Studies on persistent infections of tissue cultures. IV. Evidence for the production of an interferon in MCN cells by myxoviruses

Expression of plasmid-based shRNA against the E1 and nsP1 genes effectively silenced Chikungunya virus replication

Background

Chikungunya virus (CHIKV) is a re-emerging alphavirus that causes chikungunya fever and persistent arthralgia in humans. Currently, there is no effective vaccine or antiviral against CHIKV infection. Therefore, this study evaluates whether RNA interference which targets at viral genomic level may be a novel antiviral strategy to inhibit the medically important CHIKV infection.

Methods

Plasmid-based small hairpin RNA (shRNA) was investigated for its efficacy in inhibiting CHIKV replication. Three shRNAs designed against CHIKV Capsid, E1 and nsP1 genes were transfected to establish stable shRNA-expressing cell clones. Following infection of stable shRNA cells clones with CHIKV at M.O.I. 1, viral plaque assay, Western blotting and transmission electron microscopy were performed. The in vivo efficacy of shRNA against CHIKV replication was also evaluated in a suckling murine model of CHIKV infection.

Results

Cell clones expressing shRNAs against CHIKV E1 and nsP1 genes displayed significant inhibition of infectious CHIKV production, while shRNA Capsid demonstrated a modest inhibitory effect as compared to scrambled shRNA cell clones and non-transfected cell controls. Western blot analysis of CHIKV E2 protein expression and transmission electron microscopy of shRNA E1 and nsP1 cell clones collectively demonstrated similar inhibitory trends against CHIKV replication. shRNA E1 showed non cell-type specific anti-CHIKV effects and broad-spectrum silencing against different geographical strains of CHIKV. Furthermore, shRNA E1 clones did not exert any inhibition against Dengue virus and Sindbis virus replication, thus indicating the high specificity of shRNA against CHIKV replication. Moreover, no shRNA-resistant CHIKV mutant was generated after 50 passages of CHIKV in the stable cell clones. More importantly, strong and sustained anti-CHIKV protection was conferred in suckling mice pre-treated with shRNA E1.

Conclusion

Taken together, these data suggest the promising efficacy of anti-CHIKV shRNAs, in particular, plasmid-shRNA E1, as a novel antiviral strategy against CHIKV infection.

Mechanism of Polykaryocytosis Associated with Noncytopathic Infection by Measles Virus

Atherton, John G. (University of Southern California, Los Angeles), Sotiros G. Chaparas, Martha Cremer, and Irving Gordon. Mechanism of polykaryocytosis associated with noncytopathic infection by measles virus. J. Bacteriol. 90:213-219. 1965.-Infection with a measles virus variant resulted not only in formation of polykaryocytes (PK) but also in formation of multicellular immunofluorescent foci (IFF) in which no cytopathic effect could be detected. The ratio of IFF to PK changed from 27 to 4 during the first passage and remained 4 after a second passage. PK were plaques. Plaque assay was linear in the presence of IFF. To investigate the mechanism of PK formation, radioautography was done on cells pulse-labeled with tritiated thymidine before virus multiplication began. The results showed that PK were formed by fusion; there were no PK whose nuclei contained no label, and the proportion of labeled nuclei (32%) and distribution of grain counts was the same in PK as in uninvolved cells, ruling out nuclear replication without concomitant cytoplasmic membrane formation as the mechanism of formation of these PK. Early in PK development, neutral red uptake was markedly increased ("red" plaques). As PK matured, hyperchromicity disappeared ("white" plaques). This sequence provided an index of rate of evolution of PK. Rate of PK maturation was more rapid at 37 than at 32 C.

The role of interferon in vaccinia virus infection of mouse embryo tissue culture

The production of an interferon-like substance by vaccinia virus is described. The physical properties of vaccinia interferon are shown to be similar to those of previously reported interferons. Data defining the role of vaccinia interferon in cell resistance and in establishment and maintenance of a carrier culture are presented. Elimination of virus from an infected culture is demonstrated and the role of interferon in the recovery process is considered.

