A neutralizing monoclonal antibody against poliovirus and its reaction with related antigens

Embracing complexity: organicism for the 21st century

Organicism (materialistic holism) has provided the philosophical underpinnings for embryology since the time of Kant. It had influenced the founders of developmental mechanics, and the importance of organicism to embryology was explicitly recognized by such figures as O. Hertwig, H. Spemann, R. Harrison, A. M. Dalq, J. Needham, and C. H. Waddington. Many of the principles of organicism remain in contemporary developmental biology, but they are rarely defined as such. A combination of genetic reductionism and the adoption of holism by unscientific communities has led to the devaluation of organicism as a fruitful heuristic for research. This essay attempts to define organicism, provide a brief history of its importance to experimental embryology, outline some sociologically based reasons for its decline, and document its value in contemporary developmental biology. Based on principles or organicism, developmental biology should become a science of emerging complexity. However, this does mean that some of us will have to learn calculus.

Detection of coxsackie B virus infection using a rapid screening method

A Western blot method (WB) was adapted for rapid screening of antibodies against coxsackie virus B1-B6 in sera from patients with newly diagnosed insulin-dependent diabetes mellitus, myocarditis or febrile syndrome of suspected coxsackie viral aetiology. The use of a mixture of all 6 coxsackie virus B serotypes as the common antigen permitted a very rapid and inexpensive detection of antibody-positive sera for preliminary diagnosis and further detailed assay. Comparison of the results with those obtained in parallel run virus-neutralization tests showed a higher sensitivity and comparable specificity of WB.

RNA binding properties of poliovirus subviral particles

The mechanism of encapsidation of the RNA genome of poliovirus and other picornaviruses is unknown. To test whether any of the putative assembly intermediates of poliovirus could interact directly with the poliovirus RNA genome, poliovirus RNA was attached to magnetic streptavidin beads and incubated with partially purified extracts containing 35S-labeled 14S pentamer and 75S empty-capsid subviral particles from infected cells. The amount of labeled protein bound to the beads was monitored, thus testing the RNA-binding activities of only the labeled viral proteins in the preparations. In this assay, nonspecific RNA-binding activity was displayed by the 14S pentameric particles and mature virons. 75S empty capsids displayed no propensity to associate with RNA. 14S pentamers were demonstrated to form rapidly sedimenting complexes and to undergo a conformational alteration upon RNA binding. These findings are consistent with a direct role for the 14S pentameric particles in RNA packaging during poliovirus morphogenesis.

Role and mechanism of the maturation cleavage of VP0 in poliovirus assembly: structure of the empty capsid assembly intermediate at 2.9 A resolution

The crystal structure of the P1/Mahoney poliovirus empty capsid has been determined at 2.9 A resolution. The empty capsids differ from mature virions in that they lack the viral RNA and have yet to undergo a stabilizing maturation cleavage of VP0 to yield the mature capsid proteins VP4 and VP2. The outer surface and the bulk of the protein shell are very similar to those of the mature virion. The major differences between the 2 structures are focused in a network formed by the N-terminal extensions of the capsid proteins on the inner surface of the shell. In the empty capsids, the entire N-terminal extension of VP1, as well as portions corresponding to VP4 and the N-terminal extension of VP2, are disordered, and many stabilizing interactions that are present in the mature virion are missing. In the empty capsid, the VP0 scissile bond is located some 20 A away from the positions in the mature virion of the termini generated by VP0 cleavage. The scissile bond is located on the rim of a trefoil-shaped depression in the inner surface of the shell that is highly reminiscent of an RNA binding site in bean pod mottle virus. The structure suggests plausible (and ultimately testable) models for the initiation of encapsidation, for the RNA-dependent autocatalytic cleavage of VP0, and for the role of the cleavage in establishing the ordered N-terminal network and in generating stable virions.

