The most developments in biotechnology originated for their potential applications in health care in both human and animals. And it is in this sector that the contributions of biotechnology are more frequent, more notable and more rewarding (both financially and psychologically). The biotechnology is very used in health care as like, disease prevention, disease detection and therapeutic agents, correction of genetic diseases, fertility control and forensic medicine.
The prevention of disease is most desirable, most convenient and highly effective approach to health; this is achieved by vaccination or immunization using biological preparations called vaccines. Vaccine represent an invaluable contribution of biotechnology as they provide protection against even such diseases for which effective cures are not yet available. The effectiveness of vaccines may be appreciated from the fact that small pox, once a dreaded disease the world over, has been completely eradicated from the world; the last case of small pox was reported in 1977. The term vaccine is derived from the Latin term ‘vacca’ which means ‘cow’.
The vaccinia virus, a relative of variola virus that causes smallpox, has been used as a live vaccine for small pox since 1796, when Edward Jenner realized that this virus is harmless to humans, but it can stimulate immunity against the smallpox virus. The various vaccine scan be grouped under the following types:
An ideal vaccine or vaccination protocol should have the following features.
Conventional vaccines consist of whole pathogenic organisms, which may either be killed (most bacterial vaccines and some viral vaccines); or live (the virulence of pathogens is greatly reduced, mostly viral vaccines). The conventional vaccines, although highly effective and relatively easy to produce at low cost, suffer from following limitations.
These vaccines are based on purified antigens isolated from the concerned pathogens, i.e., these are non-recombinant. Since they do not contain the organism, the risk of pathogenicity is avoided. However, their cost is higher due to the steps involved in purification and vaccine preparation, and many of the isolated antigens are poorly immunogenic. Successful examples of such vaccine are mostly from bacteria. Many bacteria produce exotoxins, which are highly immunogenic. But these toxins produce toxic effects, the intensity which decreases with storage and this decline is accelerated by heat, formaldehyde and other chemicals. Fortunately most exotoxins that have lost their toxicity retain their immunogenicity; they are called toxoids and are used as effective vaccines, toxoids of pathogens causing tetanus, diphtheria, gangrene,etc. precipitation of toxoids with alum enhances their immunogenicity. The toxoid vaccines are quite effective and cheap.
Some toxoids are good adjuvants i.e., increase the immunogenicity of other antigens, e.g., diphtheria toxoids. The B-polysaccharide of haemophilus influenzae is poorly immunogenic. But when the B-polysaccharide is combined with diphtheria toxoid, its immunogenicity is greatly increased. In many cases, such adjuvant activities can be used to great advantage since most of the isolated antigens from pathogens are poorly immunogenic.
Recombinant vaccines contain either a protein or a gene coding protein of a pathogen origin that is immunogenic and critical to the pathogen function; the vaccine is produced using recombinant DNA technology. The vaccines based on recombinant proteins (proteins produced by recombinant DNA technology) are also called subunit vaccines. The logic of such vaccines, in simple terms, is as follows. Proteins are generally immunogenic, and many of them are critical for pathogenic organisms. The genes encoding such proteins can be identified and isolated from pathogens and expressed in E.coli or some other suitable host for mass production of proteins. The concerned proteins are then purified and mixed with suitable stabilizers and adjuvant, if required, and used for immunization.
Generally whole protein molecule is not necessary for immunogenicity; the immunogenic property is usually confined to a small portion of protein molecule. for eg, the immunogenicity of foot-and –mouth disease virus coat protein is due to its amino acids 114-160,and also 201-213. The segment of proteins containing either of these two amino acids sequences are effective in immunization; the induce antibodies, which neutralize the virus and thereby provide protection against the foot-and –mouth disease. In some cases, the immunogenic protein may be composed of two or more distinct polypeptides. In such cases, it may be desirable to use only one of the polypeptides as a vaccine for various reasons. For eg, the cholera enterotoxin consists of three polypeptides, viz., A1,A2 and B polypeptides. The A-polypeptides are toxic, while – polypeptide is nontoxic, but immunogenic. The gene encoding B-polypeptide has been cloned, and the recombinant B-polypeptide thus produced is being used, in combination with inactivated cholera pathogen cells, as an oral vaccine in the place of the conventional injectable cholera vaccine.
Recombinant protein or polypeptide vaccines are very safe since whole organisms are not involved. They are also of high efficacy. But (i) there cost is very high and often prohibitive, since they are produced by either bacterial fermentation or in animal cell cultures. (ii) They have to be stored at low temperature since heat destabilizes the proteins. (iii) This makes their storage and transportation, especially in developing countries, problematic and often limiting.
The gene encoding the relevant immunogenic protein is isolated, cloned and then integrated into a suitable expression vector. This preparation is introduced into the individual to be immunized. The gene is ultimately express in the vaccinated individual and the immunogenic protein is expressed in sufficient quantities to invoke both humoral and cell-mediated immunities. It may pointed out that cell mediated immune response is essential for recovery from infectious diseases. The various approaches for DNA vaccines are as follow: (i) injection of pure DNA preparation into muscle; (ii) use of vectors (e.g, vaccinia viruses, adenoviruses, retroviruses, E.coli, Salmonella typhimurium, herpesviruses, etc.) for delivery of the gene; (iii) reimplantation of autologus cells (cells of the individual to be vaccinated) into which the gene has been transferred, and (iv) particle gun delivery of plasmid DNA, which contain the gene in an expression cassette.
Injection of pure DNA into the skeleton muscle leads to the uptake and expression of DNA in muscle cells. When a gene encoding an immunogenic protein is introduced, its expression also results in immunization of the individuals. This approach has potential for delivery of DNA vaccines. Another approach is to remove cells from the body of an individual into which the concerned immunogen encoding gene is introduced and expressed. These cells are reintroduced into the body of the individual in variety of ways, e.g., simple infusion, implantation, encapsulation, etc. the immunogen encoding may be interrogated into an expression plasmid, which is purified, coated on gold or tungsten particles and introduced into the skin cells by a particle gun. Antigen encoding gene introduced into the skin of mice and guinea pigs elicited humoral immune response. The plasmid DNA is noninfectious, heat stable and offers other advantages over viral/bacterial vectors. DNA vaccines offer the following advantages: