B cell

Summary

B cells, also known as B lymphocytes, are a type of white blood cell of the lymphocyte subtype. They function in the humoral immunity component of the adaptive immune system. B cells produce antibody molecules which may be either secreted or inserted into the plasma membrane where they serve as a part of B-cell receptors. When a naïve or memory B cell is activated by an antigen, it proliferates and differentiates into an antibody-secreting effector cell, known as a plasmablast or plasma cell. Additionally, B cells present antigens (they are also classified as professional antigen-presenting cells (APCs)) and secrete cytokines. In mammals, B cells mature in the bone marrow, which is at the core of most bones. In birds, B cells mature in the bursa of Fabricius, a lymphoid organ where they were first discovered by Chang and Glick, which is why the 'B' stands for bursa and not bone marrow as commonly believed. B cells, unlike

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Bruton's tyrosine kinase (Btk) is a key component of BCR signaling required for B-cell maturation and activation. Loss-of-function mutations throughout the Btk protein cause human X-linked agammaglobulinemia (XLA), a heritable genetic disorder characterized by the absence of mature B-cells and lack of adaptive immune response. In contrast, Btk signaling sustains the growth of several B-cell neoplasms, which may be treated with small-molecule tyrosine kinase inhibitors (TKIs) targeting the ATP-site of the kinase. No alternative targeted therapy is available for patients who acquire resistance to current Btk inhibitors. Whereas a number of intramolecular interactions amongst the PH, SH3, SH2, and kinase domain (KD) are well resolved and stabilize a compact autoinhibited conformation, the molecular mechanisms governing Btk activation are not fully understood. Using a combination of in silico, structural, cellular, and molecular approaches, we discovered an allosteric interface between the SH2 and the N-lobe of the KD that is critical for kinase activation in vitro and cells. In contrast to the low Btk activity of the KD alone, the presence of the SH2 domain is sufficient to increase Btk phosphorylation at Y551 and multiple other sites. This provides the structural mechanism by which a subset of XLA mutations mapped to the SH2 domain near the interface region strongly perturbs Btk activation, even though the SH2 canonical function is preserved. Furthermore, we demonstrated that the active Btk SH2-kinase adopts an elongated conformation with a dynamic contact interface structurally distinct than to the ones observed for other tyrosine kinases, such as Abl, Fes, and Csk. As allosteric interactions provide unique targeting opportunities far from the ATP-site, we developed an engineered repebody protein binding to the human Btk SH2 domain. The crystal structure of the SH2-repebody complex combined with other structural approaches demonstrated that the repebody disrupts the SH2-kinase interaction. The repebody prevented activation of wild-type and TKI-resistant Btk in vitro and cells seen by a significant decrease in Btk phosphorylation. In parallel, sole allosteric targeting inhibited Btk-dependent signaling and reduced proliferation of malignant B-cells dependent on the BCR signaling. Therefore, the SH2-kinase interface is critical for Btk activation and is the first targetable site able to trigger allosteric inhibition. As Btk targeted therapy has a great impact on several conditions, including B-cell malignancies and autoimmune diseases, this study provides a rationale to support the development of alternative Btk inhibitors. This approach for targeted inhibition could particularly benefit patients with current therapy-resistant Btk variants.

