The trypsin-sensitive RVER domain in the capsid proteins of minute virus of mice is required for efficient cell binding and viral infection but not for proteolytic processing in vivo

Differential phosphorylation and n-terminal configuration of capsid subunits in parvovirus assembly and viral trafficking

The T1 parvovirus Minute Virus of Mice (MVM) was used to study the roles that phosphorylation and N-terminal domains (Nt) configuration of capsid subunits may play in icosahedral nuclear viruses assembly. In synchronous MVM infection, capsid subunits newly assembled as two types of cytoplasmic trimeric intermediates (3VP2, and 1VP1:2VP2) harbored a VP1 phosphorylation level fivefold higher than that of VP2, and hidden Nt. Upon nuclear translocation at S phase, VP1-Nt became exposed in the heterotrimer and subsequent subviral assembly intermediates. Empty capsid subunits showed a phosphorylation level restored to VP1:VP2 stoichiometry, and the Nt concealed in their interior. However ssDNA-filled virus maturing at S/G2 lacked VP1 phosphorylation and one major VP2 phosphopeptide, and exposed VP2-Nt. Endosomal VP2-Nt cleavage resulted in VP3 subunits devoid of any phospholabel, implying that incoming viral particles specifically harbor a low phosphorylation status. Phosphorylation provides a mechanistic coupling of parvovirus nuclear assembly to the cell cycle.

Protoparvovirus Cell Entry

The Protoparvovirus (PtPV) genus of the Parvoviridae family of viruses includes important animal pathogens and reference molecular models for the entire family. Some virus members of the PtPV genus have arisen as promising tools to treat tumoral processes, as they exhibit marked oncotropism and oncolytic activities while being nonpathogenic for humans. The PtPVs invade and replicate within the nucleus making extensive use of the transport, transcription and replication machineries of the host cells. In order to reach the nucleus, PtPVs need to cross over several intracellular barriers and traffic through different cell compartments, which limit their infection efficiency. In this review we summarize molecular interactions, capsid structural transitions and hijacking of cellular processes, by which the PtPVs enter and deliver their single-stranded DNA genome into the host cell nucleus. Understanding mechanisms that govern the complex PtPV entry will be instrumental in developing approaches to boost their anticancer therapeutic potential and improving their safety profile.

Late Maturation Steps Preceding Selective Nuclear Export and Egress of Progeny Parvovirus

Although the mechanism is not well understood, growing evidence indicates that the nonenveloped parvovirus minute virus of mice (MVM) may actively egress before passive release through cell lysis. We have dissected the late maturation steps of the intranuclear progeny with the aims of confirming the existence of active prelytic egress and identifying critical capsid rearrangements required to initiate the process. By performing anion-exchange chromatography (AEX), we separated intranuclear progeny particles by their net surface charges. Apart from empty capsids (EC), two distinct populations of full capsids (FC) arose in the nuclei of infected cells. The earliest population of FC to appear was infectious but, like EC, could not be actively exported from the nucleus. Further maturation of this early population, involving the phosphorylation of surface residues, gave rise to a second, late population with nuclear export potential. While capsid surface phosphorylation was strictly associated with nuclear export capacity, mutational analysis revealed that the phosphoserine-rich N terminus of VP2 (N-VP2) was dispensable, although it contributed to passive release. The reverse situation was observed for the incoming particles, which were dephosphorylated in the endosomes. Our results confirm the existence of active prelytic egress and reveal a late phosphorylation event occurring in the nucleus as a selective factor for initiating the process.

IMPORTANCE

In general, the process of egress of enveloped viruses is active and involves host cell membranes. However, the release of nonenveloped viruses seems to rely more on cell lysis. At least for some nonenveloped viruses, an active process before passive release by cell lysis has been reported, although the underlying mechanism remains poorly understood. By using the nonenveloped model parvovirus minute virus of mice, we could confirm the existence of an active process of nuclear export and further characterize the associated capsid maturation steps. Following DNA packaging in the nucleus, capsids required further modifications, involving the phosphorylation of surface residues, to acquire nuclear export potential. Inversely, those surface residues were dephosphorylated on entering capsids. These spatially controlled phosphorylation-dephosphorylation events concurred with the nuclear export-import potential required to complete the infectious cycle.

