Genetic basis of abnormal B cell development

Immunogenetics of marsupial B-cells

Marsupials and eutherians are mammals that differ in their physiological traits, predominately their reproductive and developmental strategies; eutherians give birth to well-developed young, while marsupials are born highly altricial after a much shorter gestation. These developmental traits also result in differences in the development of the immune system of eutherian and marsupial species. In eutherians, B-cells are the key to humoral immunity as they are found in multiple lymphoid organs and have the unique ability to mediate the production of antigen-specific antibodies in the presence of extracellular pathogens. The development of B-cells in marsupials has been reported and hypothesised to be similar to that of eutherians, except that haematopoiesis occurs in the liver, postpartum, until the bone marrow fully matures. In eutherians, specific genes are linked to specific stages in B-cell development, maturation, and differentiation processes, and have been identified including immunoglobulins (heavy and light chains), cluster of differentiation markers (CD10, 19, 34 and CD79α/β), signal transduction molecules (BTK, Lyn and Syk) and transcriptional regulators (EBF1, E2A, and Pax5). This review aims to discuss the known similarities and differences between marsupial and eutherian B-cells, in regards to their genetic presence, homology, and developmental stages, as well as to highlight the areas requiring further investigation. By enhancing our understanding of the genes that are involved with B-cells in the marsupial lineage, it will, in turn, aid our understanding of the marsupial immune system and support the development of specific immunological reagents for research and wildlife conservation purposes.

Flow Cytometry for Diagnosis of Primary Immune Deficiencies-A Tertiary Center Experience From North India

Flow cytometry has emerged as a useful technology that has facilitated our understanding of the human immune system. Primary immune deficiency disorders (PIDDs) are a heterogeneous group of inherited disorders affecting the immune system. More than 350 genes causing various PIDDs have been identified. While the initial suspicion and recognition of PIDDs is clinical, laboratory tools such as flow cytometry and genetic sequencing are essential for confirmation and categorization. Genetic sequencing, however, are prohibitively expensive and not readily available in resource constrained settings. Flow cytometry remains a simple, yet powerful, tool for multi-parametric analysis of cells. While it is confirmatory of diagnosis in certain conditions, in others it helps in narrowing the list of putative genes to be analyzed. The utility of flow cytometry in diagnosis of PIDDs can be divided into four major categories: (a) Enumeration of lymphocyte subsets in peripheral blood. (b) Detection of intracellular signaling molecules, transcription factors, and cytokines. (c) Functional assessment of adaptive and innate immune cells (e.g., T cell function in severe combined immune deficiency and natural killer cell function in familial hemophagocytic lymphohistiocytosis). (d) Evaluation of normal biological processes (e.g., class switching in B cells by B cell immunophenotyping). This review focuses on use of flow cytometry in disease-specific diagnosis of PIDDs in the context of a developing country.

X-linked agammaglobulinemia (XLA):Phenotype, diagnosis, and therapeutic challenges around the world

BACKGROUND

X-linked agammaglobulinemia is an inherited immunodeficiency recognized since 1952. In spite of seven decades of experience, there is still a limited understanding of regional differences in presentation and complications. This study was designed by the Primary Immunodeficiencies Committee of the World Allergy Organization to better understand regional needs, challenges and unique patient features.

METHODS

A survey instrument was designed by the Primary Immunodeficiencies Committee of the World Allergy Organization to collect both structured and semi-structured data on X-linked agammaglobulinemia. The survey was sent to 54 centers around the world chosen on the basis of World Allergy Organization participation and/or registration in the European Society for Immunodeficiencies. There were 40 centers that responded, comprising 32 countries.

RESULTS

This study reports on 783 patients from 40 centers around the world. Problems with diagnosis are highlighted by the reported delays in diagnosis>24 months in 34% of patients and the lack of genetic studies in 39% of centers Two infections exhibited regional variation. Vaccine-associated paralytic poliomyelitis was seen only in countries with live polio vaccination and two centers reported mycobacteria. High rates of morbidity were reported. Acute and chronic lung diseases accounted for 41% of the deaths. Unusual complications such as inflammatory bowel disease and large granular lymphocyte disease, among others were specifically enumerated, and while individually uncommon, they were collectively seen in 20.3% of patients. These data suggest that a broad range of both inflammatory, infectious, and autoimmune conditions can occur in patients. The breadth of complications and lack of data on management subsequently appeared as a significant challenge reported by centers. Survival above 20 years of age was lowest in Africa (22%) and reached above 70% in Australia, Europe and the Americas. Centers were asked to report their challenges and responses (n = 116) emphasized the difficulties in access to immunoglobulin products (16%) and reflected the ongoing need for education of both patients and referring physicians.

CONCLUSIONS

This is the largest study of patients with X-linked agammaglobulinemia and emphasizes the continued morbidity and mortality of XLA despite progress in diagnosis and treatment. It presents a world view of the successes and challenges for patients and physicians alike. A pivotal finding is the need for education of physicians regarding typical symptoms suggesting a possible diagnosis of X-linked agammaglobulinemia and sharing of best practices for the less common complications.

B-cell subsets, signaling and their roles in secretion of autoantibodies

B cells play a pivotal role in the pathogenesis of autoimmune diseases. In patients with systemic lupus erythematosus (SLE), the percentages of plasmablasts and IgD−CD27− double-negative memory B cells in peripheral blood are significantly increased, while IgD+CD27+ IgM memory B cells are significantly decreased compared to healthy donors. The phenotypic change is significantly associated with disease activity and concentration of autoantibodies. Treatment of B-cell depletion using rituximab results in the reconstitution of peripheral B cells in SLE patients with subsequent improvement in disease activity. Numerous studies have described abnormalities in B-cell receptor (BCR)-mediated signaling in B cells of SLE patients. Since differences in BCR signaling are considered to dictate the survival or death of naïve and memory B cells, aberrant BCR signal can lead to abnormality of B-cell subsets in SLE patients. Although Syk and Btk function as key molecules in BCR signaling, their pathological role in SLE remains unclear. We found that Syk and Btk do not only transduce activation signal through BCR, but also mediate crosstalk between BCR and Toll-like receptor (TLR) as well as BCR and JAK-STAT pathways in human B cells in vitro. In addition, pronounced Syk and Btk phosphorylation was observed in B cells of patients with active SLE compared to those of healthy individuals. The results suggest the involvement of Syk and Btk activation in abnormalities of BCR-mediated signaling and B-cell phenotypes during the pathological process of SLE and that Syk, Btk and JAK are potential therapeutic targets in SLE.

