CD40 ligand expression deficiency in a female carrier of the X-linked hyper-IgM syndrome as a result of X chromosome lyonization

Role of Skewed X-Chromosome Inactivation in Common Variable Immunodeficiency

The term common variable immunodeficiency (CVID) encompasses a clinically diverse group of disorders, mainly characterized by hypogammaglobulinemia, insufficient specific antibody production, and recurrent infections. The genetics of CVID is complex, and monogenic defects account for only a portion of cases, typically <30%. Other proposed mechanisms include digenic, oligogenic, or polygenic inheritance and epigenetic dysregulation. In this study, we aimed to assess the role of skewed X-chromosome inactivation (XCI) in CVID. Within our cohort of 131 genetically analyzed CVID patients, we selected female patients with rare variants in CVID-associated genes located on the X-chromosome. Four patients harboring heterozygous variants in BTK (n = 2), CD40LG (n = 1), and IKBKG (n = 1) were included in the study. We assessed XCI status using the HUMARA assay and an NGS-based method to quantify the expression of the 2 alleles in mRNA. Three of the 4 patients (75%) exhibited skewed XCI, and the mutated allele was predominantly expressed in all cases. Patient 1 harbored a hypomorphic variant in BTK (p.Tyr418His), patient 3 had a pathogenic variant in CD40LG (c.288+1G>A), and patient 4 had a hypomorphic variant in IKBKG (p.Glu57Lys) and a heterozygous splice variant in TNFRSF13B (TACI) (c.61+2T>A). Overall, the analysis of our cohort suggests that CVID in a small proportion of females (1.6% in our cohort) is caused by skewed XCI and highly penetrant gene variants on the X-chromosome. Additionally, skewed XCI may contribute to polygenic effects (3.3% in our cohort). These results indicate that skewed XCI may represent another piece in the complex puzzle of CVID genetics.

Advanced computational analysis of CD40LG variants in atypical X-linked hyper-IgM syndrome

X-Linked Hyper-IgM Syndrome is caused by pathogenic variants in CD40LG. Three patients with atypical clinical and immunological features were identified with variants in CD40LG requiring further characterization. Flow cytometry was used to evaluate CD40L protein expression and binding capacity to a surrogate receptor, CD40-muIg. Though functional anomalies were observed, there was still a lack of clarity regarding the underlying mechanism. We developed structural models for wild-type and the three variants of CD40L protein observed in these patients (p. Lys143Asn, Leu225Ser and Met36Arg) to evaluate structural alterations by molecular mechanic calculations, and assess protein movement by molecular dynamic simulations. These studies demonstrate that functional analysis of variants of unknown significance in CD40LG can be supplemented by advanced computational analysis in atypical clinical contexts. These studies in combination identify the deleterious effects of these variants and potential mechanisms for protein dysfunction.

HSCT using carrier donors for CD40L deficiency results in excellent immune function and higher CD40L expression in cTfh

Data are limited regarding the immune status of CD40 ligand (CD40L)-deficient carriers and hematopoietic stem cell transplantation (HSCT) outcomes using them as donors for CD40L-deficient patients. Therefore, we studied the immune profiles of 7 carriers, 4 of whom were HSCT donors for family members with CD40L deficiency, and we characterized their HSCT outcomes. Immunoglobulin profiles, CD4, CD8, circulating T-follicular helper (cTfh) cells, and regulatory T cells (Tregs) in carriers were comparable to those in healthy controls. CD40L expression in carriers ranged from 37% to 78%. cTfh cells from carriers expressed higher CD40L compared with total CD4 cells or the memory CD4 compartment, suggesting a potential advantage to CD40L-expressing cTfh cells. Tregs had minimal CD40L expression in carriers and healthy controls. So we postulated that HSCT using donors who were CD40L carriers may result in excellent immune reconstitution without immune dysregulation. Four CD40L-deficient patients underwent HSCT from carriers who had CD40L expression from 37% to 63%. All patients engrafted, achieved excellent immune reconstitution with lack of opportunistic infections, graft-versus-host disease, and immune dysregulation; stable CD40L expression mimicked that of donors 1 to 5 years after HSCT. Immunoglobulin independence was achieved in 3 of the 4 patients. We demonstrated higher CD40L expression in the cTfh compartment of carriers and excellent immune reconstitution using donors who were CD40L carriers in CD40L-deficient patients.

