Temporal maturation of neutralizing antibodies in COVID-19 convalescent individuals improves potency and breadth to circulating SARS-CoV-2 variants.

Antibody titers against SARS-CoV-2 slowly wane over time. Here, we examined how time affects antibody potency. To assess the impact of antibody maturation on durable neutralizing activity against original SARS-CoV-2 and emerging variants of concern (VOCs), we analyzed receptor binding domain (RBD)-specific IgG antibodies in convalescent plasma taken 1-10 months after SARS-CoV-2 infection. Longitudinal evaluation of total RBD IgG and neutralizing antibody revealed declining total antibody titers but improved neutralization potency per antibody to original SARS-CoV-2, indicative of antibody response maturation. Neutralization assays with authentic viruses revealed that early antibodies capable of neutralizing original SARS-CoV-2 had limited reactivity toward B.1.351 (501Y.V2) and P.1 (501Y.V3) variants. Antibodies from late convalescents exhibited increased neutralization potency to VOCs, suggesting persistence of cross-neutralizing antibodies in plasma. Thus, maturation of the antibody response to SARS-CoV-2 potentiates cross-neutralizing ability to circulating variants, suggesting that declining antibody titers may not be indicative of declining protection.

A SARS-CoV-2 antibody broadly neutralizes SARS-related coronaviruses and variants by coordinated recognition of a virus-vulnerable site.

Potent neutralizing SARS-CoV-2 antibodies often target the spike protein receptor-binding site (RBS), but the variability of RBS epitopes hampers broad neutralization of multiple sarbecoviruses and drifted viruses. Here, using humanized mice, we identified an RBS antibody with a germline VH gene that potently neutralized SARS-related coronaviruses, including SARS-CoV and SARS-CoV-2 variants. X-ray crystallography revealed coordinated recognition by the heavy chain of non-RBS conserved sites and the light chain of RBS with a binding angle mimicking the angiotensin-converting enzyme 2 (ACE2) receptor. The minimum footprints in the hypervariable region of RBS contributed to the breadth of neutralization, which was enhanced by immunoglobulin G3 (IgG3) class switching. The coordinated binding resulted in broad neutralization of SARS-CoV and emerging SARS-CoV-2 variants of concern. Low-dose therapeutic antibody treatment in hamsters reduced the virus titers and morbidity during SARS-CoV-2 challenge. The structural basis for broad neutralizing activity may inform the design of a broad spectrum of therapeutics and vaccines.

Characterization of SARS-CoV-2 Omicron BA.4 and BA.5 isolates in rodents.

The BA.2 sublineage of the SARS-CoV-2 Omicron variant has become dominant in most countries around the world; however, the prevalence of BA.4 and BA.5 is increasing rapidly in several regions. BA.2 is less pathogenic in animal models than previously circulating variants of concern1-4. Compared with BA.2, however, BA.4 and BA.5 possess additional substitutions in the spike protein, which play a key role in viral entry, raising concerns that the replication capacity and pathogenicity of BA.4 and BA.5 are higher than those of BA.2. Here we have evaluated the replicative ability and pathogenicity of BA.4 and BA.5 isolates in wild-type Syrian hamsters, human ACE2 (hACE2) transgenic hamsters and hACE2 transgenic mice. We have observed no obvious differences among BA.2, BA.4 and BA.5 isolates in growth ability or pathogenicity in rodent models, and less pathogenicity compared to a previously circulating Delta (B.1.617.2 lineage) isolate. In addition, in vivo competition experiments revealed that BA.5 outcompeted BA.2 in hamsters, whereas BA.4 and BA.2 exhibited similar fitness. These findings suggest that BA.4 and BA.5 clinical isolates have similar pathogenicity to BA.2 in rodents and that BA.5 possesses viral fitness superior to that of BA.2.

Macacine alphaherpesvirus 1 (B Virus) Infection in Humans, Japan, 2019.

Two human patients with Macacine alphaherpesvirus 1 infection were identified in Japan in 2019. Both patients had worked at the same company, which had a macaque facility. The rhesus-genotype B virus genome was detected in cerebrospinal fluid samples from both patients.

Impact of Reinfection with SARS-CoV-2 Omicron Variants in Previously Infected Hamsters.

