Stern blotting was performed as described previously (26) with minor modifications. In brief, 1.5 g of the recombinant proteins was separated by SDS-PAGE on a 15 Tris/HCl gel (Bio-Rad) and electrotransferred onto a nitrocellulose membrane. Following blocking, the membranes were incubated with a 1:5000 dilution of preblocked sera, and bound antibodies were detected with secondary goat anti-bovine