Causative mechanisms and clinical impact of immunoglobulin deficiencies in ataxia telangiectasia.

BACKGROUND: Ataxia telangiectasia (AT) is characterized by cerebellar ataxia, telangiectasia, immunodeficiency, and increased cancer susceptibility and is caused by mutations in the ataxia telangiectasia mutated (ATM) gene. The immunodeficiency comprises predominantly immunoglobulin deficiency, mainly IgA and IgG2, with a variable severity. So far, the exact mechanisms underlying the immunoglobulin deficiency, especially the variable severity, remain unelucidated. OBJECTIVE: We characterized the clinical impact of immunoglobulin deficiencies in AT and elucidated their mechanisms in AT. METHODS: We analyzed long-term immunoglobulin levels, immunophenotyping, and survival time in our cohort (n = 87, median age 16 years; maximum 64 years). Somatic hypermutation and class-switch junctions in B cells were analyzed by next-generation sequencing. Furthermore, an in vitro class-switching induction assay was performed, followed by RNA sequencing, to assess the effect of ATM inhibition. RESULTS: Only the hyper-IgM AT phenotype significantly worsened survival time, while IgA or IgG2 deficiencies did not. The immunoglobulin levels showed predominantly decreased IgG2 and IgA. Moreover, flow cytometric analysis demonstrated reduced naive B and T lymphocytes and a deficiency of class-switched IgG2 and IgA memory B cells. Somatic hypermutation frequencies were lowered in IgA- and IgG2-deficient patients, indicating hampered germinal center reaction. In addition, the microhomology of switch junctions was elongated, suggesting alternative end joining during class-switch DNA repair. The in vitro class switching and proliferation were negatively affected by ATM inhibition. RNA sequencing analysis showed that ATM inhibitor influenced expression of germinal center reaction genes. CONCLUSION: Immunoglobulin deficiency in AT is caused by disturbed development of class-switched memory B cells. ATM deficiency affects both germinal center reaction and choice of DNA-repair pathway in class switching.

ATM: Translating the DNA Damage Response to Adaptive Immunity.

ATM is often dubbed the master regulator of the DNA double stranded break (DSB) response. Since proper induction and repair of DNA DSBs forms the core of immunological diversity, it is surprising that patients with ataxia telangiectasia generally have a mild immunodeficiency in contrast to other DSB repair syndromes. In this review, we address this discrepancy by delving into the functions of ATM in DSB repair and cell cycle control and translate these to adaptive immunity. We conclude that ATM, despite its myriad functions, is not an absolute requirement for acquiring sufficient levels of immunological diversity to prevent severe viral and opportunistic infections. There is, however, a more clinically pronounced antibody deficiency in ataxia telangiectasia due to disturbed class switch recombination.

Nicotinamide Riboside Improves Ataxia Scores and Immunoglobulin Levels in Ataxia Telangiectasia.

BACKGROUND: Treatment of animal models with ataxia telangiectasia (A-T) with nicotinamide riboside (NR) improved their neurological outcome and survival. OBJECTIVE: The aim of this study is to investigate the effects of NR in patients with A-T. METHODS: In this open-label, proof-of-concept study, 24 patients with A-T were treated with NR during four consecutive months. The effects of NR on ataxia, dysarthria, quality of life, and laboratory parameters were analyzed. RESULTS: During treatment, ataxia scores improved; mean total Scale for the Assessment and Rating of Ataxia and International Cooperative Ataxia Rating Scale scores decreased to 2.4 and 10.1 points, respectively. After NR withdrawal, ataxia scores worsened. In immunodeficient patients, the mean serum IgG concentration increased substantially until the end of the study period with 0.52 g/L. Untargeted metabolomics analysis revealed increased plasma levels of NR metabolites and purine nucleosides during treatment. Adverse effects did not occur. CONCLUSIONS: Treatment with NR is tolerated well and associated with improvement in ataxia and serum immunoglobulin concentrations in patients with A-T. © 2021 The Authors. Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson and Movement Disorder Society.

