Immunity elicited by hepatitis C virus.

ABSTRACT

Hepatitis C virus (HCV), the major causative agent of post-transfusion and community-acquired non-A, non-B (NANB), is a single-stranded RNA virus characterized by a high degree of genetic heterogeneity. HCV is endemic worldwide and is a major cause of chronic liver disease and hepatocellular carcinoma. The development of a broadly reactive vaccine is a high priority for the control of HCV infection. In recent years, however, serious concerns have been raised regarding the degree of protective immunity elicited by HCV in the host. Several observations, both in patients and in the chimpanzee model, have suggested a lack of protective immunity against HCV. Chronic HCV infection develops in more than 80% of patients, suggesting that in most cases the immune response of the host fails to mediate resolution of the infection. Cross-challenge studies demonstrated that convalescent chimpanzees are not protected against re-infection with homologous or heterologous HCV strains. Similar evidence has been obtained in polytransfused beta-thalassemic children, in whom re-infection with HCV was associated with multiple episodes of acute hepatitis. Although most of the evidence thus far accumulated suggests that HCV does not elicit a protective immune response, recent studies have provided experimental evidence, both in vitro and in vivo, that HCV infection induces a neutralizing antibody response in humans. However, such antibodies are isolate-restricted and ineffective against variant HCV strains emerging in vivo. Recently, using recombinant envelope proteins of HCV, a successful vaccination of chimpanzees against challenge with a homologous viral strain was reported. Whether this vaccine can provide protection against challenge with a higher infectious dose of the homologous virus or against challenge with heterologous strains of HCV remains to be established. Overall, the data hitherto accumulated indicate that the genetic heterogeneity of HCV will be a major impediment for the development of a broadly reactive vaccine for the control of HCV infection.