Resolution and characterization of pro-B and pre-pro-B cell stages in normal mouse bone marrow

LNK/SH2B3 regulates IL-7 receptor signaling in normal and malignant B-progenitors

Philadelphia chromosome-like acute lymphoblastic leukemia (Ph-like ALL) is a high-risk ALL commonly associated with alterations that affect the tyrosine kinase pathway, tumor suppressors, and lymphoid transcription factors. Loss-of-function mutations in the gene-encoding adaptor protein LNK (also known as SH2B3) are found in Ph-like ALLs; however, it is not clear how LNK regulates normal B cell development or promotes leukemogenesis. Here, we have shown that combined loss of Lnk and tumor suppressors Tp53 or Ink4a/Arf in mice triggers a highly aggressive and transplantable precursor B-ALL. Tp53-/-Lnk-/- B-ALLs displayed similar gene expression profiles to human Ph-like B-ALLs, supporting use of this model for preclinical and molecular studies. Preleukemic Tp53-/-Lnk-/- pro-B progenitors were hypersensitive to IL-7, exhibited marked self-renewal in vitro and in vivo, and were able to initiate B-ALL in transplant recipients. Mechanistically, we demonstrated that LNK regulates pro-B progenitor homeostasis by attenuating IL-7-stimuated JAK/STAT5 signaling via a direct interaction with phosphorylated JAK3. Moreover, JAK inhibitors were effective in prolonging survival of mice transplanted with Lnk-/-Tp53-/- leukemia. Additionally, synergistic administration of PI3K/mTOR and JAK inhibitors further abrogated leukemia development. Hence, our results suggest that LNK suppresses IL-7R/JAK/STAT signaling to restrict pro-/pre-B progenitor expansion and leukemia development, providing a pathogenic mechanism and a potential therapeutic approach for B-ALLs with LNK mutations.

Rpl22 Loss Impairs the Development of B Lymphocytes by Activating a p53-Dependent Checkpoint

Although ribosomal proteins facilitate the ribosome’s core function of translation, emerging evidence suggests that some ribosomal proteins are also capable of performing tissue-restricted functions either from within specialized ribosomes or from outside of the ribosome. In particular, we have previously demonstrated that germline ablation of the gene encoding ribosomal protein Rpl22 causes a selective and p53-dependent arrest of ab T cell progenitors at the b-selection checkpoint. We have now identified a crucial role for Rpl22 during early B cell development. Germline ablation of Rpl22 results in a reduction in the absolute number of B-lineage progenitors in the bone marrow beginning at the pro–B cell stage. Although Rpl22-deficient pro–B cells are hyporesponsive to IL-7, a key cytokine required for early B cell development, the arrest of B cell development does not result from disrupted IL-7 signaling. Instead, p53 induction appears to be responsible for the developmental defects, as Rpl22 deficiency causes increased expression of p53 and activation of downstream p53 target genes, and p53 deficiency rescues the defect in B cell development in Rpl22-deficient mice. Interestingly, the requirement for Rpl22 in the B cell lineage appears to be developmentally restricted, because Rpl22-deficient splenic B cells proliferate normally in response to Ag receptor and Toll receptor stimuli and undergo normal class-switch recombination. These results indicate that Rpl22 performs a critical, developmentally restricted role in supporting early B cell development by preventing p53 induction.

B-cell survival and development controlled by the coordination of NF-κB family members RelB and cRel

Targeted deletion of BAFF causes severe deficiency of splenic B cells. BAFF-R is commonly thought to signal to nuclear factor κ-light-chain-enhancer of activated B cells (NF-κB)-inducing kinase dependent noncanonical NF-κB RelB. However, RelB-deficient mice have normal B-cell numbers. Recent studies showed that BAFF also signals to the canonical NF-κB pathway, and we found that both RelB and cRel are persistently activated, suggesting BAFF signaling coordinates both pathways to ensure robust B-cell development. Indeed, we report now that combined loss of these 2 NF-κB family members leads to impaired BAFF-mediated survival and development in vitro. Although single deletion of RelB and cRel was dispensable for normal B-cell development, double knockout mice displayed an early B-cell developmental blockade and decreased mature B cells. Despite disorganized splenic architecture in Relb(-/-)cRel(-/-) mice, generation of mixed-mouse chimeras established the developmental phenotype to be B-cell intrinsic. Together, our results indicate that BAFF signals coordinate both RelB and cRel activities to ensure survival during peripheral B-cell maturation.

γδ T Cells Shape Preimmune Peripheral B Cell Populations

We previously reported that selective ablation of certain γδ T cell subsets, rather than removal of all γδ T cells, strongly affects serum Ab levels in nonimmunized mice. This type of manipulation also changed T cells, including residual γδ T cells, revealing some interdependence of γδ T cell populations. For example, in mice lacking Vγ4(+) and Vγ6(+) γδ T cells (B6.TCR-Vγ4(-/-)/6(-/-)), we observed expanded Vγ1(+) cells, which changed in composition and activation and produced more IL-4 upon stimulation in vitro, increased IL-4 production by αβ T cells as well as spontaneous germinal center formation in the spleen, and elevated serum Ig and autoantibodies. We therefore examined B cell populations in this and other γδ-deficient mouse strains. Whereas immature bone marrow B cells remained largely unchanged, peripheral B cells underwent several changes. Specifically, transitional and mature B cells in the spleen of B6.TCR-Vγ4(-/-)/6(-/-) mice and other peripheral B cell populations were diminished, most of all splenic marginal zone (MZ) B cells. However, relative frequencies and absolute numbers of Ab-producing cells, as well as serum levels of Abs, IL-4, and BAFF, were increased. Cell transfers confirmed that these changes are directly dependent on the altered γδ T cells in this strain and on their enhanced potential of producing IL-4. Further evidence suggests the possibility of direct interactions between γδ T cells and B cells in the splenic MZ. Taken together, these data demonstrate the capability of γδ T cells of modulating size and productivity of preimmune peripheral B cell populations.

PTH Signaling in Osteoprogenitors Is Essential for B-Lymphocyte Differentiation and Mobilization

Cells of the osteoblast lineage provide critical support for B lymphopoiesis in the bone marrow (BM). Parathyroid hormone (PTH) signaling in osteoblastic cells through its receptor (PPR) is an important regulator of hematopoietic stem cells; however, its role in regulation of B lymphopoiesis is not clear. Here we demonstrate that deletion of PPR in osteoprogenitors results in a significant loss of trabecular and cortical bone. PPR signaling in osteoprogenitors, but not in mature osteoblasts or osteocytes, is critical for B-cell precursor differentiation via IL-7 production. Interestingly, despite a severe reduction in B-cell progenitors in BM, mature B-lymphocytes were increased 3.5-fold in the BM of mice lacking PPR in osteoprogenitors. This retention of mature IgD(+) B cells in the BM was associated with increased expression of vascular cell adhesion molecule 1 (VCAM1) by PPR-deficient osteoprogenitors, and treatment with VCAM1 neutralizing antibody increased mobilization of B lymphocytes from mutant BM. Our results demonstrate that PPR signaling in early osteoblasts is necessary for B-cell differentiation via IL-7 secretion and for B-lymphocyte mobilization via VCAM1.

