Biological Characterization of Yellow Fever Viruses Isolated From Non-human Primates in Brazil With Distinct Genomic Landscapes

A safer cell-based yellow fever live attenuated vaccine protects mice against YFV infection

The live attenuated yellow fever vaccine (YF17D) has caused controversial safety issues in history with low-yield problems, which has led to a large population being unable to be vaccinated and vaccine shortage in facing recent outbreaks. Here, we report a safer live attenuated vaccine candidate, YF17D-Δ77, which contains 77 nucleotides deletion in the 3' untranslated region (3' UTR) of the YF17D genome. YF17D-Δ77 exhibited no neurotropism and decreased viscerotropism and caused significantly lower lethality in mice compared to YF17D. Mechanistically, the deletion enhanced the sensitivity of the virus to type I and type II interferon responses, which hindered viral replication. Encouragingly, YF17D-Δ77 provided comparable immune protection in mice as did YF17D. Even 10 PFU of YF17D-Δ77 completely protected mice against YFV-Asibi challenge. In addition, the Δ77 mutation showed excellent stability after successive passages in Vero cells. Collectively, the data suggest that further development of YF17D-Δ77 as vaccine candidate is warranted.

Evaluation of humoral immune response after yellow fever infection: an observational study on patients from the 2017-2018 sylvatic outbreak in Brazil

Between 2016 and 2018, Brazil experienced major sylvatic yellow fever (YF) outbreaks that caused hundreds of casualties, with Minas Gerais (MG) being the most affected state. These outbreaks provided a unique opportunity to assess the immune response triggered by the wild-type (WT) yellow fever virus (YFV) in humans. The plaque reduction neutralization test (PRNT) is currently the standard method to assess the humoral immune response to YFV by measuring neutralizing antibodies (nAbs). The present study aimed to evaluate the humoral immune response of patients from the 2017-2018 sylvatic YF outbreak in MG with different disease outcomes by using PRNTs with a WT YFV strain, isolated from the 2017-2018 outbreak, and a vaccine YFV strain. Samples from naturally infected YF patients were tested, in comparison with healthy vaccinees. Results showed that both groups presented different levels of nAb against the WT and vaccine strains, and the levels of neutralization against the strains varied homotypically and heterotypically. Results based on the geometric mean titers (GMTs) suggest that the humoral immune response after a natural infection of YFV can reach higher levels than that induced by vaccination (GMT of patients against WT YFV compared to GMT of vaccinees, P < 0.0001). These findings suggest that the humoral immune responses triggered by the vaccine and WT strains of YFV are different, possibly due to genetic and antigenic differences between these viruses. Therefore, current means of assessing the immune response in naturally infected YF individuals and immunological surveillance methods in areas with intense viral circulation may need to be updated.IMPORTANCEYellow fever is a deadly febrile disease caused by the YFV. Despite the existence of effective vaccines, this disease still represents a public health concern worldwide. Much is known about the immune response against the vaccine strains of the YFV, but recent studies have shown that it differs from that induced by WT strains. The extent of this difference and the mechanisms behind it are still unclear. Thus, studies aimed to better understand the immune response against this virus are relevant and necessary. The present study evaluated levels of neutralizing antibodies of yellow fever patients from recent outbreaks in Brazil, in comparison with healthy vaccinees, using plaque reduction neutralization tests with WT and vaccine YFV strains. Results showed that the humoral immune response in naturally infected patients was higher than that induced by vaccination, thus providing new insights into the immune response triggered against these viruses.

Aedes-borne orthoflavivirus infections in neotropical primates - Ecology, susceptibility, and pathogenesis

Arboviral diseases comprise a group of important infectious diseases imposing a heavy burden to public health in many locations of the world. Orthoflaviviruses are viruses belonging to the genus Orthoflavivirus; this genus includes some of the most relevant arboviruses to human health. Orthoflaviviruses can infect several different hosts, with some species being transmitted in cycles involving birds and anthropophilic mosquitoes and others transmitted between mammals and mostly Aedes sp. mosquitoes. Some of the most important sylvatic reservoirs of orthoflaviviruses are non-human primates (NHPs). Many flaviviruses that infect NHPs in nature have the potential to cause epidemics in humans, as has been observed in the cases of Orthoflavivirus denguei (dengue virus - DENV), Orthoflavivirus flavi (yellow fever virus - YFV), and Orthoflavivirus zikaense (Zika virus - ZIKV). In this minireview, we discuss important aspects regarding history, ecology involving NHP, distribution, disease outcome, and pathogenesis of these three major orthoflaviviruses that affect humans and NHP and relate this information to the potential of using NHP as experimental models. In addition, we suggest some orthoflaviviruses that could be better investigated, both in nature and in experimental studies, in light of the recent revolution in molecular biology.

Therapeutic neutralizing monoclonal antibody administration protects against lethal yellow fever virus infection

Yellow fever virus (YFV) is a reemerging global health threat, driven by several factors, including increased spread of the mosquito vector and rapid urbanization. Although a prophylactic vaccine exists, vaccine hesitancy, supply deficits, and distribution difficulties leave specific populations at risk of severe YFV disease, as evidenced by recent outbreaks in South America. To establish a treatment for patients with severe YFV infection, we tested 37 YFV-specific monoclonal antibodies isolated from vaccinated humans and identified two capable of potently neutralizing multiple pathogenic primary YFV isolates. Using both hamster and nonhuman primate models of lethal YFV infection, we demonstrate that a single administration of either of these two potently neutralizing antibodies during acute infection fully controlled viremia and prevented severe disease and death in treated animals. Given the potential severity of YFV-induced disease, our results show that these antibodies could be effective in saving lives and fill a much-needed void in managing YFV cases during outbreaks.

