VACCINES 

Vaccines are chemical substances prepared from the antigens of pathogenic organisms, which confer immunity to the animal from that pathogen.

Vaccines for Hepatitis B Virus

Hepatitis B Virus (HBV) is wide spread in man and induces several chronic liver disorders such as fulminant chronic hepatitis, cirrhosis and primary cancer. HBV DNA is a double stranded circular molecule of about 3 kb size and has a large single stranded gap which must be required with an endogenous polymerase before digestion with an 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 amounts of antigens. Three types of viral proteins are recognized to be antigenic; 1. Viral surface antigen (HBsAg), 2. Viral core antigen (HBcAg), and 3. 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 in pBR322 near b-galactosidase gene. 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.

Recombinant vaccine for Hepatitis B Virus

After infections, HBV fails to grow and even in cultured cells it does not grow. This property is attributed to inhibition of its molecular expression. Recombinant vaccine for HBV was produced by cloning HBsAg gene of the virus in yeast cells. The yeast system has its complex membrane and ability of secreting glycosylated protein. This have made it possible to build an autonomously replicating plasmid containing HBsAg gene near the yeast alcohol dehydrogenase (ADH) I promoter. The HBsAg gene contains 6 bp long sequences preceding the AUG that synthesises N-terminal methionine. This is joined to ADH promoter cloned in the yeast vector PMA -56. The recombinant plasmid inserted into yeast cells. The transformed yeast cells are multiplied in tryptophan 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 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.

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 USA, France and Belgium) to develop this highly advanced vaccine. The indigenous yeast derived 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 imported vaccine. The Drug Controller General of India has permitted it for commercial manufacture.

DNA vaccines:

For the first time Wolf et al., (1990) injected naked DNA into the muscles of mice which led to expression of encoded marker protein. Thereafter, there has been a surge to use this approach to generate DNA vaccines against a variety of infectious diseases. Thus DNA vaccines are giving hope of a third vaccine evolution.

The first published report from India indicates modest success in the development of DNA vaccines against rabies and Japanese encephalitis virus (JEV) in experimental animals. The efficacy whereas in the case of JEV envelope protein, cell mediated immunity appears to be the major mechanism of protection. There is great hope that DNA vaccines can protect against serious infectious diseases. Most likely it is to become a tool to benefit mankind in 21^st century as such or in combination with recombinant / cell culture vaccines or as an adjunct to chemotherapy (Padmanaban, 1999).

In case of malaria, DNA vaccines have distinct advantage, where plasmid DNA encoding different antigens, prepared by the same genentic procedures can be mixed and administered. A mixture of 4 plasmid DNA (pfCSP, pfSSP2, pfEXP-1 and pfLSA-1) has been injected into rhesus monkeys and found to elicit multiple antigen specific cytotoxic T-lymphocytes.

Lowrie et al (1999) have reported that a DNA vaccine coding for a mycobacterial heat shock protein of Mr65000 (Hsp 65) when administered in 4 doses to mice, 8 weeks after intravenous injection of virulent M.tuberculosis H37RV, leads to a dramatic decrease in number of live bacteria in spleen and lungs 2 months and 5 months after the first dose of DNA. Certain other mycobacterial antigens and BCG did not have this effect.