The diversity of SARS-CoV-2 mutations raises the possibility of reinfection of individuals previously infected with earlier variants, and this risk is further increased by the emergence of the B.1.1.529 Omicron variant. In this study, we used an in vivo, hamster infection model to assess the potential for individuals previously infected with SARS-CoV-2 to be reinfected with Omicron variant and we also investigated the pathology associated with such infections. Initially, Syrian hamsters were inoculated with a lineage A, B.1.1.7, B.1.351, B.1.617.2 or a subvariant of Omicron, BA.1 strain and then reinfected with the BA.1 strain 5 weeks later. Subsequently, the impact of reinfection with Omicron subvariants (BA.1 and BA.2) in individuals previously infected with the BA.1 strain was examined. Although viral infection and replication were suppressed in both the upper and lower airways, following reinfection, virus-associated RNA was detected in the airways of most hamsters. Viral replication was more strongly suppressed in the lower respiratory tract than in the upper respiratory tract. Consistent amino acid substitutions were observed in the upper respiratory tract of infected hamsters after primary infection with variant BA.1, whereas diverse mutations appeared in hamsters reinfected with the same variant. Histopathology showed no acute pneumonia or disease enhancement in any of the reinfection groups and, in addition, the expression of inflammatory cytokines and chemokines in the airways of reinfected animals was only mildly elevated. These findings are important for understanding the risk of reinfection with new variants of SARS-CoV-2. IMPORTANCE The emergence of SARS-CoV-2 variants and the widespread use of COVID-19 vaccines has resulted in individual differences in immune status against SARS-CoV-2. A decay in immunity over time and the emergence of variants that partially evade the immune response can also lead to reinfection. In this study, we demonstrated that, in hamsters, immunity acquired following primary infection with previous SARS-CoV-2 variants was effective in preventing the onset of pneumonia after reinfection with the Omicron variant. However, viral infection and multiplication in the upper respiratory tract were still observed after reinfection. We also showed that more diverse nonsynonymous mutations appeared in the upper respiratory tract of reinfected hamsters that had acquired immunity from primary infection. This hamster model reveals the within-host evolution of SARS-CoV-2 and its pathology after reinfection, and provides important information for countermeasures against diversifying SARS-CoV-2 variants.

Significant role of host sialylated glycans in the infection and spread of severe acute respiratory syndrome coronavirus 2.

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has been transmitted across all over the world, in contrast to the limited epidemic of genetically- and virologically-related SARS-CoV. However, the molecular basis explaining the difference in the virological characteristics among SARS-CoV-2 and SARS-CoV has been poorly defined. Here we identified that host sialoglycans play a significant role in the efficient spread of SARS-CoV-2 infection, while this was not the case with SARS-CoV. SARS-CoV-2 infection was significantly inhibited by α2-6-linked sialic acid-containing compounds, but not by α2-3 analog, in VeroE6/TMPRSS2 cells. The α2-6-linked compound bound to SARS-CoV-2 spike S1 subunit to competitively inhibit SARS-CoV-2 attachment to cells. Enzymatic removal of cell surface sialic acids impaired the interaction between SARS-CoV-2 spike and angiotensin-converting enzyme 2 (ACE2), and suppressed the efficient spread of SARS-CoV-2 infection over time, in contrast to its least effect on SARS-CoV spread. Our study provides a novel molecular basis of SARS-CoV-2 infection which illustrates the distinctive characteristics from SARS-CoV.

Characterization of a new SARS-CoV-2 variant that emerged in Brazil.

The spike (S) protein of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) plays a key role in viral infectivity. It is also the major antigen stimulating the host's protective immune response, specifically, the production of neutralizing antibodies. Recently, a new variant of SARS-CoV-2 possessing multiple mutations in the S protein, designated P.1, emerged in Brazil. Here, we characterized a P.1 variant isolated in Japan by using Syrian hamsters, a well-established small animal model for the study of SARS-CoV-2 disease (COVID-19). In hamsters, the variant showed replicative abilities and pathogenicity similar to those of early and contemporary strains (i.e., SARS-CoV-2 bearing aspartic acid [D] or glycine [G] at position 614 of the S protein). Sera and/or plasma from convalescent patients and BNT162b2 messenger RNA vaccinees showed comparable neutralization titers across the P.1 variant, S-614D, and S-614G strains. In contrast, the S-614D and S-614G strains were less well recognized than the P.1 variant by serum from a P.1-infected patient. Prior infection with S-614D or S-614G strains efficiently prevented the replication of the P.1 variant in the lower respiratory tract of hamsters upon reinfection. In addition, passive transfer of neutralizing antibodies to hamsters infected with the P.1 variant or the S-614G strain led to reduced virus replication in the lower respiratory tract. However, the effect was less pronounced against the P.1 variant than the S-614G strain. These findings suggest that the P.1 variant may be somewhat antigenically different from the early and contemporary strains of SARS-CoV-2.

