Mutations in Igalpha (CD79a) result in a complete block in B-cell development

B cell deficient mice are protected from biliary obstruction in the rotavirus-induced mouse model of biliary atresia

A leading theory regarding the pathogenesis of biliary atresia (BA) is that bile duct injury is initiated by a virus infection, followed by an autoimmune response targeting bile ducts. In experimental models of autoimmune diseases, B cells have been shown to play an important role. The aim of this study was to determine the role of B cells in the development of biliary obstruction in the Rhesus rotavirus (RRV)-induced mouse model of BA. Wild-type (WT) and B cell-deficient (Ig-α(-/-)) mice received RRV shortly after birth. Ig-α(-/-) RRV-infected mice had significantly increased disease-free survival rate compared to WT RRV-infected BA mice (76.8% vs. 17.5%). In stark contrast to the RRV-infected BA mice, the RRV-infected Ig-α(-/-) mice did not have hyperbilirubinemia or bile duct obstruction. The RRV-infected Ig-α(-/-) mice had significantly less liver inflammation and Th1 cytokine production compared to RRV-infected WT mice. In addition, Ig-α(-/-) mice had significantly increased numbers of regulatory T cells (Tregs) at baseline and after RRV infection compared to WT mice. However, depletion of Tregs in Ig-α(-/-) mice did not induce biliary obstruction, indicating that the expanded Tregs in the Ig-α(-/-) mice were not the sole reason for protection from disease. Conclusion : B cell deficient Ig-α(-/-) mice are protected from biliary obstruction in the RRV-induced mouse model of BA, indicating a primary role of B cells in mediating disease pathology. The mechanism of protection may involve lack of B cell antigen presentation, which impairs T-cell activation and Th1 inflammation. Immune modulators that inhibit B cell function may be a new strategy for treatment of BA.

B cell receptor accessory molecule CD79α: characterisation and expression analysis in a cartilaginous fish, the spiny dogfish (Squalus acanthias)

CD79α (also known as Igα) is a component of the B cell antigen receptor complex and plays an important role in B cell signalling. The CD79α protein is present on the surface of B cells throughout their life cycle, and is absent on all other healthy cells, making it a highly reliable marker for B cells in mammals. In this study the spiny dogfish (Squalus acanthias) CD79α (SaCD79α) is described and its expression studied under constitutive and stimulated conditions. The spiny dogfish CD79α cDNA contains an open reading frame of 618 bp, encoding a protein of 205 amino acids. Comparison of the SaCD79α gene with that of other species shows that the gross structure (number of exons, exon/intron boundaries, etc.) is highly conserved across phylogeny. Additionally, analysis of the 5' flanking region shows SaCD79α lacks a TATA box and possesses binding sites for multiple transcription factors implicated in its B cell-specific gene transcription in other species. Spiny dogfish CD79α is most highly expressed in immune tissues, such as spleen, epigonal and Leydig organ, and its transcript level significantly correlates with those of spiny dogfish immunoglobulin heavy chains. Additionally, CD79α transcription is up-regulated, to a small but significant degree, in peripheral blood cells following stimulation with pokeweed mitogen. These results strongly indicate that, as in mammals, spiny dogfish CD79α is expressed by shark B cells where it associates with surface-bound immunoglobulin to form a fully functional BCR, and thus may serve as a pan-B cell marker in future shark immunological studies.

The genetic theory of infectious diseases: a brief history and selected illustrations

Until the mid-nineteenth century, life expectancy at birth averaged 20 years worldwide, owing mostly to childhood fevers. The germ theory of diseases then gradually overcame the belief that diseases were intrinsic. However, around the turn of the twentieth century, asymptomatic infection was discovered to be much more common than clinical disease. Paradoxically, this observation barely challenged the newly developed notion that infectious diseases were fundamentally extrinsic. Moreover, interindividual variability in the course of infection was typically explained by the emerging immunological (or somatic) theory of infectious diseases, best illustrated by the impact of vaccination. This powerful explanation is, however, best applicable to reactivation and secondary infections, particularly in adults; it can less easily account for interindividual variability in the course of primary infection during childhood. Population and clinical geneticists soon proposed a complementary hypothesis, a germline genetic theory of infectious diseases. Over the past century, this idea has gained some support, particularly among clinicians and geneticists, but has also encountered resistance, particularly among microbiologists and immunologists. We present here the genetic theory of infectious diseases and briefly discuss its history and the challenges encountered during its emergence in the context of the apparently competing but actually complementary microbiological and immunological theories. We also illustrate its recent achievements by highlighting inborn errors of immunity underlying eight life-threatening infectious diseases of children and young adults. Finally, we consider the far-reaching biological and clinical implications of the ongoing human genetic dissection of severe infectious diseases.

B-cell biology and development

B cells develop from hematopoietic precursor cells in an ordered maturation and selection process. Extensive studies with many different mouse mutants provided fundamental insights into this process. However, the characterization of genetic defects causing primary immunodeficiencies was essential in understanding human B-cell biology. Defects in pre-B-cell receptor components or in downstream signaling proteins, such as Bruton tyrosine kinase and B-cell linker protein, arrest development at the pre-B-cell stage. Defects in survival-regulating proteins, such as B-cell activator of the TNF-α family receptor (BAFF-R) or caspase recruitment domain-containing protein 11 (CARD11), interrupt maturation and prevent differentiation of transitional B cells into marginal zone and follicular B cells. Mature B-cell subsets, immune responses, and memory B-cell and plasma cell development are disturbed by mutations affecting Toll-like receptor signaling, B-cell antigen receptor coreceptors (eg, CD19), or enzymes responsible for immunoglobulin class-switch recombination. Transgenic mouse models helped to identify key regulatory mechanisms, such as receptor editing and clonal anergy, preventing the activation of B cells expressing antibodies recognizing autoantigens. Nevertheless, the combination of susceptible genetic backgrounds with the rescue of self-reactive B cells by T cells allows the generation of autoreactive clones found in patients with many autoimmune diseases and even in those with primary immunodeficiencies. The rapid progress of functional genomic research is expected to foster the development of new tools that specifically target dysfunctional B lymphocytes to treat autoimmunity, B-cell malignancies, and immunodeficiency.

