Synthetic peptides of Venezuelan equine encephalomyelitis virus E2 glycoprotein. I. Immunogenic analysis and identification of a protective peptide

Human Antibodies Protect against Aerosolized Eastern Equine Encephalitis Virus Infection

Eastern equine encephalitis virus (EEEV) is one of the most virulent viruses endemic to North America. No licensed vaccines or antiviral therapeutics are available to combat this infection, which has recently shown an increase in human cases. Here, we characterize human monoclonal antibodies (mAbs) isolated from a survivor of natural EEEV infection with potent (<20 pM) inhibitory activity of EEEV. Cryo-electron microscopy reconstructions of two highly neutralizing mAbs, EEEV-33 and EEEV-143, were solved in complex with chimeric Sindbis/EEEV virions to 7.2 Å and 8.3 Å, respectively. The mAbs recognize two distinct antigenic sites that are critical for inhibiting viral entry into cells. EEEV-33 and EEEV-143 protect against disease following stringent lethal aerosol challenge of mice with highly pathogenic EEEV. These studies provide insight into the molecular basis for the neutralizing human antibody response against EEEV and can facilitate development of vaccines and candidate antibody therapeutics.

A humanised murine monoclonal antibody with broad serogroup specificity protects mice from challenge with Venezuelan equine encephalitis virus

In murine models of Venezuelan equine encephalitis virus (VEEV) infection, the neutralising monoclonal antibody 1A3B-7 has been shown to be effective in passive protection from challenge by the aerosol route with serogroups I, II and Mucambo virus (formally VEE complex subtype IIIA). This antibody is able to bind to all serogroups of the VEEV complex when used in ELISA and therefore is an excellent candidate for protein engineering in order to derive a humanised molecule suitable for therapeutic use in humans. A Complementarity Determining Region (CDR) grafting approach using human germline IgG frameworks was used to produce a panel of humanised variants of 1A3B-7, from which a single candidate molecule with retained binding specificity was identified. Evaluation of humanised 1A3B-7 (Hu1A3B-7) in in vitro studies indicated that Hu1A3B-7 retained both broad specificity and neutralising activity. Furthermore, in vivo experiments showed that Hu1A3B-7 successfully protected mice against lethal subcutaneous and aerosol challenges with VEEV strain TrD (serogroup I). Hu1A3B-7 is therefore a promising candidate for the future development of a broad-spectrum antiviral therapy to treat VEEV disease in humans.

The first human epitope map of the alphaviral E1 and E2 proteins reveals a new E2 epitope with significant virus neutralizing activity

Background

Venezuelan equine encephalitis virus (VEEV) is responsible for VEE epidemics that occur in South and Central America and the U.S. The VEEV envelope contains two glycoproteins E1 (mediates cell membrane fusion) and E2 (binds receptor and elicits virus neutralizing antibodies). Previously we constructed E1 and E2 epitope maps using murine monoclonal antibodies (mMAbs). Six E2 epitopes (E2^c,d,e,f,g,h) bound VEEV-neutralizing antibody and mapped to amino acids (aa) 182–207. Nothing is known about the human antibody repertoire to VEEV or epitopes that engage human virus-neutralizing antibodies. There is no specific treatment for VEE; however virus-neutralizing mMAbs are potent protective and therapeutic agents for mice challenged with VEEV by either peripheral or aerosol routes. Therefore, fully human MAbs (hMAbs) with virus-neutralizing activity should be useful for prevention or clinical treatment of human VEE.

Methods

We used phage-display to isolate VEEV-specific hFabs from human bone marrow donors. These hFabs were characterized by sequencing, specificity testing, VEEV subtype cross-reactivity using indirect ELISA, and in vitro virus neutralization capacity. One E2-specific neutralizing hFAb, F5n, was converted into IgG, and its binding site was identified using competitive ELISA with mMAbs and by preparing and sequencing antibody neutralization-escape variants.

Findings

Using 11 VEEV-reactive hFabs we constructed the first human epitope map for the alphaviral surface proteins E1 and E2. We identified an important neutralization-associated epitope unique to the human immune response, E2 aa115–119. Using a 9 Å resolution cryo-electron microscopy map of the Sindbis virus E2 protein, we showed the probable surface location of this human VEEV epitope.

