Molecular pathogenesis of influenza A virus infection and virus-induced regulation of cytokine gene expression.

Despite vaccines and antiviral substances influenza still causes significant morbidity and mortality world wide. Better understanding of the molecular mechanisms of influenza virus replication, pathogenesis and host immune responses is required for the development of more efficient means of prevention and treatment of influenza. Influenza A virus, which replicates in epithelial cells and leukocytes, regulates host cell transcriptional and translational systems and activates, as well as downregulates apoptotic pathways. Influenza A virus infection results in the production of chemotactic (RANTES, MIP-1 alpha, MCP-1, MCP-3, and IP-10), pro-inflammatory (IL-1 beta, IL-6, IL-18, and TNF-alpha), and antiviral (IFN-alpha/beta) cytokines. Cytokine gene expression is associated with the activation of NF-kappa B, AP-1, STAT and IRF signal transducing molecules in influenza A virus-infected cells. In addition of upregulating cytokine gene expression, influenza A virus infection activates caspase-1 enzyme, which is involved in the proteolytic processing of proIL-1 beta and proIL-18 into their biologically active forms. Influenza A virus-induced IFN-alpha/beta is essential in host's antiviral defence by activating the expression of antiviral Mx, PKR and oligoadenylate synthetase genes. IFN-alpha/beta also prolongs T cell survival, upregulates IL-12 and IL-18 receptor gene expression and together with IL-18 stimulates NK and T cell IFN-gamma production and the development of Th1-type immune response.

Interferon-alpha activates multiple STAT proteins and upregulates proliferation-associated IL-2Ralpha, c-myc, and pim-1 genes in human T cells.

Interferon-alpha (IFN-alpha) is a pleiotropic cytokine that has antiviral, antiproliferative, and immunoregulatory functions. There is increasing evidence that IFN-alpha has an important role in T-cell biology. We have analyzed the expression of IL-2Ralpha, c-myc, and pim-1 genes in anti-CD3-activated human T lymphocytes. The induction of these genes is associated with interleukin-2 (IL-2)-induced T-cell proliferation. Treatment of T lymphocytes with IFN-alpha, IL-2, IL-12, and IL-15 upregulated IL-2Ralpha, c-myc, and pim-1 gene expression. IFN-alpha also sensitized T cells to IL-2-induced proliferation, further suggesting that IFN-alpha may be involved in the regulation of T-cell mitogenesis. When we analyzed the nature of STAT proteins capable of binding to IL-2Ralpha, pim-1, and IRF-1 GAS elements after cytokine stimulation, we observed IFN-alpha-induced binding of STAT1, STAT3, and STAT4, but not STAT5 to all of these elements. Yet, IFN-alpha was able to activate binding of STAT5 to the high-affinity IFP53 GAS site. IFN-alpha enhanced tyrosine phosphorylation of STAT1, STAT3, STAT4, STAT5a, and STAT5b. IL-12 induced STAT4 and IL-2 and IL-15 induced STAT5 binding to the GAS elements. Taken together, our results suggest that IFN-alpha, IL-2, IL-12, and IL-15 have overlapping activities on human T cells. These findings thus emphasize the importance of IFN-alpha as a T-cell regulatory cytokine.

The proximal interferon-stimulated response elements are essential for interferon responsiveness: a promoter analysis of the antiviral MxA gene.

Interferon (IFN)-inducible human MxA protein mediates resistance against influenza and several other RNA viruses. The MxA gene is under the control of type I IFN and, in certain cell types, is also directly activated by viruses. Here we show that in human macrophages, MxA mRNA levels are upregulated by very low doses of IFN-alpha in a dose-dependent manner. A similar, albeit much weaker, dose-dependent induction was seen with IFN-gamma. The induction was rapid and independent of protein synthesis. Interleukin-6 (IL-6) or tumor necrosis factor-alpha (TNF-alpha) did not influence MxA mRNA levels alone or in combination with IFNs, in spite of the presence of putative response elements of these cytokines in the MxA promoter. We show that the promoter of the MxA gene contains two functional IFN-stimulated response elements (ISRE) near the transcription start site and one homologous ISRE-like element, which is apparently nonfunctional, further upstream. The two proximal ISRE sites are essential for IFN-alpha-induced transcription and appear to be binding sites for IFN-stimulated gene factor 3 complex. In addition, EMSA and DNAse I footprinting analysis demonstrated that Spl binds with high affinity to a region encompassing nucleotides -25 and -50 and, thus, may provide means of interaction with the basal transcriptional machinery.

