Epistemic_Statements  

CORD-19:01548ef6d46d6e6d72afe8cbf4e231552b9d2bd7 JSONTXT 9 Projects

Annnotations TAB TSV DIC JSON TextAE

Id Subject Object Predicate Lexical cue
T1 244-716 Epistemic_statement denotes The latter can come from medium ingredients that have a biological origin, such as serum (Kniazeff, this volume) or trypsin (Hallauer et al., 1971) , can result from accidental contamination of the cultures by the technicians who handle them (Collier, 1957) , or can be introduced in association with the cells used to initiate the cultures (Hsiung, 1968) , the latter being particularly likely to occur when primary tissue cultures are used (Kalter and Heberling, 1968 ).
T2 717-998 Epistemic_statement denotes The first two sorts of viral contamination can be considered trivial from a biological point of view, because they constitute a peculiar hazard of the circumstances in which tissue culture is carried out and can be expected to be eliminated when a foolproof methodology is adopted.
T3 999-1181 Epistemic_statement denotes The third source from which viruses can be unknowingly introduced, namely those viruses inherent to the cultured tissues themselves, is the one with which this chapter is con-cerned.
T4 1182-1489 Epistemic_statement denotes It cannot be considered trivial, because the large extent of its probable occurrence raises the question of whether the presence of viruses can be considered an inevitable, or perhaps essential, accompaniment of the propagation of cells in vitro, especially when continuous cell lines are to be established.
T5 1490-1694 Epistemic_statement denotes Very many animal species have hidden virus infections, that is, viruses that appear to exist in close association with particular tissues or organs of the species involved, without causing overt symptoms.
T6 1695-1896 Epistemic_statement denotes It follows that primary cultures prepared from almost any tissue may be found infected with viruses in vitro simply because the organ from which they were prepared was infected in vivo (Hsiung, 1968) .
T7 2414-2563 Epistemic_statement denotes Primates have been more intensively studied in this respect that any other group of species, perhaps because of the widespread use of viral vaccines.
T8 2793-3021 Epistemic_statement denotes By reason of their capacity for continuous synthesis and replication, tumor cells have been assumed to provide a favorable milieu for the multiplication of viruses and therefore to be especially likely to harbor such passengers.
T9 3022-3307 Epistemic_statement denotes At the present time, it may be preferable to restrict the use of the term passenger to viruses carried into culture with the explanted tissue in the form of fully infectious extracellular virions that subsequently become capable of invading and replicating within cells of the culture.
T10 3308-3589 Epistemic_statement denotes In the most obvious cases, this type of association will eventually be manifested as a frank cytopathic effect, as seems to occur with passenger adenoviruses in human adenoids (Rowe et al, 1956 ) and the foamy viruses that are often found in monkey kidney cultures (Hsiung, 1968) .
T11 3590-3775 Epistemic_statement denotes The cytopathic effects observed in vitro may sometimes result from the acquisition of susceptibility to infection and lysis by cells that were originally resistant to the virus in vivo.
T12 3928-4131 Epistemic_statement denotes When monkey kidneys are infected in vivo with polio virus, no cytopathic effect can be observed and no virus can be recovered following subsequent perfusion of the kidney or homogenization of its tissue.
T13 4132-4412 Epistemic_statement denotes However, if parts of an infected kidney found negative for virus by such tests are reduced to cell suspensions by trypsin treatment and used to set up conventional monolayer cultures, cell destruction eventually takes place, accompanied by the appearance of fully infective virus.
T14 4553-4748 Epistemic_statement denotes The nature of the mechanisms involved-whether an actual change in the cells themselves or merely their removal from a protected environment, such as would be provided by antibody-remains unknown.
T15 4749-4886 Epistemic_statement denotes The so-called conditional lethal mutants are further examples of how extraneous conditions can affect the response of cells to infection.
T16 5048-5193 Epistemic_statement denotes Such a virus will not be apparent at the higher temperature but will emerge, accompanied by a cytopathic effect, when the temperature is lowered.
