Abstract CD20 is a four-helix transmembrane protein specifically expressed in B-cells that serves as a prominent target of therapeutic anti-CD20 antibodies. It is localized in a membrane nanocluster harboring the B-cell antigen receptor of IgD class (IgD-BCR), where it functions to maintain the resting state of naïve B-lymphocytes. How CD20 exerts this resting B-cell gatekeeper function is not yet known. Using Ramos and human peripheral blood B-cells, we show here that the serine/threonine kinase PKCδ constitutively phosphorylates serine residues in the two cytosolic tails of CD20. Phosphorylated CD20 becomes a binding target for 14-3-3 adaptor proteins, which link it to the RhoA GDP/GTP exchange factor GEF-H1 and the microtubule network, supporting the function of the IgD-BCR nanocluster. Binding of anti-CD20 antibodies results in microtubule dissociation and replacement of the GEF-H1/CD20 complex with a RhoA-GTP/ROCK1/CD20 complex, which promotes actomyosin contractility. Our study suggests that CD20 not only maintains the resting state of B-lymphocytes by anchoring the microtubule network and controlling the stability of the IgD-BCR nanocluster, but also mediates the microtubule/actin switch in active B-lymphocytes. These findings could have important implications for anti-CD20 antibody treatment and the optimization of therapeutic protocols.

IntroductionCalcium (Ca2+) signaling plays a pivotal role in determining B cell fate, shaping processes such as activation, differentiation, anergy or apoptosis. Upon B cell antigen receptor activation, Ca2+ is rapidly mobilized from the endoplasmic reticulum and supplemented by Ca2+ influx from the extracellular space, ultimately driving activation of various signaling pathways required for appropriate B cell responses. Although mitochondria also harbor significant levels of Ca2+, how mitochondrial Ca2+ dynamics are regulated in B cells in response to activation or other cues remains unknown, as do the functional consequences of altered mitochondrial Ca2+ levels.MethodsChemical dyes as well as a genetically encoded Ca2+ sensor with a mitochondrial targeting sequence were used to study mitochondrial Ca2+ dynamics in response to various stimuli. Proximity ligation assays were performed to assess interaction between mitochondria and the endoplasmic reticulum. Primary mouse B cells and the Burkitt lymphoma cell line Ramos were used to study functional consequences of the loss of the Mitochondrial Calcium Uniporter.ResultsHere, we show that mitochondrial Ca2+ levels dynamically respond to cell activation, stress and metabolic cues and that mitochondrial Ca2+ uptake is largely dependent on the Mitochondrial Calcium Uniporter. Reduced mitochondrial Ca2+ uptake has a negative impact on mitochondrial activity and also affects cell signaling. These findings demonstrate that changes in mitochondrial Ca2+ contribute to shaping functional B cell responses.DiscussionThe spatial and temporal dynamics of Ca2+ accumulation within distinct subcellular compartments, particularly the cytosol, endoplasmic reticulum and mitochondria, are essential for translating extracellular and intracellular signals into specific cellular outcomes. Our study provides new insights into the regulation of Ca2+ homeostasis in B cells.

CD20 is a four-transmembrane protein expressed at the surface of B cells from late pro-B cells to memory B cells, with the exception of plasma cells. Its expression pattern makes it an attractive therapeutic target for different B cell malignancies and autoimmune diseases. Despite the clinical success of CD20-targeting antibodies, the biology of the CD20 protein is still not well understood. We investigated CD20 binding partners in the membrane of human B cells using immunoprecipitation followed by mass spectrometry analysis. We identified a molecular interaction between CD70 and CD20, and confirmed this using proximity ligation assays. CD20-CD70 spatiotemporal colocalization was validated at the plasma membrane of B cells using high-resolution microscopy. Cell surface expression of CD70 was found to be enhanced upon CD20 overexpression, suggesting a role for CD20 in stabilizing CD70 at the B cell membrane. Moreover, we observed impaired B-T cell synapse formation and defective recruitment of CD70 to the immunological synapse in the absence of CD20. Impaired synapse formation was confirmed by deleting CD20 in primary B cells, and analysis of B cells from a CD20-deficient patient. Finally, CD20-deletion resulted in diminished T cell activation and cytokine secretion. Together, this study demonstrates that CD20 interacts with CD70 at the B cell membrane, and that CD20 is required for immune synapse formation between B and T cells and consequent T cell activation.

