Endogenous protease-dependent replication of human influenza viruses in two MDCK cell lines

Multi-cycle replication and plaque formation of influenza A and B viruses and cleavage activation of their hemagglutinin (HA) by an endogenous protease(s) were examined in two MDCK cell lines, MDCK(-) and MDCK(+). No exogenous trypsin was required for multi-cycle replication and plaque formation of all the influenza A viruses tested in the MDCK(+) cell, while those of the viruses in the MDCK(-) cell were completely trypsin-dependent. In both cell lines, on the other hand, influenza B viruses grew well in the absence of trypsin. The capability of multiple replication and plaque formation of the influenza viruses correlated with cleavage of the HA precursor (HA0) to HA1 and HA2, indicating that both cell lines express an HA activating endoprotease(s); that of the MDCK(+) cell activates the HA of influenza A and B viruses, and that of the MDCK(-) cell does only the HA of influenza B virus. Furthermore, the protease of the MDCK(+) cell was strongly suggested to be present on the cell surface and a serine protease. The MDCK(+) cell would be useful for isolation of influenza viruses from clinical specimens and for screening of protease inhibitors for anti-influenza virus drugs.     

NMR studies of Borrelia burgdorferi OspA, a 28 kDa protein containing a single-layer beta-sheet

Viruses such as HIV, influenza, picornavirus and others are known stimulators of apoptosis. This individual cellular elimination is a preferential host defense in regenerative tissues. In contrast, if this death occurred in nonregenerating cells, such as neurons of the central nervous system, may result in disease. The target cell for rabies virus is the neuron. Here we studied the outcome of the interaction between rabies virus (CVS-11) and mouse brain cells. Replication of rabies virus in suckling mouse brain cells resulted in brain cell apoptosis, detected by DNA fragmentation and in situ apoptosis within 25 h after infection and before evidence of intracerebral immune activation. Cell death occurred simultaneously with rabies virus replication. There were clinical signs of illness in infected newborn mice within 24 h after the appearance of DNA fragmentation and before infiltration by lymphocytes. This suggested that onset of illness started independently of the immune function. This conclusion was supported by the occurrence of massive apoptosis followed by paralysis in rabies virus-infected immunosuppressed mice. Direct, viral-induced, neuronal apoptosis was the earliest death mechanism detected in these mice. We propose that pathogenesis of this fixed strain of rabies virus in mice begins with the induction of apoptosis by rabies virus replication. Cerebral damage may then be amplified by immunological mechanisms plus an additional unidentified factor. This is followed by increased permeability of the blood brain barrier.     

Antiviral activity of plant flavonoid, 5,7,4'-trihydroxy-8-methoxyflavone, from the roots of Scutellaria baicalensis against influenza A (H3N2) and B viruses

We investigated effects of isoscutellarein-8-methylether (5,7,4'-trihydroxy-8-methoxyflavone, F36) from the roots of Scutellaria baicalensis on the single-cycle replication of mouse-adapted influenza viruses A/Guizhou/54/89 (H3N2 subtype) and B/Ibaraki/2/85 in Madin-Darby canine kidney (MDCK) cells. The agent suppressed replication of these viruses from 6 to 12 h after incubation in a dose-dependent manner by 50% at 20 microM and 90% at 40 microM, respectively. F36 (50 microM) reduced the release of B/Ibaraki virus in the medium by 90-93% when it was added to the MDCK cells at 0 to 4 h after incubation. The cell-associated virus determined by sialidase activity was also reduced by the treatment at 0 to 4 h. F36 (120 microM) inhibited the low pH-dependent membrane fusion of both the viruses with the liposome containing mixed gangliosides from bovine brain. However, the agent little affected the hemagglutination and RNA-dependent RNA polymerase activities of these viruses in vitro. These results suggest that F36 inhibits the replication of A/Guizhou and B/Ibaraki viruses at least partly by inhibiting the fusion of viral envelopes with the endosome/lysosome membrane which occurs at the early stage of the virus infection cycle. F36 (0.5 mg/kg) showed no antiviral activity against A/Guizhou and B/Ibaraki viruses in mice when administered intranasally 5 min prior to virus inoculation, whereas it significantly inhibited their proliferation in the mouse lung when administered intranasally 7 times (total 3.5 mg/kg) from 18 h before to 54 h after virus infection.     

