Activation-induced cytidine deaminase (AID) initiates a process generating DNA mutations and breaks in germinal center (GC) B cells that are necessary for somatic hypermutation and class-switch ...recombination. GC B cells can "tolerate" DNA damage while rapidly proliferating because of partial suppression of the DNA damage response by BCL6. In this study, we develop a model to study the response of mouse GC B cells to endogenous DNA damage. We show that the base excision repair protein apurinic/apyrimidinic endonuclease (APE) 2 protects activated B cells from oxidative damage in vitro. APE2-deficient mice have smaller GCs and reduced Ab responses compared with wild-type mice. DNA double-strand breaks are increased in the rapidly dividing GC centroblasts of APE2-deficient mice, which activate a p53-independent cell cycle checkpoint and a p53-dependent apoptotic response. Proliferative and/or oxidative damage and AID-dependent damage are additive stresses that correlate inversely with GC size in wild-type, AID-, and APE2-deficient mice. Excessive double-strand breaks lead to decreased expression of BCL6, which would enable DNA repair pathways but limit GC cell numbers. These results describe a nonredundant role for APE2 in the protection of GC cells from AID-independent damage, and although GC cells uniquely tolerate DNA damage, we find that the DNA damage response can still regulate GC size through pathways that involve p53 and BCL6.
The rate of pathogen clearance is a critical determinant of morbidity and mortality. We sought to characterize the immune response responsible for the remarkably rapid clearance of individual ...episodes of bacteremia caused by the relapsing fever bacterium, Borrelia hermsii. SCID or Rag(-/-) mice were incapable of resolving B. hermsii infection, indicating a critical role for T and/or B cells. TCR(-/-) mice, which lack T cells, and IL-7(-/-) mice, which are deficient in both T cells and follicular B cells, but not in B1 cells and splenic marginal zone (MZ) B cells, efficiently cleared B. hermsii. These findings suggested that B1 cells and/or MZ B cells, two B cell subsets that are known to participate in rapid, T-independent responses, might be involved. The efficient resolution of the episodes of moderate level bacteremia by splenectomized mice suggested that MZ B cells do not play the primary role in clearance of this bacterium. In contrast, xid mice, which are deficient in B1 cells, suffered more severe episodes of bacteremia than wild-type mice. The hypothesis that B1 cells are critical for clearance of B. hermsii was further supported by a selective expansion of the B1b (i.e., IgM(high), IgD(-/low), Mac1(+) CD23(-), and CD5(-)) cell subset in infected xid mice, which coincided with the eventual resolution of infection. Finally, mice selectively incapable of secreting IgM, the dominant isotype produced by B1 cells, were completely unable to clear B. hermsii. Together these results support the model that B1b cells generate the T-independent IgM required for the control and resolution of relapsing fever borreliosis.
BLyS and B cell homeostasis Woodland, Robert T.; Schmidt, Madelyn R.; Thompson, Craig B.
Seminars in Immunology,
10/2006, Letnik:
18, Številka:
5
Journal Article
Recenzirano
Naïve peripheral B cells survive
in vivo because of active stimulation by the TNF superfamily ligand B lymphocyte stimulator (BLyS/BAFF). Although the survival promoting properties of BLyS are well ...known, the signal pathways and molecular effectors that characterize this stimulation are still being elucidated. In this communication, we discuss the signal cascades that effect BLyS dependent survival and the regulation of BLyS induced signaling. We also examine the role of BLyS as a growth factor and propose that BLyS induced metabolic enhancement optimizes the B cell response to BCR and TLR-dependent signaling.