CD20 is a B cell-specific four-helix transmembrane protein and a prominent target of therapeutic anti-CD20 antibodies. We have recently described that CD20 is localized within a membrane nanocluster harboring the IgD class B cell antigen receptor (IgD-BCR) where it functions as a gatekeeper for the resting state of naïve human B lymphocytes. How CD20 exerts its gatekeeper function is not yet known. Using Ramos and human peripheral blood B cells, we show here that the serine/threonine kinase PKCδ, constitutively phosphorylates serine residues at the cytosolic tails of CD20. The phosphorylated CD20 becomes a target for 14-3-3 adaptor proteins that link CD20 to the RhoA GDP/GTP exchange factor GEF-H1 and the microtubule (MT) network controlling the stability of the IgD-BCR nanocluster. Binding of anti-CD20 antibodies results in MT disassembly and the replacement of the GEF-H1/CD20 complex by a RhoA-GTP/ROCK1/CD20 complex which drives actomyosin contractility. Our study suggests that CD20 not only maintains the resting state, but also orchestrates the MT/actin switch in active B lymphocytes. This could have implications for treatment with anti-CD20 antibodies and may help to optimize therapeutic protocols. ![Figure][1] ### Competing Interest Statement The authors have declared no competing interest. R01 Human Vaccine institute, A031503 Roche (Switzerland), https://ror.org/00by1q217, ZVK-2023101202 Deutsche Forschungsgemeinschaft, 31778431, FOR2743, 450392965 Nanoscale Microscopy and Molecular Physiology of the Brain Cluster of Excellence 171 – DFG Research Center 103, https://ror.org/02wkzzr31, 541758684 DFG TRR130, C01 University of Freiburg, 2023/A2-Fol, 2021/B3-Fol DKTK [1]: pending:yes

How different classes of the B cell antigen receptor (BCR) sense viral antigens used in vaccination protocols is poorly understood. Here, we study antigen binding and sensing of human Ramos B cells expressing a BCR of either the IgM or IgG1 class with specificity for the CD4-binding-site of the envelope (Env) protein of the HIV-1. Both BCRs carry an identical antigen binding site derived from the broad neutralizing antibody (bnAb) CH31. We find a five times higher expression of the IgG1-BCR in comparison to the IgM-BCR on the surface of transfected Ramos B cells. The two BCR classes also differ from each other in their interaction with cognate HIV Env antigens in that the IgG1-BCR and IgM-BCR bind preferentially to polyvalent and monovalent antigens, respectively. By generating an IgM/IgG1 chimeric BCR, we found that the class-specific BCR expression and antigen-sensing behavior can be transferred with the CH1γ domain from the IgG1-BCR to the IgM-BCR. Thus, the class of CH1 domain has an impact on BCR assembly and expression as well as on antigen sensing.

IntroductionThe early transcription unit 3 (E3) of human adenoviruses (HAdVs) encodes several immunoevasins, including the E3/49K protein, which is unique for species D of HAdVs. It is expressed as surface transmembrane protein and shed. E3/49K of HAdV-D64 binds to the protein tyrosine phosphatase surface receptor CD45, thereby modulating activation of T and NK cells.MethodsConsidering that E3/49K represents the most polymorphic viral protein among species D HAdVs, we demonstrate here that all tested E3/49K orthologs bind to the immunologically important regulator CD45. Thus, this feature is conserved regardless of the pathological associations of the respective HAdV types.ResultsIt appeared that modulation of CD45 is a unique property restricted to HAdVs of species D. Moreover, E3/49K treatment inhibited B cell receptor (BCR) signaling and impaired BCR signal phenotypes. The latter were highly comparable to B cells having defects in the expression of CD45, suggesting E3/49K as a potential tool to investigate CD45 specific functions.ConclusionWe identified B cells as new direct target of E3/49K-mediated immune modulation, representing a novel viral immunosubversive mechanism.

