The polymeric immunoglobulin receptor. A model protein to study transcytosis

Understanding Fc Receptor Involvement in Inflammatory Diseases: From Mechanisms to New Therapeutic Tools

Fc receptors (FcRs) belong to the ITAM-associated receptor family. FcRs control the humoral and innate immunity which are essential for appropriate responses to infections and prevention of chronic inflammation or auto-immune diseases. Following their crosslinking by immune complexes, FcRs play various roles such as modulation of the immune response by released cytokines or of phagocytosis. Here, we review FcR involvement in pathologies leading notably to altered intracellular signaling with functionally relevant consequences to the host, and targeting of Fc receptors as therapeutic approaches. Special emphasis will be given to some FcRs, such as the FcαRI, the FcγRIIA and the FcγRIIIA, which behave like the ancient god Janus depending on the ITAM motif to inhibit or activate immune responses depending on their targeting by monomeric/dimeric immunoglobulins or by immune complexes. This ITAM duality has been recently defined as inhibitory or activating ITAM (ITAMi or ITAMa) which are controlled by Src family kinases. Involvement of various ITAM-bearing FcRs observed during infectious or autoimmune diseases is associated with allelic variants, changes in ligand binding ability responsible for host defense perturbation. During auto-immune diseases such as rheumatoid arthritis, lupus or immune thrombocytopenia, the autoantibodies and immune complexes lead to inflammation through FcR aggregation. We will discuss the role of FcRs in autoimmune diseases, and focus on novel approaches to target FcRs for resolution of antibody-mediated autoimmunity. We will finally also discuss the down-regulation of FcR functionality as a therapeutic approach for autoimmune diseases.

PIP5KIβ selectively modulates apical endocytosis in polarized renal epithelial cells

Localized synthesis of phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P₂] at clathrin coated pits (CCPs) is crucial for the recruitment of adaptors and other components of the internalization machinery, as well as for regulating actin dynamics during endocytosis. PtdIns(4,5)P₂ is synthesized from phosphatidylinositol 4-phosphate by any of three phosphatidylinositol 5-kinase type I (PIP5KI) isoforms (α, β or γ). PIP5KIβ localizes almost exclusively to the apical surface in polarized mouse cortical collecting duct cells, whereas the other isoforms have a less polarized membrane distribution. We therefore investigated the role of PIP5KI isoforms in endocytosis at the apical and basolateral domains. Endocytosis at the apical surface is known to occur more slowly than at the basolateral surface. Apical endocytosis was selectively stimulated by overexpression of PIP5KIβ whereas the other isoforms had no effect on either apical or basolateral internalization. We found no difference in the affinity for PtdIns(4,5)P₂-containing liposomes of the PtdIns(4,5)P₂ binding domains of epsin and Dab2, consistent with a generic effect of elevated PtdIns(4,5)P₂ on apical endocytosis. Additionally, using apical total internal reflection fluorescence imaging and electron microscopy we found that cells overexpressing PIP5KIβ have fewer apical CCPs but more internalized coated structures than control cells, consistent with enhanced maturation of apical CCPs. Together, our results suggest that synthesis of PtdIns(4,5)P₂ mediated by PIP5KIβ is rate limiting for apical but not basolateral endocytosis in polarized kidney cells. PtdIns(4,5)P₂ may be required to overcome specific structural constraints that limit the efficiency of apical endocytosis.

The epithelial sodium channel (ENaC) establishes a trafficking vesicle pool responsible for its regulation

The epithelial sodium channel (ENaC) is the rate-limiting step for sodium reabsorption across tight epithelia. Cyclic-AMP (cAMP) stimulation promotes ENaC trafficking to the apical surface to increase channel number and transcellular Na(+) transport. Removal of corticosteroid supplementation in a cultured cortical collecting duct cell line reduced ENaC expression. Concurrently, the number of vesicles trafficked in response to cAMP stimulation, as measured by a change in membrane capacitance, also decreased. Stimulation with aldosterone restored both the basal and cAMP-stimulated ENaC activity and increased the number of exocytosed vesicles. Knocking down ENaC directly decreased both the cAMP-stimulated short-circuit current and capacitance response in the presence of aldosterone. However, constitutive apical recycling of the Immunoglobulin A receptor was unaffected by alterations in ENaC expression or trafficking. Fischer Rat Thyroid cells, transfected with α,β,γ-mENaC had a significantly greater membrane capacitance response to cAMP stimulation compared to non-ENaC controls. Finally, immunofluorescent labeling and quantitation revealed a smaller number of vesicles in cells where ENaC expression was reduced. These findings indicate that ENaC is not a passive passenger in regulated epithelial vesicle trafficking, but plays a role in establishing and maintaining the pool of vesicles that respond to cAMP stimulation.

Intestinal epithelial cells and their role in innate mucosal immunity

The mucosal surfaces of the respiratory, gastrointestinal and urogenital tracts are covered by a layer of epithelial cells that are responsible for sensing and promoting a host immune response in order to establish the limits not only for commensal microorganisms but also for foreign organisms or particles. This is a remarkable task as the human body represents a composite of about 10 trillion human-self cells plus non-self cells from autochthonous or indigenous microbes that outnumber human cells 10:1. Hence, the homeostasis of epithelial cells that line mucosal surfaces relies on a fine-tuned immune system that patrols the boundaries between human and microbial cells. In the case of the intestine, the epithelial layer is composed of at least six epithelial cell lineages that act as a physiological barrier in addition to aiding digestion and the absorption of nutrients, water and electrolytes. In this review, we highlight the immense role of the intestinal epithelium in coordinating the mucosal innate immune response.