Ecological, Genetic, and Phylogenetic Aspects of YFV 2017-2019 Spread in Rio de Janeiro State

In Brazil, a yellow fever (YF) outbreak was reported in areas considered YF-free for decades. The low vaccination coverage and the increasing forest fragmentation, with the wide distribution of vector mosquitoes, have been related to yellow fever virus (YFV) transmission beyond endemic areas since 2016. Aiming to elucidate the molecular and phylogenetic aspects of YFV spread on a local scale, we generated 43 new YFV genomes sampled from humans, non-human primates (NHP), and primarily, mosquitoes from highly heterogenic areas in 15 localities from Rio de Janeiro (RJ) state during the YFV 2016-2019 outbreak in southeast Brazil. Our analysis revealed that the genetic diversity and spatial distribution of the sylvatic transmission of YFV in RJ originated from at least two introductions and followed two chains of dissemination, here named the YFV RJ-I and YFV RJ-II clades. They moved with similar dispersal speeds from the north to the south of the RJ state in parallel directions, separated by the Serra do Mar Mountain chain, with YFV RJ-I invading the north coast of São Paulo state. The YFV RJ-I clade showed a more significant heterogeneity across the entire polyprotein. The YFV RJ-II clade, with only two amino acid polymorphisms, mapped at NS1 (I1086V), present only in mosquitoes at the same locality and NS4A (I2176V), shared by all YFV clade RJ-II, suggests a recent clustering of YFV isolates collected from different hosts. Our analyses strengthen the role of surveillance, genomic analyses of YVF isolated from other hosts, and environmental studies into the strategies to forecast, control, and prevent yellow fever outbreaks.

Amino Acid Polymorphisms on the Brazilian Strain of Yellow Fever Virus Methyltransferase Are Related to the Host's Immune Evasion Mediated by Type I Interferon

Since late 2016, a yellow fever virus (YFV) variant carrying a set of nine amino acid variations has circulated in South America. Three of them were mapped on the methyltransferase (MTase) domain of viral NS5 protein. To assess whether these changes affected viral infectivity, we synthesized YFV carrying the MTase of circulating lineage as well as its isoform with the residues of the previous strains (NS5 K101R, NS5 V138I, and NS5 G173S). We observed a slight difference in viral growth properties and plaque phenotype between the two synthetic YFVs. However, the MTase polymorphisms associated with the Brazilian strain of YFV (2016-2019) confer more susceptibility to the IFN-I. In addition, in vitro MTase assay revealed that the interaction between the YFV MTase and the methyl donor molecule (SAM) is altered in the Brazilian MTase variant. Altogether, the results reported here describe that the MTase carrying the molecular signature of the Brazilian YFV circulating since 2016 might display a slight decrease in its catalytic activity but virtually no effect on viral fitness in the parameters comprised in this study. The most marked influence of these residues stands in the immune escape against the antiviral response mediated by IFN-I.

Scratching the Surface Takes a Toll: Immune Recognition of Viral Proteins by Surface Toll-like Receptors

Early innate viral recognition by the host is critical for the rapid response and subsequent clearance of an infection. Innate immune cells patrol sites of infection to detect and respond to invading microorganisms including viruses. Surface Toll-like receptors (TLRs) are a group of pattern recognition receptors (PRRs) that can be activated by viruses even before the host cell becomes infected. However, the early activation of surface TLRs by viruses can lead to viral clearance by the host or promote pathogenesis. Thus, a plethora of research has attempted to identify specific viral ligands that bind to surface TLRs and mediate progression of viral infection. Herein, we will discuss the past two decades of research that have identified specific viral proteins recognized by cell surface-associated TLRs, how these viral proteins and host surface TLR interactions affect the host inflammatory response and outcome of infection, and address why controversy remains regarding host surface TLR recognition of viral proteins.

Phenotypic and Genetic Studies of the Viral Lineage Associated with the Recent Yellow Fever Outbreak in Brazil

Yellow fever virus (YFV) caused an outbreak in the Brazilian Southeast from 2016 to 2019, of the most significant magnitude since the 1900s. An investigation of the circulating virus revealed that most of the genomes detected in this period carried nine unique amino acid polymorphisms, with eight located in the non-structural proteins NS3 and NS5, which are pivotal for viral replication. To elucidate the effect of these amino acid changes on viral infection, we constructed viruses carrying amino acid alterations in NS3 and NS5, performed infection in different cells, and assessed their neurovirulence in BALB/c mice and infected AG129 mice. We observed that the residues that compose the YFV 2016-2019 molecular signature in the NS5 protein might have been related to an attenuated phenotype, and that the alterations in the NS3 protein only slightly affected viral infection in AG129 mice, increasing to a low extent the mortality rate of these animals. These results contributed to unveiling the role of specific naturally occurring amino acid changes in the circulating strain of YFV in Brazil.