Monensin inhibits recycling of macrophage mannose-glycoprotein receptors and ligand delivery to lysosomes

Mannose receptor (MRC1) mediates uptake of dextran by bone marrow-derived macrophages

Macrophages survey their environment using receptor-mediated endocytosis and pinocytosis. Receptor-mediated endocytosis allows internalization of specific ligands, whereas pinocytosis nonselectively internalizes extracellular fluids and solutes. CRISPR/Cas9 whole-genome screens were used to identify genes regulating constitutive and growth factor-stimulated dextran uptake in murine bone marrow-derived macrophages (BMDM). The mannose receptor c-type 1 (MRC1/CD206) was a top hit in the screen. Targeted gene disruptions of Mrc1 reduced dextran uptake but had little effect on fluid-phase uptake of Lucifer yellow. Other screen hits also differentially affected the uptake of dextran and Lucifer yellow, indicating internalization by separate mechanisms. Visualization of dextran and Lucifer yellow uptake by microscopy showed enrichment of dextran in small puncta, which was inhibitable by mannan, a ligand of MRC1. In contrast, Lucifer yellow predominantly was internalized in larger macropinosomes. In addition, IL4-treated BMDMs internalized more dextran than untreated BMDM correlating with increased MRC1 expression. Therefore, dextran is not an effective marker for pinocytosis in BMDMs since it is internalized by receptor-mediated process. Numerous genes that regulate dextran internalization in primary murine macrophages were identified in the whole-genome screens, which can inform understanding of the regulation of MRC1 expression and MRC1-mediated uptake in macrophages.

Mannose receptor (MRC1) mediates uptake of dextran in macrophages via receptor-mediated endocytosis

Macrophages maintain surveillance of their environment using receptor-mediated endocytosis and pinocytosis. Receptor-mediated endocytosis allows macrophages to recognize and internalize specific ligands whereas macropinocytosis non-selectively internalizes extracellular fluids and solutes. Here, CRISPR/Cas9 whole-genome screens were used to identify genes regulating constitutive and growth factor-stimulated dextran uptake in murine bone-marrow derived macrophages (BMDM). The endocytic mannose receptor c-type 1 ( Mrc1 , also known as CD206) was a top hit in the screen. Targeted gene disruptions of Mrc1 reduced dextran uptake but had little effect on uptake of Lucifer yellow, a fluid-phase marker. Other screen hits also differentially affected the uptake of dextran and Lucifer yellow, indicating the solutes are internalized by different mechanisms. We further deduced that BMDMs take up dextran via MRC1-mediated endocytosis by showing that competition with mannan, a ligand of MRC1, as well as treatment with Dyngo-4a, a dynamin inhibitor, reduced dextran uptake. Finally, we observed that IL4-treated BMDM internalize more dextran than untreated BMDM by upregulating MRC1 expression. These results demonstrate that dextran is not an effective marker for the bulk uptake of fluids and solutes by macropinocytosis since it is internalized by both macropinocytosis and receptor-mediated endocytosis in cells expressing MRC1. This report identifies numerous genes that regulate dextran internalization in primary murine macrophages and predicts cellular pathways and processes regulating MRC1. This work lays the groundwork for identifying specific genes and regulatory networks that regulate MRC1 expression and MRC1-mediated endocytosis in macrophages.

Significance Statement

Macrophages constantly survey and clear tissues by specifically and non-specifically internalizing debris and solutes. However, the molecular mechanisms and modes of regulation of these endocytic and macropinocytic processes are not well understood. Here, CRISPR/Cas9 whole genome screens were used to identify genes regulating uptake of dextran, a sugar polymer that is frequently used as a marker macropinocytosis, and compared with Lucifer yellow, a fluorescent dye with no known receptors. The authors identified the mannose receptor as well as other proteins regulating expression of the mannose receptor as top hits in the screen. Targeted disruption of Mrc1 , the gene that encodes mannose receptor, greatly diminished dextran uptake but had no effect on cellular uptake of Lucifer yellow. Furthermore, exposure to the cytokine IL4 upregulated mannose receptor expression on the cell surface and increased uptake of dextran with little effect on Lucifer yellow uptake. Studies seeking to understand regulation of macropinocytosis in macrophages will be confounded by the use of dextran as a fluid-phase marker. MRC1 is a marker of alternatively activated/anti-inflammatory macrophages and is a potential target for delivery of therapeutics to macrophages. This work provides the basis for mechanistic underpinning of how MRC1 contributes to the receptor-mediated uptake of carbohydrates and glycoproteins from the tissue milieu and distinguishes genes regulating receptor-mediated endocytosis from those regulating the bona fide fluid-phase uptake of fluids and solutes by macropinocytosis.

Macropinocytosis Is the Principal Uptake Mechanism of Antigen-Presenting Cells for Allergen-Specific Virus-like Nanoparticles

Virus-like nanoparticles (VNP) are regarded as efficient vaccination platforms and have proven to be useful for the non-anaphylactogenic delivery of allergen-specific immunotherapy in preclinical models previously. Herein, we sought to determine the mode of VNP uptake by antigen presenting cells (APC). Accordingly, we screened a collection of substances known to inhibit different uptake pathways by APC. The human leukemia monocytic cell line THP-1 and the murine dendritic cell line DC 2.4 were examined for the uptake of fluorescently labelled VNP in the presence or absence of inhibitors. The inhibitory effect of candidate substances that blocked VNP uptake in APC lines was subsequently evaluated in studies with primary APC present in splenocyte and lung cell homogenates in vitro and upon intratracheal application of VNP in vivo. The uptake of allergen-specific VNP in vitro and in vivo was mainly observed by macrophages and CD103+ dendritic cells and was sensitive to inhibitors that block macropinocytosis, such as hyperosmolarity induced by sucrose or the polyphenol compound Rottlerin at low micromolar concentrations but not by other inhibitors. Also, T-cell proliferation induced by allergen-specific VNP was significantly reduced by both substances. In contrast, substances that stimulate macropinocytosis, such as Heparin and phorbol myristate acetate (PMA), increased VNP-uptake and may, thus, help modulate allergen-specific T-cell responses. We have identified macropinocytosis as the principal uptake mechanism of APC for allergen-specific VNP in vitro and in vivo, paving the way for further improvement of VNP-based therapies, especially those that can be used for tolerance induction in allergy, in the future.

Sulfation at Glycopolymer Side Chains Switches Activity at the Macrophage Mannose Receptor (CD206) In Vitro and In Vivo

The mannose receptor (CD206) is an endocytic receptor expressed by selected innate immune cells and nonvascular endothelium, which plays a critical role in both homeostasis and pathogen recognition. Although its involvement in the development of several diseases and viral infections is well established, molecular tools able to both provide insight on the chemistry of CD206-ligand interactions and, importantly, effectively modulate its activity are currently lacking. Using novel SO4-3-Gal-glycopolymers targeting its cysteine-rich lectin ectodomain, this study uncovers and elucidates a previously unknown mechanism of CD206 blockade involving the formation of stable intracellular SO4-3-Gal-glycopolymer-CD206 complexes that prevents receptor recycling to the cell membrane. Further, we show that SO4-3-Gal glycopolymers inhibit CD206 both in vitro and in vivo, revealing hitherto unknown receptor function and demonstrating their potential as CD206 modulators within future immunotherapies.

Targeting and Modulation of Liver Myeloid Immune Cells by Hard-Shell Microbubbles

Poly n-butylcyanoacrylate (PBCA)-based hard-shell microbubbles (MB) have manifold biomedical applications, including targeted drug delivery or contrast agents for ultrasound (US)-based liver imaging. MB and their fragments accumulate in phagocytes, especially in the liver, but it is unclear if MB affect the function of these immune cells. We herein show that human primary monocytes internalize different PBCA-MB by phagocytosis, which transiently inhibits monocyte migration in vertical chemotaxis assays and renders monocytes susceptible to cytotoxic effects of MB during US-guided destruction. Conversely, human macrophage viability and function, including cytokine release and polarization, remain unaffected after MB uptake. After i.v. injection in mice, MB predominantly accumulate in liver, especially in hepatic phagocytes (monocytes and Kupffer cells). Despite efficiently targeting myeloid immune cells in liver, MB or MB after US-elicited burst do not cause overt hepatotoxicity or inflammation. Furthermore, MB application with or without US-guided burst does not aggravate the course of experimental liver injury in mice or the inflammatory response to liver injury in vivo. In conclusion, PBCA-MB have immunomodulatory effects on primary human myeloid cells in vitro, but do not provoke hepatotoxicity, inflammation or altered response to liver injury in vivo, suggesting the safety of these MB for diagnostic and therapeutic purposes.

