Deletions in the ligand for CD40 in X-linked immunoglobulin deficiency with normal or elevated IgM (HIGMX-1)

Gene regulation in inborn errors of immunity: Implications for gene therapy design and efficacy

Inborn errors of immunity (IEI) present a unique paradigm in the realm of gene therapy, emphasizing the need for precision in therapeutic design. As gene therapy transitions from broad-spectrum gene addition to careful modification of specific genes, the enduring safety and effectiveness of these therapies in clinical settings have become crucial. This review discusses the significance of IEIs as foundational models for pioneering and refining precision medicine. We explore the capabilities of gene addition and gene correction platforms in modifying the DNA sequence of primary cells tailored for IEIs. The review uses four specific IEIs to highlight key issues in gene therapy strategies: X-linked agammaglobulinemia (XLA), X-linked chronic granulomatous disease (X-CGD), X-linked hyper IgM syndrome (XHIGM), and immune dysregulation, polyendocrinopathy, enteropathy, X-linked (IPEX). We detail the regulatory intricacies and therapeutic innovations for each disorder, incorporating insights from relevant clinical trials. For most IEIs, regulated expression is a vital aspect of the underlying biology, and we discuss the importance of endogenous regulation in developing gene therapy strategies.

Mitochondrial DNA insert into CD40 ligand gene-associated X-linked hyper-IgM syndrome

BACKGROUND

X-linked hyper-IgM (X-HIGM), which results from mutations in the CD40LG gene located on chromosome Xq26.3, is the most common form of HIGM. To date, more than 130 variants of the CD40L gene have been reported. We described a patient with novel de novo nuclear mitochondrial DNA sequences (NUMTs) in the CD40LG gene that have resulted in X-HIGM.

METHODS

Whole-exome sequencing (WES) analysis was used to screen for causal variants in the genome, and the candidate breakpoint was confirmed by Sanger sequencing.

RESULTS

A new mutation of CD40LG, which deletes A at position 17 followed by a 147-nucleotide from mitochondrial DNA copies insertion in exon 1, was detected in a 20-month-old boy harbouring an X-HIGM combined with immunodeficiency syndrome.

CONCLUSION

This is one of the few cases of a human genetic disease caused by nuclear mitochondrial DNA sequences (NUMTs). The presented data serve to demonstrate that de novo NUMT transfer of nucleic acid is a novel mechanism of X-HIGM.

The TNF Family of Ligands and Receptors: Communication Modules in the Immune System and Beyond

The tumor necrosis factor (TNF) and TNF receptor (TNFR) superfamilies (TNFSF/TNFRSF) include 19 ligands and 29 receptors that play important roles in the modulation of cellular functions. The communication pathways mediated by TNFSF/TNFRSF are essential for numerous developmental, homeostatic, and stimulus-responsive processes in vivo. TNFSF/TNFRSF members regulate cellular differentiation, survival, and programmed death, but their most critical functions pertain to the immune system. Both innate and adaptive immune cells are controlled by TNFSF/TNFRSF members in a manner that is crucial for the coordination of various mechanisms driving either co-stimulation or co-inhibition of the immune response. Dysregulation of these same signaling pathways has been implicated in inflammatory and autoimmune diseases, highlighting the importance of their tight regulation. Investigation of the control of TNFSF/TNFRSF activities has led to the development of therapeutics with the potential to reduce chronic inflammation or promote anti-tumor immunity. The study of TNFSF/TNFRSF proteins has exploded over the last 30 yr, but there remains a need to better understand the fundamental mechanisms underlying the molecular pathways they mediate to design more effective anti-inflammatory and anti-cancer therapies.

Clinical and molecular features of X-linked hyper IgM syndrome - An experience from North India

X-linked hyper IgM Syndrome (XLHIGM), the most frequent form of the Hyper IgM syndromes is a primary immune deficiency resulting from a mutation in the CD40 ligand gene (CD40LG). We analyzed the clinical and laboratory features of ten patients with XLHIGM, who were diagnosed at a tertiary care hospital in North India. Most common infections were sinopulmonary infections (80%) and diarrhea (50%). Sclerosing cholangitis and necrotising fasciitis were noted in one patient each. Three novel mutations in CD40LG (c.429_429 delA, p. G144DfsX5; c.500 G > A, p.G167E and c.156 G > C, p.K52 N) were detected. In addition, we found one missense mutation, two splice site mutations and two large deletions, which have been previously reported. Four (4) patients had expired at the time of analysis. We report the first series of XLHIGM from North India where we have documented unique features such as pulmonary alveolar proteinosis and infections with Mycobacterium sp.

A Novel de Novo Mutation in the CD40 Ligand Gene in a Patient With a Mild X-Linked Hyper-IgM Phenotype Initially Diagnosed as CVID: New Aspects of Old Diseases

Mutations in the CD40 ligand (CD40L) gene (CD40LG) lead to X-linked hyper-IgM syndrome (X-HIGM), which is a primary immunodeficiency (PID) characterized by decreased serum levels of IgG and IgA and normal or elevated IgM levels. Although most X-HIGM patients become symptomatic during the first or second year of life, during which they exhibit recurrent infections, some patients exhibit mild phenotypes, which are usually associated with hypomorphic mutations that do not abrogate protein expression or function. Here, we describe a 28-year-old man who initially presented with recurrent infections since the age of 7 years, when he exhibited meningitis caused by Cryptococcus neoformans. The patient had no family history of immunodeficiency, and based on clinical and laboratory presentation, he was initially diagnosed with common variable immunodeficiency (CVID). In subsequent years, he displayed several sporadic episodes of infection, including pneumonia, pharyngotonsillitis, acute otitis media, rhinosinusitis, fungal dermatosis, and intestinal helminthiasis. The evaluation of CD40L expression on the surface of activated CD3⁺CD4⁺ T cells from the patient showed decreased expression of CD40L. Genetic analysis revealed a novel de novo mutation consisting of a 6-nucleotide insertion in exon 1 of CD40LG, which confirmed the diagnosis of X-HIGM. In this report, we describe a novel mutation in the CD40L gene and highlight the complexities of PID diagnosis in light of atypical phenotypes and hypomorphic mutations as well as the importance of the differential diagnosis of PIDs.

