The Research Institute for Children

Transepithelial Trafficking of IgG by FcRn

The neonatal Fc gamma receptor, FcRn
Image courtesy of Dr. Pamela Bjorkman

FcRn-dependent IgG transcytosis

Epithelial barriers function to prevent the passive movement of pathogens, toxins, and other noxious agents into the sterile, internal environment of the body. At the same time, however, the epithelium must selectively transport macromolecules both into the body (the interstitium) and in the opposite direction, delivering proteins onto mucosal surfaces. One mechanism by which this transport occurs is by moving cargos directly into and across the epithelial cell in a process termed “transcytosis”, or “transcellular transport”. The focus of our research is to study the biology of the neonatal Fc gamma receptor, FcRn, which functions to transcytose immunoglobulin G (IgG) in both directions across epithelia.

We have recently discovered that FcRn is present in the epithelial cells that form the lining of the gastrointestinal tract. We have also shown that FcRn can transport IgG across polarized epithelial cells, suggesting that this receptor may be responsible for transport of IgG onto mucosal surfaces where, like IgA, it may protect the host by immune exclusion. It is also possible that FcRn transports IgG back across the intestine thereby moving IgG-antigen complexes in the opposite direction. In this way, FcRn may deliver lumenal antigens bound to IgG to immune cells resident in the underlying tissue such that an immune response may be mounted before an infection can occur (immunosurveillance).

Calmodulin regulates FcRn Transcytosis

To understand how FcRn affects mucosal immunity, it is necessary to understand the molecular mechanisms by which the receptor traffics in polarized epithelial cells. To do this, we have expressed human FcRn in MDCK cells and demonstrate FcRn-dependent bidirectional transcytosis of IgG across cell monolayers. With this model system, we have discovered that a calcium sensing protein, calmodulin, binds to FcRn in a calcium dependent and reversible fashion. FcRn-mutants with single residue substitutions that cannot bind calmodulin transport IgG more slowly across MDCK monolayers than wild-type FcRn, at rates similar to FcRn-mutants lacking the cytoplasmic tail entirely. These data suggest that calmodulin may regulate the trafficking of FcRn in polarized epithelial cells and thus mediate the transport of IgG across mucosal surfaces.

We are now working to identify the exact step in trafficking affected by calmodulin. Specifically, our studies will determine whether calmodulin (1) is the trigger that prevents the lysosomal degradation of FcRn, (2) mediates the exocytosis of transcytotic vesicles containing FcRn, (3) signals FcRn to leave the recycling endosome and enter the transcytotic pathway, (4) controls the phosphorylation state of FcRn and thus trafficking in one or more of these pathways, or (5) regulates association of the FcRn cytoplasmic tail with biological membranes. These studies will identify a mechanism for the physiologic regulation of transepithelial IgG transport by cycles of calmodulin binding to FcRn.

Mechanism of the cholera toxin and heat-labile enterotoxin adjuvanticity

Structure of CT and LT
Image courtesy of Dr. Wim Hol
and Dr. Ethan Merritt

A fundamental principle of mammalian physiology is the formation and maintenance of epithelial barriers. Such barriers line the tissues of organ systems that interface with the environment and are found at all mucosal surfaces including the gastrointestinal, respiratory, and genitourinary systems. Structurally, these barriers are created by a continuous monolayer of polarized epithelial cells with distinct apical and basolateral membrane domains, each containing unique protein and lipid components. This polarity in cell structure defines the opposing lumenal (apical) and serosal (basolateral) functions of mucosal surfaces and accounts for the vectorial transport of specific solutes, gases, and water across epithelial barriers such as that found in the intestine, lung, and genitourinary tract.

To generate barrier function, the individual epithelial cells lining these tissues must also assemble circumferential intercellular tight junctions that seal one cell to another. The tight junction represents the rate-limiting barrier that restricts passive diffusion and convection of solutes, molecules, and water between cells. In this way, the single cell thick monolayer that defines the mucosal surface functions to establish and maintain biological homeostasis between the outside and inside environments. Ultimately, the epithelial barrier protects the host from microbial invasion and penetration of other noxious agents. However, some proteins, such as bacterial toxins, are able to breech this barrier to cause disease.

CT and LT function as potent mucosal adjuvants

The cholera enterotoxin (CT) and the related heat-labile enterotoxin (LT) produced by Vibrio cholerae and enterotoxigenic strains of Escherichia coli, respectively, must breech the intestinal epithelial barrier to cause disease. Both toxins enter intestinal epithelial cells and in doing so, stimulate chloride and water secretion into the intestinal lumen resulting in the watery diarrhea associated with cholera and traveler’s diarrhea. These toxins also cross the mucosal barrier by transcytosis and enter the lamina propria as fully folded and functional proteins.

It is well accepted that CT and LT represent the most potent mucosal immunogens and adjuvants recognized to date. While the mechanism by which CT and LT function as mucosal adjuvants is unknown, their adjuvant properties depends on toxin transcytosis across intestinal epithelial cells, an event that facilitates toxin access to relevant antigen-presenting cells in the lamina propria. We are examining the immunological consequences of toxin transcytosis to determine how CT and LT function as mucosal adjuvants so that “smart” drugs and pharmaceuticals may be developed to mimic the adjuvant effect observed with these toxins without the resulting enterotoxicity.