STUDIES ON THE TOXICITY OF INFLUENZA VIRUSES : II. THE EFFECT OF INTRA-ABDOMINAL AND INTRAVENOUS INJECTION OF INFLUENZA VIRUSES

Understanding the Variability of Certain Biological Properties of H1N1pdm09 Influenza Viruses

The influenza virus continually evolves because of the high mutation rate, resulting in dramatic changes in its pathogenicity and other biological properties. This study aimed to evaluate the evolution of certain essential properties, understand the connections between them, and find the molecular basis for the manifestation of these properties. To that end, 21 A(H1N1)pdm09 influenza viruses were tested for their pathogenicity and toxicity in a mouse model with a ts/non-ts phenotype manifestation and HA thermal stability. The results demonstrated that, for a strain to have high pathogenicity, it must express a toxic effect, have a non-ts phenotype, and have a thermally stable HA. The ancestor A/California/07/2009 (H1N1)pdm influenza virus expressed the non-ts phenotype, after which the cycling trend of the ts/non-ts phenotype was observed in new strains of A(H1N1)pdm09 influenza viruses, indicating that the ratio of the ts phenotype will increase in the coming years. Of the 21 tested viruses, A/South Africa/3626/2013 had the high pathogenicity in the mouse model. Sequence alignment analysis showed that this virus has three unique mutations in the polymerase complex, two of which are in the PB2 gene and one that is in the PB1 gene. Further study of these mutations might explain the distinguishing pathogenicity.

Neurologic aspects of influenza viruses

Although influenza A and B viruses are primarily known as respiratory viruses and mainly infected only the upper respiratory tract in humans, patients with influenza often develop signs and symptoms that are not due to the respiratory system. Frequently individuals with influenza develop headaches, meningismus, and even seizures in addition to their typical respiratory symptoms. In the past decades, influenza viruses have also been associated with serious non-respiratory signs. The famous 1918 strain of influenza was associated with von Economo's encephalitis lethargica and postencephalitic parkinsonism. In the 1960s influenza virus infections in children were associated with Reye's syndrome characterized often by fatty non-inflammatory hepatic disease and an encephalopathy with marked non-inflammatory cerebral edema. Intermittently children with influenza develop focal myalgia and myositis. Guillain–Barré syndrome was epidemiologically associated with the 1978 killed influenza vaccine but not subsequent vaccines. Although occasional children with influenza have developed encephalopathy, from 2000 through 2004 there was an increase in the number of serious cases of acute necrotizing encephalopathy accompanying infection with the influenza A 2009 strain. The current H5N1 strain of bird influenza occasionally infects humans with a high mortality rate and some appear to have central nervous signs. This chapter explores what is known about these influenza neurologic associations.

The Role of Animal Models In Influenza Vaccine Research

A major challenge for research on influenza vaccines is the selection of an appropriate animal model that accurately reflects the disease and the protective immune response to influenza infection in humans. Vaccines for seasonal influenza have been available for decades and there is a wealth of data available on the immune response to these vaccines in humans, with well-established correlates of protection for inactivated influenza virus vaccines. Many of the seminal studies on vaccines for epidemic influenza have been conducted in human subjects. Studies in humans are performed less frequently now than they were in the past. Therefore, as the quest for improved influenza vaccines continues, it is important to consider the use of animal models for the evaluation of influenza vaccines, and a major challenge is the selection of an appropriate animal model that accurately reflects the disease and the protective immune response to influenza infection in humans.

The emergence of highly pathogenic H5N1 avian influenza (AI) viruses and the threat of a pandemic caused by AI viruses of this or another subtype has resulted in a resurgence of interest in influenza vaccine research. The development of vaccines for pandemic influenza presents a unique set of obstacles, not the least of which is that the demonstration of efficacy in humans is not possible. As the correlates of protection from pandemic influenza are not known, we rely on extrapolation of the lessons from seasonal influenza vaccines and on data from the evaluation of pandemic influenza vaccines in animal models to guide our decisions on vaccines for use in humans. The features and contributions of commonly used animal models for influenza vaccine research are discussed. The recent emergence of the pandemic 2009 H1N1 influenza virus underscores the unpredictable nature of influenza viruses and the importance of pandemic preparedness.