Myosin 1g Contributes to CD44 Adhesion Protein and Lipid Rafts Recycling and Controls CD44 Capping and Cell Migration in B Lymphocytes

Cell migration and adhesion are critical for immune system function and involve many proteins, which must be continuously transported and recycled in the cell. Recycling of adhesion molecules requires the participation of several proteins, including actin, tubulin, and GTPases, and of membrane components such as sphingolipids and cholesterol. However, roles of actin motor proteins in adhesion molecule recycling are poorly understood. In this study, we identified myosin 1g as one of the important motor proteins that drives recycling of the adhesion protein CD44 in B lymphocytes. We demonstrate that the lack of Myo1g decreases the cell-surface levels of CD44 and of the lipid raft surrogate GM1. In cells depleted of Myo1g, the recycling of CD44 was delayed, the delay seems to be caused at the level of formation of recycling complex and entry into recycling endosomes. Moreover, a defective lipid raft recycling in Myo1g-deficient cells had an impact both on the capping of CD44 and on cell migration. Both processes required the transportation of lipid rafts to the cell surface to deliver signaling components. Furthermore, the extramembrane was essential for cell expansion and remodeling of the plasma membrane topology. Therefore, Myo1g is important during the recycling of lipid rafts to the membrane and to the accompanied proteins that regulate plasma membrane plasticity. Thus, Myosin 1g contributes to cell adhesion and cell migration through CD44 recycling in B lymphocytes.

Revisiting the value of competition assays in folate receptor-mediated drug delivery

Polymeric nanoparticles have been studied for gene and drug delivery. These nanoparticles can be modified to utilize a targeted delivery approach to selectively deliver their payload to specific cells, while avoiding unwanted delivery to healthy cells. One commonly over-expressed receptor which can be targeted by ligand-conjugated nanoparticles is the folate receptor alpha (FRα). The ability to target FRα remains a promising concept, and therefore, understanding the binding dynamics of the receptor with the ligand of the nanoparticle therapeutic can provide valuable insight. This manuscript focuses on the interaction between self-assembled nanoparticles decorated with a folic acid (FA) ligand and FRα. The nanoparticles consist of micelles formed with a FA conjugated triblock copolymer (PEI-g-PCL-b-PEG-FA) which condensed siRNA to form micelleplexes. By combining biological and biophysical approaches, this manuscript explores the binding kinetics and force of the targeted siRNA containing nanoparticles to FRα in comparison with free FA. We demonstrate via flow cytometry and atomic force microscopy that multivalent micelleplexes bind to FRα with a higher binding probability and binding force than monovalent FA. Furthermore, we revisited why competitive inhibition studies of binding of multivalent nanoparticles to their respective receptor are often reported in literature to be inconclusive evidence of effective receptor targeting. In conclusion, the results presented in this paper suggest that multivalent targeted nanoparticles display strong receptor binding that a monovalent ligand may not be able to compete with under in vitro conditions and that high concentrations of competing monovalent ligands can lead to measurement artifacts.

Design of an Anticancer Copper(II) Prodrug Based on the Lys199 Residue of the Active Targeting Human Serum Albumin Nanoparticle Carrier

We not only modified the types and numbers of coordinated ligands in a metal agent to enhance its anticancer activity, but we also designed a metal prodrug based on the N-donor residues of the human serum albumin (HSA) IIA subdomain to improve its delivery efficiency and selectivity in vivo. However, there may be a conflict in simultaneously achieving the two goals because Lys199 and His242 in the IIA subdomain of HSA can replace its two coordinated ligands, which will decrease its anticancer activity relative to the original metal agent. Thus, to improve the delivery efficiency of the metal agent and simultaneously avoid decreasing its anticancer activity in vivo, we decided to develop an anticancer metal prodrug by regulating its pharmacophore ligand so that it would not be displaced by the Lys199 residue of the folic acid (FA)-functionalized HSA nanoparticle (NP) carrier. To this end, we first synthesized two (E)-N'-(5-chloro-2-hydroxybenzylidene)benzohydrazide Schiff base (HL) Cu(II) compounds by designing a second ligand with a different coordinating atom with Cu²⁺/Cu(L)(QL)(Br) [C1, QL = quinolone] and Cu(L)(DMF)(Br) [C2, DMF = N,N-dimethylformamide]. As revealed by the structures of the two HSA complexes, the Cu compounds bind to the hydrophobic cavity in the HSA IIA subdomain. The QL ligand of C1 is replaced by Lys199, which coordinates with Cu²⁺, whereas the DMF ligand of C2 is kept intact and His242 is replaced with Br^- of C2 and coordinates with Cu²⁺. The cytotoxicity of the Cu compounds was enhanced by the FA-HSA NPs in the Bel-7402 cells approximately 2-4-fold; however, they raise the cytotoxicity levels in the normal cells in vitro, and the FA-HSA NPs did not. Importantly, the in vivo data showed that FA-HSA-C2 NPs increased selectivity and the capacity to inhibit tumor growth and were less toxic than HSA-C2 NPs and C2. Moreover, C2/HSA-C2 NPs/FA-HSA-C2 NPs induced Bel-7402 cell death by potentially multiple mechanisms.

γ-Tilmanocept, a New Radiopharmaceutical Tracer for Cancer Sentinel Lymph Nodes, Binds to the Mannose Receptor (CD206)

γ-Tilmanocept ((99m)Tc-labeled-tilmanocept or [(99m)Tc]-tilmanocept) is the first mannose-containing, receptor-directed, radiolabeled tracer for the highly sensitive imaging of sentinel lymph nodes in solid tumor staging. To elucidate the mannose-binding receptor that retains tilmanocept in this microenvironment, human macrophages were used that have high expression of the C-type lectin mannose receptor (MR; CD206). Cy3-labeled tilmanocept exhibited high specificity binding to macrophages that was nearly abolished in competitive inhibition experiments. Furthermore, Cy3-tilmanocept binding was markedly reduced on macrophages deficient in the MR by small interfering RNA treatment and was increased on MR-transfected HEK 293 cells. Finally, confocal microscopy revealed colocalization of Cy3-tilmanocept with the macrophage membrane MR and binding of labeled tilmanocept to MR(+) cells (macrophages and/or dendritic cells) in human sentinel lymph node tissues. Together these data provide strong evidence that CD206 is a major binding receptor for γ-tilmanocept. Identification of CD206 as the γ-tilmanocept-binding receptor enables opportunities for designing receptor-targeted advanced imaging agents and therapeutics for cancer and other diseases.

Stable Expression and Characterization of an Optimized Mannose Receptor

The mannose receptor (MR) is a macrophage surface receptor that recognizes pathogen associated molecular patterns (PAMPs) from a diverse array of bacterial, fungal and viral pathogens. Functional studies of the MR are hampered by the scarcity of human cell lines that express the receptor. Current model systems available for the study of MR biology often demonstrate low levels of expression and do not retain many of the classical MR properties. Although several laboratories have reported transient and stable expression of MR from plasmids, preliminary data from our laboratory suggests that these plasmids produce a protein that lacks critical domains and is often not stable over time. In this current report we describe the generation and characterization of a novel human codon-optimized system for transient and stable MR expression. Rare codons and sequences that contribute to mRNA instability were modified to produce mRNA that is qualitatively and quantitatively improved. Confocal imaging of the transient and stably expressed optimized receptor demonstrates a distribution consistent with previous reports. To demonstrate the functional characteristics of the optimized receptor, we further show that the introduction of codon-optimized MR plasmid can confer MR-associated phagocytosis of S. aureus to non-phagocytic HeLa cells. We show that three molecules participate in the engagement and internalization of S. aureus. MR was found to colocalize with Toll-like receptor 2 (TLR2) and Rab5 following exposure to pHrodo-stained S. aureus, suggesting cooperation among the three molecules to engage and internalize the bacterial particle. This study describes a transfection capable, optimized MR receptor with functional characteristics similar to the wild type receptor and further demonstrates a new system for the continued study of MR biology and function.