Hyper IgM Syndrome: a Report from the USIDNET Registry

PURPOSE

The United States Immunodeficiency Network (USIDNET) patient registry was used to characterize the presentation, genetics, phenotypes, and treatment of patients with Hyper IgM Syndrome (HIGM).

METHODS

The USIDNET Registry was queried for HIGM patient data collected from October 1992 to July 2015. Data fields included demographics, criteria for diagnosis, pedigree analysis, mutations, clinical features, treatment and transplant records, laboratory findings, and mortality.

RESULTS

Fifty-two physicians entered data from 145 patients of ages 2 months to 62 years (median 12 years); 131 were males. Using patients' age at last entry, data from 2072 patient years are included. Mutations were recorded for 85 subjects; 82 were in CD40LG. Eighteen subjects had non-X-linked HIGM. 40 % had a normal serum IgM and 15 %, normal IgA. Infections were reported for 91 %, with pulmonary, ear, and sinus infections being the most common. 42 % had Pneumocystis jirovecii pneumonia; 6 % had Cryptosporidium. 41 % had neutropenia. 78 % experienced non-infectious complications: chronic diarrhea (n = 22), aphthous ulcers (n = 28), and neoplasms (n = 8) including colon cancer, adrenal adenoma, liver adenocarcinoma, pancreatic carcinoid, acute myeloid leukemia, hepatoma, and, in a female with an autosomal dominant gain of function mutation in PIK3CD, an ovarian dysgerminoma. Thirteen patients had a hematopoietic marrow or stem cell transplant; three had solid organ transplants. Thirteen were known to have died (median age = 14 years).

CONCLUSIONS

Analysis of the USIDNET Registry provides data on the common clinical features of this rare syndrome, and in contrast with previously published data, demonstrates longer survival times and reduced gastrointestinal manifestations.

A novel CD40LG deletion causes the hyper-IgM syndrome with normal CD40L expression in a 6-month-old child

The X-linked hyper-IgM syndrome (XHIGM) is the most common form of HIGM. Patients are clinically diagnosed on the basis of recurrent sinopulmonary infections, accompanied with low levels of IgG and IgA, normal to elevated levels of IgM, and the presence of peripheral B cells. Here, we have reported a novel deletion of four nucleotides in CD40LG exon 3, c.375_378delCAAA, which led to a frameshift mutation with a premature stop codon, p.Asn101*126. The deletion resulted in a truncated protein, in which majority of the extracellular domain was lost. However, detection of surface CD40L was still possible as the intracellular, transmembrane, and part of the extracellular domains were not affected. This indicated that this mutation did not affect protein stability and that immunodetection of CD40L expression is not enough for the diagnosis of XHIGM. Our study strongly suggests that genetic diagnosis for XHIGM should always be performed when clinical data support this diagnosis and CD40L protein is present.

Primary immune deficiency disorders in the South Pacific: the clinical utility of a customized genetic testing program in New Zealand

Primary immune deficiency disorders (PIDs) are a group of diseases associated with a genetic susceptibility to recurrent infections, malignancy, autoimmunity, and allergy. The molecular basis of many of these disorders has been identified in the last two decades. Most are inherited as single gene defects. As discussed in this paper, identifying the underlying genetic defect plays a critical role in many areas-including patient management, diagnosis, identifying atypical presentations, family studies, providing prognostic information, prenatal diagnosis, and defining new diseases. New Zealand is a geographically isolated, developed country in the South Pacific. We have introduced a dedicated customized genetic testing service for PID patients in New Zealand. This accredited diagnostic program offers rapid turnaround times for genetic tests and minimizes the risk of laboratory errors. Here we review the clinical indications for genetic testing for PIDs based on cases referred to the molecular immunology diagnostic service at Auckland City Hospital.

Immunology

The concept of forbidden foods that should not be eaten goes back to the Garden of Eden and apart from its religious meanings it may also have foreshadowed the concept of foods that can provoke adverse reactions. Thus we could say that allergic diseases have plagued mankind since the beginning of life on earth. The prophet Job was affected by a condition that following the rare symptoms described by the Holy Bible might be identified as a severe form of atopic dermatitis (AD). The earliest record of an apparently allergic reaction is 2621 B.C., when death from stinging insects was first described by hieroglyphics carved into the walls of the tomb of Pharaoh Menes depicting his death following the sting of a wasp. In 79 A.D., the death of the Roman admiral Pliny the Elder was ascribed to the SO₂-rich gases emanating from the eruption of Mount Vesuvius. Hippocrates (460–377 B.C.) was probably the first to describe how cow’s milk (CM) could cause gastric upset and hives, proposing dietetic measures including both treatment and prevention for CM allergy.

Reduced pathology following infection with transgenic Leishmania major expressing murine CD40 ligand

Leishmanization is the inoculation of live Leishmania into the host to vaccinate against subsequent infections. This approach has been largely discontinued due to safety concerns. We have previously shown that combining CD40 ligand (CD40L) with Leishmania antigen preferentially induces a type 1 immune response and provides some protection to vaccinated mice (G. Chen, P. A. Darrah, and D. M. Mosser, Infect. Immun. 69:3255-3263, 2001). In the present study, we developed transgenic L. major organisms which express and secrete the extracellular portion of CD40L (L. major CD40LE). We hypothesized that these organisms would be less virulent but more immunogenic than wild-type organisms and therefore be more effective at leishmanization. Transgenic parasites expressing CD40L mRNA and protein were developed. BALB/c mice infected with these parasites developed significantly smaller lesions containing fewer parasites than animals infected with wild-type organisms. Infection of resistant C57BL/6 mice with low doses of transgenic parasites induced a significant amount of protection against subsequent high-dose infection with wild-type organisms. These results demonstrate that transgenic organisms expressing CD40L are less virulent than wild-type organisms while retaining full immunogenicity.

