HIV-1 infection

Cellular immune responses play a crucial role in the control of viral replication in HIV-infected individuals. However, the virus succeeds in exploiting the immune system to its advantage and therefore, the host ultimately fails to control the virus leading to development of terminal AIDS. The virus adopts numerous evasion mechanisms to hijack the host immune system. We and others recently described the expression of inhibitory molecules on T cells as a contributing factor for suboptimal T-cell responses in HIV infection both in vitro and in vivo. The expression of these molecules that negatively impacts the normal functions of the host immune armory and the underlying signaling pathways associated with their enhanced expression need to be discussed. Targets to restrain the expression of these molecular markers of immune inhibition is likely to contribute to development of therapeutic interventions that augment the functionality of host immune cells leading to improved immune control of HIV infection. In this review, we focus on the functions of inhibitory molecules that are expressed or secreted following HIV infection such as BTLA, CTLA-4, CD160, IDO, KLRG1, LAG-3, LILRB1, PD-1, TRAIL, TIM-3, and regulatory cytokines, and highlight their significance in immune inhibition. We also highlight the ensemble of transcriptional factors such as BATF, BLIMP-1/PRDM1, FoxP3, DTX1 and molecular pathways that facilitate the recruitment and differentiation of suppressor T cells in response to HIV infection.

Liver sinusoidal endothelial cell lectin inhibits CTL-dependent virus clearance in mouse models of viral hepatitis

Liver sinusoidal endothelial cell lectin (LSECtin) was recently reported to suppress intrahepatic T cell immunity and to limit immune-mediated liver injury. However, its role in the outcome and pathogenesis of viral infection has not yet been elucidated. Using a mouse model infected with a hepatotropic adenovirus, we found that the absence of LSECtin led to a higher frequency of intrahepatic effector CTLs. These cells produced higher levels of antiviral cytokines and cytotoxic factors and exhibited an increased expression of the transcription factors T-bet and Runx3. This phenotype observed in the LSECtin-knockout cells mediated a more efficient virus-specific cytotoxicity compared with that of wild-type cells. As a consequence, LSECtin deficiency significantly accelerated liver adenovirus clearance. In contrast, LSECtin upregulation in the liver delayed viral clearance; this delayed clearance was accompanied by the downregulation of the antiviral activity of CTLs. We further constructed an immunocompetent mouse model of acute hepatitis B viral infection to demonstrate that LSECtin significantly delayed the clearance of hepatitis B virus from blood and infected hepatocytes by limiting the frequency of hepatitis B virus-specific IFN-γ-producing cells. Consistent with this function, LSECtin was upregulated in the liver of mouse models of viral hepatitis. Taken together, our results suggest that LSECtin may facilitate the reduction of liver inflammation at the cost of delaying virus clearance and that this effect might be hijacked by the virus as an escape mechanism.

Immunity and hepatitis C: a review

Hepatitis C virus (HCV) is a major cause of chronic hepatitis and hepatocellular carcinoma worldwide. Due to shared transmission routes, the prevalence of HCV is especially high among individuals infected with HIV. HIV uninfected individuals spontaneously clear HCV approximately 30 % of the time, while the rate of control in HIV infected individuals who subsequently acquire HCV is substantially lower. In addition, complications of HCV are more frequent in those with HIV infection, making liver disease the leading cause of non-AIDS-related death in HIV infected individuals. This review summarizes recent advances in understanding the role of the innate and adaptive immune responses to HCV in those with and without HIV. Further defining the interaction between hepatitis C and the host immune system will potentially reveal insights into HCV pathogenesis and the host's ability to prevent persistent infection, as well as direct the development of vaccines.

HIV-specific CD8+ T cells from elite controllers are primed for survival

HIV-specific cytotoxic T lymphocytes (CTL) are preferentially primed for apoptosis, and this may represent a viral escape mechanism. We hypothesized that HIV-infected individuals that control virus to undetectable levels without antiretroviral therapy (ART) (elite controllers [EC]) have the capacity to upregulate survival factors that allow them to resist apoptosis. To address this, we performed cross-sectional and longitudinal analysis of proapoptotic (cleaved caspase-3) and antiapoptotic (Bcl-2) markers of cytomegalovirus (CMV) and HIV-specific CD8 T cells in a cohort of HIV-infected subjects with various degrees of viral control on and off ART. We demonstrated that HIV-specific CTL from EC are more resistant to apoptosis than those with pharmacologic control (successfully treated patients [ST]), despite similar in vivo conditions. Longitudinal analysis of chronically infected persons starting ART revealed that the frequency of HIV-specific T cells prone to death decreased, suggesting that this phenotype is partially reversible even though it never achieves the levels present in EC. Elucidating the apoptotic factors contributing to the survival of CTL in EC is paramount to our development of effective HIV-1 vaccines. Furthermore, a better understanding of cellular markers that can be utilized to predict response durability in disease- or vaccine-elicited responses will advance the field.