INFLUENZA : III. RAPID ALTERATIONS IN THE RESPIRATORY RATE OF EMBRYONATED EGGS APPARENTLY CAUSED BY INFLUENZA VIRUS TOXIN

Allantoic fluid from embryonated eggs infected with influenza A virus contains a toxic agent which can be demonstrated and quantitatively measured by its rapid effect on oxygen consumption when it is introduced in new series of fertile eggs. The effects were measured 90 minutes after the injection of the infected fluid, and were seen following both intra-allantoic injection and injection into the yolk sac. This toxin, in concentrations resulting from the injection of 0.5 cc. or less of the infected fluid, has no effect on oxygen consumption. The injection of 0.75 to 2.0 cc. of the fluid strikingly increases the oxygen consumption of the fertile eggs, while the injection of 3.0 cc. markedly depresses respiration. A similar reversal and eventual loss of the effect of the toxin on respiration were noted when the concentration of toxin was progressively diminished by heat inactivation. The toxic agent is slowly inactivated by heating at 56°C., but is effective long after infectivity and hemagglutinating ability have been destroyed. In this respect the agent differs from rickettsial and lymphogranuloma venereum virus toxins. The method described may be of value in studying the physiological effects of other toxic agents.

Studies on host-virus interactions in the chick embryo-influenza virus system; adsorption and recovery of seed virus

Upon injection of active influenza A or B virus into the allantoic cavity of the developing chick embryo, an average of only 70 per cent of the agent was adsorbed onto the tissue, as measured by the difference between the quantity of virus injected and that found free in the allantoic fluid of the injected eggs during the constant period. The degree of adsorption was similar, regardless of whether 10⁹ or 10² ID₅₀ of active virus was injected. Attempts to demonstrate the adsorbed virus in suspensions of the infected tissue met with partial success only in that not more than 1 to 5 per cent of the amount calculated to be adsorbed was actually found. All efforts to increase the yield of virus have failed. These results led to the suggestion that the seed virus, which participates in the propagation, becomes altered in such a way that it no longer may be demonstrated by infectivity titrations, whereas the active virus found represents superficially adsorbed virus, which does not multiply.

The production of fever by influenzal viruses; factors influencing the febrile response to single injections of virus

The intravenous injection of the PR8 strain of influenza A virus, the Lee strain of influenza B, and the "B" strain of Newcastle disease virus produces fever in rabbits. This phenomenon has been studied in relation to certain in vitro properties of these viruses. Saline suspensions of virus prepared by centrifugation or elution from chicken erythrocytes produced fever. Fluids from which most of the virus particles had been removed were non-pyrogenic. Exposure to temperatures which destroyed the infectivity of the virus for chick embryos did not prevent fever. However, heating sufficient to destroy the hemagglutinin also rendered virus non-pyrogenic. The injection of erythrocytes onto which virus had been adsorbed produced fever. Heated virus adsorbed onto erythrocytes, which failed to elute, produced no elevation of temperature, although heated virus alone was pyrogenic. Neutralization of virus with specific immune serum prevented fever. Antipyrine was capable of abolishing the febrile response to virus. Certain differences between the febrile response in rabbits to the injection of viruses and that following bacterial pyrogens were noted. The period between injection and beginning of temperature rise is longer with virus than with bacterial pyrogens. Relatively low temperatures inactivate the fever-producing capacity of viruses, whereas bacterial pyrogens withstand prolonged autoclaving, and the neutralization of viral fever by specific immune serum contrasts sharply with the failure of antibody to affect the response to bacterial pyrogens. Certain previous observations on the lymphopenia produced in rabbits by the injection of influenzal viruses were confirmed. The capacity of virus preparations to induce fever in rabbits closely parallels their capacity to induce lymphopenia. It was concluded that the fever-producing property of influenzal viruses is closely associated with the capacity to agglutinate erythrocytes.

