Interactions of pseudorabies virus with swine alveolar macrophages: effects of virus infection on cell functions

In order to assess the effect of pseudorabies virus (PRV) infection on the function of swine alveolar macrophages (AM), lung lavage cells were cultured, infected with one of six strains of PRV, and various activities were measured. Activity measurement included viability, phagocytosis of yeast, phagosome-lysosome fusion, phagocytosis of opsonized particles, and superoxide release. AM were infected with 5 x 10(-3) PFU/cell, and the comparative assessment of functions was performed at 18-20 h postinfection. Cell viability in PRV-infected cultures ranged from 79 to 94% of the viability in noninfected cultures. Phagocytosis of yeast was significantly reduced only in the AM cultures infected with the strain S-62. Phagosome-lysosome fusion was depressed in cultures infected with the strains S-62, 4892, 3816, and BUK. The phagocytosis of opsonized sheep red blood cells showed significant differences between noninfected and PRV-infected cultures in all cases except cultures infected with the strain PRV-C. The O2 release after stimulation with opsonized zymosan was significantly reduced in all the PRV-infected cultures. The effect of PRV infection on AM functions that are related to the bacterial activity of such cells suggests that PRV-induced AM dysfunction might have a role in the increased susceptibility of PRV-infected pigs to bacterial pneumonia.     

Influenza virus replication in human alveolar macrophages

Studies with animal models suggest that alveolar macrophages may be important cells in some respiratory virus infections, but little is known about the role of these cells in virus infections in man. In this study human alveolar macrophages were obtained by fibreoptic bronchoscopy and infected in vitro with a variety of influenza viruses. After infection with the NWS strain of influenza virus the haemagglutinin and nucleoprotein viral antigens were demonstrated in >90% of cells at 24 h by immunofluorescence with specific antisera. There was no cytopathic effect at this time, and no virus release was detected by plaque assay of culture fluids on MDCK cells. Alveolar macrophages were also infected with a human vaccine strain (H₁N₁) of influenza virus and with two recent isolates (H₁N₁ and H₃N₂). In each case viral nucleoprotein antigen was produced in 10–20% of the cells by 24 h postinfection, but there was no release of infectious virus. There was no cytopathic effect and the phagocytosis of IgG-coated latex beads was unimpaired 24 h after in vitro infection.     

Murine alveolar macrophages limit replication of vaccinia virus

Because of concerns about zoonotic transmission of monkeypox to humans and the bioterrorism threat posed by orthopoxviruses, there is renewed interest in probing cellular and molecular mechanisms of host defense to these pathogens. In particular, it is essential to understand viral-host interactions in the respiratory tract, which is the route of infection for smallpox and a likely route of transmission for monkeypox. In this study, we analyze functions of alveolar macrophages in poxvirus infection, using a recombinant vaccinia virus expressing firefly luciferase to quantify infection in mice and cell culture. Depletion of alveolar macrophages with liposomal clodronate worsens the overall severity of infection in mice, including greater replication and systemic dissemination of vaccinia as determined by bioluminescence imaging. Absence of alveolar macrophages increases total numbers of granulocytes and granulocytes/monocyte progenitor cells in the lungs during vaccinia infection, indicating that protective effects of alveolar macrophages may be mediated in part by reducing the host inflammation. Alveolar macrophages also limit vaccinia infection in respiratory epithelium, as shown by a co-culture model of cell lines derived from alveolar macrophages and lung epithelium. Collectively, these data demonstrate that alveolar macrophages are key determinants of host defense against local and systemic infection with poxviruses.     

Interactions between calf alveolar macrophages and parainfluenza-3 virus.

