Viral epitopes and monoclonal antibodies: isolation of blocking antibodies that inhibit virus neutralization

Antibody-mediated neutralization of African swine fever virus: myths and facts

Almost all viruses can be neutralized by antibodies. However, there is some controversy about antibody-mediated neutralization of African swine fever virus (ASFV) with sera from convalescent pigs and about the protective relevance of antibodies in experimentally vaccinated pigs. At present, there is no vaccine available for this highly lethal and economically relevant virus and all classical attempts to generate a vaccine have been unsuccessful. This failure has been attributed, in part, to what many authors describe as the absence of neutralizing antibodies. The findings of some studies clearly contradict the paradigm of the impossibility to neutralize ASFV by means of monoclonal or polyclonal antibodies. This review discusses scientific evidence of these types of antibodies in convalescent and experimentally immunized animals, the nature of their specificity, the neutralization-mediated mechanisms demonstrated, and the potential relevance of antibodies in protection.

Neutralization interfering antibodies: a "novel" example of humoral immune dysfunction facilitating viral escape?

The immune response against some viral pathogens, in particular those causing chronic infections, is often ineffective notwithstanding a robust humoral neutralizing response. Several evasion mechanisms capable of subverting the activity of neutralizing antibodies (nAbs) have been described. Among them, the elicitation of non-neutralizing and interfering Abs has been hypothesized. Recently, this evasion mechanism has acquired an increasing interest given its possible impact on novel nAb-based antiviral therapeutic and prophylactic approaches. In this review, we illustrate the mechanisms of Ab-mediated interference and the viral pathogens described in literature as able to adopt this "novel" evasion strategy.

Non-apical membrane antigen 1 (AMA1) IgGs from Malian children interfere with functional activity of AMA1 IgGs as judged by growth inhibition assay

BACKGROUND

Apical membrane antigen 1 (AMA1) is one of the best-studied blood-stage malaria vaccine candidates. When an AMA1 vaccine was tested in a malaria naïve population, it induced functionally active antibodies judged by Growth Inhibition Assay (GIA). However, the same vaccine failed to induce higher growth-inhibitory activity in adults living in a malaria endemic area. Vaccination did induce functionally active antibodies in malaria-exposed children with less than 20% inhibition in GIA at baseline, but not in children with more than that level of baseline inhibition.

METHODS

Total IgGs were purified from plasmas collected from the pediatric trial before and after immunization and pools of total IgGs were made. Another set of total IgGs was purified from U.S. adults immunized with AMA1 (US-total IgG). From these total IgGs, AMA1-specific and non-AMA1 IgGs were affinity purified and the functional activity of these IgGs was evaluated by GIA. Competition ELISA was performed with the U.S.-total IgG and non-AMA1 IgGs from malaria-exposed children.

RESULTS

AMA1-specific IgGs from malaria-exposed children and U.S. vaccinees showed similar growth-inhibitory activity at the same concentrations. When mixed with U.S.-total IgG, non-AMA1 IgGs from children showed an interference effect in GIA. Interestingly, the interference effect was higher with non-AMA1 IgGs from higher titer pools. The non-AMA1 IgGs did not compete with anti-AMA1 antibody in U.S.-total IgG in the competition ELISA.

CONCLUSION

Children living in a malaria endemic area have a fraction of IgGs that interferes with the biological activity of anti-AMA1 antibody as judged by GIA. While the mechanism of interference is not resolved in this study, these results suggest it is not caused by direct competition between non-AMA1 IgG and AMA1 protein. This study indicates that anti-malaria IgGs induced by natural exposure may interfere with the biological effect of antibody induced by an AMA1-based vaccine in the target population.