T17 5194-5499 Epistemic_statement denotes It is not suggested that temperature-dependent viruses of this type are widespread in nature-most of those known are selected laboratory mutants-but they are mentioned here to illustrate the point that appropriate culture conditions may allow hidden viruses to emerge that would not otherwise be detected.
T18 5500-5749 Epistemic_statement denotes A passenger association of a similar kind, although a less obvious one, occurs when only a small number of the cells in the culture are or become susceptible to the virus, although the latter is carried into the culture in its fully infectious form.
T19 5889-5941 Epistemic_statement denotes However, the virus never enters a truly latent form.
T20 5942-6071 Epistemic_statement denotes An association of this type between the paramyxovirus SV5 and human fibroblasts has been described by Wong and Kilbourne (1961) .
T21 6915-7065 Epistemic_statement denotes It is doubtful, though, whether the term passenger should be extended to include every virus that appears fortuitously in primary cultures of tissues.
T22 7342-7545 Epistemic_statement denotes Latent herpes simplex virus infection can be activated by a number of nonspecific stimuli, after which completed virions are released by infected skin cells that show a cytopathic effect (Fenner, 1968) .
T23 7546-7748 Epistemic_statement denotes It is unlikely that the herpes-type viruses that appear predominantly in primary tissue cultures all represent populations stemming from complete virions introduced to the cultures along with the cells.
T24 7749-7853 Epistemic_statement denotes Many may represent latent herpesvirus genomes activated as a result of the stress of culture initiation.
T25 7959-8212 Epistemic_statement denotes Primary cultures of cells from children with Burkitt's African lymphoma rarely release Epstein-Barr virus (EBV); its presence cannot be demonstrated with the electron miscroscope, either in the tumor itself or in the early stages of its culture history.
T26 8340-8578 Epistemic_statement denotes However, following a number of transfers in vitro, cells from many such cultures can be shown to contain microscopically identifiable viral structures and a proportion of them now show positive immunofluorescence (Henle and Henle, 1966) .
T27 8848-9066 Epistemic_statement denotes Acquisition of an identifiable herpes-like virus following passage is not a unique property of cells from Burkitt's African lymphoma and has been shown to occur with many floating lymphoid cell lines of varied origin .
T28 9189-9566 Epistemic_statement denotes In this instance, the recent establishment from cases of Hodgkin's disease (Eisinger et al., 1971 ) of lymphoblastoid cell lines, capable of growth in suspension and carrying a herpes-like virus resembling, but not identical with, Epstein-Barr virus, is of interest, although there is as yet no evidence of a connection between this virus and the etiology of Hodgkin's disease.
T29 9567-9776 Epistemic_statement denotes In certain other human tumor culture systems, prolonged cultivation may be necessary before antigenic and other changes, possibly indicative of the emergence of viruslike agents, can take place (Sabin, 1965) .
T30 9777-10368 Epistemic_statement denotes It is therefore proposed that the term indigenous rather than passenger be used to describe this group of viruses, indigenous viruses being defined as those that are frankly pathogenic in the appropriate circumstances [Epstein-Barr virus has been identified as the cause of infectious mononucleosis (Henle et al, 1968; Hirshaut et al., 1969) ; herpesvirus Β is latent in the rhesus monkey but causes fatal disease in man (Andrewes, 1964) ] but that are normally introduced into tissue cultures within latently infected cells, later to be induced to proliferate by stimuli at present unknown.
T31 10369-10473 Epistemic_statement denotes Cells in tissue culture may have viruses associated with them in a third way, which we may term cryptic.
T32 10716-10840 Epistemic_statement denotes Naturally, this is a broad class that includes associations between viruses and cells of widely varying degrees of intimacy.
T33 11036-11184 Epistemic_statement denotes Examples are the type C viral particles that frequently appear in cultures of mouse tissues following prolonged cultivation (Aaronson etal., 1969) .
T34 11440-11878 Epistemic_statement denotes An intermediate category is typified by the association between SV40 virus and nonpermissive mouse cells or between Polyomavirus and hamster kidney cells (BHK 21 ) (Dulbecco, 1968) in which the viral presence is signaled not only by altered cellular properties but also by the manufacture of virally specified antigens and the possibility of rescuing complete extracellular virus by fusion with permissive cells (Koprowski et al., 1967) .