Oncolytic parvoviruses: from basic virology to clinical applications

Accumulated evidence gathered over recent decades demonstrated that some members of the Parvoviridae family, in particular the rodent protoparvoviruses H-1PV, the minute virus of mice and LuIII have natural anticancer activity while being nonpathogenic to humans. These studies have laid the foundations for the launch of a first phase I/IIa clinical trial, in which the rat H-1 parvovirus is presently undergoing evaluation for its safety and first signs of efficacy in patients with glioblastoma multiforme. After a brief overview of the biology of parvoviruses, this review focuses on the studies which unraveled the antineoplastic properties of these agents and supported their clinical use as anticancer therapeutics. Furthermore, the development of novel parvovirus-based anticancer strategies with enhanced specificity and efficacy is discussed, in particular the development of second and third generation vectors and the combination of parvoviruses with other anticancer agents. Lastly, we address the key challenges that remain towards a more rational and efficient use of oncolytic parvoviruses in clinical settings, and discuss how a better understanding of the virus life-cycle and of the cellular factors involved in virus infection, replication and cytotoxicity may promote the further development of parvovirus-based anticancer therapies, open new prospects for treatment and hopefully improve clinical outcome.

A slender tract of glycine residues is required for translocation of the VP2 protein N-terminal domain through the parvovirus MVM capsid channel to initiate infection

Viruses constitute paradigms to study conformational dynamics in biomacromolecular assemblies. Infection by the parvovirus MVM (minute virus of mice) requires a conformational rearrangement that involves the intracellular externalization through capsid channels of the 2Nt (N-terminal region of VP2). We have investigated the role in this process of conserved glycine residues in an extended glycine-rich tract located immediately after 2Nt. Based on the virus structure, residues with hydrophobic side chains of increasing volume were substituted for glycine residues 31 or 33. Mutations had no effect on capsid assembly or stability, but inhibited virus infectivity. All mutations, except those to alanine residues which had minor effects, impaired 2Nt externalization in nuclear maturing virions and in purified virions, to an extent that correlated with the side chain size. Different biochemical and biophysical analyses were consistent with this result. Importantly, all of the tested glycine residue replacements impaired the capacity of the virion to initiate infection, at ratios correlating with their restrictive effects on 2Nt externalization. Thus small residues within the evolutionarily conserved glycine-rich tract facilitate 2Nt externalization through the capsid channel, as required by this virus to initiate cell entry. The results demonstrate the exquisite dependence on geometric constraints of a biologically relevant translocation event in a biomolecular complex.

Essential role of the unordered VP2 n-terminal domain of the parvovirus MVM capsid in nuclear assembly and endosomal enlargement of the virion fivefold channel for cell entry

The unordered N-termini of parvovirus capsid proteins (Nt) are translocated through a channel at the icosahedral five-fold axis to serve for virus traffick. Heterologous peptides were genetically inserted at the Nt of MVM to study their functional tolerance to manipulations. Insertion of a 5T4-single-chain antibody at VP2-Nt (2Nt) yielded chimeric capsid subunits failing to enter the nucleus. The VEGFR2-binding peptide (V1) inserted at both 2Nt and VP1-Nt efficiently assembled in virions, but V1 disrupted VP1 and VP2 entry functions. The VP2 defect correlated with restricted externalization of V1-2Nt out of the coat. The specific infectivity of MVM and wtVP-pseudotyped mosaic MVM-V1 virions, upon heating and/or partial 2Nt cleavage, demonstrated that some 2Nt domains become intracellularly translocated out of the virus shell and cleaved to initiate entry. The V1 insertion defines a VP2-driven endosomal enlargement of the channel as an essential structural rearrangement performed by the MVM virion to infect.

Parvoviral host range and cell entry mechanisms

Parvoviruses elaborate rugged nonenveloped icosahedral capsids of approximately 260 A in diameter that comprise just 60 copies of a common core structural polypeptide. While serving as exceptionally durable shells, capable of protecting the single-stranded DNA genome from environmental extremes, the capsid also undergoes sequential conformational changes that allow it to translocate the genome from its initial host cell nucleus all the way into the nucleus of its subsequent host. Lacking a duplex transcription template, the virus must then wait for its host to enter S-phase before it can initiate transcription and usurp the cell's synthetic pathways. Here we review cell entry mechanisms used by parvoviruses. We explore two apparently distinct modes of host cell specificity, first that used by Minute virus of mice, where subtle glycan-specific interactions between host receptors and residues surrounding twofold symmetry axes on the virion surface mediate differentiated cell type target specificity, while the second involves novel protein interactions with the canine transferrin receptor that allow a mutant of the feline leukopenia serotype, Canine parvovirus, to bind to and infect dog cells. We then discuss conformational shifts in the virion that accompany cell entry, causing exposure of a capsid-tethered phospholipase A2 enzymatic core that acts as an endosomolytic agent to mediate virion translocation across the lipid bilayer into the cell cytoplasm. Finally, we discuss virion delivery into the nucleus, and consider the nature of transcriptionally silent DNA species that, escaping detection by the cell, might allow unhampered progress into S-phase and hence unleash the parvoviral Trojan horse.

