Differential structure-function requirements of the transmembranal domain of the B cell antigen receptor

Structural principles of B cell antigen receptor assembly

The B cell antigen receptor (BCR) is composed of a membrane-bound class M, D, G, E or A immunoglobulin for antigen recognition1-3 and a disulfide-linked Igα (also known as CD79A) and Igβ (also known as CD79B) heterodimer (Igα/β) that functions as the signalling entity through intracellular immunoreceptor tyrosine-based activation motifs (ITAMs)4,5. The organizing principle of the BCR remains unknown. Here we report cryo-electron microscopy structures of mouse full-length IgM BCR and its Fab-deleted form. At the ectodomain (ECD), the Igα/β heterodimer mainly uses Igα to associate with Cµ3 and Cµ4 domains of one heavy chain (µHC) while leaving the other heavy chain (µHC') unbound. The transmembrane domain (TMD) helices of µHC and µHC' interact with those of the Igα/β heterodimer to form a tight four-helix bundle. The asymmetry at the TMD prevents the recruitment of two Igα/β heterodimers. Notably, the connecting peptide between the ECD and TMD of µHC intervenes in between those of Igα and Igβ to guide TMD assembly through charge complementarity. Weaker but distinct density for the Igβ ITAM nestles next to the TMD, suggesting potential autoinhibition of ITAM phosphorylation. Interfacial analyses suggest that all BCR classes utilize a general organizational architecture. Our studies provide a structural platform for understanding B cell signalling and designing rational therapies against BCR-mediated diseases.

Structural and immunogenomic insights into B-cell receptor activation

B cells express B-cell receptors (BCRs) which recognize antigen to trigger signaling cascades for B-cell activation and subsequent antibody production. BCR activation has a crucial influence on B-cell fate. How BCR is activated upon encountering antigen remains to be solved, although tremendous progresses have been achieved in the past few years. Here, we summarize the models that have been proposed to explain BCR activation, including the cross-linking model, the conformation-induced oligomerization model, the dissociation activation model, and the conformational change model. Especially, we elucidate the partially resolved structures of antibodies and/or BCRs by far and discusse how these current structural and further immunogenomic messages and more importantly the future studies may shed light on the explanation of BCR activation and the relevant diseases in the case of dysregulation.

B cell receptor accessory molecule CD79 gets involved in response against Streptococcus agalactiae infection and BCR signaling in Nile tilapia (Oreochromis niloticus)

CD79, composed of two distinct chains called CD79a and CD79b, is a transmembrane protein that forms a B cell antigen receptor with membrane immunoglobulin, and generates a signal following antigen recognition by the B cell receptor. In this study, the CD79a (OnCD79a) and CD79b (OnCD79b) were cloned and identified from Nile tilapia (Oreochromis niloticus). The cDNA of ORF for OnCD79a and OnCD79b are 669 and 627 bp, coding 222 and 208 amino acids, respectively. The deduced protein analysis showed that both CD79a andCD79b contain an immunoreceptor tyrosine-based activation motif in their intracellular tails that used to propagate a signal in a B cell. Expression analysis revealed that both CD79a and CD79b expressed at high levels in immune tissues, such as anterior kidney and spleen, and in IgM⁺ B cells. Upon Streptococcus agalactiae (S. agalactiae) infection, the expressions of OnCD79a and OnCD79b were significantly up-regulated in anterior kidney and spleen. The significant up-regulations of OnCD79a and OnCD79b were also detected in leukocytes after in vitro challenge with S. agalactiae. Further, stimulations of LPS and anti-OnIgM monoclonal antibody induced significant up-regulations of OnCD79a and OnCD79b in leukocytes. Taken together, the results of this study indicated that CD79 molecule, playing roles in BCR signaling, was likely to get involved in host defense against bacterial infection in Nile tilapia.

