Host-specific parvovirus evolution in nature is recapitulated by in vitro adaptation to different carnivore species

Canine parvovirus (CPV) emerged as a new pandemic pathogen of dogs in the 1970s and is closely related to feline panleukopenia virus (FPV), a parvovirus of cats and related carnivores. Although both viruses have wide host ranges, analysis of viral sequences recovered from different wild carnivore species, as shown here, demonstrated that >95% were derived from CPV-like viruses, suggesting that CPV is dominant in sylvatic cycles. Many viral sequences showed host-specific mutations in their capsid proteins, which were often close to sites known to control binding to the transferrin receptor (TfR), the host receptor for these carnivore parvoviruses, and which exhibited frequent parallel evolution. To further examine the process of host adaptation, we passaged parvoviruses with alternative backgrounds in cells from different carnivore hosts. Specific mutations were selected in several viruses and these differed depending on both the background of the virus and the host cells in which they were passaged. Strikingly, these in vitro mutations recapitulated many specific changes seen in viruses from natural populations, strongly suggesting they are host adaptive, and which were shown to result in fitness advantages over their parental virus. Comparison of the sequences of the transferrin receptors of the different carnivore species demonstrated that many mutations occurred in and around the apical domain where the virus binds, indicating that viral variants were likely selected through their fit to receptor structures. Some of the viruses accumulated high levels of variation upon passage in alternative hosts, while others could infect multiple different hosts with no or only a few additional mutations. Overall, these studies demonstrate that the evolutionary history of a virus, including how long it has been circulating and in which hosts, as well as its phylogenetic background, has a profound effect on determining viral host range.

Canine parvovirus (CPV) is an important example of a viral pathogen that evolved by cross-species transmission and mutation to initiate a disease pandemic. Carnivore parvoviruses infect many species, and their passage in different hosts may select mutations that facilitate host jumping; for example, natural passage of CPV in raccoons may have facilitated its adaptation to dogs. Conversely, some raccoon-adapted viruses are non-infectious to dogs, illustrating that host range barriers exist among different carnivores. Here we demonstrate that these barriers can be overcome by only a few mutations in the virus that likely alter host receptor binding, and that host adaptation can differ dramatically among very similar viruses. Importantly, we also show that passage of viruses in cell cultures of different hosts results in mutations at the same sites that vary in nature and confer fitness increases, strongly suggesting that they are adaptively important. These findings demonstrate that parvoviruses may cross species barriers to infect less susceptible hosts through single or only a few mutations, and that differences in the genetic background, host range, and/or evolutionary history of the viruses influence their propensity to jump hosts. Overall, these discoveries help reveal the mechanisms that control host switching and viral emergence.

Improving pandemic influenza risk assessment

Assessing the pandemic risk posed by specific non-human influenza A viruses is an important goal in public health research. As influenza virus genome sequencing becomes cheaper, faster, and more readily available, the ability to predict pandemic potential from sequence data could transform pandemic influenza risk assessment capabilities. However, the complexities of the relationships between virus genotype and phenotype make such predictions extremely difficult. The integration of experimental work, computational tool development, and analysis of evolutionary pathways, together with refinements to influenza surveillance, has the potential to transform our ability to assess the risks posed to humans by non-human influenza viruses and lead to improved pandemic preparedness and response.

Infection and pathogenesis of canine, equine, and human influenza viruses in canine tracheas

Influenza A viruses (IAVs) can jump species barriers and occasionally cause epidemics, epizootics, pandemics, and panzootics. Characterizing the infection dynamics at the target tissues of natural hosts is central to understanding the mechanisms that control host range, tropism, and virulence. Canine influenza virus (CIV; H3N8) originated after the transfer of an equine influenza virus (EIV) into dogs. Thus, comparing CIV and EIV isolates provides an opportunity to study the determinants of influenza virus emergence. Here we characterize the replication of canine, equine, and human IAVs in the trachea of the dog, a species to which humans are heavily exposed. We define a phenotype of infection for CIV, which is characterized by high levels of virus replication and extensive tissue damage. CIV was compared to evolutionarily distinct EIVs, and the early EIV isolates showed an impaired ability to infect dog tracheas, while EIVs that circulated near the time of CIV emergence exhibited a CIV-like infection phenotype. Inoculating dog tracheas with various human IAVs (hIAVs) showed that they infected the tracheal epithelium with various efficiencies depending on the virus tested. Finally, we show that reassortant viruses carrying gene segments of CIV and hIAV are viable and that addition of the hemagglutinin (HA) and neuraminidase (NA) of CIV to the 2009 human pandemic virus results in a virus that replicates at high levels and causes significant lesions. This provides important insights into the role of evolution on viral emergence and on the role of HA and NA as determinants of pathogenicity.

IMPORTANCE

Influenza A viruses (IAVs) have entered new host species in recent history, sometimes with devastating consequences. Canine influenza virus (CIV) H3N8 originated from a direct transfer of an equine influenza virus (EIV) in the early 2000s. We studied the infection patterns of IAVs that circulate in dogs or to which dogs are commonly exposed and showed that CIV emergence was likely caused by an adaptive driver, as evolutionarily distinct EIVs display distinct infection phenotypes. We also showed that many human viruses can infect dog tracheas and that reassortment with CIV results in viable viruses. Finally, we showed that the hemagglutinin and neuraminidase of CIV act as virulence factors. Our findings have significant implications because they show that dogs might act as "mixing vessels" in which novel viruses with pandemic potential could emerge and also provide experimental evidence supporting the role of viral evolution in influenza virus emergence.

