Polymeric IgA binding to the human pIgR elicits intracellular signalling, but fails to stimulate pIgR-transcytosis.

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-C gamma 1 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.

Molecular maturation and functional expression of mouse polymeric immunoglobulin receptor.

Mouse polymeric immunoglobulin receptor (pIgR) cDNA was stably introduced into a hamster-derived fibroblastic cell line, Chinese hamster ovary (CHO) cell, by the calcium phosphate method. Surface expression of pIgR was detected by immunostaining and FACS analysis. The immunoprecipitated products of cell lysates revealed that the molecular mass of the most mature form of pIgR was approximately 120 kDa. Western blotting and metabolic labeling experiments followed by immunoprecipitation with an anti-mouse secretory component (SC) Ab demonstrated the existence of a 110 kDa immature form of pIgR. The reason for the existence of two forms of pIgR molecule was examined by conducting pulse-chase experiments which revealed the pIgR underwent molecular maturation. During this process, the 110 kDa form of pIgR was converted into a 120 kDa form by glycosylation. Moreover, tunicamycin treatment revealed the core form of pIgR had a molecular mass of approximately 100 kDa. The pIgR expressed on the surface of the transfectant could specifically bind and take up mouse polymeric IgA (MOPC 315), suggesting that, at least in this mouse system, cell type-specific molecules are not necessary for surface pIgR expression and polymeric immunoglobulin (pIg) binding and uptake.

Neuroendocrinimmune modulation of secretory component production by rat lacrimal, salivary, and intestinal epithelial cells.

PURPOSE: To evaluate the kinetics, receptor specificity, molecular basis, and site selectivity of the endocrine and neural regulation of secretory component (SC) synthesis by rat lacrimal gland acinar cells. METHODS: Acinar cells from the rat lacrimal and submandibular glands, as well as epithelial cells (IEC-6) from the rat small intestine, were cultured in supplemented, serum-free media and treated with dihydrotestosterone, cholera toxin, carbachol, vehicle, or other agents for varying time periods. Media SC levels were measured by radioimmunoassay. RESULTS: The authors' findings with lacrimal gland acinar cells demonstrate that: a significant, temporal delay exists between the initiation of stimulatory or inhibitory signals and the eventual cellular SC response to regulatory compounds; the parasympathetic analogue, carbachol, exerts a dual effect on SC output, i.e., an early stimulation (hours) followed by an extended suppression (days); the androgen and cholinergic control of SC is receptor-mediated; and the androgen modulation of SC may involve the induction of gene expression. In addition, the authors' results show that distinct, tissue-specific variations occur in the nature of SC regulation: Compounds that control SC output by lacrimal acinar cells do not necessarily alter SC production by epithelial cells from the rat submandibular gland or small intestine. CONCLUSIONS: These findings advance the authors' understanding of the neuroendocrine regulation of SC synthesis in acinar cells from the lacrimal gland. Moreover, the authors' results indicate that the nature of the neural, endocrine, and immune control of lacrimal SC may be unique.