Inhibition of influenza virus hemagglutinin-mediated membrane fusion by a compound related to podocarpic acid

Entry of influenza virus into the host cell is dependent on the fusion of the viral envelope with the endosomal membrane and is mediated by a low-pH-induced change of the viral hemagglutinin (HA) to a conformation that is fusogenic. A compound related to podocarpic acid (180299) was identified that inhibits multicycle replication of influenza A/Kawasaki/86 (H1N1) virus in culture. Treatment of Madin-Darby canine kidney (MDCK) cells with 180299 at 1 h before infection resulted in the inhibition of viral protein synthesis. Addition of 20 microgram of 180299/ml at 1 h p.i. had no effect, indicating that 180299 affects an early step of the influenza viral replication cycle. Genetic analysis of reassortants between sensitive and resistant viruses demonstrated that hemagglutinin (HA) conferred the 180299-resistant (180299(r)) phenotype. Twelve independent isolates of influenza A/Kawasaki/86 were selected for resistance to 180299, and sequence analysis revealed that each of these viruses contained amino acid substitutions in the HA. These mutations are dispersed throughout the HA primary amino acid sequence and cluster in one of two regions: the interface between HA1 and HA2 and in a region near the fusion domain of HA2. When compared with the parent virus, the pH-of-inactivation of the resistant mutants was increased by 0.3 to 0.6 pH unit, suggesting that the mutant HAs undergo the conformational change at an elevated pH. Fusion of human erythrocytes to MDCK cells infected with parent influenza A/Kawasaki/86 was inhibited by 180299 (0.1-10 microgram/ml) in a concentration-dependent manner, whereas fusion of erythrocytes to MDCK cells infected with 180299(r) mutants was not affected. These results suggest that 180299 interacts with the neutral pH conformation of influenza A HA and prevents the low-pH-induced change of HA to its fusogenic conformation.     

Inhibition of the infectivity of influenza virus by tea polyphenols

(-)Epigallocatechin gallate (EGCg) and theaflavin digallate (TF3) (1-10 microM) inhibited the infectivity of both influenza A virus and influenza B virus in Madin-Darby canine kidney (MDCK) cells in vitro. Study by electron microscope revealed that EGCg and TF3 (1 mM) agglutinated influenza viruses as well as did antibody, and that they prevented the viruses from adsorbing to MDCK cells. EGCg and TF3 more weakly inhibited adsorption of the viruses to MDCK cells. EGCg and TF3 (1-16 microM) also inhibited haemagglutination by influenza viruses. These findings suggest that tea polyphenols bind to the haemagglutinin of influenza virus, inhibit its adsorption to MDCK cells, and thus block its infectivity.     

Role of early and late replication events in induction of apoptosis by baculoviruses

Autographa californica nuclear polyhedrosis virus (AcMNPV) mutants that lack the apoptotic suppressor gene p35 cause apoptosis in Spodoptera frugiperda SF21 cells. To identify a viral signal(s) that induces programmed cell death, we first defined the timing of apoptotic events during infection. Activation of a P35-inhibitable caspase, intracellular fragmentation of host and AcMNPV DNA, and cell membrane blebbing coincided with the initiation of viral DNA synthesis between 9 and 12 h after infection and thus suggested that apoptotic signaling begins at or before this time. Virus entry was required since binding of budded virus to host cell receptors alone was insufficient to induce apoptosis. To therefore determine the contribution of early and late replication events to apoptotic signaling, we used the AcMNPV mutant ts8 with a temperature-sensitive lesion in the putative helicase gene p143. At the nonpermissive temperature at which viral DNA synthesis was conditionally blocked, ts8 caused extensive apoptosis of the SF21 cell line p3576D, which dominantly interferes with anti-apoptotic function of viral P35. Confirming that apoptosis can be induced in the absence of normal viral DNA synthesis, parental SF21 cells also underwent apoptosis when infected with a ts8 p35 deletion mutant at the nonpermissive temperature. However, maximum levels of ts8 p35 deletion mutant-induced apoptosis required a temperature-sensitive event(s) that included the initiation of viral DNA synthesis. Collectively, these data suggested that baculovirus-induced apoptosis can be triggered by distinct early (pre-DNA synthesis) and late replicative events, including viral DNA synthesis or late gene expression.     