Parvovirus particles and movement in the cellular cytoplasm and effects of the cytoskeleton

Cell infection by parvoviruses requires that capsids be delivered from outside the cell to the cytoplasm, followed by genome trafficking to the nucleus. Here we microinject capsids into cells that lack receptors and followed their movements within the cell over time. In general the capsids remained close to the positions where they were injected, and most particles did not move to the vicinity of or enter the nucleus. When 70 kDa-dextran was injected along with the capsids that did not enter the nucleus in significant amounts. Capsids conjugated to peptides containing the SV40 large T-antigen nuclear localization signal remained in the cytoplasm, although bovine serum albumen conjugated to the same peptide entered the nucleus rapidly. No effects of disruption of microfilaments, intermediate filaments, or microtubules on the distribution of the capsids were observed. These results suggest that movement of intact capsids within cells is primarily associated with passive processes.

Canine parvovirus 2c infection in a cat with severe clinical disease

Canine parvovirus 2 (CPV-2) is considered the main pathogen responsible for acute gastroenteritis in dogs, causing vomiting and hemorrhagic enteritis mainly. However, infection in cats by CPV variants causes clinical signs similar to Feline panleukopenia virus. The current study reports a case of CPV-2c in a domestic cat, in Portugal. The findings suggest that more surveys are needed to know the true prevalence and significance of cats in CPV epidemiology worldwide.

Feline panleukopenia virus: its interesting evolution and current problems in immunoprophylaxis against a serious pathogen

Vaccination of cats against feline panleukopenia virus (FPV) has been a routine part of feline medicine for the past 40 or more years, and many of the same vaccines that were first developed in the 1960s are still in routine use today. However, there has been significant evolution of the virus in the last 40 years, in particular the emergence of canine parvovirus (CPV) in dogs in the late 1970s, which was a host range variant of the FPV-like virus, and the world-wide spread of the CPV-derived viruses since 1978. FPV and the various antigenic types of CPV have been isolated from cats, raccoons, and many different wild and captive carnivores. The consequences of these changes in the viral populations have not been investigated, and the effectiveness of the current vaccine protocols have not been reported. Here we review the recent findings about the evolution of the viruses in carnivores including cats, and describe a study that looks at the efficiency of vaccination of kittens using the standard protocols, which shows that many cats are not protected by those approaches.

Canine and feline parvoviruses preferentially recognize the non-human cell surface sialic acid N-glycolylneuraminic acid

Feline panleukopenia virus (FPV) is a pathogen whose canine-adapted form (canine parvovirus (CPV)) emerged in 1978. These viruses infect by binding host transferrin receptor type-1 (TfR), but also hemagglutinate erythrocytes. We show that hemagglutination involves selective recognition of the non-human sialic acid N-glycolylneuraminic acid (Neu5Gc) but not N-acetylneuraminic acid (Neu5Ac), which differs by only one oxygen atom from Neu5Gc. Overexpression of α2-6 sialyltransferase did not change binding, indicating that both α2-3 and α2-6 linkages are recognized. However, Neu5Gc expression on target cells did not enhance CPV or FPV infection in vitro. Thus, the conserved Neu5Gc-binding preference of these viruses likely plays a role in the natural history of the virus in vivo. Further studies must clarify relationships between virus infection and host Neu5Gc expression. As a first step, we show that transcripts of CMAH (which generates Neu5Gc from Neu5Ac) are at very low levels in Western dog breed cells.

Prediction and prevention of the next pandemic zoonosis

Most pandemics--eg, HIV/AIDS, severe acute respiratory syndrome, pandemic influenza--originate in animals, are caused by viruses, and are driven to emerge by ecological, behavioural, or socioeconomic changes. Despite their substantial effects on global public health and growing understanding of the process by which they emerge, no pandemic has been predicted before infecting human beings. We review what is known about the pathogens that emerge, the hosts that they originate in, and the factors that drive their emergence. We discuss challenges to their control and new efforts to predict pandemics, target surveillance to the most crucial interfaces, and identify prevention strategies. New mathematical modelling, diagnostic, communications, and informatics technologies can identify and report hitherto unknown microbes in other species, and thus new risk assessment approaches are needed to identify microbes most likely to cause human disease. We lay out a series of research and surveillance opportunities and goals that could help to overcome these challenges and move the global pandemic strategy from response to pre-emption.

Effects of canine parvovirus strain variations on diagnostic test results and clinical management of enteritis in dogs

OBJECTIVE

To estimate the prevalence of canine parvovirus (CPV) strains among dogs with enteritis admitted to a referral hospital in the southwestern United States during an 11-month period and to compare diagnostic test results, disease severity, and patient outcome among CPV strains.

DESIGN

Prospective observational study.

ANIMALS

72 dogs with histories and clinical signs of parvoviral enteritis.