Specific HDAC6 inhibition by ACY-738 reduces SLE pathogenesis in NZB/W mice

We sought to determine if a selective HDAC6 inhibitor (ACY-738) decreases disease in NZB/W mice. From 22 to 38weeks-of-age, mice were injected intraperitoneally with 5 or 20mg/kg of ACY-738, or vehicle control. Body weight and proteinuria were measured every 2weeks, while sera anti-dsDNA, Ig isotypes, and cytokine levels were measured every 4weeks. Kidney disease was determined by evaluation of sera, urine, immune complex deposition, and renal pathology. Flow cytometric analysis assessed thymic, splenic, bone marrow, and peripheral lymphocyte differentiation patterns. Our results showed HDAC6 inhibition decreased SLE disease by inhibiting immune complex-mediated glomerulonephritis, sera anti-dsDNA levels, and inflammatory cytokine production and increasing splenic Treg cells. Inhibition of HDAC6 increased the percentage of cells in the early-stage developmental fractions of both pro- and pre-B cells. These results suggest that specific HDAC6 inhibition may be able to decrease SLE disease by altering aberrant T and B cell differentiation.

Autophagy is dispensable for B-cell development but essential for humoral autoimmune responses

To gain new insight into the role of B-cell autophagy, we generated two novel mouse models deficient for the autophagy-related gene (Atg)5, one from the outset pro-B cell stage (Atg5(f/-) Mb1 cre) and the other in mature B cells only (Atg5(f/-) CD21 cre). We show that autophagy is dispensable for pro- to pre-B cell transition, but necessary at a basal level to maintain normal numbers of peripheral B cells. It appears non-essential for B-cell activation under B-cell receptor stimulation but required for their survival after lipopolysaccharide stimulation that drives plasmablast differentiation and for specific IgM production after immunization. Results obtained using Atg5(f/-) CD21 cre × C57BL/6(lpr/lpr) autoimmune-prone mice show that B-cell autophagy is involved in the maintenance of anti-nuclear antibody secretion, elevated number of long-lived plasma cells, and sustains IgG deposits in the kidneys. Thus, treatments specifically targeting autophagy might be beneficial in systemic autoimmune diseases.

ZFP521 contributes to pre-B-cell lymphomagenesis through modulation of the pre-B-cell receptor signaling pathway

ZFP521 was previously identified as a putative gene involved in induction of B-cell lymphomagenesis. However, the contribution of ZFP521 to lymphomagenesis has not been confirmed. In this study, we sought to elucidate the role of ZFP521 in B-cell lymphomagenesis. To this end, we used a retroviral insertion method to show that ZFP521 was a target of mutagenesis in pre-B-lymphoblastic lymphoma cells. The pre-B-cell receptor (pre-BCR) signaling molecules BLNK, BTK and BANK1 were positively regulated by the ZFP521 gene, leading to enhancement of the pre-BCR signaling pathway. In addition, c-myc and c-jun were upregulated following activation of ZFP521. Stimulation of pre-BCR signaling using anti-Vpreb antibodies caused aberrant upregulation of c-myc and c-jun and of Ccnd3, which encodes cyclin D3, thereby inducing the growth of pre-B cells. Stimulation with Vpreb affected the growth of pre-B cells, and addition of interleukin (IL)-7 receptor exerted competitive effects on pre-B-cell growth. Knockdown of BTK and BANK1, targets of ZFP521, suppressed the effects of Vpreb stimulation on cell growth. Furthermore, in human lymphoblastic lymphoma, analogous to pre-B-cell lymphoma in mice, the expression of ZNF521, the homolog of ZFP521 in humans, was upregulated. In conclusion, our data showed that the ZFP521 gene comprehensively induced pre-B-cell lymphomagenesis by modulating the pre-B-cell receptor signaling pathway.

Perspectives on fetal derived CD5+ B1 B cells

CD5(+) B-cell origins and their predisposition to lymphoma are long-standing issues. Transfer of fetal and adult liver BM Pro-B cells generates B cells with distinct phenotypes: fetal cells generate IgM(high) IgD(low) CD5(+) , whereas adult cells IgM(low) IgD(high) CD5(-) . This suggests a developmental switch in B lymphopoiesis, similar to the switch in erythropoiesis. Comparison of mRNA and miRNA expression in fetal and adult Pro-B cells revealed differential expression of Lin28b mRNA and Let-7 miRNA, providing evidence that this regulatory axis functions in the switch. Recent work has shown that Arid3a is a key transcription factor mediating fetal-type B-cell development. Lin28b-promoted fetal development generates CD5(+) B cells as a consequence of positively selected self-reactivity. CD5(+) B cells play important roles in clearance of apoptotic cells and in protective immune responses, but also pose a risk of progression to leukemia/lymphoma. Differential Lin28b expression in fetal and adult human B-cell precursors showed that human B-cell development may resemble mouse, with self-reactive "innate-like" B cells generated early in life. It remains to be determined whether such human B cells have a higher propensity to leukemic progression. This review describes our recent research with CD5(+) B cells and presents our perspective on their role in disease.

Inhibition of B Lymphopoiesis by Adipocytes and IL-1-Producing Myeloid-Derived Suppressor Cells

B lymphopoiesis declines with age, and this decline correlates with increased adipose tissue in the bone marrow (BM). Also, adipocyte-derived factors are known to inhibit B lymphopoiesis. Using cocultures of mouse BM cells with OP9 stromal cells, we found that adipocyte-conditioned medium induces the generation of CD11b(+)Gr1(+) myeloid cells, which inhibit B cell development in vitro. Adipocyte-conditioned medium-induced CD11b(+)Gr1(+) cells express Arg1 (arginase) and Nos2 (inducible NO synthase) and suppress CD4(+) T cell proliferation, indicating that these cells are myeloid-derived suppressor cells (MDSCs). Blocking arginase and inducible NO synthase did not restore B lymphopoiesis, indicating that inhibition is not mediated by these molecules. Transwell and conditioned-medium experiments showed that MDSCs inhibit B lymphopoiesis via soluble factors, and by cytokine array we identified IL-1 as an important factor. Addition of anti-IL-1 Abs restored B lymphopoiesis in BM cultures containing MDSCs, showing that MDSC inhibition of B lymphopoiesis is mediated by IL-1. By treating hematopoietic precursors with IL-1, we found that multipotent progenitors are targets of IL-1. This study uncovers a novel function for MDSCs to inhibit B lymphopoiesis through IL-1. We suggest that inflammaging contributes to a decline of B lymphopoiesis in aged individuals, and furthermore, that MDSCs and IL-1 provide therapeutic targets for restoration of B lymphopoiesis in aged and obese individuals.

