Patients with Hyper-IgM Syndrome (HIGM) Are Unable to Switch Antibody Production from IgM to IgG, IgA, and IgE.
As a result, patients with this primary immunodeficiency disease have decreased serum levels of IgG and IgA, while IgM levels remain normal or elevated. B-lymphocytes can independently produce IgM antibodies, but for switching from IgM to IgG, IgA, and IgE, they require assistance from interacting T-lymphocytes. Hyper-IgM syndrome results from various genetic defects affecting the interaction between T-lymphocytes and B-lymphocytes.
The most common form of HIGM results from a defect or deficiency in a protein found on the surface of activated T-lymphocytes. This protein is called the CD40 ligand (CD40L) because it binds to the CD40 protein on B-lymphocytes. The CD40 ligand is encoded by a gene on the X chromosome, making this primary immunodeficiency disease an X-linked recessive trait, typically affecting only males. Due to CD40L deficiency, T-lymphocytes in these patients cannot instruct B-lymphocytes to switch antibody production from IgM to IgG, IgA, and IgE. Additionally, CD40L plays a crucial role in other T-lymphocyte functions, leading to additional immune deficiencies in patients with X-linked hyper-IgM syndrome (X-HIGM).
Other forms of HIGM are inherited as autosomal recessive traits and can affect both males and females. The molecular basis of some other forms of HIGM has been identified, involving genetic defects in components of the CD40 signaling pathway. CD40 gene mutations are extremely rare and have been described in only a few families. The resulting disease is nearly identical to HIGM since, despite the presence of CD40L on T-lymphocytes, CD40 on B-lymphocytes and other immune system cells is either absent or non-functional. Two additional genes (AID and UNG) are essential for B-lymphocytes to switch antibody production from IgM to IgG, IgA, or IgE. Mutations in these genes have been identified in HIGM patients. Since the function of these genes is limited to antibody switching, other T-lymphocyte functions associated with CD40L remain intact, and these patients are less susceptible to infections typically combated by T-lymphocytes.
A distinct form of HIGM is associated with a skin disorder called ectodermal dysplasia and involves a defect in another X-linked gene required for the activation of the NF-kB signaling molecule. Besides other abnormalities, these patients present with immunodeficiency, impaired hair growth, and conical tooth shape. NF-kB is activated by CD40 and is necessary for the signaling pathway leading to antibody switching. It is also activated by other pathways critical for infection defense. Therefore, boys with this disorder are highly susceptible to severe infections.
Clinical Manifestations
Most patients with HIGM develop clinical symptoms within the first or second year of life. The most common symptom is an increased susceptibility to infections, including recurrent bacterial infections of the upper respiratory tract and bronchi. Severe infections may also be caused by other microorganisms. For instance, pneumonia caused by the opportunistic microorganism Pneumocystis jiroveci (carinii) is relatively common in the first year of life and may be the first sign of X-linked HIGM (X-HIGM). Pulmonary infections may also be viral (e.g., cytomegalovirus) or fungal (e.g., cryptococcal).
Some patients also experience gastrointestinal disorders, primarily diarrhea and malabsorption. Cryptosporidium is a major pathogen affecting the gastrointestinal tract in HIGM, potentially leading to sclerosing cholangitis—a severe liver disease.
Approximately half of the patients with HIGM develop transient or persistent neutropenia (a decrease in white blood cells—leukocytes). The cause of neutropenia is unknown, but most patients respond to treatment with granulocyte colony-stimulating factor (G-CSF). Neutropenia is often accompanied by oral ulcers, proctitis (inflammation and ulceration of the rectum), and skin infections. Lymphadenopathy (enlarged lymph nodes) is more frequent in patients with autosomal recessive HIGM than in most other primary immunodeficiencies. These patients often have enlarged tonsils, spleen, liver, and lymph nodes.
Autoimmune disorders are also common in HIGM patients, with manifestations including chronic arthritis, thrombocytopenia (low platelet count), hemolytic anemia, hypothyroidism, and kidney disease.
Diagnosis
X-linked hyper-IgM syndrome (X-HIGM) is suspected in any male patient with hypogammaglobulinemia characterized by low or absent IgG and IgA levels, alongside normal or elevated IgM levels. A hallmark feature is the absence of CD40 ligand expression on activated T cells. However, some patients with other forms of immunodeficiency may have significantly reduced CD40L expression despite a normal CD40L gene. Thus, the final diagnosis of X-HIGM depends on detecting a mutation in the CD40L gene, which can be performed in specialized laboratories.
Autosomal recessive forms of HIGM should be considered in female patients with HIGM-like symptoms or in cases where the CD40L gene and its expression on activated T-lymphocytes are normal.
Ectodermal dysplasia with immunodeficiency—another X-linked form of HIGM—should be suspected in patients with ectodermal dysplasia (e.g., sparse hair and conical teeth) and recurrent infections, along with normal or elevated IgM levels and low IgG, IgA, and IgE levels.
Genetic testing for mutations in specific genes can confirm the diagnosis of various forms of autosomal recessive HIGM or ectodermal dysplasia with immunodeficiency.
Inheritance
X-linked HIGM (X-HIGM) and ectodermal dysplasia with immunodeficiency are inherited as X-linked recessive traits, meaning they affect only males. Since these disorders are inherited as X-linked recessive traits, similar clinical features may be observed in the patient’s brothers or maternal uncles. Like other X-linked disorders, they may not be present in other family members.
Autosomal recessive forms of HIGM require mutations in both copies of the affected gene, making them less common than X-linked disorders.
If a specific gene mutation is identified in a family, prenatal diagnosis or carrier screening for family members is possible.
Treatment
Patients with HIGM suffer from severe IgG deficiency. Regular immunoglobulin replacement therapy, administered every 3 to 4 weeks, is effective in reducing infections. This therapy replenishes IgG levels and often lowers or normalizes serum IgM levels.
Since HIGM patients are particularly susceptible to pneumonia caused by Pneumocystis jiroveci (carinii), many physicians recommend prophylactic treatment with trimethoprim-sulfamethoxazole (Bactrim, Septra) for young children immediately after diagnosis. Intravenous immunoglobulin therapy may also help alleviate neutropenia.
Patients with persistent neutropenia may benefit from granulocyte colony-stimulating factor (G-CSF), though its prolonged use is generally not recommended. HIGM patients, like others with primary immunodeficiencies, should not receive live viral vaccines due to the risk of vaccine-related disease.
Avoiding contaminated water is crucial to prevent Cryptosporidium infections, which can cause severe gastrointestinal issues and chronic liver damage. Families should consult local water authorities regarding water safety and Cryptosporidium monitoring.
Beyond antibody deficiency, patients with HIGM have T-cell dysfunction, and those with ectodermal dysplasia with immunodeficiency have additional immune system defects. Immunoglobulin therapy alone may not fully protect against infections.
In recent years, bone marrow or umbilical cord blood stem cell transplantation has been proposed as a treatment option. Over ten HIGM patients have successfully undergone bone marrow transplantation from HLA-matched siblings, leading to complete immune recovery. Transplantation from matched unrelated donors has shown similar success rates. Due to the potential for severe T-cell reactions to organ transplants, including bone marrow, immunosuppressive therapy or low-dose radiation may be necessary.
Prognosis
Despite IgG and IgA deficiencies, as well as T-lymphocyte dysfunction in X-HIGM, several effective treatments allow these children to grow into happy and successful adults.
Based on materials from IPOPI (International Patient Organisation for Primary Immunodeficiencies).