X-linked agammaglobulinemia (XLA) is caused by mutations in the Bruton’s tyrosine kinase (BTK
) gene. BTK is a member of the Tec family of cytoplasmic tyrosine kinases and is required for the maturation of B-cell precursors in the bone marrow.30
Mutations in BTK therefore cause an arrest of B-cell development at the pre-B-cell stage leading to virtual absence of circulating B cells in the peripheral
blood. Mutations that only partially interfere with the enzymatic function of BTK have also been described and are associated with milder forms of the disease that only have defects in the generation of specific antibody responses.
XLA is typically suspected in male patients with recurrent bacterial sinopulmonary infections and <2% circulating CD19+ B cells. Other infections that occur relatively frequently in patients with XLA prior to the initiation of IgG replacement therapy include skin infections (furunculosis, pyoderma, and cellulitis) and sepsis. The diagnosis can be confirmed either by identifying a mutation in the BTK gene or by demonstrating absence of the BTK protein in monocytes or platelets. A positive family history suggestive of an X-linked recessive mode of inheritance increases the suspicion for XLA. It is uncommon for patients with XLA to develop symptoms in the first months of life because newborns are protected from most infections by transplacentally acquired maternal IgG. There are few distinguishing physical features of XLA that can provide clues to the diagnosis but absence of visible tonsils or adenoids (by X-ray or CT scan) is a useful clue.
In addition to the common sinopulmonary pathogens, patients with XLA are also susceptible to infections by particular opportunistic organisms that are more rare and fastidious, which can cause unusual clinical syndromes. For example, Helicobacter cinaedi
can cause a syndrome of dermatomyositis and cellulitis that presents with cutaneous ulcerations, particularly on the lower legs.31
The organism can sometimes be recovered from the blood but is fastidious and difficult to culture using usual methods. The ability to culture the organism and evaluate its antibiotic sensitivity is crucial in many cases because it is frequently resistant to various antibiotics. A combination of antibiotics is often needed to clear the infection effectively.33
species, including M. hominis,
can cause lung, abdominal, or bone infections that are remarkably hard to eradicate and Ureaplasma urealyticum
infections are a rare cause of arthritis, urethritis, and pneumonia.34
Prior to the widespread use of IgG supplementation in these patients, opportunistic viral infections, particularly with viruses that require an extracellular phase, were especially problematic. For example, echovirus encephalitis was estimated to be the cause of death in ˜10% of boys with XLA in the 1970s but that number has fallen dramatically with aggressive use of IgG supplementation. There continue to be rare cases of echovirus encephalitis in patients with XLA, even in those on adequate IgG replacement therapy, but these are thought to be caused by viral strains for which there may not be high antibody titers in the particular IgG preparation being used.35
Similarly, a mink astrovirus strain was recently identified by deep sequencing from the brain of an XLA patient who developed a neurodegenerative disorder as a teen.37
Interestingly, he had lived next to a mink farm as a child but was started on immunosuppression early in his teens for inflammatory bowel disease, which may have allowed the virus to escape control. Lastly, infections with vaccine-strain poliovirus were pathogenic in undiagnosed patients with XLA who were immunized with the live-viral vaccine after transplacentally acquired maternal antibody had waned. The shift from live attenuated to killed poliovirus for immunization has essentially eliminated new cases.
Hyperimmunoglobulin M Syndromes
Under normal circumstances, binding of antigen to cell surface IgM on naive B cells induces activation of the B-cell. The antigen that is bound to surface IgM on the B-cell is ingested, proteolytically digested, and antigenic peptide fragments are displayed on the B-cell surface in MHC Class II molecules. Antigen-specific T cells then engage the B-cell via MHC II/TCR interactions. Once engaged, the activated helper T-cell (Th) provides additional costimulatory signals to the B-cell that are critical to promote immunoglobulin class-switching from IgM to IgG, IgA, and IgE. The most important of these costimulatory signals comes via the interaction of CD40 ligand on activated T cells with CD40 on B cells. Additional costimulatory signals come from ICOS on activated T cells and ICOS ligand (B7-H2) on B cells. Activation of the B-cell through CD40 and cytokines that are secreted by the helper T-cell cause it to undergo class-switch recombination (CSR) during which the µ-heavy chain gene segment within the immunoglobulin gene, is replaced by either a γ, α, or ε gene segment. This is accomplished by nicking and double-strand breakage of the DNA in the immunoglobulin heavy-chain locus, which requires a series of enzymatic steps that involve activation-induced cytidine deaminase (AID), uracil DNA glycosylase (UNG), and others. Genetic defects that affect CD40 ligand (CD40L), CD40, AID, or UNG can therefore prevent class-switch recombination thus thwarting the B-cell’s ability to make significant amounts of any antibody isotype in addition to IgM.
