Research-based flow cytometry assays for pathogenic assessment in the human B-cell biology of gene variants revealed in the diagnosis of inborn errors of immunity: a Bruton's tyrosine kinase case-study.

Introduction: Inborn errors of immunity (IEI) are an expanding group of rare diseases whose field has been boosted by next-generation sequencing (NGS), revealing several new entities, accelerating routine diagnoses, expanding the number of atypical presentations and generating uncertainties regarding the pathogenic relevance of several novel variants. Methods: Research laboratories that diagnose and provide support for IEI require accurate, reproducible and sustainable phenotypic, cellular and molecular functional assays to explore the pathogenic consequences of human leukocyte gene variants and contribute to their assessment. We have implemented a set of advanced flow cytometry-based assays to better dissect human B-cell biology in a translational research laboratory. We illustrate the utility of these techniques for the in-depth characterization of a novel (c.1685G>A, p.R562Q) de novo gene variant predicted as probably pathogenic but with no previous insights into the protein and cellular effects, located in the tyrosine kinase domain of the Bruton's tyrosine kinase (BTK) gene, in an apparently healthy 14-year-old male patient referred to our clinic for an incidental finding of low immunoglobulin (Ig) M levels with no history of recurrent infections. Results and discussion: A phenotypic analysis of bone marrow (BM) revealed a slightly high percentage of pre-B-I subset in BM, with no blockage at this stage, as typically observed in classical X-linked agammaglobulinemia (XLA) patients. The phenotypic analysis in peripheral blood also revealed reduced absolute numbers of B cells, all pre-germinal center maturation stages, together with reduced but detectable numbers of different memory and plasma cell isotypes. The R562Q variant allows Btk expression and normal activation of anti-IgM-induced phosphorylation of Y551 but diminished autophosphorylation at Y223 after anti IgM and CXCL12 stimulation. Lastly, we explored the potential impact of the variant protein for downstream Btk signaling in B cells. Within the canonical nuclear factor kappa B (NF-κB) activation pathway, normal IκBα degradation occurs after CD40L stimulation in patient and control cells. In contrast, disturbed IκBα degradation and reduced calcium ion (Ca2+) influx occurs on anti-IgM stimulation in the patient's B cells, suggesting an enzymatic impairment of the mutated tyrosine kinase domain.

Defective Bcl-2 expression in memory B cells from common variable immunodeficiency patients.

Common variable immunodeficiency (CVID) is a primary immunodeficiency characterized by hypogammaglobulinemia and different degrees of B cell compartment alteration. Memory B cell differentiation requires the orchestrated activation of several intracellular signaling pathways that lead to the activation of a number of factors, such as nuclear factor kappa B (NF-κB) which, in turn, promote transcriptional programs required for long-term survival. The aim of this study was to determine if disrupted B cell differentiation, survival and activation in B cells in CVID patients could be related to defects in intracellular signaling pathways. For this purpose, we selected intracellular readouts that reflected the strength of homeostatic signaling pathways in resting cells, as the protein expression levels of the Bcl-2 family which transcription is promoted by NF-κB. We found reduced Bcl-2 protein levels in memory B cells from CVID patients. We further explored the possible alteration of this crucial prosurvival signaling pathway in CVID patients by analysing the expression levels of mRNAs from anti-apoptotic proteins in naive B cells, mimicking T cell-dependent activation in vitro with CD40L and interleukin (IL)-21. BCL-XL mRNA levels were decreased, together with reduced levels of AICDA, after naive B-cell activation in CVID patients. The data suggested a molecular mechanism for this tendency towards apoptosis in B cells from CVID patients. Lower Bcl-2 protein levels in memory B cells could compromise their long-term survival, and a possible less activity of NF-κB in naive B cells, may condition an inabilityto increase BCL-XL mRNA levels, thus not promoting survival in the germinal centers.

Cross-cutting view of current challenges in paediatric solid organ and haematopoietic stem cell transplantation in Europe: the European Reference Network TransplantChild.

The low prevalence of European paediatric transplanted patients and scarcity of resources and expertise led to the need for a multidisciplinary network able to improve the quality of life of paediatric patients and families requiring a solid organ or haematopoietic stem cell transplantation. The European Reference Network (ERN) TransplantChild is one of the 24 ERNs established in a European legal framework to improve the care of patients with rare diseases. ERN TransplantChild is the only ERN focused on both solid organ and haematopoietic stem cell paediatric transplantation, based on the understanding of paediatric transplantation as a complex and highly specialised process where specific complications appear regardless the organ involved, thus linking the skills and knowledge of different organ disciplines. Gathering European centres of expertise in paediatric transplantation will give access to a correct and timely diagnosis, share expertise and knowledge and collect a critical mass of patients and data that increases the speed and value of clinical research outcomes. Therefore, the ERN TransplantChild aims for a paediatric Pan-European, Pan-transplant approach.

Mutations in PIK3R1 can lead to APDS2, SHORT syndrome or a combination of the two.

