POSSIBILITIES OF FALSE IMMUNOCYTOCHEMICAL RESULTS GENERATED BY THE USE OF MONOCLONAL-ANTIBODIES - THE EXAMPLE OF THE ANTI-PROINSULIN ANTIBODY

The immunocytochemical application of a monoclonal antibody (MAb) agai nst the Arg-Arg region at the junction of the C-peptide and the insuli n beta-chain of the human proinsulin molecule on rat pancreatic tissue resulted in positive immunogold labeling over the insulin- as well as the glucagon-secreting cells, In the insulin cells, the Golgi cistern ae and the immature clathrin-coated granules were intensely labeled, a nd in the glucagon cells all the granules were labeled, Control experi ments using the anti-proinsulin MAb adsorbed with proinsulin, insulin, and glucagon have confirmed the crossreactivity of this proinsulin an tibody with glucagon, Furthermore, the anti-proinsulin MAb appears to crossreact not only with rat but also with human glucagon and with bov ine and porcine insulin and glucagon, Examination of the amino acid se quences of proinsulin and proglucagon has shown that both molecules di splay this Arg-Arg dipeptide sequence, which could explain the labelin g obtained on both cell types, In addition, other propeptides also dis play such a sequence and, indeed, positive cells were detected in rat pituitary, These results demonstrate that by immunocytochemistry this MAb, although providing very specific results, reveals non-related mol ecules, This is due to the fact that these molecules exhibit similar s hort determinants. MAbs raised against very short amino acid sequences can reveal this determinant in various molecules, generating false-po sitive results.