Dysarthria in children and adults with ataxia telangiectasia.

AIM: To investigate the characteristics and severity of dysarthria in children and adults with ataxia telangiectasia. METHOD: All children and adults with ataxia telangiectasia who visited our multidisciplinary outpatient clinic for ataxia telangiectasia were asked to participate in this study, which took place in March 2019. To evaluate dysarthria, we used the Radboud Dysarthria Assessment in adults (older than 18y) and the paediatric Radboud Dysarthria Assessment in children (5-18y), including the observational tasks 'conversation' and 'reading', and the speech-related maximum performance tasks 'repetition rate', 'phonation time', 'fundamental frequency range', and 'phonation volume'. Speech intelligibility was measured using the Intelligibility in Context Scale. RESULTS: Twenty-two individuals (15 children [5-17y], seven adults [19-47y]; 14 males and eight females; mean age 19y, SD 15y 2mo) participated. Dysarthria was present in all participants and characterized by ataxic components in adults and similar uncontrolled movements in children. In most participants, speech was mildly to mildly/severely affected. Almost all participants had an abnormal score for at least one maximum performance task. INTERPRETATION: Dysarthria in ataxia telangiectasia is characterized by uncontrolled, ataxic, and involuntary movements, resulting in monotonous, unstable, slow, hypernasal, and chanted speech. WHAT THIS PAPER ADDS: Dysarthria in ataxia telangiectasia is characterized by uncontrolled, ataxic, and involuntary movements. Dysarthria in ataxia telangiectasia results in monotonous, unstable, slow, hypernasal, and chanted speech. Dysarthria in ataxia telangiectasia can be assessed using the Radboud Dysarthria Assessment and the paediatric Radboud Dysarthria Assessment.

Genotype, extrapyramidal features, and severity of variant ataxia-telangiectasia.

OBJECTIVE: Variant ataxia-telangiectasia is caused by mutations that allow some retained ataxia telangiectasia-mutated (ATM) kinase activity. Here, we describe the clinical features of the largest established cohort of individuals with variant ataxia-telangiectasia and explore genotype-phenotype correlations. METHODS: Cross-sectional data were collected retrospectively. Patients were classified as variant ataxia-telangiectasia based on retained ATM kinase activity. RESULTS: The study includes 57 individuals. Mean age at assessment was 37.5 years. Most had their first symptoms by age 10 (81%). There was a diagnostic delay of more than 10 years in 68% and more than 20 years in one third of probands. Disease severity was mild in one third of patients, and 43% were still ambulant 20 years after disease onset. Only one third had predominant ataxia, and 18% had a pure extrapyramidal presentation. Individuals with extrapyramidal presentations had milder neurological disease severity. There were no significant respiratory or immunological complications, but 25% of individuals had a history of malignancy. Missense mutations were associated with milder neurological disease severity, but with a higher risk of malignancy, compared to leaky splice site mutations. INTERPRETATION: Individuals with variant ataxia-telangiectasia require malignancy surveillance and tailored management. However, our data suggest the condition may sometimes be mis- or underdiagnosed because of atypical features, including exclusive extrapyramidal symptoms, normal eye movements, and normal alpha-fetoprotein levels in some individuals. Missense mutations are associated with milder neurological presentations, but a particularly high malignancy risk, and it is important for clinicians to be aware of these phenotypes. ANN NEUROL 2019;85:170-180.

Genotype-phenotype correlations in ataxia telangiectasia patients with ATM c.3576G>A and c.8147T>C mutations.

