Altered receptor specificity of coxsackievirus B3 after growth in rhabdomyosarcoma cells

Vaccines against Group B Coxsackieviruses and Their Importance

The group B coxsackieviruses (CVBs) exist in six serotypes (CVB1 to CVB6). Disease associations have been reported for most serotypes, and multiple serotypes can cause similar diseases. For example, CVB1, CVB3, and CVB5 are generally implicated in the causation of myocarditis, whereas CVB1 and CVB4 could accelerate the development of type 1 diabetes (T1D). Yet, no vaccines against these viruses are currently available. In this review, we have analyzed the attributes of experimentally tested vaccines and discussed their merits and demerits or limitations, as well as their impact in preventing infections, most importantly myocarditis and T1D.

Chemical Evolution of Rhinovirus Identifies Capsid-Destabilizing Mutations Driving Low-pH-Independent Genome Uncoating

Rhinoviruses (RVs) cause recurrent infections of the nasal and pulmonary tracts, life-threatening conditions in chronic respiratory illness patients, predisposition of children to asthmatic exacerbation, and large economic cost. RVs are difficult to treat. They rapidly evolve resistance and are genetically diverse. Here, we provide insight into RV drug resistance mechanisms against chemical compounds neutralizing low pH in endolysosomes. Serial passaging of RV-A16 in the presence of the vacuolar proton ATPase inhibitor bafilomycin A1 (BafA1) or the endolysosomotropic agent ammonium chloride (NH4Cl) promoted the emergence of resistant virus populations. We found two reproducible point mutations in viral proteins 1 and 3 (VP1 and VP3), A2526G (serine 66 to asparagine [S66N]), and G2274U (cysteine 220 to phenylalanine [C220F]), respectively. Both mutations conferred cross-resistance to BafA1, NH4Cl, and the protonophore niclosamide, as identified by massive parallel sequencing and reverse genetics, but not the double mutation, which we could not rescue. Both VP1-S66 and VP3-C220 locate at the interprotomeric face, and their mutations increase the sensitivity of virions to low pH, elevated temperature, and soluble intercellular adhesion molecule 1 receptor. These results indicate that the ability of RV to uncoat at low endosomal pH confers virion resistance to extracellular stress. The data endorse endosomal acidification inhibitors as a viable strategy against RVs, especially if inhibitors are directly applied to the airways. IMPORTANCE Rhinoviruses (RVs) are the predominant agents causing the common cold. Anti-RV drugs and vaccines are not available, largely due to rapid evolutionary adaptation of RVs giving rise to resistant mutants and an immense diversity of antigens in more than 160 different RV types. In this study, we obtained insight into the cell biology of RVs by harnessing the ability of RVs to evolve resistance against host-targeting small chemical compounds neutralizing endosomal pH, an important cue for uncoating of normal RVs. We show that RVs grown in cells treated with inhibitors of endolysosomal acidification evolved capsid mutations yielding reduced virion stability against elevated temperature, low pH, and incubation with recombinant soluble receptor fragments. This fitness cost makes it unlikely that RV mutants adapted to neutral pH become prevalent in nature. The data support the concept of host-directed drug development against respiratory viruses in general, notably at low risk of gain-of-function mutations.

Development of Group B Coxsackievirus as an Oncolytic Virus: Opportunities and Challenges

Oncolytic viruses have emerged as a promising strategy for cancer therapy due to their dual ability to selectively infect and lyse tumor cells and to induce systemic anti-tumor immunity. Among various candidate viruses, coxsackievirus group B (CVBs) have attracted increasing attention in recent years. CVBs are a group of small, non-enveloped, single-stranded, positive-sense RNA viruses, belonging to species human Enterovirus B in the genus Enterovirus of the family Picornaviridae. Preclinical studies have demonstrated potent anti-tumor activities for CVBs, particularly type 3, against multiple cancer types, including lung, breast, and colorectal cancer. Various approaches have been proposed or applied to enhance the safety and specificity of CVBs towards tumor cells and to further increase their anti-tumor efficacy. This review summarizes current knowledge and strategies for developing CVBs as oncolytic viruses for cancer virotherapy. The challenges arising from these studies and future prospects are also discussed in this review.

Slow Infection due to Lowering the Amount of Intact versus Empty Particles Is a Characteristic Feature of Coxsackievirus B5 Dictated by the Structural Proteins

Enterovirus B species typically cause a rapid cytolytic infection leading to efficient release of progeny viruses. However, they are also capable of persistent infections in tissues, which are suggested to contribute to severe chronic states such as myocardial inflammation and type 1 diabetes. In order to understand the factors contributing to differential infection strategies, we constructed a chimera by combining the capsid proteins from fast-cytolysis-causing echovirus 1 (EV1) with nonstructural proteins from coxsackievirus B5 (CVB5), which shows persistent infection in RD cells. The results showed that the chimera behaved similarly to parental EV1, leading to efficient cytolysis in both permissive A549 and semipermissive RD cells. In contrast to EV1 and the chimera, CVB5 replicated slowly in permissive cells and showed persistent infection in semipermissive cells. However, there was no difference in the efficiency of uptake of CVB5 in A549 or RD cells in comparison to the chimera or EV1. CVB5 batches constantly contained significant amounts of empty capsids, also in comparison to CVB5's close relative CVB3. During successive passaging of batches containing only intact CVB5, increasing amounts of empty and decreasing amounts of infective capsids were produced. Our results demonstrate that the increase in the amount of empty particles and the lowering of the amount of infective particles are dictated by the CVB5 structural proteins, leading to slowing down of the infection between passages. Furthermore, the key factor for persistent infection is the small amount of infective particles produced, not the high number of empty particles that accumulate.IMPORTANCE Enteroviruses cause several severe diseases, with lytic infections that lead to rapid cell death but also persistent infections that are more silent and lead to chronic states of infection. Our study compared a cytolytic echovirus 1 infection to persistent coxsackievirus B5 infection by making a chimera with the structural proteins of echovirus 1 and the nonstructural proteins of coxsackievirus B5. Coxsackievirus B5 infection was found to lead to the production of a high number of empty viruses (empty capsids) that do not contain genetic material and are unable to continue the infection. Coinciding with the high number of empty capsids, the amount of infective virions decreased. This characteristic property was not observed in the constructed chimera virus, suggesting that structural proteins are in charge of these phenomena. These results shed light on the mechanisms that may cause persistent infections. Understanding events leading to efficient or inefficient infections is essential in understanding virus-caused pathologies.

Directed evolution as a tool for the selection of oncolytic RNA viruses with desired phenotypes

Viruses have some characteristics in common with cell-based life. They can evolve and adapt to environmental conditions. Directed evolution can be used by researchers to produce viral strains with desirable phenotypes. Through bioselection, improved strains of oncolytic viruses can be obtained that have better safety profiles, increased specificity for malignant cells, and more efficient spread among tumor cells. It is also possible to select strains capable of killing a broader spectrum of cancer cell variants, so as to achieve a higher frequency of therapeutic responses. This review describes and analyses virus adaptation studies performed with members of four RNA virus families that are used for viral oncolysis: reoviruses, paramyxoviruses, enteroviruses, and rhabdoviruses.

