A novel dominant-negative variant of IRF8 in a mother and son: Clinical, phenotypic and biological characteristics.

BACKGROUND: The few reported patients with pathogenic IRF8 variants have manifested 2 distinct phenotypes: (1) an autosomal recessive severe immunodeficiency with significant neutrophilia and absence of or significant decrease in monocytes and dendritic cells and (2) a dominant-negative form with only a decrease in conventional type 2 dendritic cells (cDC2s) and susceptibility to mycobacterial disease. OBJECTIVES: Genetic testing of a child with persistent EBV viremia identified a novel IRF8 variant: c.1279dupT (p.∗427Leuext∗42). The variant was also found in his mother, who was subsequently diagnosed with a human papillomavirus-positive tumor. We sought to examine the pathogenicity of the identified IRF8 variant and its phenotypic and functional characteristics. METHODS: Immunophenotypic and functional flow cytometry, natural killer cell cytotoxicity, matrix-assisted laser desorption/ionization-time of flight mass spectrometry, T-cell receptor Vβ spectratyping, Sanger sequencing, RNA-sequencing, Olink proteomics, immunoblotting, molecular cloning, dual-luciferase reporter assay, immunofluorescence microscopy, and image analysis. RESULTS: The 42 amino acid C-terminal extension of the mutant IRF8 (∼4 kDa heavier than wild type) impaired IRF8 nuclear localization in a dominant-negative manner and inhibited IRF1/IRF8-mediated transcriptional activities. Both patients had a decrease in plasmacytoid dendritic cells (pDCs) and in cDC1s, a mild neutrophilia and a mild monocytosis. Their existing pDCs had impaired IFN-α production. On TLR engagement, the production of IL-1β, IL-6, IL-10, and IL-12 by their monocytes and of IL-12 by their myeloid DCs were within normal limits. Natural killer cell development and cytolytic activity were essentially normal. RNA-sequencing and proteomic approaches bolstered the phenotypic and functional findings. CONCLUSIONS: This study defines the pathogenic nature of the c.1279dupT (p.∗427Leuext∗42) IRF8 variant, determines its dominant-negative mechanism of action, and broadens the existing phenotype of human IRF8 immunodeficiency.