Molecular basis of antigenic structures of poliovirus: implications for their evolution during morphogenesis

Neutralizing monoclonal antibodies against poliovirus type 1 were obtained after conventional immunization or combined in vivo-in vitro immunization. Antibody binding sites were determined by sequence analysis of neutralization-resistant mutants. Site 3 variants had several amino acid substitutions in previously unidentified positions for neutralization resistance. Evidence for a linkage of subsites 3a and 3b is presented. Some site 3b antibodies as defined previously precipitated 14S subunits, although with reduced titers.

Immunocytochemical localization of capsid-related particles in subcellular fractions of poliovirus-infected cells

The structural proteins of poliovirus can assemble into a series of different configurations (capsid-related particles, CRP). Only some seem to be true capsid precursors and the role of most CRP in morphogenesis is unclear. We used electron microscopic immunocytochemistry with monoclonal antibodies recognizing different CRP [protomers, pentamers, 65S empty capsids (EC), 74S-EC, and virions] to locate CRP in subcellular fractions containing virus-induced vesicles associated with the viral replication complex. We found pentamer antigenic CRP to be associated with the replication complex. The same pentamer antigenicity was exhibited by novel, "capsid-like" structures attached to the surface of the virus-induced vesicles. Upon solubilization of the vesicular fraction, mainly 65S-EC and only negligible amounts of pentamers were found by sucrose gradient analysis and by immunoprecipitation. We show that the pentamer antigenic particles are converted into 65S-EC when their membranous support is dissolved. We propose that the vesicular membrane prevents the assembly of 65S-EC and keeps the pentamer antigenic CRP in the appropriate concentration and configuration for association with the nascent progeny RNA.

Detection of immunoglobulin G, M, and A antibodies to enterovirus structural proteins by immunoblot technique in echovirus type 4-infected patients

Paired serum specimens from 24 patients with echovirus (EV) type 4 infection by virus isolation were tested by the immunoblot technique for the presence of IgG, IgM, and IgA antibodies to EV4 structural proteins. Single sera from 20 patients without neutralizing enterovirus IgM were used as controls. All the sera from EV4-infected patients had IgG antibodies to VP1 of EV4 but also 13 out of the 20 controls. 23 out of 24 EV4-infected patients elicited IgM and IgA specific antibodies to VP1, a pattern highly significant as compared with controls (3/20 for IgM and 8/20 for IgA). In 16 out of the 24 EV4-infected patients, the IgM antibodies were also directed against VP2 (versus 2 out of 20 in the control group). Anti-VP2 IgA were detected in 4 out of the 24 EV4 patients (versus 0 in controls). The 24 paired sera from EV4-infected subjects were also tested by immunoblot technique against three other enteroviruses: EV21, coxsackievirus A9 and poliovirus 1. Cross-reactivities were observed to a large extent against VP1 and VP2 proteins with the three classes of antibodies. These results confirm the data of previous studies on the reactivity of IgM antibodies to various structural proteins that IgG antibodies react exclusively to VP1. Furthermore, this study demonstrates the occurrence of circulating IgA antibodies directed to VP1 and sometimes VP2 in the course of enterovirus infection. The potential interest of this latter finding for diagnosis requires further investigation.

Poliovirus vaccines. A continuing challenge

The control of poliomyelitis remains a provocative challenge. Alternative vaccination schedules, continuing research toward better vaccines, and ongoing international scientific, epidemiologic, and economic collaboration may make it possible to provide effective immunization for all children of the world and eventually may eradicate poliomyelitis worldwide, a goal set forward by the Expanded Programme on Immunization of the World Health Organization.

Construction and characterization of hybrid hepatitis B antigen particles carrying a poliovirus immunogen

