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Published in final edited form as: Prog Mol Biol Transl Sci. 2022 Sep 3;194:159–177. doi: 10.1016/bs.pmbts.2022.08.003

Chapter 12: B Cell Receptor (BCR) Endocytosis

Aleah D Roberts 1, Justin W Taraska 1,*
PMCID: PMC11851869  NIHMSID: NIHMS2054819  PMID: 36631191

Abstract

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.

Keywords: endocytosis, B cell receptor, clathrin, phagocytosis, lipid raft, lymphoma, leukemia

Introduction to the B Cell Receptor

B cells are a part of the adaptive immune response that generates antigen specific antibodies. Antibodies (also known as immunoglobulins) are proteins that can bind pathogens directly to prevent infection, or bind infected cells to facilitate pathogen clearance by phagocytosis. The B cell receptor (BCR) covers the surface of mature B cells, and initiates cellular responses that lead to B cell activation and antibody production. Each B cell expresses a unique B cell receptor with specificity for a unique antigen. The BCR can bind its cognate antigen encountered directly or presented on the surface of an antigen presenting cell (APC). In either case, antigen engagement leads to BCR crosslinking and triggers activation of signaling pathways and receptor-mediated endocytosis of the BCR-antigen complex. The antigen is then degraded and inserted into major histocompatibility complex class II (MHCII) proteins that are trafficked to the plasma membrane. At the membrane, MHCII molecules present antigen to CD4+ helper T cells, which stimulate the B cell to proliferate and ultimately differentiate into an antibody producing plasma cell. In some cases, B cells may also initiate T-mediated immune responses that do not involve antibodies.

B cell Receptor Structure

Structurally, the BCR is composed of a membrane anchored immunoglobulin (mIg) that is paired with two coreceptors: CD79a and CD79b (also called Igα and Igβ; Figure 1). CD79a and CD79b form a covalently linked heterodimer that interacts with the mIg through non-covalent interactions (charged residues) on the extracellular domains of both proteins [1]. The mIg contains the receptor’s antigen interaction domain, while CD79a and CD79b mediate transduction of downstream signaling and endocytosis of the receptor-antigen complex. The extracellular portion of the mIg consists of the same basic structure as secreted antibodies: 2 heavy chain proteins and two light chain proteins (Figure 1). The N-terminus region of the heavy and light chains together determine the antigen specificity of the BCR. This region is known as the variable region. Inside the variable region, there are three 5–7 amino acid sequence stretches that make up the ‘hypervariable regions’ [2]. These hypervariable regions are where somatic hypermutation occurs in activated B cells. Somatic hypermutation of the hypervariable regions allows the BCR to increase its affinity for a ligand, so that secreted antibodies are highly specific.

Figure 1.

Figure 1.

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Structure of the B cell receptor complex.

The B cell receptor complex contains an antigen interacting domain (membrane immunoglobulin: mIg) that consists of two heavy chain and two light chain proteins. The N-terminus of the heavy and light chains contains a variable region that undergoes somatic hypermutation upon B cell activation. Central tyrosine residues in the ITAMs (immunoreceptor tyrosine-based activation motif) of CD79a/b are phosphorylated to induce receptor internalization and initiate downstream signaling pathways. The CD79a/b coreceptor is associated with all isotypes of the mIg on naïve and memory B cells.

There are multiple isotypes of the mIg on B cells. The extracellular domain of mature naïve B cell receptors may be mIgM or mIgD, while B cells that have been previously activated (memory B cells) express an irreversibly class switched mIgG. mIg isotypes vary in the number of covalent bonds between their heavy and light chain subunits [2]. While mIgM and mIgG have some differences in the structure of their extracellular domains, they function similarly in their ability to bind antigen and trigger BCR clustering [3]. The cytoplasmic tails of naïve (mIgM) and memory (mIgG) B cell receptors however, are quite different in length and function. The cytoplasmic domain of the naïve B cell receptor is only 3 amino acids long, while the cytoplasmic domain of mIgG is 28 amino acids. The longer tail sequence of mIgG is required for the increased proliferative capacity of memory B cells upon antigen stimulation [4 5]. The cytoplasmic regions of CD79a and CD79b also play an important role as phosphorylation substrates for kinases that transduce downstream signaling events and mediate endocytosis of the activated B cell Receptor.

