东方马脑炎E2蛋白原核表达及免疫活性初步研究

为构建东方马脑炎病毒E2基因原核表达载体,完成E2蛋白表达及其免疫活性研究。利用PCR方法扩增E2编码全基因,大小为1 260 bp,将酶切后目的片段连接到原核表达载体pET-30a(+)上,构建成重组质粒pET30a(+)-EEEV-E2,采用酶切和测序分析方法鉴定正确的重组质粒转化到大肠杆菌BL21中,诱导E2蛋白表达,并用SDS-PAGE电泳和Western-blotting分析和鉴定目的蛋白;最后,用纯化的E2蛋白免疫BALB/c小鼠,小鼠随机分成4组:PBS对照组、弗氏佐剂对照组、E2蛋白免疫组和E2蛋白+弗氏佐剂免疫组,每组小鼠免疫2次,两次免疫间隔时间为14天,免疫剂量均为100μL/只;小鼠初次免疫后第10天,用细胞因子ELISA试剂盒检测血清中IL-6、IL-12与TNF-α的浓度,加强免疫后第14天,用EEEV的假病毒检测血清中E2蛋白抗体的中和作用。结果表明完成了E2基因的原核表达载体pET30a(+)-E2构建和成功表达了带有His标签的E2融合蛋白,蛋白以包涵体形式存在,大小为53.0 kDa;免疫小鼠血清中产生了高水平的IL-6、IL-12与TNF-α和具有较强中和作用的E2蛋白抗体。研究结果为今后E2蛋白作为基因工程亚单位疫苗的研究提供了重要参考。     

Comprehensive Mapping Antigenic Epitopes of NS1 Protein of Japanese Encephalitis Virus with Monoclonal Antibodies

Japanese encephalitis virus (JEV) non-structural protein 1 (NS1) contributes to virus replication and elicits protective immune responses during infection. JEV NS1-specific antibody responses could be a target in the differential diagnosis of different flavivirus infections. However, the epitopes on JEV NS1 are poorly characterized. The present study describes the full mapping of linear B-cell epitopes in JEV NS1. We generated eleven NS1-specific monoclonal antibodies from mice immunized with recombinant NS1. For epitope mapping of monoclonal antibodies, a set of 51 partially-overlapping peptides covering the entire NS1 protein were expressed with a GST-tag and then screened using monoclonal antibodies. Through enzyme-linked immunosorbent assay (ELISA), five linear epitope-containing peptides were identified. By sequentially removing amino acid residues from the carboxy and amino terminal of peptides, the minimal units of the five linear epitopes were identified and confirmed using monoclonal antibodies. Five linear epitopes are located in amino acids residues ⁵AIDITRK¹¹, ⁷²RDELNVL⁷⁸, ²⁵¹KSKHNRREGY²⁶⁰, ²⁶⁹DENGIVLD²⁷⁶, and ³⁴¹DETTLVRS³⁴⁸. Furthermore, it was found that the epitopes are highly conserved among JEV strains through sequence alignment. Notably, none of the homologous regions on NS1 proteins from other flaviviruses reacted with the MAbs when they were tested for cross-reactivity, and all five epitope peptides were not recognized by sera against West Nile virus or Dengue virus. These novel virus-specific linear B-cell epitopes of JEV NS1 would benefit the development of new vaccines and diagnostic assays.     

Possible Evidence for Interference with Venezuelan Equine Encephalitis Virus Vaccination of Equines by Pre-Existing Antibody to Eastern or Western Equine Encephalitis Virus, or Both

During 1971, an epizootic of Venezuelan equine encephalitis (VEE) reached the United States. Laboratory tests were performed on a large number of sick, healthy, unvaccinated, and vaccinated horses. Neutralization (N) tests in cell cultures revealed that 153 of 193 (79.3%) equines outside the state of Texas and 175 of 204 (85.8%) within Texas (82.6% overall) had detectable N antibody to VEE virus a week or more after vaccination. Twenty-six of 40 (65%) non-Texas equines and 18 of 29 (62%) Texas equines which had no detectable antibody against VEE virus a week or more after vaccination had N antibody against Eastern equine encephalitis (EEE) or Western equine encephalitis (WEE) virus or both, whereas only 50 of 153 (32.7%) non-Texas equines and 82 of 175 (46.9%) Texas equines with demonstrable N antibody against VEE also had N antibody against EEE and/or WEE virus. In vaccinated equines, significant negative correlations were found between the occurrence of antibody to VEE and antibody to EEE and/or WEE virus. These findings support the hypothesis that pre-existing antibody to EEE and/or WEE virus may modify or interfere with infection by VEE virus. The epizoologic significance of this possibility is discussed briefly.     