T35 12027-12199 Epistemic_statement denotes Cells incubated at the nonpermissive temperature appear normal; they do not have the transformed morphology and mode of growth that stamp them as carrying the viral genome.
T36 12388-12762 Epistemic_statement denotes Treatment of cultured cells with certain nucleoside analogs (bromoor iododeoxyuridine) can cause the activation of apparently cryptic viral RNA genomes, resulting in the appearance of products varying from special antigens thought to be virally specified (Weiss et al., 1971) up to complete infective type C oncornavirus particles (Lowy et al., 1971; Aaronson etal., 1971) .
T37 13016-13236 Epistemic_statement denotes host cells have been taken from cases in which the association has been deliberately established by treating cells with viruses in tissue culture and then showing later that a cryptic type of association has been set up.
T38 13237-13464 Epistemic_statement denotes There is, however, no a priori reason for assuming that associations of this kind do not occur in vivo and that cells detached from whole organisms and subsequently cultivated in vitro will not prove to be cryptically infected.
T39 13465-13802 Epistemic_statement denotes That this may be so is indicated by the observation that cells derived from tumors caused by the inoculation of polyoma or SV40 viruses into infant hamsters, and subsequently cultured do not manufacture virus but can be shown, nevertheless, to contain viral genomes because complete virus may be rescued by fusion with permissive cells .
T40 13803-13893 Epistemic_statement denotes The transfer of cells from the in vitro to an in vivo situation can show a similar effect.
T41 13894-14238 Epistemic_statement denotes Syrian (Vigier, 1970) or Chinese hamster (Hlozanek et al., 1966) or rat cells (Svoboda, 1964) transformed morphologically in vitro by means of Rous sarcoma virus show a cryptic association between virus and cell, for their injection into chickens results in the growth of a tumor consisting of chicken cells that release the same sarcoma virus.
T42 14239-14385 Epistemic_statement denotes For over 10 years it has been widely believed that cell cultures initiated from normal tissues have a limited lifespan in vitro (Hayflick, 1965) .
T43 14386-14455 Epistemic_statement denotes Two main hypotheses have been advanced to account for this condition.
T44 14459-14718 Epistemic_statement denotes It can result from inadequacy of the medium used and/or the unnatural stresses to which cells are subject during manipulation in culture (Puck et al., 1958; Todaro and Green, 1964) , as well as from the particular schedule adopted for subculture (Hay, 1970) .
T45 14893-14991 Epistemic_statement denotes Each tissue cell is conceived as having an inborn potential for a finite number of cell divisions.
T46 15178-15267 Epistemic_statement denotes At the present time, it is still uncertain whether either of these hypotheses is correct.
T47 15392-15605 Epistemic_statement denotes Human diploid fibroblasts appear generally to have a limited lifetime in vitro (Hayflick, 1965) ; those from rats appear capable of long-term cultivation without loss of growth potential (Petursson et al., 1964) .
T48 15606-15940 Epistemic_statement denotes Cells from mice, hamsters, and many other species frequently give rise to cell lines capable of continuous cultivation, the properties of which, such as saturation density and oncogenic potential, depend on the seeding density and transfer interval adopted for their propagation (Macpherson and Stoker, 1962; Todaro and Green, 1963) .
T49 16058-16345 Epistemic_statement denotes Tissues from other species, such as the Chinese hamster (Yerganian and Leonard, 1961; Hsu et al., 1964) , also readily give cell lines that can be continuously propagated in vitro, although here the cells tend to become pseudodiploid rather than heteroploid when continuously maintained.
T50 16346-16504 Epistemic_statement denotes The possibility of establishing continuous lines with cells from these species does not seem to depend on their physiological age at the time of explantation.
T51 16772-16925 Epistemic_statement denotes The mode of origin of the abnormal chromosome complements of continuous cell lines, whether by selection or by some other means, is at present uncertain.
T52 16926-17148 Epistemic_statement denotes However, it has been claimed that it can itself be a consequence of employing defective media and that by using the proper regimen, continuous lines of euploid cells can be established, even from humans (Puck et ai, 1958).