BACKGROUND: Ataxia telangiectasia (A-T) is a neurodegenerative disorder. While patients with classic A-T generally die in their 20s, some patients with variant A-T, who have residual ataxia-telangiectasia mutated (ATM) kinase activity, have a milder phenotype. We noticed two commonly occurring ATM mutations that appeared to be associated with prolonged survival and decided to study patients carrying one of these mutations. METHODS: Data were retrospectively collected from the Dutch, Italian, German and French A-T cohorts. To supplement these data, we searched the literature for patients with identical genotypes. RESULTS: This study included 35 patients who were homozygous or compound heterozygous for the ATM c.3576G>A; p.(Ser1135_Lys1192del58) mutation and 24 patients who were compound heterozygous for the ATM c.8147T>C; p.(Val2716Ala) mutation. Compared with 51 patients with classic A-T from the Dutch cohort, patients with ATM c.3576G>A had a longer survival and were less likely to develop cancer, respiratory disease or immunodeficiency. This was also true for patients with ATM c.8147T>C, who additionally became wheelchair users later in life and had fewer telangiectasias. The oldest patient with A-T reported so far was a 78-year-old patient who was compound heterozygous for ATM c.8147T>C. ATM kinase activity was demonstrated in cells from all patients tested with the ATM c.8147T>C mutant protein and only at a low level in some patients with ATM c.3576G>A. CONCLUSION: Compared with classic A-T, the presence of ATM c.3576G>A results in a milder classic phenotype. Patients with ATM c.8147T>C have a variant phenotype with prolonged survival, which in exceptional cases may approach a near-normal lifespan.

NBS1 recruits RAD18 via a RAD6-like domain and regulates Pol η-dependent translesion DNA synthesis.

Translesion DNA synthesis, a process orchestrated by monoubiquitinated PCNA, is critical for DNA damage tolerance. While the ubiquitin-conjugating enzyme RAD6 and ubiquitin ligase RAD18 are known to monoubiquitinate PCNA, how they are regulated by DNA damage is not fully understood. We show that NBS1 (mutated in Nijmegen breakage syndrome) binds to RAD18 after UV irradiation and mediates the recruitment of RAD18 to sites of DNA damage. Disruption of NBS1 abolished RAD18-dependent PCNA ubiquitination and Polη focus formation, leading to elevated UV sensitivity and mutation. Unexpectedly, the RAD18-interacting domain of NBS1, which was mapped to its C terminus, shares structural and functional similarity with the RAD18-interacting domain of RAD6. These domains of NBS1 and RAD6 allow the two proteins to interact with RAD18 homodimers simultaneously and are crucial for Polη-dependent UV tolerance. Thus, in addition to chromosomal break repair, NBS1 plays a key role in translesion DNA synthesis.

IgM Augments Complement Bactericidal Activity with Serum from a Patient with a Novel CD79a Mutation.

Antibody replacement therapy for patients with antibody deficiencies contains only IgG. As a result, concurrent IgM and IgA deficiency present in a large proportion of antibody deficient patients persists. Especially patients with IgM deficiency remain at risk for recurrent infections of the gastrointestinal and respiratory tract. The lack of IgM in the current IgG replacement therapy is likely to contribute to the persistence of these mucosal infections because this antibody class is especially important for complement activation on the mucosal surface. We evaluated whether supplementation with IgM increased serum bactericidal capacity in vitro. Serum was collected from a patient with agammaglobulinemia and supplemented with purified serum IgM to normal levels. Antibody and complement deposition on the bacterial surface was determined by multi-color flow cytometry. Bacterial survival in serum was determined by colony-forming unit counts. We present a patient previously diagnosed with agammaglobulinemia due to CD79A (Igα) deficiency revealing a novel pathogenic insertion variant in the CD79a gene (NM_001783.3:c.353_354insT). Despite IgG replacement therapy and antibiotic prophylaxis, this patient developed a Campylobacter jejuni spondylodiscitis of lumbar vertebrae L4-L5. We found that serum IgM significantly contributes to complement activation on the bacterial surface of C. jejuni. Furthermore, supplementation of serum IgM augmented serum bactericidal activity significantly. In conclusion, supplementation of intravenous IgG replacement therapy with IgM may potentially offer greater protection against bacterial infections, also in the context of increasing antibiotic resistance.

Immunodeficiency in Bloom's Syndrome.

