Polymeric IgA Research Articles

An autonomous signal for basolateral sorting in the cytoplasmic domain of the polymeric immunoglobulin receptor

The polymeric immunoglobulin receptor is normally delivered from the Golgi to the basolateral surface of epithelial cells and then transports polymeric IgA and IgM to the apical surface. We now report that a 14 residue segment of the 103 amino acid cytoplasmic domain, proximal to the plasma membrane, directs the receptor to the basolateral surface. A mutant receptor lacking these 14 amino acids is sorted directly to the apical surface from the Golgi. Furthermore, this sequence is sufficient to redirect an apical membrane protein, placental alkaline phosphatase, to the basolateral plasma membrane. We conclude that this sequence contains an autonomous signal, which specifies sorting from the Golgi to the basolateral surface, a process previously postulated to occur by default.

The human transmembrane secretory component (poly-Ig receptor): molecular cloning, restriction fragment length polymorphism and chromosomal sublocalization

The human transmembrane secretory component (SC) mediates glandular translocation of polymeric IgA and IgM into exocrine secretions. A 2898-bp cDNA clone, encoding the entire sequence of the human transmembrane SC, was isolated from a colonic adenocarcinoma cell line cDNA library. The deduced amino-acid sequence had a length of 764 residues and showed an overall similarity of 56% and 64% with the rabbit and rat counterpart, respectively. A restriction fragment length polymorphism (RFLP) was found with PvuII, revealing a two-alle RFLP with an autosomal codominant inheritance pattern and allele frequencies of 0.65 and 0.35. Southern blot analysis of human-rodent somatic hybrid panels, including hybrids with translocation chromosomes carrying different parts of chromosome 1, assigned the SC gene to 1q31-q42, thus confirming a previously reported provisional assignment.

Selective Elevation of Monomeric IgA1 in IgA Nephropathy Patients With Normal Renal Function

The molecular form of the pathognomonic IgA in IgA nephropathy (IgAN) remains controversial. Because characterization of the molecular form of IgA molecules can lend insight into their origin (systemic v mucosal), we developed immunoassays to measure both total and J chain-containing (polymeric) IgA1 and IgA2. These assays were used to measure IgA in sera from two groups of IgAN patients (with normal or decreased renal function), as well as from a group of normal individuals. IgA1 levels were higher in both groups of patients with IgAN when compared with the controls. The elevation appeared to be restricted to non-J chain-containing (monomeric) IgA1 in patients with normal renal function, whereas polymeric IgA1 was also slightly elevated in patients whose renal function was diminished. While there were no significant differences between the groups in terms of the levels of total IgA2, the patient group with normal kidney function appeared to have lower levels of polymeric IgA2. The observation that the elevation in serum IgA appears to be restricted to the monomeric form of IgA1, at least when renal function is normal, implies a systemic origin of the pathognomonic IgA in IgAN and further suggests an abnormality in the regulation of IgA secretion by immunoglobulin-producing cells in bone marrow, the site of systemic IgA synthesis.

Effect of Dextrans of Various Molecular Weights on Mesangial Transport in ddY Mice Pretreated with Sheep Anti‐type IV Collagen Serum

The authors' previous report concluded that increased mesangial IgA deposition seen in ddY mice pretreated with mesangiotropic anti-type IV collagen serum might be due to a dysfunction of mesangial transport of macromolecules such as polymeric autologous IgA. In the present study we examined the effect of macromolecules upon mesangial transport in similarly treated ddY mice, using intravenously administered FITC-labeled dextrans of various molecular weights as tracers. The intensity of each resulting mesangial dextran deposit inspected directly by immunofluorescence was measured periodically and compared with the deposition of autologous mouse IgA examined using rhodamine-labeled rabbit anti-mouse IgA. High-molecular-weight dextran of 2,000 kDa showed prolonged mesangial deposition which paralleled the intensity and distribution of the autologous mouse IgA deposition. In contrast, medium- and low-molecular-weight dextrans of 500 and 150 kDa, respectively, disappeared earlier from the mesangium, unlike the autologous IgA deposition which persisted. Based on these results, it was concluded that the macromolecularity of certain substances and a transport dysfunction of the mesangium may both be major factors in prolonged mesangial deposition. These findings may provide some clues to help clarify the pathogenesis of human IgA nephritis.

