A measles-vectored vaccine candidate expressing prefusion-stabilized SARS-CoV-2 spike protein brought to phase I/II clinical trials: protection of African green monkeys from COVID-19 disease.

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) emerged at the end of 2019 and is responsible for the largest human pandemic in 100 years. Thirty-four vaccines are currently approved for use worldwide, and approximately 67% of the world population has received a complete primary series of one, yet countries are dealing with new waves of infections, variant viruses continue to emerge, and breakthrough infections are frequent secondary to waning immunity. Here, we evaluate a measles virus (MV)-vectored vaccine expressing a stabilized prefusion SARS-CoV-2 spike (S) protein (MV-ATU3-S2PΔF2A; V591) with demonstrated immunogenicity in mouse models (see companion article [J. Brunet, Z. Choucha, M. Gransagne, H. Tabbal, M.-W. Ku et al., J Virol 98:e01693-23, 2024, https://doi.org/10.1128/jvi.01693-23]) in an established African green monkey model of disease. Animals were vaccinated with V591 or the control vaccine (an equivalent MV-vectored vaccine with an irrelevant antigen) intramuscularly using a prime/boost schedule, followed by challenge with an early pandemic isolate of SARS-CoV-2 at 56 days post-vaccination. Pre-challenge, only V591-vaccinated animals developed S-specific antibodies that had virus-neutralizing activity as well as S-specific T cells. Following the challenge, V591-vaccinated animals had lower infectious virus and viral (v) RNA loads in mucosal secretions and stopped shedding virus in these secretions earlier. vRNA loads were lower in these animals in respiratory and gastrointestinal tract tissues at necropsy. This correlated with a lower disease burden in the lungs as quantified by PET/CT at early and late time points post-challenge and by pathological analysis at necropsy.IMPORTANCESevere acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is responsible for the largest human pandemic in 100 years. Even though vaccines are currently available, countries are dealing with new waves of infections, variant viruses continue to emerge, breakthrough infections are frequent, and vaccine hesitancy persists. This study uses a safe and effective measles vaccine as a platform for vaccination against SARS-CoV-2. The candidate vaccine was used to vaccinate African green monkeys (AGMs). All vaccinated AGMs developed robust antigen-specific immune responses. After challenge, these AGMs produced less virus in mucosal secretions, for a shorter period, and had a reduced disease burden in the lungs compared to control animals. At necropsy, lower levels of viral RNA were detected in tissue samples from vaccinated animals, and the lungs of these animals lacked the histologic hallmarks of SARS-CoV-2 disease observed exclusively in the control AGMs.

Rift Valley Fever Virus Encephalitis: Viral and Host Determinants of Pathogenesis.

Rift Valley fever virus (RVFV) is a mosquito-borne virus endemic to Africa and the Middle East. RVFV infection can cause encephalitis, which is associated with significant morbidity and mortality. Studies of RVFV encephalitis following percutaneous inoculation, as would occur following a mosquito bite, have historically been limited by a lack of consistent animal models. In this review, we describe new insights into the pathogenesis of RVFV and the opportunities provided by new mouse models. We underscore the need to consider viral strain and route of inoculation when interpreting data obtained using animal models. We discuss the trafficking of RVFV and the role of host genetics and immunity in modulating the pathogenesis of RVFV encephalitis. We also explore potential strategies to prevent and treat central nervous system disease caused by RVFV and discuss remaining knowledge gaps.

Limiting viral replication in hepatocytes alters Rift Valley fever virus disease manifestations.