Mannose receptor mediated phagocytosis of bacteria in macrophages of blunt snout bream (Megalobrama amblycephala) in a Ca(2+)-dependent manner

Mannose receptor (MR) is an important pattern-recognition receptor in macrophages and plays a critical role in immune responses. It is has been reported that mammalian macrophages are able to engulf a wide range of microorganisms mediated by Ca(2+)-dependent MR binding to terminal mannose residues which are frequently found on the pathogen surfaces. However, little is known about the MR-mediated phagocytosis in macrophages of fish. In this report, the distributions of MR in the macrophage and head kidney tissue from blunt snout bream were examined using MaMR specific antibody generated in our lab. Mannan and MaMR specific antibody inhibition experiments results collectively showed that MR was involved in the GFP-expressed E. coli engulfed in the macrophages, resulting in respiratory burst, nitric oxide production as well as inflammatory cytokines secretion, and the MaMR-mediated phagocytosis was Ca(2+)-dependent. These results will shed a new light on the immune functions of teleost MRs.

Mechanisms of regulation of olfactory transduction and adaptation in the olfactory cilium

Olfactory adaptation is a fundamental process for the functioning of the olfactory system, but the underlying mechanisms regulating its occurrence in intact olfactory sensory neurons (OSNs) are not fully understood. In this work, we have combined stochastic computational modeling and a systematic pharmacological study of different signaling pathways to investigate their impact during short-term adaptation (STA). We used odorant stimulation and electroolfactogram (EOG) recordings of the olfactory epithelium treated with pharmacological blockers to study the molecular mechanisms regulating the occurrence of adaptation in OSNs. EOG responses to paired-pulses of odorants showed that inhibition of phosphodiesterases (PDEs) and phosphatases enhanced the levels of STA in the olfactory epithelium, and this effect was mimicked by blocking vesicle exocytosis and reduced by blocking cyclic adenosine monophosphate (cAMP)-dependent protein kinase (PKA) and vesicle endocytosis. These results suggest that G-coupled receptors (GPCRs) cycling is involved with the occurrence of STA. To gain insights on the dynamical aspects of this process, we developed a stochastic computational model. The model consists of the olfactory transduction currents mediated by the cyclic nucleotide gated (CNG) channels and calcium ion (Ca²⁺)-activated chloride (CAC) channels, and the dynamics of their respective ligands, cAMP and Ca²⁺, and it simulates the EOG results obtained under different experimental conditions through changes in the amplitude and duration of cAMP and Ca²⁺ response, two second messengers implicated with STA occurrence. The model reproduced the experimental data for each pharmacological treatment and provided a mechanistic explanation for the action of GPCR cycling in the levels of second messengers modulating the levels of STA. All together, these experimental and theoretical results indicate the existence of a mechanism of regulation of STA by signaling pathways that control GPCR cycling and tune the levels of second messengers in OSNs, and not only by CNG channel desensitization as previously thought.

Characterization of functional mannose receptor in a continuous hybridoma cell line

Background

The mannose receptor is the best described member of the type I transmembrane C-type lectins; however much remains unanswered about the biology of the receptor. One difficulty has been the inability to consistently express high levels of a functional full length mannose receptor cDNA in mammalian cells. Another difficulty has been the lack of a human macrophage cell line expressing a fully functional receptor. Commonly used human macrophage cell lines such as U937, THP-1, Mono-Mac and HL60 do not express the mannose receptor. We have developed a macrophage hybridoma cell line (43^MR cells) created by fusion of U937 cells with primary human monocyte-derived macrophages, resulting in a non-adherent cell line expressing several properties of primary macrophages. The purpose of this study was to identify and select mannose receptor-expressing cells using fluorescence-activated cell sorting and to characterize the expression and function of the receptor.

Results

In the current study we show that the mannose receptor found on this novel cell has endocytic characteristics consistent with and similar to the mannose receptor found on the surface of monocyte-derived human macrophages and rat bone marrow-derived macrophages. In addition, we demonstrate that these cells engage and internalize pathogen particles such as S. aureus and C. albicans. We further establish the transfectability of these cells via the introduction of a plasmid expressing influenza A hemagglutinin.

Conclusions

The 43^MR cell line represents the first naturally expressed MR-positive cell line derived from a human macrophage background. This cell line provides an important cell model for other researchers for the study of human MR biology and host-pathogen interactions.

Caveolin targeting to late endosome/lysosomal membranes is induced by perturbations of lysosomal pH and cholesterol content

Caveolin-1 traffics to late endosomal/lysosomal membranes in response to manipulations of the cholesterol content of cells, suggesting that caveolin functions in the egress of cholesterol from this organelle. Cavicles associate with the periphery of the lysosome as they do with caveosomes, but these are separate organelles.

Caveolin-1 is an integral membrane protein of plasma membrane caveolae. Here we report that caveolin-1 collects at the cytosolic surface of lysosomal membranes when cells are serum starved. This is due to an elevation of the intralysosomal pH, since ionophores and proton pump inhibitors that dissipate the lysosomal pH gradient also trapped caveolin-1 on late endosome/lysosomes. Accumulation is both saturable and reversible. At least a portion of the caveolin-1 goes to the plasma membrane upon reversal. Several studies suggest that caveolin-1 is involved in cholesterol transport within the cell. Strikingly, we find that blocking cholesterol export from lysosomes with progesterone or U18666A or treating cells with low concentrations of cyclodextrin also caused caveolin-1 to accumulate on late endosome/lysosomal membranes. Under these conditions, however, live-cell imaging shows cavicles actively docking with lysosomes, suggesting that these structures might be involved in delivering caveolin-1. Targeting of caveolin-1 to late endosome/lysosomes is not observed normally, and the degradation rate of caveolin-1 is not altered by any of these conditions, indicating that caveolin-1 accumulation is not a consequence of blocked degradation. We conclude that caveolin-1 normally traffics to and from the cytoplasmic surface of lysosomes during intracellular cholesterol trafficking.

Antigen stored in dendritic cells after macropinocytosis is released unprocessed from late endosomes to target B cells

B lymphocytes can be triggered in lymph nodes by nonopsonized antigens (Ag), potentially in their native form. However, the mechanisms that promote encounter of B lymphocytes with unprocessed antigens in lymph nodes are still elusive. We show here that antigens are detected in B cells in the draining lymph nodes of mice injected with live, but not fixed, dendritic cells (DCs) loaded with antigens. This highlights active processes in DCs to promote Ag transfer to B lymphocytes. In addition, antigen-loaded DCs found in the draining lymph node were CD103+. Using 3 different model Ag, we then show that immature DCs efficiently take up Ag by macropinocytosis and store the internalized material in late endocytic compartments. We find that DCs have a unique ability to release antigens from these compartments in the extracellular medium, which is controlled by Rab27. B cells take up the regurgitated Ag and the chemokine CXCL13, essential to attract B cells in lymph nodes, enhances this transfer. Our results reveal a unique property of DCs to regurgitate unprocessed Ag that could play an important role in B-cell activation.

The increase in mannose receptor recycling favors arginase induction and Trypanosoma cruzi survival in macrophages

The macrophage mannose receptor (MR) is a pattern recognition receptor of the innate immune system that binds to microbial structures bearing mannose, fucose and N-acetylglucosamine on their surface. Trypanosoma cruzi antigen cruzipain (Cz) is found in the different developmental forms of the parasite. This glycoprotein has a highly mannosylated C-terminal domain that participates in the host-antigen contact. Our group previously demonstrated that Cz-macrophage (Mo) interaction could modulate the immune response against T. cruzi through the induction of a preferential metabolic pathway. In this work, we have studied in Mo the role of MR in arginase induction and in T. cruzi survival using different MR ligands. We have showed that pre-incubation of T. cruzi infected cells with mannose-Bovine Serum Albumin (Man-BSA, MR specific ligand) biased nitric oxide (NO)/urea balance towards urea production and increased intracellular amastigotes growth. The study of intracellular signals showed that pre-incubation with Man-BSA in T. cruzi J774 infected cells induced down-regulation of JNK and p44/p42 phosphorylation and increased of p38 MAPK phosphorylation. These results are coincident with previous data showing that Cz also modifies the MAPK phosphorylation profile induced by the parasite. In addition, we have showed by confocal microscopy that Cz and Man-BSA enhance MR recycling. Furthermore, we studied MR behavior during T. cruzi infection in vivo. MR was up-regulated in F4/80+ cells from T. cruzi infected mice at 13 and 15 days post infection. Besides, we investigated the effect of MR blocking antibody in T. cruzi infected peritoneal Mo. Arginase activity and parasite growth were decreased in infected cells pre-incubated with anti-MR antibody as compared with infected cells treated with control antibody. Therefore, we postulate that during T. cruzi infection, Cz may contact with MR, increasing MR recycling which leads to arginase activity up-regulation and intracellular parasite growth.

