Activated PI3K-delta syndrome (also known as APDS) is a disorder that impairs the immune system. Individuals with this condition often have low numbers of white blood cells (lymphopenia), particularly B cells and T cells. Normally, these cells recognize and attack foreign invaders, such as viruses and bacteria, to prevent infection. The severity of activated PI3K-delta syndrome varies widely. Some people may have multiple, severe infections while others show mild symptoms to none at all.

Most commonly, people with activated PI3K-delta syndrome develop recurrent infections that begin in childhood, particularly in the lungs, sinuses, and ears. Over time, recurrent respiratory tract infections can lead to a condition called bronchiectasis, which damages the passages leading from the windpipe to the lungs (bronchi) and can cause breathing problems. People with activated PI3K-delta syndrome may also have chronic active viral infections, such as Epstein-Barr virus, herpes simplex virus, or cytomegalovirus infections.

Another possible feature of activated PI3K-delta syndrome is abnormal clumping of white blood cells. These clumps can lead to enlarged lymph nodes (lymphadenopathy) or an enlarged spleen (splenomegaly). The white blood cells can also build up to form solid masses (nodular lymphoid hyperplasia), usually in the moist lining of the airways or intestines. While nodular lymphoid hyperplasia is not cancerous (benign), activated PI3K-delta syndrome increases the risk of developing forms of blood cancer called Hodgkin lymphoma and non-Hodgkin lymphoma.

Some people with activated PI3K-delta syndrome develop autoimmunity, which occurs when the body attacks its own tissues and organs by mistake.

There are two types of activated PI3K-delta syndrome, each with different genetic causes.

Activated PI3K-delta syndrome is considered a rare disorder, but its exact prevalence is unknown.

Activated PI3K-delta syndrome is caused by variants (also called mutations) in the PIK3CD gene and the PIK3R1 gene. Activated PI3K-delta syndrome type 1 is caused by variants in the PIK3CD gene, which provides instructions for making a protein called p110 delta (p110δ). Activated PI3K-delta syndrome type 2 is caused by variants in the PIK3R1 gene. This gene provides instructions for making slightly different versions of another protein; the most common version is called p85 alpha (p85α). 

Both p110δ and p85α are pieces (subunits) of an enzyme called phosphatidylinositol 3-kinase (PI3K), which turns on signaling pathways within cells. The version of PI3K that contains the p110δ and p85α subunits is called PI3K delta. PI3K delta is found in white blood cells, including B cells and T cells. PI3K-delta signaling is involved in the growth and division (proliferation) of white blood cells, and it helps direct B cells and T cells to mature (differentiate) into different types, each of which has a distinct function in the immune system.

The PIK3CD gene variants that cause activated PI3K-delta syndrome lead to the production of an altered p110δ protein. PIK3R1 gene variants lead to an altered p85α protein, sometimes causing it to be abnormally short. These variants are classified as gain-of-function variants because a PI3K-delta enzyme that contains either of the altered subunits is frequently turned on (overactive). 

Studies indicate that overactive PI3K-delta signaling alters the differentiation of B cells and T cells, producing cells that cannot respond to infections and that die earlier than usual. The lack of functioning B cells and T cells makes it difficult for people with activated PI3K-delta syndrome to fight off bacterial and viral infections. Overactive PI3K-delta signaling can also stimulate the abnormal proliferation of white blood cells, leading to lymphadenopathy and nodular lymphoid hyperplasia in some affected individuals. An increase in B cell proliferation in combination with reduced immune system function may contribute to the development of lymphoma in people with activated PI3K-delta syndrome.