Functional knock down of VCAM1 in mice mediated by endoplasmatic reticulum retained intrabodies

Functional knockdowns mediated by endoplasmatic reticulum-retained antibodies (ER intrabodies) are a promising tool for research because they allow functional interference on the protein level. We demonstrate for the first time that ER intrabodies can induce a knock-down phenotype in mice. Surface VCAM1 was suppressed in bone marrow of heterozygous and homozygous ER intrabody mice (iER-VCAM1 mice). iER-VCAM1 mice did not have a lethal phenotype, in contrast to the constitutive knockout of VCAM1, but adult mice exhibited physiological effects in the form of aberrant distribution of immature B-cells in blood and bone marrow. The capability to regulate knock-down strength may spark a new approach for the functional study of membrane and plasma proteins, which may especially be valuable for generating mouse models that more closely resemble disease states than classic knockouts do.

B Cell-Intrinsic Expression of the HuR RNA-Binding Protein Is Required for the T Cell-Dependent Immune Response In Vivo

The HuR RNA-binding protein posttranscriptionally controls expression of genes involved in cellular survival, proliferation, and differentiation. To determine roles of HuR in B cell development and function, we analyzed mice with B lineage-specific deletion of the HuR gene. These HuRΔ/Δ mice have reduced numbers of immature bone marrow and mature splenic B cells, with only the former rescued by p53 inactivation, indicating that HuR supports B lineage cells through developmental stage-specific mechanisms. Upon in vitro activation, HuRΔ/Δ B cells have a mild proliferation defect and impaired ability to produce mRNAs that encode IgH chains of secreted Abs, but no deficiencies in survival, isotype switching, or expression of germinal center (GC) markers. In contrast, HuRΔ/Δ mice have minimal serum titers of all Ab isotypes, decreased numbers of GC and plasma B cells, and few peritoneal B-1 B cells. Moreover, HuRΔ/Δ mice have severely decreased GCs, T follicular helper cells, and high-affinity Abs after immunization with a T cell-dependent Ag. This failure of HuRΔ/Δ mice to mount a T cell-dependent Ab response contrasts with the ability of HuRΔ/Δ B cells to become GC-like in vitro, indicating that HuR is essential for aspects of B cell activation unique to the in vivo environment. Consistent with this notion, we find in vitro stimulated HuRΔ/Δ B cells exhibit modestly reduced surface expression of costimulatory molecules whose expression is similarly decreased in humans with common variable immunodeficiency. HuRΔ/Δ mice provide a model to identify B cell-intrinsic factors that promote T cell-dependent immune responses in vivo.

Dynamic regulation of CD24 expression and release of CD24-containing microvesicles in immature B cells in response to CD24 engagement

The glycophosphatidylinositol-anchored cell surface receptor CD24 (also called heat-stable antigen) promotes the apoptosis of progenitor and precursor B-lymphocytes. However, the immediate proximal events that occur after engagement of CD24 in B cells are not precisely understood. Using a bioinformatics analysis of mouse (Mus musculus) gene expression data from the Immunological Genome Project, we found that known vesicle trafficking and cellular organization genes have similar expression patterns to CD24 during B-cell development in the bone marrow. We therefore hypothesized that CD24 regulates vesicle trafficking. We first validated that antibody-mediated engagement of CD24 induces apoptosis in the mouse WEHI-231 cell line and mouse primary bone marrow-derived B cells. We next found that CD24 surface protein expression is rapidly and dynamically regulated in both WEHI-231 cells and primary immature B cells in response to engagement of CD24. The change in surface expression was not mediated by classical endocytosis or exocytosis. However, we found that CD24-bearing plasma membrane-derived extracellular microvesicles were released in response to CD24 engagement. Furthermore, in response to CD24 engagement we observed a clear exchange of CD24 between different populations of B cells. Hence, we show that engagement of CD24 in immature B cells results in a dynamic regulation of surface CD24 protein and a redistribution of CD24 within the population.

The Cell-Intrinsic Circadian Clock Is Dispensable for Lymphocyte Differentiation and Function

Circadian rhythms regulate many aspects of physiology, ranging from sleep-wake cycles and metabolic parameters to susceptibility to infection. The molecular clock, with transcription factor BMAL1 at its core, controls both central and cell-intrinsic circadian rhythms. Using a circadian reporter, we observed dynamic regulation of clock activity in lymphocytes. However, its disruption upon conditional Bmal1 ablation did not alter T- or B-cell differentiation or function. Although the magnitude of interleukin 2 (IL-2) production was affected by the time of bacterial infection, it was independent of cell-intrinsic expression of BMAL1. The circadian gating of the IL-2 response was preserved in Bmal1-deficient T cells, despite a slight reduction in cytokine production in a competitive setting. Our results suggest that, contrary to the prevailing view, the adaptive immune response is not affected by the cell-intrinsic clock but is likely influenced by cell-extrinsic circadian cues operating across multiple cell types.

CD38 expression in early B-cell precursors contributes to extracellular signal-regulated kinase-mediated apoptosis

CD38 is a 45,000 molecular weight transmembrane protein that is expressed in immature and mature lymphocytes. However, the expression and function of CD38 during B-cell differentiation in mice is poorly understood. Here, we report that CD38 is expressed from the earliest stages of B-cell development. Pre-pro-B, pro-B, pre-B and immature B cells from murine bone marrow all stained positive for CD38. Interestingly, CD38 expression increases with B-cell maturation. To assess the role of CD38 during B-cell maturation, CD38-deficient mice were analysed. CD38(-/-) mice showed a significant increase in both the frequency of B-lineage cells and the absolute numbers of pre-pro-B cells in bone marrow; however, no other differences were observed at later stages. CD38 cross-linking in Ba/F3 cells promoted apoptosis and marked extracellular signal-regulated kinase (ERK) phosphorylation, and these effects were reduced by treatment with the mitogen-activated protein kinase/ERK kinase inhibitor PD98059, and similar effects were observed in B-cell precursors from bone marrow. These data demonstrate that B-cell precursors in mouse bone marrow express functional CD38 and implicate the early ligation of CD38 in the ERK-associated regulation of the B-lineage differentiation pathway.

Dynamic upregulation of CD24 in pre-adipocytes promotes adipogenesis

The development of mature adipocytes from pre-adipocytes is a highly regulated process. CD24 is a glycophosphatidylinositol-linked cell surface receptor that has been identified as a critical cell surface marker for identifying pre-adipocytes that are able to reconstitute white adipose tissue (WAT) in vivo. Here, we examined the role and regulation of CD24 during adipogenesis in vitro. We found that CD24 mRNA and protein expression is upregulated early during adipogenesis in the 3T3-L1 pre-adipocytes and in murine primary pre-adipocytes isolated from subcutaneous and visceral WAT, followed by downregulation in mature adipocytes. CD24 mRNA expression was found to be dependent on increased transcription due to increased promoter activity in response to activation of a pre-existing transcriptional regulator. Furthermore, either intracellular cAMP or dexamethasone were sufficient to increase expression in pre-adipocytes, while both additively increased CD24 expression. Preventing the increase in CD24 expression, by siRNA-mediated knock-down, resulted in fewer mature lipid-laden adipocytes and decreased expression of mature adipogenic genes. Therefore, conditions experienced during adipogenesis in vitro are sufficient to increase CD24 expression, which is necessary for differentiation. Overall, we conclude that the dynamic upregulation of CD24 actively promotes adipogenesis in vitro.

