Middle East respiratory syndrome coronavirus (MERS-CoV) causes severe respiratory infections that can be life-threatening. To establish an infection and spread, MERS-CoV, like most other viruses, must navigate through an intricate network of antiviral host responses. Besides the well-known type I interferon (IFN-α/β) response, the protein kinase R (PKR)-mediated stress response is being recognized as an important innate response pathway. Upon detecting viral dsRNA, PKR phosphorylates eIF2α, leading to the inhibition of cellular and viral translation and the formation of stress granules (SGs), which are increasingly recognized as platforms for antiviral signaling pathways. It is unknown whether cellular infection by MERS-CoV activates the stress response pathway or whether the virus has evolved strategies to suppress this infection-limiting pathway. Here, we show that cellular infection with MERS-CoV does not lead to the formation of SGs. By transiently expressing the MERS-CoV accessory proteins individually, we identified a role of protein 4a (p4a) in preventing activation of the stress response pathway. Expression of MERS-CoV p4a impeded dsRNA-mediated PKR activation, thereby rescuing translation inhibition and preventing SG formation. In contrast, p4a failed to suppress stress response pathway activation that is independent of PKR and dsRNA. MERS-CoV p4a is a dsRNA binding protein. Mutation of the dsRNA binding motif in p4a disrupted its PKR antagonistic activity. By inserting p4a in a picornavirus lacking its natural PKR antagonist, we showed that p4a exerts PKR antagonistic activity also under infection conditions. However, a recombinant MERS-CoV deficient in p4a expression still suppressed SG formation, indicating the expression of at least one other stress response antagonist. This virus also suppressed the dsRNA-independent stress response pathway. Thus, MERS-CoV interferes with antiviral stress responses using at least two different mechanisms, with p4a suppressing the PKR-dependent stress response pathway, probably by sequestering dsRNA. MERS-CoV p4a represents the first coronavirus stress response antagonist described. Human coronaviruses generally cause relatively mild respiratory disease. In the past 15 years, the world has witnessed the emergence of two coronaviruses with high mortality rates in humans; severe acute respiratory syndrome coronavirus (SARS-CoV) in 2002 and Middle East respiratory syndrome coronavirus (MERS-CoV) in 2012, both originating from animal reservoirs. Successful infection of a host not only depends on the presence of an appropriate receptor but also on the ability of a virus to evade innate antiviral host responses, which constitute the first line of defense against invading viruses. MERS-CoV has been reported to actively suppress the IFN-α/β response, but it is unknown whether it also interferes with another important innate antiviral response, the stress response pathway. Activation of this pathway by a kinase, PKR, curtails virus infection by shutting off cellular and viral protein synthesis. To date, no coronavirus protein has been recognized to suppress the stress response pathway. Here, we show that the accessory protein 4a of MERS-CoV is a potent stress antagonist that prevents PKR activation by sequestering its ligand, dsRNA. This finding furthers our understanding of the molecular mechanism used by MERS-CoV to evade infection-limiting antiviral host responses and may provide new avenues for therapeutic intervention.

Middle East respiratory syndrome coronavirus (MERS-CoV) is a novel human coronavirus that emerged in 2012, causing severe pneumonia and acute respiratory distress syndrome (ARDS), with a case fatality rate of ~36%. When expressed in isolation, CoV accessory proteins have been shown to interfere with innate antiviral signaling pathways. However, there is limited information on the specific contribution of MERS-CoV accessory protein 4b to the repression of the innate antiviral response in the context of infection. We found that MERS-CoV 4b was required to prevent a robust NF-κB dependent response during infection. In wild-type virus infected cells, 4b localized to the nucleus, while NF-κB was retained in the cytoplasm. In contrast, in the absence of 4b or in the presence of cytoplasmic 4b mutants lacking a nuclear localization signal (NLS), NF-κB was translocated to the nucleus leading to the expression of pro-inflammatory cytokines. This indicates that NF-κB repression required the nuclear import of 4b mediated by a specific NLS. Interestingly, we also found that both in isolation and during infection, 4b interacted with α-karyopherin proteins in an NLS-dependent manner. In particular, 4b had a strong preference for binding karyopherin-α4 (KPNA4), which is known to translocate the NF-κB protein complex into the nucleus. Binding of 4b to KPNA4 during infection inhibited its interaction with NF-κB-p65 subunit. Thereby we propose a model where 4b outcompetes NF-κB for KPNA4 binding and translocation into the nucleus as a mechanism of interference with the NF-κB-mediated innate immune response. Middle East respiratory syndrome coronavirus (MERS-CoV) is a highly pathogenic human CoV that continues to cause lethal human infections, primarily in the Middle East. Virus accessory genes are potential contributors to pathology, possibly by interfering with the innate immune response. However, understanding their interactions with host proteins in the context of infection is rudimentary. Here, we provide evidence that the MERS-CoV accessory protein 4b functioned, at least in part, to prevent a robust NF-κB dependent response during infection. This effect depended on the nuclear localization of 4b, which was associated with the cytoplasmic retention of NF-κB. We show that 4b interacted with α-karyopherin proteins (importins) involved in the nuclear import of NF-κB, inhibiting the binding of α-karyopherin to NF-κB-p65 subunit. We propose that 4b contributes to the evasion of the innate immune response by binding α-karyopherin proteins, leading to the inhibition of NF-κB nuclear transport.

The emergence of SARS-CoV-2 in late 2019 led to the COVID-19 pandemic all over the world. When the virus was first isolated and its genome was sequenced in the early months of 2020, the efforts to develop a vaccine began. Based on prior well-known knowledge about coronavirus, the SARS-CoV-2 spike (S) protein was selected as the main target. Currently, more than one hundred vaccines are being investigated and several of them are already authorized by medical agencies. This review summarizes and compares the current knowledge about main approaches for vaccine development, focusing on those authorized and specifically their immunogenicity, efficacy preventing severe disease, adverse side effects, protection, and ability to cope with emergent SARS-CoV-2 variants.