Foxp3+ regulatory T Cells Maintain Bone Marrow Microenvironment for B Cell Differentiation from Hematopoietic Stem Cells

Abstract Background: Foxp3+regulatory T cells (Treg) are a subpopulation of T cells, which regulate the immune system, maintain self-tolerance and enhance immune tolerance after transplantation. It was also reported that recipient derived Treg could provide immune privilege niche to allogeneic hematopoietic stem cells (HSC) after transplantation. However, the precise role of Treg in hematopoiesis has not been fully elucidated. Methods: We used Foxp3-DTR mice (B6, CD45.2) for in vivo depletion of Treg through diphtheria toxin (DT) injection and investigated whether Treg depletion would affect hematopoiesis derived from HSC. To investigate whether Treg depletion affects the function of the bone marrow microenvironment, we transplanted wild type bone marrow cells into lethally irradiated Foxp3-DTR mice after Treg depletion. Results: We found 1) a significant defect on B cell progenitors including mature B cells (IgM+B220+, P<0.001), pre-B cells (IgM-B220+CD19+cKit-, P<0.001) and pro-B cells (IgM-B220+CD19+cKit+, P<0.05), 2) LT-HSC population (CD34-/lowFlit3-cKit+Sca1+Lin-) was significantly expanded (p<0.01) and entered into cell cycle, 3) the residual Foxp3-CD4+ or CD8+ T cells in the bone marrow had an activated immune phenotype and clustered at sinusoids when bone marrow cells from Treg depleted mice were analyzed. Expanded LT-HSC from Treg depleted mice had reduced long-term reconstitution capacity when we performed competitive repopulation experiments using purified LT-HSC from Foxp3-DTR mice with or without Treg depletion (100 cells/mice, CD45.2), total bone marrow cells (2x10e5/mice, B6-F1, CD45.1/CD45.2) and congenic recipient mice (lethally irradiated B6, CD45.1). B cell reconstitution was also severely abrogated following transplantation using Treg depleted mice as recipients (p<0.01). In those mice, we observed a significant reduction of IL-7 production (p<0.01). Interestingly, we found that a subpopulation of CD45-TER119-CD31- ICAM1+ perivascular stromal cells are a major source of IL-7 in the bone marrow. ICAM1+ perivascular stromal cells also secrete SCF and CXCL12, which is crucial for the maintenance of LT-HSC. In Treg depleted BM cells, a significant reduction in IL-7 secretion from ICAM1+ perivascular stromal cells was observed, suggesting that this population is the target of activated T cells after Treg depletion. Conclusions: These data demonstrate that Treg play a key role in B cell differentiation from HSCs by maintaining the immunological homeostasis in the bone marrow microenvironment. These data provide new insights into Treg biology and function in normal and stress hematopoiesis. Disclosures Negrin: Stanford University: Patents & Royalties.

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Rescue of Murine IL-7 Receptor Deficiency with Human IL-7 Receptor Gene Therapy

Blood ◽

10.1182/blood-2021-151040 ◽

2021 ◽

Vol 138 (Supplement 1) ◽

pp. 3131-3131

Author(s):

Michael Triebwasser ◽

Danuta Jadwiga Jarocha ◽

Laura Breda ◽

Megan Fedorky ◽

Stefano Rivella

Keyword(s):