Structural Model of the mIgM B-Cell Receptor Transmembrane Domain From Self-Association Molecular Dynamics Simulations

Antigen binding to B-cell antigen receptors (BCRs) followed by signaling initiates the humoral immune response. The signaling is intimately coupled to nanoclustering of BCRs and their sorting to specific membrane domains, a process that is ruled by interactions between the BCR transmembrane domain and lipids. While the structure of the extracellular domains of BCRs has been resolved, little is known about the configuration of the constituting four immunoglobulin domains spanning the membrane. Here, we modeled the structure of the transmembrane (TM) domain of the IgM B-cell receptor using self-assembly coarse-grained molecular dynamics simulations. The obtained quaternary structure was validated against available experimental data and atomistic simulations. The IgM-BCR-TM domain configuration shows a 1:1 stoichiometry between the homodimeric membrane-bound domain of IgM (mIgM) and a Ig-α/Ig-β heterodimer. The mIgM homodimer is based on an asymmetric association of two mIgM domains. We show that a specific site of the Ig-α/Ig-β heterodimer is responsible for the association of IgM-BCRs with lipid rafts. Our results further suggest that this site is blocked in small-sized IgM-BCR clusters. The BCR TM structure provides a molecular basis for the previously suggested dissociation activation model of B-cell receptors. Self-assembly molecular dynamics simulations at the coarse-grained scale here proved as a versatile tool in the study of receptor complexes.

Structural and functional properties of membrane and secreted IgD

More than 35 years ago, study of an unknown immunoglobulin (Ig) in the serum from a myeloma patient led to the discovery of IgD. Subsequently, the finding that it also exists as a membrane-bound Ig stimulated a large number of studies during the 70s. Then, the interest on IgD shrank, largely because of the lack of known function of secretory IgD (secIgD) and of a stagnating knowledge of the functions of surface IgD. In the recent years, very significant advances followed the tremendous accumulation of data on the physiology of the B cell receptor, of which IgD is the major component, on the role of secIgD in normal and diseased individuals. This review, which is focused on human IgD but integrates data in the mouse and other species when needed, summarizes present data on the structure, synthesis and functions of both membrane and secIgD, IgD receptors and the involvement of IgD in various diseases, especially the hyperIgD syndrome.

Role of transmembrane domains in the functions of B- and T-cell receptors

The antigen receptors on the surface of B- and T-lymphocytes are complexes of several integral membrane proteins, essential for their proper expression and function. Recent studies demonstrated that transmembrane (TM) domains of the components of these receptors play a critical role in their association and function. It was specifically demonstrated that in many cases point mutations in the TM domains can partially or completely disrupt the receptor surface expression and function. Here we review studies of the TM domains of B- and T-cell receptors. Furthermore, we use a novel method, PHDtopology, to provide estimates of the exact locations and lengths of the TM domains of the subunit components of these receptors. Most previous studies used single residue hydrophobicity as a criterion for determining the position and length of the TM domains. In contrast, PHDtopology utilizes a system of neural networks and the evolutionary information contained in multiple alignments of related sequences to predict the location, length, and orientation of transmembrane helices. Present results significantly differ from most published estimates of the TM domains of the B- and T-cell receptor components, primarily in the length of the TM domains. These results may lead to modification of putative TM motifs and re-interpretation of the results of studies using mutated TM domains. The availability of PHDtopology on the Internet would make it a valuable tool in the future studies of the TM domains of integral membrane proteins.

Hybrid Membrane IgM with the Transmembrane Region of I-Aα Facilitates Enhanced Presentation of Distinct Epitopes to T Cells

The role of B cell Ag receptors (membrane Ig or mIg) in the efficient Ag presentation to T cells, including the requirement of mIgM-associated Igα/Igβ, remains unclear. We report here that mIgM, substituted with greater than two-thirds of the NH2-terminal Aα transmembrane (TM) regions of the MHC class II molecule, are capable of mediating the efficient presentation of specific Ag to some (Group 1) but not all (Group 2) T cell hybridomas. In contrast, the generation of epitopes recognized by the Group 2 hybridomas can be mediated only by the wild-type mIgM. Tyrosine phosphorylation appears to be necessary for the enhanced Ag presentation to Group 2 hybridomas, while it does not for Group 1 hybridomas. In addition, differential sensitivity of Ag processing to leupeptin, different duration required for epitope generation/presentation, as well as the involvement of distinct epitopes for stimulation of these groups of T cell hybridomas were observed. These results suggest that transport of the mIgM/Ag complexes to an endocytic compartment(s) for generation of certain T cell epitopes may be mediated by the N-terminal TM sequence of mIgM, independent of Igα/Igβ association. This function can be replaced by two-thirds of the NH2-terminal TM region of Aα chain of class II molecules.