Immunodetection of Canine Parvovirus (CPV) in clinical samples by polyclonal antisera against CPV-VP2 protein expressed in Esherichia coli as an antigen

The entire virion protein 2 (VP2) gene of Canine Parvovirus (CPV) was amplified by polymerase chain reaction (PCR) and engineered to be expressed by a bacterial expression vector pET-28a, under the control of the IPTG-inducible T7lac promoter. SDS-PAGE gel revealed that VP2 expressed as a 67kDa, and found mainly in the pellet of the bacterial lysates, suggesting that cytoplasmic expression is not preferred. The recombinant protein VP2 fused with His-tag was purified from Esherichia coli using Ni-NTA resin under denaturing conditions. SDS-PAGE analysis also showed the high expression of several lower molecular weight (LMW) bands. Western blot analysis showed that polyclonal antisera produced by rabbit against E. coli-VP2 protein reacted specifically with the purified VP2 protein as well as two other LMW bands. Some of the resulting LMW products failed to keep their antigenic site in the N-terminal region of the VP2. The degradation of recombinant VP2 protein in E. coli could be due to the action of host proteases. The immunodetection ability of the polyclonal antisera was compared with that of a commercial monoclonal antibody to test numerous clinical specimens by immuno-dot blot assays. There were distinctive differences in the degree of immunodetection ability of polyclonal antisera and monoclonal antibody to react with CPV antigens. The reaction time of polyclonal antisera was much faster in visual color appearance than that of monoclonal antibody during NBT/BCIP staining. The result from diagnostic PCR assay confirmed the presence of CPV in 44 out of 46 specimens collected, consistent with polyclonal antisera-positive result. Therefore, the polyclonal antisera can be used for CPV detection in the faeces of diarrhoeic dogs, which was found to be more rapid, sensitive, broad but less specific than the monoclonal antibody.

VP2 cleavage and the leucine ring at the base of the fivefold cylinder control pH-dependent externalization of both the VP1 N terminus and the genome of minute virus of mice

Cylindrical projections surrounding the fivefold-symmetry axes in minute virus of mice (MVM) harbor central pores that penetrate through the virion shell. In newly released DNA-containing particles, these pores contain residues 28 to 38 belonging to a single copy of VP2, disposed so that its extreme N-terminal domain projects outside the particle. Virions are metastable, initially sequestering internally the N termini of all copies of the minor capsid protein, VP1, that is essential for entry. This VP1 domain can be externalized in vitro in response to limited heating, and we show here that the efficiency of this transition is greatly enhanced by proteolysis of VP2 N termini to yield VP3. This step also renders the VP1 rearrangement pH dependent, indicating that VP2 cleavage is a maturation step required to prime subsequent emergence of the VP1 "entry" domain. The tightest constriction within the cylinder is created by VP2 leucine 172, the five symmetry-related copies of which form a portal that resembles an iris diaphragm across the base of the pore. In MVMp, threonine substitution at this position, L172T, yields infectious particles following transfection at 37 degrees C, but these can initiate infection only at 32 degrees C, and this process can be blocked by exposing virions to a cellular factor(s) at 37 degrees C during the first 8 h after entry. At 32 degrees C, the mutant particle is highly infectious, and it remains stable prior to VP2 cleavage or following cleavage at pH 5.5 or below. However, upon exposure to neutral pH following VP2 cleavage, its VP1-specific sequences and genome are extruded even at room temperature, underscoring the significance of the VP2 cleavage step for MVM particle dynamics.