The overwhelming majority of patients with hyper-IgM syndrome have the X-linked form caused by X-linked recessive mutations in the CD40 ligand (CD40L/CD154), which is encoded by the CD40L
gene on the X-chromosome.38
CD40L and its receptor CD40 are members of the tumor necrosis factor (TNF) superfamily of ligands and receptors. In lymphocytes, CD40L is expressed only on activated T cells but is also expressed on platelets where its role is unknown. Affected boys may present with recurrent bacterial sinopulmonary infections caused by low IgG, IgA, and IgE, whereas IgM is normal or elevated. In addition to the usual bacterial pathogens, patients with CD40L mutations also demonstrate unique susceptibilities to fungal infections, particularly Pneumocystis jiroveci
(PJ) that causes pneumonia, and to a protozoan, Cryptosporidium parvum
(CP) that causes bowel infections. B lymphocytes are present, and T-cell numbers are generally normal. In almost all cases, the diagnosis can be made by using flow cytometry to evaluate the expression and function of the CD40L protein on activated T cells. Expression is evaluated using antibodies specific to the CD40L protein and the function is evaluated by measuring the binding of a CD40-Ig heavy chain fusion protein to the expressed CD40L. Gene sequencing can then be performed in order to identify a specific mutation.
The susceptibility to Pneumocystis
and possibly other fungal pathogens has been somewhat of a puzzle because patients do not have other signs of a significant cellular immune defect such as severe or recurrent viral infections. Interestingly, the susceptibility to Pneumocysitis jiroveci
pneumonia (PJP) appears to go away in most patients by the age of 5. PJP is almost always diagnosed by staining bronchoalveolar lavage fluid for the presence of the organism. PJP can be readily prevented by prophylactic trimethoprim-sulfamethoxazole administration and active disease is amenable to treatment using higher doses of the same drug. Recent data have suggested that the fungal susceptibility in CD40L deficiency may be a result of defective CD40L signaling into dendritic cells and monocytes that express CD40.39
bowel infections are more difficult to diagnose and manage in patients with CD40L deficiency. CP may cause abdominal pain, bloating, diarrhea, malabsorption, and weight loss. It may require multiple stool samples to identify the oocysts and occasionally, the diagnosis can only be made on endoscopically obtained biopsy specimens. Treatment with paromomycin or nitazoxanide can clear the infection although prolonged courses are typically needed in hyper IgM patients and treatment failures are not uncommon. CP infections can result in chronic inflammation of the gut and biliary tree, which seems a likely contributor to the high incidence of bile duct cancers seen in these patients.40
Treatment involves the use of IgG replacement therapy combined with prophylactic antibiotics for prevention of PJP at least until the age of 5. The role of bone marrow transplantation for CD40L deficiency is still being evaluated. Even though a number of patients have undergone successful bone marrow
transplantation, the role of BMT remains somewhat controversial in this disease although in patients with ongoing CP infection, the prognosis for the patients who develop bile duct disease is so poor that the risks of transplantation are well justified.42
CD40 deficiency is inherited as an autosomal recessive defect that has been described primarily in two cohorts from Italy and the Middle East. It results in a syndrome that is almost identical to CD40L deficiency in which both sexes are affected. Homozygosity for a null mutation occurs, but heterozygosity is more common and can result in a partial phenotype because CD40 functions as a trimer (defects of even one chain of the trimer interfere with its function).44
Autosomal recessive mutations in AID and UNG also cause a hyper-IgM phenotype but it tends to be milder than either CD40L or CD40 deficiency, likely because the defect is limited to B cells, whereas CD40L/CD40 signaling plays a role in other cell types including dendritic cells and monocytes.45
Patients with AID or UNG typically live into adulthood and do not demonstrate the same susceptibility to PJP and CP bowel infections. Patients with mutations in AID do, however, have a significant propensity to develop autoimmunity affecting various organ systems. Patients are typically treated with IgG replacement therapy and antibiotics for acute infections. There are no reports of bone marrow transplantation for AID or UNG deficiency.
