Report of the Italian Cohort with Activated Phosphoinositide 3-Kinase δ Syndrome in the Target Therapy Era

Background: Activated Phosphoinositide 3-Kinase (PI3K) δ Syndrome (APDS), an inborn error of immunity due to upregulation of the PI3K pathway, leads to recurrent infections and immune dysregulation (lymphoproliferation and autoimmunity). Methods: Clinical and genetic data of 28 APDS patients from 25 unrelated families were collected from fifteen Italian centers. Results: Patients were genetically confirmed with APDS-1 (n = 20) or APDS-2 (n = 8), with pathogenic mutations in the PIK3CD or PIK3R1 genes. The median age at diagnosis was 15.5 years, with a median follow-up of 74 months (range 6-384). The main presenting symptoms were respiratory tract infections alone (57%) or associated with lymphoproliferation (17%). Later, non-clonal lymphoproliferation was the leading clinical sign (86%), followed by respiratory infections (79%) and gastrointestinal complications (43%). Malignant lymphoproliferative disorders, all EBV-encoding RNA (EBER)-positive at the histological analysis, occurred in 14% of patients aged 17–19 years, highlighting the role of EBV in lymphomagenesis in this disorder. Diffuse large B-cell lymphoma was the most frequent. Immunological work-up revealed combined T/B cell abnormalities in most patients. Treatment strategies included immunosuppression and PI3K/Akt/mTOR inhibitor therapy. Rapamycin, employed in 36% of patients, showed efficacy in controlling lymphoproliferation, while selective PI3Kδ inhibitor leniolisib, administered in 32% of patients, was beneficial on both infections and immune dysregulation. Additionally, three patients underwent successful HSCT due to recurrent infections despite ongoing prophylaxis or lymphoproliferation poorly responsive to Rapamycin. Conclusions: This study underscores the clinical heterogeneity and challenging diagnosis of APDS, highlighting the importance of multidisciplinary management tailored to individual needs and further supporting leniolisib efficacy.