Low pH-dependent endosomal processing of the incoming parvovirus minute virus of mice virion leads to externalization of the VP1 N-terminal sequence (N-VP1), N-VP2 cleavage, and uncoating of the full-length genome

Minute virus of mice (MVM) enters the host cell via receptor-mediated endocytosis. Although endosomal processing is required, its role remains uncertain. In particular, the effect of low endosomal pH on capsid configuration and nuclear delivery of the viral genome is unclear. We have followed the progression and structural transitions of DNA full-virus capsids (FC) and empty capsids (EC) containing the VP1 and VP2 structural proteins and of VP2-only virus-like particles (VLP) during the endosomal trafficking. Three capsid rearrangements were detected in FC: externalization of the VP1 N-terminal sequence (N-VP1), cleavage of the exposed VP2 N-terminal sequence (N-VP2), and uncoating of the full-length genome. All three capsid modifications occurred simultaneously, starting as early as 30 min after internalization, and all of them were blocked by raising the endosomal pH. In particles lacking viral single-stranded DNA (EC and VLP), the N-VP2 was not exposed and thus it was not cleaved. However, the EC did externalize N-VP1 with kinetics similar to those of FC. The bulk of all the incoming particles (FC, EC, and VLP) accumulated in lysosomes without signs of lysosomal membrane destabilization. Inside lysosomes, capsid degradation was not detected, although the uncoated DNA of FC was slowly degraded. Interestingly, at any time postinfection, the amount of structural proteins of the incoming virions accumulating in the nuclear fraction was negligible. These results indicate that during the early endosomal trafficking, the MVM particles are structurally modified by low-pH-dependent mechanisms. Regardless of the structural transitions and protein composition, the majority of the entering viral particles and genomes end in lysosomes, limiting the efficiency of MVM nuclear translocation.

SAT: a late NS protein of porcine parvovirus

The genomes of all members of the Parvovirus genus were found to contain a small open reading frame (ORF), designated SAT, with a start codon four or seven nucleotides downstream of the VP2 initiation codon. Green fluorescent protein or FLAG fusion constructs of SAT demonstrated that these ORFs were expressed. Although the SAT proteins of the different parvoviruses are not particularly conserved, they were all predicted to contain a membrane-spanning helix, and mutations in this hydrophobic stretch affected the localization of the SAT protein. SAT colocalized with calreticulin in the membranes of the endoplasmic reticulum and the nucleus. A knockout mutant (SAT(-)), with an unmodified VP sequence, showed a "slow-spreading" phenotype. These knockout mutants could be complemented with VP2(-) SAT(+) mutant. The SAT protein is a late nonstructural (NS) protein, in contrast to previously identified NS proteins, since it is expressed from the same mRNA as VP2.

Mutational analysis of narrow pores at the fivefold symmetry axes of adeno-associated virus type 2 capsids reveals a dual role in genome packaging and activation of phospholipase A2 activity

Adeno-associated virus type 2 (AAV2) capsids show 12 pores at the fivefold axes of symmetry. We mutated amino acids which constitute these pores to investigate possible functions of these structures within the AAV2 life cycle. Mutants with alterations in conserved residues were impaired mainly in genome packaging or infectivity, whereas few mutants were affected in capsid assembly. The packaging phenotype was characterized by increased capsid-per-genome ratios. Analysis of capsid-associated DNA versus encapsidated DNA revealed that this observation was due to reduced and not partial DNA encapsidation. Most mutants with impaired infectivity showed a decreased capability to expose their VP1 N termini. As a consequence, the activation of phospholipase A2 (PLA2) activity, which is essential for efficient infection, was affected on intact capsids. In a few mutants, the exposure of VP1 N termini and the development of PLA2 activity were associated with enhanced capsid instability, which is obviously also deleterious for virus infection. Therefore, PLA2 activity seems to be required on intact capsids for efficient infection. In conclusion, these results suggest that the pores at the fivefold axes function not only as portals for AAV2 single-stranded DNA packaging but also as channels for presentation of the PLA2 domain on AAV2 virions during infection.