Gene Therapy ◽

Bone Marrow ◽

T Cells ◽

B Cells ◽

B Cell ◽

Board Of Directors ◽

Bone Marrow Cells ◽

Advisory Committees ◽

Il7r Gene ◽

Marrow Cells

Abstract In humans, interleukin 7 (IL-7) receptor (IL-7R) deficiency causes approximately 10% of cases of severe combined immunodeficiency (SCID). IL-7R deficient SCID is a T-B+NK+ SCID and is caused by autosomal recessive deficiency of the IL-7R alpha chain gene (IL7R). IL-7R is a heterodimeric receptor comprised of the alpha chain and the IL-2 receptor common gamma chain (IL2RG). In both mouse and human, IL-7R is a marker of the common lymphoid progenitor cell, and IL-7 signaling leads to STAT5 phosphorylation and proliferation of developing T and B cells. Mice lacking IL7R, Il7r -/-, lack both T and B cells (Peschon, JJ, et al. J Exp Med. 1994). T cells do not progress to TCR beta chain rearrangement and B cell development is halted at the pre-pro-B cell stage. Similar to the mouse, IL-7 signaling in humans is required for T cell receptor beta gene rearrangement and T cell maintenance, however humans lacking IL-7R can develop B cells. A prior attempt to rescue murine IL-7R deficiency utilized a retroviral vector (mouse stem cell virus, MSCV), the MSCV retroviral promoter, and the murine Il7r gene (Jiang, Q, et al. Gene Therapy. 2005). This strategy did restore T cells and had variable restoration of B cells. However, retroviral-based gene addition of Il7r led to a myeloproliferative condition with significant neutrophilia and splenomegaly. Transduced bone marrow cells formed myeloid progenitors in response to IL-7 in vitro. We evaluated a novel gene therapy for IL-7R deficient SCID that utilizes the human IL7R gene. To prevent lineage skewing, we sought to limit ectopic expression of IL7R in non-lymphoid cells by utilizing the endogenous enhancers and promoters of IL7R. These sequences were identified as sites of high sequence conservation across species and DNA accessibility/hypersensitivity (DHS) in human lymphocytes. We are testing these sequences alone or in combination with the constitutive phosphoglycerate kinase promoter (PGK) in VSV-G pseudotyped lentiviral vectors (LV): vPGK_DHS_hIL7R and vDHS_hIL7R. Here we present the first data evaluating the ability of the human IL-7R protein to functionally replace the murine IL-7R protein and the ability of IL7R gene addition to rescue the murine Il7r -/- immunodeficient phenotype in vivo. Transduction of Il7r -/- bone marrow cells with IL7R encoding LV rescued the formation of lymphocyte precursors from murine bone marrow cells in colony forming unit (CFU) assays (pre-B CFU with human IL-7), with the most robust response seen with vPGK_DHS_hIL7R. Mouse bone marrow from Il7r -/- animals transduced ex vivo engrafted in lethally irradiated (8 Gy) Il7r -/-oppositegender recipients and there were no significant aberrations in absolute neutrophil count, hemoglobin or platelet count. Absolute lymphocyte counts in mice receiving transduced Il7r -/-bone marrow cells was higher (mean 2555/μL) than in mice receiving untransduced bone marrow (mean 1410/μL). The proportion of leukocytes that were T cells was 4.2-fold and 9.8-fold higher at 1 and 2 months post-transplant, respectively. B cells were only seen in mice receiving vPGK_DHS_hIL7R: 7.4% of leukocytes versus 1.5% in controls. A reciprocal decrease in the fraction of Gr1+ cells (neutrophils and monocytes) was seen at two months post-transplant in transduced marrow recipients compared to untransduced controls: 36.5% versus 63% Gr1+, respectively. Lymphocyte subsets are being further analyzed, bone marrow and thymic lymphoid precursors assessed, and T and B cell function in response to immunizations are in progress. Further evaluation in human derived IL7R deficient human cells is warranted. For individuals with IL-7R deficient SCID, but no HLA matched hematopoietic stem cell (HSC) donor, there is a difficult choice between the risks of GVHD with a mismatched HSC donor and supportive care with the hope of identifying a matched HSC donor in the future. In immunodeficiencies however age and serious infection are both associated with increased mortality (Pai, SY, et al. NJEM. 2014). This novel approach to IL7R gene replacement has the potential to be a therapeutic and expedient option for those without a matched donor. Additionally, this would be an ideal disorder for HSC conditioning with less toxic, HSC-targeted strategies given gene corrected lymphocytes and progenitors will preferentially expand post-transplant. Disclosures Rivella: Disc Medicine: Consultancy, Membership on an entity's Board of Directors or advisory committees; Keros Therapeutics: Consultancy, Membership on an entity's Board of Directors or advisory committees; Celgene Corporation: Consultancy; Ionis Pharmaceuticals: Consultancy, Membership on an entity's Board of Directors or advisory committees; MeiraGTx: Consultancy, Membership on an entity's Board of Directors or advisory committees; Forma Theraputics: Consultancy; Incyte: Consultancy.

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Transformation of murine bone marrow cells with combined v-raf-v-myc oncogenes yields clonally related mature B cells and macrophages

Molecular and Cellular Biology ◽

10.1128/mcb.10.7.3562-3568.1990 ◽

1990 ◽

Vol 10 (7) ◽

pp. 3562-3568

Author(s):

M Principato ◽

J L Cleveland ◽

U R Rapp ◽

K L Holmes ◽

J H Pierce ◽

...