Constitutive endocytic recycling and protein kinase C-mediated lysosomal degradation control K(ATP) channel surface density

Pancreatic ATP-sensitive potassium (K(ATP)) channels control insulin secretion by coupling the excitability of the pancreatic beta-cell to glucose metabolism. Little is currently known about how the plasma membrane density of these channels is regulated. We therefore set out to examine in detail the endocytosis and recycling of these channels and how these processes are regulated. To achieve this goal, we expressed K(ATP) channels bearing an extracellular hemagglutinin epitope in human embryonic kidney cells and followed their fate along the endocytic pathway. Our results show that K(ATP) channels undergo multiple rounds of endocytosis and recycling. Further, activation of protein kinase C (PKC) with phorbol 12-myristate 13-acetate significantly decreases K(ATP) channel surface density by reducing channel recycling and diverting the channel to lysosomal degradation. These findings were recapitulated in the model pancreatic beta-cell line INS1e, where activation of PKC leads to a decrease in the surface density of native K(ATP) channels. Because sorting of internalized channels between lysosomal and recycling pathways could have opposite effects on the excitability of pancreatic beta-cells, we propose that PKC-regulated K(ATP) channel trafficking may play a role in the regulation of insulin secretion.

Macrophage receptors for influenza A virus: role of the macrophage galactose-type lectin and mannose receptor in viral entry

Although sialic acid has long been recognized as the primary receptor determinant for attachment of influenza virus to host cells, the specific receptor molecules that mediate viral entry are not known for any cell type. For the infection of murine macrophages by influenza virus, our earlier study indicated involvement of a C-type lectin, the macrophage mannose receptor (MMR), in this process. Here, we have used direct binding techniques to confirm and characterize the interaction of influenza virus with the MMR and to seek additional macrophage surface molecules that may have potential as receptors for viral entry. We identified the macrophage galactose-type lectin (MGL) as a second macrophage membrane C-type lectin that binds influenza virus and is known to be endocytic. Binding of influenza virus to MMR and MGL occurred independently of sialic acid through Ca(2+)-dependent recognition of viral glycans by the carbohydrate recognition domains of the two lectins; influenza virus also bound to the sialic acid on the MMR. Multivalent ligands of the MMR and MGL inhibited influenza virus infection of macrophages in a manner that correlated with expression of these receptors on different macrophage populations. Influenza virus strain A/PR/8/34, which is poorly glycosylated and infects macrophages poorly, was not recognized by the C-type lectin activity of either the MMR or the MGL. We conclude that lectin-mediated interactions of influenza virus with the MMR or the MGL are required for the endocytic uptake of the virus into macrophages, and these lectins can thus be considered secondary or coreceptors with sialic acid for infection of this cell type.

Aberrant trafficking of NSCLC-associated EGFR mutants through the endocytic recycling pathway promotes interaction with Src

BACKGROUND

Epidermal growth factor receptor (EGFR) controls a wide range of cellular processes, and altered EGFR signaling contributes to human cancer. EGFR kinase domain mutants found in non-small cell lung cancer (NSCLC) are constitutively active, a trait critical for cell transformation through activation of downstream pathways. Endocytic trafficking of EGFR is a major regulatory mechanism as ligand-induced lysosomal degradation results in termination of signaling. While numerous studies have examined mutant EGFR signaling, the endocytic traffic of mutant EGFR within the NSCLC milieu remains less clear.

RESULTS

This study shows that mutant EGFRs in NSCLC cell lines are constitutively endocytosed as shown by their colocalization with the early/recycling endosomal marker transferrin and the late endosomal/lysosomal marker LAMP1. Notably, mutant EGFRs, but not the wild-type EGFR, show a perinuclear accumulation and colocalization with recycling endosomal markers such as Rab11 and EHD1 upon treatment of cells with endocytic recycling inhibitor monensin, suggesting that mutant EGFRs preferentially traffic through the endocytic recycling compartments. Importantly, monensin treatment enhanced the mutant EGFR association and colocalization with Src, indicating that aberrant transit through the endocytic recycling compartment promotes mutant EGFR-Src association.

CONCLUSION

The findings presented in this study show that mutant EGFRs undergo aberrant traffic into the endocytic recycling compartment which allows mutant EGFRs to engage in a preferential interaction with Src, a critical partner for EGFR-mediated oncogenesis.

An analysis of the role of a retroendocytosis pathway in ABCA1-mediated cholesterol efflux from macrophages

The ATP binding cassette transporter A-1 (ABCA1) is critical for apolipoprotein-mediated cholesterol efflux, an important mechanism employed by macrophages to avoid becoming lipid-laden foam cells, the hallmark of early atherosclerotic lesions. It has been proposed that lipid-free apolipoprotein A-I (apoA-I) enters the cell and is resecreted as a lipidated particle via a retroendocytosis pathway during ABCA1-mediated cholesterol efflux from macrophages. To determine the functional importance of such a pathway, confocal microscopy was used to characterize the internalization of a fully functional apoA-I cysteine mutant containing a thiol-reactive fluorescent probe in cultured macrophages. ApoA-I was also endogenously labeled with (35)S-methionine to quantify cellular uptake and to determine the metabolic fate of the internalized protein. It was found that apoA-I was specifically taken inside macrophages and that a small amount of intact apoA-I was resecreted from the cells. However, a majority of the label that reappeared in the media was degraded. We estimate that the mass of apoA-I retroendocytosed is not sufficient to account for the HDL produced by the cholesterol efflux reaction. Furthermore, we have demonstrated that lipid-free apoA-I-mediated cholesterol efflux from macrophages can be pharmacologically uncoupled from apoA-I internalization into cells. On the basis these findings, we present a model in which the ABCA1-mediated lipid transfer process occurs primarily at the membrane surface in macrophages, but still accounts for the observed specific internalization of apoA-I.

Experimental colitis: decreased Octn2 and Atb0+ expression in rat colonocytes induces carnitine depletion that is reversible by carnitine-loaded liposomes

Carnitine transporters have recently been implicated in susceptibility to inflammatory bowel disease (IBD). Because carnitine is required for beta-oxidation, it was suggested that decreased carnitine transporters, and hence reduced carnitine uptake, could lead to impaired fatty acid oxidation in intestinal epithelial cells, and to cell injury. We investigated this issue by examining the expression of the carnitine transporters OCTN2 and ATB0+, and butyrate metabolism in colonocytes in a rat model of IBD induced by trinitrobenzene sulfonic acid (TNBS). We found that Octn2 and Atb0+ expression was decreased in inflammatory samples at translational and functional level. Butyrate oxidation, evaluated based on CO2 production and acetyl-coenzyme A synthesis, was deranged in colonocytes from TNBS-treated rats. Treatment with carnitine-loaded liposomes corrected the butyrate metabolic alterations in vitro and reduced the severity of colitis in vivo. These results suggest that carnitine depletion in colonocytes is associated with the inability of mitochondria to maintain normal butyrate beta-oxidation. Our data indicate that carnitine is a rate-limiting factor for the maintenance of physiological butyrate oxidation in colonic cells. This hypothesis could also explain the contradictory therapeutic efficacy of butyrate supplementation observed in clinical trials of IBD.

Penetratin tandemly linked to a CTL peptide induces anti-tumour T-cell responses via a cross-presentation pathway

Recently there has been increasing evidence to suggest that membrane translocating peptides enter cells by a receptor-dependent pathway. There have been some studies on the mechanism of major histocompatibility complex (MHC) class I presentation of membrane translocating peptides incorporating cytotoxic T lymphocyte epitopes. However, these have been on different cell lines and only a limited number of inhibitors of the antigen presentation pathway were used. Herein, we demonstrate a comprehensive study utilizing a full spectrum of inhibitors to various pathways of MHC class I to elucidate the mechanism of the membrane translocating peptide, penetratin from Antennapedia (Int). It is clear that Int, RQIKIWFQNRRMKWKK when tandemly linked to a cytotoxic T lymphocyte peptide of ovalbumin, SIINFEKL (IntSIIN) is endocytosed via phagocytosis or macropinocytosis by dendritic cells in an ATP-dependent manner and is processed by a proteasome- and tapasin-independent pathway for presentation and loading to MHC class I molecules. In addition, the majority of antigen is taken up by negatively charged receptors. IntSIIN activates T cells in vitro and in vivo and protects mice against challenge with an ovalbumin-expressing tumour.

