The immunoglobulin heavy chain super enhancer controls class switch recombination in developing B cells

Class switch recombination (CSR) plays an important role in adaptive immune response by enabling mature B cells to replace the initial IgM by another antibody class (IgG, IgE or IgA). CSR is preceded by transcription of the IgH constant genes and is controlled by the super-enhancer 3' regulatory region (3'RR) in an activation-specific manner. The 3'RR is composed of four enhancers (hs3a, hs1-2, hs3b and hs4). In mature B cells, 3'RR activity correlates with transcription of its enhancers. CSR can also occur in primary developing B cells though at low frequency, but in contrast to mature B cells, the transcriptional elements that regulate the process in developing B cells are ill-known. In particular, the role of the 3'RR in the control of constant genes' transcription and CSR has not been addressed. Here, by using a mouse line devoid of the 3'RR and a culture system that highly enriches in pro-B cells, we show that the 3'RR activity is indeed required for switch transcription and CSR, though its effect varies in an isotype-specific manner and correlates with transcription of hs4 enhancer only.

The Autoimmune Manifestations in Patients with Genetic Defects in the B Cell Development and Differentiation Stages

PURPOSE

Primary B cell defects manifesting as predominantly antibody deficiencies result from variable inborn errors of the B cell lineage and their development, including impairments in early bone marrow development, class switch recombination (CSR), or terminal B cell differentiation. In this study, we aimed to investigate autoimmunity in monogenic patients with B cell development and differentiation defects.

METHODS

Patients with known genetic defects in the B cell development and differentiation were recruited from the Iranian inborn errors of immunity registry.

RESULTS

A total of 393 patients with a known genetic defect in the B cell development and differentiation (257 males; 65.4%) with a median age of 12 (6-20) years were enrolled in this study. After categorizing patients, 109 patients had intrinsic B cell defects. More than half of the patients had defects in one of the ATM (85 patients), BTK (76 patients), LRBA (34 patients), and DOCK8 (33 patients) genes. Fifteen patients (3.8%) showed autoimmune complications as their first manifestation. During the course of the disease, autoimmunity was reported in 81 (20.6%) patients at a median age of 4 (2-7) years, among which 65 patients had mixed intrinsic and extrinsic and 16 had intrinsic B cell defects. The comparison between patients with the mentioned four main gene defects showed that the patient group with LRBA defect had a significantly higher frequency of autoimmunity compared to those with other gene defects. Based on the B cell defect stage, 13% of patients with early B cell defect, 17% of patients with CSR defect, and 40% of patients who had terminal B cell defect presented at least one type of autoimmunity.

CONCLUSION

Our results demonstrated that gene mutations involved in human B cell terminal stage development mainly LRBA gene defect have the highest association with autoimmunity.

Hyper IgM in tricho-hepato-enteric syndrome due to TTC37 mutation

Tricho-hepato-enteric syndrome (THES) (OMIM #222,470) is a rare autosomal recessive syndromic enteropathy whose primary manifestations are dysmorphism, intractable diarrhea, failure to thrive, hair abnormalities, liver disease, and immunodeficiency with low serum IgG concentrations. THES is caused by mutations of either Tetratricopeptide Repeat Domain 37 (TTC37) or Ski2 like RNA Helicase (SKIV2L), genes that encode two components of the human SKI complex. Here, we report a patient with a TTC37 homozygous mutation phenotypically typical for tricho-hepato-enteric syndrome in whom extremely elevated IgM with low IgG was present at the time of diagnosis. These manifestations were not previously described in THES patients and this raised our index of suspicion towards "atypical" hyper IgM syndrome. Although the pathogenesis of immunoglobulin production dysfunction in THES is still elusive, this disorder should be considered in the differential diagnosis in patients with elevated IgM and syndromic features.

Staphylococcus aureus peptidoglycan (PGN) induces pathogenic autoantibody production via autoreactive B cell receptor clonal selection, implications in systemic lupus erythematosus

OBJECTIVES

There is an intricate interplay between the microbiome and the immune response impacting development of normal immunity and autoimmunity. However, we do not fully understand how the microbiome affects production of natural-like and pathogenic autoantibodies. Peptidoglycan (PGN) is a component of the bacterial cell wall which is highly antigenic. PGNs from different bacteria can differ in their immune regulatory activities.

METHODS

C57BL/6 and MRL/lpr mice were intraperitoneally injected with saline or PGN from Staphylococcus aureus or Bacillus subtilis. Spleen anti-double-stranded DNA (dsDNA) IgG + B cells were sorted for B-cell receptor sequencing. Serum autoantibody levels and kidney damage were analyzed. Further, the association between plasma S. aureus translocation and systemic lupus erythematosus (SLE) pathogenesis was assessed in women.

RESULTS

Administration of B. subtilis PGN induced natural-like anti-dsDNA autoantibodies (e.g., IgM, short lived IgG response, and no tissue damage), whereas S. aureus PGN induced pathogenic anti-dsDNA autoantibodies (e.g., prolonged IgG production, low IgM, autoantibody-mediated kidney damage) in C57BL/6 and/or MRL/lpr mice. However, serum total IgG did not differ. S. aureus PGN induced antibodies with reduced clonality and greater hypermutation of IGHV3-74 in splenic anti-dsDNA IgG + B cells from C57BL/6 mice. Further, S. aureus PGN promoted IgG class switch recombination via toll-like receptor 2. Plasma S. aureus DNA levels were increased in women with SLE versus control women and correlated with levels of lupus-related autoantibodies and renal involvement.

CONCLUSIONS

S. aureus PGN induces pathogenic autoantibody production, whereas B. subtilis PGN drives production of natural nonpathogenic autoantibodies.