Naive peripheral B cells are maintained in sufficient numbers and diversity to mount effective immune responses against infectious agents. However, the size and repertoire of this B cell pool is ...constantly diminished by normal cell turnover and Ag activation. Homeostatic (Ag-independent) proliferation in response to B cell depletion is one mechanism to compensate for this cell loss. We have used purified CFSE-labeled B cells and an adoptive transfer model system to show that immature and mature B cells divide in a variety of B cell-deficient (scid, xid, IL-7(-/-), and sublethally irradiated) hosts. Homeostatic B cell proliferation is T cell independent, and B cells that have replicated by this mechanism retain the antigenic phenotype of naive B cells. Replication is significantly reduced in B cell-sufficient normal or B cell-reconstituted immunodeficient recipients by the action of competing mature follicular B cells. Using xid mice and transcription factor knockouts, we show that the activation signal(s) that lead to homeostatic B cell proliferation require Bruton's tyrosine kinase; however, c-Rel, a Bruton's tyrosine kinase-induced NF-kappaB/Rel transcription factor critical for Ag and mitogen stimulation, is dispensable, indicating the uniqueness of this activation pathway. Survival and replication signals can also be separated, because the transcription factor p50 (NF-kappaB1), which is required for the survival of peripheral B cells, is not necessary for homeostatic replication. Homeostatic B cell proliferation provides an Ag-independent mechanism for the maintenance and expansion of naive B cells selected into the mature B cell pool.
Homeostatic proliferation of B cells Woodland, Robert T.; Schmidt, Madelyn R.
Seminars in immunology,
06/2005, Letnik:
17, Številka:
3
Journal Article
Recenzirano
Naïve B cells introduced into a lymphopenic host undergo antigen-independent proliferation which is inhibited in a cell dose dependent manner by feedback from mature B cells. Homeostatic ...proliferation is a generalized lymphocyte property with B cells sharing many of the inductive and regulatory characteristics established for naïve and memory CD4
+ and CD8
+ T cells and NK cells. In this communication we discuss the cytokine requirements for B cell HP, extend the murine studies to human cells, and propose the hypothesis that B cell HP may provide an antigen-independent mechanism for maintaining B cell memory.
Abstract
The Provirus Integration site for Moloney murine leukemia virus (Pim) family of oncogenic serine/threonine kinases regulates a variety of metabolic functions in hematopoietic cells and ...lymphocytes. We examined the importance of Pim kinases in B cell activation following antigen stimulation. Humoral immune responses were significantly impaired in Pim 1 and 2 deficient mice (Pim 1-/-2-/-) T cell dependent responses were compromised while T cell independent type 1 and 2 antigen responses were virtually absent. Humoral deficiencies were owing to reduced production of antibody secreting cells due to the diminished induction of BLIMP-1 expression. The central role of Pim in these responses is consistent with our observation that Pims are induced downstream of multiple B cell activation receptors including CD40, BCR, and TLR4. We suggest that Pim 1 and 2 dependent signaling pathways are involved in B cell activation and their loss requires increased signaling through other pathways to reach an activation threshold. This hypothesis is further supported by the initial higher affinity of BCRs on Pim deficient B cells activated in a TD response and their inability to proliferate or undergo isotype switching in culture at the same rate as WT B cells when activator concentrations are decreased. Taken together, these data demonstrate a critical role for Pim kinases in affinity maturation, isotype switching, proliferation and the production of antibody secreting cells by B lymphocytes. .
Human X-linked agammaglobulinemia (XLA) and murine X-linked immune defect (XID) are both immunodeficiencies mediated by mutations in Bruton's tyrosine kinase (Btk), yet the developmental stage(s) ...affected remain controversial. To further refine the placement of the XID defect(s), we used bromodeoxyuridine labeling to determine turnover, production and transition rates of developing B cell subsets in normal, xid and xid mice expressing a human Bcl-2 transgene (xid/bcl-2). We find the xid mutation manifest at two stages of B cell development. The first is early, reducing pre-B cell production by restricting pro-B to pre-B cell transit. Surprisingly, this impairment is offset by increased survival of cells progressing from the pre- to immature B cell pool, suggesting that Btk-independent homeostatic mechanisms act to maintain this compartment. The second point of action is late, substantially reducing mature B cell production. Together, these findings reconcile apparent discrepancies in the developmental stage affected by the murine versus human lesions and suggest previously unappreciated homeostatic processes that act at the pre-B to immature B cell transition. Finally, Btk likely functions differently at these two checkpoints, since ectopic Bcl-2 expression fails to directly complement the early xid lesion, yet reverses the defect impeding final B cell maturation.
