VIRAL INTERFERENCE. SOME CONSIDERATIONS OF BASIC MECHANISMS AND THEIR POTENTIAL RELATIONSHIP TO HOST RESISTANCE

Coinfection and Interference Phenomena Are the Results of Multiple Thermodynamic Competitive Interactions

Biological, physical and chemical interaction between one (or more) microorganisms and a host organism, causing host cell damage, represents an infection. Infection of a plant, animal or microorganism with a virus can prevent infection with another virus. This phenomenon is known as viral interference. Viral interference is shown to result from two types of interactions, one taking place at the cell surface and the other intracellularly. Various viruses use different receptors to enter the same host cell, but various strains of one virus use the same receptor. The rate of virus–receptor binding can vary between different viruses attacking the same host, allowing interference or coinfection. The outcome of the virus–virus–host competition is determined by the Gibbs energies of binding and growth of the competing viruses and host. The virus with a more negative Gibbs energy of binding to the host cell receptor will enter the host first, while the virus characterized by a more negative Gibbs energy of growth will overtake the host metabolic machine and dominate. Once in the host cell, the multiplication machinery is shared by the competing viruses. Their potential to utilize it depends on the Gibbs energy of growth. Thus, the virus with a more negative Gibbs energy of growth will dominate. Therefore, the outcome can be interference or coinfection, depending on both the attachment kinetics (susceptibility) and the intracellular multiplication machinery (permittivity). The ratios of the Gibbs energies of binding and growth of the competing viruses determine the outcome of the competition. Based on this, a predictive model of virus–virus competition is proposed.

Improving Phage-Biofilm In Vitro Experimentation

Bacteriophages or phages, the viruses of bacteria, are abundant components of most ecosystems, including those where bacteria predominantly occupy biofilm niches. Understanding the phage impact on bacterial biofilms therefore can be crucial toward understanding both phage and bacterial ecology. Here, we take a critical look at the study of bacteriophage interactions with bacterial biofilms as carried out in vitro, since these studies serve as bases of our ecological and therapeutic understanding of phage impacts on biofilms. We suggest that phage-biofilm in vitro experiments often may be improved in terms of both design and interpretation. Specific issues discussed include (a) not distinguishing control of new biofilm growth from removal of existing biofilm, (b) inadequate descriptions of phage titers, (c) artificially small overlying fluid volumes, (d) limited explorations of treatment dosing and duration, (e) only end-point rather than kinetic analyses, (f) importance of distinguishing phage enzymatic from phage bacteriolytic anti-biofilm activities, (g) limitations of biofilm biomass determinations, (h) free-phage interference with viable-count determinations, and (i) importance of experimental conditions. Toward bettering understanding of the ecology of bacteriophage-biofilm interactions, and of phage-mediated biofilm disruption, we discuss here these various issues as well as provide tips toward improving experiments and their reporting.

In-vitro and in-vivo study of the interference between Rift Valley fever virus (clone 13) and Sheeppox/Limpy Skin disease viruses

Viral interference is a common occurrence that has been reported in cell culture in many cases. In the present study, viral interference between two capripox viruses (sheeppox SPPV and lumpy skin disease virus LSDV in cattle) with Rift Valley fever virus (RVFV) was investigated in vitro and in their natural hosts, sheep and cattle. A combination of SPPV/RVFV and LSDV/RVFV was used to co-infect susceptible cells and animals to detect potential competition. In-vitro interference was evaluated by estimating viral infectivity and copies of viral RNA by a qPCR during three serial passages in cell cultures, whereas in-vivo interference was assessed through antibody responses to vaccination. When lamb testis primary cells were infected with the mixture of capripox and RVFV, the replication of both SPPV and LSDV was inhibited by RVFV. In animals, SPPV/RVFV or LSDV/RVFV combinations inhibited the replication SPPV and LSDV and the antibody response following vaccination. The combined SPPV/RVFV did not protect sheep after challenging with the virulent strain of SPPV and the LSDV/RVFV did not induce interferon Gamma to LSDV, while immunological response to RVFV remain unaffected. Our goal was to assess this interference response to RVFV/capripoxviruses’ coinfection in order to develop effective combined live-attenuated vaccines as a control strategy for RVF and SPP/LSD diseases. Our findings indicated that this approach was not suitable for developing a combined SPPV/LSDV/RVFV vaccine candidate because of interference of replication and the immune response among these viruses.

