IRF4 Has a Unique Role in Early B Cell Development and Acts Prior to CD21 Expression to Control Marginal Zone B Cell Numbers

Strict control of B lymphocyte development is required for the ability to mount humoral immune responses to diverse foreign antigens while remaining self-tolerant. In the bone marrow, B lineage cells transit through several developmental stages in which they assemble a functional B cell receptor in a stepwise manner. The immunoglobulin heavy chain gene is rearranged at the pro-B stage. At the large pre-B stage, cells with a functional heavy chain expand in response to signals from IL-7 and the pre-BCR. Cells then cease proliferation at the small pre-B stage and rearrange the immunoglobulin light chain gene. The fully formed BCR is subsequently expressed on the surface of immature B cells and autoreactive cells are culled by central tolerance mechanisms. Once in the periphery, transitional B cells develop into mature B cell subsets such as marginal zone and follicular B cells. These developmental processes are controlled by transcription factor networks, central to which are IRF4 and IRF8. These were thought to act redundantly during B cell development in the bone marrow, with their functions diverging in the periphery where IRF4 limits the number of marginal zone B cells and is required for germinal center responses and plasma cell differentiation. Because of IRF4’s unique role in mature B cells, we hypothesized that it may also have functions earlier in B cell development that cannot be compensated for by IRF8. Indeed, we find that IRF4 has a unique role in upregulating the pre-B cell marker CD25, limiting IL-7 responsiveness, and promoting migration to CXCR4 such that IRF4-deficient mice have a partial block at the pre-B cell stage. We also find that IRF4 acts in early transitional B cells to restrict marginal zone B cell development, as deletion of IRF4 in mature B cells with CD21-cre impairs plasma cell differentiation but has no effect on marginal zone B cell numbers. These studies highlight IRF4 as the dominant IRF family member in early B lymphopoiesis.

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Induction of metabolic quiescence defines the transitional to follicular B cell switch

Science Signaling ◽

10.1126/scisignal.aaw5573 ◽

2019 ◽

Vol 12 (604) ◽

pp. eaaw5573 ◽

Cited By ~ 3

Author(s):

Jocelyn R. Farmer ◽

Hugues Allard-Chamard ◽

Na Sun ◽

Maimuna Ahmad ◽

Alice Bertocchi ◽

...

Keyword(s):

B Cells ◽

Cell Surface ◽

B Cell ◽

Cell Development ◽

B Cell Development ◽

Ampk Activation ◽

Cell Stage ◽

Extracellular Adenosine ◽

Transitional B Cells ◽

Follicular B Cells

Transitional B cells must actively undergo selection for self-tolerance before maturing into their resting follicular B cell successors. We found that metabolic quiescence was acquired at the follicular B cell stage in both humans and mice. In follicular B cells, the expression of genes involved in ribosome biogenesis, aerobic respiration, and mammalian target of rapamycin complex 1 (mTORC1) signaling was reduced when compared to that in transitional B cells. Functional metabolism studies, profiling of whole-cell metabolites, and analysis of cell surface proteins in human B cells suggested that this transition was also associated with increased extracellular adenosine salvage. Follicular B cells increased the abundance of the cell surface ectonucleotidase CD73, which coincided with adenosine 5′-monophosphate–activated protein kinase (AMPK) activation. Differentiation to the follicular B cell stage in vitro correlated with surface acquisition of CD73 on human transitional B cells and was augmented with the AMPK agonist, AICAR. Last, individuals with gain-of-function PIK3CD (PI3Kδ) mutations and increased pS6 activation exhibited a near absence of circulating follicular B cells. Together, our data suggest that mTORC1 attenuation may be necessary for human follicular B cell development. These data identify a distinct metabolic switch during human B cell development at the transitional to follicular stages, which is characterized by an induction of extracellular adenosine salvage, AMPK activation, and the acquisition of metabolic quiescence.

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Transitional B Cells in Early Human B Cell Development – Time to Revisit the Paradigm?

Frontiers in Immunology ◽

10.3389/fimmu.2016.00546 ◽

2016 ◽

Vol 7 ◽

Cited By ~ 24

Author(s):

Victoria G. Martin ◽

Yu-Chang Bryan Wu ◽

Catherine L. Townsend ◽

Grace H. C. Lu ◽

Joselli Silva O’Hare ◽

...