Antibody-mediated synergy and interference in the neutralization of SARS-CoV at an epitope cluster on the spike protein

Incomplete neutralization of virus, especially when it occurs in the presence of excess neutralizing antibody, represents a biological phenomenon that impacts greatly on antibody-mediated immune prophylaxis of viral infection and on successful vaccine design. To understand the mechanism by which a virus escapes from antibody-mediated neutralization, we have investigated the interactions of non-neutralizing and neutralizing antibodies at an epitope cluster on the spike protein of severe acute respiratory syndrome coronavirus (SARS-CoV). The epitope cluster was mapped at the C-terminus of the spike protein; it consists of structurally intertwined epitopes recognized by two neutralizing monoclonal antibodies (mAbs), 341C and 540C, and a non-neutralizing mAb, 240C. While mAb 341C binds to a mostly linear epitope composed of residues ⁵⁰⁷PAT⁵⁰⁹ and V³⁴⁹, mAb 240C binds to an epitope that partially overlaps the former by at least two residues (P⁵⁰⁷ and A⁵⁰⁸). The epitope corresponding to mAb 540C is a conformational one, involving residues L⁵⁰⁴ and N⁵⁰⁵. In neutralization assays, non-neutralizing 240C disrupted virus neutralization by mAb 341C and/or mAb 540C, whereas a combination of mAbs 341C and 540C blocked virus infectivity synergistically. These findings indicate that the epitope cluster on the spike protein may serve as an evolutionarily conserved platform at which a dynamic interplay between neutralizing and non-neutralizing antibodies occurs, thereby determining the outcome of SARS-CoV infection.

A Human Monoclonal Antibody Cocktail as a Novel Component of Rabies Postexposure Prophylaxis*

The currently recommended treatment for individuals exposed to rabies virus is the combined administration of rabies vaccine and rabies immune globulin (RIG). This review sets out the criteria used to guide development of a cocktail of human monoclonal antibodies as a replacement for RIG. Using this process as a model, the general requirements for development of safe and efficacious monoclonal antibody alternatives to currently used polyclonal serum products are discussed.

Occupancy and mechanism in antibody-mediated neutralization of animal viruses

Neutralization of virus infectivity by antibodies is an important component of immunity to several virus infections. Here, the immunochemical basis for the action of neutralizing antibodies, and what role their induction of conformational changes in the antigen might play, is reviewed. Theories of the mechanisms by which antibodies neutralize virus infectivity in vitro are also presented. The theoretical and empirical foundation of the hypothesis that viruses are neutralized by a single antibody per virion is critically reviewed. The relationship between antibody occupancy on virions and the mechanism of neutralization is explored. Examples of neutralization mediated through antibody interference with virus attachment and entry are discussed and test implications of refined theories of neutralization by antibody coating of virions are formulated.

Additive effects characterize the interaction of antibodies involved in neutralization of the primary dualtropic human immunodeficiency virus type 1 isolate 89.6

Human immunodeficiency virus-type I (HIV-1) infection elicits antibodies (Abs) directed against several regions of the gp120 and gp41 envelope glycoproteins. Many of these Abs are able to neutralize T-cell-line-adapted strains (TCLA) of HIV-1, but only a few effectively neutralize primary HIV-1 isolates. The nature of HIV-1 neutralization has been carefully studied using human monoclonal Abs (MAbs), and the ability of such MAbs to act in synergy to neutralize HIV-1 has also been extensively studied. However, most synergy studies have been conducted using TCLA strains. To determine the nature of Ab interaction in HIV-1 primary isolate neutralization, a panel of 12 anti-HIV-1 human immunoglobulin G (IgG) MAbs, specific for epitopes in gp120 and gp41, were used. Initial tests showed that six of these MAbs, as well as sCD4, used individually, were able to neutralize the dualtropic primary isolate HIV-1(89.6); MAbs giving significant neutralization at 2 to 10 microg/ml included 2F5 (anti-gp41), 50-69 (anti-gp41), IgG1b12 (anti-gp120(CD4bd)), 447-52D (anti-gp120(V3)), 2G12 (anti-gp120), and 670-D (anti-gp120(C5)). For studies of reagent interaction, 16 binary combinations of reagents were tested for their ability to neutralize HIV-1(89.6). Reagent combinations tested included one neutralizing MAb with sCD4, six pairs consisting of two neutralizing MAbs, and nine pairs consisting of one neutralizing MAb with another non-neutralizing MAb. To assess the interaction of the latter type of combination, a new mathematical treatment of reagent interaction was developed since previously used methods could be used only when both reagents neutralize. Synergy was noted between sCD4 and a neutralizing anti-gp120(V3) MAb. Antagonism was noted between two pairs of anti-gp41 MAbs (one neutralizing and one non-neutralizing). All of the other 13 pairs of MAbs tested displayed only additive effects. These studies suggest that Abs rarely act in synergy to neutralize primary isolate HIV-1(89.6); many anti-HIV-1 Abs act additively to mediate this biological function.

