Mapping and characterization of a sequential epitope on the rabies virus glycoprotein which is recognized by a neutralizing monoclonal antibody, RG719

Lyssavirus Vaccine with a Chimeric Glycoprotein Protects across Phylogroups

Rabies is nearly 100% lethal in the absence of treatment, killing an estimated 59,000 people annually. Vaccines and biologics are highly efficacious when administered properly. Sixteen rabies-related viruses (lyssaviruses) are similarly lethal, but some are divergent enough to evade protection from current vaccines and biologics, which are based only on the classical rabies virus (RABV). Here we present the development and characterization of LyssaVax, a vaccine featuring a structurally designed, functional chimeric glycoprotein (G) containing immunologically important domains from both RABV G and the highly divergent Mokola virus (MOKV) G. LyssaVax elicits high titers of antibodies specific to both RABV and MOKV Gs in mice. Immune sera also neutralize a range of wild-type lyssaviruses across the major phylogroups. LyssaVax-immunized mice are protected against challenge with recombinant RABV and MOKV. Altogether, LyssaVax demonstrates the utility of structural modeling in vaccine design and constitutes a broadened lyssavirus vaccine candidate.

A Cost Effective Easy Competitive Enzyme-Linked Immunosorbent Assay Suitable for Monitoring Protective Immunity against the Rabies Virus in the Serum of Humans and Dogs

The coverage of rabies vaccinations has been reported at 70-80% of dogs in annual reports. However, there are still outbreaks of rabies among humans and dogs in Thailand, thus indicating the necessity of ensuring seroprevalence in vaccinated dogs and efficacy of human immunization. A cost effective easy competitive enzyme-linked immunosorbent assay (CEE-cELISA) was developed here for monitoring protective immunity against the rabies virus in human and dog serum samples using monoclonal antibody clone 1-46-12, which recognizes a conformational epitope of the rabies G protein. The ELISA plate is coated with the whole viral antigen from a commercial vaccine. The serotiter measured by the CEE-cELISA and by the gold standard assay (rapid fluorescent focus inhibition test), detecting the neutralizing antibody, showed a strong correlation, with an R value of 0.958 and 0.931 in humans and dogs, respectively. These correlations were detected in the serum samples from humans and dogs at antibody concentrations up to 100 and 10 IU/ml, respectively. This CEE-cELISA could be an alternative assay for evaluating mass rabies vaccination rapidly at a low cost as well as for detecting antirabies antibodies in the serum of not only humans but also other animal species.

Antigenic site changes in the rabies virus glycoprotein dictates functionality and neutralizing capability against divergent lyssaviruses

Lyssavirus infection has a near 100 % case fatality rate following the onset of clinical disease, and current rabies vaccines confer protection against all reported phylogroup I lyssaviruses. However, there is little or no protection against more divergent lyssaviruses and so investigation into epitopes within the glycoprotein (G) that dictate a neutralizing response against divergent lyssaviruses is warranted. Importantly, the facilities required to work with these pathogens, including wild-type and mutated forms of different lyssaviruses, are scarcely available and, as such, this type of study is inherently difficult to perform. The relevance of proposed immunogenic antigenic sites within the lyssavirus glycoprotein was assessed by swapping sites between phylogroup-I and -II glycoproteins. Demonstrable intra- but limited inter-phylogroup cross-neutralization was observed. Pseudotype viruses (PTVs) presenting a phylogroup-I glycoprotein containing phylogroup-II antigenic sites (I, II III or IV) were neutralized by antibodies raised against phylogroup-II PTV with the site II (IIb, aa 34-42 and IIa, aa 198-200)-swapped PTVs being efficiently neutralized, whilst site IV-swapped PTV was poorly neutralized. Specific antibodies raised against PTV-containing antigenic site swaps between phylogroup-I and -II glycoproteins neutralized phylogroup-I PTVs efficiently, indicating an immunodominance of antigenic site II. Live lyssaviruses containing antigenic site-swapped glycoproteins were generated and indicated that specific residues within the lyssavirus glycoprotein dictate functionality and enable differential neutralizing antibody responses to lyssaviruses.

