Characterized animal models are needed for studying the pathogenesis of and evaluating medical countermeasures for persisting Middle East respiratory syndrome-coronavirus (MERS-CoV) infections. Here, we further characterized a lethal transgenic mouse model of MERS-CoV infection and disease that globally expresses human CD26 (hCD26)/DPP4. The 50% infectious dose (ID₅₀) and lethal dose (LD₅₀) of virus were estimated to be <1 and 10 TCID₅₀ of MERS-CoV, respectively. Neutralizing antibody developed in the surviving mice from the ID₅₀/LD₅₀ determinations, and all were fully immune to challenge with 100 LD₅₀ of MERS-CoV. The tissue distribution and histopathology in mice challenged with a potential working dose of 10 LD₅₀ of MERS-CoV were subsequently evaluated. In contrast to the overwhelming infection seen in the mice challenged with 10⁵ LD₅₀ of MERS-CoV, we were able to recover infectious virus from these mice only infrequently, although quantitative reverse transcription-PCR (qRT-PCR) tests indicated early and persistent lung infection and delayed occurrence of brain infection. Persistent inflammatory infiltrates were seen in the lungs and brain stems at day 2 and day 6 after infection, respectively. While focal infiltrates were also noted in the liver, definite pathology was not seen in other tissues. Finally, using a receptor binding domain protein vaccine and a MERS-CoV fusion inhibitor, we demonstrated the value of this model for evaluating vaccines and antivirals against MERS. As outcomes of MERS-CoV infection in patients differ greatly, ranging from asymptomatic to overwhelming disease and death, having available both an infection model and a lethal model makes this transgenic mouse model relevant for advancing MERS research.

IMPORTANCE Fully characterized animal models are essential for studying pathogenesis and for preclinical screening of vaccines and drugs against MERS-CoV infection and disease. When given a high dose of MERS-CoV, our transgenic mice expressing hCD26/DPP4 viral receptor uniformly succumbed to death within 6 days, making it difficult to evaluate host responses to infection and disease. We further characterized this model by determining both the ID₅₀ and the LD₅₀ of MERS-CoV in order to establish both an infection model and a lethal model for MERS and followed this by investigating the antibody responses and immunity of the mice that survived MERS-CoV infection. Using the estimated LD₅₀ and ID₅₀ data, we dissected the kinetics of viral tissue distribution and pathology in mice challenged with 10 LD₅₀ of virus and utilized the model for preclinical evaluation of a vaccine and drug for treatment of MERS-CoV infection. This further-characterized transgenic mouse model will be useful for advancing MERS research.