Mini Review: Virus Interference: History, Types and Occurrence in Crustaceans

Virus interference is a phenomenon in which two viruses interact within a host, affecting the outcome of infection of at least one of such viruses. The effect of this event was first observed in the XVIII century and it was first recorded even before virology was recognized as a distinct science from microbiology. Studies on virus interference were mostly done in the decades between 1930 and 1960 in viruses infecting bacteria and different vertebrates. The systems included in vivo experiments and later, more refined assays were done using tissue and cell cultures. Many viruses involved in interference are pathogenic to humans or to economically important animals. Thus the phenomenon may be relevant to medicine and to animal production due to the possibility to use it as alternative to chemical therapies against virus infections to reduce the severity of disease/mortality caused by a superinfecting virus. Virus interference is defined as the host resistance to a superinfection caused by a pathogenic virus causing obvious signs of disease and/or mortality due to the action of an interfering virus abrogating the replication of the former virus. Different degrees of inhibition of the superinfecting virus can occur. Due to the emergence of novel pathogenic viruses in recent years, virus interference has recently been revisited using different pathogens and hosts, including commercially important farmed aquatic species. Here, some highly pathogenic viruses affecting farmed crustaceans can be affected by interference with other viruses. This review presents data on the history of virus interference in hosts including bacteria and animals, with emphasis on the known cases of virus interference in crustacean hosts. Life Science Identifiers (LSIDs) Escherichia coli [(Migula 1895) Castellani & Chalmers 1919] Aedes albopictus (Skuse 1894) Liocarcinus depurator (Linnaeus 1758): urn:lsid:marinespecies.org:taxname:107387 Penaeus duorarum (Burkenroad 1939): urn:lsid:marinespecies.org:taxname:158334 Carcinus maenas (Linnaeus 1758): urn:lsid:marinespecies.org:taxname:107381 Macrobrachium rosenbergii (De Man 1879): urn:lsid:marinespecies.org:taxname:220137 Penaeus vannamei (Boone 1931): urn:lsid:zoobank.org:pub:C30A0A50-E309-4E24-851D-01CF94D97F23 Penaeus monodon (Fabricius 1798): urn:lsid:zoobank.org:act:3DD50D8B-01C2-48A7-B80D-9D9DD2E6F7AD Penaeus stylirostris (Stimpson 1874): urn:lsid:marinespecies.org:taxname:584982.

Would the interference phenomenon be applied as an alternative option for prophylaxis against COVID-19?

The coronavirus disease 2019 (COVID-19) is an emerged infectious disease characterized by a severe pneumonia leading to death in some cases. Currently, no licensed vaccines, drugs, or biologics have been confirmed to be absolutely effective in prophylaxis or treatment of this novel infection. Therefore, the treatment of this highly contagious disease remains a global concern and emergency. The viral interference is a competition phenomenon by which a primary virus infecting a cell prohibits the infection of the same cell by another (secondary) virus. The phenomenon has recently been indicated to be exploited for antiviral strategies. This strategy, particularly when there is no efficient drug against a viral infection, is of high importance. Some researchers have studied the application of the phenomenon among different viruses. In this paper, I discussed the possibility of the application of interference phenomenon in prophylaxis of the disease.

Game-Theoretical Modeling of Interviral Conflicts Mediated by Mini-CRISPR Arrays

All cellular organisms coevolve with multiple viruses, so that both virus-host and intervirus conflicts are major factors of evolution. Accordingly, hosts evolve multiple, elaborate defense systems and viruses respond by evolving means of antidefense. Although less thoroughly characterized, several dedicated mechanisms of intervirus competition have been described as well. Recently, the genomes of some bacterial and archaeal viruses have been shown to harbor CRISPR mini-arrays that typically contain a single spacer targeting a closely related virus. The involvement of mini-arrays in an intervirus conflict has been experimentally demonstrated for a pair of archaeal viruses. We model the evolution of virus-encoded CRISPR mini-arrays using a game theoretical approach. Analysis of the model reveals multiple equilibria that include mutual targeting, unidirectional targeting, no targeting, cyclic polymorphism, and bistability. The choice between these evolutionary regimes depends on the model parameters including the coinfection frequency, differential productivity of the conflicting viruses, and the fitness cost of mini-arrays. At high coinfection frequencies, the model becomes a version of the Prisoner's dilemma in which defection, i.e., mutual targeting between the competing viruses, is the winning strategy.