Selection and characterization of a neuraminidase-minus mutant of influenza virus and its rescue by cloned neuraminidase genes

A neuraminidase (NA)-deficient mutant, designated NWS-Mvi, of the reassortant influenza virus A/NWS/33HA-A/tern/Australia/G70c/75NA (H1N9), was selected by passaging virus in MDCK cells in a medium containing neuraminidase from the bacterium Micromonospora viridifaciens and polyclonal antiserum against the influenza NA. Growth of the resulting mutant virus is dependent on the addition of neuraminidase to the medium. Western blot analysis showed that the neuraminidase protein was absent from the mutant virus particles, and Northern hybridization showed that RNA segment 6, which contains the coding information for the NA, had undergone massive deletion. Viral protein synthesis in cells infected with the mutant virus was not dependent on the addition of neuraminidase. In the absence of a functional NA, the NWS-Mvi mutant virus can infect MDCK cells with normal cytopathic effects. This neuraminidase-minus influenza virus serves as an excellent source of parent virus for reverse genetics experiments involving genes that encode a functional neuraminidase.     

Successful replication of parvovirus B19 in the human megakaryocytic leukemia cell line MB-02

The pathogenic human parvovirus B19 has been shown to undergo productive replication in the erythroid lineage in primary normal human hematopoietic progenitor cells. However, none of the established erythroleukemia cell lines has allowed B19 virus replication in vitro. The remarkable erythroid tissue tropism of B19 virus was evaluated with a human megakaryocytic leukemia cell line, MB-02, which is dependent on the growth factor granulocyte-macrophage colony-stimulating factor but can be induced to undergo erythroid differentiation following treatment with erythropoietin (Epo). Whereas these cells did not support B19 virus DNA replication in the presence of granulocyte-macrophage colony-stimulating factor alone, active viral DNA replication was observed if the cells were exposed to Epo for 5 to 10 days prior to B19 virus infection, as detected by the presence of the characteristic B19 virus DNA replicative intermediates on Southern blots. No replication occurred if the cells were treated with Epo for 3 days or less. In addition, complete expression of the B19 virus genome also occurred in Epo-treated MB-02 cells, as detected by Northern blot analysis. B19 progeny virions were released into culture supernatants that were biologically active in secondary infection of normal human bone marrow cells. The availability of the only homogeneous permanent cell line in which induction of erythroid differentiation leads to a permissive state for B19 virus replication in vitro promises to yield new and useful information on the molecular basis of the erythroid tissue tropism as well as parvovirus B19-induced pathogenesis.     

Inhibition of influenza virus replication by phosphorothioate and liposomally endocapsulated oligonucleotides

We have demonstrated that antisense phosphorothioate oligonucleotides (S-ODNs) inhibit influenza virus A replication in MDCK cells. Phosphorothioate and liposomally encapsulated oligonucleotides with two target sites (PB1 and PB2) were synthesized and tested for virus-induced cytopathogenicity effects by a MTT assay using MDCK cells. The liposomally encapsulated S-ODNs complementary to the sites of the PB2-AUG initiation codon showed highly inhibitory effects. On the other hand, the inhibitory effect of the liposomally encapsulated S-ODNs targeted to PB1 was considerably decreased in comparison with the PB2 target sites. The liposomally encapsulated oligonucleotides exhibited higher inhibitory activity than the free oligonucleotides. The activities of the modified oligonucleotides are effectively enhanced by using the liposomal carrier.     

Influenza virus neuraminidase activates latent transforming growth factor beta

Transforming growth factor beta (TGF-beta) is a family of proteins secreted by virtually all cells in a biologically inactive form. TGF-beta levels increase during many pathophysiological situations, including viral infection. The mechanism for increased TGF-beta activity during viral infection is not understood. We observed an increase in active TGF-beta levels within 1 day in mice infected with influenza virus. Further studies showed that the neuraminidase glycoprotein of influenza A and B viruses directly activates latent TGF-beta in vitro. There are sufficient levels of TGF-beta activated by virus to induce apoptosis in cells. In addition, influenza virus-induced apoptosis is partially inhibited by TGF-beta-specific antibodies. These novel findings suggest a potential role for activation of TGF-beta during the host response to influenza virus infection, specifically apoptosis. This is the first report showing direct activation of latent TGF-beta by a viral protein.     