Keyword(s):

B Cells ◽

B Cell ◽

Development Time ◽

Cell Development ◽

B Cell Development ◽

Transitional B Cells ◽

Early Human ◽

Human B Cell

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B Cell Lineage-Specific Deletion of Icsbp/IRF8 Reveals Roles for IRF8 in the Regulation of Marginal Zone and Follicular B Cell Development.

Blood ◽

10.1182/blood.v110.11.1334.1334 ◽

2007 ◽

Vol 110 (11) ◽

pp. 1334-1334

Author(s):

Hongsheng Wang ◽

Jianxun Feng ◽

Chang Hoon Lee ◽

Herbert Morse

Keyword(s):

B Cells ◽

B Cell ◽

Marginal Zone ◽

Cell Development ◽

B Cell Development ◽

Conditional Knockout ◽

Myelogenous Leukemia ◽

Exon 2 ◽

B Lineage ◽

B Lineage Cells

Abstract Interferon regulatory factor 8 (IRF8), also known as interferon consensus sequence-binding protein (ICSBP), is a transcription factor that expresses in T cells, B cells and macrophages and plays a role in myeloid development. Targeted deletion of IRF8 in mice (IRF8−/−) induced progressive increase in the numbers of granulocytes in various lymphoid organs and development of a syndrome similar to human chronic myelogenous leukemia. In addition to defective development of macrophages and dendritic cells, B cell development was also impaired in IRF8−/− mice. This includes decreased numbers of early B cells, expanded marginal zone (MZ) B cells and diminished follicular (OF) B2 cells. Because abnormal myeloid cells could alter microenvironment required for normal B cell development, we have generated IRF8 conditional knockout mice to specifically investigate the function of IRF8 in B lineage cells. Mice were engineered to have exon 2, encoding the DNA binding domain of IRF8, flanked by loxP sites (designated IRF8f/+). These mice were then crossed with the CD19Cre strain in which the expression of Cre-recombinase is controlled by the endogenous CD19 locus. Homozygous mice (designated (IRF8f/f x Cre)F1) underwent germline excision of IRF8 in CD19+ B lineage cells. As a result, there was no detectable mRNA and protein of IRF8 in their splenic B cells. Flow cytometry analysis revealed expanded MZ B cells and reduced OF B2 cells in the spleen of (IRF8f/f x Cre)F1 mice. Interestingly, the expression level of CD23 on OF B cells was significantly decreased in (IRF8f/f x Cre)F1 mice, indicating that IRF8 is required for maintaining a normal OF phenotype. In the peritoneum of (IRF8f/f x Cre)F1 mice, while the numbers of B1a and B2 cells were slightly decreased, the number of B1b cells was slightly increased. Furthermore, BXH2 mice carrying a mutation (C915T) in the Icsbp1 gene exhibited similar expansion of MZ B cells and low expression of CD23 in OF B cells. Taken together, these analyses indicate that IRF8 is required for development of normal MZ and B2 cells.

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The Proto-Oncogene LRF Is Essential for Normal Mature B Cell Development and Germinal Center Formation.

Blood ◽

10.1182/blood.v112.11.1788.1788 ◽

2008 ◽

Vol 112 (11) ◽

pp. 1788-1788

Author(s):

Nagisa Sakurai ◽

Manami Maeda ◽

Sung-UK Lee ◽

Julie Teruya-Feldstein ◽

Takahiro Maeda

Keyword(s):