Constitutive and Protein Kinase C-Induced Internalization of the Dopamine Transporter is Mediated by a Clathrin-Dependent Mechanism

The amount of dopamine transporter (DAT) present at the cell surface is rapidly regulated by the rates of DAT internalization to endosomes and DAT recycling back to the plasma membrane. The re-distribution of the transporter from the cell surface to endosomes was induced by phorbol ester activation of protein kinase C in porcine aortic endothelial cells stably expressing the human DAT. Inhibition of DAT recycling with the carboxylic ionophore monensin also caused significant accumulation of DAT in early endosomes and a concomitant loss of DAT from the cell surface, due to protein kinase C-independent constitutive internalization of DAT in the absence of recycling. Such monensin-induced relocation of DAT to endosomes was therefore utilized as a measure of the constitutive internalization of DAT. Knock-down of clathrin heavy chain or dynamin II by small interfering RNAs dramatically inhibited both constitutive and protein kinase C-mediated internalization of DAT. In contrast, neither monensin-dependent nor phorbol ester-induced re-distribution of DAT were affected by inhibitors of endocytosis through cholesterol-rich membrane microdomains. Mutational analysis revealed the potential importance of amino acid residues 587-597 in DAT internalization. Altogether, the data suggest that both constitutive and protein kinase C-mediated internalization of DAT utilize the clathrin-dependent endocytic pathway, but likely involve unconventional mechanisms.

Toll-like receptor 2 stimulation decreases IFN-gamma receptor expression in mouse RAW264.7 macrophages

Interferon-gamma (IFN-gamma) is a key cytokine in the immune defense against mycobacteria. IFN-gamma activates macrophages to resist the growth of mycobacteria and induces expression of MHC class II molecules required for antigen presentation. Macrophages infected with mycobacteria or stimulated by the interaction of mycobacterial products with toll-like receptor 2 (TLR2) have reduced responses to IFN-gamma. Previous research has shown that infection of mouse macrophages with Mycobacterium avium causes decreased expression of the IFN-gamma receptor (IFNGR). In the present study, we show that TLR2 stimulation of RAW264.7 macrophages with a synthetic lipoprotein, Pam3CSK4, also causes rapid decrease in expression of IFNGR-1 protein, with little change in IFNGR-2 protein levels. The decrease in IFNGR-2 expression in TLR2-stimulated cells required receptor internalization and proteasomal degradation. The level of IFNGR-1 mRNA also decreased in TLR2-stimulated RAW264.7 cells and M. avium-infected cells. The decrease in IFNGR-1 mRNA was shown to be due to decreased transcription. In spite of the decrease in IFNGR-2 receptor expression, activation of Stat1 activation by an optimal dose of IFN-gamma was identical between control and TLR2-stimulated RAW264.7 cells. However, at low suboptimal doses of IFN-gamma, Stat1 activation was decreased in TLR2-stimulated cells.

Pulmonary collectins enhance phagocytosis of Mycobacterium avium through increased activity of mannose receptor

Collectins, including surfactant proteins A (SP-A) and D (SP-D) and mannose binding lectin (MBL), are the important constituents of the innate immune system. Mycobacterium avium, a facultative intracellular pathogen, has developed numerous mechanisms for entering mononuclear phagocytes. In this study, we investigated the interactions of collectins with M. avium and the effects of these lectins on phagocytosis of M. avium by macrophages. SP-A, SP-D, and MBL exhibited a concentration-dependent binding to M. avium. The binding of SP-A to M. avium was Ca(2+)-dependent but that of SP-D and MBL was Ca(2+)-independent. SP-A and SP-D but not MBL enhanced the phagocytosis of FITC-labeled M. avium by rat alveolar macrophages and human monocyte-derived macrophages. Excess mannan, zymosan, and lipoarabinomannan derived from the M. avium-intracellular complex, significantly decreased the collectin-stimulated phagocytosis of M. avium. Enhanced phagocytosis was not affected by the presence of cycloheximide or chelation of Ca(2+). The mutated collectin, SP-A(E195Q, R197D) exhibited decreased binding to M. avium but stimulated phagocytosis to a level comparable to wild-type SP-A. Enhanced phagocytosis by cells persisted even after preincubation and removal of SP-A or SP-D. Rat alveolar macrophages that had been incubated with SP-A or SP-D also exhibited enhanced uptake of (125)I-mannosylated BSA. Analysis by confocal microscopy and flow cytometry revealed that the lung collectins up-regulated the cell surface expression of mannose receptor on monocyte-derived macrophages. These results provide compelling evidence that SP-A and SP-D enhance mannose receptor-mediated phagocytosis of M. avium by macrophages.

Mannose receptor contribution to Candida albicans phagocytosis by murine E-clone J774 macrophages

Mannoproteins, as the main constituents of the outer layer of yeast cell walls, are able to interact with phagocytic cells in an opsonin-independent manner through the mannose receptor (MR) and to induce yeast ingestion by the professional phagocytes. Moreover, the MR also mediates endocytosis of soluble ligands through clathrin-coated pits. Here, we studied some aspects of the interaction between the MR and Candida albicans using murine E-clone macrophages and the consequences on MR trafficking. Using a pull-down assay involving mixture E-clone macrophage detergent lysate with mannosylated Sepharose beads and glutaraldehyde-fixed, heat-killed (HK) C. albicans, we found that binding of solubilized MR to mannosylated particles occurred with characteristics similar to the receptor's cell-surface mannose-binding activity. We then demonstrated that MR expressed on E-clone macrophages contributed to phagocytosis of unopsonized, HK C. albicans and that yeast phagocytosis induced a decrease in MR endocytic activity without concomitant degradation of the receptor in the time lapse studied.

Oligolysine-based oligosaccharide clusters: selective recognition and endocytosis by the mannose receptor and dendritic cell-specific intercellular adhesion molecule 3 (ICAM-3)-grabbing nonintegrin

Dendritic cells are potent antigen-presenting cells that express several membrane lectins, including the mannose receptor and DC-SIGN (dendritic cell-specific ICAM-3-grabbing nonintegrin). To identify highly specific ligands for these dendritic cell receptors, oligosaccharides were converted into glycosynthons (Os1) and were used to prepare oligolysine-based glycoclusters, Os-[Lys(Os)]n-Ala-Cys-NH2. Clusters containing two to six dimannosides as well as clusters containing four or five pentasaccharides (Lewisa or Lewisx) or hexasaccharides (Lewisb) were synthesized. The thiol group of the appended cysteine residue allows easy tagging by a fluorescent probe or convenient substitution with an antigen. Surface plasmon resonance was used to determine the affinity of the different glycoclusters for purified mannose receptor and DC-SIGN, whereas flow cytometry and confocal microscopy analysis allowed assessment of cell uptake of fluoresceinyl-labeled glycoclusters. Dimannoside clusters are recognized by the mannose receptor with an affinity constant close to 106 liter.mol-1 but have a very low affinity for DC-SIGN (less than 104 liter x mol-1). Conversely, Lewis clusters have a higher affinity toward DC-SIGN than toward the mannose receptor. Dimannoside clusters are efficiently taken up by human dendritic cells as well as by rat fibroblasts expressing the mannose receptor but not by HeLa cells or rat fibroblasts expressing DC-SIGN; DC-SIGN-expressing cells take up Lewis clusters. The results suggest that ligands containing dimannoside clusters can be used specifically to target the mannose receptor, whereas ligands containing Lewis clusters will be targeted to DC-SIGN.

Development of vasculature targeting strategies for the treatment of cancer and chronic inflammatory diseases

Endothelial cells play a pathological role in cancer and chronic inflammation and are therefore attractive targets for therapeutic intervention. This review focuses on endothelial cell specific drug targeting strategies for the treatment of these diseases. The cellular and molecular processes involved in the activation of endothelial cells in angiogenesis and inflammation will be reviewed. Various target epitopes expressed by activated endothelium suitable for targeting purposes, design and development of drug-carrier complexes, drugs of interest which might interfere with endothelial cell activation, as well as in vitro and in vivo experimental approaches to study (intra) cellular drug delivery will be discussed.

