Lipid raft-independent B cell receptor-mediated antigen internalization and intracellular trafficking

B cell receptor (BCR) endocytosis

The B cell receptor (BCR) interacts with foreign antigens to mediate B cell activation and secretion of antibodies. B cell activation begins with initiation of signaling pathways, such as NFAT, NF-κB, and MAPK, and endocytosis of the BCR-antigen complex. Many studies have investigated the signaling pathways associated with BCR activation, and this work has led to significant advances in drug therapies to treat cancer and autoimmune diseases that are linked to aberrant BCR signaling. Less is known, however, about the mechanisms of BCR endocytosis and the role endocytosis plays in B cell pathogenesis. This chapter will review key characteristics of the BCR, including a review of signaling pathways, and endocytic mechanisms associated with the activated BCR. We will also review recent studies investigating the role of BCR endocytosis disease pathogenesis.

Lipid Rafts Interaction of the ARID3A Transcription Factor with EZRIN and G-Actin Regulates B-Cell Receptor Signaling

Several diseases originate via dysregulation of the actin cytoskeleton. The ARID3A/Bright transcription factor has also been implicated in malignancies, primarily those derived from hematopoietic lineages. Previously, we demonstrated that ARID3A shuttles between the nucleus and the plasma membrane, where it localizes within lipid rafts. There it interacts with components of the B-cell receptor (BCR) to reduce its ability to transmit downstream signaling. We demonstrate here that a direct component of ARID3A-regulated BCR signal strength is cortical actin. ARID3A interacts with actin exclusively within lipid rafts via the actin-binding protein EZRIN, which confines unstimulated BCRs within lipid rafts. BCR ligation discharges the ARID3A-EZRIN complex from lipid rafts, allowing the BCR to initiate downstream signaling events. The ARID3A-EZRIN interaction occurs almost exclusively within unpolymerized G-actin, where EZRIN interacts with the multifunctional ARID3A REKLES domain. These observations provide a mechanism by which a transcription factor directly regulates BCR signaling via linkage to the actin cytoskeleton with consequences for B-cell-related neoplasia.

The immunobiology of ubiquitin-dependent B cell receptor functions

MHC class II-restricted antigen presentation by dendritic cells is necessary for activation of naïve CD4 T cells, whereas class II-restricted antigen presentation by B lymphocytes and macrophages is important for the recruitment of CD4+ helper and regulatory T cells. Antigen presentation by B cells is also important for induction of T cell tolerance. B cells are unique among these three types of MHC class II-expressing antigen presenting cells (APC) as they constitutively express high levels of cell surface class II molecules and express a clonally restricted antigen specific receptor, the B cell receptor (BCR). Here, I review our current understanding of three major steps that underlie the processing and presentation of BCR-bound cognate antigen: (1) endocytosis of antigen-BCR (Ag-BCR) complexes, (2) Ag-BCR trafficking to intracellular antigen processing compartments and (3) generation of antigenic peptide-MHC class II complexes, with a particular focus on the role of BCR ubiquitination in each. I will highlight potential topics for future research and briefly discuss the impact of the cell biology of BCR-mediated antigen processing on the response of the B cell and T cell to the cell-cell interactions mediated by B cell-expressed peptide-class II complexes.

Internalization of large particles by turbot (Scophthalmus maximus) IgM+ B cells mainly depends on macropinocytosis

Increasing evidence has demonstrated support for the endocytic capacities of teleost B cells. In the present study, the ability of turbot IgM⁺ B cells to ingest microspheres of different sizes and the corresponding internalization pathways were investigated. The results showed that IgM⁺ B cells exhibited relatively high endocytic capacities for 0.5 μm and 1 μm latex beads, and that different mechanisms were employed for IgM⁺ and IgM^- cells to uptake 0.5 μm and 1 μm beads. For 0.5 μm beads, IgM⁺ B cells apparently employed macropinocytosis-dependent endocytic pathway, whereas IgM^- cells utilized a different process involving both clathrin- and caveolae-mediated pathways. For the uptake of 1 μm beads, IgM⁺ cells relied mainly on macropinocytosis and partially on caveolae-mediated pathway, while IgM^- cells utilized the routes similar to that of internalizing 0.5 μm beads. Consistently, the internalized microspheres were co-localized with high-molecular-mass dextran in IgM⁺ phagocytic cells. In addition to latex beads, IgM⁺ B cells could also ingest inactivated bacteria predominately through macropinocytosis and caveolae-mediated endocytosis. These results collectively indicated that macropinocytosis is principally responsible for particle uptake by turbot IgM⁺ B cells.

PGE2 /EP4 Signaling Controls the Transfer of the Mammary Stem Cell State by Lipid Rafts in Extracellular Vesicles

Prostaglandin E₂ (PGE₂ )-initiated signaling contributes to stem cell homeostasis and regeneration. However, it is unclear how PGE₂ signaling controls cell stemness. This study identifies a previously unknown mechanism by which PGE₂ /prostaglandin E receptor 4 (EP₄ ) signaling regulates multiple signaling pathways (e.g., PI3K/Akt signaling, TGFβ signaling, Wnt signaling, EGFR signaling) which maintain the basal mammary stem cell phenotype. A shift of basal mammary epithelial stem cells (MaSCs) from a mesenchymal/stem cell state to a non-basal-MaSC state occurs in response to prostaglandin E receptor 4 (EP₄ ) antagonism. EP₄ antagonists elicit release of signaling components, by controlling their trafficking into extracellular vesicles/exosomes in a lipid raft/caveolae-dependent manner. Consequently, EP₄ antagonism indirectly inactivates, through induced extracellular vesicle/exosome release, pathways required for mammary epithelial stem cell homeostasis, e.g. canonical/noncanonical Wnt, TGFβ and PI3K/Akt pathways. EP₄ antagonism causes signaling receptors and signaling components to shift from non-lipid raft fractions to lipid raft fractions, and to then be released in EP₄ antagonist-induced extracellular vesicles/exosomes, resulting in the loss of the stem cell state by mammary epithelial stem cells. In contrast, luminal mammary epithelial cells can acquire basal stem cell properties following ingestion of EP₄ antagonist-induced stem cell extracellular vesicles/exosomes, and can then form mammary glands. These findings demonstrate that PGE₂ /EP₄ signaling controls homeostasis of mammary epithelial stem cells through regulating extracellular vesicle/exosome release. Reprogramming of mammary epithelial cells can result from EP₄ -mediated stem cell property transfer by extracellular vesicles/exosomes containing caveolae-associated proteins, between mammary basal and luminal epithelial cells. Stem Cells 2017;35:425-444.