Protective efficacy of a bivalent live attenuated vaccine against duck hepatitis A virus types 1 and 3 in ducklings

Duck hepatitis A virus (DHAV) infection is characterized by an acute, rapidly spreading that affects young ducklings. DHAV-1 or DHAV-3 infection is prevalent, and simultaneous co-infection with both viruses has recently become increasingly frequent in the domestic duck farms. In this study, we developed a bivalent live attenuated vaccine (DHV-HSBP100 and AP-04203P100) for DHAV-1 and DHAV-3 and reported the protective efficacy and safety of the vaccine. At 1-day-old, the ducklings received a bivalent vaccine via intramuscular injection. The immunized ducklings showed effective and rapid protection against virulent DHAV-1 and DHAV-3 at 2 or 3 days post vaccination. Moreover, the ducklings showed a potent humoral immune response that peaked at 3 weeks and were maintained at 6 weeks after vaccination. The bivalent vaccine was safe; ducklings administered 10 doses of bivalent vaccines showed no clinical signs, mortality, gross lesions, and body weight changes compared with those observed in the negative controls. Ducklings vaccinated with a bivalent vaccine were evaluated for tissue tropism and viral replication of vaccine strains. Both bivalent vaccine strains were detected in various organs, and the highest virus replication was detected in the kidneys, among the tested organs. No interference occurred during the replication of both vaccine strains. Thus, these experiments suggest that bivalent vaccines would be useful as a promising and practical strategy for control DHAV outbreaks caused by DHAV-1 and DHAV-3 in duck farms.

Derivation of chicken induced pluripotent stem cells tolerant to Newcastle disease virus-induced lysis through multiple rounds of infection

Background

Newcastle disease (ND), caused by Newcastle disease virus (NDV), is a devastating disease of poultry and wild birds. ND is prevented by rigorous biocontainment and vaccination. One potential approach to prevent spread of the virus is production of birds that show innate resistance to NDV-caused disease. Induced pluripotent stem cell (iPSC) technology allows adult cells to be reprogrammed into an embryonic stem cell-like state capable of contributing to live offspring and passing on unique traits in a number of species. Recently, iPSC approaches have been successfully applied to avian cells. If chicken induced pluripotent stem cells (ciPSCs) are genetically or epigenetically modified to resist NDV infection, it may be possible to generate ND resistant poultry. There is limited information on the potential of ciPSCs to be infected by NDV, or the capacity of these cells to become resistant to infection. The aim of the present work was to assess the characteristics of the interaction between NDV and ciPSCs, and to develop a selection method that would increase tolerance of these cells to NDV-induced cellular damage.

Results

Results showed that ciPSCs were permissive to infection with NDV, and susceptible to virus-mediated cell death. Since ciPSCs that survived infection demonstrated the ability to recover quickly, we devised a system to select surviving cells through multiple infection rounds with NDV. ciPSCs that sustained 9 consecutive infections had a statistically significant increase in survival (up to 36 times) compared to never-infected ciPSCs upon NDV infection (tolerant cells). Increased survival was not caused by a loss of permissiveness to NDV replication. RNA sequencing followed by enrichment pathway analysis showed that numerous metabolic pathways where differentially regulated between tolerant and never-infected ciPSCs.

Conclusions

Results demonstrate that ciPSCs are permissive to NDV infection and become increasingly tolerant to NDV under selective pressure, indicating that this system could be applied to study mechanisms of cellular tolerance to NDV.

Electronic supplementary material

The online version of this article (doi:10.1186/s12985-016-0659-3) contains supplementary material, which is available to authorized users.