Acceleration of intracellular targeting of antigen by the B-cell antigen receptor: importance depends on the nature of the antigen-antibody interaction

The B-cell antigen receptor (BCR) internalizes bound antigen such that antigen-derived peptides become associated with emigrating major histocompatibility complex (MHC) class II molecules for presentation to T cells. Experiments with B-cell transfectants reveal that BCR confers a specificity of intracellular targeting since chimeric antigen receptors which internalize antigen by virtue of a heterologous cytoplasmic domain do not necessarily give rise to presentation. In contrast, however, previous studies have shown that antigen binding to irrelevant cell surface molecules (e.g. transferrin receptor, MHC class I) can ultimately lead to presentation. The solution to this paradox appears to be that the intracellular targeting by BCR actually reflects an acceleration of antigen delivery. Depending on the nature of the BCR-antigen interaction, this accelerated targeting can be essential in determining whether or not internalization leads to significant presentation. Physiologically, the accelerated delivery of antigen by BCR could prove of particular importance early in the immune response when antigen-BCR interaction is likely to be poor.

Capture and processing of exogenous antigens for presentation on MHC molecules

Class I and class II MHC molecules bind peptides during their biosynthetic maturation and provide a continuously updated display of intracellular and environmental protein composition, respectively, for scrutiny by T cells. Receptor-mediated endocytosis, phagocytosis, and macropinocytosis all contribute to antigen uptake by class II MHC-positive antigen-presenting cells. Capture of antigenic peptides by class II MHC molecules is facilitated because antigen catabolism and class II MHC maturation take place in the same compartments or in communicating compartments of the endosome/lysosome system. These class II MHC-rich, multivesicular endosomes receive incoming antigen and can support not only antigen processing and class II MHC peptide loading but also the export of peptide/class II MHC complexes to the cell surface. A balance between production and destruction of antigenic peptides is achieved by the activity of local proteases and may be influenced by binding of antigen to other proteins both prior to the onset of processing (e.g. antibodies) and during antigen unfolding (e.g. MHC molecules). T cell determinants that can be released for MHC binding without a substantial processing requirement may be able to utilize a distinct minor population of cell surface class II MHC molecules that become available during peripheral recycling. Although peptides derived from exogenous protein sources are usually excluded from presentation on class I MHC molecules, recent evidence shows that this embargo may be lifted in certain professional antigen-presenting cells to increase the spectrum of antigens that may be displayed on class I MHC.

Compartmentalization of B-cell antigen receptor functions

Receptor tyrosine kinases (RTK), like the PDGF-receptor, translate information from the extracellular environment into cytoplasmic signals that regulate a spectrum of cellular functions. RTK molecules consist of ligand binding extracellular domains, cytoplasmic kinase domains and tyrosine phosphorylation sites [Ullrich and Schlessinger, 1990 (Cell 61, 203-212); Heldin, 1992 (EMBO J. 11, 4251-4259)]. Upon ligand-induced RTK oligomerization, the kinase domains will become activated and induce auto(trans)phosphorylation of a number of cytoplasmic tyrosine residues. These phosphorylated tyrosine residues are incorporated in distinct sequence motifs and act as specific docking sites for SH2 domain-containing proteins [Songyang et al., 1993 (Cell 72, 767-778)]. In contrast to single- or oligo-chain RTK, immunological receptors such as antigen receptors, FcR and cytokine receptors are multi-chain complexes in which distinct receptor functions appear to be compartmentalized in distinct polypeptides. Here, we summarize current knowledge on the structural and functional characteristics of the B-cell antigen receptor complex (BCR) and address the specific ability of accessory molecules to recruit intracellular signaling intermediates towards the activated receptor complex.

