• Investigating the Physical Effects in Bacterial Therapies for Avascular Tumors.

      Mascheroni, Pietro; Meyer-Hermann, Michael; Hatzikirou, Haralampos (Frontiers, 2020-06-04)
      Tumor-targeting bacteria elicit anticancer effects by infiltrating hypoxic regions, releasing toxic agents and inducing immune responses. Although current research has largely focused on the influence of chemical and immunological aspects on the mechanisms of bacterial therapy, the impact of physical effects is still elusive. Here, we propose a mathematical model for the anti-tumor activity of bacteria in avascular tumors that takes into account the relevant chemo-mechanical effects. We consider a time-dependent administration of bacteria and analyze the impact of bacterial chemotaxis and killing rate. We show that active bacterial migration toward tumor hypoxic regions provides optimal infiltration and that high killing rates combined with high chemotactic values provide the smallest tumor volumes at the end of the treatment.We highlight the emergence of steady states in which a small population of bacteria is able to constrain tumor growth. Finally, we show that bacteria treatment works best in the case of tumors with high cellular proliferation and low oxygen consumption.
    • In Vivo Killing Capacity of Cytotoxic T Cells Is Limited and Involves Dynamic Interactions and T Cell Cooperativity.

      Halle, Stephan; Keyser, Kirsten Anja; Stahl, Felix Rolf; Busche, Andreas; Marquardt, Anja; Zheng, Xiang; Galla, Melanie; Heissmeyer, Vigo; Heller, Katrin; Boelter, Jasmin; et al. (2016-02-16)
      According to in vitro assays, T cells are thought to kill rapidly and efficiently, but the efficacy and dynamics of cytotoxic T lymphocyte (CTL)-mediated killing of virus-infected cells in vivo remains elusive. We used two-photon microscopy to quantify CTL-mediated killing in mice infected with herpesviruses or poxviruses. On average, one CTL killed 2-16 virus-infected cells per day as determined by real-time imaging and by mathematical modeling. In contrast, upon virus-induced MHC class I downmodulation, CTLs failed to destroy their targets. During killing, CTLs remained migratory and formed motile kinapses rather than static synapses with targets. Viruses encoding the calcium sensor GCaMP6s revealed strong heterogeneity in individual CTL functional capacity. Furthermore, the probability of death of infected cells increased for those contacted by more than two CTLs, indicative of CTL cooperation. Thus, direct visualization of CTLs during killing of virus-infected cells reveals crucial parameters of CD8(+) T cell immunity.
    • A least microenvironmental uncertainty principle (LEUP) as a generative model of collective cell migration mechanisms.

      Barua, Arnab; Nava-Sedeño, Josue M; Meyer-Hermann, Michael; Hatzikirou, Haralampos; BRICS, Braunschweiger Zentrum für Systembiologie, Rebenring 56,38106 Braunschweig, Germany. (Nature research, 2020-12-22)
      Collective migration is commonly observed in groups of migrating cells, in the form of swarms or aggregates. Mechanistic models have proven very useful in understanding collective cell migration. Such models, either explicitly consider the forces involved in the interaction and movement of individuals or phenomenologically define rules which mimic the observed behavior of cells. However, mechanisms leading to collective migration are varied and specific to the type of cells involved. Additionally, the precise and complete dynamics of many important chemomechanical factors influencing cell movement, from signalling pathways to substrate sensing, are typically either too complex or largely unknown. The question is how to make quantitative/qualitative predictions of collective behavior without exact mechanistic knowledge. Here we propose the least microenvironmental uncertainty principle (LEUP) that may serve as a generative model of collective migration without precise incorporation of full mechanistic details. Using statistical physics tools, we show that the famous Vicsek model is a special case of LEUP. Finally, to test the biological applicability of our theory, we apply LEUP to construct a model of the collective behavior of spherical Serratia marcescens bacteria, where the underlying migration mechanisms remain elusive.
    • A mathematical model of immune activation with a unified self-nonself concept.

