Transepithelial Transport of Macromolecules

Abstract

Epithelial cells serve as a selective barrier to the diffusion of molecules between the lumenal and serosal compartments in a wide variety of tissues. The mechanisms by which small molecules such as ions and sugars cross epithelia have been the object of intense study for many years, but it has become increasingly apparent that macromolecules (ie, proteins) can also be subjected to active transepithelial transport. Perhaps the best example of this phenomenon is the transport of immunoglobulins, which occurs in several of the situations in mammals: (1) transport of maternal IgG across the fetal yolk sac or the intestinal epithelium in neonatal rats, and across the placenta in humans; and (2) transport of the polymeric immunoglobulins IgA and IgM across a variety of mucosal tissues. Both are receptor-mediated processes that take place in opposite directions across the epithelial layer. IgG is transported in the apical to basolateral direction, IgA and IgM from basolateral to apical (Fig 1). These systems, therefore, serve as valuable models to help understand the cellular and molecular mechanisms that govern the transport of macromolecules across epithelia.