Nfix expression critically modulates early B lymphopoiesis and myelopoiesis

The commitment of stem and progenitor cells toward specific hematopoietic lineages is tightly controlled by a number of transcription factors that regulate differentiation programs via the expression of lineage restricting genes. Nuclear factor one (NFI) transcription factors are important in regulating hematopoiesis and here we report an important physiological role of NFIX in B- and myeloid lineage commitment and differentiation. We demonstrate that NFIX acts as a regulator of lineage specification in the haematopoietic system and the expression of Nfix was transcriptionally downregulated as B cells commit and differentiate, whilst maintained in myeloid progenitor cells. Ectopic Nfix expression in vivo blocked early B cell development stage, coincident with the stage of its downregulation. Furthermore, loss of Nfix resulted in the perturbation of myeloid and lymphoid cell differentiation, and a skewing of gene expression involved in lineage fate determination. Nfix was able to promote myeloid differentiation of total bone marrow cells under B cell specific culture conditions but not when expressed in the hematopoietic stem cell (HSPC), consistent with its role in HSPC survival. The lineage choice determined by Nfix correlated with transcriptional changes in a number of genes, such as E2A, C/EBP, and Id genes. These data highlight a novel and critical role for NFIX transcription factor in hematopoiesis and in lineage specification.

Identification of a negative regulatory role for spi-C in the murine B cell lineage

Spi-C is an E26 transformation-specific family transcription factor that is highly related to PU.1 and Spi-B. Spi-C is expressed in developing B cells, but its function in B cell development and function is not well characterized. To determine whether Spi-C functions as a negative regulator of Spi-B (encoded by Spib), mice were generated that were germline knockout for Spib and heterozygous for Spic (Spib(-/-)Spic(+/-)). Interestingly, loss of one Spic allele substantially rescued B cell frequencies and absolute numbers in Spib(-/-) mouse spleens. Spib(-/-)Spic(+/-) B cells had restored proliferation compared with Spib(-/-) B cells in response to anti-IgM or LPS stimulation. Investigation of a potential mechanism for the Spib(-/-)Spic(+/-) phenotype revealed that steady-state levels of Nfkb1, encoding p50, were elevated in Spib(-/-)Spic(+/-) B cells compared with Spib(-/-) B cells. Spi-B was shown to directly activate the Nfkb1 gene, whereas Spi-C was shown to repress this gene. These results indicate a novel role for Spi-C as a negative regulator of B cell development and function.

Identification of FOXM1 as a therapeutic target in B-cell lineage acute lymphoblastic leukaemia

Despite recent advances in the cure rate of acute lymphoblastic leukaemia (ALL), the prognosis for patients with relapsed ALL remains poor. Here we identify FOXM1 as a candidate responsible for an aggressive clinical course. We show that FOXM1 levels peak at the pre-B-cell receptor checkpoint but are dispensable for normal B-cell development. Compared with normal B-cell populations, FOXM1 levels are 2- to 60-fold higher in ALL cells and are predictive of poor outcome in ALL patients. FOXM1 is negatively regulated by FOXO3A, supports cell survival, drug resistance, colony formation and proliferation in vitro, and promotes leukemogenesis in vivo. Two complementary approaches of pharmacological FOXM1 inhibition-(i) FOXM1 transcriptional inactivation using the thiazole antibiotic thiostrepton and (ii) an FOXM1 inhibiting ARF-derived peptide-recapitulate the findings of genetic FOXM1 deletion. Taken together, our data identify FOXM1 as a novel therapeutic target, and demonstrate feasibility of FOXM1 inhibition in ALL.

Leishmania infantum amastigotes trigger a subpopulation of human B cells with an immunoregulatory phenotype

Visceral leishmaniasis is caused by the protozoan parasites Leishmania infantum and Leishmania donovani. This infection is characterized by an uncontrolled parasitization of internal organs which, when left untreated, leads to death. Disease progression is linked with the type of immune response generated and a strong correlation was found between disease progression and serum levels of the immunosuppressive cytokine IL-10. Other studies have suggested a role for B cells in the pathology of this parasitic infection and the recent identification of a B-cell population in humans with regulatory functions, which secretes large amounts of IL-10 following activation, have sparked our interest in the context of visceral leishmaniasis. We report here that incubation of human B cells with Leishmania infantum amastigotes resulted in upregulation of multiple cell surface activation markers and a dose-dependent secretion of IL-10. Conditioned media from B cells incubated with Leishmania infantum amastigotes were shown to strongly inhibit CD4⁺ T-cell activation, proliferation and function (i.e. as monitored by TNF and IFNγ secretion). Blockade of IL-10 activity using a soluble IL-10 receptor restored only partially TNF and IFNγ production to control levels. The parasite-mediated IL-10 secretion was shown to rely on the activity of Syk, phosphatidylinositol-3 kinase and p38, as well as to require intracellular calcium mobilization. Cell sorting experiments allowed us to identify the IL-10-secreting B-cell subset (i.e. CD19⁺CD24⁺CD27^-). In summary, exposure of human B cells to Leishmania infantum amastigotes triggers B cells with regulatory activities mediated in part by IL-10, which could favor parasite dissemination in the organism.

Leishmaniasis is an infection caused by protozoan parasites of the genus Leishmania and is a significant neglected tropical disease, with 350 million people in 98 countries at risk of developing one of the forms of the disease. Visceral leishmaniasis is characterized by an uncontrolled parasitization of internal organs, which leads to death when left untreated. Disease progression is linked with the type of immune response generated and a strong correlation was found between disease progression and serum levels of the immunosuppressive cytokine IL-10. We demonstrate that a contact between human B cells with Leishmania infantum amastigotes resulted in upregulation of multiple cell surface activation markers and a dose-dependent secretion of IL-10. Conditioned media from B cells incubated with Leishmania infantum amastigotes were shown to strongly inhibit CD4⁺ T-cell activation, proliferation and function (i.e. TNF and IFNγ production). Blockade of IL-10 activity using a soluble IL-10 receptor restored to some degree TNF and IFNγ secretion. Cell sorting experiments allowed us to identify a major IL-10-secreting B cell subset characterized as CD24⁺ and CD27^-. Exposure of human B cells to Leishmania infantum amastigotes thus triggers B cells with regulatory activities mediated in part by IL-10, which could promote parasite dissemination in the organism.

Cdc42 is a key regulator of B cell differentiation and is required for antiviral humoral immunity

The small Rho GTPase Cdc42, known to interact with Wiskott-Aldrich syndrome (WAS) protein, is an important regulator of actin remodeling. Here, we show that genetic ablation of Cdc42 exclusively in the B cell lineage is sufficient to render mice unable to mount antibody responses. Indeed Cdc42-deficient mice are incapable of forming germinal centers or generating plasma B cells upon either viral infection or immunization. Such severe immune deficiency is caused by multiple and profound B cell abnormalities, including early blocks during B cell development; impaired antigen-driven BCR signaling and actin remodeling; defective antigen presentation and in vivo interaction with T cells; and a severe B cell-intrinsic block in plasma cell differentiation. Thus, our study presents a new perspective on Cdc42 as key regulator of B cell physiology.