PROCEDURES

For each dog, a fecal sample or rectal swab specimen was evaluated for CPV antigen via an ELISA. Subsequently, fecal samples (n = 42 dogs) and pharyngeal swab specimens (16) were obtained and tested for CPV antigen via an ELISA and CPV DNA via a PCR assay. For specimens with CPV-positive results via PCR assay, genetic sequencing was performed to identify the CPV strain.

RESULTS

56 dogs tested positive for CPV via ELISA or PCR assay. For 42 fecal samples tested via both ELISA and PCR assay, 27 had positive results via both assays, whereas 6 had positive PCR assay results only. Ten pharyngeal swab specimens yielded positive PCR assay results. Genetic sequencing was performed on 34 fecal or pharyngeal swab specimens that had CPV-positive PCR assay results; 25 (73.5%) were identified as containing CPV type-2c, and 9 (26.5%) were identified as containing CPV type-2b. No association was found between CPV strain and disease severity or clinical outcome.

CONCLUSIONS AND CLINICAL RELEVANCE

CPV type-2b and CPV type-2c posed similar health risks for dogs; therefore, genetic sequencing of CPV does not appear necessary for clinical management of infected patients. The diagnostic tests used could detect CPV type-2c.

Examining the cross-reactivity and neutralization mechanisms of a panel of mAbs against adeno-associated virus serotypes 1 and 5

Neutralizing antibodies play a central role in the prevention and clearance of viral infections, but can be detrimental to the use of viral capsids for gene delivery. Antibodies present a major hurdle for ongoing clinical trials using adeno-associated viruses (AAVs); however, relatively little is known about the antigenic epitopes of most AAV serotypes or the mechanism(s) of antibody-mediated neutralization. We developed panels of AAV mAbs by repeatedly immunizing mice with AAV serotype 1 (AAV1) capsids, or by sequentially immunizing with AAV1 followed by AAV5 capsids, in order to examine the efficiency and mechanisms of antibody-mediated neutralization. The antibodies were not cross-reactive between heterologous AAV serotypes except for a low level of recognition of AAV1 capsids by the AAV5 antibodies, probably due to the initial immunization with AAV1. The neutralization efficiency of different IgGs varied and Fab fragments derived from these antibodies were generally poorly neutralizing. The antibodies appeared to display various alternative mechanisms of neutralization, which included inhibition of receptor-binding and interference with a post-attachment step.

Anticipating the species jump: surveillance for emerging viral threats

Zoonotic disease surveillance is typically triggered after animal pathogens have already infected humans. Are there ways to identify high‐risk viruses before they emerge in humans? If so, then how and where can identifications be made and by what methods? These were the fundamental questions driving a workshop to examine the future of predictive surveillance for viruses that might jump from animals to infect humans. Virologists, ecologists and computational biologists from academia, federal government and non‐governmental organizations discussed opportunities as well as obstacles to the prediction of species jumps using genetic and ecological data from viruses and their hosts, vectors and reservoirs. This workshop marked an important first step towards envisioning both scientific and organizational frameworks for this future capability. Canine parvoviruses as well as seasonal H3N2 and pandemic H1N1 influenza viruses are discussed as exemplars that suggest what to look for in anticipating species jumps. To answer the question of where to look, prospects for discovering emerging viruses among wildlife, bats, rodents, arthropod vectors and occupationally exposed humans are discussed. Finally, opportunities and obstacles are identified and accompanied by suggestions for how to look for species jumps. Taken together, these findings constitute the beginnings of a conceptual framework for achieving a virus surveillance capability that could predict future species jumps.

Bridging taxonomic and disciplinary divides in infectious disease

Pathogens traverse disciplinary and taxonomic boundaries, yet infectious disease research occurs in many separate disciplines including plant pathology, veterinary and human medicine, and ecological and evolutionary sciences. These disciplines have different traditions, goals, and terminology, creating gaps in communication. Bridging these disciplinary and taxonomic gaps promises novel insights and important synergistic advances in control of infectious disease. An approach integrated across the plant-animal divide would advance our understanding of disease by quantifying critical processes including transmission, community interactions, pathogen evolution, and complexity at multiple spatial and temporal scales. These advances require more substantial investment in basic disease research.

Bovine leukaemia virus infection in New Zealand cattle

Bovine Leukaemia Virus (BLV) infection in New Zealand cattle was investigated. In a national survey of 5000 sera from 500 herds, BLV antibody was not detected. An additional 1062 sera from 140 herds were tested and 3 sera were positive. In the herd of origin of one of these 3 sera, 22.6% of cattle were serologically positive for BLV. Where cases of bovine lymphosarcoma had been diagnosed, 38 of 39 herds tested were negative for BLV antibody. Within the remaining herd, 36% of cows tested were serologically-positive. BLV was isolated from 2 serologically positive cows in this herd.

