Electron microscopical and biochemical characterization of infectious pancreatic necrosis virus

Aquabirnavirus polyploidy: a new strategy to modulate virulence?

One of the main research issues regarding infectious pancreatic necrosis virus (IPNV) is its virulence mechanisms. The basis for understanding the molecular virulence determinants of this virus was established over the last decade when it was demonstrated that certain amino acid domains in the VP2 and VP2-NS inter-region determined the level of virulence of IPNV. However, certain variability was still inexplicable and therefore other factors may also be involved. To this end, it was demonstrated recently that infectious bursal disease virus (IBDV), a virus in a different genus of the same family as IPNV, can package more than two dsRNA segments, and that polyploidy may be associated with virulence. In the present report, we analysed the viral fractions obtained after gradient centrifugation to demonstrate that IPNV virions can also package more than two segments, thus indicating that polyploidy is a common birnavirus trait. The differential replication ex vivo and virulence in vivo additionally suggested that such a characteristic is involved in the modulation of virus infectivity. However, although the ex vivo results clearly demonstrated that the replication capacity was enhanced as the viral ploidy increased, the in vivo results could not strongly support a direct relationship between ploidy and virulence to the host, thus suggesting that other virulence determinants are also involved.

Genome assembly and particle maturation of the birnavirus infectious pancreatic necrosis virus

In this study, we have analyzed the morphogenesis of the birnavirus infectious pancreatic necrosis virus throughout the infective cycle in CHSE-214 cells by using a native agarose electrophoresis system. Two types of viral particles (designated A and B) were identified, isolated, and characterized both molecularly and biologically. Together, our results are consistent with a model of morphogenesis in which the genomic double-stranded RNA is immediately assembled, after synthesis, into a large (66-nm diameter) and uninfectious particle A, where the capsid is composed of both mature and immature viral polypeptides. Upon maturation, particles A yield particles B through the proteolytic cleavage of most of the remaining viral precursors within the capsid, the compaction of the particle (60-nm diameter), and the acquisition of infectivity. These studies will provide the foundation for further analyses of birnavirus particle assembly and RNA replication.

Capsid symmetry of viruses of the proposed Birnavirus group

The capsid architecture of three members of the tentatively formed group of bisegmented ds RNA containing animal viruses (Birnavirus group) was studied in comparison with the fine structure of reovirus type 1 using high resolution negative staining and shadowing techniques followed by rotational enhancement analysis. IPNV of the trout, IBDV of the chicken, and eel virus (Berlin) are single-shelled particles with a capsid diameter of 61-65 nm and thus clearly distinguishable from the double-shelled reovirus being 80-85 nm in size. The capsid symmetries of both groups of viruses are skew, with a triangulation number T=13. The handedness of the capsid is dextro for IBDV and EV, and laevo for IPNV and reovirus type 1.

The haemagglutinating properties of infectious pancreatic necrosis virus

The haemagglutinating properties of infectious pancreatic necrosis virus have been investigated. Erythrocytes from a wide range of animals were used in these tests but only cells derived from mice (Balb/c and Manchester strain) gave reproducible haemagglutination. The tests were carried out at various temperatures but haemagglutination only occurred at 4 degrees and 22 degrees C. The haemagglutination was pH dependent with an optimum being between pH 5.75 and 6.0. Variable results were obtained at the same pH values with erythrocytes derived from rats (Sprague Dawley) but the reaction in this case appeared to be related to individual batches of erythrocytes.

Poikilotherm vertebrate cell lines and viruses: a current listing for fishes

A survey of the literature and of work being done in vertebrate cell culture shows that there are currently in existence and available to investigators some 61 cell lines representing 17 families and 36 species of fish. The literature of fish virology shows that at least 17 fish viruses have been isolated and that at least 15 others have been visualized by electron microscopy. A minimum of four major virus groups--rhabdovirus, orthomyxovirus, iridovirus and herpesvirus--are known from fish. Original references, key reviews and sources of cell lines are given.

The viral diseases of fish: a review through 1978. Part 1: Diseases of proven viral etiology

In this review, a survey is made of the published literature on the viral diseases of fish available up to and including the year 1978. It is divided into two main sections. Part 1 describes 11 diseases where a virus has been isolated and proven to be the causative agent. Part 2 discusses 16 diseases where there is reason to suspect viral etiology because of evidence deriving from electron microscopy or transmission experiments with bacteria-free filtrates of homogenates of diseased tissue, but where final proof of a causative relationship is lacking. The review attempts to provide the most significant information on the disease process itself, in most cases including external signs, fish species susceptible, pathology, geographic distribution, existence of carriers, methods of transmission, and control. It also gives the most recent and significant data concerning the nature of the causative virus, including its cultural, biological, and physicochemical properties, where such information is available.

