Hepatitis is an acute inflammatory disease of the liver caused by viruses. A but Hepatitis B virus are RNA viruses. Hepatitis B virus is a DNA virus, k least six hepatitis viruses are known till to date.
These are named by English alphabets, A, B, C, D, E and G (named in short as HAV, HBV, HCV, HDV, HEV an HG V, respectively). There are a few other viruses, which also infect the liver and magnified similar types of symptoms.
These have been referred to as non-A – non-G to distinguish them from the more common hepatitis viruses of all hepatitis viruses, A, B and C at more prevalent. Although the liver is the target organ of infection, the transmission route differs from class to class. In the past, when there was no effective cure for this disease, it had taken a heavy toll of human life.
Thank God that genetically engineered vaccines for hepatitis A and B are now available. HCV reproduces to produce viral particle that are slightly different from each other in their genetic make-ups. Scientists believe this genetic diversity allows the HCV to evade the host’s immune system.
This may be the reason for not having a successful vaccine against HCV. Patients suffering from HC1 infection are treated with interferon. Most countries have taken up exclusive immunization programmes to decrease the incidence of hepatitis A and B.
Benjamin Hall and Gustav Ammerer developed an effective Hepatitis B vaccine using yeast cell culture at the University of Washington.
It is the first genetically engineered vaccine and is considered as one of the greatest triumphs of biotechnology. Ironically, Hall and Ammerer did not set out to develop a hepatitis vaccine. In the late 70s and early 80s, they were studying the basic mechanisms by which yeast cells express their own genetic information. By this time, expression of a few genetically engineered proteins was achieved in the Escherichia coli expression host.
In collaboration with the scientists of Genentech, Hall and Ammerer synthesized a heterologous protein, human interferon in the yeast cell. Following this, they set out to express the surface proteins (antigens) of the HBV in the yeast cell. These proteins were harmless and when injected into a subject elicit an immune response and consequently, kill and eliminate hepatitis B viruses that enter into the body. Hall and Ammerer began a collaborative work with William Rutter and Pablo Valenzuela of California University with an active support of the Virology Research Laboratory of Merck & Co.
The Hall-Ammerer method will be described in the subsequent section with an illustration. After appropriate testing, the Merck group received a commercial license for this product, the first genetically engineered vaccine against a human disease, the first vaccine against a sexually transmitted disease and the first vaccine against a virus that leads to cancer. In 1996, this technology was licensed to 16 firms, including Merck, Smith Kline Beecham Biologicals, Genentech, Immunex and American Cyanamid among others.
Prior to the advent of recombinant DNA technology, two types of vaccines against pathogenic viruses were used: inactivated and attenuated. Inactivated vaccine: were chemically killed derivatives of the actual infectitious agent and attenuated vaccines were live viruses, altered such that they could not be pathogenic.
Both these vaccines worked by presenting surface antigens to the T and B lymphocytes, so that they were primed to mount an immune response to the virus. Hepatitis A vaccine is prepared from chemically inactivated HAV. No part of this vaccine is live.
However, these vaccines were potentially dangerous, since they were contaminated by other infectitious agents and molecules. Recombinant DNA technology was the obvious answer. Hepatitis B vaccine is genetically engineered from the envelop proteins of the virus. It has been shown that the virus has an outer envelope made up 01 proteins (outer envelope proteins) followed by a capsid, also made up of proteins.
The purified envelop proteins or the capsid proteins are sufficient to elicit an immune response and produce antibodies in the host organism. Vaccines that use the components of a pathogenic organism, rather than the whole organism are called sub-unit vaccines. Now recombinant DNA technology is in practice for producing such proteins in large quantity.
Hall and Ammerer (Recombinant DNA) method
The first sub-unit vaccine was produced against the HBV. The virus is coated with surface proteins (antigens), known as HBsAg. Infected persons carry large aggregates of these proteins in their blood. The early thinking was to collect these aggregates that could be used as a vaccine.
Would this method have sufficient HBsAg to be used in community immunization programme? The answer is certainly negative. Recombinant DNA technology was the obvious answer. Scintists turned to the HBV genome. The genome was cloned and the HBsAg gene identified. Initial attempts for cloning and expressing this gene in Escherichia coli failed.
The gene was inserted into a high copy number yeast (,Saccharomyces cerevisiae) expression vector and the resultant recombinant vector into a yeast host cell for expression such that the translated HBsAg were not secreted into the medium.
The transformed yeast cells were selected by plating on a semisolid media lacking leucine. The positive colonies were grown in a fermenter. The yeast cells were isolated by centrifugation and broken. The HBsAg is purified and packaged for vaccination.