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Limfòcits b; Metilació de l'ADN; Genoma Linfocitos b; Metilación de ADN; Genoma B-lymphocytes; DNA methylation; Genome Activation-induced deaminase (AID) initiates antibody diversification in germinal center B cells by deaminating cytosines, leading to somatic hypermutation and class-switch recombination. Loss-of-function mutations in AID lead to hyper-IgM syndrome type 2 (HIGM2), a rare human primary antibody deficiency. AID-mediated deamination has been proposed as leading to active demethylation of 5-methycytosines in the DNA, although evidence both supports and casts doubt on such a role. In this study, using whole-genome bisulfite sequencing of HIGM2 B cells, we investigated direct AID involvement in active DNA demethylation. HIGM2 naïve and memory B cells both display widespread DNA methylation alterations, of which ∼25% are attributable to active DNA demethylation. For genes that undergo active demethylation that is impaired in HIGM2 individuals, our analysis indicates that AID is not directly involved. We demonstrate that the widespread alterations in the DNA methylation and expression profiles of HIGM2 naïve B cells result from premature overstimulation of the B-cell receptor prior to the germinal center reaction. Our data support a role for AID in B cell central tolerance in preventing the expansion of autoreactive cell clones, affecting the correct establishment of DNA methylation patterns. Spanish Ministry of Science, Innovation and Universities [SAF2017-88086-R to E.B.]; cofunded by FEDER funds/European Regional Development Fund (ERDF)—a way to build Europe. E.B is supported by Instituto de Salud Carlos III (ISCIII), Ref. AC18/00057, associated with i-PAD project (ERARE European Union program); P.L. and C.P. are supported by the German Cancer Aid project CO-CLL [70113869]; B.G. is funded by the Deutsche Forschungsgemeinschaft [GR1617/14-1/iPAD, SFB1160/2_B5, RESIST–EXC 2155–Project ID 390874280, CIBSS–EXC-2189–Project ID 390939984]; BMBF [GAIN 01GM1910A]. Funding for open access charge: Spanish Ministry of Science, Innovation and Universities [SAF2017-88086-R].

Biallelic inactivating mutations in IL21R causes a combined immunodeficiency that is often complicated by cryptosporidium infections. While eight IL-21R-deficient patients have been reported previously, the natural course, immune characteristics of disease, and response to hematopoietic stem cell transplantation (HSCT) remain to be comprehensively examined. In our study, we have collected clinical histories of 13 patients with IL-21R deficiency from eight families across seven centers worldwide, including five novel patients identified by exome or NGS panel sequencing. Eight unique mutations in IL21R were identified in these patients, including two novel mutations. Median age at disease onset was 2.5 years (0.5–7 years). The main clinical manifestations were recurrent bacterial (84.6%), fungal (46.2%), and viral (38.5%) infections; cryptosporidiosis-associated cholangitis (46.2%); and asthma (23.1%). Inflammatory skin diseases (15.3%) and recurrent anaphylaxis (7.9%) constitute novel phenotypes of this combined immunodeficiency. Most patients exhibited hypogammaglobulinemia and reduced proportions of memory B cells, circulating T follicular helper cells, MAIT cells and terminally differentiated NK cells. However, IgE levels were elevated in 50% of IL-21R-deficient patients. Overall survival following HSCT (6 patients, mean follow-up 1.8 year) was 33.3%, with pre-existing organ damage constituting a negative prognostic factor. Mortality of non-transplanted patients (n = 7) was 57.1%. Our detailed analysis of the largest cohort of IL-21R-deficient patients to date provides in-depth clinical, immunological and immunophenotypic features of these patients, thereby establishing critical non-redundant functions of IL-21/IL-21R signaling in lymphocyte differentiation, humoral immunity and host defense against infection, and mechanisms of disease pathogenesis due to IL-21R deficiency. Outcome following HSCT depends on prior chronic infections and organ damage, which should thus be considered as early as possible following molecular diagnosis. Supplementary Information The online version contains supplementary material available at 10.1007/s10875-021-01031-5.

Predominantly antibody deficiencies (PADs) encompass a heterogeneous group of disorders characterized by low immunoglobulin serum levels in the presence or absence of peripheral B cells. Clinical presentation of affected patients may include recurrent respiratory and gastrointestinal infections, invasive infections, autoimmune manifestations, allergic reactions, lymphoproliferation, and increased susceptibility to malignant transformation. In the last decades, several genetic alterations affecting B-cell development/maturation have been identified as causative of several forms of PADs, adding important information on the genetic background of PADs, which in turn should lead to a better understanding of these disorders and precise clinical management of affected patients. This review aimed to present a comprehensive overview of the known and potentially involved molecules in the etiology of PADs to elucidate the pathogenesis of these disorders and eventually offer a better prognosis for affected patients.

Activation-induced deaminase (AID) is the major actor of immunoglobulin (Ig) gene diversification in germinal center B-cells. From its first description, it was considered as mandatory for class switch recombination (CSR), and this discovery initiated a long quest for all of the AID-interacting factors controlling its activity. The mechanisms focusing AID-mediated DNA lesions to given target sequences remain incompletely understood with regards the detailed characterization of optimal substrates in which cytidine deamination will lead to double strand breaks (DSBs) and chromosomal cleavage. In an effort to reconsider whether such CSR breaks absolutely require AID, we herein provide evidence, based on deep-sequencing approaches, showing that this dogma is not absolute in both human and mouse B lymphocytes. In activated B-cells from either AID-deficient mice or human AID-deficient patients, we report an intrinsic ability of the IgH locus to undergo “on-target” cleavage and subsequent synapsis of broken regions in conditions able to yield low-level CSR. DNA breaks occur in such conditions within the same repetitive S regions usually targeted by AID, but their repair follows a specific pathway with increased usage of microhomology-mediated repair. These data further demonstrate the role of AID machinery as not initiating de novo chromosomal cleavage but rather catalyzing a process which spontaneously initiates at low levels in an appropriately conformed IgH locus.

Mutations in activation induced deaminase (AID) lead to hyper-IgM syndrome type 2 (HIGM2), a rare human primary antibody deficiency. AID-mediated cytosine deamination has been proposed as mediating active demethylation, although evidences both support and cast doubt on such a role. We here made use of HIGM2 B cells to investigate direct AID involvement in active DNA demethylation. HIGM2 naïve and memory B cells both display widespread DNA methylation defects, of which approximately 25% of these defects correspond to active events. For genes that undergo active demethylation that is impaired in HIGM2 individuals, we did not observe AID involvement but a participation of TET enzymes. DNA methylation alterations in HIGM2 naïve B cells are related to premature overstimulation of the B-cell receptor prior to the germinal center reaction. Our data supports a role for AID in B cell central tolerance in preventing the expansion of autoreactive cell clones, affecting the correct establishment of DNA methylation patterns.

