Structural modeling and functional characterization of a novel gain-of-function TLR8 variant causing severe inflammatory syndrome

Nikolaos T Skenteris ¹, Elisa Luttermann ², Sanjana Nair ^{3 4}, Ioannis Evangelakos ⁵, Maria Pujantell ^{1 6}, Marie Eggers ^{7 8}, Fabian Hausmann ^{3 4}, Marleen Bérouti ⁹, Benedetta Padoan ¹, Felix J Flomm ¹, Janna M Claussen ¹, Benjamin Grünhagel ¹, Anika Salfelder ¹⁰, Brigitte Beifuss ¹⁰, Saskia Biskup ^{10 11}, Patrick Blümke ¹², Katrin Rading ⁵, Heike Hildebrandt ¹, Urte Matschl ¹, Silke Giesemann-Jansen ¹, Jana Hennesen ¹, Viacheslav O Nikolaev ¹³, Michael Kutsche ⁵, Christian Kubisch ⁵, Friedrich Koch-Nolte ⁶, Nicola M Tomas ^{7 8 14}, Eva Tolosa ^{6 15}, Marc Lütgehetmann ¹⁶, Felix R Stahl ⁶, Veit Hornung ^{9 17}, Madeleine J Bunders ^{1 7 14 15}, Christian Schlein ⁵, Maya Topf ^{3 4 18}, Ina Kötter ^{2 7}, Marcus Altfeld ^{1 6 19}

With the increasing use of genetic sequencing to investigate inborn errors of immunity, rare variants are frequently identified, yet their clinical relevance often remains uncertain. Establishing pathogenicity requires a multidisciplinary approach that integrates genetic, structural, functional, and clinical data. Here, we used such a strategy to investigate a previously unreported hemizygous missense variant – alanine (A) to threonine (T) at residue 518 – in Toll-like receptor 8 (TLR8), identified in 2 male siblings with recurrent infections and systemic inflammation, characterized by a proinflammatory immune signature and B cell dysregulation. Functional studies showed that the TLR8 A518T variant enhanced NF-κB activation and increased secretion of proinflammatory cytokines compared with WT TLR8 upon stimulation, consistent with a gain-of-function effect. Protein degradation and turnover assays revealed reduced abundance of the mutant TLR8 protein due to faster turnover and increased proteasomal degradation. Computational modeling predicted enhanced structural stabilization of the active TLR8 homodimer interface via additional water-mediated hydrogen bonds introduced by the A518T substitution. Together, these findings integrating structural modeling with functional assays identify a novel TLR8 ligand-specific gain-of-function mutation resulting in complex immunopathology in 2 siblings.