A novel digital PCR-based method to quantify (switched) B cells reveals the extent of allelic involvement in different recombination processes in the IGH locus

B cells fulfill an important role in the adaptive immunity. Upon activation and immunoglobulin (IG) class switching, these cells function in the humoral immunity compartment as plasma cells. For clinical applications, it can be important to quantify (switched) B cells accurately in a variety of body fluids and tissues of benign, inflammatory and malignant origin. For decades, flow cytometry and immunohistochemistry (IHC) have been the preferred methods for quantification. Although these methods are widely used, both depend on the accessibility of B cell epitopes and therefore require intact (fixed) cells. Whenever samples are low in quantity and/or quality, accurate quantification can be difficult. By shifting the focus from epitopes to DNA markers, quantification of B cells remains achievable. During differentiation and maturation, B cells are subjected to programmed genetic recombination processes like VDJ rearrangements and class switch recombination (CSR), which result in deletion of specific sequences of the IGH locus. These cell type-specific DNA "scars" (loss of sequences) in IG genes can be exploited as B cell markers in digital PCR (dPCR) based quantification methods. Here, we describe a novel, specific and sensitive digital PCR-based method to quantify mature and switched B cells in DNA specimens of benign and (copy number unstable) malignant origin. We compared this novel way of B cell quantitation with flow cytometric and immunohistochemical methods. Through cross-validation with flow cytometric sorted B cell subpopulations, we gained quantitative insights into allelic involvement in different recombination processes in the IGH locus. Our newly developed method is accurate and independent of the cellular context, offering new possibilities for quantification, even for (limited) small samples like liquid biopsies.

Identification of RNA-DNA Hybrids Associated with R-Loops at the IgH Switch Sequence in Activated B Cells

During transcription and replication, R-loops that contain RNA-DNA hybrids are generated across numerous genomic loci and contribute to many biological events. Using S9.6, a monoclonal antibody against RNA-DNA hybrids, accelerated the study of R-loop biology. An outpouring of recent studies has implicated various contributions of R-loop in physiological cellular functions. Earlier studies using nondenaturing sodium bisulfite probing also supported existence of DNA-RNA hybrids formation in mammalian cells. In activated B cells, RNA-DNA hybrids formation at IgH gene locus of B cells is crucial for class switch recombination that ensure the proper effector function of the antibody. Here, we describe the identification of R-loops associated with the IgH locus using RNA-DNA hybrid immunoprecipitation sequencing and nondenaturing sodium bisulfite probing. This will be helpful for future studies of R-loops status on whole genome as well as on IgH locus in B cells.

Context-dependent regulation of immunoglobulin mutagenesis by p53

p53 plays a major role in genome maintenance. In addition to multiple p53 functions in the control of DNA repair, a regulation of DNA damage bypass via translesion synthesis has been implied in vitro. Somatic hypermutation of immunoglobulin genes for affinity maturation of antibody responses is based on aberrant translesion polymerase action and must be subject to stringent control to prevent genetic alterations and lymphomagenesis. When studying the role of p53 in somatic hypermutation in vivo, we found altered translesion polymerase-mediated A:T mutagenesis in mice lacking p53 in all organs, but notably not in mice with B cell-specific p53 inactivation, implying that p53 functions in non-B cells may alter mutagenesis in B cells. During class switch recombination, when p53 prevents formation of chromosomal translocations, we in addition detected a B cell-intrinsic role for p53 in altering G:C and A:T mutagenesis. Thus, p53 regulates translesion polymerase activity and shows differential activity during somatic hypermutation versus class switch recombination in vivo. Finally, p53 inhibition leads to increased somatic hypermutation in human B lymphoma cells. We conclude that loss of p53 function may promote genetic instability via multiple routes during antibody diversification in vivo.

Human Germinal Center Reaction: Assessing T Follicular Helper-B Cells Interaction In Vitro

Antibody responses deeply rely on the interaction of antigen-primed B cells and CD4 helper T cells in the context of germinal center reactions, through signals provided by costimulatory molecules and cytokines. B-cell proliferation and differentiation in antibody-secreting plasma cells are processes that critically depend on the helper function of a specific CD4 T-cell subset, known as follicular helper T cells (Tfh). Here, we describe a method that mimics in vitro the cross talk between Tfh and B cells occurring in the germinal center. The procedure is based on setting up a coculture system with B cells and Tfh isolated from blood of healthy donors, or tonsils removed upon surgical intervention, in order to recapitulate in vitro the Tfh-dependent mechanisms leading to B cells' activation, proliferation, and differentiation.

Repair of programmed DNA lesions in antibody class switch recombination: common and unique features

The adaptive immune system can diversify the antigen receptors to eliminate various pathogens through programmed DNA lesions at antigen receptor genes. In immune diversification, general DNA repair machineries are applied to transform the programmed DNA lesions into gene mutation or recombination events with common and unique features. Here we focus on antibody class switch recombination (CSR), and review the initiation of base damages, the conversion of damaged base to DNA double-strand break, and the ligation of broken ends. With an emphasis on the unique features in CSR, we discuss recent advances in the understanding of DNA repair/replication coordination, and ERCC6L2-mediated deletional recombination. We further elaborate the application of CSR in end-joining, resection and translesion synthesis assays. In the time of the COVID-19 pandemic, we hope it help to understand the generation of therapeutic antibodies.

Impact of HIV-1 Vpr manipulation of the DNA repair enzyme UNG2 on B lymphocyte class switch recombination

Background

HIV-1 Vpr encodes a 14 kDa protein that has been implicated in viral pathogenesis through modulation of several host cell functions. In addition to pro-apoptotic and cytostatic properties, Vpr can redirect cellular E3 ubiquitin ligases (such as DCAF1-Cul4A E3 ligase complex) to target many host proteins and interfere with their functions. Among them, Vpr binds the uracil DNA glycosylase UNG2, which controls genome uracilation, and induces its specific degradation leading to loss of uracil removal activity in infected cells. Considering the essential role of UNG2 in antibody diversification in B-cells, we evaluated the impact of Vpr on UNG2 fate in B lymphocytes and examined the functional consequences of UNG2 modulations on class switch recombination (CSR).

Methods

The impact of Vpr-induced UNG2 deregulation on CSR proficiency was evaluated by using virus-like particles able to deliver Vpr protein to target cells including the murine model CSR B cell line CH12F3 and mouse primary B-cells. Co-culture experiments were used to re-examine the ability of Vpr to be released by HIV-1 infected cells and to effectively accumulate in bystander B-cells. Vpr-mediated UNG2 modulations were monitored by following UNG2 protein abundance and uracil removal enzymatic activity.

Results

In this study we report the ability of Vpr to reduce immunoglobulin class switch recombination (CSR) in immortalized and primary mouse B-cells through the degradation of UNG2. We also emphasize that Vpr is released by producing cells and penetrates bystander B lymphocytes.

Conclusions

This work therefore opens up new perspectives to study alterations of the B-cell response by using Vpr as a specific CSR blocking tool. Moreover, our results raise the question of whether extracellular HIV-1 Vpr detected in some patients may manipulate the antibody diversification process that engineers an adapted response against pathogenic intruders and thereby contribute to the intrinsic B-cell humoral defect reported in infected patients.