EPFL2020

The evolutionarily conserved Notch signalling pathway controls a broad spectrum of cell fate decisions in organisms as diverse as fruit flies and mice. Whithin the murine haematopoietic system, Notch1 is indispensable for T cell development as conditional inactivation of Notch1 in bone marrow (BM) precursors results in a complete block in T-cell development and ectopic B-cell development in the thymus. Conversely, constitutive activation of Notch1 signalling plays a causative role in the development of acute T-cell lymphoblastic leukaemia (T-ALL). Numerous mutations within the Notch1 gene, which result in aberrent activation of Notch1 signalling, have been identified in > 50% of human T-ALL patients. Therefore, gain-of-function studies in murine models of T-ALL have been employed by many laboratories to elucidate the molecular mechanisms acting either cooperatively or downstream of oncogenic Notch1 signalling in development and progression of T-ALL. The best-characterized Notch1 target gene to date is a member of the bHLH family of transcriptional repressors, Hes1. This study focuses on the role of Hes1 as a mediator of Notch1 signalling in adult murine haematopoietic development, as well as the requirement for Hes1 in T-ALL development and maintenance. Conditional inactivation of Hes1 in adult murine BM cells results in severe thymic hypoplasia, while myeloid and B cell development remain unperturbed. Upon transplantation in congenic recipients, Hes1-/- progenitors give rise to only a small number of mature T cells. Hes1-inactivation does not however cause ectopic B-cell development in the thymus, and therefore does not phenocopy the loss of Notch1. Thus, while Notch1 signalling is indispensable for T versus B-cell fate specification, this function is not mediated exclusively via Hes1. Strikingly, in a competitive BM chimera setting, Hes1-/- progenitors completely fail to generate T cells. The mechanism by which Hes1 inactivation results in depleted T cell numbers remains to be elucidated, although we have shown that loss of Hes1 does not result in enhanced cell death. In addition, preliminary data are indicative of cell-cycle perturbation in Hes1-deficient immature T cells. Finally, we have demonstrated that upon IT transplantation, Hes1-deficient progenitors successfully give rise to T-cells at all stages of development. This data strongly indicates that Hes1 deficiency does not result in an intrinsic developmental defect but rather in the inability of T cell progenitors to efficiently home to the thymus from the BM. Upregulation of Hes1 expression has been observed in both human T-ALL cell lines as well as murine primary Notch1-induced T-ALL samples. However, the function of Hes1 as an effector of oncogenic Notch1 signalling in modulating disease onset or progression remained to be investigated. The requirement for Hes1 as a mediator of Notch1-induced T-ALL was addressed by inducing simultaneous deletion of Hes1 and expression of a dominant active form of Notch1 (NICD) in a murine model of T-ALL. Our data demonstrate that Hes1 is essential for the development of NICD-induced T-ALL. Loss of Hes1 prevents ectopic development of DP (CD4+CD8+) leukemic T cells in the BM and the subsequent accumulation of DP T cells in peripheral tissues, a hallmark of T-ALL, and consequently results in 10-fold increased life expectancy of experimental animals. We have further determined that Hes1 is required for the maintenance of T-ALL by inactivating Hes1 in a retrovirally pre-induced T-ALL model. Deletion of Hes1 in leukemic DP cells results in rapid cell death and enhanced survival. These data demonstrate that Hes1 is required as a mediator of Notch1 signalling in normal T cell development, and is indispensable to mediate the oncogenic effect of constitutive Notch1 activation in the induction of T-ALL.

EPFL2010

Over the last few years, protein drugs have steadily gained importance in the clinic. Compared to small molecule drugs, protein drugs are taken up by antigen presenting cells, which degrade the proteins into peptides that are then presented on major histocompatability complex I and II to CD8+ respectively CD4+ T-cells. CD4+ T-cells are then able to activate B-cells, which upon activation will produce anti-drug antibodies. While anti-drug antibodies have mostly been viewed as a safety risk leading to drug injection related side effects, they have recently been proven to diminish the efficacy of the treatment. High antibody titers against Humira, a clinically approved monoclonal antibody used to treat rheumatoid arthritis, have been associated with low serum drug concentration and high disease activity score, ultimately leading to treatment failure. Two approaches to induce tolerance were developed in the Hubbell lab. One approach utilizes protein variants that are designed to bind to circulating erythrocytes. As 1 % of erythrocytes undergo apoptosis each day and are taken up by antigen presenting cells, along with the bound protein, they are presented to the immune system in a non-inflammatory tolerogenic fashion. Immunological tolerance is thereby induced against the apoptotic erythrocyte debris and the associated bound protein. The second approach uses synthetic glucose, galactose or mannose polymers conjugated to the target protein. The polymers target C-type lectins, which are known to clear apoptotic debris, leading to presentation of the protein by hepatic antigen presenting cells in a tolerogenic liver microenvironment. This thesis investigates the potential of these approaches to induce tolerance towards protein drugs. Arylsulfatase B was used as a model protein to investigate the potential of tolerance induction by targeting circulating erythrocytes, as 97% of patients receiving arylsulfatase B develop anti-drug antibodies. An erythrocyte binding variant of arylsufatase B was created by chemically conjugating the erythrocyte binding peptide ERY1 to arylsulfatse B. In mice receiving two doses of ERY1-arylsulfatase B one week apart followed by weekly doses of arylsulfatase B a significant delay of four week in the production of anti-drug Antibodies was found when compared to mice receiving weekly doses of only arylsulfatase B. To investigate the ability of synthetic glyco-polymers to induce tolerance towards protein drugs we used asparaginase as model protein. Between 37.5 % to 75% of patients treated with asparaginase develop anti-drug antibodies. Administering three intravenous injections of asparaginase-conjugated to glucose polymers before eight weeks of treatment with asparaginase significantly reduced anti-drug antibody titers by a factor of 125 when compared to animals only receiving eight weeks of treatment with asparaginase. Three pre-injections with asparaginase conjugated to the galactose or mannose polymer led to a reduction in anti-drug antibody titers by a factor of 12 respectively 400 after 5 weeks of treatment with asparaginase when compared to mice receiving only asparaginase for 5 weeks. These results indicates that synthetic glyco polymers, especially mannose and glucose based ones, are able to induce long term tolerance towards asparaginase. Synthetic glucose and mannose polymers therefore represent an excellent molecular approach to induce tolerance towards protein drugs.

EPFL2018

EPFL2020

EPFL2010

EPFL2018