Bioselection of coxsackievirus B6 strain variants with altered tropism to human cancer cell lines

Cancer cells develop increased sensitivity to members of many virus families and, in particular, can be efficiently infected and lysed by many low-pathogenic human enteroviruses. However, because of their great genetic heterogeneity, cancer cells display different levels of sensitivity to particular enterovirus strains, which may substantially limit the chances of a positive clinical response. We show that a non-pathogenic strain of coxsackievirus B6 (LEV15) can efficiently replicate to high titers in the malignant human cell lines C33A, DU145, AsPC-1 and SK-Mel28, although it displays much lower replication efficiency in A431 and A549 cells and very limited replication ability in RD and MCF7 cells, as well as in the normal lung fibroblast cell line MRC-5 and the immortalized mammary epithelial cell line MCF10A. By serial passaging in RD, MCF7 and A431 cells, we obtained LEV15 strain variants that had acquired high replication capacity in the appropriate carcinoma cell lines without losing their high replication capability in the original set of cancer cell lines and had limited replication capability in untransformed cells. The strains demonstrated improved oncolytic properties in nude-mouse xenografts. We identified nucleotide changes responsible for the phenotypes and suggest a bioselection approach for a generation of oncolytic virus strains with a wider spectrum of affected tumors.

Three capsid amino acids notably influence coxsackie B3 virus stability

Coxsackievirus B3 strain 28 (CVB3/28) is less stable at 37 °C than eight other CVB3 strains with which it has been compared, including four in this study. In a variant CVB3/28 population selected for increased stability at 37 °C, the capsid proteins of the stable variant differed from the parental CVB3/28 by two mutations in Vp1 and one mutation in Vp3, each of which resulted in altered protein sequences. Each of the amino acid changes was individually associated with a more stable virus. Competition between CVB3/28 and a more stable derivative of the strain showed that propagation of the less stable virus was favoured in receptor-rich HeLa cells.

Efficient replication of recombinant Enterovirus B types, carrying different P1 genes in the coxsackievirus B5 replicative backbone

Recombination is an important feature in the evolution of the Enterovirus genus. Phylogenetic studies of enteroviruses have revealed that the capsid genomic region (P1) is type specific, while the parts of the genome coding for the non-structural proteins (P2-P3) are species specific. Hence, the genome may be regarded as consisting of two modules that evolve independently. In this study, it was investigated whether the non-structural coding part of the genome in one type could support replication of a virus with a P1 region from another type of the same species. A cassette vector (pCas) containing a full-length cDNA copy of coxsackievirus B5 (CVB5) was used as a replicative backbone. The P1 region of pCas was replaced with the corresponding part from coxsackievirus B3 Nancy (CVB3N), coxsackievirus B6 Schmitt (CVB6S), and echovirus 7 Wallace (E7W), all members of the Enterovirus B species. The replication efficiency after transfection with clone-derived in vitro transcribed RNA was studied and compared with that of pCas. All the recombinant viruses replicated with similar efficiencies and showed threshold cycle (Ct) values, tissue culture infectivity dose 50 %, and plaque-forming unit titers comparable to viruses generated from the pCas construct. In addition to this, a clone without the P1 region was also constructed, and Western Blot and immunofluorescence staining analysis showed that the viral genome could be translated and replicated despite the lack of the structural protein-coding region. To conclude, the replicative backbone of the CVB5 cassette vector supports replication of intraspecies constructs with P1 regions derived from other members of the Enterovirus B species. In addition to this, the replicative backbone can be both translated and replicated without the presence of a P1 region.

Expression of human decay-accelerating factor on intestinal epithelium of transgenic mice does not facilitate infection by the enteral route

In vitro, infection of polarized human intestinal epithelial cells by coxsackievirus B3 (CVB3) depends on virus interaction with decay-accelerating factor (DAF), a receptor expressed on the apical cell surface. Although mice are highly susceptible to CVB3 infection when virus is delivered by intraperitoneal injection, infection by the enteral route is very inefficient. Murine DAF, unlike human DAF, does not bind virus, and we hypothesized that the absence of an accessible receptor on the intestinal surface is an important barrier to infection by the oral route. We generated transgenic mice that express human DAF specifically on intestinal epithelium and measured their susceptibility to infection by a DAF-binding CVB3 isolate. Human DAF permitted CVB3 to bind to the intestinal surface ex vivo and to infect polarized monolayers of small-intestinal epithelial cells derived from DAF transgenic mice. However, expression of human DAF did not facilitate infection by the enteral route either in immunocompetent animals or in animals deficient in the interferon alpha/beta receptor. These results indicate that the absence of an apical receptor on intestinal epithelium is not the major barrier to infection of mice by the oral route.

IMPORTANCE

CVB3 infection of human intestinal epithelial cells depends on DAF at the apical cell surface, and expression of human DAF on murine intestinal epithelial cells permits their infection in vitro. However, expression of human DAF on the intestinal surface of transgenic mice did not facilitate infection by the oral route. Although the role of intestinal DAF in human infection has not been directly examined, these results suggest that DAF is not the critical factor in mice.

Specificity of coxsackievirus B3 interaction with human, but not murine, decay-accelerating factor: replacement of a single residue within short consensus repeat 2 prevents virus attachment

Many coxsackievirus B (CVB) isolates bind to human decay-accelerating factor (DAF) as well as to the coxsackievirus and adenovirus receptor (CAR). However, the virus does not interact with murine DAF. To understand why CVB3 binds specifically to human DAF, we constructed a series of chimeric molecules in which specific regions of the human DAF molecule were replaced by the corresponding murine sequences. We found that replacement of human short consensus repeat 2 (SCR2) with murine SCR2 ablated virus binding to human DAF, as did deletion of human SCR2. Although replacement of human SCR4 had a partial inhibitory effect, deletion of SCR4 had no effect. Within human SCR2, replacement of serine 104 (S104) with the proline residue found in murine DAF eliminated virus binding. On the basis of the structure of the CVB3-DAF complex determined by cryo-electron microscopy, DAF S104 is in close contact with a viral capsid residue, a threonine at VP1 position 271. Replacement of this capsid residue with larger amino acids specifically eliminated virus attachment to human DAF but had no effect on attachment to CAR or replication in HeLa cells. Taken together, these results support the current model of virus-DAF interaction and point to a specific role for VP1 T271 and DAF S104 at the virus-DAF interface.

IMPORTANCE

The results of the present study point to a specific role for VP1 T271 and DAF S104 at the interface between CVB3 and DAF, and they demonstrate how subtle structural changes can dramatically influence virus-receptor interactions. In addition, the results support a recent pseudoatomic model of the CVB3-DAF interaction obtained by cryo-electron microscopy.