Generation and validation of canine single chain variable fragment phage display libraries

Single chain variable region fragments (scFvs) are composed of an immunoglobulin (Ig) variable heavy (VH) and variable light (VL) chain joined by a flexible serine-glycine linker. They represent the smallest antibody fragments that maintain antigen specificity and they hold significant potential for therapeutic antigen targeting in vivo. Here we report on the design and validation of a series of degenerate primers that amplify the recombined variable regions of canine Ig heavy and light chain genes from lymphocyte cDNA. We show that these VH and VL amplicons can be randomly combined by a flexible linker using splicing by overlap extension PCR to form scFv constructs that can be expressed on the surface of M13 bacteriophage. To demonstrate that scFvs with specificity for previously encountered antigens are contained within these scFv phage display libraries we used simple panning procedures to isolate canine parvovirus (CPV) specific scFvs from a library made from the splenocytes of a dog immunized against CPV. These studies reveal the feasibility of this approach for generating diverse canine scFv libraries and pave the way toward future studies to isolate canine antigen-specific scFv of interest that may be tested as targeting agents for the treatment of infectious, inflammatory and neoplastic diseases in the dog.

The emergence of parvoviruses of carnivores

The emergence of canine parvovirus (CPV) represents a well-documented example highlighting the emergence of a new virus through cross-species transmission. CPV emerged in the mid-1970s as a new pathogen of dogs and has since become endemic in the global dog population. Despite widespread vaccination, CPV has remained a widespread disease of dogs, and new genetic and antigenic variants have arisen and sometimes reached high frequency in certain geographic regions or throughout the world. Here we review our understanding of this emergence event and contrast it to what is known about the emergence of a disease in mink caused by mink enteritis virus (MEV). In addition, we summarize the evolution of CPV over the past 30 years in the global dog population, and describe the epidemiology of contemporary parvovirus infections of dogs and cats. CPV represents a valuable model for understanding disease emergence through cross-species transmission, while MEV provides an interesting comparison.

Gastroparesis and functional dyspepsia: excerpts from the AGA/ANMS meeting

BACKGROUND

Despite the relatively high prevalence of gastroparesis and functional dyspepsia, the aetiology and pathophysiology of these disorders remain incompletely understood. Similarly, the diagnostic and treatment options for these two disorders are relatively limited despite recent advances in our understanding of both disorders.

PURPOSE

This manuscript reviews the advances in the understanding of the epidemiology, pathophysiology, diagnosis, and treatment of gastroparesis and functional dyspepsia as discussed at a recent conference sponsored by the American Gastroenterological Association (AGA) and the American Neurogastroenterology and Motility Society (ANMS). Particular focus is placed on discussing unmet needs and areas for future research.

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.

Phylogenetic analysis reveals the emergence, evolution and dispersal of carnivore parvoviruses

Canine parvovirus (CPV), first recognized as an emerging virus of dogs in 1978, resulted from a successful cross-species transmission. CPV emerged from the endemic feline panleukopenia virus (FPV), or from a closely related parvovirus of another host. Here we refine our current understanding of the evolution and population dynamics of FPV and CPV. By analysing nearly full-length viral sequences we show that the majority of substitutions distinguishing CPV from FPV are located in the capsid protein gene, and that this gene is under positive selection in CPV, resulting in a significantly elevated rate of molecular evolution. This provides strong phylogenetic evidence for a prominent role of the viral capsid in host adaptation. In addition, an analysis of the population dynamics of more recent CPV reveals, on a global scale, a strongly spatially subdivided CPV population with little viral movement among countries and a relatively constant population size. Such limited viral migration contrasts with the global spread of the virus observed during the early phase of the CPV pandemic, but corresponds to the more endemic nature of current CPV infections.

Cross-species virus transmission and the emergence of new epidemic diseases

Host range is a viral property reflecting natural hosts that are infected either as part of a principal transmission cycle or, less commonly, as "spillover" infections into alternative hosts. Rarely, viruses gain the ability to spread efficiently within a new host that was not previously exposed or susceptible. These transfers involve either increased exposure or the acquisition of variations that allow them to overcome barriers to infection of the new hosts. In these cases, devastating outbreaks can result. Steps involved in transfers of viruses to new hosts include contact between the virus and the host, infection of an initial individual leading to amplification and an outbreak, and the generation within the original or new host of viral variants that have the ability to spread efficiently between individuals in populations of the new host. Here we review what is known about host switching leading to viral emergence from known examples, considering the evolutionary mechanisms, virus-host interactions, host range barriers to infection, and processes that allow efficient host-to-host transmission in the new host population.

Evaluation of a vectored equine herpesvirus type 1 (EHV-1) vaccine expressing H3 haemagglutinin in the protection of dogs against canine influenza

In 2004, canine influenza virus (CIV) was identified as a respiratory pathogen of dogs for the first time and found to be closely related to H3N8 equine influenza virus (EIV). We generated a recombinant vectored vaccine that expresses H3 of a recent isolate of EIV using equine herpesvirus type 1 (EHV-1) as the delivery vehicle. This EHV-1 vectored vaccine exhibited robust and stable EIV H3 expression and induced a strong influenza virus-specific response in both mice and dogs upon intranasal or subcutaneous administration. Furthermore, upon challenge with the recent CIV isolate A/canine/PA/10915-07, protection of vaccinated dogs could be demonstrated by a significant reduction in clinical sings, and, more importantly, by a significant reduction in virus shedding. We concluded that the EHV-1/H3 recombinant vector can be a valuable alternative for protection of dogs against clinical disease induced by CIV and can significantly reduce virus spread.

