Recombinant Receptor Binding Domain Protein Induces Partial Protective Immunity in Rhesus Macaques Against Middle East Respiratory Syndrome Coronavirus Challenge()

Lan, Jiaming; Yao, Yanfeng; Deng, Yao; Chen, Hong; Lu, Guangwen; Wang, Wen; Bao, Linlin; Deng, Wei; Wei, Qiang; Gao, George F.; Qin, Chuan; Tan, Wenjie

The MERS virus, which emerged in 2012, is now considered a threat to global public health. Developing an effective vaccine is important to control MERS-CoV infection. To date, virus-vector vaccines, such as recombinant modified vaccinia virus Ankara (MVA), which expresses full-length MERS-CoV spike protein (MVA-MERS-S), and recombinant adenoviral vectors encoding the full-length MERS-CoV S protein (Ad5.MERS-S) and the S1 extracellular domain of S protein (Ad5.MERS-S1), have been developed and tested for their ability to induce virus-neutralising antibodies in mice (Kim et al., 2014; Song et al., 2013; Volz et al., 2015) . Besides, the rRBD subunit vaccine confers a highly potent neutralising antibody and T-cell immune response in mice . However, few studies have been assessed in response to MERS-CoV challenge to verify their efficacy, due to a lack of animal models. For example, mice (Coleman et al., 2014; Scobey et al., 2013 ), hamsters (de Wit et al., 2013a and ferrets had not been proved to infect with MERS-CoV naturally. Recently, a small animal model of MERS-CoV infection was developed by transducing mice with an adenovirus vector expressing human DPP4 . However, MERS-CoV infection in this model is highly dependent on the transduction of cells and the level of DPP4 expression from the adenovirus vector, and therefore does not necessarily reflect the natural disease process (Falzarano et al., 2014) . More recently a transgenic mouse model of MERS-CoV has been published (Agrawal et al., 2015) . However, in this model, all cells of the mouse express human DPP4. This kind of non-physiological expression patterns resulted in extensive brain infection of MERS-CoV and rapidly succumbs to infection in mice. Moreover, a novel humanized mouse model of MERS-CoV infection was developed, though the mice were not acquired by routine technology (Pascal et al., 2015) . Encouragingly, rhesus macaque models are naturally permissive to MERS-CoV disease, and more closely mimic the disease course in human patients (de Wit et al., 2013a,b; Munster et al., 2013; Yao et al., 2013) . Furthermore, the effects of interferon-α2b and ribavirin treatment have been evaluated in these models (Falzarano et al., 2013a,b) . Therefore, we herein assessed rRBD vaccine efficacy in a NHP model. Similar as the rRBD protein of SARS-CoV which induced high titres of protective anti-RBD antibody responses in NHPs (Wang et al., 2012) , the rRBD vaccination of MERS-CoV induced effective IgG and neutralisation antibodies in our rhesus macaque model. Besides, the induced IgG and neutralisation antibodies maintained for at least 17 weeks without obvious attenuation. Following a subsequent boost, antibody titres reached almost their previous peak level. There was no evident difference for induction of the humoral immunity between high and low dose groups. However, high-dose vaccination induced a stronger T-cell response than the low-dose vaccination. Following MERS-CoV challenge, rhesus macaques exhibited a transient lower respiratory tract infection, in accordance with previous reports (de Wit et al., 2013a,b; Munster et al., 2013; Yao et al., 2013) . Although infection was also detected in rRBDimmunised animals, clinical signs were alleviated. Pathological changes in the lungs and tracheas of rRBD-immunised animals indicated reduced, but not fully absent lesions. Using qRT-PCR assay, viral loads in the lungs, trachea and oropharyngeal swabs of the rRBD vaccination groups were de