A hepatitis B virus core antigen-based virus-like particle vaccine expressing SARS-CoV-2 B and T cell epitopes induces epitope-specific humoral and cell-mediated immune responses but confers limited protection against SARS-CoV-2 infection.

The hepatitis B virus core antigen (HBcAg) tolerates insertion of foreign epitopes and maintains its ability to self-assemble into virus-like particles (VLPs). We constructed a ∆HBcAg-based VLP vaccine expressing three predicted severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) B and T cell epitopes and determined its immunogenicity and protective efficacy. The recombinant ∆HBcAg-SARS-CoV-2 protein was expressed in Escherichia coli, purified, and shown to form VLPs. K18-hACE2 transgenic C57BL/6 mice were immunized intramuscularly with ∆HBcAg VLP control (n = 15) or ∆HBcAg-SARS-CoV-2 VLP vaccine (n = 15). One week after the 2nd booster and before virus challenge, five ∆HBcAg-SARS-CoV-2 vaccinated mice were euthanized to evaluate epitope-specific immune responses. There is a statistically significant increase in epitope-specific Immunoglobulin G (IgG) response, and statistically higher interleukin 6 (IL-6) and monocyte chemoattractant protein-1 (MCP-1) expression levels in ∆HBcAg-SARS-CoV-2 VLP-vaccinated mice compared to ∆HBcAg VLP controls. While not statistically significant, the ∆HBcAg-SARS-CoV-2 VLP mice had numerically more memory CD8+ T-cells, and 3/5 mice also had numerically higher levels of interferon gamma (IFN-γ) and tumor necrosis factor (TNF). After challenge with SARS-CoV-2, ∆HBcAg-SARS-CoV-2 immunized mice had numerically lower viral RNA loads in the lung, and slightly higher survival, but the differences are not statistically significant. These results indicate that the ∆HBcAg-SARS-CoV-2 VLP vaccine elicits epitope-specific humoral and cell-mediated immune responses but they were insufficient against SARS-CoV-2 infection.

Stability of Aerosolized SARS-CoV-2 on Masks and Transfer to Skin.

The potential for masks to act as fomites in the transmission of SARS-CoV-2 has been suggested but not demonstrated experimentally or observationally. In this study, we aerosolized a suspension of SARS-CoV-2 in saliva and used a vacuum pump to pull the aerosol through six different types of masks. After 1 h at 28 °C and 80% RH, SARS-CoV-2 infectivity was not detectable on an N95 and surgical mask, was reduced by 0.7 log10 on a nylon/spandex mask, and was unchanged on a polyester mask and two different cotton masks when recovered by elution in a buffer. SARS-CoV-2 RNA remained stable for 1 h on all masks. We pressed artificial skin against the contaminated masks and detected the transfer of viral RNA but no infectious virus to the skin. The potential for masks contaminated with SARS-CoV-2 in aerosols to act as fomites appears to be less than indicated by studies involving SARS-CoV-2 in very large droplets.

Correction: Differential pathogenesis of Usutu virus isolates in mice.

[This corrects the article DOI: 10.1371/journal.pntd.0008765.].

North American House Sparrows Are Competent for Usutu Virus Transmission.

