Tumor necrosis factor-alpha production of influenza A virus-infected macrophages and potentiating effect of lipopolysaccharides

Influenza A virus infections are commonly associated with symptoms that suggest involvement of TNF-alpha. In this study, we exposed human monocytes, rat alveolar macrophages, and murine PU5-1.8 macrophages to influenza A virus, strain Puerto Rico 8. We observed a productive infection that was accompanied by TNF-alpha mRNA accumulation, TNF-alpha release and subsequent cell death. TNF-alpha production was dependent on exposure to live virus, in contrast to IFN release that was also induced by UV-inactivated virus. Most strikingly, low amounts of LPS (1 to 10 ng/ml) from Escherichia coli or Haemophilus influenzae were capable of strongly potentiating TNF-alpha production from virus-infected macrophages. The potentiating effect of LPS was neither due to increased survival of macrophages nor to altered virus multiplication, enhanced TNF-alpha gene expression, discharge of intracellular TNF-alpha stores, or shifts in the kinetics of TNF-alpha release. Thus, low amounts of LPS, which could easily be present in vivo, may serve as a potent trigger signal for TNF-alpha production from macrophages that have been primed by influenza A virus infection. These data suggest that the frequently observed serious complications of combined influenza A virus and bacterial infections may be partially due to a high TNF-alpha production.     

Lipopolysaccharide stimulation of ERK1/2 increases TNF-alpha production via Egr-1

Lipopolysaccharide (LPS) is a potent activator of tumor necrosis factor-alpha (TNF-alpha) production by macrophages. LPS stimulates the phosphorylation of extracellular signal-regulated kinase (ERK) 1/2 and increases TNF-alpha mRNA and protein accumulation in RAW 264.7 murine macrophages. However, the role of ERK1/2 activation in mediating LPS-stimulated TNF-alpha production is not well understood. Inhibition of ERK1/2 activation with PD-98059 or overexpression of dominant negative ERK1/2 decreased LPS-induced TNF-alpha mRNA quantity. LPS rapidly increased early growth response factor (Egr)-1 binding to the TNF-alpha promoter; this response was blunted in cells treated with PD-98059 or transfected with dominant-negative ERK1/2. Using a chloramphenicol acetyltransferase reporter gene linked to the Egr-1 promoter, we show that LPS increased Egr-1 promoter activity via an ERK1/2-dependent mechanism. These results delineate the role of ERK1/2 activation of Egr-1 activity in mediating LPS-induced increases in TNF-alpha mRNA expression in macrophages.     

Virus clearance through apoptosis-dependent phagocytosis of influenza A virus-infected cells by macrophages

Some cultured cell lines undergo typical apoptosis upon infection with influenza virus. However, the release of replicated virus into the culture medium does not change when apoptosis is inhibited. Since apoptotic cells are heterophagically eliminated at early stages of the apoptosis pathway, we anticipated that the coexistence of phagocytic cells with virus-infected cells affects the extent of virus growth. When influenza A virus-infected HeLa cells were mixed with activated mouse peritoneal macrophages, efficient phagocytosis, which was abrogated in the presence of a caspase inhibitor, occurred. At the same time, the release of virus into the culture medium was completely inhibited, and this required direct contact between virus-infected cells and macrophages. Furthermore, an immunoelectron microscopic analysis detected influenza virus particles associated with phagosome-like structures within macrophages. These results indicate that apoptosis-dependent phagocytosis of virus-infected cells may lead to direct elimination of the pathogen.     

IgG-coated erythrocytes augment LPS-stimulated TNF-alpha secretion, TNF-alpha mRNA levels, and TNF-alpha mRNA stability in macrophages

Previous studies have shown that IgG-coated erythrocytes (EIgG) augment the LPS-stimulated increase in serum TNF-alpha levels in animals and the LPS-stimulated secretion of TNF-alpha by isolated macrophages. The present study evaluated the mechanism for the effect of EIgG on LPS-stimulated TNF-alpha secretion in the murine macrophage cell line, RAW 264.7. Incubation of the macrophages with EIgG or IgG-coated glass beads caused a dose-dependent augmentation of LPS-stimulated TNF-alpha secretion. The addition of EIgG increased the rate of LPS-stimulated TNF-alpha protein secretion between 2 and 4 hr after LPS. Accordingly, EIgG increased the levels of TNF-alpha mRNA at 2 and 3 hr after LPS. The increase in the LPS-stimulated TNF-alpha mRNA levels caused by EIgG was associated with an increase in TNF-alpha mRNA stability. Thus, the augmentation of LPS-stimulated TNF-alpha secretion by EIgG was associated with an increase in TNF-alpha mRNA levels which at least partly resulted from an increase in the stability of TNF-alpha mRNA.     