An autonomous signal for basolateral sorting in the cytoplasmic domain of the polymeric immunoglobulin receptor

The polymeric immunoglobulin receptor is normally delivered from the Golgi to the basolateral surface of epithelial cells and then transports polymeric IgA and IgM to the apical surface. We now report that a 14 residue segment of the 103 amino acid cytoplasmic domain, proximal to the plasma membrane, directs the receptor to the basolateral surface. A mutant receptor lacking these 14 amino acids is sorted directly to the apical surface from the Golgi. Furthermore, this sequence is sufficient to redirect an apical membrane protein, placental alkaline phosphatase, to the basolateral plasma membrane. We conclude that this sequence contains an autonomous signal, which specifies sorting from the Golgi to the basolateral surface, a process previously postulated to occur by default.

Complement enhances the clearance of large‐sized soluble IgA aggregates in rats

In the present study the involvement of the complement system (C) in the clearance of soluble IgA aggregates in the rat was studied. Monoclonal monomeric IgA (mIgA) antibody (which does not activate C) or aggregated polymeric IgA (aIgA; which activates C) were administered intravenously to phosphate-buffered saline-treated and complement-depleted [Cobra venom factor (CVF)-treated] rats and assessed for clearance from the circulation. In control rats, mIgA was cleared in a biphasic fashion with a first half-life (T1/2) of 29.5 +/- 14.2 min and a second T1/2 of 230 +/- 176 min. No differences were observed in clearance of mIgA in CVF-treated rats as compared to PBS-treated rats. In PBS-treated rats, aIgA with a size between 20 S and 150 S disappeared very rapidly from the circulation with a first T1/2 of 1.1 +/- 0.4 min and a second T1/2 of 23.2 +/- 11.3 min. In CVF-treated rats the clearance of aIgA was significantly delayed as compared to that in control rats, namely with a first T1/2 of 7.3 +/- 2.6 min and a second T1/2 of 64.2 +/- 19.4 min. Immunohistochemical studies of the liver (which is the main site of clearance of aIgA) revealed that Kupffer cells (KC) are mainly responsible for the uptake of aIgA. Furthermore, in PBS-treated rats aIgA deposition was accompanied by C3 deposition in the KC. In CVF-treated rats, the percentage of KC containing aIgA was significantly lower during the first 16 min after aIgA administration as compared to PBS treated rats. In addition no detectable C3 was found in KC of CVF-treated rats. These results indicate that KC play an important role in the clearance of large molecular weight IgA in rats and that C facilitates the clearance of these complexes from the circulation.

IgA triggers tumor necrosis factor α secretion by monocytes: A study in normal subjects and patients with alcoholic cirrhosis

Under endotoxin-free conditions, peripheral blood mononuclear cells and purified monocytes isolated from healthy control subjects and patients with alcoholic cirrhosis disclose elevated tumor necrosis factor α messenger RNA level and produce tumor necrosis factor α in response to stimulation by either soluble polymeric IgA or monomeric IgA bound to the surface of culture dishes but not by soluble monomeric IgA. Polymeric IgA induces tumor necrosis factor α secretion in a dose-dependent fashion. These results suggest that cross-linking of Fcα receptors on human monocytes induces the messenger RNA accumulation and the secretion of the cytotoxic and immunoregulatory cytokine tumor necrosis factor α. Furthermore, it is shown that lipopolysaccharide-induced tumor necrosis factor α secretion by peripheral blood mononuclear cells is synergistically enhanced in the presence of solid phase monomeric IgA but not in the presence of either soluble monomeric or polymeric IgA. Although increased lipopolysaccharide-induced tumor necrosis factor α secretion is observed at baseline in alcoholic cirrhotic patients, this synergism is also expressed in this group of patients. These observations could be of pathophysiological relevance in alcoholic cirrhosis because monomeric IgA deposits along the liver sinusoids and increased serum levels of polymeric IgA are common even in the early stages of this disease. (HEPATOLOGY 1991;13:670–675.)