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.

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.

STUDIES ON PERSISTENT INFECTIONS OF TISSUE CULTURES. V. THE INITIAL STAGES OF INFECTION OF L(MCN) CELLS BY NEWCASTLE DISEASE VIRUS

The initial stages of infection of L(MCN) cell populations with standard Newcastle disease virus (NDV_ST) were analyzed in an effort to elucidate the steps leading to survival of the cultures and to the indefinite persistence of the infectious process at a low level. Cells were exposed in suspension to NDV at varying multiplicities and the monolayer cultures derived from such cells assayed at intervals for cellular growth rates, percentage of infected cells as determined by immunofluorescence, yields of viral progeny and of interferon, and, on occasion, resistance to superinfection with vesicular stomatitis virus. The percentage of cells calculated to be initially infected on the basis of adsorption data was found to match closely the percentage of immunofluorescent cells resulting from the first infectious cycle (up to 24 hours). Cells initially infected with NDV_ST produced a mixed progeny of infectious virus (from 15 to 40 pfu/cell) and about 10 times as many non-infectious particles in 24 hours [NDV_L(MCN)], but little or no interferon. If all cells were infected the cultures ultimately died. At multiplicities of infection (m) of 2 or less the cultures survived with increasing ease as the percentage of infected cells was reduced. The number of pfu per infected cell was of the above order during the first 3 days; it declined thereafter. Limited secondary spread of the infection was noted by 48 hours and no further cycling was noted thereafter. As m decreased from 2.0 to 0.1 there was an increase in the yields of interferon and the time at which peak titers were reached. Addition of anti-NDV serum 2 hours after infection prevented measurable production of interferon. In contrast, following exposure of cells to NDV_L(MCN) at multiplicities ranging from 20.0 to 0.2 (based on infectious virus) all cultures survived, no secondary spread was noted, the number of pfu per infected cells was reduced at the higher multiplicities, and the yields of interferon were similar and maximal by 24 hours and not affected by anti-NDV serum added after an adsorption period of 2 hours. It is concluded that the non-infectious virus particles in the progeny released from NDV_ST-infected cells induce resistance in remaining cells or, if adsorbed simultaneously with infectious virus, abort the intracellular infectious process. In both instances interferon is produced which may then render additional cells resistant. The non-infectious component is considered an incomplete or defective product of viral replication and not merely thermally inactivated virus. NDV_ST partially or completely inactivated at 37°C induced neither cellular resistance nor synthesis of interferon. The incomplete viral component behaved in all respects like ultraviolet-inactivated NDV_ST except that it was significantly more efficient in inducing interferon synthesis. On the basis of the presented data a scheme has been devised and discussed which appears to explain satisfactorily the events which take place on initial infection of L(MCN) cells with NDV and which lead to the persistence of the infectious process.

Role of interferon in murine lactic dehydrogenase virus infection, in vivo and in vitro

The induction of interferon (IF) by lactic dehydrogenase virus (LDV) and the action of exogenous IF on LDV multiplication have been studied in vivo and in vitro. In vivo the induction of serum IF by LDV was shown to correspond with the virus challenge dose. In vitro LDV-infected macrophages did not produce detectable IF. As to the action of IF in vivo, the prophylactic or therapeutic administration of IF to mice reduced the LDV serum titers only temporarily. In vitro, (i) the sensitivity of LDV to IF in macrophage culture was 30 times less than that of vesicular stomatitis virus, and 5 times less than that of Sendai virus, and (ii) prolonged exposure of LDV-infected macrophages to high IF concentrations did not cure the chronic infection, but only decreased the virus titer. The probable role of IF in the establishment of the early phase of a persistent but reduced viremia in mice is discussed.

