Humoral and cellular immune responses to proteins encoded by the hepatitis C virus

The hepatitis C virus (HCV) was the first described human pathogen of the hepaciviruses, a new genus within the flaviviridae family. A characteristic feature of HCV is that it has a high genetic heterogenecity. It is believed that the immune response is of vital importance to prevent, control, and to eradicate HCV infections. However, in most infected persons the HCV infection progress to chronicity. The present research project was aimed at further characterizing the interaction between HCV and the infected host.

The molecular basis for the antibody cross-reactivity between the HCV core and a newly described host-derived gene product, GOR, was investigated. We made two important observations. First, the relative antibody avidity was in higher for the HCV core, than for the GOR peptide. Second, the essential residues for antibody binding were conserved between the HCV core and the GOR peptide. Thus, antibodies specific for HCV core may cross-react with the GOR peptide and their possible role in autoimmunity remains unclear. Different genotypes of HCV may induce type-specific antibodies. We established a method for typing HCV-specific antibodies using differences in the avidity of specific and cross reactive antibody reactivities to type-specific synthetic peptides. Type-specific antibodies were found in 89% of patients with chronic HCV infection. The study confirmed that at least three distinct serotypes of antibodies to HCV exist in Sweden.

A major problem in the diagnosis of HCV acute infections is almost total absence of HCV specific IgM. We analyzed the kinetics of the antibody response in the acute and chronic phases of HCV infection. The serologic profile during the acute phase indicated that both the numbers and absorbance levels of HCV core peptide reactivities increased with time. In contrast, patients with chronic HCV infections had multiple high level HCV core peptide reactivities. Thus, patients with HCV RNA may be classified as acute or chronic phase by using this assay. Little is known about the basic immunogenicity and antigenicity of HCV proteins. Therefore, the immunogenicity and antigenicity of the HCV NS3 protein were characterized in mice. All tested haplotypes were antibody responders to NS3.

The finding of the multiple and cross reactive T-cell recognition sites may explain the high frequency of antibody responders to NS3 in both infected humans and immunized mice. The murine study was extended to analyze the NS3-specific T cell response in HCV infected patients before during and after IFN-a therapy. All patients showed an increased rNS3-specific CD4+ T cell proliferation after IFN-a therapy regardless of the infecting HCV genotype and the outcome of therapy. Murine T cell responses were analyzed to the short NS4A protein. Both human and murine antibodies recognize the same region within NS4A. We also found that, in contrast to NS3, the combination of viral genotype and host MHC profoundly influences the ability to mount an HCV NS4A-specific immune response. Finally, a NS3-specific single-chain antibody was constructed and expressed in both bacterial and mammalian cells which recognized the HCV NS3 protein in vitro. This suggested that NS3-specific single-chain antibody could be evaluated for the ability to disturb HCV replication in vitro and in vivo.