Mechanism and regulation of class switch recombination by IgH transcriptional control elements

Class switch recombination (CSR) plays an important role in humoral immunity by generating antibodies with different effector functions. CSR to a particular antibody isotype is induced by external stimuli, and occurs between highly repetitive switch (S) sequences. CSR requires transcription across S regions, which generates long non-coding RNAs and secondary structures that promote accessibility of S sequences to activation-induced cytidine deaminase (AID). AID initiates DNA double-strand breaks (DSBs) intermediates that are repaired by general DNA repair pathways. Switch transcription is controlled by various regulatory elements, including enhancers and insulators. The current paradigm posits that transcriptional control of CSR involves long-range chromatin interactions between regulatory elements and chromatin loops-stabilizing factors, which promote alignment of partner S regions in a CSR centre (CSRC) and initiation of CSR. In this review, we focus on the role of IgH transcriptional control elements in CSR and the chromatin-based mechanisms underlying this control.

The recent advances in non-homologous end-joining through the lens of lymphocyte development

Lymphocyte development requires ordered assembly and subsequent modifications of the antigen receptor genes through V(D)J recombination and Immunoglobulin class switch recombination (CSR), respectively. While the programmed DNA cleavage events are initiated by lymphocyte-specific factors, the resulting DNA double-strand break (DSB) intermediates activate the ATM kinase-mediated DNA damage response (DDR) and rely on the ubiquitously expressed classical non-homologous end-joining (cNHEJ) pathway including the DNA-dependent protein kinase (DNA-PK), and, in the case of CSR, also the alternative end-joining (Alt-EJ) pathway, for repair. Correspondingly, patients and animal models with cNHEJ or DDR defects develop distinct types of immunodeficiency reflecting their specific DNA repair deficiency. The unique end-structure, sequence context, and cell cycle regulation of V(D)J recombination and CSR also provide a valuable platform to study the mechanisms of, and the interplay between, cNHEJ and DDR. Here, we compare and contrast the genetic consequences of DNA repair defects in V(D)J recombination and CSR with a focus on the newly discovered cNHEJ factors and the kinase-dependent structural roles of ATM and DNA-PK in animal models. Throughout, we try to highlight the pending questions and emerging differences that will extend our understanding of cNHEJ and DDR in the context of primary immunodeficiency and lymphoid malignancies.

YY1 control of mitochondrial-related genes does not account for regulation of immunoglobulin class switch recombination in mice

Immunoglobulin class switch recombination (CSR) occurs in activated B cells with increased mitochondrial mass and membrane potential. Transcription factor Yin Yang 1 (YY1) is critical for CSR and for formation of the DNA loops involved in this process. We therefore sought to determine if YY1 knockout impacts mitochondrial gene expression and mitochondrial function in murine splenic B cells, providing a potential mechanism for regulating CSR. We identified numerous genes in splenic B cells differentially regulated when cells are induced to undergo CSR. YY1 conditional knockout caused differential expression of 1129 genes, with 59 being mitochondrial-related genes. ChIP-seq analyses showed YY1 was directly bound to nearly half of these mitochondrial-related genes. Surprisingly, at the time when YY1 knockout dramatically reduces DNA loop formation and CSR, mitochondrial mass and membrane potential were not significantly impacted, nor was there a significant change in mitochondrial oxygen consumption, extracellular acidification rate, or mitochondrial complex I or IV activities. Our results indicate that YY1 regulates numerous mitochondrial-related genes in splenic B cells, but this does not account for the impact of YY1 on CSR or long-distance DNA loop formation.

Effects of senataxin and RNA exosome on B-cell chromosomal integrity

Loss of function of senataxin (SETX), a bona-fide RNA/DNA helicase, is associated with neuronal degeneration leading to Ataxia and Ocular Apraxia (AOA) in human patients. SETX is proposed to promote transcription termination, DNA replication, DNA repair, and to unwind deleterious RNA:DNA hybrids in the genome. In all the above-mentioned mechanisms, SETX unwinds transcription complex-associated nascent RNA which is then degraded by the RNA exosome complex. Here we have used B cells isolated from a SETX mutant mouse model and compared genomic instability and immunoglobulin heavy chain locus (IgH) class switch recombination (CSR) to evaluate aberrant and programmed genomic rearrangements, respectively. Similar to RNA exosome mutant primary B cells, SETX mutant primary B cells display genomic instability but a modest decrease in efficiency of CSR. Furthermore, knockdown of Setx mRNAs from CH12–F3 B-cell lines leads to a defect in IgA CSR and accumulation of aberrant patterns of mutations in IgH switch sequences. Given that SETX mutant mice do not recapitulate the AOA neurodegenerative phenotype, it is possible that some aspects of SETX biology are rescued by redundant helicases in mice. Overall, our study provides new insights into the role of the SETX/RNA exosome axis in suppressing genomic instability so that programmed DNA breaks are properly orchestrated.

B cells from young and old mice switch isotypes with equal frequencies after ex vivo stimulation

To determine whether old B cells have the same capacity to switch isotypes as young cells, we purified splenic follicular, marginal zone, and age-associated B cell subsets from C57BL/6 mice. Cells were stimulated in culture with interleukin 4 and either lipopolysaccharide or anti-CD40, and switching to IgG1 was measured by flow cytometry of surface immunoglobulin. The results show that switching was robust in follicular and marginal zone B cells from old mice and was comparable to their young counterparts. However, age-associated B cells from old mice switched poorly relative to the other subsets. Expression of activation-induced deaminase, which initiates switching, was quantified by qPCR of mRNA, and it was equal between young and old follicular B cells. Thus, in this ex vivo system, the follicular and marginal zone cells from young and old mice behaved similarly, showing that the molecular machinery to perform switching is intact in old B cells.

Clinical Manifestations, Immunological Characteristics and Genetic Analysis of Patients with Hyper-Immunoglobulin M Syndrome in Iran

BACKGROUND

Hyper-immunoglobulin M (HIGM) syndrome is a rare heterogeneous group of primary immunodeficiency disorders characterized by low or absent serum levels of IgG and IgA along with normal or elevated serum levels of IgM.

METHODS

Clinical and immunological data were collected from the 75 patients' medical records diagnosed in Children's Medical Center affiliated to Tehran University Medical Sciences and other Universities of Medical Sciences in Iran. Among 75 selected patients, 48 patients (64%) were analyzed genetically using targeted and whole-exome sequencing.