B Cells ◽

B Cell ◽

Cell Development ◽

B Cell Development ◽

Cell Stage ◽

Transitional B Cells ◽

Notch Pathways ◽

Secondary Lymphoid Organs

Abstract LRF (Leukemia/Lymphoma Related Factor, also known as Pokemon, FBI-1, OCZF and ZBTB7a) was originally identified as an interaction partner of the oncoprotein BCL6. LRF can act as a proto-oncogene by repressing the tumor suppressor ARF and cooperates with BCL6 in MEF (mouse embryonic fibroblasts) immortalization. It is highly expressed in human Non-Hodgkin Lymphoma (NHL) cases, in the pathogenesis of which BCL6 is known to be involved (Maeda et al. Nature 2005). Inducible inactivation of the LRF gene in mouse Hematopoietic Stem Cells (HSCs) results in complete block of early B cell development at the HSC/progenitor stages and concomitant development of double positive (DP) T cells in the bone marrow (BM) (Maeda et al. Science 2007). While these findings clearly illustrate key roles of LRF in normal and malignant B cell development, it is not fully identified as to which B cell stages LRF is required during normal B cell development. To elucidate the role of LRF in B cells in vivo, we established and characterized B cell-specific LRF conditional knockout (KO) mice. We took advantage of mb-1 Cre knock-in mice, in which Cre expression is restricted to the B cells after the ProB cell stage. B cell compartments in the BM (PreProB, ProB, PreB and immatureB) are grossly normal in LRFF/ Fmb1-Cre mice. The LRF gene was efficiently eliminated in BM CD19+ B cells revealed by quantitative real-time PCR assay. Furthermore, LRF protein was not detected in purified CD19+ B cells, but seen in CD19-non-B cells, confirming the specific inactivation of the LRF gene in B cells. Thus, despite its critical role at the HSC/progenitor stages, LRF was found to be dispensable for the survival of normal BM B cells. These findings are consistent with the fact that GSI treatment (Maeda et al. Science 2007) or Notch1 loss (Lee and Maeda, unpublished) rescues the defects in early B cell development seen in LRFF/FMx1-Cre+ mice. Notch signaling is necessary for the transitional B cells to commit to the marginal zone B cells (MZB). Inactivation of the component of the Notch pathways in mice results in no MZB development. On the contrary, deletion of the MINT/SHARP gene, a suppressor of Notch signaling, leads to increase of MZB cells and concomitant reduction of follicular B (FOB) cells, indicating that Notch induces MZB cell fate at the transitional B cell stage. Given that LRF is a potent Notch suppressor at the HSC/progenitor stages, we hypothesized that LRF opposes Notch pathway in mature B cells as well. To test this hypothesis, we characterized mature B cell development in LRFF/Fmb1-Cre mice. While transitional B cells were largely unaffected in LRFF/Fmb1-Cre mice, we observed a slight but statistically significant reduction of follicular (FO) B cells (B220+CD19+AA4.1-CD1d-CD23+) and concomitant increase of MZB cells (B220+CD19+AA4.1-CD1d+CD23-) as seen in MINT/SHARP knockout mice. Thus, LRF may also oppose Notch pathways at the branching point for the FOB vs. MZB fate decision. Finally, to determine the role of LRF in Germinal Center (GC) formation in vivo, we characterized secondary lymphoid organs of LRFF/Fmb1-Cre mice after antigen stimulation. Both spleen and Peyer’s Patches were analyzed two weeks after immunization with Chicken Gamma Globulin (NP-CGG). While a GC reaction was robustly induced in control mice upon immunization, GC formation was significantly impaired in LRFF/Fmb1-Cre mice as revealed by immuno-histochemical analysis (IHC) and FACS. Only few GC cells (B220+CD19+FAS+CD38-PNA+) were observed in spleens, and the absolute numbers of GC cells were drastically reduced in LRFF/Fmb1-Cre mice. Residual LRF-deficient GC B cells were mostly negative for CXCR4, which is predominantly expressed in proliferating centroblasts within GCs, suggesting that LRF-deficient GC B cells may have defects in cellular proliferation in response to antigen stimuli. Our data indicates that LRF plays key roles in mature B cell development in the secondary lymphoid organs, but dispensable for the maintenance of early BM B cells.

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Precursor B Cell Acute Lymphoblastic Leukemia Induces Pro-Leukemic Changes in the Bone Marrow Microenvironment That Contribute to Therapeutic Resistance

Blood ◽

10.1182/blood.v120.21.1466.1466 ◽

2012 ◽

Vol 120 (21) ◽

pp. 1466-1466

Author(s):

Christopher D Chien ◽

Elizabeth D Hicks ◽

Paul P Su ◽

Haiying Qin ◽

Terry J Fry

Keyword(s):