Gag Protein Epitopes Recognized by ELA-A-Restricted Cytotoxic T Lymphocytes from Horses with Long-Term Equine Infectious Anemia Virus Infection

Most equine infectious anemia virus (EIAV)-infected horses have acute clinical disease, but they eventually control the disease and become lifelong carriers. Cytotoxic T lymphocytes (CTL) are considered an important immune component in the control of infections with lentiviruses including EIAV, but definitive evidence for CTL in the control of disease in carrier horses is lacking. By using retroviral vector-transduced target cells expressing different Gag proteins and overlapping synthetic peptides of 16 to 25 amino acids, peptides containing at least 12 Gag CTL epitopes recognized by virus-stimulated PBMC from six long-term EIAV-infected horses were identified. All identified peptides were located within Gag matrix (p15) and capsid (p26) proteins, as no killing of target cells expressing p11 and p9 occurred. Each of the six horses had CTL recognizing at least one Gag epitope, while CTL from one horse recognized at least eight different Gag epitopes. None of the identified peptides were recognized by CTL from all six horses. Two nonamer peptide epitopes were defined from Gag p26; one (18a) was likely restricted by class I equine leukocyte alloantigen A5.1 (ELA-A5.1) molecules, and the other (28b-1) was likely restricted by ELA-A9 molecules. Sensitization of equine kidney target cells for CTLm killing required 10 nM peptide 18a and 1 nM 28b-1. The results demonstrated that diverse CTL responses against Gag epitopes were generated in long-term EIAV-infected horses and indicated that ELA-A class I molecules were responsible for the diversity of CTL epitopes recognized. This information indicates that multiple epitopes or whole proteins will be needed to induce CTL in horses with different ELA-A alleles in order to evaluate their role in controlling EIAV.Equine infectious anemia virus (EIAV) belongs to the Lentivirus genus, which includes human immunodeficiency virus type 1 (HIV-1), simian immunodeficiency virus (SIV), and several other animal viruses. EIAV causes disease in horses which is characterized by recurrent febrile episodes associated with viremia, anemia, and thrombocytopenia (10). Most infected horses are able to eventually control the disease and become lifelong EIAV carriers (9). The ability of horses to restrict EIAV replication to very low levels and to remain free of clinical disease provides an opportunity to determine the immunologic mechanisms involved in this lentivirus control.Immune responses are required for the termination of the acute viremia during EIAV infection since foals with severe combined immunodeficiency cannot control the initial viremia following EIAV infection, in contrast to normal foals (41). Results suggesting that immune responses are involved in the control of EIAV in carrier horses include the observation that corticosteroid- and cyclophosphamide-treated carrier horses have recurrent viremia and disease (24). Neutralizing antibody can be an important component of the protective immune response against lentiviral infections (12). Type-specific neutralizing antibody appears following the episodes of plasma viremia in EIAV-infected horses (25); however, there is evidence suggesting that the presence of the neutralizing antibody does not necessarily relate to the occurrence and control of viremic episodes (8, 25). Detectable neutralizing antibodies to the variant isolated during a disease episode can appear after the episode is controlled (8). Neutralizing antibody-escape variants are isolated from EIAV carrier horses as early as 5 days after corticosteroid treatment, when the antibody levels have not significantly changed (24). Further, the viremic episode induced by corticosteroid treatment can be terminated before the appearance of neutralizing antibody to the variant causing viremia (24). Other evidence implicating immune responses other than neutralizing antibody in EIAV control includes the following: (i) EIAV carrier horses can resist challenge with a heterologous strain in the absence of detectable neutralizing antibody to the challenge virus (23), and (ii) some horses immunized with an inactivated virus vaccine resist homologous strain challenge without detectable levels of neutralizing antibody but with virus-specific cell-mediated immune responses (17).Accumulating evidence suggests that major histocompatibility complex (MHC) class I-restricted virus-specific cytotoxic T lymphocytes (CTL) may play an important role in the immune control of diseases caused by HIV-1 and SIV infection (5, 26, 51). CTL appear to be involved in both the clearance of the primary viremia in HIV-1 infection (26) and the prevention of disease progression to AIDS (42). In EIAV infection, the appearance of activated CD8⁺ CTL (effectors) correlated with the control of the initial viremic episodes (33). Although the CTL effectors decline to low levels when plasma viremias become undetectable, a high frequency of memory CTL (CTLm) has been detected in some carrier horses (34), and these CTLm recognize either EIAV Env or Gag/Pr proteins or both (15, 34). Both CD8⁺ and CD4⁺ CTL activities have been detected in some EIAV-infected horses (15), but their roles in disease control are not known.The epitopes recognized by CD8⁺ CTL are usually peptides of 8 to 11 amino acids (aa) presented by MHC class I molecules on the target cell surface. Identifying the CTL epitopes and the MHC class I molecules that restrict responses is necessary in order to determine how CTL are involved in the control of disease and to stimulate CTL by vaccination. However, the occurrence of escape mutants which are no longer recognized by CTL is one of the major difficulties for inducing effective CTL responses against different variants (6). Gag protein epitopes recognized by CTL may be of importance because Gag proteins are relatively conserved among EIAV strains (21, 32, 40, 48). In this study, at least 12 peptides with CTL epitopes were recognized by stimulated peripheral blood mononuclear cells (PBMC) from six long-term EIAV-infected horses with different ELA-A alleles. These peptides were identified by using retroviral vectors expressing individual Gag proteins and synthetic overlapping peptides from recognized proteins. We identified two nonamer peptides, one apparently restricted by ELA-A5.1, and another by ELA-A9, molecules.     

