An amino-terminal segment of hantavirus nucleocapsid protein presented on hepatitis B virus core particles induces a strong and highly cross-reactive antibody response in mice

Corrigendum: Hantavirus: an overview and advancements in therapeutic approaches for infection

[This corrects the article DOI: 10.3389/fmicb.2023.1233433.].

Hantavirus: an overview and advancements in therapeutic approaches for infection

Hantaviruses are a significant and emerging global public health threat, impacting more than 200,000 individuals worldwide each year. The single-stranded RNA viruses belong to the Hantaviridae family and are responsible for causing two acute febrile diseases in humans: Hantavirus pulmonary syndrome (HPS) and hemorrhagic fever with renal syndrome (HFRS). Currently, there are no licensed treatments or vaccines available globally for HTNV infection. Various candidate drugs have shown efficacy in increasing survival rates during the early stages of HTNV infection. Some of these drugs include lactoferrin, ribavirin, ETAR, favipiravir and vandetanib. Immunotherapy utilizing neutralizing antibodies (NAbs) generated from Hantavirus convalescent patients show efficacy against HTNV. Monoclonal antibodies such as MIB22 and JL16 have demonstrated effectiveness in protecting against HTNV infection. The development of vaccines and antivirals, used independently and/or in combination, is critical for elucidating hantaviral infections and the impact on public health. RNA interference (RNAi) arised as an emerging antiviral therapy, is a highly specific degrades RNA, with post-transcriptional mechanism using eukaryotic cells platform. That has demonstrated efficacy against a wide range of viruses, both in vitro and in vivo. Recent antiviral methods involve using small interfering RNA (siRNA) and other, immune-based therapies to target specific gene segments (S, M, or L) of the Hantavirus. This therapeutic approach enhances viral RNA clearance through the RNA interference process in Vero E6 cells or human lung microvascular endothelial cells. However, the use of siRNAs faces challenges due to their low biological stability and limited in vivo targeting ability. Despite their successful inhibition of Hantavirus replication in host cells, their antiviral efficacy may be hindered. In the current review, we focus on advances in therapeutic strategies, as antiviral medications, immune-based therapies and vaccine candidates aimed at enhancing the body's ability to control the progression of Hantavirus infections, with the potential to reduce the risk of severe disease.

Advances and perspectives in the development of vaccines against highly pathogenic bunyaviruses

Increased human activities around the globe and the rapid development of once rural regions have increased the probability of contact between humans and wild animals. A majority of bunyaviruses are of zoonotic origin, and outbreaks may result in the substantial loss of lives, economy contraction, and social instability. Many bunyaviruses require manipulation in the highest levels of biocontainment, such as Biosafety Level 4 (BSL-4) laboratories, and the scarcity of this resource has limited the development speed of vaccines for these pathogens. Meanwhile, new technologies have been created, and used to innovate vaccines, like the mRNA vaccine platform and bioinformatics-based antigen design. Here, we summarize current vaccine developments for three different bunyaviruses requiring work in the highest levels of biocontainment: Crimean-Congo Hemorrhagic Fever Virus (CCHFV), Rift Valley Fever Virus (RVFV), and Hantaan virus (HTNV), and provide perspectives and potential future directions that can be further explored to advance specific vaccines for humans and livestock.

Hemorrhagic Fever with Renal Syndrome in Asia: History, Pathogenesis, Diagnosis, Treatment, and Prevention

Hemorrhagic Fever with Renal Syndrome (HFRS) is the most frequently diagnosed zoonosis in Asia. This zoonotic infection is the result of exposure to the virus-contaminated aerosols. Orthohantavirus infection may cause Hemorrhagic Fever with Renal Syndrome (HRFS), a disease that is characterized by acute kidney injury and increased vascular permeability. Several species of orthohantaviruses were identified as causing infection, where Hantaan, Puumala, and Seoul viruses are most common. Orthohantaviruses are endemic to several Asian countries, such as China, South Korea, and Japan. Along with those countries, HFRS tops the list of zoonotic infections in the Far Eastern Federal District of Russia. Recently, orthohantavirus circulation was demonstrated in small mammals in Thailand and India, where orthohantavirus was not believed to be endemic. In this review, we summarized the current data on orthohantaviruses in Asia. We gave the synopsis of the history and diversity of orthohantaviruses in Asia. We also described the clinical presentation and current understanding of the pathogenesis of orthohantavirus infection. Additionally, conventional and novel approaches for preventing and treating orthohantavirus infection are discussed.

Plant-Derived Recombinant Vaccines against Zoonotic Viruses

Emerging and re-emerging zoonotic diseases cause serious illness with billions of cases, and millions of deaths. The most effective way to restrict the spread of zoonotic viruses among humans and animals and prevent disease is vaccination. Recombinant proteins produced in plants offer an alternative approach for the development of safe, effective, inexpensive candidate vaccines. Current strategies are focused on the production of highly immunogenic structural proteins, which mimic the organizations of the native virion but lack the viral genetic material. These include chimeric viral peptides, subunit virus proteins, and virus-like particles (VLPs). The latter, with their ability to self-assemble and thus resemble the form of virus particles, are gaining traction among plant-based candidate vaccines against many infectious diseases. In this review, we summarized the main zoonotic diseases and followed the progress in using plant expression systems for the production of recombinant proteins and VLPs used in the development of plant-based vaccines against zoonotic viruses.

