The modification of poliovirus antigens by heat and ultraviolet light

Protease-Independent Production of Poliovirus Virus-like Particles in Pichia pastoris: Implications for Efficient Vaccine Development and Insights into Capsid Assembly

The production of enterovirus virus-like particles (VLPs) that lack the viral genome have great potential as vaccines for a number of diseases, such as poliomyelitis and hand, foot, and mouth disease. These VLPs can mimic empty capsids, which are antigenically indistinguishable from mature virions, produced naturally during viral infection. Both in infection and in vitro, capsids and VLPs are generated by the cleavage of the P1 precursor protein by a viral protease. Here, using a stabilized poliovirus 1 (PV-1) P1 sequence as an exemplar, we show the production of PV-1 VLPs in Pichia pastoris in the absence of the potentially cytotoxic protease, 3CD, instead using the porcine teschovirus 2A (P2A) peptide sequence to terminate translation between individual capsid proteins. We compare this to protease-dependent production of PV-1 VLPs. Analysis of all permutations of the order of the capsid protein sequences revealed that only VP3 could be tagged with P2A and maintain native antigenicity. Transmission electron microscopy of these VLPs reveals the classic picornaviral icosahedral structure. Furthermore, these particles were thermostable above 37°C, demonstrating their potential as next generation vaccine candidates for PV. Finally, we believe the demonstration that native antigenic VLPs can be produced using protease-independent methods opens the possibility for future enteroviral vaccines to take advantage of recent vaccine technological advances, such as adenovirus-vectored vaccines and mRNA vaccines, circumventing the potential problems of cytotoxicity associated with 3CD, allowing for the production of immunogenic enterovirus VLPs in vivo. IMPORTANCE The widespread use of vaccines has dramatically reduced global incidence of poliovirus infections over a period of several decades and now the wild-type virus is only endemic in Pakistan and Afghanistan. However, current vaccines require the culture of large quantities of replication-competent virus for their manufacture, thus presenting a potential risk of reintroduction into the environment. It is now widely accepted that vaccination will need to be extended posteradication into the foreseeable future to prevent the potentially catastrophic reintroduction of poliovirus into an immunologically naive population. It is, therefore, imperative that novel vaccines are developed which are not dependent on the growth of live virus for their manufacture. We have expressed stabilized virus-like particles in yeast, from constructs that do not require coexpression of the protease. This is an important step in the development of environmentally safe and commercially viable vaccines against polio, which also provides some intriguing insights into the viral assembly process.

Production and Characterisation of Stabilised PV-3 Virus-like Particles Using Pichia pastoris

Following the success of global vaccination programmes using the live-attenuated oral and inactivated poliovirus vaccines (OPV and IPV), wild poliovirus (PV) is now only endemic in Afghanistan and Pakistan. However, the continued use of these vaccines poses potential risks to the eradication of PV. The production of recombinant PV virus-like particles (VLPs), which lack the viral genome offer great potential as next-generation vaccines for the post-polio world. We have previously reported production of PV VLPs using Pichia pastoris, however, these VLPs were in the non-native conformation (C Ag), which would not produce effective protection against PV. Here, we build on this work and show that it is possible to produce wt PV-3 and thermally stabilised PV-3 (referred to as PV-3 SC8) VLPs in the native conformation (D Ag) using Pichia pastoris. We show that the PV-3 SC8 VLPs provide a much-improved D:C antigen ratio as compared to wt PV-3, whilst exhibiting greater thermostability than the current IPV vaccine. Finally, we determine the cryo-EM structure of the yeast-derived PV-3 SC8 VLPs and compare this to previously published PV-3 D Ag structures, highlighting the similarities between these recombinantly expressed VLPs and the infectious virus, further emphasising their potential as a next-generation vaccine candidate for PV.