Flow Cytometry, a Versatile Tool for Diagnosis and Monitoring of Primary Immunodeficiencies

Genetic defects of the immune system are referred to as primary immunodeficiencies (PIDs). These immunodeficiencies are clinically and immunologically heterogeneous and, therefore, pose a challenge not only for the clinician but also for the diagnostic immunologist. There are several methodological tools available for evaluation and monitoring of patients with PIDs, and of these tools, flow cytometry has gained prominence, both for phenotyping and functional assays. Flow cytometry allows real-time analysis of cellular composition, cell signaling, and other relevant immunological pathways, providing an accessible tool for rapid diagnostic and prognostic assessment. This minireview provides an overview of the use of flow cytometry in disease-specific diagnosis of PIDs, in addition to other broader applications, which include immune phenotyping and cellular functional measurements.

Molecular Diagnostic Challenges and Complex Management of Consecutive Twin Pregnancies in a Family with CD40 Ligand Deficiency

X-linked hyper-IgM syndrome (XHIGM) is a primary immunodeficiency disorder (PID) caused by mutation in the gene encoding the CD40 ligand (CD40L) expressed on activated T cells. Prenatal genotyping in carriers with twin pregnancies is more challenging than in women with singleton pregnancies. In addition, women with twin pregnancies may decide on selective termination for which the risk of loss of the healthy foetus may exceed 7%. We report here on a family affected by XHIGM. Diagnosis of the disease was made in a male patient as late as 33 years of age. After family screening, the sister of the proband conceived male twins in two consecutive pregnancies. In the first pregnancy, one of the male foetuses was hemizygous for the c.521A>G (Q174R) mutation in the CD40L gene. In the second pregnancy, ultrasound scan showed one foetus to have exencephaly and karyotyping revealed this foetus to have trisomy 18. Several options were discussed, but the parents decided on selective termination in both pregnancies. The interventions were successful in both cases, and the mother now has two healthy sons. This report demonstrates the way in which advanced technologies in molecular medicine and obstetric interventions may assist families with decisions about possible selective termination in case of life-threatening molecular or chromosomal disorders. Diagnosis of CD40L deficiency at the age of 33 years in the proband was striking and indicated that PIDs are still neglected as disease entities in the evaluation of patients with recurrent severe infectious diseases.

Clinical consequences of defects in B-cell development

Abnormalities in humoral immunity typically reflect a generalized or selective failure of effective B-cell development. The developmental processes can be followed through analysis of cell-surface markers, such as IgM, IgD, CD10, CD19, CD20, CD21, and CD38. Early phases of B-cell development are devoted to the creation of immunoglobulin and testing of B-cell antigen receptor signaling. Failure leads to the absence of B cells and immunoglobulin in the blood from birth. As the developing B cells begin to express a surface B-cell receptor, they become subject to negative and positive selection pressures and increasingly depend on survival signals. Defective signaling can lead to selective or generalized hypogammaglobulinemia, even in the presence of normal numbers of B cells. In the secondary lymphoid organs some B cells enter the splenic marginal zone, where preactivated cells lie ready to rapidly respond to T-independent antigens, such as the polysaccharides that coat some microorganisms. Other cells enter the follicle and, with the aid of cognate follicular T cells, divide to help form a germinal center (GC) after their interaction with antigen. In the GC B cells can undergo the processes of class switching and somatic hypermutation. Failure to properly receive T-cell signals can lead to hyper-IgM syndrome. B cells that leave the GC can develop into memory B cells, short-lived plasma cells, or long-lived plasma cells. The latter ultimately migrate back to the bone marrow, where they can continue to produce protective antigen-specific antibodies for decades.

[IgA subclass and IgA deficiency]

There are two subclasses of IgA, IgA1 and IgA2, and its heavy chains are encoded by two different genes, alpha1 and alpha2 genes. These two subclasses play important roles in the first line of defense, and the amount ratio of these molecules in secretions varies. IgA deficiency (IgAD) is the most common immunodeficiency, however the pathogenesis in most cases of IgAD is unknown. The class switch disorder in IgA producing B lymphocytes is one of the important factors in IgAD patients. The decreased expression levels of Ialpha germline transcripts before a class switch may be the cause of selective IgAD. The alpha1 and alpha2 gene expression levels are low in most IgAD patients. Using RT-PCR method in which alpha1 and alpha2 mRNAs can be separately evaluated, we identified the second case of alpha1 gene deletion in Japan. Longitudinal change in the serum IgA of the patient with alpha1 gene deletion showed the pattern of the partial IgAD. Patients with alpha1 gene deletion can be considered as having partial IgAD.

The tec family tyrosine kinase Btk Regulates RANKL-induced osteoclast maturation

A spontaneous mutation in Bruton's tyrosine kinase (Btk) induces a defect in B-cell development that results in the immunodeficiency diseases X-linked agammaglobulinemia in humans and X-linked immunodeficiency (Xid) in mice. Here we show an unexpected role of Btk in osteoclast formation. When bone marrow cells derived from Xid mice were stimulated with receptor activator of NF-kappaB ligand, an osteoclast differentiation factor, they did not completely differentiate into mature multinucleated osteoclasts. Moreover, we found that the defects appeared to occur at the stage in which mononuclear preosteoclasts fuse to generate multinucleated cells. Supporting this notion, macrophages from Xid mice also failed to form multinucleated foreign body giant cells. The fusion defect of the Xid mutant osteoclasts was caused by decreased expression of nuclear factor of activated T cells c1 (NFATc1), a master regulator of osteoclast differentiation, as well as reduced expression of various osteoclast fusion-related molecules, such as the d2 isoform of vacuolar H(+)-ATPase V0 domain and the dendritic cell-specific transmembrane protein. This deficiency was completely rescued by the introduction of a constitutively active form of NFATc1 into bone marrow-derived macrophages. Our data provide strong evidence that Btk plays a critical role in osteoclast multinucleation by modulating the activity of NFATc1.