      Khailaie, Sahamoddin; Bahrami, Fariba; Janahmadi, Mahyar; Milanez-Almeida, Pedro; Huehn, Jochen; Meyer-Hermann, Michael (2013)
      The adaptive immune system reacts against pathogenic nonself, whereas it normally remains tolerant to self. The initiation of an immune response requires a critical antigen(Ag)-stimulation and a critical number of Ag-specific T cells. Autoreactive T cells are not completely deleted by thymic selection and partially present in the periphery of healthy individuals that respond in certain physiological conditions. A number of experimental and theoretical models are based on the concept that structural differences discriminate self from nonself. In this article, we establish a mathematical model for immune activation in which self and nonself are not distinguished. The model considers the dynamic interplay of conventional T cells, regulatory T cells (Tregs), and IL-2 molecules and shows that the renewal rate ratio of resting Tregs to naïve T cells as well as the proliferation rate of activated T cells determine the probability of immune stimulation. The actual initiation of an immune response, however, relies on the absolute renewal rate of naïve T cells. This result suggests that thymic selection reduces the probability of autoimmunity by increasing the Ag-stimulation threshold of self reaction which is established by selection of a low number of low-avidity autoreactive T cells balanced with a proper number of Tregs. The stability analysis of the ordinary differential equation model reveals three different possible immune reactions depending on critical levels of Ag-stimulation: a subcritical stimulation, a threshold stimulation inducing a proper immune response, and an overcritical stimulation leading to chronic co-existence of Ag and immune activity. The model exhibits oscillatory solutions in the case of persistent but moderate Ag-stimulation, while the system returns to the homeostatic state upon Ag clearance. In this unifying concept, self and nonself appear as a result of shifted Ag-stimulation thresholds which delineate these three regimes of immune activation.
    • A mathematical model of T lymphocyte calcium dynamics derived from single transmembrane protein properties.

      Schmeitz, Christine; Hernandez-Vargas, Esteban Abelardo; Fliegert, Ralf; Guse, Andreas H; Meyer-Hermann, Michael; Department of Systems Immunology, Helmholtz Centre for Infection Research , Braunschweig , Germany. (2013)
      Fate decision processes of T lymphocytes are crucial for health and disease. Whether a T lymphocyte is activated, divides, gets anergic, or initiates apoptosis depends on extracellular triggers and intracellular signaling. Free cytosolic calcium dynamics plays an important role in this context. The relative contributions of store-derived calcium entry and calcium entry from extracellular space to T lymphocyte activation are still a matter of debate. Here we develop a quantitative mathematical model of T lymphocyte calcium dynamics in order to establish a tool which allows to disentangle cause-effect relationships between ion fluxes and observed calcium time courses. The model is based on single transmembrane protein characteristics which have been determined in independent experiments. This reduces the number of unknown parameters in the model to a minimum and ensures the predictive power of the model. Simulation results are subsequently used for an analysis of whole cell calcium dynamics measured under various experimental conditions. The model accounts for a variety of these conditions, which supports the suitability of the modeling approach. The simulation results suggest a model in which calcium dynamics dominantly relies on the opening of channels in calcium stores while calcium entry through calcium-release activated channels (CRAC) is more associated with the maintenance of the T lymphocyte calcium levels and prevents the cell from calcium depletion. Our findings indicate that CRAC guarantees a long-term stable calcium level which is required for cell survival and sustained calcium enhancement.
    • A mathematical model of the impact of insulin secretion dynamics on selective hepatic insulin resistance.

      Zhao, Gang; Wirth, Dagmar; Schmitz, Ingo; Meyer-Hermann, Michael; Braunschweiger Zentrum für Systembiologie, Rebenring 56, 38106, Germany. (2017-11-08)
      Physiological insulin secretion exhibits various temporal patterns, the dysregulation of which is involved in diabetes development. We analyzed the impact of first-phase and pulsatile insulin release on glucose and lipid control with various hepatic insulin signaling networks. The mathematical model suggests that atypical protein kinase C (aPKC) undergoes a bistable switch-on and switch-off, under the control of insulin receptor substrate 2 (IRS2). The activation of IRS1 and IRS2 is temporally separated due to the inhibition of IRS1 by aPKC. The model further shows that the timing of aPKC switch-off is delayed by reduced first-phase insulin and reduced amplitude of insulin pulses. Based on these findings, we propose a sequential model of postprandial hepatic control of glucose and lipid by insulin, according to which delayed aPKC switch-off contributes to selective hepatic insulin resistance, which is a long-standing paradox in the field.
    • Mathematical Model Shows How Sleep May Affect Amyloid-β Fibrillization.