Thirty years of primary immunodeficiencies in Turkey

Turkey, with its population of some 75 million, has a high rate of consanguineous marriages. Because the majority of the primary immunodeficiencies (PIDs) are inherited as autosomal recessive (AR) forms, the high consanguinity rate leads to a high prevalence of PID diseases in Turkey. The first pediatric immunology division was established in 1972, since then over 10 other immunology divisions have been established in different cities. Approximately 4,000 patients with possible PID are referred to these centers annually. The percentages of some of the major immunodeficiency groups and individual disease numbers among these patients differ somewhat in comparison with Western countries, likely because the relative incidences of PIDs with AR inheritance and of rare diseases are higher. These characteristics of the patient population, and our determination of differences in disease presentation and unusual features, have led us to undertake studies in collaboration with various centers in Western countries. These collaborations have contributed to the identification of the genes responsible for some rare immunodeficiencies, to the resolution of the genetic heterogeneity underlying conventional phenotypes, and to the description of new clinical phenotypes.

New frontiers of primary antibody deficiencies

Primary antibody deficiencies (PAD) form the largest group of inherited disorders of the immune system. They are characterized by a marked reduction or absence of serum immunoglobulins (Ig) due to disturbed B cell differentiation and by a poor response to vaccination. PAD can be divided into agammaglobulinemia, Ig class switch recombination deficiencies, and idiopathic hypogammaglobulinemia. Over the past 20 years, defects have been identified in 18 different genes, but in many PAD patients the underlying gene defects have not been found. Diagnosis of known PAD and discovery of new PAD is important for good patient care. In this review, we present the effects of genetic defects in the context of normal B cell differentiation, and we discuss how new technical developments can support understanding and discovering new genetic defects in PAD.

Defective calcium signaling and disrupted CD20-B-cell receptor dissociation in patients with common variable immunodeficiency disorders

BACKGROUND

B cells of patients with common variable immunodeficiency (CVID) disorders display impairment in production of immunoglobulin class-switched antibodies, which is possibly contributed to by defects in early B-cell activation. On resting B cells, B-cell receptors (BCRs) are organized in oligomers that are signaling inactive. Their triggering by cognate antigen causes the lateral reorganization of BCRs and associated proteins into signalosomes, resulting in BCR-activated calcium entry. In resting cells the B-cell surface antigen CD20 is associated with the BCR but dissociates on signalosome formation.

OBJECTIVE

We sought to determine whether CD20 dissociation from the BCR during early B-cell activation might contribute to the development of CVID disorders.

METHODS

We evaluated BCR signalosome formation, internalization, and signaling in primary B cells of pediatric patients with CVID disorders and healthy control subjects.

RESULTS

In many pediatric patients with CVID disorders, B cells exhibit significant deficits in BCR triggering-mediated calcium entry in the cytosol, which correlates with impaired plasmablast differentiation in vitro. These alterations did not originate from upregulation of CD22 or defects in calcium channels and did not involve gene mutations in phospholipase Cγ2 or Bruton tyrosine kinase. Instead, B cells from patients with CVID disorders exhibited reduced BCR dissociation from CD20. BCR or CD20 cross-linking induced less BCR internalization, and antibody-mediated CD20 triggering elicited less BCR downstream signaling, as measured based on secondary fluxes.

CONCLUSIONS

We propose that CD20 dissociation from the BCR signalosome is pivotal to BCR-mediated calcium mobilization in the cytosol. Defects in CD20/BCR signalosome conformation might predispose to the spectrum of CVID disorders.

Synovial infiltration with CD79a-positive B cells, but not other B cell lineage markers, correlates with joint destruction in rheumatoid arthritis

OBJECTIVE

The efficacy of B cell depletion in the treatment of patients with rheumatoid arthritis (RA) has revitalized interest in the pathogenic role(s) of B cells in RA. We evaluated the distribution of synovial B lineage cells and their correlation with histologic disease activity and joint destruction in RA.

METHODS

Synovial tissue samples were obtained by closed-needle biopsy from 69 Chinese patients with active RA, from 14 patients with osteoarthritis (OA), and from 15 with orthopedic arthropathies (OrthA) as disease controls. Serial tissue sections were stained immunohistochemically for CD79a (pro-B cell to plasma cell), CD20 (B cells), CD38 (plasma cells), CD21 (follicular dendritic cells), CD68 (macrophages), CD3 (T cells), and CD34 (endothelial cells). Densities of positive-staining cells were determined and correlated with histologic disease activity (Krenn 3-component synovitis score) and radiographic joint destruction (Sharp score).

RESULTS

Mean sublining CD79a-positive cell density was significantly higher in RA than in OA (p <0.001) or OrthA (p = 0.003). Receiver operating characteristic curve analysis showed that CD79a-positive cell density differentiated RA well from OA [area under the curve (AUC) = 0.79] or OrthA (AUC = 0.75). Spearman's rank order correlation showed significant correlations between sublining CD79a-positive cell density and the synovitis score (r = 0.714, p < 0.001), total Sharp score (r = 0.490, p < 0.001), and the erosion subscore (r = 0.545, p < 0.001), as well as the joint space narrowing subscore (r = 0.468, p = 0.001) in RA.

CONCLUSION

Synovial CD79a-positive B cells may be a helpful biomarker for histologic disease activity in RA and may be involved in the pathogenesis of joint destruction in RA.

Agammaglobulinemia associated with BCR⁻ B cells and enhanced expression of CD19

Expression of a BCR is critical for B-cell development and survival. We have identified 4 patients with agammaglobulinemia and markedly reduced but detectable B cells in the peripheral circulation. These B cells have an unusual phenotype characterized by increased expression of CD19 but no BCR. The cells are positive for CD20, CD22, and CD38, but not for Annexin 5 or activation markers, including CD69, CD83, or CD86. EBV lines derived from these B cells lack functionally rearranged immunoglobulin heavy-chain transcripts, as shown by PCR-rapid amplification of cDNA ends (PCR-RACE). Analysis of BM from 2 of the patients showed a severe reduction in the number of pro-B cells as well as pre-B cells. Functionally rearranged heavy-chain transcripts were identified, indicating that machinery to rearrange immunoglobulin genes was intact. Flow cytometry of B-lineage cells suggested accelerated acquisition of maturation markers in early B-cell precursors and increased phosphorylation of signal transduction molecules. Further, expression of TdT, a molecule that is normally down-regulated by a functional pre-BCR complex, was decreased. We hypothesize that the accelerated maturation, increased expression of CD19, and lack of a BCR were due to the constitutive activation of the BCR signal transduction pathway in these patients.