Release of canine parvovirus from endocytic vesicles

Canine parvovirus (CPV) is a small nonenveloped virus with a single-stranded DNA genome. CPV enters cells by clathrin-mediated endocytosis and requires an acidic endosomal step for productive infection. Virion contains a potential nuclear localization signal as well as a phospholipase A(2) like domain in N-terminus of VP1. In this study we characterized the role of PLA(2) activity on CPV entry process. PLA(2) activity of CPV capsids was triggered in vitro by heat or acidic pH. PLA(2) inhibitors inhibited the viral proliferation suggesting that PLA(2) activity is needed for productive infection. The N-terminus of VP1 was exposed during the entry, suggesting that PLA(2) activity might have a role during endocytic entry. The presence of drugs modifying endocytosis (amiloride, bafilomycin A(1), brefeldin A, and monensin) caused viral proteins to remain in endosomal/lysosomal vesicles, even though the drugs were not able to inhibit the exposure of VP1 N-terminal end. These results indicate that the exposure of N-terminus of VP1 alone is not sufficient to allow CPV to proliferate. Some other pH-dependent changes are needed for productive infection. In addition to blocking endocytic entry, amiloride was able to block some postendocytic steps. The ability of CPV to permeabilize endosomal membranes was demonstrated by feeding cells with differently sized rhodamine-conjugated dextrans together with the CPV in the presence or in the absence of amiloride, bafilomycin A(1), brefeldin A, or monensin. Dextran with a molecular weight of 3000 was released from vesicles after 8 h of infection, while dextran with a molecular weight of 10,000 was mainly retained in vesicles. The results suggest that CPV infection does not cause disruption of endosomal vesicles. However, the permeability of endosomal membranes apparently changes during CPV infection, probably due to the PLA(2) activity of the virus. These results suggest that parvoviral PLA(2) activity is essential for productive infection and presumably utilized in membrane penetration process of the virus, but CPV also needs other pH-dependent changes or factors to be released to the cytoplasm from endocytic vesicles.

Cytoplasmic trafficking of minute virus of mice: low-pH requirement, routing to late endosomes, and proteasome interaction

The cytoplasmic trafficking of the prototype strain of minute virus of mice (MVMp) was investigated by analyzing and quantifying the effect of drugs that reduce or abolish specific cellular functions on the accumulation of viral macromolecules. With this strategy, it was found that a low endosomal pH is required for the infection, since bafilomycin A(1) and chloroquine, two pH-interfering drugs, were similarly active against MVMp. Disruption of the endosomal network by brefeldin A interfered with MVMp infection, indicating that viral particles are routed farther than the early endocytic compartment. Pulse experiments with endosome-interfering drugs showed that the bulk of MVMp particles remained in the endosomal compartment for several hours before its release to the cytosol. Drugs that block the activity of the proteasome by different mechanisms, such as MG132, lactacystin, and epoxomicin, all strongly blocked MVMp infection. Pulse experiments with the proteasome inhibitor MG132 indicated that MVMp interacts with cellular proteasomes after endosomal escape. The chymotrypsin-like but not the trypsin-like activity of the proteasome is required for the infection, since the chymotrypsin inhibitors N-tosyl-L-phenylalanine chloromethyl ketone and aclarubicin were both effective in blocking MVMp infection. However, the trypsin inhibitor Nalpha-p-tosyl-L-lysine chloromethyl ketone had no effect. These results suggest that the ubiquitin-proteasome pathway plays an essential role in the MVMp life cycle, probably assisting at the stages of capsid disassembly and/or nuclear translocation.

Caspase activation is required for permissive replication of Aleutian mink disease parvovirus in vitro

Aleutian mink disease parvovirus (ADV) is distinct among the parvoviruses as infection in vivo is persistent, restricted, and noncytopathic. In contrast, infections with other more prototypic parvoviruses, like mink enteritis virus (MEV), are acute, cytopathic, and characterized by permissive replication in vivo. Although apoptosis results in the death of cells acutely infected by parvoviruses, the role of apoptosis in ADV infections is unknown. Permissive infection of ADV resulted in apoptosis of Crandell feline kidney (CrFK) cells as indicated by TUNEL staining, Annexin-V staining, and characteristic changes in cell morphology. Pretreatment of infected cells with caspase 3 or broad-spectrum caspase inhibitors prevented apoptosis. In addition, treatment of infected cells with these inhibitors caused a 2 log(10) reduction in the yield of infectious virus compared to untreated cultures. This block in replication preceded substantial viral DNA amplification and gene expression. However, inhibitors of caspases 1, 6, and 8 did not have this effect. MEV also induced caspase-dependent apoptosis following infection of CrFK cells, although production of infectious progeny was not affected by inhibition of apoptosis. Thus, permissive replication of ADV in vitro depended upon activation of specific caspases. If ADV infection of cells in vivo fails to initiate caspase activation, the requirement of caspase activity for replication may not be met, thus providing a possible mechanism for persistent, restricted infection.