Intracellular targeting of antigens internalized by membrane immunoglobulin in B lymphocytes

An important function of membrane immunoglobulin (mIg), the B cell antigen receptor, is to endocytose limiting quantities of antigen for efficient presentation to class II-restricted T cells. We have used a panel of mIg mutants to analyze the mechanism of mIg-mediated antigen presentation, and specifically to explore the ability of mIg to target internalized antigen to intracellular processing compartments. Transfected mIgs carrying substitutions for the transmembrane Tyr587 residue fail to efficiently present specifically bound antigen. However, these mutants internalize antigen normally, and their defect cannot be attributed to a lack of mIg-associated Ig alpha/Ig beta molecules. A novel functional assay for detecting antigenic peptides in subcellular fractions shows that wild-type mIg transfectants generate class II-peptide complexes intracellularly, whereas only free antigenic peptides are detectable in the mutant mIg transfectants. Furthermore, an antigen competition assay reveals that antigen internalized by the mutant mIgs fails to enter the intracellular processing compartment accessed by wild-type mIg. Therefore, mIg specifically targets bound and endocytosed antigen to the intracellular compartment where processed peptides associate with class II molecules, and the transmembrane Tyr587 residue plays an obligatory role in this process. Targeting of internalized antigen may be mediated by receptor-associated chaperones, and may be a general mechanism for optimizing the presentation of specifically bound and endocytosed antigens in b lymphocytes and other antigen-presenting cells.

The B cell antigen receptor complex: mechanisms and implications of tyrosine kinase activation

The B cell receptor is a multimeric receptor complex whose constituent chains appear to mediate distinct and possibly interrelated functions. In this review we have focused on how one chain, immunoglobulin (Ig)-alpha, may function to activate tyrosine kinases and the consequences of that activation. The cytoplasmic domain of Ig-alpha contains a consensus sequence, the antigen recognition homology 1 (ARH 1) motif, which is found in Ig-beta and other antigen recognition receptor associated chains. We argue that this conserved structure reflects an underlying conserved mechanism of secondary effector activation. Our data also indicates that the specificity of each motif (i.e., the elements which restrict secondary effector binding to particular motifs) is encoded within divergent sequences found in each ARH 1 motif. In the particular case of kinase activation by Ig-alpha, the subsequent phosphorylation of multiple tyrosines on Ig-alpha, Ig-beta, CD19, CD22 and possibly other functionally related chains form recruitment sites for a myriad of secondary signal transducers. In this model, proximal tyrosine kinases and phosphatases do not function so much to mediate the linear transfer of information as to establish and modulate an interrelated network of signal transducers capable of driving complicated cellular responses.

Surface immunoglobulins mediate efficient transport of antigen to lysosomal compartments resulting in enhanced specific antigen presentation by B cells

A BCL1 immunoglobulin (Ig) transfectant, expressing wild-type surface (s)IgM with the TEPC-15 idiotype (T15-Id) and anti-phosphorylcholine (PC) specificity, was previously shown to present PC-conjugated hen egg-white lysozyme (PC-HEL) to a HEL-specific T cell hybridoma at a lower antigen (Ag) concentration than that required for native HEL. Two variant Ig transfectants, expressing T15-Id sIgM with substitutions either in the entire spacer, transmembrane (TM) domain and cytoplasmic tail (B186 variant) or in the NH2-terminal third of TM domain only (TM2 variant), failed to display this sIgM-mediated, enhanced presentation of PC-HEL at low concentrations. However, prolonged treatment with anti-T15-Id monoclonal antibody (mAb) led to a reduction of surface expression of the T15-Id sIgM in the wild-type and TM2 variant, but not in the B186 variant sIgM transfectants. Treatment with anti-T15-Id mAb also resulted in an increased intracellular accumulation of T15-Id sIgM in the wild-type transfectant, but not in the B186 variant. Subcellular fractionation analysis revealed that the ligands bound to the T15-Id sIgM are not efficiently transported to the dense lysosomal compartments in both B186 and TM2 transfectants, as compared to the wild-type sIgM transfectant. A significant increase in tyrosine phosphorylation after cross-linking of the T15-Id sIgM was observed only in the wild-type sIgM transfectant. These results suggest that, while the NH2-terminal third of the TM region is not involved in the process responsible for the ligand-induced reduction of surface expression of sIgM, it appears to be essential for subsequent transport of sIgM/ligand complexes to the lysosomal compartments, as well as efficient activation of tyrosine kinases. These results strongly suggest that sIg-mediated enhancement of specific antigen presentation reflects the ability of sIg to efficiently transport antigen to the lysosomal compartments, and possibly the activation of protein tyrosine kinases.

