Defect in IgV gene somatic hypermutation in common variable immuno-deficiency syndrome

Autoimmune disorders associated with common variable immunodeficiency: prediction, diagnosis, and treatment

INTRODUCTION

Common variable immunodeficiency (CVID) is the most common symptomatic primary immunodeficiency. Due to the wide spectrum of the CVID manifestations, the differential diagnosis becomes complicated, ends in a diagnostic delay and increased morbidity and mortality rates. Autoimmunity is one of the important complications associated with CVID. While immunoglobulin replacement therapy has considerably decreased the mortality rate in CVID patients, mainly infection-related mortality, other complications such as autoimmunity appeared prevalent and, in some cases, life threatening.

AREAS COVERED

In this article, genetics, responsible immune defects, autoimmune manifestations in different organs, and the diagnosis and treatment processes in CVID patients are reviewed, after searching the literature about these topics.

EXPERT OPINION

Considering the many phenotypes of CVID and the fact that it remained undiagnosed until older ages, it is important to include various manifestations of CVID in the differential diagnosis. Due to the different manifestations of CVID, including autoimmune diseases, interdisciplinary collaboration of physicians from different fields is highly recommended, as discussed in the manuscript. Meanwhile, it is important to determine which patients could benefit from genetic diagnostic studies since such studies are not necessary for establishing the diagnosis of CVID.

Genomic crossroads between non-Hodgkin’s lymphoma and common variable immunodeficiency

Common variable immunodeficiency (CVID) represents the largest group of primary immunodeficiencies that may manifest with infections, inflammation, autoimmunity, and cancer, mainly B-cell non-Hodgkin’s lymphoma (NHL). Indeed, NHL may result from chronic or recurrent infections and has, therefore, been recognized as a clinical phenotype of CVID, although rare. The more one delves into the mechanisms involved in CVID and cancer, the stronger the idea that both pathologies can be a reflection of the same primer events observed from different angles. The potential effects of germline variants on specific somatic modifications in malignancies suggest that it might be possible to anticipate critical events during tumor development. In the same way, a somatic alteration in NHL could be conditioning a similar response at the transcriptional level in the shared signaling pathways with genetic germline alterations in CVID. We aimed to explore the genomic substrate shared between these entities to better characterize the CVID phenotype immunodeficiency in NHL. By means of an in-silico approach, we interrogated the large, publicly available datasets contained in cBioPortal for the presence of genes associated with genetic pathogenic variants in a panel of 50 genes recurrently altered in CVID and previously described as causative or disease-modifying. We found that 323 (25%) of the 1,309 NHL samples available for analysis harbored variants of the CVID spectrum, with the most recurrent alteration presented in NHL occurring in PIK3CD (6%) and STAT3 (4%). Pathway analysis of common gene alterations showed enrichment in inflammatory, immune surveillance, and defective DNA repair mechanisms similar to those affected in CVID, with PIK3R1 appearing as a central node in the protein interaction network. The co-occurrence of gene alterations was a frequent phenomenon. This study represents an attempt to identify common genomic grounds between CVID and NHL. Further prospective studies are required to better know the role of genetic variants associated with CVID and their reflection on the somatic pathogenic variants responsible for cancer, as well as to characterize the CVID-like phenotype in NHL, with the potential to influence early CVID detection and therapeutic management.

Members of the Regulatory Lymphocyte Club in Common Variable Immunodeficiency

The role of CD4 T regulatory cells is well established in peripheral tolerance and the pathogenesis of the murine model and human autoimmune diseases. CD4 T regulatory cells (CD4 Tregs) have been investigated in common variable immunodeficiency (CVID). Recently, additional members have been added to the club of regulatory lymphocytes. These include CD8 T regulatory (CD8 Tregs), B regulatory (Bregs), and T follicular helper regulatory (T_FR) cells. There are accumulating data to suggest their roles in both human and experimental models of autoimmune disease. Their phenotypic characterization and mechanisms of immunoregulation are evolving. Patients with CVID may present or are associated with an increased frequency of autoimmunity and autoimmune diseases. In this review, we have primarily focused on the characteristics of CD4 Tregs and new players of the regulatory club and their changes in patients with CVID in relation to autoimmunity and emphasized the complexity of interplay among various regulatory lymphocytes. We suggest future careful investigations of phenotypic and functional regulatory lymphocytes in a large cohort of phenotypic and genotypically defined CVID patients to define their role in the pathogenesis of CVID and autoimmunity associated with CVID.

Serum Free Immunoglobulins Light Chains: A Common Feature of Common Variable Immunodeficiency?

Serum free light chain (sFLC) is a recently proposed biomarker for CVID diagnosis. Most CVID patients present low or undetectable sFLC up to 10-fold lower compared to other primary antibody deficiencies. Given that κ and λ light chains are normally secreted in excess with respect to immunoglobulins, this finding points to an intrinsic defect of B cell differentiation in CVID. sFLC levels were prospectively evaluated in a cohort of 100 primary immunodeficiency (PID) patients and in 49 patients with secondary immunodeficiency to haematological malignancy (SID). CVID patients had significantly lower κ and/or λ values (mean: κ: 1.39 ± 1.7 mg/L and λ: 1.97 ± 2.24 mg/L) compared to "other PIDs" (κ: 13.97 ± 5.88 mg/L and λ: 12.92 ± 7.4 mg/L, respectively, p < 0.001 both), and SID (κ 20.9 ± 22.8 mg/L and λ 12.8 ± 8.7 mg/L, respectively, p < 0.001 both). The sum of kappa and lambda (sum κ + λ) in CVID patients (7.25 ± 7.90 mg/L) was significantly lower respect to other PIDs (26.44 ± 13.25 mg/L, p < 0.0001), and to SID patients (28.25 ± 26.24 mg/L, p = 0.0002). ROC analysis of the sum κ + λ disclosed an area under the curve (AUC) of 0.894 for CVID diagnosis (SD 0.031; 95% CI: 0.83-0.95, p < 0.0001), with optimal cut-off of 16.7 mg/L, giving the highest combination of sensitivity (92%), specificity (75%) and NPV (98%). The Relative Risk (RR) for patients presenting a sum κ + λ below 16.7 mg/L was 20.35-fold higher (95%, CI: 5.630-75.93) for CVID than below this threshold. A similar behavior of the sFLC in our CVID cohort with respect to previously published studies was observed. We propose a cut-off of sum κ + λ 16.7 with diagnostic application in CVID patients, and discuss potential specific defects converging in low or undetectable sFLC.