Comparison of FcRn- and pIgR-mediated transport in MDCK cells by fluorescence confocal microscopy

Protein delivery across polarized epithelia is controlled by receptor-mediated transcytosis. Many studies have examined basolateral-to-apical trafficking of polymeric IgA (pIgA) by the polymeric immunoglobulin receptor (pIgR). Less is known about apical-to-basolateral transcytosis, the direction the neonatal Fc receptor (FcRn) transports maternal IgGs across intestinal epithelia. To compare apical-to-basolateral and basolateral-to-apical transcytosis, we co-expressed FcRn and pIgR in Madin-Darby canine kidney (MDCK) cells and used pulse-chase experiments with confocal microscopy to examine transport of apically applied IgG Fcgamma and basolaterally applied pIgA. Fcgamma and pIgA trafficking routes were initially separate but intermixed at later chase times. Fcgamma was first localized near the apical surface, but became more equally distributed across the cell, consistent with concomitant transcytosis and recycling. By contrast, pIgA transport was strongly unidirectional: pIgA shifted from near the basolateral surface to an apical location with increasing time. Some Fcgamma and pIgA fluorescence colocalized in early (EEA1-positive), recycling (Rab11a-positive), and transferrin (Tf)-positive common/basolateral recycling endosomes. Fcgamma became more enriched in Tf-positive endosomes with time, whereas pIgA was sorted from these compartments. Live-cell imaging revealed that vesicles containing Fcgamma or pIgA shared similar mobility characteristics and were equivalently affected by depolymerizing microtubules, indicating that both trafficking routes depended to roughly the same extent on intact microtubules.

GP41-specific antibody blocks cell-free HIV type 1 transcytosis through human rectal mucosa and model colonic epithelium

Monostratified epithelial cells translocate HIV type 1 (HIV-1) from the apical to the basolateral surface via vesicular transcytosis. Because acutely transmitted HIV-1 is almost exclusively CCR5-tropic and human intestinal epithelial cells preferentially transcytose CCR5-tropic virus, we established epithelial monolayers using polarized HT-29 cells transduced to express CCR5, and an explant system using normal human rectal mucosa, to characterize biological parameters of epithelial cell transcytosis of HIV-1 and assess antiviral Ab blockade of transcytosis. The amount of cell-free HIV-1 transcytosed through the epithelial monolayer increased linearly in relation to the amount of virus applied to the apical surface, indicating transcytosis efficiency was constant (r(2) = 0.9846; p < 0.0001). The efficiency of HIV-1 transcytosis ranged between 0.05 and 1.21%, depending on the virus strain, producer cell type and gp120 V1-V3 loop signature. Inoculation of HIV-1 neutralizing Abs to the immunodominant region (7B2) or the conserved membrane proximal external region (2F5) of gp41 or to cardiolipin (IS4) onto the apical surface of epithelial monolayers prior to inoculation of virus significantly reduced HIV-1 transcytosis. 2F5 was the most potent of these IgG1 Abs. Dimeric IgA and monomeric IgA, but not polymeric IgM, 2F5 Abs also blocked HIV-1 transcytosis across the epithelium and, importantly, across explanted normal human rectal mucosa, with monomeric IgA substantially more potent than dimeric IgA in effecting transcytosis blockade. These findings underscore the potential role of transcytosis blockade in the prevention of HIV-1 transmission across columnar epithelium such as that of the rectum.

HIV-1 gp41-specific monoclonal mucosal IgAs derived from highly exposed but IgG-seronegative individuals block HIV-1 epithelial transcytosis and neutralize CD4(+) cell infection: an IgA gene and functional analysis

AIDS is mainly a sexually transmitted disease, and accordingly, mucosal tissues are the primary sites of natural human immunodeficiency virus type-1 (HIV-1) transmission. Mucosal immunoglobulin A (IgA) antibody specific for HIV-1 envelope gp41 subunit is one correlate of protection in individuals who are highly sexually exposed to HIV-1 but remain persistently IgG seronegative (HEPS). Understanding these peculiar IgAs at the gene and functional level is possible only with monoclonal IgAs. We have constructed a mucosal Fab IgA library from HEPS and have characterized a series of HIV-1 IgAs specific for gp41 that, in vitro, are transcytosis-blocking and infection-neutralizing. Characterization of their IgA genes shows that Fab specific for the gp41 membrane-proximal region harbors a long heavy-chain CDR3 loop (CDRH3) similar to the two broadly neutralizing IgG monoclonal antibodies, 2F5 and 4E10. Furthermore, the selected Fab IgA shows extensive somatic mutations that cluster in the CDR regions, indicating that affinity maturation due to an antigen-driven process had occurred in HEPS individuals, presumably upon multiple exposures to HIV. This analysis of HEPS monoclonal IgA gives a unique opportunity to correlate an antibody function (resistance to a pathogen in vivo) with an antibody gene. Such neutralizing monoclonal IgAs could be used in microbicide formulation.

Transcytosis of polymeric immunoglobulin a in polarized Madin-Darby canine kidney cells

The transcytotic pathway allows for the bidirectional transport of endocytosed solutes, lipids, and proteins between the two membrane domains of polarized epithelial cells while maintaining the functional integrity of the epithelial tissue. A method is described to measure basolateral-to-apical transcytosis of immunoglobulin A (IgA) in polarized Madin-Darby canine kidney (MDCK) cells expressing the polymeric immunoglobulin receptor (pIgR). The cells are grown on porous Transwell filter supports, and radiolabeled (125)I-immunoglobulin A (IgA) is internalized from the basolateral pole of MDCK cells. During a subsequent 2-h chase, the amount of (125)I-IgA that is recycled, degraded, or transcytosed is quantified. This assay can be adapted to follow the postendocytic fate of other (125)I-labeled ligands and proteins.

The IgA system: a comparison of structure and function in different species

The predominant immunoglobulin isotype on most mucosal surfaces is secretory immunoglobulin A (SIgA), a polypeptide complex comprising two IgA monomers, the connecting J chain, and the secretory component. The molecular stability and strong anti-inflammatory properties make SIgA particularly well suited to provide protective immunity to the vulnerable mucosal surfaces by preventing invasion of inhaled and ingested pathogens. In contrast to SIgA, IgA in serum functions as an inflammatory antibody through interaction with FcalphaR on immune effector cells. Although IgA appears to share common features and protective functions in different species, significant variations exist within the IgA systems of different species. This review will give an overview of the basic concepts underlying mucosal IgA defence which will focus on the variations present among species in structure, antibody repertoire development, pIgR-mediated transport, colostral IgA content, hepatobiliary transport, and function with particular emphasis on the IgA system of the pig and dog. These interspecies variations emphasise the importance of elucidating and analysing the IgA system within the immune system of the species of interest rather than inferring roles from conclusions made in human and mouse studies.

