Human inborn errors of immunity (IEIs) are rare diseases caused by damaging genetic variants. IEI can be dominant or recessive, autosomal or X-linked inheritance, and the clinical phenotype can be complete or incomplete penetrance. The main clinical manifestations of these rare diseases are susceptibility to infection, as well as combining with autoimmune, autoinflammatory, allergic and/or malignant diseases. They now comprise 485 genetic disorders listed in the 2022 International Union of Immunological Societies (IUIS) classical classification [1].

Activated phosphoinositide3-kinase-δ (PI3Kδ) syndrome (APDS) is a recently reported IEI. PI3K molecules are composed of a p110 catalytic subunit and a regulatory subunit. Autosomal dominant gain-of-function (GOF) mutation in the PIK3CD gene encoding the PI3Kδ catalytic subunit p110δ cause APDS1, whereas autosomal dominant loss-of-function (LOF) mutation in the PIK3R1 gene encoding the regulatory subunit p85α cause APDS2 [2]. The hyperactivation of PI3Kδ results in the activation of the AKT-mTOR pathway, which further phosphorifies AKT and ribosomal S6 kinases in lymphocytes. The hyperactivated PI3K-AKT-mTOR pathway leads to functional defects of B lymphocytes, T lymphocytes and natural killer (NK) cells [3,4,5]. APDS1 patients develop a wide spectrum of clinical phenotypes, including recurrent respiratory tract infections, chronic epstein barr virus (EBV) and/or cytomegalovirus (CMV) viremia, benign lymphoproliferation, increased risk of lymphoma and autoimmunity [6]. Immunological features of these patients include low/normal serum level of immunoglobulin A (IgA) and IgG, normal or increased IgM [7], reduced class-switched memory B cells and CD4+ lymphopenia [8].

To date, 12 different activating missense mutations of PIK3CD gene have been revealed in APDS1, such as c.3061G > A, c.1002C > A, c.1246 T > C, c.1573G > A, and et al., of which c.3061G > A appears to be the most prevalent, accounting for approximately 85% [9,10,11,12,13]. Herein, we presented the clinical features and genotypes of two pediatric patients to show the diversity of APDS1 and to discuss the effect of sirolimus in curing APDS1. Especially, a novel variant in PIK3CD was reported.

Patient 1 was a 1 year and 5 months old male that presented with hepatosplenomegaly for more than one year and vomiting and diarrhea for one week. The medical history included recurrent respiratory tract infections, but no recurrent abdominal pain or diarrhea. When admitted to our hospital, the patient weighed 8.5 kg (< 3rd percentile) and had enlarged axilla lymph nodes. Abdominal ultrasonography showed hepatosplenomegaly, such that the liver was 2 cm below the right subcostal and the spleen was 3 cm below the umbilicus. Complete blood count values were as follows: white blood cells (WBC) 6.27 × 109/L (normal, 4–10 × 109/L), hemoglobin (Hb) 109 g/L (normal, 110–140 g/L), platelets (PLT) 76 × 109/L (normal, 85–303 × 109/L), and C-reactive protein (CRP) 3.04 mg/L (normal, 0–8 mg/L). Immunological values were as follows: CD3 919/μl (normal, 744–879/μl), CD4 422/μl (normal, 349–469/μl), CD8 496/μl (normal, 240–341/μl), CD4/CD8 0.9 (normal, 1.0–1.9), and CD19 61/μl (normal, 85–211/μl). Serum IgG, A, and M levels were 3.75 g/L (normal, 3.82–10.58 g/L), 0.8 g/L (normal, 0.14–1.14 g/L), and 1.59 g/L (normal, 0.4–1.28 g/L), respectively (Table 1). The EBV and CMV DNA load in the plasma were negative. Bone marrow biopsy excluded the possibility of malignancy. Computed tomography (CT) findings showed enlargement of multiple axilla nodes and hepatosplenomegaly. The whole exome sequencing of venous blood was performed on the child and his parents. A heterozygous missense mutation c.3061G > A (p.E1021K, Fig. 1, Table 2) of PIK3CD gene was revealed in this patient and his mother. His mother has not any clinical symptoms. After treatment with piperacillin and glucocorticoids for one week, symptoms of vomiting and diarrhea improved, but hepatosplenomegaly persisted. Subsequently, sirolimus (1 mg/m2) was started and sirolimus levels were maintained around 5.6–6.2 ng/mL. During follow-up, diarrheal episodes decreased, body weight increased, and hepatosplenomegaly significantly improved. However, the low IgG level and high IgM level have not fully recovered. Other serious adverse events were not noted.