Conclusions

The VEEV-neutralizing capacity of the hMAb F5 nIgG is similar to that exhibited by the humanized mMAb Hy4 IgG. The Hy4 IgG has been shown to limit VEEV infection in mice both prophylactically and therapeutically. Administration of a cocktail of F5n and Hy4 IgGs, which bind to different E2 epitopes, could provide enhanced prophylaxis or immunotherapy for VEEV, while reducing the possibility of generating possibly harmful virus neutralization-escape variants in vivo.

Although the murine immune response to Venezuelan equine encephalitis virus (VEEV) is well-characterized, little is known about the human antibody response to VEEV. In this study we used phage display technology to isolate a panel of 11 VEEV-specfic Fabs from two human donors. Seven E2-specific and four E1-specific Fabs were identified and mapped to five E2 epitopes and three E1 epitopes. Two neutralizing Fabs were isolated, E2-specific F5 and E1-specific L1A7, although the neutralizing capacity of L1A7 was 300-fold lower than F5. F5 Fab was expressed as a complete IgG1 molecule, F5 native (n) IgG. Neutralization-escape VEEV variants for F5 nIgG were isolated and their structural genes were sequenced to determine the theoretical binding site of F5. Based on this sequence analysis as well as the ability of F5 to neutralize four neutralization-escape variants of anti-VEEV murine monoclonal antibodies (mapped to E2 amino acids 182–207), a unique neutralization domain on E2 was identified and mapped to E2 amino acids 115–119.

Alpha-beta T cells provide protection against lethal encephalitis in the murine model of VEEV infection

We evaluated the safety and immunogenicity of a chimeric alphavirus vaccine candidate in mice with selective immunodeficiencies. This vaccine candidate was highly attenuated in mice with deficiencies in the B and T cell compartments, as well as in mice with deficient gamma-interferon responsiveness. However, the level of protection varied among the strains tested. Wild type mice were protected against lethal VEEV challenge. In contrast, alpha/beta (αβ) TCR-deficient mice developed lethal encephalitis following VEEV challenge, while mice deficient in gamma/delta (γδ) T cells were protected. Surprisingly, the vaccine potency was diminished by 50% in animals lacking interferon-gamma receptor alpha chain (R1)-chain and a minority of vaccinated immunoglobulin heavy chain-deficient (μMT) mice survived challenge, which suggests that neutralizing antibody may not be absolutely required for protection. Prolonged replication of encephalitic VEEV in the brain of pre-immunized mice is not lethal and adoptive transfer experiments indicate that CD3⁺ T cells are required for protection.

Intranasal immunisation with defective adenovirus serotype 5 expressing the Venezuelan equine encephalitis virus E2 glycoprotein protects against airborne challenge with virulent virus

There is no vaccine licensed for human use to protect laboratory or field workers against infection with Venezuelan equine encephalitis virus (VEEV). Infection of these groups is most likely to occur via the airborne route and there is evidence to suggest that protection against airborne infection may require high antibody levels and the presence of antibody on the mucosal surface of the respiratory tract. Recombinant defective type 5 adenoviruses, expressing the E3E26K structural genes of VEEV were examined for their ability to protect mice against airborne challenge with virulent virus. After intranasal administration, good protection was achieved against the homologous serogroup 1A/B challenge virus (strain Trinidad donkey). There was less protection against enzootic serogroup II and III viruses, indicating that inclusion of more than one E3E26K sequence in a putative vaccine may be necessary. These studies confirm the potential of recombinant adenoviruses as vaccine vectors for VEEV and will inform the development of a live replicating adenovirus-based VEEV vaccine, deliverable by a mucosal route and suitable for use in humans.

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.

Sequencing of prototype viruses in the Venezuelan equine encephalitis antigenic complex

The 5' nontranslated region (5'NTR) and nonstructural region nucleotide sequences of nine enzootic Venezuelan equine encephalitis (VEE) virus strains were determined, thus completing the genomic RNA sequences of all prototype strains. The full-length genomes, representing VEE virus antigenic subtypes I-VI, range in size from 11.3 to 11.5 kilobases, with 48-53% overall G+C contents. Size disparities result from subtype-related differences in the number and length of direct repeats in the C-terminal nonstructural protein 3 (nsP3) domain coding sequence and the 3'NTR, while G+C content disparities are attributable to strain-specific variations in base composition at the wobble position of the polyprotein codons. Highly-conserved protein components and one nonconserved protein domain constitute the VEE virus replicase polyproteins. Approximately 80% of deduced nsP1 and nsP4 amino acid residues are invariant, compared to less than 20% of C-terminal nsP3 domain residues. In two enzootic strains, C-terminal nsP3 domain sequences degenerate into little more than repetitive serine-rich blocks. Nonstructural region sequence information drawn from a cross-section of VEE virus subtypes clarifies features of alphavirus conserved sequence elements and proteinase recognition signals. As well, whole-genome comparative analysis supports the reclassification of VEE subtype-variety IF and subtype II viruses.