The mouse B cell-specific mb-1 gene encodes an immunoreceptor tyrosine-based activation motif (ITAM) protein that may be evolutionarily conserved in diverse species by purifying selection

The B-lymphocyte accessory molecule Ig-alpha (Ig-α) is encoded by the mouse B cell-specific gene (mb-1), and along with the Ig-beta (Ig-β) molecule and a membrane bound immunoglobulin (mIg) makes up the B-cell receptor (BCR). Ig-α and Ig-β form a heterodimer structure that upon antigen binding and receptor clustering primarily initiates and controls BCR intracellular signaling via a phosphorylation cascade, ultimately triggering an effector response. The signaling capacity of Ig-α is contained within its immunoreceptor tyrosine-based activation motif (ITAM), which is also a key component for intracellular signaling initiation in other immune cell-specific receptors. Although numerous studies have been devoted to the mb-1 gene product, Ig-α, and its signaling mechanism, an evolutionary analysis of the mb-1 gene has been lacking until now. In this study, mb-1 coding sequences from 19 species were compared using Bayesian inference. Analysis revealed a gene phylogeny consistent with an expected species divergence pattern, clustering species from the primate order separate from lower mammals and other species. In addition, an overall comparison of non-synonymous and synonymous nucleotide mutational changes suggests that the mb-1 gene has undergone purifying selection throughout its evolution.

B-cell receptors and heavy chain diseases: guilty by association?

Heavy chain diseases (HCDs) are B-cell proliferative disorders characterized by the production of monoclonal, incomplete, immunoglobulin (Ig) heavy chains (HCs) without associated light chains (LCs). These abnormal HCs are produced as a consequence of HC gene alterations in the neoplastic B cells. HC gene alterations will also impact on surface HC, which is part of the B-cell receptor (BCR), a crucial player in lymphocyte activation by antigen. The selective advantage conferred to mutant cells by abnormal BCR without an antigen-binding domain may be explained by activation of ligand-independent signaling, in analogy to what has been shown for mutated oncogenic growth factor receptors. Here we review data obtained from mouse models showing abnormal, constitutive activity of HCD-BCR, and we discuss the possible mechanism involved, namely, aberrant spontaneous self-aggregation. This self-aggregation might occur as a consequence of escape from the chaperone immunoglobulin binding protein (BiP) and from the anti-aggregation effect of LC association. The concept of misfolding-induced signaling elaborated here may extend to other pathologies termed conformational diseases.

Antibody deficiency due to a missense mutation in CD19 demonstrates the importance of the conserved tryptophan 41 in immunoglobulin superfamily domain formation

Immunoglobulin superfamily (IgSF) domains are conserved structures present in many proteins in eukaryotes and prokaryotes. These domains are well-capable of facilitating sequence variation, which is most clearly illustrated by the variable regions in immunoglobulins (Igs) and T cell receptors (TRs). We studied an antibody-deficient patient suffering from recurrent respiratory infections and with impaired antibody responses to vaccinations. Patient's B cells showed impaired Ca(2+) influx upon stimulation with anti-IgM and lacked detectable CD19 membrane expression. CD19 sequence analysis revealed a homozygous missense mutation resulting in a tryptophan to cystein (W52C) amino acid change. The affected tryptophan is CONSERVED-TRP 41 located on the C-strand of the first extracellular IgSF domain of CD19 and was found to be highly conserved, not only in mammalian CD19 proteins, but in nearly all characterized IgSF domains. Furthermore, the tryptophan is present in all variable domains in Ig and TR and was not mutated in 117 Ig class-switched transcripts of B cells from controls, despite an overall 10% amino acid change frequency. In vitro complementation studies and CD19 western blotting of patient's B cells demonstrated that the mutated protein remained immaturely glycosylated. This first missense mutation resulting in a CD19 deficiency demonstrates the crucial role of a highly conserved tryptophan in proper folding or stability of IgSF domains.

Premature replacement of mu with alpha immunoglobulin chains impairs lymphopoiesis and mucosal homing but promotes plasma cell maturation

Sequentially along B cell differentiation, the different classes of membrane Ig heavy chains associate with the Ig alpha/Ig beta heterodimer within the B cell receptor (BCR). Whether each Ig class conveys specific signals adapted to the corresponding differentiation stage remains debated. We investigated the impact of the forced expression of an IgA-class receptor throughout murine B cell differentiation by knocking in the human C alpha Ig gene in place of the S mu region. Despite expression of a functional BCR, homozygous mutant mice showed a partial developmental blockade at the pro-B/pre-BI and large pre-BII cell stages, with decreased numbers of small pre-BII cells. Beyond this stage, peripheral B cell compartments of reduced size developed and allowed specific antibody responses, whereas mature cells showed constitutive activation and a strong commitment to plasma cell differentiation. Secreted IgA correctly assembled into polymers, associated with the murine J chain, and was transported into secretions. In heterozygous mutants, cells expressing the IgA allele competed poorly with those expressing IgM from the wild-type allele and were almost undetectable among peripheral B lymphocytes, notably in gut-associated lymphoid tissues. Our data indicate that the IgM BCR is more efficient in driving early B cell education and in mucosal site targeting, whereas the IgA BCR appears particularly suited to promoting activation and differentiation of effector plasma cells.