Immunogenic Properties of MVs Containing Structural Hantaviral Proteins: An Original Study

Hemorrhagic fever with renal syndrome (HFRS) is an emerging infectious disease that remains a global public health threat. The highest incidence rate is among zoonotic disease cases in Russia. Most cases of HFRS are reported in the Volga region of Russia, which commonly identifies the Puumala virus (PUUV) as a pathogen. HFRS management is especially challenging due to the lack of specific treatments and vaccines. This study aims to develop new approaches for HFRS prevention. Our goal is to test the efficacy of microvesicles (MVs) as PUUV nucleocapsid (N) and glycoproteins (Gn/Gc) delivery vehicles. Our findings show that MVs could deliver the PUUV N and Gn/Gc proteins in vitro. We have also demonstrated that MVs loaded with PUUV proteins could elicit a specific humoral and cellular immune response in vivo. These data suggest that an MV-based vaccine could control HFRS.

Immune response during hantavirus diseases: implications for immunotherapies and vaccine design

Orthohantaviruses, previously named hantaviruses, cause two emerging zoonotic diseases: haemorrhagic fever with renal syndrome (HFRS) in Eurasia and hantavirus cardiopulmonary syndrome (HCPS) in the Americas. Overall, over 200 000 cases are registered every year worldwide, with a fatality rate ranging between 0·1% and 15% for HFRS and between 20% and 40% for HCPS. No specific treatment or vaccines have been approved by the U.S. Food and Drug Administration (FDA) to treat or prevent hantavirus-caused syndromes. Currently, little is known about the mechanisms at the basis of hantavirus-induced disease. However, it has been hypothesized that an excessive inflammatory response plays an essential role in the course of the disease. Furthermore, the contributions of the cellular immune response to either viral clearance or pathology have not been fully elucidated. This article discusses recent findings relative to the immune responses elicited to hantaviruses in subjects suffering HFRS or HCPS, highlighting the similarities and differences between these two clinical diseases. Also, we summarize the most recent data about the cellular immune response that could be important for designing new vaccines to prevent this global public health problem.

Hantavirus Infections-Treatment and Prevention

Purpose of review

Hantavirus infection is an emerging zoonosis and there are two main clinical presentations, hemorrhagic fever with renal syndrome (HFRS) and Hantavirus pulmonary syndrome (HPS). Although Hantavirus infections have a worldwide distribution with a high mortality rate, a safe and effective vaccine or an antiviral drug against the Hantavirus disease is yet to be available. This review summarizes all the efforts undertaken to develop medical countermeasures in vitro, in vivo, and human clinical trials against Hantavirus infections.

Recent findings

Multiple antivirals are shown to be effective with limited evidence and recent studies on immunotherapy were not very conclusive. There are multiple vaccine candidates with evidence of conferring long protective immunity against Hantaviruses. Some of these had been already trialed on humans.

Summary

At present, severe HPS or HFRS case management is purely based on supportive treatments, often in an intensive care unit. Rodent control and public health education and promotion play a major role in preventing Hantavirus infection.

Hepatitis B Virus Core Protein Domains Essential for Viral Capsid Assembly in a Cellular Context

Hepatitis B virus (HBV) core protein (HBc) is essential to the formation of the HBV capsid. HBc contains two domains: the N-terminal domain corresponding to residues 1-140 essential to form the icosahedral shell and the C-terminal domain corresponding to a basic and phosphorylated peptide, and required for DNA replication. The role of these two domains for HBV capsid assembly was essentially studied in vitro with HBc purified from mammalian or non-mammalian cell lysates, but their respective role in living cells remains to be clarified. We therefore investigated the assembly of the HBV capsid in Huh7 cells by combining fluorescence lifetime imaging microscopy/Förster's resonance energy transfer, fluorescence correlation spectroscopy and transmission electron microscopy approaches. We found that wild-type HBc forms oligomers early after transfection and at a sub-micromolar concentration. These oligomers are homogeneously diffused throughout the cell. We quantified a stoichiometry ranging from ~170 to ~230 HBc proteins per oligomer, consistent with the visualization of eGFP-containingHBV capsid shaped as native capsid particles by transmission electron microscopy. In contrast, no assembly was observed when HBc-N-terminal domain was expressed. This highlights the essential role of the C-terminal domain to form capsid in mammalian cells. Deletion of either the third helix or of the 124-135 residues of HBc had a dramatic impact on the assembly of the HBV capsid, inducing the formation of mis-assembled oligomers and monomers, respectively. This study shows that our approach using fluorescent derivatives of HBc is an innovative method to investigate HBV capsid formation.