The hepatitis B surface antigen (HBsAg) has the unique property of assembling with cellular lipids into spherical or elongated particles of 22 nm diameter which are secreted by mammalian cells expressing HBsAg. We have studied the structural requirements for particle formation and secretion by creating in-phase insertions into different regions of the S gene of the hepatitis B virus, coding for HBsAg. Modified genes were integrated into an appropriate vector and expressed in mouse L cells. Various single and double inserts in the two major hydrophilic domains of HBsAg were compatible with particle synthesis and secretion. The level of secretion was influenced by the length of the insert, its primary structure, and the site of insertion into the HBsAg molecule. One of the inserted sequences was a synthetic DNA fragment encoding a continuous type 1 poliovirus neutralization epitope (the C3 epitope). Mammalian cells expressing the modified hepatitis B virus S gene secreted hybrid particles carrying the poliovirus antigen. The hybrid polio-HBsAg particles reacted with a monoclonal antibody specific for the C3 epitope and induced poliovirus neutralizing antibodies at low, but significant, titers in mice and at high titers in rabbits. However, the immune response to HBsAg was weaker to hybrid particles than to unmodified HBsAg particles. By cotransfection with two different plasmids carrying either modified or unmodified genes, we obtained phenotypically mixed particles containing both polio-HBsAg and HBsAg molecules. Inoculated into rabbits, the mixed particles induced high antibody titers against both poliovirus and HBsAg.

Three-dimensional structure of poliovirus serotype 1 neutralizing determinants

Antigenic mutants of poliovirus (Sabin strain, serotype 1) were isolated by the resistance of the virus to anti-Sabin neutralizing monoclonal antibodies. The amino acid replacements within the capsid protein sequence causing the altered antigenicity were identified for each of 63 isolates. The mutations cluster into distinct nonoverlapping peptide segments that group into three general immunological phenotypes on the basis of cross-neutralization analyses with 15 neutralizing anti-Sabin monoclonal antibodies. Location of the mutated amino acid residues within the three-dimensional structure of the virion indicates that the majority of these amino acid residues are highly exposed and located within prominent structural features of the viral surface. Those mutated amino acid residues that are less accessible to antibody interaction are often involved in hydrogen bonds or salt bridges that would stabilize the local tertiary structure of the antigenic site. The interactions of the peptide segments that form these neutralizing sites suggest specific models for the generation of neutralization-resistant variants and for the interaction between the viral surface and antibody.

Intertypic cross-neutralization of polioviruses by human monoclonal antibodies

Human B cell lines producing monoclonal antibodies (mAbs) reactive with poliovirus type 1 were generated by transformation with Epstein-Barr virus (EBV) B 95-8 of tonsillar lymphocytes from several immune donors. EBV-transformed cells were cloned in semisolid agarose. Some neutralizing (Nt) human mAbs recognized and neutralized only poliovirus type 1, whereas other Nt mAbs neutralized either poliovirus type 1 and 2 or all three serotypes. mAbs reactive with poliovirus type 1 and 3 but not with type 2 were not detected. Immunoprecipitation of radiolabeled poliovirus type 1 with cross-reactive human Nt mAbs was inhibited competitively by preincubation of mAbs with cold poliovirus type 3 and/or 2.

Binding site of neutralizing monoclonal antibodies obtained after in vivo priming with purified VP1 of poliovirus type 1 is located between amino acid residues 93 and 104 of VP1

Three hybridomas obtained after in vitro stimulation of spleen cells of mice primed in vivo with purified VP1 of poliovirus type 1 (Mahoney) with the homologous virus produced antibodies which reacted with VP1 and immunoprecipitated and neutralized only the homologous virus. Evidence for the location of their binding sites was obtained by inhibition of virus neutralization and virus binding by a synthetic peptide comprising the amino acid sequence 93-104 of VP1 of poliovirus type 1 (Mahoney).

A pH-dependent dissociation of poliovirus procapsids

At pH 7.6-8.2, poliovirus procapsids are dissociated in the cold to 14 S subunits which exhibit the same antigenicity and ability to be reassembled to empty capsids as naturally occurring 14 S subunits. The dissociation of the procapsids at pH 8.2 follows exponential kinetics. At pH 8.4 or 8.5, the procapsids are converted to 80 S H-antigenic, empty capsids.