Experimental Piscine orthoreovirus infection mediates protection against pancreas disease in Atlantic salmon (Salmo salar)

Viral diseases are among the main challenges in farming of Atlantic salmon (Salmo salar). The most prevalent viral diseases in Norwegian salmon aquaculture are heart and skeletal muscle inflammation (HSMI) caused by Piscine orthoreovirus (PRV), and pancreas disease (PD) caused by Salmonid alphavirus (SAV). Both PRV and SAV target heart and skeletal muscles, but SAV additionally targets exocrine pancreas. PRV and SAV are often present in the same locations and co-infections occur, but the effect of this crosstalk on disease development has not been investigated. In the present experiment, the effect of a primary PRV infection on subsequent SAV infection was studied. Atlantic salmon were infected with PRV by cohabitation, followed by addition of SAV shedder fish 4 or 10 weeks after the initial PRV infection. Histopathological evaluation, monitoring of viral RNA levels and host gene expression analysis were used to assess disease development. Significant reduction of SAV RNA levels and of PD specific histopathological changes were observed in the co-infected groups compared to fish infected by SAV only. A strong correlation was found between histopathological development and expression of disease related genes in heart. In conclusion, experimentally PRV infected salmon are less susceptible to secondary SAV infection and development of PD.

Complexities in Isolation and Purification of Multiple Viruses from Mixed Viral Infections: Viral Interference, Persistence and Exclusion

Successful purification of multiple viruses from mixed infections remains a challenge. In this study, we investigated peste des petits ruminants virus (PPRV) and foot-and-mouth disease virus (FMDV) mixed infection in goats. Rather than in a single cell type, cytopathic effect (CPE) of the virus was observed in cocultured Vero/BHK-21 cells at 6^th blind passage (BP). PPRV, but not FMDV could be purified from the virus mixture by plaque assay. Viral RNA (mixture) transfection in BHK-21 cells produced FMDV but not PPRV virions, a strategy which we have successfully employed for the first time to eliminate the negative-stranded RNA virus from the virus mixture. FMDV phenotypes, such as replication competent but noncytolytic, cytolytic but defective in plaque formation and, cytolytic but defective in both plaque formation and standard FMDV genome were observed respectively, at passage level BP8, BP15 and BP19 and hence complicated virus isolation in the cell culture system. Mixed infection was not found to induce any significant antigenic and genetic diversity in both PPRV and FMDV. Further, we for the first time demonstrated the viral interference between PPRV and FMDV. Prior transfection of PPRV RNA, but not Newcastle disease virus (NDV) and rotavirus RNA resulted in reduced FMDV replication in BHK-21 cells suggesting that the PPRV RNA-induced interference was specifically directed against FMDV. On long-term coinfection of some acute pathogenic viruses (all possible combinations of PPRV, FMDV, NDV and buffalopox virus) in Vero cells, in most cases, one of the coinfecting viruses was excluded at passage level 5 suggesting that the long-term coinfection may modify viral persistence. To the best of our knowledge, this is the first documented evidence describing a natural mixed infection of FMDV and PPRV. The study not only provides simple and reliable methodologies for isolation and purification of two epidemiologically and economically important groups of viruses, but could also help in establishing better guidelines for trading animals that could transmit further infections and epidemics in disease free nations.

Attenuation of tick-borne encephalitis virus using large-scale random codon re-encoding

Large-scale codon re-encoding (i.e. introduction of a large number of synonymous mutations) is a novel method of generating attenuated viruses. Here, it was applied to the pathogenic flavivirus, tick-borne encephalitis virus (TBEV) which causes febrile illness and encephalitis in humans in forested regions of Europe and Asia. Using an infectious clone of the Oshima 5–10 strain ("wild-type virus"), a cassette of 1.4kb located in the NS5 coding region, was modified by randomly introducing 273 synonymous mutations ("re-encoded virus"). Whilst the in cellulo replicative fitness of the re-encoded virus was only slightly reduced, the re-encoded virus displayed an attenuated phenotype in a laboratory mouse model of non-lethal encephalitis. Following intra-peritoneal inoculation of either 2.10⁵ or 2.10⁶ TCID50 of virus, the frequency of viraemia, neurovirulence (measured using weight loss and appearance of symptoms) and neuroinvasiveness (detection of virus in the brain) were significantly decreased when compared with the wild-type virus. Mice infected by wild-type or re-encoded viruses produced comparable amounts of neutralising antibodies and results of challenge experiments demonstrated that mice previously infected with the re-encoded virus were protected against subsequent infection by the wild-type virus. This constitutes evidence that a mammalian species can be protected against infection by a virulent wild-type positive-stranded RNA virus following immunisation with a derived randomly re-encoded strain. Our results demonstrate that random codon re-encoding is potentially a simple and effective method of generating live-attenuated vaccine candidates against pathogenic flaviviruses.