Highly pathogenic avian influenza virus H5N1 infects alveolar macrophages without virus production or excessive TNF-alpha induction

Highly pathogenic avian influenza virus (HPAIV) of the subtype H5N1 causes severe, often fatal pneumonia in humans. The pathogenesis of HPAIV H5N1 infection is not completely understood, although the alveolar macrophage (AM) is thought to play an important role. HPAIV H5N1 infection of macrophages cultured from monocytes leads to high percentages of infection accompanied by virus production and an excessive pro-inflammatory immune response. However, macrophages cultured from monocytes are different from AM, both in phenotype and in response to seasonal influenza virus infection. Consequently, it remains unclear whether the results of studies with macrophages cultured from monocytes are valid for AM. Therefore we infected AM and for comparison macrophages cultured from monocytes with seasonal H3N2 virus, HPAIV H5N1 or pandemic H1N1 virus, and determined the percentage of cells infected, virus production and induction of TNF-alpha, a pro-inflammatory cytokine. In vitro HPAIV H5N1 infection of AM compared to that of macrophages cultured from monocytes resulted in a lower percentage of infected cells (up to 25% vs up to 84%), lower virus production and lower TNF-alpha induction. In vitro infection of AM with H3N2 or H1N1 virus resulted in even lower percentages of infected cells (up to 7%) than with HPAIV H5N1, while virus production and TNF-alpha induction were comparable. In conclusion, this study reveals that macrophages cultured from monocytes are not a good model to study the interaction between AM and these influenza virus strains. Furthermore, the interaction between HPAIV H5N1 and AM could contribute to the pathogenicity of this virus in humans, due to the relative high percentage of infected cells rather than virus production or an excessive TNF-alpha induction.     

NF-kappa B modulates TNF-alpha production by alveolar macrophages in asymptomatic HIV-seropositive individuals

Local TNF-alpha production in different organs may affect HIV replication and pathogenesis. Alveolar macrophages (AMs) obtained by bronchoalveolar lavage from asymptomatic HIV-seropositive and HIV-seronegative individuals did not spontaneously release TNF-alpha, but LPS stimulation of these cells significantly increased TNF-alpha production. We tested whether NF-kappa B affects TNF-alpha production by AMs using N-tosyl-l -phenylalanine chloromethylketone (TPCK) or N-benzoyl-l -tyrosine ethyl ester (BTEE), which inhibit the degradation of I kappa B, or tricyclodecan-9-yl-xanthogenate-potassium (D609), which inhibits phospholipase C. Alveolar macrophages were exposed to LPS alone and with the chemical protease inhibitors TPCK, BTEE, and D609. NF-kappa B DNA binding induced by LPS treatment of AMs was inhibited by TPCK, BTEE, and D609. These agents also inhibited TNF-alpha mRNA and TNF-alpha protein production. After 24 h, the levels of TNF-alpha mRNA reached equilibrium, as assessed by RT-PCR. The levels of NF-kappa B mRNA remained constant under all conditions. The levels of I kappa B-alpha mRNA were similar after 30, 60, and 180 min, but the I kappa B-beta mRNA concentration was initially low and increased over time under all conditions. I kappa B-alpha and I kappa B-beta protein production was not affected by the chemical protease inhibitors. Our data show that TNF-alpha production by LPS-stimulated AMs from asymptomatic HIV-seropositive and -seronegative individuals is regulated via the phospholipase C pathway and by NF-kappa B DNA binding activity without obvious changes in I kappa B-alpha or I kappa B-beta protein concentrations.     

Alveolar epithelial cells direct monocyte transepithelial migration upon influenza virus infection: impact of chemokines and adhesion molecules

