IgA deficiency affects 1 in 600 people; CVID is less common but more severe. Children and adults with either condition suffer relentlessly recurring ear infections, sinus infections, bronchitis, pneumonias and gastrointestinal infections. IgA deficiency and CVID can occur in the same family, and also predispose people to autoimmunity, particularly affecting the thyroid gland and resulting in thyroid hormone insufficiency. Finally, people with CVID are susceptible to B-cell lymphomas.
The researchers, led by Raif Geha, MD, and Emanuela Castigli, PhD, in the Division of Immunology at Children's Hospital Boston, found mutations in a gene known as TACI in 4 of 19 unrelated patients with CVID and in 1 of 16 unrelated patients with IgA deficiency. None of 50 healthy people tested had a TACI mutation. Four of the 5 patients with TACI mutations were studied further, and all 4 had relatives with the same mutations. Eleven of the 12 identified relatives with TACI or IgA deficiency reported a history of recurrent infections and were found to have low levels of immunoglobulin A (IgA), immunoglobulin G (IgG) or both.
TACI mutations interfere with two aspects of the immune response that involve maturation of B cells, the white blood cells that make immunoglobulins, which function as antibodies, to fight infections. Normally, TACI triggers B cells to switch from making immunoglobulin M (IgM), an antibody produced early in the body's immune response, to making other immunoglobulins like IgA and IgG. More important, TACI signals B ce lls to produce antibodies with a high affinity for specific attackers. Because TACI mutations are dominant, people with even one copy of the mutation will be unable to mount a strong antibody response.
"A test for TACI would allow for diagnosis of more children and their relatives," says Geha, senior author of the study and a professor of pediatrics at Harvard Medical School. "Many children who are sick are now missed, because they can have normal IgA and IgG levels, yet they still have poor antibody responses and get the same bacteria and viruses again and again."
The gene discovery will immediately not change therapy, Geha adds. "For the time being, it's prophylactic antibiotics or IV immunoglobulin infusions every three weeks," he says.
Geha's team previously demonstrated that TACI binds to two other proteins made by cells in the respiratory and gastrointestinal lining, known as APRIL and BAFF, to trigger the signal for B cells to mature. Geha believes that additional cases of immune deficiency result from mutations in the APRIL and BAFF genes, and plans to verify this in further studies.