Structure and Characterization of Crimean-Congo Hemorrhagic Fever Virus GP38

Immunogenicity of NSDV GP38 and the role of furin in GP38 proteolytic processing

Nairobi sheep disease virus (NSDV) is a tick-borne orthonairovirus, which is genetically related to Crimean-Congo hemorrhagic fever virus (CCHFV), and causes severe hemorrhagic gastroenteritis in infected sheep. CCHFV GP38, a cleavage product of the CCHFV glycoprotein precursor (GPC), has recently attracted attention: not only has GP38 been reported to elicit detectable anti-GP38 antibodies in CCHFV-infected patients, but anti-GP38 antibodies have also been shown to protect mice from lethal CCHFV challenge. While proteolytic cleavage of CCHFV GP38 has been described to involve the proprotein convertases furin and subtilisin/kexin-isozyme-1 (SKI-1), little is known about the processing of NSDV GPC, or the occurrence and immunogenicity of NSDV GP38 in infected sheep. Here, we provide the first evidence for the presence and immunogenicity of NSDV GP38 in infected sheep demonstrating seroconversion by the detection of anti-GP38 antibodies over the course of infection. To further characterize GPC processing in vitro, we investigated the impact of furin overexpression and the effect of a furin inhibitor on NSDV glycoprotein expression, cleavage, and viral infectivity. While virus infectivity remained unaffected, our results suggest that other proteases besides furin may play a role in the proteolytic processing of NSDV GPC at a cleavage site that remains to be explored. Taken together, our findings highlight the immunogenicity of NSDV GP38 in sheep and warrant further research into the similarities and differences in proteolytic cleavage between the glycoproteins of NSDV and other orthonairoviruses, such as CCHFV.

IMPORTANCE

Nairobi sheep disease virus (NSDV) is a zoonotic orthonairovirus causing severe and often fatal hemorrhagic gastroenteritis in small ruminants. Its genetic relationship to human-pathogenic Crimean-Congo hemorrhagic fever virus (CCHFV) and striking similarities in the clinical picture between CCHFV-infected patients and NSDV-infected ruminants have led to the idea that NSDV could serve as a model organism to study CCHFV pathogenesis. However, knowledge on NSDV-host interactions has been limited. While CCHFV GP38 has recently attracted attention as vaccine candidate and possible virulence factor, the occurrence and role of putative GP38 in other orthonairoviruses has been unclear. This study provides first evidence for the presence and immunogenicity of NSDV GP38 in infected sheep. Furthermore, our data indicate that other proteases besides furin may be involved in the proteolytic cleavage of NSDV GPC. Future studies are needed to determine the proteases involved and to investigate the possible functional role of GP38 in NSDV pathogenesis.

Virucidal Approaches for Hemorrhagic Fever Viruses

We have reviewed the primary literature on the virucidal efficacy of microbicidal active ingredients, formulated microbicides, and physical inactivation approaches (heat, irradiation) for hemorrhagic fever viruses (HFVs) (arenaviruses, filoviruses, flaviviruses, hantaviruses, nairoviruses, and phenuiviruses), and for two non-typical HFV paramyxoviruses. As each of these HFVs are large, lipid-enveloped RNA viruses, their susceptibilities to virucidal agents are informed by the so-called hierarchy of susceptibility of pathogens to microbicides. The unique susceptibility of lipid-enveloped viruses to most classes of microbicides is based on the common mechanisms of action of envelope-disrupting microbicides. Despite this, due to the relatively great lethality of these viruses, it is prudent (where possible) to confirm the expected efficacies of inactivation approaches in testing involving the HFVs themselves (as opposed to less lethal surrogate viruses) using field-relevant methods. Empirical data for virucidal activities of microbicidal active ingredients, formulated microbicides, and physical inactivation approaches, such as heat, ultraviolet light, and gamma irradiation, that were collected specifically for HFVs have been reviewed and summarized in this paper. These empirical data for surface and hand hygiene approaches, liquid inactivation approaches, and approaches for rendering diagnostic samples safe to handle inform non-pharmaceutical interventions intended to mitigate transmission risk associated with these HFVs.

A Longitudinal Analysis of Memory Immune Responses in Convalescent Crimean-Congo Hemorrhagic Fever Survivors in Uganda

Evaluating the adaptive immune responses to natural infection with Crimean-Congo hemorrhagic fever (CCHF) virus (CCHFV) in human survivors is critical to the development of medical countermeasures. However, the correlates of protection are unknown. As the most prevalent tick-borne human hemorrhagic fever virus with case fatality rates of 5%-30% and worldwide distribution, there is an urgent need to fill these knowledge gaps. Here, we describe adaptive immune responses in a cohort of Ugandan CCHF survivors via serial sampling over 6 years. We demonstrate persistent antibodies after infection and cross-neutralization against various clades of authentic CCHFV, as well as potent effector function. Moreover, we show for the first time persistent, polyfunctional antigen-specific memory T-cell responses to multiple CCHFV proteins up to 9 years after infection. Together, this data provides immunological benchmarks for evaluating CCHFV medical countermeasures and information that can be leveraged toward vaccine immunogen design and viral target identification for monoclonal antibody therapies.

Antibodies targeting Crimean-Congo hemorrhagic fever virus GP38 limit vascular leak and viral spread

Crimean-Congo hemorrhagic fever virus (CCHFV) is a priority pathogen transmitted by tick bites, with no vaccines or specific therapeutics approved to date. Severe disease manifestations include hemorrhage, endothelial dysfunction, and multiorgan failure. Infected cells release the viral glycoprotein GP38, whose extracellular function is presently unknown. GP38 is considered an important target for vaccine and therapeutic design because GP38-specific antibodies can protect against severe disease in animal models, albeit through an unknown mechanism of action. Here, we showed that GP38 induces endothelial barrier dysfunction in vitro by disrupting the endothelial glycocalyx layer and triggering hyperpermeability. We also demonstrated that GP38 alone can cause vascular leak in a mouse model. We found that CCHFV infection leads to vascular leak in vivo, which was exacerbated by exogenous administration of GP38, facilitating dissemination of CCHFV into target tissues such as the liver. Protective antibodies that recognized specific antigenic sites on GP38, but not a protective neutralizing antibody binding the structural protein Gc, potently inhibited endothelial hyperpermeability in vitro and vascular leak in vivo during CCHFV infection. This work uncovers a function of the circulating viral protein GP38 as a viral toxin in CCHFV pathogenesis and elucidates a potential mode of action of nonneutralizing yet protective GP38-specific antibodies.