Fractionation of Eastern Equine Encephalitis Virus by Density Gradient Centrifugation in CsCl

When partially purified Eastern equine encephalitis (EEE) virus was centrifuged to equilibrium in CsCl, three virus specific bands were observed. A hemagglutinin was detected at a buoyant density of 1.18 g/cm³. Infectious EEE virus banded in two positions; most of the virus banded at 1.20 g/cm³ and a lesser amount banded at 1.22 to 1.23 g/cm³. Analysis of radioactive profiles of CsCl-fractionated EEE virus labeled with either ³²PO₄ or ³H-uridine suggested that the hemagglutinin was stripped from the intact EEE virion. The viral origin of the hemagglutinin was verified by inhibition with specific antiserum. Attempts to differentiate between infectious EEE virus of the different buoyant densities showed that the denser particle was neither a virus contaminant nor a density mutant. No evidence was obtained to indicate that the denser particle was an immature form of EEE virus. The two infectious EEE species obtained after CsCl fractionation were indistinguishable antigenically. Furthermore, unfractionated as well as CsCl-fractionated EEE virus sedimented at about 260S in sucrose gradients. These results together with the results of rebanding experiments suggested that the denser EEE species (1.23 g/cm³) results from a salt (CsCl)-induced alteration or breakdown of the EEE virion (1.20 g/cm³), and that it arises as the hemagglutinin is stripped from the surface of the EEE virion.     

Identification of Immunodominant Neutralizing Epitopes on the Hemagglutinin Protein of Rinderpest Virus

The immunodominant epitopes on the hemagglutinin protein of rinderpest virus (RPV-H) were determined by analyzing selected monoclonal antibody (MAb)-resistant mutants and estimating the level of antibody against each epitope in five RPV-infected rabbits with the competitive enzyme-linked immunosorbent assay (c-ELISA). Six neutralizing epitopes were identified, at residues 474 (epitope A), 243 (B), 548 to 551 (D), 587 to 592 (E), 310 to 313 (G), and 383 to 387 (H), from the data on the amino acid substitutions of hemagglutinin protein of MAb-resistant mutants and the reactivities of MAbs against RPV-H to the other morbilliviruses. The epitopes identified in this study are all positioned on the loop of the propeller-like structure in a hypothetical three-dimensional model of RPV-H (J. P. M. Langedijk et al., J. Virol. 71:6155-6167, 1997). Polyclonal sera obtained from five rabbits infected experimentally with RPV were examined by c-ELISA using a biotinylated MAb against each epitope as a competitor. Although these rabbit sera hardly blocked binding of each MAb to epitopes A and B, they moderately blocked binding of each MAb to epitopes G and D and strongly blocked binding of each MAb to epitopes E and H. These results suggest that epitopes at residues 383 to 387 and 587 to 592 may be immunodominant in humoral immunity to RPV infection.     