T53 17149-17314 Epistemic_statement denotes It has not been ruled out, however, that the ability of tissues to generate continuous cell lines may be linked with the occurrence of indigenous or cryptic viruses.
T54 17443-17992 Epistemic_statement denotes A few that contain no virus detectable by immunofluorescence or electron microscopy [e.g., Raji cells derived from a Burkitt lymphoma (Pulvertaft, 1965) , NHDL 3 cell cultures from normal human leukocytes (Gerber and Monroe, 1968) , NC 37 from leukocytes of a patient with pneumonia (see Gerber, 1972) ] have been shown to contain cryptic virus by cloning (Maurer et ai, 1969) , nucleic acid hybridization (Zur Hausen andScholte-Holthausen, 1970), complement fixation (Vonka et al., 1970) , or activation by bromodeoxyuridine (BrdU) (Gerber, 1972) .
T55 17993-18145 Epistemic_statement denotes The readiness with which murine tissues give rise to continuous cell lines can be correlated with the presence of cryptic genomes of type C RNA viruses.
T56 18146-18345 Epistemic_statement denotes Activation of the latter can take place spontaneously in cells propagated in vitro (Aaronson et al, 1969) and in mice as the result either of aging or of certain carcinogenic stimuli (Kaplan, 1967) .
T57 18346-18564 Epistemic_statement denotes Treatment with halogenated pyrimidines can be used to reveal the presence of such viruses in clonal lines of cells from both high and low leukemia incidence strains of mice (Lowy et 0/., 1971 ; Aaronson et al., 1971) .
T58 18686-18818 Epistemic_statement denotes In vivo they develop a capacity to produce virus, for cells from newborns or older mice can be shown to release virus when cultured.
T59 18819-19101 Epistemic_statement denotes Although the above information indicates strongly that mouse cells may contain cryptic type C viral genomes that can be activated by various means, there is as yet no direct evidence that activation of the latter confers upon the cells a capacity to propagate indefinitely in vitro.
T60 19102-19194 Epistemic_statement denotes An indirect suggestion that this may be so, however, is given by the work of Aaronson et al.
T61 19566-19756 Epistemic_statement denotes The question thus remains open as to whether indigenous or cryptic viruses invariably accompany an ability to grow continuously in cell culture, and its answer must await further experiment.
T62 19757-20047 Epistemic_statement denotes In the meantime, it must be kept in mind that the presence of viral particles of abnormal morphology, such as the corona viruses common to hamster cells (Bernhard and Tournier, 1964) , may be a normal feature of cells in culture and not the result of contamination by some extraneous agent.
T63 20048-20238 Epistemic_statement denotes It is also necessary to be cautious regarding the relationship between the acquisition by cells of hetero-or pseudodiploidy and the onset of the ability to propagate continuously in culture.
T64 20239-20598 Epistemic_statement denotes Viral infection of cells has been shown in many instances to be accompanied by karyotypic changes (Hampar and Ellison, 1963; Nichols et al., 1962; Shein and Enders, 1962; Koprowski etal., 1962; Black and Rowe, 1963) , so that the chromosomal alterations observed at the initiation of continuous cell lines may be secondary to the real cause of the phenomenon.
T65 20599-20834 Epistemic_statement denotes Apart from the possible role of viruses in the establishment of normal tissue cultures, it is necessary to consider the extent to which the presence of passenger, indigenous, or cryptic viruses can invalidate experimental observations.
T66 20835-20944 Epistemic_statement denotes To the virologist, in particular the viral geneticist, the absence of passenger viruses would seem essential.
T67 21159-21419 Epistemic_statement denotes A great many viruses are now known that can be associated with no known disease, and with some of them it is quite uncertain whether they originated from the tissues used to set up the cultures used or from the materials that were later used to inoculate them.
T68 21420-21822 Epistemic_statement denotes Not only are passenger viruses an obvious source of error when tissue cultures are employed in attempts to isolate viruses from clinical material but extraneous agents can also confuse the results of experiments undertaken to study the behavior of human viruses in tissue culture, by suppressing (Hoggan et al., 1966) or even stimulating (Rabsoneia/., 1964) the multiplication of the agent under study.