HeLa cell heterogeneity and coxsackievirus B3 cytopathic effect: implications for inter-laboratory reproducibility of results

Concerns over cell line identities and contamination have led investigators to acquire fresh stocks of HeLa CCL-2 cells, but results with the HeLa CCL-2 cells do not always reproduce results with HeLa cells that have long history in the laboratory. When used for TCID(50) assays of Coxsackievirus B3/28 (CVB3/28), HeLa CCL-2 cells returned titers for CVB3/28 that were more than ten-fold lower than titers obtained using laboratory HeLa cells. The viral cytopathic effect was less distinct in the HeLa CCL-2 cultures, suggestive of a mixed population of cells with varied susceptibility to viral cytopathic effect. Analysis of short tandem repeat markers confirmed the identities of the cell lines as HeLa. Subpopulations in the HeLa CCL-2 culture, separated easily based on the speed with which they were released by trypsin-EDTA, differed in their susceptibilities to CVB3/28 cytopathic effect, and in their expression of the Coxsackievirus and adenovirus receptor (CAR). The distinctions between Lab HeLa and HeLa CCL-2 cells were less obvious when infected with CVB3/RD, a strain selected for growth in RD cells. Results that differ among laboratories may be due to the use of HeLa cell strains with different histories, and experiments using HeLa CCL-2 available from the American Type Culture Collection are probably incapable of reproducing many of the published studies of Coxsackievirus that have used HeLa cells with laboratory-dependent histories.

The crystal structure of a coxsackievirus B3-RD variant and a refined 9-angstrom cryo-electron microscopy reconstruction of the virus complexed with decay-accelerating factor (DAF) provide a new footprint of DAF on the virus surface

The coxsackievirus-adenovirus receptor (CAR) and decay-accelerating factor (DAF) have been identified as cellular receptors for coxsackievirus B3 (CVB3). The first described DAF-binding isolate was obtained during passage of the prototype strain, Nancy, on rhabdomyosarcoma (RD) cells, which express DAF but very little CAR. Here, the structure of the resulting variant, CVB3-RD, has been solved by X-ray crystallography to 2.74 Å, and a cryo-electron microscopy reconstruction of CVB3-RD complexed with DAF has been refined to 9.0 Å. This new high-resolution structure permits us to correct an error in our previous view of DAF-virus interactions, providing a new footprint of DAF that bridges two adjacent protomers. The contact sites between the virus and DAF clearly encompass CVB3-RD residues recently shown to be required for binding to DAF; these residues interact with DAF short consensus repeat 2 (SCR2), which is known to be essential for virus binding. Based on the new structure, the mode of the DAF interaction with CVB3 differs significantly from the mode reported previously for DAF binding to echoviruses.

Persistent infection of human intestinal Caco-2 cell line by Coxsackieviruses B

Coxsackieviruses B (CV-B, Picornaviridae family, genus Enterovirus) are characterized by their ability to cause cytopathic effects in tissue culture. These viruses are considered highly cytolytic, but can establish persistence/latency in susceptible cells, indicating that a regulatory mechanism may exist to shut off viral protein synthesis and replication under certain situations. The persistence of coxsackieviral B RNA is of great interest because of its implication in the pathogenesis of several chronic human diseases. However, a few studies have dealt with the persistence of these viruses at the intestinal level. The aim of this study is to test the capacity of the six CV-B serotypes to establish persistent infection in human intestinal Caco-2 cell line. Ten CV-B isolates, including CV-B3 prototype strain (Nancy) and a recombinant isolate (B3-B4), were tested. Six CV-B isolates were found to establish persistent infections in Caco-2 cell line. Persistent replication was proved by the detection of viral RNA from cell cultures, VP1 capsid protein detection by immunofluorescence (IF) staining, and the release of infectious particles up to two months and a half after infection without any obvious cytolysis. In addition, our results suggest that the establishment of a persistent infection is serotype-independent.

Single amino acid changes in the virus capsid permit coxsackievirus B3 to bind decay-accelerating factor

Many coxsackievirus B isolates bind to human decay-accelerating factor (DAF) as well as to the coxsackievirus and adenovirus receptor (CAR). The first-described DAF-binding isolate, coxsackievirus B3 (CB3)-RD, was obtained during passage of the prototype strain CB3-Nancy on RD cells, which express DAF but very little CAR. CB3-RD binds to human DAF, whereas CB3-Nancy does not. To determine the molecular basis for the specific interaction of CB3-RD with DAF, we produced cDNA clones encoding both CB3-RD and CB3-Nancy and mutated each of the sites at which the RD and Nancy sequences diverged. We found that a single amino acid change, the replacement of a glutamate within VP3 (VP3-234E) with a glutamine residue (Q), conferred upon CB3-Nancy the capacity to bind DAF and to infect RD cells. Readaptation of molecularly cloned CB3-Nancy to RD cells selected for a new virus with the same VP3-234Q residue. In experiments with CB3-H3, another virus isolate that does not bind measurably to DAF, adaptation to RD cells resulted in a DAF-binding isolate with a single amino acid change within VP2 (VP2-138 N to D). Both VP3-234Q and VP2-138D were required for binding of CB3-RD to DAF. In the structure of the CB3-RD-DAF complex determined by cryo-electron microscopy, both VP3-234Q and VP2-138D are located at the contact site between the virus and DAF.

Development of a recombinant CHO cell model for the investigation of CAR and DAF role during early steps of echovirus 6 infection

The early steps of echovirus 6 (E6) infection remain poorly understood and the only described receptor for haemagglutinating E6 strains is the decay accelerating factor (DAF). There is, however, accumulating evidence suggesting that E6 interaction with DAF is necessary but not sufficient for infection. In this report, we investigated the role of the coxsackie-adenovirus-receptor (CAR) as a potential DAF co-receptor during E6 infection. Using stably transfected Chinese Hamster Ovary (CHO) cells expressing CAR and DAF receptors, we found that DAF expression allowed attachment of both haemagglutinating and non-haemagglutinating E6 strains but was not sufficient for promoting E6 cell entry. Interestingly, the co-expression of DAF and CAR rendered 0.1-0.2% of cells permissive to some E6 strains' infection. Although our results did not show a major role of the CAR/DAF cooperation for E6 infection, it nevertheless indicated the use of CAR in the cell entry step of some minor E6 quasispecies. Moreover, the present report validates the use of recombinant CHO cells as valuable cellular model for the further characterisation of E6 receptors.

Variations of coxsackievirus B3 capsid primary structure, ligands, and stability are selected for in a coxsackievirus and adenovirus receptor-limited environment

While group B coxsackieviruses (CVB) use the coxsackievirus and adenovirus receptor (CAR) as the receptor through which they infect susceptible cells, some CVB strains are known for their acquired capacity to bind other molecules. The CVB3/RD strain that emerged from a CVB3/Nancy population sequentially passaged in the CAR-poor RD cell line binds decay-accelerating factor (DAF) (CD55) and CAR. A new strain, CVB3/RDVa, has been isolated from RD cells chronically infected with CVB3/RD and binds multiple molecules in addition to DAF and CAR. The capsid proteins of CVB3/RD differ from those of CVB3/28, a cloned strain that binds only CAR, by only four amino acids, including a glutamate/glutamine dimorphism in the DAF-binding region of the capsid. The capsid proteins of CVB3/RD and CVB3/RDVa differ by seven amino acids. The ability of CVB3/RDVa to bind ligands in addition to CAR and DAF may be attributed to lysine residues near the icosahedral 5-fold axes of symmetry. Considered with differences in the stability of the CVB3 strains, these traits suggest that in vitro selection in a CAR-limited environment selects for virus populations that can associate with molecules on the cell surface and survive until CAR becomes available to support infection.