Progress on the Prevention and Treatment of Hantavirus Disease

Hantaviruses, members of the order Bunyavirales, family Hantaviridae, have a world-wide distribution and are responsible for greater than 150,000 cases of disease per year. The spectrum of disease associated with hantavirus infection include hemorrhagic fever with renal syndrome (HFRS) and hantavirus pulmonary syndrome (HPS) also known as hantavirus cardiopulmonary syndrome (HCPS). There are currently no FDA-approved vaccines or treatments for these hantavirus diseases. This review provides a summary of the status of vaccine and antiviral treatment efforts including those tested in animal models or human clinical trials.

Immunogenicity of a Virus-Like-Particle Vaccine Containing Multiple Antigenic Epitopes of Toxoplasma gondii Against Acute and Chronic Toxoplasmosis in Mice

There is no effective protective vaccine against human toxoplasmosis, which is a potential threat to nearly a third of the world population. Vaccines based on virus-like particles (VLPs) have been highly successful in humans for many years, but have rarely been applied against Toxoplasma gondii infection. In this study, we inserted a B cell epitope (SAG1₈₂₋₁₀₂ or SAG1_301−320), a CD8⁺ cell epitope (HF10 or ROP7), and a CD4⁺ cell epitope (AS15) of T. gondii into a truncated HBc_Δ(amino acids1–149) particle to construct four chimeric VLP vaccine formulations, i.e., HBc_ΔH82, HBc_ΔH301, HBc_{Δ R82}, and HBc_{Δ R301}. When these chimeric HBc particles were expressed in Escherichia coli, they showed icosahedral morphology similar to that of the original VLPs and were evaluated as vaccine formulations against acute and chronic toxoplasmosis in a mouse model (BALB/c mice (H-2^d). All these chimeric HBc VLPs induced strong humoral and cellular immune responses with high IgG antibody titers and interferon(IFN)-γ production. Only the mice immunized with HBc_ΔH82 showed prolonged survival time (15.6 ± 3.8 vs. 5.6 ± 0.8 days) against acute infection with RH tachyzoites and decrease in brain parasite load (1,454 ± 239 vs. 2,091 ± 263) against chronic infection with Prugniuad cysts, as compared to the findings for the control group. These findings suggest that HBc VLPs would act as an effective carrier for delivering effective multiple antigenic epitopes and would be beneficial for developing a safe and long-acting vaccine against toxoplasmosis.

The nucleocapsid protein of hantaviruses: much more than a genome-wrapping protein

The nucleocapsid (N) protein of hantaviruses represents an impressive example of a viral multifunctional protein. It encompasses properties as diverse as genome packaging, RNA chaperoning, intracellular protein transport, DNA degradation, intervention in host translation, and restricting host immune responses. These functions all rely on the capability of N to interact with RNA and other viral and cellular proteins. We have compiled data on the N protein of different hantavirus species together with information of the recently published three-dimensional structural data of the protein. The array of diverse functional activities accommodated in the hantaviral N protein goes far beyond to be a static structural protein and makes it an interesting target in the development of antiviral therapeutics.

Hepatitis B core-based virus-like particles to present heterologous epitopes

Since the first effort to recombinantly express the hepatitis B core protein (HBc) in bacteria, the remarkable virion-like structure has fuelled interest in unraveling the structural and antigenic properties of this protein. Initial studies proved HBc virus-like particles to possess strong immunogenic properties, which can be conveyed to linked antigens. More than 35 years later, numerous studies have been performed using HBc as a carrier protein for antigens derived from over a dozen different pathogens and diseases. In this review, the authors highlight the intriguing features of HBc as carrier and antigen, illustrated by some examples and experimental results that underscore the value of HBc as an antigen-presenting platform. Two of these HBc fusions, targeting influenza A and malaria, have even progressed into clinical testing. In the future, the HBc-based virus-like particles platform will probably continue to be used for the display of poorly immunogenic antigens, mainly because virus-like particle formation by HBc capsomers is compatible with nearly any available recombinant gene expression system.

Virus-like particles displaying envelope domain III of dengue virus type 2 induce virus-specific antibody response in mice

OBJECTIVE

Currently, dengue represents one of the most significant arboviral disease worldwide, for which a vaccine is not yet available. Persistent challenges in live viral dengue vaccines have sparked a keen interest in exploring non-replicating dengue vaccines. We have examined the feasibility of using the methylotrophic yeast Pichia pastoris to develop a chimeric vaccine candidate displaying the dengue virus type-2 (DENV-2) envelope domain III (EDIII), implicated in host receptor binding and in the induction of virus-neutralizing antibodies, on the surface of non-infectious virus-like particles (VLP)-based on the Hepatitis B virus core antigen (HBcAg).

METHODS

We designed a fusion antigen by inserting DENV-2 EDIII into c/e1 loop of HBcAg. A codon-optimized gene encoding this fusion antigen was integrated into the genome of P. pastoris, under the control of the Alcohol Oxidase 1 promoter. The antigen was expressed by methanol induction and purified to near homogeneity by Ni(2+) affinity chromatography. The purified antigen was characterized physically and functionally to evaluate its ability to assemble into VLPs, and elicit DENV-2-specific antibodies in mice.

