Newcastle disease virus vectored vaccines as bivalent or antigen delivery vaccines

Choi, Kang-Seuk

Insertion of foreign genes into the full-length cDNA clone is a pivotal step for generating rNDV expressing foreign antigens. Manipulating cDNA clones to express foreign antigens must take several factors into account. The foreign gene of interest should be synthesized as a transcription unit, which comprises a gene start (GS) sequence, the foreign gene, and a gene end sequence (Fig. 2) . The transcription unit is then inserted into a noncoding intergenic region of the NDV genome [8] . The number of nucleotides that make up the rNDV genome must follow the "rule of six" to ensure integrity of the genome and efficient virus replication. The insertion site is one of the most important factors affecting efficient expression of the foreign gene. In general, genes closer to the 3´ terminus are transcribed more abundantly than downstream genes (the 3´ to 5´ transcription gradient) due to imperfect restart of transcription at the GS. In other words, the foreign gene is expressed more abundantly and more efficiently when placed closer to the 3´ end of the genome [8] . While the foreign gene can be placed at any intergenic site within the NDV genome, the P/M intergenic site is preferred to ensure optimal expression of foreign genes [16, 17] . The length of the inserted foreign gene also affects efficient expression of the foreign antigen. Incorporating a foreign gene increases the size of a genome and the number of transcriptional units within that genome, which often leads to retarded growth of rNDV in permissive cells [8] . A single NDV vector can accommodate foreign genes up to 4.5 kb in length with a good degree of stability and express at least three different foreign genes [8, 18] .