Mechanism of production of pulmonary lesions in mice by Newcastle disease virus (NDV)

Infectious NDV particles produce extensive pulmonary consolidation in the mouse in the absence of demonstrable virus multiplication. The lesions are indistinguishable from those of influenza A virus infection. This effect of NDV was blocked by intranasal injection of RDE or immune serum before virus inoculation, but not by immune serum injected 5 minutes or more after NDV. Influenza A virus infection did not diminish fixation of NDV in excised lungs but did interfere with the injurious action of this agent in the living mouse. The analogy between these reactions and those which take place in a progressive virus infection is pointed out, and the mechanism of production of lesions in virus pneumonias discussed.

Studies on host-virus interactions in the chick embryo-influenza virus system. VI. Evidence for multiplicity reactivation of inactivated virus

Evidence has been presented that influenza viruses both of type A and B partially inactivated by ultraviolet irradiation may regain their capacity to propagate in the allantoic membrane of the chick embryo. In using such irradiated preparations as inocula for growth curve experiments it could be shown that the development of hemagglutinins as well as of infectivity preceded at rates resembling those noted with more than 10 times the amount of infective virus actually found in the irradiated seed. Partial inactivation of the inocula by heating to 56°C. gave similar results. The phenomenon was observed only with seed irradiated for short periods of time so that the virus particles sustained only few hits of radiation. On prolonged exposure resulting in numerous hits per virus particle the capacity of reactivation was lost. Likewise, an irradiated preparation capable of reactivation in the allantoic membrane, could not be diluted more than about 30-fold and still clearly produce this phenomenon. This indicated that reactivation is obtained only when one host cell adsorbs more than one non-infective virus particle but not upon adsorption of a single particle. These data are in striking agreement with the phenomenon of "multiplicity reactivation" observed in the bacteriophage-E. coli system by Luria and Dulbecco.

Reactivation of non-infective virus in a cortisone-injected host

The administration of cortisone to chick embryos inoculated with large quantities of inactive influenza B virus results in a rate of viral increase greater than is concommittantly observed with inocula of comparable infectivity which are devoid of inactive particles. Thus, more than a mere negation of autointerference is effected. It is concluded that in the presence of cortisone reactivation has occurred of non-infective virus to a state in which it can participate in viral synthesis. Cortisone-induced viral reactivation is dependent upon a high partide/cell ratio and is thus analogous to the previously described phenomenon of "multiplicity reactivation." Cortisone does not influence either homologous or heterologous viral interference unless reactivation of the inactive interfering virus occurs. Virus reactivable with cortisone possesses both interfering and enzymatic properties. Reactivation of virus with cortisone cannot be effected in vitro but is mediated by the host cell. Two hypotheses concerning the action of cortisone are presented.

Viral double-stranded RNA, cytokines, and the flu

The symptoms of the flu, such as fever, drowsiness, and malaise, are the sole means by which this common clinical syndrome is defined. The syndrome is usually the first clinical manifestation of both acute bacterial and viral infections. In the case of acute bacterial infections, several proinflammatory cytokines induced by bacterial products have been implicated as the causative agents of the flu syndrome. Viruses induce similar cytokines to bacteria, plus substantial amounts of interferon-alpha (IFN-alpha), although the direct association of these cytokines with the viral flu syndrome is less clear. Furthermore, the viral inducer(s) of cytokines has not been defined. The best candidate cytokine inducer associated with a majority of viral infections is virus-associated double-stranded RNA (dsRNA). This review examines the essential physical properties of toxic dsRNA, the cytokines induced by it, its viral and cellular sources, evidence for its presence in infected cells, its quantities in normal and infected cells, its cytotoxic mechanisms, and its cell-penetration properties. Toxic effects of viruses and dsRNA are compared. Energetics and extraction artifact issues are also discussed. Whereas most research on dsRNA toxicity has employed synthetic dsRNA, studies with virus-associated dsRNA are featured when available. Finally, a model for how viral dsRNA might initiate systemic disease is presented.