Selection and Neutral Mutations Drive Pervasive Mutability Losses in Long-Lived Anti-HIV B-Cell Lineages

High-affinity antibodies arise within weeks of infection from the evolution of B-cell receptors under selection to improve antigen recognition. This rapid adaptation is enabled by the distribution of highly mutable "hotspot" motifs in B-cell receptor genes. High mutability in antigen-binding regions (complementarity determining regions [CDRs]) creates variation in binding affinity, whereas low mutability in structurally important regions (framework regions [FRs]) may reduce the frequency of destabilizing mutations. During the response, loss of mutational hotspots and changes in their distribution across CDRs and FRs are predicted to compromise the adaptability of B-cell receptors, yet the contributions of different mechanisms to gains and losses of hotspots remain unclear. We reconstructed changes in anti-HIV B-cell receptor sequences and show that mutability losses were ∼56% more frequent than gains in both CDRs and FRs, with the higher relative mutability of CDRs maintained throughout the response. At least 21% of the total mutability loss was caused by synonymous mutations. However, nonsynonymous substitutions caused most (79%) of the mutability loss in CDRs. Because CDRs also show strong positive selection, this result suggests that selection for mutations that increase binding affinity contributed to loss of mutability in antigen-binding regions. Although recurrent adaptation to evolving viruses could indirectly select for high mutation rates, we found no evidence of indirect selection to increase or retain hotspots. Our results suggest mutability losses are intrinsic to both the neutral and adaptive evolution of B-cell populations and might constrain their adaptation to rapidly evolving pathogens such as HIV and influenza.

Identification of CVID Patients With Defects in Immune Repertoire Formation or Specification

Common variable immune deficiency disorder (CVID) is the most clinically relevant cause of antibody failure. It is a highly heterogeneous disease with different underlying etiologies. CVID has been associated with a quantitative B cell defect, however, little is known about the quality of B cells present. Here, we studied the naïve and antigen selected B-cell receptor (BCR) repertoire in 33 CVID patients using next generation sequencing, to investigate B cells quality. Analysis for each individual patient revealed whether they have a defect in immune repertoire formation [V(D)J recombination] or specification (somatic hypermutation, subclass distribution, or selection). The naïve BCR repertoire was normal in most of the patients, although alterations in repertoire diversity and the junctions were found in a limited number of patients indicating possible defects in early B-cell development or V(D)J recombination in these patients. In contrast, major differences were found in the antigen selected BCR repertoire. Here, most patients (15/17) showed a reduced frequency of somatic hypermutation (SHM), changes in subclass distribution and/or minor alterations in antigen selection. Together these data show that in our CVID cohort only a small number of patients have a defect in formation of the naïve BCR repertoire, whereas the clear majority of patients have disturbances in their antigen selected repertoire, suggesting a defect in repertoire specification in the germinal centers of these patients. This highlights that CVID patients not only have a quantitative B cell defect, but that also the quality of, especially post germinal center B cells, is impaired.

Patients with common variable immunodeficiency with autoimmune cytopenias exhibit hyperplastic yet inefficient germinal center responses

BACKGROUND

The lack of pathogen-protective, isotype-switched antibodies in patients with common variable immunodeficiency (CVID) suggests germinal center (GC) hypoplasia, yet a subset of patients with CVID is paradoxically affected by autoantibody-mediated autoimmune cytopenias (AICs) and lymphadenopathy.

OBJECTIVE

We sought to compare the physical characteristics and immunologic output of GC responses in patients with CVID with AIC (CVID+AIC) and without AIC (CVID-AIC).

METHODS

We analyzed GC size and shape in excisional lymph node biopsy specimens from 14 patients with CVID+AIC and 4 patients with CVID-AIC. Using paired peripheral blood samples, we determined how AICs specifically affected B-and T-cell compartments and antibody responses in patients with CVID.

RESULTS

We found that patients with CVID+AIC displayed irregularly shaped hyperplastic GCs, whereas GCs were scarce and small in patients with CVID-AIC. GC hyperplasia was also evidenced by an increase in numbers of circulating follicular helper T cells, which correlated with decreased regulatory T-cell frequencies and function. In addition, patients with CVID+AIC had serum endotoxemia associated with a dearth of isotype-switched memory B cells that displayed significantly lower somatic hypermutation frequencies than their counterparts with CVID-AIC. Moreover, IgG⁺ B cells from patients with CVID+AIC expressed VH4-34-encoded antibodies with unmutated Ala-Val-Tyr and Asn-His-Ser motifs, which recognize both erythrocyte I/i self-antigens and commensal bacteria.

CONCLUSIONS

Patients with CVID+AIC do not contain mucosal microbiota and exhibit hyperplastic yet inefficient GC responses that favor the production of untolerized IgG⁺ B-cell clones that recognize both commensal bacteria and hematopoietic I/i self-antigens.

B-cell receptor repertoire sequencing in patients with primary immunodeficiency: a review

The advent of next-generation sequencing (NGS) now allows a detailed assessment of the adaptive immune system in health and disease. In particular, high-throughput B-cell receptor (BCR) repertoire sequencing provides detailed information about the functionality and abnormalities of the B-cell system. However, it is mostly unknown how the BCR repertoire is altered in the context of primary immunodeficiencies (PID) and whether findings are consistent throughout phenotypes and genotypes. We have performed an extensive literature search of the published work on BCR repertoire sequencing in PID patients, including several forms of predominantly antibody disorders and combined immunodeficiencies. It is somewhat surprising that BCR repertoires, even from severe clinical phenotypes, often show only mild abnormalities and that diversity or immunoglobulin gene segment usage is generally preserved to some extent. Despite the great variety of wet laboratory and analytical methods that were used in the different studies, several findings are common to most investigated PIDs, such as the increased usage of gene segments that are associated with self-reactivity. These findings suggest that BCR repertoire characteristics may be used to assess the functionality of the B-cell compartment irrespective of the underlying defect. With the use of NGS approaches, there is now the opportunity to apply BCR repertoire sequencing to multiple patients and explore the PID BCR repertoire in more detail. Ultimately, using BCR repertoire sequencing in translational research could aid the management of PID patients by improving diagnosis, estimating functionality of the immune system and improving assessment of prognosis.

Somatic Hypermutation Defects in Common Variable Immune Deficiency

Common variable immunodeficiency (CVID) is a heterogeneous disorder characterized by impaired antibody production and recurrent infections. In the last 20 years, several groups have reported that B cells from CVID patients have an impaired somatic hypermutation (SHM). The reported frequency of this defect among CVID patient cohorts is highly variable and so is the methodology used to evaluate this process. Interestingly, the low level of SHM on B cells from CVID patients has been correlated with the presence of infectious and non-infectious complications. In this review, an overview of the studies regarding SHM in CVID patients is presented. We highlight the importance of SHM studies in CVID patients as a clinical tool due to the reported association with clinical complications by several groups. We also considered SHM measurement useful to guide future investigations in order to identify genetic defects involved in the development of the disease.

