In this study, we have investigated the characteristics of secreted IgA and other classes of Ig induced after vaccination of sows with transmissible gastroenteritis virus (TGEV) or the antigenically related porcine respiratory coronavirus (PRCV). Both viruses induced the secretion of neutralizing antibodies of different classes in the sows' milk, but these protected suckling piglets against TGEV to different degrees. Quantitative differences in the induction of IgA by both viruses were found among the different viral antigenic sites and subsites of glycoprotein S. In TGEV-vaccinated sows, antigenic subsite A was the best inducer of IgA, followed by antigenic site D. After vaccination with PRCV, lower levels of IgA were detected on colostrum and milk, antigenic site D and subsite Ab being the immunodominant sites. This quantitative difference in epitope recognition could explain the differences in newborn piglet protection found using Ig classes purified from the milk of sows immunized with both viruses. Apparently only IgA recognizing at least antigenic sites A and D confers good protection , whereas any Ig class recognizing only one antigenic site may neutralize the virus in cell culture. These results indicate that the formulation of a subunit vaccine against TGEV has to consider the inclusion of more than one antigenic site involved in virus neutralization.


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