Immunogenicity of multi-epitope-based vaccine candidates administered with the adjuvant Gp96 against rabies

Rabies, a zoonotic disease, causes > 55,000 human deaths globally and results in at least 500 million dollars in losses every year. The currently available rabies vaccines are mainly inactivated and attenuated vaccines, which have been linked with clinical diseases in animals. Thus, a rabies vaccine with high safety and efficacy is urgently needed. Peptide vaccines are known for their low cost, simple production procedures and high safety. Therefore, in this study, we examined the efficacy of multi-epitope-based vaccine candidates against rabies virus. The ability of various peptides to induce epitope-specific responses was examined, and the two peptides that possessed the highest antigenicity and conservation, i.e., AR16 and hPAB, were coated with adjuvant canine-Gp96 and used to prepare vaccines. The peptides were prepared as an emulsion of oil in water (O/W) to create three batches of bivalent vaccine products. The vaccine candidates possessed high safety. Virus neutralizing antibodies were detected on the day 14 after the first immunization in mice and beagles, reaching 5-6 IU/mL in mice and 7-9 IU/mL in beagles by day 28. The protective efficacy of the vaccine candidates was about 70%-80% in mice challenged by a virulent strain of rabies virus. Thus, a novel multi-epitope-based rabies vaccine with Gp96 as an adjuvant was developed and validated in mice and dogs. Our results suggest that synthetic peptides hold promise for the development of novel vaccines against rabies.

Association between RABV G Proteins Transported from the Perinuclear Space to the Cell Surface Membrane and N-Glycosylation of the Sequon Asn(204)

In this study, G proteins of the rabies virus (RABV) Kyoto strain were detected in the cytoplasm but not distributed at the cell membrane of mouse neuroblastoma (MNA) cells. G proteins of CVS-26 were detected in both the cell membrane and perinuclear space of MNA cells. We found that N-glycosylation of street RABV G protein by the insertion of the sequon Asn(204) induced the transfer of RABV G proteins to the cell surface membrane. Fixed RABV budding from the plasma membrane has been found to depend not only on G protein but also on other structural proteins such as M protein. However, the differing N-glycosylation of G protein could be associated with the distinct budding and antigenic features of RABV in street and fixed viruses. Our study of the association of N-glycan of G protein at Asn(204) with the transport of RABV G protein to the cell surface membrane contributes to the understanding of the evolution of fixed virus from street virus, which in turn would help for determine the mechanism underlying RABV budding and enhanced host immune responses.

Re-emergence of rabies in the Guangxi province of Southern China

BACKGROUND

Human rabies cases in the Guangxi province of China decreased from 839 in 1982 to 24 in 1995, but subsequently underwent a sharp increase, and has since maintained a high level.

METHODOLOGY/PRINCIPAL FINDINGS

3,040 brain samples from normal dogs and cats were collected from 14 districts of Guangxi and assessed by RT-PCR. The brain samples showed an average rabies virus (RV) positivity rate of 3.26%, but reached 4.71% for the period Apr 2002 to Dec 2003. A total of 30 isolates were obtained from normal dogs and 28 isolates from rabid animals by the mouse inoculation test (MIT). Six representative group I and II RV isolates showed an LD50 of 10-5.35/ml to 10-6.19/ml. The reactivity of monoclonal antibodies (MAbs) to group I and II RV isolates from the Guangxi major epidemic showed that eight anti-G MAbs showed strong reactivity with isolates of group I and II with titers of ≥10,000; however, the MAbs 9-6, 13-3 and 12-14 showed lower reactivity. Phylogenetic analysis based on the G gene demonstrated that the Guangxi RV isolates have similar topologies with strong bootstrap values and are closely bonded. Alignment of deduced amino acids revealed that the mature G protein has four substitutions A96S, L132F, N436S, and A447I specific to group I, and 13 substitutions T90M, Y168C, S204G, T249I, P253S, S289T, V332I, Q382H, V427I, L474P, R463K Q486H, and T487N specific to group II, coinciding with the phylogenetic analysis of the isolates.

CONCLUSIONS

Re-emergence of human rabies has mainly occurred in rural areas of Guangxi since 1996. The human rabies incidence rate increased is related with RV positive rate of normal dogs. The Guangxi isolates tested showed a similar pathogenicity and antigenicity. The results of phylogenetic analysis coincide with that of alignment of deduced amino acids.