The arbovirus Tick-borne encephalitis virus (TBEV; genus Flavivirus) is transmitted by ticks of the Ixodes genus. TBEV causes febrile illness and encephalitis in humans in forested regions of Europe and Asia. The incidence of TBE is increasing across Central and Eastern European countries despite the availability of several licensed inactivated vaccines and appropriate vaccination programmes. Large-scale codon re-encoding, a recently developed attenuation method that modifies viral RNA nucleotide composition of large coding regions without alteration of the encoded proteins, has been successfully applied to a variety of RNA viruses. In contrast with previous empirical methods of generating live attenuated vaccines, large-scale codon re-encoding facilitates rapid generation of vaccine candidates using reverse genetics methods, by direct control of the attenuation phenotype. Additional benefits include reduced costs and induction of long-term immunity. Here, we have applied the large-scale codon re-encoding method to the TBEV to demonstrate the principle of developing a live attenuated virus vaccine which protects mice against subsequent infection with the wild type virulent virus. This study therefore illustrates that codon re-encoding is potentially an easily derived and effective method of producing live attenuated vaccine candidates against positive-stranded RNA viruses.

Respiratory viruses in hospitalized children with influenza-like illness during the h1n1 2009 pandemic in Sweden [corrected]

BACKGROUND

The swine-origin influenza A(H1N1)pdm09 pandemic of 2009 had a slower spread in Europe than expected. The human rhinovirus (HRV) has been suggested to have delayed the pandemic through viral interference. The importance of co-infections over time during the pandemic and in terms of severity of the disease needs to be assessed.

OBJECTIVE

The aim of this study was to investigate respiratory viruses and specifically the presence of co-infections with influenza A(H1N1)pdm09 (H1N1) in hospitalized children during the H1N1 pandemic. A secondary aim was to investigate if co-infections were associated with severity of disease.

METHODS

A retrospective study was performed on 502 children with influenza-like illness admitted to inpatient care at a pediatric hospital in Stockholm, Sweden during the 6 months spanning the H1N1 pandemic in 2009. Respiratory samples were analyzed for a panel of 16 viruses by real-time polymerase chain reaction.

RESULTS

One or more viruses were detected in 61.6% of the samples. Of these, 85.4% were single infections and 14.6% co-infections (2-4 viruses). The number of co-infections increased throughout the study period. H1N1 was found in 83 (16.5%) children and of these 12 (14.5%) were co-infections. HRV and H1N1 circulated to a large extent at the same time and 6.0% of the H1N1-positive children were also positive for HRV. There was no correlation between co-infections and severity of disease in children with H1N1.

CONCLUSIONS

Viral co-infections were relatively common in H1N1 infected hospitalized children and need to be considered when estimating morbidity attributed to H1N1. Population-based longitudinal studies with repeated sampling are needed to improve the understanding of the importance of co-infections and viral interference.

Cytochemical Assay of Interferon Produced by Duck Hepatitis Virus

A microscopic, cytochemical technique for assay of interferon is described in which psittacosis virus is the indicator agent. Interferon, produced in tissue culture cells in response to duck hepatitis virus, is used to illustrate the procedure.

Interference between viable strains of Newcastle disease virus

Durand, D. P. (University of Missouri, Columbia). Interference between viable strains of Newcastle disease virus. J. Bacteriol. 82:979-983. 1961.-Strains of Newcastle disease virus (NDV) which differed in their ability to produce plaques on monolayers of chicken embryo cells were studied during conditions of dual infection. Interference with NDV plaque formation by a nonplaque-forming NDV strain was observed. The primary mechanism involved with this type of interference appears to be due to the viable infectious virus, and is not associated with interferon production or the hemagglutinin of NDV.

