Antiviral Host Immunity: Relevance to Vaccine Development
Following acute infection with HCV, approximately 20% of people will spontaneously clear the infection. This is in stark contrast to HIV where infection inevitably persists. The distinct clinical outcomes that follow acute HCV infection allow comparative analysis of antiviral immunity between these clinical groups – this has been the major focus of many research groups over the last decade. Although the exact mechanisms behind successful viral clearance are still not yet fully elucidated, it appears that multiple components of the immune system, both innate and adaptive, play a crucial role in this process (Figure 1). A key observation is that strong, broad adaptive immune responses are detected during acute infection and these persist in those who resolve infection – whereas persistent infection is associated with a weak, frequently undetectable HCV-specific T-cell response.
Viral–host immune interactions during acute and chronic HCV infection.
HCV: Hepatitis C virus; TCR: T-cell receptor.
Is there any evidence, then, that the induction of adaptive immune responses during acute-resolved infection protects the host from subsequent viral challenge? If so, a HCV vaccine that mimics these responses may well afford protection. Findings from early seminal work demonstrated that chimpanzees that had recovered from HCV infection could be reinfected with the same inoculum.[20,21] However, we now have evidence that both chimpanzees and humans who have previously cleared HCV are at least partially protected against reinfection in the majority of cases.[22–25] Rather than preventing acute infection with sterilizing immunity, typically this form of protective immunity works by preventing progression to chronic infection following repeat HCV exposure. These findings suggest that a vaccine that induces and exploits similar immunogenic responses may ultimately succeed in preventing chronic HCV infection. Since chronic infection, and not acute infection, is associated with significant morbidity and mortality, prevention of chronicity is an acceptable end point.
Anti-HCV-specific T-cell Responses
The HCV-specific T-cell response has been shown to play a crucial role in determining the outcome of primary HCV infection. First, comparative studies in man have demonstrated that a broad and sustained CD8+ and CD4+ T-cell response targeting multiple HCV regions is associated with spontaneous viral clearance. Conversely, a weak and narrowly targeted T-cell response is a hallmark of persistent infection.[26–30] Second, immunogenetic studies from single-source outbreaks and from mixed populations have shown a clear association between different HLA class I and II alleles and viral clearance.[31,32] Both HLA B27 and HLA A3 were shown to be protective against the development of chronic infection following an outbreak of HCV genotype 1b infection in Irish women in 1977. Third, evidence from the chimpanzee model has shown that once protective responses are induced, depletion of either CD4+ and CD8+ T cells leads to loss of control over repeated HCV challenge.[27,33]
In comparison to those who clear acute HCV infection, studies examining patients who develop chronic infection suggest that both the quality and quantity of their CD4+ and CD8+ T-cell responses are impaired.[34,35] It is not yet clear, however, to what extent this is a cause or a consequence of chronic infection. Proposed mechanisms by which the T-cell response is attenuated include:
Viral escape from T-cell recognition: T cells recognize short viral peptides (T-cell epitopes) bound in the groove of MHC molecules. In the presence of selective pressure driven by T cells, viral variation in T-cell epitopes that arise during viral replication may abrogate this recognition – so called 'viral escape'. To varying degrees, viral escape mutations may be associated with reduced viral fitness. T-cell escape in HCV infection has clearly been demonstrated such that many T-cell responses detected during chronic infection target a 'historical' epitope that is no longer found in circulating virus. However, in many cases T cells clearly do target autologous circulating virus – therefore, T-cell escape cannot fully explain the paucity of responses seen in chronic infection. This also means that therapeutic vaccination may be able to rescue responses that are still able to target the host's circulating virus;
T-cell exhaustion: chronic antigen stimulation results in a reduction in antigen-specific T-cell frequency and function. This concept was first recognized in murine models of chronic viral infection. T-cell exhaustion is now thought to play a role in chronic HCV infection.[36–39] The mechanisms underlying exhaustion are poorly understood, but include expression of inhibitory receptors such as the molecules programmed death receptor-1 and cytotoxic T-lymphocyte-associated protein 4, which mediate T-cell function. These receptors are normally involved in self tolerance, but in the setting of chronic infections may be upregulated, presumably to evade immunopathology. One of the key suggestions from murine models of persistent viral infection is that repair of these 'defective' T-cell responses is most readily achieved in the setting of a lowered viral load. Thus, the idea of vaccination or immunomodulation as an adjunct to conventional antiviral therapy has recently emerged;
Regulatory immune cell subsets: Tregs (FOXP3+) that may actively suppress CD8+ and CD4+ T-cell responses are increased in chronic HCV infection. Indeed, the depletion of Treg activity does lead to a consistent boost in T-cell function during HCV infection. This may well be a consequence of chronicity rather than a cause, but nevertheless it may serve to limit the efficacy of immunotherapy unless it is reversed;
Finally, the liver itself is thought to represent a tolerogenic environment. In evolutionary terms this would serve to protect the liver, which is constantly exposed to antigens via the portal tract, from chronic inflammation. Interestingly, in organ transplantation the liver is the only organ where HLA matching between donor and recipient is not required. This observation raises the intriguing possibility that antiviral T cells primed in the periphery during vaccination may induce T cells that are of a 'superior' quality to those primed in the liver during natural infection.
