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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 specific transcytosis of polypeptides, demonstrated in the nasal respiratory mucosa of the rabbit, seems to be involved in antigen sampling at the airway entry, since absorption has been shown only to occur if lymphoid aggregates are present beneath the epithelium and to be proportional to aggregate volume. Nanoparticles and many polypeptides besides the two previously tested (i.e. carbocalcitonin (CCT) and adrenocorticotropic hormone) should be transportable, in agreement with the vesicular transcytosis and antigen sampling hypothesis. Thus unidirectional mucosa-submucosa and opposite fluxes (Jms, Jsm) and the corresponding net fluxes (Jnet) of uncoated or polypeptide-coated polystyrene nanospheres (diameter: about 0.5 micrometer) have been measured with the aid of spectrophotometry and quantitative dark-field microscopy. No net transport has been observed for uncoated beads, whereas it has always been shown for polypeptide-coated beads, although to different extents. The selectivity sequence for the polypeptides tested is as follows: BSA congruent with enkephalinanti-BSA IgG congruent with IgA congruent with CCT congruent with insulin/=anti-insulin IgG. With the exception of BSA and enkephalin-coated beads, whose Jnet is very small, in all the other cases the apparent affinities for receptors seem to be equal or similar; just over 6% polypeptide coating on the nanosphere is sufficient to elicit maximal transport; finally, transport seems to require many cooperating binding sites between the single nanosphere and receptors or one or many non-cooperating binding sites, but with a threshold number of polypeptide molecules adsorbed on the nanosphere to reach a minimal binding probability.

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.

An active absorption of polypeptides (elcatonin = CCT; adrenocorticotropic hormone) had been previously observed in the nasal respiratory mucosa of the rabbit. Its saturation kinetics and the parallel absence of a net transfer of other non-polypeptidic organic markers excluded the involvement of a simple pinocytosis. This absorption has been now better localized and further characterized. Unidirectional CCT fluxes (determined with radioimmunoassay) have been concomitantly monitored with transepithelial electric potential difference (Vms). Although the mucosae covering the ectoturbinal A and the lower and upper conchae displayed similar Vms, the active CCT transport was only evidenced in the upper concha. In this region cytochalasin B (which by disassembling actin microfilaments prevents the apical formation of vesicles in epithelial cells) and monensin (which prevents the split of the ligand-receptor complex in the endosomes) both eliminated the net CCT absorption, however, also permanently increasing the passive CCT junctional permeability. Aluminium fluoride (which prevents the fusion of endocytic vesicles into endosomes) and colchicine (which disrupts microtubules along which vesicles move in the cytoplasm) also permanently abolished net CCT transport, without affecting, or shortly and transiently affecting, passive permeability. On the whole these results are in favor of an active CCT transport supported by a specific vesicular transport.

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.