Antigen-specific immunoglobulin E+ B cells are preferentially localized within germinal centres

Allergen-specific immunoglobulin E (IgE) mediates immediate-type hypersensitivity reactions and plays a central role in allergic diseases. Although antigen-driven B-cell maturation and isotype switching occur within germinal centres (GCs), the role of GCs in IgE production is poorly understood. In view of this, we investigated the development of IgE-expressing cells within GCs in response to an extensively characterized antigen, 2-phenyloxazolone (phOx). The phOx-specific IgE-expressing cells localized within GCs 7 days after immunization, and peaked in number on day 11. Surprisingly, very few IgE-positive cells were found in the T-cell areas of the lymph node. Flow cytometric studies confirmed that IgE was expressed by B cells and was not the result of trapping by follicular dendritic cells. The specificity of the antibody response was confirmed by microdissection and reverse transcription-polymerase chain reaction using phOx-specific IgE primers. IgE-positive cells were primarily found within GCs while, in contrast, many IgG1-positive cells could also be detected outside GCs in the T-cell areas. Taken together, these data highlight the importance of GCs in the production of antigen-specific IgE antibody.

Classification of mutations in the human CD40 ligand, gp39, that are associated with X-linked hyper IgM syndrome

The interaction between the T cell activation antigen gp39 and CD40, its receptor CD40 on B cells, plays a critical role in the regulation of humoral immune responses. Using a detailed three-dimensional model of the gp39 extracellular region, we have analyzed 20 mutations in gp39 that were, with one exception, isolated from patients with X-linked hyper IgM (XHIM) syndrome. On the basis of this analysis, the mutations were classified according to their predicted locations and effects. Twelve mutations are thought to compromise the gp39 structure by affecting interactions in hydrophobic core regions or at monomer interfaces, whereas seven others map closely to gp39 residues important for interaction with CD40. The latter mutations may thus, directly or indirectly, interfere with CD40 binding. One naturally occurring mutant whose carrier displays normal immune responses maps to a solvent-exposed position in a loop region of the molecule.

Non-functional immunoglobulin G transcripts in a case of hyper-immunoglobulin M syndrome similar to type 4

Summary 86% of immunoglobulin G (IgG) heavy-chain gene transcripts were found to be non-functional in the peripheral blood B cells of a patient initially diagnosed with common variable immunodeficiency, who later developed raised IgM, whereas no non-functionally rearranged transcripts were found in the cells of seven healthy control subjects. All the patient's IgM heavy-chain and kappa light-chain transcripts were functional, suggesting that either non-functional rearrangements were being selectively class-switched to IgG, or that receptor editing was rendering genes non-functional after class-switching. The functional gamma-chain sequences showed a normal rate of somatic hypermutation while non-functional sequences contained few somatic mutations, suggesting that most came from cells that had no functional gene and therefore were not receiving signals for hypermutation. However, apoptosis of peripheral blood lymphocytes was not impaired. No defects have been found in any of the genes currently known to be responsible for hyper-IgM syndrome but the phenotype fits best to type 4.

Costimulatory molecules and T-cell-B-cell interactions

This article focuses on activating and inhibiting costimulatory signals that are delivered to the T cell from antigen-presenting cells, mediating and modulating T-cell clonal expansion and development of effector functions, as well as costimulatory signals that are delivered by activated T cells to interacting target cells. The coordinated expression and interaction of these molecules regulates responses to foreign antigens and avoidance of response to self-antigens. Knowledge of the structure and function of these costimulatory molecules can be used to manipulate immune function and inhibit autoimmunity and inflammation in the setting of disease.

C4b-binding protein (C4BP) activates B cells through the CD40 receptor

We demonstrate that the alpha chain of human C4b binding protein (C4BP) binds directly to CD40 on human B cells at a site that differs from that used by CD40 ligand. C4BP induces proliferation, upregulation of CD54 and CD86 expression, and IL4-dependent IgE isotype switching in normal B cells but not in B cells from patients with CD40 or IKKgamma/NEMO deficiencies. Furthermore, C4BP colocalized with B cells in the germinal centers of human tonsils. These observations suggest that C4BP is an activating ligand for CD40 and establish a novel interface between complement and B cell activation.

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.

Apoptosis as a scaffold for building up the B cell repertoire

Control of cell number is determined by a balance between cell proliferation and cell death, both of which are highly regulated processes, with numerous checks and balances. Cells control their own death through activation of an internally coded suicide program that, when activated, initiates a characteristic form of cell death called apoptosis. This type of regulation allows elimination of cells that have been produced in excess, that have developed improperly, or that have sustained genetic damage. Apoptosis is, therefore, the most common physiological form of cell death and occurs during embryonic development, tissue remodeling, immune regulation, cell activation and tumor regression.

Glucocorticoids upregulate CD40 ligand expression and induce CD40L-dependent immunoglobulin isotype switching

IL-4 and CD40 ligation are essential for IgE synthesis by B cells. We have shown previously that hydrocortisone (HC) induces IgE synthesis in IL-4-stimulated human B cells. In this study we demonstrate that HC fails to induce IgE synthesis in B cells from CD40 ligand-deficient (CD40L-deficient) patients. Disruption of CD40L-CD40 interactions by soluble CD40-Ig fusion protein or anti-CD40L mAb blocked the capacity of HC to induce IgE synthesis in normal B cells. HC upregulated CD40L mRNA expression in PBMCs and surface expression of CD40L in PBMCs as well as in purified populations of T and B cells. Upregulation of CD40L mRNA in PBMCs occurred 3 hours after stimulation with HC and was inhibited by actinomycin D. Upregulation of CD40L mRNA and induction of IgE synthesis by HC were inhibited by the steroid hormone receptor antagonist RU-486. These results indicate that ligand-mediated activation of the glucocorticoid receptor upregulates CD40L expression in human lymphocytes.