Biochemical studies with infectious bursal disease virus: comparison of some of its properties with infectious pancreatic necrosis virus

Infectious bursal disease (IBD) virus was purified from the bursae of infected chickens. Two morphologically indistinguishable populations of virus particles were separated in sucrose gradients and possessed sedimentation coefficients of 295S and 460S. Both populations contained RNA and had identical polypeptide compositions. IBD virus banded at a density of 1.31 g/ml in CsCl and at 1.24 g/ml in sodium potassium tartrate. IBD virus contained two RNA segments with mol. wts. of 2.4X10(6) and 2.2X10(6) as estimated by polyacrylamide-agarose gel electrophoresis, but sedimented in sucrose gradients at 15S. Virus RNA was resistant to 0.1 micrograms/ml ribonuclease treatment under conditions in which ribosomal RNA was completely hydrolysed, but was sensitive to 1.0 and 10 micrograms/ml treatments. These results suggest that the RNA consists of either double-stranded or highly ordered single-stranded molecules. IBD virus contained seven polypeptides with mol. wts. in the range 97,000 to 24,000. Two polypeptides were absent in empty particles of IBD virus. IBD and infectious pancreatic necrosis (IPN) viruses were morphologically indistinguishable. IPN virus possessed a sedimentation coefficient of 440S and banded at a density of 1.32 g/ml in CsCl. In addition the electrophoretic mobilities of IBD and IPN virus RNAs were almost identical. Polyacrylamide slab gel electrophoresis showed that while the number and size of the polypeptides were different for each virus there were similarities in the overall pattern.

Isolation and characterization of infectious pancreatic necrosis virus from pike (Esox lucius)

A nonenveloped icosahedral virus measuring 60 (+/-5)nm in diameter has been isolated from pike fry (Esox lucius). The isolate is stable to heat, lipid solvent and acid. Iododeoxyuridine has no effect on its replication. Serologically, the virus related to the major European strains of infectious pancreatic necrosis virus (IPN) Sp and Ab. The isolate is neutralized by normal trout serum and is not pathogenic for trout fry or for young pike. A line of CHSE-214 cells persistently infected with the isolate was established and has been passed 30 times.

Quantitative analysis of defective interfering particles in infectious pancreatic necrosis virus preparations

Infectious pancreatic necrosis virus exhibited an interference phenomenon that resulted in the survival of the infected cell with one hit kinetics. The responsible factor was found to co-purify with standard virus through a purification regime that employed two CsCl gradients and a sucrose gradient. This result suggested that a defective interfering (DI) viral particle was involved. It was possible to estimate the number of DI particles by a statistical method using the Poisson distribution that related cell survival to input DI/cell, which indicated that virus samples from dilute passage contained as many DI particles as samples from undiluted passage; this means that multiple undiluted virus passage did not increase the yield of DI particles. In isopycnic CsCl gradient centrifugation, the DI particles were found in a broad band superimposed over the standard virus peak and extending above it, such that the ratio DI/PFU varied from 0.3--20 in different fractions. These centrifugation methods did not completely separate DI particles from standard virus.

Peptide map comparison of infectious pancreatic necrosis virus-specific polypeptides