Mapping of Functional Elements in the Stem-Anchor Region of Tick-Borne Encephalitis Virus Envelope Protein E

Envelope protein E of the flavivirus tick-borne encephalitis virus mediates membrane fusion, and the structure of the N-terminal 80% of this 496-amino-acid-long protein has been shown to differ significantly from that of other viral fusion proteins. The structure of the carboxy-terminal 20%, the stem-anchor region, is not known. It contains sequences that are important for membrane anchoring, interactions with prM (the precursor of membrane protein M) during virion assembly, and low-pH-induced structural changes associated with the fusion process. To identify specific functional elements in this region, a series of C-terminal deletion mutants were constructed and the properties of the resulting truncated recombinant E proteins were examined. Full-length E proteins and proteins lacking the second of two predicted transmembrane segments were secreted in a particulate form when coexpressed with prM, whereas deletion of both segments resulted in the secretion of soluble homodimeric E proteins. Sites located within a predicted alpha-helical region of the stem (amino acids 431 to 449) and the first membrane-spanning region (amino acids 450 to 472) were found to be important for the stability of the prM-E heterodimer but not essential for prM-mediated intracellular transport and secretion of soluble E proteins. A separate site in the stem, also corresponding to a predicted alpha-helix (amino acids 401 to 413), was essential for the conversion of soluble protein E dimers to a homotrimeric form upon low-pH treatment, a process resembling the transition to the fusogenic state in whole virions. This functional mapping will aid in the understanding of the molecular mechanisms of membrane fusion and virus assembly.     

Dynamics of Vector-Host Interactions in Avian Communities in Four Eastern Equine Encephalitis Virus Foci in the Northeastern U.S.

Background

Eastern equine encephalitis (EEE) virus (Togaviridae, Alphavirus) is a highly pathogenic mosquito-borne zoonosis that is responsible for occasional outbreaks of severe disease in humans and equines, resulting in high mortality and neurological impairment in most survivors. In the past, human disease outbreaks in the northeastern U.S. have occurred intermittently with no apparent pattern; however, during the last decade we have witnessed recurring annual emergence where EEE virus activity had been historically rare, and expansion into northern New England where the virus had been previously unknown. In the northeastern U.S., EEE virus is maintained in an enzootic cycle involving the ornithophagic mosquito, Culiseta melanura, and wild passerine (perching) birds in freshwater hardwood swamps. However, the identity of key avian species that serve as principal virus reservoir and amplification hosts has not been established. The efficiency with which pathogen transmission occurs within an avian community is largely determined by the relative reservoir competence of each species and by ecological factors that influence contact rates between these avian hosts and mosquito vectors.

Methodology and principle findings

Contacts between vector mosquitoes and potential avian hosts may be directly quantified by analyzing the blood meal contents of field-collected specimens. We used PCR-based molecular methods and direct sequencing of the mitochondrial cytochrome b gene for profiling of blood meals in Cs. melanura, in an effort to quantify its feeding behavior on specific vertebrate hosts, and to infer epidemiologic implications in four historic EEE virus foci in the northeastern U.S. Avian point count surveys were conducted to determine spatiotemporal host community composition. Of 1,127 blood meals successfully identified to species level, >99% of blood meals were from 65 avian hosts in 27 families and 11 orders, and only seven were from mammalian hosts representing three species. We developed an empirically informed mathematical model for EEE virus transmission using Cs. melanura abundance and preferred and non-preferred avian hosts. To our knowledge this is the first mathematical model for EEE virus, a pathogen with many potential hosts, in the northeastern U.S. We measured strong feeding preferences for a number of avian species based on the proportion of mosquito blood meals identified from these bird species in relation to their observed frequencies. These included: American Robin, Tufted Titmouse, Common Grackle, Wood Thrush, Chipping Sparrow, Black-capped Chickadee, Northern Cardinal, and Warbling Vireo. We found that these bird species, most notably Wood Thrush, play a dominant role in supporting EEE virus amplification. It is also noteworthy that the competence of some of the aforementioned avian species for EEE virus has not been established. Our findings indicate that heterogeneity induced by mosquito host preference, is a key mediator of the epizootic transmission of vector-borne pathogens.

