FAS Ligand, Bcl-2, Granulocyte Colony-Stimulating Factor, and p38 Mitogen-Activated Protein Kinase: Regulators of Distinct Cell Death and Survival Pathways in Granulocytes

The short life span of granulocytes, which limits many inflammatory responses, is thought to be influenced by the Bcl-2 protein family, death receptors such as CD95 (Fas/APO-1), stress-activated protein kinases such as p38 mitogen-activated protein kinase (MAPK), and proinflammatory cytokines like granulocyte colony-stimulating factor (G-CSF). To clarify the roles of these various regulators in granulocyte survival, we have investigated the spontaneous apoptosis of granulocytes in culture and that induced by Fas ligand or chemotherapeutic drugs, using cells from normal, CD95-deficient lpr, or vav-bcl-2 transgenic mice. CD95-induced apoptosis, which required receptor aggregation by recombinant Fas ligand or the membrane-bound ligand, was unaffected by G-CSF treatment or Bcl-2 overexpression. Conversely, spontaneous and drug-induced apoptosis occurred normally in lpr granulocytes but were suppressed by G-CSF treatment or Bcl-2 overexpression. Although activation of p38 MAPK has been implicated in granulocyte death, their apoptosis actually was markedly accelerated by specific inhibitors of this kinase. These results suggest that G-CSF promotes granulocyte survival largely through the Bcl-2–controlled pathway, whereas CD95 regulates a distinct pathway to apoptosis that is not required for either their spontaneous or drug-induced death. Moreover, p38 MAPK signaling contributes to granulocyte survival rather than their apoptosis

TET enzymes control antibody production and shape the mutational landscape in germinal centre B cells

Upon activation by antigen, B cells form germinal centers where they clonally expand and introduce affinity-enhancing mutations into their B cell receptor genes. Somatic mutagenesis and class switch recombination in germinal center B cells are initiated by the activation-induced cytidine deaminase (AID). Upon germinal center exit, B cells differentiate into antibody-secreting plasma cells. Germinal center maintenance and terminal fate choice require transcriptional reprogramming that associates with a substantial reconfiguration of DNA methylation patterns. Here we examine the role of TET proteins, enzymes that facilitate DNA demethylation and promote a permissive chromatin state by oxidizing 5-methylcytosine, in antibody-mediated immunity. Using a conditional gene ablation strategy, we show that TET2 and TET3 guide the transition of germinal center B cells to antibody-secreting plasma cells. Optimal AID expression requires TET function, and TET2 and TET3 double-deficient germinal center B cells show defects in class switch recombination. However, TET2/TET3 double-deficiency does not prevent the generation and selection of high-affinity germinal center B cells. Rather, combined TET2 and TET3 loss-of-function in germinal center B cells favors C-to-T and G-to-A transition mutagenesis, a finding that may be of significance for understanding the etiology of B cell lymphomas evolving in conditions of reduced TET function

Canonical NF-κB signaling is uniquely required for the long-term persistence of functional mature B cells

Although canonical NF-κB signaling is crucial to generate a normal mature B-cell compartment, its role in the persistence of resting mature B cells is controversial. To resolve this conflict, we ablated NF-κB essential modulator (NEMO) and I{kappa}B kinase 2 (IKK2), two essential mediators of the canonical pathway, either early on in B-cell development or specifically in mature B cells. Early ablation severely inhibited the generation of all mature B-cell subsets, but follicular B-cell numbers could be largely rescued by ectopic expression of B-cell lymphoma 2 (Bcl2), despite a persisting block at the transitional stage. Marginal zone (MZ) B and B1 cells were not rescued, indicating a possible role of canonical NF-κB signals beyond the control of cell survival in these subsets. When canonical NF-κB signaling was ablated specifically in mature B cells, the differentiation and/or persistence of MZ B cells was still abrogated, but follicular B-cell numbers were only mildly affected. However, the mutant cells exhibited increased turnover as well as functional deficiencies upon activation, suggesting that canonical NF-κB signals contribute to their long-term persistence and functional fitness

Apoptosis Is Essential for Neutrophil Functional Shutdown and Determines Tissue Damage in Experimental Pneumococcal Meningitis