Characterization of a Slow-Migrating Component of the Rabies Virus Matrix Protein Strongly Associated with the Viral Glycoprotein

We investigated multiple forms of rabies virus matrix (M) protein. Under non-reducing electrophoretic conditions, we detected, in addition to major bands of monomer forms (23- and 24-kDa) of M protein, an M antigen-positive slow-migrating minor band (about 54 kDa) in both the virion and infected cells. Relative contents of the 54-kDa and monomer components in the virion were about 20-30% and 70-80% of the whole M protein, respectively, while the content of the 54-kDa component was smaller (about 10-20% of the total M protein) in the cell than in the virion. The 54-kDa components could be extracted from the infected cells with sodium deoxycholate, but they were quite resistant to extraction with 1% nonionic detergents by which most monomer components were solubilized. The 54-kDa component was precipitated more efficiently than the monomer by a monoclonal antibody (mAb; #3-9-16), which recognized a linear epitope located at the N-terminal of the M protein. The mAb #3-9-16 coprecipitated the viral glycoprotein (G), which was demonstrated to be due to strong association between the G and 54-kDa component of the M protein. Monomers and the 54-kDa polypeptide migrated to the same isoelectric point (pI) in twodimensional (2-D) gel electrophoresis, implicating that the 54-kDa component was composed of component(s) of the same pI as that of the M protein monomers. From these results, we conclude that the M antigen-positive 54-kDa polypeptide is a homodimer of M protein, taking an N-terminal-exposed conformation, and is strongly associated with the viral glycoprotein. Possible association with a membrane microdomain of the cell will be discussed.

Studies on the Conditions Required for Structural and Functional Maturation of Rabies Virus Glycoprotein (G) in G cDNA-Transfected Cells

When the rabies virus G cDNA was expressed with the help of T7 RNA polymerase provided by a recombinant vaccinia virus (RVV-T7), functional G proteins were produced in terms of their ability to induce low pH-dependent syncytium formation and the formation of conformational epitopes, including the acid-sensitive epitope recognized by mAb #1-30-44. Such an ability and the 1-30-44 epitope formation, however, were not associated with the G gene products when G cDNA was expressed without the help of RVV-T7 using a tetracycline-regulated expression vector (pTet-G), although they were normally transported to the surface of established G protein-producing BHK-21 (G-BHK) cells. But, when the G-BHK cells were treated with 2.5 m M sodium butyrate (NaB) after the removal of tetracycline, we could observe not only a much increased frequency of G protein-producing cells, but also the greatly enhanced maturation of the protein. Another short acylate, sodium propionate (NaP), similarly induced increased G protein synthesis at a concentration of 2.5 m M as NaB; however, such proteins were mostly not endowed with the fusion activity nor the 1-30-44 epitope, while NaP at a higher concentration as 5.0 m M did induce similarly the increased production and enhanced maturation of G protein, including the 1-30-44 epitope formation. From these results, we conclude that functional maturation of G protein to acquire fusogenic activity is correlated with 1-30-44 epitope formation, and 2.5 m M NaB not only stimulates G protein production, but also provides such cellular conditions as are required for the structural and functional maturation of the protein.

Monoclonal Antibody #3-9-16 Recognizes One of the Two Isoforms of Rabies Virus Matrix Protein That Exposes Its N-Terminus on the Virion Surface

We investigated behaviors of the rabies virus matrix (M) protein using a monoclonal antibody (mAb), #3-9-16, that recognized a linear epitope located at the N-terminus of the protein. Based on the reactivity with this mAb, M proteins could be divided into at least two isoforms; an ordinary major form (Malpha) whose 3-9-16 epitope is hidden, and an N-terminal-exposed epitope-positive form (Mbeta). The Mbeta protein accounted for about 25-30% of the total M proteins in the virion, while its content in the cell ranged from 10 to 15% of total M protein. Fluorescent antibody (FA) staining showed that the Mbeta antigen distributed in the Golgi area where the colocalized viral glycoprotein antigen was also detected. Mbeta antigen was shown to be exposed on the surface of infected cells by both immunoprecipitation and FA staining with the mAb, whereby the cells might have become sensitive to the mAb-dependent complement-mediated cytolysis. Similarly, the Mbeta antigen was shown to be exposed on the virion surface, and the infectivity of the virus was destroyed by the mAb in the presence of a complement. Together with these results, we think that the M protein molecule takes either of two conformations, one (Mbeta) of which exposes the 3-9-16 epitope located in the N-terminal region of the M protein, that are also exposed on the surface of the virion and infected cells, whereby it might play a certain important role(s) in the virus replication process differently from the other form (Malpha), probably through its intimate association with the Golgi area and/or the cell membrane.