IgH sequences in common variable immune deficiency reveal altered B cell development and selection

Common variable immune deficiency (CVID) is the most common symptomatic primary immune deficiency, affecting ~1 in 25,000 persons. These patients suffer from impaired antibody responses, autoimmunity, and susceptibility to lymphoid cancers. To explore the cellular basis for these clinical phenotypes, we conducted high-throughput DNA sequencing of immunoglobulin heavy chain gene rearrangements from 93 CVID patients and 105 control subjects and sorted naïve and memory B cells from 13 of the CVID patients and 10 of the control subjects. The CVID patients showed abnormal VDJ rearrangement and abnormal formation of complementarity-determining region 3 (CDR3). We observed a decreased selection against antibodies with long CDR3s in memory repertoires and decreased variable gene replacement, offering possible mechanisms for increased patient autoreactivity. Our data indicate that patient immunodeficiency might derive from both decreased diversity of the naïve B cell pool and decreased somatic hypermutation in memory repertoires. The CVID patients also exhibited an abnormal clonal expansion of unmutated B cells relative to the controls. Although impaired B cell germinal center activation is commonly viewed as causative in CVID, these data indicate that CVID B cells diverge from controls as early as the pro-B stage, cell and suggest possible explanations for the increased incidence of autoimmunity, immunodeficiency, and lymphoma CVID patients.

Levels of somatic hypermutations in B cell receptors increase during childhood

Somatic hypermutation (SHM) is an important step in antigen-driven B cell development creating B lymphocytes expressing high-affinity antibody receptors. It is known that the peripheral B lymphocyte compartments of healthy children and adults differ considerably. However, the development of SHM with age has not been studied in detail previously. Therefore, we used the immunoglobulin (Ig)κ-restriction enzyme hot-spot mutation assay (Igκ-REHMA) to gain an estimation of SHM levels in different age groups in order to relate this to the size of the memory B lymphocyte subpopulations. We show that the level of SHM increases rapidly during the first 2 years of life. This reflects the changes of the memory B cell subpopulations, but also changes in the SHM within memory B cell subsets, probably reflecting an increase of secondary memory B cell responses.

Exhaustion of bacteria-specific CD4 T cells and microbial translocation in common variable immunodeficiency disorders

Common variable immunodeficiency (CVID) is characterized by abnormally low levels of antibodies in the blood and dysfunctional immune cells called CD4⁺ T cells. Perreau et al. now show evidence that bacteria-fighting CD4⁺ T cells in these patients are in a state of exhaustion due to a constant leakage of normal gut bacteria into the bloodstream, possibly due to insufficient antibody levels.

In the present study, we have investigated the functional profile of CD4 T cells from patients with common variable immunodeficiency (CVID), including production of cytokines and proliferation in response to bacteria and virus-derived antigens. We show that the functional impairment of CD4 T cells, including the reduced capacity to proliferate and to produce IFN-γ and IL-2, was restricted to bacteria-specific and not virus-specific CD4 T cells. High levels of endotoxins were found in the plasma of patients with CVID, suggesting that CD4 T cell dysfunction might be caused by bacterial translocation. Of note, endotoxemia was associated with significantly higher expression of programmed death 1 (PD-1) on CD4 T cells. The blockade of the PD-1–PD-L1/2 axis in vitro restored CD4 T cell proliferation capacity, thus indicating that PD-1 signaling negatively regulates CD4 T cell functions. Finally, we showed that intravenous immunoglobulin G (IVIG) treatment significantly reduced endotoxemia and the percentage of PD-1⁺ CD4 T cells, and restored bacteria-specific CD4 T cell cytokine production and proliferation. In conclusion, the present study demonstrates that the CD4 T cell exhaustion and functional impairment observed in CVID patients is associated with bacterial translocation and that IVIG treatment resolves bacterial translocation and restores CD4 T cell functions.

Expression of activation-induced cytidine deaminase gene in B lymphocytes of patients with common variable immunodeficiency

OBJECTIVE

Common variable immunodeficiency (CVID) is a heterogeneous disorder characterized by reduced serum level of IgG, IgA or IgM and recurrent bacterial infections. Class switch recombination (CSR) as a critical process in immunoglobulin production is defective in a group of CVID patients. Activation-induced cytidine deaminase (AID) protein is an important molecule involving CSR process. The aim of this study was to investigate the AID gene mRNA production in a group of CVID patients indicating possible role of this molecule in this disorder.

METHODS

Peripheral blood mononuclear cells (PBMC) of 29 CVID patients and 21 healthy controls were isolated and stimulated by CD40L and IL-4 to induce AID gene expression. After 5 days AID gene mRNA production was investigated by real time polymerase chain reaction.

FINDINGS

AID gene was expressed in all of the studied patients. However the mean density of extracted AID mRNA showed higher level in CVID patients (230.95±103.04 ng/ml) rather than controls (210.00±44.72 ng/ml; P=0.5). CVID cases with lower level of AID had decreased total level of IgE (P=0.04) and stimulated IgE production (P=0.02); while cases with increased level of AID presented higher level of IgA (P=0.04) and numbers of B cells (P=0.02) and autoimmune disease (P=0.02).

CONCLUSION

Different levels of AID gene expression may have important roles in dysregulation of immune system and final clinical presentation in CVID patients. Therefore investigating the expression of AID gene can help in classifying CVID patients.

Long-term follow-up on affinity maturation and memory B-cell generation in patients with common variable immunodeficiency

PURPOSE

Common variable immunodeficiency (CVID) comprises a heterogeneous group of primary immunodeficiency disorders. Immunophenotyping of memory B cells at the time of diagnosis is increasingly used for the classification of patients into subgroups with different clinical prognoses. The EUROclass classification is a widely used method. Levels of somatic hypermutation (SHM) have proven useful as a prognostic marker for recurrent respiratory tract infections. As time of presentation and diagnosis is highly variable in CVID patients, and diagnostic delay is a common problem, it is important to know whether classification parameters are stable over time. The purpose of the study was to address this question in a cohort of 33 CVID patients followed from 3 to 19 years after diagnosis (average follow-up 8.8 years).

METHODS

Levels of class-switched memory B cells were analyzed using flow cytometric immunophenotyping, and patients were classified according to the EUROclass criteria. Affinity maturation of B cells was measured using Igκ-REHMA, which assesses somatic hypermutation in kappa light chain transcripts. Clinical manifestations in terms of splenomegaly, autoimmune disease and granulomatous disease were also determined.

RESULTS

Switched memory B cells and levels of SHM were not consistently stable markers in a long-term follow-up setting. At a given time during follow-up, 60% of the patients were assigned to the EUROclass group SmB- (less than 2% switched memory B cells), but only 23% were consistently assigned to this group. Associations between clinical manifestations and levels of switched memory B cells or SHM were not observed in our study.

CONCLUSION

Based on our findings, we suggest that immunologic characteristics in CVID patients should be evaluated several times after diagnosis using internationally standardized methods.