During acute bacterial infections such as meningitis, neutrophils enter the tissue where they combat the infection before they undergo apoptosis and are taken up by macrophages. Neutrophils show pro-inflammatory activity and may contribute to tissue damage. In pneumococcal meningitis, neuronal damage despite adequate chemotherapy is a frequent clinical finding. This damage may be due to excessive neutrophil activity. We here show that transgenic expression of Bcl-2 in haematopoietic cells blocks the resolution of inflammation following antibiotic therapy in a mouse model of pneumococcal meningitis. The persistence of neutrophil brain infiltrates was accompanied by high levels of IL-1β and G-CSF as well as reduced levels of anti-inflammatory TGF-β. Significantly, Bcl-2-transgenic mice developed more severe disease that was dependent on neutrophils, characterized by pronounced vasogenic edema, vasculitis, brain haemorrhages and higher clinical scores. In vitro analysis of neutrophils demonstrated that apoptosis inhibition completely preserves neutrophil effector function and prevents internalization by macrophages. The inhibitor of cyclin-dependent kinases, roscovitine induced apoptosis in neutrophils in vitro and in vivo. In wild type mice treated with antibiotics, roscovitine significantly improved the resolution of the inflammation after pneumococcal infection and accelerated recovery. These results indicate that apoptosis is essential to turn off activated neutrophils and show that inflammatory activity and disease severity in a pyogenic infection can be modulated by targeting the apoptotic pathway in neutrophils

Overexpression of Mcl-1 exacerbates lymphocyte accumulation and autoimmune kidney disease in lpr mice

Cell death by apoptosis has a critical role during embryonic development and in maintaining tissue homeostasis. In mammals, there are two converging apoptosis pathways: the ‘extrinsic’ pathway, which is triggered by engagement of cell surface ‘death receptors’ such as Fas/APO-1; and the ‘intrinsic’ pathway, which is triggered by diverse cellular stresses, and is regulated by prosurvival and pro-apoptotic members of the Bcl-2 family of proteins. Pro-survival Mcl-1, which can block activation of the proapoptotic proteins, Bax and Bak, appears critical for the survival and maintenance of multiple haemopoietic cell types. To investigate the impact on haemopoiesis of simultaneously inhibiting both apoptosis pathways, we introduced the vavP-Mcl-1 transgene, which causes overexpression of Mcl-1 protein in all haemopoietic lineages, into Faslpr/lpr mice, which lack functional Fas and are prone to autoimmunity. The combined mutations had a modest impact on myelopoiesis, primarily an increase in the macrophage/monocyte population in Mcl-1tg/lpr mice compared with lpr or Mcl-1tg mice. The impact on lymphopoiesis was striking, with a marked elevation in all major lymphoid subsets, including the non-conventional double-negative (DN) T cells (TCRβ+ CD4– CD8– B220+ ) characteristic of Faslpr/lpr mice. Of note, the onset of autoimmunity was markedly accelerated in Mcl-1tg/lpr mice compared with lpr mice, and this was preceded by an increase in immunoglobulin (Ig)-producing cells and circulating autoantibodies. This degree of impact was surprising, given the relatively mild phenotype conferred by the vavP-Mcl-1 transgene by itself: a two- to threefold elevation of peripheral B and T cells, no significant increase in the non-conventional DN T-cell population and no autoimmune disease. Comparison of the phenotype with that of other susceptible mice suggests that the development of autoimmune disease in Mcl-1tg/lpr mice may be influenced not only by Ig-producing cells but also other haemopoietic cell types

p14–MP1-MEK1 signaling regulates endosomal traffic and cellular proliferation during tissue homeostasis

The extracellular signal-regulated kinase (ERK) cascade regulates proliferation, differentiation, and survival in multicellular organisms. Scaffold proteins regulate intracellular signaling by providing critical spatial and temporal specificity. The scaffold protein MEK1 (mitogen-activated protein kinase and ERK kinase 1) partner (MP1) is localized to late endosomes by the adaptor protein p14. Using conditional gene disruption of p14 in mice, we now demonstrate that the p14–MP1-MEK1 signaling complex regulates late endosomal traffic and cellular proliferation. This function its essential for early embryogenesis and during tissue homeostasis, as revealed by epidermis-specific deletion of p14. These findings show that endosomal p14–MP1-MEK1 signaling has a specific and essential function in vivo and, therefore, indicate that regulation of late endosomal traffic by extracellular signals is required to maintain tissue homeostasis

Combined loss of the BH3-only proteins Bim and Bmf restores B-cell development and function in TACI-Ig transgenic mice.