RESULTS

The ratio of male to female was 2.9:1. The median age at the onset of the disease, time of diagnosis, and diagnostic delay were 10.5, 50, and 24 months, respectively. Pneumonia and lower respiratory tract infections (61.3%) were the most common complications. Responsible genes were identified in 35 patients (72.9%) out 48 genetically analyzed patients. Cluster of differentiation 40 ligand gene was the most mutated gene observed in 24 patients (68.5%) followed by activation-induced cytidine deaminase gene in 7 patients, lipopolysaccharide-responsive and beige-like anchor (1 patient), nuclear factor-kappa-B essential modulator (1 patient), phosphoinositide-3-kinase regulatory subunit 1 (1 patient), and nuclear factor kappa B subunit 1 (1 patient) genes. Nineteen (25.3%) patients died during the study period, and pneumonia was the major cause of death occurred in 6 (31.6%) patients.

CONCLUSION

Physicians in our country should carefully pay attention to respiratory tract infections and pneumonia, particularly in patients with a positive family history. Further investigations are required for detection of new genes and pathways resulting in HIGM phenotype.

Light-chain plasma cell myeloma caused by 14q32/IGH translocation and loss of the other allele

Light-chain plasma cell myeloma (LC-PCM) is a PCM subtype in which only immunoglobulin light-chain is secreted. However, the absence of immunoglobulin heavy-chain (IGH) production in this condition has not been fully elucidated. To address this issue, we retrospectively analyzed patients at our center with LC-PCM and found a group who had only split signals of IGH gene derived from 14q32/IGH translocations by fluorescence in situ hybridization (FISH). Six patients were identified with only split signals of the IGH gene derived from 14q32/IGH translocations. Five of these patients were newly diagnosed, while one had IgG-λ PCM at presentation, which transformed to λ LC-PCM after treatment. The translocation partners were identified in four patients: two cases of (11;14)(q13;q32) and two cases of (4;14)(p16;q32). The development of LC-PCM appears to be explained by the application of allelic exclusion in these patients, such that 14q32/IGH translocation in one allele contributes to the pathogenesis of PCM and the subsequent loss of the other allele is responsible for the loss of IGH production. These findings suggest that a FISH pattern of IGH with "split and loss" may constitute a unique subgroup of LC-PCM.

Cis- and trans-factors affecting AID targeting and mutagenic outcomes in antibody diversification

Antigen receptor diversification is a hallmark of adaptive immunity which allows specificity of the receptor to particular antigen. B cell receptor (BCR) or its secreted form, antibody, is diversified through antigen-independent and antigen-dependent mechanisms. During B cell development in bone marrow, BCR is diversified via V(D)J recombination mediated by RAG endonuclease. Upon stimulation by antigen, B cell undergo somatic hypermutation (SHM) to allow affinity maturation and class switch recombination (CSR) to change the effector function of the antibody. Both SHM and CSR are initiated by activation-induced cytidine deaminase (AID). Repair of AID-initiated lesions through different DNA repair pathways results in diverse mutagenic outcomes. Here, we focus on discussing cis- and trans-factors that target AID to its substrates and factors that affect different outcomes of AID-initiated lesions. The knowledge of mechanisms that govern AID targeting and outcomes could be harnessed to elicit rare functional antibodies and develop ex vivo antibody diversification approaches with diversifying base editors.

Signaling control of antibody isotype switching

Class switch recombination (CSR) generates isotype-switched antibodies with distinct effector functions essential for mediating effective humoral immunity. CSR is catalyzed by activation-induced deaminase (AID) that initiates DNA lesions in the evolutionarily conserved switch (S) regions at the immunoglobulin heavy chain (Igh) locus. AID-initiated DNA lesions are subsequently converted into DNA double stranded breaks (DSBs) in the S regions of Igh locus, repaired by non-homologous end-joining to effect CSR in mammalian B lymphocytes. While molecular mechanisms of CSR are well characterized, it remains less well understood how upstream signaling pathways regulate AID expression and CSR. B lymphocytes express multiple receptors including the B cell antigen receptor (BCR) and co-receptors (e.g., CD40). These receptors may share common signaling pathways or may use distinct signaling elements to regulate CSR. Here, we discuss how signals emanating from different receptors positively or negatively regulate AID expression and CSR.

The Role for the DSB Response Pathway in Regulating Chromosome Translocations

In response to DNA double strand breaks (DSB), mammalian cells activate the DNA Damage Response (DDR), a network of factors that coordinate their detection, signaling and repair. Central to this network is the ATM kinase and its substrates at chromatin surrounding DSBs H2AX, MDC1 and 53BP1. In humans, germline inactivation of ATM causes Ataxia Telangiectasia (A-T), an autosomal recessive syndrome of increased proneness to hematological malignancies driven by clonal chromosomal translocations. Studies of cancers arising in A-T patients and in genetically engineered mouse models (GEMM) deficient for ATM and its substrates have revealed complex, multilayered roles for ATM in translocation suppression and identified functional redundancies between ATM and its substrates in this context. "Programmed" DSBs at antigen receptor loci in developing lymphocytes employ ubiquitous DDR factors for signaling and repair and have been particularly useful for mechanistic studies because they are region-specific and can be monitored in vitro and in vivo. In this context, murine thymocytes deficient for ATM recapitulate the molecular events that lead to transformation in T cells from A-T patients and provide a widely used model to study the mechanisms that suppress RAG recombinase-dependent translocations. Similarly, analyses of the fate of Activation induced Cytidine Deaminase (AID)-dependent DSBs during mature B cell Class Switch Recombination (CSR) have defined the genetic requirements for end-joining and translocation suppression in this setting. Moreover, a unique role for 53BP1 in the promotion of synapsis of distant DSBs has emerged from these studies.

The hyper IgM syndromes: Epidemiology, pathogenesis, clinical manifestations, diagnosis and management

Hyper Immunoglobulin M syndrome (HIGM) is a rare primary immunodeficiency disorder characterized by low or absent levels of serum IgG, IgA, IgE and normal or increased levels of serum IgM. Various X-linked and autosomal recessive/dominant mutations have been reported as the underlying cause of the disease. Based on the underlying genetic defect, the affected patients present a variety of clinical manifestations including pulmonary and gastrointestinal complications, autoimmune disorders, hematologic abnormalities, lymphoproliferation and malignancies which could be controlled by multiple relevant therapeutic approaches. Herein, the epidemiology, pathogenesis, clinical manifestations, diagnosis, management, prognosis and treatment in patients with HIGM syndrome have been reviewed.

