Viruses as toxins. With special reference to paramyxoviruses. Brief review

It's a Small World After All: The Remarkable but Overlooked Diversity of Venomous Organisms, with Candidates Among Plants, Fungi, Protists, Bacteria, and Viruses

Numerous organisms, including animals, plants, fungi, protists, and bacteria, rely on toxins to meet their needs. Biological toxins have been classified into three groups: poisons transferred passively without a delivery mechanism; toxungens delivered to the body surface without an accompanying wound; and venoms conveyed to internal tissues via the creation of a wound. The distinctions highlight the evolutionary pathways by which toxins acquire specialized functions. Heretofore, the term venom has been largely restricted to animals. However, careful consideration reveals a surprising diversity of organisms that deploy toxic secretions via strategies remarkably analogous to those of venomous animals. Numerous plants inject toxins and pathogenic microorganisms into animals through stinging trichomes, thorns, spines, prickles, raphides, and silica needles. Some plants protect themselves via ants as venomous symbionts. Certain fungi deliver toxins via hyphae into infected hosts for nutritional and/or defensive purposes. Fungi can possess penetration structures, sometimes independent of the hyphae, that create a wound to facilitate toxin delivery. Some protists discharge harpoon-like extrusomes (toxicysts and nematocysts) that penetrate their prey and deliver toxins. Many bacteria possess secretion systems or contractile injection systems that can introduce toxins into targets via wounds. Viruses, though not "true" organisms according to many, include a group (the bacteriophages) which can inject nucleic acids and virion proteins into host cells that inflict damage rivaling that of conventional venoms. Collectively, these examples suggest that venom delivery systems-and even toxungen delivery systems, which we briefly address-are much more widespread than previously recognized. Thus, our understanding of venom as an evolutionary novelty has focused on only a small proportion of venomous organisms. With regard to this widespread form of toxin deployment, the words of the Sherman Brothers in Disney's iconic tune, It's a Small World, could hardly be more apt: "There's so much that we share, that it's time we're aware, it's a small world after all".

Early induction of proinflammatory cytokine and type I interferon mRNAs following Newcastle disease virus, poly [rI:rC], or low-dose LPS challenge of the mouse

Numerous cytokines induce symptoms characteristic of the flu syndrome common to acute viral infections. To better characterize the cytokine mRNA profile associated with the early phase of this syndrome, we examined the induction of cytokine mRNAs in spleens of mice 1, 2, and 4 h following intraperitoneal inoculation of Newcastle disease virus (NDV). The reverse transcriptase-polymerase chain reaction was used to detect mRNAs for mouse proinflammatory cytokines [interleukin (IL)-1 alpha, IL-1 beta, IL-6, tumor necrosis factor-alpha (TNF-alpha), macrophage colony-stimulating factor (M-CSF), and interferon (IFN)-gamma] and type I IFNs (IFN-alpha 4 and IFN-beta). We observed a rapid (within 2 h) induction of most of these cytokine mRNAs in the mouse spleen following challenge with live NDV or the viral stimulant poly[rI:rC]. IL-1 beta, M-CSF, and IFN-gamma mRNAs were also induced by heat-inactivated NDV, suggesting the possibility of endotoxin contamination of the virus (confirmed by Limulus lysate assay). Examination of cytokine induction by comparable doses of lipopolysaccharide indicated that endotoxin contamination could account for the cytokine mRNA-inducing activity of the heat-inactivated virus. These studies point to a critical control (heat-inactivated virus) for viral cytokine studies. In addition, they indicate that certain cytokine mRNAs (IL-1 alpha, IL-6, M-CSF, IFN-gamma, IFN-alpha, and IFN-beta) are rapidly induced in the spleen when live virus is inoculated intraperitoneally, independently of contaminating endotoxin.