Use of recombinant nucleoproteins in enzyme-linked immunosorbent assays for detection of virus-specific immunoglobulin A (IgA) and IgG antibodies in influenza virus A- or B-infected patients

The nucleoprotein genes of influenza virus A/Netherlands/018/94 (H3N2) and influenza virus B/Harbin/7/94 were cloned into the bacterial expression vector pMalC to yield highly purified recombinant influenza virus A and B nucleoproteins. With these recombinant influenza nucleoproteins, enzyme-linked immunosorbent assays (ELISAs) were developed for the detection of influenza virus A- and B-specific immunoglobulin A (IgA) and IgG serum antibodies. Serum samples were collected at consecutive time points after the onset of clinical symptoms from patients with confirmed influenza virus A or B infections. Nucleoprotein-specific IgA antibodies were detected in 41.2% of influenza virus A-infected patients and in 66. 7% of influenza virus B-infected patients on day 6 after the onset of clinical symptoms. In serum samples taken on day 21 (influenza virus A-infected patients) or day 28 (influenza virus B-infected patients), nucleoprotein-specific IgA antibodies could be detected in 58.8 and 58.3% of influenza virus A- and B-infected patients, respectively. At the same time, IgG antibody rises were detected in 88.2% of influenza virus A-infected patients and in 95.8% of influenza virus B-infected patients. On comparison, hemagglutination inhibition assays detected antibody titer rises in 81.3 and 72.7% of patients infected with influenza viruses A and B, respectively. In contrast to the detection of nucleoprotein-specific IgG antibodies or hemagglutination-inhibiting antibodies, the detection of nucleoprotein-specific IgA antibodies does not require paired serum samples and therefore can be considered an attractive alternative for the rapid serological diagnosis of influenza.     

Apoptosis of CD4+ T cells induced after contact with HIV1-infected or non-infected macrophages

The hallmark of human immunodeficiency virus type 1 (HIV1) infection is the relentless destruction of CD4+ T lymphocytes. Indirect cell killing mechanisms are thought to play an outstanding role in lymphocyte depletion. One such proposed mechanism is the induction of apoptosis through cross-linking of the CD4 receptor by anti-CD4 antibodies or by the HIV1 envelope protein expressed at the surface of infected cells. Here we provide evidence that apoptosis is triggered in CD4+ lymphoblastoid cells (MT4) following cocultivation with monocyte-derived macrophages (MDMs) productively infected with the monocytotropic isolate HIV1 PAR. Blocking virus replication by AZT abrogates apoptosis of MT4 cells. Infected MDMs do not transmit virus infection to target cells. DNA nucleosomal fragmentation occurs at 46-66 h after starting cocultures. It is inhibited by the addition of neutralizing anti-gp120 monoclonal antibody (mAb), implying that the gp120/CD4 interaction triggers the apoptotic process. Cocultivating with MDMs, either infected or not, in the presence of anti-CD4 mAb Leu-3a, also leads to MT4 cell death, with DNA fragmentation being detected at 24-40 h. Leu-3a induced apoptosis does not require cross-linking of CD4 by anti-immunoglobulin, showing that MDMs provide an alternative to conventional cross-linking. Both the infected and the non-infected MDMs were found to stimulate 3H-thymidine incorporation in cocultivated MT4 cells.     

Primary influenza A virus infection induces cross-reactive antibodies that enhance uptake of virus into Fc receptor-bearing cells

Sera of young children who had had a primary infection with influenza A virus or were immunized with a live attenuated influenza A virus vaccine were examined for antibody responses that neutralized virus or enhanced uptake of virus into Fc receptor-bearing cells, because antibodies that enhance uptake of influenza virus into Fc receptor-bearing cells have been reported using mouse immune serum and monoclonal antibodies. The neutralizing antibody titers to the homologous infecting virus and to another H1N1 virus isolated several years later were higher after natural infection than after infection with the live attenuated virus. Natural infection and the attenuated vaccine induced antibodies that enhanced uptake of homologous virus and H1N1 virus isolated several years later. These results demonstrate that primary influenza A virus infection results in the induction of infection-enhancing antibodies.     

