Viral Cancers
Hepatitis C Virus
Updated 8 February2010 by Ellie Barnes UK
Disease Burden
The need for an HCV vaccine
Hepatitis C virus (HCV) in a virus that infects the liver. Most people that are infected develop persistent infection. A proportion of people (20-50%) develop progressive liver disease leading ultimately to liver cirrhosis, liver failure and hepatocellular carcinoma
[1].
HCV is globally distributed and it is estimated that up to 170 million people (3% of the worlds population) are infected world wide
[2].
Hepatitis C is transmitted via infected blood. In parts of Asia and Africa 2-10% of people are infected and infection is frequently due to the use of contaminated blood products, medical instruments and tattooing. In Western Europe and North America less than 1% of the population are infected and infection is largely confined to at risk populations including those that received blood transfusions before the screening of infected blood products and intra-venous drug users. Sexual transmission and peri-natal transmission is unusual occurring in approximately 5% of cases. In 20% of people no cause of infection can be established.
A vaccine that prevents and treats HCV infection is urgently required. The target population would be at risk groups in developed countries and the entire population in many developing countries. No such vaccine currently exists but a number of approaches are currently in development.
A therapeutic vaccine would also be an invaluable adjunct to current treatment options for HCV. Today's gold-standard treatment for HCV consists of pegylated-interferon and ribavirin
[3]. This has been a remarkable advance is the treatment of HCV and may result in the permanent eradication of the virus in infected people. Nevertheless, it is currently prohibitively expensive, has an extensive side effect profile and is often ineffective. The current cure rates using this treatment is 45% for genotype-1 and -4 infection, 70% for genotype-3 and 80% for genotype-2, Once cirrhosis is established, a cure is less likely still and liver transplantation may be the only option.
There is currently intense research activity to establish new antiviral agents that target the HCV polymerase and protease, similar to the drugs currently used for HIV infection. Some of these are now in an advanced stage of development and may become available for clinical use in 2011
[4]
[5]. However, these new therapies are only effective for genotype-1 infection where they are expected to increase the cure rate to 70%, will be given in addition to interferon and ribavirin, and are likely to add to the cost of a treatment that is already unaffordable to most countries.
The challenges facing an HCV vaccine
One of the major challenges facing the development of a vaccine for HCV is the high degree of genetic diversity that is exhibited by the virus, estimated to be 10 fold higher than that seen in HIV. This is because the viral polymerase lacks proof reading capacity. The envelope protein, which is the major target for HCV antibodies, is particularly diverse. Based on the degree of genetic similarity, HCV has been classified into six major genotypes, that may be further subdivided into subtypes
[6]
[7]. Sequence homology between genotypes is approximately 80%. Specific genotypes are in general located in distinct geographical locations, whilst a small number of subtypes (1a, 1b, 2a and 3a) have recently become more widely distributed associated with modern practices such as medical injections, blood products and intravenous drug use. Other factors that have hindered vaccine development for HCV include the lack of an accessible animal model and the fact that the virus cannot be easily grown in the laboratory. It may not be possible to develop a vaccine that targets all HCV genotypes, but genotype specific vaccines that are administered in regions where specific genotypes dominate is a realistic goal.
Following primary (acute) infection with HCV a significant proportion of people will spontaneously eradicate the infection
[1].
This is quite different from HIV where viral persistence is inevitable following infection. In this setting, the induction of an effective immune response against HCV is thought to play a crucial role in eradicating the virus. Once chronic infection is established the antibody response persists, but the T cell response is weak or undetectable. It is not clear if the loss of T cell responsiveness is the cause or the consequence of chronic infection.
The exacts correlates of a successful immune response are not yet fully defined and probably involve interplay between multiple components of the immune system. This would include cytokines -chemicals such as interferons that inhibit virus production in infected cells, antibodies that bind to the virus and block viral entry, and T cells that target and kill infected cells.
To date, all successful vaccines against other infections work by inducing a protective antibody response. However, in HCV the predominant target for HCV antibodies is the highly variable envelope region and whilst it has been shown that these antibodies may prevent infection in chimpanzee models, these are only effective against a limited number of HCV strains
[8].