Role of influenza B virus in hepatic steatosis and mitochondrial abnormalities in a mouse model of Reye syndrome

The hepatic steatosis observed in the influenza B virus mouse model of Reye syndrome has been attributed to infectious virus or, alternately, to decreased food intake in the virus-treated mice or impurities in the virus preparation. To resolve this issue, 4- to 6-wk-old male Balb C mice were given, by intravenous injection, 12,800 hemagglutination units of influenza B Lee/40 virus in phosphate buffered saline/1% bovine serum albumin using virus prepared by ultra-centrifugation from infected allantoic fluid, by sucrose density-gradient purification of virus prepared by ultracentrifugation from infected allantoic fluid or by irradiation of virus prepared by ultracentrifugation from infected allantoic fluid to inactivate virus. The infectivity titer of virus prepared by ultracentrifugation from infected allantoic fluid was much higher than that of sucrose density-gradient purified virus prepared from infected allantoic fluid: 50% egg infectious dose for virus prepared by ultracentrifugation from infected allantoic fluid was 3.9 x 10(4)/hemagglutination unit vs. 8.7 50% egg infectious dose/hemagglutination unit for sucrose density-gradient purified virus prepared from infected allantoic fluid. Control mice received phosphate-buffered saline/1% bovine serum albumin or uninfected allantoic fluid diluted in phosphate-buffered saline/1% bovine serum albumin. Mice were fasted to eliminate dietary variation, and livers were obtained 36 hr after virus administration. Of the above treatments, only virus prepared by ultracentrifugation from infected allantoic fluid caused clinical illness and increased hepatic triglycerides (p less than 0.02) compared with controls. Hepatic triglycerides in virus prepared by ultracentrifugation from infected allantoic fluid correlated with histopathological vacuolization scores (r = 0.5773; p less than 0.03).(ABSTRACT TRUNCATED AT 250 WORDS)

The influenza B virus mouse model of Reye's syndrome: clinical, virologic and morphologic studies of the encephalopathy

The influenza B virus mouse model of Reye's syndrome was studied to learn more about the encephalopathy in Reye's syndrome. One to 3 days after intravenous influenza B/Lee virus, Balb/c mice became lethargic, seized and lapsed into a fatal coma. Wide-spread cerebral edema without inflammation developed 1-3 days after virus inoculation. Swollen astrocytic foot processes containing increased glial fibrillary acidic protein were located around capillaries. Viral particles were not seen by electron microscopy and complete viral replication did not occur. Immunohistochemical studies demonstrated influenza B viral antigen within many endothelial cells but not within other brain cells. Qualitative (Evans blue dye) and quantitative (percent brain water and technetium -99 pertechnetate) studies of the blood-brain barrier demonstrated abnormalities. This model reproduced many clinical, virologic and pathologic features of the Reye's syndrome encephalopathy. In addition, a non-permissive viral infection of brain endothelial cells occurred which may be important in the pathogenesis of the mouse encephalopathy and may participate in the encephalopathy of Reye's syndrome.

The effect of virus particle size on chemiluminescence induction by influenza and Sendai viruses in mouse spleen cells