Bone Marrow ◽

Acute Lymphoblastic Leukemia ◽

B Cells ◽

B Cell ◽

Lymphoblastic Leukemia ◽

Cell Development ◽

B Cell Development ◽

Bone Marrow Microenvironment ◽

Therapeutic Resistance

Abstract Abstract 1466 Pediatric acute lymphoblastic leukemia (ALL) is the most common childhood malignancy. Although cure rates for this disease are approximately 90%, ALL remains one of the leading causes cancer-related deaths in children. Thus, new treatments are needed for those patients that do not respond to or recur following standard chemotherapy. Understanding the mechanisms underlying resistance of pediatric ALL to therapy offers one approach to improving outcomes. Recent studies have demonstrated the importance of communication between cancer cells and their microenvironment and how this contributes to the progression and therapeutic resistance but this has not been well studied in the context of ALL. Since the bone marrow is presumed to be the site of initiation of B precursor ALL we set out in our study to determine how ALL cells utilize the bone marrow milieu in a syngeneic transplantable model of preB cell ALL in immunocompetent mice. In this model, intravenously injected preB ALL develops first in the bone marrow, followed by infiltration into the spleen, lymph node, and liver. Using flow cytometry to detect the CD45.2 isoform following injection into B6CD45.1+ congenic recipients, leukemic cells can be identified in the bone marrow as early as 5 days after IV injection with a sensitivity of 0.01%-0.1%. The pre-B ALL line is B220+/CD19+/CD43+/BP1+/IL-7Ralpha (CD127)+/CD25-/Surface IgM-/cytoplasmic IgM+ consistent with a pre-pro B cell phenotype. We find that increasing amounts of leukemic infiltration in the bone marrow leads to an accumulation of non-malignant developing B cells at stages immediately prior to the pre-pro B cell (CD43+BP1-CD25-) and a reduction in non-malignant developing pre B cells at the developmental stage just after to the pre-pro B cell stage (CD43+BP1+CD25+). These data potentially suggest occupancy of normal B cell developmental niches by leukemia resulting in block in normal B cell development. Further supporting this hypothesis, we find significant reduction in early progression of ALL in aged (10–12 month old) mice known to have a deficiency in B cell developmental niches. We next explored whether specific factors that support normal B cell development can contribute to progression of precursor B cell leukemia. The normal B cell niche has only recently been characterized and the specific contribution of this niche to early ALL progression has not been extensively studied. Using a candidate approach, we examined the role of specific cytokines such as Interleukin-7 (IL-7) and thymic stromal lymphopoietin (TSLP) in early ALL progression. Our preB ALL line expresses high levels of IL-7Ralpha and low but detectable levels of TLSPR. In the presence of IL-7 (0.1 ng/ml) and TSLP (50 ng/ml) phosphSTAT5 is detectable indicating that these receptors are functional but that supraphysiologic levels of TSLP are required. Consistent with the importance of IL-7 in leukemia progression, preliminary data demonstrates reduced lethality of pr-B cell ALL in IL-7 deficient mice. Overexpression of TSLP receptor (TSLPR) has been associated with high rates of relapse and poor overall survival in precursor B cell ALL. We are currently generating a TSLPR overepressing preBALL line to determine the effect on early ALL progression and are using GFP-expressing preB ALL cells to identify the initial location of preB ALL occupancy in the bone marrow. In conclusion, or model of early ALL progression provides insight into the role of the bone marrow microenvironment in early ALL progression and provides an opportunity to examine how these microenvironmental factors contribute to therapeutic resistance. Given recent advances in immunotherapy for hematologic malignancies, the ability to study this in an immunocompetent host will be critical. Disclosures: No relevant conflicts of interest to declare.

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RNAi Screening Identifies A Novel Role for A-Kinase Anchoring Protein 12 (AKAP12) in B Cell Development and Function

Blood ◽

10.1182/blood.v120.21.855.855 ◽

2012 ◽

Vol 120 (21) ◽

pp. 855-855 ◽

Cited By ~ 1

Author(s):

Mutlu Kartal-Kaess ◽

Luisa Cimmino ◽

Simona Infantino ◽

Mehmet Yabas ◽

Jian-Guo Zhang ◽

...

Keyword(s):