      Hoore, Masoud; Khailaie, Sahamoddin; Montaseri, Ghazal; Mitra, Tanmay; Meyer-Hermann, Michael; BRICS, Braunschweiger Zentrum für Systembiologie, Rebenring 56,38106 Braunschweig, Germany; HZI,Helmholtz-Zentrum für Infektionsforschung GmbH, Inhoffenstr. 7,38124 Braunschweig, Germany. (Elsevier (CellPress), 2020-07-22)
      Deposition of amyloid-β (Aβ) fibers in the extracellular matrix of the brain is a ubiquitous feature associated with several neurodegenerative disorders, especially Alzheimer's disease (AD). Although many of the biological aspects that contribute to the formation of Aβ plaques are well addressed at the intra- and intercellular levels in short timescales, an understanding of how Aβ fibrillization usually starts to dominate at a longer timescale despite the presence of mechanisms dedicated to Aβ clearance is still lacking. Furthermore, no existing mathematical model integrates the impact of diurnal neural activity as emanated from circadian regulation to predict disease progression due to a disruption in the sleep-wake cycle. In this study, we develop a minimal model of Aβ fibrillization to investigate the onset of AD over a long timescale. Our results suggest that the diseased state is a manifestation of a phase change of the system from soluble Aβ (sAβ) to fibrillar Aβ (fAβ) domination upon surpassing a threshold in the production rate of sAβ. By incorporating the circadian rhythm into our model, we reveal that fAβ accumulation is crucially dependent on the regulation of the sleep-wake cycle, thereby indicating the importance of good sleep hygiene in averting AD onset. We also discuss potential intervention schemes to reduce fAβ accumulation in the brain by modification of the critical sAβ production rate.
    • Mechanical Control of Cell Proliferation Increases Resistance to Chemotherapeutic Agents.

      Rizzuti, Ilaria Francesca; Mascheroni, Pietro; Arcucci, Silvia; Ben-Mériem, Zacchari; Prunet, Audrey; Barentin, Catherine; Rivière, Charlotte; Delanoë-Ayari, Hélène; Hatzikirou, Haralampos; Guillermet-Guibert, Julie; et al. (American Physical Society, 2020-09-18)
      While many cellular mechanisms leading to chemotherapeutic resistance have been identified, there is an increasing realization that tumor-stroma interactions also play an important role. In particular, mechanical alterations are inherent to solid cancer progression and profoundly impact cell physiology. Here, we explore the influence of compressive stress on the efficacy of chemotherapeutics in pancreatic cancer spheroids. We find that increased compressive stress leads to decreased drug efficacy. Theoretical modeling and experiments suggest that mechanical stress decreases cell proliferation which in turn reduces the efficacy of chemotherapeutics that target proliferating cells. Our work highlights a mechanical form of drug resistance and suggests new strategies for therapy.
    • MicroRNA-155 controls affinity-based selection by protecting c-MYC+ B cells from apoptosis.

      Nakagawa, Rinako; Leyland, Rebecca; Müller, A; Lu, Dong; Turner, Martin; Arbore, Giuseppina; Phan, Tri Giang; Brink, Robert; Vigorito, Elena; Helmholtz Centre for infection research, Inhoffenstr. 7, 38124 Braunschweig, Germany. (2016-01-04)
      The production of high-affinity antibodies by B cells is essential for pathogen clearance. Antibody affinity for antigen is increased through the affinity maturation in germinal centers (GCs). This is an iterative process in which B cells cycle between proliferation coupled with the acquisition of mutations and antigen-based positive selection, resulting in retention of the highest-affinity B cell clones. The posttranscriptional regulator microRNA-155 (miR-155) is critical for efficient affinity maturation and the maintenance of the GCs; however, the cellular and molecular mechanism by which miR-155 regulates GC responses is not well understood. Here, we utilized a miR-155 reporter mouse strain and showed that miR-155 is coexpressed with the proto-oncogene encoding c-MYC in positively selected B cells. Functionally, miR-155 protected positively selected c-MYC+ B cells from apoptosis, allowing clonal expansion of this population, providing an explanation as to why Mir155 deletion impairs affinity maturation and promotes the premature collapse of GCs. We determined that miR-155 directly inhibits the Jumonji family member JARID2, which enhances B cell apoptosis when overexpressed, and thereby promotes GC B cell survival. Our findings also suggest that there is cooperation between c-MYC and miR-155 during the normal GC response, a cooperation that may explain how c-MYC and miR-155 can collaboratively function as oncogenes.
    • A minimal modeling framework of radiation and immune system synergy to assist radiotherapy planning.