B-cell activation yields abundant cell death in parallel to clonal amplification and remodeling of immunoglobulin (Ig) genes by activation-induced deaminase (AID). AID promotes affinity maturation of Ig variable regions and class switch recombination (CSR) in mature B lymphocytes. In the IgH locus, these processes are under control of the 3’ regulatory region (3’RR) super-enhancer, a region demonstrated in the mouse to be both transcribed and itself targeted by AID-mediated recombination. Alternatively to CSR, IgH deletions joining Sμ to “like-switch” DNA repeats that flank the 3’ super-enhancer can thus accomplish so-called “locus suicide recombination” (LSR) in mouse B-cells. Using an optimized LSR-seq high throughput method, we now show that AID-mediated LSR is evolutionarily conserved and also actively occurs in humans, providing an activation-induced cell death pathway in multiple conditions of B-cell activation. LSR either focuses on the functional IgH allele or is bi-allelic, and its signature is mainly detected when LSR is ongoing while it vanishes from fully differentiated plasma cells or from “resting” blood memory B-cells. Highly diversified breakpoints are distributed either within the upstream (3’RR1) or downstream (3’RR2) copies of the IgH 3’ super-enhancer and all conditions activating CSR in vitro also seem to trigger LSR although TLR ligation appeared the most efficient. Molecular analysis of breakpoints and junctions confirms that LSR is AID-dependent and reveals junctional sequences somehow similar to CSR junctions but with increased usage of microhomologies., Author summary Class switch recombination, initiated by the activation-induced deaminase enzyme rearranges immunoglobulin (Ig) genes in order to replace expression of IgM by IgG, IgA or IgE. A variant form of this event, locus suicide recombination (LSR), was previously reported in mouse B-lymphocytes and simply deletes all functional Ig constant genes, thus terminating B-cell function. This study first demonstrates that the structure of the human Ig heavy chain locus provides an ideal target for LSR, and is thus actively (but transiently) affected by this deletion process at the activated B-cell stage. LSR then yields recombined genes that do not support B-cell survival and which thus becomes almost undetectable among long-lived memory B-cells or plasma cells.

ARHGEF1 is a RhoA-specific guanine nucleotide exchange factor expressed in hematopoietic cells. We used whole-exome sequencing to identify compound heterozygous mutations in ARHGEF1, resulting in the loss of ARHGEF1 protein expression in 2 primary antibody–deficient siblings presenting with recurrent severe respiratory tract infections and bronchiectasis. Both ARHGEF1-deficient patients showed an abnormal B cell immunophenotype, with a deficiency in marginal zone and memory B cells and an increased frequency of transitional B cells. Furthermore, the patients’ blood contained immature myeloid cells. Analysis of a mediastinal lymph node from one patient highlighted the small size of the germinal centers and an abnormally high plasma cell content. On the molecular level, T and B lymphocytes from both patients displayed low RhoA activity and low steady-state actin polymerization (even after stimulation of lysophospholipid receptors). As a consequence of disturbed regulation of the RhoA downstream target Rho-associated kinase I/II (ROCK), the patients’ lymphocytes failed to efficiently restrain AKT phosphorylation. Enforced ARHGEF1 expression or drug-induced activation of RhoA in the patients’ cells corrected the impaired actin polymerization and AKT regulation. Our results indicate that ARHGEF1 activity in human lymphocytes is involved in controlling actin cytoskeleton dynamics, restraining PI3K/AKT signaling, and confining B lymphocytes and myelocytes within their dedicated functional environment.

To assess how B cell phenotype analysis correlates with antigen responses in patients with class switch recombination defects (CSRD) we quantified memory B cells by flow-cytometry and immunized CSRD patients with the neoantigen bacteriophage phiX174 (phage). CSRD patients showed uniformly absent or markedly reduced switched memory B cells (IgM-IgD-CD27+). CD40L patients had reduced CD27+ memory B cells (both non-switched and switched). In NEMO patients, results varied depending on the IKKγ gene variant. Three of four AID patients had normal percentages of CD27+ memory B cells while CD27+IgM-IgD- switched memory B cells were markedly reduced in all AID patients. Antibody response to phage was remarkably decreased with lack of memory amplification and class-switching in immunized CD40L, UNG deficient, and NEMO patients. Distinct B-cell phenotype pattern correlated with abnormal antibody responses to a T-cell dependent neoantigen, representing a powerful tool to identify CSRD patients.

The genetic investigation of a family presenting with a dominant form of hyper IgM syndrome published in 1963 and 1975 revealed a R190X nonsense mutation in activation-induced cytidine deaminase. This report illustrates the progress made over 6 decades in the characterization of primary immunodeficiencies, from immunochemistry to whole-exome sequencing.

B-cell activation yields abundant cell death in parallel to clonal amplification and remodeling of immunoglobulin (Ig) genes by activation-induced deaminase (AID). AID promotes affinity maturation of Ig variable regions and class switch recombination (CSR) in mature B lymphocytes. In the IgH locus, these processes are under control by the 3’ regulatory region (3’RR) super-enhancer, a region demonstrated in the mouse to be both transcribed and itself targeted by AID-mediated recombination. Alternatively to CSR, IgH deletions joining Sμ to “like-switch” DNA repeats that flank the 3’ super-enhancer can thus accomplish so-called “locus suicide recombination” (LSR) in mouse B-cells. We now show that AID-mediated LSR also actively occurs in humans, and provides an activation-induced cell death pathway in multiple conditions of B-cell activation. LSR deletions either focus on the functional IgH allele or are bi-allelic, since they can only be detected when they are ongoing and their signature vanishes from fully differentiated plasma cells or from “resting” blood memory B-cells, but readily reappears when such memory B-cells are re-stimulatedin vitro. Highly diversified breakpoints are distributed either within the upstream (3’RR1) or downstream (3’RR2) copies of the IgH 3’ super-enhancer and all conditions activating CSRin vitroalso seem to trigger LSR.Author SummaryClass switch recombination, initiated by the activation-induced deaminase enzyme rearranges immunoglobulin (Ig) genes in order to replace expression of IgM by IgG, IgA or IgE. A variant form of this event, locus suicide recombination (LSR), was previously reported in mouse B-lymphocytes and simply deletes all functional Ig constant genes, thus terminating B-cell function. This study first demonstrates that the structure of the human Ig heavy chain locus provides an ideal target for LSR, and is thus actively (but transiently) affected by this deletional process at the activated B-cell stage. LSR then yields recombined genes that do not support B-cell survival and which thus become undetectable among long-lived memory B-cells or plasma cells.

Ataxia telangiectasia (AT) is a rare primary immunodeficiency disorder with various clinical manifestations. Increased serum levels of IgM and recurrent infections, mainly sinopulmonary infections, can be the presenting feature in a number of AT patients and may be initially misdiagnosed as hyper-IgM (HIgM) syndrome. This study was designed to investigate class switch recombination (CSR) as a critical mechanism in B lymphocytes' maturation to produce different isotypes of antibody in response to antigen stimulation in AT cases with HIgM presentation. Quantitative IgE production after stimulation by IL-4 and CD40L was considered as an indicator for CSR function. We also compared their results with sex and age matched AT patients without HIgM presentation. We report four AT patients with recurrent infections during infancy and high serum levels of IgM. Laboratory evaluations revealed defective CSR while none of the three AT patients without HIgM presentation had a defect in the CSR process. The characterized defect in AT is a mutation in the ataxia telangiectasia mutated (ATM) gene. This gene may result in CSR defects due to impaired DNA break repair. A special association between AT and HIgM may indicate a new subgroup of AT patients according to their clinical phenotype and CSR condition.