The antiviral activity of antibodies in vitro and in vivo

This chapter discusses in vitro and in vivo antiviral activities of antibody. Since experimentation is far easier in vitro, researchers have been sought to develop in vitro assays that are expected to predict activity in vivo. This could be important in both vaccine design and in passive antibody administration. The proposed mechanisms of in vitro neutralization range from those requiring binding of a single antibody molecule to virus to those requiring substantially complete antibody coating of virus. In vitro, antiviral activity can be separated into activity against virions and activity against infected cells. The activity against virions most often considered is neutralization that can be defined as the loss of infectivity, which ensues when antibody molecule(s) bind to a virus particle, and occurs without the involvement of any other agency. In vivo, it is conventional to distinguish phenomenologically between two types of antibody antiviral activity. One of them is the ability of antibody to protect against infection when it is present before or immediately following infection. Evidence for a number of viruses in vitro indicates that lower antibody concentrations are required to inhibit infection propagated by free virus than are required to inhibit infection propagated by cell-to-cell spread.

Blocking antibodies inhibit complete African swine fever virus neutralization

A persistent non-neutralized African swine fever virus (ASFV) fraction is found with most convalescent swine sera in in vitro neutralization assays. To study this phenomenon, antisera from convalescent pigs infected with different virus isolates and showing complete or incomplete virus neutralization were used. Different experiments determined that incomplete neutralization of ASFV is caused neither by virus aggregation, nor low affinity or stability of virus-antibody complexes. Additionally, attempts to purify antigenic escape mutant viruses from the persistent fraction was also unsuccessful. Nevertheless, competition experiments between sera demonstrated that antibodies present in sera showing persistent fraction inhibited the complete neutralization mediated by antibodies present in sera which neutralize 100% of virus infectivity. These results suggest that induction of blocking antibodies during ASFV infection could represent the main cause for the persistent surviving virus fraction observed in neutralization assays and could also explain the persistent infections observed in some convalescent pigs.

Epitopes functional in neutralization of varicella-zoster virus

By competition neutralization assay using monoclonal antibodies (MAbs) to varicella-zoster virus (VZV) glycoproteins (gps), we attempted to determine the topographical relationship of epitopes which are functional in VZV neutralization. MAbs against gpI interfered moderately to strongly with neutralization of MAbs against gpIII, and one antigenic domain with two distinct epitopes was identified on gpIII. Competition neutralization assays performed with MAbs to gpI revealed at least three distinct antigenic domains: the first contained two complement-dependent neutralizing epitopes; the second contained five complement-dependent neutralizing, overlapping epitopes and one nonneutralizing, nonoverlapping epitope; and the third contained one complement-enhanced neutralizing epitope. Competition neutralization assays performed with MAbs to gpIV showed one antigenic domain with two distinct epitopes which competed with nonneutralizing gpI MAbs. gpII did not interfere with neutralization of gpI, gpIII, or gpIV. Our data suggest that neutralizing and nonneutralizing MAbs can interfere with the action of viral neutralization either by inhibition or by enhancement. This report describes the epitope mapping of VZV gps by a functional biological assay.

Virus envelope-based peptide vaccines against virus-induced mammary tumors

Previous studies by us and others established that mammary tumors induced by murine mammary tumor virus (MuMTV) could be prevented to various extents by prior vaccination with MuMTV-containing or subviral component immunogens. In this report, four predicted surface-accessible peptide regions (EP-1 to EP-4) of the major viral envelope glycoprotein (gp52) of C3H-MuMTV were tested as carrier-conjugated vaccines for the protection of Balb/c mice against a live virus challenge. With tumor incidence as an endpoint, vaccination with one of these synthetic peptides (EP-3) resulted in a significant reduction in the frequency of early onset tumors and 67% of the test animals remained tumor-free for the entire observation period (16 months). In contrast, only marginal protection was obtained by immunization with the intact glycoprotein (gp52). Immunologic interference may explain the lower protective efficacy of gp52, as compared to EP-3.