RESULTS

This fusion antigen was expressed successfully to high yields and purified to near homogeneity. Electron microscopy and competitive ELISA analyses showed that it formed VLPs in which the EDIII moiety was accessible to different EDIII-specific antibodies. These VLPs were immunogenic in mice, stimulating the production of antibodies that could specifically recognize DENV-2 and neutralize its infectivity. However, virus-neutralizing antibody titers were modest.

CONCLUSIONS

Our data show: (i) insertion of EDIII into the c/e1 loop of HBcAg does not compromise particle assembly; and (ii) the chimeric VLPs elicit a specific humoral response against DENV-2. The strategy of displaying dengue virus EDIII using a VLP platform will need further optimization before it may be developed into a viable alternative option.

Native agarose gel electrophoresis and electroelution: A fast and cost-effective method to separate the small and large hepatitis B capsids

Hepatitis B core antigen (HBcAg) expressed in Escherichia coli is able to self-assemble into large and small capsids comprising 240 (triangulation number T = 4) and 180 (triangulation number T = 3) subunits, respectively. Conventionally, sucrose density gradient ultracentrifugation and SEC have been used to separate these capsids. However, good separation of the large and small particles with these methods is never achieved. In the present study, we employed a simple, fast, and cost-effective method to separate the T = 3 and T = 4 HBcAg capsids by using native agarose gel electrophoresis followed by an electroelution method (NAGE-EE). This is a direct, fast, and economic method for isolating the large and small HBcAg particles homogenously based on the hydrodynamic radius of the spherical particles. Dynamic light scattering analysis demonstrated that the T = 3 and T = 4 HBcAg capsids prepared using the NAGE-EE method are monodisperse with polydispersity values of ∼15% and ∼13%, respectively. ELISA proved that the antigenicity of the capsids was not affected in the purification process. Overall, NAGE-EE produced T = 3 and T = 4 capsids with a purity above 90%, and the recovery was 34% and 50%, respectively (total recovery of HBcAg is ∼84%), and the operation time is 15 and 4 times lesser than that of the sucrose density gradient ultracentrifugation and SEC, respectively.

Chimeric Hepatitis B core antigen virus-like particles displaying the envelope domain III of dengue virus type 2

Background

Dengue is a global public health problem for which no drug or vaccine is available. Currently, there is increasing interest in developing non-replicating dengue vaccines based on a discrete antigenic domain of the major structural protein of dengue viruses (DENVs), known as envelope domain III (EDIII). The use of bio-nanoparticles consisting of recombinant viral structural polypeptides, better known as virus-like particles (VLPs), has emerged as a potential platform technology for vaccine development. This work explores the feasibility of developing nanoparticles based on E. coli-expressed recombinant Hepatitis B virus core antigen (HBcAg) designed to display EDIII moiety of DENV on the surface.

Findings

We designed a synthetic gene construct encoding HBcAg containing an EDIII insert in its c/e1 loop. The fusion antigen HBcAg-EDIII-2 was expressed in E. coli, purified to near homogeneity using Ni⁺² affinity chromatography and demonstrated to assemble into discrete 35–40 nm VLPs by electron microscopy. Competitive ELISA analyses showed that the EDIII-2 moieties of the VLPs are accessible to anti-EDIII-2-specific monoclonal and polyclonal antibodies, suggesting that they are surface-displayed. The VLPs were highly immunogenic eliciting high titer anti-EDIII-2 antibodies that were able to recognize, bind and neutralize infectious DENV based on ELISA, immunofluorescence and virus-neutralization assays.

Conclusion

This work demonstrates that HBcAg-derived nanoparticles can serve as a useful platform for the display of DENV EDIII. The EDIII-displaying nanoparticles may have potential applications in diagnostics/vaccines for dengue.

Structures of hepatitis B virus cores presenting a model epitope and their complexes with antibodies

The core shell of hepatitis B virus is a potent immune stimulator, giving a strong neutralizing immune response to foreign epitopes inserted at the immunodominant region, located at the tips of spikes on the exterior of the shell. Here, we analyze structures of core shells with a model epitope inserted at two alternative positions in the immunodominant region. Recombinantly expressed core protein assembles into T = 3 and T = 4 icosahedral shells, and atomic coordinates are available for the T = 4 shell. Since the modified protein assembles predominantly into T = 3 shells, a quasi-atomic model of the native T = 3 shell was made. The spikes in this T = 3 structure resemble those in T = 4 shells crystallized from expressed protein. However, the spikes in the modified shells exhibit an altered conformation, similar to the DNA containing shells in virions. Both constructs allow full access of antibodies to the foreign epitope, DPAFR from the preS1 region of hepatitis B virus surface antigen. However, one induces a 10-fold weaker immune response when injected into mice. In this construct, the epitope is less constrained by the flanking linker regions and is positioned so that the symmetry of the shell causes pairs of epitopes to come close enough to interfere with one another. In the other construct, the epitope mimics the native epitope conformation and position. The interaction of native core shells with an antibody specific to the immunodominant epitope is compared to the constructs with an antibody against the foreign epitope. Our findings have implications for the design of vaccines based on virus-like particles.