CD20 deficiency in humans results in impaired T cell-independent antibody responses

CD20 was the first B cell differentiation antigen identified, and CD20-specific mAbs are commonly used for the treatment of B cell malignancies and autoantibody-mediated autoimmune diseases. Despite this the role of CD20 in human B cell physiology has remained elusive. We describe here a juvenile patient with CD20 deficiency due to a homozygous mutation in a splice junction of the CD20 gene (also known as MS4A1) that results in "cryptic" splicing and nonfunctional mRNA species. Analysis of this patient has led us to conclude that CD20 has a central role in the generation of T cell-independent (TI) antibody responses. Key evidence to support this conclusion was provided by the observation that although antigen-independent B cells developed normally in the absence of CD20 expression, antibody formation, particularly after vaccination with TI antigens, was strongly impaired in the patient. Consistent with this, TI antipolysaccharide B cell responses were severely impeded in CD20-deficient mice. Our study therefore identifies what we believe to be a novel type of humoral immunodeficiency caused by CD20 deficiency and characterized by normal development of antigen-independent B cells, along with a reduced capacity to mount proper antibody responses.

Pre-B cell receptor signaling in acute lymphoblastic leukemia

B cell lineage ALL represents by far the most frequent malignancy in children and is also common in adults. Despite significant advances over the past four decades, cytotoxic treatment strategies have recently reached a plateau with cure rates at 80 percent for children and 55 percent for adults. Relapse after cytotoxic drug treatment, initial drug-resistance and dose-limiting toxicity are among the most frequent complications of current therapy approaches. For this reason, pathway-specific treatment strategies in addition to cytotoxic drug treatment seem promising to further improve therapy options for ALL patients. In a recent study on 111 cases of pre-B cell-derived human ALL, we found that ALL cells carrying a BCR-ABL1-gene rearrangement lack expression of a functional pre-B cell receptor in virtually all cases. In a proof-of-principle experiment, we studied pre-B cell receptor function during progressive leukemic transformation of pre-B cells in BCR-ABL1-transgenic mice: Interestingly, signaling from the pre-B cell receptor and the oncogenic BCR-ABL1 kinase are mutually exclusive and only "crippled" pre-B cells that fail to express a functional pre-B cell receptor are permissive to transformation by BCR-ABL1.

Primary B cell immunodeficiencies: comparisons and contrasts

Sophisticated genetic tools have made possible the identification of the genes responsible for most well-described immunodeficiencies in the past 15 years. Mutations in Btk, components of the pre-B cell and B cell receptor (lambda5, Igalpha, Igbeta), or the scaffold protein BLNK account for approximately 90% of patients with defects in early B cell development. Hyper-IgM syndromes result from mutations in CD40 ligand, CD40, AID, or UNG in 70-80% of affected patients. Rare defects in ICOS or CD19 can result in a clinical picture that is consistent with common variable immunodeficiency, and as many as 10% of patients with this disorder have heterozygous amino acid substitutions in TACI. For all these disorders, there is considerable clinical heterogeneity in patients with the same mutation. Identifying the genetic and environmental factors that influence the clinical phenotype may enhance patient care and our understanding of normal B cell development.

Hypogammaglobulinaemia

This article reviews the primary immunodeficiencies that result in hypogammaglobulinemia or predominantly antibody deficiency disorders. This group makes up the largest proportion of patients with primary immunodeficiency. Significant advances have been made in understanding the molecular basis and clinical characteristics of patients with the more severe forms of antibody deficiency in the last 6 years. Recognition of these disorders remains poor with significant diagnostic delay. The milder forms of antibody deficiency disorders, especially those with normal total serum immunoglobulin G levels, remain poorly characterized and understood. Further work remains to be done in understanding and recognizing these syndromes to benefit patient care and foster further knowledge of the immune system.

Primary antibody deficiency syndromes

The primary antibody deficiency syndromes are a group of rare disorders characterized by an inability to produce clinically effective immunoglobulin responses. Some of these disorders result from genetic mutations in genes involved in B cell development, whereas others appear to be complex polygenic disorders. They most commonly present with recurrent infections due to encapsulated bacteria, although in the most common antibody deficiency, Common Variable Immunodeficiency, systemic and organ-specific autoimmunity can be a presenting feature. Diagnostic delay in this group of disorders remains a problem, and the laboratory has a vital role in the detection of abnormalities in immunoglobulin concentration and function. It is critical to distinguish this group of disorders from secondary causes of hypogammaglobulinaemia, in particular lymphoid malignancy, and appropriate laboratory investigations are of critical importance. Treatment of primary antibody deficiencies involves immunoglobulin replacement therapy, either via the intravenous or subcutaneous route. Patients remain at risk of a wide variety of complications, not all linked to diagnostic delay and inadequate therapy. In common variable immunodeficiency (CVID) in particular, patients remain at significantly increased risk of lymphoid malignancy, and regular clinical and laboratory monitoring is required. This review aims to give an overview of these conditions for the general reader, covering pathogenesis, clinical presentation, laboratory investigation, therapy and clinical management.

Autosomal recessive agammaglobulinemia: novel insights from mutations in Ig-beta

Agammaglobulinemia is a rare primary immuno-deficiency characterized by an early block of B-cell development in the bone marrow resulting in the absence of peripheral B cells and low/absent immunoglobulin serum levels. Mutations in the Bruton tyrosine kinase and in components of the pre-B-cell receptor (pre-BCR), such as mu heavy chain, surrogate light chain, and Igalpha have been found in 85% to 90% of patients affected by this disease. Here we review the recent advances in the characterization of molecular defects underlying an early block in B-cell development, focusing on the novel finding of the first two patients with agammaglobulinemia caused by mutations in Igbeta, the transmembrane protein that associates with Igalpha as part of the pre-BCR complex. Characterization of novel genetic defects involving components of the pre-BCR is crucial for a better understanding of the biology of early B-cell development and may have therapeutic and prognostic implications.