Engineering and structures of Crimean-Congo hemorrhagic fever virus glycoprotein complexes

Crimean-Congo hemorrhagic fever virus (CCHFV) is a tickborne virus that can cause severe disease in humans with case fatality rates of 10%-40%. Although structures of CCHFV glycoproteins GP38 and Gc have provided insights into viral entry and defined epitopes of neutralizing and protective antibodies, the structure of glycoprotein Gn and its interactions with GP38 and Gc have remained elusive. Here, we use structure-guided protein engineering to produce a stabilized GP38-Gn-Gc heterotrimeric glycoprotein complex (GP38-GnH-DS-Gc). A cryo-electron microscopy (cryo-EM) structure of this complex provides the molecular basis for GP38's association on the viral surface, reveals the structure of Gn, and demonstrates that GP38-Gn restrains the Gc fusion loops in the prefusion conformation, facilitated by an N-linked glycan attached to Gn. Immunization with GP38-GnH-DS-Gc conferred 40% protection against lethal IbAr10200 challenge in mice. These data define the architecture of a GP38-Gn-Gc protomer and provide a template for structure-guided vaccine antigen development.

Emerging and reemerging infectious diseases: global trends and new strategies for their prevention and control

To adequately prepare for potential hazards caused by emerging and reemerging infectious diseases, the WHO has issued a list of high-priority pathogens that are likely to cause future outbreaks and for which research and development (R&D) efforts are dedicated, known as paramount R&D blueprints. Within R&D efforts, the goal is to obtain effective prophylactic and therapeutic approaches, which depends on a comprehensive knowledge of the etiology, epidemiology, and pathogenesis of these diseases. In this process, the accessibility of animal models is a priority bottleneck because it plays a key role in bridging the gap between in-depth understanding and control efforts for infectious diseases. Here, we reviewed preclinical animal models for high priority disease in terms of their ability to simulate human infections, including both natural susceptibility models, artificially engineered models, and surrogate models. In addition, we have thoroughly reviewed the current landscape of vaccines, antibodies, and small molecule drugs, particularly hopeful candidates in the advanced stages of these infectious diseases. More importantly, focusing on global trends and novel technologies, several aspects of the prevention and control of infectious disease were discussed in detail, including but not limited to gaps in currently available animal models and medical responses, better immune correlates of protection established in animal models and humans, further understanding of disease mechanisms, and the role of artificial intelligence in guiding or supplementing the development of animal models, vaccines, and drugs. Overall, this review described pioneering approaches and sophisticated techniques involved in the study of the epidemiology, pathogenesis, prevention, and clinical theatment of WHO high-priority pathogens and proposed potential directions. Technological advances in these aspects would consolidate the line of defense, thus ensuring a timely response to WHO high priority pathogens.

Crimean Congo hemorrhagic fever virus nucleoprotein and GP38 subunit vaccine combination prevents morbidity in mice

Immunizing mice with Crimean-Congo hemorrhagic fever virus (CCHFV) nucleoprotein (NP), glycoprotein precursor (GPC), or with the GP38 domain of GPC, can be protective when the proteins are delivered with viral vectors or as a DNA or RNA vaccine. Subunit vaccines are a safe and cost-effective alternative to some vaccine platforms, but Gc and Gn glycoprotein subunit vaccines for CCHFV fail to protect despite eliciting high levels of neutralizing antibodies. Here, we investigated humoral and cellular immune responses and the protective efficacy of recombinant NP, GP38, and GP38 forms (GP85 and GP160) associated with the highly glycosylated mucin-like (MLD) domain, as well as the NP + GP38 combination. Vaccination with GP160, GP85, or GP38 did not confer protection, and vaccination with the MLD-associated GP38 forms blunted the humoral immune responses to GP38, worsened clinical chemistry, and increased viral RNA in the blood compared to the GP38 vaccination. In contrast, NP vaccination conferred 100% protection from lethal outcome and was associated with mild clinical disease, while the NP + GP38 combination conferred even more robust protection by reducing morbidity compared to mice receiving NP alone. Thus, recombinant CCHFV NP alone is a promising vaccine candidate conferring 100% survival against heterologous challenge. Moreover, incorporation of GP38 should be considered as it further enhances subunit vaccine efficacy by reducing morbidity in surviving animals.

Crimean-Congo hemorrhagic fever survivors elicit protective non-neutralizing antibodies that target 11 overlapping regions on glycoprotein GP38

Crimean-Congo hemorrhagic fever virus can cause lethal disease in humans yet there are no approved medical countermeasures. Viral glycoprotein GP38, exclusive to Nairoviridae, is a target of protective antibodies and is a key antigen in preclinical vaccine candidates. Here, we isolate 188 GP38-specific antibodies from human survivors of infection. Competition experiments show that these antibodies bind across 5 distinct antigenic sites, encompassing 11 overlapping regions. Additionally, we show structures of GP38 bound with 9 of these antibodies targeting different antigenic sites. Although these GP38-specific antibodies are non-neutralizing, several display protective efficacy equal to or better than murine antibody 13G8 in two highly stringent rodent models of infection. Together, these data expand our understanding regarding this important viral protein and may inform the development of broadly effective CCHFV antibody therapeutics.