Presence and role of cytosine methylation in DNA viruses of animals

Nucleotide composition varies greatly among DNA viruses of animals, yet the evolutionary pressures and biological mechanisms driving these patterns are unclear. One of the most striking discrepancies lies in the frequency of CpG (the dinucleotide CG, linked by a phosphate group), which is underrepresented in most small DNA viruses (those with genomes below 10 kb) but not in larger DNA viruses. Cytosine methylation might be partially responsible, but research on this topic has focused on a few virus groups. For several viruses that integrate their genome into the host genome, the methylation status during this stage has been studied extensively, and the relationship between methylation and viral-induced tumor formation has been examined carefully. However, for actively replicating viruses—particularly small DNA viruses—the methylation status of CpG motifs is rarely known and the effects on the viral life cycle are obscure. In vertebrate host genomes, most cytosines at CpG sites are methylated, which in vertebrates acts to regulate gene expression and facilitates the recognition of unmethylated, potentially pathogen-associated DNA. Here we briefly introduce cytosine methylation before reviewing what is currently known about CpG methylation in DNA viruses.

Comparative analysis reveals frequent recombination in the parvoviruses

Parvoviruses are small single-stranded DNA viruses that are ubiquitous in nature. Infections with both autonomous and helper-virus dependent parvoviruses are common in both human and animal populations, and many animals are host to a number of different parvoviral species. Despite the epidemiological importance of parvoviruses, the presence and role of genome recombination within or among parvoviral species has not been well characterized. Here we show that natural recombination may be widespread in these viruses. Different genome regions of both porcine parvoviruses and Aleutian mink disease viruses have conflicting phylogenetic histories, providing evidence for recombination within each of these two species. Further, the rodent parvoviruses show complex evolutionary histories for separate genomic regions, suggesting recombination at the interspecies level.

Minute virus of mice, a parvovirus, in complex with the Fab fragment of a neutralizing monoclonal antibody

The structure of virus-like particles of the lymphotropic, immunosuppressive strain of minute virus of mice (MVMi) in complex with the neutralizing Fab fragment of the mouse monoclonal antibody (MAb) B7 was determined by cryo-electron microscopy to 7-A resolution. The Fab molecule recognizes a conformational epitope at the vertex of a three-fold protrusion on the viral surface, thereby simultaneously engaging three symmetry-related viral proteins in binding. The location of the epitope close to the three-fold axis is consistent with the previous analysis of MVMi mutants able to escape from the B7 antibody. The binding site close to the symmetry axes sterically forbids the binding of more than one Fab molecule per spike. MAb as well as the Fab molecules inhibits the binding of the minute virus of mice (MVM) to permissive cells but can also neutralize MVM postattachment. This finding suggests that the interaction of B7 with three symmetry-related viral subunits at each spike hinders structural transitions in the viral capsid essential during viral entry.

Asymmetric binding of transferrin receptor to parvovirus capsids

Although many viruses are icosahedral when they initially bind to one or more receptor molecules on the cell surface, such an interaction is asymmetric, probably causing a breakdown in the symmetry and conformation of the original infecting virion in preparation for membrane penetration and release of the viral genome. Cryoelectron microscopy and biochemical analyses show that transferrin receptor, the cellular receptor for canine parvovirus, can bind to only one or a few of the 60 icosahedrally equivalent sites on the virion, indicating that either canine parvovirus has inherent asymmetry or binding of receptor induces asymmetry. The asymmetry of receptor binding to canine parvovirus is reminiscent of the special portal in tailed bacteriophages and some large, icosahedral viruses. Asymmetric interactions of icosahedral viruses with their hosts might be a more common phenomenon than previously thought and may have been obscured by averaging in previous crystallographic and electron microscopic structure determinations.

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.

From structure of the complex to understanding of the biology

The most extensive structural information on viruses relates to apparently icosahedral virions and is based on X-ray crystallography and on cryo-electron microscopy (cryo-EM) single-particle reconstructions. Both techniques lean heavily on imposing icosahedral symmetry, thereby obscuring any deviation from the assumed symmetry. However, tailed bacteriophages have icosahedral or prolate icosahedral heads that have one obvious unique vertex where the genome can enter for DNA packaging and exit when infecting a host cell. The presence of the tail allows cryo-EM reconstructions in which the special vertex is used to orient the head in a unique manner. Some very large dsDNA icosahedral viruses also develop special vertices thought to be required for infecting host cells. Similarly, preliminary cryo-EM data for the small ssDNA canine parvovirus complexed with receptor suggests that these viruses, previously considered to be accurately icosahedral, might have some asymmetric properties that generate one preferred receptor-binding site on the viral surface. Comparisons are made between rhinoviruses that bind receptor molecules uniformly to all 60 equivalent binding sites, canine parvovirus, which appears to have a preferred receptor-binding site, and bacteriophage T4, which gains major biological advantages on account of its unique vertex and tail organelle.

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.

Treatment of gastroparesis: a multidisciplinary clinical review

This clinical review on the treatment of patients with gastroparesis is a consensus document developed by the American Motility Society Task Force on Gastroparesis. It is a multidisciplinary effort with input from gastroenterologists and other specialists who are involved in the care of patients with gastroparesis. To provide practical guidelines for treatment, this document covers results of published research studies in the literature and areas developed by consensus agreement where clinical research trials remain lacking in the field of gastroparesis.

