Studies on persistent infections of tissue cultures. II. Nature of the resistance to vesicular stomatitis virus

Fueling influenza and the immune response: Implications for metabolic reprogramming during influenza infection and immunometabolism

Recent studies support the notion that glycolysis and oxidative phosphorylation are rheostats in immune cells whose bioenergetics have functional outputs in terms of their biology. Specific intrinsic and extrinsic molecular factors function as molecular potentiometers to adjust and control glycolytic to respiratory power output. In many cases, these potentiometers are used by influenza viruses and immune cells to support pathogenesis and the host immune response, respectively. Influenza virus infects the respiratory tract, providing a specific environmental niche, while immune cells encounter variable nutrient concentrations as they migrate in response to infection. Immune cell subsets have distinct metabolic programs that adjust to meet energetic and biosynthetic requirements to support effector functions, differentiation, and longevity in their ever-changing microenvironments. This review details how influenza coopts the host cell for metabolic reprogramming and describes the overlap of these regulatory controls in immune cells whose function and fate are dictated by metabolism. These details are contextualized with emerging evidence of the consequences of influenza-induced changes in metabolic homeostasis on disease progression.

Newcastle Disease Virus Establishes Persistent Infection in Tumor Cells In Vitro: Contribution of the Cleavage Site of Fusion Protein and Second Sialic Acid Binding Site of Hemagglutinin-Neuraminidase

Newcastle disease virus (NDV) is an oncolytic virus being developed for the treatment of cancer. Following infection of a human ovarian cancer cell line (OVCAR3) with a recombinant low-pathogenic NDV, persistent infection was established in a subset of tumor cells. Persistently infected (PI) cells exhibited resistance to superinfection with NDV and established an antiviral state, as demonstrated by upregulation of interferon and interferon-induced genes such as myxoma resistance gene 1 (Mx1) and retinoic acid-inducing gene-I (RIG-I). Viruses released from PI cells induced higher cell-to-cell fusion than the parental virus following infection in two tumor cell lines tested, HT1080 and HeLa, and remained attenuated in chickens. Two mutations, one in the fusion (F) protein cleavage site, F117S (F_117S), and another in hemagglutinin-neuraminidase (HN), G169R (HN_169R), located in the second sialic acid binding region, were responsible for the hyperfusogenic phenotype. F_117S improves F protein cleavage efficiency, facilitating cell-to-cell fusion, while HN_169R possesses a multifaceted role in contributing to higher fusion, reduced receptor binding, and lower neuraminidase activity, which together result in increased fusion and reduced viral replication. Thus, establishment of persistent infection in vitro involves viral genetic changes that facilitate efficient viral spread from cell to cell as a potential mechanism to escape host antiviral responses. The results of our study also demonstrate a critical role in the viral life cycle for the second receptor binding region of the HN protein, which is conserved in several paramyxoviruses.IMPORTANCE Oncolytic Newcastle disease virus (NDV) could establish persistent infection in a tumor cell line, resulting in a steady antiviral state reflected by constitutively expressed interferon. Viruses isolated from persistently infected cells are highly fusogenic, and this phenotype has been mapped to two mutations, one each in the fusion (F) and hemagglutinin-neuraminidase (HN) proteins. The F_117S mutation in the F protein cleavage site improved F protein cleavage efficiency while the HN_169R mutation located at the second receptor binding site of the HN protein contributed to a complex phenotype consisting of a modest increase in fusion and cell killing, lower neuraminidase activity, and reduced viral growth. This study highlights the intricate nature of these two mutations in the glycoproteins of NDV in the establishment of persistent infection. The data also shed light on the critical balance between the F and HN proteins required for efficient NDV infection and their role in avian pathogenicity.