The B-cell antigen receptor complex: structure and signal transduction

The specificity of the immune response is determined by the interaction between the B-cell receptor (BCR) and its cognate structure, antigen. Recent studies have provided considerable insight into the compartmentalization of function within this extremely versatile hetero-oligomeric receptor complex. In this article, Christopher Pleiman, Daniele D'Ambrosio and John Cambier consolidate new findings regarding BCR structure and signal transduction.

The antigen-specific immunoglobulin G receptor is more sensitive to stimulation than the IgM receptor in transfected B cells

A murine lymphoma cell line (M12.4) was transfected with immunoglobulin (Ig) genes encoding a T15+ (idiotype) IgM antibody or an idiotypically identical IgG antibody. Three transfectant clones of each class which showed similar (albeit distinguishable) levels of membrane expression of the transfected genes were used in the study. The response of each cell population to stimulation with anti-T15 antibodies was followed by measurement of the change in the intracellular Ca++ concentration. The IgG transfectants were found to be significantly more responsive to such stimulation than the IgM cells. In contrast, there was no difference in their response to a nonspecific reagent, the calcium ionophore A23187.

The internalization of the IgG2a antigen receptor does not require the association with Ig-alpha and Ig-beta but the activation of protein tyrosine kinases does

The B cell antigen receptor of class IgM is a multimeric protein complex containing the membrane-bound immunoglobulin molecule and a heterodimer of the two B cell-specific transmembrane proteins Ig-alpha and Ig-beta. The B cell antigen receptor fulfills a dual role on the surface of B cells. First, it is a signal transduction complex which can activate protein tyrosine kinases and induce the release of Ca2+ ions from intracellular stores. Second, its internalization mediates the specific uptake of bound antigens, which are processed intracellularly and presented as major histocompatibility complex-bound peptides on the cell surface. In case of the IgM antigen receptor, the association with the heterodimer is necessary for expression of large amounts of IgM on the surface. We show here that the IgG2a antigen receptor can be expressed on the surface of myeloma cells in two structurally different forms: either with or without the Ig-alpha/Ig-beta heterodimer. A functional comparison of the two forms of antigen receptors demonstrates that the Ig-alpha and Ig-beta molecules are required for the activation of protein tyrosine kinases after cross-linking of the B cell antigen receptor. In contrast, both forms of IgG2a are equally well internalized. This suggests that Ig-alpha and Ig-beta are essential for signal transduction through the IgG2a antigen receptor, whereas internalization can occur independently of the heterodimer.

Ig alpha and Ig beta are functionally homologous to the signaling proteins of the T-cell receptor

Signal transduction by antigen receptors and some Fc receptors requires the activation of a family of receptor-associated transmembrane accessory proteins. One common feature of the cytoplasmic domains of these accessory molecules is the presence is at least two YXXA repeats that are potential sites for interaction with Src homology 2 domain-containing proteins. However, the degree of similarity between the different receptor-associated proteins varies from that of T-cell receptor (TCR) zeta and Fc receptor RIIIA gamma chains, which are homologous, to the distantly related Ig alpha and Ig beta proteins of the B-cell antigen receptor. To determine whether T- and B-cell antigen receptors are in fact functionally homologous, we have studied signal transduction by chimeric immunoglobulins bearing the Ig alpha or Ig beta cytoplasmic domain. We found that Ig alpha and Ig beta cytoplasmic domains were able to activate Ca2+ flux, interleukin-2 secretion, and phosphorylation of the same group of cellular substrates as the TCR in transfected T cells. Chimeric proteins were then used to examine the minimal requirements for activation of the Fyn, Lck, and ZAP kinases in T cells. Both Ig alpha and Ig beta were able to trigger Fyn, Lck, and ZAP directly without involvement of TCR components. Cytoplasmic tyrosine residues in Ig beta were required for recruitment and activation of ZAP-70, but these amino acids were not essential for the activation of Fyn and Lck. We conclude that Fyn and Lck are able to recognize a clustered nonphosphorylated immune recognition receptor, but activation of these kinases is not sufficient to induce cellular responses such as Ca2+ flux and interleukin-2 secretion. In addition, the molecular structures involved in antigen receptor signaling pathways are conserved between T and B cells.