Bloom's syndrome (BS) is an autosomal recessive disease, caused by mutations in the BLM gene. This gene codes for BLM protein, which is a helicase involved in DNA repair. DNA repair is especially important for the development and maturation of the T and B cells. Since BLM is involved in DNA repair, we aimed to study if BLM deficiency affects T and B cell development and especially somatic hypermutation (SHM) and class switch recombination (CSR) processes. Clinical data of six BS patients was collected, and immunoglobulin serum levels were measured at different time points. In addition, we performed immune phenotyping of the B and T cells and analyzed the SHM and CSR in detail by analyzing IGHA and IGHG transcripts using next-generation sequencing. The serum immunoglobulin levels were relatively low, and patients had an increased number of infections. The absolute number of T, B, and NK cells were low but still in the normal range. Remarkably, all BS patients studied had a high percentage (20-80%) of CD4+ and CD8+ effector memory T cells. The process of SHM seems normal; however, the Ig subclass distribution was not normal, since the BS patients had more IGHG1 and IGHG3 transcripts. In conclusion, BS patients have low number of lymphocytes, but the immunodeficiency seems relatively mild since they have no severe or opportunistic infections. Most changes in the B cell development were seen in the CSR process; however, further studies are necessary to elucidate the exact role of BLM in CSR.

Ataxia-telangiectasia: Immunodeficiency and survival.

Ataxia-telangiectasia (AT) is a neurodegenerative disorder characterized by ataxia, telangiectasia, and immunodeficiency. An increased risk of malignancies and respiratory diseases dramatically reduce life expectancy. To better counsel families, develop individual follow-up programs, and select patients for therapeutic trials, more knowledge is needed on factors influencing survival. This retrospective cohort study of 61 AT patients shows that classical AT patients had a shorter survival than variant patients (HR 5.9, 95%CI 2.0-17.7), especially once a malignancy was diagnosed (HR 2.5, 95%CI 1.1-5.5, compared to classical AT patients without malignancy). Patients with the hyper IgM phenotype with hypogammaglobulinemia (AT-HIGM) and patients with an IgG2 deficiency showed decreased survival compared to patients with normal IgG (HR 9.2, 95%CI 3.2-26.5) and patients with normal IgG2 levels (HR 7.8, 95%CI 1.7-36.2), respectively. If high risk treatment trials will become available for AT, those patients with factors indicating the poorest prognosis might be considered for inclusion first.

Primary immunodeficiency caused by an exonized retroposed gene copy inserted in the CYBB gene.

Retrotransposon-mediated insertion of a long interspersed nuclear element (LINE)-1 or an Alu element into a human gene is a well-known pathogenic mechanism. We report a novel LINE-1-mediated insertion of a transcript from the TMF1 gene on chromosome 3 into the CYBB gene on the X-chromosome. In a Dutch male patient with chronic granulomatous disease, a 5.8-kb, incomplete and partly exonized TMF1 transcript was identified in intron 1 of CYBB, in opposite orientation to the host gene. The sequence of the insertion showed the hallmarks of a retrotransposition event, with an antisense poly(A) tail, target site duplication, and a consensus LINE-1 endonuclease cleavage site. This insertion induced aberrant CYBB mRNA splicing, with inclusion of an extra 117-bp exon between exons 1 and 2 of CYBB. This extra exon contained a premature stop codon. The retrotransposition took place in an early stage of fetal development in the mother of the patient, because she showed a somatic mosaicism for the mutation that was not present in the DNA of her parents. However, the mutated allele was not expressed in the patient's mother because the insertion was found only in the methylated fraction of her DNA.

Mutations in ZBTB24 are associated with immunodeficiency, centromeric instability, and facial anomalies syndrome type 2.