PU.1 opposes IL-7-dependent proliferation of developing B cells with involvement of the direct target gene bruton tyrosine kinase

Deletion of genes encoding the E26 transformation-specific transcription factors PU.1 and Spi-B in B cells (CD19-CreΔPB mice) leads to impaired B cell development, followed by B cell acute lymphoblastic leukemia at 100% incidence and with a median survival of 21 wk. However, little is known about the target genes that explain leukemogenesis in these mice. In this study we found that immature B cells were altered in frequency in the bone marrow of preleukemic CD19-CreΔPB mice. Enriched pro-B cells from CD19-CreΔPB mice induced disease upon transplantation, suggesting that these were leukemia-initiating cells. Bone marrow cells from preleukemic CD19-CreΔPB mice had increased responsiveness to IL-7 and could proliferate indefinitely in response to this cytokine. Bruton tyrosine kinase (BTK), a negative regulator of IL-7 signaling, was reduced in preleukemic and leukemic CD19-CreΔPB cells compared with controls. Induction of PU.1 expression in cultured CD19-CreΔPB pro-B cell lines induced Btk expression, followed by reduced STAT5 phosphorylation and early apoptosis. PU.1 and Spi-B regulated Btk directly as shown by chromatin immunoprecipitation analysis. Ectopic expression of BTK was sufficient to induce apoptosis in cultured pro-B cells. In summary, these results suggest that PU.1 and Spi-B activate Btk to oppose IL-7 responsiveness in developing B cells.

A crucial role for the homeodomain transcription factor Hhex in lymphopoiesis

The hematopoietically expressed homeobox gene, Hhex, is a transcription factor that is important for development of definitive hematopoietic stem cells (HSCs) and B cells, and that causes T-cell leukemia when overexpressed. Here, we have used an Hhex inducible knockout mouse model to study the role of Hhex in adult hematopoiesis. We found that loss of Hhex was tolerated in HSCs and myeloid lineages, but resulted in a progressive loss of B lymphocytes in the circulation. This was accompanied by a complete loss of B-cell progenitors in the bone marrow and of transitional B-cell subsets in the spleen. In addition, transplantation and in vitro culture experiments demonstrated an almost complete failure of Hhex-null HSCs to contribute to lymphoid lineages beyond the common lymphoid precursor stage, including T cells, B cells, NK cells, and dendritic cells. Gene expression analysis of Hhex-deleted progenitors demonstrated deregulated expression of a number of cell cycle regulators. Overexpression of one of these, cyclin D1, could rescue the B-cell developmental potential of Hhex-null lymphoid precursors. Thus, Hhex is a key regulator of early lymphoid development, functioning, at least in part, via regulation of the cell cycle.

Miz-1 regulates translation of Trp53 via ribosomal protein L22 in cells undergoing V(D)J recombination

To be effective, the adaptive immune response requires a large repertoire of antigen receptors, which are generated through V(D)J recombination in lymphoid precursors. These precursors must be protected from DNA damage-induced cell death, however, because V(D)J recombination generates double-strand breaks and may activate p53. Here we show that the BTB/POZ domain protein Miz-1 restricts p53-dependent induction of apoptosis in both pro-B and DN3a pre-T cells that actively rearrange antigen receptor genes. Miz-1 exerts this function by directly activating the gene for ribosomal protein L22 (Rpl22), which binds to p53 mRNA and negatively regulates its translation. This mechanism limits p53 expression levels and thus contains its apoptosis-inducing functions in lymphocytes, precisely at differentiation stages in which V(D)J recombination occurs.

B cell-intrinsic TLR7 signaling is essential for the development of spontaneous germinal centers

Spontaneous germinal center (Spt-GC) B cells and follicular helper T cells generate high-affinity autoantibodies that are involved in the development of systemic lupus erythematosus. TLRs play a pivotal role in systemic lupus erythematosus pathogenesis. Although previous studies focused on the B cell-intrinsic role of TLR-MyD88 signaling on immune activation, autoantibody repertoire, and systemic inflammation, the mechanisms by which TLRs control the formation of Spt-GCs remain unclear. Using nonautoimmune C57BL/6 (B6) mice deficient in MyD88, TLR2, TLR3, TLR4, TLR7, or TLR9, we identified B cell-intrinsic TLR7 signaling as a prerequisite to Spt-GC formation without the confounding effects of autoimmune susceptibility genes and the overexpression of TLRs. TLR7 deficiency also rendered autoimmune B6.Sle1b mice unable to form Spt-GCs, leading to markedly decreased autoantibodies. Conversely, B6.yaa and B6.Sle1b.yaa mice expressing an extra copy of TLR7 and B6.Sle1b mice treated with a TLR7 agonist had increased Spt-GCs and follicular helper T cells. Further, TLR7/MyD88 deficiency led to compromised B cell proliferation and survival after B cell stimulation both in vitro and in vivo. In contrast, TLR9 inhibited Spt-GC development. Our findings demonstrate an absolute requirement for TLR7 and a negative regulatory function for TLR9 in Spt-GC formation under nonautoimmune and autoimmune conditions. Our data suggest that, under nonautoimmune conditions, Spt-GCs initiated by TLR7 produce protective Abs. However, in the presence of autoimmune susceptibility genes, TLR7-dependent Spt-GCs produce pathogenic autoantibodies. Thus, a single copy of TLR7 in B cells is the minimal requirement for breaking the GC-tolerance checkpoint.

The BiP cochaperone ERdj4 is required for B cell development and function

ERdj4 is a BiP cochaperone regulated by the unfolded protein response to facilitate degradation of unfolded and/or misfolded proteins in the endoplasmic reticulum. As the unfolded protein response plays a critical role in B cell maturation and antibody production, ERdj4 gene trap mice were generated to determine if this chaperone was required for B cell homeostasis. Homozygosity for the trapped allele resulted in hypomorphic expression of ERdj4 in bone marrow cells and abnormal development of hematopoietic lineages in the bone marrow. The number of myeloid cells was increased, while the number of erythroid and B lymphoid cells was reduced in ERdj4 gene trap mice compared to controls. An intrinsic B cell defect was identified that decreased survival of B cell precursors including large and small pre-B, and immature B cells. Consistent with impaired B lymphopoiesis, the number of mature follicular B cells was reduced in both the bone marrow and spleen of ERdj4 gene trap mice. Paradoxically, unchallenged ERdj4 gene trap mice showed non-specific hypergammaglobulinemia and gene trap B cells exhibited increased proliferation, survival and isotype switching in response to LPS stimulation. Although ERdj4 gene trap mice responded normally to T cell-independent antigen, they failed to mount a specific antibody response to T cell-dependent antigen in vivo. Collectively, these findings demonstrate that the chaperone activity of ERdj4 is required for survival of B cell progenitors and normal antibody production.

