Identification of endogenous peroxidase-containing cells as eosinophils in the gastrointestinal system.

Endogenous peroxidase (EPX) activity in certain cells in the gastrointestinal system interferes with immunohistochemical methods based on the horseradish peroxidase-catalyzed substrate deposition. We studied the distribution and characteristics of these cells. We also report an effective and antigen-preserving EPX blocking method, to make possible the evaluation of immunoperoxidase stainings in cryostat sections. The EPX-containing cells (EPX cells) are present in every part of the gastrointestinal tract, predominantly in the tunica propria. We identified them as eosinophil cells in May-Grünwald-Giemsa stained sections. The complete match was confirmed by different fluorescence techniques. Firstly, the EPX cells were labeled by a red fluorochrome-conjugated substrate of peroxidase enzymes, rhodamine-tyramide, whereas the eosinophil cells were labeled by the green fluorochrome, l-hydroxy-3,6,8-pyrenetrisulfonic acid, which is known to label exclusively eosinophilic granules at pH 10. Secondly, all the EPX cells reacted with a monoclonal antibody against the eosinophil peroxidase enzyme. Finally, a set of commercially available leukocyte markers was used to characterize the EPX cells colabeled by fluorochrome-tyramides. Neither macrophages nor mast cells showed EPX activity. Increased numbers and altered distribution were seen in stressed rats and in ulcerated human stomach.