Hyper-IgM syndrome with susceptibility to opportunistic infections

Hyper-IgM syndrome with susceptibility to opportunistic infections (also known as Hyper-IgM syndrome type 1, or X-linked Hyper-IgM syndrome when caused by CD40LG mutations) is a group of rare primary immunodeficiency disorders characterized by normal or elevated levels of immunoglobulin M (IgM) in the blood, with markedly decreased or absent levels of immunoglobulin G (IgG), immunoglobulin A (IgA), and immunoglobulin E (IgE). This defect results from impaired class-switch recombination, the process by which B cells switch from producing IgM to other antibody types. The most common form is caused by mutations in the CD40LG gene (CD40 ligand, also called CD154), which is located on the X chromosome, making it predominantly affect males. Other genetic forms involve defects in genes such as AICDA, UNG, or CD40 itself. The immune system is profoundly affected, leaving patients highly vulnerable not only to bacterial infections but also to opportunistic infections, particularly Pneumocystis jirovecii pneumonia, Cryptosporidium infections of the gastrointestinal and biliary tract, and infections caused by Toxoplasma, Histoplasma, and Cryptococcus species. Cryptosporidium infection can lead to sclerosing cholangitis and liver disease, which is a major cause of morbidity and mortality. Patients also frequently experience recurrent sinopulmonary infections, chronic diarrhea, oral ulcers, and lymphoid hyperplasia. There is an increased risk of autoimmune cytopenias (such as neutropenia, hemolytic anemia, and thrombocytopenia) and certain malignancies, particularly lymphomas and hepatobiliary cancers. Treatment includes immunoglobulin replacement therapy (intravenous or subcutaneous) to reduce the frequency and severity of bacterial infections, prophylactic antibiotics (particularly trimethoprim-sulfamethoxazole for Pneumocystis prevention), and avoidance of Cryptosporidium exposure through safe water practices. Granulocyte colony-stimulating factor (G-CSF) may be used for associated neutropenia. Hematopoietic stem cell transplantation (HSCT) is the only curative treatment and is recommended particularly for the X-linked form, ideally performed early in life before the development of organ damage such as chronic liver disease. Gene therapy approaches are under investigation but remain experimental.

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