The locations of the variants in the PIK3CD gene. Black site is a hot spot variant in the PIK3CD gene of patient 1, and red site is a new variant in the PIK3CD gene of patient 2

Patient 2 was a 4-month-old male that presented with coughing for two days. He had a history of recurrent respiratory tract infections since birth. Complete blood count values were as follows: WBC 10.46 × 109/L (normal, 4–10 × 109/L), Hb 117 g/L (normal, 110–140 g/L), PLT 10 × 109/L (normal, 85–303 × 109/L), and CRP 0.81 mg/L (normal, 0–8 mg/L). Immunological values were as follows: CD3 3749/μl (normal, 4275–5056/μl), CD4 1614/μl (normal, 2008–2695/μl), CD8 1647/μl (normal, 1381–1961/μl), CD4/CD8 1.00 (normal, 1.0–1.9), and CD19 1427/μl (normal, 487–1214/μl). Serum IgG, A, and M levels were 13.31 g/L (normal, 3.22–7.18 g/L), 0.44 g/L (normal, 0.13–0.35 g/L), and 1.96 g/L (normal, 0.23–0.91 g/L), respectively. Serum C3 and C4 were 0.60 g/L (normal, 0.79–1.52 g/L) and 0.01 g/L (normal, 0.16–0.38 g/L), respectively (Table 1). Coombs test was positive, antinuclear antibodies (ANA) were positive (1:1000), and anti-dsDNA showed negative. The EBV and CMV DNA load in the plasma were negative. After treatment with glucocorticoids (1 mg/kg) and immunoglobulin (2 g/kg), significant improvement of thrombocytopenia and respiratory infections were noted after five months. However, over the next year, the patient was hospitalized twice for thrombocytopenia and was given tacrolimus (0.5 g, qd) and mycophenolate mofetil (0.25 g, bid) orally during treatment. Four months ago, the patient was referred to our clinic for thrombocytopenia again. Complete blood count values were as follows: WBC 8.36 × 109/L (normal, 4–10 × 109/L), Hb 112 g/L (normal, 120–140 g/L), PLT 5 × 109/L (normal, 100–300 × 109/L), and CRP 2.33 mg/L (normal, 0–8 mg/L). Serum C3 and C4 were 0.62 g/L (normal, 0.85–1.93 g/L) and 0.08 g/L (normal, 0.12–0.36 g/L), respectively. ANA was positive (1:100) and anti-dsDNA showed positive (+ + +) (Table 1). According to the EULAR/ACR 2019 SLE classification standard, the patient was diagnosed with SLE. The whole exome sequencing of venous blood was performed on the child and his parents. A heterozygous missense mutation c.2314G > A (p.G772S, Fig. 1, Table 2) of PIK3CD gene was revealed in this patient and his father. His father has not any clinical symptoms. His father has the identical mutation to the child. However, there is not any family history of APDS1 or of other immune deficiency (Fig. 2). Subsequently, tacrolimus was discontinued and sirolimus was started. Sirolimus level was maintained approximately 4 ng/mL at a dose of 1 mg/m2. Upon treatment, thrombocytopenia significantly improved and maintained stability. Currently, the patient has shown no signs of complications.

Sanger sequencing images of the patient 2 and his parents. Sanger sequencing chromatogram of PIK3CD showed c.2314G > A heterozygous mutation in patient 2 and his father had the same site mutation. The arrows indicated the site of PIK3CD gene mutation