CD154 variants associated with hyper-IgM syndrome can form oligomers and trigger CD40-mediated signals

X-linked hyper-IgM syndrome is a rare immunodeficiency disorder resulting from mutations in the gene encoding the CD40 ligand (CD154) molecule. These mutations are very heterogeneous, ranging from a single point mutation to a large deletion in the open reading frame. To investigate the molecular mechanisms that are responsible for the functional defect of these mutants, we examined the biochemical properties of 14 hyper-IgM-related CD154 mutant proteins produced by transient expression in COS7 cells. We show that deletion mutants lacking a significant portion of the tumor necrosis factor homologous domain cannot be stably produced. In contrast, point mutants can be detected as oligomers. Surprisingly, gene products of two point mutants, Thr-211 --> Asp and Met-36 --> Arg, can bind to the receptor, CD40. For Thr-211 --> Asp variant, it is comparable to the wild-type protein in its surface expression level, biochemical structure, and functional activities. Thus, it appears that this mutation is a polymorphism of CD154 gene. For Met-36 --> Arg variant, although it is interactive with CD40, it has a much lower surface expression level than wild-type protein. We propose that Met-36 --> Arg mutant represents a prototype of a defective CD154 family whose low cell surface expression of intrinsically active protein is simply insufficient to trigger productive signals through CD40.

[Mechanisms of tumor escape from immunologic response]

INTRODUCTION

The rationale for immune control of cancer is now better defined via the immunovirology of transforming viruses, definition of human tumor antigens recognized by T-lymphocytes, and cellular and humoral components of the anticancer response. Nonetheless tumors can escape from immune surveillance. To better define immunomodulation strategies, we describe some of the various strategies developed by transformed cells to evade the immune response.

CURRENT KNOWLEDGE AND KEY POINTS

Both the lack of specific tumor antigen and down-regulation of major histocompatibility complex (MHC) molecule expression hamper recognition of neoplastic cells by T-lymphocytes. In presence of defective expression of ligands for the T-cell co-stimulatory receptors, tumor recognition may lead to the development of tolerance instead of specific cytotoxic activity. Tumor cell counter-attack against effector T-cells has also been described, using either inhibitory cytokines (IL-10), apoptosis induction (via Fas signalling), functional inactivation (disruption of normal CD40/CD40 ligand interactions), or induction of anergy.

FUTURE PROSPECTS AND PROJECTS

Despite the many different mechanisms of tumor escape, the immune system has developed efficient counter-attacks. For instance, natural killer cells may detect and destroy tumor cells that lack class 1 MHC molecules and thus escape from specific T-lymphocyte cytolysis. Moreover, immunogenicity can be restored, at least in vitro, by different means such as tumor cell stimulation by cytokines or CD40, suggesting that therapeutic strategies will soon be developed in order to stimulate an efficient antitumoral immune response.

Responsiveness of peripheral blood B cells to recombinant CD40 ligand in patients with systemic lupus erythematosus

OBJECTIVE

To investigate the immunopathological significance of CD40/CD40 ligand system for B cell hyperactivation in SLE patients, the expression and the function of CD40 on B cells were compared with those of normal controls.

METHODS

Expression of CD40 was evaluated by flow cytometry. DNA synthesis of B cells were measured by 3H - TdR incorporation. Antibody production was assessed by ELISA.

RESULTS

There was no significant difference between SLE and normal controls in CD40 expression on peripheral blood B cells. Recombinant CD40 ligand-leucine zipper fusion protein (CD40L-LZ) significantly enhanced 3H - TdR incorporation by both SLE and normal B cells (P<0.01). 3H - TdR incorporation of SLE B cells without stimuli (P<0.001) and with CD40L-LZ stimulation (P<0.05) were significantly lower in SLE patients compared with normal controls. Active SLE B cells spontaneously produced significantly larger amounts of total IgG than normal B cells (P<0.05). CD40L-LZ significantly increased the production of total IgG by SLE B cells (P<0.05), but not by normal B cells. Active SLE B cells spontaneously produced IgG anti-dsDNA and IgG anti-ssDNA antibodies. CD40L-LZ significantly increased the production of these autoantibodies by SLE B cells (P<0.05). B cells from normal controls do not produce these autoantibodies spontaneously nor in response to CD40L-LZ.

CONCLUSION

These findings indicate that signalling via CD40 plays an important role in B cell proliferation and autoantibody production in human SLE.

Elevated levels and functional capacity of soluble CD40 ligand in systemic lupus erythematosus sera

OBJECTIVE

To measure soluble CD40 ligand (sCD40L) in sera from patients with systemic lupus erythematosus (SLE) and to study the functional capacity of sCD40L in mediating B cell activation.

METHODS

A 2-site enzyme-linked immunosorbent assay (ELISA) was used to measure sCD40L in the sera of 66 SLE patients, 30 disease control patients, and 23 healthy subjects. Induction of B cell activation antigen expression was used to assess the functional capacity of sCD40L in SLE sera.