Improved protection against Venezuelan equine encephalitis by genetic engineering of a recombinant vaccinia virus

An improved vaccine is needed against Venezuelan equine encephalitis (VEE) virus because the existing live attenuated vaccine, TC-83, causes a high incidence of adverse effects, and the Formalin-inactivated vaccine, C-84, does not protect against airborne infection. A recombinant vaccine had previously been constructed in which the VEE structural proteins were expressed by vaccinia virus. Although protection against subcutaneous challenge with VEE was achieved, the vaccine had limited efficacy against aerosolized virus. We made a similar construct (WR100) and compared its performance with that of a recombinant vaccinia virus which had been altered in two ways (WR103) in order to improve its performance as a vaccine: a synthetic promoter was inserted upstream of the VEE coding sequence to increase the amount of VEE proteins produced, and a single nucleotide in the E2 glycoprotein gene was altered to enhance immunogenicity. The WR103 virus expressed greater amounts of VEE proteins on the surface of infected cells than did WR100, and this difference was found to correspond to a 3.5-fold increase in VEE protein production. Sera from mice immunized with WR103 contained elevated levels of antibody to VEE, and enhanced protection against subcutaneous challenge with the pathogenic Trinidad donkey strain was achieved. This altered construct could form the basis for a better vaccine against VEE.

Hypervariable epitope constructs representing variability in envelope glycoprotein of SIV induce a broad humoral immune response in rabbits and rhesus macaques

Using synthetic peptides, we developed an approach to account for protein epitope variability. We have prepared, in a single synthesis, a cocktail of peptides we have designated a hypervariable epitope construct (HEC), which collectively represents much of the in vivo variability seen in an epitope. Eight HECs representing the in vivo variability seen throughout the envelope glycoprotein of the simian immunodeficiency virus (SIV) were designed and synthesized. The constructs were collectively conjugated to KLH (HEC-KLH) or recombinant gp130 (HEC-rgp130) and used to immunize rabbits and rhesus macaques, respectively. Using sera collected from rabbits immunized with HEC-KLH, we demonstrated that individual components of the immunogen were recognized as antigen in ELISAs, and that the induced antibodies cross-reacted with several strains of SIV as well as with a strain of HIV-2. Following immunization of macaques with HEC-rgp130 antiviral antibodies were induced. These antibodies were still present 9.5 months after the last boost and were also capable of recognizing several different strains of SIV, including SIVmac239, SIVmac251, and SIVsmH3, as well as a strain of HIV-2 (HIV-2ROD). In addition, the antibodies were also capable of neutralizing SIV viral infectivity in vitro. Peripheral blood lymphocytes (PBLs) from immunized macaques proliferated in response to whole proteins and virus. Finally, sera from monkeys immunized with SIV, rgp130, and HIV-2 as well as sera from HIV-2-positive humans recognized HECs in ELISAs, demonstrating the relevance of these epitopes in vivo. This approach can be used as an effective method for generating a strong, broadly cross-reactive humoral response against HIV and can serve as an important component of combination vaccines against HIV and AIDS.

Suppressors of cleavage-site mutations in the p62 envelope protein of Semliki Forest virus reveal dynamics in spike structure and function

The E2 spike glycoprotein of Semliki Forest virus is produced as a p62 precursor protein, which is cleaved by host proteases to its mature form, E2. Cleavage is not necessary for particle formation or release but is necessary for infectivity. Previous results had shown that phenotypic revertants of cleavage-deficient p62 mutants are generated, and here we show that these may contain second-site suppressor mutations in the vicinity of the cleavage site. These hot-spot sites were mutated to abolish the generation of such suppressor mutations; however, secondary mutations in another distant domain of the E2 protein appeared instead, all of which still caused cleavage-deficient mutations. Such mutants grew very poorly and were inefficient in virus entry and release. The mutated sites define domains of the spike protein which probably interact to regulate its structure and function. Because of their highly attenuated phenotype and the lower probability of reversion, the new mutations close to the cleavage site were used to make new helper vectors for packaging of recombinant RNA into infectious particles, thus increasing further the biosafety of the vector system based on the Semliki Forest virus replicon.