Enhancement of Lytic Activity by Leptin Is Independent From Lipid Rafts in Murine Primary Splenocytes

Leptin, a pleiotropic adipokine, is known as a regulator of food intake, but it is also involved in inflammation, immunity, cell proliferation, and survival. Leptin receptor is integrated inside cholesterol-rich microdomains called lipid rafts, which, if disrupted or destroyed, could lead to a perturbation of lytic mechanism. Previous studies also reported that leptin could induce membrane remodeling. In this context, we studied the effect of membrane remodeling in lytic activity modulation induced by leptin. Thus, primary mouse splenocytes were incubated with methyl-β-cyclodextrin (β-MCD), a lipid rafts disrupting agent, cholesterol, a major component of cell membranes, or ursodeoxycholic acid (UDCA), a membrane stabilizer agent for 1 h. These treatments were followed by splenocyte incubation with leptin (absence, 10 and 100 ng/ml). Unlike β-MCD or cholesterol, UDCA was able to block leptin lytic induction. This result suggests that leptin increased the lytic activity of primary spleen cells against syngenic EO771 mammary cancer cells independently from lipid rafts but may involve membrane fluidity. Furthermore, natural killer cells were shown to be involved in the splenocyte lytic activity. To our knowledge it is the first publication in primary culture that provides the link between leptin lytic modulation and membrane remodeling. J. Cell. Physiol. 232: 101-109, 2017. © 2016 Wiley Periodicals, Inc.

Coalescence of B cell receptor and invariant chain MHC II in a raft-like membrane domain

The BCR binds antigen for processing and subsequent presentation on MHC II molecules. Polyvalent antigen induces BCR clustering and targeting to endocytic processing compartments, which are also accessed by Ii-MHC II. Here, we report that clustered BCR is able to team up with Ii-MHC II already at the plasma membrane of mouse B-lymphocytes. Colocalization of BCR and Ii-MHC II on the cell surface required clustering of both types of molecules. The clustering of only one type did not trigger the recruitment of the other. Ii-bound MIF (a ligand of Ii) also colocalized with clustered BCR upon oligomerization of MIF on the surface of the B cell. Abundant surface molecules, such as B220 or TfnR, did not cocluster with the BCR. Some membrane raft-associated molecules, such as peptide-loaded MHC II, coclustered with the BCR, whereas others, such as GM1, did not. The formation of a BCR- and Ii-MHC II-containing membrane domain by antibody-mediated clustering was independent of F-actin and led to the coendocytosis of its constituents. With a rapid Brij 98 extraction method, it was possible to capture this membrane domain biochemically as a DRM. Ii and clustered BCR were present on the same DRM, as shown by immunoisolation. The coalescence of BCR and Ii-MHC II increased tyrosine phosphorylation, indicative of enhanced BCR signaling. Our work suggests a novel role for MIF and Ii-MHC II in BCR-mediated antigen processing.

Cis and trans regulatory mechanisms control AP2-mediated B cell receptor endocytosis via select tyrosine-based motifs

Following antigen recognition, B cell receptor (BCR)-mediated endocytosis is the first step of antigen processing and presentation to CD4+ T cells, a crucial component of the initiation and control of the humoral immune response. Despite this, the molecular mechanism of BCR internalization is poorly understood. Recently, studies of activated B cell-like diffuse large B cell lymphoma (ABC DLBCL) have shown that mutations within the BCR subunit CD79b leads to increased BCR surface expression, suggesting that CD79b may control BCR internalization. Adaptor protein 2 (AP2) is the major mediator of receptor endocytosis via clathrin-coated pits. The BCR contains five putative AP2-binding YxxØ motifs, including four that are present within two immunoreceptor tyrosine-based activation motifs (ITAMs). Using a combination of in vitro and in situ approaches, we establish that the sole mediator of AP2-dependent BCR internalization is the membrane proximal ITAM YxxØ motif in CD79b, which is a major target of mutation in ABC DLBCL. In addition, we establish that BCR internalization can be regulated at a minimum of two different levels: regulation of YxxØ AP2 binding in cis by downstream ITAM-embedded DCSM and QTAT regulatory elements and regulation in trans by the partner cytoplasmic domain of the CD79 heterodimer. Beyond establishing the basic rules governing BCR internalization, these results illustrate an underappreciated role for ITAM residues in controlling clathrin-dependent endocytosis and highlight the complex mechanisms that control the activity of AP2 binding motifs in this receptor system.

Macropinocytosis is responsible for the uptake of pathogenic and non-pathogenic mycobacteria by B lymphocytes (Raji cells)

Background

The classical roles of B cells include the production of antibodies and cytokines and the generation of immunological memory, these being key factors in the adaptive immune response. However, their role in innate immunity is currently being recognised. Traditionally, B cells have been considered non-phagocytic cells; therefore, the uptake of bacteria by B cells is not extensively documented. In this study, we analysed some of the features of non-specific bacterial uptake by B lymphocytes from the Raji cell line. In our model, B cells were infected with Mycobacterium tuberculosis (MTB), Mycobacterium smegmatis (MSM), and Salmonella typhimurium (ST).