Mammalian expression of virus-like particles as a proof of principle for next generation polio vaccines

Global vaccination programs using live-attenuated oral and inactivated polio vaccine (OPV and IPV) have almost eradicated poliovirus (PV) but these vaccines or their production pose significant risk in a polio-free world. Recombinant PV virus-like particles (VLPs), lacking the viral genome, represent safe next-generation vaccines, however their production requires optimisation. Here we present an efficient mammalian expression strategy producing good yields of wild-type PV VLPs for all three serotypes and a thermostabilised variant for PV3. Whilst the wild-type VLPs were predominantly in the non-native C-antigenic form, the thermostabilised PV3 VLPs adopted the native D-antigenic conformation eliciting neutralising antibody titres equivalent to the current IPV and were indistinguishable from natural empty particles by cryo-electron microscopy with a similar stabilising lipidic pocket-factor in the VP1 β-barrel. This factor may not be available in alternative expression systems, which may require synthetic pocket-binding factors. VLPs equivalent to these mammalian expressed thermostabilized particles, represent safer non-infectious vaccine candidates for the post-eradication era.

Comparative Molecular Biology Approaches for the Production of Poliovirus Virus-Like Particles Using Pichia pastoris

Although the current poliovirus immunization program has been extremely successful in reducing the number of cases of paralytic polio worldwide, now more cases are caused by vaccine-derived polioviruses than by wild poliovirus. Switching to inactivated poliovirus vaccines will reduce this over time; however, their production requires the growth of large amounts of virus. This biosafety concern can be addressed by producing just the virus capsid. The capsid serves to protect the genetic material, which causes disease when introduced into a cell. Therefore, empty capsids (virus-like particles [VLPs]), which lack the viral RNA genome, are safe both to make and to use. We exploit yeast as a versatile model expression system to produce VLPs, and here we specifically highlight the potential of this system to supply next-generation poliovirus vaccines to secure a polio-free world for the future.

For enteroviruses such as poliovirus (PV), empty capsids, which are antigenically indistinguishable from mature virions, are produced naturally during viral infection. The production of such capsids recombinantly, in heterologous systems such as yeast, have great potential as virus-like particle (VLP) vaccine candidates. Here, using PV as an exemplar, we show the production of VLPs in Pichia pastoris by coexpression of the structural precursor protein P1 and the viral protease 3CD. The level of expression of the potentially cytotoxic protease relative to that of the P1 precursor was modulated by three different approaches: expression of the P1 precursor and protease from different transcription units, separation of the P1 and protease proteins using the Thosea asigna virus (TaV) 2A translation interruption sequence, or separation of the P1 and protease-coding sequences by an internal ribosome entry site sequence from Rhopalosiphum padi virus (RhPV). We also investigate the antigenicity of VLPs containing previously characterized mutations when produced in Pichia. Finally, using transmission electron microscopy and two-dimensional classification, we show that Pichia-derived VLPs exhibited the classical icosahedral capsid structure displayed by enteroviruses.

IMPORTANCE Although the current poliovirus immunization program has been extremely successful in reducing the number of cases of paralytic polio worldwide, now more cases are caused by vaccine-derived polioviruses than by wild poliovirus. Switching to inactivated poliovirus vaccines will reduce this over time; however, their production requires the growth of large amounts of virus. This biosafety concern can be addressed by producing just the virus capsid. The capsid serves to protect the genetic material, which causes disease when introduced into a cell. Therefore, empty capsids (virus-like particles [VLPs]), which lack the viral RNA genome, are safe both to make and to use. We exploit yeast as a versatile model expression system to produce VLPs, and here we specifically highlight the potential of this system to supply next-generation poliovirus vaccines to secure a polio-free world for the future.