Divergent replication kinetics of two phenotypically different parvoviruses of rats

Rat virus (RV) is an important infectious agent of laboratory rats because of its high prevalence and capacity to disrupt research. Additionally, RV infection serves as a model for characterizing virus-host interactions during acute, persistent and prenatal infection. Our research has examined the pathogenesis of two RV strains, RV-UMass and RV-Y. RV-UMass is more pathogenic, causes a higher level of persistent infection and transmits to the foetus after oronasal inoculation of the pregnant dam. To determine in vitro distinctions between the strains that may account for these differences and to provide a benchmark for characterizing virus replication in vivo, synchronized in vitro replication of both RV strains was defined and compared. The results demonstrated that RV replication has replicative intermediates, virus transcripts and proteins similar to those reported for the prototype parvovirus, minute virus of mice. However, the replicative cycle of RV-UMass was 12 h compared with 24 h for RV-Y, and RV-UMass and RV-Y differed in kinetics of virus DNA replication, transcription and protein accumulation. Additionally, in situ analysis correlated well with kinetics data as determined by Southern and Northern blot analysis. Sequence comparisons between the strains also determined coding differences that may contribute to phenotypic differences.

Phosphorylation status of the parvovirus minute virus of mice particle: mapping and biological relevance of the major phosphorylation sites

The core of the VP-1 and VP-2 proteins forming the T=1 icosahedral capsid of the prototype strain of the parvovirus minute virus of mice (MVMp) share amino acids sequence and a common three-dimensional structure; however, the roles of these polypeptides in the virus infection cycle differ. To gain insights into this paradox, the nature, distribution, and biological significance of MVMp particle phosphorylation was investigated. The VP-1 and VP-2 proteins isolated from purified empty capsids and from virions containing DNA harbored phosphoserine and phosphothreonine amino acids, which in two-dimensional tryptic analysis resulted in complex patterns reproducibly composed by more than 15 unevenly phosphorylated peptides. Whereas secondary protease digestions and comigration of most weak peptides in the fingerprints revealed common phosphorylation sites in the VP-1 and VP-2 subunits assembled in capsids, the major tryptic phosphopeptides were remarkably characteristic of either polypeptide. The VP-2-specific peptide named B, containing the bulk of the (32)P label of the MVMp particle in the form of phosphoserine, was mapped to the structurally unordered N-terminal domain of this polypeptide. Mutations in any or all four serine residues present in peptide B showed that the VP-2 N-terminal domain is phosphorylated at multiple sites, even though none of them was essential for capsid assembly or virus formation. Chromatographic analysis of purified wild-type (wt) and mutant peptide B digested with a panel of specific proteases allowed us to identify the VP-2 residues Ser-2, Ser-6, and Ser-10 as the main phosphate acceptors for MVMp capsid during the natural viral infection. Phosphorylation at VP-2 N-terminal serines was not necessary for the externalization of this domain outside of the capsid shell in particles containing DNA. However, the plaque-forming capacity and plaque size of VP-2 N-terminal phosphorylation mutants were severely reduced, with the evolutionarily conserved Ser-2 determining most of the phenotypic effect. In addition, the phosphorylated amino acids were not required for infection initiation or for nuclear translocation of the expressed structural proteins, and thus a role at a late stage of MVMp life cycle is proposed. This study illustrates the complexity of posttranslational modification of icosahedral viral capsids and underscores phosphorylation as a versatile mechanism to modulate the biological functions of their protein subunits.

Assaying for structural variation in the parvovirus capsid and its role in infection

The capsid of canine parvovirus (CPV) was assayed for susceptibility to proteases and for structural variation. The natural cleavage of VP2 to VP3 in CPV full (DNA containing) particles recovered from tissue culture occurred within the sequence Arg-Asn-Glu-Arg Ala-Thr. Trypsin, chymotrypsin, bromelain, and cathepsin B all cleaved >90% of the VP2 to VP3 in full but not in empty capsids and did not digest the capsid further. Digestion with proteinase K, Pronase, papain, or subtilisin cleaved the VP2 to VP3 and also cleaved at additional internal sites, causing particle disintegration and protein degradation. Several partial digestion products produced by proteinase K or subtilisin were approximately 31-32.5 kDa, indicating cleavage within loop 3 of the capsid protein as well as other sites. Protease treatment of capsids at pH 5.5 or 7.5 did not significantly alter their susceptibility to digestion. The isoelectric point of CPV empty capsids was pH 5.3, and full capsids were 0.3 pH more acidic, but after proteolysis of VP2 to VP3, the pI of the full capsids became the same as that of the empty capsids. Antibodies against various capsid protein sequences showed the amino termini of most VP2 molecules were on the outside of full but not empty particles, that the VP1-unique sequence was internal, and that the capsid could be disintegrated by heat or urea treatment to expose the internal sequences. Capsids added to cells were localized within the cell cytoplasm in vesicles that appeared to be lysosomes. Microinjected capsids remained primarily in the cytoplasm, although a small proportion was observed to be in the nucleus after 2 h. After CPV capsids labeled with [35S]methionine were bound to cells at 0 degrees C and the cells warmed, little cleavage of VP1 or VP2 was observed even after prolonged incubation. Inoculation of cells with virus in the presence of proteinase inhibitors did not significantly reduce the infection.