Gene Conversion-Like Events in the Diversification of Human Rearranged IGHV3-23*01 Gene Sequences

Gene conversion (GCV), a mechanism mediated by activation-induced cytidine deaminase (AID) is well established as a mechanism of immunoglobulin diversification in a few species. However, definitive evidence of GCV-like events in human immunoglobulin genes is scarce. The lack of evidence of GCV in human rearranged immunoglobulin gene sequences is puzzling given the presence of highly similar germline donors and the presence of all the enzymatic machinery required for GCV. In this study, we undertook a computational analysis of rearranged IGHV3-23(*)01 gene sequences from common variable immunodeficiency (CVID) patients, AID-deficient patients, and healthy individuals to survey "GCV-like" activities. We analyzed rearranged IGHV3-23(*)01 gene sequences obtained from total PBMC RNA and single-cell polymerase chain reaction of individual B cell lysates. Our search identified strong evidence of GCV-like activity. We observed that GCV-like tracts are flanked by AID hotspot motifs. Structural modeling of IGHV3-23(*)01 gene sequence revealed that hypermutable bases flanking GCV-like tracts are in the single stranded DNA (ssDNA) of stable stem-loop structures (SLSs). ssDNA is inherently fragile and also an optimal target for AID. We speculate that GCV could have been initiated by the targeting of hypermutable bases in ssDNA state in stable SLSs, plausibly by AID. We have observed that the frequency of GCV-like events is significantly higher in rearranged IGHV3-23-(*)01 sequences from healthy individuals compared to that of CVID patients. We did not observe GCV-like events in rearranged IGHV3-23-(*)01 sequences from AID-deficient patients. GCV, unlike somatic hypermutation (SHM), can result in multiple base substitutions that can alter many amino acids. The extensive changes in antibody affinity by GCV-like events would be instrumental in protecting humans against pathogens that diversify their genome by antigenic shift.

TLR-mediated B cell defects and IFN-α in common variable immunodeficiency

Common variable immune deficiency (CVID) B cells have impaired responses to TLR7 and TLR9 agonists including poor cell proliferation, loss of cytokine production, and failure to produce IgG or IgA. We show that TLR7- or 9-activated B cells from CVID subjects with >0.5% peripheral isotype-switched CD27(+) B cells (group 2) have increased mature Cγ1 and Cγ2 heavy-chain mRNA transcripts compared to subjects who have <0.5% isotype-switched cells (group 1). While TLR-stimulated CVID plasmacytoid dendritic cells for all subjects had impaired IFN-α production, TLR7 or TLR9 stimulation in the presence IFN-α normalized isotype-switched CD27(+) B cells, enhanced activation-induced cytidine deaminase mRNA, and significantly improved IgG production only for group 2 subjects. IFN-α also upregulated TLR7 and TLR9 mRNA expression comparable to normal levels in B cells of group 2 subjects, indicating that the loss of IFN-α could be a significant component of the B-cell defect for these subjects.

Altered spectrum of somatic hypermutation in common variable immunodeficiency disease characteristic of defective repair of mutations

Pathogenic common variable immunodeficiency diseases (CVID) are genetic, usually inherited diseases for which a limited number of genetic defects have been implicated. As CVID presents with a wide range of clinical characteristics, there are likely diverse and for the most part unidentified genetic causes. In some individuals, defects in somatic hypermutation (SHM) have been suggested as the underlying cause of CVID. To address the mechanisms of SHM defects in CVID, we conducted a comprehensive mutational analysis of immunoglobulin heavy chain sequences from CVID patients. We identified several remarkably specific alterations in the spectra of SHM in comparison to healthy individuals. We provide evidence that some CVID cases are associated with defective repair of AID-induced mutations by the DNA mismatch repair (MMR) machinery. Our findings together with reports of increased chromosomal radiosensitivity and associated lymphoproliferative disorders amongst CVID patients, suggest that altered DNA damage repair may be a cause of CVID.

Role of B cells in common variable immune deficiency

Common variable immune deficiency is a heterogeneous immune deficiency characterized by reduced serum immunoglobulins and a lack of antibodies. As the name implies, B-cell defects are variably defective. In particular, peripheral blood isotype-switched CD27(+) memory B cells are reduced in number and have been the basis of several classification schemes. A lack of these B cells has been associated with selected clinical conditions, including immune cytopenias, splenomegaly, granulomatous disease and lymphadenopathy. Genetic defects in ICOS, CD19 and TACI have been described. In addition to defects in the production or survival of memory B cells, in most subjects, B cells have defects in Toll-like receptor signaling.

Pathogenesis and treatment of gastrointestinal disease in antibody deficiency syndromes

Primary humoral immune deficiencies are characterized by limited antibody responses secondary to either impaired B-lymphocyte development or B-cell responses to T-lymphocyte signals. Given that the gastrointestinal tract is the largest lymphoid organ in the body, it is not surprising that intestinal diseases are common in immunodeficiency. These gastrointestinal diseases can be classified into one of 4 groups, infection, malignancy, inflammatory, and autoimmune, and can mimic other known disease processes, such as inflammatory bowel disease and celiac sprue. The exact pathogenesis of these gastrointestinal disorders in the setting of systemic immunodeficiency is still under investigation. However, studies suggest that defects in antibody deficiency alone do not result in gastrointestinal disease but rather that defects in cellular immunity are also involved. Treatment is difficult given an already immunocompromised state, and often therapy with immunomodulators is required for more severe processes.

B-cell activating factor receptor deficiency is associated with an adult-onset antibody deficiency syndrome in humans

B-cell survival depends on signals induced by B-cell activating factor (BAFF) binding to its receptor (BAFF-R). In mice, mutations in BAFF or BAFF-R cause B-cell lymphopenia and antibody deficiency. Analyzing BAFF-R expression and BAFF-binding to B cells in common variable immunodeficiency (CVID) patients, we identified two siblings carrying a homozygous deletion in the BAFF-R gene. Removing most of the BAFF-R transmembrane part, the deletion precludes BAFF-R expression. Without BAFF-R, B-cell development is arrested at the stage of transitional B cells and the numbers of all subsequent B-cell stages are severely reduced. Both siblings have lower IgG and IgM serum levels but, unlike most CVID patients, normal IgA concentrations. They also did not mount a T-independent immune response against pneumococcal cell wall polysaccharides but only one BAFF-R-deficient sibling developed recurrent infections. Therefore, deletion of the BAFF-R gene in humans causes a characteristic immunological phenotype but it does not necessarily lead to a clinically manifest immunodeficiency.