Evolutionary basis of codon usage and nucleotide composition bias in vertebrate DNA viruses

Understanding the extent and causes of biases in codon usage and nucleotide composition is essential to the study of viral evolution, particularly the interplay between viruses and host cells or immune responses. To understand the common features and differences among viruses we analyzed the genomic characteristics of a representative collection of all sequenced vertebrate-infecting DNA viruses. This revealed that patterns of codon usage bias are strongly correlated with overall genomic GC content, suggesting that genome-wide mutational pressure, rather than natural selection for specific coding triplets, is the main determinant of codon usage. Further, we observed a striking difference in CpG content between DNA viruses with large and small genomes. While the majority of large genome viruses show the expected frequency of CpG, most small genome viruses had CpG contents far below expected values. The exceptions to this generalization, the large gammaherpesviruses and iridoviruses and the small dependoviruses, have sufficiently different life-cycle characteristics that they may help reveal some of the factors shaping the evolution of CpG usage in viruses.

The origins of new pandemic viruses: the acquisition of new host ranges by canine parvovirus and influenza A viruses

Transfer of viruses between hosts to create a new self-sustaining epidemic is rare; however, those new viruses can cause severe outbreaks. Examples of such viruses include three pandemic human influenza A viruses and canine parvovirus in dogs. In each case one virus made the original transfer and spread worldwide, and then further adaptation resulted in the emergence of variants worldwide. For the influenza viruses several changes were required for growth and spread between humans, and the emergence of human H2N2 and H3N2 strains in 1957 and 1968 involved the acquisition of three or two new genomic segments, respectively. Adaptation to humans involved several viral genes including the hemagglutinin, the neuraminidase, and the replication proteins. The canine adaptation of the parvoviruses involved capsid protein changes altering the recognition of the host transferrin receptors, allowing canine transferrin receptor binding and its use as a receptor for cell infection.

High rate of viral evolution associated with the emergence of carnivore parvovirus

Canine parvovirus (CPV) is an emerging DNA virus that was first observed to cause disease in canines in 1978 and has since become a ubiquitous pathogen worldwide. CPV emerged from feline panleukopenia parvovirus (FPLV) or a closely related virus, differing at several key amino acid residues. Here we characterize the evolutionary processes underlying the emergence of CPV. Although FPLV has remained an endemic infection in its host populations, we show that, since the 1970s, the newly emerged CPV has undergone an epidemic-like pattern of logistic/exponential growth, effectively doubling its population size every few years. This rapid population growth was associated with a lineage of CPV that acquired a broader host range and greater infectivity. Recombination played no role in the emergence of CPV. Rather, any preexisting variation in the donor species and the subsequent rapid adaptation of the virus to canines were likely dependent on a high rate of mutation and the positive selection of mutations in the major capsid gene. Strikingly, although these single-stranded viruses have a DNA genome and use cellular replication machinery, their rate of nucleotide substitution is closer to that of RNA viruses than to that of double-stranded DNA viruses.

Parvovirus infection of cells by using variants of the feline transferrin receptor altering clathrin-mediated endocytosis, membrane domain localization, and capsid-binding domains

The feline and canine transferrin receptors (TfRs) bind canine parvovirus to host cells and mediate rapid capsid uptake and infection. The TfR and its ligand transferrin have well-described pathways of endocytosis and recycling. Here we tested several receptor-dependent steps in infection for their role in virus infection of cells. Deletions of cytoplasmic sequences or mutations of the Tyr-Thr-Arg-Phe internalization motif reduced the rate of receptor uptake from the cell surface, while polar residues introduced into the transmembrane sequence resulted in increased degradation of transferrin. However, the mutant receptors still mediated efficient virus infection. In contrast, replacing the cytoplasmic and transmembrane sequences of the feline TfR with those of the influenza virus neuraminidase (NA) resulted in a receptor that bound and endocytosed the capsid but did not mediate viral infection. This chimeric receptor became localized to detergent-insoluble membrane domains. To test the effect of structural virus receptor interaction on infection, two chimeric receptors were prepared which contained antibody-variable domains that bound the capsid in place of the TfR ectodomain. These chimeric receptors bound CPV capsids and mediated uptake but did not result in cell infection. Adding soluble feline TfR ectodomain to the virus during that uptake did not allow infection.

[Evolution and host variation of the canine parvovirus: molecular basis for the development of a new virus]

Canine parvovirus (CPV) evolved as a new pathogen in dogs between 1976 and 1978 from feline panleukopenia virus (FPV). The new virus hit an unprotected population, caused a dramatic pandemic and infected virtually all populations of domestic and wild carnivores worldwide. The great similarity between the two viruses and their differences in host range, both in vivo as well as in vitro, make it a good model system for emerging diseases and host range shifts of viruses. Recent results showed that CPV expanded its host range by binding to the canine transferrin receptor (Tfr). Residues in the capsid protein that had been defined as host range controlling regions also control the binding to the canine transferrin receptor. These residues are located on a raised region of the capsid at the three-fold axis of symmetry. Interestingly, adaption of the new virus to the new host appears to correlate with an improved binding to the Tfr receptor.