Bruton’s Tyrosine Kinase Mediates NF-κB Activation and B Cell Survival by B Cell-Activating Factor Receptor of the TNF-R Family

Loss of Bruton's tyrosine kinase (Btk) function results in mouse Xid disease characterized by a reduction in mature B cells and impaired humoral immune responses. These defects have been mainly attributed to impaired BCR signaling including reduced activation of the classical NF-kappaB pathway. In this study we show that Btk also couples the receptor for B cell-activating factor (BAFF) of the TNF family (BAFF-R) to the NF-kappaB pathway. Loss of Btk results in defective BAFF-mediated activation of both classical and alternative NF-kappaB pathways. Btk appears to regulate directly the classical pathway in response to BAFF such that Btk-deficient B cells exhibit reduced kinase activity of IkappaB kinase gamma-containing complexes and defective IkappaBalpha degradation. In addition, Btk-deficient B cells produce reduced levels of NF-kappaB2 (p100) basally and in response to stimulation via the BCR or BAFF-R, resulting in impaired activation of the alternative NF-kappaB pathway by BAFF. These results suggest that Btk regulates B cell survival by directly regulating the classical NF-kappaB pathway under both BCR and BAFF-R, as well as by inducing the expression of the components of alternative pathway for sustained NF-kappaB activation in response BAFF. Thus, impaired BCR- and BAFF-induced signaling to NF-kappaB may contribute to the observed defects in B cell survival and humoral immune responses in Btk-deficient mice.

Relapsing Campylobacter coli bacteremia with reactive arthritis in a patient with X-linked agammaglobulinemia

A patient genetically diagnosed with X-linked agammaglobulinemia repeatedly developed bacteremia due to Campylobacter coli (C. coli) for one year and seven months in spite of immunoglobulin replacement therapy. Throughout the clinical course, C. coli with identical genetic patterns was repeatedly isolated from both blood and stool cultures, thus indicating that the patient had latent intestinal infection. The bacteremia was always accompanied by reactive arthritis. Since the immunoglobulin level was extremely low with severe B cell deficiency, the reactive arthritis must have been induced in a humoral immunity-independent manner. Adding oral minocycline following intravenous meropenem was very effective; the stool cultures became negative and the patient has been well for more than one year without relapse of bacteremia.

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.

X-Linked Agammaglobulinemia Diagnosed in Adulthood: A Case Report

X-linked agammaglobulinemia (XLA) is a humoral immunodeficiency caused by mutations in Bruton's tyrosine kinase (BTK). Patients typically become symptomatic during infancy or early childhood and develop recurrent bacterial infections. We report a Japanese case of XLA diagnosed in a patient who was 27 years of age and who had no history of severe infection. The patient's serum immunoglobulin (Ig) G, IgA, and IgM levels were 132,7, and 17 mg/dL, respectively. The percentage of positive cells for CD19 and CD20 was 0.03% and 0.02%, respectively. The patient's brother and sister had no abnormalities. Flow cytometric analysis showed a partially reduced expression of BTK protein in the patient's peripheral monocytes. Sequencing of the BTK. gene revealed a missense mutation (230C>T,T33I). Given this data, this patient was diagnosed as having rare, late onset XLA with a missense mutation in the BTK gene.

Interleukin-10 polymorphisms in Spanish IgA deficiency patients: a case-control and family study

BACKGROUND

IgA deficiency (IgAD) is the most common primary immunodeficiency in Caucasians. Genetic and environmental factors are suspected to be involved in the development of the disease. Interleukin-10 (IL-10) is a cytokine with stimulatory activity on immunoglobulin production and it may be an important regulator in IgAD pathogenesis. The IL-10 gene contains several single nucleotide polymorphisms (SNPs) and two polymorphic microsatellites located in the 5'-flanking region. Our aim was to ascertain if any of these polymorphic markers are associated or linked to IgAD in Spanish patients.

METHODS

We genotyped 278 patients with IgAD and 573 ethnically matched controls for the microsatellites IL-10R and IL-10G and for three single nucleotide polymorphisms at positions -1082, -819 and -592 in the proximal promoter of the gene. We also included in this study the parents of 194 patients in order to study the IL-10 haplotypes transmitted and not transmitted to the affected offspring.

RESULTS

The only allele where a significant difference was observed in the comparison between IgA deficiency patients and controls was the IL-10G12 allele (OR = 1.58 and p = 0.021). However, this p value could not withstand a Bonferroni correction. None of the IL-10R or promoter SNP alleles was found at a different frequency when patients were compared with controls.

CONCLUSION

Our data do not show any significant difference in IL-10 polymorphism frequencies between control and IgAD patient samples. Their haplotype distribution among patients and controls was also equivalent and therefore these microsatellites and SNPs do not seem to influence IgAD susceptibility.

A rare intestinal manifestation in a patient with common variable immunodeficiency and strongyloidiasis

We report an immunodeficient patient with a rare gastrointestinal manifestation. A 26-year-old male with common variable immunodeficiency (CVID) and bronchiolitis obliterans, who was on intravenous gamma-globulin and prednisone, presented diffuse abdominal pain, nausea, vomiting and constipation of 3 days' duration. He reported 5 years of recurrent respiratory infections and diarrhea with negative stool tests, including tests for Strongyloides stercoralis. A physical exam revealed a poor general condition, anemia, dehydration and a distended painful abdomen with guarding, without abdominal sounds. The radiological study showed marked dilation of the small bowel that was edematous. Resection of the affected loop was performed and the histopathologic study showed transmural infection with S. stercoralis and hemorrhagic necrosis of the muscular layer, without mucosal destruction. The patient developed malabsorption syndrome and septic shock; he was treated with antibiotics and thiabendazole and was finally discharged in a good general condition. CVID is a rare disease and its association with systemic strongyloidiasis is very uncommon, but it has been reported in patients on corticosteroids. Hemorrhagic necrosis of the muscular layer without mucosal destruction was not found in the literature studied.