      Montaseri, Ghazal; Alfonso, Juan Carlos López; Hatzikirou, Haralampos; Meyer-Hermann, Michael (Elsevier, 2020-02-07)
    • Modeling Influenza Virus Infection: A Roadmap for Influenza Research.

      Boianelli, Alessandro; Nguyen, Van Kinh; Ebensen, Thomas; Schulze, Kai; Wilk, Esther; Sharma, Niharika; Stegemann-Koniszewski, Sabine; Bruder, Dunja; Toapanta, Franklin R; Guzmán, Carlos A; et al. (2015-10)
      Influenza A virus (IAV) infection represents a global threat causing seasonal outbreaks and pandemics. Additionally, secondary bacterial infections, caused mainly by Streptococcus pneumoniae, are one of the main complications and responsible for the enhanced morbidity and mortality associated with IAV infections. In spite of the significant advances in our knowledge of IAV infections, holistic comprehension of the interplay between IAV and the host immune response (IR) remains largely fragmented. During the last decade, mathematical modeling has been instrumental to explain and quantify IAV dynamics. In this paper, we review not only the state of the art of mathematical models of IAV infection but also the methodologies exploited for parameter estimation. We focus on the adaptive IR control of IAV infection and the possible mechanisms that could promote a secondary bacterial coinfection. To exemplify IAV dynamics and identifiability issues, a mathematical model to explain the interactions between adaptive IR and IAV infection is considered. Furthermore, in this paper we propose a roadmap for future influenza research. The development of a mathematical modeling framework with a secondary bacterial coinfection, immunosenescence, host genetic factors and responsiveness to vaccination will be pivotal to advance IAV infection understanding and treatment optimization.
    • Modeling the effect of intratumoral heterogeneity of radiosensitivity on tumor response over the course of fractionated radiation therapy.

      Alfonso, J C L; Berk, L; BRICS, Braunschweiger Zentrum für Systembiologie, Rebenring 56,38106 Braunschweig, Germany. (BioMed Central (BMC), 2019-05-30)
      Standard radiobiology theory of radiation response assumes a uniform innate radiosensitivity of tumors. However, experimental data show that there is significant intratumoral heterogeneity of radiosensitivity. Therefore, a model with heterogeneity was developed and tested using existing experimental data to show the potential effects from the presence of an intratumoral distribution of radiosensitivity on radiation therapy response over a protracted radiation therapy treatment course.
    • Modeling the three stages in HIV infection.

      Hernandez-Vargas, Esteban A; Middleton, Richard H; SIMM, Helmholtz Zentrum für Infektionsforschung, Inhoffenstraße 7, D-38124 Braunschweig, Germany. (2013-03-07)
      A typical HIV infection response consists of three stages: an initial acute infection, a long asymptomatic period and a final increase in viral load with simultaneous collapse in healthy CD4+T cell counts. The majority of existing mathematical models give a good representation of either the first two stages or the last stage of the infection. Using macrophages as a long-term active reservoir, a deterministic model is proposed to explain the three stages of the infection including the progression to AIDS. Simulation results illustrate how chronic infected macrophages can explain the progression to AIDS provoking viral explosion. Further simulation studies suggest that the proposed model retains its key properties even under moderately large parameter variations. This model provides important insights on how macrophages might play a crucial role in the long term behavior of HIV infection.
    • Modelling collective cell motion: are on- and off-lattice models equivalent?

      Nava-Sedeño, Josué Manik; Voß-Böhme, Anja; Hatzikirou, Haralampos; Deutsch, Andreas; Peruani, Fernando (2020-07-27)
    • The molecular basis of synergism between carboplatin and ABT-737 therapy targeting ovarian carcinomas.