Characterization and role in morphogenesis of a new subviral particle (55S) isolated from poliovirus-infected cells

A previously undetected subviral particle, designated the 55S particle because of its position in sucrose density gradients, has been found in cytoplasmic extracts of poliovirus-infected cells. It contains no RNA, is composed of equimolar amounts of the structural polypeptides P1AB, P1C, and P1D, and is stable in vitro under a variety of conditions: presence or absence of EDTA, dilution in low- or high-ionic-strength buffers, suspension in buffers up to pH 10, incubation at 37 degrees C, and centrifugation to equilibrium in CsCl gradients (where it bands at a density of 1.285 g/cm3). Conventional pulse-chase experiments show that 55S particles are the products of the assembly of 14S subunits and the precursors of virions. These data led to the formulation of a model of poliovirus morphogenesis in which the conversion of capsomers into 73S empty capsids does not occur directly, but through the formation of an intermediate structure, the 55S particle.

Host cell modulation of foot-and-mouth disease virus procapsid synthesis

BHK21 (clone 13S) cells of high (BHK-SH) and low (BHK-SL) passage number were infected with foot-and-mouth disease virus (FMDV) subtypes A24, A25 and C3. While the amount of virus specific RNA produced in BHK-SH cells was 25% of that in BHK-SL cells and the virion production was 27% (C3) to 53% (A24) lower, the synthesis of viral proteins was comparable, associated with an accumulation of procapsids in BHK-SH cells. The results suggest that changes in viral infection pattern with increasing BHK21 cell passage number should be considered in FMDV vaccine production.

Capsid intermediates assembled in a foot-and-mouth disease virus genome RNA-programmed cell-free translation system and in infected cells

Structural protein complexes sedimenting at 140S, 70S (empty capsids), and 14S were isolated from foot-and-mouth disease virus-infected cells. The empty capsids were stable, while 14S complexes were relatively short-lived. Radioimmune binding assays involving the use of neutralizing monoclonal antibodies to six distinct epitopes on type A12 virus and polyclonal antisera to A12 structural proteins demonstrated that native empty capsids were indistinguishable from virus. Infected cell 14S particles possessed all the neutralizing epitopes and reacted with VP2 antiserum. Cell-free structural protein complexes sedimenting at 110S, 60S, and 14S containing capsid proteins VP0, VP3, and VP1 are assembled in a rabbit reticulocyte lysate programmed with foot-and-mouth viral RNA. These structures also contain the six epitopes, and cell-free 14S structures like their in vivo counterparts reacted with VP2 antiserum. Capsid structures from infected cells and the cell-free complexes adsorbed to susceptible cells, and this binding was inhibited, to various degrees, by saturating levels of unlabeled virus. These assays and other biochemical evidence indicate that capsid assembly in the cell-free system resembles viral morphogenesis in infected cells. In addition, epitopes on the virus surface possibly involved in interaction with cellular receptor sites are found early in virion morphogenesis.

Binding of neutralizing monoclonal antibodies to empty capsids of poliovirus can be blocked by monospecific antisera to structural polypeptides VP1 and VP2

Binding of two neutralizing monoclonal antibodies (Nt-mAbs) to natural empty capsids (NEC) of poliovirus, type 1, was blocked to the extent of 83 per cent to 98 per cent by monospecific rabbit antisera directed against the structural polypeptides VP1 and VP2. Monospecific antisera against VP3 or VP4, however, did not show this blocking effect. It is therefore assumed that VP1 and VP2 are located close together at the antigenic sites for the two mAbs.

Trypsin sensitivity of the Sabin strain of type 1 poliovirus: cleavage sites in virions and related particles

Treatment of the Sabin strain of type 1 poliovirus with trypsin produced two stable fragments of capsid protein VP1 which remained associated with the virions. Trypsinized virus was fully infectious and was neutralized by type-specific antisera. The susceptible site in the Sabin 1 strain was between the lysine at position 99 and the asparagine at position 100. A similar tryptic cleavage occurred in the Leon and Sabin strains of type 3 poliovirus, probably at the arginine at position 100, but not in the type 1 Mahoney strain, which lacks a basic residue at either position 99 or position 100. Tryptic treatment of heat-treated virus and 14S assembly intermediates produced unique stable fragments which were different from those produced in virions. The implications of our results for future characterization of the surface structures of these particles and structural rearrangements in the poliovirus capsid are discussed.