Usutu virus (USUV, Flaviviridae) is an emerging mosquito-borne virus that has been implicated in neuroinvasive disease in humans and epizootic deaths in wild birds. USUV is maintained in an enzootic cycle between ornithophilic mosquitoes, primarily Culex spp., and wild birds, predominantly passerine species. However, limited experimental data exist on the species competent for USUV transmission. Here, we demonstrate that house sparrows are susceptible to multiple USUV strains. Our study also revealed that Culex quinquefasciatus mosquitoes are susceptible to USUV, with a significantly higher infection rate for the Netherlands 2016 USUV strain compared to the Uganda 2012 USUV strain at 50% and 19%, respectively. To assess transmission between avian host and mosquito vector, we allowed mosquitoes to feed on either juvenile chickens or house sparrows inoculated with USUV. Both bird models transmitted USUV to C. quinquefasciatus mosquitoes. Linear regression analyses indicated that C. quinquefasciatus infection rates were positively correlated with avian viremia levels, with 3 to 4 log10 PFU/mL representing the minimum avian viremia threshold for transmission to mosquitoes. Based on the viremia required for transmission, house sparrows were estimated to more readily transmit the Netherlands 2016 strain compared to the Uganda 2012 strain. These studies provide insights on a competent reservoir host of USUV. IMPORTANCE Usutu virus (USUV) is a zoonotic mosquito-borne virus that can cause neuroinvasive disease, including meningitis and encephalitis, in humans and has resulted in hundreds of thousands of deaths in wild birds. The perpetuation of USUV in nature is dependent on transmission between Culex spp. mosquitoes and various avian species. To date, few experimental data exist for determining which bird species are important for the maintenance of USUV. Our studies showed that house sparrows can transmit infectious Usutu virus, indicating their role as a competent host species. By identifying reservoir species of USUV, we can predict areas of USUV emergence and mitigate its impacts on global human and wildlife health.

Infectious SARS-CoV-2 Is Emitted in Aerosol Particles.

Respiratory viruses such as SARS-CoV-2 are transmitted in respiratory droplets and aerosol particles, which are released during talking, breathing, coughing, and sneezing. Noncontact transmission of SARS-CoV-2 has been demonstrated, suggesting transmission via virus carried through the air. Here, we demonstrate that golden Syrian hamsters produce infectious SARS-CoV-2 in aerosol particles prior to and concurrent with the onset of mild clinical signs of disease. The average emission rate in this study was 25 infectious virions/hour on days 1 and 2 postinoculation, with average viral RNA levels 200-fold higher than infectious virus in aerosol particles. The majority of virus was contained within particles <5 µm in size. Thus, we provide direct evidence that, in hamsters, SARS-CoV-2 is an airborne virus. IMPORTANCE SARS-CoV-2 is a respiratory virus and has been isolated from the air near COVID-19 patients. Here, using a hamster model of infection, we demonstrate that SARS-CoV-2 is emitted in aerosol particles prior to and concurrent with the onset of mild disease. Virus is contained primarily within aerosol particles <5 µm in size, which can remain airborne and be inhaled. These findings indicate that SARS-CoV-2 is an airborne virus and support the use of ventilation to reduce SARS-CoV-2 transmission.

Development and characterization of infectious clones of two strains of Usutu virus.

Usutu virus (USUV; genus Flavivirus; family Flaviviridae) is a mosquito-borne, positive-sense RNA virus that is currently causing significant die-offs in numerous bird species throughout Europe and has caused infections in humans. Currently, there are no molecular clones for USUV, hence, hindering studies on the pathogenesis and transmission of USUV. Here, we demonstrate the development and characterization of infectious clones for two modern strains of USUV isolated from Europe and Africa. We show that the infectious clone-derived viruses replicated similarly to the parental strains in mammalian and insect cells. Additionally, we observed similar levels of replication and disease in two mouse models. These clones will aid the study of USUV infection, transmission, diagnostics, and vaccines.

West Nile Virus Vaccination Protects against Usutu Virus Disease in Mice.

West Nile virus (WNV) and Usutu virus (USUV) are mosquito-borne flaviviruses that can cause neuroinvasive disease in humans. WNV and USUV circulate in both Africa and Europe and are closely related. Due to antigenic similarity, WNV-specific antibodies and USUV-specific antibodies have the potential to bind heterologous viruses; however, it is unclear whether this interaction may offer protection against infection. To investigate how prior WNV exposure would influence USUV infection, we used an attenuated WNV vaccine that contains the surface proteins of WNV in the backbone of a dengue virus 2 vaccine strain and protects against WNV disease. We hypothesized that vaccination with this attenuated WNV vaccine would protect against USUV infection. Neutralizing responses against WNV and USUV were measured in vitro using sera following vaccination. Sera from vaccinated CD-1 and Ifnar1-/- mice cross-neutralized with WNV and USUV. All mice were then subsequently challenged with an African or European USUV strain. In CD-1 mice, there was no difference in USUV titers between vaccinated and mock-vaccinated mice. However, in the Ifnar1-/- model, vaccinated mice had significantly higher survival rates and significantly lower USUV viremia compared to mock-vaccinated mice. Our results indicate that exposure to an attenuated form of WNV protects against severe USUV disease in mice and elicits a neutralizing response to both WNV and USUV. Future studies will investigate the immune mechanisms responsible for the protection against USUV infection induced by WNV vaccination, providing critical insight that will be essential for USUV and WNV vaccine development.