      Jain, Harsh Vardhan; Müller, A (2011-02-01)
      Resistance to standard chemotherapy (carboplatin + paclitaxel) is one of the leading causes of therapeutic failure in ovarian carcinomas. Emergence of chemoresistance has been shown to be mediated in part by members of the Bcl family of proteins including the antiapoptotic protein Bcl-x(L), whose expression is correlated with shorter disease-free intervals in recurrent disease. ABT-737 is an example of one of the first small-molecule inhibitors of Bcl-2/Bcl-x(L) that has been shown to increase the sensitivity of ovarian cancer cells to carboplatin. To exploit the therapeutic potential of these two drugs and predict optimal doses and dose scheduling, it is essential to understand the molecular basis of their synergistic action. Here, we build and calibrate a mathematical model of ABT-737 and carboplatin action on an ovarian cancer cell line (IGROV-1). The model suggests that carboplatin treatment primes cells for ABT-737 therapy because of an increased dependence of cells with DNA damage on Bcl-x(L) for survival. Numerical simulations predict the existence of a threshold of Bcl-x(L) below which these cells are unable to recover. Furthermore, co- plus posttreatment of ABT-737 with carboplatin is predicted to be the best strategy to maximize synergism between these two drugs. A critical challenge in chemotherapy is to strike a balance between maximizing cell-kill while minimizing patient drug load. We show that the model can be used to compute minimal doses required for any desired fraction of cell kill. These results underscore the potential of the modeling work presented here as a valuable quantitative tool to aid in the translation of novel drugs such as ABT-737 from the experimental to clinical setting and highlight the need for close collaboration between modelers and experimental scientists.
    • Molecular mechanism of Ena/VASP-mediated actin-filament elongation.

      Breitsprecher, Dennis; Kiesewetter, Antje K; Linkner, Joern; Vinzenz, Marlene; Stradal, Theresia E B; Small, John Victor; Curth, Ute; Dickinson, Richard B; Faix, Jan (2011-02-02)
      Ena/VASP proteins are implicated in a variety of fundamental cellular processes including axon guidance and cell migration. In vitro, they enhance elongation of actin filaments, but at rates differing in nearly an order of magnitude according to species, raising questions about the molecular determinants of rate control. Chimeras from fast and slow elongating VASP proteins were generated and their ability to promote actin polymerization and to bind G-actin was assessed. By in vitro TIRF microscopy as well as thermodynamic and kinetic analyses, we show that the velocity of VASP-mediated filament elongation depends on G-actin recruitment by the WASP homology 2 motif. Comparison of the experimentally observed elongation rates with a quantitative mathematical model moreover revealed that Ena/VASP-mediated filament elongation displays a saturation dependence on the actin monomer concentration, implying that Ena/VASP proteins, independent of species, are fully saturated with actin in vivo and generally act as potent filament elongators. Moreover, our data showed that spontaneous addition of monomers does not occur during processive VASP-mediated filament elongation on surfaces, suggesting that most filament formation in cells is actively controlled.
    • A molecular theory of germinal center B cell selection and division.

      Meyer-Hermann, Michael; BRICS, Braunschweiger Zentrum für Systembiologie, Rebenring 56,38106 Braunschweig, Germany. (Cell Press, 2021-08-24)
      The selection of B cells (BCs) in germinal centers (GCs) is pivotal to the generation of high-affinity antibodies and memory BCs, but it lacks global understanding. Based on the idea of a single Tfh-cell signal that controls BC selection and division, experiments appear contradictory. Here, we use the current knowledge on the molecular pathways of GC BCs to develop a theory of GC BC selection and division based on the dynamics of molecular factors. This theory explains the seemingly contradictory experiments by the separation of signals for BC fate decision from signals controlling the number of BC divisions. Three model variants are proposed and experiments are predicted that allow one to distinguish those. Understanding information processing in molecular BC states is critical for targeted immune interventions, and the proposed theory implies that selection and division can be controlled independently in GC reactions.
    • Multidimensional Analysis Integrating Human T-Cell Signatures in Lymphatic Tissues with Sex of Humanized Mice for Prediction of Responses after Dendritic Cell Immunization.

