STUDIES WITH HUMAN INFLUENZA VIRUS CULTIVATED IN ARTIFICIAL MEDIUM

Influenza vaccines: Past, present, and future

Globally, infection by seasonal influenza viruses causes 3-5 million cases of severe illness and 290,000-650,000 respiratory deaths each year. Various influenza vaccines, including inactivated split- and subunit-type, recombinant and live attenuated vaccines, have been developed since the 1930s when it was discovered that influenza viruses could be cultivated in embryonated eggs. However, the protection rate offered by these vaccines is rather low, especially in very young children and the elderly. In this review, we describe the history of influenza vaccine development, the immune responses induced by the vaccines and the adjuvants applied. Further, we suggest future directions for improving the effectiveness of influenza vaccines in all age groups. This includes the development of an influenza vaccine that induces a balanced T helper cell type 1 and type 2 immune responses based on the understanding of the immune system, and the development of a broad-spectrum influenza vaccine that can increase effectiveness despite antigen shifts and drifts, which are characteristics of the influenza virus. A brighter future can be envisaged if the development of an adjuvant that is safe and effective is realized.

The Role of Tissue Oxygen Tension in Dengue Virus Replication

Low oxygen tension exerts a profound effect on the replication of several DNA and RNA viruses. In vitro propagation of Dengue virus (DENV) has been conventionally studied under atmospheric oxygen levels despite that in vivo, the tissue microenvironment is hypoxic. Here, we compared the efficiency of DENV replication in liver cells, monocytes, and epithelial cells under hypoxic and normoxic conditions, investigated the ability of DENV to induce a hypoxia response and metabolic reprogramming and determined the underlying molecular mechanism. In DENV-infected cells, hypoxia had no effect on virus entry and RNA translation, but enhanced RNA replication. Overexpression and silencing approaches as well as chemical inhibition and energy substrate exchanging experiments showed that hypoxia-mediated enhancement of DENV replication depends on the activation of the key metabolic regulators hypoxia-inducible factors 1α/2α (HIF-1α/2α) and the serine/threonine kinase AKT. Enhanced RNA replication correlates directly with an increase in anaerobic glycolysis producing elevated ATP levels. Additionally, DENV activates HIF and anaerobic glycolysis markers. Finally, reactive oxygen species were shown to contribute, at least in part through HIF, both to the hypoxia-mediated increase of DENV replication and to virus-induced hypoxic reprogramming. These suggest that DENV manipulates hypoxia response and oxygen-dependent metabolic reprogramming for efficient viral replication.

Hypoxia enhances antibody-dependent dengue virus infection

Dengue virus (DENV) has been found to replicate in lymphoid organs such as the lymph nodes, spleen, and liver in post‐mortem analysis. These organs are known to have low oxygen levels (~0.5–4.5% O₂) due to the vascular anatomy. However, how physiologically low levels of oxygen affect DENV infection via hypoxia‐induced changes in the immune response remains unknown. Here, we show that monocytes adapted to 3% O₂ show greater susceptibility to antibody‐dependent enhancement of DENV infection. Low oxygen level induces HIF1α‐dependent upregulation of fragment crystallizable gamma receptor IIA (FcγRIIA) as well as HIF1α‐independent alterations in membrane ether lipid concentrations. The increased FcγRIIA expression operates synergistically with altered membrane composition, possibly through increase membrane fluidity, to increase uptake of DENV immune complexes for enhanced infection. Our findings thus indicate that the increased viral burden associated with secondary DENV infection is antibody‐dependent but hypoxia‐induced and suggest a role for targeting hypoxia‐induced factors for anti‐dengue therapy.