B lymphocytes recognize bacterial or viral antigens via different classes of the B cell antigen receptor (BCR). Protrusive structures termed microvilli cover lymphocyte surfaces, and are thought to perform sensory functions in screening antigen-bearing surfaces. Here, we have used lattice light-sheet microscopy in combination with tailored custom-built 4D image analysis to study the cell-surface topography of B cells of the Ramos Burkitt's Lymphoma line and the spatiotemporal organization of the IgM-BCR. Ramos B-cell surfaces were found to form dynamic networks of elevated ridges bridging individual microvilli. A fraction of membrane-localized IgM-BCR was found in clusters, which were mainly associated with the ridges and the microvilli. The dynamic ridge-network organization and the IgM-BCR cluster mobility were linked, and both were controlled by Arp2/3 complex activity. Our results suggest that dynamic topographical features of the cell surface govern the localization and transport of IgM-BCR clusters to facilitate antigen screening by B cells.

SUMMARY How different classes of the B cell antigen receptor (BCR) sense viral antigens used in vaccination protocols is poorly understood. Here we study antigen binding and sensing of Ramos B cells expressing BCRs of either the IgM or IgG1 class with a specificity for the CD4-binding-site of the envelope (Env) protein of the human immune deficiency virus-1 (HIV-1). We find that, in spite of their identical antigen binding site, the two BCR classes differ drastically from each other in that the IgM-BCR and IgG1-BCR bind preferentially to monovalent and polyvalent antigens, respectively. By generating an IgM/IgG1 chimeric BCR we found that the class-specific antigen-sensing behavior can be transferred with the CH1γ domain from the IgG1-BCR to the IgM-BCR. Our results indicate that the class-switching process not only results in the production of antibody classes with different effector functions but also alters the antigen sensing of secondary B lymphocytes. These findings suggest that antigen valency in existing vaccination protocols should be modified and altered between primary versus secondary (booster) immunization.

HIV-1 Envelope (Env) proteins designed to induce neutralizing antibody responses allow study of the role of affinities (equilibrium dissociation constant, KD) and kinetic rates (association/dissociation rates) on B cell antigen recognition. It is unclear whether affinity discrimination during B cell activation is based solely on Env protein binding KD, and whether B cells discriminate between proteins of similar affinities but that bind with different kinetic rates. Here we used a panel of Env proteins and Ramos B cell lines expressing IgM BCRs with specificity for CD4 binding-site broadly neutralizing (bnAb) or a precursor antibody to study the role of antigen binding kinetic rates on both early (proximal/distal signaling) and late events (BCR/antigen internalization) in B cell activation. Our results support a kinetic model for B cell activation in which Env protein affinity discrimination is based not on overall KD, but on sensing of association rate and a threshold antigen-BCR half-life.

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. Cryo-electron microscopy structures of the mouse class M immunoglobulin B cell receptor complex provide insight into B cell receptor subunit assembly, domain architecture and signalling mechanism.

CD20 is a B cell-specific membrane protein and represents an attractive target for therapeutic antibodies. Despite widespread usage of anti-CD20 antibodies for B cell depletion therapies, the biological function of their target remains unclear. Here, we demonstrate that CD20 controls the nanoscale organization of receptors on the surface of resting B lymphocytes. CRISPR/Cas9-mediated ablation of CD20 in resting B cells resulted in relocalization and interaction of the IgM-class B cell antigen receptor with the coreceptor CD19. This receptor rearrangement led to a transient activation of B cells, accompanied by the internalization of many B cell surface marker proteins. Reexpression of CD20 restored the expression of the B cell surface proteins and the resting state of Ramos B cells. Similarly, treatment of Ramos or naive human B cells with the anti-CD20 antibody rituximab induced nanoscale receptor rearrangements and transient B cell activation in vitro and in vivo. A departure from the resting B cell state followed by the loss of B cell identity of CD20-deficient Ramos B cells was accompanied by a PAX5 to BLIMP-1 transcriptional switch, metabolic reprogramming toward oxidative phosphorylation, and a shift toward plasma cell development. Thus, anti-CD20 engagement or the loss of CD20 disrupts membrane organization, profoundly altering the fate of human B cells.