IgA triggers tumor necrosis factor α secretion by monocytes: A study in normal subjects and patients with alcoholic cirrhosis

Under endotoxin-free conditions, peripheral blood mononuclear cells and purified monocytes isolated from healthy control subjects and patients with alcoholic cirrhosis disclose elevated tumor necrosis factor alpha messenger RNA level and produce tumor necrosis factor alpha in response to stimulation by either soluble polymeric IgA or monomeric IgA bound to the surface of culture dishes but not by soluble monomeric IgA. Polymeric IgA induces tumor necrosis factor alpha secretion in a dose-dependent fashion. These results suggest that cross-linking of Fc alpha receptors on human monocytes induces the messenger RNA accumulation and the secretion of the cytotoxic and immunoregulatory cytokine tumor necrosis factor alpha. Furthermore, it is shown that lipopolysaccharide-induced tumor necrosis factor alpha secretion by peripheral blood mononuclear cells is synergistically enhanced in the presence of solid phase monomeric IgA but not in the presence of either soluble monomeric or polymeric IgA. Although increased lipopolysaccharide-induced tumor necrosis factor alpha secretion is observed at baseline in alcoholic cirrhotic patients, this synergism is also expressed in this group of patients. These observations could be of pathophysiological relevance in alcoholic cirrhosis because monomeric IgA deposits along the liver sinusoids and increased serum levels of polymeric IgA are common even in the early stages of this disease.

In vivo response of secretory component in the rat uterus to antigen, IFN-gamma, and estradiol.

Intrauterine immunization of ovariectomized rats with SRBC is known to elicit pronounced IgA and IgG antibody responses in uterine secretions of immunized uteri. To determine whether secretory component (SC), the receptor for transporting polymeric IgA from tissues to mucosal surfaces, was also influenced by Ag, ovariectomized rats were immunized and boosted by placing SRBC into the lumena of individual uterine horns. In response to Ag, the levels of polymeric IgA, as well as free SC and SC bound to polymeric IgA, increased in uterine secretions. When ovariectomized animals were treated with estradiol, a fivefold increase in SC levels was observed in the immunized horns, indicating that a hormone response is superimposed on the Ag-induced stimulation of uterine SC. To determine whether IFN-gamma influences the presence of SC in uterine secretions, IFN-gamma was placed in the uterine lumena of ovariectomized nonimmunized rats. When uterine secretions were analyzed, significantly higher levels of SC were found in IFN-gamma-exposed uteri than were present in saline treated control animals. In contrast, intrauterine instillation of IFN-gamma had no effect on the levels of IgA in uterine secretions. This response was specific for IFN-gamma in that IFN-alpha/beta had no effect on uterine SC or IgA levels. These results indicate that intrauterine instillation of Ag, in addition to evoking pronounced antibody responses, stimulates the production of SC, which may be responsible for the transport of polymeric IgA from tissue to uterine secretions. Furthermore, they indicate that IFN-gamma placed in the uterine lumen stimulates SC production and suggest that the uterine SC response to Ag may be mediated by the action of IFN-gamma on uterine epithelial cells.

Passive transfer of local immunity to influenza virus infection by IgA antibody.