The mammalian cell-virus relationship. VI. Sustained infection of HeLa cells by Coxsackie B3 virus and effect on superinfection

Sustained infection of HeLa cells by Coxsackie B3 virus, dependent on presence of viral inhibitor in culture medium, was achieved. Persistent treatment of carrier cultures with anti-Coxsackie B3 hyperimmune monkey serum eventually eliminated virus from carrier cultures indicating that a lysogenic virus-cell relationship was not operative. Free virus was produced continuously by carrier cultures despite washing and neutralization with antiserum to eliminate free virus temporarily. In carrier cultures, about 1.5 to 1.9 plaque-forming units of virus per cell were cell-associated; approximately 6 per cent of this cell-associated virus was not neutralizable by antiserum. In growth medium containing anti-B3 antibody, cells from carrier cultures formed colonies as efficiently as cells from B3-cured cultures. Assays of carrier cultures for infectious centers indicated that less than 1 per cent of cells produced free infectious virus. The Coxsackie B3 virus-carrier state appeared to represent surface residence of B3 virus on the majority of carrier cells with restriction of productive infection to a small proportion of the population. Coxsackie B3 carrier HeLa cultures, unlike control cultures, were not destroyed by challenge with Coxsackie B1, B3, or B5 viruses. The B3 carrier state did not interfere with superinfection by herpes, vaccinia, and types 1 to 3 polioviruses. In contrast to parental or B3-cured lines, B3-carrier HeLa cultures superinfected with Coxsackie B1 virus produced no significant virus, and cultures superinfected with B5 viruses produced new virus to a limited extent only. Specific interference with Coxsackie virus superinfection by the B3-carrier state of HeLa cells was shown to be attributable to failure of attachment in the instance of Coxsackie B1 virus, and failure of penetration and/or eclipse in the instance of B5 virus. The interfering effect was circumvented successfully by superinfection of carrier cells with ribonucleic acid extracted from Coxsackie B1 and B5 viruses.

Assay of interferon activity

At present there is a lack of standard criteria for the identification and evaluation of activity of antiviral compounds. Interferon was used to explore comparatively several laboratory methods. Interferon was produced in chick embryos and in chorioallantoic membranes suspended in vitro. Evaluation of interferon activity was performed by several methods: (i) percentage of inhibition of plaque-forming units; (ii) hemagglutinin reduction of challenge virus; (iii) titer of cytopathic effect of challenge virus; and (iv) plaque-inhibition test. The suggested methods for measurement are those which express the titer of challenge virus in plaque-forming units or in hemagglutinating units.

Biochemical basis for alterations in structure and function of HeLa cells infected with Newcastle disease virus

The ability of NDV-infected HeLa cells to synthesize DNA, protein, and RNA was investigated by measuring the incorporation of tritiated precursors into these substances at intervals after infection of cells with a virus/cell multiplicity of 500:1. A significant decrease in incorporation of precursors into DNA and protein was first observed at 3¼ hours after infection. By 4½ hours, an 80 to 90 per cent decrease had occurred, and by 5¼ hours, incorporation of precursors into DNA and protein was almost completely inhibited. Incorporation of precursor into RNA decreased gradually following infection; by the 10th hour, a 40 per cent decrease had occurred. These results, integrated with earlier observations on biological aspects of infection, suggest the following causal relationships among events in NDV-infected cells: (a) The cessation of virus production is probably caused by inhibition of protein or RNA synthesis, and is not due to inhibition of DNA synthesis or to interferon. (b) The production of infective virus does not per se interfere with the ability of an infected cell to divide, nor is inhibition of mitosis caused by either inhibition of DNA synthesis or development of marked degenerative changes in infected cells. Inhibition of mitosis may be the result of inhibition of protein or RNA synthesis, (c) Marked cell damage could have been caused by inhibition of protein, DNA, or RNA synthesis, (d) Interference by NDV with the multiplication of influenza virus was probably due to the inhibitory effects of NDV on cellular biosynthetic activities.