T69 21823-21926 Epistemic_statement denotes It is, however, in genetic experiments that their presence is most likely to lead to false conclusions.
T70 21927-22150 Epistemic_statement denotes Hybridization, at the level of the genetic material, between viruses growing in the same cell is well known and subsequent replication of the hybrid agent can give rise to virions of altered properties (Rapp et al., 1965) .
T71 22151-22384 Epistemic_statement denotes Virus stocks containing virions with drastically changed properties can also arise by transcapsidation (Hanafusa et al., 1964) , a condition where the genetic material of one virus becomes enclosed within the protein coat of another.
T72 22385-22477 Epistemic_statement denotes Compared with passenger viruses, indigenous and cryptic agents are more difficult to detect.
T73 22478-22563 Epistemic_statement denotes Indeed, in some cases they may remain undetected during many tissue culture passages.
T74 22564-22966 Epistemic_statement denotes The recovery of viruses following prolonged cultivation of material from human tumors or as a result of exposure of the latter's cells to an alien environment, such as that provided by the tissues of another species (McAllister et al., 1971) , can give rise to false hopes that a tumor etiologic agent has been isolated, whereas in reality an indigenous latent virus has been stimulated to proliferate.
T75 22967-23252 Epistemic_statement denotes Positive chemical stimulation of tumor cells to release hitherto cryptic agents is likewise of doubtful significance, unless it can be shown that normal cells from the same tissue in the same individual do not also respond by the production of virus and/or antigens when so stimulated.
T76 23253-23492 Epistemic_statement denotes Like passenger viruses, stimulated indigenous or activated cryptic viruses have potential to affect the replication of known viruses added to the culture and can also be concerned in hybridization and transcapsidation (Rapp et al., 1965) .
T77 23493-23682 Epistemic_statement denotes The experimental virologist thus has the need to know that his cells are free from all three classes of agents before he can make an unequivocal interpretation of his experimental findings.
T78 23683-23788 Epistemic_statement denotes Methods for the recognition of indigenous and cryptic virus infections have been suggested by Fogh et al.
T79 23789-23831 Epistemic_statement denotes (1971) and will be mentioned in Section V.
T80 23832-24237 Epistemic_statement denotes The possibility that cryptic viral genomes are already present in the cells used to support virus growth raises difficulties in the interpretation of cell transformation and viral replication brought about by type C oncornaviruses, in particular when an attempt is made to discover the part played by RNA-dependent DNA polymerases (Baltimore, 1970; Temin andMizutani 1970) with respect to these processes.
T81 24238-24645 Epistemic_statement denotes Following demonstration in cell-free systems, it is widely accepted that these enzymes, by means of an RNA-DNA hybrid intermediate, can transcribe part or all of the information in the viral RNA into a DNA copy that then becomes integrated into the host cell DNA, causing cell transformation followed by maintenance of the trans-formed state when this material is replicated in unison with the cellular DNA.
T82 24646-24757 Epistemic_statement denotes In productive infection, it may also serve as a template for the production of further copies of the viral RNA.
T83 24758-25089 Epistemic_statement denotes In support of the theory originally put forward by Huebner and , there is now evidence that normal mouse cells, even from strains with a low incidence of leukemia (Lowy et al, 1971; Aaronson et al, 1971 ), already contain the information for the production of MuLV virions for whose replication they are commonly used as substrate.
T84 25090-25294 Epistemic_statement denotes Similarly, cells from uninfected embryos of the kind used for studying the behavior of avian sarcoma viruses can be induced to form virally specified antigens by treatment with IUdr (Weiss et al., 1971) .
T85 25550-25689 Epistemic_statement denotes In the case of the type C RNA viruses, it is even possible to suggest that perhaps cancer causes viruses rather than the other way around !
T86 25690-25939 Epistemic_statement denotes The presumably common occurrence of indigenous and cryptic infection among cells from continuous cell lines as well as early passage cell strains suggests that it is probably safer for the experimenter to assume their presence rather than otherwise.
T87 26016-26178 Epistemic_statement denotes Once the presence of a particular virus is presumed, inoculation of homogenates of suspected cultures into cells known to respond cytopathically should reveal it.