An investigation of virus-specific protein synthesis in infectious pancreatic necrosis virus (IPNV)-infected rainbow trout gonad cells was undertaken to find a relationship between the coding capacity of the virus genome (two segments of double-stranded RNA of 2.5 x 10(6) and 2.3 x 10(6) molecular weight) and the sizes and relative amounts of virus-specific proteins. Using polyacrylamide slabgel electrophoresis and autoradiography, eight distinct virus-specific polypeptides were detected in infected, [(35)S]methionine-labeled cells. These proteins may be grouped into three size classes on the basis of molecular weight: (i) large, alpha (90,000); (ii) medium, beta(1) (59,000), beta(2) (58,000), and beta(3) (57,000); and (iii) small, gamma(1) (29,000), gamma(1A) (28,000), gamma(2) (27,000), and gamma(3) (25,000). The combined molecular weight of these polypetides (373,000) is beyond the coding capacity of the virus genome. Purified IPNV contained polypeptides alpha, beta(3), gamma(1), and gamma(1A). Pulse-chase experiments and tryptic peptide map comparisons revealed that only four of the eight intracellular proteins were primary gene products, namely, alpha, beta(1), gamma(1), and beta(2), with a combined molecular weight of 205,000. Of these primary gene products only the alpha polypeptide was found to be stable, whereas the other three underwent intracellular proteolytic cleavage during virus morphogenesis. Polypeptide beta(1) was cleaved to generate beta(2) and beta(3); gamma(1) was trimmed to produce gamma(1A), and the only nonstructural primary gene product, gamma(2), was found to be a precursor of gamma(3). These results suggest that IPNV possesses a unique mechanism to synthesize three size classes of proteins using mRNA transcripts from two high-molecular-weight double-stranded RNA genome segments.

Biophysical studies of infectious pancreatic necrosis virus

The molecular weight of infectious pancreatic necrosis virus (IPNV) has been determined by analytical ultracentrifugation and dynamic light scattering. The sedimentation coefficient of the virus was found to be 435S. The average value for molecular weight is (55 +/- 7) x 106. The virus genome consists of two segments of double-stranded RNA (molecular weights, 2.5 x 106 and 2.3 x 106), which represents 8.7% of the virion mass. The capsid protein moiety of IPNV consists of four species of polypeptides, as determined by polyacrylamide gel electrophoresis. The number of molecules of each polypeptide in the virion has been determined. There are 22 molecules of the internal polypeptide alpha (molecular weight, 90,000), 544 molecules of the outer capsid polypeptide beta (molecular weight, 57,000), and 550 and 122 molecules, respectively, of the internal polypeptides gamma1 (molecular weight, 29,000) and gamma2 (molecular weight, 27,000). IPNV top component contains only the beta polypeptide species, and its molecular weight is estimated to be 31 x 106. The hydrodynamic diameter and electron microscopic diameter (calculated by catalase crystal-calibrated electron microscopy) of IPNV was compared with those of reovirus and encephalomyocarditis virus. Due to the swelling of the outer capsid, reovirus particles were found to be much larger when hydrated (96-nm diameter) than when dehydrated (76-nm diameter), having a large water content content and low average density. In contrast, IPNV particles are more rigid, having nearly the same average diameter under hydrous (64 nm) as under anhydrous conditions (59.3 nm). Encephalomyocarditis virus has a very low water content and does not shrink at all when prepared for electron microscopy.

Virus-specific protein synthesis in cells infected by infectious pancreatic necrosis virus

A study of virus-specific protein synthesis in infectious pancreatic necrosis virus-infected RTG-2 cells was undertaken to find a relationship between the coding capacity of virus genome (two segments of double-stranded RNA of 2.5 X10(6) and 2.3 X 10(6) molecular weight) and the sizes and relative amounts of polypeptides in the virion and in infected cells. The time course of virus-specific protein synthesis was followed by pulse labeling infected UV-irradiated cells with [35S]methionine and analyzing the labeled proteins by polyacrylamide gel electrophoresis followed by autoradiography. Three size classes of virus-specific polypeptides were synthesized, in the same relative proportion, throughout the infectious cycle, beginning 3 h postinfection. Their designation and molecular weight was as follows: alpha1, 1000,000; alpha2, 90,000; beta1, 59,000; beta2, 56,000; gamma1, 32,000; gamma2, 30,000; and gamma3, 28,000. Experiments using amino acid analogues, protease inhibitors, ZnCl2, and supraoptimal temperatures showed that polypeptides of the beta and gamma families did not arise from the alpha polypeptides by post-translational cleavage. Slow cleavage late in the infectious cycle could be demonstrated, since during 12-h period radioactivity was chased from beta1 via beta3 to beta4 (molecular weight 50,000) and beta5 (molecular weight, 49,000). During the chase most of gamma2 was degraded, whereas radioactivity could not be chased from the remaining virus-specific polypeptides. Purified virus contained polypeptides alpha1, alpha2, beta4, beta5, and gamma1. The beta polypeptides made up over 60% of the virion proteins. The results suggest that infectious pancreatic necrosis vibrus possesses a unique mechanism for synthesis of three size-classes of proteins using mRNA transcripts from two high-molecular-weight double-stranded RNA genome segments.