Dendritic cells of IgA nephropathy patients have an impaired capacity to induce IgA production in naïve B cells

BACKGROUND

IgA nephropathy (IgAN) is the most common primary glomerulonephritis worldwide, characterized by mesangial IgA1 deposits. We have previously demonstrated that IgAN patients have a hampered IgA immune response after mucosal challenge with a neoantigen. Dendritic cells are critically involved in the initiation of humoral immune responses, not only via activation of T-helper cells, but also via direct effect on naïve B cells. The aim of this study was to investigate the capacity of dendritic cells from IgAN patients to regulate IgA production.

METHODS

Dendritic cells were generated by culturing monocytes for 7 days in the presence of interleukin (IL)-4 and granulocyte macrophage-colony-stimulating factor (GM-CSF). Dendritic cells from either IgAN patients (N= 12) or controls (N= 12) were cultured for 14 days with naïve B cells in the presence of CD40L-transfected mouse fibroblasts (L-CD40L cells) and medium with or without IL-2 or IL-10. Supernatants were tested for the presence of immunoglobulins by specific enzyme-linked immunosorbent assay (ELISA).

RESULTS

In the presence of CD40L and IL-10, dendritic cells were able to increase immunoglobulin production by naïve B cells. Dendritic cells of IgAN patients induced significantly (P= 0.026) less IgA production than dendritic cells of control persons (2.30 microg/mL vs. 5.24 microg/mL), whereas no differences were found in the IgG and IgM production. When dendritic cells were replaced by supernatant of CD40L-stimulated dendritic cells of patients and controls, IgA production was increased, but no difference was seen between the two groups.

CONCLUSION

In the present study we show that dendritic cells of IgAN patients have an impaired capacity to induce IgA production in naïve B cells, which might explain the observed IgA hyporesponse upon mucosal challenge with a neoantigen.

Gluten sensitive enteropathy and other causes of small intestinal lymphocytosis

Recent studies on gluten sensitive enteropathy (GSE) have heightened the pathologist's awareness of intraepithelial lymphocytes in duodenal biopsies. Mild histologic forms of GSE are now recognized with increased IELs as the only histologic change. In this review, both the classic morphology and "minimal deviation" GSE are discussed in detail, with special emphasis on the differential diagnosis and how to avoid overdiagnosing this condition. The Marsh-Oberhuber classification is described and contrasted with a more practical diagnostic terminology that is recommended for use in pathology reports. Other important conditions associated with duodenal lymphocytosis, such as refractory sprue, autoimmune enteropathy, and Helicobacter pylori-associated duodenitis, are also presented.

The molecular basis and biological significance of VH replacement

First observed in mouse pre-B-cell lines and then in knock-in mice carrying self-reactive IgH transgenes, VH replacement has now been shown to contribute to the primary B-cell repertoire in humans. Through recombination-activating gene (RAG)-mediated recombination between a cryptic recombination signal sequence (RSS) present in almost all VH genes and the flanking 23 base pair RSS of an upstream VH gene, VH replacement renews the entire VH-coding region, while leaving behind a short stretch of nucleotides as a VH replacement footprint. In addition to extending the CDR3 region, the VH replacement footprints preferentially contribute charged amino acids. VH replacement rearrangement in immature B cells may either eliminate a self-reactive B-cell receptor or contribute to the generation of self-reactive antibodies. VH replacement may also rescue non-productive or dysfunctional VHDJH rearrangement in pro-B and pre-B cells. Conversely, VH replacement of a productive immunoglobulin H gene may generate non-productive VH replacement to disrupt or temporarily reverse the B-cell differentiation process. VH replacement can thus play a complex role in the generation of the primary B-cell repertoire.

Natural antibodies sustain differentiation and maturation of human dendritic cells

The differentiation and maturation of dendritic cells (DCs) is governed by various signals in the microenvironment. Monocytes and DCs circulate in peripheral blood, which contains high levels of natural antibodies (NAbs). NAbs are germ-line-encoded and occur in the absence of deliberate immunization or microbial aggression. To assess the importance of NAbs in the milieu on DC development, we examined the status of DCs in patients with X-linked agammaglobulinemia, a disease characterized by paucity of B cells and circulating antibodies. We demonstrate that the in vitro differentiation of DCs is severely impaired in these patients, at least in part because of low levels of circulating NAbs. We identified NAbs reactive with the CD40 molecule as an important component that participates in the development of DCs. CD40-reactive NAbs restored normal phenotypes of DCs in patients. The maturation process induced by CD40-reactive NAbs was accompanied by an increased IL-10 and decreased IL-12 production. The transcription factor analysis revealed distinct signaling pathways operated by CD40-reactive NAbs compared to those by CD40 ligand. These results suggest that B cells promote bystander DC development through NAbs and the interaction between NAbs and DCs may play a role in steady-state migration of DCs.

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.

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.

