Mutant spike protein extending the tissue tropism of infectious bronchitis virus (IBV)

A nucleic acid comprising a nucleotide sequence encoding an infectious bronchitis virus (IBV) spike protein (S protein) which comprises an S1 domain of an IBV strain with restricted tissue tropism that is unable to grow on Vero cells and an S2 domain which has at least 98% sequence identity to the S2 domain of the S protein from an IBV strain with restricted tissue tropism that is unable to grow on Vero cells, but which comprises the sequence XBBXBX (SEQ ID NO:

• 3) in the part of the S2 domain corresponding to residues 686 to 691 of the sequence of SEQ ID NO:
• 2, where B is a basic residue and X is any amino acid;
• and which comprises at least one of the following amino acid substitutions with reference to the position numbering of SEQ ID NO:
• 2:
• Leucine (L) to Phenylalanine (F) at position 578;
• Asparagine (N) to Serine (S) at position 617;
• Asparagine (N) to Serine (S) at position 826;
• Leucine (L) to Phenylalanine (F) at position 857;
• and
• Isoleucine (I) to Valine (V) at position 1000;
• such that an IBV virus comprising the S protein is able to grow on said Vero cells.