BP8, a novel peptide from avian immune system, modulates B cell developments

The bursa of Fabricius (BF) is the key humoral immune organ unique to birds, and is critical for early B-lymphocyte proliferation and differentiation. However, the molecular basis and mechanisms through which the BF regulates B cell development are not fully understood. In this study, we isolated and identified a new bursal peptide (BP8, AGHTKKAP) by RP-HPLC and MALDI-TOF-MS. BP8 promoted colony-forming pre-B formation, bound B cell precursor, regulated B cell development in vitro as well as in vivo, upstream of the EBF-E2A-Pax5 regulatory complex and increased immunoglobulin secretion. These data revealed a bursal-derived multifunctional factor BP8 as a novel biomaterial which is essential for the development of the immune system. This study elucidates further the mechanisms involved in humoral immune system and has implications in treating human diseases.

Deoxycytidine kinase augments ATM-Mediated DNA repair and contributes to radiation resistance

Efficient and adequate generation of deoxyribonucleotides is critical to successful DNA repair. We show that ataxia telangiectasia mutated (ATM) integrates the DNA damage response with DNA metabolism by regulating the salvage of deoxyribonucleosides. Specifically, ATM phosphorylates and activates deoxycytidine kinase (dCK) at serine 74 in response to ionizing radiation (IR). Activation of dCK shifts its substrate specificity toward deoxycytidine, increases intracellular dCTP pools post IR, and enhances the rate of DNA repair. Mutation of a single serine 74 residue has profound effects on murine T and B lymphocyte development, suggesting that post-translational regulation of dCK may be important in maintaining genomic stability during hematopoiesis. Using [(18)F]-FAC, a dCK-specific positron emission tomography (PET) probe, we visualized and quantified dCK activation in tumor xenografts after IR, indicating that dCK activation could serve as a biomarker for ATM function and DNA damage response in vivo. In addition, dCK-deficient leukemia cell lines and murine embryonic fibroblasts exhibited increased sensitivity to IR, indicating that pharmacologic inhibition of dCK may be an effective radiosensitization strategy.

Pax5 loss imposes a reversible differentiation block in B-progenitor acute lymphoblastic leukemia

Loss-of-function mutations in hematopoietic transcription factors occur in most cases of B-progenitor acute lymphoblastic leukemia (B-ALL). Here, Liu et al. used transgenic RNAi to reversibly suppress endogenous Pax5 expression in the hematopoietic compartment of mice. Restoring Pax5 expression in established B-ALL triggers immunophenotypic maturation and durable disease remission by engaging a transcriptional program reminiscent of normal B-cell differentiation. Similar findings in human B-ALL cell lines establish that Pax5 hypomorphism promotes B-ALL self-renewal by impairing a differentiation program that can be re-engaged despite the presence of additional oncogenic lesions.

Loss-of-function mutations in hematopoietic transcription factors including PAX5 occur in most cases of B-progenitor acute lymphoblastic leukemia (B-ALL), a disease characterized by the accumulation of undifferentiated lymphoblasts. Although PAX5 mutation is a critical driver of B-ALL development in mice and humans, it remains unclear how its loss contributes to leukemogenesis and whether ongoing PAX5 deficiency is required for B-ALL maintenance. Here we used transgenic RNAi to reversibly suppress endogenous Pax5 expression in the hematopoietic compartment of mice, which cooperates with activated signal transducer and activator of transcription 5 (STAT5) to induce B-ALL. In this model, restoring endogenous Pax5 expression in established B-ALL triggers immunophenotypic maturation and durable disease remission by engaging a transcriptional program reminiscent of normal B-cell differentiation. Notably, even brief Pax5 restoration in B-ALL cells causes rapid cell cycle exit and disables their leukemia-initiating capacity. These and similar findings in human B-ALL cell lines establish that Pax5 hypomorphism promotes B-ALL self-renewal by impairing a differentiation program that can be re-engaged despite the presence of additional oncogenic lesions. Our results establish a causal relationship between the hallmark genetic and phenotypic features of B-ALL and suggest that engaging the latent differentiation potential of B-ALL cells may provide new therapeutic entry points.

A reporter mouse reveals lineage-specific and heterogeneous expression of IRF8 during lymphoid and myeloid cell differentiation

The IFN regulatory factor family member 8 (IRF8) regulates differentiation of lymphoid and myeloid lineage cells by promoting or suppressing lineage-specific genes. How IRF8 promotes hematopoietic progenitors to commit to one lineage while preventing the development of alternative lineages is not known. In this study, we report an IRF8-EGFP fusion protein reporter mouse that revealed previously unrecognized patterns of IRF8 expression. Differentiation of hematopoietic stem cells into oligopotent progenitors is associated with progressive increases in IRF8-EGFP expression. However, significant induction of IRF8-EGFP is found in granulocyte-myeloid progenitors and the common lymphoid progenitors but not the megakaryocytic-erythroid progenitors. Surprisingly, IRF8-EGFP identifies three subsets of the seemingly homogeneous granulocyte-myeloid progenitors with an intermediate level of expression of EGFP defining bipotent progenitors that differentiation into either EGFP(hi) monocytic progenitors or EGFP(lo) granulocytic progenitors. Also surprisingly, IRF8-EGFP revealed a highly heterogeneous pre-pro-B population with a fluorescence intensity ranging from background to 4 orders above background. Interestingly, IRF8-EGFP readily distinguishes true B cell committed (EGFP(int)) from those that are noncommitted. Moreover, dendritic cell progenitors expressed extremely high levels of IRF8-EGFP. Taken together, the IRF8-EGFP reporter revealed previously unrecognized subsets with distinct developmental potentials in phenotypically well-defined oligopotent progenitors, providing new insights into the dynamic heterogeneity of developing hematopoietic progenitors.

Leukocyte β7 integrin targeted by Krüppel-like factors

Constitutive expression of Krüppel-like factor 3 (KLF3, BKLF) increases marginal zone (MZ) B cell numbers, a phenotype shared with mice lacking KLF2. Ablation of KLF3, known to interact with serum response factor (SRF), or SRF itself, results in fewer MZ B cells. It is unknown how these functional equivalences result. In this study, it is shown that KLF3 acts as transcriptional repressor for the leukocyte-specific integrin β7 (Itgb7, Ly69) by binding to the β7 promoter, as revealed by chromatin immunoprecipitation. KLF2 overexpression antagonizes this repression and also binds the β7 promoter, indicating that these factors may compete for target sequence(s). Whereas β7 is identified as direct KLF target, its repression by KLF3 is not connected to the MZ B cell increase because β7-deficient mice have a normal complement of these and the KLF3-driven increase still occurs when β7 is deleted. Despite this, KLF3 overexpression abolishes lymphocyte homing to Peyer's patches, much like β7 deficiency does. Furthermore, KLF3 expression alone overcomes the MZ B cell deficiency when SRF is absent. SRF is also dispensable for the KLF3-mediated repression of β7. Thus, despite the shared phenotype of KLF3 and SRF-deficient mice, cooperation of these factors appears neither relevant for the formation of MZ B cells nor for the regulation of β7. Finally, a potent negative regulatory feedback loop limiting KLF3 expression is shown in this study, mediated by KLF3 directly repressing its own gene promoter. In summary, KLFs use regulatory circuits to steer lymphocyte maturation and homing and directly control leukocyte integrin expression.