Protection of chimpanzees from infection by HIV-1 after vaccination with recombinant glycoprotein gp120 but not gp160

The development of a vaccine to provide protective immunity to human immunodeficiency virus type 1 (HIV-1), the virus causing AIDS, would be the most practical method to control its spread. Subunit vaccines consisting of virus envelope glycoproteins, produced by recombinant DNA technology, are effective in preventing viral infections. We have now used this approach in the development of a candidate AIDS vaccine. Chimpanzees were immunized with recombinant forms of the HIV-1 glycoproteins gp120 and gp160 produced in Chinese hamster ovary cells, and then challenged with HIV-1. The control and the two animals immunized with the gp160 variant became infected within 7 weeks of challenge. The two animals immunized with the gp120 variant have shown no signs of infection after more than 6 months. These studies demonstrate that recombinant gp120, formulated in an adjuvant approved for human use, can elicit protective immunity against a homologous strain of HIV-1.

Identification of a neutralizing epitope on glycoprotein gp58 of human cytomegalovirus

Human cytomegalovirus contains an envelope glycoprotein of 58 kilodaltons (gp58). The protein, which is derived from a glycosylated precursor molecule of 160 kilodaltons via proteolytic cleavage, is capable of inducing neutralizing antibodies. We have mapped the epitopes recognized by the neutralizing monoclonal antibody 7-17 and a second antibody (27-287) which is not neutralizing. Overlapping fragments of the carboxy-terminal part of the open reading frame coding for gp58 were expressed in Escherichia coli as beta-galactosidase fusion proteins. The reactivities of antibodies 7-17 and 27-287 were determined by Western blot (immunoblot) analysis. Both antibodies recognized sequences between amino acids 608 and 625 of the primary gp58 translation product. The antibodies almost completely inhibited one another in a competitive binding assay with intact virus as antigen. Moreover, antibody 27-287 was able to inhibit the complement-independent neutralizing activity of antibody 7-17.

Neutralization of respiratory syncytial virus by individual and mixtures of F and G protein monoclonal antibodies

We identified several types of neutralization effected by F and G protein monoclonal antibodies (MAbs) reacted individually or as mixtures against respiratory syncytial virus (RSV). Neutralizing activity was identified by a microneutralization test in which virus replication was determined by enzyme immunoassay. Complete neutralization was seen only with MAbs against the F protein. Strain-specific neutralization, complete neutralization against some strains of RSV, and no neutralization against other strains were seen with an additional MAb against the F protein. Partial neutralization, virus replication significantly reduced but still present, and no neutralization were seen with MAbs against both the F and G proteins. Enhanced neutralization, enhanced efficacy of neutralization, or increased neutralizing titer with a mixture of two MAbs over that for the individual MAbs was seen with all MAbs against the F protein and all but three MAbs against the G protein. Most (10 of 13) of the MAbs that exhibited neutralizing activity reacted with some but not all strains of RSV in an enzyme immunoassay. The epitopes corresponding to these 10 MAbs probably contribute to the strain-specific component of the neutralizing antibody response to RSV. Our results suggest that interpretation of RSV neutralization with MAbs is complex and that studies of such neutralization should include mixtures of MAbs and multiple RSV strains.

Epitope analysis of human cytomegalovirus glycoprotein complexes using murine monoclonal antibodies

A panel of 10 monoclonal antibodies reactive with human cytomegalovirus (HCMV) glycoproteins was generated. These antibodies immunoprecipitated disulfide-linked complexes which contained glycoproteins with molecular weights of 130,000, 93,000, and 52,000. These complexes were designated gC-I. Epitope analysis of gC-I was done with a simultaneous two antibody binding assay. A network of epitopes was revealed which clustered in three major domains designated I, II, and III. Antibodies within individual domains I and II showed strong mutual inhibition of each other's binding. However, there were multiple antibody interactions between domains I and II. For example, the binding of most antibodies in domain I was augmented to some extent by antibodies from domain II. However, the binding of only one antibody from domain II was augmented by all antibodies from domain I. The augmentation in binding between two antibodies was dependent on the native structure of gC-I and was sensitive to conformational changes due to nonionic detergent extraction of gC-I and/or disruption of disulfide bonds. A synergistic effect was also observed between antibodies in domains I and II in a virus neutralization assay. A neutralizing antibody had a much greater neutralizing activity in the presence of a nonneutralizing antibody, which also enhanced the binding of the neutralizing antibody in the simultaneous two antibody binding assay. Also, two antibodies which were nonneutralizing individually were neutralizing when used in combination. Such antibodies also augmented each other's binding in the simultaneous two antibody binding assay. Finally, domain III consisted of a nonneutralizing antibody that inhibited the binding of all antibodies in domains I and II. This antibody also inhibited the neutralizing activity of a neutralizing antibody in a virus neutralizing assay.