PRODUCTION OF AN INTERFERON BY L CELLS INFECTED WITH WESTERN EQUINE ENCEPHALOMYELITIS VIRUS

Lockart, Royce Z., Jr. (The University of Texas, Austin). Production of an interferon by L cells infected with Western equine encephalomyelitis virus. J. Bacteriol. 85:556-566. 1963.-Two strains of Western equine encephalomyelitis virus (WEE), WEE (L+) and WEE (L-), which differed with respect to their cytopathogenicity for L cells were isolated. Both strains reproduced in L cells, and both induced the production of an interferon distinct from virus particles. L-cell monolayers were protected from degeneration by prior addition of interferon. By use of the absence of cytopathic effects (CPE) as an end point, interferon content was assayed. Monolayers failing to show CPE consistently produced less than 2% as much virus as control monolayers, indicating that virus synthesis was also inhibited. The use of this assay method was facilitated by the use of horse serum that appeared to contain antibodies against WEE and that permitted interferon to act selectively in the presence of active virus. It was found that interferon was produced during the time in which active virus was produced, and not significantly later. No interferon could be found in fluids from cells treated with inactive virus, although these are known to act as interfering agents. Interferon production was inhibited by pretreatment of L cells with sufficient amounts of interferon. It is concluded that interferon production is closely connected with WEE virus synthesis in L cells. The question is raised as to whether interferon need be a necessary intermediate for interference in L cells.

Interferon-mediated inhibition of virus penetration

Pretreatment of mouse L cells with mouse interferon (IFN) inhibits the penetration of vesicular stomatitis virus without affecting viral adsorption. The inhibition of virus uptake by IFN is dose dependent and, at the highest dose tested (1,000 units/ml), reaches 65%; 24 hr of treatment with IFN are required for maximal effect. A similar inhibition of uptake of virus occurs in human diploid fibroblasts and primary chicken embryo fibroblasts treated with homologous IFN. No significant inhibition occurs when cells are treated with heterologous IFN. These results document a previously unrecognized antiviral effect of IFN--namely, inhibition at the level of viral uptake.

Effect of Corynebacterium acnes on interferon production in mice

Exposure to Corynebacterium acnes, the most prominent member of our normal skin flora, produces stimulation of lymphoid tissue and certain reticuloendothelial system functions, as well as the immune response. Alteration of the host response is extended by these studies to include changes in the pattern of interferon production in response to a representative group of inducing agents. Serum interferon levels induced by the injection of endotoxin in mice are enhanced, whereas interferon production after injection of Newcastle disease virus, Chikungunya virus, and polyinosinic:polycytidylic acid is depressed in animals inoculated with viable or nonviable C. acnes organisms.

Evaluation of gentamicin for use in virology and tissue culture

Data are presented comparing gentamicin to penicillin and streptomycin (Pen-Strep) in tissue culture medium with respect to a number of parameters associated with virology and tissue culture. Unlike Pen-Strep, gentamicin was stable at pH 2 to 10 for 15 days at 37 C in tissue culture medium, and its activity was unaffected by the presence of serum. Moreover, it was stable to autoclaving. Twenty cell types replicated normally at the suggested concentration of 50 mug/ml, and all cells were unaffected by 20 times this concentration. Evidence for its practical use in virus studies was demonstrated in that (i) it was not viricidal to ribonucleic acid or deoxyribonucleic acid viruses at 40 times the suggested concentration at 37 C, (ii) the size and number of plaques were not affected by 20 times the suggested concentration, (iii) interferon assays and production were unaffected by 20 times the suggested concentrations. Gentamicin may be uniquely useful for shipment of clinical specimens and long-term tissue culture and virus studies.

Interferon and transcription of early virus-specific RNA in cells infected with simian virus 40

Treatment with interferon reduced the content of early virus-specific RNA, as well as the content of an early viral protein (T antigen), in monkey cells acutely infected with simian virus 40 (SV40). This unexpected finding suggests either that the action of interferon involves inhibition of the transcription of early SV40 messenger RNA, or that the SV40 genome contains a "proto-early" gene whose product is required for the transcription of the remaining early genes.

Factors Modifying Host Resistance to Virus Infection: II. Enhanced Susceptibility of Mice to Encephalomyocarditis Virus Infection During Pregnancy

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Characteristics of serially propagated monkey kidney cell cultures with persistent rubella infection

Maassab, H. F. (University of Michigan, Ann Arbor), and J. A. Veronelli. Characteristics of serially propagated monkey kidney cell cultures with persistent rubella infection. J. Bacteriol. 91:436-441. 1966.-A persistent infection of LLC-MK(2) cells with rubella virus has been established and maintained for over 3 years. This "carrier culture," designated as LLC-MK(2)-RAL, possesses distinct morphological and biological characteristics when compared with the original uninfected LLC-MK(2) line. The mechanism of viral persistance has not been entirely elucidated, but available data suggest a regulated infection with transmission of the virus directly from cell to cell or through cell division. Interferon was isolated from RAL (rubella-associated line) culture, which explains partly the wide spectrum of resistance to superinfecting viruses. Amantadine, although inhibiting cultures of LLC-MK(2) cells infected with rubella virus, failed to cure the "carrier culture."