Pyelonephritic Escherichia coli expressing P fimbriae decrease immune response of the mouse kidney

P fimbriae are proteinaceous appendages on the surface of Escherichia coli bacteria that mediate adherence to uroepithelial cells. E. coli that express P fimbriae account for the majority of ascending urinary tract infections in women with normal urinary tracts. The hypothesis that P fimbriae on uropathic E. coli attach to renal epithelia and may regulate the immune response to establish infection was investigated. The polymeric Ig receptor (pIgR), produced by renal epithelia, transports IgA into the urinary space. Kidney pIgR and urine IgA levels were analyzed in a mouse model of ascending pyelonephritis, using E. coli with (P+) and without (P-) P fimbriae, to determine whether P(+) E. coli regulate epithelial pIgR expression and IgA transport into the urine. (P+) E. coli establish infection and persist to a greater amount than P(-) E. coli. P(+)-infected mice downregulate pIgR mRNA and protein levels compared with P(-)-infected or PBS controls at > or =48 h. The decrease in pIgR was associated with decreased urinary IgA levels in the P(+)-infected group at 48 h. pIgR mRNA and protein also decline in P(+) E. coli-infected LPS-hyporesponsive mice. These studies identify a novel virulence mechanism of E. coli that express P fimbriae. It is proposed that P fimbriae decrease pIgR expression in the kidney and consequently decrease IgA transport into the urinary space. This may explain, in part, how E. coli that bear P fimbriae exploit the immune system of human hosts to establish ascending pyelonephritis.

N-glycosylation and microtubule integrity are involved in apical targeting of prostate-specific membrane antigen: implications for immunotherapy

Prostate-specific membrane antigen (PSMA) is an important biomarker expressed in prostate cancer cells with levels proportional to tumor grade. The membrane association and correlation with disease stage portend a promising role for PSMA as an antigenic target for antibody-based therapies. Successful application of such modalities necessitates a detailed knowledge of the subcellular localization and trafficking of target antigen. In this study, we show that PSMA is expressed predominantly in the apical plasma membrane in epithelial cells of the prostate gland and in well-differentiated Madin-Darby canine kidney cells. We show that PSMA is targeted directly to the apical surface and that sorting into appropriate post-Golgi vesicles is dependent upon N-glycosylation of the protein. Integrity of the microtubule cytoskeleton is also essential for delivery and retention of PSMA at the apical plasma membrane domain, as destabilization of microtubules with nocodazole or commonly used chemotherapeutic Vinca alkaloids resulted in the basolateral expression of PSMA and increased the uptake of anti-PSMA antibody from the basolateral domain. These results may have important relevance to PSMA-based immunotherapy and imaging strategies, as prostate cancer cells can maintain a well-differentiated morphology even after metastasis to distal sites. In contrast to antigens on the basolateral surface, apical antigens are separated from the circulation by tight junctions that restrict transport of molecules across the epithelium. Thus, antigens expressed on the apical plasma membrane are not exposed to intravenously administered agents. The ability to reverse the polarity of PSMA from apical to basolateral could have significant implications for the use of PSMA as a therapeutic target.

Membrane dynamics and the regulation of epithelial cell polarity

Plasma membranes of epithelial cells consist of two domains, an apical and a basolateral domain, the surfaces of which differ in composition. The separation of these domains by a tight junction and the fact that specific transport pathways exist for intracellular communication between these domains and distinct intracellular compartments relevant to cell polarity development, have triggered extensive research on issues that focus on how the polarity is generated and maintained. Apart from proper assembly of tight junctions, their potential functioning as landmark for the transport machinery, cell-cell adhesion is obviously instrumental in barrier formation. In recent years, distinct endocytic compartments, defined as subapical compartment or common endosome, were shown to play a prominent role in regulating membrane trafficking to and from polarized membrane domains. Sorting devices remain to be determined but likely include distinct rab proteins, and evidence is accumulating to indicate that signaling events may direct intracellular membrane transport, intimately involved in the biogenesis and maintenance of polarized membrane domains and hence the development of cell polarity.

Axonal transport and neuronal transcytosis of trophic factors, tracers, and pathogens

Neurons can specifically internalize macromolecules, such as trophic factors, lectins, toxins, and other pathogens. Upon internalization in terminals, proteins can move retrogradely along axons, or, upon internalization at somatodendritic domains, they can move into an anterograde axonal transport pathway. Release of internalized proteins from neurons after either retrograde or anterograde axonal transport results in transcytosis and trafficking of proteins across multiple synapses. Recent studies of binding properties of several such proteins suggest that pathogens and lectins may utilize existing transport machineries designed for trafficking of trophic factors. Specific pathways may protect trophic factors, pathogens, and toxins from degradation after internalization and may target the trophic or pathogenic cargo for transcytosis after either retrograde or anterograde transport along axons. Elucidating the molecular mechanisms of sorting steps and transport pathways will further our understanding of trophic signaling and could be relevant for an understanding and possible treatment of neurological diseases such as rabies, Alzheimer's disease, and prion encephalopathies. At present, our knowledge is remarkably sparse about the types of receptors used by pathogens for trafficking, the signals that sort trophins or pathogens into recycling or degradation pathways, and the mechanisms that regulate their release from somatodendritic domains or axon terminals. This review intends to draw attention to potential convergences and parallels in trafficking of trophic and pathogenic proteins. It discusses axonal transport/trafficking mechanisms that may help to understand and eventually treat neurological diseases by targeted drug delivery.

Recombinant human protein C expression in the milk of transgenic pigs and the effect on endogenous milk immunoglobulin and transferrin levels

Colostrum and milk are natural vehicles for acquiring passive immunity and are valuable tools for decreasing neonatant mortality from diarrheal disease. The effects of recombinant human protein C (rhPC) expression levels on endogenous immunoglobulin and transferrin content of the milk of different lineages of transgenic pigs were studied. The levels of rhPC in the milk ranged from 40 to 1200 microg/ml. Transgenic pigs with rhPC expression levels less than 500 microg/ml had no significant differences in milk protein composition with respect to nontransgenic pigs. A line of transgenic pigs having rhPC expression levels of 960-1200 microg/ml had two- to three-fold higher IgG, IgM, and secretory IgA concentrations compared to other transgenic and nontransgenic pig groups (P < 0.05), and four- to five-fold higher transferrin levels than nontransgenic pigs (P < 0.05). Changes in milk protein composition were not associated with mastitis or other pathologic disruption of epithelial cell junctions as indicated by normal casein and albumin levels in milk. Since IgG, IgM, secretory IgA, and transferrin are transported into the milk by transcytosis, higher levels of these proteins indicate that transcyctosis in the mammary epithelial cell was likely upregulated in pigs having high rhPC expression levels. This study is the first that shows a statistically significant example that mammary tissue specific expression of a heterologous protein can enhance endogenous phenotypic characteristics of milk.