Variants Disrupting CD40L Transmembrane Domain and Atypical X-Linked Hyper-IgM Syndrome: A Case Report With Leishmaniasis and Review of the Literature

X-linked hyper-IgM (XHIGM) syndrome is caused by mutations of the CD40LG gene, encoding the CD40L protein. The clinical presentation is characterized by early-onset infections, with profound hypogammaglobulinemia and often elevated IgM, susceptibility to opportunistic infections, such as Pneumocystis jirovecii pneumonia, biliary tract disease due to Cryptosporidium parvum, and malignancy. We report a 41-year-old male presenting with recurrent leishmaniasis, hypogammaglobulinemia, and myopathy. Whole-exome sequencing (WES) identified a missense variant in the CD40LG gene (c.107T>A, p.M36K), involving the transmembrane domain of the protein and a missense variant in the carnitine palmitoyl-transferase II (CPT2; c.593C>G; p.S198C) gene, leading to the diagnosis of hypomorphic XHIGM and CPT2 deficiency stress-induced myopathy. A review of all the previously reported cases of XHIGM with variants in the transmembrane domain showcased that these patients could present with atypical clinical features. Variants in the transmembrane domain of CD40LG act as hypomorphic generating a protein with a lower surface expression. Unlike large deletions or extracellular domain variants, they do not abolish the interaction with CD40, therefore preserving some biological activity.

The effect of mutatio-type on proteo-phenotype and clinico-phenotype in selected primary immunodeficiencies

In the diagnosis of primary immunodeficiencies which are heterogeneous groups of genetic disorders, next-generation sequencing strategies take an important place. Protein expression analyses and some functional studies which are fundamental to determine the pathogenicity of the mutation are also performed to accelerate the diagnosis of PIDs before sequencing. However, protein expressions and functions do not always reflect the genetic and clinical background of the disease even the existence of a pathogenic variant or vice versa. In this study, it was aimed to understand genotype-proteophenotype-clinicophenotype correlation by investigating the effect of mutation types on protein expression, function, and clinical severity in X-linked, autosomal dominant, and autosomal recessive forms of PIDs. It was searched in PubMed and Web of Science without any restrictions on study design and publication time. Totally, 1178 patients with PIDs who have 553 different mutations were investigated from 174 eligible full-text articles. For all mutations, the effect of mutation type on protein expressions and protein functions was analyzed. Furthermore, the most frequent missense and nonsense mutations that were identified in patients with PIDs were evaluated to determine the genotype-clinicophenotype correlation. Protein expressions and functions were changed depending on the mutation type and the affected domain. A significant relationship was observed between protein expression level and clinical severity among all investigated patients. There was also a positive correlation between clinical severity and the affected domains. Mutation types and affected domains should be carefully evaluated with respect to protein expression levels and functional changes during the evaluation of clinico-phenotype.

Immunity and Genetics at the Revolving Doors of Diagnostics in Primary Immunodeficiencies

Primary immunodeficiencies (PIDs) are a large and growing group of disorders commonly associated with recurrent infections. However, nowadays, we know that PIDs often carry with them consequences related to organ or hematologic autoimmunity, autoinflammation, and lymphoproliferation in addition to simple susceptibility to pathogens. Alongside this conceptual development, there has been technical advancement, given by the new but already established diagnostic possibilities offered by new genetic testing (e.g., next-generation sequencing). Nevertheless, there is also the need to understand the large number of gene variants detected with these powerful methods. That means advancing beyond genetic results and resorting to the clinical phenotype and to immunological or alternative molecular tests that allow us to prove the causative role of a genetic variant of uncertain significance and/or better define the underlying pathophysiological mechanism. Furthermore, because of the rapid availability of results, laboratory immunoassays are still critical to diagnosing many PIDs, even in screening settings. Fundamental is the integration between different specialties and the development of multidisciplinary and flexible diagnostic workflows. This paper aims to tell these evolving aspects of immunodeficiencies, which are summarized in five key messages, through introducing and exemplifying five clinical cases, focusing on diseases that could benefit targeted therapy.

Aberrant X chromosome skewing and acquired clonal hematopoiesis in adult-onset common variable immunodeficiency

Advances in genomic medicine have elucidated an increasing number of genetic etiologies for patients with common variable immunodeficiency (CVID). However, there is heterogeneity in clinical and immunophenotypic presentations and a limited understanding of the underlying pathophysiology of many cases. The primary defects in CVID may extend beyond the adaptive immune system, and the combined defect in both the myeloid and lymphoid compartments suggests the mechanism may involve bone marrow output and earlier progenitors. Using the methylation profile of the human androgen receptor (AR) gene as a surrogate epigenetic marker for bone marrow clonality, we examined the hematopoietic compartments of patients with CVID. Our data show that clonal hematopoiesis is common among patients with adult-onset CVID who do not have associated noninfectious complications. Nonblood tissues did not show a skewed AR methylation status, supporting a model of an acquired clonal hematopoietic event. Attenuation of memory B cell differentiation into long-lived plasma cells (CD20^–CD27⁺CD38⁺CD138⁺) was associated with marked changes in the postdifferentiation methylation profile, demonstrating the functional consequence of clonal hematopoiesis on humoral immunity in these patients. This study sheds light on a potential etiology of a subset of patients with CVID, and the findings suggest that it is a stage of an acquired lymphocyte maturation disorder.