There is now strong evidence to show that a major component of a protective immune reaction includes the generation of robust CD8+ and CD4+ T cell responses that target parts of the HCV virus that is presented to T cells in association with self (HLA class I and II) molecules
[9]
[10]
[11]
[12]
[13]
. To some extent T cells must also deal with the problem of variability, but these may also target the more conserved "internal" proteins of the virus. Broadly reactive T cells that target multiple parts of the virus, a strong T cell response, and a "functional" T cell response that produces the right kind of cytokines and proliferate well will be required.
A number of groups are currently working to develop both T cell and antibody based vaccines to prevent and also to treat HCV infection. As a treatment it is thought that a vaccine may be more effective in the presence of a lower viral load. For this reason, the concept of vaccination as an adjunct to current therapy in the presence of a lowered viral load has emerged.
Vaccine development
Many different approaches have been tried in HCV vaccine development. However, only a small fraction of animal and primate studies have progressed to human studies.
Recombinant protein vaccines
Recombinant HCV envelope (E1/E2) vaccines
The first candidate therapeutic vaccine was administered to humans in 2003, and consisted of recombinant HCV-E1 protein in alum adjuvant (InnoVAC-C developed by Innogenetics, Ghent, Belgium). The rationale for this was that pre-existing antibodies to HCV envelope proteins have been associated with a better response to interferon therapy. Following multiple injections, this vaccine induced both antibody and T cell responses in healthy and treatment naïve HCV infected patients
[14]
[15]
. Twenty-four patients received two courses of 6 injections and liver histology was assessed before and after vaccination. Plasma virus levels did not change but liver biopsy showed histological improvement in 9 patients. The observed increase in HCV-E1 antibody levels correlated with a decline in alanine transaminase levels (a measure of liver inflammation) and new E1 specific T cell responses were generated in 21 patients. Further studies on this vaccine have not been published and the company stopped its HCV vaccine programme in 2008.
A phase I clinical trials in healthy subjects of an E1/E2 heterodimer was conducted by Chiron/Novartis (Emeryville, California), following successful challenge experiments in chimpanzees (http://clinicaltrials.gov/ct2/show/study/NCT00500747). This study is currently unpublished. The same company is also developing a T cell vaccine using HCV core protein produced in yeast adsorbed onto immunostimulating complex matrix (ISCOM), following studies in macaques showing induction of both CD8+ and CD4+ T cell responses
[16]
.
Peptide Vaccines
A candidate therapeutic vaccine, IC41, was developed by Intercell AG (Vienna, Austria), based on 5 synthetic peptides containing 5 HCV HLA A2 restricted CD8+T cell epitopes, and 3 CD4+ T cell epitopes, in addition to a T cell adjuvant poly-L-arginine. This vaccine has been shown to generate proliferative HCV specific T cell responses and also IFN-gamma specific responses in ELISpot assays in both healthy and chronically infected HCV infected patients in phase I studies
[17]
. A larger phase II study included 60 HLA -A2 patients with genotype 1 infection, who had previously failed to respond to interferon based therapy
[18]
. Of these, 36 patients received 6 vaccinations with IC41 whilst the remainder received peptides or adjuvant alone. T cell proliferative responses and IFN-gamma ELISpot responses were observed in 67% and 42% of the IC41 vaccinees respectively. In 3 patients a decline of > 1 log viraemia was observed. The observed T cell responses were generally weak. More recently the same vaccine was administered to patients during combination pegylated-interferon and ribavirin therapy. Weak T cell responses were observed in vaccinated patients. However, no control arm of unvaccinated patients was included for comparison
[19]
.
A further study using a "personalised" approach has assessed the immunogenicity of four CD8+ A24 peptides in Freund's adjuvant administered to 12 patients with HCV, that had previously failed to respond to interferon therapy
[20]
. Antibody and CD8+ T cell responses to the peptides were assessed pre-vaccination in each individual and only those peptides that induced an immune response were then used for a further 14 vaccinations given 2 weeks apart. Augmentation of peptide specific T cell responses were reported in the majority of patients following 7 vaccinations, the first time point at which responses were assessed.