Thin silk fibroin films as a dried format for temperature stabilization of inactivated polio vaccine

Current inactivated polio vaccine (IPV) products are sensitive to both freezing and elevated temperatures and therefore must be shipped and stored between 2 °C and 8 °C, a requirement that imposes financial and logistical challenges for global distribution. As such, there is a critical need for a robust, thermally stable IPV to support global polio eradication and post-eradication immunization needs. Here, we present the development of air-dried thin films for temperature stabilization of IPV using the biomaterial silk fibroin. Thin-film product compositions were optimized for physical properties as well as poliovirus D-antigen recovery and were tested under accelerated and real-time stability storage conditions. Silk fibroin IPV films maintained 70% D-antigen potency after storage for nearly three years at room temperature, and greater than 50% potency for IPV-2 and IPV-3 serotypes at 45 °C for one year. The immunogenicity of silk fibroin IPV films after 2-week storage at 45 °C was assessed in Wistar rats and the stressed films generated equivalent neutralizing antibody responses to commercial vaccine for IPV-1 and IPV-2. However, the absence of IPV-3 responses warrants further investigation into the specificity of ELISA for intact IPV-3 D-antigen. By demonstrating immunogenicity post-storage, we offer the air-dried silk film format as a means to increase IPV vaccine access through innovative delivery systems such as microneedles.

Structure, Immunogenicity, and Protective Mechanism of an Engineered Enterovirus 71-Like Particle Vaccine Mimicking 80S Empty Capsid

Enterovirus 71 (EV71) is the major causative agent of severe hand, foot, and mouth disease, which affects millions of young children in the Asia-Pacific region annually. In this study, we engineered a novel EV71 virus-like particle (VLP) that lacks VP4 (therefore designated VLP_ΔVP4) and investigated its structure, antigenicity, and vaccine potential. The cryo-electron microscopy (cryo-EM) structure of VLP_ΔVP4 was reconstructed to 3.71-Å resolution. Results from structural and biochemical analyses revealed that VLP_ΔVP4 resembles the end product of the viral uncoating process, the 80S empty capsid. VLP_ΔVP4 is able to elicit high-titer neutralizing antibodies and to fully protect mice against lethal viral challenge. Mechanistic studies showed that, at the cellular level, the anti-VLP_ΔVP4 sera exert neutralization effects at both pre- and postattachment stages by inhibiting both virus attachment and internalization, and at the molecular level, the antisera can block multiple interactions between EV71 and its key receptors. Our study gives a better understanding of EV71 capsid assembly and provides important information for the design and development of new-generation vaccines for EV71, and perhaps for other enteroviruses, as well.IMPORTANCE Enterovirus 71 (EV71) infection may lead to severe hand, foot, and mouth disease, with significant morbidity and mortality. Knowledge regarding EV71 particle assembly remains limited. Here, we report the generation and characterization of a novel EV71 virus-like particle that lacks the VP4 capsid subunit protein. This particle, termed VLP_ΔVP4, structurally mimics the 80S empty capsid, which is the end stage of EV71 uncoating. We further show that VLP_ΔVP4 exhibits desirable immunogenicity and protective efficacy in proof-of-concept studies. In addition, the inhibitory mechanisms of the VLP_ΔVP4-induced antibodies are unraveled at both the cellular and molecular levels. Our work provides the first evidence of picornaviral particle assembly in the complete absence of VP4 and identifies VLP_ΔVP4 as an improved EV71 vaccine candidate with desirable traits. These findings not only enhance our understanding of particle assembly and uncoating of picornaviruses, but also provide important information for structure-guided vaccine design for EV71 and other enteroviruses.

Plant-made polio type 3 stabilized VLPs-a candidate synthetic polio vaccine

Poliovirus (PV) is the causative agent of poliomyelitis, a crippling human disease known since antiquity. PV occurs in two distinct antigenic forms, D and C, of which only the D form elicits a robust neutralizing response. Developing a synthetically produced stabilized virus-like particle (sVLP)-based vaccine with D antigenicity, without the drawbacks of current vaccines, will be a major step towards the final eradication of poliovirus. Such a sVLP would retain the native antigenic conformation and the repetitive structure of the original virus particle, but lack infectious genomic material. In this study, we report the production of synthetically stabilized PV VLPs in plants. Mice carrying the gene for the human PV receptor are protected from wild-type PV when immunized with the plant-made PV sVLPs. Structural analysis of the stabilized mutant at 3.6 Å resolution by cryo-electron microscopy and single-particle reconstruction reveals a structure almost indistinguishable from wild-type PV3.Despite the success of current vaccination against poliomyelitis, safe, cheap and effective vaccines remain sought for continuing eradication effort. Here the authors use plants to express stabilized virus-like particles of type 3 poliovirus that can induce a protective immune response in mice transgenic for the human poliovirus receptor.