Rift Valley fever virus (RVFV) causes mild to severe disease in humans and livestock. Outbreaks of RVFV have been reported throughout Africa and have spread outside Africa since 2000, calling for urgent worldwide attention to this emerging virus. RVFV directly infects the liver, and elevated transaminases are a hallmark of severe RVFV infection. However, the specific contribution of viral replication in hepatocytes to pathogenesis of RVFV remains undefined. To address this, we generated a recombinant miRNA-targeted virus, RVFVmiR-122, to limit hepatocellular replication. MicroRNAs are evolutionarily conserved non-coding RNAs that regulate mRNA expression by targeting them for degradation. RVFVmiR-122 includes an insertion of four target sequences of the liver-specific miR-122. In contrast to control RVFVmiR-184, which contains four target sequences of mosquito-specific miR-184, RVFVmiR-122 has restricted replication in vitro in primary mouse hepatocytes. RVFVmiR-122-infected C57BL/6 mice survived acute hepatitis and instead developed late-onset encephalitis. This difference in clinical outcome was eliminated in Mir-122 KO mice, confirming the specificity of the finding. Interestingly, C57BL/6 mice infected with higher doses of RVFVmiR-122 had a higher survival rate which was correlated with faster clearance of virus from the liver, suggesting a role for activation of host immunity in the phenotype. Together, our data demonstrate that miR-122 can specifically restrict the replication of RVFVmiR-122 in liver tissue both in vitro and in vivo, and this restriction alters the clinical course of disease following RVFVmiR-122 infection. IMPORTANCE Rift Valley fever virus (RVFV) is a hemorrhagic fever virus that causes outbreaks in humans and livestock throughout Africa and has spread to continents outside Africa since 2000. However, no commercial vaccine or treatment is currently available for human use against RVFV. Although the liver has been demonstrated as a key target of RVFV, the contribution of viral replication in hepatocytes to overall RVFV pathogenesis is less well defined. In this study we addressed this question by using a recombinant miRNA-targeted virus with restricted replication in hepatocytes. We gained a better understanding of how this individual cell type contributes to the development of disease caused by RVFV. Techniques used in this study provide an innovative tool to the RVFV field that could be applied to study the consequences of limited RVFV replication in other target cells.

Impact of maternal SARS-CoV-2 booster vaccination on blood and breastmilk antibodies.

Maternal COVID-19 vaccination could protect infants who are ineligible for vaccine through antibody transfer during pregnancy and lactation. We measured the quantity and durability of SARS-CoV-2 antibodies in human milk and infant blood before and after maternal booster vaccination. Prospective cohort of lactating women immunized with primary and booster COVID-19 vaccines during pregnancy or lactation and their infants. Milk and blood samples from October 2021 to April 2022 were included. Anti-nucleoprotein (NP) and anti-receptor binding domain (RBD) IgG and IgA in maternal milk and maternal and infant blood were measured and compared longitudinally after maternal booster vaccine. Forty-five lactating women and their infants provided samples. 58% of women were anti-NP negative and 42% were positive on their first blood sample prior to booster vaccine. Anti-RBD IgG and IgA in milk remained significantly increased through 120-170 days after booster vaccine and did not differ by maternal NP status. Anti-RBD IgG and IgA did not increase in infant blood after maternal booster. Of infants born to women vaccinated in pregnancy, 74% still had positive serum anti-RBD IgG measured on average 5 months after delivery. Infant to maternal IgG ratio was highest for infants exposed to maternal primary vaccine during the second trimester compared to third trimester (0.85 versus 0.29; p<0.001). Maternal COVID-19 primary and booster vaccine resulted in robust and long-lasting transplacental and milk antibodies. These antibodies may provide important protection against SARS-CoV-2 during the first six months of life.

Immune correlates of protection following Rift Valley fever virus vaccination.

Rift Valley fever virus (RVFV) is a hemorrhagic fever virus with the potential for significant economic and public health impact. Vaccination with an attenuated strain, DelNSsRVFV, provides protection from an otherwise lethal RVFV challenge, but mechanistic determinants of protection are undefined. In this study, a murine model was used to assess the contributions of humoral and cellular immunity to DelNSsRVFV-mediated protection. Vaccinated mice depleted of T cells were protected against subsequent challenge, and passive transfer of immune serum from vaccinated animals to naïve animals was also protective, demonstrating that T cells were dispensable in the presence of humoral immunity and that humoral immunity alone was sufficient. Animals depleted of B cells and then vaccinated were protected against challenge. Total splenocytes, but not T cells alone, B cells alone, or B + T cells harvested from vaccinated animals and then transferred to naïve animals were sufficient to confer protection, suggesting that multiple cellular interactions were required for effective cellular immunity. Together, these data indicate that humoral immunity is sufficient to confer vaccine-mediated protection and suggests that cellular immunity plays a role in protection that requires the interaction of various cellular components.

Rift Valley Fever Virus Infects the Posterior Segment of the Eye and Induces Inflammation in a Rat Model of Ocular Disease.