Keyword(s):

Bone Marrow ◽

B Cells ◽

B Cell ◽

Cell Lines ◽

Bone Marrow Cells ◽

Hematopoietic Differentiation ◽

Developmental Relationships ◽

Murine Bone Marrow ◽

B Lineage ◽

Marrow Cells

Murine bone marrow cells infected with replication-defective retroviruses containing v-raf alone or v-myc alone yielded transformed pre-B cell lines, while a retroviral construct containing both v-raf and v-myc oncogenes produced clonally related populations of mature B cells and mature macrophages. The genealogy of these transformants demonstrates that mature myeloid cells were derived from cells with apparent B-lineage commitment and functional immunoglobulin rearrangements. This system should facilitate studies of developmental relationships in hematopoietic differentiation and analysis of lineage determination.

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IL-21 and IL-6 Mediate Interactions Between T Cells and Malignant B Cells in the Bone Marrow Microenvironment in Waldenstrom's Macroglobulinemia

Blood ◽

10.1182/blood.v120.21.1554.1554 ◽

2012 ◽

Vol 120 (21) ◽

pp. 1554-1554

Author(s):

Lucy S. Hodge ◽

Steve Ziesmer ◽

Frank J Secreto ◽

Zhi-Zhang Yang ◽

Anne Novak ◽

...

Keyword(s):

Bone Marrow ◽

Flow Cytometry ◽

T Cells ◽

T Cell ◽

B Cells ◽

Tumor Microenvironment ◽

B Cell ◽

Bone Marrow Microenvironment ◽

Waldenström’S Macroglobulinemia ◽

Waldenström's Macroglobulinemia

Abstract Abstract 1554 T cells in the tumor microenvironment influence the biology of malignant cells in many hematologic malignancies, often through cytokine-mediated interactions. Recent studies involving healthy B cells and CD4+T cells identified an interplay between IL-6 and IL-21, whereby IL-6 increased IL-21 production by T cells, driving the differentiation and IL-6 secretion of nearby B cells. In addition to their known effects on healthy B cell function, IL-6 and IL-21 have also been implicated in the pathology of various lymphomas. In Waldenstrom's macroglobulinemia (WM), IL-6 is elevated in the bone marrow and is associated with increased IgM production. However, the function of IL-21 in the WM tumor microenvironment and its relationship to IL-6 is poorly understood. Our objective in this study was to characterize IL-21 production and function in WM and to examine the role of IL-6 and IL-21 in regulating interactions between malignant B cells and T cells in the tumor microenvironment. Immunohistochemistry revealed significant IL-21 staining in bone marrows of patients with WM (n=5), but the areas of infiltration by WM in the bone marrow sections appeared negative for IL-21 staining. To better understand the origin of IL-21 in in the tumor microenvironment, IL-21 expression was assessed by PCR in the CD19−CD138− fraction of cells remaining in patient bone marrow aspirates after positive selection for malignant B cells (n=5). IL-21 transcript was detected in 4/5 samples. CD19−CD138− cells activated with anti-CD3 and anti-CD28 antibodies expressed higher levels of IL-21 transcript and secreted significantly higher levels of IL-21 protein compared to unstimulated cells, suggesting that IL-21 in the WM bone marrow is derived from activated T cells. Intracellular expression of IL-21 protein was confirmed in CD4+ and CD8+ cells within the CD19−CD138− population using flow cytometry. Furthermore, dual staining of WM bone marrow sections with antibodies against IL-21 and CD3 or CD20 revealed co-staining of IL-21 with CD3+ T cells but not with CD20+ B cells. The response of WM B cells to T-cell derived IL-21 was then assessed in positively selected CD19+CD138+ WM B cells (n=5) and in the MWCL-1 cell line. Using flow cytometry, both the IL-21 receptor and the required common gamma chain subunit were detected on all patient samples as well as on MWCL-1 cells. Treatment of MWCL-1 cells with IL-21 (100 ng/mL) for 72 h increased proliferation by 35% (p<0.05) and IgM secretion by 80% (p<0.005). Similarly, in primary CD19+CD138+ WM cells (n=5), proliferation increased on average by 38% and IgM secretion by 71%. No apoptotic effects were associated with IL-21 in WM. Characterization of STAT activation in response to IL-21 revealed significant phosphorylation of STAT3 in both CD19+CD138+ WM cells and MWCL-1 cells and was associated with increases in BLIMP-1 and XBP-1 protein and decreases in PAX5. As STAT3 activation is known to regulate IL-6, we assessed the effect of IL-21 on B cell-mediated IL-6 secretion using ELISA. IL-21 significantly increased IL-6 secretion by both primary CD19+CD138+ WM cells (n=4) and MWCL-1 cells (87.9 +/− 10.9 ng/mL vs. 297.8 +/− 129.2 ng/mL, p<0.05). Treatment with IL-6 and IL-21 together had no additional effect over IL-21 alone on proliferation or IgM secretion in MWCL-1 cells, but culturing anti-CD3/anti-CD28-activated CD19−CD138−cells from WM bone marrows with IL-6 significantly increased IL-21 secretion (n=3). Overall, these data indicate that T-cell derived IL-21 significantly promotes growth and immunoglobulin production by malignant WM B cells and that subsequent IL-6 secretion by malignant B cells may enhance the secretion of IL-21 by T cells within the bone marrow microenvironment. Disclosures: No relevant conflicts of interest to declare.