Role of VP30 Phosphorylation in Ebola Virus Nucleocapsid Assembly and Transport.

Ebola virus (EBOV) VP30 regulates viral genome transcription and replication by switching its phosphorylation status. However, the importance of VP30 phosphorylation and dephosphorylation in other viral replication processes such as nucleocapsid and virion assembly is unclear. Interestingly, VP30 is predominantly dephosphorylated by cellular phosphatases in viral inclusions, while it is phosphorylated in the released virions. Thus, uncertainties regarding how VP30 phosphorylation in nucleocapsids is achieved and whether VP30 phosphorylation provides any advantages in later steps in viral replication have arisen. In the present study, to characterize the roles of VP30 phosphorylation in nucleocapsid formation, we used electron microscopic analyses and live cell imaging systems. We identified VP30 localized to the surface of protrusions surrounding nucleoprotein (NP)-forming helical structures in the nucleocapsid, suggesting the involvement in assembly and transport of nucleocapsids. Interestingly, VP30 phosphorylation facilitated its association with nucleocapsid-like structures (NCLSs). On the contrary, VP30 phosphorylation does not influence the transport characteristics and NCLS number leaving from and coming back into viral inclusions, indicating that the phosphorylation status of VP30 is not a prerequisite for NCLS departure. Moreover, the phosphorylation status of VP30 did not cause major differences in nucleocapsid transport in authentic EBOV-infected cells. In the following budding step, the association of VP30 and its phosphorylation status did not influence the budding efficiency of virus-like particles. Taken together, it is plausible that EBOV may utilize the phosphorylation of VP30 for its selective association with nucleocapsids, without affecting nucleocapsid transport and virion budding processes. IMPORTANCE Ebola virus (EBOV) causes severe fevers with unusually high case fatality rates. The nucleocapsid provides the template for viral genome transcription and replication. Thus, understanding the regulatory mechanism behind its formation is important for the development of novel therapeutic approaches. Previously, we established a live-cell imaging system based on the ectopic expression of viral fluorescent fusion proteins, allowing the visualization and characterization of intracytoplasmic transport of nucleocapsid-like structures. EBOV VP30 is an essential transcriptional factor for viral genome synthesis, and, although its role in viral genome transcription and replication is well understood, the functional importance of VP30 phosphorylation in assembly of nucleocapsids is still unclear. Our work determines the localization of VP30 at the surface of ruffled nucleocapsids, which differs from the localization of polymerase in EBOV-infected cells. This study sheds light on the novel role of VP30 phosphorylation in nucleocapsid assembly, which is an important prerequisite for virion formation.

Reverse Genetics System for Heartland Bandavirus: NSs Protein Contributes to Heartland Bandavirus Virulence.

Heartland bandavirus (HRTV), which is an emerging tick-borne virus first identified in Missouri in 2009, causes fever, fatigue, decreased appetite, headache, nausea, diarrhea, and muscle or joint pain in humans. HRTV is genetically close to Dabie bandavirus, which is the causative agent of severe fever with thrombocytopenia syndrome (SFTS) in humans and is known as SFTS virus (SFTSV). The generation of infectious HRTV entirely from cloned cDNAs has not yet been reported. The absence of a reverse genetics system for HRTV has delayed efforts to understand its pathogenesis and to generate vaccines and antiviral drugs. Here, we developed a reverse genetics system for HRTV, which employs an RNA polymerase I-mediated expression system. A recombinant nonstructural protein (NSs)-knockout HRTV (rHRTV-NSsKO) was generated. We found that NSs interrupted signaling associated with innate immunity in HRTV-infected cells. The rHRTV-NSsKO was highly attenuated, indicated by the apparent absence of symptoms in a mouse model of HRTV infection. Moreover, mice immunized with rHRTV-NSsKO survived a lethal dose of HRTV. These findings suggest that NSs is a virulence factor of HRTV and that rHRTV-NSsKO could be a vaccine candidate for HRTV. IMPORTANCE Heartland bandavirus (HRTV) is a tick-borne virus identified in the United States in 2009. HRTV causes fever, fatigue, decreased appetite, headache, nausea, diarrhea, and muscle or joint pain in humans. FDA-approved vaccines and antiviral drugs are unavailable. The lack of a reverse genetics system hampers efforts to develop such antiviral therapeutics. Here, we developed a reverse genetics system for HRTV that led to the generation of a recombinant nonstructural protein (NSs)-knockout HRTV (rHRTV-NSsKO). We found that NSs interrupted signaling associated with innate immunity in HRTV-infected cells. Furthermore, rHRTV-NSsKO was highly attenuated and immunogenic in a mouse model. These findings suggest that NSs is a virulence factor of HRTV and that rHRTV-NSsKO could be a vaccine candidate for HRTV.

