Mutations in the mu heavy-chain gene in patients with agammaglobulinemia

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.

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.

Gross deletions involving IGHM, BTK, or Artemis: a model for genomic lesions mediated by transposable elements

Most genetic disruptions underlying human disease are microlesions, whereas gross lesions are rare with gross deletions being most frequently found (6%). Similar observations have been made in primary immunodeficiency genes, such as BTK, but for unknown reasons the IGHM and DCLRE1C (Artemis) gene defects frequently represent gross deletions ( approximately 60%). We characterized the gross deletion breakpoints in IGHM-, BTK-, and Artemis-deficient patients. The IGHM deletion breakpoints did not show involvement of recombination signal sequences or immunoglobulin switch regions. Instead, five IGHM, eight BTK, and five unique Artemis breakpoints were located in or near sequences derived from transposable elements (TE). The breakpoints of four out of five disrupted Artemis alleles were located in highly homologous regions, similar to Ig subclass deficiencies and Vh deletion polymorphisms. Nevertheless, these observations suggest a role for TEs in mediating gross deletions. The identified gross deletion breakpoints were mostly located in TE subclasses that were specifically overrepresented in the involved gene as compared to the average in the human genome. This concerned both long (LINE1) and short (Alu, MIR) interspersed elements, as well as LTR retrotransposons (ERV). Furthermore, a high total TE content (>40%) was associated with an increased frequency of gross deletions. Both findings were further investigated and confirmed in a total set of 20 genes disrupted in human disease. Thus, to our knowledge for the first time, we provide evidence that a high TE content, irrespective of the type of element, results in the increased incidence of gross deletions as gene disruption underlying human disease.

Primary Immunodeficiencies

Primary immunodeficiencies (PIDs), once considered to be very rare, are now increasingly recognized because of growing knowledge in the immunological field and the availability of more sophisticated diagnostic techniques and therapeutic modalities [161]. However in a database of >120,000 inpatients of a general hospital for conditions suggestive of ID 59 patients were tested, and an undiagnosed PID was found in 17 (29%) of the subjects tested [107]. The publication of the first case of agammaglobulinemia by Bruton in 1952 [60] demonstrated that the PID diagnosis is first done in the laboratory. However, PIDs require specialized immunological centers for diagnosis and management [33]. A large body of epidemiological evidence supports the hypothesis of the existence of a close etiopathogenetic relation between PID and atopy [73]. In particular, an elevated frequency of asthma, food allergy (FA), atopic dermatitis and enteric pathologies can be found in various PIDs. In addition we will discuss another subject that is certainly of interest: the pseudo-immunodepressed child with recurrent respiratory infections (RRIs), an event that often requires medical intervention and that very often leads to the suspicion that it involves antibody deficiencies [149].

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.

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.

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.

Class switch recombination: a comparison between mouse and human

Humans and mice separated more than 60 million years ago. Since then, evolution has led to a multitude of changes in their genomic sequences. The divergence of genes has resulted in differences both in the innate and adaptive immune systems. In this chapter, we focus on species difference with regard to immunoglobulin class switch recombination (CSR). We have compared the immunoglobulin constant region gene loci from human and mouse, with an emphasis on the switch regions, germ line transcription promoters, and 3' enhancers. We have also compared pathways/factors that are involved in CSR. Although there are remarkable similarities in the cellular machinery involved in CSR, there are also a number of unique features in each species.

Memory switched B cell percentage and not serum immunoglobulin concentration is associated with clinical complications in children and adults with specific antibody deficiency and common variable immunodeficiency

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

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.

Cellular and genetic basis of primary immune deficiencies

Knowledge of the genetic mutations of primary immune deficiency syndromes has grown significantly over the last 30 years. In this article the authors present an overview of the clinical aspects, laboratory evaluation, and genetic defects of primary immunodeficiencies, with an emphasis on the pathophysiology of the known molecular defects. This article is designed to give the primary pediatrician a general knowledge of this rapidly expanding field.

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.