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Mesenchymal Stromal Cells Enhance G-CSF-Induced Mobilization Of Hematopoietic Stem- and Progenitor Cells

Blood ◽

10.1182/blood.v122.21.2460.2460 ◽

2013 ◽

Vol 122 (21) ◽

pp. 2460-2460

Author(s):

Evert-Jan F. M. de Kruijf ◽

Ingmar van Hengel ◽

Jorge M Perez-Galarza ◽

Willem E. Fibbe ◽

Melissa van Pel

Keyword(s):

Bone Marrow ◽

B Lymphocytes ◽

Mesenchymal Stromal Cells ◽

Stromal Cells ◽

Peripheral Blood ◽

Bone Marrow Cells ◽

Calf Serum ◽

Intraperitoneal Administration ◽

Hematopoietic Stem ◽

Marrow Cells

Abstract Hematopoietic stem- and progenitor cell (HSPC) mobilization is a property of most hematopoietic growth factors, such as Granulocyte Colony Stimulating Factor (G-CSF). Not all donors mobilize equally well and therefore the number of HSPC that are obtained following mobilization may be limited. Mesenchymal stromal cells (MSC) have the capacity to differentiate into cells of the mesodermal lineage and have immunomodulatory properties in vivo and in vitro. Here, we have investigated the effect of MSC co-administration on G-CSF-induced HSPC mobilization. MSC were obtained from bone marrow cells (bone marrow-derived) or bone fragments (bone-derived) and were expanded in alpha-MEM containing 10% fetal calf serum until sufficient cell numbers were obtained. Bone marrow or bone-derived MSC were administered intravenously for three days at a dose of 200 x103 cells per day to male C57BL/6 recipients that were simultaneously mobilized with G-CSF (10 μg per day intraperitoneally for 3 days) or PBS as a control. Co-injection of G-CSF and MSC lead to a 2-fold increase in HSPC mobilization compared to G-CSF alone (8,563 ± 3,309 vs. 4,268 ± 1,314 CFU-C per ml peripheral blood respectively; n=13, p<0.01). Administration of MSC alone did not induce HSPC mobilization (273 ± 229 CFU-C/ml blood; n=13). Furthermore, co-injection of splenocytes and G-CSF did not enhance HSPC mobilization, showing that the administration of exogeneous cells as such is not sufficient for enhancement of HSPC mobilization. It has been reported that G-CSF-induced HSPC mobilization is associated with a decrease in the number of osteal macrophages, B lymphocytes and erythroid progenitors. Administration of MSC alone induced a significant decrease in the frequency of osteal macrophages (7.9 ± 1.2 vs 6.2 ± 1.4% bone marrow cells for PBS vs. MSC respectively; n=8, p<0.05), but did not affect osteoblast numbers. Furthermore, the frequency of B lymphocytes was significantly decreased following MSC administration (29.9 ± 4.0 vs. 16.5 ± 4.9% bone marrow cells for PBS vs. MSC respectively; n=13, p<0.0001). No differences were observed in erythroid numbers following MSC administration. To investigate the mechanisms underlying these observations, the migratory capacity of luciferase transduced MSC was studied through bioluminescence imaging. Following intravenous injection, MSC were detected in the lungs, but not in other organs. In addition, no difference in MSC migration was observed between G-CSF and PBS treated mice. Moreover, intraperitoneal administration of G-CSF and MSC resulted in increased HSPC mobilization compared to G-CSF alone (10,178 ±3,039 vs. 5,158 ± 2,436 CFU-C per ml peripheral blood; n=5-12). Together, these data point to an endocrine effect of MSC on G-CSF-induced HSPC mobilization. No differences in IL-6, CXCL-12 or M-CSF levels in bone marrow extracellular fluid were observed. In conclusion, G-CSF-induced HSPC mobilization is enhanced by injection of MSC. We hypothesize that the MSC-induced partial depletion of B lymphocytes and osteal macrophages in the bone marrow are crucial factors involved in the enhancement of G-CSF-induced HSPC mobilization. Disclosures: No relevant conflicts of interest to declare.