Bone Marrow ◽

B Cells ◽

B Cell ◽

Leukemia Cell ◽

Cell Development ◽

B Cell Development ◽

Leukemia Cell Line ◽

B Lymphopoiesis ◽

Catalytic Subunits ◽

B Cell Leukemia

Abstract Abstract 855 The cAMP signaling pathway has emerged as a key regulator of hematopoietic cell proliferation, differentiation, and apoptosis. Signal specificity is achieved through local activation of signaling enzymes that are anchored to subcellular organelles and membranes. In particular, A-kinase anchoring proteins (AKAPs) coordinate and control cAMP responsive events. AKAPs were originally classified based on their ability to bind cAMP-dependent protein kinase (protein kinase A; PKA). The activity of PKA is regulated by its two regulatory subunits, which from a dimer that binds to the two catalytic subunits. Binding of cAMP to the regulatory dimer dissociates the catalytic subunits and activates PKA. Anchoring of PKA by AKAPs constrains PKA activity to a relevant subset of potential substrates. Thus, AKAPs contribute to the precision of intracellular signaling events by directing anchored enzyme pools to a subset of their physiological substrates at specific subcellular localizations. Using an in vitro short hairpin RNA (shRNA) screen against potentially druggable targets, we have uncovered a requirement for AKAP12 in the proliferation of a cultured pre-B cell leukemia cell line. In the hematopoietic system of mice and humans, expression of AKAP12 is tightly restricted to the pro/pre/immature stages of B lymphopoiesis, suggesting a potential role in pre-B cell receptor (pre-BCR) or BCR signaling. We find that retroviral knockdown or germline knockout of AKAP12 in mice leads to an increase in pre B and immature B cells in the bone marrow. In contrast, B cell numbers in the spleen are significantly reduced, as are recirculating B cells in the bone marrow. Transplantation of AKAP12 null hematopoietic stem and progenitor cells from fetal liver into wildtype recipients demonstrates an autonomous defect in the development of AKAP12−/− B cells. Competitive bone marrow transplantations confirm that this defect is cell autonomous and not due to a defective bone marrow environment or secretion of a B cell inhibitory factor. To identify AKAP12 interaction partners, we overexpressed FLAG-epitope tagged AKAP12 in a pre-B cell leukemia cell line. Affinity purification of AKAP12 showed a repeated co-immunoprecipitation of poorly characterized RIO kinase 1 (RIOK1). Our current efforts are focused on investigating the interaction between RIOK1 and AKAP12 and their role in the control of B cell development and cell cycle progression. Further, we are focusing on a likely role for AKAP12 in the scaffolding of PKA, PKC and phosphodiesterases by analyzing the activation of signaling cascades in cultured primary wildtype and AKAP12−/− pre B cells. Additionally, we are investigating the role of the BCR in vivo by testing if enforced expression of BCR components rescue B cell development in a AKAP12−/− BCR transgenic mouse model (SWHEL mouse). In summary, we have confirmed a novel role for AKAP12 in B lymphopoiesis. Disclosures: No relevant conflicts of interest to declare.

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B Cell Development in the Bone Marrow Is Regulated by Homeostatic Feedback Exerted by Mature B Cells

Frontiers in Immunology ◽

10.3389/fimmu.2016.00077 ◽

2016 ◽

Vol 7 ◽

Cited By ~ 9

Author(s):

Gitit Shahaf ◽

Simona Zisman-Rozen ◽

David Benhamou ◽

Doron Melamed ◽

Ramit Mehr

Keyword(s):

Bone Marrow ◽

B Cells ◽

B Cell ◽

Cell Development ◽

B Cell Development

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Regulated Expression of Human Histocompatibility Leukocyte Antigen (HLA)-DO During Antigen-dependent and Antigen-independent Phases of B Cell Development

Journal of Experimental Medicine ◽

10.1084/jem.20012066 ◽

2002 ◽

Vol 195 (8) ◽

pp. 1053-1062 ◽

Cited By ~ 47

Author(s):

Xinjian Chen ◽

Oskar Laur ◽

Taku Kambayashi ◽

Shiyong Li ◽

Robert A. Bray ◽

...

Keyword(s):

Bone Marrow ◽

B Cells ◽

B Cell ◽

Peripheral Blood ◽

Class Ii ◽

Cell Development ◽

B Cell Development ◽

Negative Regulator ◽

Leukocyte Antigen ◽

Peptide Loading

Human histocompatibility leukocyte antigen (HLA)-DO, a lysosomal resident major histocompatibility complex class II molecule expressed in B cells, has previously been shown to be a negative regulator of HLA-DM peptide loading function. We analyze the expression of DO in human peripheral blood, lymph node, tonsil, and bone marrow to determine if DO expression is modulated in the physiological setting. B cells, but not monocytes or monocyte-derived dendritic cells, are observed to express this protein. Preclearing experiments demonstrate that ∼50% of HLA-DM is bound to DO in peripheral blood B cells. HLA-DM and HLA-DR expression is demonstrated early in B cell development, beginning at the pro-B stage in adult human bone marrow. In contrast, DO expression is initiated only after B cell development is complete. In all situations, there is a striking correlation between intracellular DO expression and cell surface class II–associated invariant chain peptide expression, which suggests that DO substantially inhibits DM function in primary human B cells. We report that the expression of DO is markedly downmodulated in human germinal center B cells. Modulation of DO expression may provide a mechanism to regulate peptide loading activity and antigen presentation by B cells during the development of humoral immune responses.