Release of intracellular calcium stores facilitates coxsackievirus entry into polarized endothelial cells

Group B coxsackieviruses (CVB) are associated with viral-induced heart disease and are among the leading causes of aseptic meningitis worldwide. Here we show that CVB entry into polarized brain microvasculature and aortic endothelial cells triggers a depletion of intracellular calcium stores initiated through viral attachment to the apical attachment factor decay-accelerating factor. Calcium release was dependent upon a signaling cascade that required the activity of the Src family of tyrosine kinases, phospholipase C, and the inositol 1,4,5-trisphosphate receptor isoform 3. CVB-mediated calcium release was required for the activation of calpain-2, a calcium-dependent cysteine protease, which controlled the vesicular trafficking of internalized CVB particles. These data point to a specific role for calcium signaling in CVB entry into polarized endothelial monolayers and highlight the unique signaling mechanisms used by these viruses to cross endothelial barriers.

Enteroviruses are associated with a number of diverse syndromes such as myocarditis, febrile illness, and are the main causative agents of aseptic meningitis. No effective therapeutics exist to combat non-poliovirus enterovirus infections. A better understanding of the mechanisms by which these viruses infect host cells could lead to the design of effective therapeutic interventions. In this study, we found that intracellular calcium stores in polarized endothelial monolayers are depleted upon exposure to coxsackievirus B (CVB) and that this release is mediated by viral attachment to its receptor decay-accelerating factor. We also discovered that the calcium release requires the activation of signaling molecules involved in calcium signaling such as Src tyrosine kinases, phospholipase C, and the inositol 1,4,5-trisphosphate receptor isoform 3 on the ER membrane. Furthermore, we found that a calcium-activated cystein protease, calpain-2, was activated and necessary for proper viral trafficking inside the cell. Interestingly, we found that this signaling cascade was critical for CVB internalization into the endothelium, but was not involved in CVB entry into the epithelium. This is an important advance in our understanding of how enteroviruses hijack host endothelial cell signaling mechanisms in order to facilitate their entry and eventual spread.

Characterization of a putative ancestor of coxsackievirus B5

Like other RNA viruses, coxsackievirus B5 (CVB5) exists as circulating heterogeneous populations of genetic variants. In this study, we present the reconstruction and characterization of a probable ancestral virion of CVB5. Phylogenetic analyses based on capsid protein-encoding regions (the VP1 gene of 41 clinical isolates and the entire P1 region of eight clinical isolates) of CVB5 revealed two major cocirculating lineages. Ancestral capsid sequences were inferred from sequences of these contemporary CVB5 isolates by using maximum likelihood methods. By using Bayesian phylodynamic analysis, the inferred VP1 ancestral sequence dated back to 1854 (1807 to 1898). In order to study the properties of the putative ancestral capsid, the entire ancestral P1 sequence was synthesized de novo and inserted into the replicative backbone of an infectious CVB5 cDNA clone. Characterization of the recombinant virus in cell culture showed that fully functional infectious virus particles were assembled and that these viruses displayed properties similar to those of modern isolates in terms of receptor preferences, plaque phenotypes, growth characteristics, and cell tropism. This is the first report describing the resurrection and characterization of a picornavirus with a putative ancestral capsid. Our approach, including a phylogenetics-based reconstruction of viral predecessors, could serve as a starting point for experimental studies of viral evolution and might also provide an alternative strategy for the development of vaccines.

Dynamin- and lipid raft-dependent entry of decay-accelerating factor (DAF)-binding and non-DAF-binding coxsackieviruses into nonpolarized cells

Group B coxsackieviruses (CVB) use the CVB and adenovirus receptor (CAR) to enter and infect cells. Some CVB also bind to decay-accelerating factor (DAF), but that interaction alone is insufficient for infection. We previously found that CVB3 entry into polarized human intestinal cells (Caco-2) occurs by a caveolin-dependent but dynamin-independent mechanism that requires DAF-mediated tyrosine kinase signals. In this study, we examined how CVB enter and infect nonpolarized HeLa cells and how DAF binding affects these processes. Using immunofluorescence microscopy and a combination of dominant-negative proteins, small interfering RNAs, and drugs targeting specific endocytic pathways, we found that both DAF-binding and non-DAF-binding virus isolates require dynamin and lipid rafts to enter and infect cells. Unlike what we observed in Caco-2 cells, CVB3 entered HeLa cells with CAR. We found no role for clathrin, endosomal acidification, or caveolin. Inhibition of tyrosine kinases blocked an early event in infection but did not prevent entry of virus into the cell. These results indicate that CVB3 entry into nonpolarized HeLa cells differs significantly from entry into polarized Caco-2 cells and is not influenced by virus binding to DAF.

Replication of Ljungan virus in cell culture: The genomic 5′-end, infectious cDNA clones and host cell response to viral infections

Ljungan virus (LV) is a picornavirus recently isolated from bank voles (Clethrionomys glareolus). The previously uncharacterised 5'-end sequence of the LV genome was determined. Infectious cDNA clones were constructed of the wild type LV prototype strain 87-012 and of the cytolytically replicating cell culture adapted variant 87-012G. Virus generated from cDNA clones showed identical growth characteristics as uncloned virus stocks. Cell culture adapted LV, 87-012G, showed a clear cytopathic effect (CPE) at 3-4 days post-infection (p.i.). Virus titers, determined by plaque titration, increased however only within the first 18h p.i. Replication of LV (+) strand RNA was determined by real-time PCR and corresponded in time with increasing titers. In contrast, the amounts of the replication intermediate, the (-) strand, continued to increase until the cells showed CPE. This indicates separate controlling mechanisms for replication of LV (+) and (-) genome strands. Replication was also monitored by immunofluorescence (IF) staining. IF staining of both prototype 87-012 and the CPE causing 87-012G showed groups of 5-25 infected cells at 48h p.i., suggesting a, for picornaviruses, not previously described direct cell-to-cell transmission.

Endogenous low-level expression of the coxsackievirus and adenovirus receptor enables coxsackievirus B3 infection of RD cells

Cells in which the appropriate viral receptor cannot be detected may paradoxically act as a host to the virus. For example, RD cells are often considered to be non-permissive for infection with coxsackievirus and adenovirus receptor (CAR)-dependent group B coxsackieviruses (CVB), insofar as inoculated cell monolayers show little or no cytopathic effect (CPE) and immunohistological assays for CAR have been consistently negative. Supernatants recovered from RD cells exposed to CVB, however, contained more virus than was added in the initial inoculum, indicating that productive virus replication occurred in the monolayer. When infected with a recombinant CVB type 3 (CVB3) chimeric strain expressing S-Tag within the viral polyprotein, 4–11 % of RD cells expressed S-Tag over 48 h. CAR mRNA was detected in RD cells by RT-PCR, and CAR protein was detected on Western blots of RD lysates; both were detected at much lower levels than in HeLa cells. Receptor blockade by an anti-CAR antibody confirmed that CVB3 infection of RD cells was mediated by CAR. These results show that some RD cells in the culture population express CAR and can thereby be infected by CVB, which explains the replication of CAR-dependent CVB in cell types that show little or no CPE and in which CAR has not previously been detected. Cells within cultures of cell types that have been considered non-permissive may express receptor transiently, leading to persistent replication of virus within the cultured population.