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BRAFV600E Mutations Occur In The Hematopoietic Stem Cell Compartment In Hairy Cell Leukemia

Blood ◽

10.1182/blood.v122.21.816.816 ◽

2013 ◽

Vol 122 (21) ◽

pp. 816-816

Author(s):

Stephen S. Chung ◽

Jae H. Park ◽

Eunhee Kim ◽

Young Rock Chung ◽

Wenhuo Hu ◽

...

Keyword(s):

Bone Marrow ◽

Cell Proliferation ◽

B Cells ◽

B Cell ◽

Bone Marrow Cells ◽

Hairy Cell ◽

Cell Leukemia ◽

Hematopoietic Stem ◽

Mutant Braf

Abstract Hairy cell leukemia (HCL) is a chronic lymphoproliferative disorder recently found to be characterized by somatic BRAFV600E mutations. The malignant cell in HCL exhibits features consistent with a mature B-lymphocyte, including cell-surface expression of the pan-B-cell marker CD19 and monotypic surface immunoglobulins with clonal rearrangements of immunoglobulin heavy and light chains. Despite possessing these stereotypic features, the cell of origin of HCL has been long debated, and no cell type along the continuum of developing B-lymphocytes has been definitively identified as the normal counterpart of HCL cells. We hypothesized that HCL may originate from immature hematopoietic cells, and therefore investigated the hematopoietic-stem/progenitor cell (HSPC) compartment in HCL patients. We found that HCL patients exhibited a significantly increased frequency of immunophenotypically defined long-term hematopoietic stem cells (LT-HSCs; lineage-negative (Lin-neg) CD34+CD38-CD90+CD45RA- cells), pro-B cells (Lin-neg CD10+ cells), and CD34-CD38+ CD10+CD19+ hematogones, as well as a decreased frequency of granulocyte-macrophage progenitor cells (Lin-neg CD34+CD38+CD45RA+CD123+) relative to age-matched normal controls. Sequencing of cDNA from highly pure FACS-sorted cell populations from the bone marrow of HCL patients revealed the presence of the BRAFV600E allele in LT-HSCs and in pro-B cells (Figure). Transplantation of LT-HSCs from the pretreatment bone marrow of HCL patients into NOD/SCID/IL2r-gnull mice resulted in stable human grafts characterized by an expanded B-progenitor population and development of a clonal population of hCD19+hCD103+hCD25+ B cells characteristic of HCL 6 months after transplantation. Together, these data suggest that HCL arises from HSCs that then differentiate into committed B-cells which ultimately give rise to the characteristic clonal B-cell proliferation of HCL. Given the human HSC genetic and functional cell data, we conditionally expressed BRafV600E from its endogenous locus at different stages of hematopoiesis, including in HSPCs and committed B cells. Mice with conditional expression of BRafV600E in Mx1Cre+ BRafV600E knock-in mice died of a lethal hematopoietic malignancy characterized by features of human HCL including splenomegaly, anemia, thrombocytopenia, increased circulating sCD25, and increased clonogenic capacity of B-lineage cells (evidenced by infinite serial replating in the presence of IL-7) (Figure). This disorder was transplantable into lethally-irradiated recipient mice. In contrast, mice with expression of BRafV600E restricted to the B-cell lineage with Cd19 Cre manifested no overt malignant phenotype up to one year of age. Stimulation of these mice with alloantigen through injections of sheep red blood cells resulted in germinal center B-cell hyperplasia, but still did not result in development of a clonal B-cell proliferation. Recent case reports have noted that refractory HCL patients respond to mutant BRAF inhibition with vemurafenib. We investigated the effect of vemurafenib on HSPCs and hematopoiesis in patients treated on a phase II study of the mutant BRAF inhibitor vemurafenib for relapsed/refractory HCL as well as in our in vivo murine models. Flow cytometric analysis of bone marrow cells from vemurafenib treated HCL patients revealed normalization of HSPC frequencies within three months of starting therapy, concomitant with an improvement in peripheral blood counts. Consistent with this, evaluation of the in vitro clonogenic capacity of sorted LT-HSC's from the bone marrow of HCL patients revealed a significant increase in myeloid/erythroid colony formation in HCL patients treated for 3 months with vemurafenib compared to their pretreatment marrows. Likewise, treatment of wildtype mice transplanted with Mx1Cre+ BRafV600E mutant bone marrow cells revealed improvement in anemia and hepatosplenomegaly with in vivo therapy. Overall, these findings link the pathogenesis of HCL to a specific somatic genetic abnormality present in HSCs and provide evidence that mature B-cell malignancies can initiate in the HSC compartment. Moreover, these data suggest that the use of therapies targeting MAP kinase signaling in HCL may lead to durable remissions not only by eliminating the mature leukemic cells but also through targeted inhibition of signaling and survival in HCL initiating cells. Disclosures: No relevant conflicts of interest to declare.

