DISTRIBUTION OF MACROPHAGES AND GRANULOCYTES EXPRESSING L1 PROTEIN (CALPROTECTIN) IN HUMAN PEYERS-PATCHES COMPARED WITH NORMAL ILEAL LAMINAPROPRIA AND MESENTERIC LYMPH-NODES

Antibodies to the cytosolic leucocyte L1 protein (or calprotectin) wer e examined for reactivity with macrophages, neutrophils, and eosinophi ls identified by paired immunofluorescence staining in sections of nor mal human ileal mucosa, including Peyer's patches. Macrophages were re cognised by expression of the myelomonocytic antigen CD68 (monoclonal antibody KP1). Neutrophilic granulocytes were identified by their cont ent of neutrophil elastase, and eosinophilic granulocytes by monoclona l antibody EG2. Virtually all CD68+ macrophages in normal lamina propr ia and Peyer's patches were L1- and the same was true for most extrava sated macrophages in normal peripheral lymph nodes. Some mesenteric ly mph nodes, however, and all peripheral lymph nodes with overt patholog ical processes (malignant lymphoma) contained many CD68+L1+ macrophage s. Numerous L1+ cells were also localised to the crypt region and to s ome extent beneath the villous epithelium in normal lamina propria, bu t they.were mainly identified as EG2+ eosinophils. Such cells were rem arkably scarce or absent beneath the follicle associated epithelium in the dome region of Peyer's patches, where CD68+L1-macrophages were ab undant. Also subepithetial and interfollicular CD68- interdigitating d endritic cells in Peyer's patches (recognised by antibody to S-100 pro tein) 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 immun osuppression postulated to take place in lamina propria. The virtual a bsence of L1 producing cells beneath the follicle associated epitheliu m in Peyer's patches may support the immunostimulatory function of the se macrophage rich structures, which are held to be crucial for induct ion of specific mucosal immunity.