Terminal differentiation of B cells depends on two interconnected survival pathways, elicited by the B-cell receptor (BCR) and the BAFF receptor (BAFF-R), respectively. Loss of either signaling pathway arrests B-cell development. Although BCR-dependent survival depends mainly on the activation of the v-AKT murine thymoma viral oncogene homolog 1 (AKT)/PI3-kinase network, BAFF/BAFF-R-mediated survival engages non-canonical NF-κB signaling as well as MAPK/extracellular-signal regulated kinase and AKT/PI3-kinase modules to allow proper B-cell development. Plasma cell survival, however, is independent of BAFF-R and regulated by APRIL that signals NF-κB activation via alternative receptors, that is, transmembrane activator and CAML interactor (TACI) or B-cell maturation (BCMA). All these complex signaling events are believed to secure survival by increased expression of anti-apoptotic B-cell lymphoma 2 (Bcl2) family proteins in developing and mature B cells. Curiously, how lack of BAFF- or APRIL-mediated signaling triggers B-cell apoptosis remains largely unexplored. Here, we show that two pro-apoptotic members of the 'Bcl2 homology domain 3-only' subgroup of the Bcl2 family, Bcl2 interacting mediator of cell death (Bim) and Bcl2 modifying factor (Bmf), mediate apoptosis in the context of TACI-Ig overexpression that effectively neutralizes BAFF as well as APRIL. Surprisingly, although Bcl2 overexpression triggers B-cell hyperplasia exceeding the one observed in Bim(-/-)Bmf(-/-) mice, Bcl2 transgenic B cells remain susceptible to the effects of TACI-Ig expression in vivo, leading to ameliorated pathology in Vav-Bcl2 transgenic mice. Together, our findings shed new light on the molecular machinery restricting B-cell survival during development, normal homeostasis and under pathological conditions. Our data further suggest that Bcl2 antagonists might improve the potency of BAFF/APRIL-depletion strategies in B-cell-driven pathologies

Destruction of tumor vasculature and abated tumor growth upon VEGF blockade is driven by proapoptotic protein Bim in endothelial cells

VEGF deprivation induces Bim expression in tumor endothelial cells, and Bim is needed for anti-VEGF–driven endothelial cell death and tumor shrinkage

Neither loss of Bik alone, nor combined loss of Bik and Noxa, accelerate murine lymphoma development or render lymphoma cells resistant to DNA damaging drugs

The pro-apoptotic BH3-only protein, BIK, is widely expressed and although many critical functions in developmental or stress-induced death have been ascribed to this protein, mice lacking Bik display no overt abnormalities. It has been postulated that Bik can serve as a tumour suppressor, on the basis that its deficiency and loss of apoptotic function have been reported in many human cancers, including lymphoid malignancies. Evasion of apoptosis is a major factor contributing to c-Myc-induced tumour development, but despite this, we found that Bik deficiency did not accelerate Eμ-Myc-induced lymphomagenesis. Co-operation between BIK and NOXA, another BH3-only protein, has been previously described, and was attributed to their complementary binding specificities to distinct subsets of pro-survival BCL-2 family proteins. Nevertheless, combined deficiency of Bik and Noxa did not alter the onset of Eμ-Myc transgene induced lymphoma development. Moreover, although p53-mediated induction of Bik has been reported, neither Eμ-Myc/Bik−/− nor Eμ-Myc/Bik−/−Noxa−/− lymphomas were more resistant than control Eμ-Myc lymphomas to killing by DNA damaging drugs, either in vitro or in vivo. These results suggest that Bik, even in combination with Noxa, is not a potent suppressor of c-Myc-driven tumourigenesis or critical for chemotherapeutic drug-induced killing of Myc-driven tumours