ETS1 and PAX5 transcription factors recruit AID to Igh DNA

B lymphocytes optimize antibody responses by class switch recombination (CSR), which changes the expressed constant region exon of the immunoglobulin heavy chain (IgH), and by somatic hypermutation (SH) that introduces point mutations in the variable regions of the antibody genes. Activation-induced cytidine deaminase (AID) is the key mutagenic enzyme that initiates both these antibody diversification processes by deaminating cytosine to uracil. Here we asked the question if transcription factors can mediate the specific targeting of the antibody diversification by recruiting AID. We have recently reported that AID is together with the transcription factors E2A, PAX5 and IRF4 in a complex on key sequences of the Igh locus. Here we report that also ETS1 is together with AID in this complex on key sequences of the Igh locus in splenic B cells of mice. Furthermore, we show that both ETS1 and PAX5 can directly recruit AID to DNA sequences from the Igh locus with the specific binding site for the transcription factor. Taken together, our findings support the notion of a targeting mechanism for the selective diversification of antibody genes with limited genome wide mutagenesis by recruitment of AID by PAX5 and ETS1 in a transcription factor complex.

DNA/RNA hybrid substrates modulate the catalytic activity of purified AID

Activation-induced cytidine deaminase (AID) converts cytidine to uridine at Immunoglobulin (Ig) loci, initiating somatic hypermutation and class switching of antibodies. In vitro, AID acts on single stranded DNA (ssDNA), but neither double-stranded DNA (dsDNA) oligonucleotides nor RNA, and it is believed that transcription is the in vivo generator of ssDNA targeted by AID. It is also known that the Ig loci, particularly the switch (S) regions targeted by AID are rich in transcription-generated DNA/RNA hybrids. Here, we examined the binding and catalytic behavior of purified AID on DNA/RNA hybrid substrates bearing either random sequences or GC-rich sequences simulating Ig S regions. If substrates were made up of a random sequence, AID preferred substrates composed entirely of DNA over DNA/RNA hybrids. In contrast, if substrates were composed of S region sequences, AID preferred to mutate DNA/RNA hybrids over substrates composed entirely of DNA. Accordingly, AID exhibited a significantly higher affinity for binding DNA/RNA hybrid substrates composed specifically of S region sequences, than any other substrates composed of DNA. Thus, in the absence of any other cellular processes or factors, AID itself favors binding and mutating DNA/RNA hybrids composed of S region sequences. AID:DNA/RNA complex formation and supporting mutational analyses suggest that recognition of DNA/RNA hybrids is an inherent structural property of AID.

BAFF augments IgA2 and IL-10 production by TLR7/8 stimulated total peripheral blood B cells

Class‐switching of B cells to IgA can be induced via both T‐cell‐dependent and T‐cell‐independent mechanisms. IgA is most predominantly produced mucosally and is important for combating infections and allergies. In contrast to mice, humans have two forms of IgA; IgA1 and IgA2 with diverse tissue distribution. In early life, IgA levels might be sub‐optimal especially during the fall season when bacterial and viral infections are more common. Therefore, we investigated using human B cells whether T‐cell‐independent factors ‐promoting cell survival, class switching and immunoglobulin secretion‐ BAFF, APRIL, IL‐10 and retinoic acid can boost IgA production in the context of viral or bacterial infection. To this end total and naive peripheral blood B cells were stimulated with these factors for 6 days in the presence or absence of TLR7/8 agonist R848 (mimicking viral infection) or TLR9 agonist CpG‐ODN (mimicking bacterial infection). We show that BAFF significantly augments IgA2 production in TLR7/8 stimulated mature, but not naïve B cells. In addition, BAFF augments IL‐10 production and viability in TLR7/8 and TLR9 stimulated mature B cells. These data warrant further investigation of its role in immune regulation both in the periphery and mucosal tissues in early life or during disease.

Molecular analysis of immunoglobulin variable genes supports a germinal center experienced normal counterpart in primary cutaneous diffuse large B-cell lymphoma, leg-type

BACKGROUND

Immunophenotype of primary cutaneous diffuse large B-cell lymphoma, leg-type (PCLBCL-LT) suggests a germinal center-experienced B lymphocyte (BCL2+ MUM1+ BCL6+/-).

OBJECTIVES

As maturation history of B-cell is "imprinted" during B-cell development on the immunoglobulin gene sequence, we studied the structure and sequence of the variable part of the genes (IGHV, IGLV, IGKV), immunoglobulin surface expression and features of class switching in order to determine the PCLBCL-LT cell of origin.

METHODS

Clonality analysis with BIOMED2 protocol and VH leader primers was done on DNA extracted from frozen skin biopsies on retrospective samples from 14 patients. The clonal DNA IGHV sequence of the tumor was aligned and compared with the closest germline sequence and homology percentage was calculated. Superantigen binding sites were studied. Features of selection pressure were evaluated with the multinomial Lossos model.

RESULTS

A functional monoclonal sequence was observed in 14 cases as determined for IGHV (10), IGLV (2) or IGKV (3). IGV mutation rates were high (>5%) in all cases but one (median:15.5%), with superantigen binding sites conservation. Features of selection pressure were identified in 11/12 interpretable cases, more frequently negative (75%) than positive (25%). Intraclonal variation was detected in 3 of 8 tumor specimens with a low rate of mutations. Surface immunoglobulin was an IgM in 12/12 cases. FISH analysis of IGHM locus, deleted during class switching, showed heterozygous IGHM gene deletion in half of cases. The genomic PCR analysis confirmed the deletions within the switch μ region. IGV sequences were highly mutated but functional, with negative features of selection pressure suggesting one or more germinal center passage(s) with somatic hypermutation, but superantigen (SpA) binding sites conservation. Genetic features of class switch were observed, but on the non functional allele and co-existing with primary isotype IgM expression.

CONCLUSION

These data suggest that cell-of origin is germinal center experienced and superantigen driven selected B-cell, in a stage between germinal center B-cell and plasma cell.