Next-generation DNA sequencing has accelerated the genetic characterization of many human primary immunodeficiency diseases (PIDs). These discoveries can be lifesaving for the affected patients and also provide a unique opportunity to study the effect of specific genes on human immune function. In the past 18 months, a number of independent groups have begun to define novel PIDs caused by defects in the caspase recruitment domain family, member 11 (CARD11) –B-cell chronic lymphocytic leukemia/lymphoma 10 (BCL10) –mucosa-associated lymphoid tissue lymphoma translocation gene 1 (MALT1 [CBM]) signalosome complex. The CBM complex forms an essential molecular link between the triggering of cell-surface antigen receptors and nuclear factor κB activation. Germline mutations affecting the CBM complex are now recognized as the cause of novel combined immunodeficiency phenotypes, which all share abnormal nuclear factor κB activation and dysregulated B-cell development as defining features. For this "Current perspectives" article, we have engaged experts in both basic biology and clinical immunology to capture the worldwide experience in recognizing and managing patients with PIDs caused by CBM complex mutations.

Activation-induced deaminase (AID) triggers antibody class switch recombination (CSR) in B cells by initiating DNA double strand breaks that are repaired by nonhomologous end-joining pathways. A role for AID at the repair step is unclear. We show that specific inactivation of the C-terminal AID domain encoded by exon 5 (E5) allows very efficient deamination of the AID target regions but greatly impacts the efficiency and quality of subsequent DNA repair. Specifically eliminating E5 not only precludes CSR but also, causes an atypical, enzymatic activity-dependent dominant-negative effect on CSR. Moreover, the E5 domain is required for the formation of AID-dependent Igh-cMyc chromosomal translocations. DNA breaks at the Igh switch regions induced by AID lacking E5 display defective end joining, failing to recruit DNA damage response factors and undergoing extensive end resection. These defects lead to nonproductive resolutions, such as rearrangements and homologous recombination that can antagonize CSR. Our results can explain the autosomal dominant inheritance of AID variants with truncated E5 in patients with hyper-IgM syndrome 2 and establish that AID, through the E5 domain, provides a link between DNA damage and repair during CSR.

Answers from Exomes Exome sequencing, which targets only the protein-coding regions of the genome, has the potential to identify the underlying genetic causes of rare inherited diseases. Angulo et al. (p. 866 , published online 17 October; see Perspective by Conley and Fruman ) performed exome sequencing of individuals from seven unrelated families with severe, recurrent respiratory infections. The patients carried the same mutation in the gene coding for the catalytic subunit of phosphoinositide 3-kinase δ (PI3Kδ). The mutation caused aberrant activation of this kinase, which plays a key role in immune cell signaling. Drugs inhibiting PI3Kδ are already in clinical trials for other disorders.

Memory B cells, unlike naive B cells, require a reduced level of STAT3 activation to differentiate into antibody-secreting plasmablasts in response to IL-10 and IL-21; however, this process requires IL-21R expression in both naive and memory cells., Long-lived antibody memory is mediated by the combined effects of long-lived plasma cells (PCs) and memory B cells generated in response to T cell–dependent antigens (Ags). IL-10 and IL-21 can activate multiple signaling pathways, including STAT1, STAT3, and STAT5; ERK; PI3K/Akt, and potently promote human B cell differentiation. We previously showed that loss-of-function mutations in STAT3, but not STAT1, abrogate IL-10– and IL-21–mediated differentiation of human naive B cells into plasmablasts. We report here that, in contrast to naive B cells, STAT3-deficient memory B cells responded to these STAT3-activating cytokines, differentiating into plasmablasts and secreting high levels of IgM, IgG, and IgA, as well as Ag-specific IgG. This was associated with the induction of the molecular machinery necessary for PC formation. Mutations in IL21R, however, abolished IL-21–induced responses of both naive and memory human B cells and compromised memory B cell formation in vivo. These findings reveal a key role for IL-21R/STAT3 signaling in regulating human B cell function. Furthermore, our results indicate that the threshold of STAT3 activation required for differentiation is lower in memory compared with naive B cells, thereby identifying an intrinsic difference in the mechanism underlying differentiation of naive versus memory B cells.

We report a male with atypical severe combined immunodeficiency caused by heterozygous compound mutations c.256-257del and c.C1331T in RAG1 gene. The patient presents with recurrent bronchopneumonias with obstruction, chronic fibrosing alveolitis, complicated by respiratory failure, pulmonary hypertension and hepatosplenomegaly. He was diagnosed with agammaglobulinemia at the age of 9. His condition was complicated by granulomatous skin disease at the age of 12 despite regular IVIg substitution. Immunological presentation included profound hypogammaglobulinemia and absence of B cells. Under immunoglobulin substitution for 5 years patient has permanent lymphopenia, skewed phenotype of T cells and diminished number of recent thymic emigrants.

The present study focuses on a large family with an X-linked immunodeficiency in which there are variable clinical and laboratory phenotypes, including recurrent viral and bacterial infections, hypogammaglobulinemia, Epstein-Barr virus–driven lymphoproliferation, splenomegaly, colitis, and liver disease. Molecular and genetic analyses revealed that affected males were carriers of a hypomorphic hemizygous mutation in XIAP (XIAPG466X) that cosegregated with a rare polymorphism in CD40LG (CD40 ligandG219R). These genes are involved in the X-linked lymphoproliferative syndrome 2 and the X-linked hyper-IgM syndrome, respectively. Single expression of XIAPG466X or CD40LG219R had no or minimal effect in vivo, although in vitro, they lead to altered functional activities of their gene products, which suggests that the combination of XIAP and CD40LG mutations contributed to the expression of clinical manifestations observed in affected individuals. Our report of a primary X-linked immunodeficiency of oligogenic origin emphasizes that primary immunodeficiencies are not caused by a single defective gene, which leads to restricted manifestations, but are likely to be the result of an interplay between several genetic determinants, which leads to more variable clinical phenotypes.

Maturation of the secondary antibody repertoire is generated by means of immunoglobulin (Ig) class-switch recombination (CSR) and somatic hypermutation (SHM). The molecular mechanisms underlying these important processes have long-remained obscure. Inherited defects in CSR, variably associated to defects in SHM, are a group of phenotypically and genetically heterogeneous diseases, the characterization of which has allowed to recognize that T cell–B cell interaction (resulting in CD40-mediated signaling), intrinsic B cell mechanisms, and complex DNA repair machinery are involved in CSR and SHM. Elucidation of the molecular defects underlying these disorders has been essential to better understand the molecular basis of Ig diversification, and has offered the opportunity to define the clinical spectrum of these diseases and to prompt more accurate diagnostic and therapeutic approaches. Here we describe the currently known genetic defects leading to Ig CSR deficiency and classify them due to their physiopathogeny.

