Enhanced AKT phosphorylation of circulating B cells in patients with activated PI3Kδ syndrome

Activated PI3Kδ syndrome (APDS) is a primary immunodeficiency characterized by recurrent respiratory tract infections, lymphoproliferation, and defective IgG production. Heterozygous mutations in PIK3CD, PIK3R1, or PTEN, which are related to the hyperactive phosphoinositide 3-kinase (PI3K) signaling, were recently presented to cause APDS1 or APDS2 (APDSs), or APDS-like (APDS-L) disorder. In this study, we examined the AKT phosphorylation of peripheral blood lymphocytes and monocytes in patients with APDSs and APDS-L by using flow cytometry. CD19⁺ B cells of peripheral blood in APDS2 patients showed the enhanced phosphorylation of AKT at Ser473 (pAKT) without any specific stimulation. The enhanced pAKT in CD19⁺ B cells was normalized by the addition of a p110δ inhibitor. In contrast, CD3⁺ T cells and CD14⁺ monocytes did not show the enhanced pAKT in the absence of stimulation. These findings were similarly observed in patients with APDS1 and APDS-L. Among CD19⁺ B cells, enhanced pAKT was prominently detected in CD10⁺ immature B cells compared with CD10^- mature B cells. Enhanced pAKT was not observed in B cells of healthy controls, patients with common variable immunodeficiency, and hyper IgM syndrome due to CD40L deficiency. These results suggest that the enhanced pAKT in circulating B cells may be useful for the discrimination of APDS1, APDS2, and APDS-L from other antibody deficiencies.