Mitogen responses and interferon production after exposure of human macrophages to infectious and inactivated influenza viruses

Diverse and Unexpected Roles of Human Monocytes/Macrophages in the Immune Response to Influenza Virus

Human monocytes/macrophages play a central role in the immune response and defense of the host from influenza virus infection. They classically act as antigen-presenting cells for lymphocytes in the context of an immune cell cluster. In that setting, however, monocytes/macrophages exhibit additional, unexpected, roles. They are required for influenza virus infection of the lymphocytes in the cluster, and they are responsible for lymphocyte apoptosis via their synthesis and expression of the viral neuraminidase. Surprisingly, human alveolar macrophages, expected to be among the first cells to encounter the virus, are not susceptible to direct infection by a human influenza virus but can be infected when the virus is complexed with an antibody. Such monocyte/macrophage responses to influenza virus challenge should be considered part of a very complex but quite effective defense, since the common outcome is recovery of the host with development of immunity to the challenging strain of virus.

Influenza virus activates human immunodeficiency virus type-1 gene expression in human CD4-expressing T cells through an NF-κB-dependent pathway

Influenza virus infection can cause severe complications in human immunodeficiency virus type-1 (HIV-1)-infected individuals leading to an increased risk of complications and death compared to that seen in uninfected individuals. We assessed the capacity of influenza virus (Flu) to modulate transcription of the HIV-1 long terminal repeat (LTR) in human CD4+ T cells. We found that Flu is able to promote expression of both the transiently transfected and stably integrated HIV-1 LTR-driven reporter gene. Experiments performed with Arthrobacter-derived neuraminidase and ammonium chloride revealed that Flu-dependent activation of HIV-1 transcription required an intimate contact between Flu and the target cell and efficient entry of Flu inside human CD4+ T cells. Amplification of a Flu-specific mRNA by RT-PCR indicated that human T cells were indeed productively infected with Flu. Virus preparations rendered noninfectious after UV irradiation could no longer upregulate HIV-1 LTR activity. Furthermore, experiments conducted with wild type and NF-kappaB-mutated HIV-1 LTR-directed reporter vectors suggested that the positive action of Flu on HIV-1 LTR activity was mediated through the induction of NF-kappaB. Our data show that fully competent Flu can lead to NF-kappaB-dependent activation of HIV-1 transcription in CD4+ T cells.

Human lymphocyte apoptosis after exposure to influenza A virus

Infection of humans with influenza A virus (IAV) results in a severe transient leukopenia. The goal of these studies was to analyze possible mechanisms behind this IAV-induced leukopenia with emphasis on the potential induction of apoptosis of lymphocytes by the virus. Analysis of lymphocyte subpopulations after exposure to IAV showed that a portion of CD3(+), CD4(+), CD8(+), and CD19(+) lymphocytes became apoptotic (terminal deoxynucleotidyltransferase-mediated dUTP-biotin nick end labeling positive). The percentage of cells that are infected was shown to be less than the percentage of apoptotic cells, suggesting that direct effects of cell infection by the virus cannot account fully for the high level of cell death. Removal of monocytes-macrophages after IAV exposure reduced the percent of lymphocytes that were apoptotic. Treatment of virus-exposed cultures with anti-tumor necrosis factor alpha did not reduce the percentage of lymphocytes that were apoptotic. In virus-exposed cultures treated with anti-FasL antibody, recombinant soluble human Fas, Ac-DEVD-CHO (caspase-3 inhibitor), or Z-VAD-FMK (general caspase inhibitor), apoptosis and production of the active form of caspase-3 was reduced. The apoptotic cells were Fas-high-density cells while the nonapoptotic cells expressed a low density of Fas. The present studies showed that Fas-FasL signaling plays a major role in the induction of apoptosis in lymphocytes after exposure to IAV. Since the host response to influenza virus commonly results in recovery from the infection, with residual disease uncommon, lymphocyte apoptosis likely represents a part of an overall beneficial immune response but could be a possible mechanism of disease pathogenesis.

Human macrophage responses to vaccine strains of influenza virus: synthesis of viral proteins, interleukin-1 beta, interleukin-6, tumour necrosis factor-alpha and interleukin-1 inhibitor

Interactions between influenza viruses and human macrophages were examined to detect potential mechanisms for enhanced febrile reactions previously associated with administration of an avian-human H1N1 reassortant vaccine. Cells exposed to that strain were compared with cells exposed to wild-type and cold-adapted H1H1 and H3H2 strains and an avian-human H3N2 strain. Cells exposed to the avian-human H1N1 virus showed increased synthesis of viral neuraminidase, previously reported to induce fever-producing cytokines, but no detectable increase in production of interleukin-1 beta, interleukin-6 and tumour necrosis factor-alpha measured by immunoassay, or decrease in interleukin-1 inhibitor activity by bioassay.

Early identification and retrieval or deletion of human lymphocyte subpopulations responding to influenza virus or respiratory syncytial virus challenge

Differences in immune responses of human mononuclear leukocytes (MNL) have been demonstrated following exposure in vitro to influenza virus or respiratory syncytial virus (RSV). In the current studies, we sought to identify early differences in reactive subpopulations that emerge from within the heterogeneous resting MNL pool after challenge. MNL were sham-exposed or exposed to influenza virus or RSV, separated, and retrieved by countercurrent centrifugal elutriation after 3 d. Exposure to influenza virus caused a relative decline in the number of large MNL, but an increase in small lymphocytes. Large cells that remained included primitive lymphoblasts, rare plasma cells, and typical lymphocytes of progressively greater volume. Exposure to RSV increased the number of large MNL, but diminished the number of small lymphocytes. The subpopulation of large cells consisted of atypical and large granular lymphocytes. Furthermore, deletion of the latter large, reactive lymphocytes led to abrogation of an RSV-specific proliferative response upon subsequent challenge. Thus, the specific and different subpopulations reactive after infectious virus challenge could be identified, retrieved, and manipulated without dependence on a priori, phenotypic markers.