Mapping the antigenic structure of porcine parvovirus at the level of peptides

The antigenic structure of the capsid proteins of porcine parvovirus (PPV) was investigated. A total of nine linear epitopes were identified by Pepscan using porcine or rabbit anti-PPV antisera. No sites were identified with a panel of neutralising monoclonal antibodies (MAbs). All epitopes were located in the region corresponding to the major capsid protein VP2. Based on this information, and on analogy to other autonomous parvoviruses, 24 different peptides were synthesised, coupled to keyhole limpet haemocyanin (KLH) and used to immunise rabbits. Most antisera were able to bind viral protein. Only peptides from the N-terminal part of VP2 were able to induce virus-neutralising antibodies, although at low levels. A similar neutralising activity could be obtained in pigs. The exposure of the N-terminus was shown in full virions, both by immunoelectron microscopy and absorption experiments. It is concluded that in PPV, the VP2 N-terminus is involved in virus neutralisation (VN) and peptides from this region are therefore primary targets for developing peptide-based vaccines against this virus.

Identification of domains in canine parvovirus VP2 essential for the assembly of virus-like particles

Canine parvovirus capsids are composed of 60 copies of VP2 and 6 to 10 copies of VPl. To locate essential sites of interaction between VP2 monomers, we have analyzed the effects of a number of VP2 deletion mutants representing the amino terminus and the four major loops of the surface, using as an assay the formation of virus-like particles (VLPs) expressed by recombinant baculoviruses. For the amino terminus we constructed three mutants with progressively larger deletions, i.e., 9, 14, and 24 amino acids. Deletions of 9 and 14 amino acids did not affect the morphology and assembly capabilities of the mutants. However, the mutant with the 24-amino-acid deletion did not show hemagglutination properties or correct VLP morphology, stressing again the relevance of the RNER domain in canine parvovirus functionality. Three of the four mutants with deletions in the loops failed to make correct VLPs, indicating that these regions are essential for correct capsid assembly and morphology. Only the mutant with the deletion in loop 2 was able to assemble in regular VLPs, suggesting that this loop has little or no effect in capsid morphogenesis. Further research has demonstrated that this region can tolerate the insertion of foreign epitopes that are correctly exposed in the surface of the capsid. This result opens the door to the use of these VLPs for antigen delivery.

Peptide vaccine against canine parvovirus: identification of two neutralization subsites in the N terminus of VP2 and optimization of the amino acid sequence

The N-terminal domain of the major capsid protein VP2 of canine parvovirus was shown to be an excellent target for development of a synthetic peptide vaccine, but detailed information about number of epitopes, optimal length, sequence choice, and site of coupling to the carrier protein was lacking. Therefore, several overlapping peptides based on this N terminus were synthesized to establish conditions for optimal and reproducible induction of neutralizing antibodies in rabbits. The specificity and neutralizing ability of the antibody response for these peptides were determined. Within the N-terminal 23 residues of VP2, two subsites able to induce neutralizing antibodies and which overlapped by only two glycine residues at positions 10 and 11 could be discriminated. The shortest sequence sufficient for neutralization induction was nine residues. Peptides longer than 13 residues consistently induced neutralization, provided that their N termini were located between positions 1 and 11 of VP2. The orientation of the peptides at the carrier protein was also of importance, being more effective when coupled through the N terminus than through the C terminus to keyhole limpet hemocyanin. The results suggest that the presence of amino acid residues 2 to 21 (and probably 3 to 17) of VP2 in a single peptide is preferable for a synthetic peptide vaccine.