Hypoxia and HIF-1 activation in bacterial infections

For most of the living beings, oxygen is one of the essential elements required to sustain life. Deprivation of oxygen causes tissue hypoxia and this severely affects host cell and organ functions. Tissue hypoxia is a prominent microenvironmental condition occurring in infections and there is a body of evidence that hypoxia and inflammation are interconnected with each other. The primary key factor mediating the mammalian hypoxic response is hypoxia inducible factor (HIF)-1, which regulates oxygen homeostasis on cellular, tissue and organism level. Recent studies show that HIF-1 plays a central role in angiogenesis, cancer and cardiovascular disease but also in bacterial infections. Activation of HIF-1 depends on the nature of the pathogen and the characteristics of infections in certain hosts. Up to date, it is not completely clear whether the phenomenon of HIF-1 activation in infections has a protective or detrimental effect on the host. In this review, we give an overview of whether and how hypoxia and HIF-1 affect the course of infections.

Virus-host interactions under hypoxia

Oxygen tension can exert a significant effect on viral propagation in vitro and possibly in vivo. In general, hypoxia restricts the replication of viruses that naturally infect tissues exposed to ambient oxygen and induces the growth of viruses that naturally target tissues exposed to low oxygen. Some viruses can reprogram cell bioenergetics towards lowering cellular respiration and therefore oxygen consumption in order to support their replication. Aim of this review is to summarize findings on the interplay between viral infection and oxygen levels, highlighting the implicated oxygen tension-sensitive elements and metabolic determinants and concluding with possible therapeutic approaches targeting these mediators.

Oxygen tension level and human viral infections

The role of oxygen tension level is a well-known phenomenon that has been studied in oncology and radiotherapy since about 60 years. Oxygen tension may inhibit or stimulate propagation of viruses in vitro as well as in vivo. In turn modulating oxygen metabolism may constitute a novel approach to treat viral infections as an adjuvant therapy. The major transcription factor which regulates oxygen tension level is hypoxia-inducible factor-1 alpha (HIF-1α). Down-regulating the expression of HIF-1α is a possible method in the treatment of chronic viral infection such as human immunodeficiency virus infection, chronic hepatitis B and C viral infections and Kaposi sarcoma in addition to classic chemotherapy. The aim of this review is to supply an updating concerning the influence of oxygen tension level in human viral infections and to evoke possible new therapeutic strategies regarding this environmental condition.

THE PROLONGED COEXISTENCE OF VACCINIA VIRUS IN HIGH TITRE AND LIVING CELLS IN ROLLER TUBE CULTURES OF CHICK EMBRYONIC TISSUES

1. The virus of vaccinia in so called roller tube cultures of mixed embryonic chick tissue rapidly increases to maximal titre. 2. Under these conditions the quantity of virus in the tissue remains at or near the maximum for at least 9 weeks and considerable amounts are present in the fluids removed each day. 3. The same results are obtained when only fragments of embryonic chick heart are employed. 4. Many, though not necessarily all, of the cells in infected cultures remain alive and retain the capacity to proliferate. The presence of these living cells is essential for the persistence of the virus. 5. No apparent differences in the rate or amount of growth of cells in infected as contrasted with non-infected cultures can be discerned in the gross. 6. It is suggested, but not proved, that virus is continually being produced rather than simply preserved throughout the period.

THE ACTION OF IMMUNE SERUM ON HUMAN INFLUENZA VIRUS IN VITRO

Studies have been conducted on the effect of immune serum upon a strain of human influenza virus (PR8) grown in chick embryo tissue culture medium. The results have demonstrated (a) that when cells are exposed to the action of immune serum of high titer and subsequently washed freely, these cells support the growth of virus as well as cells treated with normal serum; (b) that, in agreement with the results of other workers, when virus is added to cell suspensions before the addition of immune serum of low titer, virus survives in the cells; (c) that when mixtures of immune serum of low titer and virus are added to cells, there is little evidence of survival or multiplication of the virus. Furthermore, when immune serum of high titer is used the virus is inactivated regardless of whether the cells are first exposed to virus or immune serum. Finally, virus mixed with a strong immune serum is inactivated in the absence of cells, as shown by the fact that centrifugation at high speeds of such serum-virus mixtures yields no active virus, whereas normal serum-virus mixtures yield fully active virus.