Finally, a T cell vaccine using HCV core protein produced in yeast adsorbed onto immunostimulating complex matrix (ISCOM) is under development by Novartis, following studies in macaques showing induction of both CD8+ and CD4+ T cell responses
[16].
Yeast based protein vaccine
Heat-killed recombinant Saccharomyces cerevisiae expressing a core-NS3 fusion protein (GI-5005) is developed as a candidate HCV vaccine by Globeimmune (Louisville, Colorado). A phase II clinical trial including 140 treatment naïve and prior non-responders to interferon recently reported an increase in rapid virological response and end of treatment response when vaccination is administered with interferon and ribavirin compared to a control group of standard therapy alone. No immunological data to support these findings has been published
(http://www.globeimmune.com)
.
DNA Vaccination:
ChronVac-C is a DNA based vaccine using plasmids expressing HCV proteins NS3 and 4a made by Tripep AB (Stockholm, Sweden). This is co-administered intra-muscularly with electroporation (EP) as a delivery system. EP consists of a number of short electrical pulses, which are said to be painful but shortlived. EP has been shown to enhance cellular responses probably through creation of pores in the cell membranes of target cells that enhance vaccine delivery. Additionally the damage to the cell membranes is thought to enhance a local inflammatory response at the site of vaccination
[21]
[22]
[23]
. EP, at least in mice is said to enhance the immunogenic response induced by DNA vaccination 10 fold
[24]
. A phase I clinical trial in 12 treatment naïve, genotype-1 HCV infected patients, with low viral load (< 800,000 IU/mL) is currently underway. Interim results suggest that 4/6 showed a decline in viral load of >0.5 logs with a concomitant increase in T cell reactivity in 3 of these patients
[25]
.
DNA vaccination using plasmids encoding structural proteins, has been combined with core protein (CIGB-230, Centro de Ingenieria Genetica y Biotecnologia, Havana, Cuba) in a recent trial of 6 intramuscular injections, 4 weeks apart in 15 patients with genotype-1 infection, who were non-responders to previous interferon therapy
[26]
. Weak anti-HCV proliferative responses were generated in some patients. However, whilst some patients showed a small increase in IFN-gamma ELISpot responses, others showed a reduction in responses following the last vaccination.
Virally vectored vaccines:
Transgene S.A. (Strasbourg, France) are developing a candidate therapeutic vaccine (TG4040) based on modified vaccinia Ankara (MVA) encoding the HCV NS3/4/5B proteins. Initial chimpanzee studies used a heterologous prime boost regimen of DNA encoding core-E2 and NS3 that showed reduced viremia upon viral challenge, although 3/4 animals developed persistent infection
[27]
. Phase I studies of 15 HCV infected human subjects are currently underway, in a dose escalation study involving 3 subcutaneous injections given weekly of MVA encoding NS3-5, followed by a boost with the same vector a month later in a sub-group of patients
[28]
. Early results show that three patients developed HCV specific T cell responses in response to vaccination.
Replicative defective adenoviral vectors that are genetically engineered to encode the non-structural proteins (NS3-NS5B) of a genotype-1b HCV strain have been developed by Okairos (Rome, Italy), and are currently in phase I clinical trials (Oxford, UK). This follows earlier studies in chimpanzees using a heterologous prime boost regimen of adenoviral/DNA (electroporated plasmid) vectors encoding the HCV NS proteins. This strategy induced broad CD8+ and CD4+ HCV specific T cell responses in 4/5 animals that were protected from heterologous viral challenge
[29]
. Since anti-adenoviral antibodies can limit the efficacy of these vectors, a number of adenoviral vectors derived from rare human adenoviral serotypes and also from chimpanzee adenovirus, to which humans have been rarely exposed, have been developed. A Phase I study of healthy volunteers, using a double prime/heterologous boost with two different adenoviral vectors has recently been shown to be highly immunogenic. A therapeutic vaccine approach using the same vectors in combination with interferon and ribavirin is currently underway in Oxford, UK.