Regulation of HLA class I and II expression by interferons and influenza A virus in human peripheral blood mononuclear cells.

HLA class I and II molecules play a central role in regulating host immune responses against microbial infections because they present foreign antigens to CD8+ and CD4+ T lymphocytes, respectively. Many cytokines, especially interferons (IFN), are known to upregulate human leucocyte antigen (HLA) class I and II gene expression, but the kinetics, expression levels and viral regulation of HLA genes in primary human cells have not been well documented. Stimulation of peripheral blood mononuclear cells (PBMC) with IFN-alpha and IFN-gamma resulted in a 1.5- to twofold increase in HLA class I and beta 2-microglobulin expression in lymphocytes and monocytes. Lymphocytes did not express any detectable HLA class II either basally or after IFN induction. In monocytes, instead, a high basal class II expression was found and it was further induced by IFN-alpha (up to twofold) and especially by IFN-gamma (up to fivefold). In granulocyte-macrophage colony-stimulating factor (GM-CSF) differentiated human macrophages, basal HLA class I and II protein expression levels were high but IFN-gamma stimulation was able to further enhance their expression. Accordingly, class I and II mRNA expression was elevated by IFN-gamma, whereas IFN-alpha practically had no effect on HLA class I mRNA levels. Influenza A virus infection of macrophages resulted in temporary increases in HLA class I, beta 2-microglobulin and class II antigen expression. Neutralization of virus-induced IFN production by antibodies against type I and II IFNs prevented the virus-induced upregulation of HLA antigens. At late times of infection, as analysed by steady-state mRNA expression, both HLA class I and II mRNA were strongly reduced. These results suggest that IFNs are important regulators of HLA genes and responsible for a temporary increase in HLA antigen expression during influenza A virus infection.

Retinoic acid induces signal transducer and activator of transcription (STAT) 1, STAT2, and p48 expression in myeloid leukemia cells and enhances their responsiveness to interferons.

IFNs are antiproliferative cytokines that have growth-inhibitory effects on various normal and malignant cells. Therefore, they have been used in the treatment of certain forms of cancer, such as chronic myelogenous leukemia and hairy cell leukemia. However, there is little evidence that IFNs would be effective in the treatment of acute myelogenous leukemia, and molecular mechanisms underlying IFN unresponsiveness have not been clarified. Here we have studied the activation and induction of IFN-specific transcription factors signal transducer and activator of transcription (STAT) 1, STAT2, and p48 in all-trans-retinoic acid (ATRA)-differentiated myeloid leukemia cells using promyelocytic NB4, myeloblastic HL-60, and monoblastic U937 cells as model systems. These cells respond to ATRA by growth inhibition and differentiation. We show that in undifferentiated NB4 cells, 2',5'-oligoadenylate synthetase and MxB gene expression is not activated by IFN-alpha, possibly due to a relative lack of signaling molecules, especially p48 protein. However, during ATRA-induced differentiation, steady-state STAT1, STAT2, and especially p48 mRNA and corresponding protein levels were elevated both in NB4 and U937 cells, apparently correlating to an enhanced responsiveness of these cells to IFNs. ATRA treatment of NB4 cells sensitized them to IFN action as seen by increased IFN-gamma activation site DNA-binding activity or by efficient formation of IFN-alpha-specific ISGF3 complex and subsequent oligoadenylate synthetase and MxB gene expression. Lack of p48 expression could be one of the mechanisms of promyelocytic leukemia cell escape from growth-inhibitory effects of IFN-alpha.

Regulation of IFN-alpha/beta, MxA, 2',5'-oligoadenylate synthetase, and HLA gene expression in influenza A-infected human lung epithelial cells.

The epithelial cells of the respiratory tract are the primary sites of virus replication in influenza A virus infections. We infected human alveolar epithelium-like A549 cells and fibroblast-like human fetal lung (HFL1) cells with a pathogenic influenza A virus (A/Beijing/353/89), and studied the kinetics of infection and the expression of host IFN-alpha/beta, MxA, OAS (2',5'-oligoadenylate synthetase), and HLA class I and II genes. Viral mRNA and protein synthesis was clearly seen in virus-infected lung cells. A549 and HFL1 cells produced only small amounts of IFN-alpha/beta, whereas virus-infected macrophages produced type I IFN very efficiently. The kinetics of IFN-beta gene expression in A549 cells was rapid, as shown by reverse-transcriptase PCR, and IFN-beta mRNA expression levels correlated well to the kinetics of nuclear factor-kappa B transcription factor activation. In influenza A virus-infected A549 and HFL1 cells, MxA gene induction was mediated by IFN-alpha/beta released into the cell culture supernatant, and was prevented by anti-type I IFN Abs. HLA class I Ag expression, which could be activated by IFN in noninfected A549 and HFL1 cells, was not induced in these cells by virus infection. The results suggest that type I IFN are essential for the activation of the antiviral response in lung epithelial cells.