Identification of rabies virus mimotopes screened from a phage display peptide library with purified dog anti-rabies virus serum IgG

The rabies virus glycoprotein (G) is a key protein for both virus infectivity and eliciting protective immunity as an antigen. What is more, the nucleoprotein (N) is also a significant rabies virus antigen. In this study, purified anti-rabies virus IgG from dogs immunized with the standard CVS-11 strain was used to screen the Ph.D.-12™ Phage Display Peptide Library for peptides that correspond to or mimic native G and N epitopes. In contrast to previous reports that use monoclonal antibodies or human anti-rabies virus serum, this study describes the first use of dog serum to screen for epitopes. After three rounds of biopanning, selected phage clones were identified by plaque screening, western blotting (WB), and ELISA. Positive phage clones were sequenced, and their amino acid sequences were deduced. Alignment of the peptide sequences to G and N indicated that the epitope peptides matched well with G amino acids at positions 34-42, 198-200, 226-264, 296-371, and 330-343, as well as to N amino acids at positions 22-168 (N-terminal) and 262-450 (C-terminal), confirming that the sequences were indeed mimicking epitopes. Thirty percent of the selected clones matched reported antigenic regions located at sites II and III of the glycoprotein. Two sequences, LEPKGRYDDPWT and ATRYDDIWASTA, that have no homology to the known antigenic sites of either the G or N exhibited a common RYDD-W-T motif that is highly homologous to the amino acid residues at positions 126-141 of the G. This finding indicates that this motif may be a new potential RABV G B cell epitope. Amino acids 126-141 containing the RYDD-W-T motif may become a novel key epitope region and allow the development of a rabies vaccine or diagnostic reagents for the treatment of rabies.

Generation and Characterization of an scFv Directed against Site II of Rabies Glycoprotein

Recombinant antibody phage display technology is a vital tool that facilitates identification of specific binding molecules to a target enabling the rapid generation and selection of high affinity, fully human, or mouse antibody product candidates essentially directed towards disease target appropriate for antibody therapy. In this study, a recombinant single-chain Fv antibody fragment (scFv) A11 was isolated from immune spleen cells obtained from mice immunized with inactivated rabies virus (Pasteur strain) using standard methodology and was characterized for its specificity towards the rabies virus glycoprotein. Epitope mapping using peptide libraries and truncated glycoprotein polypeptides suggested that A11 bound to the antigenic site II of rabies glycoprotein against which a majority of rabies virus neutralizing antibodies are directed. The use of the above technology could, therefore, allow development of scFvs with different specificities against the rabies glycoprotein as an alternative to the more cumbersome protocols used for the development of monoclonal antibodies.

Fine mapping and interaction analysis of a linear rabies virus neutralizing epitope

A novel human antibody AR16, targeting the G5 linear epitope of rabies virus glycoprotein (RVG) was shown to have promising antivirus potency. Using AR16, the minimal binding region within G5 was identified as HDFR (residues 261-264), with key residues HDF (residues 261-263) identified by alanine replacement scanning. The key HDF was highly conserved within phylogroup I Lyssaviruses but not those in phylogroup II. Using computer-aided docking and interaction models, not only the key residues (Asp30, Asp31, Tyr32, Trp53, Asn54, Glu99, Ile101, and Trp166) of AR16 that participated in the interaction with G5 were identified, the van der Waals forces that mediated the epitope-antibody interaction were also revealed. Seven out of eight presumed key residues (Asp30, Asp31, Tyr32, Trp53, Asn54, Glu99, and Ile101) were located at the variable regions of AR16 heavy chains. A novel mAb cocktail containing AR16 and CR57, has the potential to recognize non-overlapping, non-competing epitopes, and neutralize a broad range of rabies virus.

A molecular epidemiological study targeting the glycoprotein gene of rabies virus isolates from China

A group of 31 rabies viruses (RABVs), recovered primarily from dogs, one deer and one human case, were collected from various areas in China between 1989 and 2006. Complete G gene sequences determined for these isolates indicated identities of nucleotide and amino acid sequences of >or=87% and 93.8%, respectively. Phylogenetic analysis of these and some additional Chinese isolates clearly supported the placement of all Chinese viruses in Lyssavirus genotype 1 and divided all Chinese isolates between four distinct groups (I-IV). Several variants identified within the most commonly encountered group I were distributed according to their geographical origins. A comparison of representative Chinese viruses with other isolates retrieved world-wide indicated a close evolutionary relationship between China group I and II viruses and those of Indonesia while China group III viruses formed an outlying branch to variants from Malaysia and Thailand. China group IV viruses were closely related to several vaccine strains. The predicted glycoprotein sequences of these RABVs variants are presented and discussed with respect to the utility of the anti-rabies biologicals currently employed in China.

