MAKING VECTOR VACCINES USING HOMOLOGOUS RECOMBINATION
Another method of displaying a foreign antigen for use as a vaccine is the vector vaccine. Here genetic engineering is used to express a disease-causing antigen on the surface of a nonpathogenic virus or bacterium. When this infects a person, it induces immunity both to the nonpathogenic microorganism and to the attached antigen. For example, vaccinia virus is a nonpathogenic relative of the smallpox virus. Using vaccinia virus is so effective that smallpox was eradicated. If vaccinia virus were engineered to express an antigen from another deadly virus, the person vaccinated would gain immunity to smallpox and the other virus at the same time. Indeed, multiple genes could be inserted, conferring resistant to multiple diseases. The benefit of using vaccinia virus is that it is very potent and stimulates development of both B cells and T cells. In contrast, many other vaccines, particularly subunit vaccines, stimulate only a B-cell response.
Inserting genes into the vaccinia genome is awkward because the genome has very few restriction enzyme sites. However, the vaccinia genome sequence is known. This allows genes to be added to the genome using homologous recombination (Fig. 6.23). In homologous recombination, two segments of similar or homologous DNA align, and one strand of each DNA helix is broken and exchanged to form a crossover. A single crossover creates a hybrid molecule; if two crossovers occur close together, entire regions of DNA are exchanged. During homologous recombination in vaccinia, a region of single-stranded DNA is generated from a double-stranded break in the incoming new gene. The single-stranded region invades the double helix of the vaccinia genome to form a triple helix. One of the strands from vaccinia then is free to hybridize to the single-stranded homologous region on the incoming gene. If this occurs on both sides, the foreign gene is inserted into the vaccinia genome.