Rapid and sustained autologous neutralizing response leading to early spontaneous recovery after HCV infection

Evolutionary modeling reveals enhanced mutational flexibility of HCV subtype 1b compared with 1a

Hepatitis C virus (HCV) is a leading cause of liver-associated disease and liver cancer. Of the major HCV subtypes, patients infected with subtype 1b have been associated with having a higher risk of developing chronic infection and hepatocellular carcinoma. However, underlying reasons for this increased disease severity remain unknown. Here, we provide an evolutionary rationale, based on a comparative study of fitness landscape and in-host evolutionary models of the E2 glycoprotein of HCV subtypes 1a and 1b. Our analysis demonstrates that a higher chronicity rate of 1b may be attributed to lower fitness constraints, enabling 1b viruses to more easily escape antibody responses. More generally, our results suggest that differences in evolutionary constraints between HCV subtypes may be an important factor in mediating distinct disease outcomes. Our analysis also identifies antibodies that appear escape-resistant against both subtypes 1a and 1b, providing directions for designing HCV vaccines having cross-subtype protection.

Circulating CXCR3+ Tfh cells positively correlate with neutralizing antibody responses in HCV-infected patients

Circulating T follicular helper (cTfh) cells have been identified as counterparts of germinal center Tfh (GC Tfh) cells in humans and can support T-dependent B cell maturation and antibody production in vitro. However, the role of cTfh cells in neutralizing antibody (nAb) responses in HCV infection remains unclear. Here, we characterized the phenotype and function of cTfh cells and demonstrated the associations of cTfh cells and their subsets with nAb responses in HCV infection. A total of 38 HCV-infected individuals and 28 healthy controls were enrolled from a pool of injection drug users. The frequency and function of blood Tfh cells were analyzed by flow cytometry. The titers and breadths of serum nAbs were measured using HCV pseudo-particle neutralization assays. Herein, we report several key observations. First, HCV infection skewed cTfh toward CXCR3⁺ cTfh cell differentiation. Second, the frequency of CXCR3⁺ cTfh cells positively correlated with HCV nAb titers and breadths. Third, CXCR3⁺ cTfh cells showed higher expression of Tfh-associated molecules (PD-1, ICOS, IL-21, Bcl-6) compared with CXCR3⁻ cTfh cells from individuals with HCV infection. Coculture of cTfh cells and autologous memory B cells in vitro indicated that CXCR3⁺ cTfh cells show a superior ability to support HCV E2-specific B cell expansion compared with CXCR3⁻ cTfh cells from individuals with HCV infection. HCV infection skews cTfh cells toward CXCR3-biased Tfh cell differentiation, which positively correlates with the magnitude and breadth of the HCV nAb response. It is our hope that these findings will provide insights for the rational design of a nAb-based HCV vaccine.

Innate and Adaptive Immune Responses in Chronic HCV Infection

Hepatitis C virus (HCV) remains a public health problem of global importance, even in the era of potent directly-acting antiviral drugs. In this chapter, I discuss immune responses to acute and chronic HCV infection. The outcome of HCV infection is influenced by viral strategies that limit or delay the initiation of innate antiviral responses. This delay may enable HCV to establish widespread infection long before the host mounts effective T and B cell responses. HCV's genetic agility, resulting from its high rate of replication and its error prone replication mechanism, enables it to evade immune recognition. Adaptive immune responses fail to keep up with changing viral epitopes. Neutralizing antibody epitopes may be hidden by decoy structures, glycans, and lipoproteins. T cell responses fail due to changing epitope sequences and due to exhaustion, a phenomenon that may have evolved to limit immune-mediated pathology. Despite these difficulties, innate and adaptive immune mechanisms do impact HCV replication. Immune-mediated clearance of infection is possible, occurring in 20-50% of people who contract the disease. New developments raise hopes for effective immunological interventions to prevent or treat HCV infection.

Type III Interferons in Hepatitis C Virus Infection

The interferon (IFN)-λ family of type III cytokines includes the closely related interleukin (IL)-28A (IFN-λ2), IL-28B (IFN-λ3), and IL-29 (IFN-λ1). They signal through the Janus kinases (JAK)-signal transducers and activators of transcription pathway and promote an antiviral state by the induction of expression of several interferon-stimulated genes (ISGs). Contrary to type I IFNs, the effect of IFN-λ cytokines is largely limited to epithelial cells due to the restricted pattern of expression of their specific receptor. Several genome-wide association studies have established a strong correlation between polymorphism in the region of IL-28B gene (encoding for IFN-λ3) and both spontaneous and therapeutic IFN-mediated clearance of hepatitis C virus (HCV) infection, but the mechanism(s) underlying this enhanced viral clearance are not fully understood. IFN-λ3 directly inhibits HCV replication, and in vitro studies suggest that polymorphism in the IFN-λ3 and its recently identified overlapping IFN-λ4 govern the pattern of ISGs induced upon HCV infection of hepatocytes. IFN-λ can also be produced by dendritic cells, and apart from its antiviral action on hepatocytes, it can regulate the inflammatory response of monocytes/macrophages, thus acting at the interface between innate and adaptive immunity. Here, we review the current state of knowledge about the role of IFN-λ cytokines in mediating and regulating the immune response during acute and chronic HCV infections.

