The S Genome Segment Is Sufficient to Maintain Pathogenicity in Intra-Clade Lassa Virus Reassortants in a Guinea Pig Model.

Genome reassortment in Lassa virus (LASV) has been reported in nature, but phenotypic consequences of this phenomenon are not well described. Here we characterize, both and , reassortment between 2 LASV strains: the prototypic 1976 Josiah strain and a more recently isolated 2015 Liberian strain. analysis showed that although cis- and trans-acting elements of viral RNA synthesis were compatible between strains, reassortants demonstrated different levels of viral replication. These differences were also apparent , as reassortants varied in pathogenicity in the guinea pig model of LASV infection. The reassortant variant containing the Josiah S segment retained the virulence of the parental Josiah strain, but the reassortant variant containing the S segment of the Liberian isolate was highly attenuated compared to both parental strains. Contrary to observations in reassortants between LASV and other arenavirus species, which suggest that L segment-encoded factors are responsible for virulence, these studies highlight a role for S segment-encoded virulence factors in disease, and also suggest that inefficient interactions between proteins of heterologous strains may limit the prevalence of reassortant LASV variants in nature.