Perspectives for a hepatitis C virus vaccine

Abstract

Background: Natural hepatitis C virus (HCV) infection elicits poor immunity. Although HCV proteins elicit immune responses in virtually all cases of infection, the great majority of HCV infections become chronic. Currently, no vaccine is available for HCV despite an estimated incidence of approximately 50 000 new cases per year in the USA alone.

Objectives: To discuss how the problems associated with developing a vaccine against HCV infection may be overcome and describe recent progress made towards overcoming these problems and developing a vaccine.

Study design: A cytofluorimetric assay that can assess the ability of a serum sample to neutralise the binding of the HCV-envelope glycoprotein E2 to human cells (neutralisation of binding or NOB assay) was developed. The assay was used to assess the levels of antibodies capable of neutralising E2 binding in the sera of vaccinated and carrier chimpanzees.

Results: Low titres of NOB antibodies were found in the majority of chimpanzees challenged with HCV infection. Chimpanzees immunised with the E1/E2 heterodimer developed NOB antibodies and high levels of neutralising antibodies. These chimpanzees were not protected from challenge with heterologous virus but were protected from subsequent chronic infection.

Conclusions: A subunit vaccine composed of recombinant HCV proteins may protect from infection or chronic infection by different HCV genotypes.

Introduction

Hepatitis C virus (HCV) is a positive strand RNA virus of the Flaviviridae family which is the leading causative agent of blood-borne chronic hepatitis (Kuo et al., 1989). Its genome is 9.5 kb in length with one open reading frame that is translated as a single polyprotein. The polyprotein is processed by host and virus proteases to produce structural proteins (Core, Envelope 1 [E1] and Envelope 2 [E2]) and non-structural proteins with various enzymatic activities. HCV has a high mutation rate, and at least six major genotypes have been defined based on the nucleotide sequence of conserved and non-conserved regions. There is no vaccine for HCV and the only treatment which has proven efficacious is interferon (IFN)-α therapy. However, the success rate of this treatment is exceedingly low with only 10–20% of patients responding with sustained viral clearance (Alberti, 1997). Given that approximately 200 million chronic HCV infections have been estimated worldwide (WHO, 1997), there is a pressing need to develop not only new therapies, but also vaccination strategies. It is likely that the development of such strategies will not be feasible without a more complete picture of the immune response occurring with HCV infection.

HCV copes very well with the host’s immune system. Once HCV enters the body, it is unlikely that it will be cleared naturally. Only a minority of HCV infections begin with acute disease, most are asymptomatic, before becoming chronic and being diagnosed (Van der Poel, 1994). The majority of chronic HCV infections result in chronic hepatitis, which often causes liver failure, cirrhosis or hepatocellular carcinoma (Houghton, 1996). The high percentage of chronic HCV infections may be due to viral escape from a protective immune response or the failure of HCV to induce one. While most HCV infections do elicit immune responses, there is little evidence of immunity (Abrignani, 1997).

An ideal HCV vaccine should prime cross-neutralising anti-envelope antibodies, wide helper and inflammatory CD4+ T cell responses, and a wide cytotoxic CD8+ T cell response. However, several problems arise in designing a vaccine against HCV infection. First, HCV exhibits low viraemia in vivo and is only readily detected as RNA by polymerase chain reaction (PCR) (Houghton, 1996). Second, low-fidelity RNA replicase leads to mutations within the viral genome resulting in a population of genetically varying viruses (quasispecies) in a single individual (Weiner et al., 1991). Third, the only species that can be infected by HCV are humans and chimpanzees (Houghton, 1996). Finally, the virus does not replicate efficiently in vitro, and, therefore, little is known about the appearance of HCV (Houghton, 1996).

We are interested in developing an efficacious HCV vaccine. It is the aim of this paper to discuss how one might by-pass the above mentioned problems and develop candidate HCV vaccines.

Obviously, an ideal vaccine should protect from infection, in that it should elicit sterilising immunity. However, this is quite an ambitious goal in the PCR era. Even the antibody response to the hepatitis B surface antigen (HBsAg), when reassessed using PCR as read-out for virus clearance, has been shown to protect from disease but not low-level persistent infection in a sizeable fraction of cases (Rehermann et al., 1996). There are three possible options in designing a vaccine against a virus which induces, in most cases, chronic infection and then chronic disease (Table 1). In the case of HCV, where infection can only be assessed by PCR, a more realistic goal, than complete protection, might be to look for vaccines capable of protecting from chronic infection or at least from the chronic disease. After all, acute HCV infection is not a serious public health problem and most of us would be satisfied with a vaccine that allowed subclinical acute infection which, after a few months, would be cleared by the immune response. The final possibility would be a vaccine or vaccine-based therapy which, although incapable of clearing the infection at any stage, would prevent evolution of the infection into chronic disease. Theoretically, this implies that infection would not be eradicated, but the symptoms of the disease would be targetted.

The final aspect of HCV which must be considered in any vaccine strategy, is the high genetic variability of the viral envelope proteins (E1/E2) (Weiner et al., 1991). It has been proposed that neutralising antibodies are elicited in patients with HCV. These fail to provide protection because the virus is able to constantly alter its antigenic surface (Farci et al., 1994). Thus, due to the substantial variation in HCV genotypes and subspecies, most neutralising antibodies induced in response to vaccination would only protect against subsequent challenge with homologous virus. Given the likely event that many vaccinated individuals would come into contact with a different HCV strain during their lifetime, these antibodies would be practically useless.

Overall, our view is that a candidate HCV vaccine for most developed countries would be one that protects from infection and/or progression to chronic infection by the major HCV genotypes: 1a and 1b.

Correlate(s) of immunity can be established by either: identifying the immune responses associated with either clearance or benign outcome of natural infection in humans, or by demonstrating protection from experimental challenge in animals in which a given immune response has been induced or transferred.

Although little information is available on correlates of immunity in HCV infection, there is some evidence from clinical and experimental studies for the existence of HCV-specific neutralising antibodies. For humans, it has been shown that immunoglobulin (Ig) containing anti-HCV antibodies has some prophylactic effects on hepatitis C (Knodell et al., 1976, Sanchez-Quijano et al., 1988, Piazza et al., 1997). In chimpanzees, passive immunisation with such Ig preparations prolonged the incubation period of acute hepatitis C, and HCV infection was prevented after in vitro neutralisation with plasma of a chronically infected patient (Farci et al., 1994, Krawczynski et al., 1996).

Whilst it appears that effects of passive immunisation are limited, and complete clearance of HCV is not usually accomplished, vaccination experiments have been more successful. When vaccinated with recombinant envelope proteins, chimpanzees developed high serum titers of anti-E2 antibodies and were protected from subsequent challenge with homologous isolates (Choo et al., 1994).