Integrin expression, enterocyte maturation, and bacterial internalization.

Little is known about the molecular mechanisms involved in the translocation of enteric bacteria. Adhesion molecules mediate interactions between some enteric pathogens and mammalian cells, but no such interactions have been identified for enterocytes and normal enteric bacteria. Using enteric pathogens, adhesion molecule expression has been linked to bacterial internalization and to enterocyte differentiation. Therefore, experiments were designed to study enterocyte integrin expression and differentiation, as well as enterocyte internalization of Salmonella typhimurium, Proteus mirabilis, and Escherichia coli. Relative expression of the alpha2, alpha3, and beta1 integrin subunits on Caco-2 and HT-29 enterocytes (mature and immature) was measured by ELISA. Bacteria-enterocyte surface interactions were observed by light and scanning electron microscopy. Bacterial internalization by enterocytes was quantified using the gentamicin protection assay. Expression of the alpha2, alpha3, and beta1 integrin subunits was consistently increased in immature compared to mature Caco-2 enterocytes; however, compared to mature enterocytes, immature HT-29 enterocytes had similar expression of alpha3 and beta1 but decreased alpha2. Compared to untreated mature enterocytes, bacterial internalization was increased in immature enterocytes as well as mature enterocytes with lateral membranes artifactually exposed. However, there was no difference in bacterial internalization between immature enterocytes and mature enterocytes treated to expose the lateral membrane. Bacterial internalization by enterocytes appeared to be due to factors other than integrin expression or enterocyte differentiation. Exposure of the lateral enterocyte membrane may play an important role in facilitating bacterial internalization by enterocytes.