Variant nucleic acid sequences encoding naturally occurring non human C3d polypeptides with immunostimulatory activity are provided. The variant sequencs are non-identical to the naturally occurring DNA sequences encoding the C3d polypeptides by virtue of third base redundancy and/or other variations permissible within an amino acid codon. The variant nucleic acid sequences can be included in veterinary immunisation vectors fused to one or more sequences encoding an antigen to enhance the immunogenicity of the antigen in the host.