Ataxia telangiectasia-mutated (ATM) and ataxia telangiectasia and Rad3-related (ATR) kinases are two key regulators of DNA-damage responses (DDR) that are mainly activated in response to DNA double-strand breaks and single-stranded DNA damages, respectively. Seckel syndrome, a rare genetic disorder characterized by a microcephaly and a markedly reduced body size, has been associated with defective ATR-dependent DNA damage signaling. However, the only human genetic ATR defect reported so far is a hypomorphic splicing mutation identified in five related individuals with Seckel syndrome. Here, we report the first case of primary microcephaly with compound heterozygous mutations in ATR: a 540 kb genomic deletion on one allele and a missense mutation leading to splice dysregulation on the other, which ultimately lead to a sharp decrease in ATR expression. DNA combing technology revealed a profound spontaneous alteration of several DNA replication parameters in patient's cells and FISH analyses highlighted the genomic instability caused by ATR deficiency. Collectively, our results emphasize the crucial role for ATR in the control of DNA replication, and reinforce the complementary and nonredundant contributions of ATM and ATR in human cells to face DNA damages and warrant genome integrity.

Homozygous missense mutations in POLE1 caused an inherited disorder characterized by facial dysmorphism, immunodeficiency, livedo, and short stature., DNA polymerase ε (Polε) is a large, four-subunit polymerase that is conserved throughout the eukaryotes. Its primary function is to synthesize DNA at the leading strand during replication. It is also involved in a wide variety of fundamental cellular processes, including cell cycle progression and DNA repair/recombination. Here, we report that a homozygous single base pair substitution in POLE1 (polymerase ε 1), encoding the catalytic subunit of Polε, caused facial dysmorphism, immunodeficiency, livedo, and short stature (“FILS syndrome”) in a large, consanguineous family. The mutation resulted in alternative splicing in the conserved region of intron 34, which strongly decreased protein expression of Polε1 and also to a lesser extent the Polε2 subunit. We observed impairment in proliferation and G1- to S-phase progression in patients’ T lymphocytes. Polε1 depletion also impaired G1- to S-phase progression in B lymphocytes, chondrocytes, and osteoblasts. Our results evidence the developmental impact of a Polε catalytic subunit deficiency in humans and its causal relationship with a newly recognized, inherited disorder.

We report on a child with a de novo deletion of approximately 12 Mb detected through array comparative genomic hybridization (CGH). The deletion involved chromosome bands 13q12.3 to 13q14.11 and determined the loss of ≥ 50 genes. A second deletion on chromosome 12p11.3p11.22 of 43-167 kb, including about 12 genes, was unlikely of clinical relevance because inherited from the asymptomatic father. The child had developmental delay, dysmorphisms and many features reminiscent of ataxia-telangiectasia (A-T), as cerebellar ataxia, oculocutaneus telangiectasia and recurrent upper airway infections. Atraumatic fractures of the metatarsus were noted. Moreover, this is a rare case of 13q deletion syndrome associated with peripheral blood white cells radiosensitivity to bleomycin, reminiscent of what previously reported on X-ray hypersensitivity of fibroblasts from patients with alterations of this chromosome. The immunological evaluation revealed increased IgM serum levels and a low proliferative response to mitogens, PHA and CD3 cross-linking (CD3 XL). After 12 years of age only a mild dysmetria persisted, while the proliferative response to mitogens became normal by 9 years of age.

International audience; ABSTRACT: Autosomal dominant deficiency of signal transducer and activator of transcription 3 (STAT3) is the main genetic etiology of hyper-immunoglobulin (Ig) E syndrome. We documented the molecular, cellular, and clinical features of 60 patients with heterozygous STAT3 mutations from 47 kindreds followed in France. We identified 11 known and 13 new mutations of STAT3. Low levels of interleukin (IL)-6-dependent phosphorylation and nuclear translocation (or accumulation) of STAT3 were observed in Epstein-Barr virus-transformed B lymphocytes (EBV-B cells) from all STAT3-deficient patients tested. The immunologic phenotype was characterized by high serum IgE levels (96% of the patients), memory B-cell lymphopenia (94.5%), and hypereosinophilia (80%). A low proportion of IL-17A-producing circulating T cells was found in 14 of the 15 patients tested. Mucocutaneous infections were the most frequent, typically caused by Staphylococcus aureus (all patients) and Candida albicans (85%). Up to 90% of the patients had pneumonia, mostly caused by Staph. aureus (31%) or Streptococcus pneumoniae (30%). Recurrent pneumonia was associated with secondary bronchiectasis and pneumatocele (67%), as well as secondary aspergillosis (22%). Up to 92% of the patients had dermatitis and connective tissue abnormalities, with facial dysmorphism (95%), retention of decidual teeth (65%), osteopenia (50%), and hyperextensibility (50%). Four patients developed non-Hodgkin lymphoma. The clinical outcome was favorable, with 56 patients, including 43 adults, still alive at the end of study (mean age, 21 yr; range, 1 mo to 46 yr). Only 4 patients died, 3 from severe bacterial infection (aged 1, 15, and 29 yr, respectively). Antibiotic prophylaxis (90% of patients), antifungal prophylaxis (50%), and IgG infusions (53%) improved patient health, as demonstrated by the large decrease in pneumonia recurrence. Overall, the prognosis of STAT3 deficiency may be considered good, provided that multiple prophylactic measures, including IgG infusions, are implemented.

Mutations of the CD40 gene have been found in patients with autosomal recessive hyper-immunoglobulin M (HIGM) syndrome type 3. Five patients from four unrelated families with CD40 mutation have been reported so far. Clinical manifestations include recurrent sinopulmonary infections, Pneumocystis carinii pneumonia and Cryptosporidium parvum infection. Affected patients typically have very low levels of IgG and IgA and normal or high levels of IgM. Flow cytometry analysis of these five patients demonstrated that peripheral blood B lymphocytes lacked expression of surface CD40. Herein, we present two siblings from second-degree consanguineous Turkish parents with homozygous CD40 deletion of four nucleotides including the stop codon resulting presumably to a longer protein. Clinical and immunological profile of these patients is similar to the already reported HIGM3 patients except normal CD40 expression on B lymphocytes. This observation emphasizes the requirement of CD40 mutation analysis for definite diagnosis of HIGM3 despite normal flow cytometric expression of CD40, particularly if the immunological and clinical profile is suggestive for HIGM3.

In human cell nuclei, UNG2 is the major uracil-DNA glycosylase initiating DNA base excision repair of uracil. In activated B cells it has an additional role in facilitating mutagenic processing of AID-induced uracil at Ig loci and UNG-deficient patients develop hyper-IgM syndrome characterized by impaired class-switch recombination and disturbed somatic hypermutation. How UNG2 is recruited to either error-free or mutagenic uracil processing remains obscure, but likely involves regulated interactions with other proteins. The UNG2 N-terminal domain contains binding motifs for both proliferating cell nuclear antigen (PCNA) and replication protein A (RPA), but the relative contribution of these interactions to genomic uracil processing is not understood. Interestingly, a heterozygous germline single-nucleotide variant leading to Arg88Cys (R88C) substitution in the RPA-interaction motif of UNG2 has been observed in humans, but with unknown functional relevance. Here we demonstrate that UNG2-R88C protein is expressed from the variant allele in a lymphoblastoid cell line derived from a heterozygous germ line carrier. Enzyme activity as well as localization in replication foci of UNG2-R88C was similar to that of WT. However, binding to RPA was essentially abolished by the R88C substitution, whereas binding to PCNA was unaffected. Moreover, we show that disruption of the PCNA-binding motif impaired recruitment of UNG2 to S-phase replication foci, demonstrating that PCNA is a major factor for recruitment of UNG2 to unperturbed replication forks. Conversely, in cells treated with hydroxyurea, RPA mediated recruitment of UNG2 to stalled replication forks independently of functional PCNA binding. Modulation of PCNA- versus RPA-binding may thus constitute a functional switch for UNG2 in cells subsequent to genotoxic stress and potentially also during the processing of uracil at the immunoglobulin locus in antigen-stimulated B cells.