T88 26179-26229 Epistemic_statement denotes In some instances, such cells may be hard to find.
T89 26230-26367 Epistemic_statement denotes Murine leukemia viruses multiply exuberantly in cultures of mouse fibroblasts without cytopathic effect and could thus remain undetected.
T90 26628-26860 Epistemic_statement denotes Direct examination of both cultured cells and gradient-centrifuged tissue culture supernatants with the electron microscope, using both thin sectioning and negative staining techniques, should also serve to reveal passenger viruses.
T91 26861-26954 Epistemic_statement denotes However, the relative insensitivity of such methods may fail to reveal minimal contamination.
T92 26955-27271 Epistemic_statement denotes Passenger viruses that do not have an obvious cytopathic effect, such as certain myxoviruses, might be detected by their ability to induce interferon production, causing the appearance of resistance to infection by an interferon-sensitive cytopathic agent, such as vesicular stomatitis or encephalomyocarditis virus.
T93 27272-27396 Epistemic_statement denotes Myxo-or paramyxoviruses may also be demonstrated directly through hemadsorption (Kilbourne, 1969; Marcus and Carver, 1969) .
T94 27397-27484 Epistemic_statement denotes The revelation of indigenous or cryptic viruses may require more sophisticated methods.
T95 27584-27775 Epistemic_statement denotes Other techniques might include the cocultivation and/or fusion (using ß-propiolactone-inactivated Sendai virus) of cells with indicator cells thought to be permissive for the virus suspected.
T96 27776-28167 Epistemic_statement denotes In this instance, properties of the virus other than its ability to replicate and cause cell destruction could be used to devise a detection system; for example, such cells as skin fibroblasts from human cases of Fanconi's anemia (Todaro et al., 1966) , Down's syndrome (Todaro and Martin, 1967) or Bloom's syndrome (German et al., 1965) may be sensitive indicators for transforming viruses.
T97 28168-28472 Epistemic_statement denotes Cocultivation and cell fusion, combined with chemical activation by nucleoside analogs and, perhaps, water-soluble mutagens and carcinogens Igel etal., 1969) , may prove useful for the detection of cryptic viruses, as will the use of helper viruses to call forth hidden viral genomes from cultured cells.
T98 28473-28618 Epistemic_statement denotes Feline leukemia virus (FeLV) would be an obvious choice, from its ability to multiply without cytopathic effect in many types of mammalian cells.
T99 28619-28769 Epistemic_statement denotes Its most important property isthat it can form a hybrid with murine sarcoma virus that can then infect cat cells and be sarcomagenic for this species.
T100 28770-28888 Epistemic_statement denotes FeLV may thus be used to call forth cryptic sarcoma virus genomes that could then be detected by their effect in cats.
T101 28889-29121 Epistemic_statement denotes More direct tests for indigenous or cryptic viruses or their intracellular manifestations will be to search, by immunofluorescence, for the presence of antigens associated with a specific group of viruses, using selected antiserums.
T102 29122-29270 Epistemic_statement denotes Immunofluorescence may also be used to detect the results of activating cryptic viral genomes by any of the methods suggested in the last paragraph.
T103 29271-29360 Epistemic_statement denotes Less certain, but giving suggestive results, is the technique of molecular hybridization.
T104 29361-29388 Epistemic_statement denotes It can be used in two ways.
T105 29392-29548 Epistemic_statement denotes The first is to see whether messenger RNA (mRNA) from cells suspected of harboring a cryptic virus hybridizes with nucleic acids from viruses already known.
T106 29958-30214 Epistemic_statement denotes Possibly the best way to determine whether a passenger, indigenous, or activated cryptic virus in a particular tissue culture system is complicating the results of experiments is to determine whether the viral yields comprise a mixed population of virions.
T107 30215-30335 Epistemic_statement denotes This can be done by analyzing the infective dose-response curve to see whether it obeys single-or multiple-hit kinetics.
T108 30473-30733 Epistemic_statement denotes However, it should be pointed out that even in these systems, the dose-response curve has the appearance of being single-hit once the helper virus is present in great excess, so that its presence will not be suspected if this is the only means of testing used.