Radioimmunoassay of Human Jejunal Iga Secretion 1

Abstract

Secretory immunoglobulin-A (S-IgA) has been assigned a major protective role for a variety of epithelial surfaces. In the gastrointestinal tract, S-IgA inhibits adsorption of bacteria to mucosal cells and neutralizes viral antigens. Intestinal S-IgA is synthesized as a dimer in the lamina propria by immunocytes adjacent to epithelial cells, is conjugated with secretory component at the lateral membrane, enters the cell by pinocytosis, and is subsequently secreted into the lumen (1). Immunofluorescent studies indicate that S-IgA is synthesized throughout the gut. It has been estimated that there are from 350,000 to 980,000 IgA immunocytes per mm³ of jejunal mucosa (2-5) and similar numbers in the ileum (6), colon (2), and rectum (2, 7). From 50% to 85% of gut immunocytes produce IgA (2, 4, 6). Jejunal fluid concentrations of S-IgA range from 21 to 41 mg/dl (7-11) but free secretory component has not been detected in jejunal aspirates (12). Eating, gallbladder contraction, pancreatic secretion, and swallowed IgA from oropharyngeal secretions alter intestinal concentrations (13). Recently, semiquantitative measurements of S-IgA synthesis have been reported in man (14) and synthesis and secretion have been documented in rabbits (15). To enable quantitative determination of human S-IgA secretion by intestinal mucosa, we developed a radioimmunoassay technique applicable to organ culture of jejunal biopsies from man.

Methods. Subjects. Five normal human volunteers between the ages of 40 and 57 gave informed consent for these investigations which were approved by the University of Vermont Committee on Human Experimentation. Upper G.I. and small bowel series, barium enema, sigmoidoscopy, rectal biopsy, quantitative jejunal bacterial cultures, ⁵¹Cr-albumin clearance, quantitative fat balance, and small-bowel biopsies were normal in each subject.