B Cell Receptor Signaling And Regulation

The intracellular domains of CD79a and CD79b contain ITAM (immunoreceptor tyrosine-based activation motif) consensus sequences. BCR aggregation triggers phosphorylation of specific tyrosines in the ITAM motifs of CD79a and CD79b by Src-family Protein Tyrosine Kinases (PTKs), including Fyn, Blk, Lyn, and Lck [6 7]. Association of Src-family PTKs with the BCR is further enhanced by lipid raft microdomains on the plasma membrane. These lipid raft domains contain increased concentrations of Src-family PTKs, which amplify BCR signaling [8]. The tyrosine kinase Syk is recruited to phosphorylated ITAM motifs through its phosphotyrosine binding domain (SH2 domain) and auto phosphorylates. Syk then acts as a docking site for other SH2 domain containing proteins, including the B cell linker protein (BLNK) and the Tec-family kinase Btk. BLNK is an adaptor protein that nucleates transcriptional activation of multiple signaling pathways that are involved in B cell activation and proliferation [6 7 9]. In brief, BLNK recruits Phospholipase C-γ2 (PLC-γ2), which is activated by Btk to generate inositol 1,4,5-trisphosphate (IP3). IP3 binds the endoplasmic reticulum, which triggers calcium release, and culminates in activation of the NFAT (nuclear factor of activated T cells) pathway. PLC-γ2 also contributes to activation of the NF-κB pathway by stimulating PKC (protein kinase C) activity. Finally, Diacylglycerol (DAG) produced by PLC-γ2, recruits a guanine nucleotide exchange factor protein that activates the small GTPase Ras, and leads to activation of the MAPK family of pathways [10]. A simplified overview of these signaling events is shown in Figure 2.

Figure 2.

Figure 2.

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Overview of B cell receptor signaling

Upon antigen stimulation the BCR clusters at the plasma membrane and triggers recruitment of Src-family protein tyrosine kinases (PTKs). Src-family PTKs phosphorylate the ITAM motifs on the BCR and recruit Syk (a tyrosine protein kinase), BLNK (B cell linker protein), and Btk (Burton’s tyrosine kinase). BLNK is an adaptor protein that nucleates activation of multiple downstream signaling pathways, including MAPK (mitogen-activated protein kinase), NF-ĸB (nuclear Factor ĸ B), and NFAT (nuclear factor of activated T cells).

PLCγ2 – phospholipase C gamma two; DAG – diacylglycerol; IP3 – 1,4,5-triphosphate

The regulation of BCR signaling events is coordinated at multiple levels by accessory protein complexes. The three most well characterized accessory complexes are CD19, CD22, and FcγRIIB [6 7]. The CD19 co-receptor complex includes complement receptor 2 (CR2) and CD81, and is a positive regulator of BCR activation. Antigens that have bound complement bind CR2 in addition to the BCR, which augments BCR clustering in the early phase of B cell activation. CD81 then is a chaperone to facilitate proper segregation of the CD19 complex with the BCR [11]. CD22 and FcγRIIB are both negative regulators of BCR activation. Both receptors contain immunoreceptor tyrosine-based inhibitory motifs (ITIMs) that are phosphorylated by Lyn, a Src-family PTK.