Characterization of monoclonal antibodies against hantavirus nucleocapsid protein and their use for immunohistochemistry on rodent and human samples

Monoclonal antibodies are important tools for various applications in hantavirus diagnostics. Recently, we generated Puumala virus (PUUV)-reactive monoclonal antibodies (mAbs) by immunisation of mice with chimeric polyomavirus-derived virus-like particles (VLPs) harbouring the 120-amino-acid-long amino-terminal region of the PUUV nucleocapsid (N) protein. Here, we describe the generation of two mAbs by co-immunisation of mice with hexahistidine-tagged full-length N proteins of Sin Nombre virus (SNV) and Andes virus (ANDV), their characterization by different immunoassays and comparison with the previously generated mAbs raised against a segment of PUUV N protein inserted into VLPs. All of the mAbs reacted strongly in ELISA and western blot tests with the antigens used for immunization and cross-reacted to varying extents with N proteins of other hantaviruses. All mAbs raised against a segment of the PUUV N protein presented on chimeric VLPs and both mAbs raised against the full-length AND/SNV N protein reacted with Vero cells infected with different hantaviruses. The reactivity of mAbs with native viral nucleocapsids was also confirmed by their reactivity in immunohistochemistry assays with kidney tissue specimens from experimentally SNV-infected rodents and human heart tissue specimens from hantavirus cardiopulmonary syndrome patients. Therefore, the described mAbs represent useful tools for the immunodetection of hantavirus infection.

Generation of E3-deleted canine adenovirus type 2 expressing the Gc glycoprotein of Seoul virus by gene insertion or deletion of related terminal region sequences

Seoul virus (SEOV) is one of the four hantaviruses known to cause haemorrhagic fever with renal syndrome. The medium genome segment encodes the Gn/Gc glycoproteins of SEOV, which form the major structural part of the virus envelope. Gc and/or Gn are the candidate antigens of hantavirus for induction of a highly immunogenic response for hantavirus vaccine. In this study, the immune response induced by a replication-competent recombinant canine adenovirus type 2 expressing the Gc protein of SEOV was evaluated in BALB/c mice. Sera from immunized mice contained neutralizing antibodies that could specifically recognize SEOV and neutralize its infectivity in vitro. Moreover, the recombinant virus induced complete protection against an intensive infectious challenge with approximately 1000 50 % infective doses for SEOV strain CC-2. Protective-level neutralizing antibodies were maintained for at least 20 weeks. This recombinant virus is therefore a potential alternative to the inactivated vaccine.

N-terminally His-tagged hepatitis B core antigens: construction, expression, purification and antigenicity

The core antigen of the hepatitis B virus (HBcAg) has been used widely as a diagnostic reagent for the identification of the viral infection. However, purification using the conventional sucrose density gradient ultracentrifugation is time consuming and costly. To overcome this, HBcAg particles displaying His-tag on their surface were constructed and produced in Escherichia coli. The recombinant His-tagged HBcAgs were purified using immobilized metal affinity chromatography. Transmission electron microscopy and enzyme-linked immunosorbent assay (ELISA) revealed that the displayed His-tag did not impair the formation of the core particles and the antigenicity of HBcAg.

Recent approaches in hantavirus vaccine development

Rodent-borne hantaviruses are associated with two main clinical disorders in humans: hemorrhagic fever with renal syndrome and hantavirus cardiopulmonary syndrome. Although hantavirus diseases can be life threatening and numerous research efforts are focused on the development of hantavirus prevention, no specific antiviral therapy is yet available and, at this time, no WHO-approved vaccine has gained widespread acceptance. This review will summarize the current knowledge and recent progress as well as new speculative approaches in the development of hantavirus vaccines.

NMR structure of the N-terminal coiled coil domain of the Andes hantavirus nucleocapsid protein

The hantaviruses are emerging infectious viruses that in humans can cause a cardiopulmonary syndrome or a hemorrhagic fever with renal syndrome. The nucleocapsid (N) is the most abundant viral protein, and during viral assembly, the N protein forms trimers and packages the viral RNA genome. Here, we report the NMR structure of the N-terminal domain (residues 1-74, called N1-74) of the Andes hantavirus N protein. N1-74 forms two long helices (alpha1 and alpha2) that intertwine into a coiled coil domain. The conserved hydrophobic residues at the helix alpha1-alpha2 interface stabilize the coiled coil; however, there are many conserved surface residues whose function is not known. Site-directed mutagenesis, CD spectroscopy, and immunocytochemistry reveal that a point mutation in the conserved basic surface formed by Arg22 or Lys26 lead to antibody recognition based on the subcellular localization of the N protein. Thus, Arg22 and Lys26 are likely involved in a conformational change or molecular recognition when the N protein is trafficked from the cytoplasm to the Golgi, the site of viral assembly and maturation.