Pulmonary surfactant protein A up-regulates activity of the mannose receptor, a pattern recognition receptor expressed on human macrophages

Inhaled particulates and microbes are continually cleared by a complex array of lung innate immune determinants, including alveolar macrophages (AMs). AMs are unique cells with an enhanced capacity for phagocytosis that is due, in part, to increased activity of the macrophage mannose receptor (MR), a pattern recognition receptor for various microorganisms. The local factors that "shape" AM function are not well understood. Surfactant protein A (SP-A), a major component of lung surfactant, participates in the innate immune response and can enhance phagocytosis. Here we show that SP-A selectively enhances MR expression on human monocyte-derived macrophages, a process involving both the attached sugars and collagen-like domain of SP-A. The newly expressed MR is functional. Monocyte-derived macrophages on an SP-A substrate demonstrated enhanced pinocytosis of mannose BSA and phagocytosis of Mycobacterium tuberculosis lipoarabinomannan-coated microspheres. The newly expressed MR likely came from intracellular pools because: 1) up-regulation of the MR by SP-A occurred by 1 h, 2) new protein synthesis was not necessary for MR up-regulation, and 3) pinocytosis of mannose BSA via MR recycling was increased. AMs from SP-A(-/-) mice have reduced MR expression relative to SP-A(+/+). SP-A up-regulation of MR activity provides a mechanism for enhanced phagocytosis of microbes by AMs, thereby enhancing lung host defense against extracellular pathogens or, paradoxically, enhancing the potential for intracellular pathogens to enter their intracellular niche. SP-A contributes to the alternative activation state of the AM in the lung.

Biochemical evidence for transcytotic absorption of polyaspartamide from the rat lung: effects of temperature and metabolic inhibitors

Airway-to-perfusate polyhydroxyethylaspartamide (PHEA) absorption was studied in the isolated perfused rat lung at a reduced temperature and by the use of metabolic inhibitors, to kinetically clarify the mechanisms and cellular pathways of its active absorption. Fluorophore-labeled PHEA (F-PHEA; 7.4 kDa) was administered into the airways, and its absorption followed with time at 25 degrees C and in the presence of 2,4-dinitrophenol (DNP), ouabain (OUA), monensin (MON), and nocodazole (NOC). Across-dose absorption profiles were analyzed using a kinetic model incorporating active (V(max,P) and K(m,P)) and passive (k(a,P)) absorption from the pulmonary lung region alongside the competing, pulmonary-to-bronchial mucociliary escalator (k(E)). The model was validated at 25 degrees C and a lack of perturbation on the k(a,P) and k(E) values for passively absorbed solutes confirmed by studying the disposition of sodium fluorescein and 4.4 kDa fluorescein isothiocyanate-labeled dextran. F-PHEA absorption was significantly suppressed at 25 degrees C, compared with 37 degrees C, because of a significant decrease in the value of the maximum rate of active absorption, V(max,P) (4.37 --> 0.67 microg/min; p < 0.05), whereas the carrier-affinity term, K(m,P), was statistically unchanged. F-PHEA's active absorption was also significantly inhibited by DNP (> or =0.5 mM), OUA (> or =50 microM), MON (> or =10 microM), and NOC (> or =1 microM), whereas these inhibitors had no significant effect on the values for k(a,P) and k(E). Thus, F-PHEA's pulmonary active absorption in the rat lung was temperature- and adenosine 5'-triphosphate-derived intracellular energy-dependent (DNP and OUA inhibition) and apparently mediated via transcytosis through cytoplasmic endosomes and microtubules (MON and NOC inhibition).

Fusogenic potential of prokaryotic membrane lipids. Implication in vaccine development

Development of protective immunity against many pathogens, particularly viruses, requires fine orchestration of both humoral- and cell mediated-immunity. The immunization of animals with soluble antigens usually leads to the induction of humoral immune responses. In contrast, the activation of a cell-mediated immune response against exogenous antigens has always been a challenge, requiring special strategies to expose them to the proteasome, a multifunctional protease complex in the cytosol of the target cells. The degradation of the protein by the cytosolic proteolytic system forms a cardinal step for the induction of cytotoxic T lymphocytes (CTLs). In the present study, we report that a potent primary CTL response against a soluble protein, ovalbumin, can be induced in mice by encapsulating it in the liposomes comprised of Escherichia coli membrane lipids. These lipids were shown to induce strong membrane-membrane fusion as evident from resonance energy transfer and content mixing assays. Furthermore, the fusion of these liposomes with living cells (J774 A1) was demonstrated to result in effective transfer of a fluorescent lipid probe to the plasma membrane of the cells. Moreover, ricin A, a protein synthesis inhibitor that does not cross plasma membrane, was demonstrated to gain access to the cytosol when it was encapsulated in these liposomes. Finally, the liposomes were demonstrated to behave like efficient vehicles for the in vivo delivery of the antigens to the target cells resulting in the elicitation of antigen reactive CD8+ T cell responses.

Endogenous ligands of carbohydrate recognition domains of the mannose receptor in murine macrophages, endothelial cells and secretory cells; potential relevance to inflammation and immunity

The macrophage mannose receptor (MR) has an established role in the phagocytosis of a wide range of microbes, and also functions in viral endocytosis, and clearance of a number of endogenous glycoproteins from the circulation. Its broad ligand specificity is mediated by tandemly linked carbohydrate recognition domains (CRDs). Recent studies suggest that binding or internalization of both natural and synthetic ligands of MR CRDs may modulate macrophage (MPhi ) function, for example to increase cidal capacity or cytokine synthesis. To identify endogenous ligands in the normal mouse we used an Fc-fusion protein (CRD4-7Fc) bearing four of the CRDs of MR. CRD4-7Fc recognized endocytic compartments of cultured MPhi, consistent with lysosomal enzymes being major ligands of MR. CRD4-7Fc also recognized MPhi and some endothelial cells in tissues, and intensely labeled secretory cells of the exocrine pancreas, salivary gland and thyroid. Strongly MR-positive interstitial cells were found in close proximity to the ligand-rich secretory cells, suggesting a role for MR in uptake of secretory glycoproteins, including thyroglobulin which was identified as a novel ligand in vitro. Endocytosis of these ligands by MR may have implications for tissue homeostasis and immunity, including antigen presentation, in secretory organs.

Cell-specific basolateral membrane sorting of the human liver Na(+)-dependent bile acid cotransporter

The human Na(+)-taurocholate cotransporting polypeptide (Ntcp) is located exclusively on the basolateral membrane of hepatocyte, but the mechanisms underlying its membrane sorting domain have not been fully elucidated. In the present study, a green fluorescent protein-fused human NTCP (NTCP-GFP) was constructed using the polymerase chain reaction and was stably transfected into Madin-Darby canine kidney (MDCK) and Caco-2 cells. Taurocholate uptake studies and confocal microscopy demonstrated that the polarity of basolateral surface expression of NTCP-GFP was maintained in MDCK cells but was lost in Caco-2 cells. Nocodazole (33 microM), an agent that causes microtubular depolymerization, partially disrupted the basolateral localization of NTCP-GFP by increasing apical surface expression to 33.5% compared with untreated cells (P < 0.05). Brefeldin A (BFA; 1-2 microM) disrupted the polarized basolateral localization of NTCP, but monensin (1.4 microM) had no affect on NTCP-GFP localization. In addition, low-temperature shift (20 degrees C) did not affect the polarized basolateral surface sorting of NTCP-GFP and repolarization of this protein after BFA interruption. In summary, these data suggest that the polarized basolateral localization of human NTCP is cell specific and is mediated by a novel sorting pathway that is BFA sensitive and monensin and low-temperature shift insensitive. The process may also involve microtubule motors.