The AM14 BCR, derived from an autoimmune MRL/lpr mouse, binds autologous IgG2a
a/j
with low affinity, and as a result AM14 B cells only proliferate in response to IgG2a immune complexes that ...incorporate DNA, RNA, or nucleic acid binding proteins that serve as autoadjuvants. As such, AM14 B cells have served as a useful model for demonstrating the importance of BCR/TLR co-engagement in the activation of autoreactive B cells. We now show that the same receptor recognizes an additional murine encoded antigen, expressed by B6 splenocytes, with sufficient avidity to induce a TLR-independent proliferative response of BALB/c AM14 Vκ8 B cells both in vivo and in vitro. Moreover, detection of this cross-reactive antigen by B6 AM14 Vκ8 B cells promotes an anergic phenotype as reflected by suboptimal responses to BCR-crosslinking and the absence of mature B cells in the bone marrow. The B6 antigen further impacts B cell development as shown by a dramatically expanded MZ compartment and extensive receptor editing in B6 AM14 Vκ8 mice, but not BALB/c AM14 Vκ8 mice. Despite their anergic phenotypes, B6 AM14 Vκ8 B cells can respond robustly to autoantigen/autoadjuvant immune complexes and could therefore participate in both autoimmune responses and host defense.
Replication-defective adenoviruses are effective vehicles for gene transfer, both for the repair of defective genes and for studies of gene function in primary cells. Many cell types, including ...lymphocytes, are refractory to adenovirus infection because they lack the Coxsackie/adenovirus receptor (CAR) needed for virus attachment. To extend the advantages of adenovirus-mediated gene transfer to primary lymphoid populations and other cell types lacking endogenous CAR, we produced a mouse that expresses human (h) CAR as a transgene under control of a murine MHC class I promoter. hCAR protein is expressed on T and B lymphocytes from a variety of organs (spleen, lymph node, bone marrow, thymus, and peritoneum). These lymphocytes are susceptible to adenovirus infection, as demonstrated by reporter green fluorescent protein gene expression, with the fraction of expressing cells as high as 70%. Some lymphocyte subpopulations required stimulation subsequent to adenovirus infection for reporter expression. This activation requirement is a restriction imposed by the promoter used in the adenovirus construct. In subpopulations requiring activation, the elongation factor 1 promoter was far superior to a hCMV promoter for directing green fluorescent protein production. We also find that hCAR mRNA is produced in nonlymphoid tissues from all founder lines, including tissues that do not express endogenous murine CAR, suggesting the opportunity for effecting gene delivery to and testing gene function in a wide variety of primary cell types previously resistant to gene transfer.
B cell susceptibility to Fas-mediated apoptosis is regulated in a receptor-specific fashion. CD40 engagement produces marked sensitivity to Fas killing, whereas surface Ig (sIg) engagement blocks Fas ...signaling for cell death in otherwise sensitive, CD40-stimulated B cell targets, and thus, induces a state of Fas resistance. The signaling mediator, Bruton's tyrosine kinase (Btk), is required for certain sIg-triggered responses, and Btk is reported to directly bind Fas and block Fas-mediated apoptosis. For these reasons, the role of Btk as a mediator of sIg-induced Fas resistance was examined. Dysfunction of Btk through mutation, and absence of Btk through deletion did not interfere with induction of Fas resistance by anti-Ig. This may be due, at least in part, to induction of Btk-dependent Bcl-2 family members by anti-Ig after CD40 ligand treatment. However, the susceptibility to Fas-mediated apoptosis of B cell targets stimulated by CD40 ligand alone was increased in the absence of Btk. These results indicate that Fas resistance produced by sIg triggering does not require Btk, but suggests that in certain situations Btk modulates B cell susceptibility to Fas killing.
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