Glycoproteins E2 of the Venezuelan and eastern equine encephalomyelitis viruses contain multiple cross-reactive epitopes

Enzyme immunoassay (EIA) with sixty types of monoclonal antibodies (MAbs) was used to study cross-reactive epitopes on the attenuated and virulent strains of the Eastern equine encephalomyelitis (EEE) and Venezuelan equine encephalomyelitis (VEE) viruses. All three structural proteins of the EEE and VEE viruses were demonstrated to have both cross-reactive and specific antigenic determinants. The glycoprotein E1 of EEE and VEE viruses possesses three cross-reactive epitopes for binding to MAbs. The glycoprotein E2 has a cluster of epitopes for 20 cross-reacting MAbs produced to EEE and VEE viruses. Cross-reactive epitopes were localised within five different sites of glycoprotein E2 of VEE virus and within four sites of that of the EEE virus. There are no cross-neutralising MAbs to the VEE and EEE viruses. Only one type of the protective Mabs was able to cross-protect mice against lethal infection by the virulent strains of the VEE and EEE viruses. Eight MAbs blocked the hemagglutination activity (HA) of both viruses. Antigenic alterations of neutralising and protective sites were revealed for all attenuated strains of the VEE and EEE viruses. Comparative studies of the E2 proteins amino acid sequences show that the antigenic modifications observed with the attenuated strains of the VEE virus may be caused by multiple amino acid changes in positions 7, 62, 120, 192 and 209-213. The escape-variants of the VEE virus obtained with cross-reactive MAbs 7D1, 2D4 and 7A6 have mutations of the E2 protein at positions 59, 212-213 and 232, respectively. Amino acid sequences in these regions of the VEE and EEE viruses are not homologous. These observations indicate that cross-reactive MAbs are capable of recognising discontinuous epitopes on the E2 glycoprotein.

Localization of a protective epitope on a Venezuelan equine encephalomyelitis (VEE) virus peptide that protects mice from both epizootic and enzootic VEE virus challenge and is immunogenic in horses

In order to define more precisely the protective epitope encoded within the first 25 amino acids (aa) of the E2 glycoprotein of the Trinidad donkey strain of Venezuelan equine encephalomyelitis (VEE) virus, we examined the immunogenicity of smaller peptides within the first 19 aa. pep1-9 and pep3-10 elicited virus-reactive antibody, but failed to protect mice from virus challenge. Additionally, pep3-10 was identified by a competitive binding assay using overlapping peptide octamers as the putative binding site of the antipeptide monoclonal antibody (mAb) 1A2B-10. Since the E2 amino-terminal sequence for all VEE subtype viruses is conserved, we tested the protective capacity in mice of passively transferred mAb 1A2B-10 and found it to protect from both epizootic and enzootic VEE virus challenge. Since horses are an important natural host for VEE virus, pep1-19 was used to immunize horses and was found to be immunogenic and to elicit virus-reactive antibody.

The amino-terminal residue of Sindbis virus glycoprotein E2 influences virus maturation, specific infectivity for BHK cells, and virulence in mice

The E2 glycoprotein of Sindbis virus is synthesized as a precursor, PE2, which is cleaved by furin or a furin-like host cell protease at a late stage of maturation. The four-residue PE2 cleavage signal conforms to the basic amino acid-X-basic-basic motif which is present in many other viral and cellular glycoproteins which are processed by the cellular enzyme(s). In this report, we present evidence that the amino acid which immediately follows the signal, the N-terminal residue of E2, can influence protease recognition, binding, and/or cleavage of PE2. Constructs encoding nine different amino acids at E2 position 1 (E2 1) were produced by site-directed mutagenesis of the full-length cDNA clone of our laboratory strain of Sindbis virus AR339 (pTRSB). Viruses derived from clones encoding Arg (TRSB), Asp, Ser, Phe, His, and Asn in a nonglycosylated form at E2 1 contained predominantly E2. Viruses encoding Ile, Leu, or Val at E2 1 contained the uncleaved form of PE2. The specific infectivity of TRSB (E2 Arg-1) for baby hamster kidney (BHK-21) cells was from 5- to greater than 100-fold higher than those of isogenic constructs with other residues at E2 1, suggesting that E2 Arg-1 represents a BHK-21 cell adaptive mutation in our laboratory strain. In newborn CD-1 mice, TRSB was more virulent than the PE2-containing viruses but less virulent than other PE2-cleaving viruses with alternative amino acids at E2 1. These results indicate that in TRSB, E2 Arg-1 increased the efficiency of virus-cell interactions in cultured BHK-21 cells but simultaneously decreased the ability of virus to mediate in vivo virus-cell interactions critical for the induction of disease. This suggests that the N terminus of E2 may participate in or be associated with virion domains which mediate these viral functions.