Conclusion and significance

Detailed knowledge of the vector-host interactions of mosquito populations in nature is essential for evaluating their vectorial capacity and for assessing the role of individual vertebrates as reservoir hosts involved in the maintenance and amplification of zoonotic agents of human diseases. Our study clarifies the host associations of Cs. melanura in four EEE virus foci in the northeastern U.S., identifies vector host preferences as the most important transmission parameter, and quantifies the contribution of preference-induced contact heterogeneity to enzootic transmission. Our study identifies Wood Thrush, American Robin and a few avian species that may serve as superspreaders of EEE virus. Our study elucidates spatiotemporal host species utilization by Cs. melanura in relation to avian host community. This research provides a basis to better understand the involvement of Cs. melanura and avian hosts in the transmission and ecology of EEE virus and the risk of human infection in virus foci.     

Discovery of a Novel Compound with Anti-Venezuelan Equine Encephalitis Virus Activity That Targets the Nonstructural Protein 2

Alphaviruses present serious health threats as emerging and re-emerging viruses. Venezuelan equine encephalitis virus (VEEV), a New World alphavirus, can cause encephalitis in humans and horses, but there are no therapeutics for treatment. To date, compounds reported as anti-VEEV or anti-alphavirus inhibitors have shown moderate activity. To discover new classes of anti-VEEV inhibitors with novel viral targets, we used a high-throughput screen based on the measurement of cell protection from live VEEV TC-83-induced cytopathic effect to screen a 340,000 compound library. Of those, we identified five novel anti-VEEV compounds and chose a quinazolinone compound, CID15997213 (IC₅₀ = 0.84 µM), for further characterization. The antiviral effect of CID15997213 was alphavirus-specific, inhibiting VEEV and Western equine encephalitis virus, but not Eastern equine encephalitis virus. In vitro assays confirmed inhibition of viral RNA, protein, and progeny synthesis. No antiviral activity was detected against a select group of RNA viruses. We found mutations conferring the resistance to the compound in the N-terminal domain of nsP2 and confirmed the target residues using a reverse genetic approach. Time of addition studies showed that the compound inhibits the middle stage of replication when viral genome replication is most active. In mice, the compound showed complete protection from lethal VEEV disease at 50 mg/kg/day. Collectively, these results reveal a potent anti-VEEV compound that uniquely targets the viral nsP2 N-terminal domain. While the function of nsP2 has yet to be characterized, our studies suggest that the protein might play a critical role in viral replication, and further, may represent an innovative opportunity to develop therapeutic interventions for alphavirus infection.     

Natural Variation in the Heparan Sulfate Binding Domain of the Eastern Equine Encephalitis Virus E2 Glycoprotein Alters Interactions with Cell Surfaces and Virulence in Mice

Recently, we compared amino acid sequences of the E2 glycoprotein of natural North American eastern equine encephalitis virus (NA-EEEV) isolates and demonstrated that naturally circulating viruses interact with heparan sulfate (HS) and that this interaction contributes to the extreme neurovirulence of EEEV (C. L. Gardner, G. D. Ebel, K. D. Ryman, and W. B. Klimstra, Proc. Natl. Acad. Sci. U. S. A., 108:16026–16031, 2011). In the current study, we have examined the contribution to HS binding of each of three lysine residues in the E2 71-to-77 region that comprise the primary HS binding site of wild-type (WT) NA-EEEV viruses. We also report that the original sequence comparison identified five virus isolates, each with one of three amino acid differences in the E2 71-to-77 region, including mutations in residues critical for HS binding by the WT virus. The natural variant viruses, which possessed either a mutation from lysine to glutamine at E2 71, a mutation from lysine to threonine at E2 71, or a mutation from threonine to lysine at E2 72, exhibited altered interactions with heparan sulfate and cell surfaces and altered virulence in a mouse model of EEEV disease. An electrostatic map of the EEEV E1/E2 heterotrimer based upon the recent Chikungunya virus crystal structure (J. E. Voss, M. C. Vaney, S. Duquerroy, C. Vonrhein, C. Girard-Blanc, E. Crublet, A. Thompson, G. Bricogne, and F. A. Rey, Nature, 468:709–712, 2010) showed the HS binding site to be at the apical surface of E2, with variants affecting the electrochemical nature of the binding site. Together, these results suggest that natural variation in the EEEV HS binding domain may arise during EEEV sylvatic cycles and that this variation may influence receptor interaction and the severity of EEEV disease.     