DNA repair and systemic lupus erythematosus

Systemic lupus erythematosus (SLE) is a chronic autoimmune disease with no known cure that affects at least five million people worldwide. Monozygotic twin concordance and familial aggregation studies strongly suggest that lupus results from genetic predisposition along with environmental exposures including UV light. The majority of the common risk alleles associated with genetic predisposition to SLE map to genes associated with the immune system. However, evidence is emerging that implicates a role for aberrant DNA repair in the development of lupus. Here we summarize our current knowledge of the potential association of lupus with mutations in DNA repair genes. We also discuss how defective or aberrant DNA repair could lead to the development of lupus.

Recent advances in the study of immunodeficiency and DNA damage response

DNA breaks can be induced by exogenous stimuli or by endogenous stress, but are also generated during recombination of V, D, and J genes (V(D)J recombination), immunoglobulin class switch recombination (CSR). Among various DNA breaks generated, DNA double strand break (DSB) is the most deleterious one. DNA damage response (DDR) is initiated when DSBs are detected, leading to DNA break repair by non-homologous end joining (NHEJ). The process is critically important for the generation of diversity for foreign antigens; and failure to exert DNA repair leads to immunodeficiency such as severe combined immunodeficiency and hyper-IgM syndrome. In V(D)J recombination, DSBs are induced by RAG1/2; and generated post-cleavage hairpins are resolved by Artemis/DNA-PKcs/KU70/KU80. DDR is initiated by ataxia-telangiectasia mutated as a master regulator together with MRE11/RAD50/NBS1 complex. Finally, DSBs are repaired by NHEJ. The defect of one of the molecules shows various degree of immunodeficiency and radiosensitivity. Upon CSR inducing signal, DSBs induced by activation-induced cytidine deaminase and endonucleases elicit DDR. Broken ends are repaired either by NHEJ or by mismatch repair system. Patients with radiosensitive SCID require hematopoietic cell transplantation as a curative therapy; but the procedures for eradication of recipient hematopoietic cells are often associated with severe toxicity.

An efficient method to enrich for knock-out and knock-in cellular clones using the CRISPR/Cas9 system

Clustered Regularly Interspaced Short Palindromic Repeats-associated protein 9 nuclease (CRISPR/Cas9) and Transcription Activator-Like Effector Nucleases (TALENs) are versatile tools for genome editing. Here we report a method to increase the frequency of Cas9-targeted cellular clones. Our method is based on a chimeric construct with a Blasticidin S Resistance gene (bsr) placed out-of-frame by a surrogate target sequence. End joining of the CRISPR/Cas9-induced double-strand break on the surrogate target can place the bsr in frame, thus providing temporary resistance to Blasticidin S: this is used to enrich for cells where Cas9 is active. By this approach, in a real experimental setting, we disrupted the Aicda gene in ~70% of clones from CH12F3 lymphoma cells (>40% biallelically). With the same approach we knocked in a single nucleotide to reconstruct the frame of Aicda in these null cells, restoring the function in ~37% of the clones (less than 10% by the standard approach). Targeting of single nucleotide changes in other genes yielded analogous results. These results support our enrichment method as an efficient tool in genome editing.

Robust immunoglobulin class switch recombination and end joining in Parp9-deficient mice

To mount highly specific and adapted immune responses, B lymphocytes assemble and diversify their antibody repertoire through mechanisms involving the formation of programmed DNA damage. Immunoglobulin class switch recombination (CSR) is triggered by DNA lesions induced by activation-induced cytidine deaminase, which are processed to double-stranded DNA break (DSB) intermediates. These DSBs activate the cellular DNA damage response and enroll numerous DNA repair factors, involving poly(ADP-ribose) polymerases Parp1, Parp2, and Parp3 to promote appropriate DNA repair and efficient long-range recombination. The macroParp Parp9, which is overexpressed in certain lymphomas, has been recently implicated in DSB repair, acting together with Parp1. Here, we examine the contribution of Parp9 to the resolution of physiological DSBs incurred during V(D)J recombination and CSR by generating Parp9^-/- mice. We find that Parp9-deficient mice are viable, fertile, and do not show any overt phenotype. Moreover, we find that Parp9 is dispensable for B-cell development. Finally, we show that CSR and DNA end-joining are robust in the absence of Parp9, indicating that Parp9 is not essential in vivo to achieve physiological DSB repair, or that strong compensatory mechanisms exist.

RNA Exosome and Non-coding RNA-Coupled Mechanisms in AID-Mediated Genomic Alterations

The eukaryotic RNA exosome is a well-conserved protein complex with ribonuclease activity implicated in RNA metabolism. Various families of non-coding RNAs have been identified as substrates of the complex, underscoring its role as a non-coding RNA processing/degradation unit. However, the role of RNA exosome and its RNA processing activity on DNA mutagenesis/alteration events have not been investigated until recently. B lymphocytes use two DNA alteration mechanisms, class switch recombination (CSR) and somatic hypermutation (SHM), to re-engineer their antibody gene expressing loci until a tailored antibody gene for a specific antigen is satisfactorily generated. CSR and SHM require the essential activity of the DNA activation-induced cytidine deaminase (AID). Causing collateral damage to the B-cell genome during CSR and SHM, AID induces unwanted (and sometimes oncogenic) mutations at numerous non-immunoglobulin gene sequences. Recent studies have revealed that AID's DNA mutator activity is regulated by the RNA exosome complex, thus providing an example of a mechanism that relates DNA mutagenesis to RNA processing. Here, we review the emergent functions of RNA exosome during CSR, SHM, and other chromosomal alterations in B cells, and discuss implications relevant to mechanisms that maintain B-cell genomic integrity.

Novel AICDA mutation in a case of autosomal recessive hyper-IgM syndrome, growth hormone deficiency and autoimmunity

BACKGROUND

The Hyper-immunoglobulin M syndromes (HIGM) are a heterogeneous group of genetic disorders, which have been rarely reported to be associated with growth hormone deficiency (GHD).

METHODS AND RESULTS

A nine-year-old girl with recurrent urinary tract infections, diarrhoea, sinopulmonary infections, and failure to thrive since the age of six months had normal CD3+, CD4+, CD8+T lymphocytes, and CD19+B lymphocytes and natural killer (NK) cells, but extremely elevated IgM and significantly decreased IgG and IgA. In view of the patient's short stature, growth hormone evaluation was carried out and growth hormone deficiency established. The patient underwent Ig replacement therapy and received growth hormone therapy in addition to antibiotics and responded well. Furthermore, the patient developed benign cervical lymphadenopathy, as well as elevated erythrocyte sedimentation rate, positive autoantibodies to SSA-Ro, and severely dry eyes, which partially responded to both the punctate occlusion and systemic corticosteroids, at the age of seven years. Sequencing analysis of the exons from activation-induced cytidine deaminase (AICDA) gene revealed that the patient was homozygous for a single T to C transversion at position 455 in exon 4, which replaces a Valine with an Alanine.