Preferential cytotoxic effect of Newcastle disease virus on lymphoma cells

Susceptibility of lymphoma cells (Daudi, HD-Mar) to Newcastle disease virus toxicity was found to be higher than that of lymphoblastoid cells (Milstein) and of resting peripheral blood lymphocyte (PBL). Phytohemagglutinin- and/or pokeweed-mitogen-activated PBL however, exhibited, elevated sensitivity, similar to that of lymphoma cells. The level of cytotoxicity was monitored by cell viability, inhibition of DNA synthesis and release of 51Cr. When Daudi cells were mixed with PBL they were significantly more sensitive to the killing effect of the virus (70% mortality compared to 30% 30 h after infection, P < 0.05). The degree of sensitivity to viral cytotoxicity was unrelated to the efficacy of adsorption, which was similar for all cell lines as shown by immunofluorescent staining and flow cytometry. Also an influenza strain A/PR/8/34 (H1N1) adsorbed but did not affect the viability of any of the cells tested. Our results demonstrate that New-castle disease virus caused preferential damage to lymphoma cells as compared to non-cancerous normal cells.

Activation and deactivation of membrane currents in human fibroblasts following infection with human cytomegalovirus

The whole cell patch clamp technique was used to study the effects on membrane currents of infection of cultured human embryonic lung (HEL) fibroblasts with human cytomegalovirus (CMV). Four types of membrane currents were found in uninfected HEL cells, namely: Ca(2+)-activated potassium current, inward rectifier potassium current, delayed rectifier potassium current and voltage-dependent CMV. Voltage-dependent sodium current was detected in 30% of uninfected HEL cells whenever they were examined up to 72 h after seeding; however this current had completely disappeared by 18 h after infection with CMV. The delayed rectifier potassium current was detectable in 8% of uninfected HEL cells but, after infection, the proportion of cells expressing this current gradually increased from 20% at 18-24 h post-infection to 100% at 48 h and 72 h. Pharmacological agents known to regulate the activity of ion channels, via cellular secondary messengers, did not alter the frequency at which either current was detected in uninfected and infected cells. Phosphonoformate, an inhibitor of CMV DNA polymerase, caused 95% block of expression of CMV 'late' proteins in infected cells but did not prevent the switching off of the sodium current or the increased expression of the potassium current. The results indicate an association between the expression of CMV 'immediate-early' or 'early' proteins and the down-regulation of the sodium current and up-regulation of the potassium current.

The effect of a mesogenic and a lentogenic Newcastle disease virus strain on Burkitt lymphoma Daudi cells

The destructive effect of Newcastle disease virus (NDV) strains on Burkitt lymphoma Daudi cells was investigated. Interaction of an active and UV-inactivated mesogenic strain (Roakin), as well as an active attenuated lentogenic strain (B1), grown in the allantoic sac of embryonated eggs, at high multiplicity, caused inhibition in cellular DNA synthesis and arrest in cell multiplication, eventually killing of the cells. The lentogenic strain cultivated in chicken fibroblasts exhibited only a moderate activity. The mechanism of the cytolytic effect is presumably linked to the increase in cell membrane permeability indicated by the elevation in 51Cr release. Thus it appears that the massive adsorption and/or penetration of viral particles, active or UV-inactivated (or possibly a toxic component that resides in the virion), damages the plasma membrane and may be responsible for the killing of the cells.

The fusion of artificial lipid membranes induced by the synthetic arenavirus 'fusion peptide'

The fusing activity of the synthetic 23 amino-acid fragment (fusion peptide, FP) of the fusion protein of the Lassa arenavirus membrane was tested in a model liposomal system. The resonance energy transfer between two fluorescent phospholipid probes was monitored in order to detect dioleoylphosphatidylcholine liposome fusion induced by the peptide. Fusion rates were compared at different pH values, ionic strength and calcium concentrations. FP demonstrated fusing activity at pH 4.5-5.5, indicating that the protonated form of the FP is the active one. A transmembrane proton-gradient induced by acidification was not relevant to the fusion process, since its elimination with nigericin did not affect the FP-mediated fusion. Both Ca2+ (8 mM) and the increase of the ionic strength (1 M NaCl) inhibited liposome fusion. The efficacy of liposome fusion depended also on the lipid-to-lipid ratio. Non-linear dependence was observed at a saturation ratio of 10 mol lipid per mol peptide. A model of 'side insertion' is suggested, describing FP interaction with the membrane.