Detergent insoluble microdomains are not involved in transcytosis of polymeric Ig receptor in FRT and MDCK cells

In polarized epithelial cells, sorting of proteins and lipids to the apical or basolateral domain of the plasma membrane can occur via direct or indirect (transcytotic) pathways from the trans Golgi network (TGN). The 'rafts' hypothesis postulates that the key event for direct apical sorting of some transmembrane proteins and the majority of GPI-anchored proteins depends on their association with glycosphingolipid and cholesterol enriched microdomains (rafts). However, the mechanism of indirect sorting to the apical membrane is not clear. The polyimmunoglobulin receptor (pIgR) is one of the best studied proteins that follow the transcytotic pathway. It is normally delivered from the TGN to the basolateral surface of polarized Madin-Darby Canine Kidney (MDCK) cells from where it transports dIgA or dIgM to the apical surface. We have studied the intracellular trafficking of pIgR in Fischer rat thyroid cells (FRT), and have investigated the sorting machinery involved in transcytosis of this receptor in both FRT and MDCK cells. We found that, in contrast with MDCK cells, a significant amount (approximately 30%) of pIgR reaches the apical surface by a direct pathway. Furthermore, in both cell lines it does not associate with Triton X-100 insoluble microdomains, suggesting that at least in these cells 'rafts' are not involved in basolateral to apical transcytosis.

Modulation of endocytic traffic in polarized Madin-Darby canine kidney cells by the small GTPase RhoA

Efficient postendocytic membrane traffic in polarized epithelial cells is thought to be regulated in part by the actin cytoskeleton. RhoA modulates assemblies of actin in the cell, and it has been shown to regulate pinocytosis and phagocytosis; however, its effects on postendocytic traffic are largely unexplored. To this end, we expressed wild-type RhoA (RhoAWT), dominant active RhoA (RhoAV14), and dominant inactive RhoA (RhoAN19) in Madin-Darby canine kidney (MDCK) cells expressing the polymeric immunoglobulin receptor. RhoAV14 expression stimulated the rate of apical and basolateral endocytosis, whereas RhoAN19 expression decreased the rate from both membrane domains. Polarized basolateral recycling of transferrin was disrupted in RhoAV14-expressing cells as a result of increased ligand release at the apical pole of the cell. Degradation of basolaterally internalized epidermal growth factor was slowed in RhoAV14-expressing cells. Although apical recycling of immunoglobulin A (IgA) was largely unaffected in cells expressing RhoAV14, transcytosis of basolaterally internalized IgA was severely impaired. Morphological and biochemical analyses demonstrated that a large proportion of IgA internalized from the basolateral pole of RhoAV14-expressing cells remained within basolateral early endosomes and was slow to exit these compartments. RhoAN19 and RhoAWT expression had little effect on these postendocytic pathways. These results indicate that in polarized MDCK cells activated RhoA may modulate endocytosis from both membrane domains and postendocytic traffic at the basolateral pole of the cell.

Shiga toxins 1 and 2 translocate differently across polarized intestinal epithelial cells

Shiga toxin-producing Escherichia coli (STEC) is an important food-borne pathogen that causes hemolytic-uremic syndrome. Following ingestion, STEC cells colonize the intestine and produce Shiga toxins (Stx), which appear to translocate across the intestinal epithelium and subsequently reach sensitive endothelial cell beds. STEC cells produce one or both of two major toxins, Stx1 and Stx2. Stx2-producing STEC is more often associated with disease for reasons as yet undetermined. In this study, we used polarized intestinal epithelial cells grown on permeable filters as a model to compare Stx1 and Stx2 movement across the intestinal epithelium. We have previously shown that biologically active Stx1 is able to translocate across cell monolayers in an energy-dependent, saturable manner. This study demonstrates that biologically active Stx2 is also capable of movement across the epithelium without affecting barrier function, but significantly less Stx2 crossed monolayers than Stx1. Chilling the monolayers to 4 degrees C reduced the amount of Stx1 and Stx2 movement by 200-fold and 20-fold respectively. Stx1 movement was clearly directional, favoring an apical-to-basolateral translocation, whereas Stx2 movement was not. Colchicine reduced Stx1, but not Stx2, translocation. Monensin reduced the translocation of both toxins, but the effect was more pronounced with Stx1. Brefeldin A had no effect on either toxin. Excess unlabeled Stx1 blocks the movement of (125)I-Stx1. Excess Stx2 failed to have any effect on Stx1 movement. Our data suggests that, despite the many common physical and biochemical properties of the two toxins, they appear to be crossing the epithelial cell barrier by different pathways.

Infectious human immunodeficiency virus can rapidly penetrate a tight human epithelial barrier by transcytosis in a process impaired by mucosal immunoglobulins

Mucosal surfaces are the main natural site of entry into the body for human immunodeficiency virus (HIV). Herein, an alternative mechanism for virus spread is described. The mechanism, which involves transcytosis of endosome-internalized HIV-particles, was generated by contact of HIV-infected cells with the apical surface of an epithelial cell line. Transcytosed viruses rapidly (in 20-30 min) access the serosal side of the epithelial barrier without infecting the epithelium itself. In turn, transcytosed HIV could infect host submucosal mononucleated target cells, and thus the infection could spread. An investigation was done to determine whether mucosal antibodies could block HIV transcytosis. Both secretory IgA (S-IgA) and IgG that were purified from colostrum from HIV-seropositive women impaired HIV transcytosis, irrespective of the level of the recombinant HIV envelope anti-gp160-specific activities in an ELISA. However, specific S-IgAs were more efficient than IgG. Therefore, mucosal-specific S-IgA to HIV-1 could be relevant to reducing infectivity of HIV-1 in corporeal fluids.