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A monoclonal antibody that recognizes B cells and B cell precursors in mice.

Journal of Experimental Medicine ◽

10.1084/jem.153.2.269 ◽

1981 ◽

Vol 153 (2) ◽

pp. 269-279 ◽

Cited By ~ 139

Author(s):

R L Coffman ◽

I L Weissman

Keyword(s):

Monoclonal Antibody ◽

Bone Marrow ◽

B Cells ◽

B Cell ◽

Plasma Cells ◽

Bone Marrow Cells ◽

Cell Lineage ◽

Hematopoietic Stem ◽

B Cell Precursors

The monoclonal antibody, RA3-2C2, appears to be specific for cells within the B cell lineage. This antibody does not recognize thymocytes, peripheral T cells, or nonlymphoid hematopoietic cells in the spleen or bone marrow. Nor does it recognize the pluripotent hematopoietic stem cells, the spleen colony-forming unit, All sIg+ B cells and most plasma cells are RA3-2C2+. In addition, approximately 20% of nucleated bone marrow cells are RA3-2C2+ but sIg-. This population contains B cell precursors that can give rise to sIg+ cells within 2 d in vitro.

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G-CSF-Induced Alterations in the Bone Marrow Microenvironment Suppress B Lymphopoiesis

Blood ◽

10.1182/blood.v120.21.1246.1246 ◽

2012 ◽

Vol 120 (21) ◽

pp. 1246-1246

Author(s):

Ryan B. Day ◽

Adam Greenbaum ◽

Daniel C. Link

Keyword(s):