Suspensions of orthomyxo- and paramyxoviruses are composed of pleomorphic particles ranging from large filaments to small spheres. Influenza and Sendai viruses were separated according to size by gel filtration and the induction of luminol-dependent chemiluminescence (CL) by particles of similar size was studied in suspensions of mouse spleen cells known to contain phagocytes. CL reflects the generation by the cells of reactive oxygen species. CL induction decreased with particle size for both viruses. Compared with small spheres, large influenza filaments were approximately 10 times as efficient in activating cellular light emission while the ratio between large and small Sendai viruses was 3:1. Small Sendai virus particles were also less efficient in lysing red cells and had lower neuraminidase activity. By contrast, with influenza virus, only neuraminidase and not the hemolytic activity decreased with the virus size. When influenza virus filaments were broken into smaller particles by sonication, the capacity to induce chemiluminescence dropped markedly while the hemolytic and hemagglutinating activities increased and neuraminidase activity remained unaltered. These results suggest that the presentation of influenza virus hemagglutinin and neuraminidase glycoproteins in a large particle, leading to extensive receptor crosslinking, may be an important factor in the efficient activation of CL by filamentous influenza virus. We suggest that radical generation as reflected in cellular CL may relate to the toxic in vivo effects that contribute to the pathogenesis of influenza and infections with paramyxoviruses.

Virus-induced formation of reactive oxygen intermediates in phagocytic cells

Viruses cause disease by a wide variety of mechanisms. These include the impairment of differentiated host cell functions and the killing of infected cells. The latter is referred to as cytopathic effect and is exemplified by Polio virus infection where paralysis results from the loss of neurons killed by the virus. Host immune response as a factor contributing to disease is evident in the skin rashes in measles and rubella. Virus-immune complexes occur in many infections and may be associated with glomerulonephritis and arthropathy. We describe two mechanisms by which viruses activate the generation of reactive oxygen intermediates (ROI) in polymorphonuclear leukocytes. The first is mediated by antiviral antibody and hence is controlled by the immune system. The second mechanism depends on a direct interaction of viral antigen with the plasma membrane of the phagocyte. It is suggested that the direct activation of ROI generation by paramyxo- and influenza viruses may be related to their well-known toxic effects in vivo.

Influenza B virus model of Reye's syndrome in mice: the effect of aspirin

Aspirin is a possible cofactor with influenza B virus in Reye's syndrome. In an attempt to determine whether single or multiple doses of acetylsalicylic acid (ASA) alter mortality in an influenza B virus mouse model of Reye's syndrome, either ASA (300 to 667 mg/kg) or saline was given intraperitoneally to 3-week-old Balb/c mice. Two hours later, one median lethal dose (LD50) of influenza B/Lee virus was given intravenously. Mortality was not significantly increased by the single ASA dose. Blood ASA levels rose as high as 68 mg/dl. The average ASA blood levels in mice 12 hours after receiving the virus plus 667 mg/kg ASA was 22.6 mg/dl, which was significantly higher than the 3.4 mg/dl for mice who received saline plus ASA. No significant increase in mortality occurred in mice given 67 mg/kg ASA twice daily for 7 days and one LD50 dose of virus on day 4. We conclude that a single dose of ASA at up to half the median lethal dose or multiple doses of ASA do not significantly increase mortality in this influenza virus model of Reye's syndrome. However, influenza B virus infection appears to interfere with ASA metabolism.

Extrapulmonary manifestations of adenovirus type 7 pneumonia simulating Reye syndrome and the possible role of an adenovirus toxin

Three children developed extensive extrapulmonary disease in the course of fatal adenovirus type 7 pneumonia. Several clinical features, including the unexpected onset of coma, suggested the development of Reye syndrome, but biochemical and histopathologic findings were inconsistent with this diagnosis. Virologic and pathologic studies did not reveal evidence of extrapulmonary adenovirus infection, despite clinical involvement of the liver, skeletal muscle, and central nervous system. The detection in premortem sera from all three patients of adenovirus penton antigen, known to be cytotoxic in vitro, suggests a possible mechanism for the production of extrapulmonary pathology in the absence of extrapulmonary virus infection.