Proteins of lymphocytic choriomeningitis virus: antigenic topography of the viral glycoproteins

Topographical relationships among antigenic sites on the envelope glycoproteins of lymphocytic choriomeningitis virus (LCMV) were established using a panel of monoclonal antibodies (MAb) directed against viral GP-1 and GP-2. Purified MAb were radioiodinated and used as probes in a solid phase competitive binding assay. Epitopes on LCMV GP-1 were found to cluster in four antigenic sites. Five neutralizing MAb raised by immunization with the WE strain of LCMV reacted with a single topographic site, termed GP-1A, which was present on four strains of LCMV examined in this study. A second site, GP-1B, was characterized by two MAb which partially competed with one another and with a subset of neutralizing antibodies. This site appeared to be close to site A and was found to be nonneutralizing. The third site, GP-1C, contained sequential epitopes and was also nonneutralizing. Antibodies binding to site B enhanced the binding of MAb at site C, presumably through a conformational change. In addition to the common neutralizing site A, LCMV Armstrong strain (LCMV-Arm) GP-1 contained a second topographically related neutralizing site, GP-1D, which was specific for LCMV-Arm, absent in WE, and appeared to be the major immunogenic epitope on GP-1 of this virus. Analysis of MAb binding to LCMV GP-2 demonstrated the presence of three overlapping binding sites. GP-2A was defined by two antibodies while GP-2B and C represented binding sites of one antibody each. Guinea pigs primed with LCMV-Arm and challenged with LCMV-WE developed a significant immune response which was directed toward the common major neutralizing site, GP-1A, but had poor responses to the LCMV-Arm specific neutralizing site GP-1D. Immune sera contained antibody to site GP-1B but lacked detectable antibody to GP-1C.

Possible immunoenhancement of persistent viremia by feline leukemia virus envelope glycoprotein vaccines in challenge-exposure situations where whole inactivated virus vaccines were protective

Kittens immunized with purified native FeLV-gp70 or -gp85 envelope proteins developed ELISA, but not virus neutralizing, antibodies in their serum to both whole FeLV and FeLV-gp70. Kittens vaccinated with envelope proteins and infected with feline sarcoma virus (FeSV) developed smaller tumors than nonvaccinates, but a greater incidence of persistent retroviremia. Similarly, FeLV-gp70 and -gp85 vaccinated kittens were more apt to become persistently retroviremic following virulent FeLV challenge exposure than nonvaccinates. Kittens vaccinated with inactivated whole FeLV developed smaller tumors after FeSV inoculation and had a lower incidence of persistent retroviremia than nonvaccinates. The protective effect of inactivated whole FeLV vaccine against persistent retroviremia was also seen with FeLV challenge-exposed cats. Protection afforded by inactivated whole FeLV vaccine was not associated with virus neutralizing antibodies, although ELISA antibodies to both whole FeLV and FeLV-gp70 were induced by vaccination.

E1 glycoprotein of rubella virus carries an epitope that binds a neutralizing antibody

We identified by immunoprecipitation and Western blot analysis, using a monoclonal antibody that neutralizes rubella virus, that E1 glycoprotein carries an epitope linked with neutralization. Glycosidase treatment of virus does not prevent blotting of this monoclonal antibody with the E1 glycoprotein, dissociating this epitope from the hemagglutination epitope which is linked with the oligosaccharide side chains. We also investigated by Western blot analysis human serum reactivity toward E1 glycoprotein and the two other structural proteins of rubella virus, E2 and C: all positive sera detected E1 and C, irrespective of their titers, indicating the importance of glycoprotein E1 in immunity. Frequent lack of reactivity against E2 might suggest that this glycoprotein is either less exposed or less immunogenic.