Clonal hematopoiesis is common among a subset of patients with common variable immunodeficiency (CVID), suggesting that CVID may be a stage of lymphoid dysplasia.

Clinical Phenotypes of Hyper-IgM Syndromes

The primary immunodeficiency (PID) diseases comprise a heterogeneous group of inherited disorders of immune function. Technical advancements in whole-genome, whole-exome, and RNA-sequencing have seen the explosion of genetic discoveries in the field of PIDs. The present review aims to focus on a group of immunodeficiency disorders associated with elevated levels of IgM (hyper IgM; HIGM) and provides a clinical differential diagnosis. Most patients present for evaluation of immunodeficiency due to recurrent infections, and laboratory studies show either a clear isolated elevation of serum immunoglobulin M (IgM) with low or absent IgG, IgA, and IgE. Alternatively, IgM levels may be normal or moderately elevated while other serum immunoglobulins are reported below the norms for age but not absent. Mechanistically, these disorders are recognized as defects in immunoglobulin (Ig) class switch recombination (CSR). Importantly, to safeguard genetic stability, CSR utilizes elements of the DNA repair machinery including multi-protein complexes involved in mismatch repair (MMR). Therefore, it is not uncommon for defects in the DNA repair machinery, to present with laboratory findings of HIGM. This review will discuss clinical phenotypes associated with congenital defects associated with HIGM. Clinical manifestations, relevant immunologic testing, inheritance pattern, molecular diagnosis, presumed pathogenesis, and OMIM number, when annotated are compiled. Accepted therapeutic options, when available, are reviewed for each condition discussed.

X-linked primary immunodeficiency associated with hemizygous mutations in the moesin (MSN) gene

BACKGROUND

We investigated 7 male patients (from 5 different families) presenting with profound lymphopenia, hypogammaglobulinemia, fluctuating monocytopenia and neutropenia, a poor immune response to vaccine antigens, and increased susceptibility to bacterial and varicella zoster virus infections.

OBJECTIVE

We sought to characterize the genetic defect involved in a new form of X-linked immunodeficiency.

METHODS

We performed genetic analyses and an exhaustive phenotypic and functional characterization of the lymphocyte compartment.

RESULTS

We observed hemizygous mutations in the moesin (MSN) gene (located on the X chromosome and coding for MSN) in all 7 patients. Six of the latter had the same missense mutation, which led to an amino acid substitution (R171W) in the MSN four-point-one, ezrin, radixin, moesin domain. The seventh patient had a nonsense mutation leading to a premature stop codon mutation (R533X). The naive T-cell counts were particularly low for age, and most CD8⁺ T cells expressed the senescence marker CD57. This phenotype was associated with impaired T-cell proliferation, which was rescued by expression of wild-type MSN. MSN-deficient T cells also displayed poor chemokine receptor expression, increased adhesion molecule expression, and altered migration and adhesion capacities.

CONCLUSION

Our observations establish a causal link between an ezrin-radixin-moesin protein mutation and a primary immunodeficiency that could be referred to as X-linked moesin-associated immunodeficiency.

Dysfunctional BLK in common variable immunodeficiency perturbs B-cell proliferation and ability to elicit antigen-specific CD4+ T-cell help

Common Variable Immunodeficiency (CVID) is the most prevalent primary antibody deficiency, and characterized by defective generation of high-affinity antibodies. Patients have therefore increased risk to recurrent infections of the respiratory and intestinal tract. Development of high-affinity antigen-specific antibodies involves two key actions of B-cell receptors (BCR): transmembrane signaling through BCR-complexes to induce B-cell differentiation and proliferation, and BCR-mediated antigen internalization for class-II MHC-mediated presentation to acquire antigen-specific CD4(+) T-cell help.We identified a variant (L3P) in the B-lymphoid tyrosine kinase (BLK) gene of 2 related CVID-patients, which was absent in healthy relatives. BLK belongs to the Src-kinases family and involved in BCR-signaling. Here, we sought to clarify BLK function in healthy human B-cells and its association to CVID.BLK expression was comparable in patient and healthy B-cells. Functional analysis of L3P-BLK showed reduced BCR crosslinking-induced Syk phosphorylation and proliferation, in both primary B-cells and B-LCLs. B-cells expressing L3P-BLK showed accelerated destruction of BCR-internalized antigen and reduced ability to elicit CD40L-expression on antigen-specific CD4(+) T-cells.In conclusion, we found a novel BLK gene variant in CVID-patients that causes suppressed B-cell proliferation and reduced ability of B-cells to elicit antigen-specific CD4(+) T-cell responses. Both these mechanisms may contribute to hypogammaglobulinemia in CVID-patients.