CONCLUSIONS

To our knowledge, this is a new AICDA mutation, which has not been reported previously in HIGM. The mutation analysis could improve diagnosis of HIGM patients and also elaborating on the spectrum of AICDA mutations.

Effects of HDACi on Immunological Functions

Histone deacetylase inhibitors (HDACi) are used as therapeutics for several B cell-derived malignancies. Furthermore, they have been shown to modulate the response of the immune system, like the B cell function. HDACi treatment affects differentiation, proliferation, and survival of B cells. Here we describe how to investigate the effects of HDACi treatment on naïve B cells regarding class-switch recombination (CSR) in vitro using flow cytometry.

Activation induced cytidine deaminase mutant (AID-His130Pro) from Hyper IgM 2 patient retained mutagenic activity on SHM artificial substrate

Activation induced cytidine deaminase (AID) is an essential enzyme for class switch recombination (CSR) and somatic hypermutation (SHM) during secondary immune response. Mutations in the AICDA gene are responsible for Hyper IgM 2 syndrome where both CSR and SHM or only CSR are affected. Indeed, triggering either of the two mechanisms requires the DNA deamination activity of AID. Besides, different domains of AID may be differentially involved in CSR and SHM through their interaction with specific cofactors. Herein, we studied the AID-induced SHM activity of the AID-His130Pro mutant identified in a patient with Hyper IgM 2 syndrome. AID mutagenic activity was monitored by the reversion of nonsense mutations of the EGFP gene assessed by flow cytometry. We found that the His130Pro mutation, which affects CSR, preserves AID mutagenic activity. Indeed, the His130 residue is located in a putative specific CSR region in the APOBEC-like domain, known to involve CSR specific cofactors that probably play a major role in AID physiological activities.

Binding of estrogen receptors to switch sites and regulatory elements in the immunoglobulin heavy chain locus of activated B cells suggests a direct influence of estrogen on antibody expression

Females and males differ in antibody isotype expression patterns and in immune responses to foreign- and self-antigens. For example, systemic lupus erythematosus is a condition that associates with the production of isotype-skewed anti-self antibodies, and exhibits a 9:1 female:male disease ratio. To explain differences between B cell responses in males and females, we sought to identify direct interactions of the estrogen receptor (ER) with the immunoglobulin heavy chain locus. This effort was encouraged by our previous identification of estrogen response elements (ERE) in heavy chain switch (S) regions. We conducted a full-genome chromatin immunoprecipitation analysis (ChIP-seq) using DNA from LPS-activated B cells and an ERα-specific antibody. Results revealed ER binding to a wide region of DNA, spanning sequences from the JH cluster to Cδ, with peaks in Eμ and Sμ sites. Additional peaks of ERα binding were coincident with hs1,2 and hs4 sites in the 3' regulatory region (3'RR) of the heavy chain locus. This first demonstration of direct binding of ER to key regulatory elements in the immunoglobulin locus supports our hypothesis that estrogen and other nuclear hormone receptors and ligands may directly influence antibody expression and class switch recombination (CSR). Our hypothesis encourages the conduct of new experiments to evaluate the consequences of ER binding. A better understanding of ER:DNA interactions in the immunoglobulin heavy chain locus, and respective mechanisms, may ultimately translate to better control of antibody expression, better protection against pathogens, and prevention of pathologies caused by auto-immune disease.

Regulating infidelity: RNA-mediated recruitment of AID to DNA during class switch recombination

The mechanism by which the DNA deaminase activation-induced cytidine deaminase (AID) is specifically recruited to repetitive switch region DNA during class switch recombination is still poorly understood. Work over the past decade has revealed a strong link between transcription and RNA polymerase-associated factors in AID recruitment, yet none of these processes satisfactorily explain how AID specificity is affected. Here, we review a recent finding wherein AID is guided to switch regions not by a protein factor but by an RNA moiety, and especially one associated with a noncoding RNA that has been long thought of as being inert. This work explains the long-standing requirement of splicing of noncoding transcripts during class switching, and has implications in both B cell-mediated immunity as well as the underlying pathological syndromes associated with the recombination reaction.

Antibody diversification caused by disrupted mismatch repair and promiscuous DNA polymerases

The enzyme activation-induced deaminase (AID) targets the immunoglobulin loci in activated B cells and creates DNA mutations in the antigen-binding variable region and DNA breaks in the switch region through processes known, respectively, as somatic hypermutation and class switch recombination. AID deaminates cytosine to uracil in DNA to create a U:G mismatch. During somatic hypermutation, the MutSα complex binds to the mismatch, and the error-prone DNA polymerase η generates mutations at A and T bases. During class switch recombination, both MutSα and MutLα complexes bind to the mismatch, resulting in double-strand break formation and end-joining. This review is centered on the mechanisms of how the MMR pathway is commandeered by B cells to generate antibody diversity.

Novel and recurrent AID mutations underlie prevalent autosomal recessive form of HIGM in consanguineous patients

Immunoglobulin class switch recombination deficiencies (Ig-CSR-D) are characterized by normal or elevated serum IgM level and absence of IgG, IgA, and IgE. Most reported cases are due to X-linked CD40L deficiency. Activation-induced cytidine deaminase deficiency is the most frequent autosomal recessive form, whereas CD40 deficiency is more rare. Herein, we present the first North African study on hyper IgM (HIGM) syndrome including 16 Tunisian patients. Phenotypic and genetic studies allowed us to determine their molecular basis. Three CD40LG mutations have been identified including two novels (c.348_351dup and c.782_*2del) and one already reported mutation (g.6182G>A). No mutation has been found in another patient despite the lack of CD40L expression. Interestingly, three AICDA mutations have been identified in 11 patients. Two mutations were novel (c.91T>C and c.389A>C found in one and five patients respectively), and one previously reported splicing mutation (c.156+1T>G) was found in five patients. Only one CD40-deficient patient, bearing a novel mutation (c.109T>G), has been identified. Thus, unlike previous reports, AID deficiency is the most frequent underlying molecular basis (68%) of Ig-CSR-D in Tunisian patients. This finding and the presence of specific recurrent mutations are probably due to the critical role played by inbreeding in North African populations.

