Vaccine‐induced hepatitis C virus‐specific CD8+ T cells do not always help
Authors: Dominik Wieland, Robert Thimme
Journal: Hepatology (2015)

Potential conflict of interest: Nothing to report. See Articles on Pages 1442 and 1455 Chronic hepatitis C viral (HCV) infection is characterized by a failure of virus‐specific CD8+ T cells to control viremia.1 Two major mechanisms contribute to this virus‐specific CD8+ T‐cell failure: first, mutational escape of major histocompatibility complex class I epitopes resulting in lack of viral recognition and, second, functional exhaustion of virus‐specific CD8+ T cells due to persistent antigen stimulation. The two mechanisms can be discriminated by phenotypical features. Virus‐specific CD8+ T cells targeting viral sequence variations are characterized by high expression of CD127 and low expression of inhibitory receptors (e.g., PD1), while exhausted CD8+ T cells targeting intact viral antigen show reduced CD127 and higher inhibitory receptor expression.3 Previous approaches to induce virus‐specific CD8+ T cells by therapeutic vaccination failed to significantly suppress HCV viremia despite the partial priming and expansion of HCV‐specific T‐cell immunity. The reasons for this failure of vaccine‐induced T cells to control viremia and its correlation with the presence or absence of viral sequence variations are currently not well understood. These important questions have now been addressed by two elegant studies published in this issue of hepatology. Kelly et al. vaccinated 35 chronically genotype 1 HCV‐infected patients using heterologous adenoviral vectors encoding HCV nonstructural proteins NS3‐5b in a dose‐escalation prime/boost regimen.5 Callendret et al. vaccinated two chronically HCV‐infected chimpanzees also with an adenoviral vector‐based genetic vaccine encoding HCV NS3‐5b.6 Vaccination was combined with either direct‐acting antiviral (DAA)‐based (Callendret et al.) or interferon (IFN)‐based therapy (Kelly et al.) to suppress viral load during vaccination. Importantly, although different species, vaccination strategies, and antiviral therapies were used, the main conclusions from the two studies are remarkably similar: vaccination resulted in the induction of HCV‐specific CD8+ T‐cell responses that, however, did not have a significant impact on viral loads, and sequence analyses revealed that vaccine‐induced HCV‐specific CD8+ T cells primarily targeted viral sequence variations and thus failed to recognize the circulating virus. These results give important novel insights into T‐cell immunity in chronic HCV infection. First, the finding that HCV‐specific CD8+ T cells can principally be induced by vaccination indicates that antigen delivery, endogenous antigen processing, class I presentation, and priming of T cells are not severely impaired in chronically HCV‐infected subjects. However, in contrast to vaccinated healthy volunteers, the magnitude and breadth of HCV‐specific CD8+ T‐cell responses following vaccination in chronically HCV‐infected patients were remarkably reduced. One explanation might be that a considerable proportion of naive HCV‐specific T cells had been primed during acute HCV infection but rendered dysfunctional and thus unresponsive to vaccination during chronic infection. In addition, the findings may reflect the presence of an immunosuppressive milieu or the lack of sufficient CD4+ T‐cell help. Indeed, absence of sufficient CD4+ T‐cell help is a hallmark of HCV‐specific CD8+ T‐cell failure.7 Thus, it is noteworthy that in both studies only very weak HCV‐specific CD4+ T‐cell responses were detectable after vaccination. Second, the finding that the induced and expanded functional HCV‐specific CD8+ T cells targeted primarily viral sequence variations clearly suggests that vaccine‐mediated HCV‐specific T‐cell induction primarily occurs in the absence of autologous antigen stimulation (Fig. 1). Accordingly, these CD8+ T cells did not have a significant impact on viral load and, following vaccine‐induced expansion, even transitioned from an activated to a memory‐like phenotype characterized by high CD127 expression, again indicating lack of antigen recognition.3 The profound exhaustion of HCV‐specific CD8+ T‐cell responses, however, that target intact HCV epitopes is most likely maintained by ongoing antigen recognition and cannot be simply overcome by vaccination alone (Fig. 1). This is supported by the missing induction of novel functional CD8+ T‐cell responses to intact epitopes and the