We describe the results of two vaccinations of a self-experimenting healthy volunteer with SARS-CoV-2-derived peptides performed in March and April 2020, respectively. The first set of peptides contained eight peptides predicted to bind to the individual's HLA molecules. The second set consisted of ten peptides predicted to bind promiscuously to several HLA-DR allotypes. The vaccine formulation contained the new TLR 1/2 agonist XS15 and was administered as an emulsion in Montanide as a single subcutaneous injection. Peripheral blood mononuclear cells isolated from blood drawn before and after vaccinations were assessed using Interferon-γ ELISpot assays and intracellular cytokine staining. We detected vaccine-induced CD4 T cell responses against six out of 11 peptides predicted to bind to HLA-DR after 19 days, following vaccination, for one peptide already at day 12. We used these results to support the design of a T-cell-inducing vaccine for application in high-risk patients, with weakened lymphocyte performance. Meanwhile, an according vaccine, incorporating T cell epitopes predominant in convalescents, is undergoing clinical trial testing.

Objective Autoantibodies against antigens carrying distinct post-translational modifications (PTMs), such as citrulline, homocitrulline or acetyllysine, are hallmarks of rheumatoid arthritis (RA). The relation between these anti-modified protein antibody (AMPA)-classes is poorly understood as is the ability of different PTM-antigens to activate B-cell receptors (BCRs) directed against citrullinated proteins (CP). Insights into the nature of PTMs able to activate such B cells are pivotal to understand the ‘evolution’ of the autoimmune response conceivable underlying the disease. Here, we investigated the cross-reactivity of monoclonal AMPA and the ability of different types of PTM-antigens to activate CP-reactive BCRs. Methods BCR sequences from B cells isolated using citrullinated or acetylated antigens were used to produce monoclonal antibodies (mAb) followed by a detailed analysis of their cross-reactivity towards PTM-antigens. Ramos B-cell transfectants expressing CP-reactive IgG BCRs were generated and their activation on stimulation with PTM-antigens investigated. Results Most mAbs were highly cross-reactive towards multiple PTMs, while no reactivity was observed to the unmodified controls. B cells carrying CP-reactive BCRs showed activation on stimulation with various types of PTM-antigens. Conclusions Our study illustrates that AMPA exhibit a high cross-reactivity towards at least two PTMs indicating that their recognition pattern is not confined to one type of modification. Furthermore, our data show that CP-reactive B cells are not only activated by citrullinated, but also by carbamylated and/or acetylated antigens. These data are vital for the understanding of the breach of B-cell tolerance against PTM-antigens and the possible contribution of these antigens to RA-pathogenesis.

The major function of B lymphocytes is to sense antigens and to produce protective antibodies after activation. This function requires the expression of a B-cell antigen receptor (BCR), and evolutionary conserved mechanisms seem to exist that ensure that B cells without a BCR do not develop nor survive in the periphery. Here, we show that the loss of BCR expression on Burkitt lymphoma cells leads to decreased mitochondrial function and impaired metabolic flexibility. Strikingly, this phenotype does not result from the absence of a classical Syk-dependent BCR signal but rather from compromised ER expansion. We show that the reexpression of immunoglobulins (Ig) in the absence of the BCR signaling subunits Igα and Igβ rescues the observed metabolic defects. We demonstrate that immunoglobulin expression is needed to maintain ER homeostasis not only in lymphoma cells but also in resting B cells. Our study provides evidence that the expression of BCR components, which is sensed in the ER and shapes mitochondrial function, represents a novel mechanism of metabolic control in B cells.

In contrast to other B-cell antigen receptor (BCR) classes, the function of IgD BCR on mature B cells remains largely elusive as mature B cells co-express IgM, which is sufficient for development, survival, and activation of B cells. Here, we show that IgD expression is regulated by the forkhead box transcription factor FoxO1, thereby shifting the responsiveness of mature B cells towards recognition of multivalent antigen. FoxO1 is repressed by phosphoinositide 3-kinase (PI3K) signaling and requires the lipid phosphatase Pten for its activation. Consequently, Pten-deficient B cells expressing knock-ins for BCR heavy and light chain genes are unable to upregulate IgD. Furthermore, in the presence of autoantigen, Pten-deficient B cells cannot eliminate the autoreactive BCR specificity by secondary light chain gene recombination. Instead, Pten-deficient B cells downregulate BCR expression and become unresponsive to further BCR-mediated stimulation. Notably, we observed a delayed germinal center (GC) reaction by IgD-deficient B cells after immunization with trinitrophenyl-ovalbumin (TNP-Ova), a commonly used antigen for T-cell-dependent antibody responses. Together, our data suggest that the activation of IgD expression by Pten/FoxO1 results in mature B cells that are selectively responsive to multivalent antigen and are capable of initiating rapid GC reactions and T-cell-dependent antibody responses.