A highly attenuated vaccinia virus strain LC16m8-based vaccine for severe fever with thrombocytopenia syndrome.

Severe fever with thrombocytopenia syndrome (SFTS) caused by a species Dabie bandavirus (formerly SFTS virus [SFTSV]) is an emerging hemorrhagic infectious disease with a high case-fatality rate. One of the best strategies for preventing SFTS is to develop a vaccine, which is expected to induce both humoral and cellular immunity. We applied a highly attenuated but still immunogenic vaccinia virus strain LC16m8 (m8) as a recombinant vaccine for SFTS. Recombinant m8s expressing SFTSV nucleoprotein (m8-N), envelope glycoprotein precursor (m8-GPC), and both N and GPC (m8-N+GPC) in the infected cells were generated. Both m8-GPC- and m8-N+GPC-infected cells were confirmed to produce SFTSV-like-particles (VLP) in vitro, and the N was incorporated in the VLP produced by the infection of cells with m8-N+GPC. Specific antibodies to SFTSV were induced in mice inoculated with each of the recombinant m8s, and the mice were fully protected from lethal challenge with SFTSV at both 103 TCID50 and 105 TCID50. In mice that had been immunized with vaccinia virus strain Lister in advance of m8-based SFTSV vaccine inoculation, protective immunity against the SFTSV challenge was also conferred. The pathological analysis revealed that mice immunized with m8-GPC or m8-N+GPC did not show any histopathological changes without any viral antigen-positive cells, whereas the control mice showed focal necrosis with inflammatory infiltration with SFTSV antigen-positive cells in tissues after SFTSV challenge. The passive serum transfer experiments revealed that sera collected from mice inoculated with m8-GPC or m8-N+GPC but not with m8-N conferred protective immunity against lethal SFTSV challenge in naïve mice. On the other hand, the depletion of CD8-positive cells in vivo did not abrogate the protective immunity conferred by m8-based SFTSV vaccines. Based on these results, the recombinant m8-GPC and m8-N+GPC were considered promising vaccine candidates for SFTS.

Successful Treatment with Letermovir in a Heart Transplant Recipient with UL97 Mutation Ganciclovir-Resistant Cytomegalovirus Colitis and Viremia.

Currently available anti-cytomegalovirus (CMV) agents are sometimes poorly tolerated, owing to their side effects. Letermovir is a novel anti-CMV drug that is only approved for CMV prophylaxis in hematopoietic stem cell transplant recipients, with fewer side effects. We report the case of a heart transplant recipient with UL97 mutation (L595F) ganciclovir-resistant cytomegalovirus colitis who was successfully treated with off-label use of letermovir. In treating CMV infection or disease with letermovir, a transient rise or lag in the clearance of CMV-DNA polymerase chain reaction levels has been observed. Our case suggests that CMV-pp65 antigenemia can be an additional marker of treatment efficacy.

Heparan sulfate 3-O-sulfotransferase 4 is genetically associated with herpes zoster and enhances varicella-zoster virus-mediated fusogenic activity.

Acute pain that is associated with herpes zoster (HZ) can become long-lasting neuropathic pain, known as chronic post-herpetic neuralgia (PHN), especially in the elderly. HZ is caused by the reactivation of latent varicella-zoster virus (VZV), whereas PHN is not attributed to ongoing viral replication. Although VZV infection reportedly induces neuronal cell fusion in humans, the pathogenesis of PHN is not fully understood. A genome-wide association study (GWAS) revealed significant associations between PHN and the rs12596324 single-nucleotide polymorphism (SNP) of the heparan sulfate 3-O-sulfotransferase 4 (HS3ST4) gene in a previous study. To further examine whether this SNP is associated with both PHN and VZV reactivation, associations between rs12596324 and a history of HZ were statistically analyzed using GWAS data. HZ was significantly associated with the rs12596324 SNP of HS3ST4, indicating that HS3ST4 is related to viral replication. We investigated the influence of HS3ST4 expression on VZV infection in cultured cells. Fusogenic activity after VZV infection was enhanced in cells with HS3ST4 expression by microscopy. To quantitatively evaluate the fusogenic activity, we applied cytotoxicity assay and revealed that HS3ST4 expression enhanced cytotoxicity after VZV infection. Expression of the VZV glycoproteins gB, gH, and gL significantly increased cytotoxicity in cells with HS3ST4 expression by cytotoxicity assay, consistent with the fusogenic activity as visualized by fluorescence microscopy. HS3ST4 had little influence on viral genome replication, revealed by quantitative real-time polymerase chain reaction. These results suggest that HS3ST4 enhances cytotoxicity including fusogenic activity in the presence of VZV glycoproteins without enhancing viral genome replication.