RESULTS

The mean concentration of sCD40L was statistically significantly higher (P < 0.0001) in SLE patients than in disease controls or healthy subjects, and segregation of SLE patients by severe, moderate, or mild extent of disease showed a relationship between disease severity and sCD40L concentration. Western blot analysis demonstrated the presence of the 18-kd band of sCD40L in SLE sera, and the results of a 1-site ELISA protocol suggested that some of the product in SLE sera was present in dimer or trimer form. Functional studies showed that 10 ng/ml of recombinant CD40L, a level present in some SLE sera, induced increased expression of CD95 on B cells. Several SLE sera also induced CD95 or CD86 on Ramos B cells, a result that was inhibited by anti-CD40L monoclonal antibodies.

CONCLUSION

The soluble form of CD40L is present in the sera of most patients with SLE and may have the capacity to mediate B cell activation. Aberrant expression of CD40L might be predicted to result in activation of bystander B cells, including those that have encountered self antigens, and to contribute to autoantibody secretion.

Defects of T-cell effector function and post-thymic maturation in X-linked hyper-IgM syndrome

X-linked hyper-IgM syndrome (XHIM) results from mutations in the gene encoding for CD40 ligand (CD154). Patients with the syndrome suffer from infections with opportunistic pathogens such as Cryptosporidium and Pneumocystis carinii. In this study, we demonstrate that activated T cells from patients with XHIM produce markedly reduced levels of IFN-gamma, fail to induce antigen-presenting cells to synthesize IL-12, and induce greatly reduced levels of TNF-alpha. In addition, we show that the patients' circulating T lymphocytes of both the CD4(+) and CD8(+) subsets contain a markedly reduced antigen-primed population, as determined by CD45RO expression. Finally, we demonstrate that the defects in antigen priming are likely due to the lack of CD154 expression and insufficient costimulation of T cells by CD80/CD86 interactions. Taken together, this study offers a basis for the increased susceptibility of patients with XHIM to certain opportunistic infections.

The role of polar interactions in the molecular recognition of CD40L with its receptor CD40

CD40 Ligand (CD40L) is transiently expressed on the surface of T-cells and binds to CD40, which is expressed on the surface of B-cells. This binding event leads to the differentiation, proliferation, and isotype switching of the B-cells. The physiological importance of CD40L has been demonstrated by the fact that expression of defective CD40L protein causes an immunodeficiency state characterized by high IgM and low IgG serum levels, indicating faulty T-cell dependent B-cell activation. To understand the structural basis for CD40L/CD40 association, we have used a combination of molecular modeling, mutagenesis, and X-ray crystallography. The structure of the extracellular region of CD40L was determined by protein crystallography, while the CD40 receptor was built using homology modeling based upon a novel alignment of the TNF receptor superfamily, and using the X-ray structure of the TNF receptor as a template. The model shows that the interface of the complex is composed of charged residues, with CD40L presenting basic side chains (K143, R203, R207), and CD40 presenting acidic side chains (D84, E114, E117). These residues were studied experimentally through site-directed mutagenesis, and also theoretically using electrostatic calculations with the program Delphi. The mutagenesis data explored the role of the charged residues in both CD40L and CD40 by switching to Ala (K143A, R203A, R207A of CD40L, and E74A, D84A, E114A, E117A of CD40), charge reversal (K143E, R203E, R207E of CD40L, and D84R, E114R, E117R of CD40), mutation to a polar residue (K143N, R207N, R207Q of CD40L, and D84N, E117N of CD40), and for the basic side chains in CD40L, isosteric substitution to a hydrophobic side chain (R203M, R207M). All the charge-reversal mutants and the majority of the Met and Ala substitutions led to loss of binding, suggesting that charged interactions stabilize the complex. This was supported by the Delphi calculations which confirmed that the CD40/CD40L residue pairs E74-R203, D84-R207, and E117-R207 had a net stabilizing effect on the complex. However, the substitution of hydrophilic side chains at several of the positions was tolerated, which suggests that although charged interactions stabilize the complex, charge per se is not crucial at all positions. Finally, we compared the electrostatic surface of TNF/TNFR with CD40L/CD40 and have identified a set of polar interactions surrounded by a wall of hydrophobic residues that appear to be similar but inverted between the two complexes.

Heteromultimeric complexes of CD40 ligand are present on the cell surface of human T lymphocytes

CD40 ligand (CD40L), a 33-kDa type II membrane glycoprotein expressed primarily on activated CD4+ T lymphocytes, is responsible for the helper function of T cells on resting B cells in a non-antigen-dependent, non-major histocompatability complex-restricted fashion. Interaction of CD40L with its receptor CD40 induces proliferation of and isotype switching in B lymphocytes. Recently we solved the x-ray structure of recombinant soluble CD40L and showed that, similar to other members of the tumor necrosis factor family, CD40L indeed exists as a trimer. We now report that, under normal physiological conditions, CD40L molecules exist as heteromultimeric complexes. These CD40L complexes, made of the full length and smaller fragments of CD40L, are present on the cell surface of T lymphocytes and are capable of interacting with CD40 molecule. A prominent fragment with a mass of 31 kDa accounts for as much as half of the CD40L on the surface of Jurkat cells. N-terminal sequence data revealed that this fragment lacks the cytoplasmic tail. A minor 18-kDa fragment of CD40L was also characterized which lacks the cytoplasmic tail, transmembrane region, and stalk region of the extracellular domain. The presence of CD40L heteromultimeric variants implies an additional regulation of the functional activity of this ligand complex.

Molecular pathology of the X-linked hyper-immunoglobulin M syndrome: detection of wild-type transcripts in a patient with a complex splicing defect of the CD40 ligand

The X-linked hyper-immunoglobulin M syndrome (XHIM) is a primary immune deficiency disorder characterized by an inability to produce immunoglobulin isotypes other than immunoglobulin M (IgM) and IgD. Recently, a B-cell surface antigen (CD40) and its conjugate T-cell counterstructure (CD40 ligand) were shown to mediate immunoglobulin isotype switching in the presence of cytokines such as interleukin 4. Most patients with XHIM have been shown to have mutations of the extracellular domain of the CD40 ligand. Here we describe a novel point mutation of an intronic splice acceptor site which results in a complex splicing defect of the CD40 ligand in a patient with XHIM. In addition to two species of deleted transcripts, wild-type transcripts were also detected in this individual. The demonstration of wild-type CD40 ligand transcripts may be an explanation for previous observations suggesting that some XHIM patients are able to undergo immunoglobulin isotype switching in vivo.