Coxsackievirus entry across epithelial tight junctions requires occludin and the small GTPases Rab34 and Rab5

The major group B coxsackievirus (CVB) receptor is a component of the epithelial tight junction (TJ), a protein complex that regulates the selective passage of ions and molecules across the epithelium. CVB enters polarized epithelial cells from the TJ, causing a transient disruption of TJ integrity. Here we show that CVB does not induce major reorganization of the TJ, but stimulates the specific internalization of occludin-a TJ integral membrane component-within macropinosomes. Although occludin does not interact directly with virus, depletion of occludin prevents CVB entry into the cytoplasm and inhibits infection. Both occludin internalization and CVB entry require caveolin but not dynamin; both are blocked by inhibitors of macropinocytosis and require the activity of Rab34, Ras, and Rab5, GTPases known to regulate macropinocytosis. Thus, CVB entry depends on occludin and occurs by a process that combines aspects of caveolar endocytosis with features characteristic of macropinocytosis.

Virus-induced Abl and Fyn kinase signals permit coxsackievirus entry through epithelial tight junctions

Group B coxsackieviruses (CVBs) must cross the epithelium as they initiate infection, but the mechanism by which this occurs remains uncertain. The coxsackievirus and adenovirus receptor (CAR) is a component of the tight junction and is inaccessible to virus approaching from the apical surface. Many CVBs also interact with the GPI-anchored protein decay-accelerating factor (DAF). Here, we report that virus attachment to DAF on the apical cell surface activates Abl kinase, triggering Rac-dependent actin rearrangements that permit virus movement to the tight junction. Within the junction, interaction with CAR promotes conformational changes in the virus capsid that are essential for virus entry and release of viral RNA. Interaction with DAF also activates Fyn kinase, an event that is required for the phosphorylation of caveolin and transport of virus into the cell within caveolar vesicles. CVBs thus exploit DAF-mediated signaling pathways to surmount the epithelial barrier.

Cytolytic replication of coxsackievirus B2 in CAR-deficient rhabdomyosarcoma cells

The six coxsackievirus B serotypes (CVB1-6) use the coxsackie- and adenovirus receptor (CAR) for host cell entry. Four of these serotypes, CVB1, 3, 5 and 6, have also shown the capacity to replicate and cause cytolysis in rhabdomyosarcoma (RD) cells, a CAR-deficient cell line. This extended tropism has been associated with an acquired ability to bind decay accelerating factor (DAF). In this study, we have adapted the CVB2 prototype strain Ohio-1 (CVB2/O) to replicate in RD cells. Two types of infection were identified: (I) an enterovirus-typical, lytic infection, and (II) a non-lytic infection. Both CVB2/O-RD variants retained the prototype-ability to cause cytopathic effect in HeLa cells using CAR as receptor. Phenotypic and genotypic changes in the CVB2/O-RD-variants were determined and compared to the prototype cultured in HeLa cells. Inhibition studies using antibodies against CAR and DAF revealed a maintained ability of the CVB2/O-RD-variants to bind CAR, but no binding to DAF was observed. In addition, neither the prototype nor the CVB2/O-RD-variants were able to cause hemagglutination in human red blood cells, an enterovirus feature associated with affinity for DAF. Sequence analysis of the CVB2/O-RD-variants showed acquired mutations in the capsid region, suggesting extended receptor usage towards an alternative, yet unidentified, receptor for CVB2.

Interaction with coxsackievirus and adenovirus receptor, but not with decay-accelerating factor (DAF), induces A-particle formation in a DAF-binding coxsackievirus B3 isolate

Although many coxsackie B viruses interact with decay accelerating factor (DAF), attachment to DAF by itself is not sufficient to initiate infection. We examined the early events in infection that follow virus interaction with DAF, and with the coxsackievirus and adenovirus receptor (CAR). Interaction with soluble CAR in a cell-free system, or with CAR on the surfaces of transfected cells, induced the formation of A particles; interaction with soluble or cell surface DAF did not. The results suggest that CAR, but not DAF, is capable of initiating the conformational changes in the viral capsid that lead to release of viral nucleic acid.

Enterovirus infection in human pancreatic islet cells, islet tropism in vivo and receptor involvement in cultured islet beta cells

AIMS/HYPOTHESIS

It is thought that enterovirus infections cause beta-cell damage and contribute to the development of Type 1 diabetes by replicating in the pancreatic islets. We sought evidence for this through autopsy studies and by investigating known enterovirus receptors in cultured human islets.

METHODS

Autopsy pancreases from 12 newborn infants who died of fulminant coxsackievirus infections and from 65 Type 1 diabetic patients were studied for presence of enteroviral ribonucleic acid by in situ hybridisation. Forty non-diabetic control pancreases were included in the study. The expression and role of receptor candidates in cultured human islets were investigated with receptor-specific antibodies using immunocytochemistry and functional assays.

RESULTS

Enterovirus-positive islet cells were found in some of both autopsy specimen collections, but not in control pancreases. No infected cells were seen in exocrine tissue. The cell surface molecules, poliovirus receptor and integrin alphavbeta3, which act as enterovirus receptors in established cell lines, were expressed in beta cells. Antibodies to poliovirus receptor, human coxsackievirus and adenovirus receptor and integrin alphavbeta3 protected islets and beta cells from adverse effects of poliovirus, coxsackie B viruses, and several of the arginine-glycine-aspartic acid motifs containing enteroviruses and human parechovirus 1 respectively. No evidence was found for expression of the decay-accelerating factor which acts as a receptor for several islet-cell-replicating echoviruses in established cell lines.

CONCLUSIONS/INTERPRETATION

The results show a definite islet-cell tropism of enteroviruses in the human pancreas. Some enteroviruses seem to use previously identified cell surface molecules as receptors in beta cells, whereas the identity of receptors used by other enteroviruses remains unknown.

Coxsackievirus B3-associated myocardial pathology and viral load reduced by recombinant soluble human decay-accelerating factor in mice

Coxsackievirus B3 (CVB3) infection can result in myocarditis, which in turn may lead to a protracted immune response and subsequent dilated cardiomyopathy. Human decay-accelerating factor (DAF), a binding receptor for CVB3, was synthesized as a soluble IgG1-Fc fusion protein (DAF-Fc). In vitro, DAF-Fc was able to inhibit complement activity and block infection by CVB3, although blockade of infection varied widely among strains of CVB3. To determine the effects of DAF-Fc in vivo, 40 adolescent A/J mice were infected with a myopathic strain of CVB3 and given DAF-Fc treatment 3 days before infection, during infection, or 3 days after infection; the mice were compared with virus alone and sham-infected animals. Sections of heart, spleen, kidney, pancreas, and liver were stained with hematoxylin and eosin and submitted to in situ hybridization for both positive-strand and negative-strand viral RNA to determine the extent of myocarditis and viral infection, respectively. Salient histopathologic features, including myocardial lesion area, cell death, calcification and inflammatory cell infiltration, pancreatitis, and hepatitis were scored without knowledge of the experimental groups. DAF-Fc treatment of mice either preceding or concurrent with CVB3 infection resulted in a significant decrease in myocardial lesion area and cell death and a reduction in the presence of viral RNA. All DAF-Fc treatment groups had reduced infectious CVB3 recoverable from the heart after infection. DAF-Fc may be a novel therapeutic agent for active myocarditis and acute dilated cardiomyopathy if given early in the infectious period, although more studies are needed to determine its mechanism and efficacy.