B cells from Cernunnos-deficient patients contain aberrant class switch recombination junctions, and a dominant-negative Cernunnos mutation was detected in a diffuse large B cell lymphoma sample., Cernunnos is involved in the nonhomologous end-joining (NHEJ) process during DNA double-strand break (DSB) repair. Here, we studied immunoglobulin (Ig) class switch recombination (CSR), a physiological process which relies on proper repair of the DSBs, in B cells from Cernunnos-deficient patients. The pattern of in vivo generated CSR junctions is altered in these cells, with unusually long microhomologies and a lack of direct end-joining. The CSR junctions from Cernunnos-deficient patients largely resemble those from patients lacking DNA ligase IV, Artemis, or ATM, suggesting that these factors are involved in the same end-joining pathway during CSR. By screening 269 mature B cell lymphoma biopsies, we also identified a somatic missense Cernunnos mutation in a diffuse large B cell lymphoma sample. This mutation has a dominant-negative effect on joining of a subset of DNA ends in an in vitro NHEJ assay. Translocations involving both Ig heavy chain loci and clonal-like, dynamic IgA switching activities were observed in this tumor. Collectively, our results suggest a link between defects in the Cernunnos-dependent NHEJ pathway and aberrant CSR or switch translocations during the development of B cell malignancies.

Hyper-IgM syndromes are characterized by normal or elevated serum IgM levels with the absence or reduced levels of other immunoglobulins. There are some patients with defective class-switch recombination (CSR) who do not have CD40L, CD40, AID, and UNG defects. The aim of this study is to determine the B-cell functions of patients with Hyper-IgM type 4 phenotype. Ten patients (seven males and three females) 84.2 ± 16.5 months of age with initial low serum IgG and IgA and high or normal IgM levels were included. Clinically, 50% had recurrent upper respiratory tract, 10% urinary tract, 10% lower respiratory tract infections, and 30% had mixed type infections. Lymphoid hyperplasia, overt autoimmune manifestations, or malignancy was not noted. Seven of 10 patients were studied twice; at the age of 34.2 ± 13.7 and at 86.6 ± 12.3 months. Absolute lymphocyte counts and lymphocyte subsets were normal in all cases. All of them had normal expression of CD40 on B cells and CD40L on activated T cells for males. At first examination, all patients had normal in vitro sCD40L+rIL-4-induced B cell proliferation response and somatic hypermutation but CSR towards IgE was absent. AID and UNG genes did not show any abnormalities. All showed improvement in both clinical findings and Ig levels during the follow-up period of 55.8 ± 14.8 months. Ages for normalization of IgG and IgA were 68.2 ± 8.7 and 70.2 ± 21.6 months, respectively. During the second evaluation: In vitro sCD40L+rIL-4-induced B-cell proliferation was normal in all cases, whereas CSR was still abnormal in five of eight patients. Two of the patients had an increase in in vitro CSR response but still low IgG2 subclass levels. Three patients with initially absent in vitro CSR response also normalized. Conclusion: Clinical manifestations and immunoglobulin levels of the patients with Hyper-IgM type 4 phenotype recovered in late childhood at about 6 years of age. There was a transient CSR defect which was not observed in cases with transient hypogammaglobulinemia of infancy. Detection of a non-AID or non-UNG associated CSR defect in infancy should be confirmed later on since spontaneous recovery may occur.

Immunoglobulin class-switch recombination deficiencies (Ig-CSR-Ds) are rare primary immunodeficiencies characterized by defective switched isotype (IgG/IgA/IgE) production. Depending on the molecular defect in question, the Ig-CSR-D may be combined with an impairment in somatic hypermutation (SHM). Some of the mechanisms underlying Ig-CSR and SHM have been described by studying natural mutants in humans. This approach has revealed that T cell-B cell interaction (resulting in CD40-mediated signaling), intrinsic B-cell mechanisms (activation-induced cytidine deaminase-induced DNA damage), and complex DNA repair machineries (including uracil-N-glycosylase and mismatch repair pathways) are all involved in class-switch recombination and SHM. However, several of the mechanisms required for full antibody maturation have yet to be defined. Elucidation of the molecular defects underlying the diverse set of Ig-CSR-Ds is essential for understanding Ig diversification and has prompted better definition of the clinical spectrum of diseases and the development of increasingly accurate diagnostic and therapeutic approaches.

SummaryThe biological response to DNA double-strand breaks acts to preserve genome integrity. Individuals bearing inactivating mutations in components of this response exhibit clinical symptoms that include cellular radiosensitivity, immunodeficiency, and cancer predisposition. The archetype for such disorders is Ataxia-Telangiectasia caused by biallelic mutation in ATM, a central component of the DNA damage response. Here, we report that the ubiquitin ligase RNF168 is mutated in the RIDDLE syndrome, a recently discovered immunodeficiency and radiosensitivity disorder. We show that RNF168 is recruited to sites of DNA damage by binding to ubiquitylated histone H2A. RNF168 acts with UBC13 to amplify the RNF8-dependent histone ubiquitylation by targeting H2A-type histones and by promoting the formation of lysine 63-linked ubiquitin conjugates. These RNF168-dependent chromatin modifications orchestrate the accumulation of 53BP1 and BRCA1 to DNA lesions, and their loss is the likely cause of the cellular and developmental phenotypes associated with RIDDLE syndrome.

Immunoglobulin (Ig) class switch recombination (CSR) deficiencies are rare primary immunodeficiencies characterized by the lack of switched isotype (IgG/IgA/IgE) production. In some cases, CSR deficiencies can be associated with abnormal somatic hypermutation. Analysis of CSR deficiencies has helped reveal the key functions of CSR-triggering molecules, i.e., CD40L, CD40, and effector molecules such as activation-induced cytidine deaminase and uracil N-glycosylase. We report a new form of B cell–intrinsic CSR deficiency found in three patients with deleterious, homozygous mutations in the gene encoding the PMS2 component of the mismatch repair machinery. CSR was found partially defective in vivo and markedly impaired in vitro. It is characterized by the defective occurrence of double-strand DNA breaks (DSBs) in switch regions and abnormal formation of switch junctions. This observation strongly suggests a role for PMS2 in CSR-induced DSB generation.