Follicular lymphoma in a X-linked lymphoproliferative syndrome carrier female

X-linked lymphoproliferative (XLP) syndrome is a rare primary immune-deficiency disorder caused by mutations of the SH2D1A or XIAP genes. Males with the disorder are usually in good health until contracting Epstein-Barr virus (EBV) whereupon the majority of patients die from fulminant infectious mononucleosis, lymphoma or hypogammaglobulinaemia. This report describes a female carrier with an XLP phenotype who was retrospectively identified after her grandson died from the disorder. Subsequent genetic testing identified the patient's mother and affected maternal grandmother as XLP carriers. The family's medical records were significant. The proband had lymphoma at ages 2 and 8 and made a full recovery following treatment. Both the maternal grandmother and uncle died of non-Hodgkin's lymphoma. We were concerned that the XLP carrier mother may be predisposed to lymphoma if the normal X chromosome is skewed towards inactivation. The human androgen receptor assay detected random X chromosome inactivation in the carrier mother. EBV was not detected in the lymphoma tissues of the proband and his grandmother, confirming previous findings that EBV is not always associated with lymphoma in XLP. More significantly, our study highlights the importance of identifying XLP in families with a high incidence of lymphoma.

Molecular cloning and expression analysis of pig CD79alpha

The CD79alpha (immunoglobulin alpha, Igalpha), a part of B cell receptor (BCR) complex, forms a heterodimer with CD79beta (Igbeta) and plays an important role in the B cell signaling. In this study, we have cloned pig Cd79a cDNA using RT-PCR and determined the complete cDNA sequence of pig Cd79a. Pig Cd79a cDNA contains an open reading frame (672bp) encoding 223 amino acids. The putative amino acid identity of pig CD79alpha with those of human, cattle and mouse are 70.4, 81.4, and 67.7%, respectively. Alignment of the CD79alpha amino acid sequence with those of mammalian species showed that the extracellular domain is the most divergent, whereas transmembrane region and cytoplasmic tail including immunoreceptor tyrosine-based activation motif (ITAM) are largely conserved. Pig Cd79a mRNA was detected mainly in lymphoid tissues by RT-PCR. The highest level of Cd79a mRNA expression was observed in mesenteric lymph node and spleen. Relatively low level of Cd79a mRNA expression was observed in lung, thymus and small intestine. The lowest level of Cd79a mRNA expression was observed in large intestine. Flow cytometry analyses demonstrated that human CD79alpha antibody recognizes a CD79alpha in pig B cells. Further, immunohistochemistry analysis using human CD79alpha antibody on pig spleen was revealed that CD79alpha is strongly expressed in the follicular mantle zone rather than in the germinal center. Future study will be focused on defining the functional role of CD79alpha during the course of pig infectious diseases and the formation of neoplasm.

Antibody deficiency diseases

Primary immunodeficiency diseases are rare disorders characterized by quantitative or qualitative defects in cells or components in the immune system, resulting in a high degree of susceptibility to various types of infections. During differentiation, stem cells undergo a series of discrete steps, governed by a large number of different genes. Mutations/deletions in these genes will result in a block in differentiation of the affected cell lineage(s), leading to immunodeficiency. To date, more than 150 different types of disorders have been described. In this review, we will focus on novel findings in antibody deficiency syndromes.

Cutting edge: a hypomorphic mutation in Igbeta (CD79b) in a patient with immunodeficiency and a leaky defect in B cell development

Although null mutations in Igalpha have been identified in patients with defects in B cell development, no mutations in Igbeta have been reported. We recently identified a patient with a homozygous amino acid substitution in Igbeta, a glycine to serine at codon 137, adjacent to the cysteine required for the disulfide bond between Igalpha and Igbeta. This patient has a small percentage of surface IgM(dim) B cells in the peripheral circulation (0.08% compared with 5-20% in healthy controls). Using expression vectors in 293T cells or Jurkat T cells, we show that the mutant Igbeta can form disulfide-linked complexes and bring the mu H chain to the cell surface as part of the BCR but is inefficient at both tasks. The results show that minor changes in the ability of the Igalpha/Igbeta complex to bring the BCR to the cell surface have profound effects on B cell development.

Mutations of the Igbeta gene cause agammaglobulinemia in man

Agammaglobulinemia is a rare primary immunodeficiency characterized by an early block of B cell development in the bone marrow, resulting in the absence of peripheral B cells and low/absent immunoglobulin serum levels. So far, mutations in Btk, mu heavy chain, surrogate light chain, Igalpha, and B cell linker have been found in 85-90% of patients with agammaglobulinemia. We report on the first patient with agammaglobulinemia caused by a homozygous nonsense mutation in Igbeta, which is a transmembrane protein that associates with Igalpha as part of the preBCR complex. Transfection experiments using Drosophila melanogaster S2 Schneider cells showed that the mutant Igbeta is no longer able to associate with Igalpha, and that assembly of the BCR complex on the cell surface is abrogated. The essential role of Igbeta for human B cell development was further demonstrated by immunofluorescence analysis of the patient's bone marrow, which showed a complete block of B cell development at the pro-B to preB transition. These results indicate that mutations in Igbeta can cause agammaglobulinemia in man.

Dominant-negative mutations in the DNA-binding domain of STAT3 cause hyper-IgE syndrome

Hyper-immunoglobulin E syndrome (HIES) is a compound primary immunodeficiency characterized by a highly elevated serum IgE, recurrent staphylococcal skin abscesses and cyst-forming pneumonia, with disproportionately milder inflammatory responses, referred to as cold abscesses, and skeletal abnormalities. Although some cases of familial HIES with autosomal dominant or recessive inheritance have been reported, most cases of HIES are sporadic, and their pathogenesis has remained mysterious for a long time. Here we show that dominant-negative mutations in the human signal transducer and activator of transcription 3 (STAT3) gene result in the classical multisystem HIES. We found that eight out of fifteen unrelated non-familial HIES patients had heterozygous STAT3 mutations, but their parents and siblings did not have the mutant STAT3 alleles, suggesting that these were de novo mutations. Five different mutations were found, all of which were located in the STAT3 DNA-binding domain. The patients' peripheral blood cells showed defective responses to cytokines, including interleukin (IL)-6 and IL-10, and the DNA-binding ability of STAT3 in these cells was greatly diminished. All five mutants were non-functional by themselves and showed dominant-negative effects when co-expressed with wild-type STAT3. These results highlight the multiple roles played by STAT3 in humans, and underline the critical involvement of multiple cytokine pathways in the pathogenesis of HIES.