Increasing Type 1 Poliovirus Capsid Stability by Thermal Selection

Poliomyelitis is a highly infectious disease caused by poliovirus (PV). It can result in paralysis and may be fatal. Integrated global immunization programs using live-attenuated oral (OPV) and/or inactivated (IPV) PV vaccines have systematically reduced its spread and paved the way for eradication. Immunization will continue posteradication to ensure against reintroduction of the disease, but there are biosafety concerns for both OPV and IPV. They could be addressed by the production and use of virus-free virus-like particle (VLP) vaccines that mimic the "empty" capsids (ECs) normally produced in viral infection. Although ECs are antigenically indistinguishable from mature virus particles, they are less stable and readily convert into an alternative conformation unsuitable for vaccine purposes. Stabilized ECs, expressed recombinantly as VLPs, could be ideal candidate vaccines for a polio-free world. However, although genome-free PV ECs have been expressed as VLPs in a variety of systems, their inherent antigenic instability has proved a barrier to further development. In this study, we selected thermally stable ECs of type 1 PV (PV-1). The ECs are antigenically stable at temperatures above the conversion temperature of wild-type (wt) virions. We have identified mutations on the capsid surface and in internal networks that are responsible for EC stability. With reference to the capsid structure, we speculate on the roles of these residues in capsid stability and postulate that such stabilized VLPs could be used as novel vaccines.

IMPORTANCE

Poliomyelitis is a highly infectious disease caused by PV and is on the verge of eradication. There are biosafety concerns about reintroduction of the disease from current vaccines that require live virus for production. Recombinantly expressed virus-like particles (VLPs) could address these inherent problems. However, the genome-free capsids (ECs) of wt PV are unstable and readily change antigenicity to a form not suitable as a vaccine. Here, we demonstrate that the ECs of type 1 PV can be stabilized by selecting heat-resistant viruses. Our data show that some capsid mutations stabilize the ECs and could be applied as candidates to synthesize stable VLPs as future genome-free poliovirus vaccines.

Crystal structures of enterovirus 71 (EV71) recombinant virus particles provide insights into vaccine design

Hand-foot-and-mouth disease (HFMD) remains a major health concern in the Asia-Pacific regions, and its major causative agents include human enterovirus 71 (EV71) and coxsackievirus A16. A desirable vaccine against HFMD would be multivalent and able to elicit protective responses against multiple HFMD causative agents. Previously, we have demonstrated that a thermostable recombinant EV71 vaccine candidate can be produced by the insertion of a foreign peptide into the BC loop of VP1 without affecting viral replication. Here we present crystal structures of two different naturally occurring empty particles, one from a clinical C4 strain EV71 and the other from its recombinant virus containing an insertion in the VP1 BC loop. Crystal structure analysis demonstrated that the inserted foreign peptide is well exposed on the particle surface without significant structural changes in the capsid. Importantly, such insertions do not seem to affect the virus uncoating process as illustrated by the conformational similarity between an uncoating intermediate of another recombinant virus and that of EV71. Especially, at least 18 residues from the N terminus of VP1 are transiently externalized. Altogether, our study provides insights into vaccine development against HFMD.

