Canine and feline parvoviruses can use human or feline transferrin receptors to bind, enter, and infect cells

Canine parvovirus: current perspective

Canine parvovirus 2 (CPV-2) has been considered to be an important pathogen of domestic and wild canids and has spread worldwide since its emergence in 1978. It has been reported from Asia, Australia, New Zealand, the Americas and Europe. Two distinct parvoviruses are now known to infect dogs-the pathogenic CPV-2 and CPV-1 or the minute virus of canine (MVC). CPV-2, the causative agent of acute hemorrhagic enteritis and myocarditis in dogs, is one of the most important pathogenic viruses with high morbidity (100%) and frequent mortality up to 10% in adult dogs and 91% in pups. The disease condition has been complicated further due to emergence of a number of variants namely CPV-2a, CPV-2b and CPV-2c over the years and involvement of domestic and wild canines. There are a number of different serological and molecular tests available for prompt, specific and accurate diagnosis of the disease. Further, both live attenuated and inactivated vaccines are available to control the disease in animals. Besides, new generation vaccines namely recombinant vaccine, peptide vaccine and DNA vaccine are in different stages of development and offer hope for better management of the disease in canines. However, new generation vaccines have not been issued license to be used in the field condition. Again, the presence of maternal antibodies often interferes with the active immunization with live attenuated vaccine and there always exists a window of susceptibility in spite of following proper immunization regimen. Lastly, judicious use of the vaccines in pet dogs, stray dogs and wild canids keeping in mind the new variants of the CPV-2 along with the proper sanitation and disinfection practices must be implemented for the successful control the disease.

Mechanisms of cell death in canine parvovirus-infected cells provide intuitive insights to developing nanotools for medicine

Viruses have great potential as nanotools in medicine for gene transfer, targeted gene delivery, and oncolytic cancer virotherapy. Here we have studied cell death mechanisms of canine parvovirus (CPV) to increase the knowledge on the CPV life cycle in order to facilitate the development of better parvovirus vectors. Morphological studies of CPV-infected Norden laboratory feline kidney (NLFK) cells and canine fibroma cells (A72) displayed characteristic apoptotic events. Apoptosis was further confirmed by activation of caspases and cellular DNA damage. However, results from annexin V-propidium iodide (PI) labeling and membrane polarization assays indicated disruption of the plasma membrane uncommon to apoptosis. These results provide evidence that secondary necrosis followed apoptosis. In addition, two human cancer cell lines were found to be infected by CPV. This necrotic event over apoptotic cell death and infection in human cells provide insightful information when developing CPV as a nanotool for cancer treatments.

Leukoencephalopathy associated with parvovirus infection in Cretan hound puppies

Leukoencephalopathies in dogs encompass presumably inherited conditions such as leukodystrophies, hypomyelination or spongiform degeneration, but other causes, such as virus infections and toxic or nutritional factors, might also play a contributory role. In this report, we provide evidence of parvovirus infection and replication in the brains of five 6-week-old Cretan hound puppies suffering from a puppy shaker syndrome and leukoencephalopathy. Although these puppies belonged to two different litters, they were closely related, tracing back two generations to the same sire. Histologically, a mild to moderate lymphohistiocytic meningitis, with focal lymphohistiocytic leukoencephalitis in two animals, and a mild to moderate vacuolation with myelin loss, mainly in the white matter of the cerebellum was detected. Vacuolation was also found in the corpus callosum, fimbria hippocampi, mesencephalon, capsula interna, basal ganglia, and hypothalamus. By immunohistology and in situ hybridization, either parvoviral antigen, DNA, mRNA, or replicative intermediate DNA were detected in the cerebellum, hippocampus, periventricular areas, corpus callosum, cerebral cortex, medulla oblongata, and spinal cord. Parvovirus antigen, DNA, and mRNA were present in cells of the outer granular layer of the cerebellum and in periventricular cells, most likely representing spongioblasts, glial cells, neurons, endothelial cells, occasional macrophages, and ependymal cells. Sequencing revealed canine parvovirus type 2 stretches. Thus, an association of parvovirus infection with the leukoencephalopathy seems likely, possibly facilitated by a genetic predisposition due to the mode of inbreeding in this particular dog breed.

Canine parvovirus: current perspective

Canine parvovirus 2 (CPV-2) has been considered to be an important pathogen of domestic and wild canids and has spread worldwide since its emergence in 1978. It has been reported from Asia, Australia, New Zealand, the Americas and Europe. Two distinct parvoviruses are now known to infect dogs-the pathogenic CPV-2 and CPV-1 or the minute virus of canine (MVC). CPV-2, the causative agent of acute hemorrhagic enteritis and myocarditis in dogs, is one of the most important pathogenic viruses with high morbidity (100%) and frequent mortality up to 10% in adult dogs and 91% in pups. The disease condition has been complicated further due to emergence of a number of variants namely CPV-2a, CPV-2b and CPV-2c over the years and involvement of domestic and wild canines. There are a number of different serological and molecular tests available for prompt, specific and accurate diagnosis of the disease. Further, both live attenuated and inactivated vaccines are available to control the disease in animals. Besides, new generation vaccines namely recombinant vaccine, peptide vaccine and DNA vaccine are in different stages of development and offer hope for better management of the disease in canines. However, new generation vaccines have not been issued license to be used in the field condition. Again, the presence of maternal antibodies often interferes with the active immunization with live attenuated vaccine and there always exists a window of susceptibility in spite of following proper immunization regimen. Lastly, judicious use of the vaccines in pet dogs, stray dogs and wild canids keeping in mind the new variants of the CPV-2 along with the proper sanitation and disinfection practices must be implemented for the successful control the disease.

Multiple pathways involved in porcine parvovirus cellular entry and trafficking toward the nucleus

Porcine parvovirus (PPV) is a major cause of reproductive failure in swine. The mechanisms implicated in the first steps of infection that lead to the delivery of the PPV genome to the nucleus are poorly understood. In the present work, a panel of chemical inhibitors was used to dissect the cellular mechanisms involved in establishing a PPV infection. The results demonstrated that following binding to sialic acids on cell surface glycoproteins, the virus used both clathrin-mediated endocytosis and macropinocytosis pathways to gain access into cells. Virus obtained from infected cells was present either as isolated particles or as aggregates, and these two forms could be separated by low-speed centrifugation. Isolated and purified particles strongly preferred entry by clathrin-mediated endocytosis, whereas aggregates clearly favored macropinocytosis. Subsequent endosomal acidification and traffic to the late endosomes were also shown to be essential for infection. The microtubule network was found to be important during the first 10 h of infection, whereas an intact actin network was required for almost the whole viral cycle. Proteasome processing was found to be essential, and capsid proteins were ubiquitinated relatively early during infection. Taken together, these results provided new insights into the first steps of PPV infection, including the use of alternative entry pathways, unique among members of this viral family.