鸡传染性支气管炎病毒E蛋白基因的表达

鸡传染性支气管炎(Infectious Bronchitis,IB)是由传染性支气管炎病毒(Infectious Bronchitis Virus,IBV)引起的鸡的一种急性、高度接触性传染病。该病是危害全球养禽业的重要传染病之一[1,2]。其感染的主要特征是气管啰音、咳嗽和打喷嚏?送猓?该病还可以引起蛋鸡产蛋量下降和蛋品质下降。雏鸡可由于呼吸道或肾脏的感染而死亡。虽然疫苗的使用对IB的流行起到了一定的预防和控制作用,但由于IBV血清型众多,不同血清型毒株之间具有较小的交叉保护性甚至无交叉保护性。因此,IB目前仍在免疫鸡群和非免疫鸡群发生和传播,给养禽业造成了严重…     

鸡传染性支气管炎病毒E蛋白基因的表达

鸡传染性支气管炎(Infectious Bronchitis,IB)是由传染性支气管炎病毒(Infectious Bronchitis Virus,IBV)引起的鸡的一种急性、高度接触性传染病.该病是危害全球养禽业的重要传染病之一[1,2].其感染的主要特征是气管啰音、咳嗽和打喷嚏.此外,该病还可以引起蛋鸡产蛋量下降和蛋品质下降.雏鸡可由于呼吸道或肾脏的感染而死亡.虽然疫苗的使用对IB的流行起到了一定的预防和控制作用,但由于IBV血清型众多,不同血清型毒株之间具有较小的交叉保护性甚至无交叉保护性.因此,IB目前仍在免疫鸡群和非免疫鸡群发生和传播,给养禽业造成了严重的经济损失.     

Vector-Host Interactions of Culiseta melanura in a Focus of Eastern Equine Encephalitis Virus Activity in Southeastern Virginia

Eastern equine encephalitis virus (EEEV) causes a highly pathogenic mosquito-borne zoonosis that is responsible for sporadic outbreaks of severe illness in humans and equines in the eastern USA. Culiseta (Cs.) melanura is the primary vector of EEEV in most geographic regions but its feeding patterns on specific avian and mammalian hosts are largely unknown in the mid-Atlantic region. The objectives of our study were to: 1) identify avian hosts of Cs. melanura and evaluate their potential role in enzootic amplification of EEEV, 2) assess spatial and temporal patterns of virus activity during a season of intense virus transmission, and 3) investigate the potential role of Cs. melanura in epidemic/epizootic transmission of EEEV to humans and equines. Accordingly, we collected mosquitoes at 55 sites in Suffolk, Virginia in 2013, and identified the source of blood meals in engorged mosquitoes by nucleotide sequencing PCR products of the mitochondrial cytochrome b gene. We also examined field-collected mosquitoes for evidence of infection with EEEV using Vector Test, cell culture, and PCR. Analysis of 188 engorged Cs. melanura sampled from April through October 2013 indicated that 95.2%, 4.3%, and 0.5% obtained blood meals from avian, mammalian, and reptilian hosts, respectively. American Robin was the most frequently identified host for Cs. melanura (42.6% of blood meals) followed by Northern Cardinal (16.0%), European Starling (11.2%), Carolina Wren (4.3%), and Common Grackle (4.3%). EEEV was detected in 106 mosquito pools of Cs. melanura, and the number of virus positive pools peaked in late July with 22 positive pools and a Maximum Likelihood Estimation (MLE) infection rate of 4.46 per 1,000 mosquitoes. Our findings highlight the importance of Cs. melanura as a regional EEEV vector based on frequent feeding on virus-competent bird species. A small proportion of blood meals acquired from mammalian hosts suggests the possibility that this species may occasionally contribute to epidemic/epizootic transmission of EEEV.     

戊型肝炎病毒衣壳蛋白中和表位间的构象诱导

重组蛋白NE2包含了戊型肝炎病毒(HEV)衣壳蛋白(pORF2)的aa394～606片段.在NE2上已鉴定出了2个HEV中和表位,并获得了3个识别中和表位的单克隆抗体(MAb)8C11、13D8和8H3.这3个MAb间的交叉阻断ELISA实验发现,8C11和13D8可以彼此完全阻断,8H3对8C11和13D8均不能阻断,而8C11非但不能阻断8H3,反而显著增强了8H3与抗原的结合.用生物传感器进行的抗体与抗原结合的动力学分析也证实了这一现象.这些结果提示,在NE2上8H3表位区域受到抗原上某些结构的掩盖,而8C11与NE2的结合引起了抗原空间结构的改变,导致了掩盖8H3表位的结构的去除和8H3表位的充分暴露.免疫捕获RT-PCR发现,8C11同样可以显著增强8H3对天然HEV病毒的捕获能力,提示这种结合诱导的衣壳蛋白空间构象改变在天然HEV病毒颗粒上同样存在.     