Liposome-mediated cytosolic delivery of macromolecules and its possible use in vaccine development

In the majority of bacterial and viral infections the generation of cytotoxic T cells is of particular interest because such pathogens are able to escape the host defence mechanisms by surviving intracellularly within the phagocytic cells. To generate a CD8+ T lymphocyte response against exogenous antigens, the prerequisite is their delivery into the cytosol followed by processing and presentation along with class I major histocompatibility complex (MHC-I) molecules. In the present study we describe the method of liposome-based delivery of antigens and other macromolecules into the cytosol of target cells. To develop safe and effective methods for generating CD8+ T lymphocytes, we exploited the fusogenic character of lipids derived from lower organisms, that is baker's yeast (Saccharomyces cerevisiae). The degree of fusion with model membrane systems using yeast lipid liposomes varied from 40-70%, as opposed to 1-8% observed with egg PtdCho liposomes, depending on the assay system used. The fusion of yeast lipid liposomes with macrophages resulted in effective delivery of the entrapped solutes into the cytoplasmic compartment. This was further supported by the inhibition of cellular protein synthesis in J774 A1 cells by ricin A, encapsulated in the yeast lipid liposomes. Interestingly, the model antigen ovalbumin, when entrapped in the yeast lipid liposomes, successfully elicited antigen reactive CD8+ T cell responses. It may be concluded that the liposomes made of lipids derived from S. cerevisiae can spontaneously fuse with macrophages, delivering a significant portion of their contents into the cytoplasmic compartment of the cells.

Langerhans-like dendritic cells generated from cord blood progenitors internalize pollen allergens by macropinocytosis, and part of the molecules are processed and can activate autologous naive T lymphocytes

BACKGROUND

The safety and efficacy of sublingual immunotherapy have been demonstrated in moderate allergic asthma and seasonal rhinitis. However, not much is known about the precise mechanism of action of the allergen when it crosses the oral mucosa.

OBJECTIVE

To define this mechanism, we investigated the role of Langerhans' cells in the capture and internalization of allergens.

METHODS

We generated dendritic cells in vitro with the phenotypic characteristics of Langerhans-like dendritic cells (LLDCs) from cord blood CD34(+) progenitors. We used two recombinant major allergens: Bet v 1 and Phl p 1 labeled with FITC.

RESULTS

Internalization of allergens and control proteins was dose- and time-dependent and related to the immature state of the cells. LLDCs internalized allergens with a high efficiency in comparison with control molecules. Allergens were only internalized by macropinocytosis, as demonstrated by the use of various inhibitors. Addition of intracellular pH-modifying molecules indicated that only a part of the allergens was accumulated in acidic vesicles, whereas the majority remained in other cytoplasmic structures. Pulse-chase experiments calculated a half-life of 4 hours, suggesting that part of the molecules were not metabolized in the lysosome. Allergen internalization by LLDCs might be followed by processing in some experiments, as demonstrated by activation of autologous T lymphocytes in 4 of 9 experiments.

CONCLUSION

These elements showed that Langerhans' cells present in mucosa might play an active role in immune responses to allergens.

Binding and Uptake of Agalactosyl IgG by Mannose Receptor on Macrophages and Dendritic Cells

Increased levels of agalactosyl IgG (G0 IgG) are found in several autoimmune diseases, including rheumatoid arthritis, in which they are correlated with severity of the disease. To investigate whether structural alteration of IgG may lead to aberrant processing and presentation of IgG peptides as autoantigens, we have studied uptake of G0 IgG by human dendritic cells and macrophages cultured from PBMC. We found that enzymatic removal of terminal galactose residues, which exposes N-acetylglucosamine residues, increases uptake of soluble IgG mediated by mannose receptor on macrophages and dendritic cells. Efficient uptake appears to require recycling of the receptor, can be blocked by saccharides or Abs reactive with mannose receptor, and is dependent upon the state of maturation of the dendritic cells. No differences between IgG isotypes in ability to be internalized by APC were identified, suggesting that uptake would not be limited to a particular subset of Abs. These results suggest a novel pathway by which Abs or Ag-Ab complexes can be taken into dendritic cells and macrophages, and potentially generate epitopes recognized by T cells. These findings may have particular relevance for autoimmune disorders characterized by high levels of G0 IgG.

Identification and functional characterization of the mannose receptor in astrocytes

The immune competence of astrocytes is still ill defined, especially their endocytic capacity, a prerequisite for efficient antigen presentation. We show that mannose receptor, a very important conduit for internalization of infectious agents and self antigens, is functionally expressed in the murine CNS. By in vitro assays, astrocytes and microglia were shown to be the prime cells expressing this receptor. Studies on astrocytes demonstrate that its expression and function are inversely regulated by anti-and pro-inflammatory compounds. Downregulation of the mannose receptor by IFN-gamma is concomitant with the induction of the invariant chain, which is also induced by GM-CSF + IL-4. Mannose receptor-expressing astrocytes may thus act as scavenger not only in CNS development but also in defense, against soluble and particulate mannosylated pathogens, presenting fragments thereof at strategic locations in the CNS. These findings unravel a new and putatively very important role of astrocytes in innate immunity and possibly development.

Recycling cell surface glycoproteins undergo limited oligosaccharide reprocessing in LEC1 mutant Chinese hamster ovary cells

The ability of particular cell surface glycoproteins to recycle and become exposed to individual Golgi enzymes has been demonstrated. This study was designed to determine whether endocytic trafficking includes significant reentry into the overall oligosaccharide processing pathway. The Lec1 mutant of Chinese hamster ovary (CHO) cells lack N -acetylglucosaminyltransferase I (GlcNAc-TI) activity resulting in surface expression of incompletely processed Man5GlcNAc2 N -linked oligosaccharides. An oligosaccharide tracer was created by exoglycosylation of cell surface glycoproteins with purified porcine GlcNAc-TI and UDP-[3H]GlcNAc. Upon reculturing, all cell surface glycoproteins that acquired [3H]GlcNAc were acted upon by intracellular mannosidase II, the next enzyme in the Golgi processing pathway of complex N -linked oligosaccharides (t1/2= 3-4 h). That all radiolabeled cell surface glycoproteins were included in this endocytic pathway indicates a common intracellular compartment into which endocytosed cell surface glycoproteins return. Significantly, no evidence was found for continued oligosaccharide processing consistent with transit through the latter cisternae of the Golgi apparatus. These data indicate that, although recycling plasma membrane glycoproteins can be reexposed to individual Golgi-derived enzymes, significant reentry into the overall contiguous processing pathway is not evident.

Transport of residual endocytosed products into terminal lysosomes occurs slowly in rat liver endothelial cells

Receptor-mediated endocytosis of circulating collagen is a major physiological scavenger function of the liver endothelial cell and an important catabolic event in the complete turnover of this abundant connective tissue protein. In the present study, transport of collagen through the endocytic pathway was investigated in cultured liver endothelial cells. Collagen conjugated to fluorescein isothiocyanate, to allow detection of the ligand by fluorescence and immunoelectron microscopy, was found sequentially in three different organelles that compose the basic degradative endocytic pathway of eukaryotic cells: early endosomes, late endosomes, and terminal lysosomes. Early endosomes were identified as vesicles positive for early endosome antigen 1 (EEA1). Late endosomes were distinguished as structures positive for the late endosomal/lysosomal marker rat lysosomal membrane glycoprotein 120, but negative for EEA1 and lysosomally targeted BSA-gold. Lysosomes were defined by their content of BSA-gold, injected 24 hours before isolation of cells. Coated pits and coated vesicles mediated an extremely rapid internalization. Shortly after internalization and during the first 20 minutes, ligand was found in early endosomes. From 20 minutes on, ligand started to appear in late endosomes (23%), and by 2 hours the transfer was largely complete (82.5%). Only 2.5% of ligand was transferred to the lysosomes after 2 hours, and this number slowly increased to 21% and 53% after 6 and 16 hours, respectively. We conclude that 1) EEA1 is a useful marker for tracing early events of endocytosis in liver endothelial cells; 2) in contrast to the rapid internalization, transit of internalized ligand through early sorting endosomes generally takes from 20 minutes to 2 hours; and 3) exit from the late endosomes is very slow, requiring several hours.

