S1 gene sequence analysis of infectious bronchitis virus vaccinal strains (H120 & H52) and their embryo-passaged derivatives

Avian infectious bronchitis is an acute and highly contagious disease that mainly causes respiratory symptoms in poultry. A number of serotypes and variants of the viral agent with poor cross-protection are the major problem to achieve desired immunity from vaccination. The S1 subunit of S glycoprotein (spike) is the major determinant of IBV so that a minor change in amino acid sequence of this protein, alters the virus strain. Therefore, characterization of the sequence of S1 gene is necessary to identify virus strains and their similarities with the vaccinal strains. In this research, the S1 sequence of H52 and H120 vaccinal strains of Razi Institute was fully characterized, and also the effect of serial passages in embryonated - eggs (5 passages beyond the master seed) on the S1 gene was investigated. The results showed that H120 and H52 strains of Razi Institute are 100% identical to the reference vaccine strains available in the GenBank. In addition, the H52 strain showed one amino acid substitution from the 3rd passage in which Glycine (G) was replaced by Valine (V) at position 118 making these passages exactly identical to the H120 strain while no change occurred for the H120 strain during these passages. Analysis of the original vaccinal strains which are widely administered in Iran, is definitely useful for prevention and control strategies against the circulating viruses. To identify the genetic change(s) responsible for attenuation of these strains during passages in embryonated-egg, characterization of other genes, especially those involved in replication is recommended.