Novel clades of tick-borne pathogenic nairoviruses in Europe

Members of the Orthonairovirus genus (family Nairoviridae) include many tick-borne viruses of significant human and animal health impact, with several recently-documented pathogenic viruses lacking sufficient epidemiological information. We screened 215 adult ticks of seven species collected in Bulgaria, Georgia, Latvia and Poland for orthonairoviruses, followed by nanopore sequencing (NS) for genome characterization. Initial generic amplification revealed Sulina virus (SULV, Orthonairovirus sulinaense), for which an updated amplification assay was used, revealing an overall prevalence of 2.7% in Ixodes ricinus ticks from Latvia. Three complete and additional partial SULV genomes were generated, that consistently formed a separate, distinct clade with further intragroup divergence in the maximum likelihood analyses. Comparisons with previously described viruses from Romania exhibited similar genome topologies, albeit with divergent motifs and cleavage sites on the glycoprotein precursor. Preliminary evidence of recombination involving the S segment was documented, in addition to variations in predicted viral glycoproteins. Generic screening further identified Tacheng tick virus 1 (TCTV1, Orthonairovirus tachengense), with documented human infections, in Dermacentor reticulatus ticks from Poland, with a prevalence of 0.9%. Subsequent NS and assembly provided the first complete TCTV1 genome outside of China, where it was originally described. Phylogenetic analysis of virus genome segments revealed TCTV1-Poland as a discrete taxon within the TCTV1 cluster in the Orthonairovirus genus, representing a geographically segregated clade. Comparable genome topology with TCTV1 from China was observed, aside from minor variations in the M segment. Similar to SULV, TCTV1 exhibited several mismatches on previously described screening primer binding sites, likely to prevent amplification. These findings indicate presence of novel TCTV1 and SULV clades in Eastern Europe, confirming the expansion of orthonairoviruses with pathogenic potential.

Vaccine approaches and treatment aspects against Crimean Congo hemorrhagic fever

Crimean-Congo hemorrhagic fever [CCHF] is a severe infectious viral disease caused by a tick borne virus which can lead to fatal hemorrhagic disease in humans. It has been reported from some continents including Africa, Asia and Europe. Virus is transmitted to human mainly through tick bite, whose acquire infection from reservoirs wild and domesticated mammalians and ostriches. Currently no approved vaccine or drug is available for CCHF and prevention is mainly based on biosecurity measures. Ribavirin is the only approved drug that has been used in some countries to treat human disease, however some new studies did not prove the Ribavirin efficacy. Different strategies to design effective vaccines, have been conducted through years, from inactivated virus to nucleotide-based ones including DNA and mRNA vaccines. In this study we review of pioneering vaccine candidate platforms.

Antibodies targeting Crimean-Congo hemorrhagic fever virus GP38 limit vascular leak and viral spread

Crimean-Congo hemorrhagic fever virus (CCHFV) is a priority pathogen transmitted by tick bites, with no vaccines or specific therapeutics approved to date. Severe disease manifestations include hemorrhage, endothelial dysfunction, and multiorgan failure. Infected cells secrete the viral glycoprotein GP38, whose extracellular function is presently unknown. GP38 is considered an important target for vaccine and therapeutic design as GP38-specific antibodies can protect against severe disease in animal models, albeit through a currently unknown mechanism of action. Here, we show that GP38 induces endothelial barrier dysfunction in vitro, and that CCHFV infection, and GP38 alone, can trigger vascular leak in a mouse model. Protective antibodies that recognize specific antigenic sites on GP38, but not a protective neutralizing antibody binding the structural protein Gc, potently inhibit endothelial hyperpermeability in vitro and vascular leak in vivo during CCHFV infection. This work uncovers a function of the secreted viral protein GP38 as a viral toxin in CCHFV pathogenesis and elucidates the mode of action of non-neutralizing GP38-specific antibodies.

Third International Conference on Crimean-Congo Hemorrhagic Fever in Thessaloniki, Greece, September 19-21, 2023

The Third International Conference on Crimean-Congo Hemorrhagic Fever (CCHF) was held in Thessaloniki, Greece, September 19-21, 2023, bringing together a diverse group of international partners, including public health professionals, clinicians, ecologists, epidemiologists, immunologists, and virologists. The conference was attended by 118 participants representing 24 countries and the World Health Organization (WHO). Meeting sessions covered the epidemiology of CCHF in humans; Crimean-Congo hemorrhagic fever virus (CCHFV) in ticks; wild and domestic animal hosts; molecular virology; pathogenesis and animal models; immune response related to therapeutics; and CCHF prevention in humans. The concluding session focused on recent WHO recommendations regarding disease prevention, control strategies, and innovations against CCHFV outbreaks. This meeting report summarizes lectures by the invited speakers and highlights advances in the field.

Engineering, structure, and immunogenicity of a Crimean-Congo hemorrhagic fever virus pre-fusion heterotrimeric glycoprotein complex

Crimean-Congo hemorrhagic fever virus (CCHFV) is a tick-borne virus that can cause severe disease in humans with case fatality rates of 10-40%. Although structures of CCHFV glycoproteins GP38 and Gc have provided insights into viral entry and defined epitopes of neutralizing and protective antibodies, the structure of glycoprotein Gn and its interactions with GP38 and Gc have remained elusive. Here, we used structure-guided protein engineering to produce a stabilized GP38-Gn-Gc heterotrimeric glycoprotein complex (GP38-GnH-DS-Gc). A cryo-EM structure of this complex provides the molecular basis for GP38's association on the viral surface, reveals the structure of Gn, and demonstrates that GP38-Gn restrains the Gc fusion loops in the prefusion conformation, facilitated by an N-linked glycan attached to Gn. Immunization with GP38-GnH-DS-Gc conferred 40% protection against lethal IbAr10200 challenge in mice. These data define the architecture of a GP38-Gn-Gc protomer and provide a template for structure-guided vaccine antigen development.

