How I treat: Diagnostic clues and treatment for activated phosphoinositide 3-kinase delta syndrome.

Activated phosphoinositide 3-kinase delta syndrome (APDS) is a form of common variable immunodeficiency (CVID) caused by gain-of-function variants in PIK3CD (APDS1) or loss-of-function variants in PIK3R1 (APDS2), resulting in hyperactivation of the PI3Kδ pathway. Clinically, it is characterized by recurrent respiratory infections, lymphadenopathy, splenomegaly, and progressive airway destruction, often leading to bronchiectasis. Enteropathy, cytopenia, and immune dysregulation have also been observed in several patients. Some patients develop malignant lymphomas, which can be fatal. Prophylactic antimicrobial therapy and regular immunoglobulin replacement therapy are administered in a manner similar to that used for CVID. However, since excessive activation of the class IA phosphoinositide 3-kinase signaling pathway is the primary pathophysiology of APDS, mammalian target of rapamycin inhibitors and selective p110δ inhibitors are expected to be licensed therapies. Early diagnosis based on diagnostic clues specific to APDS, followed by prompt treatment, is expected to improve patient prognosis.