Disruption of microtubules reveals two independent apical targeting mechanisms for G-protein-coupled receptors in polarized renal epithelial cells

G-protein-coupled receptors demonstrate differing trafficking itineraries in polarized Madin-Darby canine kidney (MDCK II) cells. The alpha2A adrenergic receptor (alpha2AAR) is directly delivered to the basolateral subdomain; the A1 adenosine receptor (A1AdoR) is apically enriched in its targeting; and the alpha2BAR subtype is randomly delivered to both domains but selectively retained basolaterally (Keefer, J. R., and Limbird, L. E. (1993) J. Biol. Chem. 268, 11340-11347; Saunders, C., Keefer, J. R., Kennedy, A. P., Wells, J. N., and Limbird, L. E. (1996) J. Biol. Chem. 271, 995-1002; Wozniak, M., and Limbird, L. E. (1996) J. Biol. Chem. 271, 5017-5024). The present studies explore the role of the polarized cytoskeleton in localization of G-protein-coupled receptors in MDCK II cells. Nocodazole or colchicine, which disrupt microtubules, did not perturb lateral localization of alpha2AR subtypes but led to a relocalization the A1AdoR to the basolateral surface, revealed by immunocytochemical and metabolic labeling strategies. Conversely, the apical component of the random delivery of alpha2BAR was not affected by these agents, suggesting microtubule-dependent and -independent apical targeting mechanisms for G-protein-coupled receptors in polarized cells. Apparent rerouting of the apically targeted A1AdoR was selective for microtubule-disrupting agents, since cytochalasin D, which disrupts actin polymerization, did not alter A1AdoR or alpha2BAR localization or targeting. These data suggest that multiple apical targeting mechanisms exist for G-protein-coupled receptors and that microtubule-disrupting agents serve as tools to probe their different trafficking mechanisms.

The Vif and Gag proteins of human immunodeficiency virus type 1 colocalize in infected human T cells

The Vif protein of human immunodeficiency virus type 1 (HIV-1) and other lentiviruses is required for efficient replication in primary cells and certain immortalized cell lines in vitro and, in all likelihood, for the establishment of pathogenic infections in vivo. Current hypotheses concerning Vif's mechanism of action posit that it operates in virus-expressing cells during virion assembly, budding, or maturation such that released virions are modified in a manner that enables them to undergo productive infection in subsequent viral challenges. To gain further insight into the mechanism of action of lentivirus Vif proteins, we have performed a variety of in situ localization and biochemical fractionation studies using cells in which Vif is essential for efficient replication. Double-label immunofluorescence analyses of cells productively infected with HIV-1 or feline immunodeficiency virus revealed dramatic patterns of colocalization between Vif and the virally encoded Gag proteins. Subcellular fractionations of human T cells expressing HIV-1 Vif performed in the absence of any detergent demonstrated that greater than 90% of Vif is associated with cellular membranes. Additional purification using a continuous density gradient indicated that the majority of the membrane-bound Vif copurifies with the plasma membrane. Taken together, these observations suggest that lentivirus Vif and Gag proteins colocalize at the plasma membrane as virion assembly and budding take place. As a result, Vif is able to exert its modulatory effect(s) on these late steps of the virus life cycle.

Binding and internalization of extracellular type-I phospholipase A2 in uterine stromal cells

The cellular uptake of extracellular type-I phospholipase A2 (PLA2) was investigated in rat uterine stromal cells (UIII) in culture, which were found to express the high-affinity binding site for mammalian type-I PLA2, with a measured KD of 6.4 nM, a Bmax of 0.1-1 pmol/mg of DNA at 4 degrees C, and a molecular mass of about 200 kDa. When UIII cells were treated with type-I PLA2 at 37 degrees C, the ligand specifically associated with the cells increased, reaching a plateau after 90 min of incubation, whose level was about 5-fold higher than that measured if cells were maintained at 4 degrees C. We could determine that the PLA2 was bound to plasma membrane receptors which were responsible for internalization of the ligand, and that the binding sites were still suitable for binding at the level of plasma membrane during UIII cell incubation at 37 degrees C. Proteolysis of internalized PLA2 could be clearly detected only after 90 min of UIII cell incubation with the ligand at 37 degrees C, and most of the intracellular PLA2 consisted of the apparently intact 14 kDa enzyme. By cross-linking studies, we found that most of the internalized PLA2 was not associated with the receptor, supporting the conclusion that in our experimental system a single pool of membrane receptors for mammalian type-I PLA2 undergoes cycles of ligand binding, intracellular transfer and release of PLA2, followed by restoration of binding sites on the plasma membrane. We calculated that the rate of internalization of the ligand by one receptor molecule in UIII cells at 37 degrees C is about three molecules of type-I PLA2 per h.

Chimerization of LL2, a rapidly internalizing antibody specific for B cell lymphoma

LL2 is a murine monoclonal antibody (MAb) that has been shown to be effective for the diagnosis and treatment of patients with non-Hodgkin's B cell lymphoma. Studies have also shown that radiolabeled murine LL2 (mLL2) or mLL2 and fragments thereof coupled to Pseudomonas exotoxin (PE) can effectively target human B cell lymphoma in mice. We have obtained the DNA sequences encoding the VK and VH domains of mLL2, an IgG2a MAb, which were combined with their respective human kappa and IgG1 constant region domains and expressed in SP2/0 cells. Like its murine counterpart, the chimeric LL2 (cLL2) antibody is glycosylated in the light chain variable region. Chimerization did not interfere with the immunoreactivity of the antibody, as determined by a competitive binding assay, where either antibody shows equivalent inhibition of the binding of its counterpart to the Raji cell membrane surface antigen, CD22. Both antibodies bind and are rapidly internalized by Raji cells, whereas an irrelevant humanized antibody did not bind and was not internalized under similar conditions. The internalization rates of the bound murine or chimeric antibodies were nearly identical, with Ke values of 0.106 and 0.118 min-1 for mLL2 and cLL2, respectively. The observed close equivalence between the murine and chimeric antibodies suggests potential advantages of the latter as a less immunogenic agent. Studies are currently underway to evaluate the chimeric antibody as a potential therapeutic immunoconjugate.

Improved intracellular delivery of a muramyl dipeptide analog by means of nanocapsules

A lipophilic derivative of muramyl dipeptide, muramyl tripeptide cholesterol, was incorporated into poly(D,L-lactide) nanocapsules and its immunomodulating properties were assessed in vitro. The nanocapsule form was more effective than the free drug in activating rat alveolar macrophages for a cytostatic effect toward syngeneic tumor cells. Induction of NO synthase correlated with anti-proliferative activity. The time course of activation and the effect of inhibitors of endocytosis suggested that this increased efficiency was due to improved intracellular delivery by phagocytosis of nanocapsules. Such nanocapsules might be useful for immunotherapy of metastases resistant to conventional treatment, since they could overcome two problems associated with soluble muramyl peptides: rapid elimination and poor uptake by macrophages.

Phagocytosis of antigens by Langerhans cells in vitro

Dendritic cells (DC) isolated from lymphoid tissues are generally thought to be nonphagocytic in culture. It has therefore been unclear how these cells could acquire particulate antigens such as microorganisms for initiation of primary immune responses. Lymphoid DC derive in part from cells that have migrated from nonlymphoid tissues, such as Langerhans cells (LC) of skin. The ability of LC to internalize a variety of particles was studied by electron, ultraviolet, phase, and differential interference contrast microscopy, and by two-color flow cytometry. Freshly isolated LC in epidermal cell suspensions phagocytosed the yeast cell wall derivative zymosan, intact Saccharomyces cerevisiae, representatives of two genera of Gram-positive bacteria, Corynebacterium parvum and Staphylococcus aureus, as well as 0.5-3.5-microns latex microspheres. During maturation in culture, the phagocytic activity of these cells was markedly reduced. Likewise, freshly isolated splenic DC were more phagocytic than cultured DC for two types of particle examined, zymosan and latex beads. Unlike macrophages, LC did not bind or internalize sheep erythrocytes before or after opsonization with immunoglobulin G or complement, and did not internalize colloidal carbon. The receptors mediating zymosan uptake by LC were examined. For this particle, C57BL/6 LC were considerably more phagocytic than BALB/c LC and exhibited a reproducible increase in phagocytic activity after 6 h of culture followed by a decline, whereas this initial rise did not occur for BALB/c LC. These differential kinetics of uptake were reflected in the pattern of zymosan binding at 4 degrees C, and endocytosis of the soluble tracer fluorescein isothiocyanate-mannose-bovine serum albumin at 37 degrees C. Zymosan uptake by LC from both strains of mice was inhibited in the presence of mannan or beta-glucan, although to different extents, but not by antibodies specific for CR3 (CD11b/CD18). These data indicate that zymosan uptake by LC can be mediated by a mannose/beta-glucan receptor(s) that is differentially expressed in the two strains of mice and that is downregulated during maturation of LC in culture.