Activation-induced cytidine deaminase (AID), the enzyme- mediating class-switch recombination (CSR) and somatic hypermutation (SHM) of immunoglobulin genes, is essential for the removal of developing autoreactive B cells. How AID mediates central B cell tolerance remains unknown. We report that AID enzymes were produced in a discrete population of immature B cells that expressed recombination-activating gene 2 (RAG2), suggesting that they undergo secondary recombination to edit autoreactive antibodies. However, most AID(+) immature B cells lacked anti-apoptotic MCL-1 and were deleted by apoptosis. AID inhibition using lentiviral-encoded short hairpin (sh)RNA in B cells developing in humanized mice resulted in a failure to remove autoreactive clones. Hence, B cell intrinsic AID expression mediates central B cell tolerance potentially through its RAG-coupled genotoxic activity in self-reactive immature B cells. United States Department of Health & Human Services National Institutes of Health (NIH) - USA NIH National Institute of Allergy & Infectious Diseases (NIAID) (AI061093) United States Department of Health & Human Services National Institutes of Health (NIH) - USA NIH National Institute of Allergy & Infectious Diseases (NIAID) (AI071087) United States Department of Health & Human Services National Institutes of Health (NIH) - USA NIH National Institute of Allergy & Infectious Diseases (NIAID) (AI082713) INSERM, CEE EUROPAD-contract 7th Framework Program (201549) Assiociation Contre le Cancer Rubicon program, Netherlands Organization for Scientific Research United States Department of Health & Human Services National Institutes of Health (NIH) - USA NIH National Institute of Allergy & Infectious Diseases (NIAID) (R01AI071087) United States Department of Health & Human Services National Institutes of Health (NIH) - USA NIH National Institute of Allergy & Infectious Diseases (NIAID) (P01AI061093) United States Department of Health & Human Services National Institutes of Health (NIH) - USA NIH National Institute of Allergy & Infectious Diseases (NIAID) (U19AI082713)

Recently, patient mutations that activate PI3K signaling have been linked to a primary antibody deficiency. Here, we used whole-exome sequencing and characterized the molecular defects in 4 patients from 3 unrelated families diagnosed with hypogammaglobulinemia and recurrent infections. We identified 2 different heterozygous splice site mutations that affect the same splice site in PIK3R1, which encodes the p85α subunit of PI3K. The resulting deletion of exon 10 produced a shortened p85α protein that lacks part of the PI3K p110-binding domain. The hypothetical loss of p85α-mediated inhibition of p110 activity was supported by elevated phosphorylation of the known downstream signaling kinase AKT in patient T cell blasts. Analysis of patient blood revealed that naive T and memory B cell counts were low, and T cell blasts displayed enhanced activation-induced cell death, which was corrected by addition of the PI3Kδ inhibitor IC87114. Furthermore, B lymphocytes proliferated weakly in response to activation via the B cell receptor and TLR9, indicating a B cell defect. The phenotype exhibited by patients carrying the PIK3R1 splice site mutation is similar to that of patients carrying gain-of-function mutations in PIK3CD. Our results suggest that PI3K activity is tightly regulated in T and B lymphocytes and that various defects in the PI3K-triggered pathway can cause primary immunodeficiencies.

Primary antibody deficiencies (PADs) are the most common inherited immunodeficiencies in humans. The use of novel approaches, such as whole-exome sequencing and mouse genetic engineering, has helped identify new genes involved in the pathogenesis of PADs and has enabled the characterization of the molecular pathways that are involved in B cell development and function. We will summarize the different PADs in terms of their known or putative mechanisms that underlie B cell development in bone marrow, migration to secondary lymphoid organs, survival, and activation, although these different mechanisms can share the same molecular pathways. The defect leading to PAD can be B cell-specific or affecting also other(s) hematopoietic or nonhematopoietic lineage(s). Finally, most of PADs remain not molecularly defined so far.

Background Inherited mutations in DNA mismatch repair genes predispose to different cancer syndromes depending on whether they are mono-allelic or bi-allelic. This supports a causal relationship between expression level in the germline and phenotype variation. As a model to study this relationship, our study aimed to define the pathogenic characteristics of a recurrent homozygous coding variant in PMS2 displaying an attenuated phenotype identified by clinical genetic testing in seven Inuit families from Northern Quebec. Methods Pathogenic characteristics of the PMS2 mutation NM_000535.5:c.2002A>G were studied using genotype–phenotype correlation, single-molecule expression detection and single genome microsatellite instability analysis. Results This PMS2 mutation generates a de novo splice site that competes with the authentic site. In homozygotes, expression of the full-length protein is reduced to a level barely detectable by conventional diagnostics. Median age at primary cancer diagnosis is 22 years among 13 NM_000535.5:c.2002A>G homozygotes, versus 8 years in individuals carrying bi-allelic truncating mutations. Residual expression of full-length PMS2 transcript was detected in normal tissues from homozygotes with cancers in their 20s. Conclusions Our genotype–phenotype study of c.2002A>G illustrates that an extremely low level of PMS2 expression likely delays cancer onset, a feature that could be exploited in cancer preventive intervention.

ICOS is expressed on activated T cells and particularly on CXCR5+ follicular Th cells in germinal centers (GC). Its deletion leads to a profound deficiency in memory B cell formation and switched Ab response in humans. Here, we show that in ICOS-deficient patients the generation of GCs is severely disturbed, and the numbers of circulating CXCR5+CD45RO+ memory CD4 T cells are significantly reduced, indicating an essential role of ICOS in the differentiation of CXCR5+CD4 T cells. The GC-specific CD57+CXCR5+ subpopulation is virtually absent. In ICOS−/− mice, the decrease of circulating CXCR5+CD4 T cells reflects the reduction of CXCR5+ follicular Th cells in lymph nodes and spleen. Therefore, in concurrence with the absence of CXCR5+ T cells in the blood of CD40L-deficient patients, these data support the hypothesis that circulating CD57+CXCR5+ T cells are GC derived and thus may serve as a surrogate marker for the presence of functional GCs in humans.

Although idiopathic humoral immunodeficiencies are arbitrarily classified into specific antibody deficiency (SAD) or common variable immunodeficiency (CVID), this distinction does not accurately predict the risk of the bronchiectasis, one of the major long-term clinical complications in these patients. In this study, clinical complications were compared with laboratory markers of cellular and humoral immunity in fifty-five consecutive patients (27 children and 28 adults) attending regional immunology clinics in Manchester, United Kingdom. Reduced CD19(+)CD27(+)IgD(-) B cell percentage but not serum immunoglobulin levels or classification of patients into SAD and CVID was associated with a significantly higher prevalence of bronchiectasis (OR 0.4 (0.2-0.8), P = 0.001), splenomegaly (OR 0.2 (0.1-0.5), P = 0.001) and autoimmunity (OR 0.4 (0.2-0.7), P = 0.003). We conclude that in patients with idiopathic humoral immunodeficiencies assessment of B cell switching more accurately predicts clinical prognosis than either classification of patients into SAD and CVID or serum immunoglobulin concentrations.