Practice parameter for the diagnosis and management of primary immunodeficiency

The American Academy of Allergy, Asthma & Immunology (AAAAI) and the American College of Allergy, Asthma & Immunology (ACAAI) have jointly accepted responsibility for establishing the "Practice parameter for the diagnosis and management of primary immunodeficiency." This is a complete and comprehensive document at the current time. The medical environment is a changing environment, and not all recommendations will be appropriate for all patients. Because this document incorporated the efforts of many participants, no single individual, including those who served on the Joint Task Force, is authorized to provide an official AAAAI or ACAAI interpretation of these practice parameters. Any request for information about or an interpretation of these practice parameters by the AAAAI or ACAAI should be directed to the Executive Offices of the AAAAI, the ACAAI, and the Joint Council of Allergy, Asthma & Immunology. These parameters are not designed for use by pharmaceutical companies in drug promotion.

Immunoglobulin class-switch recombination deficiencies

Immunoglobulin class-switch recombination deficiencies (Ig-CSR-Ds) are rare primary immunodeficiencies characterized by defective switched isotype (IgG/IgA/IgE) production. Depending on the molecular defect in question, the Ig-CSR-D may be combined with an impairment in somatic hypermutation (SHM). Some of the mechanisms underlying Ig-CSR and SHM have been described by studying natural mutants in humans. This approach has revealed that T cell-B cell interaction (resulting in CD40-mediated signaling), intrinsic B-cell mechanisms (activation-induced cytidine deaminase-induced DNA damage), and complex DNA repair machineries (including uracil-N-glycosylase and mismatch repair pathways) are all involved in class-switch recombination and SHM. However, several of the mechanisms required for full antibody maturation have yet to be defined. Elucidation of the molecular defects underlying the diverse set of Ig-CSR-Ds is essential for understanding Ig diversification and has prompted better definition of the clinical spectrum of diseases and the development of increasingly accurate diagnostic and therapeutic approaches.

Phenotypic heterogeneity in a family with a CD40 ligand intracellular domain mutation

We describe a family with the rare mutation R11X that leads to a truncated CD40 ligand (CD40L) missing the intracellular domain. The index case had detectable CD40L expression and presented at the age of 41 years with cerebral toxoplasmosis. A brother and two nephews were also identified as having the same mutation but exhibited milder and variable phenotypes. The older affected nephew had a moderately depressed immunoglobulin G level and a history of pneumonia at 4 months of age. The younger nephew suffered from sinusitis with normal immunoglobulin levels. Both nephews had absent antibody responses to a protein antigen with conserved responses to polysaccharide antigens. The two sisters of the index case are carriers who had elevated levels of IgM but remain well. This mutation may affect CD40 ligand function by reducing cell surface levels, diminishing CD40 interaction or disrupting CD40L intracellular signalling in T cells. The variable phenotype in members of this family offers an opportunity to further understand the CD40-CD40L signalling pathway in human immune responses.

Update on the hyper immunoglobulin M syndromes

The Hyper-immunoglobulin M syndromes (HIGM) are a heterogeneous group of genetic disorders resulting in defects of immunoglobulin class switch recombination (CSR), with or without defects of somatic hypermutation (SHM). They can be classified as defects of signalling through CD40 causing both a humoral immunodeficiency and a susceptibility to opportunistic infections, or intrinsic defects in B cells of the mechanism of CSR resulting in a pure humoral immunodeficiency. A HIGM picture can also be seen as part of generalized defects of DNA repair and in antibody deficiency syndromes, such as common variable immunodeficiency. CD40 signalling defects may require corrective therapy with bone marrow transplantation. Gene therapy, a potential curative approach in the future, currently remains a distant prospect. Those with a defective CSR mechanism generally do well on immunologoblulin replacement therapy. Complications may include autoimmunity, lymphoid hyperplasia and, in some cases, a predisposition to lymphoid malignancy.

Hyper-IgM syndrome in a boy with recurrent pneumonia and hepatosplenomegaly

Introduction. We present a boy diagnosed at age 14 years with hyper-immunoglobulin (Ig) M syndrome, a congenital immunodeficiency characterized by reduced plasma concentrations of IgA, IgE and IgG, with normal or elevated concentrations of IgM. This syndrome is caused by a defect of CD40 ligand (CD40L) on T-helper lymphocytes, impeding the 'second signal' during activation of B lymphocytes and interactions of T cells with dendritic cells and macrophages, resulting in the absence of secondary immune response (class switching, affinity maturation, immune memory), as well as responses to T-dependent antigens, with an impairment of cellular immunity. Case Outline. The history of the presented patient was dominated by frequent lower respiratory infections and failure to thrive. Physical examination demonstrated severe hepatosplenomegaly. The suspicion of hyper-IgM syndrome was raised by low plasma IgA (0.36 g/l) with high plasma IgM (35.5 g/l), while the concentration of IgG was within the normal range (12.1 g/l). The diagnosis was confirmed by flow cytometry, which demonstrated the absence of expression of CD40L on lymphocytes following stimulation by phorbolmyristylacetate and calcium ionophore. Since the time of diagnosis, intravenous immunoglobulin therapy has led to catch-up growth, recession of hepatosplenomegaly and reduction in the frequency of respiratory infections. Conclusion. Our report emphasizes the importance for the primary healthcare paediatrician to be well informed about the clinical presentation and pathogenesis of hyper-IgM syndrome, in order to provide early detection and increase the likelihood of success in treating this rare immunodeficiency. To the best of our knowledge, this is the first case of hyper-IgM syndrome reported in the Republic of Serbia.