BCR clustering and signaling is also regulated by the cortical actin cytoskeleton. Inhibition of actin remodeling using the f-actin stabilizing drug jasplakinolide for example blocked BCR aggregation and tyrosine phosphorylation, indicating that actin remodeling is critical for BCR activation [12 13]. In resting B cells, cortical actin is linked to the plasma membrane through ERM (ezrin-radixin-moesin) proteins and is thought to create a barrier that maintains the BCR in compartmentalized plasma membrane regions [14 15]. Antigen engagement with the BCR triggers a transient depolymerization of cortical actin, which allows for increased lateral mobility of the BCR and the formation of receptor clusters. Actin then polymerizes to reform in a polarized manner around the outer edge of BCR clusters where it plays a role in facilitating endocytosis [16 17].

Finally, antigen valency also impacts the functional response of the B cell receptor to stimulation. Specifically, monovalent antigens or Fab fragments that bind only one BCR, can stimulate calcium signaling Src-family PTK dependent manner [18]. BCR stimulation with monovalent antigens, however, results in reduced endocytosis, less calcium mobilization, and less efficient antigen presentation, when compared to oligomeric multivalent antigens[19].

Mechanism of B Cell Receptor Endocytosis

One of the earliest steps in the production of an effective antibody response to infection is antigen engagement with the BCR and endocytosis of the receptor-antigen complex. Upon endocytosis, the antigen moves from the early endosome to MHCII loading compartments [16 20–23]. MHCII loading compartments contain newly synthesized MHCII, and lysosomal enzymes that degrade the antigen to generate epitopes [22]. Antigen epitopes are then loaded onto MHCII by Human Leukocyte Antigen-DM [24]. Epitope loaded MHCII molecules are finally trafficked back to the cell surface to be presented to CD4+ helper T cells. BCR signaling and endocytic trafficking of antigens appear to be linked. Studies have shown that activation of receptor signaling induces reorganization of antigen processing compartments that contain MHCII and the fusion with vesicles containing endocytosed receptor antigen complexes [19 25 26]. B cell receptor signaling also induces polarization of the microtubule-organizing center (MTOC), which is critical for antigen processing and formation of the immune synapse between B and T cells [21 27].

Two recent studies highlight the importance of endocytic proteins in generating specific immune responses by supporting B cell proliferation and antigen presentation functions. The first is a study of the endocytic protein endophilin A2 [28]. Genetic knockdown of endophilin A2 lead to reduced antigen uptake in B cells and decreased antigen specific immune responses in mice, which was caused by a loss in the B cell’s ability to proliferate in response to stimulation [28]. Another study investigated the role of dynamin in BCR-antigen endocytosis and found that expression of a dominant negative dynamin (K44A mutant) protein significantly reduced the B cell’s ability to internalize the BCR-antigen complex and subsequently present antigens to T cells [29]. An overview of receptor mediated endocytic mechanisms that will be discussed in the following sections is presented in Figure 3.

Figure 3.

Figure 3.

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Overview of B cell receptor mediated endocytosis mechanisms.

The BCR may use clathrin mediated or clathrin independent endocytosis mechanisms to internalize the BCR-antigen complex. The method of endocytosis may depend on the concentration or size of the antigen. Many antigens are endocytosed by clathrin, which can typically accommodate receptor clusters between 56 and 126 nm in radius. High concentrations of antigen generate larger BCR clusters that are endocytosed by clathrin on smooth raised membrane structures, which can accommodate clusters of about 200 nm in radius. Finally, antigens or whole bacteria that are larger than 0.5 μm are internalized through phagocytosis. And fast endophilin mediated endocytosis also plays a role in BCR internalization the from the plasma membrane upon activation.