Kinetics of duck hepatitis B virus infection following low dose virus inoculation: one virus DNA genome is infectious in neonatal ducks

Using pooled serum from congenitally duck hepatitis B virus (DHBV)-infected ducks as inoculum, we examined the effect of virus dose on the incubation period of infection and on the patterns of spread of virus infection in the liver. The pooled serum inoculum contained 9.5 x 10(9) DHBV genomes per milliliter and had an infectivity titre (ID50) in newly hatched ducks of 1.5 x 10(10) per milliliter with a 95% confidence interval of 3.0 x 10(9) to 6.3 x 10(10) ID50/ml, indicating the equivalence between one DHBV genome and one infectious unit within the limits of the assays. The incubation period of infection was inversely related to the dose of inoculum and the onset of viraemia ranged from Day 6 with the highest dose to Day 14 or 29 with the lowest dose inoculum. To study the spread of virus infection from a low percentage of initially infected cells we inoculated newly hatched ducks intravenously with sufficient DHBV (1.5 x 10(3) ID50) to infect only approximately 0.0001% of total liver cells. DHBV infection first reached detectable levels on Day 4 postinoculation (p.i.) and was detected in approximately 0.035% of hepatocytes, most of which occurred as single cells or pairs of cells, indicating that a number of rounds of infection had occurred with the spread of virus both to adjoining cells, i.e., by cell-to-cell spread, and to cells located in other parts of the liver lobule. Despite some bird-to-bird variation in timing, the percentage of infected hepatocytes increased exponentially with a mean doubling time of 16 hr from Day 4 to Day 14 p.i., by which time replication was seen in > 95% of hepatocytes. This rapid dissemination from a small number of infected hepatocytes suggests that, in neonatal ducks, there are no major delays in virus replication within the liver, that any innate and adaptive defence mechanisms operating during the first 10 to 14 days of infection are insufficient to contain virus spread, and that even a small number of infected hepatocytes produce enough progeny to rapidly infect the remaining hepatocytes.     

Transfectant influenza A viruses with long deletions in the NS1 protein grow efficiently in Vero cells

We established a reverse genetics system for the nonstructural (NS) gene segment of influenza A virus. This system is based on the use of the temperature-sensitive (ts) reassortant virus 25A-1. The 25A-1 virus contains the NS gene from influenza A/Leningrad/134/57 virus and the remaining gene segments from A/Puerto Rico (PR)/8/34 virus. This particular gene constellation was found to be responsible for the ts phenotype. For reverse genetics of the NS gene, a plasmid-derived NS gene from influenza A/PR/8/34 virus was ribonucleoprotein transfected into cells that were previously infected with the 25A-1 virus. Two subsequent passages of the transfection supernatant at 40 degreesC selected viruses containing the transfected NS gene derived from A/PR/8/34 virus. The high efficiency of the selection process permitted the rescue of transfectant viruses with large deletions of the C-terminal part of the NS1 protein. Viable transfectant viruses containing the N-terminal 124, 80, or 38 amino acids of the NS1 protein were obtained. Whereas all deletion mutants grew to high titers in Vero cells, growth on Madin-Darby canine kidney (MDCK) cells and replication in mice decreased with increasing length of the deletions. In Vero cells expression levels of viral proteins of the deletion mutants were similar to those of the wild type. In contrast, in MDCK cells the level of the M1 protein was significantly reduced for the deletion mutants.     

The mode of death of pig kidney cells infected with cowpox virus is governed by the expression of the crmA gene

Pig kidney cells (LLC-PK1) were infected with one of three viruses: wild-type cowpox virus (Brighton red strain) expressing the crmA gene; recombinant cowpox virus A602, lacking the crmA gene; or cowpox virus A604, a revertant of virus A602, expressing the crmA gene. The wild-type virus and virus A604 produced identical cytopathic effects consistent with death by necrosis. In these cells, the structural features of the plasma membrane, the nuclear membrane, and the chromatin were maintained until lysis of the cells. In contrast, cowpox virus A602 produced cytopathic effects consistent with death by apoptosis. These effects included loss of microvilli on the cell surface, margination and condensation of the chromatin, progressive convolution of the nuclear membrane, release of dense chromatin masses on disintegration of the nucleus, fragmentation of the DNA, and the generation of apoptotic bodies. These results suggest that the crmA gene is necessary to inhibit processes of apoptosis induced in LLC-PK1 cells by infection with cowpox virus. Thus in cells of certain types, the crmA gene can act with other viral genes to control the mode of death of the virus-infected cell. This capability may be advantageous to virus replication in vivo, potentially facilitating both virus trafficking and interference with antiviral immune defenses.     