The alphaviruses: gene expression, replication, and evolution

The alphaviruses are a genus of 26 enveloped viruses that cause disease in humans and domestic animals. Mosquitoes or other hematophagous arthropods serve as vectors for these viruses. The complete sequences of the +/- 11.7-kb plus-strand RNA genomes of eight alphaviruses have been determined, and partial sequences are known for several others; this has made possible evolutionary comparisons between different alphaviruses as well as comparisons of this group of viruses with other animal and plant viruses. Full-length cDNA clones from which infectious RNA can be recovered have been constructed for four alphaviruses; these clones have facilitated many molecular genetic studies as well as the development of these viruses as expression vectors. From these and studies involving biochemical approaches, many details of the replication cycle of the alphaviruses are known. The interactions of the viruses with host cells and host organisms have been exclusively studied, and the molecular basis of virulence and recovery from viral infection have been addressed in a large number of recent papers. The structure of the viruses has been determined to about 2.5 nm, making them the best-characterized enveloped virus to date. Because of the wealth of data that has appeared, these viruses represent a well-characterized system that tell us much about the evolution of RNA viruses, their replication, and their interactions with their hosts. This review summarizes our current knowledge of this group of viruses.

Lethality of PE2 incorporation into Sindbis virus can be suppressed by second-site mutations in E3 and E2

Sindbis virions contain two glycoproteins, E1 and E2. E2 is produced initially as a precursor, PE2, from which the amino-terminal 64 amino acids are cleaved by a cellular protease at a late stage in virion maturation. A mutation at E2 position 1 (Arg to Asn) was placed into Sindbis virus AR339 by site-directed mutagenesis of a full-length AR339 cDNA clone, pTRSB, to produce pTRSB-N. The mutation created a signal for N-linked glycosylation immediately adjacent to the PE2 cleavage signal. Virions derived from pTRSB-N were glycosylated at E2 position 1, and they quantitatively incorporated PE2 in place of E2. When pTRSB-N transcripts were electroporated into BHK-21 cells, TRSB-N particles were released with nearly normal efficiency; however, the specific infectivity of TRSB-N particles was very low. Analysis of seven infectious revertants of TRSB-N revealed that reversion was linked to (i) mutations that eliminated the signal for N-linked glycosylation and thus restored the PE2 cleavage phenotype or (ii) conservation of the PE2 cleavage defect combined with incorporation of suppressor mutations in E3 or E2. The genotype of each revertant was reconstructed in the genetic background of TRSB-N, and each reverting mutation also was replaced individually into the genetic background of wild-type virus (TRSB). Each PE2-containing revertant was attenuated in newborn CD-1 mice and replicated poorly in cultured mosquito cells (C6/36). Reverting mutations in the genetic background of TRSB did not reduce virulence in mice or growth in mosquito cells, suggesting that the phenotypes of attenuation in mice and reduced growth in mosquito cells were linked to failure of PE2 cleavage and not to the reverting mutations themselves.

Localization of four antigenic sites involved in Venezuelan equine encephalomyelitis virus protection

Stable neutralization and protection escape variants of a virulent strain (Trinidad Donkey) of the VEE virus were selected by monoclonal antibodies (MAbs). Determination of nucleotide sequences of nine variants revealed a clustering of single mutations in four regions of the E1 and E2 glycoproteins. Involvement of amino acid residues 206 (site E1-1), 57 and 59 (site E2-2), 180, 182, 213, 214 and 216 (site E2-6) and 232 (site E2-3) in protective epitopes was demonstrated.