Immunocytochemical colocalization of specific immunoglobulin A with sendai virus protein in infected polarized epithelium

Immunoglobulin (Ig)A provides the initial immune barrier to viruses at mucosal surfaces. Specific IgA interrupts viral replication in polarized epithelium during receptor-mediated transport, probably by binding to newly synthesized viral proteins. Here, we demonstrate by immunoelectron microscopy that specific IgA monoclonal antibodies (mAbs) accumulate within Sendai virus-infected polarized cell monolayers and colocalize with the hemagglutinin- neuraminidase (HN) viral protein in a novel intracellular structure. Neither IgG specific for HN nor irrelevant IgA mAbs colocalize with viral protein. Treatment of cultures with viral-specific IgA but not with viral-specific IgG or irrelevant IgA decreases viral titers. These observations provide definitive ultrastructural evidence of a subcellular compartment in which specific IgA and viral envelope proteins interact, further strengthening our hypothesis of intracellular neutralization of virus by specific IgA antibodies. Our results have important implications for intracellular protein trafficking, viral replication, and viral vaccine development.

(Glyco)sphingolipids are sorted in sub-apical compartments in HepG2 cells: a role for non-Golgi-related intracellular sites in the polarized distribution of (glyco)sphingolipids

In polarized HepG2 cells, the fluorescent sphingolipid analogues of glucosylceramide (C6-NBD-GlcCer) and sphingomyelin (C6-NBD-SM) display a preferential localization at the apical and basolateral domain, respectively, which is expressed during apical to basolateral transcytosis of the lipids (van IJzendoorn, S.C.D., M.M. P. Zegers, J.W. Kok, and D. Hoekstra. 1997. J. Cell Biol. 137:347-457). In the present study we have identified a non-Golgi-related, sub-apical compartment (SAC), in which sorting of the lipids occurs. Thus, in the apical to basolateral transcytotic pathway both C6-NBD-GlcCer and C6-NBD-SM accumulate in SAC at 18 degreesC. At this temperature, transcytosing IgA also accumulates, and colocalizes with the lipids. Upon rewarming the cells to 37 degreesC, the lipids are transported from the SAC to their preferred membrane domain. Kinetic evidence is presented that shows in a direct manner that after leaving SAC, sphingomyelin disappears from the apical region of the cell, whereas GlcCer is transferred to the apical, bile canalicular membrane. The sorting event is very specific, as the GlcCer epimer C6-NBD-galactosylceramide, like C6-NBD-SM, is sorted in the SAC and directed to the basolateral surface. It is demonstrated that transport of the lipids to and from SAC is accomplished by a vesicular mechanism, and is in part microtubule dependent. Furthermore, the SAC in HepG2 bear analogy to the apical recycling compartments, previously described in MDCK cells. However, in contrast to the latter, the structural integrity of SAC does not depend on an intact microtubule system. Taken together, we have identified a non-Golgi-related compartment, acting as a "traffic center" in apical to basolateral trafficking and vice versa, and directing the polarized distribution of sphingolipids in hepatic cells.

Human IgGFc binding protein (FcgammaBP) in colonic epithelial cells exhibits mucin-like structure

Cloning a cDNA for human IgGFc binding protein (FcgammaBP) from human colonic epithelial cells reveals an mRNA and coding region of 17 and 16.2 kilobases, respectively. The predicted amino acid sequence contains 12 occurrences of a 400-amino acid cysteine-rich unit resembling that found in mucin. A motif (CGLCGN) in FcgammaBP is conserved in MUC2 and prepro-von Willebrand factor. The N-terminal 450-amino acid sequences are necessary and sufficient to confer IgG Fc binding activity. FcgammaBP mRNA is expressed only in placenta and colonic epithelial cells. These results suggest that FcgammaBP may play an important role in immune protection and inflammation in the intestines of primates.

The mechanism of Epstein-Barr virus infection in nasopharyngeal carcinoma cells

To investigate the relationship between Epstein-Barr virus (EBV) and nasopharyngeal carcinoma (NPC) cells, we examined the pathway of EBV infection in NPC cell lines. We used immunolocalization to investigate the EBV receptor (C3d-R) and polymeric immunoglobulin receptor [secretory component (SC) protein]. We incubated IgA anti-EBV and EBV particles with NPC cells and observed the EBV DNA signal by in situ polymerase chain reaction hybridization and polymerase chain reaction plus Southern blotting. We also colocalized SC protein and EBV RNA in NPC biopsy specimens. Results showed that: 1) NPC cells did not express the EBV receptor but did express SC protein in each line; 2) SC protein was also expressed in some tumor cells but not in untransformed squamous metaplastic epithelia in NPC biopsy specimens; 3) EBV could infect NPC cells through an EBV-IgA and SC complex and retained an EBV viral genome in their nuclei; SC expression could be down-regulated by EBV proteins; and 4) in biopsy specimens, a fraction of tumor cells showed SC protein expression; only a portion of tumor cells contained EBV, and of these cells only a few expressed SC protein. These findings indicate that EBV cannot infect untransformed nasopharyngeal squamous metaplastic epithelia but can enter NPC cells through IgA-mediated endocytosis.

Hepatocyte growth factor alters the polarity of Madin-Darby canine kidney cell monolayers

Hepatocyte growth factor (HGF) and E-cadherin are important for epithelial morphogenetic events. We examined the effects of HGF on E-cadherin localization and interaction with beta-catenin in polarized Madin-Darby canine kidney (MDCK) cell monolayers grown on filters. Surface biotinylation experiments showed that HGF increases apically accessible E-cadherin. Confocal immunofluorescence microscopy of HGF-treated cells showed localization of E-cadherin at membrane domains contacting the apical compartment and an increase in accessibility of apically applied antibodies to lateral E-cadherin below the tight junction. Coimmunoprecipitation of beta-catenin/E-cadherin complexes showed that the amount of E-cadherin associated with beta-catenin increased during the first 24 h of HGF treatment with a return to baseline values after 48 and 72 h. Metabolic labeling showed that HGF increased the synthetic rate of beta-catenin and the amount of newly synthesized E-cadherin associated with immunoprecipitated beta-catenin, with the peak effect occurring after 12 h of treatment and returning to baseline after 24 h. HGF treatment inhibited transcytosis of immunoglobulin A by the polymeric immunoglobulin receptor. We conclude that HGF treatment of polarized MDCK cells grown on filters decreases cell polarity and alters E-cadherin/beta-catenin interaction and synthesis.