Interaction with decay-accelerating factor facilitates coxsackievirus B infection of polarized epithelial cells

All coxsackie B (CB) viruses can initiate infection by attaching to the coxsackievirus and adenovirus receptor (CAR). Although some CB isolates also bind to decay-accelerating factor (DAF), the role of DAF interaction during infection remains uncertain. We recently observed that CAR in polarized epithelial cells is concentrated at tight junctions, where it is relatively inaccessible to virus. In the experiments reported here we found that, unlike CAR, DAF was present on the apical surface of polarized cells and that DAF-binding isolates of CB3 and CB5 infected polarized epithelial cells more efficiently than did isolates incapable of attaching to DAF. Virus attachment and subsequent infection of polarized cells by DAF-binding isolates were prevented in the presence of anti-DAF antibody. Serial passage on polarized cell monolayers selected for DAF-binding virus variants. Taken together, these results indicate that interaction with DAF on the apical surface of polarized epithelial cells facilitates infection by a subset of CB virus isolates. The results suggest a possible role for DAF in infection of epithelial cells at mucosal surfaces.

Comparative analysis of two coxsackievirus B3 strains: putative influence of virus-receptor interactions on pathogenesis

Strain-specific differences in the interaction of coxsackievirus B3 (CVB3) with the coxsackievirus-adenovirus receptor (CAR) and the decay-accelerating factor (DAF) co-receptor proteins were investigated using a non-haemagglutinating (CVB3) and a haemagglutinating (CVB3-HA) strain of CVB3. A panel of receptor-transfected hamster CHO cells, expressing either CAR (CHOCAR cells), DAF (CHODAF cells), or both receptor proteins (CHODC cells) were used to study the interplay of CAR and DAF receptor molecules with regard to binding and infection with CVB3 and CVB3-HA. Despite clear differences in their binding phenotypes, both virus strains were found to primarily depend on the CAR receptor protein for initialization of productive infections. Cytopathic effects induced by CVB3-HA were influenced by co-expression of DAF receptor proteins. The cardiotropic potential of both virus strains was investigated in A.BY/SnJ mice. Despite comparable virus replication of both CVB3 strains in individual myocytes, the number of infected heart muscle cells was significantly lower in CVB3-HA infected mice. Infections of pancreata correlated with myocardial infections. Together these data suggest that even small differences in virus-receptor interactions, influencing virus binding and virus spread, may have an impact on the pathogenesis of CVB-induced diseases.

Cell cycle status affects coxsackievirus replication, persistence, and reactivation in vitro

Enteroviral persistence has been implicated in the pathogenesis of several chronic human diseases, including dilated cardiomyopathy, insulin-dependent diabetes mellitus, and chronic inflammatory myopathy. However, these viruses are considered highly cytolytic, and it is unclear what mechanisms might permit their long-term survival. Here, we describe the generation of a recombinant coxsackievirus B3 (CVB3) expressing the enhanced green fluorescent protein (eGFP), which we used to mark and track infected cells in vitro. Following exposure of quiescent tissue culture cells to either wild-type CVB3 or eGFP-CVB3, virus production was very limited but increased dramatically after cells were permitted to divide. Studies with cell cycle inhibitors revealed that cells arrested at the G(1) or G(1)/S phase could express high levels of viral polyprotein and produced abundant infectious virus. In contrast, both protein expression and virus yield were markedly reduced in quiescent cells (i.e., cells in G(0)) and in cells blocked at the G(2)/M phase. Following infection with eGFP-CVB3, quiescent cells retained viral RNA for several days in the absence of infectious virus production. Furthermore, RNA extracted from nonproductive quiescent cells was infectious when transfected into dividing cells, indicating that CVB3 appears to be capable of establishing a latent infection in G(0) cells, at least in tissue culture. Finally, wounding of infected quiescent cells resulted in viral protein expression limited to cells in and adjacent to the lesion. We suggest that (i) cell cycle status determines the distribution of CVB3 during acute infection and (ii) the persistence of CVB3 in vivo may rely on infection of quiescent (G(0)) cells incapable of supporting viral replication; a subsequent change in the cell cycle status may lead to virus reactivation, triggering chronic viral and/or immune-mediated pathology in the host.

Receptor for the group B coxsackieviruses and adenoviruses: CAR

Considerable progress towards the characterisation of the long-sought receptor, CAR (coxsackievirus and adenovirus receptor), shared by group B coxsackieviruses (CVB) and most adenoviruses (Ad) has been made since it was isolated and cloned in 1997. The primary sequence of CAR shows that it is a member of the immunoglobulin superfamily of proteins, containing two Ig superfamily domains: an amino-terminal V-like module and a C2-like module. The CAR cytoplasmic domain, representing nearly one-third of the protein, is separated from the C2-like module by a single membrane-spanning sequence. The structure of the CAR V-like module complexed with the Ad fibre knob has been determined using recombinant proteins, and reveals three CAR modules associated with a single knob. Although recombinant CAR expressed in mammalian cells confers permissivity to CVB infection, details of the interaction between CAR and CVB remain to be elucidated. The expression of CAR appears to be highly regulated with respect to both cell type and developmental age. In rodents, CAR is expressed at high levels just before birth, and declines thereafter. Expressed levels have been found to increase in regenerating muscle and in response to immunological mediators or inflammation, and in RD cells and umbilical vein endothelial cells in response to high cell density. These studies indicate that CAR expression is highly regulated, but the mechanisms and molecules that mediate the expression remain to be discovered. The physiological function of CAR and its natural ligand also remain to be discovered. In addition, while CAR expression generally correlates with viral tropism, the relationship between the physiological function of CAR and the pathologies of CVB and Ad infections remain to be described.

Attachment of coxsackievirus B3 variants to various cell lines: mapping of phenotypic differences to capsid protein VP1

The coxsackievirus B3 (CVB3) strain Nancy P establishes a persistent carrier-state infection without visible cytopathic effect in primary human fibroblasts (HuFi H), whereas the derivative variant PD induces a complete lysis of the cell monolayer. To define the molecular basis of this exceptional growth property, the complete genomes of both viruses were sequenced and compared to all published sequences of CVB3. As a result, six unique amino acid substitutions in the VP1 capsid protein were observed. Via hybrid virus construction, the lytic phenotype was transferred to a nonlytic cDNA-generated CVB3. Mapping experiments indicate that the presence of amino acid residues K78, A80, A91, and I92 in VP1 is sufficient to induce "lytic" infections in HuFi H cells. Binding assays demonstrate that CVB3 Nancy P preferentially binds to the human coxsackievirus-adenovirus receptor (CAR), while PD exhibits a very weak interaction with CAR but strong binding to the decay accelerating factor (DAF). These results suggest that the mutated amino acid residues in VP1 are involved in receptor recognition/binding. Moreover, the lytic replication of CVB3 PD and the hybrid virus in various nonpermissive rodent cell lines indicates that cell surface molecules other than CAR and DAF may be involved in attachment of this variant to cell surfaces.