Translation control by protein kinase R restricts minute virus of mice infection: role in parvovirus oncolysis

The relevance of translational control in the gene expression and oncotropism of the autonomous parvoviruses was investigated with MVMp, the prototype strain of minute virus of mice (MVM), infecting normal and transformed rodent and human cells of different tissue origins. Mouse embryo fibroblasts (MEFs) and NIH 3T3 fibroblasts were resistant to MVMp infection, but 3T3 fibroblasts derived from double-stranded RNA (dsRNA)-dependent protein kinase R (PKR) knockout mice (PKR(o/o)) behaved in a manner that was highly permissive to productive MVMp replication. NIH 3T3 resistance correlated with significant phosphorylation of eukaryotic translation initiation factor 2 (eIF2) occurring at early time points after infection. Permissive PKR(o/o) cells were converted to MVMp-restrictive cells after reintroduction of the PKR gene by transfection. Conversely, regulated expression of the vaccinia virus E3 protein, a PKR inhibitor, in MEFs prevented eIF2alpha phosphorylation and increased MVMp protein synthesis. In vitro-synthesized genome-length R1 mRNA of MVMp was a potent activator of PKR. Virus-resistant primary MEFs and NIH 3T3 cells responded to MVMp infection with significant increases in eIF2alpha phosphorylation. In contrast, virus-permissive mouse (PKR(o/o), BHK21, and A9) and human transformed (NB324K fibroblast, U373 glioma, and HepG2 hepatoma) cells consistently showed no significant increase in the level of eIF2alpha phosphorylation following MVMp infection. The synthesis of the viral NS1 protein was inversely correlated with the steady-state PKR levels. Our results show that the PKR-mediated antiviral response is an important mechanism for control of productive MVMp infection, and its impairment in human transformed cells allowed efficient MVMp gene expression. PKR translational control may therefore contribute to the oncolysis of MVMp and other autonomous parvoviruses.

Binding site on the transferrin receptor for the parvovirus capsid and effects of altered affinity on cell uptake and infection

Canine parvovirus (CPV) and its relative feline panleukopenia virus (FPV) bind the transferrin receptor type 1 (TfR) to infect their host cells but show differences in the interactions with the feline and canine TfRs that determine viral host range and tissue tropism. We changed apical and protease-like domain residues by introducing point mutations and adding or removing glycosylation signals, and we then examined the interactions of those mutant TfRs with the capsids. Most substitutions had little effect on virus binding and uptake. However, mutations of several sites in the apical domain of the receptor either prevented binding to the capsids or reduced the affinity of receptor binding to various degrees. Glycans within the virus binding face of the apical domain also controlled capsid binding. CPV, but not the related feline parvovirus, could use receptors containing a canine TfR-specific glycosylation to mediate efficient infection, while addition of other N-linked glycosylation sites into the virus binding face of the feline apical domain reduced or eliminated both binding and infection. Replacement of critical feline TfR residue 221 with every amino acid had effects on binding and infection which were significantly associated with the biochemical properties of the residue replaced. Receptors with reduced affinities mostly showed proportional changes in their ability to mediate infection. Testing feline TfR variants for their binding and uptake patterns in cells showed that low-affinity versions bound fewer capsids and also differed in attachment to the cell surface and filopodia, but transport to the perinuclear endosome was similar.

Structures and functions of parvovirus capsids and the process of cell infection

To infect a cell, the parvovirus or adeno-associated virus (AAV) genome must be delivered from outside the plasma membrane to the nucleus, and in the process, the capsid must follow a series of binding and trafficking steps and also undergo necessary changes that result in exposure or release the ssDNA genome at the appropriate time and place within the cell. The 25 nm parvovirus capsid is comprised of two or three forms of a single protein, and although it is robust and stable, it is still sufficiently flexible to allow the exposure of several internal components at appropriate times during cell infection. The capsid can also accommodate insertion of peptides into surface loops, and capsid proteins from different viral serotypes can be shuffled to create novel functional variants. The capsids of the different viruses bind to one or more cell receptors, and for at least some viruses, the insertion of additional or alternative receptor binding sequences or structures into the capsid can expand or redirect its tropism. The infection process after cell binding involves receptor-mediated endocytosis followed by viral trafficking through the endosomal systems. That endosomal trafficking may be complex and prolonged for hours or be relatively brief. Generally only a small proportion of the particles taken up enter the cytoplasm after altering the endosomal membrane through the activity of a VP1-encoded phospholipase A2 domain that becomes released to the outside of the viral particle. Modifications to the capsid that can occur within the endosome or cytoplasm include structural changes to expose internal components, ubiquination and proteosomal processing, and possible trafficking of particles on molecular motors. It is still not clear how the genomes enter the nucleus, but nuclear pore-dependent entry of particles or permeabilization of nuclear membranes have been proposed. Those processes control the infection, pathogenesis, and host ranges of the autonomous viruses and determine the effectiveness of gene therapy using AAV capsids.

Early steps in cell infection by parvoviruses: host-specific differences in cell receptor binding but similar endosomal trafficking

Canine parvovirus (CPV) and feline panleukopenia virus (FPV) are closely related parvoviruses that differ in their host ranges for cats and dogs. Both viruses bind their host transferrin receptor (TfR), enter cells by clathrin-mediated endocytosis, and traffic with that receptor through endosomal pathways. Infection by these viruses appears to be inefficient and slow, with low numbers of virions infecting the cell after a number of hours. Species-specific binding to TfR controls viral host range, and in this study FPV and strains of CPV differed in the levels of cell attachment, uptake, and infection in canine and feline cells. During infection, CPV particles initially bound and trafficked passively on the filopodia of canine cells while they bound to the cell body of feline cells. That binding was associated with the TfR as it was disrupted by anti-TfR antibodies. Capsids were taken up from the cell surface with different kinetics in canine and feline cells but, unlike transferrin, most did not recycle. Capsids labeled with fluorescent markers were seen in Rab5-, Rab7-, or Rab11-positive endosomal compartments within minutes of uptake, but reached the nucleus. Constitutively active or dominant negative Rab mutants changed the intracellular distribution of capsids and affected the infectivity of virus in cells.