A monoclonal antibody that neutralizes poliovirus by cross-linking virions

The neutralization of type 1 poliovirus by monoclonal antibody 35-1f4 was studied. The virions were rapidly linked by antibody into oligomers and larger aggregates, followed by slow redistribution of antibody between the immune complexes. The antibody content and infectivity of immune complexes were determined. Remaining single virions were fully infectious and free of antibody. The oligomers and larger aggregates did not significantly contribute to the residual infectivity, which therefore correlated with the number of remaining single virions. Papain digestion of neutralized poliovirus released fully infectious, antibody-free virions from the immune complexes. Anti-immunoglobulin antibodies reneutralized these virions. Polymerization was shown to occur even at virus concentrations of less than 10(3) PFU per ml.

Synthetic peptides from four separate regions of the poliovirus type 1 capsid protein VP1 induce neutralizing antibodies

Peptides from different regions of the poliovirus type 1 capsid protein VP1 were synthesized. Antibodies raised against these peptides in rabbits and rats recognized the cognate peptides and denatured VP1. Peptides from four regions of VP1 generated antisera with neutralizing titers specifically against poliovirus type 1. Antisera against all other regions of VP1 failed to neutralize virus infectivity, although some of the antisera clearly bound to native virions. Thus, the neutralizing determinants on VP1 reside in specific noncontiguous regions of the protein and can be defined by specific peptides from these regions.

Neutralizing monoclonal antibodies to hepatitis A virus: partial localization of a neutralizing antigenic site

BALB/c mice were immunized with purified preparations of hepatitis A virus (HAV) isolated after 21 days of growth in LLC-MK2 cells. The HAV antigen was isolated from CsCl gradients and consisted primarily of the following three proteins as analyzed after sodium dodecyl sulfate-polyacrylamide gel electrophoresis and Coomassie blue staining: VP-1 at 33,000 daltons, VP-2 at 29,000 to 30,000 daltons, and VP-3 at 27,000 daltons. The spleen cells isolated from two BALB/c mice, immunized with two inoculations of HAV, were fused with SP 2/0 myeloma cells and grown in hypoxanthine-aminopterin-thymidine medium. Of 270 hybridomas initially screened, 72 were positive for binding HAV by a noncompetitive radioimmunoassay. All 72 were tested for the ability to neutralize the infectivity of HAV in an in vitro cell culture assay that was adapted for microtiter plates and that used detergent-treated virus for improved neutralization sensitivity and newborn cynomolgus monkey kidney cells for rapid growth. Eighteen hybridomas were positive for neutralization; 16 remained stable. Of the 16, 9 were able to compete with labeled polyclonal serum for binding to HAV. The nine competing hybridomas could be separated into two groups which appear to be directed towards two different sites on HAV and could complement each other in the competitive radioimmunoassay against polyclonal sera. Of the original 16 neutralizing hybridomas, 4 were subcloned through two cycles of limit dilutions. All four monoclonal antibodies retained their original neutralizing and competitive properties; three were immunoglobulin G2a, and one was immunoglobulin G1. All four monoclonal antibodies readily precipitate whole 125I-labeled HAV but are not able to recognize the disrupted proteins of the virus (as tested by immune precipitations of heat- and detergent-disrupted virions or Western blot analyses). However, the heterobifunctional cross-linking reagent toluene-2,4-diisocyanate was used to cross-link purified Fab fragments of two different monoclonal antibodies (2D2 and 6A5) to HAV before disruption. This reagent demonstrated a specific reaction of the monoclonal antibodies to the VP-1 of HAV, suggesting this major surface protein contains at least one of the major neutralization sites for HAV.

Monoclonal antibodies against microorganisms

The recent spread of hybridoma technology among laboratories has promoted the development of monoclonal antibodies against a wide variety of infectious disease agents. While monoclonal antibodies theoretically represent an excellent (perhaps superior) alternative to conventional antisera as diagnostic, therapeutic or laboratory reagents, traditional antisera may be preferable to monoclonal antibody in some circumstances because of the fixed affinity and specificity as well as the limited functional capacities of some antibodies. The acceptance of monoclonal antibodies by the clinical microbiologist and physician must await proof of their reliability, safety and efficacy.