A sensor-adaptor mechanism for enterovirus uncoating from structures of EV71

Enterovirus 71 (EV71), a major agent of hand-foot-and-mouth disease in children, can cause severe central nervous system disease and mortality. At present no vaccine or antiviral therapy is available. We have determined high-resolution structures for the mature virus and natural empty particles. The structure of the mature virus is similar to that of other enteroviruses, whilst the empty particles are dramatically expanded, with notable fissures, resembling elusive enterovirus uncoating intermediates not previously characterized in atomic detail. Hydrophobic capsid pockets within the EV71 capsid are collapsed in this expanded particle, providing a detailed explanation of the mechanism for receptor-binding triggered virus uncoating. The results provide a paradigm for enterovirus uncoating, in which the VP1 GH loop acts as an adaptor-sensor for the attachment of cellular receptors, converting heterologous inputs to a generic uncoating mechanism, spotlighting novel points for therapeutic intervention.

Poliovirus RNA is released from the capsid near a twofold symmetry axis

After recognizing and binding to its host cell, poliovirus (like other nonenveloped viruses) faces the challenge of translocating its genome across a cellular membrane and into the cytoplasm. To avoid entanglement with the capsid, the RNA must exit via a single site on the virion surface. However, the mechanism by which a single site is selected (from among 60 equivalents) is unknown; and until now, even its location on the virion surface has been controversial. To help to elucidate the mechanism of infection, we have used single-particle cryo-electron microscopy and tomography to reconstruct conformationally altered intermediates that are formed by the poliovirion at various stages of the poliovirus infection process. Recently, we reported icosahedrally symmetric structures for two forms of the end-state 80S empty capsid particle. Surprisingly, RNA was frequently visible near the capsid; and in a subset of the virions, RNA was seen on both the inside and outside of the capsid, caught in the act of exiting. To visualize RNA exiting, we have now determined asymmetric reconstructions from that subset, using both single-particle cryo-electron microscopy and cryo-electron tomographic methods, producing independent reconstructions at ∼50-Å resolution. Contrary to predictions in the literature, the footprint of RNA on the capsid surface is located close to a viral 2-fold axis, covering a slot-shaped area of reduced density that is present in both of the symmetrized 80S reconstructions and which extends by about 20 Å away from the 2-fold axis toward each neighboring 5-fold axis.

The use of new antigens in the complementfixation test for acute poliomyelitis

A complement-fixation test for acute poliomyelitis using unheated antigens derived from suckling mouse brain infected with poliovirus Type 1 or Type 2 is described.

The results of tests in 62 patients clinically diagnosed as cases of acute poliomyelitis in a recent epidemic and in 26 controls are recorded.

The CF tests were positive in 100% of 53 cases with poliovirus Type 1 and/or Type 2 in stool. A positive result was obtained in 23 (76%) of 30 cases whose sera were examined in the first 7 days of illness.

Negative tests of the initial serum samples were found in 15 (28·5%) of 53 cases, but all these became positive in titres of 40 or 80 on testing of convalescent serum.

In 31 (69%) of 45 cases whose sera were re-tested between the 3rd and 4th weeks of illness the CF antibody levels rose, reaching titres of 80 or 160 in most instances. Of the remaining 14 cases only one dropped in insignificant degree (from titre 320 to 160) and the 13 stationary results had been positive in titres of 40–160 on initial tests most of which were performed in the 2nd week of illness.

Homotypic CF antibody response without crossing was found in 37 (71%) of 52 cases with Type 1 or Type 2 virus in stool. In the cases of crossing the heterotypic antibody response was either transient, diminishing or stationary in all and in only low titre in most instances.

In 26 control cases there were seven positive CF tests, but one of these was nonspecific, five were in lowest titres, and one case appeared to have had recent poliomyelitis infection.

Heating the antigens did not broaden the reaction. It caused only slight loss of potency except in two cases in which the CF titre increased substantially.

The antigenic preparation described appears to be superior to antigens of other origin in the diagnosis of acute poliomyelitis by complement-fixation tests, as positive tests are recognized earlier in the illness and the titres are higher. Homotypic results were obtained in all cases and no instance of false negative occurred in this series.

