Gastroenterology
Volume 134, Issue 5 , Pages 1601-1604, May 2008

Therapeutic Vaccination for Hepatitis C: Can Protective T-Cell Responses Be Restored After Prolonged Antigen Exposure?

  • Carlo Ferrari

      Affiliations

    • Corresponding Author InformationAddress reprint requests to: Carlo Ferrari, MD, Unità Operativa di Malattie Infettive ed Epatologia, Azienda Ospedaliero-Universitaria di Parma, Via A. Gramsci 14, 43100 Parma, Italy.

Unit of Infectious Diseases and Hepatology, Laboratory of Viral Immunopathology, Azienda Ospedaliero-Universitaria di Parma, Parma, Italy

Article Outline

 

See “Therapeutic vaccination of chronic hepatitis C non-responder patients with the peptide vaccine IC41,” by Klade CS, Wedemeyer H, Berg T, et al, on page 1385.

Despite the substantial improvement of available therapies for chronic hepatitis C virus (HCV) infection over the last 20 years, alternative treatments for chronic HCV infection are needed. In alternative to compounds that can block selectively the function of viral proteins vital for the HCV life cycle, strategies aimed at improving protective T-cell responses have the potential to be successful in view of the crucial role of T cells in the control of chronic viral infections.

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Role of T-cell responses in virus control and persistence 

In HCV infection, T-cell responses associated with spontaneous virus control are generally stronger and wider in terms of targeted epitopes than those of patients who do not succeed in recovering from acute hepatitis C infection.1, 2 Thus, the goal of immune therapies is to reconstitute efficient immune control by making the dysfunctional T-cell responses of chronic patients qualitatively and quantitatively similar to those of patients able to resolve infection spontaneously.

When viral infections are successfully controlled, most effector cells are expected to die by activation induced cell death; however, some are protected from apoptosis and survive, giving rise to long-living memory T cells that can expand vigorously and can redifferentiate into protective antiviral effector cells when they reencounter the original priming antigen. In contrast, in animal models of virus infection chronic viral persistence is generally associated with a lack of memory T-cell maturation and persistence of dysfunctional T cells.3, 4 Different mechanisms have been proposed to contribute to the T-cell dysfunction of chronic virus infections, including HCV infections. They comprise inhibitory signals delivered to virus-specific T cells by over-expressed negative costimulatory pathways, such as the PD-1/PD-L1 receptor–ligand pairs5, 6, 7, 8, 9; immunoregulatory cytokines, such as interleukin (IL)-10 and transforming growth factor (TGF)-β10, 11, 12, 13; and hyperfunctional CD25+FoxP3+ regulatory T cells.14, 15, 16, 17, 18, 19, 20 The possibility of controlling lymphocytic chriomeningitis virus (LCMV) infection by restoring in vivo the antiviral CD8 function through blockade of the PD-1/PD-L15 or the IL10/IL10 receptor10, 11 pathways confirms the role of these mechanisms in establishing and maintaining chronic viral infections.

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Induction of T-cell responses by therapeutic vaccination 

To what extent can a therapeutic vaccination based on selective stimulation of HCV-specific T cells improve the protective antiviral function when there is a deep T-cell hyporesponsiveness to exogenous stimulation typical of chronic HCV infection? The results of the study published in this issue of Gastroenterology by Klade et al21 may provide some important clues to this question.

Klade et al21 treated 60 human leukocyte antigen (HLA)-A2–positive patients with chronic hepatitis C who did not respond to standard pegylated interferon alpha and ribavirin therapy or relapsed after discontinuation with a synthetic peptide vaccine containing 4 HLA-A0201–restricted CD8 epitopes, 3 promiscuous CD4 epitopes previously identified in patients with acute and chronic HCV infection, and poly-l-arginine as adjuvant. Peptides were based on a genotype 1 sequence because the majority of patients no responsive to available therapies are infected by this genotype; moreover, CD8 epitopes were selected among the viral sequences that are recognized by human CD8 cells in association with the HLA-A2 molecule because most information has been generated about responses restricted by this HLA class I allele that is also the most widely recognized among the Caucasian population (45–50%).

