Oropouche Virus: Comparative Study of Two Strains Reveals Distinct Innate Immunity Modulation

Oropouche virus (OROV), an Orthobunyavirus of the Peribunyaviridae family, is usually transmitted to humans by biting midges, while, to date, cases of human-to-human transmission have not been reported. Although OROV's clinical manifestation is usually characterized by mild symptoms, cases of Central Nervous System (CNS) infections have been reported. The non-structural protein NSs of many bunyaviruses and phleboviruses acts as a major virulence factor. Previous studies have demonstrated that OROV NSs also behaves as a suppressor of the host IFN-α/β responses. Furthermore, OROV NSs protein promotes cellular RPB1 proteasomal degradation to suppress RNA synthesis. In the present work, we perform a comparative study on two OROV strains (BeAn19991 and IRCCS-SCDC_1/2024) to further characterize the molecular mechanisms by which OROV NSs inhibits IFN-β expression. Our results demonstrate that the NSs protein mediates a reduction in IFN-β promoter activation, hindering RIG-I and IRF-3, therefore acting on multiple steps of the IFN-β signaling pathway. However, this suppression seems to reside in the blockage of RPB1 rather than a direct effect on those mediators. Moreover, we investigated the behavior of the two virus strains in a human glioblastoma cell line derived from brain (DBTRG.05MG cells) and fibroblasts (MRC-5 cells), observing different replication kinetics, degradation activity towards RPB1, and strength/timing of IFN-β modulation.