The role of costimulation in antibody deficiencies: ICOS and common variable immunodeficiency

SUMMARY

The identification of mutations in the inducible costimulator (ICOS) gene in nine patients with common variable immunodeficiency (CVID) was a major breakthrough. CVID is a complex, highly heterogeneous primary immunodeficiency disease, and the discovery of these mutations revealed a molecular basis. ICOS belongs to the CD28 family of costimulatory molecules and is expressed exclusively on activated T cells. It has at least three critical functions: germinal center formation, isotype class switching, and the development of memory B cells. The discovery of human ICOS deficiency showed that a monogenic disorder could account for the full spectrum of manifestations seen in childhood and adulthood-onset CVID, including autoimmune, inflammatory, and malignant disease complications, as well as recurrent infections. Moreover, this discovery showed that a disorder which had previously been perceived as a B-cell disease might in fact have its genetic origin in human T cells. In this article, we review the role of ICOS in the mammalian immune system and human disease, as well as the discovery and characteristics of patients with ICOS deficiency. Finally, we also discuss how these 'human knockouts' have contributed to our understanding of ICOS functions and have suggested potential avenues for using therapeutic ICOS manipulation to treat other diseases.

Transmembrane activator and calcium-modulator and cyclophilin ligand interactor mutations in common variable immunodeficiency

PURPOSE OF REVIEW

TNFRSF13B, the gene which encodes transmembrane activator and calcium-modulator and cyclophilin ligand interactor (TACI), is mutated in nearly 10% of patients with common variable immune deficiency (CVID), an antibody deficiency syndrome characterized by loss of memory B cells and plasma cells. This review discusses the normal function of TACI and the role of TACI mutants in CVID.

RECENT FINDINGS

TACI activates isotype switching, mediates immunoglobulin production in response to type II T-independent antigens, and plays an inhibitory role in B cell homeostasis. Recent evidence indicates that TACI synergizes with CD40 and Toll-like receptors for immunoglobulin secretion and promotion of plasma cell differentiation. The two most common TACI mutants associated with CVID--C104R and A181E--are primarily found as heterozygous mutations suggesting that they either cause haploinsufficiency or exert a dominant negative effect. TACI mutations in CVID are associated with an increased susceptibility to autoimmunity and lymphoproliferation.

SUMMARY

TACI has a dual function in promoting B cell antibody responses and inhibiting B cell proliferation. The observation that TACI mutations are present in healthy participants suggests that modifier genes may play an important role in the development of CVID. The discovery of these genes will help understand the pathogenesis of this disease.

Somatic diversification in the absence of antigen-driven responses is the hallmark of the IgM+ IgD+ CD27+ B cell repertoire in infants

T cell–dependent immune responses develop soon after birth, whereas it takes 2 yr for humans to develop T cell–independent responses. We used this dissociation to analyze the repertoire diversification of IgM⁺IgD⁺CD27⁺ B cells (also known as “IgM memory” B cells), comparing these cells with switched B cells in children <2 yr of age, with the aim of determining whether these two subsets are developmentally related. We show that the repertoire of IgM⁺IgD⁺CD27⁺ B cells in the spleen and blood displays no sign of antigen-driven activation and expansion on H-CDR3 spectratyping, despite the many antigenic challenges provided by childhood vaccinations. This repertoire differed markedly from those of switched B cells and splenic germinal center B cells, even at the early stage of differentiation associated with μ heavy chain expression. These data provide evidence for the developmental diversification of IgM⁺IgD⁺CD27⁺ B cells, at least in very young children, outside of T cell–dependent and –independent immune responses.

Autoimmunity in common variable immunodeficiency

BACKGROUND

Autoimmunity has been increasingly recognized as a major issue in patients with common variable immunodeficiency (CVID), the most common symptomatic primary immunodeficiency in adulthood. Different authors report high prevalences of autoimmune diseases in CVID, and several mechanisms have been proposed to explain this apparent paradox. Genetic predisposition, under current surveillance, innate and adaptive immunity deficiencies leading to persistent/recurrent infections, variable degrees of immune dysregulation, and possible failure in central and peripheral mechanisms of tolerance induction or maintenance may all contribute to increased autoimmunity.

CONCLUSIONS

Data on the clinical/immunological profile of affected patients and treatment are available mostly concerning autoimmune cytopenias, the most common autoimmune diseases in CVID. Treatment is based on conventional alternatives, in association with short experience with new agents, including rituximab and infliximab. Benefits of early immunoglobulin substitutive treatment and hypothetical premature predictors of autoimmunity are discussed as potential improvements to CVID patients' follow-up.

Common variable immune deficiency: reviews, continued puzzles, and a new registry

BACKGROUND

Common variable immune deficiency (CVID) is a clinically and immunologically heterogenous primary immune deficiency first described more than 50 years ago. The main features are hypogammaglobulinemia, recurrent infections, and other complications. While CVID is considered as a genetic immune defect, and several genes have been reported as leading to the CVID phenotype, one of the most puzzling features of CVID is the sporadic inheritance pattern and the relatively late onset. In most cases, no other family members have any immune defect. The mean age at diagnosis is between 25 and 45 years of age. These features suggest the interplay between either several or numerous genes with or without potential environmental factors.

TACI, isotype switching, CVID and IgAD

Common Variable Immunodeficiency (CVID) is the most prevalent human primary immunodeficiency requiring medical attention. Until recently the only known genetic defect specific to CVID was ICOS deficiency that accounts for about 1% of the patients analyzed. Mutations in the TNFR family member TACI (transmembrane activator and calcium-modulator and cyclophilin ligand interactor), which mediates isotype switching in B cells, were found to be present in 5% of patients with CVID. Mutations in TACI were also found in relatives of patients with CVID who suffered from IgA deficiency (IgAD) as well as in a patient with isolated IgAD. In the majority of patients described to date only one TACI allele is mutated, showing an autosomal dominant transmission of the disease. B cells from individuals with TACI mutations did not produce IgG and IgA in response to the TACI ligand, APRIL (a proliferation-inducing ligand), probably reflecting impaired isotype switching. These results suggest that TACI mutations can lead to CVID.

Deficiency of somatic hypermutation of immunoglobulin G transcripts is a better predictor of severe respiratory tract infections than lack of memory B cells in common variable immunodeficiency

Defects of memory B cells and of somatic hypermutation (SHM) are involved in the pathogenesis of common variable immunodeficiency (CVID). Here we report for the first time a systematic study of the relationship between memory B cell deficiency and SHM abnormalities in CVID, and relate these variables to prediagnostic infections. Isotype switched Vh3-23 transcripts were undetectable or low in 30% (IgG) and 63% (IgA) of the patients, but never in controls (P < 0.001). When measurable, the SHM fraction of transcripts was significantly lower in patients (IgM: median 32% vs. 56% (P = 0.0002); IgG: 72% vs. 87% (P = 0.0002); IgA: 81% vs. 88% (P = 0.04)). The concentration of switched (CD19+/CD27+/IgG+) and unswitched (CD19+/CD27+/IgM+/IgD+) memory cells was reduced in 75% and 58% of the patients, respectively. Patients with reduced concentrations of switched memory B cells had normal or low SHM, and only the IgG SHM fraction correlated with prediagnostic incidence of severe respiratory tract infections (P = 0.004).

