Studies on host-virus interactions in the chick embryo-influenza virus system. IV. The role of inhibitors of hemagglutination in the evaluation of viral multiplication

Studies on host-virus interactions in the chick embryo-influenza virus system. V. Simultaneous serial passage of the agents of influenza A and B in relation to variations in the growth cycle of influenza B virus

The combined passage of influenza A and B viruses in series, as reported by Sugg and Magill, has been confirmed. When the mixed passage materials were not too highly diluted both agents could be traced through 10 transfers. Growth curve experiments revealed that both agents developed independently, as measured by hemagglutination-inhibition tests in the presence of specific immune sera against one or the other type. However, the hemagglutinin titers of the 2 viruses in the mixed series were always substantially lower than those recorded when the strains were used individually as seed in the same concentrations as were employed in the mixed series. Assay of the infectivity titers of the individual strains in the presence of appropriate immune sera led to the demonstration that the time required for the growth cycle of influenza B virus varied with the dose of seed virus. With undiluted infected allantoic fluid as seed only 4 to 5 hours elapsed before new generations of virus were liberated. With increasing 10-fold dilution of the seed the constant period became increasingly longer until it stabilized at 8 to 10 hours. This finding offers an explanation for the seeming discrepancy between the observations on interference between the 2 viruses and the difference reported previously in the extent of their growth periods, on the one hand, and the fact that the 2 agents could be carried simultaneously in series through numerous passages in the chick embryo.

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.

Immunocytological effects of varied inocula of influenza virus

Variation in the amount and quality of influenza virus injected into the amniotic sac of the chick embryo led to differences not only in the yield of virus but also in the immunofluorescent cytology of the infection. The production of infectious virus was associated with a predominance of cells in which the virus antigens were first detected along the cell surface in contact with the amniotic fluid, later deeper in the cytoplasm, and finally, though to a lesser extent, in the nucleus. When the virus yield was primarily non-infectious hemagglutinin the virus antigens appeared in reverse sequence; i.e., nucleus first, then adjacent cytoplasm, and ultimately throughout the entire cell. Under conditions of "autointerference," the immunofluorescence seen in some of the cells failed to progress beyond the nuclear or early cytoplasmic stage, while many other cells remained unreactive throughout the experimental period. With the largest dose used a pale intranuclear reaction was localized 1 hour after injection but the embryo died shortly thereafter.

THE SUSCEPTIBILITY OF CHICK EMBRYO SKIN ORGAN CULTURES TO INFLUENZA VIRUS FOLLOWING EXCESS VITAMIN A

The conversion of chick embryonic epidermis to mucous epithelium by excess vitamin A in organ culture as reported by Fell and Mellanby (5) was shown to be accompanied by a corresponding change of susceptibility to influenza and vaccinia viruses. Untreated epidermis of 10- to 12-day chick embryos supported the growth of influenza (PR8) virus in organ cultures and a maximum infectivity (EID(50)) titer was reached 2 to 3 days after infection. At the same time) the epidermis showed squamous keratinization, beginning about the 4th day of cultivation. Addition of excess vitamin A (40 microg per ml) to the skin organ culture induced the following changes: (a) mucous metaplasia of the epidermis which was usually first evident after 4 to 5 days in the vitamin A medium, (b) increase in the daily and maximum yield of influenza virus, if the epidermis had been grown for 4 or more days in the vitamin A medium before infection took place, and (c) decrease in the production of vaccinia virus under similar conditions. The maximum yield of both viruses remained unchanged, however, if excess vitamin A was introduced to the organ culture at the time of virus inoculation. The magnitude of increase in the yield of influenza virus in this organ culture system was found to be proportionally related to the concentration of vitamin A added 4 or more days before inoculation of this virus. Increasing doses of vitamin A however, had no effect on the short-term growth of influenza virus in tissue cultures of chorio-allantoic membrane. Observation on the early period (2 to 12 hours) of influenza virus growth initiated in the 4-day organ cultures of chick embryonic skin showed no significant difference in virus production between the normal and the vitamin A medium groups. The change of virus specificity apparently is not due to the presence of excess vitamin A per se, but appears to be related to the change of differentiation produced in the organ culture system.