Increased T-bet is associated with senescence of influenza virus-specific CD8 T cells in aged humans

Aged individuals have increased morbidity and mortality following influenza and other viral infections, despite previous exposure or vaccination. Mouse and human studies suggest increased senescence and/or exhaustion of influenza virus-specific CD8 T cells with advanced age. However, neither the relationship between senescence and exhaustion nor the underlying transcriptional pathways leading to decreased function of influenza virus-specific cellular immunity in elderly humans are well-defined. Here, we demonstrate that increased percentages of CD8 T cells from aged individuals express CD57 and KLRG1, along with PD-1 and other inhibitory receptors, markers of senescence, or exhaustion, respectively. Expression of T-box transcription factors, T-bet and Eomes, were also increased in CD8 T cells from aged subjects and correlated closely with expression of CD57 and KLRG1. Influenza virus-specific CD8 T cells from aged individuals exhibited decreased functionality with corresponding increases in CD57, KLRG1, and T-bet, a molecular regulator of terminal differentiation. However, in contrast to total CD8 T cells, influenza virus-specific CD8 T cells had altered expression of inhibitory receptors, including lower PD-1, in aged compared with young subjects. Thus, our data suggest a prominent role for senescence and/or terminal differentiation for influenza virus-specific CD8 T cells in elderly subjects.

The frequency of CD127(+) hepatitis C virus (HCV)-specific T cells but not the expression of exhaustion markers predicts the outcome of acute HCV infection

T cells are exhausted and overexpress inhibitory molecules in chronic hepatitis C virus (HCV) infection. It is unclear whether this is the cause or consequence of HCV persistence. By studying serial blood and liver samples of chimpanzees during acute infection, we demonstrate that the early expression of the memory precursor marker CD127 on HCV-specific T cells, but not the expression of inhibitory molecules on those T cells or their ligands in the liver, predicts the outcome of acute infection.

Comparative analysis of the magnitude, quality, phenotype, and protective capacity of simian immunodeficiency virus gag-specific CD8+ T cells following human-, simian-, and chimpanzee-derived recombinant adenoviral vector immunization

Recombinant adenoviral vectors (rAds) are the most potent recombinant vaccines for eliciting CD8(+) T cell-mediated immunity in humans; however, prior exposure from natural adenoviral infection can decrease such responses. In this study we show low seroreactivity in humans against simian- (sAd11, sAd16) or chimpanzee-derived (chAd3, chAd63) compared with human-derived (rAd5, rAd28, rAd35) vectors across multiple geographic regions. We then compared the magnitude, quality, phenotype, and protective capacity of CD8(+) T cell responses in mice vaccinated with rAds encoding SIV Gag. Using a dose range (1 × 10(7)-10(9) particle units), we defined a hierarchy among rAd vectors based on the magnitude and protective capacity of CD8(+) T cell responses, from most to least, as: rAd5 and chAd3, rAd28 and sAd11, chAd63, sAd16, and rAd35. Selection of rAd vector or dose could modulate the proportion and/or frequency of IFN-γ(+)TNF-α(+)IL-2(+) and KLRG1(+)CD127(-)CD8(+) T cells, but strikingly ∼30-80% of memory CD8(+) T cells coexpressed CD127 and KLRG1. To further optimize CD8(+) T cell responses, we assessed rAds as part of prime-boost regimens. Mice primed with rAds and boosted with NYVAC generated Gag-specific responses that approached ∼60% of total CD8(+) T cells at peak. Alternatively, priming with DNA or rAd28 and boosting with rAd5 or chAd3 induced robust and equivalent CD8(+) T cell responses compared with prime or boost alone. Collectively, these data provide the immunologic basis for using specific rAd vectors alone or as part of prime-boost regimens to induce CD8(+) T cells for rapid effector function or robust long-term memory, respectively.