Expansion or elimination of B cells in vivo: dual roles for CD40- and Fas (CD95)-ligands modulated by the B cell antigen receptor

Signals from CD4+ T cells induce two opposite fates in B cells: clonal proliferation of B cells that bind specifically to foreign antigens and clonal deletion of equivalent B cells that bind self-antigens. This B cell fate decision is determined by the concerted action of two surface proteins on activated T cells, CD40-and Fas-ligands (CD40L and FasL), whose effects are switched by signals from the B cell antigen receptor (BCR). Foreign antigens that stimulate the BCR acutely cause CD40L and FasL to promote clonal proliferation. CD40L and FasL trigger deletion, however, when the BCRs become desensitized by chronic stimulation with self-antigens or when BCRs have not bound an antigen. The need for both Fas and CD40L to correctly regulate self-reactive B cell fate may explain the severe autoantibody disorders in Fas- or CD40L-deficient children.

Increased expression of CD40 ligand on systemic lupus erythematosus lymphocytes

The specificity of T cell help for B cell activation and differentiation is maintained by the brief expression on the T cell surface, following T cell receptor-mediated triggering, of CD40 ligand (CD40L). Interaction of T helper (Th) cell CD40L with B cell CD40 induces B cell activation, cell surface expression of activation antigens, proliferation, and initiation of immunoglobulin isotype switch. We predicted that in patients with systemic lupus erythematosus (SLE), in whom Th cell-dependent production of autoantibodies results in immune complex-mediated tissue damage, CD40L expression might be augmented, prolonged, or abnormally regulated. Baseline expression of CD40L was increased in some SLE patients studied, when compared with control subjects. While Th cells from normal subjects (n = 14) and rheumatic disease control patients (n = 9) showed maximal expression of CD40L, after in vitro activation with phorbol myristate acetate (PMA) and ionomycin, at 6 h of culture with diminished levels observed at 24 and 48 h, Th cells from SLE patients (n = 19) maintained high level cell surface expression of CD40L through 24 and 48 h of culture. The prolonged expression of CD40L was functionally significant, as 24 h-activated SLE T cells, when cocultured with target B cells, induced greater B cell surface CD80 (B7-1) expression than did 24 h-activated normal T cells. These results document impaired regulation of CD40L expression in SLE T cells and identify an important potential target for therapy in this systemic autoimmune disease.

Genomic structure and PCR-SSCP analysis of the human CD40 ligand gene: its application to prenatal screening for X-linked hyper-IgM syndrome

To develop a general method for analysis of the mutation and prenatal diagnosis of X-linked hyper-IgM syndrome (XHM), the human CD40 ligand (hCD40L) gene was cloned and sequenced with special reference to the 5' and 3' flanking regions and exon/intron boundaries. The hCD40L gene consists of five exons and four introns, as already reported by others. Two major transcription initiation sites were identified at 67 bp and 64 bp upstream from the ATG initiation codon. The hCD40L mRNA transcripts terminated at 321 bp, 327 bp and 987 bp downstream from the TGA stop codon. Based on the intronic sequences, oligonucleotide primers were designed for amplifying the coding region of each exon separately. Polymerase chain reaction--single-strand conformational polymorphism (PCR-SSCP) analysis was successfully applied to screening for the defective hCD40L gene in a family with XHM. The nonsense mutation, Trp140 (TGG)-->stop (TAG) in exon 5, was found in the mother and an affected child. We also performed prenatal diagnosis by PCR-SSCP during the first trimester of pregnancy in this family.

A single strand conformation polymorphism study of CD40 ligand. Efficient mutation analysis and carrier detection for X-linked hyper IgM syndrome

Mutations in the gene for CD40 ligand are responsible for the X-linked form of hyper IgM syndrome. However, no clinical or laboratory findings that reliably distinguish X-linked disease from other forms of hyper IgM syndrome have been reported, nor are there tests available that can be used to confidently provide carrier detection. To identify efficiently mutations in the gene for CD40 ligand, eight pairs of PCR primers that could be used to screen genomic DNA by single strand conformation polymorphism (SSCP) were designed. 11 different mutations were found in DNA from all 13 patients whose activated T cells failed to bind a recombinant CD40 construct. The exact nature of four of these mutations, a deletion and three splice defects, could not be determined by cDNA sequencing. In addition, SSCP analysis permitted rapid carrier detection in two families in whom the source of the mutation was most likely a male with gonadal chimerism who passed the disorder on to some but not all of his daughters. These studies document the utility of SSCP analysis for both mutation detection and carrier detection in X-linked hyper IgM syndrome.

Cleavable CD40Ig fusion proteins and the binding to sgp39

Recombinant immunoglobulin (Ig) fusion proteins of cell surface and intracellular proteins have wide applications. For example, fusion proteins have been used in the isolation, identification and study of ligands and the effects of binding or blocking a receptor-ligand pair, either in vivo or in vitro. For some applications, removal of the immunoglobulin Fc region is advantageous. We have developed two vectors for the expression of Ig fusion proteins that contain recognition sequences for protease cleavage using thrombin. In one vector, the sequence encoding the thrombin cleavage site is located at the junction of the DNA fragment encoding the protein or protein fragment to be studied and the hinge and constant regions of the immunoglobulin, allowing the generation of a monomeric form of the protein of interest. In the second vector, the sequence encoding the thrombin cleavage site is located between the sequences encoding the hinge and constant regions of the immunoglobulin, allowing for the generation of covalent dimers of the recombinant protein without the constant Fc domains. We have used these vectors to produce the constructs encoding two forms of the extracellular domain of CD40, CD40ThrIg and CD40HinThrIg, allowing production of a monomeric and dimeric form of recombinant CD40. Cleavage is efficient and complete. Following cleavage, there was no detectable binding of the monomeric form of CD40 to a soluble form of gp39, the ligand of CD40, while the dimeric form was able to bind. These vectors have been constructed to allow facile substitution with other sequences to generate cleavable forms of other proteins of interest.