Positively Charged Amino Acid Substitutions in the E2 Envelope Glycoprotein Are Associated with the Emergence of Venezuelan Equine Encephalitis Virus

Epidemic-epizootic Venezuelan equine encephalitis (VEE) viruses (VEEV) have emerged repeatedly via convergent evolution from enzootic predecessors. However, previous sequence analyses have failed to identify common sets of nucleotide or amino acid substitutions associated with all emergence events. During 1993 and 1996, VEEV subtype IE epizootics occurred on the Pacific Coast of the states of Chiapas and Oaxaca in southern Mexico. Like other epizootic VEEV strains, when inoculated into guinea pigs and mice, the Mexican isolates were no more virulent than closely related enzootic strains, complicating genetic studies of VEE emergence. Complete genomic sequences of 4 of the Mexican strains were determined and compared to those of closely related enzootic subtype IE isolates from Guatemala. The epizootic viruses were less than 2% different at the nucleotide sequence level, and phylogenetic relationships confirmed that the equine-virulent Mexican strains probably evolved from enzootic progenitors on the Pacific Coast of Mexico or Guatemala. Of 35 amino acids that varied among the Guatemalan and Mexican isolates, only 8 were predicted phylogenetically to have accompanied the phenotypic change. One mutation at position 117 of the E2 envelope glycoprotein, involving replacement of Glu by Lys, resulted in a small-plaque phenotype characteristic of epizootic VEEV strains. Analysis of additional E2 sequences from representative enzootic and epizootic VEEV isolates implicated similar surface charge changes in the emergence of previous South American epizootic phenotypes, indicating that E2 mutations are probably important determinants of the equine-virulent phenotype and of VEE emergence. Maximum-likelihood analysis indicated that one change at E2 position 213 has been influenced by positive selection and convergent evolution of the epizootic phenotype.     

Structure of the Recombinant Alphavirus Western Equine Encephalitis Virus Revealed by Cryoelectron Microscopy