Cells washed from the lungs of freshly killed calves (lung wash cells; LWC) were cytotoxic for calf kidney (CK) target cells infected with parainfluenzavirus type 3 (Pi-3) when assayed by chromium release. LWC collected from 25 calves, including two gnotobiotic animals that had not previously been infected with Pi-3, were all cytotoxic, giving a specific chromium release between 11 and 50%. Cytotoxicity was detected at ratios of LWC to target cell as low as 5:1. The cytotoxic reaction required viable LWC, was inhibited by Pi-3 antiserum, and was not the result of virus-induced damage to the target cells. The cytotoxic cells in the LWC population were identified as alveolar macrophages from observations on glass adherence, phagocytic activity, killing by silica and fine-structural appearance. When LWC were added to CK cells or organ cultures of bovine trachea infected with Pi-3, the yield of virus was reduced for the first 2 to 3 days. However, subsequently, Pi-3 virus replicated in the LWC. Infection of LWC with Pi-3 virus reduced their cytotoxic activity. The significance of these interactions between alveolar macrophages and Pi-3 virus is discussed.     

Production and titration of African swine fever virus in porcine alveolar macrophages

The broncho-alveolar lavage of a pig (20–40 kg) contains about 1.6 × 10⁹ alveolar cells, half of which were macrophages. The number of cells in the lavage of bacille Calmette Guerin (BCG)-treated pigs increased about 4-fold. Both African swine fever virus-infected porcine alveolar macrophages and blood monocytes produced about 1000 hemadsorption units/cell, a value 10-fold larger than that obtained in virus-infected Vero cells. Porcine alveolar cells could be stored frozen and, after thawing, they could be infected with African swine fever virus, producing the same amount of virus as the unfrozen cells. With the number of alveolar macrophages obtained from a single pig it is possible to titer about 3000 virus samples with the same stock of alveolar macrophages.     

Pseudorabies virus in wild swine: a global perspective

Suid herpesvirus 1 (SuHV1, syn. Aujeszky’s disease virus [ADV] or pseudorabies virus [PrV]), which belongs to the family Herpesviridae, subfamily Alphaherpesvirinae, genus Varicellovirus is the causative agent of Aujeszky’s disease (AD, pseudorabies), a notifiable disease, that causes substantial economic losses to the swine industry in countries, where AD is present. Members of the family Suidae (true pigs) are the only natural hosts for PrV, although the virus can infect numerous other mammals including ruminants, carnivores and rodents. Despite the tremendous progress that has been made in controlling and eliminating PrV in domestic pigs, there is mounting evidence that PrV infections are more widespread in wild swine across the world than originally thought. Unfortunately, our understanding of the extent of PrV infections in these wild populations and of the threat to domestic swine is still fragmentary. This review aims at giving a global perspective on PrV infections in wild swine by scrutinizing the current state of knowledge concerning (i) the global occurrence of PrV infections in free-living populations of wild swine, e.g., wild boar and feral swine, (ii) the molecular characterization of wild swine PrV, (iii) infection characteristics of PrV in populations of wild swine, (iv) the risk of spillover infections to domestic pigs, (v) potential risk-mitigating measures, focusing on further research needs.     

Fc receptors do not mediate African swine fever virus replication in macrophages

Titration experiments in swine macrophages have shown that African swine fever virus infectivity was not enhanced in the presence of antiviral antibodies. The early viral protein synthesis and the viral DNA replication in swine macrophages infected with virus-antibody complexes were inhibited in the presence of high doses of uv-inactivated virus, which saturated specific virus receptors, but not when Fc receptors were saturated with antibodies. These results indicate that African swine fever virus does not infect swine macrophages through Fc receptors and that the normal entry pathway through virus receptors is not bypassed by the virus-antibody complexes.     

Porcine reproductive and respiratory syndrome virus modulates apoptosis during replication in alveolar macrophages

Different viruses have evolved strategies that inhibit apoptosis of the host cell early in infection and/or induce apoptosis in the host cell late in infection. In this study, it was investigated if and when porcine reproductive and respiratory syndrome virus (PRRSV) modulates apoptosis in PRRSV-infected macrophages. The PRRSV replication cycle in macrophages was completed within 12 h post-inoculation (hpi). PRRSV-infected macrophages, treated with staurosporine at 4, 5, 6 and 8 hpi, were significantly protected against staurosporine-induced apoptosis, but PRRSV-infected macrophages, treated with staurosporine at 12 hpi, were not. In contrast, starting from 12 hpi, all PRRSV-infected macrophages died by caspase-dependent apoptosis, which culminated in secondary necrosis. Treatment of PRRSV-infected macrophages with Z-Val-DL-Asp-fluoromethylketone indicated that apoptosis late in infection was not essential for efficient virus release. Anti- and pro-apoptotic activities were also observed in PRRSV-infected Marc-145 cells. In conclusion, this study shows that PRRSV stimulates anti-apoptotic pathways in macrophages early in infection and that PRRSV-infected macrophages die by apoptosis late in infection.     