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].

X-Linked Hyper-Immunoglobulin M Syndrome: Molecular Genetic Study and Long-Time Follow-Up of Three Generations of a Chinese Family

BACKGROUND

X-linked hyper-immunoglobulin M (IgM) syndrome (XHIGM) is a rare immunodeficiency disease caused by mutations of the CD40 ligand gene. Patients are subject to recurrent infections and have normal or elevated levels of IgM but markedly decreased serum IgG.

OBJECTIVE

We describe molecular genetic studies and clinical manifestations in three generations of one family, as well as results of long-term treatment of 2 young men with the disorder.

METHODS

Of 37 living family members, mutational analysis of the CD40 ligand gene was performed in 36 members. Laboratory data for patients and carriers were reviewed.

RESULTS

Four male family members had died of unexplained causes. The 3 patients with XHIGM syndrome and the 5 carriers all had a novel mutation located at Tyr 169 Asn (T526A) in exon 5, the tumor necrosis factor domain of the CD40 ligand gene. In the 3 patients, CD40 ligand expression in activated CD4+ T cells was below 1%. In the carriers, about half of activated CD4+ cells expressed CD40 ligand. One carrier had malignant lymphoma. Long-term (>20 years) intravenous immunoglobulin therapy in 2 patients improved IgG levels but did not fully suppress the high levels of IgM, nor did it prevent late complications (bronchiectasis and sclerosing cholangitis).

CONCLUSIONS

Diagnosis of a genetic immunodeficiency, especially an X-linked disease such as XHIGM syndrome, should prompt a survey of the entire family.

Molecular, biochemical, and genetic characterization of a female patient with Lesch-Nyhan disease

Lesch-Nyhan disease (LND) is a rare X-linked recessive disorder caused by virtually complete deficiency of activity of the purine salvage enzyme hypoxanthine phosphoribosyltransferase (HPRT; EC 2.4.2.8). Human HPRT is encoded by a single structural gene located on the long arm of the X-chromosome (Xq26). The classical LND phenotype occurs almost exclusively in males, manifested in excessive purine production and characteristic neurological manifestations, including compulsive self-mutilation, choreoathetosis, spasticity, and occasionally developmental delay. Heterozygous females are usually phenotypically normal, due to the random inactivation of the X chromosome (Lyonization mechanism). However, six females were reported to be affected with the full biochemical and clinical manifestations of LND. All these cases were heterozygous for an HPRT mutation. Absence of transcription of the normal HPRT allele was attributed in all of them to non-random inactivation of the X chromosome carrying the normal allele. Here we describe an additional LND female, who presented with acute renal failure at the age of two months, in whom absence of transcription of the two HPRT alleles occurred due to as yet undescribed mechanism in LND females: the transcription of one HPRT allele was blocked due to a de novo X chromosome-autosome translocation 46,XX,t(X:2)(q26:p25), with a breaking point encompassing the HPRT gene locus, whereas the transcription of the normal allele was inhibited due to non-random inactivation of the second X-chromosome. Cultured fibroblasts from this patient exhibited the biochemical alterations in purine nucleotide metabolism characteristic of male LND fibroblasts.

A novel form of non-X-linked hyperigm associated with growth and pubertal disturbances and with lymphoma development

HyperIgM syndrome is a heterogenous immunodeficiency characterized by impaired class-switch recombination due to different molecular abnormalities. We report on two female patients affected by a novel syndrome associating HIGM, growth and pubertal disturbances, and severe lymphoid hyperplasia with eventual development into lymphomas, suggesting a DNA repair defect.