Common variable immunodeficiency in three horses with presumptive bacterial meningitis

Three adult horses were evaluated for signs of musculoskeletal pain, dullness, ataxia, and seizures. A diagnosis of bacterial meningitis was made on the basis of results of CSF analysis. Because primary bacterial meningitis is so rare in adult horses without any history of generalized sepsis or trauma, immune function testing was pursued. Flow cytometric phenotyping of peripheral blood lymphocytes was performed, and proliferation of peripheral blood lymphocytes in response to concanavalin A, phytohemagglutinin, pokeweed mitogen, and lipopolysaccharide was determined. Serum IgA, IgM, and IgG concentrations were measured by means of radial immunodiffusion, and serum concentrations of IgG isotypes were assessed with a capture antibody ELISA. Serum tetanus antibody concentrations were measured before and 1 month after tetanus toxoid administration. Phagocytosis and oxidative burst activity of isolated peripheral blood phagocytes were evaluated by means of simultaneous flow cytometric analysis. Persistent B-cell lymphopenia, hypogammaglobulinemia, and abnormal in vitro responses to mitogens were detected in all 3 horses, and a diagnosis of common variable immunodeficiency was made.

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.

Insight into B cell development and differentiation

The main topic of this article is B cell development and differentiation, with a special focus on the mechanisms and molecules that regulate the expression of humoral immunity. Molecular epidemiological analysis was performed on the genes responsible for the X-linked agammaglobulinemia (XLA) phenotype of the majority of Italian patients and their distinct mutations were characterized. Mutations in Bruton's tyrosine kinase (BTK), a member of Tec Family of protein tyrosine kinases, have been found to be mainly responsible for XLA disease. The exact function of BTK in signal transduction is not yet known; thus, the specific role of BTK in receptor-dependent calcium signaling and the pro-antiapoptotic regulatory activity was addressed by transfecting RAMOS-1, a BTK-deficient human Burkitt's/B cell leukemia line with wild-type and mutant constructs. This work may provide clues about critical sites in the molecule and give support for gene therapy as a potential successful approach to XLA. Another aspect of this research is the identification and dissection of the molecular events that are likely to be directly related to the ability to express various isotypes of immunoglobulin with differing function and certain B cell immunodeficiency, mainly common variable disease and non-X-linked hyperIgM. B cell development and maturation steps in different compartments of the immune system are tracked by the analysis of cell-surface molecules and components of the signal transduction pathways, i.e. CD40, CD30, CD27, CD38, CD22 and CD24. A few components involved in B cell development, maturation and differentiation and their specific functional role are at least partially known, but these are far from fitting into an understandable pathway at present.

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.

Semiquantitative evaluation of mRNAs for the membranous form of immunoglobulin heavy chain is useful for investigating the etiology in CVID

Common variable immunodeficiency (CVID) is a primary antibody deficiency syndrome characterized by defective B-cell maturation and antibody formation resulting in low serum antibody levels of all immunoglobulin (Ig) isotypes. To investigate the pathogenesis of CVID, we developed a set of competitive polymerase chain reaction for membrane-bound Ig heavy chain (mHC) mRNAs for IgM, IgG and IgA. Data on three children with CVID in group A of Bryant's classification were analysed. All the three mHC mRNA levels in Patient 1 were almost same as those in healthy controls. In Patient 2, mHC mRNA for IgM was detected at a level similar to that in controls, but mHC mRNAs for IgG and IgA heavy chains were not detected. In Patient 3, all the three mHC mRNAs were undetectable. Our data suggest that a different molecular basis exists in these patients with CVID even though all belong to group A of Bryant's classification. Use of our method facilitates a better understanding of molecular events in CVID patients and may be useful for precise classifications of CVID.

Therapeutic strategies in common variable immunodeficiency

The treatment of common variable immunodeficiency (CVID) is currently based on the early recognition of the condition and replacement immunoglobulin combined with prompt treatment of infections and complications. The route of administration, dose and frequency of administration of immunoglobulin still vary between centres and countries. Other interventions aimed at overcoming the immunological defects in CVID such as interleukin-2 therapy are being studied but there is as yet insufficient evidence to support their routine use. The treatment of complications such as suppurative lung disease uses principles broadly similar to those used for cystic fibrosis, whereas the granulomatous complications involving the lungs and other organ systems are in need of much more research to define optimum therapies.

