Three human polymeric IgA (pIgA) myeloma proteins of tetrameric size were compared for their J-chain content, their in vitro secretory component (SC)-binding ability, and their capacity to be transcytosed by polymeric immunoglobulin receptor (pIgR)-expressing epithelial cells in vitro and rat hepatocytes in vivo. One of the three pIgA preparations, pIgA-L, was shown to lack J chain and was unable to combine with purified free human and rat SC, whereas pIgA-G and pIgA-C contained J chain and combined readily with SC. Furthermore, pIgA-L was not transferred into rat bile after intravenous injection, and was hardly transported apically by polarized Madin-Darbey canine kidney cell monolayers expressing the human pIgR, whereas pIgA-G and pIgA-C were efficiently transported in both test systems. Together with our recent demonstration that antibodies to human J chain block the SC/pIgR-mediated epithelial transport of pIgA, these data unanimously confirm the proposed key role of J chain in the epithelial transport of polymeric immunoglobulins into exocrine secretions.

Binding of human polymeric IgA ligand to its epithelial cell polymeric Ig receptor, pIgR, has been shown to stimulate pIgR apical transcytosis in an in vitro system, based on polarized confluent MDCK cells expressing rabbit pIgR. The present study aimed at testing whether such a stimulation also occurs in vivo. Transcytosis of pIgR was monitored by rat liver output of total secretory component (SC) into bile, measured by radial immunodiffusion as the sum of free SC and pIgA-bound SC. Whereas in the perfused rat liver system addition of pIgA to the perfusate showed no effect, i.v. injection of human and rat pIgA, but not of monomeric IgA nor PBS, in living rats significantly increased total bile SC output for more than 1 h. Furthermore, depletion of the normal pIgA level circulating in the liver before injecting more pIgA was not required to show the stimulation. Our data thus strongly suggest that stimulation of liver pIgR transcytosis by pIgA ligand binding is physiologically relevant, helping to quickly adjust pIgA transport into bile to increase circulating pIgA levels, without need for increased SC/pIgR synthesis.

The respiratory tract in humans represents a large mucosal area exposed to the environment. The secretory immune system is the first, noninflammatory line of defence, protecting the respiratory tract from viral and bacterial infections. Secretory immunoglobulin A (S-IgA) is the major immunoglobulin present in human airway secretions [1]. The humoral secretory immune response depends on interactions between the B-cell system for the production of J-chain-containing polymeric IgA (pIgA) and the mucosal epithelium for the epithelial expression of secretory component (SC), respectively. SC is the soluble extracellular domain of the polymeric Ig receptor (pIgR) present on the basolateral surface of most epithelial cells. The pIgA antibodies produced by plasma cells are transported into mucosal secretions by transcytosis through epithelial cells [2,3]. The pIgR at the basolateral pole of the epithelium, with or without bound pIgA, is constitutively endocytosed and transported by vesicles to the apical pole, where pIgR vesicles fuse with the apical membrane. A proteolytic cleavage then occurs between the extracellular and membrane domains of the pIgR, resulting in the release of SC or S-IgA, a complex of mostly dimeric IgA (dIgA), J-chain and SC [4,5]. Immunohistology and immunofluorescence studies have demonstrated the presence of SC in bronchial epithelial cells and of IgA-containing plasma cells in the bronchial mucosa [6]. Moreover, pIgA and SC transcytosis have also been demonstrated by immunoelectron microscopy of bronchial mucosa [7]. In human pathology, decreased SC levels have been described in the bronchoalveolar lavage (BAL) fluid of asthmatic patients compared with normal controls [8]. Furthermore, a prospective study in children suggested a relationship between transient salivary IgA deficiency during the first year of life and the later development of bronchial hyperreactivity [9].The production of SC by cultured human tracheal epithelial cells has been demonstrated to be influenced by cell polarization [10]. The influence of cell polarization on SC production was further stressed by studies using neoplastic colonic cells, demonstrating an enhancing effect of polarization on their in vitro SC production [11]. It is concluded that human bronchial epithelial cells produce secretory component and transcytose dimeric immunoglobulin A in vitro. These processes were apically polarized and upregulated by interferon-γ. Among the cell lines studied, only CALU-3 expressed secretory component-messenger ribonucleic acid and produced detectable secretory component.

The intracellular pathway of polymeric immunoglobulin receptor (pIgR) is governed by multiple signals that lead to constitutive transcytosis. In addition, in transfected polarized MDCK cells, polymeric immunoglobulin A (pIgA) binding stimulates rabbit pIgR‐transcytosis, owing to phospholipase‐Cγ1 activation and increase of intracellular calcium. Transcytosis of rat pIgR across hepatocytes is similarly accelerated by pIgA injection. In contrast we show here that human Madrin‐Darby Canine Kidney (pIgR)‐transcytosis, in human Calu‐3 and human pIgR‐transfected MDCK cells, is not promoted by pIgA, as monitored by a continuous apical release of its secreted ectodomain. However, the incubation of cells expressing human or rabbit pIgR with pIgA induces a comparable IP3 production, and pIgR‐transcytosis of either species is accelerated by the protein kinase C (PKC)‐activator phorbol myristate acetate. Without pIgA, mimicking phospholipase‐C activation by combining low concentrations of phorbol myristate acetate with ionomycin, or high concentrations of ionomycin alone, stimulates the rabbit, but not the human, pIgR transcytosis. These data suggest that the species difference in pIgA‐induced pIgR‐transcytosis does not stem from the defective production of second messengers, but from a different sensitivity of pIgR to intracellular calcium. Our results outline the danger of extrapolating to humans the abundant data obtained from mucosal vaccination of laboratory animals.