Changes in membrane permeability during Semliki Forest virus induced cell fusion

The infection of Aedes albopictus cells by Semliki Forest virus (SFV) is a non lytic event. Exposure of infected cells to mildly acidic pH (less than 6.2) leads to syncytium formation. This polykaryon formation is accompanied by an influx of protons into the cells (Kempf et al. Biosci. Rep. 7, 761-769, 1987). We have further investigated this permeability change using various fluorescent or radiolabeled compounds. A significant, pH dependent increase of the membrane permeability to low molecular weight compounds (M(r) less than 1000) was observed when infected cells were exposed to a pH less than 6.2. The pH dependence of the permeability change was very similar to the pH dependence of cell-cell fusion. The permeability change was sensitive to divalent cations, protons and anionic antiviral drugs such as trypan blue. The nature of this virus induced, pH dependent permeability change is discussed.

Killing of Burkitt-lymphoma-derived Daudi cells by ultraviolet-inactivated vaccinia virus

Interaction of active and UV-inactivated vaccinia virus at high multiplicity caused cytological changes and inhibition in cellular protein and DNA synthesis, thus arresting the multiplication of Burkitt-lymphoma-derived Daudi cells and eventually killing the cells. Adsorption to the cells but the lack of penetration was evident by immunofluorescence, electron microscopy and [3H]thymidine-labeled virus incorporation. Viral DNA synthesis or virus replication was not demonstrated. Thus, it appears that the massive adsorption of viral particles, active or UV-inactivated, or possibly a "toxic" component that resides in the virion, damages the plasma membrane and may be responsible for killing the cells by a mechanism of lysis from without.

Perturbation of host-cell membrane is a primary mechanism of HIV cytopathology

Cytopathic viruses injure cells by a number of different mechanisms. The mechanism by which HIV-1 injures T cells was studied by temporally examining host-cell macromolecular syntheses, stages of the cell cycle, and membrane permeability following acute infection. T cells cytopathically infected at an m.o.i. of 1-5 grew normally for 24-72 hr, depending on the cell line, followed by the first manifestation of cell injury, slowing of cell division. At that time significant amounts of unintegrated HIV DNA and p24 core protein became detectable, and acridine orange flow cytometric cell cycle studies demonstrated the presence of fewer cells in the G2/M stage of the cell cycle. There was no change in the frequency of cells in the S-stage, and metabolic pulsing with radioactive precursors demonstrated that host-cell DNA, RNA, and protein syntheses were normal at that time and normal up to the time cells started to die (approximately 24 hr later), when all three decreased. Cellular lipid synthesis, however, was perturbed when cell multiplication slowed, with phospholipid synthesis reduced and neutral lipid synthesis enhanced. Permeability of the host-cell membrane to small molecules, such as Ca2+ and sucrose, was slightly enhanced early postinfection, and by the time of slowing of cell division, host membrane permeability was greatly increased to both Ca2+ and sucrose (Stokes radius 5.2 A) but not to inulin (Stokes radium 20 A). These changes in host-cell membrane permeability and phospholipid synthesis were not observed in acutely infected H9 cells, which are not susceptible to HIV cytopathology. Thus, HIV-1 appeared to predominantly injure T cells by perturbing host-cell membrane permeability and lipid synthesis, which is similar to the cytopathic mechanisms of paramyxoviruses.

Detection of toxic viral-associated double-stranded RNA (dsRNA) in influenza-infected lung

While many of the molecular events in viral replication are well studied, the molecular mechanisms by which viral infections trigger such constitutional symptoms as fever and 'malaise' are unknown. The hypothesis that these viral constitutional symptoms can be triggered by the toxic action of dsRNA associated with viral replication was investigated. Total lung RNA from mice acutely infected with PR8 influenza virus, but not from sham-infected mice, was shown to induce fever and altered sleep (excess slow-wave sleep, enhanced amplitudes of electroencephalographic slow waves, and reduced rapid eye movement sleep) when injected into the rabbit brain. Viral-associated dsRNA was shown to be responsible for the rabbit responses by differential nuclease digestion. Influenza viral dsRNA was directly demonstrated in the active lung RNA preparations by reverse transcriptase-polymerase chain reaction techniques. The time course of the responses paralleled those seen in the same model inoculated with nanogram quantities of the synthetic dsRNA polyriboinosinic-polyribocytidylic acid and suggested that they were mediated by induced cytokines. A model for the role of viral-associated dsRNA in eliciting both local cytotoxicity and viral constitutional symptoms is presented.