Comparative analysis of virus-host cell interactions of haemagglutinating and non-haemagglutinating strains of coxsackievirus B3

Decay-accelerating factor (DAF/CD55), and coxsackievirus-adenovirus receptor (CAR) have been identified as cellular receptors for coxsackie B viruses (CBV). To elucidate the interplay of DAF and CAR on the cell surface, virus-receptor interactions of two coxsackieviruses of serotype B3 (non-haemagglutinating CBV3 and haemagglutinating CBV3-HA strain) were analysed. Binding assays revealed clear differences between these viruses with regard to their interactions with DAF and CAR. However, only the combination of anti-DAF and anti-CAR antibodies resulted in complete inhibition of virus binding for both strains. In plaque-reduction assays, anti-DAF antibodies had no effect, whereas CAR-specific antibodies significantly reduced productive infection of HeLa cells by both viruses. Interestingly, a synergistic inhibitory effect of anti-DAF and anti-CAR antibodies was also observed with regard to infection. These findings support the model of preferential interactions of both strains of CBV3 with closely associated DAF and CAR proteins on HeLa cells, despite displaying clear differences in their binding phenotypes.

Mapping the binding domains on decay accelerating factor (DAF) for haemagglutinating enteroviruses: implications for the evolution of a DAF-binding phenotype

Decay accelerating factor (DAF) functions as a cell attachment receptor for a wide range of human enteroviruses, the interaction accounting for the haemagglutination phenotype exhibited by many members of this family. Haemagglutination inhibition assays using purified truncated soluble DAF (sDAF) receptors and short consensus repeat (SCR) domain-specific antibodies have been used to determine the domain(s) of DAF to which the viruses bind. Further sDAF-mediated virus neutralization and biosensor analysis have been used to confirm the virus-binding domains of DAF. Of the four distinct clusters of human enteroviruses, three contain representatives that bind DAF. The majority of DAF-binding enteroviruses occupy the 'CBV-like' cluster, and require SCR domains 2-4 for DAF binding. In contrast, the DAF-binding representatives of the 'ENV70-like' and 'PV-like' clusters require SCR1 for DAF interaction. These studies confirm that DAF binding is a widespread characteristic amongst phylogenetically divergent clusters within the enteroviruses and suggest that the ability to bind DAF may have evolved more than once within this group of viruses.

Genomic and phylogenetic characterization of coxsackievirus B2 prototype strain Ohio-1

The human picornavirus coxsackievirus B2 (CVB2) is often linked to several infections, from mild respiratory diseases to more severe illnesses such as myocarditis. In this study, we report the complete genome sequence of CVB2 prototype strain Ohio-1. The genome sequence was determined from reverse transcribed viral RNA, amplified with long distance PCR and used for non-radioactive sequencing. The full length PCR amplicons were used for in vitro transcription and the obtained cRNA was lipofected onto green monkey kidney cells, in order to confirm that the PCR generated sequence reflects a viable virus RNA. The CVB2 genome sequence shows a typical enterovirus genome organization with a total length of 7411 nucleotides. Phylogenetic analysis, using the CVB2 polyprotein in comparison with other enterovirus polyproteins, clearly shows that CVB2 clusters with the coxsackievirus B-like enteroviruses and is more related to coxsackievirus B4 (CVB4) than any other published CVB serotype. The grouping of CVB2 and CVB4 as one subgroup has earlier been reported in connection with receptor usage and ability to replicate in different cell lines. The exposed viral capsid proteins of CVB2 (VP1-VP3) show high similarity to other CVB proteins, except in regions that are likely to be surface epitopes.

Cardiovirulent coxsackieviruses and the decay-accelerating factor (CD55) receptor

Group B coxsackieviruses are etiologically linked with many human diseases including acute myocarditis and associated chronic dilated cardiomyopathy. Well-established CVB3 cardiovirulent strains (CVB3c(s)) with known phenotypic difference have been used to study the pathogenesis of virus-induced heart disease. The receptor-binding characteristics of cardiovirulent CVB3 are not known, but may represent one mechanism accounting for differences in disease virulence. In this study, interactions between CVB3c(s) and the decay-accelerating factor (DAF or CD55) cell surface receptor were examined. Anti-DAF monoclonal antibodies (MAbs) blocked virus binding and infection of susceptible HeLa cells. Virus binding was significantly reduced by treatment of these cells with phosphatidylinositol phospholipase C enzyme, which rendered them DAF-deficient CVB3c(s) exhibited a differential propensity for the DAF receptor, as several cardiovirulent strains interacted more strongly than others. However, virus binding and infection was always most effectively blocked by MAbs directed against the SCR 2 and 3 domains of DAF, suggesting that binding occurs at a similar site(s) on the molecule for all strains. Virus binding and internalization were associated with DAF down-regulation at the cell surface, as monitored by flow cytometry analysis. Cardiovirulent CVB3 did not interact with molecules functionally and/or structurally related to DAF, including CD35, CD46, Factor H, or C4-binding protein. Adenovirus type 2 (Ad2) does not use the DAF receptor. However, competitive binding assays between Ad2 and CVB1-6, CVB3c(s), anti-DAF MAbs, or DAF-reduced cells indicated that DAF is associated with Ad2 receptors on the HeLa cell membrane. In summary, this study indicates that DAF is an attachment receptor for cardiovirulent CVB3 and that DAF interaction may be important in the pathogenesis of CVB-mediated heart disease.

Integrin alpha v beta 6 enhances coxsackievirus B1 lytic infection of human colon cancer cells

Viral entry into host cells depends upon specific interactions between virus attachment proteins and cell surface receptors that enable virus binding and internalization of virus and/or the virus-receptor complex. We have recently reported that the ubiquitous cell surface molecule, decay-accelerating factor (DAF), is a major cell attachment receptor for Coxsackieviruses B1, B3, and B5. However, DAF permits only virus binding and not virus internalization, invoking the presence of secondary or accessory receptors. Among the known receptors for enteroviruses are members of the cell adhesion molecule family known as integrins. In the present study, we found that expression of the epithelial-restricted integrin, alpha v beta 6, on colonic epithelial cells significantly enhanced Coxsackievirus B1-mediated cell lysis. Importantly, the viral-mediated cell killing required the presence of the 11-amino-acid C-terminal cytoplasmic extension unique to the beta 6 subunit, providing the first evidence of regulation of viral infectivity by integrin cytoplasmic domains. These results indicate that alpha v beta 6 expression on intestinal epithelial cells critically affects Coxsackievirus B1 infectivity. This may be essential in the conversion of asymptomatic enterovirus infection into clinically apparent disease.