Expression of adhesion molecules on endothelial cells stimulated by Chlamydia pneumoniae.

Previous studies have shown that C. pneumoniae is able to infect human endothelial cells in vitro. In this report, the ability of C. pneumoniae to induce the expression of E-selectin or endothelial-leukocyte adhesion molecule 1 (ELAM-1), intercellular adhesion molecule 1 (ICAM-1) and vascular cell adhesion molecule 1 (VCAM-1) on human umbilical vein endothelial (HUVE) cell surface was investigated. C. pneumoniae was found to cause a moderate upregulation of the adhesion molecules. Maximal expression of E-selectin was noted at 6 h post infection (p.i.) and that of ICAM-1 and VCAM-1 at 20 h p.i. The capability of C. pneumoniae to grow in endothelial cells and to stimulate the expression of adhesion molecules essential for leukocyte-endothelial cell interactions suggests a role for C. pneumoniae as a local pathogenetic factor in vascular inflammatory alterations, including atherogenesis.

Human MxB protein, an interferon-alpha-inducible GTPase, contains a nuclear targeting signal and is localized in the heterochromatin region beneath the nuclear envelope.

Interferon-inducible Mx proteins belong to the family of large GTPases and are highly homologous with dynamins within their GTP-binding domain. Cytoplasmically localized human MxA protein mediates resistance to influenza and several other viruses, whereas human MxB protein has not been found to have any antiviral activity. Here we show that MxB protein is found both in the cytoplasm and in the nucleus, where it is localized in a granular pattern in the heterochromatin region beneath the nuclear envelope. Transfection experiments in COS cells of N-terminally deleted MxB constructs revealed a functional nuclear localization signal within the first 24 N-terminal amino acids. Nuclear 78-kDa and cytoplasmic 76-kDa forms of MxB protein were found in all of the cell lines studied and in human peripheral blood mononuclear cells. MxB protein proved to be a functional GTPase with activity comparable to that of MxA protein. N-terminally truncated (delta1-82) MxB protein lacking both the nuclear localization signal and a proline-rich domain had almost completely lost its GTPase activity. Analysis of peripheral blood mononuclear cells suggested that MxB protein expression is strictly regulated by interferon-alpha. This is the first documentation that human Mx protein resides in the nucleus. It also emphasizes that there are considerable differences in the localization and structure of functional domains within Mx proteins.

Retinoic acid activates interferon regulatory factor-1 gene expression in myeloid cells.

All-trans-retinoic acid (ATRA) is the drug of choice in the treatment of acute promyelocytic leukemia (APL). ATRA induces both in vitro and in vivo differentiation of APL cells into mature granulocytes. However, the molecular mechanisms involved in ATRA-dependent growth inhibition and cellular differentiation are not presently understood. The NB4 cell line, which is derived from the bone marrow of a patient with APL during relapse, can be used as a model system to study the growth and differentiation of APL cells. Because interferon (IFN) regulatory factors (IRF-1 and IRF-2) and other IFN-inducible gene products regulate cell growth, we analyzed the effects of ATRA on the expression of these genes. We show that ATRA directly activates IRF-1 gene expression, followed by activation of IRF-2 and 2'-5' oligoadenylate synthetase (OAS) gene expression with slower kinetics. In addition to NB4 cells, ATRA also activated IRF-1 gene expression in HL-60, U937, and THP-1 cells, which all respond to ATRA by growth inhibition. A more than additive increase in IRF-1 gene expression was seen with ATRA and IFN-gamma in NB4 cells. ATRA did not activate nuclear factor kappa B or signal transducer and activator of transcription (STAT) activation pathways, suggesting that an alternate mechanism is involved in IRF-1 gene activation. The ATRA-induced expression of IRF-1, an activator of transcription and repressor of transformation, may be one of the molecular mechanisms of ATRA-induced growth inhibition, and the basis for the synergistic actions of ATRA and IFNs in myeloid leukemia cells.

Activation of IFN-alpha, IFN-gamma, MxA, and IFN regulatory factor 1 genes in influenza A virus-infected human peripheral blood mononuclear cells.