Interaction of African swine fever virus with macrophages

Morphological data obtained by electron microscopy have shown that African swine fever virus adapted to VERO cells enters swine macrophages, its natural host cell, by a mechanism of receptor-mediated endocytosis. Binding studies with 3H-labeled virus and competition experiments with UV-inactivated virus have shown that the virus entry that leads to a productive infection in swine macrophages is mediated by saturable binding sites on the plasma membrane. The virus also penetrated into rabbit macrophages that do not produce infectious virus and initiated the synthesis of some early viral proteins; however, the viral replication cycle was aborted since viral DNA synthesis did not occur. The interaction of ASF virus particles with rabbit macrophages was mediated by nonsaturable binding sites, suggesting that the lack of specific receptors in these cells may be related to the absence of a productive infection. A similar abortive infection was detected in macrophages from other virus-resistant animal species.     

Porcine parvovirus: replication in and inhibition of selected cellular functions of swine alveolar macrophages and peripheral blood lymphocytes

The ability of four isolates of Porcine Parvovirus (NADL-8, NADL-2, KBSH, and Kresse) to replicate in and affect the functions of swine peripheral blood lymphocytes and alveolar macrophages was studied in vitro. V-strand and C-strand viral DNA was present in both concanavalin A- and non-treated lymphocytes as well as alveolar macrophages following infection with all four isolates. Indirect fluorescent antibody assays on swine testis cells, inoculated with cell lysates of NADL-8-infected peripheral blood lymphocytes (both concanavalin A- and non-treated) and alveolar macrophages, indicated that these immune cells supported the production of progeny virus. The quantity of viral DNA and progeny virus was dependent upon the multiplicity of infection and length of time following infection. Infection of lymphocytes and alveolar macrophages with PPV was associated with a decrease in cell viability. Peripheral blood mononuclear cells and alveolar macrophages infected with any of the four isolates demonstrated reduced lymphocyte blastogenesis and non-Fc-mediated alveolar macrophage phagocytosis, respectively.     

Pseudorabies virus infectivity for swine skin characterized in vitro

Summary The infectivity of pseudorabies virus (PrV) was demonstrated in a cell substrate derived from swine skin explant cultures designated primary porcine skin cells (^c/cSLA PPSC).^c/cSLA PPSC infected with either wild type or TK^– PrV strain Kaplan (Ka) developed typical cytopathologic changes (CPE) as early as 4 h post inoculation (p.i.). The CPE caused by PrV on^c/cSLA PPSC was specifically neutralized by covalescent swine sera. Synthesis of late viral proteins was demonstrated in PrV-infected^c/cSLA PPSC by indirect fluorescent antibody staining using monoclonal antibodies (mAbs) specific for PrV gIII. PrV induced protein synthesis was further confirmed by specific immunoprecipitation of³⁵S-methionine labeled viral polypeptides from PrV-infected^c/cSLA PPSC with PrV convalescent swine serum, PrV immune mouse serum or mAb to PrV gIII. Moreover, the virus progeny derived from^c/cSLA PPSC was shown to be infectious for MDBK cells and this infection was specifically neutralized by PrV convalescent swine serum. The capacity^c/cSLA PPSC to support a complete growth cycle of PrV and the relative ease of deriving these cells from pigs can be applied in an autologous fashion in studies of cellular immunity where the MHC needs to be matched.     