2 A crystal structure of an extracellular fragment of human CD40 ligand

BACKGROUND

The CD40 ligand (CD40L) is a member of the tumor necrosis factor (TNF) family of proteins and is transiently expressed on the surface of activated T cells. The binding of CD40L to CD40, which is expressed on the surface of B cells, provides a critical and unique pathway of cellular activation resulting in antibody isotype switching, regulation of apoptosis, and B cell proliferation and differentiation. Naturally occurring mutations of CD40L result in the clinical hyper-IgM syndrome, characterized by an inability to produce immunoglobulins of the IgG, IgA and IgE isotypes.

RESULTS

We have determined the crystal structure of a soluble extracellular fragment of human CD40L to 2 A resolution and with an R factor of 21.8%. Although the molecule forms a trimer similar to that found for other members of the TNF family, such as TNF alpha and lymphotoxin-alpha, and exhibits a similar overall fold, there are considerable differences in several loops including those predicted to be involved in CD40 binding.

CONCLUSIONS

The structure suggests that most of the hyper-IgM syndrome mutations affect the folding and stability of the molecule rather than the CD40-binding site directly. Despite the fact that the hyper-IgM syndrome mutations are dispersed in the primary sequence, a large fraction of them are clustered in space in the vicinity of a surface loop, close to the predicted CD40-binding site.

CD40-CD40 ligand (CD40L) interactions and X-linked hyperIgM syndrome (HIGMX-1)

Interactions between the B cell surface antigen CD40 and its ligand (CD40L) expressed on activated T cells play a critical role in isotype switching. This is illustrated by failure of isotype switching in patients with X-linked hyperIgM syndrome in whom the CD40L gene is mutated and by failure of isotype switching of CD40-deficient mice in response to T-cell-dependent antigens. We review these findings and discuss the signaling mechanisms of CD40 and the developmental control and transcriptional regulation of CD40L expression.

Common variable immunodeficiency: clinical aspects and recent progress in identifying the immunological defect(s)

Common variable immunodeficiency (CVID) comprises a heterogeneous group of patients with as yet undefined genetic defects. Patients with CVID have in common a decrease in the levels of one or more serum immunoglobulin isotypes and a severe defect in the production of specific antibodies. Typically, the patients suffer from recurrent infections of the upper and lower respiratory tract or the gastrointestinal tract. In consequence of these infections patients may develop severe organ damage, such as chronic pulmonary disease with bronchiectases, leading to pulmonary failure. Early diagnosis of CVID is important, as antibody deficiency can efficiently be treated by regular intravenous IgG (IVIG) substitution therapy. IVIG therapy prevents the occurrence of further acute infectious episodes and the development of long-term complications. The basic immunological defect(s) in patients with CVID are still unknown. There is currently no convincing evidence for an intrinsic B-cell defect in patients with CVID. A defect in T-cell activation due to impaired signal transduction upon T-cell receptor triggering has been described in a large subgroup of patients with CVID. Defective T-cell activation may lead to an impairment in cognate T-B-cell interaction due to impaired expression of CD40 ligand and/or abnormalities in the production T-cell-derived cytokines required for fully functional B-cell activation, proliferation and/or differentiation which could indeed explain the impairment in antibody production present in CVID patients.

Involvement of CRAF1, a relative of TRAF, in CD40 signaling

CD40 is a receptor on the surface of B lymphocytes, the activation of which leads to B cell survival, growth, and differentiation. A yeast two-hybrid screen identified a gene, CRAF1, encoding a protein that interacts directly with the CD40 cytoplasmic tail through a region of similarity to the tumor necrosis factor-alpha (TNF-alpha) receptor-associated factors. Overexpression of a truncated CRAF1 gene inhibited CD40-mediated up-regulation of CD23. A region of CRAF1 was similar to the TNF-alpha receptor-associated factors TRAF1 and TRAF2 and so defined a shared TRAF-C domain that was necessary and sufficient for CD40 binding and homodimerization. The CRAF1 sequence also predicted a long amphipathic helix, a pattern of five zinc fingers, and a zinc ring finger. It is likely that other members of the TNF receptor superfamily use CRAF-related proteins in their signal transduction processes.

Somatic mutation of human immunoglobulin V genes in the X-linked HyperIgM syndrome

Somatic mutation of Ig variable regions occurs prominently in germinal centers, but it has been debated whether the mutation process initiates in germinal centers or is activated before germinal center entry of B cells. We have analyzed for the presence of somatic mutation in Ig gene rearrangements of the nonpolymorphic human VH6 gene in the X-linked HyperIgM syndrome, which is associated with defective CD40 ligand expression and absence of germinal centers and generation of memory B lymphocytes. IgM and rare IgG VH6 productive rearrangements were isolated from PBL of patients with X-linked HyperIgM syndrome. Although the majority of both the IgM and IgG VH6 rearrangements had a germline VH6 sequence, 7 of 102 VH6 IgM and 1 of 6 IgG rearrangements had a mutated VH6 gene. The mutation frequency (mutations/bp) was 1.4% with a range of 2-9 mutations per clone, a mutation frequency lower, however, than that observed in IgM (3.2%) and IgG (5.4%) VH6 rearrangements of normal individuals. These results suggest that somatic mutation may be initiated in a CD40 ligand-independent pathway before entry of B cells into germinal centers, but fails to achieve the high mutation frequency observed in the presence of germinal centers.