Persistence of Zika virus RNA in the epididymis of the murine male reproductive tract.

Zika virus (ZIKV) can infect developing fetuses in utero and cause severe congenital defects independent of route of maternal infection. Infected men can shed ZIKV RNA in semen for over six months. Whether prolonged viral RNA shedding in semen indicates a persistent infection in the male reproductive tract is unknown. We hypothesized that if ZIKV establishes a persistent infection in the male reproductive tract (MRT), then immunosuppressant treatment should stimulate ZIKV replication and seminal shedding. Male mice were infected with ZIKV and immunosuppressed when they shed viral RNA but not infectious virus in ejaculates. Following immunosuppression, we did not detect infectious virus in ejaculates. However, we did detect ZIKV positive and negative sense RNA in the epididymal lumens of mice treated with cyclophosphamide, suggesting that ZIKV persists in the epididymis. This study provides insight into the mechanisms behind ZIKV sexual transmission, which may inform public health decisions regarding ZIKV risks.

Differential pathogenesis of Usutu virus isolates in mice.

Usutu virus (USUV; Flavivirus), a close phylogenetic and ecological relative of West Nile virus, is a zoonotic virus that can cause neuroinvasive disease in humans. USUV is maintained in an enzootic cycle between Culex mosquitoes and birds. Since the first isolation in 1959 in South Africa, USUV has spread throughout Africa and Europe. Reported human cases have increased over the last few decades, primarily in Europe, with symptoms ranging from mild febrile illness to severe neurological effects. In this study, we investigated whether USUV has become more pathogenic during emergence in Europe. Interferon α/ß receptor knockout (Ifnar1-/-) mice were inoculated with recent USUV isolates from Africa and Europe, as well as the historic 1959 South African strain. The three tested African strains and one European strain from Spain caused 100% mortality in inoculated mice, with similar survival times and histopathology in tissues. Unexpectedly, a European strain from the Netherlands caused only 12% mortality and significantly less histopathology in tissues from mice compared to mice inoculated with the other strains. Viremia was highest in mice inoculated with the recent African strains and lowest in mice inoculated with the Netherlands strain. Based on phylogenetics, the USUV isolates from Spain and the Netherlands were derived from separate introductions into Europe, suggesting that disease outcomes may differ for USUV strains circulating in Europe. These results also suggest that while more human USUV disease cases have been reported in Europe recently, circulating African USUV strains are still a potential major health concern.

Mutations present in a low-passage Zika virus isolate result in attenuated pathogenesis in mice.

Zika virus (ZIKV) infection can result in neurological disorders including Congenital Zika Syndrome in infants exposed to the virus in utero. Pregnant women can be infected by mosquito bite as well as by sexual transmission from infected men. Herein, the variants of ZIKV within the male reproductive tract and ejaculates were assessed in inoculated mice. We identified two non-synonymous variants at positions E-V330L and NS1-W98G. These variants were also present in the passage three PRVABC59 isolate and infectious clone relative to the patient serum PRVABC59 sequence. In subsequent studies, ZIKV E-330L was less pathogenic in mice than ZIKV E-330V as evident by increased average survival times. In Vero cells, ZIKV E-330L/NS1-98G outcompeted ZIKV E-330V/NS1-98W within 3 passages. These results suggest that the E-330L/NS1-98G variants are attenuating in mice and were enriched during cell culture passaging. Cell culture propagation of ZIKV could significantly affect animal model development and vaccine efficacy studies.