Defense against influenza A virus infection: essential role of the chemokine system.

Monocytes/macrophages are highly susceptible to an infection with influenza A virus. After infection, de novo virus protein synthesis is detectable but rapidly interrupted before completion of the first viral replication cycle. Within 24-48 hours the infected monocytes die by apoptosis. Before cell death, infected monocytes initiate a cell-specific immune response. This includes the transcription and subsequent release of TNF-alpha (tumor necrosis factor alpha), IL-1beta (Interleukin 1beta), IL-6, type I inferferons and CC chemokines. Enhanced cytokine mRNA expression is due to a prolonged mRNA stability and an augmented gene transcription. Activation of transcription factors such as NF-kappaB (nuclear factor kappaB) and AP-1 are involved in activation of cytokine mRNA transcription. Infection of monocytes with influenza A virus induces the selective expression of mononuclear leukocyte attracting chemokines, such as MCP-1 (monocyte chemotactic protein 1), MIP-1alpha (macrophage inflammatory protein 1alpha) and RANTES (regulated upon activation, normal T cell expressed and secreted). In striking contrast, the release of the neutrophil-specific chemokines IL-8 (interleukin 8) and GRO-alpha (growth stimulatory activity alpha) is entirely suppressed. This differentially regulated chemokine expression may explain the mononuclear cell infiltrate characteristic for virus-infected tissue. Thus, infection of monocytes/macrophages with influenza A virus primes for a rapid proinflammatory reaction and induces an enhanced immigration of mononuclear cells into infected tissue. Taken together, these mechanisms may prepare the infected host for a fast and virus-specific immune response.

Selective induction of the monocyte-attracting chemokines MCP-1 and IP-10 in vesicular stomatitis virus-infected human monocytes.

It is characteristic of viral infections that monocytes/macrophages and lymphocytes infiltrate infected tissue, and neutrophils are absent. CC and non-ELR CXC chemokines predominantly attract mononuclear leukocytes, whereas the ELR motif-expressing CXC chemokines primarily act on neutrophils. To investigate the general role of chemokines in viral diseases, we determined their release and expression patterns after infection of human monocytes with vesicular stomatitis virus (VSV). Human monocytes were productively infected by VSV. Surprisingly, VSV did not induce the release of the proinflammatory cytokines tumor necrosis factor-alpha (TNF-alpha), interleukin-1beta (IL-1beta), and IL-6. In contrast, we found a strong induction of the CC chemokine monocyte chemotactic protein-1 (MCP-1) and the non-ELR CXC chemokine interferon-gamma (IFN-gamma) inducible protein-10 (IP-10) by VSV on the gene and protein level. The expression and release of the neutrophil chemoattractants IL-8 and growth-related oncogene-alpha (GRO-alpha) remained unaffected after VSV infection. Our results indicate that the typical monocyte and lymphocyte-dominated leukocyte infiltration of virus-infected tissue is based on a selective induction of mononuclear leukocyte-attracting chemokines.

Differential mononuclear leukocyte attracting chemokine production after stimulation with active and inactivated influenza A virus.

After influenza A virus infection of human monocytes, we found a rapid and marked release of the mononuclear cell attracting chemokines MCP-1, MIP-1 alpha, and IP-10, with corresponding gene expression patterns as determined by Northern blot analysis. In striking contrast, the expression and release of the neutrophil chemoattractant IL-8 was not inducible. To determine the underlying mechanisms responsible for the induction of this differential chemokine pattern, we stimulated monocytes with UV- and heat-inactivated (56 degrees C and 100 degrees C) influenza A virus. In comparison with fully infectious influenza A, 56 degrees C-inactivated virus induced a strong production of MCP-1, MIP-1 alpha, and IP-10, while the release of MIP-1 alpha and IP-10 was substantially lower after exposure to UV-inactivated virus. No chemokine expression was found after stimulation with 100 degrees C-inactivated influenza A virus. Our data indicate that, contingent upon the chemokine examined, the maximal induction depends on the unrestricted infectivity of the virus, the unaltered hemagglutinin molecule, or the intact viral RNA. This diversified chemokine production may enable the infected host to mount an efficient antiviral response against infective and noninfective virus particles.