Humoral Immunity Towards HCV
Circulating antibodies to HCV are usually detectable within 1–3 months of HCV infection and appear to be an important component to viral control in early infection. There is a direct correlation between viral clearance during acute infection and the rapid induction of a high-titer of circulating cross-neutralizing antibodies. However, neutralizing antibodies are also found in high titers in the majority of chronically infected patients and clearly, in these cases, are unable to control infection. The envelope protein, which is the major target for HCV antibodies, displays some of the virus' highest levels of genetic heterogeneity. It is likely that variation between quasispecies in these envelope proteins allows the virus to evade host neutralizing antibodies. This contrasts to other viruses such as the genetically stable hepatitis B virus in which persistent humoral immunity has allowed successful development of a preventative vaccine.
There are several other mechanisms by which HCV evades humoral immunity including direct cell-to-cell viral transfer, induction of antibodies that interfere with neutralizing antibodies[46,48] and the shielding of neutralizing epitopes by glycosylation of defined amino acids of envelope glycoproteins. For these reasons, it seems unlikely that an antibody-mediated vaccine in isolation will successfully induce sterilizing immunity, however, such a strategy may have an important role either as an adjunct with other approaches, or in attenuating the course of acute infection.
The fact that IFN-α forms the mainstay of treatment of HCV clearly demonstrates that innate immune cytokines can eradicate the virus. However, the mechanism of action of IFN is complicated, having both direct antiviral actions through the inhibition of protein kinase R and cellular protein production, and through diverse effects on immune cellular function. Although some early studies suggested that HCV-specific T-cell responses are enhanced by IFN,[50,51] more recently, this has been challenged in studies that show that HCV-specific T cells and total T-cell counts decline in peripheral blood as treatment proceeds.[52,53] The reasons for this decline are unclear, but if they reflect a true failure of T-cell production, rather than migration, then therapeutic T-cell vaccination strategies will need to overcome this.
Recent genome-wide association studies and candidate gene studies have further highlighted the crucial importance of innate immunity during HCV infection. These studies identified a cluster of seven host single-nucleotide genetic polymorphisms linked to IFN-λ3 (also known as IL-28B) that are important in determining both spontaneous viral clearance during acute infection and also the response to standard PEG-IFN/ribavirin therapy.[54–57] Individuals homozygous for the protective alleles have viral clearance rates following treatment that are approximately three-times higher than those of patients who are homozygotes for the risk allele. As yet, the causal genetic variant has not yet been identified. These observations have created intense interest in the field, and the biological role of IFN-λ3 is currently under intense scrutiny. The mechanism by which IFN-λ3 acts during HCV infection is not yet fully elucidated, although this cytokine clearly has direct antiviral actions in vivo and readily inhibits HCV replication in hepatoma cells (Huh-7.5).
A Phase II human study of IFN-λ3 for the treatment for HCV is currently underway, and while it is not yet known if this cytokine administered to patients will be of benefit, it is already clear that HCV vaccine studies will need to stratify patients according to IFN-λ3 host genotype.
Expert Rev Vaccines. 2011;10(5):659-672. © 2011 Expert Reviews Ltd.
Cite this: Vaccination for Hepatitis C Virus - Medscape - May 01, 2011.