People infected with the mosquito-borne Rift Valley fever virus (RVFV) can suffer from eye-related problems resulting in ongoing vision issues or even permanent blindness. Despite ocular disease being the most frequently reported severe outcome, it is vastly understudied compared to other disease outcomes caused by RVFV. Ocular manifestations of RVFV include blurred vision, uveitis, and retinitis. When an infected individual develops macular or paramacular lesions, there is a 50% chance of permanent vision loss in one or both eyes. The cause of blinding ocular pathology remains unknown in part due to the lack of a tractable animal model. Using 3 relevant exposure routes, both subcutaneous (SC) and aerosol inoculation of Sprague Dawley rats led to RVFV infection of the eye. Surprisingly, direct inoculation of the conjunctiva did not result in successful ocular infection. The posterior segment of the eye, including the optic nerve, choroid, ciliary body, and retina, were all positive for RVFV antigen in SC-infected rats, and live virus was isolated from the eyes. Proinflammatory cytokines and increased leukocyte counts were also found in the eyes of infected rats. Additionally, human ocular cell lines were permissive for Lrp1-dependent RVFV infection. This study experimentally defines viral tropism of RVFV in the posterior segment of the rat eye and characterizes virally-mediated ocular inflammation, providing a foundation for evaluation of vaccines and therapeutics to protect against adverse ocular outcomes. IMPORTANCE Rift Valley fever virus (RVFV) infection leads to eye damage in humans in up to 10% of reported cases. Permanent blindness occurs in 50% of individuals with significant retinal scarring. Despite the prevalence and severity of this outcome, very little is known about the mechanisms of pathogenesis. We addressed this gap by developing a rodent model of ocular disease. Subcutaneous infection of Sprague Dawley rats resulted in infection of the uvea, retina, and optic nerve along with the induction of inflammation within the posterior eye. Infection of human ocular cells induced inflammatory responses and required host entry factors for RVFV infection similar to rodents. This work provides evidence of how RVFV infects the eye, and this information can be applied to help mitigate the devastating outcomes of RVF ocular disease through vaccines or treatments.

Genetic diversity of collaborative cross mice enables identification of novel rift valley fever virus encephalitis model.

Rift Valley fever (RVF) is an arboviral disease of humans and livestock responsible for severe economic and human health impacts. In humans, RVF spans a variety of clinical manifestations, ranging from an acute flu-like illness to severe forms of disease, including late-onset encephalitis. The large variations in human RVF disease are inadequately represented by current murine models, which overwhelmingly die of early-onset hepatitis. Existing mouse models of RVF encephalitis are either immunosuppressed, display an inconsistent phenotype, or develop encephalitis only when challenged via intranasal or aerosol exposure. In this study, the genetically defined recombinant inbred mouse resource known as the Collaborative Cross (CC) was used to identify mice with additional RVF disease phenotypes when challenged via a peripheral foot-pad route to mimic mosquito-bite exposure. Wild-type Rift Valley fever virus (RVFV) challenge of 20 CC strains revealed three distinct disease phenotypes: early-onset hepatitis, mixed phenotype, and late-onset encephalitis. Strain CC057/Unc, with the most divergent phenotype, which died of late-onset encephalitis at a median of 11 days post-infection, is the first mouse strain to develop consistent encephalitis following peripheral challenge. CC057/Unc mice were directly compared to C57BL/6 mice, which uniformly succumb to hepatitis within 2-4 days of infection. Encephalitic disease in CC057/Unc mice was characterized by high viral RNA loads in brain tissue, accompanied by clearance of viral RNA from the periphery, low ALT levels, lymphopenia, and neutrophilia. In contrast, C57BL/6 mice succumbed from hepatitis at 3 days post-infection with high viral RNA loads in the liver, viremia, high ALT levels, lymphopenia, and thrombocytopenia. The identification of a strain of CC mice as an RVFV encephalitis model will allow for future investigation into the pathogenesis and treatment of RVF encephalitic disease and indicates that genetic background makes a major contribution to RVF disease variation.

Comparative analysis of human immune responses following SARS-CoV-2 vaccination with BNT162b2, mRNA-1273, or Ad26.COV2.S.