The B cell antigen receptor controls integrin activity through Btk and PLCgamma2

Integrin-mediated adhesion and B cell antigen receptor (BCR) signaling play a critical role in B cell development and function, including antigen-specific B cell differentiation. Here we show that the BCR controls integrin α4β1 (VLA-4)-mediated adhesion of B cells to vascular cell adhesion molecule-1 and fibronectin. Molecular dissection of the underlying signaling mechanism by a combined biochemical, pharmacological, and genetic approach demonstrates that this BCR-controlled integrin-mediated adhesion requires the (consecutive) activation of Lyn, Syk, phosphatidylinositol 3-kinase, Bruton's tyrosine kinase (Btk), phospholipase C (PLC)γ2, IP3R-mediated Ca²⁺ release, and PKC. In contrast, activation of mitogen-activated protein kinase kinase (MEK) or extracellular signal–regulated kinase (ERK) is not required, and simultaneous activation of MEK, ERK, and PKB is not sufficient either. Furthermore, Btk is also involved in the control of integrin-mediated adhesion of preB cells. The control of integrin α4β1-mediated B cell adhesion by the BCR involves cytoskeletal reorganization and integrin clustering. These results reveal a novel function for the BCR and Btk, i.e., regulation of integrin α4β1 activity, thereby providing new insights into the control of B cell development and differentiation, as well as into the pathogenesis of the immunodeficiency disease X-linked agammaglobulineamia (XLA).

Bruton's tyrosine kinase targets NF-kappaB to the bcl-x promoter via a mechanism involving phospholipase C-gamma2 following B cell antigen receptor engagement

Disruption of Bruton's tyrosine kinase (BTK) function leads to x-linked immunodeficiency (xid) in mice. BTK-deficient (btk(-/-)) B cells are defective for survival. Prior studies show that BTK is required for the induction of Bcl-x(L) following B cell antigen receptor (BCR) engagement. However, the mechanism underlying Bcl-x(L) induction in response to BCR ligation remains unresolved. We now demonstrate that BTK regulates bcl-x expression by transcriptional control in response to BCR engagement. BTK targets nuclear factor-kappaB (NF-kappaB) to activate the bcl-x promoter via a phospholipase C-gamma2 (PLC-gamma2)-dependent mechanism. Perturbation of the BTK/PLC-gamma2/NF-kappaB signaling axis likely contributes to the defective expression of bcl-x and compromised survival of xid B cells.

Immunodeficiency and genetic conditions that cause arthritis in childhood

Many conditions can cause or be associated with arthritis in childhood. The authors of this paper will review the situations in which underlying immunodeficiency or defective regulation of lymphocyte homeostasis must be suspected, and discuss, for some of these diseases, the genetic bases and pathogenesis. In the second part of this article, the authors will focus on other diseases that can cause arthritis in childhood, often with other symptoms, and for which evidence of an association with genetic abnormalities has been recently discovered. Finally, the authors will discuss the implications of recent findings regarding the role of some genes as causing or modulating factors in juvenile idiopathic arthritis and related disorders, as well as observations made in adults and in animal models of inflammation and autoimmunity.

Deletion of immunoglobulin beta in developing B cells leads to cell death

Inducible gene-targeting experiments have shown that Igmu expression is essential for maintaining survival of mature B cells, but the role of Igmu expression in immature B cell survival has not been determined. To assess whether continued B cell receptor (BCR) expression is required for bone marrow B cell precursor development and survival, we developed a method for conditional gene deletion in these cells. Recombination-activating gene regulatory elements were used to express Igbeta cDNA as a transgene to complement Igbeta(-/-) mice. Transgenic Igbeta expression was found in proB and small preB cells and was extinguished in large preB and immature B cells. Igbeta deletion from large preB cells and immature B cells resulted in cell death that could be rescued by transgenic bcl-2 expression. However, transgenic bcl-2 expression was unable to restore B cell development in the absence of Igbeta. We conclude that Igbeta expression is essential to maintain preB cell and immature B cell survival and to mediate B cell differentiation. In addition, complementation of null mutations with cDNAs under the control of heterologous bacterial artificial chromosomes is a useful method for cell-type-specific and developmentally regulated gene ablation in vivo.

Protein kinase C beta controls nuclear factor kappaB activation in B cells through selective regulation of the IkappaB kinase alpha

Activation of the nuclear factor (NF)-kappaB transcription complex by signals derived from the surface expressed B cell antigen receptor controls B cell development, survival, and antigenic responses. Activation of NF-kappaB is critically dependent on serine phosphorylation of the IkappaB protein by the multi-component IkappaB kinase (IKK) containing two catalytic subunits (IKKalpha and IKKbeta) and one regulatory subunit (IKKgamma). Using mice deficient for protein kinase C beta (PKCbeta) we show an essential role of PKCbeta in the phosphorylation of IKKalpha and the subsequent activation of NF-kappaB in B cells. Defective IKKalpha phosphorylation correlates with impaired B cell antigen receptor-mediated induction of the pro-survival protein Bcl-xL. Lack of IKKalpha phosphorylation and defective NF-kappaB induction in the absence of PKCbeta explains the similarity in immunodeficiencies caused by PKCbeta or IKKalpha ablation in B cells. Furthermore, the well established functional cooperation between the protein tyrosine kinase Bruton's tyrosine kinase (Btk), which regulates the activity of NF-kappaB and PKCbeta, suggests PKCbeta as a likely serine/threonine kinase component of the Btk-dependent NF-kappaB activating signal transduction chain downstream of the BCR.

Long-lasting memory-resting and memory-effector CD4+ T cells in human X-linked agammaglobulinemia

Conflicting results obtained from animal studies suggest that B cells play a role in maintaining long-term T-cell memory and in skewing T-cell response toward a T-helper 2 (T(H)2) phenotype. X-linked agammaglobulinemia (XLA) is a genetic human disease characterized by the lack of circulating B cells due to the mutation of Bruton tyrosine kinase. This disease thus represents a unique model for studying the role of B lymphocytes in regulating T-cell functions in humans. To this aim, we analyzed hepatitis B envelope antigen (HBenvAg)-specific T-cell memory in a series of XLA patients vaccinated against hepatitis B virus (HBV). We found HBenvAg-specific T lymphocytes producing interferon-gamma, interleukin-4, or both in the peripheral blood of XLA patients up to at least 24 months after completing the standard anti-HBV immunization protocol. The HBenvAg-specific T-cell frequencies and the percentage of patients with these responses were not significantly different from healthy vaccinated controls. By combining cell purification and enzyme-linked immunospot assay, we found that effector CD27- T cells, which promptly produced cytokines in response to antigen (Ag), and memory-resting CD27+ T cells, which required Ag restimulation to perform their functions, were maintained in both XLA patients and controls for up to 24 months after the last vaccination boost. These data strongly suggest that B cells are not an absolute requirement for the generation of effective T-cell memory in humans, nor do they seem to influence T(H)1/T(H)2 balance.