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Differential Regulation of B Cell Development, Activation, and Death by the Src Homology 2 Domain–Containing 5′ Inositol Phosphatase (Ship)

Journal of Experimental Medicine ◽

10.1084/jem.191.9.1545 ◽

2000 ◽

Vol 191 (9) ◽

pp. 1545-1554 ◽

Cited By ~ 96

Author(s):

Anne Brauweiler ◽

Idan Tamir ◽

Joseph Dal Porto ◽

Robert J. Benschop ◽

Cheryl D. Helgason ◽

...

Keyword(s):

Bone Marrow ◽

B Cells ◽

B Cell ◽

Cell Development ◽

B Cell Development ◽

Inositol Phosphatase ◽

Src Homology 2 ◽

Src Homology ◽

Src Homology 2 Domain ◽

Deficient Mice

Although the Src homology 2 domain–containing 5′ inositol phosphatase (SHIP) is a well-known mediator of inhibitory signals after B cell antigen receptor (BCR) coaggregation with the low affinity Fc receptor, it is not known whether SHIP functions to inhibit signals after stimulation through the BCR alone. Here, we show using gene-ablated mice that SHIP is a crucial regulator of BCR-mediated signaling, B cell activation, and B cell development. We demonstrate a critical role for SHIP in termination of phosphatidylinositol 3,4,5-triphosphate (PI[3,4,5]P3) signals that follow BCR aggregation. Consistent with enhanced PI(3,4,5)P3 signaling, we find that splenic B cells from SHIP-deficient mice display enhanced sensitivity to BCR-mediated induction of the activation markers CD86 and CD69. We further demonstrate that SHIP regulates the rate of B cell development in the bone marrow and spleen, as B cell precursors from SHIP-deficient mice progress more rapidly through the immature and transitional developmental stages. Finally, we observe that SHIP-deficient B cells have increased resistance to BCR-mediated cell death. These results demonstrate a central role for SHIP in regulation of BCR signaling and B cell biology, from signal driven development in the bone marrow and spleen, to activation and death in the periphery.

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Differential expression of CD21 identifies developmentally and functionally distinct subsets of human transitional B cells

Blood ◽

10.1182/blood-2009-07-234799 ◽

2010 ◽

Vol 115 (3) ◽

pp. 519-529 ◽

Cited By ~ 77

Author(s):

Santi Suryani ◽

David A. Fulcher ◽

Brigitte Santner-Nanan ◽

Ralph Nanan ◽

Melanie Wong ◽

...

Keyword(s):

B Cells ◽

B Cell ◽

Differential Expression ◽

Cell Development ◽

B Cell Development ◽

Cell Repertoire ◽

Molecular Features ◽

Transitional B Cells ◽

Transitional Cells

Abstract The transitional stage of B-cell development represents an important step where autoreactive cells are deleted, allowing the generation of a mature functional B-cell repertoire. In mice, 3 subsets of transitional B cells have been identified. In contrast, most studies of human transitional B cells have focused on a single subset defined as CD24hiCD38hi B cells. Here, we have identified 2 subsets of human transitional B cells based on the differential expression of CD21. CD21hi transitional cells displayed higher expression of CD23, CD44, and IgD, and exhibited greater proliferation and Ig secretion in vitro than CD21lo transitional B cells. In contrast, the CD21lo subset expressed elevated levels of LEF1, a transcription factor highly expressed by immature lymphocytes, and produced higher amounts of autoreactive Ab. These phenotypic, functional, and molecular features suggest that CD21lo transitional B cells are less mature than the CD21hi subset. This was confirmed by analyzing X-linked agammaglobulinemia patients and the kinetics of B-cell reconstitution after stem cell transplantation, which revealed that the development of CD21lo transitional B cells preceded that of CD21hi transitional cells. These findings provide important insights into the process of human B-cell development and have implications for understanding the processes underlying perturbed B-cell maturation in autoimmune and immunodeficient conditions.

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