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The active transcytosis of many different polypeptides (either presented free or adsorbed on latex nanoparticles), found in the respiratory mucosa of the upper nasal concha, has previously been shown to be proportional to the total volume of the lymphoid aggregates present in the tissue. By combining the use of fluorescent nanoparticles, flux measurements, confocal and scanning electron microscopy and conventional histology, it is shown in this paper that: (i) the areas of epithelium overlying lymphoid aggregates are the only transporting polypeptides; (ii) the respiratory epithelium in these areas consists mainly of non-ciliated microvillar cells, with numerous ciliated cells and rare mucous goblet cells at the periphery of the area only; (iii) non-ciliated microvillar cells are distinguishable in cells with well developed finger-like microvilli and cells with an irregularly pleated apical membrane, similar to that of intestinal and bronchial antigen-sampling M-cells; (iv) groups of polypeptide-coated nanospheres are found bound to this latter type of cells, demonstrating that these are the transporting cells, detected at the first stage of the transcytotic cycle.

In rabbit nasal mucosa, polypeptides and polypeptide-coated nanospheres are actively transported from lumen to blood by M-cells present in specialized transport areas of the epithelium. The largest transport is shown here to occur when some molecules of the polypeptides coating the nanospheres, after adsorption, are bound to the specific anti-polypeptide IgG, e.g. when insulin is bound to the anti-insulin IgG. The transport kinetics of nanospheres coated by insulin bound to its antibody, as a function of bead concentration or of the antibody/insulin coating ratio, have been analyzed. On this basis it was possible to assess the maximal transport capacity of the epithelium and to calculate the percentage of M-cells involved.