Structures of host range-controlling regions of the capsids of canine and feline parvoviruses and mutants

Canine parvovirus (CPV) and feline panleukopenia virus (FPV) differ in their ability to infect dogs and dog cells. Canine cell infection is a specific property of CPV and depends on the ability of the virus to bind the canine transferrin receptor (TfR), as well as other unidentified factors. Three regions in the capsid structure, located around VP2 residues 93, 300, and 323, can all influence canine TfR binding and canine cell infection. These regions were compared in the CPV and FPV capsid structures that have been determined, as well as in two new structures of CPV capsids that contain substitutions of the VP2 Asn-93 to Asp and Arg, respectively. The new structures, determined by X-ray crystallography to 3.2 and 3.3 A resolutions, respectively, clearly showed differences in the interactions of residue 93 with an adjacent loop on the capsid surface. Each of the three regions show small differences in structure, but each appears to be structurally independent of the others, and the changes likely act together to affect the ability of the capsid to bind the canine TfR and to infect canine cells. This emphasizes the complex nature of capsid alterations that change the virus-cell interaction to allow infection of cells from different hosts.

Combinations of two capsid regions controlling canine host range determine canine transferrin receptor binding by canine and feline parvoviruses

Feline panleukopenia virus (FPV) and its host range variant, canine parvovirus (CPV), can bind the feline transferrin receptor (TfR), while only CPV binds to the canine TfR. Introducing two CPV-specific changes into FPV (at VP2 residues 93 and 323) endowed that virus with the canine TfR binding property and allowed canine cell infection, although neither change alone altered either property. In CPV the reciprocal changes of VP2 residue 93 or 323 to the FPV sequences individually resulted in modest reductions in infectivity for canine cells. Changing both residues in CPV to the FPV amino acids blocked the canine cell infection, but that virus was still able to bind the canine TfR at low levels. This shows that both CPV-specific changes control canine TfR binding but that binding is not always sufficient to mediate infection.

Residues in the apical domain of the feline and canine transferrin receptors control host-specific binding and cell infection of canine and feline parvoviruses

Canine parvovirus (CPV) and feline panleukopenia virus (FPV) capsids bind to the transferrin receptors (TfRs) of their hosts and use these receptors to infect cells. The binding is partially host specific, as FPV binds only to the feline TfR, while CPV binds to both the canine and feline TfRs. The host-specific binding is controlled by a combination of residues within a raised region of the capsid. To define the TfR structures that interact with the virus, we altered the apical domain of the feline or canine TfR or prepared chimeras of these receptors and tested the altered receptors for binding to FPV or CPV capsids. Most changes in the apical domain of the feline TfR did not affect binding, but replacing Leu221 with Ser or Asp prevented receptor binding to either FPV or CPV capsids, while replacing Leu221 with Lys resulted in a receptor that bound only to CPV but not to FPV. Analysis of recombinants of the feline and canine TfRs showed that sequences controlling CPV-specific binding were within the apical domain and that more than one difference between these receptors determined the CPV-specific binding of the canine TfR. Single changes within the canine TfR which removed a single amino acid insertion or which eliminated a glycosylation site gave that receptor the expanded ability to bind to FPV and CPV. In some cases, binding of capsids to mutant receptors did not result in infection, suggesting a structural role for the receptor in cell infection by the viruses.

Parvovirus host range, cell tropism and evolution

The past few years have seen major advances in our understanding of the controls of evolution, host range and cell tropism of parvoviruses. Notable findings have included the identification of the transferrin receptor TfR as the cell surface receptor for canine parvovirus and feline panleukopenia virus, and also the finding that specific binding to the canine TfR led to the emergence of canine parvovirus as a new pathogen in dogs. The structures of the adeno-associated virus-2 and porcine parvovirus capsids, along with those of the minute virus of mice, have also advanced our understanding of parvovirus biology. Structure-function studies have shown that in several different parvoviruses the threefold spikes or peaks of the capsid control several aspects of cell tropism and host range, and that those are subject to selective pressures leading to viral evolution. The cell and tissue tropisms of different adeno-associated virus serotypes were demonstrated to be due, in part, to specific receptor binding.

Flow cytometric characterization of perfused human bone marrow cultures: identification of the major cell lineages and correlation with the CFU-GM assay

BACKGROUND

Prolific cultures of human bone marrow mononuclear cells (BM MNCs) were recently developed that include a full spectrum of hematopoietic and accessory cells, with the presence of autofluorescent cells indicating adequate cell expansion. However, phenotypic and functional clonogenic characterizations of the autofluorescent cells and the various other subpopulations present in these cultures have not been carried out.

METHODS

Cells from a continuously perfused bioreactor inoculated with BM MNCs and cultured for 12 days in serum-containing medium with PIXY321, erythropoietin, and with or without FLT3-L were evaluated by using flow cytometry.

RESULTS

Two antibodies, CD71 and CD13, allowed the separation of the autofluorescent cells into two distinct populations. The CD71+CD13++ autofluorescent population contained the colony-forming unit (CFU) fibroblast, and the CD71++CD13++ autofluorescent population contained macrophage/dendritic like cells. The CFU-granulocyte/macrophage (CFU-GM) could not be thoroughly evaluated with CD71 and CD13. However, the number of CD13+/++Lin- cells correlated with the number of CFU-GM (r = 0.83), with approximately 1 CFU-GM for every 30 CD13+/++Lin- cells.