Linkage of autosomal dominant common variable immunodeficiency to chromosome 5p and evidence for locus heterogeneity

Common variable immunodeficiency (CVID, OMIM 240500) and selective immunoglobulin A deficiency (IgAD) are the most frequent primary immunodeficiencies in humans. Of the cases with CVID/IgAD, 20%-25% are familial, but the only previous claims of linkage or association are to the HLA region on chromosome 6p. We report the results of a genome-wide scan in three multiplex families with CVID, IgAD, and dysgammaglobulinemia, where affection is inherited in an autosomal dominant pattern. Two of the families are consistent with linkage to the telomeric region of chromosome 5p, whereas the third is consistent with linkage to the HLA region. Using a locus heterogeneity model and a conservative penetrance model, we obtained a LOD score of 3.35 for the 5p region. We sequenced the exons of one promising candidate gene within this region (PDCD6, also known as ALG-2) but found no causative mutation.

Immunodeficiency Disorders

Hematological complications occur frequently in patients with both primary and secondary immunodeficiency disorders. Anemia, thrombocytopenia or leukopenias may bring these individuals to the attention of hematologists. Conversely, evidence suggesting a lymphoproliferative disorder may be the cause for referral. This session will provide an update on the diagnosis and treatment of immunodeficiency diseases ranging from isolated defects in antibody production to the severe combined immunodeficiencies (SCID).

Immunodeficiency diseases have traditionally been defined as defects in the development and function of T and B cells, the primary effector cells of specific cellular and humoral immunity. However, it has become increasingly evident that innate immune mechanisms contribute greatly to host defense, either through acting alone or by enhancing specific T and B cell responses.

In Section I, Dr. Lewis Lanier reviews the burgeoning information on the extensive families of activating and inhibitory immunoreceptors that are expressed on NK cells, dendritic cells, T and B cells, and phagocytic cells. He provides an overview on the biological functions of these receptors in host defense.

In Section II, Dr. Mary Ellen Conley defines the spectrum of antibody deficiency disorders, the most frequently occurring types of primary immunodeficiencies. She covers the different defects in B-cell development and function that lead to antibody deficiencies, and includes diagnosis and therapy of these disorders.

In Section III, Dr. Jennifer Puck discusses the diagnosis and treatment of the different types of SCID. She describes the genetic basis for SCID, and the benefits, pitfalls, and complications of gene therapy and bone marrow transplantation in SCID patients.

Early defects in B cell development

PURPOSE OF REVIEW

Recent clinical studies in patients with genetically proven X-linked or autosomal recessive agammaglobulinemia provide some guidelines that should influence our management of patients with suspected immunodeficiency.

RECENT FINDINGS

Males who are at a high risk of having X-linked agammaglobulinemia because they have an affected brother or uncle are often not evaluated for immunodeficiency until they are hospitalized for infection. Some of those who are evaluated are not started on gammaglobulin therapy immediately. More than 10% of patients with X-linked agammaglobulinemia are hospitalized for infection at less than 6 months of age, indicating that patients with known X-linked agammaglobulinemia should be started on therapy by 2-3 months of age. In patients with sporadic X-linked agammaglobulinemia, the incidence of chronic lung disease correlates with the age at diagnosis, highlighting the importance of early diagnosis. Although almost all patients who are diagnosed as having the condition at more than 12 months of age have a history of recurrent otitis, 93% are not evaluated for immunodeficiency until they are hospitalized for infection. Because the physical exam provides a clue to the diagnosis of the condition--unusually small or absent cervical lymph nodes and tonsils--it should be possible to make an early diagnosis in a greater percentage of patients. Patients with autosomal recessive agammaglobulinemia have an earlier onset of disease compared with patients with X-linked agammaglobulinemia and they are more likely to have severe complications of the disease.

SUMMARY

There is plenty of room for improvement in the diagnosis and management of patients with defects in early B cell development.

Common variable immunodeficiency in a horse

A 12-year-old Quarter Horse mare that was nonresponsive to medical treatment was evaluated for chronic respiratory disease and hepatobiliary disease. Serum immunoglobulin concentrations were measured by use of radial immunodiffusion that revealed trace to nondetectable concentrations of IgG, IgG(T), IgM, and IgA. Use of serum protein electrophoresis confirmed agammaglobulinemia by the absence of the expected peak in the gamma region. In addition, vaccination with tetanus toxoid did not result in specific immunoglobulin production. Flow cytometric analysis of blood lymphocyte subpopulations revealed the absence of B cells in blood. Immunohistochemical analysis of tissue sections revealed the absence of B lymphocytes in bone marrow and spleen, with occasional B cells in the peripheral lymph nodes. Blood lymphocyte proliferation assays revealed weak responses to pokeweed mitogen and no response to stimulation with lipopolysaccharide. Considering the age and sex of the horse, results of the immunologic tests suggested a diagnosis of common variable immunodeficiency.

