Molecular mechanisms of B cell antigen receptor trafficking

B cells acquire a unique and differential transcriptomic profile during pregnancy

Pregnancy alters B cell development and function. B cell activation is initiated by antigens binding to the BCR leading to B cell survival, proliferation, antigen presentation and antibody production. We performed a genome-wide transcriptome profiling of splenic B cells from pregnant (P) and non-pregnant (NP) mice and identified 1136 genes exhibiting differential expression in B cells from P mice (625 up- and 511 down-regulated) compared to NP animals. In silico analysis showed that B cell activation through BCR seems to be lowered during pregnancy. RT-qPCR analysis confirmed these data. Additionally, B cells from pregnant women stimulated in vitro through BCR produced lower levels of inflammatory cytokines compared to non-pregnant women. Our results suggest that B cells acquire a state of hypo-responsiveness during gestation, probably as part of the maternal immune strategy for fetal tolerance but also open new avenues to understand why pregnant women are at highest risk for infections.

Purification and characterization of immunogenic recombinant virus-like particles of porcine circovirus type 2 expressed in silkworm pupae

Porcine circovirus type 2 (PCV2) is a primary causative agent of postweaningmultisystemic wasting syndrome (PMWS), which has a significant economic impact on the swine industry. The capsid protein (Cap) encoded by ORF2 of the viral genome has been used effectively as a vaccine against PCV2 infection. The Cap protein can spontaneously assemble into virus-like particles (VLPs) that are safe and highly immunogenic for vaccine applications. Several expression systems, including bacteria, yeast and insect cells, have been utilized to produce PCV2 VLPs. However, in some cases, the recombinant Cap (rCap) proteins produced in bacteria and yeast do not assemble spontaneously. In this study, we expressed rCap protein using a silkworm-baculovirus expression vector system (silkworm-BEVS) for mass production of PCV2 VLPs and established a simple three-step protocol for its purification from pupae: extraction by detergent, ammonium sulfate precipitation and anion exchange column chromatography. Size-exclusion chromatography (SEC) analysis and transmission electron microscope (TEM) observation showed that purified rCap proteins formed VLPs with a similar morphology to that of the original virus. Furthermore, the VLPs produced in silkworms were capable of inducing neutralizing antibodies against PCV2 in mice. Our results demonstrated that the silkworm system is a powerful tool for the production of PCV2 VLPs and will be useful for the development of a reliable and cost-effective PCV2 vaccine.

B Cell Receptor Signaling and Compartmentalization by Confocal Microscopy

Binding of antigen to the B cell receptor (BCR) triggers both BCR signaling and endocytosis simultaneously. BCR signaling pathways and their regulation have been studied extensively by both biochemical methods and flow cytometry, resulting in a comprehensive understanding of the temporal dynamics of the signaling enzymes and effector proteins. However, spatial regulation of these signaling pathways in subcellular pathways is relatively poorly understood. Here, we describe a method to study the spatio-temporal distribution of phosphorylated-kinases in antigen-activated B cells by confocal microscopy. This method can also be applied to other cell types where it is of interest to understand the spatial distribution of signaling enzymes and their effector proteins.

Piperine, a Pungent Alkaloid from Black Pepper, Inhibits B Lymphocyte Activation and Effector Functions

Piperine has several well-documented anti-inflammatory properties; however, little is known regarding its effect on humoral immunity. In this study, we describe the immunosuppressive effect of piperine on B lymphocytes, which are integral to the humoral immune response. Mouse B cells were cultured in the absence or presence of non-cytotoxic concentrations (25, 50, and 100 μM) of piperine during T-dependent or T-independent stimulation. Piperine inhibited B cell proliferation by causing G0/G1 phase cell cycle arrest in association with reduced expression of cyclin D2 and D3. The inhibitory effect of piperine was not mediated through transient receptor potential vanilloid-1 ion channel (TRPV1) because piperine also inhibited the proliferation of B cells from TRPV1-deficient mice. Expression of class II major histocompatibility complex molecules and costimulatory CD40 and CD86 on B lymphocytes was reduced in the presence of piperine, as was B cell-mediated antigen presentation to syngeneic T cells. In addition, piperine inhibited B cell synthesis of interleukin (IL)-6 and IL-10 cytokines, as well as IgM, IgG2b, and IgG3 immunoglobulins. The inhibitory effect of piperine on B lymphocyte activation and effector function warrants further investigation for possible application in the treatment of pathologies related to inappropriate humoral immune responses. Copyright © 2017 John Wiley & Sons, Ltd.