Results

Our observations revealed that the Raji B cells were readily infected by the three bacteria that were studied. All of the infections induced changes in the cellular membrane during bacterial internalisation. M. smegmatis and S. typhimurium were able to induce important membrane changes that were characterised by abundant filopodia and lamellipodia formation. These membrane changes were driven by actin cytoskeletal rearrangements. The intracellular growth of these bacteria was also controlled by B cells. M. tuberculosis infection also induced actin rearrangement-driven membrane changes; however, the B cells were not able to control this infection. The phorbol 12-myristate 13-acetate (PMA) treatment of B cells induced filopodia and lamellipodia formation, the production of spacious vacuoles (macropinosomes), and the fluid-phase uptake that is characteristic of macropinocytosis. S. typhimurium infection induced the highest fluid-phase uptake, although both mycobacteria also induced fluid uptake. A macropinocytosis inhibitor such as amiloride was used and abolished the bacterial uptake and the fluid-phase uptake that is triggered during the bacterial infection.

Conclusions

Raji B cells can internalise S. typhimurium and mycobacteria through an active process, such as macropinocytosis, although the resolution of the infection depends on factors that are inherent in the virulence of each pathogen.

The Syk-binding ubiquitin ligase c-Cbl mediates signaling-dependent B cell receptor ubiquitination and B cell receptor-mediated antigen processing and presentation

B cell receptor (BCR)-mediated antigen (Ag) processing and presentation lead to B cell-T cell interactions, which support affinity maturation and immunoglobulin class switching. These interactions are supported by generation of peptide-MHC class II complexes in multivesicular body-like MIIC compartments of B cells. Previous studies have shown that trafficking of Ag·BCR complexes to MVB-like MIIC occurs via an ubiquitin-dependent pathway and that ubiquitination of Ag·BCR complexes occurs by an Src family kinase signaling-dependent mechanism that is restricted to lipid raft-resident Ag·BCR complexes. This study establishes that downstream Syk-dependent BCR signaling is also required for BCR ubiquitination and BCR-mediated antigen processing and presentation. Knockdown studies reveal that of the two known Syk-binding E3 ubiquitin ligases c-Cbl and Cbl-b, only c-Cbl appears to have a central role in BCR ubiquitination, trafficking to MIIC, and ubiquitin-dependent BCR-mediated antigen processing and presentation. These results establish the novel role for Syk signaling and the Syk-binding ubiquitin ligase c-Cbl in the BCR-mediated processing and presentation of cognate antigen and define one mechanism by which antigen-induced BCR ubiquitination is modulated to impact the initiation and maturation of the humoral immune response.

Membrane microdomain sphingolipids are required for anti-CD20-induced death of chronic lymphocytic leukemia B cells

BACKGROUND

Chronic lymphocytic leukemia remains incurable, despite the addition of rituximab to chemotherapy as an available means of treatment. The resistance of certain patients to this monoclonal antibody prompted us to set up in vitro studies of another CD20-specific monoclonal antibody, B1 (later termed tositumomab). We hypothesized that the membrane lipid organization of leukemic B cells might be instrumental in the cells' sensitivity to the B1 monoclonal antibody.

DESIGN AND METHODS

B lymphocytes from 36 patients with chronic lymphocytic leukemia and 13 patients with non-Hodgkin's lymphoma were investigated for B1-triggered cell death. Membrane components, such as sphingomyelin and ganglioside M1, were investigated by flow cytometry, immunofluorescence and co-immunoprecipitation, together with the Csk-binding protein.

RESULTS

Chronic lymphocytic leukemia patients segregated into two groups: B cells from one group were sensitive to B1, whereas those from the second group were not. Further results ascribed the resistance of these latter cases to a defective recruitment of Csk-binding protein, resulting in a lack of sphingomyelin and ganglioside M1 at the outer leaflet of the plasma membrane of their malignant B cells. Sphingolipids were indeed retained in the cytoplasm, because of lowered activity of P-glycoprotein. Supporting this mechanism, rifampicin, an inducer of P-glycoprotein, improved the activity of this transmembrane efflux pump, normalized the quantity of sphingomyelin within the membrane, and thereby restored the efficacy of the B1 monoclonal antibody in the formerly B1-resistant cases of chronic lymphocytic leukemia.

CONCLUSIONS

The lipid organization of membranes of B cells from patients with chronic lymphocytic leukemia differs from one patient to another. In practice, given the relevance of the membrane lipid distribution to the efficacy of biotherapies, this observation is of potential importance.

The Ia.2 epitope defines a subset of lipid raft-resident MHC class II molecules crucial to effective antigen presentation

Previous work established that binding of the 11-5.2 anti-I-A(k) mAb, which recognizes the Ia.2 epitope on I-A(k) class II molecules, elicits MHC class II signaling, whereas binding of two other anti-I-A(k) mAbs that recognize the Ia.17 epitope fail to elicit signaling. Using a biochemical approach, we establish that the Ia.2 epitope recognized by the widely used 11-5.2 mAb defines a subset of cell surface I-A(k) molecules predominantly found within membrane lipid rafts. Functional studies demonstrate that the Ia.2-bearing subset of I-A(k) class II molecules is critically necessary for effective B cell-T cell interactions, especially at low Ag doses, a finding consistent with published studies on the role of raft-resident class II molecules in CD4 T cell activation. Interestingly, B cells expressing recombinant I-A(k) class II molecules possessing a β-chain-tethered hen egg lysosome peptide lack the Ia.2 epitope and fail to partition into lipid rafts. Moreover, cells expressing Ia.2(-) tethered peptide-class II molecules are severely impaired in their ability to present both tethered peptide or peptide derived from exogenous Ag to CD4 T cells. These results establish the Ia.2 epitope as defining a lipid raft-resident MHC class II conformer vital to the initiation of MHC class II-restricted B cell-T cell interactions.