Bibliometric analysis for the identification of main limitations and future directions of vaccines for the control of ticks and tick-borne pathogens in Uganda

Ticks and tick-borne diseases (TBD) are a growing threat for human and animal health worldwide with high incidence in African countries such as Uganda where it affects cattle health and production. Considering recent advances in bibliometric analysis, in this review we used a bibliometric descriptive approach for the analysis of publications and patents in the fields of ticks, TBD, and vaccines in Uganda. The results showed that major gaps and limitations are associated with (i) low contributions from Ugandan institutions, (ii) limited international collaborations, (iii) poor impact of translational research, and (iv) little research on tick control vaccines. The results were then used to propose future directions to approach these limitations in Uganda. Although ongoing initiatives and international collaborations are contributing to address major gaps and limitations, future directions should advance in these collaborative projects together with new initiatives addressing (i) basic and translational research on TBD such as CCHF and ASF, (ii) participation of Ugandan institutions in new international consortia in this area, (iii) promoting communication of these initiatives to Ugandan cattle holders and general population to attract support from public and private sectors, (iv) stimulate and support scientific publications and patents with participation of Ugandan scientists, and (v) build and implement production capacity for vaccines in Uganda. These results contribute to guiding Ugandan scientists and national authorities to face challenges posed by ticks and TBD with implications for other African countries.

The Adaptive Immune Response against Bunyavirales

The Bunyavirales order includes at least fourteen families with diverse but related viruses, which are transmitted to vertebrate hosts by arthropod or rodent vectors. These viruses are responsible for an increasing number of outbreaks worldwide and represent a threat to public health. Infection in humans can be asymptomatic, or it may present with a range of conditions from a mild, febrile illness to severe hemorrhagic syndromes and/or neurological complications. There is a need to develop safe and effective vaccines, a process requiring better understanding of the adaptive immune responses involved during infection. This review highlights the most recent findings regarding T cell and antibody responses to the five Bunyavirales families with known human pathogens (Peribunyaviridae, Phenuiviridae, Hantaviridae, Nairoviridae, and Arenaviridae). Future studies that define and characterize mechanistic correlates of protection against Bunyavirales infections or disease will help inform the development of effective vaccines.

Crimean-Congo Hemorrhagic Fever Survivors Elicit Protective Non-Neutralizing Antibodies that Target 11 Overlapping Regions on Viral Glycoprotein GP38

Crimean-Congo hemorrhagic fever virus can cause lethal disease in humans yet there are no approved medical countermeasures. Viral glycoprotein GP38, unique to Nairoviridae, is a target of protective antibodies, but extensive mapping of the human antibody response to GP38 has not been previously performed. Here, we isolated 188 GP38-specific antibodies from human survivors of infection. Competition experiments showed that these antibodies bind across five distinct antigenic sites, encompassing eleven overlapping regions. Additionally, we reveal structures of GP38 bound with nine of these antibodies targeting different antigenic sites. Although GP38-specific antibodies were non-neutralizing, several antibodies were found to have protection equal to or better than murine antibody 13G8 in two highly stringent rodent models of infection. Together, these data expand our understanding regarding this important viral protein and inform the development of broadly effective CCHFV antibody therapeutics.

Nucleocapsid protein-specific monoclonal antibodies protect mice against Crimean-Congo hemorrhagic fever virus

Crimean-Congo hemorrhagic fever virus (CCHFV) is a WHO priority pathogen. Antibody-based medical countermeasures offer an important strategy to mitigate severe disease caused by CCHFV. Most efforts have focused on targeting the viral glycoproteins. However, glycoproteins are poorly conserved among viral strains. The CCHFV nucleocapsid protein (NP) is highly conserved between CCHFV strains. Here, we investigate the protective efficacy of a CCHFV monoclonal antibody targeting the NP. We find that an anti-NP monoclonal antibody (mAb-9D5) protected female mice against lethal CCHFV infection or resulted in a significant delay in mean time-to-death in mice that succumbed to disease compared to isotype control animals. Antibody protection is independent of Fc-receptor functionality and complement activity. The antibody bound NP from several CCHFV strains and exhibited robust cross-protection against the heterologous CCHFV strain Afg09-2990. Our work demonstrates that the NP is a viable target for antibody-based therapeutics, providing another direction for developing immunotherapeutics against CCHFV.

Neutralizing monoclonal antibodies against the Gc fusion loop region of Crimean-Congo hemorrhagic fever virus

Crimean-Congo hemorrhagic fever virus (CCHFV) is a highly pathogenic tick-borne virus, prevalent in more than 30 countries worldwide. Human infection by this virus leads to severe illness, with an average case fatality of 40%. There is currently no approved vaccine or drug to treat the disease. Neutralizing antibodies are a promising approach to treat virus infectious diseases. This study generated 37 mouse-derived specific monoclonal antibodies against CCHFV Gc subunit. Neutralization assays using pseudotyped virus and authentic CCHFV identified Gc8, Gc13, and Gc35 as neutralizing antibodies. Among them, Gc13 had the highest neutralizing activity and binding affinity with CCHFV Gc. Consistently, Gc13, but not Gc8 or Gc35, showed in vivo protective efficacy (62.5% survival rate) against CCHFV infection in a lethal mouse infection model. Further characterization studies suggested that Gc8 and Gc13 may recognize a similar, linear epitope in domain II of CCHFV Gc, while Gc35 may recognize a different epitope in Gc. Cryo-electron microscopy of Gc-Fab complexes indicated that both Gc8 and Gc13 bind to the conserved fusion loop region and Gc13 had stronger interactions with sGc-trimers. This was supported by the ability of Gc13 to block CCHFV GP-mediated membrane fusion. Overall, this study provides new therapeutic strategies to treat CCHF and new insights into the interaction between antibodies with CCHFV Gc proteins.

Do NSm Virulence Factors in the Bunyavirales Viral Order Originate from Gn Gene Duplication?

One-third of the nine WHO shortlisted pathogens prioritized for research and development in public health emergencies belong to the Bunyavirales order. Several Bunyavirales species carry an NSm protein that acts as a virulence factor. We predicted the structures of these NSm proteins and unexpectedly found that in two families, their cytosolic domain was inferred to have a similar fold to that of the cytosolic domain of the viral envelope-forming glycoprotein N (Gncyto) encoded on the same genome fragment. We show that although the sequence identity between the NSmcyto and the Gncyto domains is low, the conservation of the two zinc finger-forming CysCysHisCys motifs explains the predicted structural conservation. Importantly, our predictions provide a first glimpse into the long-unknown structure of NSm. Also, these predictions suggest that NSm is the result of a gene duplication event in the Bunyavirales Nairoviridae and Peribunyaviridae families and that such events may be common in the recent evolutionary history of RNA viruses.

