RESUMEN
INTRODUCTION: M cells are located in the epithelial layer covering the gut-associated lymphoid tissue and are responsible for delivery of macromolecules and microorganisms to the underlying lymphoid cells. It has been shown that the human colonic cell line Caco-2 can be converted to M cells in vitro following coculture with isolated lymphocytes from murine Peyer's patches. Studies were undertaken to evaluate and characterize the transepithelial transport of select macromolecules across these in vitro derived M cells. METHODS: Caco-2 cells were converted to M cells as reported previously. The morphology of Caco-2 cells and M cells was compared by transmission electron microscopy (TEM). The transport properties of macromolecules such as horseradish peroxidase, FITC-conjugated polystyrene beads, and radiolabeled dextrans were examined. The activation of murine antigen-specific T cells following transport of the antigen ovalbumin across the M-cell barrier was assessed by measuring cytokine production. RESULTS: M cells were shown to be irregular in shape and have fewer and shorter microvilli compared to the Caco-2 cell progenitors. These cells were still able to form tight junctions and monolayers on polycarbonate membranes. Time-course studies demonstrated that the transport of polystyrene beads and large-molecular-weight dextrans at physiological temperature across M-cell-containing monolayers was size dependent and more rapid than across Caco-2 cell monolayers. The transport of dextrans was also shown to be temperature and concentration dependent. Befitting the role of the M cell in mucosal defense, protein antigen could be delivered by these cells in order to be processed and presented to antigen-specific CD4+ T lymphocytes. DISCUSSION: The M-cell permeability model is a functional and practical system for evaluating the transport properties of macromolecules and assessing the potential for intestinal mucosal antigen sampling to elicit immunological responses.