I would like to thank the medical staff of the Cape Town City Hospital for Infectious Diseases for the trouble taken in collecting stools and paired sera, and Prof. Kipps for his interest in this work. I am indebted to Miss Karin Larssen for valuable technical assistance.

Use of the multiple copy simultaneous search (MCSS) method to design a new class of picornavirus capsid binding drugs

A combinatorial ligand design approach based on the multiple copy simultaneous search (MCSS) method and a simple scheme for joining MCSS functional group sites was applied to the binding pocket of P3/Sabin poliovirus and rhinovirus 14. The MCSS method determines where specific functional (chemical) groups have local potential energy minima in the binding site. Before the virus application, test calculations were run to determine the optimal set of input parameters to be used in evaluating the MCSS results. The MCSS minima are analysed and selected minima are connected with (CH2)n linkers to form candidate ligands, whose structures are optimized in the binding site. Estimates of the binding strength were made for the ligands and compared with those for known drugs. The results indicate that the proposed ligands should bind to P3/Sabin poliovirus at least as well as the best of the existing drugs, and that they should also bind to P1/Mahoney poliovirus and rhinovirus 14. A detailed comparison of the poliovirus and rhinovirus binding pockets and an analysis of drug binding specificity is presented.

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.

A simple quantitative protein A micro-immunoprecipitation method; assay of antibodies to the N and H antigens of poliovirus

Staphylococcus aureus (Cowan strain I) was used to absorb immune complexes from antiserum to poliovirus to which labeled N or H poliovirus antigens had been added, and the radioactivity in the pelleted organisms and in the supernatant was measured. Excellent agreement was obtained between values calculated separately from the pellet and supernatant readings, validating the use of supernatant measurements from a microtitration plate method.

Protein-RNA interaction in encephalomyocarditis virus as revealed by UV light-induced covalent linkages

UV irradiation of encephalomyocarditis virus led to an increase in the buoyant density of the virus in CsCl gradients from 1.34 to 1.46 g/cm3. Heat treatment of the irradiated virus (20 min at 54 degrees C) reduced the density to 1.40 g/cm3 and led to the loss of approximately 55% of the labeled RNA from the virions. The non-irradiated virions were converted by such heating into empty capsids. Irradiation also resulted in an increase in the accessibility of RNA inside the virions to the action of pancreatic RNase. An increase in the UV dose did not enlarge the fraction of RNA molecules covalently linked to protein; this was revealed by the lack of any secondary increase in the apparent RNase resistance of the labeled RNA in the irradiated virions. Destruction of the irradiated virus with sodium dodecyl sulfate and 2-mercaptoethanol allowed the isolation of a 40S structure containing viral RNA and RNA-linked proteins. The latter comprised no more than 2.5% of the whole protein content of the virion. Polyacrylamide gel electrophoretic analysis of the RNase-treated 40S structure revealed at least three viral structural proteins in the same ratio as was present in the intact virions.

Rhinoviruses: kinetics of ultraviolet inactivation and effects of UV and heat on immunogenicity

The kinetics of ultraviolet inactivation for two human rhinoviruses and poliovirus were compared. No major differences in the rates of ultraviolet inactivation were detectable. All viral preparations inactivated by ultraviolet irradiation induced neutralizing antibody in guinea pigs. In contrast, when guinea pigs were immunized with a heat inactivated rhinovirus preparation, little or no neutralizing antibody was elicited. Immune electron microscopy of the heated rhinovirus preparations revealed the presence of particles resembling empty capsids. These results suggest that rhinoviruses and enteroviruses are affected in a similar manner when subjected to ultraviolet or heat inactivation.