SARS-CoV-2 vaccines BNT162b2, mRNA-1273, and Ad26.COV2.S received emergency use authorization by the U.S. Food and Drug Administration in 2020/2021. Individuals being vaccinated were invited to participate in a prospective longitudinal comparative study of immune responses elicited by the three vaccines. In this observational cohort study, immune responses were evaluated using a SARS-CoV-2 spike protein receptor-binding domain ELISA, SARS-CoV-2 virus neutralization assays and an IFN- γ ELISPOT assay at various times over six months following initial vaccination. mRNA-based vaccines elicited higher magnitude humoral responses than Ad26.COV2.S; mRNA-1273 elicited the most durable humoral response, and all humoral responses waned over time. Neutralizing antibodies against the Delta variant were of lower magnitude than the wild-type strain for all three vaccines. mRNA-1273 initially elicited the greatest magnitude of T cell response, but this declined by 6 months. Declining immunity over time supports the use of booster dosing, especially in the setting of emerging variants.

SARS-CoV-2 Antibody Response Is Associated with Age and Body Mass Index in Convalescent Outpatients.

COVID-19 has had an unprecedented global impact on human health. Understanding the Ab memory responses to infection is one tool needed to effectively control the pandemic. Among 173 outpatients who had virologically confirmed SARS-CoV-2 infection, we evaluated serum Ab concentrations, microneutralization activity, and enumerated SARS-CoV-2-specific B cells in convalescent human blood specimens. Serum Ab concentrations were variable, allowing for stratification of the cohort into high and low responders. Neither participant sex, the timing of blood sampling following the onset of illness, nor the number of SARS-CoV-2 spike protein-specific B cells correlated with serum Ab concentration. Serum Ab concentration was positively associated with microneutralization activity and participant age, with participants under the age of 30 showing the lowest Ab level. These data suggest that young adult outpatients did not generate as robust Ab memory, compared with older adults. Body mass index was also positively correlated with serum Ab levels. Multivariate analyses showed that participant age and body mass index were independently associated with Ab levels. These findings have direct implications for public health policy and current vaccine efforts. Knowledge gained regarding Ab memory following infection will inform the need for vaccination in those previously infected and allow for a better approximation of population-wide protective immunity.

A Mycobacteriophage-Based Vaccine Platform: SARS-CoV-2 Antigen Expression and Display.

The explosion of SARS-CoV-2 infections in 2020 prompted a flurry of activity in vaccine development and exploration of various vaccine platforms, some well-established and some new. Phage-based vaccines were described previously, and we explored the possibility of using mycobacteriophages as a platform for displaying antigens of SARS-CoV-2 or other infectious agents. The potential advantages of using mycobacteriophages are that a large and diverse variety of them have been described and genomically characterized, engineering tools are available, and there is the capacity to display up to 700 antigen copies on a single particle approximately 100 nm in size. The phage body may itself be a good adjuvant, and the phages can be propagated easily, cheaply, and to high purity. Furthermore, the recent use of these phages therapeutically, including by intravenous administration, suggests an excellent safety profile, although efficacy can be restricted by neutralizing antibodies. We describe here the potent immunogenicity of mycobacteriophage Bxb1, and Bxb1 recombinants displaying SARS-CoV-2 Spike protein antigens.

SARS-CoV-2 Antibody Response is Associated with Age in Convalescent Outpatients.

COVID-19 has had an unprecedented global impact on human health. Understanding the antibody memory responses to infection is one tool needed to effectively control the pandemic. Among 173 outpatients who had virologically confirmed SARS-CoV-2 infection, we evaluated serum antibody concentrations, microneutralization activity, and enumerated SARS-CoV-2 specific B cells in convalescent blood specimens. Serum antibody concentrations were variable, allowing for stratification of the cohort into high and low responders. Serum antibody concentration was positively associated with microneutralization activity and participant age, with participants under the age of 30 showing the lowest antibody level. Neither participant sex, the timing of blood sampling following the onset of illness, nor the number of SARS-CoV-2 spike protein specific B cells correlated with serum antibody concentration. These data suggest that young adult outpatients did not generate as robust antibody memory, compared with older adults. Further, serum antibody concentration or neutralizing activity trended but did not significantly correlate with the number of SARS-CoV-2 memory B cells. These findings have direct implications for public health policy and current vaccine efforts. Knowledge gained regarding antibody memory following infection will inform the need for vaccination in those previously infected and allow for a better approximation of population-wide protective immunity.

Isotype-Specific Fc Effector Functions Enhance Antibody-Mediated Rift Valley Fever Virus Protection In Vivo.