Autosomal-recessive immunodeficiency, centromeric instability, and facial anomalies (ICF) syndrome is mainly characterized by recurrent, often fatal, respiratory and gastrointestinal infections. About 50% of patients carry mutations in the DNA methyltransferase 3B gene (DNMT3B) (ICF1). The remaining patients carry unknown genetic defects (ICF2) but share with ICF1 patients the same immunological and epigenetic features, including hypomethylation of juxtacentromeric repeat sequences. We performed homozygosity mapping in five unrelated ICF2 patients with consanguineous parents and then performed whole-exome sequencing in one of these patients and Sanger sequencing in all to identify mutations in the zinc-finger- and BTB (bric-a-bric, tramtrack, broad complex)-domain-containing 24 (ZBTB24) gene in four consanguineously descended ICF2 patients. Additionally, we found ZBTB24 mutations in an affected sibling pair and in one patient for whom it was not known whether his parents were consanguineous. ZBTB24 belongs to a large family of transcriptional repressors that include members, such as BCL6 and PATZ1, with prominent regulatory roles in hematopoietic development and malignancy. These data thus indicate that ZBTB24 is involved in DNA methylation of juxtacentromeric DNA and in B cell development and/or B and T cell interactions. Because ZBTB24 is a putative DNA-binding protein highly expressed in the lymphoid lineage, we predict that by studying the molecular function of ZBTB24, we will improve our understanding of the molecular pathophysiology of ICF syndrome and of lymphocyte biology in general.

Antibody deficiency in patients with ataxia telangiectasia is caused by disturbed B- and T-cell homeostasis and reduced immune repertoire diversity.

BACKGROUND: Ataxia telangiectasia (AT) is a multisystem DNA-repair disorder caused by mutations in the ataxia telangiectasia mutated (ATM) gene. Patients with AT have reduced B- and T-cell numbers and a highly variable immunodeficiency. ATM is important for V(D)J recombination and immunoglobulin class-switch recombination (CSR); however, little is known about the mechanisms resulting in antibody deficiency severity. OBJECTIVE: We sought to examine the immunologic mechanisms responsible for antibody deficiency heterogeneity in patients with AT. METHODS: In this study we included patients with classical AT plus early-onset hypogammaglobulinemia (n = 3), classical AT (n = 8), and variant AT (late onset, n = 4). We studied peripheral B- and T-cell subsets, B-cell subset replication history, somatic hypermutation frequencies, CSR patterns, B-cell repertoire, and ATM kinase activity. RESULTS: Patients with classical AT lacked ATM kinase activity, whereas patients with variant AT showed residual function. Most patients had disturbed naive B-cell and T-cell homeostasis, as evidenced by low cell numbers, increased proliferation, a large proportion CD21(low)CD38(low) anergic B cells, and decreased antigen receptor repertoire diversity. Impaired formation of T cell-dependent memory B cells was predominantly found in patients with AT plus hypogammaglobulinemia. These patients had extremely low naive CD4(+) T-cell counts, which were more severely reduced compared with those seen in patients with classical AT without hypogammaglobulinemia. Finally, AT deficiency resulted in defective CSR to distal constant regions that might reflect an impaired ability of B cells to undergo multiple germinal center reactions. CONCLUSION: The severity of the antibody deficiency in patients with AT correlates with disturbances in B- and T-cell homeostasis resulting in reduced immune repertoire diversity, which consequently affects the chance of successful antigen-dependent cognate B-T interaction.

Presence of ATM protein and residual kinase activity correlates with the phenotype in ataxia-telangiectasia: a genotype-phenotype study.

Ataxia-telangiectasia (A-T) is an autosomal recessive neurodegenerative disorder with multisystem involvement and cancer predisposition, caused by mutations in the A-T mutated (ATM) gene. To study genotype-phenotype correlations, we evaluated the clinical and laboratory data of 51 genetically proven A-T patients, and additionally measured ATM protein expression and kinase activity. Patients without ATM kinase activity showed the classical phenotype. The presence of ATM protein, correlated with slightly better immunological function. Residual kinase activity correlated with a milder and essentially different neurological phenotype, absence of telangiectasia, normal endocrine and pulmonary function, normal immunoglobulins, significantly lower X-ray hypersensitivity in lymphocytes, and extended lifespan. In these patients, cancer occurred later in life and generally consisted of solid instead of lymphoid malignancies. The genotypes of severely affected patients generally included truncating mutations resulting in total absence of ATM kinase activity, while patients with milder phenotypes harbored at least one missense or splice site mutation resulting in expression of ATM with some kinase activity. Overall, the phenotypic manifestations in A-T show a continuous spectrum from severe classical childhood-onset A-T to a relatively mild adult-onset disorder, depending on the presence of ATM protein and kinase activity. Each patient is left with a tremendously increased cancer risk.