Potentiation of platelet responses in vitro by feline infectious peritonitis virus

The effect of feline infectious peritonitis virus (FIPV) on platelet aggregation and 14C-serotonin release induced by threshold levels of four agonists (adenosine diphosphate [ADP], collagen, arachidonic acid, and epinephrine) was examined in vitro in ten specific-pathogen-free cats. Purified suspensions of FIPV added to stirred platelet suspensions (virus to platelet ratio equal to 1:320) 1 minute prior to the addition of agonist potentiated the ADP-induced aggregation response by greater than 100% in seven cats. Platelet 14C-serotonin release was increased by greater than 100% in four cats. Collagen-induced platelet aggregation was enhanced in ten cats while collagen-induced 14C-serotonin release was enhanced in eight cats. Potentiation of arachidonic acid-induced platelet aggregation was observed in three cats, two of which demonstrated enhanced platelet 14C-serotonin release. Although epinephrine-induced platelet aggregation was enhanced in five cats, the samples displayed only fine microaggregates. Enhanced 14C-serotonin release from platelets in response to epinephrine was not demonstrated. Interaction with the outer platelet membrane and internalization of viral particles within the surface-connected open canalicular system were demonstrated by electron microscopy within 5 minutes of the addition of virus to platelet suspensions with or without added agonists. Decreasing the virus concentration by ten- or one hundred-fold abolished the potentiating effect observed previously, while increasing the concentration tenfold resulted in direct platelet activation in the absence of agonist.(ABSTRACT TRUNCATED AT 250 WORDS)

Enhanced platelet reactivity in cats experimentally infected with feline infectious peritonitis virus

Platelet function was evaluated in six specific-pathogen-free cats prior to and following intraperitoneal inoculation with feline infectious peritonitis virus (FIPV). By 4 days post-inoculation, platelet samples from five of six cats responded with irreversible platelet aggregation to threshold concentrations of adenosine diphosphate (ADP). This was accompanied by enhanced platelet 14C-serotonin release (greater than 10%) in two cats. Compared to one of six baseline samples, five of five post-inoculation samples exhibited microaggregate formation in response to 20 microM epinephrine. Enhanced platelet 14C-serotonin release did not accompany these responses. Enhanced platelet responses to ADP and epinephrine were also observed on day 11 post-inoculation and day 16 (when one cat died) or 21 (the end of the study). Platelet 14C-serotonin release in response to 20 microM epinephrine increased markedly in three of five cats on day 21. Enhanced collagen-induced platelet responses were not demonstrated. Although the mechanism for the enhanced platelet responses observed on day 4 was unknown, a direct effect on the virus on platelets, mononuclear inflammatory cells, and endothelial cells must be considered.

Membrane transport and disease

Four situations in which membrane transport is altered by disease are discussed: (a) non-specific leaks induced by poreforming agents; (b) glucose transport and cellular stress; (c) Ca2+-ATPase and hypertension; (d) Na+ channels and HSV infection.

Stress-induced increase of hexose transport as a novel index of cytopathic effects in virus-infected cells: role of the L protein in the action of vesicular stomatitis virus

The VSV-specific increase in hexose transport by BHK cells has been measured by assay of the [3H]dGlc/[14C]AIB uptake ratio. The effect was abolished by uv-irradiation of the virus, indicating that viral gene expression is required. Cells infected with the T1026 R1 mutant of VSV, which causes only slight cytopathic changes, exhibited only a slight increase in hexose uptake. Cells infected with temperature-sensitive (ts) mutants of VSV that are defective in the function of the viral N, NS, G, or M proteins at the restrictive temperature (39.5 degrees) exhibited increased [3H]dGLC/[14C]AIB uptake ratios typical of wild-type virus at either restrictive (39.5 degrees) or permissive temperature (34 degrees). Cells infected with a mutant defective in the function of the viral L protein exhibited an increased [3H]dGlc/[14C]AIB uptake ratio at permissive temperature (34 degrees) only; at restrictive temperature (39.5 degrees) the uptake ratio was essentially the same as that of mock-infected cells. Temperature-shift experiments indicated that the effect on hexose transport persisted for at least 6 hr in cells which no longer expressed function L protein, and that when expression of L was restricted to the first 2 hr of infection, an almost complete stimulation of hexose transport was observed 4 hr later. These results indicate that expression of the L gene is a necessary factor for inducing an increased hexose uptake in VSV-infected BHK cells. They also suggest that the action of the L protein on hexose transport is indirect, and is presumably mediated by other cellular constituents. The studies support the concept that an increased dGlc uptake may be a useful index of the cytopathic consequences of virus infection.