Selection of unique antigenic variants of Newcastle disease virus with neutralizing monoclonal antibodies and anti-immunoglobulin

Monoclonal antibodies were used to isolate nonneutralizable antigenic variants in the hemagglutinin-neuraminidase glycoprotein of Newcastle disease virus. It had been found that a large percentage of virus retains infectivity despite binding neutralizing antibody. This high persistent fraction of nonneutralized virus precluded the isolation of variants by the standard treatment with antibody alone. Rabbit anti-mouse immunoglobulin was used to reduce the percentage of virus that remains infectious despite the presence of bound antibody. This procedure made possible the isolation of variants of two distinct types: classical variants, not neutralized because they do not bind the antibody used to select them; and unique variants that, although still capable of binding the selecting antibody, are only slightly neutralized. The general applicability of this method for the isolation of antigenic variants in nonneutralizing epitopes is also discussed.

A serum protein affecting the regulation of the immune system

A thermostable alpha 2 globulin inhibiting the immunoglobulin/anti-immunoglobulin reaction was demonstrated working with subacute sclerosing panencephalitis (SSPE) and control serum IgG. This protein was isolated from SSPE and normal human blood, it inhibits the immunoglobulin/anti-immunoglobulin reaction but no other antigen/antibody reactions when applying different immunochemical methods such as nitrocellulose immunofixation, 2 site immunoradiometric assay, solid phase radioimmunoassay in coated cups. This was demonstrated, working on the one hand with measles virus strain Edmonston or SSPE virus strain D.R. and SSPE serum and on the other hand with IgG from SSPE and control serum. This alpha 2 globulin, an inhibiting protein, appears to be related to "normal immunosuppressive protein".

Enhanced neutralization of La Crosse virus by the binding of specific pairs of monoclonal antibodies to the G1 glycoprotein

Monoclonal antibodies specific for eight antigenic sites on the G1 glycoprotein of La Crosse virus were studied in a kinetics of neutralization assay. When examined at the same protein concentration, there were differences in the rates and degree of neutralization induced by these antibodies. The maximum amount of neutralization detected in antibody excess ranged from 1.5 to 3 logs. Another 2 to 3 logs of viral infectivity could be neutralized by the addition of polyclonal anti-G1 antibody or a second monoclonal antibody. However, only certain pairs of monoclonal antibodies enhanced the neutralization the additional 2 to 3 logs. The antibody pairs enhancing neutralization were those also found to be synergistic in a competition binding assay (L. Kingsford, L. D. Ishizawa, and D. W. Hill, 1983, Virology 129, 443-455) as well as any neutralizing antibody used in combination with antibodies to epitopes within one particular site on the glycoprotein. The data indicate that antibody binding to two different specific sites on G1 is needed to neutralize La Crosse virus to the same extent that is observed with polyclonal antibody.

Location of immunodominant antigenic determinants on fragments of the tick-borne encephalitis virus glycoprotein: evidence for two different mechanisms by which antibodies mediate neutralization and hemagglutination inhibition