Discovered in 1931, Rift Valley fever virus (RVFV) is an arbovirus that causes disease in humans and livestock. In humans, disease ranges from a self-limiting febrile illness to a more severe hepatitis or encephalitis. There are currently no licensed human therapeutics for RVFV disease. Given the recent advances in the use of monoclonal antibodies (MAbs) for treating infectious disease, a panel of anti-RVFV Gn glycoprotein MAbs was developed and characterized. RVFV MAbs spanned a range of neutralizing abilities and mapped to distinct epitopes along Gn. The contribution of Fc effector functions in providing MAb-mediated protection from RVFV was assessed. IgG2a version MAbs had increased capacity to induce effector functions and conferred better protection from RVFV challenge in a lethal mouse model than IgG1 version MAbs. Overall, this study shows that Fc-mediated functions are a critical component of humoral protection from RVFV. IMPORTANCE Rift Valley fever virus (RVFV) is a mosquito-borne virus found throughout Africa and into the Middle East. It has a substantial disease burden; in areas of endemicity, up to 60% of adults are seropositive. With a case fatality rate of up to 3% and the ability to cause hemorrhagic fever and encephalitis, RVFV poses a serious threat to human health. Despite the known human disease burden and the fact that it is a NIAID category A priority pathogen and a WHO priority disease for research and development, there are no vaccines or therapeutics available for RVF. In this study, we developed and characterized a panel of monoclonal antibodies against the RVFV surface glycoprotein, Gn. We then demonstrated therapeutic efficacy in the prevention of RVF in vivo in an otherwise lethal mouse model. Finally, we revealed a role for Fc-mediated function in augmenting the protection provided by these antibodies.

Identification and Characterization of Rift Valley Fever Virus-Specific T Cells Reveals a Dependence on CD40/CD40L Interactions for Prevention of Encephalitis.

Rift Valley fever virus (RVFV) is an arbovirus found throughout Africa. It causes disease that is typically mild and self-limiting; however, some infected individuals experience severe manifestations, including hepatitis, encephalitis, or even death. Reports of RVFV encephalitis are notable among immunosuppressed individuals, suggesting a role for adaptive immunity in preventing this severe complication. This phenomenon has been modeled in C57BL/6 mice depleted of CD4 T cells prior to infection with DelNSs RVFV (RVFV containing a deletion of nonstructural protein NSs), resulting in late-onset encephalitis accompanied by high levels of viral RNA in the brain in 30% of animals. In this study, we sought to define the specific type(s) of CD4 T cells that mediate protection from RVFV encephalitis. The viral epitopes targeted by CD4 and CD8 T cells were defined in C57BL/6 mice, and tetramers for both CD4 and CD8 T cells were generated. RVFV-specific CD8 T cells were expanded and of a cytotoxic and proliferating phenotype in the liver following infection. RVFV-specific CD4 T cells were identified in the liver and spleen following infection and phenotyped as largely Th1 or Tfh subtypes. Knockout mice lacking various aspects of pathways important in Th1 and Tfh development and function were used to demonstrate that T-bet, CD40, CD40L, and major histocompatibility complex class II (MHC-II) mediated protection from RVFV encephalitis, while gamma interferon (IFN-γ) and interleukin-12 (IL-12) were dispensable. Virus-specific antibody responses correlated with protection from encephalitis in all mouse strains, suggesting that Tfh/B cell interactions modulate clinical outcome in this model. IMPORTANCE The prevention of RVFV encephalitis requires intact adaptive immunity. In this study, we developed reagents to detect RVFV-specific T cells and provide evidence for Tfh cells and CD40/CD40L interactions as critical mediators of this protection.

Multiplex assessment of SARS-CoV-2 antibodies improves assay sensitivity and correlation with neutralizing antibodies.