Female hyper IgM syndrome type 1 with a chromosomal translocation disrupting CD40LG

Hyper-IgM syndrome type 1 (HIGM1) is a primary immunodeficiency characterized by recurrent bacterial and opportunistic infections, associated with normal or high serum level of IgM and decreased serum levels of IgG, IgA and IgE due to the defect of class switch recombination. CD40LG, located in Xq26, has been reported to be mutated in male HIGM1 patients. Here, we report the second case of a female HIGM1 with the defect of CD40 ligand (CD40L) expression and of soluble serum CD40L. Clinical course and HIGM phenotype was indistinguishable from that of male HIGM1 including severe neutropenia. High-resolution chromosome banding revealed that this patient's karyotype is 46, X, t(X;14)(q26.3;q13.1), and FISH analysis demonstrated that the break point of the chromosomal translocation is within CD40LG. Using four chimeric cDNA clones obtained by 3' RACE method, the break point was identified within the intron 4 of CD40LG on X chromosome and non-coding region of chromosome 14. We also found an extremely skewed X-chromosome inactivation pattern by methylation-specific PCR. Thus, the reciprocal translocation caused the disruption of CD40LG, resulting in defective CD40L expression in the female patient with an extremely skewed X-inactivation pattern in T cells leading to the HIGM1 phenotype.

[Cellular mechanisms implicated in anti-erythrocyte alloimmunization]

In many clinical situations patients are dependent on blood transfusions. Occurrence of alloimmunization to blood group antigens (BGA) complicates the transfusion strategy and may be involved in clinical transfusion stalemate situations. B cell differentiation into antibody-secreting plasma cells is triggered by antigen and requires helper T cells which produce cytokines. Although antibodies implicated in BGA alloimmunization have been studied for many years, little is known about helper T cell responses that drive their production. Few studies on BGA specific T cell responses have been published today. This review summarizes the new developments in the field of cellular mechanisms implicated into antibody production. The definition of immunodominant peptides derived from RhD and Jk(a) BGAs, the cytokine patterns induced and the HLA class II molecules implicated in their presentation are analyzed. A tolerogenic route for RhD immunodominant peptides is experimented. Identification of such immunodominant peptides, the cytokine patterns induced and the HLA class II molecules implicated in their presentation, would facilitate the design of new therapeutic strategies including the specific control of alloimmunization with peptide antigen tolerogens or the ex-vivo induction of regulatory T cells.

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

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

The Repair of DNA Damages/Modifications During the Maturation of the Immune System: Lessons from Human Primary Immunodeficiency Disorders and Animal Models

The immune system is the site of various genotoxic stresses that occur during its maturation as well as during immune responses. These DNA lesions/modifications are primarily the consequences of specific physiological processes such as the V(D)J recombination, the immunoglobulin class switch recombination (CSR), and the generation of somatic hypermutations (SHMs) within Ig variable domains. The DNA lesions can be introduced either by specific factors (RAG1 and RAG2 in the case of V(D)J recombination and AID in the case of CSR and SHM) or during the various phases of cellular proliferation and cellular activation. All these DNA lesions are taken care of by the diverse DNA repair machineries of the cell. Several animal models as well as human conditions have established the critical importance of these DNA lesions/modifications and their repair in the physiology of the immune system. Indeed their defects have consequences ranging from immune deficiency to development of immune malignancy. The survey of human pathology has been highly instrumental in the past in identifying key factors involved in the generation of DNA modifications (AID for the Ig CSR and generation of SHM) or the repair of specific DNA damages (Artemis for V(D)J recombination). Defects in factors involved in the cell cycle checkpoints following DNA damage also have deleterious consequences on the immune system. The continuous survey of human diseases characterized by primary immunodeficiency associated with increased sensitivity to ionizing radiation should help identify other important DNA repair factors essential for the development and maintenance of the immune system.

Severe phenotype of chronic granulomatous disease presenting in a female with a de novo mutation in gp91-phox and a non familial, extremely skewed X chromosome inactivation

Chronic granulomatous disease (CGD) is an inherited immunodeficiency resulting from defects in the multienzyme complex NADPH-oxidase (phagozyte oxidase, phox), which normally produces microbicidal reactive oxygen metabolites (ROM). The reason for our patient's CGD was unusual, as revealed by the following in vitro findings in neutrophils and EBV-transformed B-cells: lack of flavocytochrome b(558) expression, restoration of significant ROM production after transduction with gp91-phox cDNA by a retrovirus vector, an 879G-->A, Trp289-->Stop mutation in one X chromosomal gp91-phox allele, a one-sided paternal X chromosome inactivation, as shown by a lyonization assay at the HUMARA locus, and the result of a dihydrorhodamine 123 flow cytometry assay revealing consistently that 1 in 2500 neutrophils produced ROM at normal levels. Our conclusion: A presumed autosomal form of CGD has been excluded. Instead, a spontaneous mutation in gp91-phox coinciding with an extreme X chromosome inactivation ratio resulted in X-linked CGD in this young woman.

