Immunocytochemical demonstration of a pancreatic secretory protein of unknown function in human duodenum

We have previously isolated from human pancreatic juice a secretory glycoprotein of 19 KD (P19), devoid of known enzymatic activity. P19 gave by proteolysis a protein of 14 KD (P14), at first named protein X and also called pancreatic thread protein or pancreatic stone protein. Specific rabbit immunosera prepared against P19 and P14 were applied to localize these proteins in human small intestine. By comparison, antibodies directed against some human pancreatic enzymes (amylase, lipase, chymotrypsin, trypsinogen 1, trypsinogen 2, and trypsin 1) were also tested. Positive immunoreactivity was observed on Paneth cells with antisera directed against trypsinogens, trypsin 1, and P19-related proteins. In addition, antisera directed against P19-related proteins stained the columnar cells located in the crypts of Lieberkühn. These original findings are a further indication of the resemblance between Paneth and pancreatic acinar cells but show that their functional analogy is only partial. On the other hand, the presence of P19-related proteins on non-mature columnar cells suggests that this differential distribution is a consequence of differentiation.