Clinical and mutational characteristics of X-linked agammaglobulinemia and its carrier identified by flow cytometric assessment combined with genetic analysis

BACKGROUND

X-linked agammaglobulinemia (XLA), caused by mutations in Bruton's tyrosine kinase (BTK ), is the most common form of inherited antibody deficiency. We previously reported that a flow cytometric evaluation of BTK expression in monocytes could easily detect XLA as well as its carrier.

OBJECTIVE

Our purpose was to perform further flow cytometric analysis in additional XLA families in Japan.

METHODS

In all, 106 hypogammaglobulinemic males (from 91 families) of various ages with a lack of mature B cells (<1%) were investigated.

RESULTS

Flow cytometric assessment revealed the deficient BTK expression status in 78 families (93 patients), and mutations in BTK were identified in 76 of 78 families with presumed XLA. Of the patients with normal BTK expression, 2 showed missense mutations in which the normal amount of altered BTK transcript would cause the XLA phenotype. As many as 30% of these patients with XLA were clinically or genetically recognized beyond 5 years of age. Higher concentrations (>300 mg/dL) of serum IgG were evident in the cases diagnosed among adults, seemingly preventing severe infections. Fifty-seven of 70 mothers of patients with BTK deficiency were diagnosed as obligate carriers on the basis of a bimodal BTK expression pattern. Nine of the remaining 13 mothers showing nonmosaic BTK expression had no mutations in 2 alleles; surprisingly, the other 4 mothers had the mutated alleles.

CONCLUSIONS

A diagnostic approach based on flow cytometric assessment for XLA should be initially considered in genetic investigation of antibody deficiencies, regardless of the patient's age.

Conventional and surrogate light chains differentially regulate Ig mu and Dmu heavy chain maturation and surface expression

Positive selection of precursor (pre-) B cells by Ig membrane mu H chains (mum HC) and counterselection mediated by the truncated HC Dmu depend on the ability of each HC to form a pre-B cell receptor (pre-BCR) signaling complex with the surrogate L chain (SLC) components lambda5 and Vpre-B. To better understand how pre-BCR signaling output is determined by its Ig components and the SLC, we investigated the regulation of pre-BCR surface expression and HC secretory maturation in a new nonlymphoid system. We took this approach as a means to distinguish B-lineage-specific effects from pre-BCR-intrinsic properties that may influence these aspects of pre-BCR homeostasis necessary for signaling. As in pre-B cells, the SLC in nonlymphoid cells supported only a limited degree of mum HC maturation and low pre-BCR surface expression levels compared with conventional LCs, indicating that this was due to an intrinsic property of the SLC. We identified the non-Ig region of lambda5 as harboring the restrictive activity responsible for this phenotype. This property of lambda5 was also evident with Dmu, but the overall SLC- and L chain-dependent requirements for Dmu maturation and surface expression were markedly different from those for mum. Surprisingly, Dmu was modified in an unusual manner that was only dependent on Vpre-B. These results establish a novel function of lambda5 in limiting surface pre-BCR levels and reveal biochemical properties of Ig molecules that may underlie the diverse consequences of pre-BCR signaling in vivo by different HCs.

Relatively normal human lymphopoiesis but rapid turnover of newly formed B cells in transplanted nonobese diabetic/SCID mice

Human B lineage lymphocyte precursors in chimeric nonobese diabetic/SCID mice transplanted with umbilical cord blood cells were directly compared with those present in normal bone marrow. All precursor subsets were represented and in nearly normal proportions. Cell cycle activity and population dynamics were investigated by staining for the Ki-67 nuclear Ag as well as by incorporation experiments using 5-bromo-2'-deoxyuridine. Again, this revealed that human B lymphopoiesis in chimeras parallels that in normal marrow with respect to replication and progression through the lineage. Moreover, sequencing of Ig gene rearrangement products showed that a diverse repertoire of V(H) genes was utilized by the newly formed lymphocytes but there was no evidence for somatic hypermutation. The newly formed B cells frequently acquired the CD5 Ag and had a short life span in the periphery. Thus, all molecular requirements for normal B lymphocyte formation are present in nonobese diabetic/SCID mice, but additional factors are needed for recruitment of B cells into a fully mature, long-lived pool. The model can now be exploited to learn about species restricted and conserved environmental cues for human B lymphocyte production.

Bruton's tyrosine kinase regulates the activation of gene rearrangements at the lambda light chain locus in precursor B cells in the mouse

Bruton's tyrosine kinase (Btk) is a nonreceptor tyrosine kinase involved in precursor B (pre-B) cell receptor signaling. Here we demonstrate that Btk-deficient mice have an ∼50% reduction in the frequency of immunoglobulin (Ig) λ light chain expression, already at the immature B cell stage in the bone marrow. Conversely, transgenic mice expressing the activated mutant Btk^E41K showed increased λ usage. As the κ/λ ratio is dependent on (a) the level and kinetics of κ and λ locus activation, (b) the life span of pre-B cells, and (c) the extent of receptor editing, we analyzed the role of Btk in these processes. Enforced expression of the Bcl-2 apoptosis inhibitor did not alter the Btk dependence of λ usage. Crossing 3-83μδ autoantibody transgenic mice into Btk-deficient mice showed that Btk is not essential for receptor editing. Also, Btk-deficient surface Ig⁺ B cells that were generated in vitro in interleukin 7-driven bone marrow cultures manifested reduced λ usage. An intrinsic defect in λ locus recombination was further supported by the finding in Btk-deficient mice of reduced λ usage in the fraction of pre-B cells that express light chains in their cytoplasm. These results implicate Btk in the regulation of the activation of the λ locus for V(D)J recombination in pre-B cells.