Regulation of lymphocyte proliferation after influenza virus infection of human mononuclear leukocytes

Previous studies demonstrated that in vitro infection with influenza A viruses altered several functions of human monocytes-macrophages but did not detectably alter functions of human lymphocytes. However, both types of cells were infected, as determined by production and surface expression of viral antigens. In the current studies, human mononuclear leukocytes were infected in vitro and assayed for both influenza virus-induced proliferation and mitogen (phytohemagglutinin [PHAl)-induced proliferation, as well as for ability to stimulate proliferative responses by normal autologous leukocytes. The leukocytes showed proliferation in response to the infectious virus, but concomitant depressed proliferative responses to PHA. Coculture experiments suggested suppression of PHA-induced responses by the virus-infected cells. However, upon coculture with fresh autologous leukocytes (without PHA stimulation), both virus-infected macrophages and virus-infected lymphocytes induced autologous lymphocyte proliferative responses. Altered proliferative responses to mitogen stimulation after exposure to the virus were not due to diminished interleukin-1 production or diminished expression of HLA-DR by monocytes-macrophages. The expression of influenza virus antigens and resulting induction of autologous proliferative responses, combined with depressed mitogen-induced proliferation, may be important in human antiviral defense.

Influenza virus infection of tracheal gland cells in culture

Influenza virus-induced tracheobronchitis causes limited epithelial deciliation but markedly decreased mucociliary transport. This suggests that virus-induced alterations in airway mucus play a role in decreased mucociliary transport. Airway submucosal glands are a primary source of mucus. Therefore, we examined virus-gland cell interactions by exposing primary cultures of isolated feline tracheal gland cells to influenza A/Scotland/840/74 H3N2 virus for 1 h at a multiplicity of infection of 0.1. Virus production and release into the culture medium first occurred between 8 and 12 h postinfection and eventually reached a steady state that continued for at least 8 days. Virus which was produced and released by infected cells infected other monolayers, resulting in viral production similar to that after infection with stock virus. Hemadsorption assays conducted 24 h after infection demonstrated that most of the cells in a monolayer became infected. The infection was nonlytic according to cell morphology, trypan blue dye exclusion, and release of lactate dehydrogenase. Because lysis of a cell subpopulation could have been masked by subsequent cell division, we compared the uptake of [3H]thymidine by infected and control monolayers. There was no increase in uptake by infected monolayers. These results demonstrate that feline tracheal gland cells in primary culture undergo productive and nonlytic infection with influenza A virus. This model provides a unique system for the study of virus-gland interactions isolated from the influence of other tissues.

Interferon-induced 2-5A synthetase activity in human peripheral blood mononuclear cells after immunization with influenza virus and rubella virus vaccines

The interferon-induced enzyme 2-5A synthetase can be a sensitive indicator of activation of the human interferon system during viral infection or interferon therapy. To determine the response of the human interferon system to viral antigens, the level of 2-5A synthetase activity was monitored in peripheral blood mononuclear cells of healthy adults before and after immunization with influenza or rubella virus vaccine. The influenza virus-vaccinated individuals demonstrated increases in enzyme activity on days 1 and 11 in vivo, whereas those vaccinated with rubella virus vaccine showed an increase only on day 11. The difference in the day 1 in vivo 2-5A synthetase response in the two vaccinated groups could be demonstrated by in vitro incubations of peripheral blood mononuclear cells isolated approximately 90 days postvaccination with the two vaccines. The day 11 increase of enzyme activity in the rubella virus group showed a positive correlation with an increase in serum antibody titer, suggesting activation of the interferon system during antibody production in vivo after human exposure to virus antigens. The demonstration of increased 2-5A synthetase activity at specific times postimmunization in this investigation indicates that the interferon system is involved in the human in vivo response to virus vaccination.

Different effects of influenza virus, respiratory syncytial virus, and Sendai virus on human lymphocytes and macrophages

Influenza virus, respiratory syncytial virus, and Sendai virus depress human cell-mediated immune responses, such as mitogen-induced lymphocyte transformation, but differ in their ability to induce other immune defense mechanisms, such as interferon production. Exposure to the different viruses resulted in depressed transformation responses to the mitogen phytohemagglutinin by affecting the function of lymphocytes, or macrophages, or both cell types.

Interferon production by human mononuclear leukocytes: differences between respiratory syncytial virus and influenza viruses

The ability of respiratory syncytial virus (RSV) to induce interferon production by human mononuclear leukocytes was compared with that of influenza viruses. Cell culture fluids were assayed for interferon activity 1, 3 and 7 days after exposure to RSV or to one of two subtypes of influenza A virus (H0N1 and H3N2). RSV induced interferon production inconsistently and in low titers. Varying the multiplicity of infection did not improve the ability of RSV to induce interferon production. In contrast, influenza viruses were effective inducers of interferon production. Seropositivity to the influenza virus strains was not associated with increased interferon titers. Interferon produced after exposure to RSV or to the influenza viruses was resistant to low pH treatment. The data suggest that interferon production may not be a major component of human immunological defense against RSV infection.