Erythrophagocytosis by plasmacytoid dendritic cells and monocytes is enhanced during inflammation

BACKGROUND

Generation of antibodies against red blood cell (RBC) antigens can be a clinically significant problem. The underlying mechanisms that regulate the production of RBC antibodies are only partially understood; however, factors such as inflammation significantly increase the rates of RBC antibody generation. Humoral alloimmunization begins with consumption of transfused RBCs by antigen-presenting cells (APCs). Recently, it has become appreciated that there are multiple different types of APCs. The relative contribution of APC subsets to RBC antibodies has not been described in either the quiescent or the inflamed states.

STUDY DESIGN AND METHODS

To evaluate the types of APCs that consume RBCs, and how inflammation affects this process, C56Bl/6 mice were treated with polyinosinic-polycytidylic acid (poly(I:C)) to induce an inflammatory response and/or were transfused with 3,3'-dihexadecyloxacarbocyanine perchlorate-labeled syngeneic RBCs. Erythrophagocytosis (both at baseline and during inflammation) was analyzed for different subsets of macrophages (MΦ), dendritic cells (DCs), B cells, and monocytes, by a combined approach using flow cytometry and fluorescent microscopy technology.

RESULTS

In four independent experiments, erythrophagocytosis at baseline was predominately performed by red pulp MΦ; however, during inflammation both plasmacytoid DCs (pDCs) and monocytes increased RBC consumption. Furthermore, pDCs up regulated MHC-II and activation markers CD80 and CD86. In addition to changing patterns of erythrophagocytosis, inflammation also led to a significant decrease in CD11c+ conventional DC populations and an increase in granulocytes.

CONCLUSIONS

The nature of APCs that consume transfused RBCs is changed by inflammation. Given that APCs initiate humoral immune responses, these findings provide potential mechanistic insight into how inflammation regulates RBC alloimmunization.

Tricomponent complex loaded with a mosquito-stage antigen of the malaria parasite induces potent transmission-blocking immunity

The development of malaria vaccines is challenging, partly because the immunogenicity of recombinant malaria parasite antigens is low. We previously demonstrated that parasite antigens integrated into a tricomponent immunopotentiating complex increase antiparasitic immunity. In this study, the B domains of a group G Streptococcus (SpG) strain and Peptostreptococcus magnus (PpL) were used to evaluate whether vaccine efficacy is influenced by the type of immunoglobulin-binding domain (IBD) in the tricomponent complex. IBDs were fused to a pentameric cartilage oligomeric matrix protein (COMP) to increase the binding avidity of the complexes for their targets. The COMP-IBD fusion proteins generated (COMP-SpG and COMP-PpL and the previously constructed COMP-Z) bound a large fraction of splenic B lymphocytes but not T lymphocytes. These carrier molecules were then loaded with an ookinete surface protein of Plasmodium vivax, Pvs25, by chemical conjugation. The administration of the tricomponent complexes to mice induced more Pvs25-specific serum IgG than did the unloaded antigen. The PpL complex, which exhibited a broad Ig-binding spectrum, conferred higher vaccine efficacy than did the Z or SpG complexes when evaluated with a membrane feed assay. This study demonstrates that this tricomponent immunopotentiating system, incorporating IBDs as the B-lymphocyte-targeting ligands, is a promising technology for the delivery of malaria vaccines, particularly when combined with an aluminum salt adjuvant.