Secretory IgA is presumed to be the mediator of mucosal immunity based on many studies that show a correlation between protection and secretory IgA titers; however, a causal relationship has not yet been established. Classically, passive transfer of antibody has been used to demonstrate causality, but the passive transfer of local immunity with physiologically transported IgA has not been previously reported. In this study mice were injected intravenously with polymeric IgA (pIgA), monomeric IgA (mIgA), or IgG1 mAb specific for the H1 hemaglutinin of PR8 influenza virus. pIgA was shown to be specifically transported into nasal secretions relative to the mIg. The transported pIgA was functional, as evidenced by its ability to bind to virus in an ELISA assay and to protect nonimmune mice against intranasal infection with H1N1 but not H3N2 influenza virus. Intravenous injection of similar virus-neutralizing doses of anti-influenza IgG1 mAb did not protect against nasal viral challenge. IgA-mediated protection could be abrogated by the intranasal administration of antiserum against the alpha chain of IgA. These data demonstrate the passive transfer of local immunity by the i.v. administration of pIgA antibody and show that the IgA in secretions can protect against influenza virus infection. This general approach could provide a model for the evaluation of the role of local IgA in host defense against other pathogens.

Evidence that the interaction between circulating IgA and fibronectin is a normal process enhanced in primary IgA nephropathy

A solid-phase ELISA was set up to measure the direct binding capacity (BC) of different, commercially available, purified human IgA preparations to plates coated with human fibronectin (FN). It was found that secretory, polymeric, and, to a much lesser extent, monomeric IgA exhibited elevated FN-BC as compared to their BC to plates coated with bovine serum albumin. This binding was specific since not observed with human IgG or IgM antibodies. In addition, we noted that this interaction was dose dependent, Ca2+ dependent, saturable, and not covalent, was inhibited by soluble FN, but not by a prior incubation of FN-coated plates with anti-human fibronectin antibodies, and appeared to involve on the dimeric FN other structures than its heparin-binding, collagen-binding, or C1q-binding domains. Similar experiments conducted with normal plasma indicated that plasma IgA, but not plasma IgG or IgM, was also capable of significant binding to FN-coated plates. In contrast, serum IgA did not significantly bind to those plates under otherwise identical experimental conditions. Thus, the coagulation process induces a strong decrease in the FN-BC of circulating IgA, which implies the necessity of using plasma rather than serum to study such interactions. The apparent molecular weight of plasma IgA interacting with FN-coated plates ranged between 450 and 900 kd, and its major binding characteristics were quite similar to those observed with purified polymeric IgA. The FN-BC of plasma IgA was then measured by the same ELISA in 30 patients with primary IgA nephropathy (IgAN) and in 23 healthy controls. The mean FN-BC of plasma IgA was significantly higher in patients than in normal controls. This enhancement was due mainly to the augmentation in the concentration of circulating "macromolecular" IgA and was significantly correlated with the plasma levels of IgA-FN complexes. However, the pathogenetic role of these findings was probably not determinant since similar observations were made in alcoholic liver cirrhosis without urinary abnormalities and since the FN-BC of plasma IgA or the plasma levels of IgA-FN complexes were not correlated with the various biological parameters of evolutivity of primary IgAN. In conclusion, these studies suggest that the ability of polymeric IgA to directly bind to FN is involved in the formation of circulating IgA-FN complexes and that this normal binding process, although enhanced in IgAN, is probably not responsible for kidney injury, at least in the patients studied.

Transcytotic vesicular carriers for polymeric iga receptors accumulate in rat hepatocytes after bile duct ligation