Parvovirus induced alterations in nuclear architecture and dynamics

The nucleus of interphase eukaryotic cell is a highly compartmentalized structure containing the three-dimensional network of chromatin and numerous proteinaceous subcompartments. DNA viruses induce profound changes in the intranuclear structures of their host cells. We are applying a combination of confocal imaging including photobleaching microscopy and computational methods to analyze the modifications of nuclear architecture and dynamics in parvovirus infected cells. Upon canine parvovirus infection, expansion of the viral replication compartment is accompanied by chromatin marginalization to the vicinity of the nuclear membrane. Dextran microinjection and fluorescence recovery after photobleaching (FRAP) studies revealed the homogeneity of this compartment. Markedly, in spite of increase in viral DNA content of the nucleus, a significant increase in the protein mobility was observed in infected compared to non-infected cells. Moreover, analyzis of the dynamics of photoactivable capsid protein demonstrated rapid intranuclear dynamics of viral capsids. Finally, quantitative FRAP and cellular modelling were used to determine the duration of viral genome replication. Altogether, our findings indicate that parvoviruses modify the nuclear structure and dynamics extensively. Intranuclear crowding of viral components leads to enlargement of the interchromosomal domain and to chromatin marginalization via depletion attraction. In conclusion, parvoviruses provide a useful model system for understanding the mechanisms of virus-induced intranuclear modifications.

Recombination between vaccine and field strains of canine parvovirus is revealed by isolation of virus in canine and feline cell cultures

Canine parvovirus type 2 (CPV) is a pathogen that causes severe hemorrhagic gastroenteritis with a high fatality rate in pups worldwide. Since CPV emerged in the late 1970s, its origin has been explored with the conclusion that CPV originated from feline panleukopenia virus or a closely related virus. Both high mutation rate and recombination are assumed to be key factors in the evolution of parvoviruses. Here we provide evidence for natural recombination in CPV isolated from dogs in cell culture. Antigenic and genetic properties of isolates from 10 diseased pups were elucidated. Six pups had been vaccinated beforehand with live combined vaccine containing original antigenic type CPV (CPV-2). Six isolates recovered from 4 vaccinated pups in cell cultures were found to contain either CPV-2 or CPV-2-like viruses. The other isolates, including all those from non-vaccinated pups, were CPV-2b viruses. Antigenic typing of two CPV-2-like isolates, 03-029/M and 1887/f, with a monoclonal antibody panel suggested they were a mixture of CPV-2 and CPV-2a (03-029/M) and a recombinant of CPV-2 and CPV-2b (1887/f). Genetic analysis of the VP1 gene indicated that isolate 03-029/M was a mixture of CPV-2, CPV-2a and a recombinant of CPV-2 and CPV-2a viruses, while isolate 1887/f was composed of a recombinant of CPV-2 and CPV-2b viruses. This is the first demonstration of natural CPV recombination in the field and suggests that recombination in the evolution of CPV is a more frequent and important process than previously believed.

Tumor targeting using canine parvovirus nanoparticles

Advances in genetics, proteomics and cellular and molecular biology are being integrated and translated to develop effective methods for the prevention and control of cancer. One such combined effort is to create multifunctional nanodevices that will specifically recognize tumors and thus enable early diagnosis and provide targeted treatment of this disease. Viral particles are being considered for this purpose since they are inherently nanostructures with well-defined geometry and uniformity, ideal for displaying molecules in a precise spatial distribution at the nanoscale level and subject to greater structural control. Viruses are presumably the most efficient nanocontainer for cellular delivery as they have naturally evolved mechanisms for binding to and entering cells. Virus-based systems typically require genetic or chemical modification of their surfaces to achieve tumor-specific interactions. Interestingly, canine parvovirus (CPV) has a natural affinity for transferrin receptors (TfRs) (both of canine and human origin) and this property could be harnessed as TfRs are overexpressed by a variety of human tumor cells. Since TfR recognition relies on the CPV capsid protein, we envisioned the use of virus or its shells as tumor targeting agents. We observed that derivatization of CPV virus-like particles (VLPs) with dye molecules did not impair particle binding to TfRs or internalization into human tumor cells. Thus CPV-based VLPs with a natural tropism for TfRs hold great promise in the development of novel nanomaterial for delivery of a therapeutic and/or genetic cargo.

Characterization of a continuous feline mammary epithelial cell line susceptible to feline epitheliotropic viruses

Mucosal epithelial cells are the primary targets for many common viral pathogens of cats. Viral infection of epithelia can damage or disrupt the epithelial barrier that protects underlying tissues. In vitro cell culture systems are an effective means to study how viruses infect and disrupt epithelial barriers, however no true continuous or immortalized feline epithelial cell culture lines are available. A continuous cell culture of feline mammary epithelial cells (FMEC UCD-04-2) that forms tight junctions with high transepithelial electrical resistance (>2000 Ω cm⁻¹) 3–4 days after reaching confluence was characterized. In addition, it was shown that FMECs are susceptible to infection with feline calicivirus (FCV), feline herpesvirus (FHV-1), feline coronavirus (FeCoV), and feline panleukopenia virus (FPV). These cells will be useful for studies of feline viral disease and for in vitro studies of feline epithelia.

Late steps of parvoviral infection induce changes in cell morphology

Previously, virus-induced non-filopodial extensions have not been encountered in connection with viral infections. Here, we report emergence of long extensions protruding from Norden laboratory feline kidney (NLFK) and A72 (canine fibroma) cells infected with canine parvovirus for 72 h. These extensions significantly differ in length and number from those appearing in control cells. The most striking feature in the extensions is the length, reaching up to 130 microm, almost twice the average length of a healthy NLFK cell. In A72 cells, the extensions were even longer, up to 200 microm. The results presented here also suggest that the events leading to the growth of these extensions start earlier in infection and abnormal extension growth is detectable already at 24-h post-infection (p.i.). These extensions may have a vital role in the cell-to-cell transmission of the virus.