Physiological-range temperature changes modulate cognate antigen processing and presentation mediated by lipid raft-restricted ubiquitinated B cell receptor molecules

BCR-mediated Ag processing and presentation is critical to the initiation and control of a humoral immune response. Trafficking of internalized Ag-BCR complexes to intracellular Ag processing compartments is driven by ubiquitination of the cytoplasmic domain of the BCR. Using a biochemical approach, it is here established that ubiquitinated Ag-BCR complexes are formed via a signaling-dependent mechanism and restricted to plasma membrane lipid rafts. Because the structure of lipid rafts is temperature sensitive, the impact of physiological-range temperature changes (PRTCs; 33-39°C) on lipid raft-dependent and -independent BCR functions was investigated. Whereas the kinetics of lipid raft-independent BCR internalization is unaffected by temperature changes within this range, raft-dependent BCR signaling and ubiquitination as well as BCR-mediated Ag processing are significantly affected. The extent and duration of Ag-BCR ubiquitination is increased and prolonged at 37-39°C (normal to febrile temperature) compared with that at 33°C (peripheral body temperature). As might be expected, increased temperature also accelerates the overall kinetics of Ag-BCR degradation. Notably, at 33°C the expression of peptide-MHC class II complexes derived from the BCR-mediated processing of cognate Ag is profoundly slowed, whereas the kinetics of expression of peptide-MHC class II complexes derived from fluid-phase Ag processing remains unchanged. These results establish the effect of PRTCs on multiple lipid raft-dependent BCR functions including the processing and presentation of cognate Ag, suggesting one mechanism by which PRTCs, such as fever, may impact the initiation and/or maturation of a humoral immune response.

Bam32/DAPP1 promotes B cell adhesion and formation of polarized conjugates with T cells

B cell Ag receptors function in both signaling activation of Ag-specific cells and in collecting specific Ag for presentation to T lymphocytes. Signaling via PI3K is required for BCR-mediated activation and Ag presentation functions; however, the relevant downstream targets of PI3K in B cells are incompletely defined. In this study, we have investigated the roles of the PI3K effector molecule Bam32/DAPP1 in BCR signaling and BCR-mediated Ag presentation functions. In mouse primary B cells, Bam32 was required for efficient activation of the GTPase Rac1 and downstream signaling to JNK, but not activation of BLNK, phospholipase C gamma2, or calcium responses. Consistent with a role of this adaptor in Rac-mediated cytoskeletal rearrangement, Bam32 was required for BCR-induced cell adhesion and spreading responses on ICAM-1 or fibronectin-coated surfaces. The function of Bam32 in promoting Rac activation and adhesion required tyrosine 139, a known site of phosphorylation by Lyn kinase. After BCR crosslinking by Ag, Bam32-deficient B cells are able to carry out the initial steps of Ag endocytosis and processing, but show diminished ability to form Ag-specific conjugates with T cells and polarize F-actin at the B-T interface. As a result, Bam32-deficient B cells were unable to efficiently activate Ag-specific T cells. Together, these results indicate that Bam32 serves to integrate PI3K and Src kinase signaling to promote Rac-dependent B cell adhesive interactions important for Ag presentation function.

Palmitic Acid Is a Novel CD4 Fusion Inhibitor That Blocks HIV Entry and Infection

The high rate of HIV-1 mutation and the frequent sexual transmission highlight the need for novel therapeutic modalities with broad activity against both CXCR4 (X4) and CCR5 (R5)-tropic viruses. We investigated a large number of natural products, and from Sargassum fusiforme we isolated and identified palmitic acid (PA) as a natural small bioactive molecule with activity against HIV-1 infection. Treatment with 100 microM PA inhibited both X4 and R5 independent infection in the T cell line up to 70%. Treatment with 22 microM PA inhibited X4 infection in primary peripheral blood lymphocytes (PBL) up to 95% and 100 microM PA inhibited R5 infection in primary macrophages by over 90%. Inhibition of infection was concentration dependent, and cell viability for all treatments tested remained above 80%, similar to treatment with 10(-6)M nucleoside analogue 2', 3'-dideoxycytidine (ddC). Micromolar PA concentrations also inhibited cell-to-cell fusion and specific virus-to-cell fusion up to 62%. PA treatment did not result in internalization of the cell surface CD4 receptor or lipid raft disruption, and it did not inhibit intracellular virus replication. PA directly inhibited gp120-CD4 complex formation in a dose-dependent manner. We used fluorescence spectroscopy to determine that PA binds to the CD4 receptor with K(d) approximately 1.5 +/- 0.2 microM, and we used one-dimensional saturation transfer difference NMR (STD-NMR) to determined that the PA binding epitope for CD4 consists of the hydrophobic methyl and methelene groups located away from the PA carboxyl terminal, which blocks efficient gp120-CD4 attachment. These findings introduce a novel class of antiviral compound that binds directly to the CD4 receptor, blocking HIV-1 entry and infection. Understanding the structure-affinity relationship (SAR) between PA and CD4 should lead to the development of PA analogs with greater potency against HIV-1 entry.