The major drawback of peptide vaccines is that they necessarily contain a limited number of preselected T-cell epitopes that may not include the most immunogenic viral sequences implicated in the induction of immunodominant protective T-cell responses. Moreover, HCV is a highly variable virus; this heterogeneity makes interindividual variations among the selected epitopes highly likely, which may preclude recognition of the peptide vaccine by CD8 cells in a proportion of HCV-infected patients. To overcome this problem and minimize the risk of viral escape from the response induced by vaccination, epitope selection was limited to conserved HCV sequences. After immunization with 6 vaccine doses administered 4weeks apart, HCV-specific T-cell responses were more frequently detectable in vaccine-treated patients than in the control groups treated with peptides without adjuvant or with adjuvant alone, showing that induction of HCV-specific responses can actually be achieved even in “difficult-to-treat” patients who are nonresponsive to interferon and ribavirin. Responses, however, were very weak and only rarely associated with a transient decline in HCV-RNA levels.

This poor induction of T-cell responses raises the issues of whether a fully protective T-cell response can be restored after a long standing chronic viral infection and how the effect of an exogenous therapeutic stimulation of a profoundly exhausted antiviral T-cell response can be improved.

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Are the HCV-specific T-cell defects of chronic HCV infection reversible? 

Antigen removal to allow T-cell resting from antigenic stimulation is believed to be a crucial requirement for reconstitution of functional antiviral T-cell responses and for memory T-cell differentiation in chronic viral infections.4, 22, 23 Therefore, a potential strategy for improving the effect of therapeutic vaccinations by circumventing defects in proliferative potential is to decrease the antigen load by inhibiting virus replication. Clear proofs of this concept, however, are lacking in HCV infection; conflicting results have been reported in patients responsive to interferon/ribavirin therapy24, 25, 26, 27 on the effect of viremia decline on T-cell responses; moreover, no evident improvement of HCV-specific T-cell responses has been observed upon decline of HCV viremia to undetectable levels caused by acute HBV infection occurring in preexisting chronic hepatitis C infection.28 These findings are consistent with the possibility suggested by studies in animal models of chronic virus infection that reconstitution of optimal proliferation capacity and optimal memory T-cell differentiation may be precluded after a certain duration of exposure to high antigen loads, even if T cells are removed from antigen exposure.3, 22 Indeed, intrinsic defects in the proliferation capacity are confirmed by the observation that LCMV-specific T cells maintain poor proliferative activity when restimulated in vitro with antigen-presenting cells from uninfected mice and when transferred into uninfected mice subsequently challenged with LCMV.23 Likewise, intrinsic defects in memory T-cell maturation in long-standing chronic viral infections are suggested by the lack of homeostatic proliferation and survival of CD8 T cells generated during chronic LCMV infection, when transferred to an antigen-free environment in naïve uninfected mice,3 these exhausted T cells are unable to complete the differentiation program for a full recovery of their proliferation machinery. These CD8 cells that have been persistently exposed to antigens express low levels of CD127 and CD122 and fail to respond to the homeostatic cytokines IL-7 and IL-15 in vitro.3 This is similar to what was observed within the liver in chronic HCV infection where HCV-specific CD8 cells express the phenotype typical of exhaustion, with high PD1 expression and low levels of CD127.8, 9, 29

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Possible strategies to facilitate restoration of protective T-cell responses 

A great deal of information has been generated in the last few years on the mechanisms that control T-cell activation and function, and that can suppress optimal antiviral T-cell responses in chronic viral infections. Understanding these mechanisms is crucial to identifying specific strategies to revert T-cell dysfunction, thereby enhancing the effect of antigen removal on T-cell exhaustion which may not be sufficient alone to allow good levels of responsiveness to exogenous stimulation provided by therapeutic vaccines.

It is well known that T-cell activation depends not only on signals released through the T-cell receptor, but also on additional receptor–ligand interactions, which can provide costimulatory signals needed to achieve full T-cell activation and survival.30 Induction or inhibition of specific costimulatory pathways may be sustained by pathogen replication and subsequent inflammation; the final balance between activating and inhibitory pathways at the site of infection may influence the antiviral T-cell function contributing to the T-cell dysfunction typical of chronic viral hepatitis. Among the described costimulatory pathways, the PD-1/PD-L1 complex has been reported to be involved in T-cell exhaustion caused by persistent exposure to high viral antigen loads,5, 6, 7, 8, 9 and recent data from different laboratories show that, in chronic HCV infection, antiviral CD4 and CD8 functions can be improved by blocking PD-1/PD-L1 engagement.7, 8, 9

Dysregulation of other costimulatory pathways, including the CTLA-4/B7, 4-1BB/4-1BBL, OX40/OX40L, and CD27/CD70 pathways, has been reported as a possible additional cause of T-cell dysfunction in different models of virus infection and human cancers.30, 31, 32 Therefore, combined blockade or stimulation of different receptor–ligand interactions may be even more effective in optimizing functional T-cell restoration, if individual costimulatory pathways preferentially act at different stages of chronic infections or express regulatory role at different stages of T-cell activation or on different subsets of T cells.