Interferon-Like Viral Inhibitor in Rabbits after Intravenous Administration of Endotoxin

Intravenous injection into rabbits of endotoxin or killed cells of Escherichia coli induced, in 1 hour, a viral inhibitor detectable in serum. The inhibitor disappeared from the serum in 7 to 24 hours, and was only active after incubation with rabbit cell cultures. Like interferon, it did not preferentially inactivate virus directly, was ineffective in chick cells, was inactivated by trypsin, and was not sedimentable. Unlike interferon, the inhibitor was heat labile. Nucleic acid or nucleotides apparently play no role in its induction.

ISOLOGOUS INTERFERENCE WITH ULTRAVIOLET AND X-RAY IRRADIATED BACTERIOPHAGE T 2

Levy, Stuart B. (Institut du Radium, Paris, France). Isologous interference with ultraviolet and X-ray irradiated bacteriophage T 2 . J. Bacteriol. 87: 1330–1338. 1964.—Qualitative and quantitative analysis of the interference capacity of an irradiated T 2 bacteriophage was made with ultraviolet and X-ray irradiation. Two different effects were found to explain the total interference picture in the ultraviolet-irradiated system: exclusion and depression. Exclusion is the absolute inhibition of infectious phage growth in the bacterial host. Depression is the diminution of burst size in instances where the infectious phage has not been excluded. Both effects were seen when the infectious phage was added after the addition of ultraviolet-irradiated phage. Doses between 1,600 and 2,200 ergs/mm 2 (survivals, ca. 10 −7 ) showed the greatest exclusion effect (70%). Exclusion was lost between 6,500 and 7,500 ergs/mm 2 . The depression effect was highest (90%) at lower doses (survivals, ca. 10 −6 ), falling off as the dose range went above 1,600 ergs/mm 2 or survivals of 10 −7 . Depression was lost at 3,000 ergs/mm 2 . X-ray irradiation (both direct and indirect) to survivals less than 10 −2 showed no interference capacity in the phage irradiated. Indirect X-ray irradiation to survivals between 5 and 10% showed 50% exclusion, but no depression.

INTERFERENCE INDUCED AGAINST VACCINIA IN AN ADENOVIRUS-RHF-1 SYSTEM

Khoobyarian, Newton (University of Illinois, Chicago). Interference induced against vaccinia in an adenovirus-RHF-1 system. J. Bacteriol. 87: 24–32. 1964.—It was observed that a continuous line of rabbit heart cell culture (strain RHF-1), when overlaid with growth medium after infection of cultures with adenovirus types 2, 3, 4, and 7, would develop resistance to vaccinia plaque formation. Data are presented to provide some information on the nature of such resistance observed specifically in an adenovirus 2-RHF-1 system. An analysis of this phenomenon indicated the following. (i) At least 6 hr were required before the start of vaccinia inhibition, and 15 to 18 hr were necessary for the maximal occurrence of inhibition; the degree of interference established varied with the concentration of adenovirus. (ii) The site of interference action was inside rather than outside the cells, since hardly any difference could be shown in the rate of vaccinia adsorption on resistant and susceptible cells. (iii) The interaction of adenovirus with RHF-1 cells not only would inhibit cell infection with vaccinia but would also suppress the 24- to 48-hr yield of virus. (iv) When RHF-1 cells were overlaid with maintenance medium after their infection with a low multiplicity of RHF-1 passaged adenovirus 2 (to establish sublethal infection), an inhibitory substance of varied activity could be detected daily over a period of at least 10 days which, when transferred to normal RHF-1 cultures, would render them resistant to vaccinia infection. (v) Although limited growth of virus appeared to be responsible for the production of the inhibitor, increase in inhibitory activity did not seem to coincide with increase in virus titer. (vi) The inhibitor seemed to resemble interferons in inhibiting virus plaque production, but its exact identity and the mode of action remain to be determined.