Detecting small changes and additional peptides in the canine parvovirus capsid structure

Parvovirus capsids are assembled from multiple forms of a single protein and are quite stable structurally. However, in order to infect cells, conformational plasticity of the capsid is required and this likely involves the exposure of structures that are buried within the structural models. The presence of functional asymmetry in the otherwise icosahedral capsid has also been proposed. Here we examined the protein composition of canine parvovirus capsids and evaluated their structural variation and permeability by protease sensitivity, spectrofluorometry, and negative staining electron microscopy. Additional protein forms identified included an apparent smaller variant of the virus protein 1 (VP1) and a small proportion of a cleaved form of VP2. Only a small percentage of the proteins in intact capsids were cleaved by any of the proteases tested. The capsid susceptibility to proteolysis varied with temperature but new cleavages were not revealed. No global change in the capsid structure was observed by analysis of Trp fluorescence when capsids were heated between 40 degrees C and 60 degrees C. However, increased polarity of empty capsids was indicated by bis-ANS binding, something not seen for DNA-containing capsids. Removal of calcium with EGTA or exposure to pHs as low as 5.0 had little effect on the structure, but at pH 4.0 changes were revealed by proteinase K digestion. Exposure of viral DNA to the external environment started above 50 degrees C. Some negative stains showed increased permeability of empty capsids at higher temperatures, but no effects were seen after EGTA treatment.

Viral infection and iron metabolism

Fundamental cellular operations, including DNA synthesis and the generation of ATP, require iron. Viruses hijack cells in order to replicate, and efficient replication needs an iron-replete host. Some viruses selectively infect iron-acquiring cells by binding to transferrin receptor 1 during cell entry. Other viruses alter the expression of proteins involved in iron homeostasis, such as HFE and hepcidin. In HIV-1 and hepatitis C virus infections, iron overload is associated with poor prognosis and could be partly caused by the viruses themselves. Understanding how iron metabolism and viral infection interact might suggest new methods to control disease.

Naturally occurring parvovirus-associated feline hypogranular cerebellar hypoplasia-- A comparison to experimentally-induced lesions using immunohistology

Three cases of feline cerebellar hypoplasia are presented. At the time of examination, the ages of the cats ranged from 2 months to 1 year. Necropsy revealed cerebellar and pons hypoplasia. Polymerase chain reaction for parvoviral deoxyribonucleic acid was positive in cerebellar tissue. Cell-specific immunolabeling was used to characterize the lesions, which were characterized into 2 types. In type 1 lesions, the cortex was nearly agranular, with an extremely thin molecular layer; the Purkinje cells were randomly placed and oriented, and their stunted main dendrite produced a thorn-covered atrophic dendritic tree; the basket cell axons ran randomly and had dysmorphic endings; and myelinated fibers were severely reduced in folia axes. In type 2 lesions, the cortex was hypogranular; the Purkinje cells were linearly organized, but their main dendrite extended too far in the molecular layer before giving up smooth, bent secondary dendrites; many basket cells were located along the cerebellar surface, and their axons ran at right angle to the surface; myelinated fibers were moderately reduced. Defects in climbing fiber synapse translocation and elimination were evident in both types of lesion. This immunohistologic study allowed a comparison between lesions in these spontaneous cerebellar hypoplasia cases with those documented when using silver impregnation studies after perinatal experimental cerebellar damage. Such a comparison is consistent with viral infection that occurs before birth in all 3 cases. Progress in parvovirus biology knowledge suggests that viral NS1 protein cytotoxicity might explain degenerative changes in the Purkinje cells that were present, in addition to the development defect.

Evaluation of the antigenic relationships among canine parvovirus type 2 variants

The antigenic relationships among the original canine parvovirus type 2 (CPV-2) and the variants CPV-2a, -2b, and -2c were evaluated. Cross-antigenic evaluation revealed clear differences among the CPV variants, which were more appreciable by serum neutralization (SN) than by hemagglutination inhibition. Antigenic differences were found mostly between the original CPV-2 and the variants, but they were also observed among the variants CPV-2a, -2b, and -2c. The variant CPV-2c exhibited a unique antigenic pattern, since it was poorly recognized by the sera of animals immunized with CPV-2, CPV-2a, and CPV-2b. However, animals immunized with CPV-2c exhibited higher SN titers to CPV-2b than to the homologous virus CPV-2c. The observed antigenic differences might drive selection of CPV strains by generating differential immune pressure in the canine population, which raises concerns about vaccine efficacy.

Cell entry by human pathogenic arenaviruses

The arenaviruses Lassa virus (LASV) in Africa and Machupo (MACV), Guanarito (GTOV) and Junin viruses (JUNV) in South America cause severe haemorrhagic fevers in humans with fatality rates of 15-35%. The present review focuses on the first steps of infection with human pathogenic arenaviruses, the interaction with their cellular receptor molecules and subsequent entry into the host cell. While similarities exist in genomic organization, structure and clinical disease caused by pathogenic Old World and New World arenaviruses these pathogens use different primary receptors. The Old World arenaviruses employ alpha-dystroglycan, a cellular receptor for proteins of the extracellular matrix, and the human pathogenic New World arenaviruses use the cellular cargo receptor transferrin receptor 1. While the New World arenavirus JUNV enters cells via clathrin-dependent endocytosis, evidence occurred for clathrin-independent entry of the prototypic Old World arenavirus lymphocytic choriomeningitis virus. Upon internalization, arenaviruses are delivered to the endosome, where pH-dependent membrane fusion is mediated by the envelope glycoprotein (GP). While arenavirus GPs share characteristics with class I fusion GPs of other enveloped viruses, unusual mechanistic features of GP-mediated membrane fusion have recently been discovered for arenaviruses with important implications for viral entry.

Penetration of nonenveloped viruses into the cytoplasm

Although the precise mechanism by which nonenveloped viruses penetrate biological membranes is unclear, a more coherent understanding of this process is starting to emerge. To initiate membrane penetration, nonenveloped viruses engage host cell factors that impart conformational changes on the viral particles, resulting in the exposure of a hydrophobic moiety or the release of a lytic factor. The viruses' interactions with the limiting membrane subsequently compromise the bilayer integrity. This reaction presumably perforates the bilayer to enable the virus to cross the membrane and reach the cytosol. Valuable insights into this process can be gleaned from the membrane transport mechanisms of enveloped viruses and bacterial toxins. To identify systematically the cellular components that facilitate nonenveloped virus membrane penetration, sensitive assays that monitor the transport event directly must first be established. Moreover, higher-resolution structures of penetration intermediates, particularly those solved in complex with membranes, would provide important molecular details into this process.

