Acute induction of IFNα is responsible for the attenuation of the live measles vaccine

The live-attenuated measles vaccine (LAMV) protects against measles in a safe, efficacious and durable manner. However, the mechanism by which LAMV is attenuated is not well understood, contributing to the increased vaccine hesitancy that has caused a worldwide resurgence of measles. Here, we provide a molecular model of attenuation that implicates increased host innate immune responses and increased LAMV susceptibility to those responses. Our study leverages both in vitro and in vivo models of infection to find that acute induction of innate immunity in immune cells is the critical determinant of LAMV attenuation. We show that LAMV, in contrast to pathogenic wild type measles virus (MeV), causes a strong IFNα response in primary human peripheral blood mononuclear cells, which restricts viral replication and spread in a post-entry manner. LAMV can enter, transcribe viral mRNA and translate viral proteins in immune cells, but is not able to form infectious particles and spread from cell to cell. Specifically, we find that mutations conserved across LAMV strains in the P/V/C and H genes are responsible for the induction of IFNα and the restriction of infectious virus production. In PBMC cultures and in rhesus macaques, LAMV infection also results in the acute induction of proinflammatory cytokines that likely play a role in the immunogenicity of the vaccine. Overall, our study provides a virological and immunological framework for LAMV attenuation which can be leveraged in repurposing the LAMV backbone and in future vaccine design.