Two different conformations of rabies virus glycoprotein taken under neutral pH conditions

We previously reported that the rabies virus glycoprotein (G) takes either of two different conformations (referred to as B and C forms) under neutral pH conditions, that could be differentiated by their reactivity to a monoclonal antibody (mAb), #1-30-44, that recognizes the acid-sensitive conformational epitope, and the formation taken is dependent on two separate regions containing Lys-202 and Asn-336 of the protein (Kankanamge et al., Microbiol. Immunol., 47, 507-519, 2003). Semi-quantitative antibody-binding assays demonstrated that only one-third to one-fourth of mature G proteins on the cell surface were taking the 1-30-44 epitope-positive B form even at pH 7.4. The ratio of B to C varied, depending on the environmental pH, but did not decrease to zero even at pH 5.8-6.2, preserving a certain content (about 15-20%) of B form. Immunoprecipitation studies demonstrated that a portion of G proteins were intimately associated with a dimer form of matrix (M) protein in terms of resistance to treatment with a mixture of 1% deoxycholate and 1% Nonidet P-40, and seemed to preserve the B form even at lower pHs. Similar results were also obtained with the virion-associated G proteins, including the intimate association of a portion of the G proteins with the M protein dimer. From these results, we assume that a certain portion of the rabies virion-associated G proteins are associated with a dimer form of M protein, keeping the 1-30-44 epitope-positive B conformation under various pH conditions, which might possibly assure the virion's recognition of host cell receptor molecules in the body.

Further studies on the soluble form (gs) of rabies virus glycoprotein (g): molecular structure of gs protein and possible mechanism of the shedding

In this study, we investigated the antigenic structures and maturation of some C-terminal-deficient derivatives of rabies virus glycoprotein (G). The Gs protein, a soluble form of G protein shed from infected cells, displayed antigenicity to most of our conformational epitope-specific anti-G mAbs, but took the 1-30-44 epitope-deficient conformation (termed G(C) form). (The 1-30-44 epitope was acid-sensitive and dependent on two separate regions, the Lys-202-containing and Asn-336-containing regions; Kankanamge et al., Microbiol. Immunol., 47: 507-519). Intact G proteins took the 1-30-44 epitope-positive form (referred to as G(B) form) on the cell surface, but not inside the cell. A deletion mutant G(1-429) (termed GDeltaTC), lacking the transmembrane (TM) and cytoplasmic domains, was shown to be accumulated in the rough endoplasmic reticulum (rER) with BiP and did not seem to be shed. Another C-terminal-deficient mutant G(1-462) (termed CT1) was deprived of the whole cytoplasmic domain except for a basic amino acid left at the C-terminus, but was transported to the cell surface, where it showed pH-dependent cell fusion activity and almost full antigenicity to most of the anti-G mAbs with the exception of very weak antigenicity to mAb #1-30-44. No Gs protein could be detected in the CT1-producing cultures. Based on these results, we think that the cytoplasmic domain was not necessary for the G protein to be transported to the cell surface, but was necessary to keep its 1-30-44 epitope-positive G(B) conformation. Gs proteins might have lost the C-terminal regions during the maturation process after being exported from the rER.

Production of rabies neutralizing antibody in hen's eggs using a part of the G protein expressed in Escherichia coli

In an attempt to produce anti-rabies immunoglobulin affordable for people living in developing countries, we have immunized layer chickens with a part of the G protein of rabies virus expressed in Escherichia coli. Immunoglobulin (IgY) was purified from the yolks of eggs layed by immunized hens. It was revealed in vitro that the antibody specifically bound to virions as well as cells infected with rabies virus. Moreover, the antibody apparently neutralized rabies virus infectivity. Inoculation of the antibody into mice infected with rabies virus reduced the mortality caused by the virus, suggesting that IgY directed to the part of the G protein expressed in E. coli could serve as a possible alternative to currently available anti-rabies human or equine immunoglobulins.

Novel rabies virus-neutralizing epitope recognized by human monoclonal antibody: fine mapping and escape mutant analysis

Anti-rabies virus immunoglobulin combined with rabies vaccine protects humans from lethal rabies infections. For cost and safety reasons, replacement of the human or equine polyclonal immunoglobulin is advocated, and the use of rabies virus-specific monoclonal antibodies (MAbs) is recommended. We produced two previously described potent rabies virus-neutralizing human MAbs, CR57 and CRJB, in human PER.C6 cells. The two MAbs competed for binding to rabies virus glycoprotein. Using CR57 and a set of 15-mer overlapping peptides covering the glycoprotein ectodomain, a neutralization domain was identified between amino acids (aa) 218 and 240. The minimal binding region was identified as KLCGVL (aa 226 to 231), with key residues K-CGV- identified by alanine replacement scanning. The critical binding region of this novel nonconformational rabies virus epitope is highly conserved within rabies viruses of genotype 1. Subsequently, we generated six rabies virus variants escaping neutralization by CR57 and six variants escaping CRJB. The CR57 escape mutants were only partially covered by CRJB, and all CRJB-resistant variants completely escaped neutralization by CR57. Without exception, the CR57-resistant variants showed a mutation at key residues within the defined minimal binding region, while the CRJB escape viruses showed a single mutation distant from the CR57 epitope (N182D) combined with mutations in the CR57 epitope. The competition between CR57 and CRJB, the in vitro escape profile, and the apparent overlap between the recognized epitopes argues against including both CR57 and CRJB in a MAb cocktail aimed at replacing classical immunoglobulin preparations.