New world clade B arenaviruses can use transferrin receptor 1 (TfR1)-dependent and -independent entry pathways, and glycoproteins from human pathogenic strains are associated with the use of TfR1

Arenaviruses are rodent-borne viruses, with five members of the family capable of causing severe hemorrhagic fevers if transmitted to humans. To date, two distinct cellular receptors have been identified that are used by different pathogenic viruses, alpha-dystroglycan by Lassa fever virus and transferrin receptor 1 (TfR1) by certain New World clade B viruses. Our previous studies have suggested that other, as-yet-unknown receptors are involved in arenavirus entry. In the present study, we examined the use of TfR1 by the glycoproteins (GPs) from a panel of New World clade B arenaviruses comprising three pathogenic and two nonpathogenic strains. Interestingly, we found that TfR1 was only used by the GPs from the pathogenic viruses, with entry of the nonpathogenic strains being TfR1 independent. The pathogenic GPs could also direct entry into cells by TfR1-independent pathways, albeit less efficiently. A comparison of the abilities of TfR1 orthologs from different species to support arenavirus entry found that the human and feline receptors were able to enhance entry of the pathogenic strains, but that neither the murine or canine forms were functional. Since the ability to use TfR1 is a characteristic feature of the human pathogens, this interaction may represent an important target in the treatment of New World hemorrhagic fevers. In addition, the ability to use TfR1 may be a useful tool to predict the likelihood that any existing or newly discovered viruses in this family could infect humans.

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.

Detection of infectious canine parvovirus type 2 by mRNA real-time RT-PCR

A TaqMan real-time RT-PCR assay was developed for detection of RNA transcripts produced by replicating CPV-2. A pair of primers and a TaqMan probe targeting the spliced NS2 mRNA were designed. A synthetic DNA fragment was constructed to mimic the spliced NS2 mRNA by PCR-based gene assembly and was used for generation of standard RNAs. The detection limit of the assay was 1 × 10² RNA copies and standard curve displayed a linear range from 1 × 10² to 1 × 10⁹ copies and a good reproducibility. The assay was then applied to determine the mRNA loads in the tissues of dogs naturally infected by CPV-2. mRNA was detected in a variety of tissues, including the central nervous system.

Dynamics and interactions of parvoviral NS1 protein in the nucleus

Nuclear positioning and dynamic interactions of viral proteins with nuclear substructures play essential roles during infection with DNA viruses. Visualization of the intranuclear interactions and motility of the parvovirus replication protein (NS1) in living cells gives insight into specific parvovirus protein-cellular structure interactions. Confocal analysis of highly synchronized infected Norden Laboratory Feline Kidney cells showed accumulation of nuclear NS1 in discrete interchromosomal foci. NS1 fused with enhanced yellow fluorescence protein (NS1-EYFP) provided a marker in live cells for dynamics of NS1 traced by photobleaching techniques. Fluorescence Recovery after Photobleaching suggested that the NS1 protein is not freely diffusing but undergoes transient interactions with nuclear compartments. Fluorescence Loss in Photobleaching demonstrated for the first time the shuttling of a parvoviral protein between the nucleus and the cytoplasm as assayed with NS1-EYFP. Finally, time-lapse imaging of infected cells revealed that the intranuclear distribution of NS1-EYFP evolves dramatically starting from the formation of NS1 foci and proceeding to a homogenous distribution extending throughout the nucleus.

Human bocavirus: a new respiratory pathogen?

A new virus was recently discovered by molecular techniques in respiratory samples collected from young children with respiratory disease. This virus is a new member of the Parvoviridae family and has been classified in the Bocavirus genus. This virus, named human bocavirus, is genetically related to the bovine parvovirus and the canine minute virus, which both belong to the Bocavirus genus. Recent studies conducted in different countries have shown that this virus is found in 3–18% of children with respiratory disease worldwide, and a seasonal distribution with a peak in winter and spring is suspected. Genetic analysis has indicated that this virus has low genetic variability. The clinical signs observed in patients infected with human bocavirus are not different from those caused by other respiratory viruses. Given the frequent association of human bocavirus with other respiratory pathogens, the exact role played by this virus in human diseases still remains unclear, and further studies including control subjects are needed to ascertain the pathogenic potential of this virus.

Transferrin receptor 1 is a cellular receptor for New World haemorrhagic fever arenaviruses

The transferrin receptor 1 (TfR1) has been identified as the cellular receptor for four New World arenaviruses — the Junin, Machupo, Guanarito and Sabia viruses. This class of arenaviruses is important because they cause fatal haemorrhagic fevers. Treating cultured cells with an antibody against TfR1 blocks viral entry and replication. Antibodies that limit arenavirus replication without interfering with host iron metabolism may be effective in controlling outbreaks of New World haemorrhagic fever.

At least five arenaviruses cause viral haemorrhagic fevers in humans. Lassa virus, an Old World arenavirus, uses the cellular receptor α-dystroglycan to infect cells¹. Machupo, Guanarito, Junin and Sabia viruses are New World haemorrhagic fever viruses that do not use α-dystroglycan². Here we show a specific, high-affinity association between transferrin receptor 1 (TfR1) and the entry glycoprotein (GP) of Machupo virus. Expression of human TfR1, but not human transferrin receptor 2, in hamster cell lines markedly enhanced the infection of viruses pseudotyped with the GP of Machupo, Guanarito and Junin viruses, but not with those of Lassa or lymphocytic choriomeningitis viruses. An anti-TfR1 antibody efficiently inhibited the replication of Machupo, Guanarito, Junin and Sabia viruses, but not that of Lassa virus. Iron depletion of culture medium enhanced, and iron supplementation decreased, the efficiency of infection by Junin and Machupo but not Lassa pseudoviruses. These data indicate that TfR1 is a cellular receptor for New World haemorrhagic fever arenaviruses.

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.

Different mechanisms of antibody-mediated neutralization of parvoviruses revealed using the Fab fragments of monoclonal antibodies

Antibody binding and neutralization are major host defenses against viruses, yet the mechanisms are often not well understood. Eight monoclonal antibodies and their Fab fragments were tested for neutralization of canine parvovirus and feline panleukopenia virus. All IgGs neutralized >85% of virus infectivity. Two Fabs neutralized when present at 5 nM, while the others gave little or no neutralization even at 20-100 nM. The antibodies bind two antigenic sites on the capsids which overlap the binding site of the host transferrin receptor (TfR). There was no specific correlation between Fab binding affinity and neutralization. All Fabs reduced capsid binding of virus to purified feline TfR in vitro, but the highly neutralizing Fabs were more efficient competitors. All partially prevented binding and uptake of capsids by feline TfR on cells. The virus appears adapted to allow some infectivity in the presence of at least low levels of antibodies.