Assessment of markers of the cell-mediated immune response after influenza virus infection in frail older adults

The purpose of this study was to determine whether measures of the cell-mediated immune response to influenza virus could be used as markers of influenza virus infection. We studied 23 subjects who developed upper respiratory, lower respiratory, or systemic symptoms during a small outbreak of influenza in a nursing home population. Influenza virus culture from nasopharyngeal swabs yielded influenza virus isolates from 7 of the 23 subjects. Only three of the subjects had a fourfold rise in antibody titer to the influenza virus antigen positivity after the infection. Granzyme B and cytokine levels were measured in peripheral blood mononuclear cells (PBMC) obtained from all subjects and stimulated with live influenza virus. Elevated granzyme B levels in virus-stimulated PBMC in combination with lower respiratory tract or systemic symptoms in study subjects was a significant predictor of culture-confirmed influenza virus infection compared to those from whom influenza virus could not be identified. Cytokine levels did not distinguish between the two groups in a similar type of analysis. Granzyme B in combination with the clinical profile of symptoms may be a useful retrospective marker for influenza virus infection.     

Synergistic anti-influenza virus A (H1N1) activities of PM-523 (polyoxometalate) and ribavirin in vitro and in vivo

A Kegin-type polyoxometalate, PM-523, in combination with ribavirin, was tested for its therapeutic effectiveness against influenza virus (FluV) A (H1N1) infection in tissue culture and in mice. PM-523 [(PriNH3)6H [PTi2W10O38(O2)2] x H2O, where Pri is isopropanol] and ribavirin individually inhibited FluV A-induced cytopathic effects in Madin-Darby canine kidney (MDCK) cells at median effective concentrations (EC50s) of 30 and 34 microM, respectively, and at 70% effective concentrations (EC70s) of 48 and 72 microM, respectively. On the other hand, a combination of PM-523 and ribavirin at a ratio of 1:16 exhibited lower EC50s and EC70s than each compound used singly, and combination indices were less than 1. A wide range of combinations of PM-523 and ribavirin at ratios of from 1:128 to 1:1 exhibited additive or synergistic anti-FluV effects in MDCK cells. When these compounds were tested for their anti-FluV A activities in vivo by aerosol exposure of mice which had been infected with a lethal dose of FluV A by an intranasal route, a 1:16 combination of PM-523 and ribavirin was found to have a significantly better therapeutic effect than a single dose of either compound used singly with respect to both the survival rate of the mice and the virus titer in the lungs of the infected mice. PM-523 was effective for the treatment of experimental FluV infection, and in combination with ribavirin, PM-523 exhibited enhanced anti-FluV effects in vitro and in vivo compared with the effect of PM-523 alone.     

Expression of the myxoma virus tumor necrosis factor receptor homologue and M11L genes is required to prevent virus-induced apoptosis in infected rabbit T lymphocytes

Myxoma virus is a leporipoxvirus that causes a highly lethal virulent disease known as myxomatosis in the European rabbit. An important aspect of myxoma virus pathogenesis is the ability of the virus to productively infect lymphocytes and spread to secondary sites via lymphatic channels. We investigated the infection of the CD4+ T lymphoma cell line RL-5 with myxoma virus and Shope fibroma virus, a related but benign leporipoxvirus, and observed that myxoma virus, but not Shope fibroma virus, was able to productively infect RL-5 cells. We also discovered that infection of RL-5 cells with Shope fibroma virus or attenuated myxoma virus mutants containing disruptions in either the T2 or the M11L gene resulted in the rapid induction of DNA fragmentation, followed by morphological changes and loss in cell integrity characteristic of cell death by apoptosis. Purified exogenous T2 protein was unable to prevent apoptosis, suggesting that T2 functions intracellularly. Thus, myxoma virus T2, originally described as a secreted homologue of the tumor necrosis factor receptor, and M11L, a novel transmembrane species with no known cellular homologue, function to extend virus host range for replication in rabbit T lymphocytes through the inhibition of apoptosis in infected T lymphocytes.