PI3K Signaling Pathway Abnormalities in Two Patients with PIK3CD (p110δ) Loss-of-Function (LOF) Variants

Background Phosphoinositide 3-kinase δ (PI3Kδ) is a lipid kinase that is composed of a catalytic subunit (p110δ, PIK3CD) and a regulatory subunit (p85, PIK3R1) and mediates signal transduction downstream of multiple immune receptors, making it essential for immune function. PI3Kδ converts phosphatidylinositol (4,5)-bisphosphate (PIP2) to phosphatidylinositol (3,4,5)-trisphosphate (PIP3), triggering activation of AKT and S6 in hematopoietic cells. Patients with PIK3CD (p110δ) loss-of-function (LOF) variants exhibit recurrent infection, immune dysregulation, with impaired T, B, and natural killer (NK) function and high mortality. In this study, we describe two patients (P1, P2) carrying novel PIK3CD (p110δ) LOF variants, their immune phenotype, and demonstrate IFNg-driven gastrointestinal inflammation, which resolved with JAK inhibition. Methods p110δ expression was evaluated by western blot in peripheral blood mononuclear cells (PBMCs), T cell blasts, and EBV-immortalized B cell lines. Downstream activation of AKT and S6 was assessed in primary B cells in response to human anti-IgM and in T cell blasts in the presence or absence of CD3/CD28 stimulation. RNAscope in situ hybridization for IFN-g, IL-12, IL-17, and CXCL9 was evaluated in gastrointestinal biopsies. Results Whole-exome sequencing of P1 revealed a novel homozygous PIK3CD frameshift variant (c.317dupA, p.D107Rfs23), whereas P2 carried compound heterozygous variants: c.2697delCAT (p.I899del) and c.346C>T (p.Q116*). Clinically, both individuals exhibited recurrent infection, immune dysregulation, and impaired T, B, and NK function, phenotypic features consistent with those previously documented in patients carrying p110δ variants. p110δ expression was absent in P2; interestingly, we observed a shorter protein in P1 (approximately 114 kDa). P1 exhibited aberrant activation of downstream mediators of PI3K activation (AKT and pS6), whereas P2 showed markedly reduced PI3K activity. Gastric biopsy in P1 demonstrated extensive lymphocytic infiltrates and a strong IFN-g signal that completely resolved after 8 weeks of ruxolitinib treatment. Conclusions Our results detail abnormal PI3K signaling caused by novel PIK3CD LOF mutations. Ruxolitinib treatment completely resolved gastrointestinal inflammation, providing further rationale for its use in patients with this autoimmune enteropathy associated with PIK3CD deficiency.