consequent absence of mutational pressure within intact class I epitopes after vaccination (Callendret et al.). Another factor that contributes to CD8+ T‐cell exhaustion is the tolerogenic liver environment.8 Indeed, Callendret et al. observed one multifunctional CD8+ T‐cell response targeting an intact viral epitope that upon liver entry transitioned to a highly exhausted phenotype.Figure 1: Responsiveness of HCV‐specific CD8+ T cells on therapeutic vaccination is limited by persistent antigen recognition in chronic HCV infection. Left panel: HCV‐specific CD8+ T cells targeting viral sequence variations expand after therapeutic vaccination. The emerging T‐cell population is multifunctional and phenotypically mimics T‐cell memory. Right panel: Exhausted HCV‐specific CD8+ T cells that recognize intact viral epitopes cannot be recovered and expanded by therapeutic vaccination.Third, antiviral therapy‐mediated suppression of viral load did not correlate with enhanced responsiveness of HCV‐specific CD8+ T cells to vaccination. This is somewhat surprising as previous findings in the lymphocytic choriomeningitis virus mouse model showed that a reduction of high viral load prior to vaccination is associated with increased effectiveness of therapeutic vaccination.9 Callendret et al. treated both chimpanzees with the MK0608 NS5b polymerase inhibitor prior to or coinciding with the first vaccination. Both animals, however, showed early rebound of viral replication within 3 weeks due to breakthrough replication of a drug‐resistant HCV variant. Kelly et al. suppressed viral load in a subgroup of patients by pegylated IFN‐α/ribavirin for 2 weeks (still high viral load at time of vaccination prime) or 14 weeks (undetectable HCV RNA in 9/10 persons at vaccination initiation) prior to vaccination. Still, no correlation between viral load at the time of vaccination and emerging T‐cell responses was found. An IFN‐mediated inhibitory effect on HCV‐specific CD8+ T cells in this setting seems rather unlikely as vector‐specific and other virus‐specific CD8+ T‐cell responses, e.g., Epstein‐Barr virus or cytomegalovirus, were not impaired during IFN administration. Although the mechanisms responsible for treatment failure of the combinational approach of vaccination and antiviral therapy are not completely understood, these findings highlight the major challenge of overcoming T‐cell exhaustion in the context of persistent antigen exposure. Important questions remain, however. For example, can these exhausted CD8+ T cells be rescued after DAA‐mediated viral elimination and long‐term sustained virological response? A first study has indicated a partial recovery of HCV‐specific CD8+ T cells in some cured patients by showing increased in vitro proliferation.10 However, in a chimpanzee that was rechallenged 2 years after DAA‐mediated cure, expansion of HCV‐specific CD8+ T cells was only reported for T cells targeting viral sequence variations but not intact viral epitopes,11 further suggesting a permanent and profound exhaustion of virus‐specific CD8+ T cells to respond to intact class I epitopes. This failure of HCV‐specific CD8+ T cells to recover after DAA‐mediated HCV cure and to respond to or even prevent secondary infection is of high clinical relevance as a significant subgroup of cured patients remains at high risk of reinfection. Strategies such as the use of checkpoint modulation or a better induction of CD4+ T‐cell help should be considered for possible restoration of CD8+ T‐cell immunity. Noteworthy, vaccination strategies including a modified vaccinia Ankara vector as boost following adenoviral vector prime might be promising as this combination has been shown to trigger a more balanced induction of CD4+/CD8+ T‐cell responses.12 Taken together, the differential impact of vaccination and/or antiviral therapy on induction of HCV‐specific CD8+ T cells depending on autologous antigen recognition is a novel important finding that extends the current knowledge of HCV‐specific T‐cell immunity. A better understanding of the mechanisms that prevent induction of T‐cell immunity specific for intact viral antigens is of high clinical and biological relevance, with implications for immunotherapeutic approaches for other chronic infections and cancers that are also characterized by T‐cell failure due to antigen persistence. These results are also critical for the design of prophylactic HCV vaccines in healthy individuals or high‐risk patients after DAA‐mediated viral elimination.