|Vaccines for Hepatitis B Virus
Hepatitis B virus (HBV)is wide spread in man and produces several chronic liver disorders such as Fulminant chronic hepatitis, cirrhosis and primary liver cancer. HBV DNA is a double stranded circular molecule of about 3Kb size and has a large single stranded gap which must be required with an endogenous polymerase before digestion with restriction enzyme for DNA cloning (Glover, 1984). After infection in human being, HBV fails to multiply and infect a large number of cells and even does not grow in cultured cells. This property has been explained to be due to hindrance of its molecular characterization and development of vaccines. Plasma of human has been detected to have varying amount of antigens. Three types of viral proteins are recognized to be antigenic : (i) viral surface antigen (HBsAg), (ii) viral core antigen (HBcAg), and (iii) the e-antigen (HBeAg).
Although the whole viral genome has been cloned and sequenced, yet there is limited information about amino acid sequence of surface and core antigens. Recently, HBV DNA has been successfully cloned in E. coli and mammalian cells, and synthesis of HBsAg and HBcAg particles has been done in the cells. Burrell et al. (1979) inserted HBcAg genes inPBR322 near b-glactosidase gene (Edman et al., 1981). Production of these genes is needed in order to get production of vaccines on a large scale. In yeast or mammalian systems, these antigens are synthesized more efficiently than in prokaryotes.
(i) Recombinant vaccine for Hepatitis B virus. After infection, HBV fails to grow and even in cultured cells it does not grow. This property has been explained to be due to inhibition of its molecular expression and development of vaccines. Recombinant vaccine for HBV was produced by cloning HBsAg gene of the virus in yeast cells. The yeast system has its complex membrence and ability of secreting glycosylate protein. This have made it possible to build an autonomously replicating plasmid containing HBsAg gene near the yeast alcohol dehydrogenase (ADH) Ipromoter (Fig.5.5). The HBsAg gene contains 6 bp long sequence preceding the AUG that synthesizes N-terminal methionine. This is joined to ADH promoter cloned in the yeast vector PMA-56. The recombinant plasmid is inserted into yeast cells. The transformed yeast cells arc multiplied in trytophan-free medium. The transformed cells are selected. The cloned yeast cells are culture for expression of HBsAg gene. This inserted gene sequence expresses and produces particles similar to the 22 mm particle of HBV as these particles are produced in serum of HBV patients. The expressed HBsAg particles have similarity in structure and immunogenicity with those isolated from HBV-infected cells of patients. Its high immunogenicity has made it possible to market the recombinant product as vaccine against HBV infection.
(ii) Indigenous Hepatitis-B vaccine. India's first genetically engineered vaccine (Guni) against HBV developed by a Hyderabad based laboratory (Shantha Biotechnics Pvt. Ltd.) was launched on August 18, 1997. India is the fourth country (after the U.S.A., France and Beligum) to develop this highly advanced vaccine. The indigenous yeast-desired HBV vaccine is one third the cost of the imported vaccine. This new vaccine had undergone human clinical trials at Nizam's Institute of Medical Sciences, Hyderabad and K.E.M. Hospital, Mumbai. The clinical trials clearly proved that the seroprotection is about 98%. It was found more effective than the imported vaccine. The Drug Controller General of India has permitted it for commercial manufacture.