Clathrin Mediated Endocytosis

Early electron microscopy studies showed that the BCR clusters when exposed to multivalent crosslinking IgM and is then endocytosed into clathrin coated pits [30 31]. Subsequent studies have confirmed these results with fluorescence microscopy [17 32 33]. Clathrin is a structural coat protein composed of heavy and light chain triskelia. Clathrin coated vesicles nucleate at specific sites on the membrane where transmembrane cargo receptors bind adaptors including the adaptor protein complex 2 (AP2). AP2 links transmembrane receptors to clathrin and other endocytic accessory proteins. Clathrin triskelia are recruited from the cytosol to the site of concentrated adaptor proteins to generate curvature of the plasma membrane and form a cargo-loaded vesicle. Vesicle scission from the membrane occurs when the GTPase dynamin is recruited to the thin neck of the vesicle to catalyze scission. After scission, the vesicle uncoats. This allows the vesicle to fuse with endosomes [34]. Stoddart et al. showed that conditional knock down of clathrin heavy chain in B cells lead to a 70% reduction in BCR internalization after stimulation with a crosslinking antibody [17]. Early studies of BCR-antigen internalization implicated the cytoplasmic domains of both CD79a and CD79b in facilitating proper internalization of the BCR through clathrin-mediated endocytosis [35–38].

A series of experiments in separate reports investigated the role of CD79a and CD79b in BCR internalization using truncation mutants or amino acid substitutions at central tyrosine residues that are required for phosphorylation of the ITAMs. Gazumyan et al. mutated the two central tyrosine residues in the ITAM motifs of both CD79a (CD79aFF) and CD79b (CD79bAA) [36]. Both mutants showed significantly decreased rates of constitutive BCR internalization, with the CD79bAA mutant exhibiting a slightly more severe phenotype than CD79aFF. The mutants were also stimulated with a crosslinking antibody to determine the effect of the mutated tyrosine residues on internalization. CD79bAA expressing cells had significantly reduced BCR internalization, while the CD79aFF mutant had only a modest effect on internalization after stimulation. Another study investigated the effects of truncating the cytoplasmic domain of CD79a (CD79-/b) or CD79b (CD79a/-) on internalization of the BCR after stimulation. In this study, the CD79a truncation mutant had a more severe effect on the cell’s response to BCR crosslinking than the CD79b truncation. These studies along with others suggest that CD79a and CD79b are both involved in BCR internalization, using different amino acid motifs to facilitate endocytosis [35 38]. Supporting this hypothesis, a study by Busman-Sahay et al. showed that the clathrin adaptor protein AP2 binds CD79b via a membrane proximal AP2 binding motif in CD79b that requires CD79a to activate AP2 binding [39].

Although clathrin is clearly required for a component BCR endocytosis, there is evidence that alternative mechanisms of clathrin independent endocytosis are also involved in driving BCR internalization after stimulation. First, as previously discussed, a conditional knockdown of the clathrin heavy chain protein in B cells only lead to a 70% reduction in endocytosis [17]. This suggest that the remaining fraction of endocytosing receptors must use an alternate mechanism. Additionally, a study of BCR cluster radius by Lee et al. found that BCR cluster radius ranges from about 60 nm in resting B cells to over 1 micron in stimulated cells [40]. The average radius of a clathrin coated pit is roughly between 56 nm and 126 nm, based on platinum replica electron microscopy studies of clathrin structures in multiple cell types [41]. The large BCR cluster radius after stimulation also suggests that an alternative mechanism of endocytosis may be required to accommodate large BCR-antigen clusters. Finally, a study of how B cells take up antigen from the APC surface found that B cells form large membrane invaginations that require clathrin to pinch off smaller antigen clusters for processing[42]. Large membrane invaginations at the APC- B cell interface are likely generated using clathrin independent mechanisms. The next section will discuss some of the clathrin independent endocytosis pathways that are active in B cells.

Clathrin Independent Endocytosis (CIE)

Phagocytosis

Phagocytosis is a process that allows cells to endocytose large particles (greater than 0.5 microns). It is classically thought to be mediated by three groups of receptors: pattern recognition receptors, opsonic receptors, or apoptotic corpse receptors [43]. Actin remodeling by Rho family GTPases is important for phagocytosis in all three groups of receptors [43 44]. Macrophages and dendritic cells are typically thought of as the primary cells responsible for phagocytosis of bacteria and antigen presentation. However, a study recently found that B cells in teleost fish (rainbow trout, specifically) are able to phagocytose bacteria [45]. This study was followed by several papers showing that human B cells are also phagocytically active [45–49], including B cell lines (Ramos, Raji) and primary human B cells [48 49]. Additionally, multiple B cell subtypes (B1 and follicular B cells) in mice have phagocytic activity[46 47].