Size and structure of the genome of infectious pancreatic necrosis virus

The genome of infectious pancreatic necrosis virus consists of two segments of dsRNA, in equimolar amounts, with molecular weights of 2.5 X 10(6) and 2.3 X 10(6) daltons, as determined by polyacrylamide gel electrophoresis and autoradiography. The viral RNA was resistant to ribonuclease, and in sucrose gradient it co-sedimented at 14S with RNase resistant RNA from virus infected cells. Upon denaturation in 98% formamide, the viral genome sedi-mented at 24S in formamide sucrose gradient and became sensitive to RNase. Denatured 24S viral RNA did revert to its undenatured 14S form upon recentrifugation in aquaeous sucrose gradient (0.1 M NaCL), but co-sedimented with the denatured large size class of reovirus 25S RNA. The same results were obtained if the native viral RNA was pre-treated with ribonuclease before denaturation, indicating the absence of exposed single strainded regions in the viral genome. Since infectious pancreatic necrosis virus contains only two dsRNA segments it does not belong to the family Reoviridae and may represent a new group of viruses.

Infectious bursal disease agent: morphology by negative stain electron microscopy

The virus of infectious bursal disease of chickens was studied by immune electron microscopy. Negatively-stained preparations revealed morphological similarities with both the bluetongue virus group, and the virus of infectious pancreatic necrosis of trout. Results indicated that the small particle found in such preparations is a degradation product of the large particle.

Inhibition of cellular DNA synthesis in cells infected with infectious pancreatic necrosis virus

In asynchronous RTG-2 cell cultures infected with infectious pancreatic necrosis (IPN) virus, inhibition of cellular DNA synthesis, but not protein synthesis, was detected 5 to 6 h postinfection and was 80 to 90% complete by 7 to 8 h. Inhibition of DNA synthesis was largely abolished by UV irradiation of the virus. Sedimentation analyses of phenol-extracted DNA indicated that native cellular DNA was not degraded during infection. Sedimentation on alkaline sucrose gradients of DNA from cells pulsed with radioactive thymidine for varying periods indicated that elongation of nascent DNA chains proceeded normally in infected cells. These and previous results suggest that IPN virus infection results in a reduction of the number of chromosomal sites active in DNA synthesis but does not affect the rate of polymerization at active sites. Cells synchronized with excess thymidine and hydroxyurea and infected with virus at the time of release from the block demonstrated an inhibition of DNA synthesis 3 h postinfection. Cells infected 4 h prior to release continued to synthesize normal amounts of DNA for 1 to 2 h after release. These results indicated that DNA synthesis in early synthetic phase is relatively insensitive to inhibition by IPN virus.

Homologous viral interference in trout and Atlantic salmon cell cultures infected with infectious pancreatic necrosis virus

The occurrence of homologous interference in the replication of infectious pancreatic necrosis virus was demonstrated after successive passages of partially purified virus at high input multiplicities in trout and Atlantic salmon cell cultures. Pretreatment of cell cultures with interfering virus inhibited the replication of homologous standard infectious virus but not unrelated viruses. The ability of infectious pancreatic interfering virus to interfere with homologous virus was abolished with UV irradiation, immune serum, and freeze-thawing.

Immunofluorescent study of the replication of infectious pancreatic necrosis virus in trout and Atlantic salmon cell cultures

Cell cultures of trout gonad tissue (RTG-2) and Atlantic salmon heart, kidney, liver, and spleen tissue were inoculated with 50 50% tissue culture infective doses (TCID(50)) of infectious pancreatic necrosis (IPN) virus per cell, and the titer of cell-associated and released virus was determined from 2 to 16 h postinoculation (PI). Cover slips were collected over the same period and stained for IPN viral antigen by the direct immunofluorescent (FA) technique. Viral replication was detected after a latent period of approximately 2 to 4 h and reached a peak titer of 10(8.2) to 10(8.4) TCID(50) per ml at 8 to 10 h PI. The release of virus was more rapid in Atlantic salmon cells than in RTG-2 cells. Viral antigen was first detected by FA from 3 to 4 h PI. Approximately 75 to 80% of the cells contained antigen in the cytoplasm 9 to 11 h PI. The direct FA technique was found to be a sensitive method for detecting IPN virus in infected cells. Three strains of IPN virus were tested for serological cross reactions by FA and virus neutralization tests.