Mononuclear blood cells have an important role in immunity as they produce different cytokines in response to microbial infections. We infected primary human mononuclear blood cells with a pathogenic influenza A virus (A/Beijing/353/89) and studied the activation of IFN-alpha, IFN-gamma, IRF-1, and MxA genes. IFN-alpha and IFN-gamma steady state mRNA levels peaked at 6 to 9 h after infection and declined rapidly thereafter. Only a modest (twofold) increase in IRF-1 mRNA was seen. MxA gene expression, normally strictly regulated by IFN-alpha/beta, had expression kinetics similar to those of IFN mRNA. Infection experiments done in the presence of cycloheximide showed that influenza virus infection could induce all genes studied in the absence of detectable protein synthesis. Pretreatment with IFN-alpha, but not with IFN-gamma, caused a dose-dependent inhibition of influenza virus replication in PBMC, and this inhibition correlated with increasing levels of MxA protein. Influenza virus replication was also inhibited in a stably transfected, MxA-expressing promonocytic U937 cell line. The results suggest that MxA protein significantly contributes to IFN-mediated host defence mechanisms against influenza A virus.

Effect of interferon-alpha and cell differentiation on Puumala virus infection in human monocyte/macrophages.

Pathogenesis of hantavirus infections is poorly understood. Puumala virus (PUU) is the etiologic agent of nephropathia epidemica, a form of hemorrhagic fever with renal syndrome common in Europe. We have studied PUU infection in primary human monocyte/macrophages and specifically the role of interferon alpha (IFN-alpha) and cell differentiation in it. PUU infection proceeded at a low level in monocyte/macrophages, and nucleocapsid (N) protein accumulation started 2 days postinfection. IFN-induced antiviral MxA protein was detected 3 days postinfection, suggesting IFN-alpha production in culture. IFN-alpha titers remained low, proposing that PUU is a poor IFN inducer. However, the PUU-induced IFN had an inhibitory effect on virus production as was shown by the effect of anti-IFN-alpha. Pretreatment of cells with IFN-alpha caused a dose-dependent inhibition of PUU N accumulation and reduced the yield of infectious virus. Monocytic U-937 cells overexpressing MxA protein were susceptible to PUU, suggesting that, unlike in some other negative strand RNA virus infections, MxA does not mediate resistance to PUU infection. Differentiation of monocyte/macrophages in culture and treatment of THP-1 promonocytic cells with phorbol 12-myristate 13-acetate made the cells more susceptible to PUU. The increased susceptibility of mature macrophages to PUU suggests that after differentiation to tissue macrophages they might function in the spread of the virus during PUU infection.

Enzymatic characterization of interferon-induced antiviral GTPases murine Mx1 and human MxA proteins.

Interferons induce a number of different proteins which mediate the antiproliferative, antiviral, and immunomodulatory functions of interferons. Interferon-induced Mx proteins, which confer resistance to influenza, vesicular stomatitis, and measles viruses, contain consensus GTPase sequence elements. Insect cell-produced purified murine Mx1 and human MxA proteins were found to hydrolyze GTP with Km = 65 microM (Vmax, 7.1 min-1) and 62 microM (Vmax, 3.1 min-1), respectively. The GTPase activity of Mx1 and MxA proteins was strictly dependent on Mg2+ ions. Murine Mx1 protein was inactivated at 10 degrees C lower temperatures than MxA protein. As analyzed, by filter binding assay, Mx1 protein (at 1 microM) showed a relatively high affinity for GDP (Kd = 1.0 x 10(-7) M) and approximately 340-fold lower affinity for guanosine 5'-3-O-(thio)triphosphate (GTP gamma S) (Kd = 3.4 x 10(-5) M). The Kd values for MxA protein were 2.0 x 10(-7) M for GDP and 5.9 x 10(-6) M for GTP gamma S, showing approximately a 30-fold affinity difference. ATP, UTP, or CTP did not inhibit the Mx protein-dependent GTPase activity, suggesting that Mx1 and MxA proteins are highly specific for guanosine nucleotides. In conclusion recombinant nuclear murine Mx1 and cytoplasmic human MxA proteins show clear differences in their enzymatic activities and nucleotide binding characteristics. How these differences influence their cellular functions and antiviral potential is presently not known.

Control of IFN-inducible MxA gene expression in human cells.