The Effect of B Cell Receptor Signaling on Antigen Endocytosis and Processing

B cell receptor (BCR)-mediated antigen processing and presentation involves both the BCR-mediated internalization and processing of cognate antigen as well as the formation and expression of antigenic peptide-MHC class II complexes. While BCR signaling is known to result in changes in the biosynthesis and intracellular trafficking of class II molecules, the effect of BCR signaling on the cell biology of antigen endocytosis and processing is less clear. Therefore, the effect of BCR signaling on the cell biology of fluid phase antigen endocytosis, processing and presentation was analyzed in both B cell lines or in normal splenic B cells. The results demonstrate that BCR signaling alters neither the global level of fluid phase antigen endocytosis nor the duration of intracellular persistence of fluid phase internalized antigen. Moreover, while BCR signal does result in an increase in the level of total cell surface MHC class II molecules as well as specific peptide-class II complexes, stimulation failed to alter the fraction of class II molecules loaded with antigen-derived peptide. These results indicate that while BCR-mediated signaling elicits an increase in the expression of antigenic peptide-class II complexes, signaling does not augment antigen presentation by profoundly altering the basic biology of antigen endocytosis and processing. These results also demonstrate that the high efficiency of BCR-mediated antigen processing (when compared to fluid phase antigen processing) is likely to occur independent of BCR signaling-induced global alterations in the biology of endocytosis, processing and presentation. This finding suggests that if BCR signaling augments the efficiency of processing of cognate antigen, it must impact unique aspects of BCR-mediated antigen processing, such as the intracellular persistence of internalized antigen-BCR complexes.

Endocytosis and intracellular trafficking of human natural killer cell receptors

Natural killer (NK) cells play a vital role in the defense against viral infections and tumor development. NK cell function is primarily regulated by the sum of signals from a broad array of activation and inhibitory receptors. Key to generating the input level of either activating or inhibitory signals is the maintenance of receptor expression levels on the cell surface. Although the mechanisms of endocytosis and trafficking for some cell surface receptors, such as transferrin receptor and certain immune receptors, are very well known, that is not the situation for receptors expressed by NK cells. Recent studies have uncovered that endocytosis and trafficking routes characteristic for specific activation and inhibitory receptors can regulate the functional responses of NK cells. In this review, we summarize the current knowledge of receptor endocytosis and trafficking, and integrate this with our current understanding of NK cell receptor trafficking.

Signalling of the BCR is regulated by a lipid rafts-localised transcription factor, Bright

Regulation of BCR signalling strength is crucial for B-cell development and function. Bright is a B-cell-restricted factor that complexes with Bruton's tyrosine kinase (Btk) and its substrate, transcription initiation factor-I (TFII-I), to activate immunoglobulin heavy chain gene transcription in the nucleus. Here we show that a palmitoylated pool of Bright is diverted to lipid rafts of resting B cells where it associates with signalosome components. After BCR ligation, Bright transiently interacts with sumoylation enzymes, blocks calcium flux and phosphorylation of Btk and TFII-I and is then discharged from lipid rafts as a Sumo-I-modified form. The resulting lipid raft concentration of Bright contributes to the signalling threshold of B cells, as their sensitivity to BCR stimulation decreases as the levels of Bright increase. Bright regulates signalling independent of its role in IgH transcription, as shown by specific dominant-negative titration of rafts-specific forms. This study identifies a BCR tuning mechanism in lipid rafts that is regulated by differential post-translational modification of a transcription factor with implications for B-cell tolerance and autoimmunity.

Endocytosis as a mechanism of regulating natural killer cell function: unique endocytic and trafficking pathway for CD94/NKG2A

Natural killer (NK) cells are lymphocytes generally recognized as sentinels of the innate immune system due to their inherent capacity to deal with diseased (stressed) cells, including malignant and infected. This ability to recognize many potentially pathogenic situations is due to the expression of a diverse panel of activation receptors. Because NK cell activation triggers an aggressive inflammatory response, it is important to have a means of throttling this response. Hence, NK cells also express a panel of inhibitory receptors that recognize ligands expressed by "normal" cells. Little or nothing is known about the endocytosis and trafficking of NK cell receptors, which are of great relevance to understanding how NK cells maintain the appropriate balance of activating and inhibitory receptors on their cell surface. In this review, we focus on the ITIM-containing inhibitory receptor CD94/NKG2A showing that it is endocytosed by a previously undescribed macropinocytic-like process that may be related to the maintenance of its surface expression.

The transmembrane tyrosine of micro-heavy chain is required for BCR destabilization and entry of antigen into clathrin-coated vesicles

The B cell antigen receptor (BCR) delivers antigen to the endocytic compartment and transduces signals that regulate the stability of the receptor complex. Previous studies showed that BCR-mediated signal transduction dissociates micro-heavy chain (microm) from Ig-alpha/Ig-beta, facilitating the delivery of antigen to clathrin-coated vesicles (CCVs). Herein, we demonstrate that the dissociation of Ig-alpha/Ig-beta from microm requires tyrosine-587 of the microm transmembrane domain. Receptors expressing a mutation at tyrosine-587 (Y587F) transduced signals that were comparable to wild type, yet they failed to dissociate microm from Ig-alpha/Ig-beta. Further, receptors harboring the Y587F mutation failed to associate with CCVs, resulting in diminished antigen in the lysosome-associated membrane protein-1 (LAMP-1(+)) compartment and severely impaired antigen presentation, indicating that endocytosis through CCVs is required for antigen presentation. Thus, the transmembrane tyrosine of mum mediates destabilization of the BCR complex, facilitating antigen processing by promoting the association of antigen with CCVs.

Lipid rafts and B cell signaling

B cells comprise an essential component of the humoral immune system. They are equipped with the unique ability to synthesize and secrete pathogen-neutralizing antibodies, and share with professional antigen presenting cells the ability to internalize foreign antigens, and process them for presentation to helper T cells. Recent evidence indicates that specialized cholesterol- and glycosphingolipid-rich microdomains in the plasma membrane commonly referred to as lipid rafts, serve to compartmentalize key signaling molecules during the different stages of B cell activation including B cell antigen receptor (BCR)-initiated signal transduction, endocytosis of BCR-antigen complexes, loading of antigenic peptides onto MHC class II molecules, MHC-II associated antigen presentation to helper T cells, and receipt of helper signals via the CD40 receptor. Here we review the recent literature arguing for a role of lipid rafts in the spatial organization of B cell function.

