A novel experimental system reveals immunoregulatory responses as mediators of persistent orthohantavirus infections in a rodent reservoir host

Orthohantaviruses are globally emerging zoonotic pathogens. Human infections are characterized by an overt immune response that is efficient at counteracting virus replication but can also cause severe tissue damage. In contrast, orthohantavirus infections in rodent reservoir hosts are persistent and asymptomatic. The mechanisms facilitating asymptomatic virus persistence in reservoir hosts are not well understood but could help to guide therapeutic strategies for human infections. Here we report on a study using in vivo and in vitro experiments to investigate immune responses associated with persistent Puumala orthohantavirus (PUUV) infections in the bank vole (Myodes glareolus), its reservoir host. We examined adaptive cellular and humoral responses by quantifying changes in T-cell related gene expression in the spleen and immunoglobulin (Ig) responses in blood, respectively. Since existing Vero E6-cell adapted hantavirus isolates have been demonstrated to have lost their wild-type infection characteristics, infections were conducted with a novel PUUV strain isolated on a bank vole cell line. Whole virus genome sequencing revealed that only minor sequence changes occurred during the isolation process, and critically, experimental infections of bank voles with the new isolate resembled natural infections. In vitro infection of bank vole splenocytes with the novel isolate demonstrated that PUUV promotes immunoregulatory responses by inducing interleukin-10, a cytokine strongly associated with chronic viral infections. A delayed virus-specific humoral response occurred in experimentally infected bank voles, which is likely to allow for initial virus replication and the establishment of persistent infections. These results suggest that host immunoregulation facilitates persistent orthohantavirus infections in reservoir hosts.\n\nImportanceOrthohantaviruses are a group of global pathogens that regularly spillover from rodent reservoirs into humans and can cause severe disease. Conversely, infections in reservoir hosts do not cause obvious adverse effects. The mechanisms responsible for persistent asymptomatic reservoir infections are unknown, and progress has been hindered by the absence of an adequate experimental system. Knowledge on these mechanisms could help provide strategies to treat human infections. We developed and validated an experimental system based on an orthohantavirus isolated in cells of its vole reservoir host. Using animal and cell culture experiments in the reservoir host system, we demonstrated that infection suppresses immunity in the vole reservoir via specific mechanisms, likely allowing the virus to take hold and preventing immune responses that can cause self-damage.