Variant B cell receptor isotype functions differ in hairy cell leukemia with mutated BRAF and IGHV genes

A functional B-cell receptor (BCR) is critical for survival of normal B-cells, but whether it plays a comparable role in B-cell malignancy is as yet not fully delineated. Typical Hairy Cell Leukemia (HCL) is a rare B-cell tumor, and unique in expressing multiple surface immunoglobulin (sIg) isotypes on individual tumor cells (mult-HCL), to raise questions as to their functional relevance. Typical mult-HCL also displays a mutated BRAF V(600)E lesion. Since wild type BRAF is a primary conduit for transducing normal BCR signals, as revealed by deletion modelling studies, it is as yet not apparent if mutated BRAF alters BCR signal transduction in mult-HCL. To address these questions, we examined BCR signalling in mult-HCL cases uniformly displaying mutated BRAF and IGHV genes. Two apparent functional sets were delineated by IgD co-expression. In sIgD^+ve mult-HCL, IgD mediated persistent Ca²⁺ flux, also evident via >1 sIgH isotype, linked to increased ERK activation and BCR endocytosis. In sIgD^−ve mult-HCL however, BCR-mediated signals and downstream effects were restricted to a single sIgH isotype, with sIgM notably dysfunctional and remaining immobilised on the cell surface. These observations reveal discordance between expression and function of individual isotypes in mult-HCL. In dual sIgL expressing cases, only a single sIgL was fully functional. We examined effects of anti-BCR stimuli on mult-HCL survival ex-vivo. Significantly, all functional non-IgD isotypes increased ERK1/2 phosphorylation but triggered apoptosis of tumor cells, in both subsets. IgD stimuli, in marked contrast retained tumor viability. Despite mutant BRAF, BCR signals augment ERK1/2 phosphorylation, but isotype dictates functional downstream outcomes. In mult-HCL, sIgD retains a potential to transduce BCR signals for tumor survival in-vivo. The BCR in mult-HCL emerges as subject to complex regulation, with apparent conflicting signalling by individual isotypes when co-expressed with sIgD. This suggests the possibility that mutant BRAF by-passes BCR constraints in mult-HCL.

Endocytosed BCRs sequentially regulate MAPK and Akt signaling pathways from intracellular compartments

Binding of antigen to the B cell antigen receptor (BCR) triggers both BCR signaling and endocytosis. How endocytosis regulates BCR signaling remains unknown. Here we report that BCR signaling was not extinguished by endocytosis of BCRs; instead, BCR signaling initiated at the plasma membrane continued as the BCR trafficked intracellularly with the sequential phosphorylation of kinases. Blocking the endocytosis of BCRs resulted in the recruitment of both proximal and downstream kinases to the plasma membrane, where mitogen-activated protein kinases (MAPKs) were hyperphosphorylated and the kinase Akt and its downstream target Foxo were hypophosphorylated, which led to the dysregulation of gene transcription controlled by these pathways. Thus, the cellular location of the BCR serves to compartmentalize kinase activation to regulate the outcome of signaling.

Tricomponent immunopotentiating system as a novel molecular design strategy for malaria vaccine development

The creation of subunit vaccines to prevent malaria infection has been hampered by the intrinsically weak immunogenicity of the recombinant antigens. We have developed a novel strategy to increase immune responses by creating genetic fusion proteins to target specific antigen-presenting cells (APCs). The fusion complex was composed of three physically linked molecular entities: (i) a vaccine antigen, (ii) a multimeric α-helical coiled-coil core, and (iii) an APC-targeting ligand linked to the core via a flexible linker. The vaccine efficacy of the tricomponent complex was evaluated using an ookinete surface protein of Plasmodium vivax, Pvs25, and merozoite surface protein-1 of Plasmodium yoelii. Immunization of mice with the tricomponent complex induced a robust antibody response and conferred substantial levels of P. vivax transmission blockade as evaluated by a membrane feed assay, as well as protection from lethal P. yoelii infection. The observed effect was strongly dependent on the presence of all three components physically integrated as a fusion complex. This system, designated the tricomponent immunopotentiating system (TIPS), onto which any recombinant protein antigens or nonproteinaceous substances could be loaded, may be a promising strategy for devising subunit vaccines or adjuvants against various infectious diseases, including malaria.