Nonmyeloablative hematopoietic stem cell transplant for X-linked hyper-immunoglobulin m syndrome with cholangiopathy

OBJECTIVE

X-linked hyper-immunoglobulin M (X-HIM) syndrome is a rare genetic immunodeficiency syndrome caused by mutations in the gene encoding CD40 ligand (CD40L, CD154). Allogeneic hematopoietic stem cell transplantation (HSCT) offers the prospect of immune reconstitution in X-HIM syndrome. Standard HSCT using high-dose chemoradiotherapy can be followed by serious hepatic problems, including veno-occlusive disease, graft-versus-host disease, and/or drug-induced hepatotoxicity. In patients whose liver function is compromised before HSCT, such as in X-HIM syndrome caused by cholangiopathy and hepatitis related to opportunistic infections, there is a higher likelihood of hepatotoxicity. We explored nonmyeloablative HSCT in 2 patients with X-HIM syndrome. Nonmyeloablative HSCT without liver transplant for X-HIM syndrome, to our knowledge, has not been described previously.

METHODS

Two children with X-HIM syndrome and persistent infections had documented cholangiopathy on liver biopsy. Both children underwent nonmyeloablative HSCT from HLA-matched siblings with fludarabine, busulfan, and anti-thymocyte globulin as their preparative regimen. Graft-versus-host disease prophylaxis consisted of cyclosporine.

RESULTS

Both children are >2 years after their HSCT. One remains a mixed chimera, and the other shows 100% donor chimerism. Both children are now free of infections and are no longer dependent on intravenous gammaglobulin. Both show response to immunizations. Both have had resolution of their cholangiopathy.

CONCLUSIONS

Nonmyeloablative HSCT from HLA-matched siblings can offer immune reconstitution without hepatotoxicity in patients with X-HIM syndrome and preexisting cholangiopathy. Even with stable mixed chimerism after allogeneic HSCT, patients may be able to enjoy a normal phenotype. Nonmyeloablative HSCT warrants additional study in X-HIM syndrome.

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

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

CD40 ligand-deficient T cells from X-linked hyper-IgM syndrome carriers have intrinsic priming capability

Deficiency in CD40 ligand (CD40L) expression is associated with impaired T cell immunity in mouse models and in humans. Previous studies have indicated that this is due to the failure of induction of extrinsic costimulatory molecules. However, other studies have suggested that CD40L is an intrinsic costimulatory molecule. The X-linked hyper-IgM syndrome (XHIM) is a primary immunodeficiency caused by mutations in CD40L, resulting in impaired Ab production and T cell immunity. CD4+ T cells from female carriers of XHIM express a variable degree of normal CD40L based on random X chromosome inactivation. We have examined T cells from XHIM carriers to investigate whether CD40L supports T cell function by acting as an intrinsic costimulator or by induction of other costimulatory molecules by examining coexpression of CD40L and markers of T lymphocyte priming. These carriers provide a unique model for comparison of CD40L-expressing and -nonexpressing lymphocytes in that all factors, including immunological experience, are equivalent between the two populations. Our results show that compared with CD40L-deficient T cells, T cells that express CD40L normally have a minimal advantage in becoming primed, as defined by CD45 RO isoform expression and production of IFN-gamma and TNF-alpha. Conversely, CD40L-deficient T lymphocytes clearly were capable of becoming primed as defined by the same parameters. These findings imply that the intrinsic costimulatory activity of CD40L is not required for attaining primed status, and that CD40L primarily supports T cell function by inducing extrinsic factors that can be shared by CD40L-deficient cells.

Terminal defects of B lymphocyte differentiation

In humans several abnormalities can occur during terminal B cell differentiation, leading to primary humoral immunodeficiencies. A recent study provided evidence of a qualitative defect of the affinity antibody maturation in some patients affected with common variable immunodeficiency syndrome, the molecular basis of which remains unknown. Several genetic defects in class switch recombination leading to a hyper-IgM syndrome have recently been delineated. Besides the well-known role of CD40-CD40 ligand interaction, they definitively demonstrate the requirement of CD40-mediated nuclear factor kappa B activation and the essential role of a newly described molecule, the activation-induced cytidine deaminase, in B cell terminal differentiation.

Human genetic defects in class-switch recombination (hyper-IgM syndromes)

Several genetic defects in class-switch recombination, leading to hyper-IgM syndromes, have been recently described in humans. Besides the well-known role of interaction between CD40-ligand and CD40, these pathological conditions definitively demonstrate the requirement of CD40-mediated NF-kappaB activation and the essential role of a newly described molecule, activation-induced cytidine deaminase (AID), in B cell terminal differentiation.

The hyper IgM syndrome

The hyper IgM syndrome is a rare, inherited immune deficiency disorder resulting from defects in the CD40 ligand/CD40-signaling pathway. X-linked hyper IgM is caused by defects in the CD40 ligand gene, while autosomal recessive hyper IgM is caused by defects in the CD40-activated RNA-editing enzyme, activation-induced cytidine deaminase, which is required for immunoglobulin isotype switching and somatic hypermutation in B cells. The loss of interaction between CD40 and its ligand in X-linked hyper IgM results in an impairment of T cell function, of B cell differentiation, and of monocyte function, while only B cell differentiation appears to be affected in autosomal recessive hyper IgM. With genetic defects in the hyper IgM syndrome identified, it is possible to diagnose patients definitely, to perform genetic screening, and to delineate the clinical manifestations of this syndrome. Further research may lead to novel and definitive therapeutic options for patients with hyper IgM syndrome.