Molecular Evolution of the Transferrin Receptor/Glutamate Carboxypeptidase II Family

The transferrin receptor family is represented by at least seven different homologous proteins in primates. Transferrin receptor (TfR1) is a type II membrane glycoprotein that, as a cell surface homodimer, binds iron-loaded transferrin as part of the process of iron transfer and uptake. Other family members include transferrin receptor 2 (TfR2), glutamate carboxypeptidase II (GCP2 or PSMA), N-acetylated alpha-linked acidic dipeptidase-like protein (NLDL), N-acetylated alpha-linked acidic dipeptidase 2 (NAALAD2), and prostate-specific membrane antigen-like protein (PMSAL/GCPIII). We compared 86 different sequences from 24 different species, from mammals to fungi. Through this comparison, we have identified several highly conserved residues specific to each family not previously associated with clinical mutations. The evolutionary history of the TfR/GCP2 family shows repeated episodes of duplications consistent with recent theories that nondispensable, slowly evolving genes are more likely to form multiple gene families.

Isolation and characterization of the potential receptor for wheat germ agglutinin from human neutrophils

Neutrophils participate in host protection and central to this process is the regulation of oxidative mechanisms. We purified by affinity chromatography the receptor for the GlcNAc-specific WGA from CD14+ CD16+ cell lysates (WGAr). The receptor is a 141 kDa glycoprotein constituted by two subunits of 78 and 63 kDa. It is mainly composed of Ser, Asx, and Gly, and, in a minor proportion, His, Cys, and Pro. Its glycan portion contains GlcNAc, Gal, and Man; NeuAc and GalNAc were identified in a minor proportion. The amino acid sequence of the WGA receptor was predicted from tryptic peptides by MALDI-TOF, both subunits showed homology with cytokeratin type II (26 and 29% for the 78 and 63 kDa subunits, respectively); the 78 kDa subunit showed also homology with the human transferrin receptor (24%). Antibodies against WGAr induce higher oxidative burst than WGA, determined by NBT reduction; however, this effect was inhibited (p < 0.05) with GlcNAc suggesting that WGAr participates as mediator in signal transduction in neutrophils.

Evolution of CPV-2 and implication for antigenic/genetic characterization

A few amino acid differences in the viral protein VP2 account for important antigenic and biological changes among feline parvovirus (FPV), canine parvovirus (CPV-2) and CPV-2 variants 2a and 2b. Several pieces of evidence suggest that CPV-2 is still evolving as additional amino acid changes occurred within the main antigenic regions of CPV-2 capsid, altering the antigenic profile of the virus and stressing the need for implementing the diagnostic assays.

Purified feline and canine transferrin receptors reveal complex interactions with the capsids of canine and feline parvoviruses that correspond to their host ranges

The cell infection processes and host ranges of canine parvovirus (CPV) and feline panleukopenia virus (FPV) are controlled by their capsid interactions with the transferrin receptors (TfR) on their host cells. Here, we expressed the ectodomains of wild-type and mutant TfR and tested those for binding to purified viral capsids and showed that different naturally variant strains of the viruses were associated with variant interactions with the receptors which likely reflect the optimization of the viral infection processes in the different hosts. While all viruses bound the feline TfR, reflecting their tissue culture host ranges, a naturally variant mutant of CPV (represented by the CPV type-2b strain) that became the dominant virus worldwide in 1979 showed significantly lower levels of binding to the feline TfR. The canine TfR ectodomain did not bind to a detectable level in the in vitro assays, but this appears to reflect the naturally low affinity of that interaction, as only low levels of binding were seen when the receptor was expressed on mammalian cells; however, that was sufficient to allow endocytosis and infection. The apical domain of the canine TfR controls the specific interaction with CPV capsids, as a canine TfR mutant altering a glycosylation site in that domain bound FPV, CPV-2, and CPV-2b capsids efficiently. Enzymatic removal of the N-linked glycans did not allow FPV binding to the canine TfR, suggesting that the protein sequence difference is itself important. The purified feline TfR inhibited FPV and CPV-2 binding and infection of feline cells but not CPV-2b, indicating that the receptor binding may be able to prevent the attachment to the same receptor on cells.

Evolution of canine parvovirus--a need for new vaccines?

Canine parvovirus (CPV) is a new virus, which is continuing to evolve, giving rise to new antigenic types and virus mutants that spread through the dog population. The most successful mutants, from an evolutionary perspective, appear to be selected by improved binding to the CPV receptor, the canine transferrin receptor, and by an extended host range, which for the newer antigenic types now includes both the dog and the cat. The new viruses also show antigenic differences that can be defined by binding of certain monoclonal antibodies; they also differ in their reactivity in virus neutralisation tests, using immune sera raised against the various antigenic types. These differences may influence the susceptibility of young animals to infection at the time when the level of maternally derived antibody decreases to the minimum protective titre. This minimum protective titre may vary depending on the infecting virus type. There is, however, a high degree of cross-protection between the virus types and the true relevance of the differences in neutralisation titer is currently not known.

Canine parvovirus-like particles, a novel nanomaterial for tumor targeting

Specific targeting of tumor cells is an important goal for the design of nanotherapeutics for the treatment of cancer. Recently, viruses have been explored as nano-containers for specific targeting applications, however these systems typically require modification of the virus surface using chemical or genetic means to achieve tumor-specific delivery. Interestingly, there exists a subset of viruses with natural affinity for receptors on tumor cells that could be exploited for nanotechnology applications. For example, the canine parvovirus (CPV) utilizes transferrin receptors (TfRs) for binding and cell entry into canine as well as human cells. TfRs are over-expressed by a variety of tumor cells and are widely being investigated for tumor-targeted drug delivery. We explored whether the natural tropism of CPV to TfRs could be harnessed for targeting tumor cells. Towards this goal, CPV virus-like particles (VLPs) produced by expression of the CPV-VP2 capsid protein in a baculovirus expression system were examined for attachment of small molecules and delivery to tumor cells. Structural modeling suggested that six lysines per VP2 subunit are presumably addressable for bioconjugation on the CPV capsid exterior. Between 45 and 100 of the possible 360 lysines/particle could be routinely derivatized with dye molecules depending on the conjugation conditions. Dye conjugation also demonstrated that the CPV-VLPs could withstand conditions for chemical modification on lysines. Attachment of fluorescent dyes neither impaired binding to the TfRs nor affected internalization of the 26 nm-sized VLPs into several human tumor cell lines. CPV-VLPs therefore exhibit highly favorable characteristics for development as a novel nanomaterial for tumor targeting.

