HOW MANY TYPES ARE KNOWN?

LIVE  DELETED VACCINES

Thanks to the development of molecular biology, a better knowledge of the different genes and the proteins that they codify in microorganisms has become possible. Thus, the genomic structure of some microorganisms has been modified (Aujeszky´s disease virus, AD). The genes that codify the proteins related to virulence have been eliminated and so attenuated strains that are safe and stable have been obtained..     

Diagram showing the virion structure, its different proteins (with the new nomenclature) and the DNA of the AD virus. 

Structure of the DNA of the Aujeszky´s virus.The genes of the proteins gE and gI are located in the Us fragment. 

In the mid-eighties, the study of the genome of the AD virus showed that some strains, used for the elaboration of live attenuated vaccines, had a deletion in the Us genomic region. This deletion affected those genes which codify the gE glycoproteins (formwerly known as gI) and gI (formerly gp 63) 

The Bartha strain had a deletion in the Us region affecting gE and gI (formerly qp 63). The NIA-4 strain also presented a delection in the Us region and that's why it did not express the gE protein. The Alfort strain presented a deletion in the Us region but it only affected the expression of the gI and not the gE. This is the reason why those vaccines produced using these strains induced a good protection in the animals, but didn't produce antibodies against protein gE; vaccinated animals could therefore be differentiated from infected ones. (The virus that causes the disease does have protein gE.) 

AD virion structure, with the location of the different proteins. 

Those findings stimulated studies of the molecular structure of the AD virus. Using genetic engineering techniques different studies were performed in order to eliminate either genes codifying for virulence-related proteins (thymidin-kinase) or proteins that not being important for the development of an immune response allowed the differentiation different strains. This way less virulent strains (eliminating the gene of the thymidin-kinase) and  of the labeled ones (eliminating the gE gene) were obtained. 

Live recombinant vaccines are based on the use of a live microorganism (virus or bacteria) that acts as a vector for the expression of genes from another organism. The new recombinant microorganism can be used as a vaccine for both organisms. 

The microorganism most frequently used as an expression vector is the vaccine virus ("vaccinia") because it has a large and well known genome that allows the insertion of foreign genes without disrupting the replication machinery. A live recombinant against rabies has been obtained inserting the gene of the G protein of the rabies virus into the genome of the vaccinia virus.The disadvantage of this vector is that it is a virus able to infect most animal species, including humans. The vaccine is not species-specific and thus could immunize any animal.  

One of the last vaccines obtained using recombinant technology, but not with the vaccinia virus, is that obtained by our research group against myxomatosis and Hemorrhagic Disease of the Rabbit. The vector used in this vaccine is the myxomatosis virus, with the gene of the protein VP60 of the hemorrhagic disease virus inserted in its genome. The result has been a vaccine capable of inducing protection against both diseases.  

In the porcine species, an experimental recombinant vaccine using the Aujeszky´s Disease virus has been obtained. The gene of the gE protein is deleted and then the gene of the glycoprotein gp 55 of the Classical Swine Fever virus is inserted in order to induce immunity against both diseases. The main advantage of this vector is its species-specificity, however, a live vaccine is not always recomended against Aujeszky´s disease. Other approaches currently under study are the use of specific porcine viruses, such as non pathogenic Porcine Parvovirus. They have the great advantage of their capability for the insertion and expression of genes; their specifity and that they are not linked to any disease..Finally, the possibility of using adenovirus as vectors is being studied, because they are not pathogenic but can replicate and express in a large number of cellular systems. They also induce a very good humoral and cellular immune response, even at the mucosa level.

Construction of a recombinant with the myxomatosis virus and the protein vp 60 of the Hemorrhagic Disease of the Rabbit.

Regarding bacterial vectors, the one most currently used is an attenuated variant of Salmonella, which induces good immune responses, even at the enteric mucosa level. In the case of the porcine species this bacteria could imply diagnostic problems.  

Electronic microscopy of viral particles of myxomatosis and Hemorrhagic virical disease of the rabbit. 

DNA VACCINES

The possibility of using a purified fraction of DNA containing the gene of the protein able to induce an effective immune response has been studied lately. This fraction of purified DNA is inserted in a plasmid that acts as a vector. Animal cells capture these plasmids (the exact process is not yet well understood) and then incorporate them into the cell nucleus, allowing the expression of the foreign gene and the production of the protein. This protein is released to the extracellular space, where it is recognized by the immune system in its natural form, just as happens during a field infection. A very efficient immune response is therefore induced.

Antigens (1) induce the production of antibodies (2) whose binding sites are complementary to those of the structure (1).  When the binding sites of (2) are inoculated into another animal, they will induce antibodies (3) whose binding sites will be complementary to (2) and identical to the first ones (1), because they will have the same structure. These binding sites could be used as a vaccine against the antibody (1).

Finally, and only experimentally at the moment, there are new types of vaccines called anti-idiotype vaccines. These vaccines have never been used in the porcine species, but they must not be forgotten. They are based in the antigen-antibody recognition site. When an animal is inoculated with an antigen a humoral response is elicited.

The induced immunoglobulins in the binding site (idiotype) of the variable region present the inverse structure of the epitope inducing the reaction. If we inoculated these binding sites into another animal, it would induce the production of antibodies against these structures, that would also be complementary (anti-idiotype antibodies). 

So, antibodies induced against an idiotype (anti-idiotype) would have the same structure as the original antigen, and thus could be useful as antigens to elicit an immune response against the first antigen that originate the reaction. Anti-idiotype antibodies can be either polyclonal antibodies or monoclonal antibodies, and could be used as vaccines, specially in those cases in which getting the immunogenic proteins or codifying their  genes is very complicated.