High affinity IgM(+) memory B cells are generated through a germinal center-dependent pathway

During a T cell-dependent immune response, B cells undergo clonal expansion and selection and the induction of isotype switching and somatic hypermutation (SHM). Although somatically mutated IgM(+) memory B cells have been reported, it has not been established whether they are really high affinity B cells. We tracked (4-hydroxy-3-nitrophenyl) acetyl hapten-specific GC B cells from normal immunized mice based on affinity of their B cell receptor (BCR) and performed BCR sequence analysis. SHM was evident by day 7 postimmunization and increased with time, such that high affinity IgM(+) as well as IgG(+) memory B cells continued to be generated up to day 42. In contrast, class-switch recombination (CSR) was almost completed by day 7 and then the ratio of IgG1(+)/IgM(+) GC B cells remained unchanged. Together these findings suggest that IgM(+) B cells undergo SHM in the GC to generate high affinity IgM(+) memory cells and that this process continues even after CSR is accomplished.

HSP90 inhibitors decrease AID levels and activity in mice and in human cells

Activation induced deaminase (AID) initiates somatic hypermutation and class switch recombination of the Ig genes in antigen-activated B cells, underpinning antibody affinity maturation and isotype switching. AID can also be pathogenic by contributing to autoimmune diseases and oncogenic mutations. Moreover, AID can exert noncanonical functions when aberrantly expressed in epithelial cells. The lack of specific inhibitors prevents therapeutic applications to modulate AID functions. Here, we have exploited our previous finding that the HSP90 molecular chaperoning pathway stabilizes AID in B cells, to test whether HSP90 inhibitors could target AID in vivo. We demonstrate that chronic administration of HSP90 inhibitors decreases AID protein levels and isotype switching in immunized mice. HSP90 inhibitors also reduce disease severity in a mouse model of acute B-cell lymphoblastic leukemia in which AID accelerates disease progression. We further show that human AID protein levels are sensitive to HSP90 inhibition in normal and leukemic B cells, and that HSP90 inhibition prevents AID-dependent epithelial to mesenchymal transition in a human breast cancer cell line in vitro. Thus, we provide proof-of-concept that HSP90 inhibitors indirectly target AID in vivo and that endogenous human AID is widely sensitive to them, which could have therapeutic applications.

AIDing cancer treatment: Reducing AID activity via HSP90 inhibition

The activation induced deaminase (AID) catalyses the two key events underlying humoral adaptive immunity: class switch recombination and somatic hypermutation of antibody genes in B lymphocytes. AID accomplishes this task by directly deaminating cytosines within the genomic immunoglobulin locus, thereby triggering a complex mutagenic process eventually leading to improved effector function of antibodies. However, it has long been noticed that AID can be aberrantly expressed in cancer and that its activity is not absolutely restricted to antibody genes, as substantial genome‐wide off‐target mutations have been observed, which contribute to tumorigenesis and clonal evolution of AID‐expressing malignancies. In this issue of the European Journal of Immunology, Montamat‐Sicotte et al. [Eur. J. Immunol. 2015. 45: 2365–2376] investigate the feasibility and efficacy of in vivo inhibition of AID with HSP90 inhibitors in a mouse model of B‐cell leukemia and in vitro with a human breast cancer cell line, thereby demonstrating that cancer patients may benefit from preventing noncanonical AID functions.

Activation-Induced Cytidine Deaminase and Switched Memory B Cells as Predictors of Effective In Vivo Responses to the Influenza Vaccine

Aging impairs humoral immune responses, leading to increased frequency and severity of infectious diseases and reduced protective effects of vaccination. We have identified B-cell biomarkers that are reduced by aging and that can be used as predictive markers of the response of an individual to vaccination. The identification of these biomarkers will have an impact on the development of effective vaccines to protect the elderly from infections and other debilitating diseases.

Refining the Neuberger model: Uracil processing by activated B cells

During the immune response, B cells undergo a programed mutagenic cascade to promote increased affinity and expanded antibody function. The two processes, somatic hypermutation (SHM) and class switch recombination (CSR), are initiated by the protein activation-induced deaminase (AID), which converts cytosine to uracil in the immunoglobulin loci. The presence of uracil in DNA promotes DNA mutagenesis though a subset of DNA repair proteins. Two distinct mechanisms have been proposed to control uracil processing. The first is through base removal by uracil DNA glycosylase (UNG), and the second is through detection by the mismatch repair (MMR) complex MSH2/6. In a study published in this issue of European Journal of Immunology, Dingler et al. [Eur. J. Immunol. 2014. 44: 1925-1935] examine uracil processing in B cells in the absence of UNG and SMUG1 glycosylases. Similar to UNG, SMUG1 is an uracil glycosylase which can remove the uracil base. While Smug1(-/-) mice show no clear deficiency in SHM or CSR, Ung(-/-) Smug1(-/-) mice display exacerbated phenotypes, suggesting a back-up role for SMUG1 in antibody diversity. This new information expands the model of uracil processing in B cells and raises several interesting questions about the dynamic relationship between base excision repair and MMR.

Glucocorticoid inhibition of activation-induced cytidine deaminase expression in human B lymphocytes

We examined whether glucocorticoids could modulate the expression of activation-induced cytidine deaminase (AICDA), the principal regulator of the processes of immunoglobulin gene somatic hypermutation and class switch recombination in B lymphocytes. Treatment of human B cells with IL-4 and anti-CD40 antibody for 18-20h resulted in induction of expression of AICDA mRNA by over 10-fold. Dexamethasone at 10nM concentration inhibited AICDA induction by an average of 51.8% (p<0.0001). These effects of glucocorticoids were found to be dose dependent in the physiologic range and were reversible by co-treatment with a glucocorticoid receptor antagonist. Human B cell viability and proliferation were unaltered by glucocorticoid treatment. These data demonstrate that physiologic concentrations of glucocorticoids can act on human B lymphocytes through glucocorticoid receptor-mediated mechanisms to diminish the expression of AICDA, a key regulator of humoral immune responses.

Ikaros in B cell development and function

The zinc finger transcription factor, Ikaros, is a central regulator of hematopoiesis. It is required for the development of the earliest B cell progenitors and at later stages for VDJ recombination and B cell receptor expression. Mature B cells rely on Ikaros to set the activation threshold for various stimuli, and to choose the correct antibody isotype during class switch recombination. Thus, Ikaros contributes to nearly every level of B cell differentiation and function.