Immunologic compensation in a patient with a large IgH constant region deletion

BACKGROUND

Deficiencies of serum Ig of the IgG isotype typically predispose individuals to recurrent infections in some but not all cases. Patients with large deletions of the Ig heavy chain genes are free of recurrent and severe infections.

OBJECTIVE

We sought to determine a mechanism of immunologic compensation that would possibly explain the reason for this patient's paucity of infection despite lacking several classes of serum Ig.

METHODS

The patient is a 50-year-old white man. Serum Ig levels and specific antibody titers were measured by using various methods, including nephelometry, enzyme immunoassay, and radial immunodiffusion. The status of the Ig heavy chain genes was examined by means of Southern blotting of genomic DNA isolated from EBV-transformed B cells.

RESULTS

The patient's serum lacked detectable IgG1, IgG2, IgG4, and IgA1 levels. Southern blot analysis demonstrated a large heavy chain constant (C) region gene deletion that included Cgamma1, Calpha1, psiCgamma, Cgamma2, and Cgamma4. Antibody responses to capsular pneumococcal and hemophilus polysaccharide antigens were essentially absent. However, IgG3 antibodies against the protein antigen tetanus toxoid were present. Relatively high antibody titers were found against pneumococcal surface proteins as well.

CONCLUSION

We conclude that our patient's relative freedom from serious infection may be as a result of production of IgG3 antibodies to pneumococcal capsular proteins.

[IgA deficiency]

Selective IgA deficiency is the most common primary immunodeficiency, with a prevalence of approximately 1/600 in whites. Most subjects are asymptomatic but some may suffer from frequent respiratory and gastrointestinal infections. Patients who suffer from frequent infections usually have a defect in antibody responses toward polysaccharides, which is often associated with IgG2 deficiency. Genetic predisposition to develop IgA deficiency has been shown to be linked to at least one locus on 6p21. Some IgA-deficient patients are also prone to develop more severe immunodeficiency called common variable immunodeficiency, which is associated with decreased IgG and sometimes IgM production as well as partial T-cell defect. In a few cases, IgA deficiency may reveal a severe disease such as ataxia-telangiectasia. Selective IgA deficiency contraindicates immunoglobulin administration. Only the minority of IgA-deficient patients who develop severe or frequent infections in association with IgG2 deficiency or impaired antibody response are candidates for prophylactic intravenous immunoglobulin substitution. Immunoglobulin preparations containing particularly low amounts of IgA are required to avoid adverse effects related to anti-IgA alloantibodies.

An essential role for Daxx in the inhibition of B lymphopoiesis by type I interferons

Interferon-alpha and -beta inhibit the interleukin-7-mediated growth and survival of T and B lymphoid progenitors via an unknown, STAT1-independent pathway. Gene expression profile analysis of interferon-beta-treated progenitor B cells revealed enhanced Daxx expression, with concomitant Daxx protein increase and nuclear body translocation. The interferon effects included downregulation of cell cycle regulating genes and cell cycle arrest, followed by Bcl-2 downregulation and apoptosis. Daxx antisense oligonucleotides rescued the interferon-treated pro-B cells from growth arrest and apoptosis in parallel with the reduction of nuclear Daxx. These findings implicate the gene repressor function of Daxx in interferon-induced apoptosis of lymphoid progenitors.

Assessment of male CVID patients for mutations in the Btk gene: how many have been misdiagnosed?

The presentation of hypogammaglobulinaemia in young males without a family history of immunodeficiency can pose a diagnostic problem. In the past, the presence of B-cells has suggested a diagnosis of common variable immunodeficiency (CVID), although genotypic analysis has now clarified that individuals with B cells may have mutations in their Btk gene. In order to address the issue of how many male individuals with a clinical diagnosis of CVID do in fact have mutations in the Btk gene, we analysed a group of 24 male patients. Single-strand conformation polymorphism (SSCP) analysis was used to screen the patient cohort for mutations in the Btk gene. Given the size of the Btk gene, the number of patients in the cohort and the amount of available DNA, multiplex PCR reactions were utilized to span the 19 exons and promoter region of the gene. Where abnormal migration patterns were observed with multiplex PCR reactions, in nine of the 24 patients, the individual Btk gene fragments were re-amplified and analysed again by SSCP. Following this analysis, four patients continued to demonstrate abnormal SSCP migration patterns. However, direct sequencing of the relevant Btk gene fragments for these four CVID patients revealed a mutation in only one patient. The mutation was the previously described polymorphism at position 2031 of Btk gene within exon 18. These results indicate that caution should be taken with the application of SSCP analysis to mutation detection. While it has a role to play in screening large patient cohorts, direct sequencing is a necessary adjunct to such analysis. Finally, the clinical diagnosis of CVID in this cohort successfully excluded males with Btk mutations.

Nucleocytoplasmic shuttling of Bruton's tyrosine kinase

Bruton's tyrosine kinase (Btk), a nonreceptor cytoplasmic tyrosine kinase belonging to the Tec family of kinases, has been shown to be critical for B cell proliferation, differentiation, and signaling. Loss-of-function mutations in the Btk gene lead to X-linked agammaglobulinemia (XLA), a primary immunodeficiency in humans, and the less severe condition xid in mice. Although Btk is mainly localized in the cytoplasm under steady state conditions, it translocates to the plasma membrane upon growth factor stimulation and cross-linking of the B cell receptor. Nevertheless, in ectopically as well as endogenously Btk-expressing cells, it can also translocate to the nucleus. Deletion of the pleckstrin homology (PH) domain (DeltaPH1) leads, however, to an even redistribution of Btk within the nucleus and cytoplasm in the majority of transfected cells. In contrast, an SH3-deleted (DeltaSH3) mutant of Btk has been found to be predominantly nuclear. We also demonstrate that the nuclear accumulation of DeltaPH1 is dependent on Src expression. This nucleocytoplasmic shuttling is sensitive to the exportin 1/CRM1-inactivating drug, leptomycin B, indicating that Btk utilizes functional nuclear export signals. In addition, while the DeltaPH1 mutant of Btk was found to be active and tyrosine-phosphorylated in vivo, DeltaSH3 displayed decreased autokinase activity and was not phosphorylated. Our findings indicate that the nucleocytoplasmic shuttling of Btk has implications regarding potential targets inside the nucleus, which may be critical in gene regulation during B cell development and differentiation.