Neuropathology in classical and variant ataxia-telangiectasia.

Ataxia-telangiectasia (A-T) is classically characterized by progressive neurodegeneration, oculocutaneous telangiectasia, immunodeficiency and elevated α-fetoprotein levels. Some patients, classified as variant A-T, exhibit a milder clinical course. In the latter patients extrapyramidal symptoms, instead of cerebellar ataxia, tend to be the dominating feature and other classical disease hallmarks, like telangiectasia, appear later or even may be absent. Some patients with variant disease have clinically pronounced anterior horn cell degeneration. Neuropathological studies of genetically proven A-T patients are lacking. The aims of our study were to describe the neuropathology of three A-T patients; in two of them the diagnosis was genetically confirmed. The neuropathological findings were compared with those of all known published autopsy findings in A-T patients up to now. Two classical A-T patients aged 19 and 22 and a 33-year-old patient with variant disease were autopsied. In line with previous reports, our patients had severe cerebellar atrophy, less pronounced degeneration of the dentate nucleus and inferior olive, degeneration of the posterior columns and neurogenic muscular atrophy. In addition, all three had anterior horn cell degeneration, which was most prominent at the lumbar level. Compared to the literature, the degenerative changes in the brain stem of the variant A-T patient were somewhat less than anticipated for his age. Degenerative changes in the cerebellum and spinal cord were comparable with those in the literature. Progeric changes were lacking. In conclusion, compared to classical A-T, the variant A-T patient showed essentially the same, only slightly milder neuropathological abnormalities, except for anterior horn degeneration.

Angiosarcoma in a patient with immunodeficiency, centromeric region instability, facial anomalies (ICF) syndrome.

The Immunodeficiency, Centromeric region instability, and Facial anomalies (ICF) syndrome (OMIM #242860) is a rare autosomal recessive disorder caused by defective DNA methylation. Hematological disease and malignancy (macrophage activation syndrome, myelodysplastic syndrome, and Hodgkin lymphoma) have been reported in three patients. To date, there have been no reports of either epithelial or mesenchymal malignancies. We present a patient with all clinical and laboratory findings of the ICF syndrome who died of a metastatic angiosarcoma of the liver. This is the first report of a non-hematological malignancy in the ICF syndrome. The young age at which our patient developed an angiosarcoma suggests an effect of the defective DNA methylation observed in the ICF syndrome. Therefore, with improvement of recognition and treatment of the ICF syndrome, malignancy could become more common in this condition.

Educational paper: syndromic forms of primary immunodeficiency.

The syndromic primary immunodeficiencies are disorders in which not only the immune system but also other organ systems are affected. Other features most commonly involve the ectodermal, skeletal, nervous, and gastrointestinal systems. Key in identifying syndromic immunodeficiencies is the awareness that increased susceptibility to infections or immune dysregulation in a patient known to have other symptoms or special features may hint at an underlying genetic syndrome. Because the extraimmune clinical features can be highly variable, it is more difficult establishing the correct diagnosis. Nevertheless, correct diagnosis at an early age is important because of the possible treatment options. Therefore, diagnostic work-up is best performed in a center with extensive expertise in this field, having immunologists and clinical geneticists, as well as adequate support from a specialized laboratory at hand. This paper provides the general pediatrician with the main clinical features that are crucial for the recognition of these syndromes.

Immunodeficiency in a child with partial trisomy 6p.