Crimean-Congo Hemorrhagic Fever Virus: Progress in Vaccine Development

Crimean-Congo hemorrhagic fever virus (CCHFV), a member of the Nairoviridae family and Bunyavirales order, is transmitted to humans via tick bites or contact with the blood of infected animals. It can cause severe symptoms, including hemorrhagic fever, with a mortality rate between 5 to 30%. CCHFV is classified as a high-priority pathogen by the World Health Organization (WHO) due to its high fatality rate and the absence of effective medical countermeasures. CCHFV is endemic in several regions across the world, including Africa, Europe, the Middle East, and Asia, and has the potential for global spread. The emergence of the disease in new areas, as well as the presence of the tick vector in countries without reported cases, emphasizes the need for preventive measures to be taken. In the past, the lack of a suitable animal model susceptible to CCHFV infection has been a major obstacle in the development of vaccines and treatments. However, recent advances in biotechnology and the availability of suitable animal models have significantly expedited the development of vaccines against CCHF. These advancements have not only contributed to an enhanced understanding of the pathogenesis of CCHF but have also facilitated the evaluation of potential vaccine candidates. This review outlines the immune response to CCHFV and animal models utilized for the study of CCHFV and highlights the progress made in CCHFV vaccine studies. Despite remarkable advancements in vaccine development for CCHFV, it remains crucial to prioritize continued research, collaboration, and investment in this field.

Comparative characterization of Crimean-Congo hemorrhagic fever virus cell culture systems with application to propagation and titration methods

Crimean-Congo hemorrhagic fever orthonairovirus (CCHFV) is a biosafety level 4 and World Health Organization top priority pathogen. Infection leads to an often fatal hemorrhagic fever disease in humans. The tick-borne virus is endemic in countries across Asia, Europe and Africa, with signs of spreading into new regions. Despite the severity of disease and the potential of CCHFV geographic expansion to cause widespread outbreaks, no approved vaccine or treatment is currently available. Critical for basic research and the development of diagnostics or medical countermeasures, CCHFV viral stocks are commonly produced in Vero E6 and SW-13 cell lines. While a variety of in-house methods are being used across different laboratories, there has been no clear, specific consensus on a standard, optimal system for CCHFV growth and titration. In this study, we perform a systematic, side-by-side characterization of Vero E6 and SW-13 cell lines concerning the replication kinetics of CCHFV under different culture conditions. SW-13 cells are typically cultured in a CO₂-free condition (SW-13 CO₂^-) according to the American Type Culture Collection. However, we identify a CO₂-compatible culture condition (SW-13 CO₂⁺) that demonstrates the highest viral load (RNA concentration) and titer (infectious virus concentration) in the culture supernatants, in comparison to SW-13 CO₂^- and Vero E6 cultures. This optimal viral propagation system also leads to the development of two titration methods: an immunostaining-based plaque assay using a commercial CCHFV antibody and a colorimetric readout, and an antibody staining-free, cytopathic effect-based median tissue culture infectious dose assay using a simple excel calculator. These are anticipated to serve as a basis for a reproducible, standardized and user-friendly platform for CCHFV propagation and titration.

Multiplex Assay for Simultaneous Detection of Antibodies against Crimean-Congo Hemorrhagic Fever Virus Nucleocapsid Protein and Glycoproteins in Ruminants

Crimean-Congo hemorrhagic fever virus (CCHFV) is a widespread tick-borne zoonotic virus that causes Crimean-Congo hemorrhagic fever (CCHF). CCHF is asymptomatic in infected animals but can develop into severe illness in humans, with high case-fatality rates. Due to complex environmental and socio-economic factors, the distribution of CCHFV vectors is changing, leading to disease occurrence in previously unaffected countries. Neither an effective treatment nor a vaccine has been developed against CCHFV; thus, surveillance programs are essential to limit and control the spread of the virus. Furthermore, the WHO highlighted the need of assays that can cover a range of CCHFV antigenic targets, DIVA (differentiating infected from vaccinated animals) assays, or assays for future vaccine evaluation. Here, we developed a multiplex assay, based on a suspension microarray, able to detect specific antibodies in ruminants to three recombinantly produced CCHFV proteins: the nucleocapsid (N) protein and two glycoproteins, GN ectodomain (GNe), and GP38. This triplex assay was used to assess the antibody response in naturally infected animals. Out of the 29 positive field sera to the N protein, 40% showed antibodies against GNe or GP38, with 11 out of these 12 samples being positive to both glycoproteins. To determine the diagnostic specificity of the test, a total of 147 sera from Spanish farms free of CCHFV were included in the study. This multiplex assay could be useful to detect antibodies to different proteins of CCHFV as vaccine target candidates and to study the immune response to CCHFV in infected animals and for surveillance programs to prevent the further spread of the virus. IMPORTANCE Crimean-Congo hemorrhagic fever virus (CCHFV) causes Crimean-Congo hemorrhagic fever, which is one of the most important tick-borne viral diseases of humans and has recently been found in previously unaffected countries such as Spain. The disease is asymptomatic in infected animals but can develop into severe illness in humans. As neither an effective treatment nor a vaccine has been developed against CCHFV, surveillance programs are essential to limit and control the spread of the virus. In this study, a multiplex assay detecting antibodies against different CCHFV antigens in a single sample and independent of the ruminant species has been developed. This assay could be very useful in surveillance studies, to control the spread of CCHFV and prevent future outbreaks, and to better understand the immune response induced by CCHFV.