Gene expression profiling of peripheral blood mononuclear cells from children with active hemophagocytic lymphohistiocytosis

Familial hemophagocytic lymphohistiocytosis (FHL) is a rare, genetically heterogeneous autosomal recessive immune disorder that results when the critical regulatory pathways that mediate immune defense mechanisms and the natural termination of immune/inflammatory responses are disrupted or overwhelmed. To advance the understanding of FHL, we performed gene expression profiling of peripheral blood mononuclear cells from 11 children with untreated FHL. Total RNA was isolated and gene expression levels were determined using microarray analysis. Comparisons between patients with FHL and normal pediatric controls (n = 30) identified 915 down-regulated and 550 up-regulated genes with more than or equal to 2.5-fold difference in expression (P ≤ .05). The expression of genes associated with natural killer cell functions, innate and adaptive immune responses, proapoptotic proteins, and B- and T-cell differentiation were down-regulated in patients with FHL. Genes associated with the canonical pathways of interleukin-6 (IL-6), IL-10 IL-1, IL-8, TREM1, LXR/RXR activation, and PPAR signaling and genes encoding of antiapoptotic proteins were overexpressed in patients with FHL. This first study of genome-wide expression profiling in children with FHL demonstrates the complexity of gene expression patterns, which underlie the immunobiology of FHL.

B cell receptor-mediated internalization of salmonella: a novel pathway for autonomous B cell activation and antibody production

The present paradigm is that primary B cells are nonphagocytosing cells. In this study, we demonstrate that human primary B cells are able to internalize bacteria when the bacteria are recognized by the BCR. BCR-mediated internalization of Salmonella typhimurium results in B cell differentiation and secretion of anti-Salmonella Ab by the Salmonella-specific B cells. In addition, BCR-mediated internalization leads to efficient Ag delivery to the MHC class II Ag-loading compartments, even though Salmonella remains vital intracellularly in primary B cells. Consequently, BCR-mediated bacterial uptake induces efficient CD4(+) T cell help, which boosts Salmonella-specific Ab production. BCR-mediated internalization of Salmonella by B cells is superior over extracellular Ag extraction to induce rapid and specific humoral immune responses and efficiently combat infection.

The B cell receptor governs the subcellular location of Toll-like receptor 9 leading to hyperresponses to DNA-containing antigens

Synergistic engagement of the B cell receptor (BCR) and Toll-like receptor 9 (TLR9) in response to DNA-containing antigens underlies the production of many autoantibodies in systemic autoimmune diseases. However, the molecular basis of this synergistic engagement is not known. Given that these receptors are spatially segregated, with the BCR on the cell surface and TLR9 in endocytic vesicles, achieving synergy must involve unique mechanisms. We show that upon antigen binding, the BCR initiates signaling at the plasma membrane and continues to signal to activate MAP kinases as it traffics to autophagosome-like compartments. The internalized BCR signals through a phospholipase-D-dependent pathway to recruit TLR9-containing endosomes to the autophagosome via the microtubular network. The recruitment of TLR9 to the autophagosomes was necessary for hyperactivation of MAP kinases. This unique mechanism for BCR-induced TLR9 recruitment resulting in B cells hyperresponses may provide new targets for therapeutics for autoimmune diseases.

Bystander B cells rapidly acquire antigen receptors from activated B cells by membrane transfer

The B cell antigen receptor (BCR) efficiently facilitates the capture and processing of a specific antigen for presentation on MHC class II molecules to antigen-specific CD4(+) T cells (1). Despite this, the majority of B cells are thought to play only a limited role in CD4(+) T cell activation because BCRs are clonotypically expressed. Here, we show, however, that activated B cells can, both in vitro and in vivo, rapidly donate their BCR to bystander B cells, a process that is mediated by direct membrane transfer between adjacent B cells and is amplified by the interaction of the BCR with a specific antigen. This results in a dramatic expansion in the number of antigen-binding B cells in vivo, with the transferred BCR endowing recipient B cells with the ability to present a specific antigen to antigen-specific CD4(+) T cells.

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.

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.

TRPML cation channels regulate the specialized lysosomal compartment of vertebrate B-lymphocytes

