Immunogenicity of a bovine viral diarrhea virus E2–C3d fusion protein containing a bovine homolog of C3d

Recently we demonstrated that attachment of three copies of murine C3d (muC3d) to the E2 envelope protein of bovine viral diarrhea virus results in a 10,000-fold increase in the immunogenicity of E2. Here we describe the cloning of the bovine homolog of C3d (boC3d), construction of an E2–boC3d expression cassette and expression and purification of the E2–boC3d fusion protein. We then show that E2, when coupled to boC3d, exhibits greatly enhanced immunogenicity. Thus, boC3d represents the second mammalian C3d homolog, thus far, shown to enhance the immunogenicity of a protein to which it has been coupled. Although the primary sequence of boC3d differs from muC3d by about 19%, we were able to demonstrate the enhanced immunogenicity of E2–boC3d using mice. The ability of boC3d to function in mice provides a less costly and more convenient animal model than cattle for the preliminary evaluation of E2–boC3d and other bovine antigen–boC3d fusion proteins.