The Leeuwenhoek lecture 2006. Microscopy goes cold: frozen viruses reveal their structural secrets

The electron microscope provides a powerful tool for investigating the structure of biological complexes such as viruses. A modern instrument is fully capable of atomic resolution on suitable non-biological specimens, but biological materials are difficult to preserve, owing to their fragility, and to image, owing to their radiation, sensitivity. The act of imaging the specimen severely damages it. Originally, samples were prepared by staining with a heavy metal salt, which provides a stable specimen but limits the amount of details that can be retrieved. Now particulate specimens, such as viruses, are prepared by rapid freezing of unstained material and observed in a frozen state with low doses of electrons. The resulting images require extensive computer processing to extract fully detailed three-dimensional information about the specimen. The whole process is referred to as single-particle electron cryomicroscopy. Using this approach, the structure of the human hepatitis B virus core was solved at the level of the protein fold. By comparing maps of RNA- and DNA-containing cores, it was possible to propose a model for the maturation and control of the envelopment of the virus during assembly. These examples show that cryomicroscopy offers great potential for understanding the structure and function of complex biological assemblies.

Electron tomography of viruses

Understanding the molecular architectures of enveloped and complex viruses is a challenging frontier in structural biology. In these viruses, the structural and compositional variation from one viral particle to another generally precludes the use of either crystallization or image averaging procedures that have been successfully implemented in the past for highly symmetric viruses. While advances in cryo electron tomography of unstained specimens provide new opportunities for identification and molecular averaging of individual subcomponents such as the surface glycoprotein spikes on purified viruses, electron tomography of stained and plunge-frozen cells is being used to visualize the cellular context of viral entry and replication. Here, we review recent developments in both areas as they relate to our understanding of the biology of heterogeneous and pleiomorphic viruses.

Purification of recombinant annexins without the use of phospholipids

Due to their involvement in a variety of physiological and pathological processes, different isoforms of annexins are being utilized as markers of some human diseases and bio-imaging of tissue injury (due to apoptosis), and have been proposed as drug delivery vehicles. These, in addition to extensive biophysical studies on the role of annexins in organizing lipid domains in biological membranes, have necessitated development of an efficient protocol for producing annexins in bulk quantities. In this paper, we report a one-step purification protocol for annexin a5 without using lipid vesicles or involving any column chromatographic step. Depending on the growth and expression condition, a fraction of recombinant annexin a5 (cloned in pET3d vector) was sequestered into inclusion bodies. When these inclusion bodies were dissolved in 6 M urea, subjected to a 10-fold snap dilution in the presence of 5 mM Ca(2+) and stored overnight at 4 degrees C, annexin a5 was precipitated as a homogenous protein as judged by SDS-PAGE. This one-step purification protocol produced about 35 mg of highly purified annexin a5 per liter of bacterial culture. The annexin a5 purified from inclusion bodies exhibited similar properties to that obtained from the soluble fraction using the conventional lipid-partitioning approach. Our purification protocol for annexin a5 elaborated herein is equally effective for purification of annexin A2, and we believe, will serve as general protocol for purifying other annexins in bulk quantities for diagnostic as well as detailed biophysical studies.

Hepatitis B virus capsid-like particles can display the complete, dimeric outer surface protein C and stimulate production of protective antibody responses against Borrelia burgdorferi infection

Hepatitis B virus capsid-like particles (CLPs), icosahedral assemblies formed by 90 or 120 core protein dimers, hold promise as immune-enhancing vaccine carriers for heterologous antigens. Insertions into the immunodominant c/e1 B cell epitope, a surface-exposed loop, are especially immunogenic. However, display of whole proteins, desirable to induce multispecific and possibly neutralizing antibody responses, can be restrained by an unsuitable structure of the foreign protein and by its propensity to undergo homomeric interactions. Here we analyzed CLP formation by core fusions with two distinct variants of the dimeric outer surface lipoprotein C (OspC) of the Lyme disease agent Borrelia burgdorferi. Although the topology of the termini in the OspC dimer does not match that of the insertion sites in the carrier dimer, both fusions, coreOspCa and coreOspCb, efficiently formed stable CLPs. Electron cryomicroscopy clearly revealed the surface disposition of the OspC domains, possibly with OspC dimerization occurring across different core protein dimers. In mice, both CLP preparations induced high-titered antibody responses against the homologous OspC variant, but with substantial cross-reactivity against the other variant. Importantly, both conferred protection to mice challenged with B. burgdorferi. These data show the principal applicability of hepatitis B virus CLPs for the display of dimeric proteins, demonstrate the presence in OspC of hitherto uncharacterized epitopes, and suggest that OspC, despite its genetic variability, may be a valid vaccine candidate.