A mouse hepatotropic variant of influenza virus

A hepatotropic variant of avian influenza virus A/Turkey/England 63 (Hav 1, Nav 3) was selected by serial passages in mouse liver. Adaptation to this organ was established after 13 in vivo passages and was found to improve during further passages as shown by increasing rates of replication in livers of ICR mice. The mutant virus finally selected was stable and differed from the original virus mainly in lethality upon intraperitoneal injection in mice, in its ability to grow to high titers in livers of susceptible animals and in plaque morphology in chick embryo fibroblasts. No differences were detected in hemagglutination inhibition and neutralization by standard mouse antisera. Pathogenicity for the liver was independent of the route of inoculation, included other laboratory animals sensitive to influenza virus and could be inhibited by amantadine. Fatal hepatitis in 50 per cent of susceptible mice by the intraperitoneal route required from 10 to 20 EID50-. Pathological changes consisted of severe necrosis of liver parenchyma accompanied by release of F antigen into the serum and were apparently due to virus replication in hepatic cells as evidenced by immunofluorescence. The main implications of this animal model for studies on experimental hepatitis and on myxovirus-host interactions in an organ not usually associated with influenza are discussed.

Studies on the pathogenesis of influenzal pneumonitis; intranasal vs. intravenous infection of mice

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Studies on host-virus interactions in the chick embryo-influenza virus system. XIV. The relation between tissue-bound and liberated virus materials under various conditions of infection

Studies have been reported concerning the relationships between virus materials found in the allantoic membranes and media of eggs deembryonated after injection of Standard (ST), heat-inactivated (37 degrees C.) standard (DeltaST), and undiluted passage (UP) seeds. It was found that the membranes always contained relatively more non-infectious hemagglutinins (NIHA) than the media and, correspondingly, the ratios between infectious virus and hemagglutinin units (ID(50)/HA) in the tissues were up to 1.5 log(10) units lower than in the liberated progeny. These differences were seen not only following inoculation of undiluted ST, DeltaST, and UP seeds, the progenies of which always contain considerable proportions of NIHA, but also when dilute ST inocula were employed which lead to the liberation of only infectious virus. Essentially similar differences in the ID(50)/HA ratios were observed also in the allantoic membranes and fluids obtained from growth curve experiments in the intact chick embryo employing the various types of seeds. In correlating the liberated virus materials in the media of deembryonated eggs to those in the membranes it was noted that in any given 2 hour interval during the phase of nearly constant production and release up to 10 times the quantity of infectious virus was shed as was present in the tissues at the onset of that period. In contrast, only about (1/4) of the hemagglutinins were released during the same time. The viral (V) and soluble (S) complement-fixing antigens were found in the tissues but no detectable quantities were released during any 2 hour interval. The NIHA in the membranes apparently is located within the cells since it could not be released by the action of RDE. Intracellular inhibitors of hemagglutination were readily inactivated following inoculation of undiluted ST, DeltaST, or UP seeds but not when ultraviolet-inactivated virus was used. The inhibitor activity decreased in proportion to the hemagglutinins produced. Transfer of infected deembryonated eggs to the cold room after production and liberation of progeny were well under way immediately halted further release but in the tissues the status quo was maintained and release was resumed on return to the 37 degrees C. incubator. The addition of potassium cyanide to the medium of deembryonated eggs at 37 degrees C. during the period of nearly constant production and release of virus material reduced immediately and to comparable extents the ID(50) and HA titers in the tissues and liberation decreased in proportion. On removal of the cyanide 2 hours later, both titers in the tissues gradually returned to those of the untreated control eggs with a corresponding increase in liberation. The ID(50)/HA ratios were not affected by these manipulations. It is concluded that the NIHA in the membranes forms part of a dynamic process. An attempt has been made in the discussion to integrate the present results with previous observations concerning the formation of incomplete forms of virus and their nature and role in the infectious process.