Studies on persistent infections of tissue cultures. I. General aspects of the system

Inoculation of the MCN and Lung-To lines of human cells in continuous culture with Newcastle disease (NDV), mumps, or 6-6 viruses led to slight cytopathic effects (CPE) if the multiplicity of infection exceeded one. On second passage or with smaller initial inocula no CPE became apparent. The viruses multiplied, however, as determined by titrations in HeLa cultures or chick embryos. Indeed, persistently infected sublines of MCN and Lung-To were readily established without resort to special manipulations and some of these have been carried now for over 18 months on the same media and schedules as the uninfected parent strains. The viruses were found to be associated mainly with the cells and only 1, or at most 10 per cent of it was detectable in the media. The titers obtained were always low in relation to the available cell population. Reduction or even omission of the horse serum component in the media or ultraviolet irradiation of the cultures did not increase the yield of virus, and CPE became apparent only when similarly treated, uninfected cultures were, likewise, affected by the manipulations. The persistently infected cultures differed from their uninfected counterparts in that they exhibited (a) decreased cellular growth rates and ultimate yields; (b) increased aerobic glycolysis; and (c) a high degree of resistance to cytopathogenic viruses, influenza A (PR8), herpes simplex and, especially vesicular stomatitis (VSV) viruses. Prolonged treatment of persistently infected cultures by addition of specific antiviral immune sera to the media reduced significantly the amount of virus present and the degree of resistance to VSV. However, upon removal of the sera after as many as 187 days of treatment the viruses reappeared in all but one instance. The cured culture, on reinfection, became again persistently infected. No evidence was obtained to indicate genetic inhomogeneity of the cell populations. Of 50 cloned MCN lines none was destroyed by NDV and all became persistently infected. None were initially resistant to VSV but all after establishment of persistent NBV infection. All 39 cloned lines derived from MCN_NDV cultures in the presence of anti-NDV serum, were free of virus and susceptible to VSV, and all acquired persistent infections and with it resistance to VSV following inoculation of NDV. NDV maintained in MCN cultures differed from the parent, chick embryo-adapted strain with respect to its plaque morphology. Whereas the former yielded only plaques on monolayers of chick embryo fibroblasts which were of pin-point size and hazy, those obtained with the latter were rarely of this type and mostly large and clear. This apparent selection of virus particles did not alter significantly their behavior with respect to cytopathogenicity for uninfected MCN cultures.

Studies on persistent infections of tissue cultures. III. Some quantitative aspects of host cell-virus interactions

Efforts were made to obtain information on some of the quantitative aspects of host cell-virus interactions in MCN cultures persistently infected with Newcastle disease, mumps, and 6-6 viruses, and to elicit the mechanism which permits simultaneous maintenance of virus and cells for indefinite periods of time. It was shown by 4 different technics that only between 10 and less than 1 per cent of the cells yield infectious virus, depending upon the agent employed and, possibly, variations in the conditions of the cultures. No evidence was found to indicate production of non-infectious virus materials. Only the cells carrying infectious virus are capable upon transfer to uninfected cultures to transmit the infection. Cells from persistently infected cultures, which are free of infectious virus at the time of transfer, failed to liberate virus at a later time during incubation periods of up to 4 weeks. The virus-producing cells contain at any given moment not more than 1 infectious unit of virus, suggesting a linear mode of production; i.e., as soon as a virus particle is completed, it is released. Upon inoculation of MCN test tube cultures with chick embryo-adapted NDV persistent infection and interference with vesicular stomatitis virus (VSV) is established with considerable delay. In contrast, following transfer of MCN-adapted NDV, in form of MCN(NDV) cells or first allantoic passage seeds derived therefrom, the number of virus-producing cells increases logarithmically, doubling every 6 to 8 hours until a total of about 10(4) is reached. Thereafter their numbers rise in proportion to the increase in total cell population; i.e., doubling approximately every 48 hours. At the time when 10(4) virus-producing cells are present in the culture interference with VSV is solidly established. In order to obtain this result about 10(6) cells must have adsorbed virus particles, or, in other words, at least 10(6) virus units must have totally been produced instead of the 10(4) measured by infectivity assay. The implications of these and previously reported data have been discussed in detail and a scheme of the course of events in persistently infected cultures has been presented.