High serum levels of BAFF, APRIL, and TACI in common variable immunodeficiency

Common variable immunodeficiency (CVID) is characterized by low levels of immune globulins and lack of antibody. Mutations in transmembrane activator and calcium-modulating cyclophilin ligand (TACI), are found in 8-10%, associated with autoimmunity and splenomegaly. Some patients with mutations had increased serum levels of TACI. Because of this, and the prevalence of autoimmunity, splenomegaly, and lymphadenopathy, we quantitated levels of TACI ligands, a proliferation inducing ligand (APRIL) and B cell activating factor (BAFF) and TACI in serum of 77 patients. CVID subjects had markedly increased serum levels of BAFF (p<0.0001), APRIL (p<0.0001), and TACI (p=0.001) but there was no relationship between levels and autoimmunity, lymphadenopathy, splenomegaly, B cell numbers, or mutations in TACI. Peripheral blood mononuclear cells of CVID subjects had increased levels of BAFF mRNA. We conclude that increased constitutive production and/or underlying immuno-regulatory or inflammatory conditions lead to enhanced release of ligands; however, the biological result remains unclear.

Deconstructing common variable immunodeficiency by genetic analysis

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

Germinal center function in the spleen during simian HIV infection in rhesus monkeys

Infection with HIV-1, SIV, or simian HIV is associated with abnormalities in the number, size, and structure of germinal centers (GCs). To determine whether these histopathologic abnormalities are associated with abnormalities in Ab development, we analyzed nucleotide sequences of Igs from splenic GCs of simian HIV-infected macaques. Virus-specific GCs were identified in frozen splenic tissue sections by inverse immunohistochemistry using rHIV-1 gp120 as a probe. B cells from envelope-specific GCs were isolated from these sections using laser capture microdissection. Their Igs were amplified from cDNA using nested PCR, then cloned and sequenced. Nucleotide sequences were recovered from nine multimember clonal lineages. Within each lineage, sequences had similar V-D-J or V-J junctions but differed by somatic mutations distributed throughout the variable domain. The clones were highly mutated, similar to that previously reported for HIV-1-specific human IgG Abs. The average clone had 37 mutations in the V region, for a frequency of 0.11 mutations/base. The mutational pattern was strikingly nonrandom, with somatic mutations occurring preferentially at RGYW/WRCY hotspots. Transition mutations were favored over transversions, with C-->T and G-->A replacements together accounting for almost one-third of all mutations. Analysis of replacement and silent mutations in the framework and CDRs suggests that the Igs were subjected to affinity selection. These data demonstrate that the process of Ab maturation is not seriously disrupted in GCs during the early stages of immunodeficiency virus infection, and that Env-specific Igs developing in GCs are subject to extensive somatic mutation and profound selection pressures.

Hypogammaglobulinémies primitives découvertes à l’âge adulte

Hypogammaglobulinemia during adulthood can lead to recurrent infectious diseases, particularly pneumonia, otitis, and sinusitis, mainly due to Streptococcus pneumoniae and Haemophilus influenzae. Physicians must discriminate between primary and secondary hypogammaglobulinemia to provide appropriate treatment. Common variable immunodeficiency (CVID) is the most common cause of primary hypogammaglobulinemia diagnosed in adulthood. Clinical features include various combinations of infectious diseases, autoimmune diseases, lymphoproliferative disorders, and gastrointestinal diseases. Mutations in genes responsible for primary hypogammaglobulinemia in children are occasionally described in adults with CVID. ICOS and TACI deficiencies have recently been reported in a few CVID patients, although most cases have no proven genetic defects. The DefI cohort is recruiting adults with CVID throughout France. It will make it possible to describe the clinical, immunological, and genetic features of French patients with hypogammaglobulinemia and will help us to understand this immune deficiency better.

TLR9 activation is defective in common variable immune deficiency

Common variable immune deficiency (CVID) is a primary immune deficiency characterized by low levels of serum immune globulins, lack of Ab, and reduced numbers of CD27+ memory B cells. Although T, B, and dendritic cell defects have been described, for the great majority, genetic causes have not been identified. In these experiments, we investigated B cell and plasmacytoid dendritic cell activation induced via TLR9, an intracellular recognition receptor that detects DNA-containing CpG motifs from viruses and bacteria. CpG-DNA activates normal B cells by the constitutively expressed TLR9, resulting in cytokine secretion, IgG class switch, immune globulin production, and potentially, the preservation of long-lived memory B cells. We found that CpG-DNA did not up-regulate expression of CD86 on CVID B cells, even when costimulated by the BCR, or induce production of IL-6 or IL-10 as it does for normal B cells. TLR9, found intracytoplasmically and on the surface of oligodeoxynucleotide-activated normal B cells, was deficient in CVID B cells, as was TLR9 mRNA. TLR9 B cell defects were not related to proportions of CD27+ memory B cells. CpG-activated CVID plasmacytoid dendritic cells did not produce IFN-alpha in normal amounts, even though these cells contained abundant intracytoplasmic TLR9. No mutations or polymorphisms of TLR9 were found. These data show that there are broad TLR9 activation defects in CVID which would prevent CpG-DNA-initiated innate immune responses; these defects may lead to impaired responses of plasmacytoid dendritic cells and loss of B cell function.

Tackling the heterogeneity of CVID

PURPOSE OF REVIEW

Common variable immunodeficiency is clinically the most relevant primary immunodeficiency of the adult. Its heterogeneity has hindered progress in the pathogenetic understanding of the majority of common variable immunodeficiency patients. This abstract summarizes recent aspects of the field and emphasizes the need for a commonly accepted approach to classify common variable immunodeficiency.

RECENT FINDINGS

In the last 2 years, the first genetic defects underlying common variable immunodeficiency, including ICOS, TACI, BAFF-R and CD19, have been identified. The analysis of dendritic cells demonstrated alterations in a majority of patients in addition to the disturbed T and B-cell function. Several changes of the adaptive immune system might be secondary to an underlying chronic inflammatory setting possibly due to a HHV8 infection in a subgroup of patients with granulomatous disease, autoimmune phenomena and T-cell dysfunction. The occurrence of granulomatous inflammation is associated with a worse prognosis compared with common variable immunodeficiency patients without granuloma.

SUMMARY

The pathogenesis of common variable immunodeficiency includes disturbances of the adaptive as well as innate immune system. Identified monogenic defects account for about 10% of cases, leaving the majority of defects undefined and certainly in part epigenetic. To combine the known aspects of the pathogenesis of common variable immunodeficiency to a conclusive picture, the clinical and immunologic phenotyping of patients needs to be standardized.

