Infection of the resultant double transgenic mice (C57BL/6 hDPP4 RAG1–/–) with two different mouse-adapted MERS-CoV viruses resulted in diminished pathogenesis and significantly increased viral lung titers 7 dpi implicating T and/or B cells as important for both the control of virus replication and driving severe disease. Here, RAG1 is linked to infection.