The capsid determinant of fibrotropism for the MVMp strain of minute virus of mice functions via VP2 and not VP1

The minute virus of mice, prototype strain MVMp, productively infects cultured murine fibroblasts but not T cells. The immunosuppressive strain, MVMi, shows the converse tropism. These reciprocal tropisms are mediated by the viral capsids, in which their determinants have been mapped to a few specific amino acids in the primary sequence shared by VP1 and VP2. Which of these proteins is relevant in presenting these determinants during infection is not known. We have approached this question using a recombinant parvovirus system in which a LuIII-derived transducing genome, containing the luciferase reporter in place of viral coding sequences, can be packaged by capsid proteins from separate helper sources. We generated transducing virions by using helper constructs expressing either VP1 or VP2, containing the MVMp or MVMi tropic determinant region, in various combinations. The virions were used to infect human NB324K cells and murine A9 fibroblasts. Transduction of the human cells (permissive for both MVMp and MVMi) required both VP1 and VP2 and was successful with all combinations of these proteins. In contrast, significant transducing activity for A9 cells was detected only with recombinant virions containing VP2 of MVMp, while the use of either source of VP1 had little effect. We conclude that VP2 from MVMp is necessary to enable infection of murine A9 fibroblasts.

Effective induction of neutralizing antibodies with the amino terminus of VP2 of canine parvovirus as a synthetic peptide

Fourteen synthetic peptides corresponding to previously mapped antigenic sites in VP2 of canine parvovirus (CPV) were used for immunization of rabbits to identify antiviral properties favourable for inclusion into a vaccine. Most antipeptide antisera obtained were reactive with viral protein, and with one of them it was possible to locate the hypothetical amino terminus of VP3 within positions 15-31 of VP2. Virus-neutralizing antibodies were only obtained with two overlapping 15-mer peptides corresponding in sequence to the amino terminus of VP2 (MSDGAVQPDGGQPAVRNERAT). Antibodies in the neutralizing sera bound most strongly to amino acids of the sequence DGGQPAV within the N-terminus of VP2, indicating that efforts to develop a synthetic vaccine against CVP should be focused on this stretch of amino acids. The two peptides induced long-lasting immunity (at least 8 months) using either Freund's adjuvant or aluminium hydroxide plus Quil A. Thus, this approach delineated the exact peptide sequence useful for vaccines applied to the amino-terminal region of VP2. These findings in experimental animals form a solid basis for exploration of a synthetic peptide vaccine against parvovirus infection in dogs, minks or cats.

First peptide vaccine providing protection against viral infection in the target animal: studies of canine parvovirus in dogs

A synthetic peptide vaccine which protects dogs against challenge with virulent canine parvovirus is described. The amino acid sequence used was discovered in previous studies on the immunogenic properties of previously mapped antigenic sites and represents the amino-terminal region of viral protein VP2. As with marker vaccines, it is possible to discriminate between vaccinated dogs that have not been exposed to the virus and dogs that have been infected with the virus. The protective mechanism can be explained by a humoral response against the peptide aided by T-cell epitopes contained in the carrier protein used for peptide coupling. This is the first example of a synthetic peptide vaccine that induces protection in target animals.

Characterization of chimeric full-length molecular clones of Aleutian mink disease parvovirus (ADV): identification of a determinant governing replication of ADV in cell culture

The ADV-G strain of Aleutian mink disease parvovirus (ADV) is nonpathogenic for mink but replicates permissively in cell culture, whereas the ADV-Utah 1 strain is highly pathogenic for mink but replicates poorly in cell culture. In order to relate these phenotypic differences to primary genomic features, we constructed a series of chimeric plasmids between a full-length replication-competent molecular clone of ADV-G and subgenomic clones of ADV-Utah 1 representing map units (MU) 15 to 88. After transfection of the plasmids into cell culture and serial passage of cell lysates, we determined that substitution of several segments of the ADV-Utah 1 genome (MU 15 to 54 and 65 to 73) within an infectious ADV-G plasmid did not impair the ability of these constructs to yield infectious virus in vitro. Like ADV-G, the viruses derived from these replication-competent clones caused neither detectable viremia 10 days after inoculation nor any evidence of Aleutian disease in adult mink. On the other hand, other chimeric plasmids were incapable of yielding infectious virus and were therefore replication defective in vitro. The MU 54 to 65 EcoRI-EcoRV fragment of ADV-Utah 1 was the minimal segment capable of rendering ADV-G replication defective. Substitution of the ADV-G EcoRI-EcoRV fragment into a replication-defective clone restored replication competence, indicating that this 0.53-kb portion of the genome, wholly located within shared coding sequences for the capsid proteins VP1 and VP2, contained a determinant that governs replication in cell culture. When cultures of cells were studied 5 days after transfection with replication-defective clones, rescue of dimeric replicative form DNA and single-stranded progeny DNA could not be demonstrated. This defect could not be complemented by cotransfection with a replication-competent construction.