Genetic Testing Information

• Genetic Testing Registry: Immunodeficiency 14
• Genetic Testing Registry: Immunodeficiency 36

Genetic and Rare Diseases Information Center

• Activated PI3K-delta syndrome

Patient Support and Advocacy Resources

• National Organization for Rare Disorders (NORD)

Clinical Trials

• ClinicalTrials.gov

Catalog of Genes and Diseases from OMIM

• IMMUNODEFICIENCY 36 WITH LYMPHOPROLIFERATION; IMD36
• IMMUNODEFICIENCY 14A WITH LYMPHOPROLIFERATION, AUTOSOMAL DOMINANT; IMD14A

Scientific Articles on PubMed

• PubMed

• Angulo I, Vadas O, Garcon F, Banham-Hall E, Plagnol V, Leahy TR, Baxendale H, Coulter T, Curtis J, Wu C, Blake-Palmer K, Perisic O, Smyth D, Maes M, Fiddler C, Juss J, Cilliers D, Markelj G, Chandra A, Farmer G, Kielkowska A, Clark J, Kracker S, Debre M, Picard C, Pellier I, Jabado N, Morris JA, Barcenas-Morales G, Fischer A, Stephens L, Hawkins P, Barrett JC, Abinun M, Clatworthy M, Durandy A, Doffinger R, Chilvers ER, Cant AJ, Kumararatne D, Okkenhaug K, Williams RL, Condliffe A, Nejentsev S. Phosphoinositide 3-kinase delta gene mutation predisposes to respiratory infection and airway damage. Science. 2013 Nov 15;342(6160):866-71. doi: 10.1126/science.1243292. Epub 2013 Oct 17. Citation on PubMed or Free article on PubMed Central
• Crank MC, Grossman JK, Moir S, Pittaluga S, Buckner CM, Kardava L, Agharahimi A, Meuwissen H, Stoddard J, Niemela J, Kuehn H, Rosenzweig SD. Mutations in PIK3CD can cause hyper IgM syndrome (HIGM) associated with increased cancer susceptibility. J Clin Immunol. 2014 Apr;34(3):272-6. doi: 10.1007/s10875-014-0012-9. Epub 2014 Mar 8. Citation on PubMed or Free article on PubMed Central
• Jou ST, Chien YH, Yang YH, Wang TC, Shyur SD, Chou CC, Chang ML, Lin DT, Lin KH, Chiang BL. Identification of variations in the human phosphoinositide 3-kinase p110delta gene in children with primary B-cell immunodeficiency of unknown aetiology. Int J Immunogenet. 2006 Oct;33(5):361-9. doi: 10.1111/j.1744-313X.2006.00627.x. Citation on PubMed
• Kracker S, Curtis J, Ibrahim MA, Sediva A, Salisbury J, Campr V, Debre M, Edgar JD, Imai K, Picard C, Casanova JL, Fischer A, Nejentsev S, Durandy A. Occurrence of B-cell lymphomas in patients with activated phosphoinositide 3-kinase delta syndrome. J Allergy Clin Immunol. 2014 Jul;134(1):233-6. doi: 10.1016/j.jaci.2014.02.020. Epub 2014 Apr 1. No abstract available. Citation on PubMed or Free article on PubMed Central
• Lucas CL, Kuehn HS, Zhao F, Niemela JE, Deenick EK, Palendira U, Avery DT, Moens L, Cannons JL, Biancalana M, Stoddard J, Ouyang W, Frucht DM, Rao VK, Atkinson TP, Agharahimi A, Hussey AA, Folio LR, Olivier KN, Fleisher TA, Pittaluga S, Holland SM, Cohen JI, Oliveira JB, Tangye SG, Schwartzberg PL, Lenardo MJ, Uzel G. Dominant-activating germline mutations in the gene encoding the PI(3)K catalytic subunit p110delta result in T cell senescence and human immunodeficiency. Nat Immunol. 2014 Jan;15(1):88-97. doi: 10.1038/ni.2771. Epub 2013 Oct 28. Citation on PubMed or Free article on PubMed Central
• Singh A, Joshi V, Jindal AK, Mathew B, Rawat A. An updated review on activated PI3 kinase delta syndrome (APDS). Genes Dis. 2019 Oct 14;7(1):67-74. doi: 10.1016/j.gendis.2019.09.015. eCollection 2020 Mar. Citation on PubMed