Effect of ultraviolet light on mengovirus: formation of uracil dimers, instability and degradation of capsid, and covalent linkage of protein to viral RNA

UV irradiation of purified mengovirus resulted in a very rapid inactivation of the infectivity of the virions (D(37) [37% survival dose] = 700 ergs/mm(2)) which correlated in time with the formation of uracil dimers in the viral RNA. During the first 2 min of irradiation, an average of 1.7 uracil dimers were formed per PFU of virus inactivated. Hemagglutination activity of the virions began to decrease only after a lag period of about 5 min and at a much lower rate (D(37) = 84,000 ergs/mm(2)). This decrease coincided in time with the appearance of altered proteins in the capsid and a structural change in the capsid. Although 10- to 20-min irradiated virions appeared intact in the electron microscope and sedimented at 150S in sucrose density gradients, the RNA of the virions became accessible to RNase and extractable by low concentrations of sodium dodecyl sulfate, and the virions broke down upon equilibrium centrifugation in CsCl gradients. During longer periods of irradiation (30 to 60 min), a progressively greater proportion of the virions were converted to 14S protein particles and 80S ribonucleoprotein particles composed of intact viral RNA and about 30% of the capsid proteins, alpha, beta, and gamma. Empty capsids were not detectable at any time during 60 min of irradiation, by which time disruption of the virions was complete. Irradiation of complete virions also resulted in an increased sedimentation rate of the viral RNA and in the covalent linkage to the viral RNA of about 1% of the total capsid protein in the form of heterogeneous low-molecular-weight polypeptides. The two observations seem to be causally related, since irradiation of isolated viral RNA did not result in an increase in sedimentation rate of the RNA, even though uracil dimer formation in viral RNA occurred at about the same rate and to the same extent whether intact virions or viral RNA were irradiated.

Inactivation of encephalomyocarditis virus in aerosols: fate of virus protein and ribonucleic acid

After aerosolization at relative humidities of 50% or lower, encephalomyocarditis virus is rapidly inactivated. In this process the protein coat of the virion is damaged. This appears as a loss of hemagglutination activity and loss of affinity for hemagglutination inhibiting antibodies. The ribonucleic acid of the virus retains its infectivity but it becomes susceptible to ribonuclease. It sediments in sucrose gradients when centrifuged at high speed with the same velocity as free infectious ribonucleic acid extracted with phenol from intact encephalomyocarditis virus.

Lack of close relationship between three strains of human rhinoviruses as determined by their RNA sequences

The possible genomic homologies between three serotypes of human rhinoviruses (HRV 1A, HRV 2, and HRV 14) were investigated. First we confirmed that these viruses were unrelated by the criterion of the absence of common antigenic determinants on the surfaces of the native virions, as detected by cross-neutralization of complementfixation. RNA-RNA hybridization was then examined with purified, highly radioactive, double-stranded, replicative-form RNA and excess single-stranded virion RNA. Single-stranded RNA showed 100% homology with the minus strand from the replicative-form RNA of the same type of virus. HRV 1A, HRV 2, and HRV 14 showed low intertypic homologies; these were not significantly greater than those found between the rhinoviruses and polivirus, which were used as a negative control. The immunological relationship and the RNA homology between HRV 1A and HRV 1B were also examined by the above techniques. It was confirmed that HRV 1A and HRV 1B share some surface determinants and it was also found that HRV 1B RNA shares 70% homology with HRV 1A RNA.

Antigenic determinants of infective and inactivated human rhinovirus type 2

Treatment of human rhinovirus type 2 (HRV 2) virions at pH 5, at 56 C or in 2 M urea, produces one or both of two types of subviral particles. These subviral particles sediment at 135S or at 80S and both share what have been designated as C-antigenic determinants; the determinants of native virions have been designated D. These sets of determinants have been contrasted by the techniques of immunodiffusion, complement fixation, and serum blocking, and the results indicate that many or most of the D-determinants are lost in the conversion to C antigenicity. Some of the HRV 2 C-determinants also react, in immunodiffusion and in complement fixation tests, with antisera produced against HRV 1A virions. The inverse reaction has also been detected by complement fixation. Purified natural top component (NTC) of HRV 2 contains C- and, to a lesser extent, D-determinants. The D-determinants of NTC are also, like those of virions, lost upon treatment at pH 5. These results are discussed in terms of a conformational model for the D- to C-antigenic conversion.