Selection of temperature-sensitive mutants during persistent infection: role in maintenance of persistent Newcastle disease virus infections of L cells

Virus mutants (NDV(pi)) recovered from L cells persistently infected with Newcastle disease virus (NDV, Herts strain) are temperature-sensitive (ts) at 43 C, although the wild-type virus (NDV(o)) which initiated the persistent infection replicates normally at that temperature. To study the relationship between the ts marker of NDV(pi) and the other properties which distinguish this virus from NDV(o), NDV(pi) ts(+) revertants were selected at the nonpermissive temperature and NDV(o) ts mutants were generated by treating NDV(o) with nitrous acid. Spontaneously-occurring ts mutants in the Herts NDV population were also isolated. The different virus populations were characterized with regard to plaque size, virulence for eggs, and thermal stability of infectivity, hemagglutinin, and neuraminidase. The NDV(pi) ts(+) revertants, although no longer temperature-sensitive, retained NDV(pi) properties, whereas both spontaneously-occurring and mutagen-induced ts mutants remained wild-type in their other properties. These findings showed that the properties which characterized NDV(pi) were independent of the ts marker. However, the ts marker and the other markers of NDV(pi) were coselected during the persistent infection, and the combination of those markers appeared to be important in the outcome of NDV infection of L cells. NDV(pi) replicated productively in L cells, whereas NDV(o), the NDV(pi) ts(+) revertants, and the spontaneously-occurring ts mutants all yielded covert infections in L cells. The role of the selection of ts mutants in persistent infection was confirmed as follows: L cells were persistently infected with NDV(pi) ts(+) revertants and NDV(o) ts mutants. Virus recovered from the persistently infected cultures after eight cell passages was always temperature-sensitive and of smaller plaque size than the parental virus in chicken embryo cell cultures. Similar results were obtained with virus recovered from L-cell cultures persistently infected with two other velogenic strains of NDV, the Texas-GB and Kansas-Man strains. These results strongly suggest that selection of ts mutants during the persistent infection was not random and played a role in establishment or maintenance of the persistent infection, or both.

Interferon induction by influenza type C

The presence of a heat-stable interferon-like inhibitor in allantoic and amniotic fluids collected from chick embryos infected with type C influenza virus was determined. This inhibitor was characterized as an interferon and the ability of both live and ultra-violet-irradiated influenza type C virus to induce the substance was examined under various conditions.

The Production of Cytopathic Changes in Canine Cell Lines by Infectious Agents: Cell Lines Derived from Melanoma and Thyroidal Carcinoma

The cytopathic effect of more than 30 viruses and Toxoplasma gondii and Histoplasma capsulatum was determined in established canine thyroid adenocarcinoma and canine melanoma cell lines. Marked effect was observed with the viruses of canine hepatitis, pseudorabies, and canine distemper, and reo type 1 and vaccinia viruses in both cell lines. Moderate effect was found with human adenovirus type 1, swine adenovirus and the viruses of swine influenza and Newcastle disease. Cytopathic changes were produced with the virus of herpes simplex in the thyroid adenocarcinoma cells. A transient giant cell formation was produced by the virus of measles in the thyroid cell line. Hemadsorption was demonstrated in both cell lines with vaccinia virus. Toxoplasma gondii multiplied in both cell lines and produced cellular degeneration and necrosis. Histoplasma capsulatum grew in the thyroid adenocarcinoma cell line with little cellular degeneration.

Studies on persistent infections of tissue culture. VI. Reversible changes in Newcastle disease virus populations as a result of passage in L cells or chick embryos

