Faculty Opinions recommendation of Class switch recombination and somatic hypermutation in early mouse B cells are mediated by B cell and Toll-like receptors.

AbstractThe Epstein-Barr virus (EBV) protein LMP1 is considered to be a functional homologue of the CD40 receptor. However, in contrast to the latter, LMP1 is a constitutively active signaling molecule. To compare B cell–specific LMP1 and CD40 signaling in an unambiguous manner, we generated transgenic mice conditionally expressing a CD40/LMP1 fusion protein, which retained the LMP1 cytoplasmic tail but has lost the constitutive activity of LMP1 and needs to be activated by the CD40 ligand. We show that LMP1 signaling can completely substitute CD40 signaling in B cells, leading to normal B-cell development, activation, and immune responses including class-switch recombination, germinal center formation, and somatic hypermutation. In addition, the LMP1-signaling domain has a unique property in that it can induce class-switch recombination to IgG1 independent of cytokines. Thus, our data indicate that LMP1 has evolved to imitate T-helper cell function allowing activation, proliferation, and differentiation of EBV-infected B cells independent of T cells.

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The immunoglobulin heavy-chain locus hs3b and hs4 3′ enhancers are dispensable for VDJ assembly and somatic hypermutation

Blood ◽

10.1182/blood-2002-12-3827 ◽

2003 ◽

Vol 102 (4) ◽

pp. 1421-1427 ◽

Cited By ~ 39

Author(s):

Caroline Le Morvan ◽

Eric Pinaud ◽

Catherine Decourt ◽

Armelle Cuvillier ◽

Michel Cogné

Keyword(s):

B Cells ◽

B Cell ◽

Heavy Chain ◽

Somatic Hypermutation ◽

Class Switch Recombination ◽

Immunoglobulin Heavy Chain ◽

Germinal Centers ◽

Affinity Maturation ◽

Class Switch ◽

Endogenous Locus

Abstract The more distal enhancers of the immunoglobulin heavy-chain 3′ regulatory region, hs3b and hs4, were recently demonstrated as master control elements of germline transcription and class switch recombination to most immunoglobulin constant genes. In addition, they were shown to enhance the accumulation of somatic mutations on linked transgenes. Since somatic hypermutation and class switch recombination are tightly linked processes, their common dependency on the endogenous locus 3′ enhancers could be an attractive hypothesis. VDJ structure and somatic hypermutation were analyzed in B cells from mice carrying either a heterozygous or a homozygous deletion of these enhancers. We find that hs3b and hs4 are dispensable both for VDJ assembly and for the occurrence of mutations at a physiologic frequency in the endogenous locus. In addition, we show that cells functionally expressing the immunoglobulin M (IgM) class B-cell receptor encoded by an hs3b/hs4-deficient locus were fully able to enter germinal centers, undergo affinity maturation, and yield specific antibody responses in homozygous mutant mice, where IgG1 antibodies compensated for the defect in other IgG isotypes. By contrast, analysis of Peyer patches from heterozygous animals showed that peanut agglutinin (PNAhigh) B cells functionally expressing the hs3b/hs4-deficient allele were dramatically outclassed by B cells expressing the wild-type locus and normally switching to IgA. This study thus also highlights the role of germinal centers in the competition between B cells for affinity maturation and suggests that membrane IgA may promote recruitment in an activated B-cell compartment, or proliferation of activated B cells, more efficiently than IgM in Peyer patches.

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MicroRNA-directed pathway discovery elucidates an miR-221/222–mediated regulatory circuit in class switch recombination

Journal of Experimental Medicine ◽

10.1084/jem.20201422 ◽

2021 ◽

Vol 218 (11) ◽

Author(s):

Eric J. Wigton ◽

Yohei Mikami ◽

Ryan J. McMonigle ◽

Carlos A. Castellanos ◽

Adam K. Wade-Vallance ◽

...