Influenza A virus pneumonia is characterized by severe lung injury and high mortality. Early infection elicits a strong recruitment of monocytes from the peripheral blood across the endo-/epithelial barrier into the alveolar air space. However, it is currently unclear which of the infected resident lung cell populations, alveolar epithelial cells or alveolar macrophages, elicit monocyte recruitment during influenza A virus infection. In the current study, we investigated whether influenza A virus infection of primary alveolar epithelial cells and resident alveolar macrophages would elicit a basal-to-apical monocyte transepithelial migration in vitro. We found that infection of alveolar epithelial cells with the mouse-adapted influenza A virus strain PR/8 strongly induced the release of monocyte chemoattractants CCL2 and CCL5 followed by a strong monocyte transepithelial migration, and this monocytic response was strictly dependent on monocyte CCR2 but not CCR5 chemokine receptor expression. Analysis of the adhesion molecule pathways demonstrated a role of ICAM-1, VCAM-1, integrin-associated protein (CD47), and junctional adhesion molecule-c on the epithelial cell surface interacting with monocyte beta(1) and beta(2) integrins and integrin-associated protein in the monocyte transmigration process. Importantly, addition of influenza A virus-infected alveolar macrophages further enhanced monocyte transmigration across virus-infected epithelium in a TNF-alpha-dependent manner. Collectively, the data show an active role for virus-infected alveolar epithelium in the regulation of CCL2/CCR2-dependent monocyte transepithelial migration during influenza infection that is essentially dependent on both classical beta(1) and beta(2) integrins but also junctional adhesion molecule pathways.     

Short-term treatment of RAW264.7 macrophages with adiponectin increases tumor necrosis factor-alpha (TNF-alpha) expression via ERK1/2 activation and Egr-1 expression: role of TNF-alpha in adiponectin-stimulated interleukin-10 production

Adiponectin is an adipokine with potent anti-inflammatory properties. However, the mechanisms by which adiponectin suppresses macrophage function are not well understood. Treatment of RAW264.7 macrophages with adiponectin for 18 h decreased lipopolysaccharide (LPS)-stimulated tumor necrosis factor-alpha (TNF-alpha) production. Here we demonstrate that globular adiponectin (gAcrp) initially increased TNF-alpha expression in RAW264.7 macrophages; this TNF-alpha then contributed to increased expression of interleukin-10, which in turn was required for the development of tolerance to subsequent LPS exposure. gAcrp-mediated increases in TNF-alpha mRNA accumulation were associated with increased TNF-alpha promoter activity. gAcrp increased the DNA binding activity of both Egr-1 and NFkappaB; mutation of either the Egr-1 or NFkappaB binding sites in the TNF-alpha promoter decreased gAcrp-stimulated promoter activity. Further, co-transfection with either dominant negative Egr-1 or the IkappaB super-repressor prevented gAcrp-stimulated TNF-alpha promoter activity. gAcrp also increased Egr-1 promoter activity, mRNA accumulation, and DNA binding activity. Inhibition of ERK1/2 with U0126 potently suppressed gAcrp-stimulated Egr-1 promoter activity, as well as TNF-alpha promoter activity. In summary, these data demonstrate that adiponectin initially increases TNF-alpha production by macrophages via ERK1/2-->Egr-1 and NFkappaB-dependent mechanisms; these increases in TNF-alpha in turn lead to increased expression of interleukin-10 and an eventual dampening of LPS-mediated cytokine production in macrophages.     

Filovirus-induced endothelial leakage triggered by infected monocytes/macrophages.

The pathogenetic mechanisms underlying hemorrhagic fevers are not fully understood, but hemorrhage, activation of coagulation, and shock suggest vascular instability. Here, we demonstrate that Marburg virus (MBG), a filovirus causing a severe form of hemorrhagic fever in humans, replicates in human monocytes/macrophages, resulting in cytolytic infection and release of infectious virus particles. Replication also led to intracellular budding and accumulation of viral particles in vacuoles, thus providing a mechanism by which the virus may escape immune surveillance. Monocytes/macrophages were activated by MBG infection as indicated by tumor necrosis factor alpha (TNF-alpha) release. Supernatants of monocyte/macrophage cultures infected with MBG increased the permeability of cultured human endothelial cell monolayers. The increase in endothelial permeability correlated with the time course of TNF-alpha release and was inhibited by a TNF-alpha specific monoclonal antibody. Furthermore, recombinant TNF-alpha added at concentrations present in supernatants of virus-infected macrophage cultures increased endothelial permeability in the presence of 10 micron H2O2. These results indicate that TNF-alpha plays a critical role in mediating increased permeability, which was identified as a paraendothelial route shown by formation of interendothelial gaps. The combination of viral replication in endothelial cells (H.-J. Schnittler, F. Mahner, D. Drenckhahn, H.-D. Klenk, and H. Feldmann, J. Clin. Invest. 19:1301-1309, 1993) and monocytes/macrophages and the permeability-increasing effect of virus-induced cytokine release provide the first experimental data for a novel concept in the pathogenesis of viral hemorrhagic fever.