GLANDULAR SECRETION OF IMMUNOGLOBULINS1

Polymeric IgA is complexed with secretory component (SC) on the surface of human intestinal epithelial cells

Viable suspensions of columnar cells were obtained from the surface and outer crypt epithelium of human colon mucosa by a combined treatment with citrate and EDTA solutions and mechanical disruption, but without the use of enzymes. A substantial fraction of the isolated cells contained free SC and secretory IgA in a cytoplasmic distribution that corresponded to previously reported immunohistochemical and immunoelectron-microscopical results obtained with tissue sections. On their surface the same cells were shown to bear SC complexed with J-chain-containing polymeric IgA, whereas only occasional traces of free SC were found. IgM was detected in the surface patches which contained the largest concentrations of SC and IgA. These findings demonstrate that in SC-producing epithelial cells SC molecules become exposed on the plasma membrane, thereby being able to bind specifically J-chain-containing IgA and IgM present in the tissue fluid. Such adsorptive complexing is probably a prerequisite for the selective pinocytotic transport of these two Ig classes through glandular epithelium in man.

Intestinal secretion of IgA and IgM: a hypothetical model

The secretory component (SC) has recently been found to be associated with IgM in external secretions, although in a less stable complex than secretory IgA. Moreover, SC combines spontaneously in vitro with both IgA and IgM. A prerequisite is that the immunoglobulins contain the J chain, which is present only in dimers and polymers. This polypeptide is essential for the formation of an SC-binding site which appears already at the cytoplasmic level in IgA- and IgM-producing immunocytes. Locally formed J-chain-containing immunoglobulins are therefore readily available for complexing with SC present in the membranes of columnar secretory epithelial cells of glandular sites. This complexing initiates pinocytosis and external transport. Immunohistochemically the gland cells are shown to contain SC, IgA and IgM in identical locations, except that SC alone appears in the Golgi zone. Locally formed IgA and IgM antibodies are thus efficiently transferred to the mucosal surface where they exert an immunological exclusion of antigens. Conversely, IgG antibodies, which are not actively drained away from the lamina propria, may rather become engaged in complement activation and cell-mediated cytotoxicity with potentially deleterious effects on the tissue. Secondary to severe inflammatory reactions, secretory epithelium may show decreased production of SC; the selective external transport of SC-stabilized secretory IgA and IgM is thus jeopardized, and a vicious circle may be set up in the mucosa.