CONCLUSIONS

The data showed that CD71 and CD13 antibodies separate the autofluorescent cells into two populations but do not separate hematopoietic cells into specific phenotypic populations. The data also showed that the number of CD13+/++Lin- cells correlated with the number of CFU-GM. These data present the initial step toward detailed phenotypic analysis of ex vivo expanded human BM MNC cultures.

The natural host range shift and subsequent evolution of canine parvovirus resulted from virus-specific binding to the canine transferrin receptor

Canine parvovirus (CPV) is a host range variant of a feline virus that acquired the ability to infect dogs through changes in its capsid protein. Canine and feline viruses both use the feline transferrin receptor (TfR) to infect feline cells, and here we show that CPV infects canine cells through its ability to specifically bind the canine TfR. Receptor binding on host cells at 37 degrees C only partially correlated with the host ranges of the viruses, and an intermediate virus strain (CPV type 2) bound to higher levels on cells than did either the feline panleukopenia virus or a later strain of CPV. During the process of adaptation to dogs the later variant strain of CPV gained the ability to more efficiently use the canine TfR for infection and also showed reduced binding to feline and canine cells compared to CPV type 2. Differences on the top and the side of the threefold spike of the capsid surface controlled specific TfR binding and the efficiency of binding to feline and canine cells, and these differences also determined the cell infection properties of the viruses.

Virologic and serologic identification of minute virus of canines (canine parvovirus type 1) from dogs in Japan

Minute virus of canines (MVC), also known as canine parvovirus type 1, was initially believed to be a nonpathogenic agent, since it was first isolated from canine fecal specimens in the late 1960s. However, subsequent pathological as well as epidemiological studies suggested that MVC is a pathogen of neonatal puppies and is widely distributed among domestic dogs in the United States. The virus also has been shown to cause fetal deaths. Nevertheless, the virus was not detected in dogs outside the United States until recently, presumably because of a lack of widespread availability of the only susceptible canine cell line, WRCC/3873D, used for MVC isolation. We examined 470 clinical specimens from 346 dogs by PCR and detected MVC-specific gene fragments from four diseased puppies (positive rate, 1.2%). Viruses were recovered from three PCR-positive rectal specimens by using WRCC/3873D and MDCK cells. The isolates possessed antigenic and genomic properties similar to those of the U.S. reference strain GA3 and were identified as MVC. In addition, seroepidemiological evidence that 5.0% of dogs possessed anti-MVC antibodies also indicated the presence of MVC infection among dogs in Japan. From this study and several recent European reports describing MVC field cases, it is evident that MVC is distributed among domestic dogs worldwide.

The canine minute virus (minute virus of canines) is a distinct parvovirus that is most similar to bovine parvovirus

We characterized the genome and proteins of the canine minute virus (the minute virus of canines (MVC)). The genome sequence showed MVC to be an autonomous parvovirus encoding a large nonstructural protein 1 gene, a smaller nonstructural protein, and overlapping VP1 and VP2 protein genes. The virus was most closely related to bovine parvovirus (BPV), with which it was 43% identical at the DNA sequence level, while the NS1 and VP1 proteins were 33.6 and 41.4% identical to those of BPV, respectively. Spliced messages of the NS1 gene transcripts were detected by RT-PCR. VP1 and VP2 proteins were detected in purified capsids, as were modified versions of each protein, and VP3 was also found in full capsids.

Evolutionary dynamics of viral attenuation

The genetic trajectory leading to viral attenuation was studied in a canine parvovirus (CPV) strain grown on dog kidney cells for 115 transfers. Consensus sequences of viral populations at passages 0, 3, 30, 50, 80, and 115 were obtained from PCR products covering 86% of the genome; clones from each of the 80th and 115th passages were also sequenced, covering 69% of the genome. Sixteen changes were fixed in the 115th-passage virus sample. Levels of polymorphism were strikingly different over time, in part because of a plaque-cloning step at passage 112 that reduced variation: passage 80 had 19 variants common among the clones, but passage 115 had only a single common variant. Several mutations increased in the culture at the same time, with most reaching fixation only after the 80th passage. The pattern of evolution was consistent with recombination and not with separate selective sweeps of individual mutations. Thirteen of the changes observed were identical to or at the same positions as changes observed in other isolates of CPV or feline panleukopenia virus.

The structure of porcine parvovirus: comparison with related viruses

The structure of baculovirus-expressed porcine parvovirus (PPV) capsids was solved using X-ray crystallography and was found to be similar to the related canine parvovirus (CPV) and minute virus of mice (MVM). The PPV capsid protein has 57 % and 49 % amino acid sequence identity with CPV and MVM, respectively, but the degree of conservation of surface-exposed residues is lower than average. Consequently, most of the structural differences are on the surface and are the probable cause of the known variability in antigenicity and host range. The NADL-2 and Kresse strains of PPV have distinct tissue tropisms and pathogenicity, which are mediated by one or more of the amino acid residues 381, 386, and 436. These residues are on or near the surface of the virus capsid, where they are likely to be associated with virus-cell interactions.