Hemokinin is a hematopoietic-specific tachykinin that regulates B lymphopoiesis

We report here the molecular cloning of a newly identified preprotachykinin gene, Pptc, which specifies the sequence for a new preprotachykinin protein and bioactive peptide designated hemokinin 1 (HK-1). PPT-C mRNA was detected primarily in hematopoietic cells in contrast to the previously described Ppta and Pptb genes, which are predominantly expressed in neuronal tissues. HK-1 has several biological activities that are similar to the most studied tachykinin, substance P, such as induction of plasma extravasation and mast cell degranulation. However, HK-1 also has properties that are indicative of a critical role in mouse B cell development. HK-1 stimulated the proliferation of interleukin 7-expanded B cell precursors, whereas substance P had no effect. HK-1, but not substance P, promoted the survival of freshly isolated bone marrow B lineage cells or cultured, lipopolysaccharide-stimulated pre-B cells. N-acetyl-L-trytophan-3,5-bistrifluromethyl benzyl ester, a tachykinin receptor antagonist, increased apoptosis of these cells and in vivo administration of this antagonist led to specific reductions of the B220lowCD43 population (the pre-B cell compartment) in the bone marrow and the IgMhighIgDlow population (the newly generated B cells) in the spleen. Thus, HK-1 may be an autocrine factor that is important for the survival of B cell precursors at a critical phase of development.

Prospective analysis suggests susceptibility genes for deficiencies of IgA and several other immunoglobulins on the [HLA-B8, SC01, DR3] conserved extended haplotype

The extended major histocompatibility complex (MHC) haplotype [HLA-B8, SC01, DR3] is increased in frequency among patients with immunoglobulin (Ig)A deficiency and common variable immunodeficiency. Because the genomic region from HLA-B to HLA-DR/DQ is virtually the same on all instances of the haplotype in the general population, we reasoned that all independent instances of [HLA-B8, SC01, DR3] carry MHC susceptibility genes for these disorders. To define immunoglobulin deficiencies determined by genes on this haplotype and their mode of expression and penetrance, serum immunoglobulin class and IgG subclass concentrations were determined prospectively in homozygotes and heterozygotes of this haplotype and in Caucasian controls. Prevalence of individual immunoglobulin deficiencies in persons with [HLA-B8, SC01, DR3] ranged from 13% to 37%, significantly higher than rates in non-carriers or general controls. We found significantly increased frequencies of IgA and IgG4 deficiency only in homozygotes (13.3% and 30%, respectively) compared with heterozygotes (1.7% and 3.4%) or non-carriers (1.6% each), suggesting recessive expression. In contrast, IgD and IgG3 deficiencies were significantly more common in both homozygotes (36.7% and 30%) and heterozygotes (20.3% and 17.5%) compared with controls (4.9% and 3.4%), suggesting dominant inheritance. These results indicate multiple distinct susceptibility genes, some recessive and others dominant, for deficiency of IgA, IgD, IgG3 or IgG4 (but not for IgE, IgG1, IgG2 or IgM) on [HLA-B8, SC01, DR3]. These observations may also help to explain the observed associations of [HLA-B8, SC01, DR3] with both IgA deficiency and common variable immunodeficiency and the common occurrence of IgG subclass deficiencies in some patients with IgA deficiency.

Engagement of the human pre-B cell receptor generates a lipid raft-dependent calcium signaling complex

Pre-B cell receptor (pre-BCR) expression is critical for B lineage development. The signaling events initiated by the pre-BCR, however, remain poorly defined. We demonstrate that lipid rafts are the major functional compartment for human pre-B cell activation. A fraction of pre-BCR was constitutively raft associated, and receptor engagement enhanced this association. These events promoted Lyn activation and Igbeta phosphorylation and led to the generation of a raft-associated signaling module composed of tyrosine phosphorylated Lyn, Syk, BLNK, PI3K, Btk, VAV, and PLCgamma2. Formation of this module was essential for pre-BCR calcium signaling. Together, these observations directly link the previously identified genetic requirement for the components of this module in B lineage development with theirfunctional role(s) in human preBCR signaling.

Fates of human B-cell precursors

Development of mammalian B-lineage cells is characterized by progression through a series of checkpoints defined primarily by rearrangement and expression of immunoglobulin genes. Progression through these checkpoints is also influenced by stromal cells in the microenvironment of the primary tissues wherein B-cell development occurs, ie, fetal liver and bone marrow and adult bone marrow. This review focuses on the developmental biology of human bone marrow B-lineage cells, including perturbations that contribute to the origin and evolution of B-lineage acute lymphoblastic leukemia and primary immunodeficiency diseases characterized by agammaglobulinemia. Recently described in vitro and in vivo models that support development and expansion of human B-lineage cells through multiple checkpoints provide new tools for identifying the bone marrow stromal cell–derived molecules necessary for survival and proliferation. Mutations in genes encoding subunits of the pre-B cell receptor and molecules involved in pre-B cell receptor signaling culminate in X-linked and non–X-linked agammaglobulinemia. A cardinal feature of these immunodeficiencies is an apparent apoptotic sensitivity of B-lineage cells at the pro-B to pre-B transition. On the other end of the spectrum is the apoptotic resistance that accompanies the development of B-lineage acute lymphoblastic leukemia, potentially a reflection of genetic abnormalities that subvert normal apoptotic programs. The triad of laboratory models that mimic the bone marrow microenvironment, immunodeficiency diseases with specific defects in B-cell development, and B-lineage acute lymphoblastic leukemia can now be integrated to deepen our understanding of human B-cell development.