The Inhaled Steroid Ciclesonide Blocks SARS-CoV-2 RNA Replication by Targeting the Viral Replication-Transcription Complex in Cultured Cells.

Here, we screened steroid compounds to obtain a drug expected to block host inflammatory responses and Middle East respiratory syndrome coronavirus (MERS-CoV) replication. Ciclesonide, an inhaled corticosteroid, suppressed the replication of MERS-CoV and other coronaviruses, including severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the cause of coronavirus disease 2019 (COVID-19), in cultured cells. The 90% effective concentration (EC90) of ciclesonide for SARS-CoV-2 in differentiated human bronchial tracheal epithelial cells was 0.55 µM. Eight consecutive passages of 43 SARS-CoV-2 isolates in the presence of ciclesonide generated 15 resistant mutants harboring single amino acid substitutions in nonstructural protein 3 (nsp3) or nsp4. Of note, ciclesonide suppressed the replication of all these mutants by 90% or more, suggesting that these mutants cannot completely overcome ciclesonide blockade. Under a microscope, the viral RNA replication-transcription complex in cells, which is thought to be detectable using antibodies specific for nsp3 and double-stranded RNA, was observed to fall in the presence of ciclesonide in a concentration-dependent manner. These observations indicate that the suppressive effect of ciclesonide on viral replication is specific to coronaviruses, highlighting it as a candidate drug for the treatment of COVID-19 patients.IMPORTANCE The outbreak of SARS-CoV-2, the cause of COVID-19, is ongoing. New and effective antiviral agents that combat the disease are needed urgently. Here, we found that an inhaled corticosteroid, ciclesonide, suppresses the replication of coronaviruses, including betacoronaviruses (murine hepatitis virus type 2 [MHV-2], MERS-CoV, SARS-CoV, and SARS-CoV-2) and an alphacoronavirus (human coronavirus 229E [HCoV-229E]), in cultured cells. Ciclesonide is safe; indeed, it can be administered to infants at high concentrations. Thus, ciclesonide is expected to be a broad-spectrum antiviral drug that is effective against many members of the coronavirus family. It could be prescribed for the treatment of MERS and COVID-19.

Characterization of a Novel Alphaherpesvirus Isolated from the Fruit Bat Pteropus lylei in Vietnam.

Herpesviruses exist in nature within each host animal. Ten herpesviruses have been isolated from bats and their biological properties reported. A novel bat alphaherpesvirus, which we propose to name "Pteropus lylei-associated alphaherpesvirus (PLAHV)," was isolated from urine of the fruit bat Pteropus lylei in Vietnam and characterized. The entire genome sequence was determined to be 144,008 bp in length and predicted to include 72 genes. PLAHV was assigned to genus Simplexvirus with other bat alphaherpesviruses isolated from pteropodid bats in Southeast Asia and Africa. The replication capacity of PLAHV in several cells was evaluated in comparison with that of herpes simplex virus 1 (HSV-1). PLAHV replicated better in the bat-originated cell line and less in human embryonic lung fibroblasts than HSV-1 did. PLAHV was serologically related to another bat alphaherpesvirus, Pteropodid alphaherpesvirus 1 (PtAHV1), isolated from a Pteropus hypomelanus-related bat captured in Indonesia, but not with HSV-1. PLAHV caused lethal infection in mice. PLAHV was as susceptible to acyclovir as HSV-1 was. Characterization of this new member of bat alphaherpesviruses, PLAHV, expands the knowledge on bat-associated alphaherpesvirology.IMPORTANCE A novel bat alphaherpesvirus, Pteropus lylei-associated alphaherpesvirus (PLAHV), was isolated from urine of the fruit bat Pteropus lylei in Vietnam. The whole-genome sequence was determined and was predicted to include 72 open reading frames in the 144,008-bp genome. PLAHV is circulating in a species of fruit bats, Pteropus lylei, in Asia. This study expands the knowledge on bat-associated alphaherpesvirology.