Immunogens of encephalitis viruses

The equine encephalitis viruses are members of the genus Alphavirus, in the family Togaviridae. Three main virus serogroups represented by western (WEE), eastern (EEE) and Venezuelan equine encephalitis (VEE) viruses cause epizootic and enzootic infection of horses throughout the western hemisphere. All equine encephalitis viruses are transmitted through the bite of an infected mosquito. The first equine encephalitis virus vaccines were produced by virus inactivation. Problems with inadequate inactivation, which may have caused a major epidemic/epizootic of VEE in central America and Texas in the 1970s, led to the development of a live attenuated VEE virus vaccine (TC-83) derived by cell culture passage. Inactivated vaccines are still used to prevent equine infections with WEE and EEE viruses. Alphaviruses are small single stranded, positive sense RNA viruses. The 12000 nucleotide genome is enclosed in an icosahedral nucleocapsid composed of multiple copies of the capsid (C) protein. The virion is enveloped. The membrane is modified by the insertion of heterodimers of two glycoproteins: E1 and E2. Monoclonal antibody analysis of the surface glycoproteins have provided a detailed understanding of important protective antigens. Recent studies comparing gene sequences from virulent and avirulent VEE viruses have begun to delineate mechanisms of alphavirus attenuation.

An antibody- and synthetic peptide-defined rubella virus E1 glycoprotein neutralization domain

We previously described a monoclonal antibody (MAb) library generated by infecting BALB/c mice with rubella virus (RV) and selected by an enzyme-linked immunosorbent assay (ELISA) using purified virion targets. Plasmid pARV02-01, which expresses the fusion protein RecA1-35-GIGDLGSP-E1(202)-E1(283)-GDP-LacZ9-1015 in Escherichia coli, was shown to be a ligand for MAbs E1-18 and E1-20 (J. S. Wolinsky, M. McCarthy, O. Allen-Cannady, W. T. Moore, R. Jin, S. N. Cao, A. Lovett, and D. Simmons, J. Virol. 65:3986-3994, 1991). Both of these MAbs neutralize RV infectivity. A series of five overlapping synthetic peptides was made to further explore the requirements of this MAb binding domain. One of these peptides (SP15; E1(208) to E1(239)) proved an effective ligand for both MAbs in the ELISA. Stepwise synthesis of SP15 defined the minimal amino-terminal requirement for binding MAb E1-18 as E1(221) and that of MAb E1-20 as E1(223); the minimal carboxyl-terminal requirement is uncertain but does not exceed E1(239). Immunization of mice and rabbits with SP15 induced polyvalent antibody reactive with SP15, with other overlapped and related but not unrelated synthetic peptides, and with RV. The rabbit anti-SP15 antibody showed neutralization activity to RV similar to that of MAbs E1-18 and E1-20 but lacked hemagglutination inhibition activity. These data define a neutralization domain on E1 and suggest that the RV epitopes conserved by SP15 may be critical for protective host humoral immune responses.

Characterization of a B-cell immunodominant epitope of human T-lymphotropic virus type 1 (HTLV-I) envelope gp46

The immune response elicited by a synthetic peptide derived from an immunodominant external envelope region (Env-5, amino acids 242-257) of human T-lymphotropic virus type 1 (HTLV-I) was tested in a rabbit model of HTLV-I infection. The synthetic peptide elicited a strong antibody response to the HTLV-I envelope protein gp46; however, these antibodies failed to inhibit HTLV-I-mediated cell fusion. Immunized rabbits were not protected from HTLV-I infection as determined by seroconversion to viral core proteins by immunoblot, HTLV-I p24 antigen detection in lymphocyte cultures and polymerase chain reaction for the HTLV-I provirus in lymphocyte DNA. Env-5 peptide immunization failed to induce T-cell lymphocyte proliferative responses in rabbits, but induced antibody responses in T-cell deficient Balb c nu/nu mice suggesting that the antigenic determinant represented by the Env-5 peptide is primarily a B-cell epitope. These results further define an immunodominant epitope of the HTLV-I envelope protein and suggest that potential synthetic peptide vaccines against HTLV-I infection must contain multiple antigens that induce both humoral and cellular immune reactivity.