Western equine encephalitis virus (WEEV; Togaviridae, Alphavirus) is an enveloped RNA virus that is typically transmitted to vertebrate hosts by infected mosquitoes. WEEV is an important cause of viral encephalitis in humans and horses in the Americas, and infection results in a range of disease, from mild flu-like illnesses to encephalitis, coma, and death. In addition to spreading via mosquito vectors, human WEEV infections can potentially occur directly via aerosol transmission. Due to its aerosol infectivity and virulence, WEEV is thus classified as a biological safety level 3 (BSL-3) agent. Because of its highly infectious nature and containment requirements, it has not been possible to investigate WEEV''s structure or assembly mechanism using standard structural biology techniques. Thus, to image WEEV and other BSL-3 agents, we have constructed a first-of-its-kind BSL-3 cryoelectron microscopy (cryoEM) containment facility. cryoEM images of WEEV were used to determine the first three-dimensional structure of this important human pathogen. The overall organization of WEEV is similar to those of other alphaviruses, consistent with the high sequence similarity among alphavirus structural proteins. Surprisingly, the nucleocapsid of WEEV, a New World virus, is more similar to the Old World alphavirus Sindbis virus than to other New World alphaviruses.The alphaviruses comprise a genus of single-stranded, plus-sense, enveloped RNA viruses that, together with rubella virus, comprise the family Togaviridae. The current classification of the genus Alphavirus includes 29 different species, with multiple subtypes and/or varieties represented within some species (30). These species can be grouped into 8 different complexes based on antigenic and/or genetic similarities (20). Most viruses from the New World are found in the Eastern, Venezuelan, and Western equine encephalitis (EEE, VEE, and WEE, respectively) complexes and cause encephalitis in humans and a variety of domesticated animals. Old World alphaviruses, on the other hand, typically cause only an arthralgia and rash syndrome that is rarely life threatening (5, 24). Among the New World alphaviruses, EEE, VEE, and WEE viruses (EEEV, VEEV, and WEEV, respectively) are potential biological weapons as well as naturally emerging pathogens and are therefore included on the category B Priority Pathogens list of the National Institute of Allergy and Infectious Diseases of the National Institutes of Health (http://www.niaid.nih.gov/topics/biodefenserelated/biodefense/research/pages/cata.aspx).Alphaviruses replicate in the cytoplasm of infected cells after entry via receptor-mediated endocytosis (8). Following internalization, fusion of the viral envelope with the endocytic membrane is mediated by a low-pH-induced conformational change that exposes a fusion peptide found in the E1 envelope glycoprotein. The nucleocapsid then disassembles upon interactions with ribosomes, and an open reading frame (ORF) found in the 5′ two-thirds of the genome is translated. The resultant polyprotein is cleaved into 4 nonstructural proteins (nsP1 to -4) that mediate viral RNA replication, RNA capping, and polyprotein processing (Fig. ​(Fig.1).1). The structural proteins, including the two envelope glycoproteins E2 and E1 as well as the capsid protein, are encoded in a second ORF that is translated from a subgenomic message often referred to as 26S RNA. Following auto-cleavage of the capsid protein in the cytoplasm, the remaining polyprotein is inserted into the endoplasmic reticulum, where it is cleaved by host cell proteases and then processed through the secretory pathway, where the glycosylation of E2 and E1 occurs. Virion maturation occurs after E2/E1 heterodimers are inserted into the plasma membrane and 240 copies of the capsid protein interact with one copy of the genomic RNA to form nucleocapsids. These nucleocapsids then interact with a cytoplasmic domain of the E2 protein to initiate budding. The mature virion thus includes 240 copies of the capsid protein and 240 E2/E1 heterodimers arranged as trimeric spikes on the surface of the virus (8).Open in a separate windowFIG. 1.Diagram of the alphavirus genome, showing the 5′ cap, 5′ untranslated region, nonstructural polyprotein open reading frame, and major functions of the individual proteins, subgenomic promoter, structural polyprotein open reading frame, 3′ untranslated region, and poly(A) tail.The structures of several different alphaviruses, including Sindbis virus (SINV) (13), Ross River virus (RRV) (3, 35), Semliki Forest virus (SFV), (11), and VEEV (16), have been solved to subnanometer resolution using cryoelectron microscopy (cryoEM), and the X-ray crystallographic structure of the E1 protein from Semliki Forest virus has been determined to atomic resolution (9). The alphaviruses are ca. 700 Å in diameter, with 80 trimeric spikes on their surfaces. By fitting the E1 crystal structure into cryoEM reconstruction maps of whole viruses, the orientations of both envelope proteins within the spikes have been estimated (36). The E1 and E2 proteins are similar in shape, and the E2 proteins extend to the tips of the spikes, where most glycosylation and antibody-binding sites have been mapped (13). The underlying T=4 icosahedral capsid is constructed from regularly ordered capsomers arranged as hexons and pentons. These pentons and hexons consist of capsid protein monomers that apparently represent only the C-terminal half of the protein. Crystal structures of alphavirus capsid proteins also indicate that only the C terminus, including the protease domain, is ordered (25). cryoEM reconstructions of VEEV nucleocapsids isolated from virions have a less ordered structure, with density redistributed from the 3-fold to the 5-fold axis, suggesting that the envelope and/or the envelope glycoproteins constrain and stabilize the nucleocapsid in a compressed structure (15). Additionally, the VEEV nucleocapsids within viruses differ from those of Old World alphaviruses, with a counterclockwise rotation of the pentameric and hexameric capsomers in VEEV (16). Similar differences were observed in the capsid of Aura virus (AURAV), another New World alphavirus (34).In addition to being an important human and equine pathogen, WEEV is one of three alphaviruses that descended from a recombinant ancestor (6, 31). This ancestor derived its nonstructural and capsid protein genes from an ancestral EEEV strain, whereas its envelope glycoprotein genes were provided from an ancestral SINV. The recombination event was apparently followed by compensatory mutations in the cytoplasmic domain of the E2 protein that restored efficient interactions with the EEEV-like capsid protein (6). If this interpretation of the WEEV ancestral recombination event is correct, its nucleocapsids, constructed from capsid proteins derived from the New World EEEV ancestor, would be expected be more similar to those of the New World VEEV than to those of the Old World SINV, RRV, and SFV. To test this hypothesis and to investigate other structural features of interest related to its recombinant history and pathogenicity, we determined the structure of WEEV to a 13-Å resolution using cryoEM image reconstruction.