Signaling through CD40 rescues IgE but not IgG or IgA secretion in X-linked immunodeficiency with hyper-IgM

The ligand for CD40 (CD40L) is a membrane protein on activated T cells that induces B cell proliferation and differentiation. Several mutations of the CD40L gene were reported responsible for defective class switching of B cells in an X-linked immunodeficiency with hyper IgM (X-HIM). We studied four affected males from three families and found three independent mutations including new mutations of CD40L gene. In every X-HIM patient tested, however, anti-CD40 plus IL-10 did not induce class switching from IgM to IgG or IgA, even in the presence of Staphylococcus aureus Cowan I strain (SAC). CD4+ T cell clones, expressing CD40L on their surface, also did not rescue IgG or IgA induction by X-HIM peripheral blood B cells in vitro. But signaling through CD40 induced both B cell proliferation and IgE secretion when IL-4 was added to the culture. Taken together, these results show that in vitro signaling through CD40 rescues IgE but not IgG or IgA secretion by peripheral blood X-HIM B cells and suggest that in vivo CD40 and CD40L interaction might be necessary for IgG and IgA differentiation in X-HIM.

Structural organization of the gene for CD40 ligand: molecular analysis for diagnosis of X-linked hyper-IgM syndrome

CD40 ligand (CD40L) on activated T cells plays a crucial role for Ig heavy-chain class switching and the mutation of the gene for this ligand in the X-chromosome causes immunodeficiency with hyper-IgM (X-HIM). We isolated and characterized the human genomic clone for CD40 L to obtain information about the transcriptional regulatory regions of the gene and to develop a molecular diagnostic method for X-HIM patients. The genomic DNA isolated was over 12 kb long containing 5 exons that cover full sequence of mRNA for the ligand. DNA motif analysis based on transcription factor database revealed the presence of a GATA 1 box at around -265 bp. The typical TATA box, CAT site or GC rich region was not found in the 5' flanking region. However, two possible TATA like sequences were found at around -27 and -136 bp. Using the oligonucleotide primers corresponding to the introns, we performed a PCR-SSCP analysis of each exon from a patient with X-HIM syndrome and detected abnormality in exon 5. When sequenced, dinucleotide deletion in this exon was found in the patient and in one X allele of his mother as the only different sequence from that of the control gene. This procedure is simple and could be used for diagnosis of the X-HIM syndrome.

Hyper IgM syndrome associated with defective CD40-mediated B cell activation

Recent studies show that most patients with X-linked hyper IgM syndrome have defects in the gene for CD40 ligand. We evaluated 17 unrelated males suspected of having X-linked hyper IgM syndrome. Activated T cells from 13 of the 17 patients failed to bind a soluble CD40 construct. In these patients, the sequence of CD40 ligand demonstrated mutations. By contrast, T cells from the remaining four patients exhibited normal binding to the CD40 construct. Sequencing of the cDNA for CD40 ligand from these patients did not show mutations. The possibility that hyper IgM syndrome in these four patients was due to abnormalities in the B cell response to CD40-mediated signals was examined. Peripheral blood lymphocytes were stimulated with anti-CD40 alone, IL4 alone or anti-CD40 plus IL4. In comparison with B cells from controls or patients with hyper IgM syndrome and mutant CD40 ligand, B cells from the patients with hyper IgM syndrome and normal CD40 ligand were defective in their ability to secrete IgE (P < 0.02) or express activation markers, CD25 and CD23 (P < 0.02) in response to stimulation with anti-CD40. The failure of these B cells to respond to CD40-mediated activation could not be attributed to a generalized deficiency in B cell activation because IL4 induced normal up-regulation of CD23 and CD25 expression. These findings indicate that hyper IgM syndrome may result from defects in expression of CD40 ligand by activated T cells or defects in CD40-mediated signal transduction in B cells.

Decreased expression of the ligand for CD40 in newborn lymphocytes

Immune responses in newborn lymphocytes show a defect in isotype switching from IgM to IgG and IgA. Immunoglobulin isotype switching in B lymphocytes requires a contact-dependent signal from T lymphocytes which is delivered by the ligand for the B cell surface antigen CD40. We investigated the capacity of newborn lymphocytes to express the CD40 ligand and to undergo CD40 ligand-dependent immunoglobulin isotype switching. After stimulation by phorbol ester and ionomycin, newborn lymphocytes expressed markedly decreased amounts of CD40 ligand on their surface compared to normal adult lymphocytes. Northern blot analysis of mRNA derived from activated cord blood lymphocytes also revealed markedly decreased amounts of CD40 ligand mRNA. Decreased expression of CD40 ligand in newborn lymphocytes was associated with a severely decreased ability to undergo T cell-dependent immunoglobulin isotype switching. Newborn lymphocytes synthesized little or no detectable IgE in response to T cell-dependent stimulation by interleukin-4 but synthesized IgE in response to T cell-independent stimulation by CD40 monoclonal antibody and interleukin-4. These results indicate that decreased expression of CD40 ligand in newborn lymphocytes may be the underlying cause of deficient immunoglobulin isotype switching in newborns.

X-linked immunodeficiencies

In the past year, researchers have identified the genes responsible for X-linked severe combined immunodeficiency (encoding a cytokine receptor protein), X-linked agammaglobulinemia (encoding a cytoplasmic tyrosine kinase) and X-linked hyper IgM syndrome (encoding the ligand for CD40). Although these three genes are completely unrelated, it is of interest that all are lineage-specific genes that are involved in the control of lymphocyte proliferation or differentiation.