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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.

The nasal respiratory mucosa of the rabbit has a leaky epithelium actively transporting polypeptides by a specific transcytosis probably involved in sampling antigens. The transfer displays saturation kinetics and is abolished by metabolic inhibitors, actin filamet and microtubule disassemblers, inhibitors of vesicle fusion, and receptor recycling;it accepts polypeptide-covered but not uncovered nanoparticles.

We have previously demonstrated that polypeptides (elcatonin and ACTH) can be actively absorbed across the rabbit nasal mucosa. In this paper we show that elcatonin is also transported when it is adsorbed onto microspheres (diameter: 0.5 micron). whereas the elcatonin-uncovered microspheres do not display any net transport. Cytochalasin D (0.1 microgram/ml) abolishes the net absorption of elcatonin presented either alone or adsorbed. At the same concentration the inhibitor does not affect cellular active ion transports (and hence metabolism); although it increases intercellular ion and elcatonin permeability, it does not affect intercellular and paracellular permeability of the elcatonin-covered microspheres. Altogether, these results show that polypeptide transport is supported by a specific vesicular transfer inhibited by cytochalasin D by disassembly of the actin cytoskeleton, probably at the apical border of the cell.

It has been suggested that the specific transcytosis of polypeptides, demonstrated in rabbit nasal mucosa (upper concha), is involved in antigen sampling at the airway entry. To test this hypothesis, unidirectional transepithelial fluxes of carbocalcitonin (CCT Mw=3362) from the mucosal to the submucosal side, and vice versa, were measured by radioimmunoassay every 30min for 120min and, from the difference, net absorption was determined in the upper concha and septum mucosae. The exposed mucosae were examined by quantitative histology; isolated scattered lymphoid cells/mm2 and volumes of lymphoid infiltrates and aggregates were quantified. CCT absorption was observed in the mucosae of the upper concha and septum provided that aggregates were present, being proportional to aggregate volume. No relationship was noted with isolated scattered lymphoid cells and infiltrates. Passive permeability was unaffected by lymphoid tissue. On this basis, the antigen sampling hypothesis seems to be at least partially substantiated.