Mutations in PIK3R1 gene have been associated to two different conditions: a primary immunodeficiency, called APDS2, of recent description and SHORT syndrome. 47 patients with APDS2 have been reported to date, only one of them sharing both PIK3R1-related phenotypes. Here we describe two more patients affected by APDS2 and SHORT syndrome, which highlights that this association may not be so infrequent. We recommend that patients with mutations in PIK3R1 gene should be assessed by both clinical immunologists and clinical geneticists.

IFITM3 and severe influenza virus infection. No evidence of genetic association.

Influenza virus infection (IVI) is typically subclinical or causes a self-limiting upper respiratory disease. However, in a small subset of patients IVI rapidly progresses to primary viral pneumonia (PVP) with respiratory failure; a minority of patients require intensive care unit admission. Inherited and acquired variability in host immune responses may influence susceptibility and outcome of IVI. However, the molecular basis of such human factors remains largely elusive. It has been proposed that homozygosity for IFITM3 rs12252-C is associated with a population-attributable risk of 5.4 % for severe IVI in Northern Europeans and 54.3 % for severe H1N1pdm infection in Chinese. A total of 148 patients with confirmed IVI were considered for recruitment; 118 Spanish patients (60 of them hospitalized with PVP) and 246 healthy Spanish individuals were finally included in the statistical analysis. PCR-RFLP was used with confirmation by Sanger sequencing. The allele frequency for rs12252-C was found to be 3.5 % among the general Spanish population. We found no rs12252-C homozygous individuals in our control group. The only Spanish patient homozygous for rs12252-C had a neurological disorder (a known risk factor for severe IVI) and mild influenza. Our data do not suggest a role of rs12252-C in the development of severe IVI in our population. These data may be relevant to recognize whether patients homozygous for rs12252-C are at risk of severe influenza, and hence require individualized measures in the case of IVI.

A mutation in X-linked inhibitor of apoptosis (G466X) leads to memory inflation of Epstein-Barr virus-specific T cells.

Mutations in the X-linked inhibitor of apoptosis (XIAP) gene have been associated with XLP-like disease, including recurrent Epstein-Barr virus (EBV)-related haemophagocytic lymphohystiocytosis (HLH), but the immunopathogenic bases of EBV-related disease in XIAP deficiency is unknown. We present the first analysis of EBV-specific T cell responses in functional XIAP deficiency. In a family of patients with a novel mutation in XIAP (G466X) leading to a late-truncated protein and varying clinical features, we identified gradual hypogammaglobulinaemia and large expansions of T cell subsets, including a prominent CD4(+) CD8(+) population. Extensive ex-vivo analyses showed that the expanded T cell subsets were dominated by EBV-specific cells with conserved cytotoxic, proliferative and interferon (IFN)-γ secretion capacity. The EBV load in blood fluctuated and was occasionally very high, indicating that the XIAP(G466X) mutation could impact upon EBV latency. XIAP deficiency may unravel a new immunopathogenic mechanism in EBV-associated disease.

NOD2 polymorphisms in clinical phenotypes of common variable immunodeficiency disorders.

Common variable immunodeficiency disorders (CVIDs) are a heterogeneous group of diseases characterized by hypogammaglobulinaemia and consequent susceptibility to infection. CVID patients commonly develop a variety of additional manifestations for which the causative factors are not fully understood. Two such manifestations are granulomatous disease and enteropathy. Because the ability to predict complications would aid clinical management, we continue to search for possible disease modifier genes. NOD2 acts a microbial sensor and is involved in proinflammatory signalling. Particular mutations of the NOD2 gene are associated with Crohn's disease including gly908arg, leu1007finsc and arg702trp polymorphisms. We hypothesized that NOD2 polymorphisms may be a disease modifier gene towards an enteropathic or granulomatous phenotype within CVIDs. Sequence-specific primers returned genotypes for 285 CVID patients from centres across the United Kingdom and Europe. We present the frequencies of the different phenotypes of patients within our international cohort. Arg702trp polymorphisms were significantly less frequent than wild-type (WT) (P = 0·038) among international CVID patients with splenomegaly. Gly908arg polymorphisms were more prevalent than WT in UK patients with autoimmune disorders (P = 0·049) or enteropathy (P = 0·049). NOD2 polymorphisms were not more prevalent than WT in CVID patients with clinical phenotypes of granulomata. UK allele frequencies of 0·014, 0·056 and 0·026 were found for gly908arg, arg702trp and leu1007finsc NOD2 polymorphisms, respectively. These do not differ significantly from UK immunocompetent controls confirming, as expected, that in addition these NOD2 polymorphisms do not confer susceptibility to CVIDs per se.

Naturally occurring Bruton's tyrosine kinase mutations have no dominant negative effect in an X-linked agammaglobulinaemia cellular model.