The recent elucidation of the molecular defects leading to hyper-IgM syndromes has provided considerable insight into the complex mechanisms that govern the antibody maturation in humans.The study of a large cohort of patients revealed unexpected clinical, immunological and genetic findings, which have significant implications on the molecular basis of immunoglobulin class switch recombination and somatic hypermutation, as shown for hypomorphic mutations in the nuclear factor-kappaB essential modulator (NEMO) gene and peculiar activation-induced cytidine deaminase defects that differently affect class switch recombination and somatic hypermutation. The description of the hyper-IgM condition due to mutations in the gene encoding uracil-N glycosylase has been essential for defining the DNA-editing activity of activation-induced cytidine deaminase. Novel findings are awaited from the study of the yet genetically undefined hyper-IgM syndromes, leading to the identification of activation-induced cytidine deaminase cofactors and proteins involved in class switch recombination-induced DNA repair. In the genetically characterized hyper-IgM syndromes, the precise identification of the molecular defect allows the evaluation of hyper-IgM complications, and thus aids assessment of prognosis and proper survey and treatment.The important contribution made by investigation of this condition improves our understanding of the physiology of the antibody response in humans.

Shaping of the secondary antibody repertoire is generated by means of class-switch recombination (CSR), which replaces IgM with other isotypes, and somatic hypermutation (SHM), which allows production of high-affinity antibodies. However, the molecular mechanisms underlying these important processes have long remained obscure. Immunodeficiency with hyper-IgM comprises a group of genetically heterogeneous defects of CSR variably associated with defects of SHM. The study of these patients has allowed us to recognize that both T-cell-B-cell interaction (resulting in CD40-mediated signaling) and intrinsic B-cell mechanisms are involved in CSR and SHM. Elucidation of the molecular defects underlying these disorders has been essential to better understand the molecular basis of Ig diversification and has offered the opportunity to define the clinical spectrum of these diseases and to prompt more accurate diagnostic and therapeutic approaches.

Activation-induced cytidine deaminase (AID; gene symbol AICDA) is the key molecule required to induce immunoglobulin (Ig) class switch recombination (CSR) and somatic hypermutation (SHM) of the variable regions of Ig genes. Its deficiency causes a form of hyper-IgM (HIGM) syndrome. The study of natural AID mutants associated with HIGM as well as engineered mutants led to the characterization of the active domains of the protein. AID, through its cytidine deaminase activity, induces a targeted DNA lesion as an early step required for both CSR and SHM. Besides its cytidine deaminase activity, AID plays a further essential role in CSR, likely by recruiting CSR-specific cofactors by its C-terminus. A similar binding of SHM-specific cofactors to the N-terminal part is suggested by the functional characteristics of N(ter) AID artificial mutants. These data require confirmation in vivo. Finally, AID acts as a homo-, di-, or multimeric complex. Together, these data strongly suggest that AID, a master molecule for antibody diversification, exerts its activity on CSR not only as a cytidine deaminase enzyme but also as a docking protein, recruiting specific cofactors to a multimeric complex.

Common variable immunodeficiency (CVID) is a heterogeneous disorder characterized by defective Ab production and recurrent bacterial infections. The largely unknown causes are likely to comprise a diverse set of genetic or acquired defects. In this study, we investigated terminal B cell differentiation in lymph nodes from CVID patients. Up to the germinal center B cell stage, B cell differentiation was normal but terminal plasma cell development was found to be impaired. Using differential Blimp-1 and Syndecan-1 expression in controls, we defined three different plasma cell subsets that correspond to progressive developmental stages locating to different sites in the lymph node. In the CVID patients, we could only detect one or two of these subsets indicating a defective differentiation. Thus, terminal plasma cell differentiation was found to be impaired despite normal expression of Blimp-1. B cells reaching only the first stage of plasma cell differentiation were further unable to undergo isotype switching and to up-regulate activation markers on B cells stimulated in vitro.

Autosomal recessive form of hyper-IgM syndrome type 2 (AR-HIGM2) is secondary to mutations affecting both alleles of AICDA gene encoding activation-induced cytidine deaminase, characterized by defects of immunoglobulin class switch recombination (CSR) and somatic hypermutation (SHM) in most of the patients. We herein report the immunological phenotype of seven patients carrying a single heterozygous R190X mutation in AICDA. Variable defect in in vivo CSR inherited as an autosomal dominant (AD) trait strongly suggests that this heterozygous AICDA mutation causes HIGM (AD-HIGM2). In AD-HIGM2 B cells, CSR was consistently found impaired in vitro. However, in contrast to AR-HIGM2, the CSR-induced double-stranded DNA breaks in the switch region of IgM heavy chain gene were detected. The SHM frequency in V regions of IgM heavy chain gene in B cells was normal in all (but one patient). The characteristics of the AD-HIGM2 phenotype indicate that the AID C-terminal region may be involved in DNA repair machinery required for CSR.

The generation of high-affinity antibodies requires somatic hypermutation (SHM) and class switch recombination (CSR) at the immunoglobulin (Ig) locus. Both processes are triggered by activation-induced cytidine deaminase (AID) and require UNG-encoded uracil-DNA glycosylase. AID has been suggested to function as an mRNA editing deaminase or as a single-strand DNA deaminase. In the latter model, SHM may result from replicative incorporation of dAMP opposite U or from error-prone repair of U, whereas CSR may be triggered by strand breaks at abasic sites. Here, we demonstrate that extracts of UNG-proficient human B cell lines efficiently remove U from single-stranded DNA. In B cell lines from hyper-IgM patients carrying UNG mutations, the single-strand–specific uracil-DNA glycosylase, SMUG1, cannot complement this function. Moreover, the UNG mutations lead to increased accumulation of genomic uracil. One mutation results in an F251S substitution in the UNG catalytic domain. Although this UNG form was fully active and stable when expressed in Escherichia coli, it was mistargeted to mitochondria and degraded in mammalian cells. Our results may explain why SMUG1 cannot compensate the UNG2 deficiency in human B cells, and are fully consistent with the DNA deamination model that requires active nuclear UNG2. Based on our findings and recent information in the literature, we present an integrated model for the initiating steps in CSR.

Mutations of the Activation-Induced Cytidine Deaminase (AID) gene have been found in patients with autosomal recessive hyper-IgM (HIGM) syndrome type 2. We retrospectively analyzed clinical, immunologic and genetic characteristics of 29 patients from 22 families with AID deficiency. Patients' median age at diagnosis and at last evaluation was 4.9 years (range: 0 to 53) and 14.2 years (range: 2.7 to 63), respectively. Most patients had suffered from recurrent and severe infections, however, intravenous immunoglobulin (IVIG) replacement therapy resulted in a dramatic decrease in the number of infections. Lymphoid hyperplasia developed in 22 patients and persisted in 7 at last follow-up. It is striking to note that six patients developed autoimmune or inflammatory disorders including diabetes mellitus, polyarthritis, autoimmune hepatitis, hemolytic anemia, immune thrombocytopenia, Crohn's disease and chronic uveitis. Fifteen distinct AID mutations were found but there was no significant genotype-phenotype correlation. In conclusion, AID-deficient patients are prone to infections and lymphoid hyperplasia, which may be prevented by early-onset IVIG replacement, but also to autoimmune and inflammatory disorders.