Generation of monoclonal antibodies of desired specificity using chimeric polyomavirus-derived virus-like particles

Foreign protein sequences presented on hamster polyomavirus (HaPyV) major capsid protein VP1-derived virus-like particles (VLPs) have been demonstrated to be highly immunogenic. The current study was aimed to evaluate VP1-derived chimeric VLPs as tools for hybridoma technology to generate monoclonal antibodies (mAbs) of desired specificity. Chimeric VLPs containing inserts of different size and origin were used as immunogens. Chimeric VLPs carrying a 9 amino acid (aa)-long cytotoxic T-cell epitope (STAPPVHNV) of human mucin 1 (MUC1) elicited a strong epitope-specific humoral immune response in mice and promoted the production of MUC1-specific mAbs. From a total of seven mAbs of IgG isotype generated against the chimeric VLPs, two mAbs were directed against the MUC1 epitope and five mAbs against the VP1-carrier. Two out of five anti-VP1 mAbs recognized epitopes located at the previously defined insertion site #2 (aa 223/224), which confirms its surface-exposed localization. Chimeric VLPs carrying a 120-aa long sequence of Puumala hantavirus (PUUV) nucleocapsid protein (NP) promoted the generation of five mAbs of IgG isotype specific to PUUV NP. All mAbs recognized the full-length NP of different PUUV strains. In contrast, no VP1-specific mAbs were obtained. The ability of chimeric VLPs to activate antigen-presenting cells was evaluated by studying the uptake of chimeric VLPs by murine spleen cell-derived dendritic cells (DCs). Efficient uptake of VLPs and activation of murine DCs were demonstrated, which may represent the basis of the strong immunogenicity of chimeric VLPs. In conclusion, chimeric VLPs effectively stimulated the production of IgG antibodies specific for foreign epitopes presented at surface-exposed regions. Thus, chimeric HaPyV VP1-derived VLPs represent efficient immunogens for hybridoma technology and provide a promising alternative to chemical coupling of synthetic peptides to carrier proteins.

Different response requirements for IFNgamma production in ELISPOT assays by CD4+ T cells from mice early and late after immunization

Antigen specific immune responses that occur early after antigen exposure differ from those that are present late in the response. The present study focused on detecting changes in production of IFNgamma by CD4+ T cells over time during chronic antigen exposure. (C3HxCB17)F1 mice were primed with recombinant hepatitis B core antigen (rHBcAg) in incomplete Freund's adjuvant to allow persistent antigen exposure. To assay the CD4+ T cell response to HBcAg, splenocytes from immunized mice were restimulated with rHBcAg for 24 or 48 h in vitro and tested for IFNgamma and IL-5 secreting cells by ELISPOT. Results showed that early after antigen exposure (7 days for primary and 3 days for secondary exposures), the maximal number of IFNgamma secreting cells was detected in the ELISPOT after 24 h of restimulation. However, late after antigen exposure (28 days for primary and 14 days for secondary exposures), the maximum number of IFNgamma secreting cells was not detected until 48 h of restimulation in this assay. This delay in IFNgamma production was related to the availability of IL-2, since addition of IL-2 allowed the delayed cells from late responses to develop peak IFNgamma production in vitro by 24 h, equivalent to that of cells from early responses. This IL-2 dependent delay occurred in Th1-type IFNgamma responses but not in Th2-type IL-5 responses. These observations indicate that, when detecting IFNgamma secreting cells it is important to screen responses at different times of restimulation or in the presence and absence of IL-2 to ensure optimal detection. This approach should prove critical, particularly when evaluating patients with chronic infections and in determining the effectiveness of vaccines since these deal with both early and late responses.

High plasticity of the hepatitis B virus capsid revealed by conformational stress

Hepatitis B virus (HBV) replicates through reverse transcription inside its icosahedral nucleocapsid. The internal genome status is signaled to the capsid surface, predicting regulated conformational changes in the capsid structure. To probe their nature and extent, we imposed local conformational stress on the outer surface of HBV capsid-like particles, and monitored its consequences by electron cryomicroscopy and image reconstruction. The capsid structure had an enormous flexibility and robustness as a whole, as well as within the subunits, whose spikes were able to rotate by as much as 40 degrees against the distal interdimer contact sites. The likely hinge for the swiveling movement was the conserved Gly111 residue at the inner surface of the capsid. The stress imposed from the outside also affected the internal capsid organization, implying a specific route for the flow of conformational information between capsid interior and exterior as required for signaling of the genome status.

A fusion product of the complete Borrelia burgdorferi outer surface protein A (OspA) and the hepatitis B virus capsid protein is highly immunogenic and induces protective immunity similar to that seen with an effective lipidated OspA vaccine formula

The immunogenicity of peptides and protein fragments can be considerably enhanced by their presentation on particulate carriers such as capsid-like particles (CLP) from hepatitis B virus (HBV). Here we tested the suitability of the HBV capsid protein as a carrier for a relevant full-length pathogen-derived protein antigen. The entire 255-amino acid ectodomain of the outer surface protein A (OspA) from Borrelia burgdorferi, the causative agent of Lyme disease, was inserted into the major B cell epitope of the HBV capsid, yielding a multimerization-competent fusion protein, termed coreOspA. CoreOspA, consisting only in part of regular CLP, induced antibodies to OspA, including the Ig isotype profile and specificity for the protective epitope LA-2, with an efficiency similar to that of recombinant lipidated OspA, the first generation vaccine against Lyme disease. Moreover, coreOspA actively and passively protected mice against subsequent challenge with B. burgdorferi. The data demonstrate the capacity of the HBV capsid protein to act as a potent immunomodulator even for full-length and structurally complex polypeptide chains and thus opens new avenues for novel vaccine designs.

