Adaptive evolution influences the infectious dose of MERS-CoV necessary to achieve severe respiratory disease

We recently established a mouse model (288-330(+/+)) that developed acute respiratory disease resembling human pathology following infection with a high dose (5 x 10(6) PFU) of mouse-adapted MERS-CoV (icMERSmal). Although this high dose conferred fatal respiratory disease in mice, achieving similar pathology at lower viral doses may more closely reflect naturally acquired infections. Through continued adaptive evolution of icMERSmal we generated a novel mouse-adapted MERS-CoV (maM35c4) capable of achieving severe respiratory disease at doses between 10(3) and 10(5) PFU. Novel mutations were identified in the maM35c4 genome that may be responsible for eliciting etiologies of acute respiratory distress syndrome at 10-1000 fold lower viral doses. Importantly, comparative genetics of the two mouse-adapted MERS strains allowed us to identify specific mutations that remained fixed through an additional 20 cycles of adaptive evolution. Our data indicate that the extent of MERS-CoV adaptation determines the minimal infectious dose required to achieve severe respiratory disease.