A model showing the topological distribution, functions, and serological specificities of eight distinct, monoclonal antibody-defined epitopes on the tick-borne encephalitis (TBE) virus glycoprotein has been presented in a previous publication (F. X. Heinz, R. Berger, W. Tuma, and Ch. Kunz (1983). Virology 126, 525-537.) In the present report the influence of conformational change, chemical modification, and fragmentation on the antigenic reactivity of each epitope has been analyzed by the use of blocking enzyme immunoassays and "Western blotting." One of the two major antigenic domains (A), composed of three different epitopes, completely lost its antigenicity upon incubation at pH 5.0 or by treatment with guanidine-HCl/urea, SDS, reduction and carboxymethylation, as well as by proteolytic (trypsin, alpha-chymotrypsin, thermolysin) and chemical (CNBr) fragmentation. The second major antigenic domain (B), however, defined by four distinct monoclonal antibodies, three of which are hemagglutination (HA)-inhibiting, neutralizing, and protective, was shown to be resistant to low pH, guanidine-HCl/urea treatment, and proteolytic cleavage of the native protein. Also, polyclonal immune sera from mice and rabbits contained antibody populations reactive with antigenic determinants which are resistant and others which are sensitive to conformational change and fragmentation. Glycoprotein fragments with molecular weights of about 9000, generated by proteolysis of the native protein, were immunoreactive with neutralizing and protective monoclonal antibodies (defining domain B) as well as with a polyclonal mouse immune serum. Thus, these fragments appear to contain antigenic determinants which are immunodominant on the native protein and play an important role in the induction of a protective immune response against TBE virus. In addition, these results show that antibody binding to antigenic domains which are topologically and structurally completely unrelated may result in neutralization and/or HA inhibition. As the presence of two receptor-binding sites is unlikely, different effector mechanisms may account for the effects of these antibodies. The antigenic reactivity of domain A is sensitive to the same treatments which also inactivate HA activity of TBE virus, whereas domain B is resistant. These treatments include a change of domain A induced by incubation at slightly acidic pH which also results in inactivation of virus infectivity. Antibodies to domain A therefore presumably block viral activities by direct binding at or near the putative receptor-binding site whereas antibodies to domain B may cause loss of biological activities by inducing a conformational change of the receptor-binding site.

A topological and functional model of epitopes on the structural glycoprotein of tick-borne encephalitis virus defined by monoclonal antibodies

A topological and functional model of eight distinct epitopes on the structural glycoprotein of tick-borne encephalitis (TBE) virus was established by the use of monoclonal antibodies. The unique specificities and spatial relationships of these antibodies were determined by variant analysis, haemagglutination inhibition (HI), neutralization, passive mouse protection, and antibody blocking assays. Seven out of the eight distinct epitopes were shown to be partially linked and to cluster in two antigenically reactive domains (A, B). Each of these domains is inhomogeneous and contains constituents with different serological specificities and functions. Domain A is defined by three HA-inhibiting antibodies, two of which are flavivirus group-reactive, whereas the third is TBE virus subtype specific. Within this domain only the subtype-specific antibody is involved in virus neutralization, thus explaining the observation that neutralization tests with flaviviruses show higher serological specificities than HI tests and that HI tests can be made type and subtype specific by antibody absorption. Domain B is composed of three TBE-complex reactive epitopes, and the corresponding antibodies inhibit HA and neutralize the virus. A fourth epitope linked to this domain is neither involved in HA nor in neutralization and the same holds true for a subtype-specific epitope which is topologically independent of domains A and B. Each of two different nonneutralizing antibodies was capable of blocking the binding of distinct neutralizing antibodies. All eight epitopes are indistinguishably present on strains of the western subtype of TBE virus isolated all over Europe in different years from different hosts, thus again confirming the great stability of this virus.

Identification of a cellular receptor for mouse mammary tumor virus and mapping of its gene to chromosome 16

Pseudotypes of vesicular stomatitis virus (VSV) containing envelope glycoproteins provided by C3H mammary tumor virus (MTV) instead of the normal VSV G-proteins were prepared and used to assay the presence of an MTV receptor on cells. The assay was specific as demonstrated by competition studies with excess MTV particles and neutralization of the pseudotypes with anti-MTV serum or monoclonal antibodies directed against MTV gp52. The MTV receptor was abundantly present on mouse cells but hardly detectable on nonmurine cells, including the Chinese hamster cell line E36. Somatic cell hybrids between E36 cells and GRS/A spontaneous leukemia cells (GRSL cells) and between E36 and GRS/A primary mammary tumor cells were made. The hybrids retained all Chinese hamster chromosomes but segregated mouse chromosomes. From the analysis of the isoenzymes and chromosomes of the hybrid cell lines we conclude that the gene for the receptor (MTVR-1) is located on mouse chromosome 16.