B-lymphocytes possess a specialized lysosomal compartment, the regulated transformation of which has been implicated in B-cell antigen presentation. Members of the mucolipin (TRPML) family of cation channels have been implicated in regulated vesicular transport in several tissues, but a role for TRPML function in lymphocyte vesicular transport physiology has not been previously described. To address the role of TRPML proteins in lymphocyte vesicular transport, we analyzed the lysosomal compartment in cultured B-lymphocytes engineered to lack TRPML1 or after expression of N- or C-terminal GFP fusion proteins of TRPML1 or TRPML2. Consistent with previous analyses of lymphocytes derived from human patients with mutations in TRPML1, we were not able to detect abnormalities in the lysosomes of TRPML1-deficient DT40 B-lymphocytes. However, while N-terminal GFP fusions of TRPML2 localized to normal appearing lysosomes, C-terminal GFP fusions of either TRPML1 or TRPML2 acted to antagonize endogenous TRPML function, localizing to large vesicular structures, the histological properties of which were indistinguishable from the enlarged lysosomes observed in affected tissues of TRPML1-deficient humans. Endocytosed B-cell receptors were delivered to these enlarged lysosomes, demonstrating that a TRPML-dependent process is required for normal regulation of the specialized lysosome compartment of vertebrate B-lymphocytes.

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.

Monovalent ligation of the B cell receptor induces receptor activation but fails to promote antigen presentation

We explored the role of antigen valency in B cell receptor (BCR) activation and rearrangement of intracellular MHC class II compartments as factors that contribute to the efficacy of antigen presentation. Using primary B cells that express a hen egg lysozyme (HEL)-specific BCR, we found that oligomeric HEL more efficiently promoted both BCR activation and internalization than did monovalent HEL, although monovalent HEL, unlike monovalent Fab fragments of anti-Ig, readily triggered the BCR. Nonetheless, oligovalent ligation positions the BCR in a membrane microdomain that is distinct from one engaged in the course of monovalent ligation, as judged by detergent extraction of the BCR. Furthermore, oligovalent HEL induced more pronounced rearrangement of MHC class II-containing antigen-processing compartments. Using videomicroscopy we observed in real time the rearrangement of MHC class II compartments as well as delivery of antigen in primary B cells. The observed increase in rearrangement of MHC class II-positive compartments and the disposition of antigen-bound BCRs therein correlates with improved presentation of a HEL-derived epitope. Although monomeric HEL efficiently engages the BCR, presentation of HEL-derived epitopes is impaired compared to oligovalent antigens. This trait may help explain the known ability of soluble, disaggregated antigen to induce a state of B cell tolerance.

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.

BCR targeting of biotin-α-galactosylceramide leads to enhanced presentation on CD1d and requires transport of BCR to CD1d-containing endocytic compartments

CD1d is a non-polymorphic MHC class I-related protein that binds and presents glycolipid antigens to T cell antigen receptors expressed by NK-like T (NKT) cells. CD1d-dependent immune responses play critical roles in infectious disease, autoimmunity, allergy and cancer. We tested the hypothesis that B cell antigen receptor (BCR) targeting of a biotin-modified version of the CD1d-binding antigen alpha-galactosylceramide (biotin-alpha-GalCer) results in enhanced murine CD1d-mediated presentation as compared with presentation of non-targeted biotin-alpha-GalCer. Presentation of BCR-targeted antigen to NKT cells was enhanced 100- to 1000-fold compared with non-targeted antigen. CD1d presentation of BCR-targeted antigen was observed after 4 h treatment, consistent with a requirement for endosomal trafficking. Furthermore, unlike non-targeted antigen, BCR-targeted antigen was not loaded directly onto cell-surface CD1d. Blocking BCR signaling with the Syk tyrosine kinase inhibitor piceatannol inhibited presentation of BCR-targeted antigen but not non-targeted antigen. Piceatannol blocked transport of the BCR to CD1d-containing endosomes, showing that intersection of BCR-targeted antigen with endosomes is required for enhanced mCD1d antigen presentation. Our data suggest that the BCR facilitates capture of low quantities of mCD1d antigens to enhance CD1d-dependent immune responses.

Ig-independent Ig beta expression on the surface of B lymphocytes after B cell receptor aggregation

In order for humoral immune responses to develop, B cells must be able to recognize, bind, and internalize Ags. These functions are performed by the BCR, which is also responsible for initiating and transducing activation signals necessary for B cell proliferation and differentiation. We have examined surface expression patterns of individual components of the BCR following anti-Ig- and Ag-induced aggregation. Specifically, the localization and expression levels of the Ag-binding component, surface Ig (sIg), and the Igbeta component of the Igalpha/Igbeta signaling unit were investigated to determine their individual participation in the internalization and signal transduction. Using primary murine B cells, we found that while >95% of the sIg is internalized following anti-Ig-induced aggregation, 20-30% of Igbeta remains on the surface. These results suggest that sIg and Igbeta may function independently following the initial stages of signal transduction.