Identification of a conserved linear epitope at the N terminus of the rabies virus glycoprotein

A novel, linear B-cell epitope has been identified at the N terminus of the rabies virus (RABV) glycoprotein. Screening of a phage-display library demonstrated that two glycoprotein-specific mAbs recognized a conserved sequence, WxxxDI, which aligned between aa 14 and 19 of the mature glycoprotein. Screening of truncated glycoprotein fragments with both mAbs confirmed the location of the epitope in the N-terminal region. Alignment of amino acid sequences from a range of RABV isolates indicated that the site was conserved in most viruses. Alignment with representatives of other lyssaviruses suggested that it is conserved within phylogroup I, which includes the European bat lyssaviruses, but not phylogroup II. A 12 aa synthetic peptide of this epitope was recognized by both mAbs and sera from a subset of rabies-vaccinated dogs. In a multimeric form, the peptide could induce an epitope-specific response following immunization in rabbits and mice.

Mapping of the low pH-sensitive conformational epitope of rabies virus glycoprotein recognized by a monoclonal antibody #1-30-44

Monoclonal antibody (mAb) #1-30-44 recognized an acid-sensitive conformational epitope of rabies virus glycoprotein (G). The antigenicity of G protein exposed on the cell surface was lost when the infected cells were exposed to pH 5.8. By comparing the deduced amino acid sequence of G protein between the HEP-Flury strain and the epitope-negative CVS strain as well as the mAb-resistant escape mutants, two distant sites that contained Lys-202 and Asn-336 were shown to be involved in the epitope formation. Lys-202 is located in the so-called neurotoxin-like sequence, while Asn-336 is included in antigenic site III and is very near the amino acid at position 333, which is known to affect greatly the neuropathogenicity of rabies virus when changed. Consistent with this finding, antigenicity of a neurovirulent revertant of the HEP-Flury strain, in which Gln-333 of G protein was replaced by Arg, was also affected as shown by its greatly decreased reactivity with mAb #1-30-44 compared to that of the original avirulent HEP virus. Based on these results, we hypothesize that the neurotoxin-like domain and some amino acids in antigenic site III come into contact with each other to form a conformational epitope for mAb #1-30-44, and such a configuration would be lost when exposed to acidic conditions to perform a certain low pH-dependent function of G protein.

Further characterization of the rabies virus glycoproteins produced by virus-infected and G cDNA-transfected cells using a monoclonal antibody, #1-30-44, which recognizes an acid-sensitive epitope

Expression of rabies virus glycoprotein (G) by G cDNA-transfected mammalian cells resulted in the production of only a fusion-negative form. Low pH-dependent fusion activity, however, was seen when the expression was done under control of the T7 promoter with the help of recombinant vaccinia virus (RVV-T7) that provided T7 RNA polymerase. Fusion-inactive G proteins were transported to the cell surface as being detected by a conformational epitope-specific monoclonal antibody (mAb; #1-46-12). The fusion-inactive G proteins were recognized by most of our 13 conformation-specific mAbs, except for one mAb, #1-30-44, that recognized the low pH-sensitive conformational epitope. When the G gene expression was done with the help of RVV-T7, although most G proteins remained in the epitope-negative form, a small fraction of G gene products were 1-30-44 epitope-positive, and cell fusion activity could be seen when cells were exposed to low pH conditions. From these results, we conclude that acquisition of low pH-dependent fusion activity is closely related to structural maturation of the G protein to form the low pH-sensitive 1-30-44 epitope. Such maturation seems to be dependent on certain rabies virus-induced cellular conditions or functions, which might also be provided in part by the vaccinia virus infection. We further assume that expression of G cDNA alone mostly results in the production of mis-folded and/or differently folded forms of G protein, and only a small fraction is correctly folded even under RVV-T7-mediated expression conditions.