THE DEMONSTRATION OF LESIONS AND VIRUS IN THE LUNGS OF MICE RECEIVING LARGE INTRA-PERITONEAL INOCULATIONS OF EPIDEMIC INFLUENZA VIRUS

Following the intraperitoneal inoculation of mice with large doses of epidemic influenza virus (50,000 to 1 million intranasal M.L.D.) it can be recovered from the lungs in high concentration, and pulmonary lesions of moderate extent may be observed. The virus reaches its highest titer in the lungs 48 to 72 hours after intraperitoneal injection and may persist for 10 days. Virus may be recovered from the blood in the first 24 hours, but is readily detected in the omentum and peritoneum for 5 to 6 days. Mice which as a result of the intraperitoneal injection of virus show a high concentration of virus in the lungs do not die but become solidly immune to intranasal infection. Moreover, as early as 24 to 48 hours after intraperitoneal inoculation of large amounts of virus the animals may exhibit resistance to infection with fatal doses of virus given intranasally. Influenza virus given intravenously to mice is rapidly removed from the blood but persists in the lungs and induces pulmonary lesions. Virus can also be recovered from the liver for several days. With subcutaneous inoculation of influenza virus, however, the virus does not reach the blood or lungs in detectable amounts although the regional lymph nodes may yield considerable quantities of the agent. A brief consideration is presented of the mechanisms of infection and resistance which may be involved.

THE ANTIBODY RESPONSE OF HUMAN SUBJECTS VACCINATED WITH THE VIRUS OF HUMAN INFLUENZA

Human influenza virus cultivated in tissue culture medium may be administered subcutaneously or intradermally to human individuals without causing evidence of infection. Subjects so treated develop a good titer of circulating antibodies effective against mouse passage virus and, if antibodies were previously present, vaccination stimulates the production of more antibody. The antibodies so induced persist for at least 5 months, although in this period of time some decline in titer may have begun. The antibody response to vaccination parallels both in extent and persistence that occurring as a result of the naturally acquired disease. The available data do not enable one to evaluate the effect of vaccination in preventing human infection with influenza. It seems not unlikely that the increase in circulating antibody will be accompanied by an increased ability to combat the natural infection.

The reversibility of the O-D type of influenza virus variation

Data are presented which enhance the idea that the influenza virus is an unstable agent. They indicate that the O-D type of variation is not a discontinuous mutation but rather is a reversible phenomenon. The O and the D forms of virus both appear to be inherent in the virus particle; the dominance of one or the other form seems to be subject to chance occurrences, but is influenced by the conditions under which the virus is propagated. The capacity of the O form of virus to agglutinate guinea pig but not chicken crythrocytes is a relative, not an absolute phenomenon; allantoic fluids which exhibit clear-cut O form hemagglutination may be made to exhibit D form merely by addition of suitable buffer to the test system. That point is of importance from the viewpoint of the mechanism of influenza virus hemagglutination.

Persistent antigenic variation of influenza A viruses after incomplete neutralization in ovo with heterologous immune serum

Antigenic variants of influenza A virus strains emerge on serial passage in ovo in the presence of immune serum against different but related strains. An old laboratory strain (PR8) which had been through hundreds of animal passages was as readily modified by this procedure as recently recovered strains. Such variants apparently can be obtained at will and show antigenic patterns which are reproducible and appear to be predictable in terms of the immune serum used for their selection. Variant strains retain their new antigenic patterns on serial passage in ovo in the absence of immune serum. Limited serial passage in ovo of strains in the absence of immune serum did not result in the emergence of antigenic variants. Similarly, serial passages of strains in ovo in the presence of immune serum against widely different strains, which failed to show significant cross-neutralization, did not lead to the appearance of antigenic variants.