Clinical and molecular analysis of patients with defects in micro heavy chain gene

Autosomal recessive disorders of B cell development are rare and heterogeneous. To determine the proportion of affected patients who have defects in the micro heavy chain (IGHM) gene, we used single-stranded conformational polymorphism analysis to screen genomic DNA from 40 unrelated patients with early onset infections, profound hypogammaglobulinemia, and absent B cells. All of the patients were genotypically normal in BTK, the gene that underlies X-linked agammaglobulinemia. Eight different mutations in the micro heavy chain were identified in 19 members of 12 unrelated families. Four of the mutations were large deletions that removed more than 40 kb of DNA in the IGHM locus. In six of the 12 families, the affected patients had an identical single base pair substitution, a G-->A, at the -1 position of the alternative splice site. Immunoglobulin haplotype analysis showed that this mutation occurred on at least three different haplotypes, indicating that this is a hot spot for mutations. Compared with patients with mutations in Btk, patients with defects in the micro heavy chain had an earlier onset of disease and more complications. Our study indicates that at least 20-30% of patients with autosomal recessive defects in B cell development have mutations in the micro heavy chain.

Diagnosis of immunodeficiency: clinical clues and diagnostic tests

Recognition of immunodeficiency allows steps to be taken to minimize morbidity and mortality. Immunodeficiency can be secondary to viral infection, most importantly secondary to HIV-1 worldwide, medications, disruption of the usual infection clearance mechanisms, or secondary to a myriad of systemic disorders. Immunodeficiency may also be due to one of the growing list of primary immunodeficiency disorders. In infancy, lymphopenia should trigger an evaluation investigating the possibility of severe combined immunodeficiency. Evaluations of children should be done keeping in mind that normal numbers of lymphocytes are higher in children than in adults, immunoglobulin levels in children are lower than in adults in younger age groups, and antibody production in response to polysaccharide antigens is not usually fully developed in the less-than 2-year-old child.

Signals mediated by FcgammaRIIA suppress the growth of B-lineage acute lymphoblastic leukemia cells

We examined Fc receptor expression and function in normal and leukemic human immature B cells. Fc receptor expression increased with normal B cell maturation: CD32(+) cells composed 8.1% +/- 1.2% (mean +/- s.d.) of the least mature (CD34(+)CD10(+)), 19.2% +/- 5.7% of intermediate (CD34(-)CD10(+)), and 82.4% +/- 5.0% of mature (CD34(-)CD10(-)) bone marrow CD19(+) B cells. Forty-five of 57 primary B-lineage acute lymphoblastic leukemia samples and all six cell lines studied expressed Fc receptors. By RT-PCR and antibody staining, FcgammaRIIA was the Fc receptor predominantly expressed in these cells. FcgammaRIIA ligation in RS4;11 and 380 cells induced tyrosine phosphorylation of CD32, CD19, CBL, SYK, P13-K p85 and SHIP, as well as RasGAP association with tyrosine-phosphorylated p62(dok). These signalling events resulted in a marked suppression of leukemia cell growth. After a 7-day exposure to anti-CD32, the recovery of ALL cells cocultured with stroma was reduced to 5.5% +/- 2.8% of control values in 380 cells (n = 14), 19.4% +/- 6.1% (n = 8) in RS4;11, and 4.0% +/- 1.3% (n = 6) in KOPN55bi. CD32 ligation also reduced cell recovery in five of seven CD32(+) primary leukemia samples. Thus, FcgammaRIIA mediates signals that suppress the growth of lymphoid leukemia cells.