Studies on the escape mutants of rabies virus which are resistant to neutralization by a highly conserved conformational epitope-specific monoclonal antibody #1-46-12

We investigated a virus-neutralizing conformational epitope of the rabies virus glycoprotein (G) that is recognized by an anti-G monoclonal antibody (mAb; #1-46-12) and shared by most of the laboratory strains of the virus. To investigate the epitope structure, we isolated escape mutants from the HEP-Flury virus (wild-type; wt) after repeated passages in culture in the presence of the mAb. Immunofluorescence studies indicated that the mutants could be classified into two groups; the Group I lacked the epitope, while Group II preserved the epitope. The latter was dominant under the passage conditions, since Group I disappeared during the continuous passages. G proteins showed different electrophoretic mobilities; G protein of Group I migrated at the same rate as wt G protein, while that of Group II migrated at a slower rate, which was shown to be due to acquisition of an additional oligosaccharide side chain. Nucleotide sequencing of the G gene strongly suggested that amino acid substitutions at Thr-36 by Pro and Ser-39 by Thr of the G protein are responsible for the escape mutations of Groups I and II, respectively. The latter is a unique mutation of the rabies virus that allows the G protein to be glycosylated additionally at Asn-37, a potential glycosylation site that is not glycosylated in the parent virus, in preserving the epitope-positive conformation. These results suggest that to keep the 1-46-12 epitope structure is of greater survival advantage for the virus to escape the neutralization than to destroy it, which could be achieved by acquiring an additional oligosaccharide chain at Asn-37.

Studies on the different conditions for rabies virus neutralization by monoclonal antibodies #1-46-12 and #7-1-9

Virus-neutralizing activity of two monoclonal antibodies (mAbs), #7-1-9 and #1-46-12, against rabies virus glycoprotein (G) was compared. Although these mAbs affected the virion's ability to bind to host cells similarly, a big difference was found in the titres of virus neutralization (1:7132 and 1:32 for mAbs #1-46-12 and #7-1-9, respectively, at a concentration of 10 micro g protein/ml). Although no big difference in virion-binding affinity between the two mAbs was found, the number of antibodies required for virus neutralization was very low, </=20 molecules for mAb #1-46-12 and >/=250 molecules for mAb #7-1-9. In the latter case, the mAbs cover a major part of the virion surface and cause steric hindrance of viral receptor-binding activity. The infectivity of an epitope-preserved escape mutant virus (R-61) was not affected by the binding of high numbers of mAb #1-46-12 to the virion, which implies that mAb binding does not mask the receptor-binding site of the viral spikes. Based on these results, it is hypothesized that mAb #1-46-12 affected virus infectivity by a mechanism different from covering the virion spikes. Possible virus-neutralizing mechanisms by low numbers of mAb #1-46-12 in comparison to that of mAb #7-1-9 are discussed.

Canine vaccine recipients recognize an immunodominant region of the rabies virus glycoprotein

To investigate the immune response to anti-rabies vaccination in the principal recipient (the domestic dog), four truncated fragments of the rabies virus glycoprotein were expressed as glutathione S-transferase fusion proteins. Immune sera from vaccinated rabbits and dogs were then used to probe for reactivity with these expressed proteins. In two rabbits and four dogs tested, the dominant antibody response to non-conformational antigenic sites appeared to be directed to a region of the glycoprotein between amino acids 222 and 332. The N-terminal fragment of the glycoprotein was also significantly antigenic. Further studies to assess whether the antibody response to the internal domain could neutralize the rabies Challenge Virus Standard (CVS) strain, using antibody depletion, suggested that this fraction did contribute to the ability of post-vaccination sera to neutralize and therefore protect against infection.

Antigenic and functional analyses of glycoprotein of rabies virus using monoclonal antibodies

Thirty-five monoclonal antibodies (MAbs) against glycoprotein (G protein) of the RC-HL strain of the rabies virus have been established. Using these MAbs, two antigenic sites (I and II) were delineated on the G protein of the RC-HL strain in a competitive binding assay. Of these, 34 MAbs recognized the epitopes on site II. Site II was further categorized into 10 subsites according to their patterns in a competitive binding assay. Each site II-specific MAb showed 5 to 23 nonreciprocal competitions. The reactivities of 35 MAbs to rabies and rabies-related viruses in an indirect immunofluorescent antibody test showed that six MAbs in group A binded to rabies and rabies-related viruses and eight MAbs in group E reacted only with rabies viruses, considering that the former represent the genus-specific of Lyssavirus and the latter are rabies virus-specific. From biological assays, 28 of the 35 MAbs showed neutralization activity, 31 showed hemagglutination inhibition (HI) activity, and 18 showed immunolysis (IL) activity. The MAbs recognizing neutralization epitopes fell into at least three groups: those exhibiting both HI and IL activity, those showing only HI activity, and those showing neither HI nor IL activity. All IL epitopes overlap with HA epitopes. Five of the nine MAbs which reacted with the antigen treated by sodium dodecyl sulfate in ELISA were not reduced, or reduced only slightly, in the titer. None of the MAbs reacted with 2-mercaptoethanol-treated antigen. Only one MAb that recognized site I reacted with the denatured G protein in a Western blotting assay, indicating that its epitope is linear. These results suggest that almost all of the epitopes on the G protein of the rabies virus are conformation-dependent and the G protein forms a complicated antigenic structure.