Analysis of the major patterns of B cell gene expression changes in response to short-term stimulation with 33 single ligands

We examined the major patterns of changes in gene expression in mouse splenic B cells in response to stimulation with 33 single ligands for 0.5, 1, 2, and 4 h. We found that ligands known to directly induce or costimulate proliferation, namely, anti-IgM (anti-Ig), anti-CD40 (CD40L), LPS, and, to a lesser extent, IL-4 and CpG-oligodeoxynucleotide (CpG), induced significant expression changes in a large number of genes. The remaining 28 single ligands produced changes in relatively few genes, even though they elicited measurable elevations in intracellular Ca(2+) and cAMP concentration and/or protein phosphorylation, including cytokines, chemokines, and other ligands that interact with G protein-coupled receptors. A detailed comparison of gene expression responses to anti-Ig, CD40L, LPS, IL-4, and CpG indicates that while many genes had similar temporal patterns of change in expression in response to these ligands, subsets of genes showed unique expression patterns in response to IL-4, anti-Ig, and CD40L.

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.

Ligand-dependent and -independent transforming growth factor-beta receptor recycling regulated by clathrin-mediated endocytosis and Rab11

Proteins in the transforming growth factor-beta (TGF-beta) family recognize transmembrane serine/threonine kinases known as type I and type II receptors. Binding of TGF-beta to receptors results in receptor down-regulation and signaling. Whereas previous work has focused on activities controlling TGF-beta signaling, more recent studies have begun to address the trafficking properties of TGF-beta receptors. In this report, it is shown that receptors undergo recycling both in the presence and absence of ligand activation, with the rates of internalization and recycling being unaffected by ligand binding. Recycling occurs as receptors are most likely internalized through clathrin-coated pits, and then returned to the plasma membrane via a rab11-dependent, rab4-independent mechanism. Together, the results suggest a mechanism wherein activated TGF-beta receptors are directed to a distinct endocytic pathway for down-regulation and clathrin-dependent degradation after one or more rounds of recycling.

Experimental arthritis in CC chemokine receptor 2-null mice closely mimics severe human rheumatoid arthritis

The prevailing paradigm is that in human rheumatoid arthritis (RA), the accumulation of monocytes and T cells in the joint, mediated in part by such CC chemokine receptors (CCRs) as CCR2 and CCR5, respectively, plays a central role in disease pathogenesis. To further validate this paradigm, we conducted proof-of-principle studies and tested the hypothesis that gene inactivation of Ccr2 or Ccr5 will ameliorate experimental RA. Contrary to our expectations, we found that in two well-established murine models of experimental RA, CCR2 expression in the hematopoietic cell compartment served as a negative regulator of autoantibody production as well as arthritic disease onset, severity, and resolution. In contrast, the RA phenotype in Ccr5-null mice was similar to that of WT mice. Remarkably, the collagen-induced arthritis phenotype of Ccr2-/- mice mimicked closely that of severe human RA, including production of rheumatoid factor, enhanced T cell production, and monocyte/macrophage accumulation in the joints. Our findings demonstrate an essential protective role of CCR2 expression in RA, indicate the existence of alternative receptors responsible for monocyte/macrophage accumulation to inflamed joints, and emphasize the need to clarify carefully the complex effects of the chemokine system in RA before they can be considered as therapeutic targets.

Membrane specializations and endosome maturation in dendritic cells and B cells

Interest in the cell biology of antigen presentation is centered on dendritic cells (DCs) as initiators of the immune response. The ability to examine primary antigen-presenting cells, as opposed to cell lines, has opened a new window for study of antigen processing and peptide acquisition by Class II major histocompatibility complex (MHC) products, especially where intracellular trafficking of peptide-Class-II complexes is concerned. Here, we review the dynamics of Class II MHC-positive intracellular structures in dendritic cells as well as B cells. We focus on the generation of multivesicular bodies, where Class II MHC products acquire antigenic peptide, on the endosomal transport of peptide-loaded Class II MHC to the cell surface and on the importance of Class II MHC localization in membrane microdomains.