Production of interferon in mice: effect of altered gaseous environments

Effects of altered gaseous environments (parabarosis) on interferon production in mice were studied, with Newcastle disease virus (NDV) as the inducer. Increased levels of interferon in lung tissue were observed when mice were exposed to 11% O(2) in N(2) for 3 days before and after, or only after, injection of NDV. However, serum interferon levels remained unchanged. Exposure of mice to 77% O(2) for up to 7 days did not affect the response to interferon induction as assayed in lungs or sera. Interferon levels were significantly depressed in mice exposed to a simulated depth of 213 ft in seawater [with normal partial pressure of O(2) (pO(2)) in N(2)] for 2 or 4 weeks. Whereas definite depression of interferon was also observed in mice maintained at a simulated altitude of 37,000 ft (with normal pO(2)) for 2 weeks, those maintained at the same condition for 4 weeks showed a normal level of interferon. The results obtained with hypoxia are compatible with other reports on the influence of O(2) tension on viral infection. The factors responsible for alterations observed in interferon level in mice kept in normal pO(2), but under altered pressure, have not yet been identified.

Virus proliferation in hypoxic mice and chick embryos

Virus proliferation in hypoxic animals was markedly reduced. Little difference in this respect between influenza virus (PR8) and mouse encephalomyelitis virus (Theiler's GD VII) was observed except that there was a longer depression of the latter virus. The possible mechanisms involved are discussed.

STUDIES ON THE PROPAGATION OF INFLUENZA AND MUMPS VIRUSES IN TISSUE CULTURE WITH CHEMICALLY-DEFINED MEDIA

Optimal conditions have been established for the propagation of influenza virus (PR8 strain) in various chick embryonic tissues cultivated in synthetic medium 199. The propagation of several other influenza strains, both standard laboratory and freshly-isolated, and of mumps virus, has also been studied. Comparative investigations with virus-infected tissues cultivated in medium 199 and in simple inorganic salt solutions have shown that the extent of virus propagation is more dependent on the intracellular material present in the tissues than on the composition of the extracellular culture medium. It has also been shown that virus propagation occurs equally well in healthy, surviving cells, in actively-growing cells, and in cells depleted of nutrients and undergoing degeneration.

Inhibition of influenza virus multiplication by N-glycosides of benzimidazoles-N

Chloro derivatives of benzimidazole were found to be 2 to 3 times more active than corresponding methyl derivatives in causing inhibition of Lee virus multiplication in chorioallantoic membrane cultures in vitro. The most active benzimidazole derivative thus far tested is 5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole (DRB); it caused 75 per cent inhibition of Lee virus multiplication in membrane cultures at a concentration of 0.38 x 10(-4)M. On the other hand, 5,6-dimethyl-1-alpha;-D-ribofuranosylbenzimidazole, the moiety present in vitamin B(12), failed to inhibit Lee virus multiplication at a concentration of 35 x 10(-4)M. Other N-glycosides of 5,6-dichlorobenzimidazole were considerably less active than DRB. In single cycle experiments, the degree of inhibition of Lee virus multiplication by DRB in membrane cultures was not dependent on the amount of virus in the inoculum. This compound did not inactivate the infectivity of extracellular Lee virus, had no effect on virus-erythrocyte interaction, did not interfere with the adsorption of the virus by the host tissue, nor affect the release of newly formed virus from the membrane. The inhibitory effect of DRB on Lee virus multiplication, in contrast to that of 2,5-dimethylbenzimidazole, persisted after transfer of infected membranes into fresh culture medium not containing the compound. Both DRB and the 2,5-dimethyl compound caused 99 per cent inhibition of Lee virus multiplication without affecting oxygen uptake of the membrane. Tissue proliferation of membrane pieces in roller tube culture was not significantly affected by DRB at inhibitory concentration, whereas at equivalent concentration the 2,5-dimethyl compound did restrict cellular growth. At higher concentrations, both compounds caused retardation of cell proliferation. This effect was reversible on removal of either compound from the medium. The multiplication of several strains of influenza A and B viruses, i.e. Lee, MB, PR8, and FM1, was inhibited to the same degree by each of the two compounds; DRB was 35 times more active than the 2,5-dimethyl compound relative to each of the strains. DRB caused inhibition of Lee virus multiplication in intact embryonated chicken eggs and in mice without causing significant signs of toxicity in either host. Some of the implications of these findings are discussed in relation to the mechanism of the inhibition of influenza virus multiplication.