Keyword(s):

B Cells ◽

B Cell ◽

Cell Fate ◽

Cell Activation ◽

Class Switch Recombination ◽

Cell Fate Decisions ◽

Class Switch ◽

Mrna Targets ◽

Regulatory Circuit ◽

Pathway Discovery

MicroRNAs (miRNAs, miRs) regulate cell fate decisions by post-transcriptionally tuning networks of mRNA targets. We used miRNA-directed pathway discovery to reveal a regulatory circuit that influences Ig class switch recombination (CSR). We developed a system to deplete mature, activated B cells of miRNAs, and performed a rescue screen that identified the miR-221/222 family as a positive regulator of CSR. Endogenous miR-221/222 regulated B cell CSR to IgE and IgG1 in vitro, and miR-221/222–deficient mice exhibited defective IgE production in allergic airway challenge and polyclonal B cell activation models in vivo. We combined comparative Ago2-HITS-CLIP and gene expression analyses to identify mRNAs bound and regulated by miR-221/222 in primary B cells. Interrogation of these putative direct targets uncovered functionally relevant downstream genes. Genetic depletion or pharmacological inhibition of Foxp1 and Arid1a confirmed their roles as key modulators of CSR to IgE and IgG1.

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Clinical, Immunological, and Functional Characterization of Six Patients with Very High IgM Levels

Journal of Clinical Medicine ◽

10.3390/jcm9030818 ◽

2020 ◽

Vol 9 (3) ◽

pp. 818 ◽

Cited By ~ 1

Author(s):

Vera Gallo ◽

Emilia Cirillo ◽

Rosaria Prencipe ◽

Alessio Lepore ◽

Luigi Del Vecchio ◽

...

Keyword(s):

B Cell ◽

Somatic Hypermutation ◽

Functional Characterization ◽

Class Switch Recombination ◽

Pathogenic Mutation ◽

Potential Candidate ◽

Class Switch ◽

Potential Candidate Gene ◽

Very High

Very high IgM levels represent the hallmark of hyper IgM (HIGM) syndromes, a group of primary immunodeficiencies (PIDs) characterized by susceptibility to infections and malignancies. Other PIDs not fulfilling the diagnostic criteria for HIGM syndromes can also be characterized by high IgM levels and susceptibility to malignancies. The aim of this study is to characterize clinical phenotype, immune impairment, and pathogenic mechanism in six patients with very high IgM levels in whom classical HIGM syndromes were ruled out. The immunological analysis included extended B-cell immunophenotyping, evaluation of class switch recombination and somatic hypermutation, and next generation sequencing (NGS). Recurrent or severe infections and chronic lung changes at the diagnosis were reported in five out of six and two out of six patients, respectively. Five out of six patients showed signs of lymphoproliferation and four patients developed malignancies. Four patients showed impaired B-cell homeostasis. Class switch recombination was functional in vivo in all patients. NGS revealed, in one case, a pathogenic mutation in PIK3R1. In a second case, the ITPKB gene, implicated in B- and T-cell development, survival, and activity was identified as a potential candidate gene. Independent of the genetic basis, very high IgM levels represent a risk factor for the development of recurrent infections leading to chronic lung changes, lymphoproliferation, and high risk of malignancies.

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Fanca deficiency reduces A/T transitions in somatic hypermutation and alters class switch recombination junctions in mouse B cells

Journal of Experimental Medicine ◽

10.1084/jem.20131637 ◽

2014 ◽

Vol 211 (6) ◽

pp. 1011-1018 ◽

Cited By ~ 17

Author(s):

Thuy Vy Nguyen ◽

Lydia Riou ◽

Saïd Aoufouchi ◽

Filippo Rosselli

Keyword(s):

B Cells ◽

Somatic Hypermutation ◽

Bone Marrow Failure ◽

Genetic Disorder ◽

Point Mutations ◽

Class Switch Recombination ◽

Loss Of Function ◽

Nhej Pathway ◽

Class Switch ◽

Increased Risk

Fanconi anemia is a rare genetic disorder that can lead to bone marrow failure, congenital abnormalities, and increased risk for leukemia and cancer. Cells with loss-of-function mutations in the FANC pathway are characterized by chromosome fragility, altered mutability, and abnormal regulation of the nonhomologous end-joining (NHEJ) pathway. Somatic hypermutation (SHM) and immunoglobulin (Ig) class switch recombination (CSR) enable B cells to produce high-affinity antibodies of various isotypes. Both processes are initiated after the generation of dG:dU mismatches by activation-induced cytidine deaminase. Whereas SHM involves an error-prone repair process that introduces novel point mutations into the Ig gene, the mismatches generated during CSR are processed to create double-stranded breaks (DSBs) in DNA, which are then repaired by the NHEJ pathway. As several lines of evidence suggest a possible role for the FANC pathway in SHM and CSR, we analyzed both processes in B cells derived from Fanca−/− mice. Here we show that Fanca is required for the induction of transition mutations at A/T residues during SHM and that despite globally normal CSR function in splenic B cells, Fanca is required during CSR to stabilize duplexes between pairs of short microhomology regions, thereby impeding short-range recombination downstream of DSB formation.