In rat hepatocytes, transcytotic vesicular carriers transport the mature 120 x 10(3) Mr form of the polymeric IgA receptor (pIgA-R), with or without its ligand, pIgA, from the sinusoidal to the biliary plasmalemma, where the ectodomain of the receptor is cleaved to produce an 80 x 10(3) Mr fragment that is secreted into the bile. Here we show that cholestasis induced by bile duct ligation results in the accumulation of transcytotic carriers, identified by the 120 x 10(3) Mr pIgA-R and pIgA, in the pericanalicular cytoplasm of hepatocytes. To determine the extent of pIgA-R accumulation, hepatic total microsomes (TM) were prepared from control and cholestatic rats. Solubilized TM proteins were separated by SDS-PAGE and receptor forms were detected by immunoblotting and autoradiography. Quantitative densitometry of these autoradiograms showed that after duct ligation the 120 x 10(3) Mr receptor accumulated to a level approximately threefold higher than the control. Concomitantly, immunologically related, novel 124, 90 and 80 x 10(3) Mr proteins (cholestatic antigens) became detectable. Immunoblot analyses of biliary and serum proteins showed that cholestasis resulted in: (1) a marked decrease in the concentrations of the 80 x 10(3) Mr receptor and pIgA in the bile, whereas albumin concentrations remained at control levels; and (2) a marked increase in the concentration of the 80 x 10(3) Mr receptor in the serum. Positive sites for pIgA-R were localized to the pericanalicular cytoplasm of hepatocytes by indirect immunofluorescence on semithin frozen sections in cholestatic hepatocytes. The sites were more numerous and the positive signal stronger than in controls. One day post-ligation, pIgA-positive sites were located to the same pericanalicular cytoplasm of hepatocytes; by three days, however, most pIgA appeared in sinusoidal endothelia and Kupffer cells. To validate the vesicular character of the receptor-positive sites, sham-operated and cholestatic livers were processed for either transmission electron microscopy (TEM) or immunogold localization of receptors on thin frozen sections. TEM verified the accumulation of pericanalicular vesicles in cholestatic hepatocytes. Immunogold tests localized pIgA-R to pleiomorphic, pericanalicular vesicles, which were increased in number, size and concentration of antigenic sites in cholestatic hepatocytes. These findings indicate that bile duct ligation provides a method for manipulating the in vivo transcytotic pathway and for accumulating previously unstudied transcytotic carriers in hepatocytes.

Immunoglobulin A: Interaction with Complement, Phagocytic Cells and Endothelial Cells

Deposits of IgA together with complement (C) in different organs support the hypothesis that IgA can trigger inflammatory mechanisms. Some inflammatory mechanisms may be caused by activation of C and phagocytic cells. Therefore, it is essential to understand the interaction of IgA with C and phagocytic cells. Studies will be described demonstrating that polymeric human serum IgA is able to activate the alternative pathway of C and that the activating principle is located in the intact F(ab')2 portion of the molecule. Activation of C is dependent on the molecular composition of IgA, as derived from results obtained with rat monoclonal IgA antibodies. Furthermore, it is demonstrated that polymeric IgA (pIgA) and dimeric IgA (dIgA) are potent activators of C in a homologous rat model, whereas monomeric IgA (mIgA) has a very poor C-activating potential. The interaction of IgA with phagocytic cells induces phagocytosis and release of H2O2 by granulocytes, which may contribute to tissue damage. Little is known about the clearance mechanism of IgA. It is shown in this report that Kupffer cells and C play an important role in the clearance of IgA immune complexes (IC). Clearance of large-sized IgA IC occurs via different receptors present on Kupffer cells. Finally, a new aspect will be described: the interaction of IgA with endothelial cells. Rat liver endothelial cells are able to eliminate IgA IC from the circulation via specific receptors when no Kupffer cells are present. These observations may contribute to our knowledge on diseases such as IgA nephropathy and Henoch-Schönlein purpura. The studies summarized and presented here illustrate the inflammatory potential of IgA.