Primary immunodeficiency mutation databases

Primary immunodeficiencies are intrinsic defects of immune systems. Mutations in a large number of cellular functions can lead to impaired immune responses. More than 80 primary immunodeficiencies are known to date. During the last years genes for several of these disorders have been identified. Here, mutation information for 23 genes affected in 14 immunodefects is presented. The proteins produced are employed in widely diverse functions, such as signal transduction, cell surface receptors, nucleotide metabolism, gene diversification, transcription factors, and phagocytosis. Altogether, the genetic defect of 2,140 families has been determined. Diseases with X-chromosomal origin constitute about 70% of all the cases, presumably due to full penetrance and because the single affected allele causes the phenotype. All types of mutations have been identified; missense mutations are the most common mutation type, and truncation is the most common effect on the protein level. Mutational hotspots in many disorders appear in CPG dinucleotides. The mutation data for the majority of diseases are distributed on the Internet with a special database management system, MUTbase. Despite large numbers of mutations, it has not been possible to make genotype-phenotype correlations for many of the diseases.

Long-term study of a female hyper-IgM immunodeficiency

Hyper-IgM immunodeficiency (HIM) is an immunological disorder characterized by normal or elevated serum IgM levels, and reduced serum IgG and IgA levels, due to the disruption of immunoglobulin class switching in B cells. X-linked hyper-IgM is caused by the defective expression of the CD40 ligand on activated T cells, which induces immunoglobulin class switching along with some cytokines, such as interleukin 4, by the signal transduction of CD40 in B cells. We report on a Japanese girl who initially showed low serum IgM, IgG and IgA levels like patients with common variable immunodeficiency; however, in the course of time, serum IgG levels became reduced and serum IgM levels increased, resulting in the typical immunoglobulin profile of HIM. Neutropenia, one of the features of X-linked HIM, was not observed. In spite of extremely low serum IgG levels, she did not show any predisposition to severe infection, even without gammaglobulin replacement therapy. No mutation of the CD40 ligand or CD40 was detected. Sequencing of the complementarity-determining region of immunoglobulin heavy-chain genes in peripheral B lymphocytes revealed that they were all in frame, and insertion of the N region was detected. These results indicate that the heavy-chain gene rearrangement in the patient's B cells is intact. Non-X-linked HIM has heterogeneous pathogenetic mechanisms, and some groups may show the resistance to infection at the healthy donor level. The underlying defects in non-X-linked HIM might be specifically involved in class switching.

CD40-CD40L independent Ig gene hypermutation suggests a second B cell diversification pathway in humans

Somatically mutated IgM(+)-only and IgM(+)IgD(+)CD27(+) B lymphocytes comprise approximately 25% of the human peripheral B cell pool. These cells phenotypically resemble class-switched B cells and have therefore been classified as postgerminal center memory B cells. X-linked hyper IgM patients have a genetic defect characterized by a mutation of the CD40L gene. These patients, who do not express a functional CD40 ligand, cannot switch Ig isotypes and do not form germinal centers and memory B cells. We report here that an IgM(+)IgD(+)CD27(+) B cell subset with somatically mutated Ig receptors is generated in these patients, implying that these cells expand and diversify their Ig receptors in the absence of classical cognate T-B collaboration. The presence of this sole subset in the absence of IgM(+)-only and switched CD27(+) memory B cells suggests that it belongs to a separate diversification pathway.

An unexpected affected female patient in a classical Lesch-Nyhan family

Lesch-Nyhan disease is a genetic disorder of purine metabolism caused by defective activity of the enzyme hypoxanthine-guanine phosphoribosyl transferase (HPRT), resulting from mutation in the corresponding gene on the long arm of the X chromosome (Xq26). The classical phenotype, which includes spasticity, involuntary movements, developmental disability, and self-injurious behavior, occurs exclusively in males, while heterozygous, carrier females are clinically normal. We analyzed an Argentine family in which there were male and female siblings with clinically identical classic features of Lesch-Nyhan disease. The mother and an older daughter were carriers and had normal phenotypes. We identified the HPRT mutation in the family. It is a C --> T transition at position 508 of the cDNA (c.508 C --> T) that changes the CGA codon for Arg(169) to the TGA stop codon (R169X). The female patient was karyotypically normal and heterozygous for the mutation. She inherited the HPRT mutation from her mother, but she also had unexpected nonrandom inactivation of the paternal X chromosome carrying the normal HPRT gene. This additional genetic alteration is the cause of the clinical expression of disease in this female patient.