Host-selected amino acid changes at the sialic acid binding pocket of the parvovirus capsid modulate cell binding affinity and determine virulence

The role of receptor recognition in the emergence of virulent viruses was investigated in the infection of severe combined immunodeficient (SCID) mice by the apathogenic prototype strain of the parvovirus minute virus of mice (MVMp). Genetic analysis of isolated MVMp viral clones (n = 48) emerging in mice, including lethal variants, showed only one of three single changes (V325M, I362S, or K368R) in the common sequence of the two capsid proteins. As was found for the parental isolates, the constructed recombinant viruses harboring the I362S or the K368R single substitutions in the capsid sequence, or mutations at both sites, showed a large-plaque phenotype and lower avidity than the wild type for cells in the cytotoxic interaction with two permissive fibroblast cell lines in vitro and caused a lethal disease in SCID mice when inoculated by the natural oronasal route. Significantly, the productive adsorption of MVMp variants carrying any of the three mutations selected through parallel evolution in mice showed higher sensitivity to the treatment of cells by neuraminidase than that of the wild type, indicating a lower affinity of the viral particle for the sialic acid component of the receptor. Consistent with this, the X-ray crystal structure of the MVMp capsids soaked with sialic acid (N-acetyl neuraminic acid) showed the sugar allocated in the depression at the twofold axis of symmetry (termed the dimple), immediately adjacent to residues I362 and K368, which are located on the wall of the dimple, and approximately 22 A away from V325 in a threefold-related monomer. This is the first reported crystal structure identifying an infectious receptor attachment site on a parvovirus capsid. We conclude that the affinity of the interactions of sialic-acid-containing receptors with residues at or surrounding the dimple can evolutionarily regulate parvovirus pathogenicity and adaptation to new hosts.

Parvovirus variation for disease: a difference with RNA viruses?

The Parvoviridae, a family of viruses with single-stranded DNA genomes widely spread from invertebrates to mammal and human hosts, display a remarkable evolutionary capacity uncommon in DNA genomes. Parvovirus populations show high genetic heterogeneity and large population sizes resembling the quasispecies found in RNA viruses. These viruses multiply in proliferating cells, causing acute, persistent or latent infections relying in the immunocompetence and developmental stage of the hosts. Some parvovirus populations in natural settings, such as carnivore autonomous parvoviruses or primate adeno associated virus, show a high degree of genetic heterogeneity. However, other parvoviruses such as the pathogenic B19 human erythrovirus or the porcine parvovirus, show little genetic variation, indicating different virus-host relationships. The Parvoviridae evolutionary potential in mammal infections has been modeled in the experimental system formed by the immunodeficient scid mouse infected by the minute virus of mice (MVM) under distinct immune and adaptive pressures. The sequence of viral genomes (close to 10(5) nucleotides) in emerging MVM pathogenic populations present in the organs of 26 mice showed consensus sequences not representing the complex distribution of viral clones and a high genetic heterogeneity (average mutation frequency 8.3 x 10(-4) substitutions/nt accumulated over 2-3 months). Specific amino acid changes, selected at a rate up to 1% in the capsid and in the NS2 nonstructural protein, endowed these viruses with new tropism and increased fitness. Further molecular analysis supported the notion that, in addition to immune pressures, the affinity of molecular interactions with cellular targets, as the Crml nuclear export receptor or the primary capsid receptor, as well as the adaptation to tissues enriched in proliferating cells, are major selective factors in the rapid parvovirus evolutionary dynamics.

Immunogenicity of an intranasally administered modified live canine parvovirus type 2b vaccine in pups with maternally derived antibodies

The ability of a modified live canine parvovirus type 2b vaccine to elicit active immunization in pups with maternally derived antibodies (MDA) by intranasal administration was evaluated. The vaccine induced seroconversion in 100% of pups with MDA titers of < or = 80 and in 51.6% of pups with titers between 160 and 320.

Two mink parvoviruses use different cellular receptors for entry into CRFK cells

Mink enteritis virus (MEV) and Aleutian mink disease parvovirus (ADV) are two mink parvoviruses that replicate permissively in Crandell feline kidney (CRFK) cells. We have used this cell model to examine if these two mink parvoviruses use the same cellular receptor. Whereas the cellular receptor for MEV is expected to be the transferrin receptor (TfR), the cellular receptor for ADV has not been clearly identified. We used short hairpin RNAs (shRNAs) produced from plasmids to trigger RNA interference (RNAi), specifically and effectively reducing TfR expression in CRFK cells. TfR expression was reduced to levels undetectable by immunofluorescence in the majority of cells. In viral infection assays, we show that TfR expression was necessary for MEV infection but was not required for ADV infection. Thus, our results demonstrate that TfR is the cellular receptor for MEV, but not the cellular receptor for ADV. The use of two different receptors by MEV and ADV to infect the same cell line is yet another difference between these two parvoviruses that may contribute to their unique pathogenesis in mink.

Immunogenicity of an intranasally administered modified live canine parvovirus type 2b vaccine in pups with maternally derived antibodies

The ability of a modified live canine parvovirus type 2b vaccine to elicit active immunization in pups with maternally derived antibodies (MDA) by intranasal administration was evaluated. The vaccine induced seroconversion in 100% of pups with MDA titers of < or = 80 and in 51.6% of pups with titers between 160 and 320.