<I>Chlamydia trachomatis</I>に対する血清IgA抗体のmonomer/polymer比

Antibody titers of monomeric and polymeric types of serum specific IgA for Chlamydia trachomatis were measured by indirect immunoperoxidase assay (Savyon kit) in male and female cases with various chlamydial urogenital infections. From these results, the ratio of monomeric to polymeric IgA (m/p ratio) was determined. All cases were positive for specific IgA and IgG antibodies for C. trachomatis, and the antigen was also detected in all of cases except for those with prostatitis. 1. Study in males: The m/p ratio (mean +/- S.E.) was 2.6 +/- 1.0 in acute chlamydial urethritis and 8.0 +/- 2.2 in chronic non-bacterial prostatitis. The result indicated monomeric IgA-predominance in the chronic stage. 2. Study in females: The m/p ratio was 5.0 +/- 1.9 in subacute chlamydial cervicitis, while it was 8.5 +/- 2.9 in pregnant women considered to have chronic chlamydial infection. The rate was 31.5 +/- 16.8 in prostitutes considered to have repeated chlamydial infections. The result suggested that monomeric IgA was predominant in patients with chronic and repeated infections. 3. As for sequential changes of IgA antibody titer, polymeric IgA alone decreased after treatment of acute chlamydial urethritis. However monomeric IgA decreased in chronic infection such as prostatitis, and a similar change was shown in IgG antibody. 4. These results suggest that polymeric IgA is predominant in the acute stage of chlamydial infections, while monomeric IgA predominates in the chronic stage.

Antiglomerular Basement Membrane Nephritis Induced by IgA1 Antibodies

A 62-year-old man was admitted for acute renal failure due to crescentic rapidly progressive glomerulonephritis. Kidney biopsy showed intense staining for IgA, IgA1 and C3 in a linear pattern along the glomerular basement membrane (GBM). Serologic tests for detecting anti-GBM antibodies were positive for IgA class and IgA1 subclass and negative for IgG and IgA2. Serum IgA anti-GBM antibodies appeared essentially in the polymeric form, an observation in agreement with recent studies demonstrating polymeric IgA response after parenteral immunization. There was no sign of pulmonary involvement. This patient is the first to present with an isolated nephritis due to anti-GBM antibodies restricted to the IgA class (IgA1 subclass). Routine tests for circulating anti-GBM antibodies should also include the IgA class.

Polymeric IgA antibody to gliadin in the serum of patients with coeliac disease

Secretory component (SC) binding assays which detect polymeric IgA (pIgA) in serum were used to measure serum antigliadin pIgA and total pIgA in patients with coeliac disease. Total IgA antigliadin antibody in serum and intestinal fluid was measured by enzyme linked immunosorbent assay (ELISA). The relationship of pIgA antibody to dietary gluten and the antigliadin IgA antibody in intestinal fluid was examined. Twenty-nine serum samples were assayed, twelve from patients ingesting gluten and seventeen from patients who had excluded gluten from their diet for 6 months. Eight of these were paired samples from 4 adults on and off gluten. In addition, paired samples of both intestinal fluid and serum were obtained from 7 children on and off gluten. Polymeric IgA antibody to gliadin was detected in 11 of 12 subjects on gluten but in only 3 of 17 who had excluded gluten. Three of the four adults from whom paired serum samples were obtained had pIgA antigliadin, but only while on gluten. Three of the seven children in whom intestinal and serum antibody were assayed had pIgA to gliadin, which could not be detected after exclusion of gluten, although their intestinal antibody level remained elevated. There was no change in total pIgA levels with diet although the levels were higher than those seen in normal subjects. We conclude that pIgA antibody to gliadin is frequently found in the serum of coeliac patients ingesting gluten. It disappears with gluten elimination at a time when the IgA antigliadin antibody in intestinal fluid has not altered.

