Health & Medical hepatitis

DNA Immunization and Hepatitis C

DNA Immunization and Hepatitis C
To induce a sustained and specific cellular immune response to hepatitis C virus (HCV), DNA immunization of mice was performed using plasmids containing the HCV nonstructural gene 3 (HCV/NS3). Plasmids were constructed such that the NS3 gene was expressed in a secreted form, a nonsecreted form or as a membrane-bound antigen. The plasmid encoding the secreted antigen induced the strongest humoral and cellular immunity and favoured the T-helper type 1 (Th1) pathway as shown by cytokine profiles and switching of antibody subclasses. Our study indicates that DNA immunization with a secreted form of HCV/NS3 is an effective means of inducing primary Th1 immune responses in the murine model.

Hepatitis C virus (HCV) infection is the major cause of chronic liver disease in the US and one of the major causes throughout the world. Although the development of a screening test for HCV antibody in 1990 virtually eliminated the spread of HCV through blood transfusion, the Centers of Disease Control (CDC) estimated that approximately 36 000 new cases of HCV infection occurred in the US in 1996 because of other routes of transmission. A total of 75-85% of people infected with HCV develop persistent infection, most with chronic hepatitis, and approximately 20% of those chronically infected develop liver cirrhosis; 1-5% of patients with cirrhosis progress to hepatocellular carcinoma.

Until recently, the sole therapeutic option for chronic HCV infection has been interferon-α (IFN-α) monotherapy and such treatment induced a sustained remission in only a minority of cases. Currently, the combination of IFN-α with ribavirin has considerably increased the response rate; nonetheless, only 30-40% of treated patients have sustained benefit from antiviral therapy. Therefore the development of a therapeutic as well as a preventive vaccine is highly desirable.

Many investigators have found a significant association between a strong and sustained HCV-specific CD4+ T-cell response and the clearance of viremia during acute HCV infection. This immune response is frequently directed against nonstructural proteins. Specifically, recognition of an immunodominant epitope within the nonstructural gene 3 (NS3) (amino acid position 1251-1259) has been associated with a favourable outcome of infection. Recent studies on virus-specific T-cell reactivity in association with IFN plus ribavirin treatment have found that the treatment-induced clearance of HCV is associated with the development of HCV-specific CD4 T-cell responses with enhanced IFN-γ T-helper type 1 (Th1) and low interleukin (IL)-10 (Th2) production. Therefore, the development of a vaccine that increased cellular immune responses, especially those directed against HCV NS3, could be valuable in overcoming established HCV infection.

DNA vaccination can induce both humoral and cellular immune responses and many studies have evaluated DNA vaccines against HCV. Previously, using HCV core as the target, we have evaluated the humoral and cellular immune responses induced by a DNA vaccine. Recently, we found that cationic liposomes can enhance and modulate the immune responses against HCV NS3 induced by DNA immunization. It is well known that the addition of a signal sequence, the truncation of transmembrane domains or the addition of localization sequences can alter the form and cellular compartmentalization of vaccine antigens, so that DNA immunization offers a unique opportunity to examine the immunogenicity of the antigens expressed in different forms. Up to now, no paradigm has been established as to the effect of antigen localization on the induction of both humoral and cellular immune responses to DNA immunization. It appears that responses may be different for individual antigens. Therefore, in this study, as a part of efforts to develop a vaccine against HCV, plasmids encoding secreted (pSecTag2-HCV/NS3), non-secreted (pCI-HCV/NS3 and pcDNA3.1-HCV/NS3) or membrane-bound (pDisplay-HCV/NS3) forms of HCV NS3 antigens were constructed and their immunogenicity compared. The results from both enzyme-linked immunospot assay (ELISPOT) and enzyme-linked immunosorbent assay (ELISA) revealed that the constructs encoding secreted HCV NS3 proteins induced the strongest immune response against HCV NS3 and favoured the Th1 pathway.

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