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AID from bony fish catalyzes class switch recombination

Journal of Experimental Medicine ◽

10.1084/jem.20051378 ◽

2005 ◽

Vol 202 (6) ◽

pp. 733-738 ◽

Cited By ~ 78

Author(s):

Vasco M. Barreto ◽

Qiang Pan-Hammarstrom ◽

Yaofeng Zhao ◽

Lennart Hammarstrom ◽

Ziva Misulovin ◽

...

Keyword(s):

B Cells ◽

Catalytic Domain ◽

Somatic Hypermutation ◽

Cytidine Deaminase ◽

Class Switch Recombination ◽

Adaptive Immune System ◽

Bony Fish ◽

Class Switching ◽

Class Switch ◽

Switch Regions

Class switch recombination was the last of the lymphocyte-specific DNA modification reactions to appear in the evolution of the adaptive immune system. It is absent in cartilaginous and bony fish, and it is common to all tetrapods. Class switching is initiated by activation-induced cytidine deaminase (AID), an enzyme expressed in cartilaginous and bony fish that is also required for somatic hypermutation. Fish AID differs from orthologs found in tetrapods in several respects, including its catalytic domain and carboxy-terminal region, both of which are essential for the switching reaction. To determine whether evolution of class switch recombination required alterations in AID, we assayed AID from Japanese puffer and zebra fish for class-switching activity in mouse B cells. We find that fish AID catalyzes class switch recombination in mammalian B cells. Thus, AID had the potential to catalyze this reaction before the teleost and tetrapod lineages diverged, suggesting that the later appearance of a class-switching reaction was dependent on the evolution of switch regions and multiple constant regions in the IgH locus.

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The mTOR-Bach2 Cascade Controls Cell Cycle and Class Switch Recombination during B Cell Differentiation

Molecular and Cellular Biology ◽

10.1128/mcb.00418-17 ◽

2017 ◽

Vol 37 (24) ◽

Cited By ~ 12

Author(s):

Toru Tamahara ◽

Kyoko Ochiai ◽

Akihiko Muto ◽

Yukinari Kato ◽

Nicolas Sax ◽

...

Keyword(s):

Cell Cycle ◽

Cell Differentiation ◽

B Cells ◽

B Cell ◽

Molecular Mechanisms ◽

Class Switch Recombination ◽

Cyclin D3 ◽

Immunoglobulin Gene ◽

B Cell Differentiation ◽

Class Switch

ABSTRACT The transcription factor Bach2 regulates both acquired and innate immunity at multiple steps, including antibody class switching and regulatory T cell development in activated B and T cells, respectively. However, little is known about the molecular mechanisms of Bach2 regulation in response to signaling of cytokines and antigen. We show here that mammalian target of rapamycin (mTOR) controls Bach2 along B cell differentiation with two distinct mechanisms in pre-B cells. First, mTOR complex 1 (mTORC1) inhibited accumulation of Bach2 protein in nuclei and reduced its stability. Second, mTOR complex 2 (mTORC2) inhibited FoxO1 to reduce Bach2 mRNA expression. Using expression profiling and chromatin immunoprecipitation assay, the Ccnd3 gene, encoding cyclin D3, was identified as a new direct target of Bach2. A proper cell cycle was lost at pre-B and mature B cell stages in Bach2-deficient mice. Furthermore, AZD8055, an mTOR inhibitor, increased class switch recombination in wild-type mature B cells but not in Bach2-deficient cells. These results suggest that the mTOR-Bach2 cascade regulates proper cell cycle arrest in B cells as well as immunoglobulin gene rearrangement.

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