Brie£ DeHnitive Report

The rapid and active transport of polymeric IgA from the blood to the bile (1, 2) is selective (3, 4), which suggests the existence of a specific receptor for this molecule. The liver cells responsible for the transfer have been identified as the hepatocytes by autoradiography and electron microscopy of the livers o f rats killed 5, 30, and 60 min after the injection of radiolabeled IgA (5). 5 rain after being injected, the radiolabeled IgA appeared to be bound to the sinusoidal membrane of the hepatocytes. We have found that intravenously injected radiolabeled IgM, IgG1, and monomeric IgA did not appear in the bile at all, and that the transport into bile of s-IgA (i.e., polymeric IgA bound to secretory component [SC]) was both considerably slower and quantitatively less than that of polymeric IgA devoid of SC. Studies of the transport into the bile o f IgG2a, IgE, IgM, monomeric and polymeric IgA, and s-IgA with isolated perfused rat livers gave similar results: only polymeric IgA that lacked SC was actively transported (6). These facts suggest that SC may be the receptor for IgA on the surface of rat hepatocytes as it is on the epithelial cells from h u m a n gut (7). The availability of hepatocytes in short-term monolayer culture (8), together with that of specific antibodies to rat SC, has enabled us to test this hypothesis by experiments in vitro.