Persistent hepatitis C virus (HCV) infection impairs HCV-specific cytotoxic T cell reactivity through Mcl-1/Bim imbalance due to CD127 down-regulation

In persistent hepatitis C virus (HCV) infection, HCV-specific cytotoxic T lymphocyte (CTL) reactivity is impaired and this affects HCV control. Interleukin-7 receptor (CD127) expression on these cells could regulate CTL reactivity through Mcl-1/Bim balance modulation. Bim is a pro-apoptotic molecule blocked by the action of Mcl-1. Mcl-1/Bim expression and T cell reactivity on HCV-specific CTLs were compared according to CD127 phenotype. Peripheral blood lymphocytes (PBL) from HLA-A2(+) HCV(+) patients were obtained. HCV-specific CTLs were visualized by staining PBL with anti-CD8 and HLA-A2/peptide pentameric complexes (pentamer). Mcl-1/Bim/CD127 phenotype of HCV-specific CTLs was tested by staining detectable CD8(+)/pentamer(+) cells with anti-Mcl-1/Bim/CD127 antibodies. HCV-specific CTL proliferation ability after specific in vitro challenge was tested in the presence and absence of pancaspase inhibitor z-VAD-fmk. All stained cells were analysed by flow cytometry. CD127(low)-expressing HCV-specific CTLs associated with high HCV viraemia, while CD127(high) correlated with undetectable viral loads (P < 0.001). Directly ex vivo, pentamer(+) cell frequency was similar according to CD127 expression level. Nevertheless, CD127(low) pentamer(+) cell proliferation after specific in vitro challenge was impaired (P < 0.05), although this was corrected by z-VAD-fmk treatment (P < 0.05). Mcl-1 expression was low directly ex vivo (P < 0.01), and Bim was up-regulated after antigen encounter (P < 0.05) of CD127(low) pentamer(+) cells. The ex vivo difference between Mcl-1 and Bim expression on pentamer(+) cells correlated positively with CD127 expression level (P < 0.001) and with pentamer(+) cell reactivity (P < 0.05). In summary, a low ex vivo Mcl-1 expression and Bim up-regulation after antigen encounter are involved in CD127(low) HCV-specific CTL hyporeactivity during chronic infection, but it can be overcome by apoptosis blockade.

Factors that determine the antiviral efficacy of HCV-specific CD8(+) T cells ex vivo

BACKGROUND & AIMS

Dysfunctional CD8(+) T cells are believed to contribute to the ability of hepatitis C virus (HCV) to evade the immune response. Most studies have focused on the effector functions of HCV-specific CD8(+) T cells or their surface expression of inhibitory receptors. There is currently no information available about the ex vivo ability of HCV-specific CD8(+) T cells to inhibit viral replication (antiviral efficacy).

METHODS

To analyze the antiviral efficacy of virus-specific CD8(+) T cells ex vivo, we used an immunologic model based on a cell line that expresses HLA-A*02 and contains a stably replicating HCV reporter replicon. We isolated HCV-specific CD8(+) T cells from 18 HLA-A*02-positive patients with chronic HCV infection and 15 subjects with resolved HCV infection (7 spontaneous, 8 after therapy). Replicon cells were labeled with virus-specific peptides; inhibition of HCV replication was determined by measuring luciferase activity after 72 hours of coculture with virus-specific CD8(+) T cells.

RESULTS

HCV-specific CD8(+) T cells from patients with chronic HCV infection had a significantly lower antiviral efficacy than influenza-, Epstein-Barr virus-, and cytomegalovirus-specific CD8(+) T cells. Antiviral efficacy was associated with the ability of virus-specific CD8(+) T cells to secrete interferon gamma. The antiviral efficacy of HCV-specific CD8(+) T cells was linked to surface expression of CD127 and PD-1. The cytokines interleukin-2, interleukin-7, and interleukin-15 increased the antiviral efficacy of CD127-positive but not of CD127-negative, HCV-specific CD8(+) T cells. Spontaneous, but not antiviral therapy-induced, viral clearance was associated with increased antiviral efficacy.

CONCLUSIONS

The ability of CD8(+) T cells to inhibit HCV replication ex vivo is associated with their ability to secrete interferon gamma and their surface expression of CD127 and PD-1.