Deconstructing common variable immunodeficiency by genetic analysis

Common variable immunodeficiency (CVID) is the most common symptomatic primary immunodeficiency. Patients have recurrent bacterial infections and an increased risk of developing autoimmune diseases, lung damage, and selected cancers. Since 2003, four genes have been shown to be mutated in CVID patients: ICOS, TNFRSF13B (encoding TACI), TNFRSF13C (encoding BAFF-R) and CD19. Heterozygous mutations in TNFRSF13B are also associated with CVID, whereas the other three genes are purely recessive. Recent genetic linkage studies have also identified possible loci for dominant CVID genes on chromosomes 4q, 5p and 16q. These findings markedly improved the genetic diagnosis of CVID and point towards new strategies for future genetic studies. In addition, some CVID genes might be relevant to more common diseases such as asthma and stroke.

Molecular analysis of the pre-BCR complex in a large cohort of patients affected by autosomal-recessive agammaglobulinemia

Autosomal-recessive agammaglobulinemia is a rare and heterogeneous disorder, characterized by early-onset infections, profound hypogammaglobulinemia of all immunoglobulin isotypes and absence of circulating B lymphocytes. To investigate the molecular basis of the disease, 23 patients with early-onset disease and no mutations in Bruton tyrosine kinase, the gene responsible for X-linked agammaglobulinemia, were selected and analyzed by direct sequencing of candidate genes. Two novel mutations in the mu heavy chain (muHC) gene (IGHM) were identified in three patients belonging to two unrelated families. A fourth patient carries a previously described G>A nucleotide substitution at the -1 position of an alternative splice site in IGHM; here, we demonstrate that this mutation is indeed responsible for aberrant splicing. Comparison of bone marrow cytofluorimetric profiles in two patients carrying different mutations in the IGHM gene suggests a genotype-phenotype correlation with the stage at which B-cell development is blocked. Several new single nucleotide polymorphisms (SNPs) both in the muHC and in the lambda5-like/VpreB-coding genes were identified. Two unrelated patients carry compound heterozygous variations in the VpreB1 gene that may be involved in disease ethiology.

Human tyrosine kinase 2 deficiency reveals its requisite roles in multiple cytokine signals involved in innate and acquired immunity

Tyrosine kinase 2 (Tyk2) is a nonreceptor tyrosine kinase that belongs to the Janus kinase (Jak) family. Here we identified a homozygous Tyk2 mutation in a patient who had been clinically diagnosed with hyper-IgE syndrome. This patient showed unusual susceptibility to various microorganisms including virus, fungi, and mycobacteria and suffered from atopic dermatitis with elevated serum IgE. The patient's cells displayed defects in multiple cytokine signaling pathways including those for type I interferon (IFN), interleukin (IL)-6, IL-10, IL-12, and IL-23. The cytokine signals were successfully restored by transducing the intact Tyk2 gene. Thus, the Tyk2 deficiency is likely to account for the patient's complex clinical manifestations, including the phenotype of impaired T helper 1 (Th1) differentiation and accelerated Th2 differentiation. This study identifies human Tyk2 deficiency and demonstrates that Tyk2 plays obligatory roles in multiple cytokine signals involved in innate and acquired immunity of humans, which differs substantially from Tyk2 function in mice.

Clinical, immunological and molecular characteristics of 37 Iranian patients with X-linked agammaglobulinemia

BACKGROUND

X-linked agammaglobulinemia (XLA) is a hereditary immunodeficiency characterized by an early onset of recurrent bacterial infections, a profound deficiency of all immunoglobulin isotypes and a markedly reduced number of peripheral B lymphocytes. Eighty-five percent of the patients with this phenotype have mutations in Bruton's tyrosine kinase (BTK) gene.

METHODS

To provide an informative outlook of clinical and immunological manifestations of XLA in Iran, 37 Iranian male patients with an age range of 1-34 years, followed over a period of 25 years, were studied. Twenty-four of the 37 patients were screened for BTK gene mutation using PCR-SSCP followed by direct sequencing. BTK protein expression assay was done by flow cytometry in 9 families.

RESULTS

All patients first presented with infectious diseases, the most common of which were respiratory tract infections. Eighteen different mutations were identified, 13 of which were novel: IVS1+5G>C, 1896G>A, 349delA, 1618C>T, 1783T>C, 2084A>G, 1346delT, 1351delGAG, 587A>G, IVS14-1G>A, IVS3+2T>C, 1482G>A, 1975C>A.

CONCLUSION

The fact that we found a great number of novel mutations in a relatively limited number of patients underlines the heterogeneity of BTK mutations in the Iranian population. The large number of new mutations indicates that extended studies in this region would be rewarding.

An antibody-deficiency syndrome due to mutations in the CD19 gene

BACKGROUND

The CD19 protein forms a complex with CD21, CD81, and CD225 in the membrane of mature B cells. Together with the B-cell antigen receptor, this complex signals the B cell to decrease its threshold for activation by the antigen.

METHODS

We evaluated four patients from two unrelated families who had increased susceptibility to infection, hypogammaglobulinemia, and normal numbers of mature B cells in blood. We found a mutation in the CD19 gene in all four patients. The CD19 gene in the patients and their first-degree relatives was sequenced, and flow-cytometric immunophenotyping of B cells, immunohistochemical staining of lymphoid tissues, and DNA and messenger RNA analysis were performed. B-cell responses on the triggering of the B-cell receptor were investigated by in vitro stimulation; the antibody response after vaccination with rabies vaccine was also studied.

RESULTS

All four patients had homozygous mutations in the CD19 gene. Levels of CD19 were undetectable in one patient and substantially decreased in the other three. Levels of CD21 were decreased, whereas levels of CD81 and CD225 were normal, in all four patients. The composition of the precursor B-cell compartment in bone marrow and the total numbers of B cells in blood were normal. However, the numbers of CD27+ memory B cells and CD5+ B cells were decreased. Secondary follicles in lymphoid tissues were small to normal in size and had a normal cellular composition. The few B cells that showed molecular signs of switching from one immunoglobulin class to another contained V(H)-C(alpha) and V(H)-C(gamma) transcripts with somatic mutations. The response of the patients' B cells to in vitro stimulation through the B-cell receptor was impaired, and in all four patients, the antibody response to rabies vaccination was poor.

