Differentiation of F4 receptor profiles in pigs based on their mucin 4 polymorphism, responsiveness to oral F4 immunization and in vitro binding of F4 to villi

F4+ enterotoxigenic Escherichia coli (F4+ ETEC) are an important cause of diarrhoea and mortality in piglets. F4+ ETEC use their F4 fimbriae to adhere to specific receptors (F4Rs) on small intestinal brush borders, resulting in colonization of the small intestine. To prevent pigs from post-weaning diarrhoea, pigs should be vaccinated during the suckling period. Previously, we demonstrated that F4acR+, but not F4acR− piglets could be orally immunized with purified F4 fimbriae resulting in a protective immunity against F4+ ETEC infections, indicating that this immune response was F4R dependent. Recently, aminopeptidase N has been identified as a glycoprotein receptor important for this oral immune response. However, in some oral immunization experiments, a few F4acR+ piglets did not show an antibody response upon oral immunization, suggesting additional receptors. Therefore, the binding profile of F4 to brush border membrane (glyco)proteins was determined for pigs differing in F4-specific antibody response upon oral immunization, in in vitro adhesion of F4+ E. coli to small intestinal villi, and in Muc4 genotype. Six groups of pigs could be identified. Only two groups positive in all three assays showed two high molecular weight (MW) glycoprotein bands (>250kDa) suggesting that these high MW bands are linked to the MUC4 susceptible genotype. The fact that these bands were absent in the MUC4 resistant group which showed a positive immune response against F4 and was positive in the adhesion test confirm that at least one or perhaps more other F4Rs exist. Interestingly, two pigs that were positive in the villous adhesion assay did not show an immune response against F4 fimbriae. This suggests that a third receptor category might exist which allows the bacteria to adhere but does not allow effective immunization with soluble F4 fimbriae. Future research will be necessary to confirm or reveal the identity of these receptors.