Populations of the Victoria strain of Newcastle disease virus (NDV), reisolated from persistently infected L-cell cultures and passed twice in the embryonated hen's egg (NDV(L-E-2)), were found to differ strikingly from the original, chick embryo-adapted virus (NDV(o)). After exposure of L cells to NDV(o) at high multiplicities of infection, all cells became abortively infected; they produced only small aggregates of viral antigen and few, if any, infectious virus particles, but they yielded large amounts of interferon. No cytopathic effects (CPE) were noted, and the cultures survived readily as viral carriers. In contrast, NDV(L-E-2) yielded under similar conditions large quantities of viral antigen and infectious virus particles, but no detectable interferon, and the cultures were rapidly destroyed. This change in "virulence" was at least partially reversible by further serial passages of NDV(L-E-2) in chick embryos, as was evident from a consecutive decrease in CPE with a concomitant increasingly rapid recovery of the L-cell cultures, gradually diminishing yields of infectious viral progeny, and the returning of a capacity to induce interferon synthesis. Thus, NDV(L-E-16) resembled NDV(o) in many aspects, except for a less striking reduction in its ability to replicate in L cells. Although a selection of viral variants under the given sets of conditions has not been entirely excluded, the establishment of "avirulence" appears to be largely explained by a gradual accumulation of noninfectious, interferon-inducing components in the course of serial passages in the embryonated hen's egg, and the acquisition of "virulence" by a loss of these components. The evidence is as follows. (i) By a step-wise decrease in the dose of virus and restriction of the analyses to the first infectious cycle, a multiplicity of infection was ultimately reached for all "avirulent" populations at which infected cells produced normal yields of infectious viral progeny; i.e., the interferon-inducing components were diluted to noneffective levels. The lowest multiplicity which resulted in a measurable reduction in infectious virus replication was also the last one to induce detectable interferon synthesis. (ii) All viral clones derived from "avirulent" populations behaved like NDV(L-E-2) rather than like the parent viral suspensions, except that some of them elicited small amounts of interferon in L cells. The interferon-inducing components were reduced or lost in the cloning procedures. The nature of the interferon-inducing components has not been established. These components, which were neutralized by rabbit sera against "virulent" NDV(L-E-2) populations, may represent largely inactive or incomplete virus particles; however, the infectious virus-hemagglutinin ratios of "avirulent" populations were mostly of an order similar to those of "virulent" populations. The interferon-inducing components aborted the infectious process in cells simultaneously invaded by infectious virus particles. The implications of these findings are discussed.

Interferon production by inactivated Newcastle disease virus in cell cultures and in mice

Youngner, Julius S. (University of Pittsburgh, Pittsburgh, Pa.), Anne W. Scott, Jules V. Hallum, and Warren R. Stinebring. Interferon production by inactivated Newcastle disease virus in cell cultures and in mice. J. Bacteriol. 92:862-868. 1966.-A comparison was made of the effects of ultraviolet (UV) irradiation or heating at 56 C on the interferon-stimulating capacity of Newcastle disease virus in primary chick embryo fibroblast (CE) cultures, in L-cell cultures, and in the intact mouse. The data obtained indicated the critical importance of the host cell system used for interferon production. Virus inactivated by UV irradiation, as well as infective virus, was an effective stimulus of interferon production in L cells and in mice, and this property persisted on continued irradiation. In contrast, in CE cell cultures, infective virus produced no interferon, whereas UV-irradiated virus produced maximal interferon titers when all infectivity was destroyed; continued irradiation resulted in a rapid loss of the interferon-stimulating capacity of the virus. Virus inactivated at 56 C did not produce interferon in CE or L-cell cultures. In the intact mouse, on the other hand, heat-inactivated virus was capable of stimulating the release of significant levels of circulating interferon.

Rubella virus carrier cultures derived from congenitally infected infants

Spontaneous rubella carrier cultures derived from tissues of infants with congenital rubella were studied in an attempt to elucidate a possible mechanism for viral persistence observed in these infants. Chronically infected cells were found to have a reduced growth rate and the cultures appeared to have a shortened life span. The rubella carrier state was not dependent on serum inhibitors or rubella antibodies. Virtually every cell in the carrier population was found to be producing virus. The carrier cultures could not be cured by rubella antibodies. The rubella-infected cells were resistant to superinfection with vesicular stomatitis virus and herpes simplex virus but were susceptible to infection with echovirus 11. The replication of vesicular stomatitis virus was apparently blocked at an intracellular site, for the virus readily adsorbed to the chronically infected cells and entered into an eclipse phase; however no infectious virus developed. No evidence of interferon production by these cells could be obtained. It is postulated that clones of rubella-infected cells in vivo, with properties similar to those in carrier cultures developed in vitro from tissues of in utero infected infants, might explain the observed viral persistence noted in congenital rubella.