OBJECTIVES: Detection of antibodies to multiple SARS-CoV-2 antigens in a single assay could increase diagnostic accuracy, differentiate vaccination from natural disease, and aid in retrospective exposure determination. Correlation of binding antibody assessment in clinical assays with neutralizing antibodies is needed to better understand the humoral response to SARS-CoV-2 infection and establish of correlates of protection. METHODS: A cohort of 752 samples was used to assess specificity, sensitivity, and comparison to 6 other Conformitè Europëenne serologic assays for the BioRad SARS-CoV-2 IgG multiplex assay which measures receptor binding domain IgG (RBD), spike-S1 IgG (S1), spike-S2 IgG (S2), and nucleocapsid IgG (N). A subset of serial specimens from 14 patients was also tested for neutralizing antibodies (n = 61). RESULTS: Specificity for RBD and S1 IgG was 99.4% (n = 170) and 100% for S2 and N IgG (n = 170) in a cohort selected for probable interference. Overall assay concordance with other assays was >93% for IgG and total antibody assays and reached 100% sensitivity for clinical concordance at >14 days as a multiplex assay. RBD and S1 binding antibody positivity demonstrated 79-95% agreement with the presence of neutralizing antibodies. CONCLUSIONS: The BioRad SARS-CoV-2 IgG assay is comparable to existing assays, and achieved 100% sensitivity when all markers were included. The ability to measure antibodies against spike and nucleocapsid proteins simultaneously may be advantageous for complex clinical presentations, epidemiologic research, and in decisions regarding infection prevention strategies. Additional independent validations are needed to further determine binding antibody and neutralizing antibody correlations.

A Cross-Sectional Study of SARS-CoV-2 Seroprevalence between Fall 2020 and February 2021 in Allegheny County, Western Pennsylvania, USA.

Seroprevalence studies are important for understanding the dynamics of local virus transmission and evaluating community immunity. To assess the seroprevalence for SARS-CoV-2 in Allegheny County, an urban/suburban county in Western PA, 393 human blood samples collected in Fall 2020 and February 2021 were examined for spike protein receptor-binding domain (RBD) and nucleocapsid protein (N) antibodies. All RBD-positive samples were evaluated for virus-specific neutralization activity. Our results showed a seroprevalence of 5.5% by RBD ELISA, 4.5% by N ELISA, and 2.5% for both in Fall 2020, which increased to 24.7% by RBD ELISA, 14.9% by N ELISA, and 12.9% for both in February 2021. Neutralization titer was significantly correlated with RBD titer but not with N titer. Using these two assays, we were able to distinguish infected from vaccinated individuals. In the February cohort, higher median income and white race were associated with serological findings consistent with vaccination. This study demonstrates a 4.5-fold increase in SARS-CoV-2 seroprevalence from Fall 2020 to February 2021 in Allegheny County, PA, due to increased incidence of both natural disease and vaccination. Future seroprevalence studies will need to include the effect of vaccination on assay results and incorporate non-vaccine antigens in serological assessments.

Rift Valley Fever Virus Infection Causes Acute Encephalitis in the Ferret.

Rift Valley fever virus (RVFV) is a pathogen of both humans and livestock in Africa and the Middle East. Severe human disease is associated with hepatitis and/or encephalitis. Current pathogenesis studies rely on rodents and nonhuman primates, which have advantages and disadvantages. We evaluated disease progression in Mustela putorius furo (the ferret) following intradermal (i.d.) or intranasal (i.n.) infection. Infected ferrets developed hyperpyrexia, weight loss, lymphopenia, and hypoalbuminemia. Three of four ferrets inoculated intranasally with RVFV developed central nervous system (CNS) disease that manifested as seizure, ataxia, and/or hind limb weakness at 8 to 11 days postinfection (dpi). Animals with clinical CNS disease had transient viral RNAemia, high viral RNA loads in the brain, and histopathological evidence of encephalitis. The ferret model will facilitate our understanding of how RVFV accesses the CNS and has utility for the evaluation of vaccines and/or therapeutics in preventing RVFV CNS disease.IMPORTANCE Animal models of viral disease are very important for understanding how viruses make people sick and for testing out drugs and vaccines to see if they can prevent disease. In this study, we identify the ferret as a model of encephalitis caused by Rift Valley fever virus (RVFV). This novel model will allow researchers to evaluate ways to prevent RVFV encephalitis.

Rift Valley Fever Virus Is Lethal in Different Inbred Mouse Strains Independent of Sex.