Common Variable Immunodeficiency Syndromes
Common variable immunodeficiency (CVID) is a heterogeneous disorder that is likely caused by a variety of molecular mechanisms that ultimately lead to a similar clinical phenotype. The European Society of Immunodeficiency (ESID) has proposed diagnostic criteria in an effort to standardize the diagnosis of CVID. These include: (1) plasma IgG levels that are less than 2 standard deviations below the mean for age combined with a “marked decrease” in either IgM or IgA; (2) age of onset of immunodeficiency >2 years of age; (3) absent isohemagglutinins or poor responses to vaccines; and (4) defined causes of hypogammaglobulinemia have been excluded.
The peak age of onset of CVID is in the second or third decade of life and 50% to 60% of patients have a clinical phenotype consisting almost exclusively of increased bacterial sinopulmonary infections. With IgG supplementation, this group of patients has a relatively benign course with long-term survival that is not unlike the normal population. The other half of patients have a complicated disease course with autoimmunity or lymphoproliferative disease that can involve the hematopoietic system, lungs, lymph nodes, liver, and bowel. The long-term outcome of this population is significantly worse, approaching only 40% survival over 40 years.47
Among the disorders seen in this population, a granulomatous, lymphoproliferative, interstitial lung disease (GLILD) affects approximately 30% to 40% of patients.48
This often presents with decreasing lung function that is manifested by cough, decreased exercise tolerance, and sometimes hypoxemia. Typical findings on chest CT scan include diffuse nodules within the lung, opacities that have a “ground glass” appearance, bronchial wall thickening, and sometimes bronchiectasis. Lung biopsy generally demonstrates a lymphocytic interstitial pneumonitis with noncaseating granulomas and a follicular bronchiolitis with lymphoid aggregates of both B and T cells. This pattern is sometimes mistaken for sarcoidosis although there are differences. Over time, this inflammatory process in the lungs will cause destruction of alveoli and will contribute to development of bronchiectasis. There continues to be some debate among providers about whether this process should be treated if the patient is not demonstrating pulmonary compromise. If left unchecked, however, there is evidence that irreversible damage and fibrosis develops in many patients. High-dose steroids are often used as first-line therapy to treat this process and in many cases they are effective but do not typically lead to a lasting remission on their own. A recent study using a combination of anti-CD20 monoclonal antibody (rituximab) therapy and azathioprine in a small cohort of CVID patients with GLILD demonstrated dramatic responses with a prolonged remission of disease in many patients.49
In addition to pulmonary symptoms, gastrointestinal complaints are common in CVID, affecting 20% to 30% of patients.47
Patients who develop disease demonstrate a hypertrophic lymphoproliferation of Peyer’s patches that causes a nodular lymphoid hyperplasia in the bowel. This is associated with abdominal discomfort, diarrhea, malabsorption, and weight loss and can cause significant morbidity. A variety of approaches have been taken to treat this process but none have offered particularly dramatic results although nonabsorbable steroid preparations have shown some benefit with minimal side effects. A more troubling complication observed in 5% to 10% of patients is a hepatitis that can cause severe hepatic dysfunction with development of hepatosplenomegaly and ascites.47
Infectious causes are almost never identified and liver biopsy demonstrates a nodular lymphoid hyperplasia in the liver parenchyma, not unlike that observed in the bowel. Liver disease is among the complications associated with a poor outcome.
Some 20% of subjects have additional clinical findings that are suggestive of autoimmunity/immune dysregulation including immune thrombocytopenia and hemolytic anemia, neuropathy, endocrinopathies, and skin disease. Skin involvement ranges from alopecia and vitiligo to psoriasis and granuloma annulare.50
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