A study of BCR mediated phagocytosis in Ramos cells, which do not express the opsonic receptors (FcγRs), found that beads coated with monoclonal antibodies against the BCR were internalized within 10–20 minutes, while beads coated in an irrelevant antibody were not internalized [48]. This study further showed that a small proportion of primary human B cells were able to phagocytose Salmonella typhimurium through direct recognition of Salmonella antigens [48]. These data suggest that phagocytosis in B cells is mediated by the BCR and is supported by studies of B1 cells in mice [46]. B1 cells are a part of the innate immune system and contribute to early antibody responses to T-cell-independent antigens. Gao et al. found that B1 cells phagocytose Staphylococcus aureus, and that phagocytic activity is enhanced by antigen specific B cell receptors [46]. These studies suggest that the classical models of phagocytosis might need to be revisited in B cells.

The details of the molecular mechanisms driving BCR mediated phagocytosis in B cells are not well characterized. Several studies have shown that actin destabilizing drugs such as cytochalasin can inhibit phagocytosis in B cells. Actin is likely an important cytoskeletal driver of phagocytosis in B cells as it is in antigen presenting cells as well [45 47–49]. Additionally, a study of follicular B cells showed that Rho G is required for BCR mediated phagocytosis [47]. Rho G is a Rho family GTPase that is involved in phagocytosis via opsonic and apoptotic corpse receptors. B cells from Rhog−/− mice had four fold less phagocytic activity when compared cells from wild type mice [47]. Future studies may determine the extent of the role B cell phagocytosis plays in natural immune responses to infection or vaccination.

Other mechanisms of CIE

Fast endophilin mediated endocytosis (FEME) is a receptor triggered mechanism of endocytosis that is mediated by endophilin proteins. Five types of endophilin proteins (A1, A2, A3, B1, and B2) in humans generate curvature by binding the plasma membrane through their BAR domains [50]. Enrichment of endophilins A1, A2, or A3 at the plasma membrane prior to receptor stimulation can mediate FEME of specific receptors [51 52]. Malinova et al. showed that knock down of endophilin A2 in Ramos cells and in primary mouse B cells lead to a reduction in antigen uptake by the BCR [28]. Furthermore, endophilin A2 colocalized with antigen and not with clathrin after receptor stimulation. Finally, genetic knock down of clathrin adaptor proteins did not affect endophilin A2 colocalization with the antigen. This study suggests that endophilin A2 mediates FEME of the BCR after antigen stimulation in a clathrin independent manner.

A study of the physical plasma membrane structures associated with crosslinked BCR clusters, identified unique Smooth Raised Membrane (SRM) structures that are significantly colocalized with large BCR clusters and may represent an additional endocytic pathway for the BCR [32]. SRM structures consistently have small clathrin lattices associated with them, and may facilitate endocytosis through a hybrid pathway that requires both clathrin and SRMs. Receptor clusters colocalized with SRMs are unlikely to be internalized by clathrin alone, however, due to their large sizes. The BCR clusters analyzed in this study were much larger than clusters typically internalized by clathrin coated pits, with an average cluster radius of about 200 nm [32]. Additionally, actin is required for efficient internalization of BCR clusters through these large membrane invaginations and forms basket like assemblies around the invaginated sites. This study suggests an alternate mechanism of BCR internalization through smooth raised membrane structures that require both actin and clathrin. Further work is required to identify additional proteins involved in this hybrid pathway of BCR internalization and to determine the role of hybrid endocytosis in BCR-antigen processing.

B cells may also internalize antigen through fluid-phase pinocytosis, similar to antigen presenting cells, although this mechanism is less efficient than receptor mediated endocytosis [53].