Epitopes on foot-and-mouth disease virus outer capsid protein VP1 involved in neutralization and cell attachment

Foot-and-mouth disease virus structural protein VP1 elicits neutralizing and protective antibody and is probably the viral attachment protein which interacts with cellular receptor sites on cultured cells. To study the relationships between epitopes on the molecule related to neutralization and cell attachment, we tested monoclonal antibodies prepared against type A12 virus, isolated A12 VP1, and a CNBr-generated A12 VP1 fragment for neutralization and effect on viral absorption. The antibodies selected for analysis neutralized viral infectivity with varying efficiencies. One group of antibodies caused a high degree of viral aggregation and inhibited the adsorption of virus to cells by 50 to 70%. A second group of antibodies caused little or no viral aggregation but inhibited the adsorption of virus to cells by 80 to 90%. One antibody, which is specific for the intact virion, caused little viral aggregation and had no effect on the binding of virus to specific cellular receptor sites. Thus, at least three antigenic areas on the surface of foot-and-mouth disease virus which were involved in neutralization were demonstrated. One of the antigenic sites appears to have been responsible for interaction with the cellular receptor sites on the surface of susceptible cells.

In vitro assembly of poliovirus empty capsids: antigenic consequences and immunological assay of the morphopoietic factor

The assembly of poliovirus 14 S particles into empty capsids was studied without cell extract (self-assembly) and in extracts of infected or uninfected HeLa cells. The products were analyzed using monoclonal antibodies specific for N1, N2, or H epitopes. The empty capsids formed in infected cell extract, and only those, possessed N2 epitopes like the procapsids formed in vivo, thus showing that a virally encoded or induced factor determines the antigenicity of the assembly product. Based on this observation, a simple immunological assay for the activity of the morphopoietic factor is presented. This factor is shown to lack serotype specificity.

The application of monoclonal antibodies in the study of viruses

This chapter discusses the applications of monoclonal antibodies in virology. A single monoclonal antibody can provide information on protein “relatedness,” structure, function, synthesis, processing, and cellular or tissue distribution and on the association among molecules. The use of monoclonal antibodies provides valuable insight into the working of the protein both as an enzyme and as a target for the host immune response, evolving in reaction to that response. Monoclonal antibodies find application in two main areas: (1) in the field of rapid diagnosis of virus disease in man, animals, and plants and (2) in the extension of virus taxonomy. Monoclonal antibodies may be used to analyze the role of a protein. This ability to distinguish related proteins can be used to provide a genetic marker in recombination experiments. Monoclonal antibodies can detect low amounts of individual virus proteins within the infected cell. They can, thus, provide information concerning the temporal and spatial separation of protein formation and accumulation, and data on protein modification and processing in the infected cell.

Epitope evolution in poliovirus maturation

Eight monoclonal antibodies specific for native (N) poliovirus antigen all recognized empty capsids extracted from infected HeLa cells; only three of them recognized 14S particles. The facts point to the existence of two different epitopes N1 and N2, only one of which (N1) is present on 14S particles (these observations confirm and extend findings by EMINI et al). Another epitope of 14S particles is recognized by antibodies to heated (H) poliovirus antigen. Infected cell extract-catalyzed polymerization of 14S particles yielded mainly empty capsids carrying the N1 and N2, but no H epitopes.

Type-specific and cross-reactive antigenicity of capsid proteins VP1 and VP2 of echovirus type 7

After disruption of echovirus type 7 virions with urea and heat, VP1 and VP2 were separated by isoelectric focusing in urea-containing sucrose gradients. Antisera to these two polypeptides were produced in guinea pigs. In complement fixation, antiserum to VP1 reacted with native and heated virions (N and H antigens, respectively) of homologous virus, and also cross-reacted with heated virions of some other enteroviruses used. Antiserum to VP2 was reactive only with heated virions of homologous and heterologous viruses. Interestingly, the anti-VP2 serum reacted neither with native nor even with heated procapsids (naturally-occurring empty capsids). Antiserum to VP1, but not VP2, showed neutralizing and hemagglutination-inhibiting activities. These results suggest that 1) both VP1 and VP2 possess cross-reactive antigenic determinants which are exposed on the surface of heated virions, and 2) type-specific determinants of VP1 are located on the surface of native virions.

