Efficiency of pig transport through secretory epithelia

Immunohistochemical studies have shown that SC is expressed mainly by serous types of epithelial cells in exocrine tissues. In the human liver, SC is restricted to the bile duct epithelium whereas abundant SC expression is shown by hepatocytes in the rat and some other animals. This species difference has been supported by Northern blot analysis of plgR mRNA (see above) and harmonizes with the fact that -90% IgA in duodena] fluid is derived from bile in the rat but <15% in humans.

Exocrine tissues constitute quantitatively the most important effector sites of specific humoral immunity with the gut as the major contributor. It has been estimated that at least 80% of all Ig-producing cells in humans and mice are located in the intestinal mucosa. Nevertheless, there is normally very little spillover of locally produced IgA from the human gut lamina propria to lymph and venous blood. The capacity of the plgR-mediated external transport through human intestinal crypt epithelium is hence remarkable; more plgA is translocated to the gut lumen by this mechanism every day (40mgkg~' body weight) than the total daily production of IgG (which is about 30 mgkg-1). An in vitro model system established in our laboratory to characterize this process has shown that the epithelial transport of plgA and plgM mediated by the human plgR is equally efficient. However, in vivo external translocation of plgA is on a molar basis favored 6-12 fold over that of plgM because the latter ligand is severely impeded when diffusing through the lamina propria and basement membrane. Nevertheless, S-IgM is an important compensatory antibody class in external secretions of newborns and patients with selective IgA deficiency.

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