Lipid Rafts

The BCR has been proposed to either aggregate into nanoscale lipid raft domains or stabilize raft domains upon clustering in the plasma membrane [54]. In either case, nanoscale domains that are enriched in cholesterol and sphingolipids, and are highly resistant to detergent induced solubilization, are likely formed [55]. As previously discussed, enrichment of Src-family kinases in lipid raft domains helps to amplify BCR signaling [8]. The role of lipid rafts in internalization, however, is more complex. Early studies implicated clathrin in facilitating BCR endocytosis from lipid rafts. Experiments using B cells stimulated with F(ab’)2 crosslinking antibody fragment found that receptor stimulation led to increased phosphorylation of clathrin heavy chain (CHC) proteins localized in lipid rafts. Additionally, CHC phosphorylation was inhibited by the Src-family PTK inhibitor PP1 [33]. These data suggested that Src-family kinases, which are enriched at lipid rafts, may coordinate both signaling and endocytosis of the BCR by phosphorylating CD79a/b and clathrin proteins [33]. A second study, however, used a polyvalent antigen model to stimulate the BCR and concluded that antigen stimulation did not induce BCR uptake from lipid rafts. This study observed no effect of Methyl-β Cyclodextrin (MBCD) treatment (a cholesterol depleting drug) on the rate or extent of BCR-antigen uptake [56]. Finally, a third report directly compared the effect of crosslinking antibody or polyvalent antigen stimulation on BCR internalization [57]. BCRs stimulated with a crosslinking antibody were more strongly associated with lipid raft microdomains than antigen stimulated BCRs, and exhibited a significant decrease in endocytosis after stimulation in the presence of MBCD [57]. Taken together, these results provide a complex picture of the relationship between lipid raft microdomains and BCR internalization. Lipid rafts or microdomains may play a role in internalization of some BCR stimuli through clathrin phosphorylation at the microdomain, but not all stimuli appear to be internalized through this pathway.

B Cell Receptor Signaling and Endocytosis in Leukemia and Lymphoma

Aberrant B cell receptor signaling is a primary driver of disease pathogenesis in Chronic Lymphocytic Leukemia (CLL) and Diffuse Large B Cell Lymphoma (DLBCL). Recent advances in drug therapies that target kinases in the BCR signaling pathway have been effective at improving clinical outcomes for some patients [58–61]. However, not all subtypes of CLL or DLBCL are equally responsive to BCR pathway inhibitors [60–62]. Additionally, aggressive cancers can acquire resistance to BCR pathway inhibitors [63]. Recent studies have found that both BCR signaling and endocytosis play a role in the pathogenesis of certain subtypes of CLL and DLBCL.

Chronic Lymphocytic Leukemia

There are two major subtypes of CLL: Mutated CLL (M-CLL) and Unmutated CLL (U-CLL). M-CLL is derived from B cells that have undergone somatic hypermutation (induced by an infection) of their Immunoglobulin Heavy Chain Variable (IGHV) gene region. The BCR of malignant B cells from M-CLL patients are anergic (cannot transduce downstream signaling in response to ligation) and this subtype is often associated with better clinical outcomes than U-CLL. In U-CLL, malignant B cells have not undergone somatic hypermutation of their IGHV gene region and express BCRs that are responsive to ligation and able to activate downstream signaling pathways. U-CLL patients typically have poor clinical outcomes, but are more responsive to BCR signaling pathway inhibitor treatment than M-CLL patients [64 65].

A recent study has shed light on the relationship between BCR signaling and endocytosis in cells from patients with both subtypes of CLL and healthy individuals[62]. This study found that B cells from M-CLL patients had significantly increased mIgM endocytic activity, compared to healthy controls, despite the anergy of the BCR and lower overall mIgM expression levels. In healthy B cells, the mIgM and CD79a/CD79b coreceptor complex are endocytosed together upon ligation of the mIg subunit. B cells from CLL patients, however, appear to endocytose mIgM independently of CD79b [62]. This study showed that in M-CLL patients, malignant B cells are able to decouple BCR signaling and endocytosis. This insight into the molecular pathogenesis of CLL is significant and could facilitate the development of new therapies targeting the aberrant endocytosis of mIgM.