A hantavirus nucleocapsid protein segment exposed on hepatitis B virus core particles is highly immunogenic in mice when applied without adjuvants or in the presence of pre-existing anti-core antibodies

Hepatitis B virus (HBV) core particles carrying the amino-terminal 120 amino acids (aa) of the nucleocapsid (N) protein of the hantaviruses Dobrava, Hantaan or Puumala have been demonstrated to be highly immunogenic in mice when complexed with adjuvants. Here we demonstrate that even without adjuvant, these chimeric particles induced high-titered, and strongly cross-reactive N-specific antibody responses in BALB/c and C57BL/6 mice. The induced N-specific antibodies represented all IgG subclasses. Pre-existing core-specific antibodies did not abrogate the induction of an N-specific immune response by a hantavirus N insert presented on core particles. Therefore, chimeric core particles should represent promising vaccine candidates even for anti-core positive humans.

Immunostimulatory potential of hepatitis B nucleocapsid preparations: lipopolysaccharide contamination should not be overlooked

The nucleocapsid of hepatitis B virus (HBV) allows insertions of heterologous peptides and even complete proteins. Because of its outstanding capacity to induce B-cell, T-helper and cytotoxic T-cell responses, this structure is considered to be an important instrument for future vaccine development. Most of the evidence for the unique immunogenic qualities of nucleocapsids has been generated in mice, which are not natural hosts of HBV. Moreover, most nucleocapsid preparations used in these studies were produced in a recombinant manner in Escherichia coli. Such preparations have been shown to contain lipopolysaccharide (LPS). Not unexpectedly, it is shown here that contaminating LPS, rather than the nucleocapsid structure itself, is responsible for the activation of human antigen-presenting cells. Careful examination of the literature dealing with the immunogenicity of HBV nucleocapsids suggests that the possible presence of LPS has been largely ignored or underestimated in several studies. This raises doubts on some of the underlying mechanisms that have been proposed to explain the unique immunogenicity of the HBV nucleocapsid.

Virus-like particles: models for assembly studies and foreign epitope carriers

Virus‐like particles (VLPs), formed by the structural elements of viruses, have received considerable attention over the past two decades. The number of reports on newly obtained VLPs has grown proportionally with the systems developed for the expression of these particles. The chapter outlines the recent achievements in two important fields of research brought about by the availability of VLPs produced in a foreign host. These are: (1) The requirements for VLP assembly and (2) the use of VLPs as carriers for foreign epitopes. VLP technology is a rapidly advancing domain of molecular and structural biology. Extensive progress in VLP studies was achieved as the insect cell based protein production system was developed. This baculovirus expression system has many advantages for the synthesis of viral structural proteins resulting in the formation of VLPs. It allows production of large amounts of correctly folded proteins while also providing cell membranes that can serve as structural elements for enveloped viruses. These features give us the opportunity to gain insights into the interactions and requirements accompanying VLP formation that are similar to the assembly events occurring in mammalian cells. Other encouraging elements are the ability to easily scale up the system and the simplicity of purification of the assembled VLPs. The growing number of VLPs carrying foreign protein fragments on their surface and studies on the successful assembly of these chimeric molecules is a promising avenue towards the development of a new technology, in which the newly designed VLPs will be directed to particular mammalian cell types by exposing specific binding domains. The progress made in modeling the surface of VLPs makes them to date the best candidates for the design of delivery systems that can efficiently reach their targets.

High yields of stable and highly pure nucleocapsid proteins of different hantaviruses can be generated in the yeast Saccharomyces cerevisiae

Recently, the high-level expression of authentic and hexahistidine (His)-tagged Puumala (strain Vranica/Hällnäs) hantavirus nucleocapsid protein derivatives in the yeast Saccharomyces cerevisiae has been reported [Dargeviciute et al., Vaccine, 20 (2002) 3523-3531]. Here we describe the expression of His-tagged nucleocapsid proteins of other Puumala virus strains (Sotkamo, Kazan) as well as Dobrava (strains Slovenia and Slovakia) and Hantaan (strain Fojnica) hantaviruses using the same system. All nucleocapsid proteins were expressed in the yeast S. cerevisiae at high levels. The nucleocapsid proteins can be easily purified by nickel chelate chromatography; the yield for all nucleocapsid proteins ranged from 0.5 to 1.5 mg per g wet weight of yeast cells. In general, long-term storage of all nucleocapsid proteins without degradation can be obtained by storage in PBS at -20 degrees C or lyophilization. The nucleocapsid protein of Puumala virus (strain Vranica/Hällnäs) was demonstrated to contain only traces of less than 10 pg nucleic acid contamination per 100 microg of protein. The yeast-expressed nucleocapsid proteins of Hantaan, Puumala and Dobrava viruses described here represent useful tools for serological hantavirus diagnostics and for vaccine development.