Reconstitution of transcytosis in SLO-permeabilized MDCK cells: existence of an NSF-dependent fusion mechanism with the apical surface of MDCK cells

Recently, it was demonstrated that delivery from the trans-Golgi network (TGN) to the basolateral surface of Madin-Darby canine kidney (MDCK) cells required N-ethylmaleimide-sensitive factor (NSF)-alpha soluble NSF attachment protein (SNAP)-SNAP receptor (SNARE) complexes, while delivery from the TGN to the apical surface was independent of NSF-alpha SNAP-SNARE. To determine if all traffic to the apical surface of this cell line was NSF independent, we reconstituted the transcytosis of pre-internalized IgA to the apical surface and recycling to the basolateral surface. Transcytosis and the recycling of IgA required ATP and cytosol, and both were inhibited by treatment with N-ethylmaleimide. This inhibition was reversed by the addition of recombinant NSF. Botulinum neurotoxin serotype E, which is known to cleave the 25,000 Da synaptosomal associated protein, inhibited both transcytosis and recycling, although incompletely. We conclude that membrane traffic to a target membrane is not determined by utilizing a single molecular mechanism for fusion. Rather, a target membrane, e.g. the apical plasma membrane of MDCK cells, may use multiple molecular mechanisms to fuse with incoming vesicle.

Antigen sampling across epithelial barriers and induction of mucosal immune responses

Epithelial barriers on mucosal surfaces at different sites in the body differ dramatically in their cellular organization, and antigen sampling strategies at diverse mucosal sites are adapted accordingly. In stratified and pseudostratified epithelia, dendritic cells migrate to the outer limit of the epithelium, where they sample antigens for subsequent presentation in local or distant organized lymphoid tissues. In simple epithelia, specialized epithelial M cells (a phenotype that occurs only in the epithelium over organized lymphoid follicles) deliver samples of foreign material by transepithelial transport from the lumen to organized lymphoid tissues within the mucosa. Certain pathogens exploit the M cell transport process to cross the epithelial barrier and invade the mucosa. Here we review the features of M cells that determine antigen and pathogen adherence and transport into mucosal lymphoid tissues.

Intracellular routing of wild-type and mutated polymeric immunoglobulin receptor in hippocampal neurons in culture

Certain epithelial cells synthesize the polymeric immunoglobulin receptor (pIgR) to transport immunoglobulins (Igs) A and M into external secretions. In polarized epithelia, newly synthesized receptor is first delivered to the basolateral plasma membrane and is then, after binding the Ig, transcytosed to the apical plasma membrane, where the receptor-ligand complex is released by proteolytic cleavage. In a previous work (Ikonen et al., 1993), we implied the existence of a dendro-axonal transcytotic pathway for the rabbit pIgR expressed in hippocampal neurons via the Semliki Forest Virus (SFV) expression system. By labeling surface-exposed pIgR in live neuronal cells, we now show (a) internalization of the receptor from the dendritic plasma membrane to the dendritic early endosomes, (b) redistribution of the receptor from the dendritic to the axonal domain, (c) inhibition of this movement by brefeldin A (BFA) and (d) stimulation by the activation of protein kinase C (PKC) via phorbol myristate acetate (PMA). In addition, we show that a mutant form of the receptor lacking the epithelial basolateral sorting signal is directly delivered to the axonal domain of hippocampal neurons. Although this mutant is internalized into early endosomes, no transcytosis to the dendrites could be observed. In epithelial Madin-Darby Canine Kidney (MDCK) cells, the mutant receptor could also be internalized into basolaterally derived early endosomes. These results suggest the existence of a dendro-axonal transcytotic pathway in neuronal cells which shares similarities with the basolateral to apical transcytosis in epithelial cells and constitute the basis for the future analysis of its physiological role.

Serum secretory IgA and secretory component in patients with non-cirrhotic alcoholic liver diseases

Elevated levels of secretory IgA in serum have been demonstrated in several liver dysfunctions such as hepatic cytolysis and cholestasis. However, these possible alterations at an early stage of liver diseases have not yet been investigated. We studied a cohort of chronic alcoholic patients without cirrhosis in order to assess the changes in serum secretory IgA and other forms of secretory component, the split product of the polymeric Ig-receptor of epithelial cells. The possible diagnostic value of these measurements in the assessment of alcoholic disease was compared to that of serum gamma-glutamyl transpeptidase activity. Serum levels of secretory IgA and IgM and free secretory component, were quantified by an enzyme-linked immunosorbent assay in 71 patients with chronic alcoholic liver disease without cirrhosis and in 45 healthy controls. Patients were divided into two groups according to the severity of the liver abnormalities. In addition, the reversibility of serum secretory IgA, IgM and free secretory component abnormalities after alcohol withdrawal was evaluated in 15 patients. Serum levels of the three molecular forms of secretory component were significantly higher than those measured in control subjects, both in the whole population of patients and in the two groups of alcoholic patients without cirrhosis. In all groups, serum secretory IgA levels were correlated to free secretory component but not to total IgA levels. Serum secretory IgA levels were as discriminative as gammaglutamyl transferase activity in distinguishing between chronic alcoholic patients without cirrhosis and non-alcoholic subjects. The abnormalities of serum secretory IgA concentrations were reversible after alcohol withdrawal.(ABSTRACT TRUNCATED AT 250 WORDS)

A distinct class of intracellular storage vesicles, identified by expression of the glucose transporter GLUT4

