Antibodies to the cytosolic leucocyte L1 protein (or calprotectin) were examined for reactivity with macrophages, neutrophils, and eosinophils identified by paired immunofluorescence staining in sections of normal human ileal mucosa, including Peyer's patches. Macrophages were recognised by expression of the myelomonocytic antigen CD68 (monoclonal antibody KP1). Neutrophilic granulocytes were identified by their content of neutrophil elastase, and eosinophilic granulocytes by monoclonal antibody EG2. Virtually all CD68+ macrophages in normal lamina propria and Peyer's patches were L1- and the same was true for most extravasated macrophages in normal peripheral lymph nodes. Some mesenteric lymph nodes, however, and all peripheral lymph nodes with overt pathological processes (malignant lymphoma) contained many CD68+L1+ macrophages. Numerous L1+ cells were also localised to the crypt region and to some extent beneath the villous epithelium in normal lamina propria, but they were mainly identified as EG2+ eosinophils. Such cells were remarkably scarce or absent beneath the follicle associated epithelium in the dome region of Peyer's patches, where CD68+L1- macrophages were abundant. Also subepithelial and interfollicular CD68- interdigitating dendritic cells in Peyer's patches (recognised by antibody to S-100 protein) were usually unreactive with L1 antibody. The L1 protein shows a broad spectrum of antimicrobial activities in vitro, and its putative antiproliferative properties are interesting in relation to the immunosuppression postulated to take place in lamina propria. The virtual absence of L1 producing cells beneath the follicle associated epithelium in Peyer's patches may support the immunostimulatory function of these macrophage rich structures, which are held to be crucial for induction of specific mucosal immunity.
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