Synthesis of IgA by human gastric mucosa in vitro

Human gastric mucosa was removed at gastrectomy for duodenal ulcers. Intact mucosal layer and homogenized mucosal scrapings were incubated in vitro with L-[U-14C]leucine and with D-[U-14C]glucose and incorporation of radioactive label into IgA was investigated. 14C of both the precursors was incorporated into tissue IgA when intact gastric mucosa was incubated indicating synthesis of IgA by the tissue in vitro. IgA isolated from the media which bathed the luminal sides of the mucosae during incubation was also radioactive implying secretion of the newly-synthesized immunoglobulin into the media. When mucosal scrapings were incubated as homogenized suspensions, radioactivity of IgA was below the level of sensitivity of our methods. For analysis, IgA was separated from the bulk of radioactive mucosal proteins of the samples by isopycnic CsCl gradient centrifugation followed by gel filtration on Sephacryl S-200 or Sephadex G200 and then Sepharose 4B. The IgA-enriched Sepharose 4B fractions were subjected (1) to immunoprecipitation in agar gels by double diffusion against specific anti alpha-chain serum and (2) to SDS-polyacrylamide gel electrophoresis after reduction. Radioactivity of precipitated IgA, and of the protein band which corresponds to the alpha-chain of IgA on SDS-PAGE gels, was demonstrated autoradiographically. It is suggested that in vivo the mucosa is involved in local synthesis of the constituent IgA of gastric mucus.