Bone Marrow ◽

B Cells ◽

B Cell ◽

Stromal Cells ◽

Cell Development ◽

B Cell Development ◽

Bone Marrow Microenvironment ◽

B Lymphopoiesis ◽

Treatment Results ◽

Cell Suppression

Abstract Abstract 1246 Infectious stress is associated with a shift in the bone marrow from lymphopoiesis to granulopoiesis. Expression of granulocyte colony-stimulating factor (G-CSF), the principal cytokine regulating granulopoiesis, is often induced during infection. We previously reported that G-CSF treatment is associated with marked suppression of B lymphopoiesis in murine bone marrow. After 5 days of G-CSF treatment (250 μg/kg), total B cells in the bone marrow were reduced 8.1 ± 0.9-fold. Pre-pro-B cells were reduced 1.6 ± 0.3-fold, pro-B cells 12.4 ± 1.9-fold, pre-B cells 5.6 ± 0.8-fold, immature B cells 7.5 ± 1.2-fold, and mature naïve B cells 83 ± 7.6-fold. B-committed lymphoid progenitors (BLP) were modestly but significantly decreased (1.4 ± 0.2-fold), while common lymphoid progenitors (CLP) were not affected by G-CSF treatment. Increased apoptosis of mature naïve B cells in the bone marrow was observed. Studies of G-CSF receptor deficient (Csf3r−/−) bone marrow chimeras show that G-CSF acts in a non-cell intrinsic fashion to suppress B lymphopoiesis. Consistent with this observation, we show that G-CSF treatment results in decreased expression in the bone marrow microenvironment of multiple B-supportive factors including CXCL12, interleukin-6, interleukin-7, and B cell activating factor (BAFF). Prior studies have established that CXCL12-abundant reticular (CAR) cells in the bone marrow play a key role in B cell development. CAR cells are perivascular stromal cells that express very high levels of CXCL12 and are in direct contact with pre-pro-B cells. G-CSF treatment did not affect CAR cell number. However, RNA expression profiling of sorted CAR cells showed that expression of several genes associated with B cell development are significantly decreased by G-CSF, including CXCL12 (4.2 ± 1.5-fold). In addition to CAR cells, other stromal cells in the bone marrow express CXCL12, including osteoblasts and endothelial cells. To assess the role of CXCL12 production by each of these cell types to B lymphopoiesis, we generated Cxcl12flox mice and crossed them with mice expressing the following tissue-specific Cre-recombinase transgenes: Osteocalcin-Cre (Oc-Cre) targeting mature mineralizing osteoblasts; Osterix-Cre (Osx-Cre) targeting CAR cells and all osteolineage cells; or Prx1-Cre targeting mesenchymal progenitors and their progeny. Deletion of Cxcl12 using Oc-Cre or Osx-Cre had a similar effect on B cell development, with an isolated loss of mature naïve B cells in the bone marrow (2.7 ± 0.5 and 4.1 ± 1.7-fold, respectively). In contrast, deletion of Cxcl12 using Prx1-Cre resulted in severe suppression of B lymphopoiesis that included a loss of CLP (3.3 ± 2.0-fold), BLP (5.6 ± 4.3-fold), and pre-pro-B cells (12.4 ± 5.1-fold). Interestingly, treatment of Prx1-Cre Cxcl12flox/- mice with G-CSF resulted in additional B cell loss, indicating that deletion of Cxcl12 in mesenchymal stromal cells is not sufficient to fully recapitulate G-CSF-induced B cell suppression. In summary, G-CSF treatment results in marked changes in the bone marrow microenvironment that lead to a suppression of B lymphopoiesis. While G-CSF-induced inhibition of CXCL12 expression from stromal cells contributes to B cell suppression, additional alterations in the microenvironment also contribute to this phenotype. Disclosures: No relevant conflicts of interest to declare.

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Reduced Hspa9 Expression Alters IL-7 Signaling In B-Cells

Blood ◽

10.1182/blood.v122.21.1569.1569 ◽

2013 ◽

Vol 122 (21) ◽

pp. 1569-1569

Author(s):

Kilannin Krysiak ◽

Justin Tibbitts ◽

Matthew J. Walter

Keyword(s):