Functional and structural requirements for the internalization of distinct BCR-ligand complexes

Antigen (Ag) binding to the BCR rapidly initiates two important events: a phosphorylation cascade that results in the production of secondary signaling intermediaries and the internalization of Ag-BCR complexes. Previous studies using anti-BCR antibodies (Ab) have suggested that BCR signaling is an essential requirement for BCR endocytosis and have further implicated lipid rafts as essential platforms for both BCR functions. However, published data from our laboratory indicate that lipid rafts and consequently raft-mediated signaling are dispensable for BCR-mediated internalization of Ag-specific BCR. Therefore, we investigated the relationship between BCR signaling and endocytosis by defining the role of early kinase signaling in the BCR-mediated internalization of a model Ag (haptenated protein). The results demonstrate that Src kinases and Syk-mediated BCR signaling are not essential for BCR-mediated Ag internalization. Moreover, by comparing Ag and Ab, it was determined that while both localize to clathrin-coated pits, the internalization of Ab-BCR complexes is more susceptible to inhibition of signaling and highly sensitive to disruption of lipid rafts and the actin cytoskeleton compared to Ag-BCR complexes. Thus, these results demonstrate that the nature of the ligand ultimately determines the functional requirements and relative contribution of lipid rafts and other membrane structures to the internalization of BCR-ligand complexes.

A mutation in Epstein-Barr virus LMP2A reveals a role for phospholipase D in B-Cell antigen receptor trafficking

Epstein-Barr virus (EBV) latent infection of B cells blocks the interrelated signaling and antigen-trafficking functions of the BCR through the activity of its latent membrane protein 2A (LMP2A). At present, the molecular mechanisms by which LMP2A exerts its control of BCR functions are only poorly understood. Earlier studies showed that in B cells expressing LMP2A containing a tyrosine mutation at position 112 in its cytoplasmic domain (Y112-LMP2A), the BCR could initiate signaling but could not properly traffic antigen for processing. Here, we show that BCR signaling in Y112-LMP2A-expressing cells is attenuated with a reduction in both the degree and duration of phosphorylation of key components of the BCR signaling cascade including Syk, BLNK, PI3K, and Btk. Notably, Y112-LMP2A expression completely blocked the BCR-induced activation of phospholipase D (PLD), a lipase implicated in the intracellular trafficking of a variety of surface receptors. We show that blocking PLD activity, by expressing Y112-LMP2A, treating cells with the PLD inhibitor 1-butanol or reducing PLD expression by siRNA, blocked BCR trafficking to class II-containing compartments. Moreover, Y112-LMP2A expression blocked the recruitment of phosphorylated forms of the downstream BCR signaling components, Erk and JNK, through both PLD-dependent and PLD-independent mechanisms. Thus, the investigation of the mechanism by which Y112-LMP2A blocks BCR function revealed an essential role for PLD in BCR trafficking for antigen processing.

BCR ubiquitination controls BCR-mediated antigen processing and presentation

BCR-mediated antigen processing occurs at immunologically relevant antigen concentrations and hinges on the trafficking of antigen-BCR (Ag-BCR) complexes to class II-containing multivesicular bodies (MVBs) termed MIICs. However, the molecular mechanism underlying the trafficking of Ag-BCR complexes to and within MIICs is not well understood. In contrast, the trafficking of the epidermal growth factor receptor (EGFR) to and within MVBs occurs via a well-characterized ubiquitin-dependent mechanism, which is blocked by acute inhibition of proteasome activity. Using a highly characterized antigen-specific model system, it was determined that the immunoglobulin heavy chain subunit of the IgM BCR of normal (ie, nontransformed) B cells is ubiquitinated. Moreover, acute inhibition of proteasome activity delays the formation of ubiquitinated ligand-BCR complexes, alters the intracellular trafficking of internalized Ag-BCR complexes, and selectively blocks the BCR-mediated processing and presentation of cognate antigen, without inhibiting the endocytosis, processing, and presentation of non-cognate antigen internalized by fluidphase endocytosis. These results demonstrate that the trafficking of Ag-BCR complexes to and within MVB-like antigen processing compartments occurs via a molecular mechanism with similarities to that used by the EGFR, and establishes the EGFR as a paradigm for the further analysis of Ag-BCR trafficking to and within MIICs.

ST6Gal-I restrains CD22-dependent antigen receptor endocytosis and Shp-1 recruitment in normal and pathogenic immune signaling

The ST6Gal-I sialyltransferase produces Siglec ligands for the B-cell-specific CD22 lectin and sustains humoral immune responses. Using multiple experimental approaches to elucidate the mechanisms involved, we report that ST6Gal-I deficiency induces immunoglobulin M (IgM) antigen receptor endocytosis in the absence of immune stimulation. This coincides with increased antigen receptor colocalization with CD22 in both clathrin-deficient and clathrin-enriched membrane microdomains concurrent with diminished tyrosine phosphorylation of Igalpha/beta, Syk, and phospholipase C-gamma2 upon immune activation. Codeficiency with CD22 restores IgM antigen receptor half-life at the cell surface in addition to reversing alterations in membrane trafficking and immune signaling. Diminished immune responses due to ST6Gal-I deficiency further correlate with constitutive recruitment of Shp-1 to CD22 in unstimulated B cells independent of Lyn tyrosine kinase activity and prevent autoimmune disease pathogenesis in the Lyn-deficient model of systemic lupus erythematosus, resulting in a significant extension of life span. Protein glycosylation by ST6Gal-I restricts access of antigen receptors and Shp-1 to CD22 and operates by a CD22-dependent mechanism that decreases the basal rate of IgM antigen receptor endocytosis in altering the threshold of B-cell immune activation.