Cross antigenicity among enteroviruses as revealed by immunoblot technique

Antigenic relationships of various human and two animal picornaviruses were investigated by the immunoblotting ("Western blot") technique. The viruses included all coxsackievirus B types (1-6), poliovirus types 1-3, several strains of echovirus 11, EMC virus, and FMDV. Antisera included human sera and sera from rabbits hyperimmunized with either purified picornaviruses, viral structural polypeptides (VP8), boiled or "sample-boiled" virions. Group-specific reactions of various extent were observed among the human picornaviruses, but not with EMC virus. These reactions were obtained with human sera (whole serum, IgG- and IgM-fraction) as well as with "monospecific" (neutralization test) rabbit antisera. Among cross reacting polypeptides VP1 was predominant with the notable exception of coxsackie B4, where VP1 (defined according to cleavage pattern) migrates in our gel system as second largest polypeptide. Antisera prepared vs VP1 had neutralizing activity as demonstrated with five different echovirus 11 strains (titers up to 2000). Antisera vs VP1 (and other VP8) exhibited cross-reactivity in the immunoblots. Antisera to the three poliovirus types (and to certain echovirus 11 strains) showed a surprisingly narrow cross-reacting spectrum which--in the case of poliovirus--could not be broadened by additional hyperimmunization of the rabbits with heated poliovirus 2. The significance of these results for a diagnostic ELISA in patients with picornavirus infections is dealt with.

Localization of a poliovirus type 1 neutralization epitope in viral capsid polypeptide VP1

Poliovirus type 1 cDNA sequences coding for viral capsid polypeptide VP1 were inserted into the beta-lactamase sequence of Escherichia coli plasmid pBR322. Resulting recombinant plasmid pSW119 expressed in Escherichia coli a VP1-beta-lactamase fusion protein that reacted with antibodies raised against poliovirus capsid polypeptide VP1 and with a monoclonal poliovirus type 1 neutralizing antibody, C3. Deletions of various lengths were generated within the VP1 sequence. The hybrid proteins expressed by the deleted plasmids did not react any more with C3 when the region of VP1 amino acids 95-110 (poliovirus nucleotides 2,754-2,806) was deleted. Therefore, the C3 epitope responsible for virus neutralization is most probably located in this region of the capsid polypeptide.

Neutralization of poliovirus by a monoclonal antibody: kinetics and stoichiometry

First-order kinetics of neutralization have usually been interpreted as evidence that a single antibody, binding at a critical site, neutralizes the infectivity of a virus particle. In such a case, if all the binding sites were critical, an average of one antibody bound per virion would be required to reduce the infectivity of a virus sample to 37% (1/e) of its initial infectivity. However, in the work reported here using a monoclonal antibody to poliovirus which inactivated with first-order kinetics, an average of four bound antibodies were required. These results are consistent with two different models: one in which only one-fourth of the antibody binding sites on the virion are critical for neutralization; the other, in which none of the sites is critical, but neutralization takes place instead in a stepwise fashion in which each bound antibody reduces the infectivity by a factor of 3/4. The maximum binding capacity of the virion for this monoclonal antibody was approximately 30 molecules. Since the 60 protein subunits of the poliovirus capsid are related by 30 twofold axes of symmetry, it is proposed that each monoclonal antibody binds bivalently to two protein subunits related by a twofold axis. Such a binding mode would crosslink pentamers, the basic structures in picornaviral assembly and dissociation. It is proposed that pentamer crosslinking is an important element in neutralization by this monoclonal antibody. Another antibody, which may neutralize by a different mechanism, is also discussed briefly.