Virus-host interactions during tick-borne bunyavirus infection

The Bunyavirales order is the largest grouping of RNA viruses, comprising emerging and re-emerging human, plant and animal pathogens. Bunyaviruses have a global distribution and many members of the order are transmitted by arthropods. They have evolved a plethora of mechanisms to manipulate the regulatory processes of the infected cell to facilitate their own replicative cycle, in hosts of disparate phylogenies. Interest in virus-vector interactions is growing rapidly. However, current understanding of tick-borne bunyavirus cellular interaction is heavily biased to studies conducted in mammalian systems. In this short review, we summarise current understandings of how tick-borne bunyaviruses utilise major cellular pathways (innate immunity, apoptosis and RNAi responses) in mammalian or tick cells to facilitate virus replication.

Structural characterization of protective non-neutralizing antibodies targeting Crimean-Congo hemorrhagic fever virus

Crimean-Congo Hemorrhagic Fever Virus (CCHFV) causes a life-threatening disease with up to a 40% mortality rate. With no approved medical countermeasures, CCHFV is considered a public health priority agent. The non-neutralizing mouse monoclonal antibody (mAb) 13G8 targets CCHFV glycoprotein GP38 and protects mice from lethal CCHFV challenge when administered prophylactically or therapeutically. Here, we reveal the structures of GP38 bound with a human chimeric 13G8 mAb and a newly isolated CC5-17 mAb from a human survivor. These mAbs bind overlapping epitopes with a shifted angle. The broad-spectrum potential of c13G8 and CC5-17 and the practicality of using them against Aigai virus, a closely related nairovirus were examined. Binding studies demonstrate that the presence of non-conserved amino acids in Aigai virus corresponding region prevent CCHFV mAbs from binding Aigai virus GP38. This information, coupled with in vivo efficacy, paves the way for future mAb therapeutics effective against a wide swath of CCHFV strains.

A comprehensive list of the Bunyavirales replication promoters reveals a unique promoter structure in Nairoviridae differing from other virus families

Members of the order Bunyavirales infect a wide variety of host species, including plants, animals and humans, and pose a threat to public health. Major families in this order have tri-segmented negative-sense RNA genomes, the 5′ and 3′ ends of which form complementary strands that serve as a replication promoter. Elucidation of the mechanisms by which viral polymerases recognize the promoter to initiate RNA synthesis is important for understanding viral replication and pathogenesis, and developing antivirals. A list of replication promoter configuration patterns may provide details on the differences in the replication mechanisms among bunyaviruses. By using public sequence data of all known bunyavirus species, we constructed a comprehensive list of the replication promoters comprising 40 nucleotides in both the 5′ and 3′ ends of the genome that form a specific complementary strand. Among tri-segmented bunyaviruses, members of the family Nairoviridae, including the highly pathogenic Crimean-Congo hemorrhagic fever virus, have evolved a GC-rich promoter structure differing from that of other families. The unique promoter structure might be related to the large genome size of the family Nairoviridae among bunyaviruses, and the large genome architecture might confer pathogenic advantages. The promoter list provided in this report is useful for predicting the virus family-specific replication mechanisms of bunyaviruses.

The structure of Crimean-Congo hemorrhagic fever virus Gc is revealed; many more still need an answer

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The structure of glycoprotein Gc, responsible for membrane fusion, is revealed, but many more mysteries remain.

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Why do only antibodies against Gc have neutralizing effect, but not the one against Gn?

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Why can NAbs against Gc only be protective in the animals in preventive settings, but not in the therapeutic administration?

Adenovirus-based vaccines-a platform for pandemic preparedness against emerging viral pathogens

Zoonotic viruses continually pose a pandemic threat. Infection of humans with viruses for which we typically have little or no prior immunity can result in epidemics with high morbidity and mortality. These epidemics can have public health and economic impact and can exacerbate civil unrest or political instability. Changes in human behavior in the past few decades-increased global travel, farming intensification, the exotic animal trade, and the impact of global warming on animal migratory patterns, habitats, and ecosystems-contribute to the increased frequency of cross-species transmission events. Investing in the pre-clinical advancement of vaccine candidates against diverse emerging viral threats is crucial for pandemic preparedness. Replication-defective adenoviral (Ad) vectors have demonstrated their utility as an outbreak-responsive vaccine platform during the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) pandemic. Ad vectors are easy to engineer; are amenable to rapid, inexpensive manufacturing; are relatively safe and immunogenic in humans; and, importantly, do not require specialized cold-chain storage, making them an ideal platform for equitable global distribution or stockpiling. In this review, we discuss the progress in applying Ad-based vaccines against emerging viruses and summarize their global safety profile, as reflected by their widespread geographic use during the SARS-CoV-2 pandemic.

Immunobiology of Crimean-Congo hemorrhagic fever

Human infection with Crimean-Congo hemorrhagic fever virus (CCHFV), a tick-borne pathogen in the family Nairoviridae, can result in a spectrum of outcomes, ranging from asymptomatic infection through mild clinical signs to severe or fatal disease. Studies of CCHFV immunobiology have investigated the relationship between innate and adaptive immune responses with disease severity, attempting to elucidate factors associated with differential outcomes. In this article, we begin by highlighting unanswered questions, then review current efforts to answer them. We discuss in detail current clinical studies and research in laboratory animals on CCHF, including immune targets of infection and adaptive and innate immune responses. We summarize data about the role of the immune response in natural infections of animals and humans and experimental studies in vitro and in vivo and from evaluating immune-based therapies and vaccines, and present recommendations for future research.

The Viral Class II Membrane Fusion Machinery: Divergent Evolution from an Ancestral Heterodimer

A key step during the entry of enveloped viruses into cells is the merger of viral and cell lipid bilayers. This process is driven by a dedicated membrane fusion protein (MFP) present at the virion surface, which undergoes a membrane–fusogenic conformational change triggered by interactions with the target cell. Viral MFPs have been extensively studied structurally, and are divided into three classes depending on their three-dimensional fold. Because MFPs of the same class are found in otherwise unrelated viruses, their intra-class structural homology indicates horizontal gene exchange. We focus this review on the class II fusion machinery, which is composed of two glycoproteins that associate as heterodimers. They fold together in the ER of infected cells such that the MFP adopts a conformation primed to react to specific clues only upon contact with a target cell, avoiding premature fusion in the producer cell. We show that, despite having diverged in their 3D fold during evolution much more than the actual MFP, the class II accompanying proteins (AP) also derive from a distant common ancestor, displaying an invariant core formed by a β-ribbon and a C-terminal immunoglobulin-like domain playing different functional roles—heterotypic interactions with the MFP, and homotypic AP/AP contacts to form spikes, respectively. Our analysis shows that class II APs are easily identifiable with modern structural prediction algorithms, providing useful information in devising immunogens for vaccine design.