CONCLUSIONS

Mutation of the CD19 gene causes a type of hypogammaglobulinemia in which the response of mature B cells to antigenic stimulation is defective.

Initiation of pre-B cell receptor signaling: Common and distinctive features in human and mouse

B cell development in the bone marrow is a highly regulated process and expression of a functional pre-BCR represents a crucial checkpoint, common to human and mouse. In this review, we discuss pre-BCR analogies and differences between the two species leading to pre-B cell differentiation and proliferation. In addition, the mechanisms triggering pre-BCR activation are reviewed, taking into account the recent report of heparan sulfates and galectin 1 as stromal cell-derived pre-BCR ligands. Finally, ligand-induced pre-BCR activation models are proposed on the bases of the differences reported for pre-BCR and IL7 dependencies in the two species.

Molecular defects in T- and B-cell primary immunodeficiency diseases

More than 120 inherited primary immunodeficiency diseases have been discovered in the past five decades, and the precise genetic defect in many of these diseases has now been identified. Increasing understanding of these molecular defects has considerably influenced both basic and translational research, and this has extended to many branches of medicine. Recent advances in both diagnosis and therapeutic modalities have allowed these defects to be identified earlier and to be more precisely defined, and they have also resulted in more promising long-term outcomes. The prospect of gene therapy continues to be included in the armamentarium of treatment considerations, because these conditions could be among the first to benefit from gene-therapy trials in humans.

Heterogeneous intracellular expression of B-cell receptor components in B-cell chronic lymphocytic leukaemia (B-CLL) cells and effects of CD79b gene transfer on surface immunoglobulin levels in a B-CLL-derived cell line

B-cell chronic lymphocytic leukaemia (B-CLL) cells display low amounts of surface immunoglobulins (sIg). To investigate the mechanisms underlying this phenomenon, we performed a thorough study of surface and intracellular expression of the B-cell receptor (BCR) components in B-CLL cells using flow cytometry. There was an heterogeneous pattern of expression. Overall, 20 of 22 samples showed reduced sIgM levels, compared with normal B cells. Among them, three (15%) had very low to undetectable intracellular IgM levels and variable amounts of CD79a and CD79b; nine (45%) had low intracellular CD79b levels but appreciable levels of IgM and CD79a; and eight (40%) had relatively normal intracellular levels of all BCR components. To investigate whether surface BCR levels could be controlled by the rate of CD79b synthesis, adenoviral vectors encoding CD79b were generated and used for gene transfer experiments. Delivery of CD79b to non-B cells transfected with IgM and CD79a lead to high-level expression of a functional BCR. Moreover, CD79b gene transfer in a B cell line derived from a B-CLL patient and characterised by low intracellular levels of endogenous CD79b consistently increased sIgM levels. These findings indicate that the phenotype of B-CLL cells in a subset of patients may depend primarily on poor CD79b expression, and suggest that upregulation of CD79b expression may correct the phenotype of these cells.

Genetic analysis of patients with defects in early B-cell development

Approximately 85% of patients with defects in early B-cell development have X-linked agammaglobulinemia (XLA), a disorder caused by mutations in the cytoplasmic Bruton's tyrosine kinase (Btk). Although Btk is activated by cross-linking of a variety of cell-surface receptors, the most critical signal transduction pathway is the one initiated by the pre-B cell and B-cell antigen receptor complex. Mutations in Btk are highly diverse, and no single mutation accounts for more than 3% of patients. Although there is no strong genotype/phenotype correlation in XLA, the specific mutation in Btk is one of the factors that influences the severity of disease. Mutations in the components of the pre-B cell and B-cell antigen receptor complex account for an additional 5-7% of patients with defects in early B-cell development. Patients with defects in these proteins are clinically indistinguishable from those with XLA. However, they tend to be younger at the time of diagnosis, and whereas most patients with XLA have a small number of B cells in the peripheral circulation, these cells are not found in patients with defects in micro heavy chain or Igalpha. Polymorphic variants in the components of the pre-B cell and B-cell receptor complex, particularly micro heavy chain and lambda5, may contribute to the severity of XLA.

The SLP-76 family of adapter proteins

Adapter molecules are multidomain proteins lacking intrinsic catalytic activity, functioning instead by nucleating molecular complexes during signal transduction. The SLP-76 family of adapters includes SH2 domain-containing leukocyte phosphoprotein of 76kDa (SLP-76), B cell linker protein (BLNK), and cytokine-dependent hematopoietic cell linker (Clnk). These proteins are critical for integration of numerous signaling cascades downstream of immunotyrosine-based activation motif (ITAM)-bearing receptors and integrins in diverse hematopoietic cell types. Mutations in genes encoding SLP-76 family adapters result in severe phenotypes, underscoring the critical role these proteins play in cellular development and function by directing formation of signaling complexes in a temporally- and spatially-specific manner.

[Adult-onset primary immunodeficiencies]

Different individuals with the same kind of primary immunodeficiency may start having symptoms from early childhood on, or alternatively much later in adult life, or never. The differences in phenotype can only partly be deduced from genotype-analysis or--in case of female patients with X-linked diseases--from age-related skewing of lyonisation. The role of compensatory immune mechanisms is less clear. The microbial spectrum of infections is usually the same for both adult and infantile forms of a special primary immunodeficiency syndrome. Yet, many of the adult forms are associated with non-infectious complications, such as granuloma formation, autoimmunity or tumors. Besides standard antibiotic treatment and IgG replacement therapy, there are now different cytokine- or enzyme-replacement regimens available for some of the primary immunodeficiencies. However, exact diagnostic classification of the immunodeficiency should be obtained before such treatment modalities are used. Adult primary immunodeficiency syndromes therefore represent a challenge to both clinicians and molecular biologists.

The genetics of hypogammaglobulinemia

Etiologies for human hypogammaglobulinemias are diverse and include genetic and nongenetic causes. Although recent reviews focus on the complex genetics of common variable immunodeficiency, in this review, we survey different causes of hypogammaglobulinemias and discuss possible mechanisms.