Effect of sonication on B cell development and immunomodulatory functions of Bursa of Fabricius

A study was initiated with the objective of evaluating the effects of sonication treatment on important quality parameters of extract of Bursa of Fabricius. Sonication of extract was done (frequency 20 kHz and various amplitude levels) at 0 °C for 10 min, 30 min, 50 min, respectively. As results, the yield of bursa peptides significantly increased (p<0.05). Then we found sonicated bursa extract promoted the content of bursin and the CFU pre-B formation, exerted immunomodulatory function on antigen-specific immune responses in C57/BL6 mice immunized with inactivated Japanese encephalitis b virus (JEV) vaccine, including enhancing JEV-specific antibody and cytokine production, T-cell immunophenotyping and lymphocyte proliferation. Findings of the present study suggested the sonication treatment of Bursa of Fabricius could improve the yield as well as the quality of bursa peptides, indicating that sonication is effective in processing of bursa extract and could be a potential process for future immuno-pharmacological use.

PDK1 regulates B cell differentiation and homeostasis

Successful B cell differentiation and prevention of cell transformation depends on balanced and fine-tuned activation of cellular signaling pathways. The phosphatidyl inositol-3 kinase (PI3K) signaling pathway has emerged as a major regulator of B lymphocyte homeostasis and function. Phosphoinositide-dependent protein kinase-1 (PDK1) is the pivotal node in the PI3K pathway, regulating the stability and activity of downstream AGC kinases (including Akt, RSK, S6K, SGK, and PKC). Although the importance of PI3K activity in B cell differentiation is well documented, the role of PDK1 and other downstream effectors is underexplored. Here we used inducible and stage-specific gene targeting approaches to elucidate the role of PDK1 in early and peripheral B cell differentiation. PDK1 ablation enhanced cell cycle entry and apoptosis of IL-7-dependent pro-B cells, blocking Ig synthesis and B cell maturation. PDK1 also was essential for the survival and activation of peripheral B cells via regulation of PKC and Akt-dependent downstream effectors, such as GSK3α/β and Foxo1. We found that PDK1 deletion strongly impaired B cell receptor (BCR) signaling, but IL-4 costimulation was sufficient to restore BCR-induced proliferation. IL-4 also normalized PKCβ activation and hexokinase II expression in BCR-stimulated cells, suggesting that this signaling pathway can act independent of PDK1 to support B cell growth. In summary, our results demonstrate that PDK1 is indispensable for B cell survival, proliferation, and growth regulation.

The role of the Syk/Shp-1 kinase-phosphatase equilibrium in B cell development and signaling

Signal transduction from the BCR is regulated by the equilibrium between kinases (e.g., spleen tyrosine kinase [Syk]) and phosphatases (e.g., Shp-1). Previous studies showed that Syk-deficient B cells have a developmental block at the pro/pre-B cell stage, whereas a B cell-specific Shp-1 deficiency promoted B-1a cell development and led to autoimmunity. We generated B cell-specific Shp-1 and Syk double-knockout (DKO) mice and compared them to the single-knockout mice deficient for either Syk or Shp-1. Unlike Syk-deficient mice, the DKO mice can generate mature B cells, albeit at >20-fold reduced B cell numbers. The DKO B-2 cells are all Syk-negative, whereas the peritoneal B1 cells of the DKO mice still express Syk, indicating that they require this kinase for their proper development. The DKO B-2 cells cannot be stimulated via the BCR, whereas they are efficiently activated via TLR or CD40. We also found that in DKO pre-B cells, the kinase Zap70 is associated with the pre-BCR, suggesting that Zap70 is important to promote B cell maturation in the absence of Syk and SHP-1. Together, our data show that a properly balanced kinase/phosphatase equilibrium is crucial for normal B cell development and function.

Induction of mixed chimerism depletes pre-existing and de novo-developed autoreactive B cells in autoimmune NOD mice

Destruction of pancreatic islet β-cells in type 1 diabetes (T1D) is mainly mediated by autoimmune T and B lymphocytes. We reported that induction of major histocompatibility complex (MHC)-mismatched mixed chimerism reversed autoimmunity and reestablished thymic negative selection of autoreactive T cells in NOD mice, but it is still unclear how mixed chimerism tolerizes autoreactive B cells. The current studies were designed to reveal the mechanisms on how mixed chimerism tolerizes autoreactive B cells in T1D. Accordingly, mixed chimerism was induced in NOD mice through radiation-free nonmyeloablative anti-CD3/CD8 conditioning and infusion of donor CD4(+) T cell-depleted spleen and whole bone marrow (BM) cells or through myeloablative total body irradiation conditioning and reconstitution with T cell-depleted BM cells from donor and host. Kinetic analysis of percentage and yield of preplasma and plasma B cells, newly developed B-cell subsets, and their apoptosis was performed 30-60 days after transplantation. Induction of MHC-mismatched mixed chimerism results in depleting host-type pre-existing preplasma and plasma B cells as well as augmenting apoptosis of immature transitional T1 B cells, including insulin-specific B cells in a donor B cell-dependent manner. Therefore, induction of MHC-mismatched mixed chimerism depletes pre-existing and de novo-developed autoreactive B cells.

Integrated genetic approaches identify the molecular mechanisms of Sox4 in early B-cell development: intricate roles for RAG1/2 and CK1ε

Commitment of hematopoietic stem cells to B lineage precursors and subsequent development of B lineage precursors into mature B cells is stringently controlled by stage-specific transcription factors. In this study, we used integrated genetic approaches and systematically determined the role of Sry-related high mobility group box (Sox) 4 and the underlying molecular mechanisms in early B-cell development. We found that Sox4 coordinates multilevel controls in the differentiation of early stage B cells. At the molecular level, Sox4 orchestrates a unique gene regulatory program, and its function was predominantly mediated through a conventional Sox4-binding motif as well as an unconventional GA-binding protein α chain binding motif. Our integrated gene network and functional analysis indicated that Sox4 functions as a bimodular transcription factor and ensures B lineage precursor differentiation through 2 distinct mechanisms. It positively induces gene rearrangements at immunoglobulin heavy chain gene loci by transcriptionally activating the Rag1 and Rag2 genes and negatively regulates Wnt signaling, which is critical for self-renewal, by inducing the expression of casein kinase 1 ε. Our findings illustrate that Sox4 mediates critical fine-tuning of the 2 opposing forces in early B-cell development and also set forth a model for characterization of critical genes whose deficiency, like Sox4 deficiency, is detrimental to this process.