Protective neutralizing antibodies from human survivors of Crimean-Congo hemorrhagic fever

Crimean-Congo hemorrhagic fever virus (CCHFV) is a World Health Organization priority pathogen. CCHFV infections cause a highly lethal hemorrhagic fever for which specific treatments and vaccines are urgently needed. Here, we characterize the human immune response to natural CCHFV infection to identify potent neutralizing monoclonal antibodies (nAbs) targeting the viral glycoprotein. Competition experiments showed that these nAbs bind six distinct antigenic sites in the Gc subunit. These sites were further delineated through mutagenesis and mapped onto a prefusion model of Gc. Pairwise screening identified combinations of non-competing nAbs that afford synergistic neutralization. Further enhancements in neutralization breadth and potency were attained by physically linking variable domains of synergistic nAb pairs through bispecific antibody (bsAb) engineering. Although multiple nAbs protected mice from lethal CCHFV challenge in pre- or post-exposure prophylactic settings, only a single bsAb, DVD-121-801, afforded therapeutic protection. DVD-121-801 is a promising candidate suitable for clinical development as a CCHFV therapeutic.

Crimean-Congo Hemorrhagic Fever Virus: Current Advances and Future Prospects of Antiviral Strategies

Crimean-Congo hemorrhagic fever virus (CCHFV) is a widespread, tick-borne pathogen that causes Crimean-Congo hemorrhagic fever (CCHF) with high morbidity and mortality. CCHFV is transmitted to humans through tick bites or direct contact with patients or infected animals with viremia. Currently, climate change and globalization have increased the transmission risk of this biosafety level (BSL)-4 virus. The treatment options of CCHFV infection remain limited and there is no FDA-approved vaccine or specific antivirals, which urges the identification of potential therapeutic targets and the design of CCHF therapies with greater effort. In this article, we discuss the current progress and some future directions in the development of antiviral strategies against CCHFV.

Development of a Novel Multi-Epitope Vaccine Against Crimean-Congo Hemorrhagic Fever Virus: An Integrated Reverse Vaccinology, Vaccine Informatics and Biophysics Approach

Crimean-Congo hemorrhagic fever (CCHF) is a highly severe and virulent viral disease of zoonotic origin, caused by a tick-born CCHF virus (CCHFV). The virus is endemic in many countries and has a mortality rate between 10% and 40%. As there is no licensed vaccine or therapeutic options available to treat CCHF, the present study was designed to focus on application of modern computational approaches to propose a multi-epitope vaccine (MEV) expressing antigenic determinants prioritized from the CCHFV genome. Integrated computational analyses revealed the presence of 9 immunodominant epitopes from Nucleoprotein (N), RNA dependent RNA polymerase (RdRp), Glycoprotein N (Gn/G2), and Glycoprotein C (Gc/G1). Together these epitopes were observed to cover 99.74% of the world populations. The epitopes demonstrated excellent binding affinity for the B- and T-cell reference set of alleles, the high antigenic potential, non-allergenic nature, excellent solubility, zero percent toxicity and interferon-gamma induction potential. The epitopes were engineered into an MEV through suitable linkers and adjuvating with an appropriate adjuvant molecule. The recombinant vaccine sequence revealed all favorable physicochemical properties allowing the ease of experimental analysis in vivo and in vitro. The vaccine 3D structure was established ab initio. Furthermore, the vaccine displayed excellent binding affinity for critical innate immune receptors: TLR2 (-14.33 kcal/mol) and TLR3 (-6.95 kcal/mol). Vaccine binding with these receptors was dynamically analyzed in terms of complex stability and interaction energetics. Finally, we speculate the vaccine sequence reported here has excellent potential to evoke protective and specific immune responses subject to evaluation of downstream experimental analysis.

Broadly neutralizing antibody cocktails targeting Nipah virus and Hendra virus fusion glycoproteins

Hendra virus (HeV) and Nipah virus (NiV) are henipaviruses (HNVs) causing respiratory illness and severe encephalitis in humans, with fatality rates of 50-100%. There are no licensed therapeutics or vaccines to protect humans. HeV and NiV use a receptor-binding glycoprotein (G) and a fusion glycoprotein (F) to enter host cells. HNV F and G are the main targets of the humoral immune response, and the presence of neutralizing antibodies is a correlate of protection against NiV and HeV in experimentally infected animals. We describe here two cross-reactive F-specific antibodies, 1F5 and 12B2, that neutralize NiV and HeV through inhibition of membrane fusion. Cryo-electron microscopy structures reveal that 1F5 and 12B2 recognize distinct prefusion-specific, conserved quaternary epitopes and lock F in its prefusion conformation. We provide proof-of-concept for using antibody cocktails for neutralizing NiV and HeV and define a roadmap for developing effective countermeasures against these highly pathogenic viruses.

A CCHFV DNA vaccine protects against heterologous challenge and establishes GP38 as immunorelevant in mice

Crimean-Congo hemorrhagic fever virus (CCHFV) is a tick-borne virus that causes severe hemorrhagic fever disease in humans. Currently, no licensed CCHF vaccines exist, and the protective epitopes remain unclear. Previously, we tested a DNA vaccine expressing the M-segment glycoprotein precursor gene of the laboratory CCHFV strain IbAr 10200 (CCHFV-M₁₀₂₀₀). CCHFV-M₁₀₂₀₀ provided >60% protection against homologous CCHFV-IbAr 10200 challenge in mice. Here, we report that increasing the dose of CCHFV-M₁₀₂₀₀ provides complete protection from homologous CCHFV challenge in mice, and significant (80%) protection from challenge with the clinically relevant heterologous strain CCHFV-Afg09-2990. We also report complete protection from CCHFV-Afg09-2990 challenge following vaccination with a CCHFV-Afg09-2990 M-segment DNA vaccine (CCHFV-M_Afg09). Finally, we show that the non-structural M-segment protein, GP38, influences CCHF vaccine immunogenicity and provides significant protection from homologous CCHFV challenge. Our results demonstrate that M-segment DNA vaccines elicit protective CCHF immunity and further illustrate the immunorelevance of GP38.