Prevalence of SAP gene defects in male patients diagnosed with common variable immunodeficiency

The molecular basis of common variable immunodeficiency (CVID) is undefined, and diagnosis requires exclusion of other diseases including X-linked lymphoproliferative disease (XLP). This rare disorder of immunedysregulation presents typically after Epstein-Barr virus infection and results from defects in the SAP (SLAM associated protein) gene. SAP mutations have been found in a few patients diagnosed previously as CVID, suggesting that XLP may mimic CVID, but no large-scale analysis of CVID patients has been undertaken. We therefore analysed 60 male CVID and hypogammaglobulinaemic patients for abnormalities in SAP protein expression and for mutations in the SAP gene. In this study only one individual, who was found later to have an X-linked family history, was found to have a genomic mutation leading to abnormal SAP cDNA and protein expression. These results demonstrate that SAP defects are rarely observed in CVID patients. We suggest that routine screening of SAP may only be necessary in patients with other suggestive clinical features.

Functional and molecular evaluation of lymphocytes

The laboratory plays a central role in the evaluation of immune function and is critical in the diagnosis and treatment of immune deficiencies. The range of options available to evaluate lymphocyte function has expanded dramatically as our understanding of the immune system has expanded. As the choices of laboratory tests increase, so also does the need to choose testing in such a way as to appropriately direct the evaluation. Typically, this approach involves starting with screening tests and, on the basis of the results of these tests, deciding whether more sophisticated and expensive testing is warranted. The remarkable developments over the past decade leading to the identification of numerous gene defects underlying a variety of immune deficiencies has moved mutation analysis into the realm of the clinical laboratory. This information could be indispensable for immune deficiency diagnosis, prenatal screening, carrier detection, and family counseling. In this review a sequential approach to evaluating lymphocyte function is presented, starting with readily available screening tests and followed by more complex in vitro testing, including the application of newer assays. The various approaches are presented from the perspective of appropriate use and information garnered, whereas actual details of test procedures are not discussed but are referenced. The evolution of immune function testing suggests that it will continue to develop, and future assays are likely to provide even more insight into specific aspects of the immune response and be linked to immune deficiencies not yet defined.

Survival of resting mature B lymphocytes depends on BCR signaling via the Igalpha/beta heterodimer

We previously showed that type I interferon-induced, Cre-mediated ablation of surface BCR expression in mature B cells through Ig-heavy chain deletion results in apoptosis of these cells. This led to the hypothesis that survival signals from the BCR are vital for mature B cells. Here, we test two critical assumptions of this model. First, we demonstrate loss of mature B cells upon induced mutation of a signaling module of the BCR, not precluding BCR surface expression. Second, we show that the cells are also lost upon BCR inactivation in the absence of an exogenous inducer like interferon, excluding that cell death depends on previous cellular activation by the latter. Kinetic data demonstrate that BCR-less mature B cells have a severely reduced lifespan, with a half-life of 3-6 days. Together these results establish that BCR signaling is required to keep resting mature B cells alive in vivo.

Role of the BCR complex in B cell development, activation, and leukemic transformation

A primary focus of signal transduction in B cells, from the pre-B cell to the mature B cell, is the B cell receptor complex. Here we describe work demonstrating the importance of signaling via the pre-B cell receptor complex (pre-BCR) to the pre-B cell transition, the central checkpoint in B-cell development. We have shown tht pre-BCR complex components Igalpha and Igbeta are critical to allowing the pre-B cell to move through this transition, but may not be required for allelic exclusion. Pre-BCR expression also directly affects the response of leukemic cells to steroid treatment, suggesting that signals initiated by the pre-BCR complex may present therapeutic targets in acute leukemia. Additionally, interleukin-7 may also modulate the response of leukemic cells arising from early B-cell stages to treatment. This observation has lead directly to proposals to test drugs which may antagonize early B-cell growth signals, such as rapamycin, in acute lymphoid leukemia.

Genes required for B cell development

Mutations in a variety of genes can cause congenital agammaglobulinemia and a failure of B cell development. The currently known genes encode components of the pre-B cell receptor or proteins that are activated by cross-linking of the pre-B cell receptor. Defects in these genes result in a block in B cell differentiation at the pro-B to pre-B cell transition. A patient with a translocation involving a previously unknown gene, LRRC8, demonstrated a block at exactly the same point in B cell differentiation (see the related article beginning on page 1707). It will be interesting to determine whether the protein encoded by this gene interacts with the pre-B cell receptor signal transduction pathway or is involved in a new pathway.

Update on primary immunodeficiency: defects of lymphocytes

The recent identification of the genes involved in many primary immunodeficiency disorders has led to a significant increase in our understanding of the pathogenesis of these defects. Many of these disorders share a clinical phenotype with common features such as recurrent infections, chronic inflammation, and autoimmunity. Although some of these immune defects have mild presentations and better outcomes, others result in severe infections and significant morbidity and mortality. For these, early diagnosis and treatment are critical. This review provides an overview of the genetic defects and clinical features of primary immune deficiencies due to defects in lymphocytes.

A congenital mutation of the novel gene LRRC8 causes agammaglobulinemia in humans

A girl with congenital agammaglobulinemia and minor facial anomalies lacked B cells in peripheral blood: karyotypic analysis of white blood cells showed balanced translocation, t(9;20)(q33.2;q12). In the current study, we isolated a novel gene, leucine-rich repeat-containing 8 (LRRC8), at the translocation site on chromosome 9. It has four transmembrane helices with one isolated and eight sequentially located leucine-rich repeats (LRRs) and constitutes a new protein family. It is expressed on T cells as well as on B-lineage cells. Translocation truncates the LRRC8 gene, resulting in deletion of the eighth, ninth, and half of the seventh LRR domains located close to the C-terminal. The truncated form of the LRRC8 gene is transcribed with sequences from the noncoding region adjacent to the truncated seventh LRR. Protein products derived from the truncated gene are coexpressed on white blood cells with the intact LRRC8 protein from the untranslocated allele. Transplantation experiments with murine bone marrow cells that were forced to express the truncated LRRC8 show that expression of the truncated protein inhibited B cell development. These results indicate that LRRC8 is responsible for the B cell deficiency in this patient and is required for B cell development.