A decay-accelerating factor-binding strain of coxsackievirus B3 requires the coxsackievirus-adenovirus receptor protein to mediate lytic infection of rhabdomyosarcoma cells

The composition of the cellular receptor complex for coxsackievirus B3 (CVB3) has been an area of much contention for the last 30 years. Recently, two individual components of a putative CVB3 cellular receptor complex have been identified as (i) decay-accelerating factor (DAF) and (ii) the coxsackievirus-adenovirus receptor protein (CAR). The present study elucidates the individual roles of DAF and CAR in cell entry of CVB3 Nancy. First, we confirm that the DAF-binding phenotype of CVB3 correlates to the presence of key amino acids located in the viral capsid protein, VP2. Second, using antibody blockade, we show that complete protection of permissive cells from infection by high input multiplicities of CVB3 requires a combination of both anti-DAF and anti-CAR antibodies. Finally, it is shown that expression of the CAR protein on the surface of nonpermissive DAF-expressing RD cells renders them highly susceptible to CVB3-mediated lytic infection. Therefore, although the majority of CVB3 Nancy attaches to the cell via DAF, only virus directly interacting with the CAR protein mediates lytic infection. The role of DAF in CVB3 cell infection may be analogous to that recently described for coxsackievirus A21 (D. R. Shafren, D. J. Dorahy, R. A. Ingham, G. F. Burns, and R. D. Barry, J. Virol. 71:4736-4743, 1997), in that DAF may act as a CVB3 sequestration site, enhancing viral presentation to the functional CAR protein.

Genetic divergence among the group B coxsackieviruses

As documented in the preceding discussion, the noncoding regions, and in particular the 5' NTR, of the CVB are tolerant of a substantial degree of nucleotide diversity while still being capable of fulfilling the life cycle requirements for these viruses. While diversity among the CVB is observed in the sequences encoding for the capsid proteins, it tends to involve predominantly those regions coding for amino acids located at the surface of the virus and not those responsible for the structural integrity of the mature virion, i.e., beta-barrels and alpha-helices. It is these capsid surface differences that define the six serotypes of the CVB and subdivide them antigenically into strains. Additionally, these proteins most likely play the major role in determining host and cellular tropism. The most conserved of the CVB proteins and, therefore those with the least diversity in their coding sequences, appear to be the nonstructural proteins. Perhaps, as speculated earlier, it is a conformational requirement imposed by the necessity to interact with host or viral substrates that maintains the high degree of amino acid identity of this group of viral proteins.

A short history and introductory background on the coxsackieviruses of group B

The past 50 years have revealed an array of significant developments in our documentation and understanding of viruses and their associated diseases. The CVB, as enteroviruses, were discovered in the search for poliomyelitis-related viruses by the inoculation of newborn mice. Future strategies for the discovery of additional viruses will undoubtedly come through the application of differentiating cell culture systems with increased susceptibility to infection by specific viruses. Developments in regulation of the cell cycle also will contribute to the better definition of events controlling persistent infections caused by the CVB. Methods utilizing molecular biological probes in situ will prove to be major aids in identifying the molecular events in CVB pathogenesis. Virology of the CVB continues to be an exciting area for research and application of preventive measures to lesson human suffering. The chapters in this book which follow will amplify most of the themes briefly presented here.

Is CD46 the cellular receptor for measles virus?

Although CD46 would appear to be the cellular receptor for vaccine strains of measles virus (MV), recently there has been an accumulation of data suggesting that CD46 does not play this role for MV wild-type strains. Clarification of the nature of the MV receptor is necessary for the development of more effective vaccines against this virus which is responsible for the deaths of nearly two million children each year in the Third World.

Expression and distribution of the receptors for coxsackievirus B3 during fetal development of the Balb/c mouse and of their brain cells in culture

This study was designed mainly to determine the relationships between the expression and distribution of the cellular receptor proteins for coxsackievirus B3 (CVB3) and susceptibility of mouse brain cells during fetal development of Balb/c mice. Immunoblot analysis of fetal extracts demonstrated that the CVB3 receptor proteins were first expressed at day 14 of the fetal stage, and that maximal expression of the cellular receptor occurred at near term or newborn stage. Results also suggested that newborn mouse brain tissue expressed much larger quantities of viral receptor proteins, compared to other tissues. In vitro studies showed that both mouse neurons and astrocytes could be infected by two CVB3 strains, pantropic CVB3 Nancy strain (CVB3N) and myocardiotropic CVB3 Woodruff strain (CVB3W). CVB3N, however, replicated and grew to high titer in primary astrocyte cultures and in primary neuron cultures, whereas, primary astrocyte cultures were relatively resistant to CVB3W. Virus binding assays revealed that CVB3N bound faster and in greater amounts to mouse brain cells than CVBW. These two virus strains, however, were found to share the same receptor specificity by virus competition assays. The number of virus binding sites for CVB3 on newborn mouse brain cells was approximately 1.8 x 10(4) per cell. The data suggested that preferential expression of the cellular receptors on newborn mouse brain cells may be related to their high susceptibilities to CVB3 infection.

Permissiveness of human embryonal fibroblasts for coxsackieviruses B3. Investigations on virus genetic markers in vitro and localization of virus receptor distribution by immunogold replica technique

In our previous paper (29) we could show, that the replication of four Coxsackie B3 virus strains in human fibroblast lines from different origin was dependent on both the virus strain and the cell line used. Although generally no cytopathic effect could be observed out of one virus strain more pathogenic virus variants could be selected able to destroy virus-sensitive as well as insensitive fibroblasts. The present study was designed to characterize these virus strains by using several in vitro genetic markers. Differences were found concerning plaque size and the temperature marker, whereas the other markers (d, DD, DEAE-D) remained constant. Furthermore, these models of persistent as well as lytic CVB3 infection were analysed by immunoelectron microscopy to study the interaction of viral ligands with cellular receptors. The qualitative and quantitative differences in adsorption of the CVB3 strains to two human fibroblast lines as well as to HeLa cells corresponded well with the virological results. They underline that even in vitro in human cell lines of different origin changes in distribution, quantity and quality of receptors were demonstrable forming the base for the various virus sensitivity.

Identification of cell membrane proteins linked to susceptibility to bovine viral diarrhoea virus infection

Three monoclonal antibodies directed against cell surface molecules of bovine cells inhibited subsequent infections with bovine viral diarrhoea virus (BVDV). They specifically blocked the infectivity of three non-cytopathogenic and three cytopathogenic BVDV strains. These results showed that an important mechanism for virus uptake was inhibited. The ligand of the monoclonal antibody BVD/CA 17, which blocked infectivity most efficiently, was found on leukocytes from a wide range of domestic and wild even-toed ungulates using flow cytometric analysis. In contrast, the monoclonal antibodies BVD/CA 26 and BVD/CA 27 appeared to be specific for bovine cells. Immunoprecipitation of labelled bovine cell surface proteins showed that the three monoclonal antibodies bound to proteins with identical relative molecular masses (M(r)). Proteins of an apparent M(r) of 93 K and 60 K were precipitated from lysates of fetal bovine kidney cells irrespectively of the MAbs used.