Studies on host-virus interactions in the chick embryo-influenza virus system. XIII. Some aspects of non-infectious virus production

Certain aspects of the formation of non-infectious hemagglutinins (NIHA) in the chick embryo infected with influenza virus have been analyzed. It was shown by the use of combined in ovo-deembryonation technics that little or no NIHA is released following infection with small doses of standard virus during the most active and constant growth periods of the virus extending to about the 36th hour of incubation in spite of the fact that multiple infection of cells must have taken place in the latter half of that period. A slight decrease in the ID(50)/HA ratios of the yields obtained after the 36th hour, coinciding with the falling off of virus production and release may possibly be explained in terms of inactivation of completed virus or leakage of as yet incompleted virus from damaged cells. Exposure of the entodermal cells of the allantois of eggs deembryonated shortiy after injection of saturation or near saturation inocula of standard seed to large quantities of infectious virus added to the media at various times after infection and not extending over more than 2 hours resulted in a decrease of the ID(50)/HA ratios of the progenies only during the first 2 or possibly 4 hours after the primary inoculation. Later addition did not influence the yields. As discussed, such sudden and heavy exposures of cells are not expected to occur during the infectious process induced by small inocula of standard seed. The possible role of destruction of cell receptors in NIHA production has been analyzed in several ways. The addition of receptor-destroying enzyme (RDE) to the media of deembryonated eggs after near saturation inocula of standard seeds, if anything, increased the ID(50)/HA ratios of the progenies, and that only when added during the first few hours following infection, presumably by reducing the changes for high multiplicity of infection of cells. In contrast, ultraviolet-inactivated virus, which retains its enzymatic activity, lowered, if anything, the ID(50)/HA ratios of the progenies, when present in the media of deembryonated eggs from the 2nd to 4th or possibly 6th hour after infection. Excessive amounts of irradiated virus may still cause some degree of interference under these conditions. Later addition of irradiated viruses were without effect with respect to NIHA production or interference. In attempts to alter the cell receptors prior to infection by potassium periodate (KIO(4)), it was noted that the addition of glycerol led to the appearance and partial retention for at least 24 hours of substances in the allantoic fluids which were capable of inactivating considerable proportions of standard virus. These data indicate that destruction of external cell receptors plays little if any role in NIHA production. The implications of these findings are discussed.

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.

Influenza virus and its mucoprotein substrate in the chorioallantoic membrane of the chick embryo. II. Stepwise inactivation of substrate and its relation to the mode of viral multiplication

The fate of heat-stable inhibitors (HI) of hemagglutination by influenza viruses in the infected chorioallantoic membrane has been studied. The amount of soluble HI extractable from the normal CAM is taken as a measure of the total mucoprotein substrate derived from the allantoic cells. It has been shown that a decrease in HI is demonstrable when all or nearly all cells of the allantois are involved in the infectious process. Once this condition is fulfilled, viral multiplication in the CAM is associated with stepwise breakdown and partial restoration of HI. The periodicity of these steps is in close agreement with earlier reports by others on the duration of primary and secondary cycles of viral multiplication. Periodicity of viral multiplication and of HI breakdown with subsequent restoration is particularly pronounced in eggs pretreated with metaperiodate or with mixtures of periodate and glucose or glycerol. The stepwise nature of the HI breakdown indicates that it is in some way related to intracellular phases of viral reproduction, and that individual cells produce virus in cycles rather than continuously. A possible mechanism of this process is proposed.

Influenza virus and its mucoprotein substrate in the chorioallantoic membrane of the chick embryo. I. Characterization and quantitative assay of soluble substrate and studies on its relation to allantoic cells

