Human inherited complete STAT2 deficiency underlies inflammatory viral diseases

**Background:** Human type I and III interferons (IFNs) are critical for antiviral immunity. STAT2 is a key transcription factor activated by these IFNs, forming the ISGF3 complex with STAT1 and IRF9 to induce interferon-stimulated genes (ISGs). Previous reports identified 11 patients with autosomal recessive (AR) STAT2 deficiency, but the full clinical spectrum and underlying mechanisms remained incompletely defined. This study aimed to comprehensively characterize the genetic, immunological, and clinical features of AR complete STAT2 deficiency.

**Methods:** The authors studied 23 patients from 10 kindreds across 7 countries (UK, Belgium, France, Morocco, Saudi Arabia, Honduras, Russia), including 11 previously reported patients and 12 new cases. Genetic analysis identified biallelic loss-of-function STAT2 variants. Functional studies included immunoblotting for STAT2 protein expression and phosphorylation, quantitative RT-PCR for ISG induction in transfected cells and patient-derived lymphoblastoid cell lines (LCLs), and in vitro HSV-1 infection assays in SV40 fibroblasts. Single-cell RNA sequencing (scRNA-Seq) was performed on PBMCs from a STAT2-deficient patient and an IFNAR2-deficient patient, both at baseline and after IFN-α stimulation, compared with healthy controls. Bulk RNA-Seq was also performed on whole blood from a STAT2-deficient patient during acute mild COVID-19 and at baseline.

**Key Results:** Seven single-nucleotide substitutions, 2 small deletions, and 2 large deletions were identified, all predicted to be loss-of-function. All mutant alleles resulted in absent or truncated STAT2 protein and absent phosphorylation after IFN-α stimulation, with impaired ISG induction. Clinically, 12 of 17 patients who received live attenuated viral vaccines (LAV) developed severe adverse reactions, including disseminated measles, atypical Kawasaki disease, encephalitis, and hemophagocytic lymphohistiocytosis (HLH). Ten of 23 patients suffered severe natural viral infections: 6 had PCR-proven influenza A pneumonia (2 with ARDS requiring mechanical ventilation), 2 had enteroviral meningitis, 1 had HSV-1 encephalitis (fatal), and 1 had severe COVID-19 pneumonia (75% lung involvement). Eight patients died in early childhood (range 2 months to 7 years), 6 from heart failure during febrile illness without identified pathogen, 1 from fulminant hepatitis, and 1 from HSV-1 encephalitis. Overall mortality was 35% in early childhood. scRNA-Seq revealed impaired basal ISG expression across all 24 leukocyte subsets in STAT2- and IFNAR2-deficient patients, with upregulation of TNF/NF-κB signaling in 8 subsets. After IFN-α stimulation, STAT2-deficient cells showed profound impairment of ISG induction, with minimal residual response in monocytes. Prolonged STAT1 phosphorylation was observed in STAT2-deficient LCLs after IFN-α stimulation, but without increased expression of IFN-γ-dependent genes IRF1 and ICAM1. USP18 expression was not induced in STAT2-deficient LCLs after IFN-α stimulation.

**Clinical Implications:** AR complete STAT2 deficiency is a newly characterized inborn error of immunity that underlies severe viral diseases and adverse reactions to LAVs, with substantial early childhood mortality. The phenotype overlaps with deficiencies of IFNAR1, IFNAR2, STAT1, and IRF9, but STAT2 deficiency appears less severe than complete STAT1 deficiency, with 15 of 23 patients surviving (median age 13 years). The high prevalence of influenza pneumonia (86% of PCR-proven cases) and the occurrence of critical COVID-19 pneumonia highlight the importance of type I IFN immunity for respiratory viruses. The hyperinflammation observed in these patients is likely driven by uncontrolled viral replication due to impaired IFN responses, rather than by specific signaling abnormalities. Early diagnosis is crucial, as antiviral prophylaxis (acyclovir, immunoglobulins) may prevent severe infections. Hematopoietic stem cell transplantation may be considered but its risks must be weighed against the natural history of the disease. These findings also have implications for vaccine safety, as LAVs (MMR, VZV, YFV) are contraindicated in these patients.