Novel E2 Glycoprotein Tetramer Detects Hepatitis C Virus-Specific Memory B Cells

Acute hepatitis C virus (HCV) infection culminates in viral persistence in the majority of cases. Abs that recognize the envelope glycoproteins E1 and E2 are generated during the late stages of acute infection, yet their contribution to spontaneous viral clearance remains controversial. Investigation of the humoral responses during acute HCV infection have been limited by the inability to directly identify and characterize HCV-specific B cells. In this study we describe the development of a novel tetramer of the E2 glycoprotein ectodomain (J6, genotype 2a strain), which allowed us to visualize E2-specific B cells longitudinally in the peripheral blood of HCV-infected individuals. HCV-specific class-switched memory B cells were detected in 3 out of 7 participants during late acute infection, with a mean frequency of 0.63% for positive samples (range 0.16-0.67%) and in 7 out of 7 participants with chronic infection with a mean frequency of 0.47% (range 0.20-0.78%). In a cross-sectional study, E2 tetramer positive population was detected in 28 out of 31 chronically infected individuals. Deep sequencing of the BCR from E2-specific class-switched memory B cells sorted from two independent participants revealed a focused repertoire suggestive of clonal selection. Tetramer-specific B cells exhibited skewed CDR3 length distribution and increased mutation frequency compared with naive B cells. This BCR profile is indicative of clonal expansion and affinity maturation. E2 tetramer allows for specific and sensitive ex vivo characterization of rare HCV-specific B cells in infected individuals, and will enable researchers to gain a better understanding of humoral immunity in HCV infection.

Anti-envelope antibody responses in individuals at high risk of hepatitis C virus who resist infection

Injection drug users uninfected by hepatitis C virus (HCV) despite likely repeated exposure through high-risk behaviour are well documented. Factors preventing infection in these individuals are incompletely understood. Here, we looked for anti-HCV-envelope antibody responses in a cohort of repeatedly exposed but uninfected subjects. Forty-two hepatitis C diagnostic antibody- and RNA-negative injection drug users at high risk of exposure were studied and findings compared to healthy controls and cases with chronic HCV infection. Purified IgGs from sera were tested by ELISA for binding to genotype 1a and 3a envelope glycoproteins E1E2 with further testing for IgG and IgM reactivity against soluble E2. Virus-neutralizing activity was assessed using an HCV pseudoparticle system. Uninfected subjects demonstrated significantly greater IgG and IgM reactivities to envelope glycoproteins than healthy controls with IgG from 6 individuals additionally showing significant neutralization. This study is the first to describe humoral immunological responses targeting the HCV envelope, important for viral neutralization, in exposed uninfected individuals. A subset of these cases also had evidence of viral neutralization via anti-envelope antibodies. In addition to confirming viral exposure, the presence of specific anti-envelope antibodies may be a factor that helps these individuals resist HCV infection.

Immune responses and immunopathology in acute and chronic viral hepatitis

Hepatitis A virus (HAV), hepatitis B virus (HBV) and hepatitis C virus (HCV) are responsible for most cases of viral hepatitis. Infection by each type of virus results in a different typical natural disease course and clinical outcome that are determined by virological and immunological factors. HCV tends to establish a chronic persistent infection, whereas HAV does not. HBV is effectively controlled in adults, although it persists for a lifetime after neonatal infection. In this Review, we discuss the similarities and differences in immune responses to and immunopathogenesis of HAV, HBV and HCV infections, which may explain the distinct courses and outcomes of each hepatitis virus infection.

Patterns of longitudinal change in hepatitis C virus neutralization titers correlate with the outcome of peginterferon and ribavirin combination therapy

In chronic hepatitis C virus (HCV) infection, combination therapy of peginterferon and ribavirin does not guarantee viral eradication. Among factors relevant to therapeutic efficacy, the role of humoral immunity has not been examined thoroughly. In the current study, HCV pseudoparticles (HCVpp) were first generated with 80 patient-derived full-length HCV envelope clones, followed by detailed characterization with regard to virus productivity, infectivity and neutralizing activity. Selective HCVpp were used to measure HCV neutralization titers in two independent patient cohorts consisting of 102 patients undergoing antiviral therapy. The HCV neutralization titers at the baseline fitted with a power-law distribution among patients. Pretreatment neutralization titers in both patient cohorts were not correlated with treatment outcomes. In the patient cohort 2 (n = 28) that had samples available at multiple time points, however, HCV neutralization titers displayed clearly distinct patterns over therapeutic course and follow-up. No virological responders (n = 10) had neutralization titers stabilized at low level while it was increased significantly in both sustained virological responders (n = 10) and relapsers (n = 8). High HCV neutralization titers were maintained only in sustained virological responders but not in relapsers after treatment cessation. Therefore, patterns of longitudinal change of HCV neutralization titers, but not pretreatment titers, correlate with the treatment outcome in patients undergoing peginterferon and ribavirin combination therapy.

The Humoral Immune Response to HCV: Understanding is Key to Vaccine Development

Hepatitis C virus (HCV) remains a global problem, despite advances in treatment. The low cost and high benefit of vaccines have made them the backbone of modern public health strategies, and the fight against HCV will not be won without an effective vaccine. Achievement of this goal will benefit from a robust understanding of virus-host interactions and protective immunity in HCV infection. In this review, we summarize recent findings on HCV-specific antibody responses associated with chronic and spontaneously resolving human infection. In addition, we discuss specific epitopes within HCV's envelope glycoproteins that are targeted by neutralizing antibodies. Understanding what prompts or prevents a successful immune response leading to viral clearance or persistence is essential to designing a successful vaccine.