Differential properties of NS1 glycoproteins in West Nile and Usutu viruses.

West Nile virus (WNV) and Usutu virus (USUV) are neurotropic orthoflaviviruses of the Flaviviridae family, transmitted primarily by Culex mosquitoes and maintained in enzootic cycles involving birds. While WNV is a well-established human pathogen causing hundreds of neuroinvasive cases annually in Europe, USUV has emerged more recently, with fewer documented human infections but increasing evidence of neurovirulence. The viral nonstructural protein 1 (NS1) plays a central role in orthoflavivirus pathogenesis by modulating host immune responses, disrupting endothelial barrier integrity, and facilitating viral dissemination. However, the functional and biochemical properties of NS1 from WNV and USUV remain poorly characterized. We combined in vitro, in vivo, and clinical approaches to compare NS1 secretion, stability, and its impact on blood-brain barrier. Our results show that WNV NS1 is secreted at significantly higher levels, exhibits greater thermal stability, and disrupts brain endothelial barrier integrity in vitro. In contrast, USUV NS1 is secreted less efficiently, is slightly less stable, and does not compromise blood-brain barrier integrity, despite inducing distinct transcriptional responses in brain endothelial cells. In mice, WNV infection led to higher serum NS1 levels and stronger systemic inflammation than USUV. Clinically, WNV NS1 was detected mainly in patients with neurological symptoms, whereas USUV NS1 remained undetectable in all cases. Altogether, these findings reveal differential NS1 properties between these closely related viruses, with key implications for orthoflavivirus diagnosis and neurovirulence mechanisms.