Some cell types have cytoplasmic storage vesicles whose fusion with the cell surface is triggered by an extracellular signal. To explore the relationship between different classes of storage vesicles, we expressed, in the neuro-endocrine cell line PC12, the facilitative glucose transporter GLUT4, which is stored in small cytoplasmic vesicles in fat and muscle cells and mobilized to the cell surface when insulin is present. PC12 cells have two known types of storage vesicles, secretory granules and synaptic vesicles, but GLUT4 is targeted to neither. It is recovered, however, in a class of small vesicles that sediment approximately twice as fast as synaptic vesicles. Immunoelectron microscopy confirmed the presence of such small vesicles in transfected PC12 cells. By velocity sedimentation analysis, GLUT4 vesicles efficiently exclude the synaptic vesicle markers synaptophysin, SV2, and synaptobrevin; the transferrin receptor, a marker of conventional endocytosis; and the polymeric immunoglobulin receptor, a marker of transcytosis. The exclusion of synaptophysin and the transferrin receptor from most of the GLUT4-containing structures was confirmed by confocal immunofluorescence microscopy. Like synaptic vesicles, therefore, GLUT4 vesicles of PC12 cells appear to be a unique type of organelle. A GLUT4-containing organelle of identical sedimentation properties was found in transfected fibroblast cell lines and in rat adipocytes. On stimulation of the adipocytes with insulin, GLUT4 was translocated from the peak of small vesicles to faster sedimenting membranes. We propose that the class of vesicles described here is present in a wide range of cell types and is involved in transient modification of the cell surface.

Mechanisms of cell polarity: sorting and transport in epithelial cells

The recent discovery of widely distributed targeting determinants, which govern the polarized cell-surface distribution of plasma membrane proteins in epithelial cells, has significantly changed our view of how polarized cells generate functionally distinct membrane domains. Together with the surprising finding that the same determinants are recognized on both the biosynthetic and the endocytic pathways, it now appears likely that a common epigenetic code may exist that controls molecular sorting of membrane proteins in a wide variety of polarized, and perhaps even non-polarized, cell types.

Gs alpha stimulates transcytosis and apical secretion in MDCK cells through cAMP and protein kinase A

Recent evidence suggests a role for heterotrimeric G proteins in vesicular transport. Cholera toxin, which activates Gs alpha by ADP-ribosylation, has been reported to stimulate both apical secretion (Pimplikar, S.W., and K. Simons. 1993. Nature (Lond.). 352:456-458) and apically directed transcytosis (Bomsel, M., and K.E. Mostov. 1993. J. Biol. Chem. 268:25824-25835) in MDCK cells, via a cAMP-independent mechanism. Here, we demonstrate that apical secretion and apically directed transcytosis are significantly stimulated by agents that elevate cellular cAMP. Forskolin, which activates adenylyl cyclase directly, and 8BrcAMP augment both transport processes in MDCK cells. The increase is not limited to receptor-mediated transport (polymeric Ig receptor), since transcytosis of ricin, a galactose-binding lectin, is similarly stimulated. The effects of elevated cellular cAMP on apical secretion and transcytosis are apparently mediated via protein kinase A (PKA), as they are inhibited by H-89, a selective PKA inhibitor. Experiments employing a 17 degrees C temperature block indicate that cAMP/PKA acts at a late, possibly rate-limiting stage in the transcytotic pathway, after translocation of internalized markers into the apical cytoplasm. However, no significant stimulus of apical recycling was observed in the presence of FSK, suggesting that cAMP/PKA either affects transcytosis at a level proximal to apical early endosomes and/or specifically increases the efficiency by which transcytosing molecules are delivered to the apical plasma membrane. Finally, we overexpressed wild-type Gs alpha and a mutant, Q227L, which constitutively activates adenylyl cyclase, in MDCK cells. Although Q227L increased transcytosis more than wild-type Gs alpha, neither construct was as effective as FSK in stimulating transcytosis, arguing against a significant role of Gs alpha in transcytosis independent of cAMP and PKA.

Receptor-mediated transcytosis of IgA in MDCK cells is via apical recycling endosomes

Classically, the polymeric immunoglobulin receptor and its ligand, IgA, are thought to be sorted from basolateral early endosomes into transcytotic vesicles that directly fuse with the apical plasma membrane. In contrast, we have found that in MDCK cells IgA is delivered from basolateral endosomes to apical endosomes and only then to the apical cell surface. When internalized from the basolateral surface of MDCK cells IgA is found to accumulate under the apical plasma membrane in a compartment that is accessible to two apically added membrane markers: anti-secretory component Fab fragments, and avidin internalized from the biotinylated apical pole of the cell. This accumulation occurs in the presence of apical trypsin, which prevents internalization of the ligand from the apical cell surface. Using a modification of the diaminobenzidine density-shift assay, we estimate that approximately 80% of basolaterally internalized IgA resides in the apical endosomal compartment. In addition, approximately 50% of basolaterally internalized transferrin, a basolateral recycling protein, has access to this apical endosomal compartment and is efficiently recycled back to the basolateral surface. Microtubules are required for the organization of the apical endosomal compartment and it is dispersed in nocodazole-treated cells. Moreover, this compartment is largely inaccessible to fluid-phase markers added to either pole of the cell, and therefore seems analogous to the recycling endosome described in nonpolarized cells. We propose a model in which transcytosis is not a specialized pathway that uses unique transcytotic vesicles, but rather combines portions of pathways used by non-transcytosing molecules.

The fodrin-ankyrin cytoskeleton of choroid plexus preferentially colocalizes with apical Na+K(+)-ATPase rather than with basolateral anion exchanger AE2

A unique feature of the choroid plexus as a single-layer epithelium is its localization of Na+K(+)-ATPase at its apical (lumenal) surface. In contrast, a band 3 (AE1)-related anion exchanger protein has been localized to the basolateral surface of the choroid plexus. Both Na+K(+)-ATPase and AE1 in other tissues have been shown to bind via ankyrin to the spectrin-actin-based membrane cytoskeleton. Since linkage of integral membrane proteins to the membrane cytoskeleton is important for their restriction to specialized domains of the cell surface, we investigated the polarity of the choroid plexus membrane cytoskeleton. We developed isoform-specific antibodies to confirm the identity of choroid plexus band 3-related polypeptide as AE2. We demonstrated that ankyrin, fodrin/spectrin, actin, myosin, and alpha-actinin are predominantly apical in choroid plexus and preferentially colocalize with apical Na+K(+)-ATPase rather than with basolateral anion exchanger AE2. Colchicine administration did not alter the polarity of apical cytoskeletal and transport proteins or basolateral AE2 in choroid plexus, suggesting that biosynthetic targeting of these proteins is not microtubule dependent. In choroid plexus papilloma, Na+K(+)-ATPase and AE2 were decreased in amount and failed to preserve their polarized distributions.