B lymphocytes have the ability to sense thousands of structurally different antigens and produce cognate antibodies against these molecules. For this they carry on their surface multiple copies of the B cell antigen receptor (BCR) comprising the membrane-bound Ig (mIg) molecule and the Igα/Igβ heterodimer functioning as antigen binding and signal transducing components, respectively. The mIg is a symmetric complex of 2 identical membrane-bound heavy chains (mHC) and 2 identical light chains. How the symmetric mIg molecule is asymmetrically associated with only one Igα/Igβ heterodimer has been a puzzle. Here we describe that Igα and Igβ both carry on one side of their α-helical transmembrane domain a conserved amino acid motif. By a mutational analysis in combination with a BCR rebuilding approach, we show that this motif is required for the retention of unassembled Igα or Igβ molecules inside the endoplasmic reticulum and the binding of the Igα/Igβ heterodimer to the mIg molecule. We suggest that the BCR forms within the lipid bilayer of the membrane a symmetric Igα-mHC:mHC-Igβ complex that is stabilized by an aromatic proline-tyrosine interaction. Outside the membrane this symmetry is broken by the disulfide-bridged dimerization of the extracellular Ig domains of Igα and Igβ. However, symmetry of the receptor can be regained by a dimerization of 2 BCR complexes as suggested by the dissociation activation model.

In mice, neonatally-developing, self-reactive B-1 cells generate steady levels of natural antibodies throughout life. B-1 cells can, however, also rapidly respond to infections with increased local antibody production. The mechanisms regulating these two seemingly very distinct functions are poorly understood, but have been linked to expression of CD5, an inhibitor of BCR-signaling. Here we demonstrate that TLR-mediated activation of CD5+ B-1 cells induced the rapid reorganization of the IgM-BCR complex, leading to the eventual loss of CD5 expression, and a concomitant increase in BCR-downstream signaling, both in vitro and in vivo after infections of mice with influenza virus and Salmonella typhimurium. Both, initial CD5 expression and TLR-mediated stimulation, were required for the differentiation of B-1 cells to IgM-producing plasmablasts after infections. Thus, TLR-mediated signals support participation of B-1 cells in immune defense via BCR-complex reorganization.

Expression of the B-cell antigen receptor (BCR) is essential not only for the development but also for the maintenance of mature B cells. Similarly, many B-cell lymphomas, including Burkitt lymphoma (BL), require continuous BCR signaling for their tumor growth. This growth is driven by immunoreceptor tyrosine-based activation motif (ITAM) and PI3 kinase (PI3K) signaling. Here, we employ CRISPR/Cas9 to delete BCR and B-cell co-receptor genes in the human BL cell line Ramos. We find that Ramos B cells require the expression of the BCR signaling component Igβ (CD79b), and the co-receptor CD19, for their fitness and competitive growth in culture. Furthermore, we show that in the absence of any other BCR component, Igβ can be expressed on the B-cell surface, where it is found in close proximity to CD19 and signals in an ITAM-dependent manner. These data suggest that Igβ and CD19 are part of an alternative B-cell signaling module that use continuous ITAM/PI3K signaling to promote the survival of B lymphoma and normal B cells.

Developing B cells undergo defined maturation steps in the bone marrow and in the spleen. The timing and the factors that control these differentiation steps are not fully understood. By targeting the B cell-restricted mb-1 locus to generate an mb-1 allele that expresses a tamoxifen inducible Cre and another allele in which mb-1 expression can be controlled by Cre, we have established a mouse model with an inducible B cell compartment. With these mice, we studied in detail the kinetics of B cell development and the consequence of BCR activation at a defined B cell maturation stage. Contrary to expectations, transitional 1-B cells exposed to anti-IgM reagents in vivo did not die but instead developed into transitional 2 (T2)-B cells with upregulated Bcl-2 expression. We show, however, that these T2-B cells had an increased dependency on the B cell survival factor B cell activating factor when compared to non-stimulated B cells. Overall, our findings indicate that the inducible mb-1 mouse strain represents a useful model, which allows studying the signals that control the selection of B cells in greater detail.