Characterization of a major neutralization domain of Ross River virus using anti-viral and anti-peptide antibodies

The E2 glycoprotein of the alphavirus Ross River virus (RRV) contains three defined neutralization epitopes (a, b1 and b2) with determinants located between amino acids 216 and 251 in the linear sequence (Vrati et al., 1988, Virology 162, 346-353). The antigenic structure of this region has been examined using hyperimmune mouse antiserum against RRV and antiserum against four synthetic peptides representing linear amino acid sequences in the neutralization region of E2. In plaque reduction neutralization tests using hyperimmune antiserum to RRV, an RRV mutant altered at all three neutralization epitopes was markedly more resistant than the parental virus; variants altered at single epitopes could not be distinguished in these tests. Sera from mice immunized with synthetic RRV E2 peptides conjugated to keyhole limpet haemocyanin reacted, in a direct ELISA, with the specific region of RRV represented by the peptide. The same sera did not neutralize or immunoprecipitate RRV in solution or bind to RRV in a capture ELISA. The RRV peptides did not prime mice to react to a subimmunogenic dose of RRV; they did not bind monoclonal or polyclonal antibodies to RRV. We conclude that a significant proportion of the neutralizing antibody response in mice is elicited by epitopes a, b1, and b2 of RRV E2 and that the sites to which neutralizing antibodies bind are formed by complex folding.

Synthetic peptides of Venezuelan equine encephalomyelitis virus E2 glycoprotein. III. Identification of a protective peptide derived from the carboxy-terminal extramembranal one-third of the protein

To complete our analysis of the E2 glycoprotein of Venezuelan equine encephalomyelitis (VEE) virus, we prepared six synthetic peptides corresponding to the extramembranal carboxy-terminal one-third of the protein. NIH-Swiss mice were immunized with the peptides, and antipeptide and antiviral titers were determined by enzyme-linked immunosorbent assay (ELISA). Challenge studies revealed that peptide 13 (amino acids 241-265) protected 60-70% of virus-challenged mice. Although the other peptides generally elicited antipeptide ELISA titers but no or low antiviral titers and did not protect mice, significant E2 reactivity was found in immunoblots. These results provide the first direct evidence that much of the E2 carboxy-terminal domain is cryptic in the VEE virion, even when virus was bound to polystyrene ELISA plates.

Synthetic peptides of the E2 glycoprotein of Venezuelan equine encephalomyelitis virus. II. Antibody to the amino terminus protects animals by limiting viral replication

A peptide composed of the amino-terminal 25 amino acids of the E2 glycoprotein of the virulent Trinidad donkey (TRD) strain of Venezuelan equine encephalomyelitis virus was found to protect peptide-immunized mice from lethal TRD virus challenge (Hunt et al., 1990). Viral growth in peptide-immunized animals was found to be limited in comparison to that in nonimmunized controls. Although both treated and control groups of mice responded to virus challenge by producing neutralizing antibody, only immunized mice with preexisting antipeptide antibody survived. Polyclonal antipeptide sera as well as a monoclonal antipeptide antibody were able to passively protect naive mice from TRD virus challenge, despite the fact that these antibodies were nonneutralizing. Passive transfer of antipeptide antibody to immunosuppressed recipients was not protective, thus indicating that survival of TRD virus challenge required an in situ immune response as well as preexisting antipeptide antibody. Binding studies of both polyclonal and monoclonal antipeptide antibodies indicated that they recognize only epitopes present on virus-infected cells or denatured virus.

Studies on linear epitopes of Semliki Forest virus in protection studies against lethal challenge virus infection

Purified reduced and non-reduced glycoproteins E1 and E2 of Semliki Forest Virus (SFV) were used to investigate the protection potency to prevent clinical disease after lethal virus challenge. In parallel synthetic oligopeptides deduced from conserved regions of the nucleotide sequences coding for the glycoproteins E1 and E2 were included. It could be demonstrated that both reduced and non-reduced glycoprotein preparations induced protection against lethal virus challenge, whereas the oligopeptides did not. The role of linear epitopes in immunity and their potential use as synthetic vaccines against Alphaviruses are critically discussed.

Use of a new synthetic-peptide-derived monoclonal antibody to differentiate between vaccine and wild-type Venezuelan equine encephalomyelitis viruses

We have prepared a murine monoclonal antibody (MAb) capable of distinguishing between wild-type Venezuelan equine encephalomyelitis (VEE) virus and the TC-83 vaccine derivative. This MAb, 1A2B-10, was derived from immunization with a synthetic peptide corresponding to the first 19 amino acids of the E2 glycoprotein of Trinidad donkey VEE virus. The MAb reacts with prototype viruses from all naturally occurring VEE subtypes except subtype 6 in an enzyme-linked immunosorbent assay. It does not react with TC-83 virus or members of the western and eastern equine encephalitis virus complex or with Semliki Forest virus. This antibody will also differentiate between TC-83 and Trinidad donkey VEE virus in indirect immunofluorescence assays with virus-infected Vero cells.