Nucleotide Sequence Analysis of S1 Gene among Iranian Avian Infectious Bronchitis Viruses Isolated during 2001-2002

Infectious bronchitis (IB) virus genome codes for four structural proteins, among which the S1 subunit of spike glycoprotein comprises the major epitopes to induce neutralizing antibodies. This study involved the comparison of the full S1 sequences of five IB viruses, namely two Massachusetts and three 793/B serotypes, isolated from IB outbreaks during 2001-2002, with all other Iranian and foreign 793/B isolates and 10 known serotypes. Analysis of S1 subunit showed three unique amino acid changes at positions 349 (V to L), 392 (T to N), and 393 (Q or R to T or K or S) for the Iranian 793/B isolates, compared to those of the foreign 793/B isolates reported before 2006 (onset of vaccination with 793/B vaccine in Iran). They were used as amino acid markers for the differentiation of Iranian 793/B isolates for years. Sequence alignment of the Iranian isolates with those of the foreign ones reported after 2006 demonstrated that amino acids 392 and 393 were no longer considered as amino acid markers, and only the change in amino acid 349 still remained specific to the Iranian 793/B isolates. Phylogenetic tree sequence analysis revealed that the Iranian 793/B isolates were closely related indicating that they came from a single source, more probably from France. There was a very close correlation between the first detection of 793/B serotype and the time of French chicken meat importation. Moreover, it was shown that one of the Massachusetts isolates was completely identical with the H120 vaccine strain. Furthermore, the other Massachusetts isolate with two amino acid changes at positions 64 (G to E) and 95 (S to R) was very similar to this vaccine strain. It seems that the latter isolate is a passaged chicken H120 vaccine strain.