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C. Meagher, H. Liu, C.P. Moore, T.E. Phillips; Characterization of an M cell in the mammalian ocular mucosa . Invest. Ophthalmol. Vis. Sci. 2004;45(13):1501.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: The Guinea pig conjunctival epithelium includes a cell that shares a striking morphological resemblance to the M cells (membranous cells) found over lymphoid follicles in other mucosa. The presence of M cells in the conjunctiva would be significant since intestinal M cells are known to initiate the mucosal immune response by sampling luminal antigens and translocating them to underlying lymphocytes and antigen–presenting cells. Since intestinal M cells are also the site of entry for opportunistic pathogens such as viruses or bacteria, conjunctival M cells would also represent a previously unrecognized site of pathogen entry. We hypothesized that if the conjunctiva contains M cells, it should be possible to selectively label their apical surface glycoconjugates with lectins as has been done with M cells in other mucosal locations. Methods: We screened a panel of lectins on aldehyde–fixed nodules and identified Maakia amurensis leukoagglutinin (MAL–I) as a potential conjunctival M cell marker. Biotinylated or fluorescein conjugated MAL–I was then instilled into Guinea pig conjunctival sacs in vivo for 15–45 min (n = 9). Following excision, washing and fixation, tissues were counterstained with colloidal gold or fluorescently conjugated secondary antibodies. Wholemounts and nodular cross–sections (5 µm cryostat or 0.08–0.5 µm acrylic resin) were examined using a combination of epi–fluorescence stereomicroscopy, confocal scanning laser microscopy, and transmission and scanning electron microscopy (TEM and SEM). Results: Well–developed lymphoid follicles are reproducibly found in the fornix region of adult Guinea pigs. Preferential labeling of the epithelium overlying these lymphoid follicles can be seen in conjunctival tissues exposed to fluorescein–labeled MAL–I in vivo. In TEM of conjunctival nodules incubated with biotinylated MAL–I, we found intense labeling on the apical membrane of cells with the morphological characteristics of intestinal M cells, including a narrow apical bridge and an underlying lymphocyte pocket. These cells were filled with many intracellular vesicles containing endocytosed MAL–I. SEM demonstrated that colloidal gold conjugated MAL–I labeling was restricted to cells with irregular microvilli which is another characteristic feature of M cells. Conclusions: MAL–I selectively labels a novel cell type in the conjunctival follicle associated epithelium. Based on this cell’s morphological characteristics and ability to actively endocytose the membrane marker, we conclude it is a conjunctival M cell.
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