As a preliminary to a study of the fate of mucoprotein substrate in tissues infected with influenza virus, some characteristics of soluble hemagglutination inhibitors (HI) extracted from chorioallantoic membranes (CAM) have been investigated. The inhibitory material was found to be heat-stable, precipitable with cold ethanol, and subject to progressive inactivation by active viruses or by receptor-destroying enzyme (RDE). Certain changes in the slope of titration curves obtained with precipitated and non-precipitated fractions upon alcohol fractionation suggested that the HI in heated CAM extracts was heterogeneous. Alcohol in low concentration precipitated preferentially the more efficient (longer?) HI leaving the "weak" component in the non-precipitated fraction. With higher concentrations of alcobol, "strong" HI were converted to "weak" ones, either by denaturation or as result of reduced solubility. These changes in slope of titration curves were reflected in significant discrepancies when densitometric titers (HI(50)) of fractions were compared with their titers in pattern tests. The action of active viruses on HI, on the other hand, did not induce qualitative changes in the composition of the inhibitory principle: Titration curves retained parallel slopes even when their position was markedly displaced from that of control curves. In its essential properties, the HI from the CAM appeared to be similar to inhibitors isolated from various other biological sources which have been identified as mucoprotein in nature. Standards for reproducibility of inhibitory titers obtained by the densitometric method of Hirst and Pickels have been presented. The relation of HI to the allantoic epithelium has been analyzed. It has been concluded that the HI is a normal constituent and secretion product of these cells. Under physiological conditions, i.e. in the intact egg, the HI contained in the mucoid outer layer of allantoic cells appears to be protected from enzymatic action from without, although adsorption of viral particles may be temporarily impeded. In deembryonated eggs, or in excised membranes, a reduction in total inhibitory substrate as a result of the action of RDE has been observed. It has been shown in experiments on adsorption of active or heat-inactivated virus on the allantoic membrane that prevention of adsorption by RDE may require the synergistic action of the active viral enzyme itself. It has been concluded that the maintenance or restoration of a normal supply of mucoprotein substrate is a function of the ability of allantoic cells to maintain homeostatic conditions under stress.

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.

Studies on host-virus interactions in the chick embryo-influenza virus system

The usefulness of the deembryonation technic has been analyzed as a tool in the study of various problems in the growth cycle of influenza virus in the entodermal cells of the allantoic of chick embryos. Various improvements in the deembryonation technic have been described. The method readily permits repeated sampling of the medium at various stages after infection (cumulative growth curves) or frequent exchanges of the medium (differential growth curve). However, the yield of infectious virus or of hemagglutinins is less than that observed in the intact chick embryo. The difference observed is greater than can be accounted for by the reduction in the available host cells and is assumed, therefore, to be due in part to interruption of blood and nutrient supply to the cells. This handicap can be overcome by the combined in ovo-deembryonation technic, in which deembryonation is performed at any desired time after infection of the intact chick embryo, and the medium is collected and analyzed after 1 to 3 hours of further incubation. The value of the technic is demonstrated by the fact that liberation of virus from infected cells can be detected earlier than in the intact egg. Furthermore, it continues at a nearly constant rate for many hours, thus proving to be erroneous previous inference which had been based upon in ovo experiments. The technic also permits readily the addition and subsequent removal of substances that might interfere with viral propagation. As an example a study was made of the effect of the receptor-destroying enzyme of V. cholerae (RDE) when added to the medium of eggs infected prior to deembryonation. By carefully grading the dose of virus and using an appropriate amount of RDE, one-step growth curves were obtained indicating that those cells not directly invaded by the seed virus were subsequently protected against infection by action of the enzyme. The smaller the amount of virus the less RDE was required in order to note a protective effect. With a decrease in the period of exposure to RDE regeneration of cell receptors became increasingly more apparent in that correspondingly greater amounts of virus were produced and liberated late in the incubation periods. These results confirmed and extended those reported by Stone. More extensive applications of these technics will be reported in subsequent papers of this series.

The influenza virus: its morphology, immunology, and kinetics of multiplication

The morphology of influenza virus is described, and the properties of its two components, the elementary body and the soluble substance, differentiated. The relationship between the filamentous and spherical structures observed in the infected allantoic fluid is still obscure, but the filaments are thought to represent either a form of influenza virus or a stage in its multiplication.The antigenic constitution of A and B viruses is discussed and variations in the pattern of individual strains over the past two decades described. The author outlines his speculations on the origin and mechanism of these variations. Other differences in serological behaviour, such as non-specific inhibition and the avidity effect, are discussed.The results of experimental investigation, with various animal hosts, of the kinetics of influenza virus multiplication are reviewed, and the effects of such factors as incubation temperature and concentration of the seed virus considered. The bearing of these studies of enzymatic virus/host-cell interactions on the chemotherapeutic control of influenza is emphasized.