Site of Catabolism of Autologous and Heterologous IgA in Non‐Human Primates

Because of similarities between the human and monkey immune systems, we considered the monkey a suitable model for studies on the catabolism of various molecular forms of IgA, for which little information is available. The residualizing label dilactitol-[125I]tyramine was coupled to monkey (Macaca fuscata) IgA and IgG, as well as to human monomeric and polymeric myeloma IgA1 and IgA2 proteins. When labelled proteins were injected intravenously into monkeys, the non-metabolizable radioiodinated tracer accumulated at the cellular site of protein degradation, allowing identification of the catabolic sites. To determine the uptake of injected proteins by various tissues, monkeys were sacrificed 6-7 days after injection of labelled proteins, when blood-associated radioactivity was less than or equal to 10% of the injected dose, as measured by plasma clearance. When monkey or human monomeric IgA, as well as human polymeric IgA, irrespective of subclass, was administered to monkeys, the liver showed the greatest tissue uptake relative to total dose injected and to organ weight, and the highest acid soluble radioactivity (degraded protein). Although both hepatocytes and non-parenchymal liver cells were involved in IgA uptake, the hepatocytes were more active. Therefore, it appears that the liver is the major site of uptake and catabolism of IgA in monkeys and possibly in humans.

Jejunal immunoglobulin and antigliadin antibody secretion in adult coeliac disease.

We compared the local intestinal immunoglobulin (Ig) secretion in six adult patients with coeliac disease and nine control subjects by perfusion of a small bowel segment under an occluding balloon and analysis of the perfusion fluid for the content of Ig and secretory component. The results were compared to the number of Ig-containing plasma cells in the test segment. There was, respectively, a two-fold and a fivefold increase in jejunal secretion rates of IgA (both monomeric and polymeric) and IgM in patients with coeliac disease compared with control subjects. The high IgA and IgM secretion rates parallel the increase of Ig-containing plasma cells in the lamina propria. In contrast, the IgG plasma cell density increase was barely significant in patients with coeliac disease and did not result in a high IgG secretion rate. The jejunal secretion rate of secretory component was significantly increased in patients with coeliac disease and no free dimeric IgA was present in the jejunal fluid. Antigliadin-IgA was detected in the serum and jejunal fluid of the six patients with coeliac disease. Antigliadin-IgA, however, was almost entirely polymeric IgA linked to secretory component in jejunal fluid, whereas 61% was dimeric IgA not linked to secretory component in serum. This result, combined with a raised secretory component secretion rate with no evidence of secretory component saturation, suggests that serum and intestinal antigliadin IgA might be of different origins in coeliac disease.

The role of IgA in local immune protection

The secretions that bathe the mucosal surfaces contain an array of host defense factors that differ in composition and concentration from those in the serum. Most of the immunoglobulins in external secretions exist as polymers of the basic four-chain Ig units; IgA is the major isotype. The unique cellular pathway that results in the production of IgA by plasma cells at the local mucosal level is linked by proximity to an epithelial mechanism that selectively transports polymeric IgA (as well as IgM) into the secretions and converts them into secretory immunoglobulins (SIgA and SIgM). Because of their unique structure, these secretory immunoglobulins are well adapted to function, along with the other secretory immune factors, in the external secretions. The rate of SIgA production may also be regulated at the level of the epithelial cell.

Exocytic transport vesicles generated in vitro from the trans-Golgi network carry secretory and plasma membrane proteins.

We have developed a cell-free assay that reproduces vesicular budding during exit from the Golgi complex. The starting preparation for the in vitro system was a rat liver stacked Golgi fraction immobilized on a magnetic solid support by means of an antibody against the cytoplasmic domain of the polymeric IgA receptor. Vesicular budding was ATP, cytosol, and temperature dependent and was inhibited by 1 mM N-ethylmaleimide. Budding was maximum within 10 min and originated preferentially from the trans-Golgi. Exocytic transport vesicles immunoisolated from the total budded population were enriched in the mature forms of secretory and membrane proteins destined to the basolateral plasma membrane and were depleted in lysosomal enzymes and galactosyl-transferase activity. The finding that a major proportion (greater than 70%) of newly synthesized, siaylated secretory and transmembrane proteins is contained in a single population of post-Golgi transport vesicles implies that, in a constitutively secreting cell, basolaterally destined proteins are sorted and packaged together into the same exocytic transport vesicles.