Rift Valley fever virus (RVFV) is a zoonotic arbovirus affecting humans and livestock in Africa and the Arabian Peninsula. The majority of human cases are mild and self-limiting; however, severe cases can result in hepatitis, encephalitis, or hemorrhagic fever. There is a lack of immunocompetent mouse models that faithfully recapitulate the varied clinical outcomes of RVF in humans. However, there are easily accessible and commonly used inbred mouse strains that have never been challenged with wild-type RVFV. Here, RVFV susceptibility and pathogenesis were evaluated across five commonly used inbred laboratory mouse strains: C57BL/6J, 129S1/SvlmJ, NOD/ShiLtJ, A/J, and NZO/HILtJ. Comparisons between different mouse strains, challenge doses, and sexes revealed exquisite susceptibility to wild-type RVFV in an almost uniform manner. Never before challenged NOD/ShiLtJ, A/J, and NZO/HILtJ mice showed similar phenotypes of Rift Valley fever disease as previously tested inbred mouse strains. The majority of infected mice died or were euthanized by day 5 post-infection due to overwhelming hepatic disease as evidenced by gross liver pathology and high viral RNA loads in the liver. Mice surviving past day 6 across all strains succumbed to late-onset encephalitis. Remarkably, sex was not found to impact survival or viral load and showed only modest effect on time to death and weight loss for any of the challenged mouse strains following RVFV infection. Regardless of sex, these inbred mouse strains displayed extreme susceptibility to wild-type RVFV down to one virus particle.

Animal models for COVID-19.

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is the aetiological agent of coronavirus disease 2019 (COVID-19), an emerging respiratory infection caused by the introduction of a novel coronavirus into humans late in 2019 (first detected in Hubei province, China). As of 18 September 2020, SARS-CoV-2 has spread to 215 countries, has infected more than 30 million people and has caused more than 950,000 deaths. As humans do not have pre-existing immunity to SARS-CoV-2, there is an urgent need to develop therapeutic agents and vaccines to mitigate the current pandemic and to prevent the re-emergence of COVID-19. In February 2020, the World Health Organization (WHO) assembled an international panel to develop animal models for COVID-19 to accelerate the testing of vaccines and therapeutic agents. Here we summarize the findings to date and provides relevant information for preclinical testing of vaccine candidates and therapeutic agents for COVID-19.

SARS-CoV-2 infection of African green monkeys results in mild respiratory disease discernible by PET/CT imaging and shedding of infectious virus from both respiratory and gastrointestinal tracts.

Vaccines are urgently needed to combat the global coronavirus disease 2019 (COVID-19) pandemic, and testing of candidate vaccines in an appropriate non-human primate (NHP) model is a critical step in the process. Infection of African green monkeys (AGM) with a low passage human isolate of SARS-CoV-2 by aerosol or mucosal exposure resulted in mild clinical infection with a transient decrease in lung tidal volume. Imaging with human clinical-grade 18F-fluoro-2-deoxy-D-glucose positron emission tomography (18F-FDG PET) co-registered with computed tomography (CT) revealed pulmonary lesions at 4 days post-infection (dpi) that resolved over time. Infectious virus was shed from both respiratory and gastrointestinal (GI) tracts in all animals in a biphasic manner, first between 2-7 dpi followed by a recrudescence at 14-21 dpi. Viral RNA (vRNA) was found throughout both respiratory and gastrointestinal systems at necropsy with higher levels of vRNA found within the GI tract tissues. All animals seroconverted simultaneously for IgM and IgG, which has also been documented in human COVID-19 cases. Young AGM represent an species to study mild/subclinical COVID-19 disease and with possible insights into live virus shedding. Future vaccine evaluation can be performed in AGM with correlates of efficacy being lung lesions by PET/CT, virus shedding, and tissue viral load.

Dynamics of human B and T cell adaptive immune responses to Kyasanur Forest disease virus infection.

Kyasanur Forest disease (KFD) is a tick-borne, acute, febrile viral illness endemic in southern India. No major studies have been done to understand the adaptive immune response during KFDV infection in humans. In this study, KFDV-positive patients were prospectively enrolled, and repeated peripheral blood collections were performed. Clinical and virologic characterization of these samples is reported along with phenotypic analysis of cellular immunity and quantitation of humoral immunity. We noted robust T and B cell responses, particularly of CD8 T cells, during KFDV infection in most of the patients. Virus clearance from the blood coincided with peak CD8 T cell activation and the appearance of KFDV-specific IgG. Increased frequency of plasmablasts and very few activated B cells were observed in the acute phase of KFD infection. Notably, only humoral immunity and activated B cell frequency in the acute phase correlated with prior KFDV vaccination, and only with 2 or more doses. This novel work has implications in KFD vaccine research as well as in understanding the pathogenesis.