Bone Marrow ◽

B Cells ◽

B Cell ◽

Bone Marrow Cells ◽

Cell Development ◽

B Cell Development ◽

Driving Mechanism ◽

Wild Type ◽

Fold Reduction ◽

Marrow Cells

Abstract Myeloid and erythroid differentiation defects and cytopenias are most commonly described in myelodysplastic syndromes (MDS), however, a reduction in B-cell progenitors exists. The genetic events contributing to this reduction are poorly understood. Interstitial deletion or loss of one copy of the long arm of chromosome 5 (del5q) is the most common cytogenetic abnormality associated with MDS. Two commonly deleted regions on del(5q) have been described and no biallelic mutations have been identified implicating haploinsufficiency of genes on this interval as a driving mechanism. We, and others, have identified several del(5q) candidate genes, including RPS14, EGR1, CTNNA1, APC, NPM1, DIAPH1, miR145, miR146a, and HSPA9. Consistent with haploinsufficiency, HSPA9 mRNA levels are 50% reduced in del(5q) patients. We previously showed that knockdown of Hspa9by shRNA in a murine bone marrow transplant model resulted in a significant reduction in murine B-cells in the bone marrow, spleen and peripheral blood. To further characterize the role of Hspa9 in hematopoiesis, we created Hspa9 heterozygous mice (Hspa9+/-). Heterozygotes express 50% less Hspa9 protein and are born at normal Mendelian frequencies (N>100). No significant differences in mature lineage markers, complete blood counts, and hematopoietic organ cellularity, have been identified up to 12 months of age. However, as early as 2 months of age, Hspa9+/- mice show a significant reduction in CFU-PreB colonies compared to their wild-type littermates, indicating B-cell progenitor defects (14 vs. 48 colonies/100,000 bone marrow cells plated, respectively, N=10 mice/genotype, p<0.001). Following long-term engraftment of transplanted bone marrow cells from Hspa9+/-or littermate controls into lethally irradiated recipients, we also observed a 5.8-fold reduction in bone marrow CFU-PreB colonies (N=7-9 mice/genotype, p=0.002), confirming the B-cell progenitor defect is hematopoietic cell-intrinsic. Despite the reduction in CFU-PreB colony numbers, frequencies of freshly isolated early B-cell progenitor and precursor populations in the bone marrow and spleen of Hspa9+/- mice are not different than wild-type littermate controls when assessed by flow cytometry (common lymphoid progenitor, Hardy fractions A-F). We hypothesized that these mice were able to compensate for B-cell alterations caused by loss of Hspa9 in vivo. Consistent with our hypothesis, the reduction in CFU-PreB colony numbers was partially rescued by increasing the concentration of IL-7 in the media. Hspa9+/- colony numbers increased 1.8 fold when the IL-7 concentration was increased from 10ng/mL to 50ng/mL compared to 0.80 fold for wild-type littermates (p=0.03, N=6 mice/genotype). This effect was unique to IL-7. Adding increasing concentrations of Flt-3 ligand, another cytokine that contributes to early B-cell development, did not alter CFU-PreB colony formation. We isolated B220+ cells from Day 7 CFU-PreB cultures for gene expression array analysis and observe reduced expression of genes promoting B-cell proliferation and activation in Hspa9+/- compared to Hspa9+/+ cells. Since IL-7 is the only supportive cytokine in the methylcellulose media, can partially rescue the reduced CFU-PreB phenotype, and is required for early B-cell development and survival, we hypothesized that Hspa9 haploinsufficiency inhibits transduction of IL-7 signaling. We tested this hypothesis using an IL-7 dependent mouse B-cell line (B7 cells; Ba/F3 cells that stably express the IL-7 receptor). Knockdown of Hspa9 by siRNAs resulted in a 8-fold reduction in cell number after 4 days in culture (p=0.004, confirmed with two independent siRNAs) and was associated with an increase in apoptosis and reduction in cells in S-phase of the cell cycle. Knockdown of Hspa9 in B7 cells resulted in reduced levels of phosphorylated Stat5, an immediate downstream target of IL-7 receptor stimulation, compared to cells treated with a non-targeting siRNA (measured at 5, 10, 15 and 30 minutes following 10ng/mL IL-7 stimulation, p≤0.03). Ongoing studies will further interrogate the effects of Hsap9 knockdown on Jak-Stat signaling. Collectively, these data implicate that loss of HSPA9 alters IL-7 signaling, potentially contributing to the reduction of B-cell progenitors observed in patients with del(5q)-associated MDS. Disclosures: No relevant conflicts of interest to declare.

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In vitro tolerance induction of neonatal murine B cells as a probe for the study of B-cell diversification.

Journal of Experimental Medicine ◽

10.1084/jem.145.5.1382 ◽

1977 ◽

Vol 145 (5) ◽

pp. 1382-1386 ◽

Cited By ~ 19

Author(s):

E S Metcalf ◽

N H Sigal ◽

N R Klinman

Keyword(s):

Bone Marrow ◽

B Cells ◽

B Cell ◽

Bone Marrow Cells ◽

Tolerance Induction ◽

Adult Bone Marrow ◽

Late Expression ◽

Adult Bone ◽

Marrow Cells

The susceptibility to in vitro tolerance induction has been implicated as a characteristic of B cells early in their development, since DNP-reactive B cells are tolerizable only during the first days after birth, and 25% of adult bone marrow cells are tolerizable. In the present study, a modification of the in vitro splenic focus technique was utilized to determine if PC-specific B cells, by virtue of their late expression (approximately 1 wk post-parturition), also display susceptibility to tolerance induction. The results demonstrate that at 7-10 days after birth, when over 90% of the DNP-specific splenic B cells are resistant to tolerance induction, the majority of PC-specific B cells are tolerizable. These results re-emphasize tolerance susceptibility as a characteristic of developing clones, confirm the late acquisition of PC-specific B cells, and support the contention that the acquisition of the specificity repertoire is a highly ordered, specifically predetermined process which is independent of antigen-driven events.

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