Recent Advances in Bunyavirus Glycoprotein Research: Precursor Processing, Receptor Binding and Structure

The Bunyavirales order accommodates related viruses (bunyaviruses) with segmented, linear, single-stranded, negative- or ambi-sense RNA genomes. Their glycoproteins form capsomeric projections or spikes on the virion surface and play a crucial role in virus entry, assembly, morphogenesis. Bunyavirus glycoproteins are encoded by a single RNA segment as a polyprotein precursor that is co- and post-translationally cleaved by host cell enzymes to yield two mature glycoproteins, Gn and Gc (or GP1 and GP2 in arenaviruses). These glycoproteins undergo extensive N-linked glycosylation and despite their cleavage, remain associated to the virion to form an integral transmembrane glycoprotein complex. This review summarizes recent advances in our understanding of the molecular biology of bunyavirus glycoproteins, including their processing, structure, and known interactions with host factors that facilitate cell entry.

A Look into Bunyavirales Genomes: Functions of Non-Structural (NS) Proteins

In 2016, the Bunyavirales order was established by the International Committee on Taxonomy of Viruses (ICTV) to incorporate the increasing number of related viruses across 13 viral families. While diverse, four of the families (Peribunyaviridae, Nairoviridae, Hantaviridae, and Phenuiviridae) contain known human pathogens and share a similar tri-segmented, negative-sense RNA genomic organization. In addition to the nucleoprotein and envelope glycoproteins encoded by the small and medium segments, respectively, many of the viruses in these families also encode for non-structural (NS) NSs and NSm proteins. The NSs of Phenuiviridae is the most extensively studied as a host interferon antagonist, functioning through a variety of mechanisms seen throughout the other three families. In addition, functions impacting cellular apoptosis, chromatin organization, and transcriptional activities, to name a few, are possessed by NSs across the families. Peribunyaviridae, Nairoviridae, and Phenuiviridae also encode an NSm, although less extensively studied than NSs, that has roles in antagonizing immune responses, promoting viral assembly and infectivity, and even maintenance of infection in host mosquito vectors. Overall, the similar and divergent roles of NS proteins of these human pathogenic Bunyavirales are of particular interest in understanding disease progression, viral pathogenesis, and developing strategies for interventions and treatments.

The interplays between Crimean-Congo hemorrhagic fever virus (CCHFV) M segment-encoded accessory proteins and structural proteins promote virus assembly and infectivity

Crimean-Congo hemorrhagic fever virus (CCHFV) is a tick-borne orthonairovirus that has become a serious threat to the public health. CCHFV has a single-stranded, tripartite RNA genome composed of L, M, and S segments. Cleavage of the M polyprotein precursor generates the two envelope glycoproteins (GPs) as well as three secreted nonstructural proteins GP38 and GP85 or GP160, representing GP38 only or GP38 linked to a mucin-like protein (MLD), and a double-membrane-spanning protein called NSm. Here, we examined the relevance of each M-segment non-structural proteins in virus assembly, egress and infectivity using a well-established CCHFV virus-like-particle system (tc-VLP). Deletion of MLD protein had no impact on infectivity although it reduced by 60% incorporation of GPs into particles. Additional deletion of GP38 abolished production of infectious tc-VLPs. The loss of infectivity was associated with impaired Gc maturation and exclusion from the Golgi, showing that Gn is not sufficient to target CCHFV GPs to the site of assembly. Consistent with this, efficient complementation was achieved in cells expressing MLD-GP38 in trans with increased levels of preGc to Gc conversion, co-targeting to the Golgi, resulting in particle incorporation and restored infectivity. Contrastingly, a MLD-GP38 variant retained in the ER allowed preGc cleavage but failed to rescue miss-localization or infectivity. NSm deletion, conversely, did not affect trafficking of Gc but interfered with Gc processing, particle formation and secretion. NSm expression affected N-glycosylation of different viral proteins most likely due to increased speed of trafficking through the secretory pathway. This highlights a potential role of NSm in overcoming Golgi retention and facilitating CCHFV egress. Thus, deletions of GP38 or NSm demonstrate their important role on CCHFV particle production and infectivity. GP85 is an essential viral factor for preGc cleavage, trafficking and Gc incorporation into particles, whereas NSm protein is involved in CCHFV assembly and virion secretion.

Orthonairoviruses, like the lethal Crimean-Congo hemorrhagic fever virus (CCHFV), encode secreted glycoproteins, such as GP38, in addition to virion envelope glycoproteins (Gn and Gc) that are processed by internal cleavage of the viral M segment encoded polyprotein. CCHFV MLD-GP38 proteins (GP160/GP85) also include an N-terminal domain encompassing a mucin-like protein that is released from GP38 by Furin. The protective effect of non-neutralizing monoclonal antibodies targeting GP38 against lethal CCHFV challenge previously highlighted the importance of GP38 in CCHFV replication. CCHFV also encodes a double-membrane-spanning protein (NSm) of unknown function, located between the Gn and Gc on the polyprotein. To investigate the roles of these so-called accessory proteins encoded by the CCHFV M-segment in virus formation and infectivity, we generated several M-segment deletion mutants and tested them in a CCHFV transcription-entry-competent virus-like particle (tc-VLP) system. Here, we demonstrate that GP38 is crucial for Gc biogenesis, interaction with Gn and trafficking to the Golgi, and that its deletion abrogates formation of infectious particles. We also show that NSm increases the rate of protein trafficking through the secretory pathway with altered N-glycosylation profiles that are advantageous for efficient virus release. These data advanced our understanding of GP38 and NSm roles and CCHFV-host interactions.