The high-affinity immunoglobulin-E receptor (FcepsilonRI) is endocytosed by an AP-2/clathrin-independent, dynamin-dependent mechanism

Aggregation of the high-affinity immunoglobulin E (IgE) receptor (FcepsilonRI), expressed on mast cells and basophils, initiates the immediate hypersensitivity reaction. Aggregated FcepsilonRI has been reported to rapidly migrate to lipid rafts in RBL-2H3 cells. We confirmed that aggregated FcepsilonRI is found in the lipid raft fractions of cellular lysates. Furthermore, we show that the cross-linked FcepsilonRI remains associated with detergent-resistant structures upon internalization. Previous morphological studies have reported that aggregated FepsiloncRI is endocytosed via clathrin-coated pits, which in general are not lipid raft associated. To address this apparent discrepancy, we employed siRNA to suppress expression of components of the clathrin-mediated internalization machinery, namely, clathrin heavy chain, and the AP-2 (alpha-adaptin or mu2-subunit). Transferrin receptor (TfR) is endocytosed by a clathrin-mediated process and, as expected, each transfected siRNA caused a two to threefold elevation of TfR surface expression and almost completely inhibited its endocytosis. In contrast, there was no effect on surface expression levels of FcepsilonRI nor on the endocytosis of the dinitrophenyl-human serum albumin (DNP-HSA)/IgE/FcepsilonRI complex. On the contrary, internalization of DNP-HSA/IgE/FcepsilonRI was inhibited by overexpression of a dominant-negative dynamin mutant. We conclude that internalization of cross-linked FcRI does not require the AP-2/clathrin complex but is dynamin-dependent and may be lipid raft mediated.

B cell antigen receptor signaling and internalization are mutually exclusive events

Engagement of the B cell antigen receptor initiates two concurrent processes, signaling and receptor internalization. While both are required for normal humoral immune responses, the relationship between these two processes is unknown. Herein, we demonstrate that following receptor ligation, a small subpopulation of B cell antigen receptors are inductively phosphorylated and selectively retained at the cell surface where they can serve as scaffolds for the assembly of signaling molecules. In contrast, the larger population of non-phosphorylated receptors is rapidly endocytosed. Each receptor can undergo only one of two mutually exclusive fates because the tyrosine-based motifs that mediate signaling when phosphorylated mediate internalization when not phosphorylated. Mathematical modeling indicates that the observed competition between receptor phosphorylation and internalization enhances signaling responses to low avidity ligands.

B cells as antigen presenting cells

Several characteristics confer on B cells the ability to present antigen efficiently: (1) they can find T cells in secondary lymphoid organs shortly after antigen entrance, (2) BCR-mediated endocytosis allows them to concentrate small amounts of specific antigen, and (3) BCR signaling and HLA-DO expression direct their antigen processing machinery to favor presentation of antigens internalized through the BCR. When presenting antigen in a resting state, B cells can induce T cell tolerance. On the other hand, activation by antigen and T cell help converts them into APC capable of promoting immune responses. Presentation of self antigens by B cells is important in the development of autoimmune diseases, while presentation of tumor antigens is being used in vaccine strategies to generate immunity. Thus, detailed understanding of the antigen presenting function of B cells can lead to their use for the generation or inhibition of immune responses.

Independent trafficking of Ig-alpha/Ig-beta and mu-heavy chain is facilitated by dissociation of the B cell antigen receptor complex

The BCR relays extracellular signals and internalizes Ag for processing and presentation. We have previously demonstrated that ligation of the BCR destabilizes Ig-alpha/Ig-beta (Ig-alphabeta) from mu-H chain (mum). In this study we report that receptor destabilization represents a physical separation of mum from Ig-alphabeta. Sucrose gradient fractionation localized Ig-alphabeta to G(M1)-containing lipid microdomains in the absence of mum. Confocal and electron microscopy studies revealed the colocalization of unsheathed mum with clathrin-coated vesicles. Furthermore, mum failed to associate with clathrin-coated vesicles when receptor destabilization was inhibited, suggesting that unsheathing of mum is required for clathrin-mediated endocytosis. In summary, we found that Ag stimulation physically separates Ig-alphabeta from mum, facilitating concomitant signal transduction and Ag delivery to the endocytic compartment.

Lipid Rafts Associate with Intracellular B Cell Receptors and Exhibit a B Cell Stage-Specific Protein Composition

Lipid rafts serve as platforms for BCR signal transduction. To better define the molecular basis of these membrane microdomains, we used two-dimensional gel electrophoresis and mass spectrometry to characterize lipid raft proteins from mature as well as immature B cell lines. Of 51 specific raft proteins, we identified a total of 18 proteins by peptide mass fingerprinting. Among them, we found vacuolar ATPase subunits alpha-1 and beta-2, vimentin, gamma-actin, mitofilin, and prohibitin. None of these has previously been reported in lipid rafts of B cells. The differential raft association of three proteins, including a novel potential signaling molecule designated swiprosin-1, correlated with the stage-specific sensitivity of B cells to BCR-induced apoptosis. In addition, MHC class II molecules were detected in lipid rafts of mature, but not immature B cells. This intriguing finding points to a role for lipid rafts in regulating Ag presentation during B cell maturation. Finally, a fraction of the BCR in the B cell line CH27 was constitutively present in lipid rafts. Surprisingly, this fraction was neither expressed at the cell surface nor fully O-glycosylated. Thus, we conclude that partitioning the BCR into lipid rafts occurs in the endoplasmic reticulum/cis-Golgi compartment and may represent a control mechanism for surface transport.