Virulent variants emerging in mice infected with the apathogenic prototype strain of the parvovirus minute virus of mice exhibit a capsid with low avidity for a primary receptor

The mechanisms involved in the emergence of virulent mammalian viruses were investigated in the adult immunodeficient SCID mouse infected by the attenuated prototype strain of the parvovirus Minute Virus of Mice (MVMp). Cloned MVMp intravenously inoculated in mice consistently evolved during weeks of subclinical infection to variants showing altered plaque phenotypes. All the isolated large-plaque variants spread systemically from the oronasal cavity and replicated in major organs (brain, kidney, liver), in sharp contrast to the absolute inability of the MVMp and small-plaque variants to productively invade SCID organs by this natural route of infection. The virulent variants retained the MVMp capacity to infect mouse fibroblasts, consistent with the lack of genetic changes across the 220-to-335 amino acid sequence of VP2, a capsid domain containing main determinants of MVM tropism. However, the capsid of the virulent variants shared a lower affinity than the wild type for a primary receptor used in the cytotoxic infection. The capsid gene of a virulent variant engineered in the MVMp background endowed the recombinant virus with a large-plaque phenotype, lower affinity for the receptor, and productive invasiveness by the oronasal route in SCID mice, eventually leading to 100% mortality. In the analysis of virulence in mice, both MVMp and the recombinant virus similarly gained the bloodstream 1 to 2 days postoronasal inoculation and remained infectious when adsorbed to blood cells in vitro. However, the wild-type MVMp was cleared from circulation a few days afterwards, in contrast to the viremia of the recombinant virus, which was sustained for life. Significantly, attachment to an abundant receptor of primary mouse kidney epithelial cells by both viruses could be quantitatively competed by wild-type MVMp capsids, indicating that virulence is not due to an extended receptor usage in target tissues. We conclude that the selection of capsid-receptor interactions of low affinity, which favors systemic infection, is a major evolutionary process in the adaptation of parvoviruses to new hosts and in the cause of disease.

Intracellular trafficking of adeno-associated viral vectors

Adeno-associated virus (AAV) has attracted considerable interest as a gene therapy vector over the past decade. In all, 85% of the current 2052 PubMed references on AAV (as of December 2004) have been published in the last 10 years. As researchers have moved forward with using this vector system for gene delivery, an increased appreciation for the complexities of AAV biology has emerged. The biology of recombinant AAV (rAAV) transduction has demonstrated considerable diversity in different cell types and target tissues. This review will summarize the current understanding of events that control rAAV transduction following receptor binding and leading to nuclear uptake. These stages are broadly classified as intracellular trafficking and have been found to be a major rate-limiting step in rAAV transduction for many cell types. Advances in understanding this area of rAAV biology will help to improve the efficacy of this vector system for the treatment of inherited and acquired diseases.

Surveillance Activity for Canine Parvovirus in Italy

Recent identification of unusual canine parvovirus (CPV) mutants in cats and dogs suggests that CPV type 2 (CPV-2), which emerged suddenly in the late 1970s, is undergoing continual genetic and antigenic variations. A peculiarity of parvoviruses is that single-nucleotide substitutions may determine drastic phenotypic changes. The effects of either natural or artificial mutations on CPV phenotypic properties have been largely investigated, and this sets up CPV as an interesting model to study virus evolution. By monitoring the evolution of CPV-2 in Italy, we observed the onset and quick spread of a Glu-426 mutant, antigenically different from the pre-existing variants that were partially displaced within a few years of the initial identification of the new mutant. The identification of CPV-2 variants raises several questions concerning their impact on the efficacy of the current CPV-2 vaccines, based on the original CPV-2 strain that no longer exists in the field.

An Annotated Historical Account of Canine Parvovirus

A brief annotated history of canine parvovirus-type 2 (CPV-2) and its variants is summarized with emphasis on the most significant contributions of individuals involved in the initial recognition of CPV-2 and subsequent discoveries that have advanced our knowledge of the nature and evolution of this novel canine virus. Time has obscured the observations of many veterinary clinicians and researchers throughout the world who sensed the presence of a new disease when CPV-2 first made its appearance in 1978 and then, within 1-2 years, spread worldwide. Since 1979, nearly 600 articles, papers, numerous text chapters and monographs have been published on the subject of CPV-2. The early history is well known by veterinary infectious diseases specialists and noteworthy publications are recorded on the National Library of Medicine (USA) website, PubMed and in review articles. Because of the great number of publications, it is not practicable to cite them individually; however, reference is made to certain individuals, reviews and selected papers that I consider particularly relevant to the history of progress in the understanding of CPV-2 and the disease it causes. The clinical disease caused by CPV-2 and its variants, the immune response to infection or vaccines, host range and the development of practical diagnostic assays are noted in historical context. The basic biological properties and the physical, molecular and antigenic structure of CPV-2 and its variants are also discussed briefly. Finally, key players who have contributed to the antigenic and DNA sequence (evolutionary) relationships between CPV-2 and the other autonomous parvoviruses of carnivores are noted and hypotheses regarding the origin and evolution of CPV-2 and its variants are mentioned.

Structure of adeno-associated virus type 4

Adeno-associated virus (AAV) is a member of the Parvoviridae, belonging to the Dependovirus genus. Currently, several distinct isolates of AAV are in development for use in human gene therapy applications due to their ability to transduce different target cells. The need to manipulate AAV capsids for specific tissue delivery has generated interest in understanding their capsid structures. The structure of AAV type 4 (AAV4), one of the most antigenically distinct serotypes, was determined to 13-A resolution by cryo-electron microscopy and image reconstruction. A pseudoatomic model was built for the AAV4 capsid by use of a structure-based sequence alignment of its major capsid protein, VP3, with that of AAV2, to which AAV4 is 58% identical and constrained by its reconstructed density envelope. The model showed variations in the surface loops that may account for the differences in receptor binding and antigenicity between AAV2 and AAV4. The AAV4 capsid surface topology also shows an unpredicted structural similarity to that of Aleutian mink disease virus and human parvovirus B19, autonomous members of the genus, despite limited sequence homology.

Viral entry

Virus entry is initiated by recognition by receptors present on the surface of host cells. Receptors can be major mediators of virus tropism, and in many cases receptor interactions occur in an apparently programmed series of events utilizing multiple receptors. After receptor interaction, both enveloped and nonenveloped viruses must deliver their genome across either the endosomal or plasma membrane for infection to proceed. Genome delivery occurs either by membrane fusion (in the case of enveloped viruses) or by pore formation or other means of permeabilizing the lipid bilayer (in the case of nonenveloped viruses). For those viruses that enter cells via endosomes, specific receptor interactions (and the signaling events that ensue) may control the particular route of endocytosis and/or the ultimate destination of the incoming virus particles. Our conception of virus entry is increasingly becoming more complex; however, the specificity involved in entry processes, once ascertained, may ultimately lead to the production of effective antiviral agents.