Passive protection experiments were conducted to determine the frequency and amounts of hyperimmune antiserum needed to block a transmissible gastroenteritis virus (TGEV) challenge infection and to identify monoclonal antibodies that are partially protective against TGEV. Hyperimmune antiserum or monoclonal antibodies were added to milk at each feeding or at selected feedings when the amount of antiserum was reduced. Three-day-old piglets were challenged with virulent virus that had been preincubated with antiserum or monoclonal antibodies. The results indicated that supplementing antiserum every other day was not efficacious for protection. Supplementing even small quantities of hyperimmune antiserum (0.5 ml) at least once a day in most cases was sufficient for piglet survival but did not prevent morbidity. Increasing the amount (>2 ml) and providing antiserum 3 times/day completely blocked the TGEV challenge infection. Two monoclonal antibodies were discovered that also provided passive protection for baby pigs. One monoclonal antibody, 5G1, had a high neutralizing titer, and the other, 6C4, was more effective in neutralizing and binding to virulent TGEV than to attenuated TGEVs. Both of these monoclonal antibodies were partially effective as supplements in milk for passive protection. Furthermore, these monoclonal antibodies were useful for boosting the efficacy of TGEV-neutralizing colostrum, which by itself was ineffective. These results show that other antigenic sites, different from the 4-well characterized epitopes on the S glycoprotein of TGEV, also are important for passive protection.
Characterization and utility of monoclonal antibodies against spike protein of transmissible gastroenteritis virus