Primary immunodeficiency disorders: antibody deficiency

As a group, antibody deficiencies represent the most common types of primary immune deficiencies in human subjects. Often symptoms do not appear until the latter part of the first year of life, as passively acquired IgG from the mother decreases to below protective levels. As with the T-cell immune deficiencies, the spectrum of antibody deficiencies is broad, ranging from the most severe type of antibody deficiency with totally absent B cells and serum Igs to patients who have a selective antibody deficiency with normal serum Ig. In addition to the increased susceptibility to infections, a number of other disease processes (eg, autoimmunity and malignancies) can be involved in the clinical presentation. Fortunately, the availability of intravenous immune serum globulin has made the management of these patients more complete. Recently, molecular immunology has led to identification of the gene or genes involved in many of these antibody deficiencies. As discussed in this review, this has led to a better elucidation of the B-cell development and differentiation pathways and a more complete understanding of the pathogenesis of many of these antibody deficiencies.

Differential surrogate light chain expression governs B-cell differentiation

Surrogate light chain expression during B lineage differentiation was examined by using indicator fluorochrome-filled liposomes in an enhanced immunofluorescence assay. Pro-B cells bearing surrogate light chain components were found in mice, but not in humans. A limited subpopulation of relatively large pre-B cells in both species expressed pre-B cell receptors. These cells had reduced expression of the recombinase activating genes, RAG-1 and RAG-2. Their receptor-negative pre-B cell progeny were relatively small, expressed RAG-1 and RAG-2, and exhibited selective down-regulation of VpreB and λ5expression. Comparative analysis of the 2 pre-B cell subpopulations indicated that loss of the pre-B cell receptors from surrogate light chain gene silencing was linked with exit from the cell cycle and light chain gene rearrangement to achieve B-cell differentiation.

Novel Igalpha (CD79a) gene mutation in a Turkish patient with B cell-deficient agammaglobulinemia

Mutations that impair early B cell development result in profound antibody deficiency, which is characterized by a paucity of mature B cells and the early onset of recurrent pyogenic infections. Among these inherited early B cell defects, X-linked agammaglobulinemia (XLA) with mutations in Bruton's tyrosine kinase (BTK) gene is mostly identified. Recent studies have shown that mutations in the gene for mu heavy chain (IGHM) and for other components of the pre-B cell receptor complex, including lambda5/14.1 (IGLL1) or Igalpha (CD79a), can cause a disorder that is clinically similar to XLA. In a genetic survey of XLA in Turkey, we examined possible mutations in the IGHM, IGLL1, and Igalpha genes in some male patients with presumed XLA who did not have identifiable BTK mutations. We found an eight-year-old boy with a novel homozygous mutation in the Igalpha gene (IVS2+1G>A) causing B cell defect. This is the second case of agammaglobulinemia due to an Igalpha (CD79a) deficiency in the world.

Composition of precursor B-cell compartment in bone marrow from patients with X-linked agammaglobulinemia compared with healthy children

X-linked agammaglobulinemia (XLA) is characterized by a severe B-cell deficiency, resulting from a differentiation arrest in the bone marrow (BM). Because XLA is clinically and immunologically heterogeneous, we investigated whether the B-cell differentiation arrest in BM of XLA patients is heterogeneous as well. First, we analyzed BM samples from 19 healthy children by flow cytometry. This resulted in a normal B-cell differentiation model with eight consecutive stages. Subsequently, we analyzed BM samples from nine XLA patients. Eight patients had amino acid substitutions in the Bruton's tyrosine kinase (BTK) domain or premature stop codons, resulting in the absence of functional BTK proteins. In seven of these eight patients a major differentiation arrest was observed at the transition between cytoplasmic Ig(mu-) pre-B-I cells and cytoplasmic Ig(mu+) pre-B-II cells, consistent with a role for BTK in pre-B-cell receptor signaling. However, one patient exhibited a very early arrest at the transition between pro-B cells and pre-B-I cells, which could not be explained by a different nature of the BTK mutation. We conclude that the absence of functional BTK proteins generally leads to an almost complete arrest of B-cell development at the pre-B-I to pre-B-II transition. The ninth XLA patient had a splice site mutation associated with the presence of low levels of wild-type BTK mRNA. His BM showed an almost normal composition of the precursor B-cell compartment, suggesting that low levels of BTK can rescue the pre-B-cell receptor signaling defect, but do not lead to sufficient numbers of mature B lymphocytes in the peripheral blood.