X-linked agammaglobulinaemia (XLA) is characterized by absence of mature B cells because of mutations in the Bruton's tyrosine kinase (Btk) gene. Btk-deficient early B cell precursors experience a block in their differentiation potentially reversible by the addition of an intact Btk gene. Btk expression was measured in 69 XLA patients with 47 different mutations and normal expression was detected in seven. We characterized these Btk mutant forms functionally by transfection into a lymphoma cell line that lacks endogenous Btk expression (Btk-/- DT40 cells) and analysed the calcium flux in response to B cell receptor stimulation. To test whether co-expression of a mutated form could compromise the function of the intact Btk transfection, studies in wild-type (WT) DT40 cells were also performed. Study reveals that none of the seven Btk mutants analysed was able to revert the absence of calcium mobilization upon IgM engagement in Btk-/- DT40 cells, as does intact Btk. In addition, calcium mobilization by anti-IgM stimulation in DT40 Btk+/+ cells was unaffected by co-expression with Btk mutants. These results suggest that gene addition would be feasible not only for patients with XLA and mutations that prevent Btk expression, but for those with expression of a mutant Btk.

Three novel mutations reflect the variety of defects causing phenotypically diverse X-linked hyper-IgM syndrome.

X-linked hyper-IgM syndrome (HIGM1) (MIM musical sharp 308230), is a severe primary immunodeficiency caused by mutations in the gene coding for CD40 ligand (CD40L or CD154), a member of the tumour necrosis factor (TNF) superfamily. The interaction of this protein with its ligand, CD40, mediates crucial processes in the immune response. The variety of defects that have been described in HIGM1 patients range from a complete lack of CD40L protein expression to missense mutations that interfere with its interaction with CD40L. In this study we describe three families - a total of seven HIGM1 patients and carriers, presenting a spectrum of severity in clinical evolution. In two of these families, patient DNA samples were available for genetic studies. In the third, carrier detection was performed on female family members. The results of immunological studies - the different patterns of CD40L expression and binding capacity as measured by flow cytometry - and molecular diagnosis are presented. Three novel mutations were identified: an intron mutation that partially interferes with the splicing process (intron 3, position + 5 G/T); a missense mutation (Ser222 Phe) located in the molecular region which interacts with the receptor and which abrogates binding capacity; and a 14 base pair deletion leading to a frameshift and a premature truncated mutation (del I 171 X 195). An attempt to correlate protein expression and function of the CD40L mutants with clinical disease evolution is described.

[Molecular diagnosis of primary immunodeficiencies]. / Diagnóstico molecular de inmunodeficiencias primarias.

Knowledge of the molecular defects responsible for some primary immunodeficiency diseases (PIDs) offers undoubted advantages in establishing a reliable diagnosis. Such knowledge would allow us not only to establish a prognosis but also to instigate the most appropriate therapy. After molecular diagnosis, some patients could benefit from gene therapy. However, apart from the diagnosis of the disease, molecular biological techniques also enable more reliable identification of carriers and, when suggested by the family history and when the familial defect is already known, prenatal diagnosis will also be possible, thus establishing the earliest possible treatment. Using the single-stranded conformational polymorphism technique followed by direct sequencing, we found 22 different mutations in 22 patients from unrelated families and with a phenotype compatible with x-linked agammaglobulinemia. Fourteen of these are new, previously undescribed mutations and the remaining eight are already included in the data base (http://www.uta.fi/imt/bioinfo/Btkbase). Analysis of the female carrier was performed in all the mothers and the mutation was de novo in only one patient. Study of the BtK gene enabled differential diagnosis with common variable immunodeficiency disease in some patients who showed absent or very low lymphocyte B counts as well as forms of autosomal recessive agammaglobulinemia. Using the same techniques, we were able to identify mutations in the CD40 ligand gene in three families in which one of the members had clinical and biological phenotype compatible with X-linked hyper-IgM. Molecular diagnosis was very useful in identifying carriers in these families as well as in making the differential diagnosis among patients with common variable immunodeficiency disease. Purely on this were we able to provide appropriate genetic counseling.

Cellular adhesion mediated by factor J, a complement inhibitor. Evidence for nucleolin involvement.

Factor J (FJ) is a complement inhibitor that acts on the classical and the alternative pathways. We demonstrated FJ-cell interactions in fluid phase by flow cytometry experiments using the cell lines Jurkat, K562, JY, and peripheral blood lymphocytes. FJ bound to plastic plates was able to induce in vitro adhesion of these cells with potency equivalent to fibronectin. As evidence for the specificity of this reaction, the adhesion was blocked by MAJ2, an anti-FJ monoclonal antibody, and by soluble FJ. Attachment of the cells required active metabolism and cytoskeletal integrity. The glycosaminoglycans heparin, heparan sulfate, or chondroitin sulfates A, B, and C inhibited to varying degrees the binding of FJ to cells, as did treatment with chondroitinase ABC. In the search for a putative receptor, a protein of 110 kDa was isolated by affinity chromatography, and microsequence analysis identified this protein as nucleolin. Confocal microscopy evidenced the presence of nucleolin in cell membrane by immunofluorescence with monoclonal (D3) and polyclonal anti-nucleolin antibodies in Jurkat cells. The interaction FJ-nucleolin was evidenced by Western blot and enzyme-linked immunosorbent assay. Furthermore, purified nucleolin and D3 inhibited adhesion of Jurkat cells to immobilized FJ, suggesting that the interaction was specific and that nucleolin mediated the binding.