Evaluation of SARS-CoV-2 neutralizing antibodies using a vesicular stomatitis virus possessing SARS-CoV-2 spike protein.

BACKGROUND: SARS-CoV-2 is a novel coronavirus that emerged in 2019 and is now classified in the genus Coronavirus with closely related SARS-CoV. SARS-CoV-2 is highly pathogenic in humans and is classified as a biosafety level (BSL)-3 pathogen, which makes manipulating it relatively difficult due to its infectious nature. METHODS: To circumvent the need for BSL-3 laboratories, an alternative assay was developed that avoids live virus and instead uses a recombinant VSV expressing luciferase and possesses the full length or truncated spike proteins of SARS-CoV-2. Furthermore, to measure SARS-CoV-2 neutralizing antibodies under BSL2 conditions, a chemiluminescence reduction neutralization test (CRNT) for SARS-CoV-2 was developed. The neutralization values of the serum samples collected from hospitalized patients with COVID-19 or SARS-CoV-2 PCR-negative donors against the pseudotyped virus infection evaluated by the CRNT were compared with antibody titers determined from an enzyme-linked immunosorbent assay (ELISA) or an immunofluorescence assay (IFA). RESULTS: The CRNT, which used whole blood collected from hospitalized patients with COVID-19, was also examined. As a result, the inhibition of pseudotyped virus infection was specifically observed in both serum and whole blood and was also correlated with the results of the IFA. CONCLUSIONS: In conclusion, the CRNT for COVID-19 is a convenient assay system that can be performed in a BSL-2 laboratory with high specificity and sensitivity for evaluating the occurrence of neutralizing antibodies against SARS-CoV-2.

Prevalence of Antibodies to Crimean-Congo Hemorrhagic Fever Virus in Ruminants, Nigeria, 2015.

Crimean-Congo hemorrhagic fever virus (CCHFV) is a highly transmissible human pathogen. Infection is often misdiagnosed, in part because of poor availability of data in disease-endemic areas. We sampled 150 apparently healthy ruminants throughout Nigeria for virus seropositivity and detected virus-specific IgG in cattle (24%) and goats (2%), highlighting the need for further investigations.

Acute Respiratory Infection in Human Dipeptidyl Peptidase 4-Transgenic Mice Infected with Middle East Respiratory Syndrome Coronavirus.

Middle East respiratory syndrome coronavirus (MERS-CoV) infection can manifest as a mild illness, acute respiratory distress, organ failure, or death. Several animal models have been established to study disease pathogenesis and to develop vaccines and therapeutic agents. Here, we developed transgenic (Tg) mice on a C57BL/6 background; these mice expressed human CD26/dipeptidyl peptidase 4 (hDPP4), a functional receptor for MERS-CoV, under the control of an endogenous hDPP4 promoter. We then characterized this mouse model of MERS-CoV. The expression profile of hDPP4 in these mice was almost equivalent to that in human tissues, including kidney and lung; however, hDPP4 was overexpressed in murine CD3-positive cells within peripheral blood and lymphoid tissues. Intranasal inoculation of young and adult Tg mice with MERS-CoV led to infection of the lower respiratory tract and pathological evidence of acute multifocal interstitial pneumonia within 7 days, with only transient loss of body weight. However, the immunopathology in young and adult Tg mice was different. On day 5 or 7 postinoculation, lungs of adult Tg mice contained higher levels of proinflammatory cytokines and chemokines associated with migration of macrophages. These results suggest that the immunopathology of MERS-CoV infection in the Tg mouse is age dependent. The mouse model described here will increase our understanding of disease pathogenesis and host mediators that protect against MERS-CoV infection.IMPORTANCE Middle East respiratory syndrome coronavirus (MERS-CoV) infections are endemic in the Middle East and a threat to public health worldwide. Rodents are not susceptible to the virus because they do not express functional receptors; therefore, we generated a new animal model of MERS-CoV infection based on transgenic mice expressing human DPP4 (hDPP4). The pattern of hDPP4 expression in this model was similar to that in human tissues (except lymphoid tissue). In addition, MERS-CoV was limited to the respiratory tract. Here, we focused on host factors involved in immunopathology in MERS-CoV infection and clarified differences in antiviral immune responses between young and adult transgenic mice. This new small-animal model could contribute to more in-depth study of the pathology of MERS-CoV infection and aid development of suitable treatments.