      Volk, Valery; Reppas, Andreas I; Robert, Philippe A; Spineli, Loukia M; Sundarasetty, Bala Sai; Theobald, Sebastian J; Schneider, Andreas; Gerasch, Laura; Deves Roth, Candida; Klöss, Stephan; et al. (2017)
      Mice transplanted with human cord blood-derived hematopoietic stem cells (HSCs) became a powerful experimental tool for studying the heterogeneity of human immune reconstitution and immune responses in vivo. Yet, analyses of human T cell maturation in humanized models have been hampered by an overall low immune reactivity and lack of methods to define predictive markers of responsiveness. Long-lived human lentiviral induced dendritic cells expressing the cytomegalovirus pp65 protein (iDCpp65) promoted the development of pp65-specific human CD8+ T cell responses in NOD.Cg-Rag1 tm1Mom -Il2rγ tm1Wj humanized mice through the presentation of immune-dominant antigenic epitopes (signal 1), expression of co-stimulatory molecules (signal 2), and inflammatory cytokines (signal 3). We exploited this validated system to evaluate the effects of mouse sex in the dynamics of T cell homing and maturation status in thymus, blood, bone marrow, spleen, and lymph nodes. Statistical analyses of cell relative frequencies and absolute numbers demonstrated higher CD8+ memory T cell reactivity in spleen and lymph nodes of immunized female mice. In order to understand to which extent the multidimensional relation between organ-specific markers predicted the immunization status, the immunophenotypic profiles of individual mice were used to train an artificial neural network designed to discriminate immunized and non-immunized mice. The highest accuracy of immune reactivity prediction could be obtained from lymph node markers of female mice (77.3%). Principal component analyses further identified clusters of markers best suited to describe the heterogeneity of immunization responses in vivo. A correlation analysis of these markers reflected a tissue-specific impact of immunization. This allowed for an organ-resolved characterization of the immunization status of individual mice based on the identified set of markers. This new modality of multidimensional analyses can be used as a framework for defining minimal but predictive signatures of human immune responses in mice and suggests critical markers to characterize responses to immunization after HSC transplantation.
    • Multiscale Modeling of Germinal Center Recapitulates the Temporal Transition From Memory B Cells to Plasma Cells Differentiation as Regulated by Antigen Affinity-Based Tfh Cell Help.

      Merino Tejero, Elena; Lashgari, Danial; García-Valiente, Rodrigo; Gao, Xuefeng; Crauste, Fabien; Robert, Philippe A; Meyer-Hermann, Michael; Martínez, María Rodríguez; van Ham, S Marieke; Guikema, Jeroen E J; et al. (Frontiers, 2021-02-05)
      Germinal centers play a key role in the adaptive immune system since they are able to produce memory B cells and plasma cells that produce high affinity antibodies for an effective immune protection. The mechanisms underlying cell-fate decisions are not well understood but asymmetric division of antigen, B-cell receptor affinity, interactions between B-cells and T follicular helper cells (triggering CD40 signaling), and regulatory interactions of transcription factors have all been proposed to play a role. In addition, a temporal switch from memory B-cell to plasma cell differentiation during the germinal center reaction has been shown. To investigate if antigen affinity-based Tfh cell help recapitulates the temporal switch we implemented a multiscale model that integrates cellular interactions with a core gene regulatory network comprising BCL6, IRF4, and BLIMP1. Using this model we show that affinity-based CD40 signaling in combination with asymmetric division of B-cells result in switch from memory B-cell to plasma cell generation during the course of the germinal center reaction. We also show that cell fate division is unlikely to be (solely) based on asymmetric division of Ag but that BLIMP1 is a more important factor. Altogether, our model enables to test the influence of molecular modulations of the CD40 signaling pathway on the production of germinal center output cells.
    • Naive- and Memory-like CD21 B Cell Subsets Share Core Phenotypic and Signaling Characteristics in Systemic Autoimmune Disorders.

      Freudenhammer, Mirjam; Voll, Reinhard E; Binder, Sebastian C; Keller, Baerbel; Warnatz, Klaus (2020-09-09)
      An expansion of CD21low B cells has been described in a variety of diseases associated with persistent immune stimulation as in chronic infection, immunodeficiency, or autoimmunity. Different developmental stages of CD21low B cells have been highlighted in specific diseases; however, a systematic comparison of distribution, phenotype, and signaling capacity of these populations has not yet been performed to delineate the pivotal character of this unusual B cell population. Screening of more than 200 patients with autoimmune disease demonstrated that the prevalence of patients with expanded CD21low B cells varies between diseases. The expansion was frequent in patients with systemic lupus erythematosus, in which it correlated to relative B cell lymphopenia and duration of disease. Different proportions of distinct developmental stages of CD21low B cells co-occur in nearly all patients with autoimmune disease. Although in most patients, naive-like and CD27- switched memory B cells were the most prominent CD21low subpopulations, there was no detectable association of the pattern with the underlying disease. Despite their distinct developmental stage, all CD21low B cells share a common core phenotype including the increased expression of inhibitory receptors, associated with an elevated constitutive phosphorylation of proximal signaling molecules downstream of the BCR but impaired Ca2+ mobilization and NF-κB activation after BCR stimulation. Further, this was accompanied by impaired upregulation of CD69, although CD86 upregulation was preserved. Beyond maturation-associated differences, the common core characteristics of all CD21low B cell populations suggests either a common ancestry or a shared sustained imprint by the environment they originated in.