The type 1 diabetes resistance locus B10 Idd9.3 mediates impaired B-cell lymphopoiesis and implicates microRNA-34a in diabetes protection

NOD.B10 Idd9.3 mice are congenic for the insulin-dependent diabetes (Idd) Idd9.3 locus, which confers significant type 1 diabetes (T1D) protection and encodes 19 genes, including microRNA (miR)-34a, from T1D-resistant C57BL/10 mice. B cells have been shown to play a critical role in the priming of autoantigen-specific CD4(+) T cells in T1D pathogenesis in non-obese diabetic (NOD) mice. We show that early B-cell development is impaired in NOD.B10 Idd9.3 mice, resulting in the profound reduction of transitional and mature splenic B cells as compared with NOD mice. Molecular analysis revealed that miR-34a expression was significantly higher in B-cell progenitors and marginal zone B cells from NOD.B10 Idd9.3 mice than in NOD mice. Furthermore, miR-34a expression in these cell populations inversely correlated with levels of Foxp1, an essential regulator of B-cell lymphopoiesis, which is directly repressed by miR-34a. In addition, we show that islet-specific CD4(+) T cells proliferated inefficiently when primed by NOD.B10 Idd9.3 B cells in vitro or in response to endogenous autoantigen in NOD.B10 Idd9.3 mice. Thus, Idd9.3-encoded miR-34a is a likely candidate in negatively regulating B-cell lymphopoiesis, which may contribute to inefficient expansion of islet-specific CD4(+) T cells and to T1D protection in NOD.B10 Idd9.3 mice.

Triplication of a 21q22 region contributes to B cell transformation through HMGN1 overexpression and loss of histone H3 Lys27 trimethylation

Down syndrome confers a 20-fold increased risk of B cell acute lymphoblastic leukemia (B-ALL)¹ and polysomy 21 is the most frequent somatic aneuploidy amongst all B-ALLs². Yet, the mechanistic links between chr.21 triplication and B-ALL remain undefined. Here we show that germline triplication of only 31 genes orthologous to human chr.21q22 confers murine progenitor B cell self-renewal in vitro, maturation defects in vivo, and B-ALL with either BCR-ABL or CRLF2 with activated JAK2. Chr.21q22 triplication suppresses H3K27me3 in progenitor B cells and B-ALLs, and “bivalent” genes with both H3K27me3 and H3K4me3 at their promoters in wild-type progenitor B cells are preferentially overexpressed in triplicated cells. Strikingly, human B-ALLs with polysomy 21 are distinguished by their overexpression of genes marked with H3K27me3 in multiple cell types. Finally, overexpression of HMGN1, a nucleosome remodeling protein encoded on chr.21q22^3–5, suppresses H3K27me3 and promotes both B cell proliferation in vitro and B-ALL in vivo.

Connective tissue growth factor is expressed in bone marrow stromal cells and promotes interleukin-7-dependent B lymphopoiesis

Hematopoiesis occurs in a complex bone marrow microenvironment in which bone marrow stromal cells provide critical support to the process through direct cell contact and indirectly through the secretion of cytokines and growth factors. We report that connective tissue growth factor (Ctgf, also known as Ccn2) is highly expressed in murine bone marrow stromal cells. In contrast, connective tissue growth factor is barely detectable in unfractionated adult bone marrow cells. While connective tissue growth factor has been implicated in hematopoietic malignancies, and is known to play critical roles in skeletogenesis and regulation of bone marrow stromal cells, its role in hematopoiesis has not been described. Here we demonstrate that the absence of connective tissue growth factor in mice results in impaired hematopoiesis. Using a chimeric fetal liver transplantation model, we show that absence of connective tissue growth factor has an impact on B-cell development, in particular from pro-B to more mature stages, which is linked to a requirement for connective tissue growth factor in bone marrow stromal cells. Using in vitro culture systems, we demonstrate that connective tissue growth factor potentiates B-cell proliferation and promotes pro-B to pre-B differentiation in the presence of interleukin-7. This study provides a better understanding of the functions of connective tissue growth factor within the bone marrow, showing the dual regulatory role of the growth factor in skeletogenesis and in stage-specific B lymphopoiesis.

Balancing Proliferation with Igκ Recombination during B-lymphopoiesis

The essential events of B-cell development are the stochastic and sequential rearrangement of immunoglobulin heavy (Igμ) and then light chain (Igκ followed by Igλ) loci. The counterpoint to recombination is proliferation, which both maintains populations of pro-B cells undergoing Igμ recombination and expands the pool of pre-B cells expressing the Igμ protein available for subsequent Igκ recombination. Proliferation and recombination must be segregated into distinct and mutually exclusive developmental stages. Failure to do so risks aberrant gene translocation and leukemic transformation. Recent studies have demonstrated that proliferation and recombination are each affected by different and antagonistic receptors. The IL-7 receptor drives proliferation while the pre-B-cell antigen receptor, which contains Igμ and surrogate light chain, enhances Igκ accessibility and recombination. Remarkably, the principal downstream proliferative effectors of the IL-7R, STAT5 and cyclin D3, directly repress Igκ accessibility through very divergent yet complementary mechanisms. Conversely, the pre-B-cell receptor represses cyclin D3 leading to cell cycle exit and enhanced Igκ accessibility. These studies reveal how cell fate decisions can be directed and reinforced at each developmental transition by single receptors. Furthermore, they identify novel mechanisms of Igκ repression that have implications for gene regulation in general.

Haematopoietic stem cells require a highly regulated protein synthesis rate

Many aspects of cellular physiology remain unstudied in somatic stem cells. For example, there are almost no data on protein synthesis in any somatic stem cell. We found that the amount of protein synthesized per hour in haematopoietic stem cells (HSCs) in vivo was lower than in most other haematopoietic cells, even if we controlled for differences in cell cycle status or forced HSCs to undergo self-renewing divisions. Reduced ribosome function in Rpl24^Bst/+ mice further reduced protein synthesis in HSCs and impaired HSC function. Pten deletion increased protein synthesis in HSCs but also reduced HSC function. Rpl24^Bst/+ cell-autonomously rescued the effects of Pten deletion in HSCs, blocking the increase in protein synthesis, restoring HSC function, and delaying leukaemogenesis. Pten deficiency thus depletes HSCs and promotes leukaemia partly by increasing protein synthesis. Either increased or decreased protein synthesis impairs HSC function.

High levels of circulating triiodothyronine induce plasma cell differentiation

The effects of hyperthyroidism on B-cell physiology are still poorly known. In this study, we evaluated the influence of high-circulating levels of 3,5,3'-triiodothyronine (T3) on bone marrow, blood, and spleen B-cell subsets, more specifically on B-cell differentiation into plasma cells, in C57BL/6 mice receiving daily injections of T3 for 14 days. As analyzed by flow cytometry, T3-treated mice exhibited increased frequencies of pre-B and immature B-cells and decreased percentages of mature B-cells in the bone marrow, accompanied by an increased frequency of blood B-cells, splenic newly formed B-cells, and total CD19(+)B-cells. T3 administration also promoted an increase in the size and cellularity of the spleen as well as in the white pulp areas of the organ, as evidenced by histological analyses. In addition, a decreased frequency of splenic B220(+) cells correlating with an increased percentage of CD138(+) plasma cells was observed in the spleen and bone marrow of T3-treated mice. Using enzyme-linked immunospot assay, an increased number of splenic immunoglobulin-secreting B-cells from T3-treated mice was detected ex vivo. Similar results were observed in mice immunized with hen egg lysozyme and aluminum adjuvant alone or together with treatment with T3. In conclusion, we provide evidence that high-circulating levels of T3 stimulate plasma cytogenesis favoring an increase in plasma cells in the bone marrow, a long-lived plasma cell survival niche. These findings indicate that a stimulatory effect on plasma cell differentiation could occur in untreated patients with Graves' disease.