Activation-induced cytidine deaminase (AID) is a 'master molecule' in immunoglobulin (Ig) class-switch recombination (CSR) and somatic hypermutation (SHM) generation, AID deficiencies are associated with hyper-IgM phenotypes in humans and mice. We show here that recessive mutations of the gene encoding uracil-DNA glycosylase (UNG) are associated with profound impairment in CSR at a DNA precleavage step and with a partial disturbance of the SHM pattern in three patients with hyper-IgM syndrome. Together with the finding that nuclear UNG expression was induced in activated B cells, these data support a model of CSR and SHM in which AID deaminates cytosine into uracil in targeted DNA (immunoglobulin switch or variable regions), followed by uracil removal by UNG.

Maturation of dendritic cells (DCs) is a critical step for the induction of an immune response. We have examined the role of retinoid nuclear receptor pathways in this process. Retinoids induce DC apoptosis, in the absence of inflammatory signals, through retinoic acid receptor (RAR)α/retinoic X receptor (RXR) heterodimers. In contrast, via a cross talk with inflammatory cytokines, retinoids increase DNA binding activity of nuclear factor κB in DCs, trigger membrane major histocompatibility complex class II and costimulatory molecule expression, induce the differentiation of immature DCs into mature DCs, and enhance antigen-specific T cell response. This maturation of DCs is mediated via a RXR-dependent/RAR-independent pathway and via an RARα/RXR pathway distinct from the one responsible for apoptosis. Apoptosis and activation, mediated through distinct nuclear retinoid receptor pathways, can be dissociated from each other with selective synthetic retinoids. We identify a novel cellular function for retinoids and suggest that selective retinoids might be of interest for controlling antigen presentation.

Maturation of the antibody repertoire is mediated by two different mechanisms: class-switch recombination (CSR) and somatic hypermutation (SHM). These two processes are T cell dependent and occur in the germinal centers of secondary lymphoid organs. CSR leads to the production of antibodies of different isotypes whereas SHM leads to the selection of B cells expressing a BCR with high affinity for antigen. The activation-induced cytidine deaminase (AID) was recently shown to play a key role in these two mechanisms, demonstrating for the first time that these maturation processes share a common mechanism. There is evidence that AID is involved in the somatic DNA alterations required for CSR and SHM. The mechanism of action of AID is unclear. As AID and APOBEC-1 display a sequence similarity, AID may act as an RNA-editing enzyme. However, the immunological abnormalities observed in uracil-N glycosylase deficiency in a recent study indirectly suggest that AID may edit DNA directly.

WOS: 000352238600023, PubMed ID: 25312759, Background: Immunoglobulin class-switch recombination defects (CSR-D) are rare primary immunodeficiencies characterized by impaired production of switched immunoglobulin isotypes and normal or elevated IgM levels. They are caused by impaired T:B cooperation or intrinsic B cell defects. However, many immunoglobulin CSR-Ds are still undefined at the molecular level. Objective: This study's objective was to delineate new causes of immunoglobulin CSR-Ds and thus gain further insights into the process of immunoglobulin class-switch recombination (CSR). Methods: Exome sequencing in 2 immunoglobulin CSR-D patients identified variations in the INO80 gene. Functional experiments were performed to assess the function of INO80 on immunoglobulin CSR. Results: We identified recessive, nonsynonymous coding variations in the INO80 gene in 2 patients affected by defective immunoglobulin CSR. Expression of wild-type INO80 in patients' fibroblastic cells corrected their hypersensitivity to high doses of gamma-irradiation. In murine CH12-F3 cells, the INO80 complex accumulates at Sa and Em regions of the IgH locus, and downregulation of INO80 as well as its partners Reptin and Pontin impaired CSR. In addition, Reptin and Pontin were shown to interact with activation-induced cytidine deaminase. Finally, an abnormal separation of sister chromatids was observed upon INO80 downregulation in CH12-F3 cells, pinpointing its role in cohesin activity. Conclusion: INO80 deficiency appears to be associated with defective immunoglobulin CSR. We propose that the INO80 complex modulates cohesin function that may be required during immunoglobulin switch region synapsis., National Institute of Health and Medical ResearchInstitut National de la Sante et de la Recherche Medicale (Inserm); le fonds de recherche clinique du Ministere de la Sante; European Union (EUROPAD) [201549]; European Union (ERC advanced grant PID-IMMUNE) [249816]; Association Contre Le Cancer; Fondation pour la Recherche MedicaleFondation pour la Recherche Medicale [ING20130526624]; la Ligue Contre le Cancer (Comite de Paris); Agence Nationale de la RechercheFrench National Research Agency (ANR) [2010-CSRD]; French Agence Nationale de la Recherche as part of the Investments for the Future programFrench National Research Agency (ANR) [ANR-10-IAHU-01]; La Ligue National contre le Cancer (France); Fondation ARC, France; Institut National du Cancer (INCA), FranceInstitut National du Cancer (INCA) France; German Federal Ministry of Education and ResearchFederal Ministry of Education & Research (BMBF) [BMBF 01 EO 0803]; Canadian Gene Cure Foundation and Finding of Rare Disease Genes (FORGE) Canada; National Institutes of HealthUnited States Department of Health & Human ServicesNational Institutes of Health (NIH) - USA [AI037526], This study was supported by the National Institute of Health and Medical Research, le fonds de recherche clinique du Ministere de la Sante, the European Union's Seventh Research and Technological Development Framework Programme (EUROPAD contract 201549 and ERC advanced grant PID-IMMUNE contract 249816), Association Contre Le Cancer, the Fondation pour la Recherche Medicale (grant number: ING20130526624), la Ligue Contre le Cancer (Comite de Paris) and the Agence Nationale de la Recherche (grant: 2010-CSRD) and a government grant managed by the French Agence Nationale de la Recherche as part of the Investments for the Future program (ANR-10-IAHU-01). This work in the Epigenetics group at the International Agency for Research on Cancer was supported by the La Ligue National contre le Cancer (France), Fondation ARC, France, and Institut National du Cancer (INCA), France. This study was also funded by the German Federal Ministry of Education and Research (grant: BMBF 01 EO 0803) and by the Canadian Gene Cure Foundation and Finding of Rare Disease Genes (FORGE) Canada. This work was supported in part by National Institutes of Health grant AI037526 to M.C.N. M.C.N. is a Howard Hughes Medical Institute investigator. S.K. is a Centre National de la Recherche Scientifique researcher.

Inherited defects in immunoglobulin class switch recombination constitute a group of genetically heterogeneous diseases. The characterization of these class switch recombination defects (which are sometimes associated with defects in the somatic hypermutation process) has prompted the realization that T:B-cell interactions (resulting in CD40-mediated signaling in B-cells), intrinsic B-cell mechanisms (including the induction of DNA lesions by activation-induced cytidine deaminase) and a complex DNA repair machinery are all involved in antibody maturation. These defects were first described in the mid-20th century but have not yet revealed all their secrets. The still on-going molecular definition of these conditions is essential for (1) better understanding the molecular basis of immunoglobulin diversification, (2) defining the clinical spectrum of these diseases and (3) developing more accurate diagnostic and therapeutic approaches.