Reproduction of influenza viruses; quantitative investigations with particle enumeration procedures on the dynamics of influenza A and B virus reproduction

Influenza A and B virus reproduction in the allantoic membrane of the intact chicken embryo was studied quantitatively with particle enumeration procedures. Virus particles were enumerated on the basis of two independent properties; capacity to infect and to cause hemagglutination. The infective property of influenza B virus (Lee) was even more unstable than that of influenza A virus (PR8). Inactivation occurred at a constant logarithmic rate which was independent of the concentration of particles and corresponded with first order reaction kinetics. In allantoic fluid at 35 degrees C. either in vitro or in vivo, Lee virus had a half-life for infectivity of only 85 minutes. In contrast, the hemagglutinating property, like that of PR8, was relatively stable and was not appreciably affected by 12 hours at 35 degrees C. On the basis that the number of non-infective particles is equal to the number of hemagglutinating particles minus the number of infective particles and that the number of cells lining the allnatoic membrane is 1.8 x 10(7), the effects of various particle-cell ratios on the reproductive process were analyzed. Adsorption of infective and non-infective Lee particles occurred at the same logarithmic rate, i.e. about 50 per cent in 72 minutes, and the rate was nearly independent of the particle-cell ratio up to a value of 55. The adsorption capacity of an allantoic cell was at least 44 Lee or 89 PR8 particles. The interval before new particles appeared in the allantoic fluid increased as the particle-cell ratio was decreased with both Lee and PR8. At ratios of 0.2 or less, the appearance time for infective particles was nearly identical to that for hemagglutinating particles with both viruses. At ratios of about 1.0, the "latent period" in the allantoic membrane per se was computed to be 150 to 160 minutes for both Lee and PR8. The number of particles, both infective and hemagglutinating, increased at a constant logarithmic rate for 6 hours or more after the adsorptive period. With Lee virus, at a particle-cell ratio of 5 or less, the doubling time was constant and had a value of 43 minutes. The dynamics of the logarithmic increase period suggest that reproduction corresponds to an autocatalytic reaction in which the rate is proportional to the amount of material produced. When the particle-cell ratio was increased to 10 or more, either with infective or non-infective (inactivated at 35 degrees C. or 22 degrees C.) particles, the doubling time increased to 65 minutes. Comparable effects from high ratios were found with PR8. Non-infective particles accumulated at a rapid rate after the interval of constant logarithmic increase regardless of the particle-cell ratio. This accumulation was even more striking with Lee than with PR8 as was expected because of the shorter half-life of the infective property. With both viruses at particle-cell ratios of 4 or more, a large proportion of the particles were non-infective within a few hours after new particles appeared. At particle-cell ratios of 0.2 or less, the maximal yield was relatively constant, i.e., about 900 to 1400 hemagglutinating particles per cell with Lee and 500 to 900 with PR8. However, even with very low ratios, i.e. 0.001 or less, it was not possible to obtain more than about 160 infective particles per cell with either virus regardless of the interval. As was expected, the lower the ratio, the longer was the interval before maximal yields were produced. At ratios of about 10, the maximal yield was reduced by 50 per cent or more with both viruses. Comparable reductions in yield were obtained whether the high particle-cell ratio was due to infective or non-infective (inactivated at 35 degrees C. or 22 degrees C.) particles. These findings indicate that there is a critical particle-cell ratio above which alterations appear in the dynamics of reproduction of influenza viruses. This ratio has a value of approximately 3. The observed alterations in the reproductive process are discussed in relation to the hypothesis that adsorption of 3 or more infective or non-infective particles per cell induces cell damage.

A non-transmissible cytopathogenic effect of influenza virus in tissue culture accompanied by formation of non-infectious hemagglutinins

Various strains of influenza virus produce a cytopathogenic effect in cultures of HeLa cells. The virus could not be passed in series. Virus partially or even completely inactivated with respect to infectivity by exposure to 37 degrees C. or ultraviolet light retained some of its cytopathogenic effect. No evidence has been obtained of an increase in infectious virus in HeLa cultures, but an increase in hemagglutinins and in both viral and soluble complement-fixing antigens became detectable during incubation. These virus materials apparently were not released from these cells prior to their destruction. These results suggested that HeLa cells are capable of supporting an incomplete reproductive cycle of influenza virus. The fact that radioactive phosphorus was readily incorporated into the hemagglutinin supplies strong evidence for this interpretation.