Immunohistochemical studies on various aspects of glandular immunoglobulin transport in man

Abstract

Gland-associated immunocyte populations have been characterized in human tissue specimens from which extracellular immunoglobulins have been removed by saline extraction. There is a striking preponderance of IgA-producing immunocytes adjacent to glands of the gastro-intestinal and respiratory tracts, in minor and major salivary glands, and in lactating mammary glands. Immunohistochemically, these cells have been found to contain dimeric IgA with incorporated J chain. Despite this local IgA production, immunohistochemical tests on alcohol-fixed specimens demonstrate that the glandular stroma is normally permeated predominantly by IgG, most of which is obviously serum-derived. However, the serous glandular cells selectively transmit dimeric IgA, which appears along their lateral borders and apically in the cytoplasm, whereas the epithelial occurrence of IgG is less conspicuous and is restricted to the interstices. The same epithelial cells produce a glycoprotein called the ‘secretory component’ (SC) which exhibits specific affinity for J chain-containing dimeric IgA and pentameric IgM. In saline-extracted tissue, IgA, but IgG, is retained regularly along the borders of SC-producing cells; this probably reflects complexing between locally formed IgA and SC in the epithelial cell membranes. SC apparently functions as a glandular receptor for dimeric IgA which thus most likely enters the epithelial cells by adsorptive pinocytosis. After covalent stabilization, the IgA-SC complexes are extruded to the gland lumen. Immunohistochemically the Golgi zone has been found to contain free SC but no IgA, whereas SC occurring more apically in the epithelial cell exhibits characteristics of being IgA-associated. Pentameric IgM is handled by the glands in a way similar to dimeric IgA, but local synthesis of IgM is normally negligible, except in the gut.