Specificity of antibodies elicited by a synthetic peptide having a sequence in common with a fragment of a virus protein, the hepatitis B surface antigen

We predicted the localization of a major hepatitis B surface antigen (HBsAg) determinant within residues 135-155 [Neurath, A.R., Strick, N. & Oleszko, W.R. (1981) J. Virol. Methods 3, 115-125]. A peptide corresponding to this sequence (P135-155) was synthesized, linked to macromolecular carriers, and used to immunize rabbits. In accordance with published data on shorter peptides within the same amino acid sequence, antibodies to HBsAg were elicited. A detailed analysis of the immune response to P135-155 revealed the following: A heterogeneous population of IgG and IgM antibodies reacting with P135-155 was detected in the antisera by a variety of radioimmunoassays and enzyme-linked fluorescence immunoassays. Only a subpopulation of these antibodies reacted with HBsAg. The equilibrium constant (K) for the reaction of the antibodies with HBsAg (K = 4 X 10(5) M-1) was approximately two orders of magnitude lower than K for the reaction with P135-155 and was below K for the reaction between HBsAg and antibodies elicited by HBsAg (K greater than 10(7) M-1). Preimmunization with P135-155 did not result in an enhanced response to subsequent immunization with HBsAg. Peptides more accurately mimicking determinants on HBsAg may have to be synthesized for possible application in antiviral prophylaxis.

Neutralizing antibody response of rabbits and goats to caprine arthritis-encephalitis virus

Rabbits were immunized with purified caprine arthritis-encephalitis virus and examined for neutralizing activity. Analysis of virus-antiserum interaction at 37 degrees C demonstrated little loss of viral infectivity after incubation with heat-inactivated rabbit antiserum for 60 min. However, sensitization of virus (as assessed by the addition of complement) occurred almost immediately and was 95% complete after 10 min. The complement-dependent neutralizing activity was associated with the immunoglobulin G fraction of rabbit antiserum. Addition of goat anti-rabbit immunoglobulin G to the immune rabbit serum-caprine arthritis-encephalitis virus mixture also resulted in neutralization of infectivity when unbound antibody was removed before addition of the anti-immunoglobulin. Serum from most caprine arthritis-encephalitis virus-infected goats contains antibody activity to the core protein p28, as demonstrated by immunodiffusion and enzyme-linked immunosorbent assay. However, attempts to demonstrate neutralizing activity in the serum of goats up to 1.5 years post-inoculation or in serum of hyperimmunized goats were unsuccessful when the sera were examined alone or in combination with complement or rabbit anti-goat immunoglobulin or both.

Monoclonal antibodies to the glycoprotein of vesicular stomatitis virus: comparative neutralizing activity

Nineteen independently isolated hybridomas producing monoclonal antibodies to the glycoprotein of vesicular stomatitis virus were isolated and studied for their capacity to neutralize viral infectivity. By measuring competitive binding of 125I-labeled monoclonal antibodies in a radioimmunoassay. 11 different, non-cross-reacting antigenic determinants were identified on the vesicular stomatitis virus G protein. All monoclonal antibodies reacting with determinants 1, 2, 3, and 4 resulted in viral neutralization, whereas those binding to the other seven determinants did not neutralize infectivity. The mixture of two monoclonal antibodies binding to different determinants resulted in a more rapid neutralization than either antibody alone, suggesting that different antibodies can exert a synergistic effect on viral neutralization. Kinetic experiments revealed biphasic neutralization curves similar to those expected for heterologous antibody. No evidence could be obtained to relate biphasic kinetics of viral neutralization to heterogeneous populations either of antibody molecules or of virus. The possible significance of the kinetic data with monoclonal antibodies is discussed.

Interaction of viruses with cell surface receptors

This chapter discusses the interaction of viruses with cell surface receptors. The rigorous characterizations of receptor–ligand interactions have been derived from binding studies of radiolabeled ligands in neuropharmacology and endocrinology. The definition of viral recognition sites as receptors involves three major criteria that are derived from models of ligand–receptor interactions: saturability, specificity, and competition. A variety of approaches have been used to study the interaction of viral particles with cell surface receptors or reception sites. A rigorous study of viral–receptor interactions requires the use of more than one technique as different approaches provide complementary information about viral binding. The chapter discusses membrane components that interact with viruses. The identification of the subviral components that are responsible for the binding of viruses to cell surfaces has preceded the structural understanding of the cellular receptors themselves. The chapter summarizes current data concerning the viral attachment protein (VAP) of selected viruses.