Hypochlorite-modified LDL: chemotactic potential and chemokine induction in human monocytes.

Within blood vessels the accumulation of monocytes/macrophages at sites of modified lipoproteins is an important feature in atherosclerosis. Recently the presence of LDL and other proteins modified by hypochlorous acid (HOCl-LDL) was demonstrated in human atherosclerotic vessels and human inflammatory kidney disease by immunohistology and protein chemistry. Chemokines contribute to a specific and directed migration of inflammatory cells. IL-8 (alpha-chemokine) attracts mainly neutrophils and distinct T-cell subsets while MCP-1 (beta-chemokine) preferentially acts on monocytes/macrophages. In the present study it was postulated that HOCl-LDL may induce and amplify inflammatory reactions by the induction of chemokine synthesis in local monocytes. After exposure of human monocytes to HOCl-LDL, it was found that mRNA and protein of the chemokine IL-8 was strongly induced, while the chemokine MCP-1 was not. HOCl-LDL itself led to a chemotactic migration of neutrophils. A chemotactic response of human monocytes toward HOCl-LDL was not detectable. We propose that HOCl-LDL may represent a form of LDL modification in the atherosclerotic process which initiates leukocyte infiltration; these mononuclear cells have been observed in the early stages of atherosclerosis.

Expression of transcription factor genes after influenza A virus infection.

Infection of human monocytes with influenza A virus induces a broad range of proinflammatory cytokines and mononuclear cell attracting chemokines before the infected cells undergo apoptosis. The underlying mechanisms by which the corresponding genes are transcriptionally initiated after virus infection are still poorly understood. Activation of NF-kappa B seems to play an important role in the regulation of many proinflammatory cytokine genes, but cannot be the only mechanism, since several cytokine genes lack respective binding sites in their promoter regions. Therefore, we additionally investigated other transcription factors of possible importance such as CREB, CTF, OTF-1, and OTF-2. To explore long-term regulatory mechanisms, we investigated the induction of transcription factors on the gene expression level which may be important to substitute for metabolized transcription factor proteins after their activation. We identified a cell-type-specific differential response: CREB, CTF, OTF-1, OFT-2, and NF-kappa B genes were strongly induced 1 to 4 hours after influenza A virus infection in the monocytic cell line Mono Mac 6, while in freshly prepared human monocytes no significant changes were detected. In infected monocytes, which die by apoptosis, the expression of CREB, CTF, and OTF-2 was rather suppressed 8 hours after infection. In conclusion, the long-term regulation of transcription factor gene expression in non-proliferating cells seems to be of minor importance after influenza infection since in apoptosisprone cells an immediate availability of transcription factor proteins is required.

Selective induction of monocyte and not neutrophil-attracting chemokines after influenza A virus infection.

It is characteristic for virus infections that monocytes/macrophages and lymphocytes infiltrate infected tissue while neutrophils are absent. To understand the mechanisms selectively attracting mononuclear cells in viral diseases, we examined in an influenza A virus model the expression and regulation of chemokines as candidate molecules responsible for the immigration of leukocytes into inflamed tissue. After influenza A virus infection of human monocytes, a rapid expression of the mononuclear cell attracting CC-chemokine genes MIP-1, MCP-1, and RANTES occurred which was followed by the release of chemokine proteins. In striking contrast to CC-chemokines, the expression of the prototype neutrophil CXC-chemoattractants IL-8 and GRO-alpha was completely suppressed after influenza A infection. The release of other neutrophil chemotactic factors was excluded by microchemotaxis assays. These results suggest that the virus-specific induction of mononuclear cell-attracting chemokines accounts for the preferential influx of mononuclear leukocytes into virus-infected tissue.