Molecular biology and the diagnosis, epidemiology and pathogenesis of infectious diseases

In this short review, the impact of molecular biology on microbiology in general is described. Specifically, molecular biology is increasingly enlarging the available choice of methods for the diagnosis of microbial disease. In situ hybridization seems to be a particularly promising procedure. In epidemiology, an interesting facet is the high mutation rate of RNA viruses. In pathogenesis, molecular biology will help to elucidate pathways of infection and the targeting of pathogenic macromolecules within the cell and within an organism.

Ion modulation of membrane permeability: effect of cations on intact cells and on cells and phospholipid bilayers treated with pore-forming agents

Leakage of ions (Na+, K+) and phosphorylated metabolites (phosphorylcholine, 2-deoxyglucose 6-phosphate) through membrane lesions in intact cells or in cells modified by 'pore-forming' agent has been studied. Leakage from intact cells is induced by protons and by divalent cations such as Cu2+, Cd2+ or Zn2+. Leakage from agent-modified cells--or across phospholipid bilayers modified by agent--is prevented by low concentrations of the same cations and by higher concentrations of Ca2+, Mn2+ or Ba2+; Mg2+, dimethonium, spermine, or spermidine are virtually ineffective. The relative efficacy of a particular cation (e.g. Ca2+) depends more on cell type than on the nature of the pore-forming agent. The predominant effect is on binding of cation to specific sites, not on surface charge. Surface charge, on the other hand, does affect leakage from agent-modified cells in that suspension in nonionic media reduces leakage, which can be restored by increasing the ionic strength: univalent (Na+, K+, Rb+, NH4+) and divalent (Mg2+, dimethonium) cations are equally effective; addition of protons or divalent cations such as Zn2+ to this system inhibits leakage. From this and other evidence here presented it is concluded that leakage across membranes is modulated by the presence of endogenous anionic components: when these are in the ionized state, leakage is favored; when unionized (as a result of protonation) or chelated (by binding to divalent cation), leakage is prevented. It is suggested that such groups are exposed at the extracellular face of the plasma membrane.

Salt-induced enhancement of measles virus yields in cultured cells

Supplementation of culture medium with MgSO4 or Na2SO4 in millimolar concentrations caused an enhanced measles virus (MV) yield from cultured quail embryo cells. MgSO4 at 25-50 mM concentrations exhibited the most pronounced and consistent stimulatory effect. MV infectivity increases ranged from 2- to 200-fold; the effect was highly reproducible for stationary monolayer, roller or microcarrier-grown cell culture types. MgSO4 also improved MV plaque development and caused MV plaque size enlargement on Vero cell monolayers. At mM concentrations MgSO4 was not operative as a MV thermostabilizing agent; rather, salt-induced enhancement of MV yields appeared to be due to intracellular events, e.g., augmented viral protein synthesis.

Cell surface effects of human immunodeficiency virus

Cell killing by human immunodeficiency virus (HIV) is thought to contribute to many of the defects of the acquired immunodeficiency syndrome (AIDS). Two types of cytopathology are observed in HIV-infected cultured cells: cell-cell fusion and killing of single cells. Both killing processes appear to involve cell surface effects of HIV. A model is proposed for the HIV-mediated cell surface processes which could result in cell-cell fusion and single cell killing. The purpose of this model is to define the potential roles of individual viral envelope and cell surface molecules in cell killing processes and to identify alternative routes to the establishment of persistently-infected cells. Elucidation of HIV-induced cell surface effects may provide the basis for a rational approach to the design of antiviral agents which are selective for HIV-infected cells.