Uptake and Intracellular Disposition of Iga by Rat Hepatocytes in Monolayer Culture

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Abstract
It is well documented that the liver provides an important source of secretory immunoglobulin A (sIgA) in the gut fluid. In the rat, liver parenchymal cells can actively sequester the dimeric form of IgA (dIgA) from plasma and transfer it attached to its receptor, secretory component (SC), via vesicles into the bile. In the present investigation, isolated rat hepatocytes in monolayer culture were studied using quantitative electron microscopic autoradiography to determine if the previous reported specific binding of dIgA by isolated cells would result in internalization of the ligand and, if so, whether its distribution within the cultured hepatocytes would be similar to that found in vivo. The specificity of the process for receptor (SC) availability was evaluated utilizing sIgA and by blocking the receptor with rabbit IgG anti–rat SC antibody. For these studies, purified dIgA and sIgA were iodinated with [125I] and added to fresh culture medium 24 hr following the isolation of the hepatocytes. After 3, 30, and 60 min of incubation with the isotope, the cells were washed, fixed, and processed for light and electron microscopic autoradiography. Binding of dlgA to the cells occurred by 3 min, and the number of autoradiographic grains per unit membrane length remained constant throughout the 60–min time period. With time, there was a progressive increase in the per cent of internalized grains. As in the previous in vivo studies, the majority of these internalized grains were localized to vesicles and smooth surface endoplasmic reticulum. There was no evidence of binding or uptake of sIgA at any time period and pretreatment with anti–SC reduced binding and internalization of dIgA to 16% of the control value. These data provide new evidence that isolated hepatocytes in monolayer culture appear functionally intact with regard to dIgA uptake and demonstrate their usefulness as an in vitro system for future studies of IgA metabolism.