Functional basis of poliovirus neutralization determined with monospecific neutralizing antibodies

Antibody-mediated poliovirus neutralization was studied by using a series of 13 monospecific neutralizing antibodies. These antibodies were found to recognize seven individual viral epitopes, several of which functionally overlap one another. Each epitope was capable of undergoing variation so that the variant virus was no longer capable of being neutralized by antibody directed against that epitope. The measured degree of variation for each site varied from -3.1 to -4.2 log10 variant PFU per wild-type PFU. Under nonsaturating but neutralizing conditions, the antibodies, with the exception of those directed to one specific epitope, failed to completely inhibit the virion's binding to the cell. Similarly, none of the neutralizing antibodies completely inhibited viral penetration, but all prevented virus-specific transcription. A strong correlation was established between the binding of each of the neutralizing antibodies, with one exception, to the virion and a significant shift in the virion's pI from 7.0 to ca. 4.0.

Detection by monoclonal antibodies of an antigenic determinant critical for poliovirus neutralization present on VP1 and on heat-inactivated virions

Hybridoma cell lines were established against poliovirus type 1 (Mahoney) heat-denatured virions (C particles). Each anti-C monoclonal antibody (McAb) immunoprecipitated specifically one of the individualized poliovirus capsid polypeptides VP1, VP2, or VP3. One of the anti-C McAb (C-3), reacting with VP1, neutralized homologous virus and immunoprecipitated infectious D particles. Its properties have been compared to those of a neutralizing anti-D McAb (D-Ic). In contrast with the C-3 antigenic site, the D-Ic epitope was not present on C particles nor on individualized structural polypeptide. This demonstrates that C-3 and D-Ic epitopes represent two independent antigenic determinants, both critical for poliovirus neutralization.

Isolated poliovirus capsid protein VP1 induces a neutralizing response in rats

Antibodies were raised in rats against the poliovirus type 1 capsid proteins, VP1, VP2, and VP3. Antibodies directed against VP1 from type 1 poliovirus (Mahoney) neutralized type 1 but not type 2 poliovirus. Antibodies raised against VP2 and VP3 failed to neutralize type 1 virus. Thus, VP1 appears to be a neutralizing antigen for poliovirus and in its denatured form presents to the immune system its neutralizing determinants.

Poliovirus neutralization epitopes: analysis and localization with neutralizing monoclonal antibodies

Two hybridomas (H3 and D3) secreting monoclonal neutralizing antibody to intact poliovirus type 1 (Mahoney strain) were established. Each antibody bound to a site qualitatively different from that to which the other antibody bound. The H3 site was located on intact virions and, to a lesser extent, on 80S naturally occurring empty capsids and 14S precursor subunits. The D3 site was found only on virions and empty capsids. Neither site was expressed on 80S heat-treated virions. The antibodies did not react with free denatured or undenatured viral structural proteins. Viral variants which were no longer capable of being neutralized by either one or the other antibody were obtained. Such variants arose during normal cell culture passage of wild-type virus and were present in the progeny viral population on the order of 10(-4) variant per wild-type virus PFU. Toluene-2,4-diisocyanate, a heterobifunctional covalent cross-linking reagent, was used to irreversibly bind the F(ab) fragments of the two antibodies to their respective binding sites. In this way, VP1 was identified as the structural protein containing both sites.

The use of monoclonal antibodies in virology

Antibodies have been used for the last five decades in the laboratory diagnosis of a wide range of diseases caused by viruses and in detailed investigations of virus structure. However, immunological and serological assays have always had problems of interpretation, reproducibility and standardization, resulting partly from the unavoidable heterogeneity of the antibodies in the test. When a mouse, for example, is immunized with a virus, the animal may easily recognise 10–20 different antigenic determinants. As many as five distinct antibodies can be produced by the mouse against each determinant and these will often differ from antibodies made by another mouse against the same antigenic determinant. The method of lymphocyte fusion and the subsequent generation of monoclonal antibodies (Kohler & Milstein, 1975; Kohler, 1980) overcomes these limitations.