AIM: We present a mentally retarded boy with partial trisomy of the short arm of chromosome 6 as a result of an interstitial tandem duplication of 6p12.2-p21.31 and immunodeficiency. Patients with mental retardation because of a chromosomal disorder or eponymous syndrome often experience recurrent respiratory tract infections as a result of their associated anatomical or neurological abnormalities. However, associated immune defects may also significantly contribute to their susceptibility to infections. Timely recognition and appropriate treatment of their immunodeficiency will greatly improve quality of life in these patients. CONCLUSION: Immunodeficiency may be the direct cause of recurrent respiratory tract infections in patients with mental retardation because of a chromosomal disorder or eponymous syndrome, even in the face of feeding difficulties and multiple episodes of aspiration, as is illustrated in this boy with partial trisomy 6p.

Normal Numbers of Stem Cell Memory T Cells Despite Strongly Reduced Naive T Cells Support Intact Memory T Cell Compartment in Ataxia Telangiectasia.

Ataxia Telangiectasia (AT) is a rare inherited disorder characterized by progressive cerebellar ataxia, chromosomal instability, cancer susceptibility and immunodeficiency. AT is caused by mutations in the ATM gene, which is involved in multiple processes linked to DNA double strand break repair. Immunologically, ATM mutations lead to hampered V(D)J recombination and consequently reduced numbers of naive B and T cells. In addition, class switch recombination is disturbed resulting in antibody deficiency causing common, mostly sinopulmonary, bacterial infections. Yet, AT patients in general have no clinical T cell associated infections and numbers of memory T cells are usually normal. In this study we investigated the naive and memory T cell compartment in five patients with classical AT and compared them with five healthy controls using a 24-color antibody panel and spectral flow cytometry. Multidimensional analysis of CD4 and CD8 TCRαß+ cells revealed that early naive T cell populations, i.e. CD4+CD31+ recent thymic emigrants and CD8+CCR7++CD45RA++ T cells, were strongly reduced in AT patients. However, we identified normal numbers of stem cell memory T cells expressing CD95, which are antigen-experienced T cells that can persist for decades because of their self-renewal capacity. We hypothesize that the presence of stem cell memory T cells explains why AT patients have an intact memory T cell compartment. In line with this novel finding, memory T cells of AT patients were normal in number and expressed chemokine receptors, activating and inhibitory receptors in comparable percentages as controls. Comparing memory T cell phenotypes by Boolean gating revealed similar diversity indices in AT compared to controls. We conclude that AT patients have a fully developed memory T cell compartment despite strongly reduced naive T cells. This could be explained by the presence of normal numbers of stem cell memory T cells in the naive T cell compartment, which support the maintenance of the memory T cells. The identification of stem cell memory T cells via our spectral flow cytometric approach is highly relevant for better understanding of T cell immunity in AT. Moreover, it provides possibilities for further research on this recently identified T cell population in other inborn errors of immunity.

The same IkappaBalpha mutation in two related individuals leads to completely different clinical syndromes.

Both innate and adaptive immune responses are dependent on activation of nuclear factor kappaB (NF-kappaB), induced upon binding of pathogen-associated molecular patterns to Toll-like receptors (TLRs). In murine models, defects in NF-kappaB pathway are often lethal and viable knockout mice have severe immune defects. Similarly, defects in the human NF-kappaB pathway described to date lead to severe clinical disease. Here, we describe a patient with a hyper immunoglobulin M-like immunodeficiency syndrome and ectodermal dysplasia. Monocytes did not produce interleukin 12p40 upon stimulation with various TLR stimuli and nuclear translocation of NF-kappaB was impaired. T cell receptor-mediated proliferation was also impaired. A heterozygous mutation was found at serine 32 in IkappaBalpha. Interestingly, his father has the same mutation but displays complex mosaicism. He does not display features of ectodermal dysplasia and did not suffer from serious infections with the exception of a relapsing Salmonella typhimurium infection. His monocyte function was impaired, whereas T cell function was relatively normal. Consistent with this, his T cells almost exclusively displayed the wild-type allele, whereas both alleles were present in his monocytes. We propose that the T and B cell compartment of the mosaic father arose as a result of selection of wild-type cells and that this underlies the widely different clinical phenotype.