Stimulation of transcytosis of the polymeric immunoglobulin receptor by dimeric IgA

The polymeric immunoglobulin receptor (pIgR) is transcytosed from the basolateral to the apical surface of polarized epithelial cells. We have previously shown that phosphorylation of Ser-664 in the cytoplasmic domain of the pIgR is a signal for its transcytosis. We now report that binding of a physiological ligand, dimeric IgA, to pIgR stimulates pIgR transcytosis. This stimulation occurs in both the presence or absence of Ser-664 phosphorylation. We have used three methods to measure transcytosis of the pIgR. (i) The pIgR was biosynthetically labeled and its cleavage to secretory component after transcytosis was measured. (ii) The pIgR was labeled with biotin at the basolateral surface. After transcytosis, release of the biotin-labeled secretory component into the apical medium was measured. (iii) Transcytosis of a ligand bound to the pIgR was measured. All three methods indicated that dimeric IgA stimulates transcytosis of the pIgR.

The covalent linkage of secretory component to IgA. Structure of sIgA

Immunoglobulin A which is secreted into external fluids is synthesized in plasma cells as an (IgA)2-J-chain complex. This complex docks on to the polyimmunoglobulin receptor which is located at the basolateral surface of epithelial cells. After docking the (IgA)2-J-receptor complex is internalized and processed. The polyimmunoglobulin receptor loses its C-terminal tail and thus becomes the secretory component. This secretory component is then covalently linked to the (IgA)2-J-chain complex by a disulfide bond, and protects the so formed sIgA from denaturation and proteolysis in external fluids. In order to establish this disulfide bond between IgA and the secretory component, sIgA, purified from human colostrum, was subjected to several enzymatic and chemical fragmentation reactions. One of the resulting polypeptides allowed us to characterize the covalent linkage of the secretory component to IgA in sIgA. IgA was found to be covalently linked to the secretory piece by a single disulfide bond between Cys 311 of one alpha-chain and Cys 467 of the secretory component. Cys 501 of the secretory component and Cys 311 of the other alpha-chain are blocked by cysteines. With this last paper of a series the structure of an entire sIgA molecule has been elucidated.

Expression of 5-HT1C receptors in transfected MDCK cells is functionally and anatomically asymmetric

Madin-Darby canine kidney (MDCK) cells were transfected with the cDNA for the rat 5-HT1C receptor (pMV7-SR1c) using electroporation. Cells that survived G418 selection medium were loaded with indo-1 and run through a fluorescence-activated cell sorter (FACS); 10% responded to serotonin (5-HT) with an increase in intracellular Ca2+ concentration ([Ca2+]i). Responding cells were separated with the FACS, grown to confluence, and resorted two more times until a clone of 100% respondents was obtained (SR-MDCK). In SR-MDCK cells grown on porous filters, [Ca2+]i increased only when 5-HT was applied to the basolateral membrane (change in [Ca2+]i = 190 +/- 43 nM); there was no response of [Ca2+]i to apical application of 5-HT. The asymmetric response to 5-HT was likely due to targeting of 5-HT1C receptors exclusively to the basolateral membrane of SR-MDCK cells; 125I-labeled lysergic acid diethylamide binding sites, a marker of high-affinity 5-HT receptors, were located only in the basolateral membrane. These experiments demonstrate that epithelial cells can be stably transfected to express G protein-linked, calcium-mobilizing receptors and that the receptors may be targeted asymmetrically to specific domains of the plasma membrane.

Hormonal control of intestinal Fc receptor gene expression and immunoglobulin transport in suckling rats

Hormonal control of immunoglobulin (Ig) absorption and of intestinal Fc receptor mRNA expression were investigated in rats to assess its potential role in the normal postsuckling inhibition of this transport system. Corticosterone and L-thyroxine therapy caused premature inhibition of the absorption of orally administered murine monoclonal antibody and of Fc receptor mRNA expression in a dose- and time-dependent manner. Low-dose corticosterone had no effect on Fc receptor mRNA synthesis after 3 d but decreased Ig transport fivefold after 7 d. High dose corticosterone resulted in a threefold reduction in Fc receptor after 3 d, and there was almost complete inhibition (> 30-fold) of transport and of Fc receptor transcript levels after 7 d. Similarly, 7 d of high-dose thyroxine decreased both serum Ig transport and Fc receptor (> 30-fold). However, adrenalectomy did not prevent the normal post-suckling declines in Ig transport or receptor synthesis. This study demonstrates that exogenous corticosteroids and thyroxine hormone inhibit Ig transport and steady-state duodenal Fc receptor mRNA levels in suckling rats. Endogenous adrenal steroids however, do not appear to be entirely responsible for the age-dependent decline in this transport system.

Molecular cloning and exon-intron mapping of the gene encoding human transmembrane secretory component (the poly-Ig receptor)

Secretory component (SC or the poly-Ig receptor) plays a crucial role in mucosal immunity by translocating polymeric IgA and IgM through secretory epithelial cells into external body fluids. Labeled restriction fragments from human SC cDNA were used to screen a human genomic leukocyte library. Three overlapping clones, spanning a total of 19 kb of the human SC gene, including 3 kb of the 5' flanking region, were characterized. The putative TATA box candidate, preceded by a CAAT-like box, was found 329 nucleotides upstream of the first exon. Altogether 11 exons covering the entire coding region were identified. The exon size ranged from 59 to 657 nucleotides and exon-intron junctions followed known consensus sequences. Three of the five extracellular Ig-related domains (D1, D4 and D5) were confined to one exon each (E3, E5 and E6), whereas D2 and D3 were encoded by the same exon (E4). The latter exon corresponds to that involved in alternate splicing of rabbit SC. The membrane-spanning segment was confined to part of one exon (E8). The cytoplasmic tail was encoded by four exons (E8-E11), whose boundaries encompassed fairly well the structural determinants proposed to be responsible for intracellular sorting of SC in the rabbit. The polymorphic restriction site reported earlier for Pvu II was localized to the third intron.