A virus-neutralizing epitope on the glycoprotein of rabies virus that contains Trp251 is a linear epitope

We have established a hybridoma producing monoclonal antibody (MAb) against a linear epitope of glycoprotein (G protein) of the RC-HL strain of rabies virus. This MAb15-13 showed almost the same neutralizing activity to all of five rabies fixed strains, including RC-HL, and reacted to the denatured G protein in western blot analysis. To characterize and map this linear epitope, an antigenic variant NR15-13 was selected from RC-HL strain in the presence of neutralizing MAb15-13. The variant reacted with MAb15-13 in an immunofluorescent antibody test but was not neutralized by the antibody and the antibody did not bind to the variant G protein in a Western blot analysis. The variant NR15-13 had an amino acid substitution at position 251 of the G protein, where tryptophan of the parental RC-HL strain was replaced by arginine. Site-directed mutagenesis analysis using the expression system in simian COS7 cells revealed that a single amino acid substitution at 251-tryptophan by arginine on the G protein of the parental RC-HL strain abolished the antigenicity of the epitope for MAb15-13 in western blot analysis, and the replacement of 251-arginine by tryptophan recovered the activity. These results strongly suggest that tryptophan at position 251 on the G protein is essential for construction of the linear epitope against MAb15-13.

Monoclonal antibody #5-2-26 recognizes the phosphatase-sensitive epitope of rabies virus nucleoprotein

We prepared monoclonal antibodies (MAbs) against the rabies virus N protein, among which one antibody (MAb 5-2-26) was shown to lack reactivity with the phosphatase-treated N protein. The MAb was able to recognize the sodium dodecyl sulfate (SDS)-denatured N protein. The MAb did not recognize the N-protein analogues produced in Escherichia coli (E. coli), indicating that the N-gene products were not normally processed in E. coli after translation. On the other hand, the MAb reacted normally with N-gene products produced in COS-7 cells, but not with those produced in the presence of K-252a (a protein kinase inhibitor of a broad spectrum). The MAb displayed weak cross-reactivity with the Triton-insoluble network structures composed of several components, while another phosphoprotein (M1) of the virus was not recognized at all. These results suggest that MAb 5-2-26 preferentially recognizes a phosphatase-sensitive linear epitope of N protein, which may enable further investigations to be conducted on the mechanism of N-protein phosphorylation and its role(s) in virus replication.

Identification of a phosphatase-sensitive epitope of rabies virus nucleoprotein which is recognized by a monoclonal antibody 5-2-26

We have investigated a phosphatase-sensitive sequential epitope of the nucleoprotein (N), one of the phosphoproteins of rabies virus, which is recognized by the monoclonal antibody (MAb) #5-2-26. The epitope was shared in common by all of the rabies virus strains we tested, including the HEP, ERA, CVS and Japanese strains (Nishigahara and Komatsukawa). Thin layer chromatography of the acid hydrolyzates of 32P-labeled N protein showed that the protein contained phosphoserine and phosphothreonine at a molar ratio of about 4 to 1, while no phosphotyrosine was detected. Immunoprecipitation studies with several deletion mutants of the N protein showed that the epitope is located in a region spanning from amino acid 344 to 415. If the phosphatase-sensitive epitope is located at or near the phosphoamino acid, the location of the latter could be narrowed further to a region from amino acid 354 to 389 by comparing the amino-acid sequences among the viral strains. To examine this assumption, point mutation was introduced by amino-acid substitution with alanine at either of five potential phosphorylation sites (i.e., positions 354, 375, 377, 386 and 389) in the 354-389 region. Among those, only one substitution, at position 389, greatly affected the antigenicity. Substitution of serine-389 by threonine also reduced the antigenicity. These results strongly suggest that serine-389 is a phosphorylation site and essential for constructing or stabilizing the antigenic structure for MAb 5-2-26.