Plasticity of B cell receptor internalization upon conditional depletion of clathrin

B cell antigen receptor (BCR) association with lipid rafts, the actin cytoskeleton, and clathrin-coated pits influences B cell signaling and antigen presentation. Although all three cellular structures have been separately implicated in BCR internalization, the relationship between them has not been clearly defined. In this study, internalization pathways were characterized by specifically blocking each potential mechanism of internalization. BCR uptake was reduced by approximately 70% in B cells conditionally deficient in clathrin heavy chain expression. Actin or raft antagonists were both able to block the residual, clathrin-independent BCR internalization. These agents also affected clathrin-dependent internalization, indicating that clathrin-coated pits, in concert with mechanisms dependent on rafts and actin, mediate the majority of BCR internalization. Clustering G(M1) gangliosides enhanced clathrin-independent BCR internalization, and this required actin. Thus, although rafts or actin independently did not mediate BCR internalization, they apparently cooperate to promote some internalization even in the absence of clathrin. Simultaneous inhibition of all BCR uptake pathways resulted in sustained tyrosine phosphorylation and activation of the extracellular signal-regulated kinase (ERK), strongly suggesting that downstream BCR signaling can occur without receptor translocation to endosomes and that internalization leads to signal attenuation.

B-cell antigen receptor signaling requirements for targeting antigen to the MHC class II presentation pathway

The ability of B lymphocytes to capture, process and present antigens to T cells is requisite for normal humoral immune responses and contributes to the pathogenesis of both B- and T-cell-mediated autoimmune diseases. B lymphocytes preferentially capture polyvalent antigens, which are capable of eliciting a coordinated series of cellular responses that ensure that even low-affinity antigens are productively captured. Polyvalency not only accelerates transit through the endocytic pathway but also induces a reorganization of the antigen-processing compartment, activates degradative pathways and determines how antigenic peptides are presented to T cells. Similar changes are observed in maturing dendritic cells, indicating that some cellular responses to foreign antigens are conserved.

Improved access to CD20 following B cell receptor cross-linking at Burkitt's lymphoma cell surfaces

Here we report that B cell receptor (BCR) engagement rapidly improves the capacity of CD20 to be accessed by cognate antibody at model Burkitt's lymphoma cell surfaces. None of eight other surface molecules demonstrated such BCR-dependent enhancement of ligand binding while the quantity of accessible CD20 remained unchanged on either CD19 or CD40 engagement. Neither the actin-depolymerizing agent cytochalasin D nor inhibitors targeting signalling pathways associated with the BCR attenuated the CD20 increase that could be uncoupled from BCR endocytosis. Instead, a role for lipid rafts was indicated both from the inhibitory actions of cholesterol-sequestering methyl-beta-cyclodextrin and direct analysis of CD20 redistribution using sucrose density gradients and confocal microscopy. Whether such observations could find application in CD20-directed therapies where success can be compromised by otherwise low-level expression of target antigen is discussed.

Differences in signaling molecule organization between naive and memory CD4+ T lymphocytes

The immunological synapse is a highly organized complex formed at the junction between Ag-specific T cells and APCs as a prelude to cell activation. Although its exact role in modulating T cell signaling is unknown, it is commonly believed that the immunological synapse is the site of cross-talk between the T cell and APC (or target). We have examined the synapses formed by naive and memory CD4 cells during Ag-specific cognate interactions with APCs. We show that the mature immunological synapse forms more quickly during memory T cell activation. We further show that the composition of the synapse found in naive or memory cell conjugates with APCs is distinct with the tyrosine phosphatase, CD45, being a more integral component of the mature synapses formed by memory cells. Finally, we show that signaling molecules, including CD45, are preassociated in discrete, lipid-raft microdomains in resting memory cells but not in naive cells. Thus, enhanced memory cell responses may be due to intrinsic properties of signaling molecule organization.

Lipid rafts in lymphocyte activation

The existence of sphingolipid- and cholesterol-rich membrane microdomains called "lipid rafts", as well as their role in lymphocyte biology, has been widely debated during the last few years. Plasma membrane microdomains seem to be primarily involved in initiation and propagation of the signal transduction cascade associated with lymphocyte activation. In this review, we discuss the recent literature suggesting that, during lymphocyte activation and chemotaxis, lipid rafts act as platforms to compartmentalise signalling and facilitate specific protein-protein interactions.

Recruitment of the Actin-binding Protein HIP-55 to the Immunological Synapse Regulates T Cell Receptor Signaling and Endocytosis

Actin cytoskeleton dynamics critically regulate T cell activation. We found that the cytoplasmic adaptor HIP-55, a Src/Syk-kinases substrate and member of the drebrin/Abp1 family of actin-binding proteins, localized to the T cell-antigen-presenting cell (APC) contact site in an antigen-dependent manner. Using green fluorescent protein fusion proteins, both Src homology 3 (SH3) and actin binding domains were found necessary for recruitment at the T cell-APC interface. HIP-55 was not implicated in conjugate formation and actin polymerization but regulated distal signaling events through binding and activation of hematopoietic progenitor kinase 1 (HPK1), a germinal center kinase (GCK) family kinase involved in negative signaling in T cells. Using RNA interference and overexpression experiments, the HIP-55-HPK1 complex was found to negatively regulate nuclear factor of activated T cell (NFAT) activation by the T cell antigen receptor. Moreover, we show that HIP-55, which partly co-localized with early endocytic compartments, promoted both basal and ligand-dependent T cell receptor (TCR) down-modulation, resulting in a decreased TCR expression. SH3 and actin-depolymerizing factor homology domains were required for this function. As controls, the expression of CD28 and the glycosylphosphatidylinositol-linked protein CD59 was not affected by HIP-55 overexpression. These results suggest that, in addition to binding to HPK1, HIP-55 might negatively regulate TCR signaling through down-regulation of TCR expression. Our findings show that HIP-55 is a key novel component of the immunological synapse that modulates T cell activation by connecting actin cytoskeleton and TCRs to gene activation and endocytic processes.