Defined blocks in terminal plasma cell differentiation of common variable immunodeficiency patients

Common variable immunodeficiency (CVID) is a heterogeneous disorder characterized by defective Ab production and recurrent bacterial infections. The largely unknown causes are likely to comprise a diverse set of genetic or acquired defects. In this study, we investigated terminal B cell differentiation in lymph nodes from CVID patients. Up to the germinal center B cell stage, B cell differentiation was normal but terminal plasma cell development was found to be impaired. Using differential Blimp-1 and Syndecan-1 expression in controls, we defined three different plasma cell subsets that correspond to progressive developmental stages locating to different sites in the lymph node. In the CVID patients, we could only detect one or two of these subsets indicating a defective differentiation. Thus, terminal plasma cell differentiation was found to be impaired despite normal expression of Blimp-1. B cells reaching only the first stage of plasma cell differentiation were further unable to undergo isotype switching and to up-regulate activation markers on B cells stimulated in vitro.

Inflammatory and autoimmune complications of common variable immune deficiency

Common variable immune deficiency (CVID) is associated with autoimmune and inflammatory complications in addition to recurrent infections. The most common conditions are idiopathic thrombocytopenia purpura, autoimmune hemolytic anemia, sarcoid-like granulomatous disease and gastrointestinal inflammation. IVIG administration reduces the frequency of infections, but does not always prevent autoimmunity or inflammation. TNF antagonists and anti-CD20 immunomodulators have shown some efficacy in CVID in a few patients; further controlled studies are needed to determine the best management of these conditions in the setting of immunodeficiency.

Reduced thymic output, increased spontaneous apoptosis and oligoclonal B cells in polyethylene glycol-adenosine deaminase-treated patients

Impairment of purine metabolism due to adenosine deaminase (ADA) deficiency is associated with a severe combined immunodeficiency (SCID). Polyethylene glycol-modified ADA (PEG-ADA) has provided noncurative, life-saving treatment for these patients, but full immune recovery is not achieved with this therapy. Since ADA-SCID is perhaps the most difficult form of SCID to handle clinically, understanding the benefits and limitations of PEG-ADA therapy may be relevant for treatment selection. To this purpose, we analyzed the rate of thymic output, T and B cell repertoires, number of T cell divisions, IFN-gamma and IL-4 production, and the extent of cell death in five ADA-SCID patients following a prolonged period of treatment with PEG-ADA. We found that thymic output was low in these patients. However, their T cell repertoire was heterogeneous, and their T lymphocytes produced cytokines upon activation and responded to mitogen stimulation, although with different kinetics. Furthermore, a high number of peripheral T lymphocytes were committed to apoptosis. Anomalies were also observed in the B cell compartment, with oligoclonal expansions of B cell clonotypes in two patients. Our data indicate that decreased thymic function, B cell oligoclonality, and increased spontaneous apoptosis may be the mechanisms by which the immunodeficiency of ADA-SCID patients persists in spite of treatment with PEG-ADA.

Common variable immunodeficiency: the immune system in chaos

Common variable immunodeficiency (CVID) is a heterogeneous disorder that is associated with low serum-immunoglobulin concentrations, defective specific-antibody production and an increased susceptibility to bacterial infections of the respiratory and gastrointestinal tracts. In spite of the identification of genes that are associated with several known primary immunodeficiencies, the basic immunologic and molecular defects of the majority of patients with CVID have remained obscure. Most of the studies aimed at understanding the immunopathogenesis of CVID suggest that this condition is primarily a T-cell disorder, although renewed attention on the genetic linkage and haplotype analysis in families of patients with CVID and on the role of dendritic cells and B cells has revealed several interesting features. This new information should assist in understanding the pathogenesis of CVID and improving the therapeutic strategies.

Measurement of peripheral B cell subpopulations in common variable immunodeficiency (CVID) using a whole blood method

Recent reports have described reduced populations of CD27+ memory B cells and increased percentages of undifferentiated B cells in peripheral blood of patients with common variable immunodeficiency (CVID). This work has prompted two attempts to classify CVID based on rapid flow cytometric quantification of peripheral blood memory B cells and immature B cells. Evidence to support the hypothesis that such in vitro B cell classification systems correlate with clinical subtypes of CVID is being sought. For the classification to be useful in routine diagnosis, it is important that the flow cytometric method can be used without prior separation of peripheral blood mononuclear cells (PBMC). We have examined 23 CVID patients and 24 controls, using both PBMC and whole blood, and find an excellent correlation between these methods. The reproducibility of the method was excellent. We classified the CVID patients by all three of the existing classifications, including secretion of immunoglobulin by B cells in vitro as described by Bryant, as well as the more recent flow cytometric classification methods. Only one patient changed classification as a result of using whole blood.

Altered dendritic cell distribution in patients with common variable immunodeficiency

Recent data suggest a critical role for dendritic cells (DCs) in the generation of immunoglobulin-secreting plasma cells. In the work reported herein, we analyzed the frequency of peripheral blood plasmacytoid DCs (pDCs) and myeloid DCs (mDCs) in a cohort of 44 adults with common variable immunodeficiency (CVID) classified according to their CD27 membrane expression status on B cells. A deep alteration in the distribution of DC subsets, especially of pDCs, in the peripheral blood of CVID patients was found. Patients with a reduced number of class-switched CD27+IgD-IgM- memory B cells and patients with granulomatous disease had a dramatic decrease in pDCs (P = 0.00005 and 0.0003 vs controls, respectively) and, to a lesser extent, of mDCs (P = 0.001 and 0.01 vs controls, respectively). In contrast, patients with normal numbers of switched memory B cells had a DC distribution pattern similar to that in controls. Taken together, our results raise the possibility that innate immunity contributes to pathogenesis in CVID.

Deficient IL-12 and dendritic cell function in common variable immune deficiency

Patients with common variable immune deficiency have reduced serum IgG, IgA, and/or IgM, defective antibody production, and many have cellular abnormalities, including proliferative defects, accelerated T cell apoptosis, and insufficient production of IL-2 and IL-10. Excess monocyte intracellular IL-12 leading to a polarized Th-1-type response which could prevent antibody production has been suggested. Here we found that dendritic cells (DCs) of CVID subjects have a significantly reduced capacity to secrete IL-12, as compared to DCs of normal subjects when cultured with physiologic simulators: LPS (P = 0.0005), TNF-alpha (P = 0.006), or CD40-L fusion protein (P = 0.0004). CVID TNF-alpha or CD40-Ligand matured DCs were also significantly impaired in antigen presentation in mixed lymphocyte culture. Deficient IL-12 production was closely correlated to lymphocyte functions in vitro and to the absolute numbers of CD4 T cells in peripheral blood. While CVID DCs appear morphologically similar to DCs of normal subjects, the lack of IL-12 production and defective antigen presentation demonstrate functional defects. Deficient DC function could lead to attenuated T cell activation and defective immunization, features characteristic of CVID.

