Genetic immunization with codon-optimized equine infectious anemia virus (EIAV) surface unit (SU) envelope protein gene sequences stimulates immune responses in ponies

In the context of DNA vaccines the native equine infectious anemia virus (EIAV)-envelope gene has proven to be an extremely weak immunogen in horses probably because the RNA transcripts are poorly expressed owing to an unusual codon-usage bias, the possession of multiple RNA splice sites and potential adenosine-rich RNA instability elements. To overcome these problems a synthetic version of sequences encoding the EIAV surface unit (SU) envelope glycoprotein was produced (SYNSU) in which the codon-usage bias was modified to conform to that of highly expressed horse and human genes. In transfected COS-1 cell cultures, the steady state expression levels of SYNSU were at least 30-fold greater than equivalent native SU sequences. More importantly, EIAV-specific humoral and lymphocyte proliferation responses were induced in ponies immunized with a mammalian expression vector encoding SYNSU. However, these immunological responses were unable to confer protection against infection with a virulent EIAV strain.