MxA is an IFN-induced human protein which is located in the cytoplasm of induced cells. MxA makes the cells resistant to infection by influenza and vesicular stomatitis viruses. In the present work we used baculovirus expression system to produce MxA protein. The protein was purified to homogeneity and highly specific polyclonal anti-MxA antibodies were prepared. In human mononuclear cells, and A549 lung carcinoma cells expression of MxA protein is induced by very low (< 1 IU/ml) doses of leukocyte IFN-alpha (nIFN-alpha), whereas IFN-gamma does not seem to induce it or potentiate the induction by IFN-alpha. In mononuclear cells stimulated with high doses of leukocyte IFN-alpha concentrations, the amount of MxA mRNA was induced 10-fold at 4 h after IFN induction and up to 10-fold higher MxA protein levels were observed at 24-48 h postinduction. The gene can be reinduced by IFN-alpha 24 h after the initial induction suggesting for a lack of negative feedback after this time point. The protein is very stable, the half-life being approximately 2.3 days. Flow cytometric analysis revealed that monocytes have higher basal and induced MxA protein levels than lymphocytes but the dose-dependency of MxA expression is very similar in both cell types. Granulocytes are producing very low amounts of MxA protein.

Interferon-induced Mx proteins form oligomers and contain a putative leucine zipper.

Interferons induce a number of different proteins that mediate the antiproliferative, antiviral, and immunomodulatory functions of interferons. At least three different proteins mediate the antiviral response, and one of them, Mx protein, specifically inhibits the replication of influenza virus and (vesicular stomatitis virus). Mouse and rat Mx1 proteins are nuclear, whereas other presently known Mx proteins are cytoplasmic. The cellular functions of Mx proteins are unknown, but all of them contain a consensus GTP binding site. Very little information is available on the structure and characteristics of the mouse Mx1 protein itself. For biochemical characterization, we expressed mouse Mx1 protein in a baculovirus system and purified it to homogeneity. The purified protein as well as the authentic murine cellular Mx1 protein exists in dimers and trimers in the presence of dissociating solvents, whereas in physiological buffers they form aggregates. Cross-linking experiments done on Mx-expressing cells from various species revealed that mouse, rat, and human Mx proteins exist predominantly in trimers. Amino acid sequence analysis shows that all known Mx proteins have conserved leucine repeats typical for a leucine zipper at their COOH-terminal end. In vitro translation of chimeric catechol O-methyltransferase-Mx1 gene constructs revealed that the leucine zipper domain of Mx1 protein is responsible for the oligomerization. The COOH terminus also functions as a nuclear localization signal. Microinjection of purified oligomers into the cell cytoplasm resulted in a fast accumulation of the protein in the resulted in a fast accumulation of the protein in the nucleus. Immunoelectron microscopy revealed that nuclear murine Mx1 protein exists in distinct, electron-dense structures separate from nuclear membrane, and chromatin, or nucleolus. These observations reveal that a COOH-terminal leucine zipper domain is an important structural element of all Mx proteins. Its relevance to the biology and functions of Mx proteins is presently not known.

Control of interleukin-1 beta expression by protein kinase C and cyclic adenosine monophosphate in myeloid leukemia cells.

We have examined the signal transduction pathways leading to the expression of the interleukin-1 beta (IL-1 beta) gene in human myeloid leukemia cells lines. Two cell lines representing different stages of differentiation were used (HL-60, promyelocytic, and THP-1, mature monocytic). In accordance with previous studies, it was observed that a protein kinase C (PKC) activator, phorbol myristate acetate (PMA), was a sufficient stimulus for induction of the IL-1 beta messenger RNA (mRNA) expression and IL-1 beta protein production in both of these cell lines. A structural analog of cyclic adenosine monophosphate (dbcAMP) or agents elevating the endogenous cAMP levels (prostaglandin E2, forskolin) were not alone able to induce IL-1 beta expression, but they strongly enhanced the PMA-induced IL-1 beta production and IL-1 beta mRNA accumulation. Nuclear run off analysis showed that this elevation in IL-1 beta mRNA levels was due to an increased rate of transcription. If dbcAMP was added 6 hours before PMA to the cultures, no enhancement in the IL-1 beta production was seen, implying that for this enhancing effect both of these signals must be present simultaneously. PKC inhibitor, H7, also blocked effectively the PMA plus dbcAMP induced IL-1 beta production, while the protein kinase A (PKA) inhibitor, HA1004, had no effect, suggesting that PKA activation is not involved in the mechanism of action of cAMP in this case. Collectively, the present findings show that cAMP-dependent signals can have a positive regulatory effect on the PKC-dependent activation of the IL-1 beta gene in cells derived from different stages of myeloid differentiation.