Bone Marrow Clonogenic Capability, Cytokine Production, and Thymic Output in Patients with Common Variable Immunodeficiency

In patients with primary Ab deficiencies, hematological and immunological abnormalities are frequently observed. A regenerative failure of hemopoietic stem/progenitor cells has been hypothesized. We evaluated in the bone marrow (BM) of 11 patients with common variable immunodeficiency, the phenotype of BM progenitors and their in vitro growth by colony-forming cell (CFC) and long-term culture (LTC) assays. A significant decrease in erythroid and mixed CFC and, to a greater extent, in primitive LTC-CFC progenitors was observed in patients compared with healthy controls. The frequency of BM pre-B and pro-B cells correlated directly with the absolute number of CD19+ lymphocytes. BM cells cultured in vitro produced spontaneously lower amounts of IL-2 and elevated levels of TNF-alpha compared with controls, indicating a skewing toward a proapoptotic cytokine pattern. In addition, stromal cells generated after BM LTC secreted less IL-7 and displayed by immunohistochemistry an altered phenotype. These findings were associated with a significant decrease in naive Th cells coexpressing CD31 in the peripheral blood. These results indicate an impaired growth and differentiation capacity of progenitor cells in patients with common variable immunodeficiency.

Specific NEMO mutations impair CD40-mediated c-Rel activation and B cell terminal differentiation

Hypomorphic mutations in the zinc finger domain of NF-kappaB essential modulator (NEMO) cause X-linked hyper-IgM syndrome with ectodermal dysplasia (XHM-ED). Here we report that patient B cells are characterized by an absence of Ig somatic hypermutation (SHM) and defective class switch recombination (CSR) despite normal induction of activation-induced cytidine deaminase (AID) and Iepsilon-Cepsilon transcripts. This indicates that AID expression alone is insufficient to support neutralizing antibody responses. Furthermore, we show that patient B cells stimulated with CD40 ligand are impaired in both p65 and c-Rel activation, and whereas addition of IL-4 can enhance p65 activity, c-Rel activity remains deficient. This suggests that these NF-kappaB components have different activation requirements and that IL-4 can augment some but not all NEMO-dependent NF-kappaB signaling. Finally, using microarray analysis of patient B cells we identified downstream effects of impaired NF-kappaB activation and candidate factors that may be necessary for CSR and SHM in B cells.

Clinical and laboratory aspects of common variable immunodeficiency

Common variable immunodeficiency (CVID) is an immunological disorder characterized by defective antibody production, recurrent infections, most notably of the respiratory tract, autoimmune phenomena and cancer. Some CVID patients may also present disturbances of the cellular immune response such as a decrease in the number and proportion of different lymphocyte populations, diminished lymphoproliferative response to mitogens and antigens, altered production of cytokines, and deficient expression of cell-surface molecules. Most Brazilian CVID patients included in this study show a decrease in T and B lymphocyte counts in the peripheral blood. Furthermore, their lymphocytes are more susceptible to apoptosis following activation than normal individuals, and they have a decrease in the expression of activation molecules like CD25, CD69, CD40L and CD70. Moreover, they show a decreased synthesis of IL-4 and IL-5 in comparison with normal individuals. The increase in susceptibility to apoptosis following activation, may also be responsible for the decrease in the expression of activation molecules and CD40L, decrease in Th2 cytokines synthesis, and in the number of T and B circulating cells. In this study we discuss some of these immunological disturbances correlating them to the patients' clinical features and comparing our patients' findings to the literature.

The genetics of hypogammaglobulinemia

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

Deficiency of somatic hypermutation of the antibody light chain is associated with increased frequency of severe respiratory tract infection in common variable immunodeficiency

Reduced levels of somatic hypermutation (SHM) have recently been described in IgG-switched immunoglobulin genes in a minority of patients with common variable immunodeficiency (CVID), demonstrating a disruption of the normal linkage between isotype switch and SHM. To see if, irrespective of isotype, there is a tendency to use unmutated immunoglobulin genes in CVID, we studied SHM in kappa light-chain transcripts using a VkappaA27-specific restriction enzyme-based hot-spot mutation assay (IgkappaREHMA). Hot-spot mutations were found in 48% (median; reference interval, 28%-62%) of transcripts from 53 healthy controls. Values were significantly lower in 31 patients (median, 7.5%; range, 0%-73%; P < .0000001) of whom 24 (77%) had levels below the reference interval. Low levels of SHM correlated with increased frequency of severe respiratory tract infection (SRTI; P < .005), but not with diarrhea (P = .8). Mannose-binding lectin (MBL) deficiency also correlated with SRTI score (P = .009). However, the correlation of SHM and SRTI was also seen when only patients with normal MBL genotypes were analyzed (n = 18, P = .006). A slight decline of mutated fractions over years was noted (P = .01). This suggests that most patients with CVID fail to recruit affinity-maturated B cells, adding a qualitative deficiency to the quantitative deficiency characterizing these patients.

DNA polymerase eta is involved in hypermutation occurring during immunoglobulin class switch recombination

Base substitutions, deletions, and duplications are observed at the immunoglobulin locus in DNA sequences involved in class switch recombination (CSR). These mutations are dependent upon activation-induced cytidine deaminase (AID) and present all the characteristics of the ones observed during V gene somatic hypermutation, implying that they could be generated by the same mutational complex. It has been proposed, based on the V gene mutation pattern of patients with the cancer-prone xeroderma pigmentosum variant (XP-V) syndrome who are deficient in DNA polymerase η (pol η), that this enzyme could be responsible for a large part of the mutations occurring on A/T bases. Here we show, by analyzing switched memory B cells from two XP-V patients, that pol η is also an A/T mutator during CSR, in both the switch region of tandem repeats as well as upstream of it, thus suggesting that the same error-prone translesional polymerases are involved, together with AID, in both processes.

Common variable immunodeficiency patient classification based on impaired B cell memory differentiation correlates with clinical aspects

Common variable immunodeficiency (CVID) is a very heterogeneous syndrome defined by impaired immunoglobulin production. The functional classification of CVID patients on the basis of in vitro immunoglobulin production is time consuming and has never shown any predictive value. We propose a classification based on the quantitative repartition of naive/memory B cells according to the dual expression of IgD and CD27. Fifty-seven patients were categorized into three groups: Group MB2 (11 patients, 19%) with normal memory B cells; Group MB1 (19 patients, 33%) with defective switched memory (IgD-CD27+) but normal nonswitched memory B cells (IgD+CD27+); Group MB0 (27 patients, 47%) with almost no memory B cells. In addition, a downexpression of activation markers (CD25, CD21, CD80, CD86) on B cells characterized the group MB1 patients and was associated with an upexpression of activation markers (HLA-DR, CD95, CD57) on T cells. This classification correlates with some clinical aspects showing a higher prevalence of splenomegaly (16/27, 59%), lymphoid proliferation (13/27, 48%) and granulomatous disease (12/27, 44%) in group MB0. Splenomegaly was also frequent in group MB1 (8/19, 42%). In contrast, autoimmunity was observed with similar prevalence in all three groups. Moreover, by analyzing B cell phenotype, immunoglobulin transcript expression, and somatic mutations, we propose different putative mechanisms responsible for impaired B cell activation and memory differentiation in this syndrome.