Recent studies in the LCMV model of infection have also highlighted the key regulatory role of the IL-10/IL-10-receptor (R) pathway because IL-10-R blockade with specific antibodies was able to restore protective antiviral responses, preventing chronic evolution of infection.10, 11 Also in human HCV infection, HCV-specific regulatory CD8 cells that are able to suppress T-cell responses by production of IL-10 or TGF-β have been isolated from the liver12 and the peripheral blood13 of patients with chronic hepatitis C. Because the impact of IL-10/IL-10-R blockade on LCMV infection was more dramatic when the pathway was blocked at early stages of infection,10, 11 studies are needed to understand whether this strategy can be envisaged for chronic infections in humans, where T cells have been exposed to viral antigens for years.

Emerging data indicate that another important negative regulatory pathway is represented by CD4+FoxP3-T cells that can suppress virus-specific T cells,14 thereby affecting quality and intensity of antiviral responses. Up-regulation of this pathway may contribute to the functional impairment of virus-specific responses in patients with a chronic evolution of HCV infection15, 16, 17, 18, 19, 20 and modulation of its suppressive effect may represent another potential strategy to revert T-cell nonresponsiveness to virus antigens in the perspective of establishing optimal conditions for maximal efficacy of therapeutic vaccination.

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How to increase the effect of therapeutic vaccination 

Is the poor intensity of the HCV-specific T-cell response induced by the peptide vaccination approach employed by Klade et al21 totally discouraging? Certainly, it is far from being comparable to what is mounted in self-limited infections. In addition, major concerns remain about the possibility to reconstitute in chronic hepatitis C a protective antiviral response able to control infection after long-term exposure to high antigen loads, as highlighted by studies in animal models of viral infections.22, 23 Do T cells that have experienced long-term exposure to antigen carry permanent changes in the T-cell differentiation program, precluding maturation of an optimal antiviral protective function? Although definitive answers are still lacking, what is promising for immunotherapy is that our knowledge of the complex mechanisms governing T-cell differentiation and influencing T-cell dysfunction in chronic hepatitis C are rapidly improving. It is becoming increasingly clear that different regulatory pathways intersect different components of the protective immunity and perhaps influence different elements of the T-cell differentiation machinery and different aspects of pathogen control. Therefore, a combined manipulation of different regulatory pathways may offer great benefits and may represent the future avenue to follow in the perspective of reverting HCV-specific T-cell nonresponsiveness, giving more chances to virus-specific vaccine therapy to be curative. Based on our knowledge of the immune mechanisms influencing HCV clearance and persistence, what we can at present envisage as the best approach to optimize the effect of peptide vaccination is a sequential strategy aimed at overcoming defects in proliferative potential by removing antigens from contact with T cells through inhibition of virus replication and blocking negative regulatory pathways able to suppress T-cell function. Only after reconstitution of T-cell responsiveness, a direct and selective stimulation of virus-specific T cells should be given by vaccination (Fig 1). Before pursuing this strategy in the clinical setting, however, additional studies are needed to define how to combine different levels of manipulation of regulatory pathways, increasing helpful protective responses without amplifying harmless immunopathology.

  • View full-size image.
  • Figure 1. 

    Optimization of vaccine therapy. Recovery of HCV-specific T-cell responsiveness by inducing decline of antigen load and manipulating different costimulatory pathways should precede T-cell stimulation by therapeutic peptide vaccines to increase the effect of immunization.

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PII: S0016-5085(08)00454-X

doi:10.1053/j.gastro.2008.03.016

Refers to article:

  • Editorial Accompanies ArticleAdditional Online Content Available Therapeutic Vaccination of Chronic Hepatitis C Nonresponder Patients With the Peptide Vaccine IC41 , 28 February 2008

    Christoph S. Klade, Heiner Wedemeyer, Thomas Berg, Holger Hinrichsen, Grazyna Cholewinska, Stefan Zeuzem, Hubert Blum, Michael Buschle, Sandra Jelovcan, Vera Buerger, Erich Tauber, Juergen Frisch, Michael P. Manns
    Gastroenterology May 2008 (Vol. 134, Issue 5, Pages 1385-1395.e1)

Refers to erratum:

  • Correction , 10 July 2008

    Gastroenterology August 2008 (Vol. 135, Issue 2, Page 713)

Gastroenterology
Volume 134, Issue 5 , Pages 1601-1604, May 2008

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