B cell antigen receptors

Antigen receptors have a dual role on the surface of B lymphocytes, namely the binding and internalization of antigens and the activation of the antigen-recognizing B cell. The structural requirements for internalization are still a matter of controversy; but as far as signalling is concerned it is becoming increasingly apparent that the antigen receptor is functionally divided into the ligand-binding immunoglobulin (Ig) molecule and the signal-transducing Ig-alpha/Ig-beta heterodimer. This knowledge has been exploited to design variant antigen receptors which combine both functions in a single molecule.

Ig alpha and Ig beta are functionally homologous to the signaling proteins of the T-cell receptor

Signal transduction by antigen receptors and some Fc receptors requires the activation of a family of receptor-associated transmembrane accessory proteins. One common feature of the cytoplasmic domains of these accessory molecules is the presence is at least two YXXA repeats that are potential sites for interaction with Src homology 2 domain-containing proteins. However, the degree of similarity between the different receptor-associated proteins varies from that of T-cell receptor (TCR) zeta and Fc receptor RIIIA gamma chains, which are homologous, to the distantly related Ig alpha and Ig beta proteins of the B-cell antigen receptor. To determine whether T- and B-cell antigen receptors are in fact functionally homologous, we have studied signal transduction by chimeric immunoglobulins bearing the Ig alpha or Ig beta cytoplasmic domain. We found that Ig alpha and Ig beta cytoplasmic domains were able to activate Ca2+ flux, interleukin-2 secretion, and phosphorylation of the same group of cellular substrates as the TCR in transfected T cells. Chimeric proteins were then used to examine the minimal requirements for activation of the Fyn, Lck, and ZAP kinases in T cells. Both Ig alpha and Ig beta were able to trigger Fyn, Lck, and ZAP directly without involvement of TCR components. Cytoplasmic tyrosine residues in Ig beta were required for recruitment and activation of ZAP-70, but these amino acids were not essential for the activation of Fyn and Lck. We conclude that Fyn and Lck are able to recognize a clustered nonphosphorylated immune recognition receptor, but activation of these kinases is not sufficient to induce cellular responses such as Ca2+ flux and interleukin-2 secretion. In addition, the molecular structures involved in antigen receptor signaling pathways are conserved between T and B cells.

Antigen presentation by the B cell antigen receptor is driven by the alpha/beta sheath and occurs independently of its cytoplasmic tyrosines

Membrane immunoglobulin functions to internalize bound antigen for its subsequent processing and presentation to T cells. Although the five immunoglobulin isotypes exhibit considerable differences in their cytoplasmic domains, we show by use of matched B lymphoma transfectants that all isotypes manifest a similar high efficacy in antigen presentation. Experiments using mutant receptors reveal that this efficacy can be ascribed to the alpha/beta sheath of the receptor, where presentation correlates with internalization of polyvalent antigen. Efficient presentation is restored to a sheathless antigen receptor by providing it with only the cytoplasmic domain of the beta sheath polypeptide. This restoration of activity does not depend on the tyrosine residues in the beta cytoplasmic tail, implying that antigen receptor-mediated presentation can occur by a pathway distinct from that used by the Fc receptor Fc gamma RIII.

Signal transduction by immunoglobulin is mediated through Ig alpha and Ig beta

Immunoglobulin (Ig) antigen receptors are composed of a noncovalently-associated complex of Ig and two other proteins, Ig alpha and Ig beta. The cytoplasmic domain of both of these Ig associated proteins contains a consensus sequence that is shared with the signaling proteins of the T cell and Fc receptor. To test the idea that Ig alpha-Ig beta heterodimers are the signaling components of the Ig receptor, we have studied Ig mutations that interfere with signal transduction. We find that specific mutations in the transmembrane domain of Ig that inactivate Ca2+ and phosphorylation responses also uncouple IgM from Ig alpha-Ig beta. These results define amino acid residues that are essential for the assembly of the Ig receptor. Further, receptor activity can be fully reconstituted in Ca2+ flux and phosphorylation assays by fusing the cytoplasmic domain of Ig alpha with the mutant Igs. In contrast, fusion of the cytoplasmic domain of Ig beta to the inactive Ig reconstitutes only Ca2+ responses. Thus, Ig alpha and Ig beta are both necessary and sufficient to mediate signal transduction by the Ig receptor in B cells. In addition, our results suggest that Ig alpha and Ig beta can activate different signaling pathways.