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

In previous reports of this series, it was shown that persistent infection of MCN cultures with certain myxoviruses rendered the cells insusceptible to superinfection by several cytopathogenic viruses. It was thought that production of an interferon might be the cause of this resistance and efforts to confirm this suggestion have been presented. Addition of ultraviolet-inactivated myxoviruses (mumps, Newcastle disease, influenza A, and Sendai) to MCN cultures for periods of 2 to 3 hours, followed by washing and refeeding of the cells, led to the subsequent release into the media of a substance which induced in fresh MCN cells a transitory resistance to infection by vesicular stomatitis virus, and prevented incomplete reproductive cycles of influenza A and Sendai viruses. Media containing this substance were free of detectable hemagglutinating activity and viral complement-fixing antigens. The substance was not neutralized by specific antiviral sera; it was not sedimentable by high speed centrifugation; it was not adsorbed onto red cells; but it was inactivated by trypsin. Thus, its properties matched those of the interferon described by Isaacs and his associates. A comparison of the extent of resistance induced in MCN cells by decreasing doses of ultraviolet-inactivated myxoviruses (interference test) and the protection afforded by the media removed from the cultures prior to challenge and transferred to fresh MCN tubes (interferon test) revealed that wherever interference became detectable in the cells, the media of the corresponding cultures contained some interferon. Interferon was obtained by inactivated myxoviruses also from primary cell cultures by the same techniques, but not from HeLa cells. Interferons derived from one type of culture may protect others equally well or show a certain degree of host specificity in that resistance in homologous cells may be somewhat more pronounced than in heterologous cultures. No resistance could be induced in HeLa cells by the interferon preparations employed. Interferon was detected also in MCN cultures, persistently infected with mumps virus. Its concentration was apparently too small in carrier cultures maintained as routine to be measurable. However, when the cells were grown in heavy sheets in roller bottles, and especially when the volume of medium was reduced for several days prior to harvest, interferon became readily detectable. These results strengthen the suggestion that interferon may play a decisive role in the establishment and maintenance of persistent infections in the system under study. Its nature, source, mode of action, and exact role in persistent infection remains to be elucidated.

CYTOPATHOGENIC MYCOPLASMAS ASSOCIATED WITH TWO HUMAN TUMORS. I. ISOLATION AND BIOLOGICAL ASPECTS

Armstrong, D. (The Children's Hospital of Philadelphia, Philadelphia, Pa.), G. Henle, N. L. Somerson, and L. Hayflick. Cytopathogenic mycoplasmas associated with two human tumors. I. Isolation and biological aspects. J. Bacteriol. 90:418-424. 1965.-Mycoplasmas were isolated from cell cultures of two benign human tumors. The first isolate contained two mycoplasmas, one a well-known human species (Mycoplasma hominis type I) and frequent tissue culture contaminant, and the other a recently reported new type. The second isolate was a mycoplasma of the newly described type. The mycoplasmas could be reisolated, after one or more passages through tissue culture, from extracts of the original tumor tissue. The relationship of the organisms to the neoplasms remains obscure. Both isolates produce cytopathic effect (CPE) and acidification of medium in a variety of tissue cultures. The CPE may be diminished, but not abolished, by increasing concentrations of arginine in the tissue culture media. Infection of various tissue cultures with the mycoplasmas did not result in interference to super-infection with vesicular stomatitis virus.

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.

Persistent cyclic herpes simplex virus infection in vitro. 3. Asynchrony in the progression of infection and cell regrowth

Hampar, Berge (National Institute of Dental Research, Bethesda, Md.). Persistent cyclic herpes simplex virus infection in vitro. III. Asynchrony in the progression of infection and cell regrowth. J. Bacteriol. 91:1965-1970. 1966.-The progression of virus-induced cytopathic effects (CPE) and virus synthesis was studied in localized areas of Chinese hamster cell cultures persistently infected with herpes simplex virus (HSV). CPE was initially evidenced by the presence of small multinucleated giant cells, followed by expanding plaquelike lesions with an occasional uninfected cell remaining within the infected areas. Cell detachment rapidly followed the appearance of viral antigen in infected cells. The surviving cells which proliferated to re-establish the cell sheet arose from two sources. The first was from viable cells which remained attached after expansion of localized areas of CPE, and the second was from reattachment of viable cells in the medium. CPE in localized areas was initiated at various times during the cycle irrespective of the virus titer in the medium. Cell regrowth in some areas and CPE in other areas occurred simultaneously throughout the cycle in an asynchronous fashion. Consequently, during periods of rising virus titers, most areas showed CPE while few areas displayed cell regrowth. As the virus titers declined, more areas showed cell regrowth and fewer areas displayed new cycles of CPE. CPE in localized areas was not initiated until cell regrowth had occurred. It is proposed that the proliferating cells were temporarily resistant to HSV infection, and that this resistance was ultimately lost in their progeny cells.