Diffuse Large B Cell Lymphoma

DLBCL is divided into two gene expression subtypes: activated B-cell-like (ABC DLBCL) and germinal center B-cell-like (GC DLBCL). DLBCL subtypes are distinguished by gene profiling analysis [66]. While GC DLBCL malignant cells appear to have a similar gene expression profile to other germinal center B cells, ABC DLBCL malignant cells have an expression profile that more closely resembles an activated B cell [67]. Studies have also shown that constitutive NF-ĸB signaling is necessary for survival of malignant cells in ABC DLBCL [68]. Tumors from ABC DLBCL patients frequently have mutations in the MYD88 oncogene, which drives continuous NF-ĸB activation, and also mutations in the BCR coreceptor CD79b [66 69]. As discussed in the previous section, CD79b plays an important role in BCR endocytosis. A study of mutations commonly found in the CD79b gene of ABC DLBCL patients showed that mutations in the first tyrosine residue of the ITAM motif of CD79b, lead to increased expression of the BCR [70]. In this study, CD79b mutants were also not readily phosphorylated by the Src-family kinase, Lyn, which is required for BCR internalization [71]. These studies suggest that CD79b mutations found in ABC DLBCL tumors contribute to pathogenesis by blocking endocytosis and increasing surface expression of the BCR, which could allow the receptor to maintain high levels of signaling. Several other mutations in CD79a and CD79b were presented in this study, and given that both coreceptors appear to be important for endocytosis, there may be other pathological mechanisms that contribute to altered clathrin mediated or independent endocytosis of the BCR.

Conclusions

B Cell Receptor interactions with foreign antigens are central to initiating the adaptive immune response and generating antibodies to combat infections. The B cell receptor complex mediates both activation of downstream signaling and endocytosis of the receptor-antigen complex. Signaling events associated with the BCR have been extensively studied and have led to important advances in leukemia and lymphoma treatments targeting the Src-family PTKs [6 60]. The mechanisms of BCR endocytosis, however, are not as well defined.

Clathrin mediated endocytosis plays an important role in both constitutive BCR endocytosis and endocytosis in response to receptor stimulation [17 30 31 33]. Clathrin mediated endocytosis is facilitated by the cytoplasmic domain of CD79a and CD79b which bind the clathrin adaptor protein AP2 [39]. Additionally, multiple clathrin independent mechanisms appear to be active in B cells and play a role in internalization of stimulated BCR clusters. Here, along with clathrin mediated endocytosis, we highlighted a series of studies of BCR internalization though phagocytosis, FEME, a novel hybrid endocytic pathway (SRM structures), and clathrin-independent lipid raft associated mechanisms [28 32 46–49]. Future studies may investigate whether the CD79a/CD79b complex also mediates clathrin independent endocytosis and determine the role of clatrin independet endocytic pathways in B cell activation and antibody production. Improving our understanding of all the mechanisms of BCR endocytosis may uncover novel interventions for the development of new lymphoma and leukemia therapies to combat drug resistant cancers that do not respond to the current set of inhibitors. B cells also play a significant role in the pathology of inflammatory autoimmune diseases such as rheumatoid arthritis and type I diabetes. B cells are particularly efficient at internalizing and processing low concentrations of antigen for presentation to auto-reactive T cells, which exacerbates pathology in inflammatory autoimmune diseases [72]. Improving our understanding of how B cells internalize and process very small amounts of antigen could also lead to more targeted therapies to eliminate pathogenic B cells in inflammatory autoimmune diseases.

Acknowledgments.

JWT is supported by the IRP of the NHBLI, NIH, Bethesda, MD.

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