Pseudorabies virus nucleocapsid antigen for skin testing in swine

A procedure is described for the production of an effective pseudorabies virus skin test antigen consisting of nucleocapsids. The nucleocapsids were prepared by disruption of pseudorabies virus-infected pig kidney cells and nonionic detergent removal of the viral envelope. This preparation was noninfectious for pig kidney cell cultures and mice. Swine that had recovered from a pseudorabies virus infection and were injected intradermally with the nucleocapsid antigen developed a delayed type hypersensitivity response similar to that induced by inactivated virion antigen. The major advantage of the nucleocapsid antigen is that the occasional problem of seroconversion that is encountered with the virion antigen is avoided.     

The replication of virulent and attenuated strains of African swine fever virus in porcine macrophages

Summary The replication of virulent and attenuated strains of African swine fever virus (ASFV) was studied in pure cultures of swine macrophages. To ensure complete destruction of the macrophage monolayers about 50–100 times more virulent ASFV was needed than attenuated virus although both isolates could be used to establish persistently infected cultures.Interferon did not appear to influence virus yields from such cultures.Fluorescent and electron microscopy studies of infected macrophages suggested that the cycle of infection of the two isolates was different.With 3 Figures     

Granulocyte macrophage colony stimulating factor is elevated in alveolar macrophages from sheep naturally infected with maedi-visna virus and stimulates maedi-visna virus replication in macrophages in vitro

Infection by maedi-visna virus, a lentivirus of sheep, leads to chronic inflammatory reactions of various tissues. In this report we have analysed the role of specific cytokines in the disease process. A significant increase in expression of interleukin-6, interleukin-10, granulocyte macrophage-colony stimulating factor (GM-CSF) and transforming growth factor-beta1 mRNA was observed in alveolar macrophages isolated from the lungs of naturally infected animals when compared with lungs of seronegative controls. Levels of GM-CSF mRNA expression in alveolar macrophages correlated with the presence of lung lesions, but there was no correlation of interleukin-10, interleukin-6, tumour necrosis factor-alpha and transforming growth factor-beta1 mRNA levels in alveolar macrophages from animals with pulmonary lesions. In vitro investigation showed that GM-CSF in the range 0.1-10 ng/ml induced a significant increase in viral p25 production after 7 days in acutely infected blood monocyte-derived macrophages. The production of p25 peaked between 7 and 14 days exposure to 10 ng/ml of GM-CSF. Quantitative polymerase chain reaction showed that the level of viral DNA in monocyte-derived macrophages was dose-dependent following GM-CSF treatment in the range 0.1-100 ng/ml after 7 days. Viral mRNA expression was also enhanced. These findings indicate a role for GM-CSF in the pathogenesis of lymphoid interstitial pneumonia in infected animals.     

Plaque assay for African swine fever virus on swine macrophages

Summary.  A plaque assay developed to detect the infection of African Swine Fever Virus on swine macrophages is described. Plaques were generated by all of the virus isolates tested. The method is suitable not only for virus titration but also for the selection of clones in protocols for isolation/purification of recombinant viruses. Received December 28, 2001; accepted February 20, 2002 Published online April 26, 2002     

Innate immune responses to replication of porcine reproductive and respiratory syndrome virus in isolated Swine alveolar macrophages

Porcine reproductive and respiratory syndrome (PRRS) is an infectious disease caused by a positive RNA strand arterivirus. PRRS virus (PRRSV) interacts primarily with lung macrophages. Identifying the genetic components involved in host resistance/susceptibility would represent an important step forward in the design of disease control programs. In this study, alveolar macrophages derived from five commercial pig lines were used to study the innate immune response to PRRSV infection in vitro. Analysis by flow cytometry has demonstrated that bronchial alveolar lavage fluid (BALF) preparations were almost exclusively composed of alveolar macrophages and that the pigs tested were free from infection. Macrophages from the Landrace line showed significantly reduced virus replication and poor growth of PRRSV during 30 h of infection. By 72 h, PRRSV viral load was down to 2.5 log(10) TCID(50) compared with an average of 5 log(10) TCID(50) for the other breeds tested. These observations suggest that factors intrinsic to the Landrace breed may be responsible for this reduced or delayed response to PRRSV. Preliminary investigation suggests that the PRRSV coreceptor, sialoadhesin, may not be responsible for the Landrace macrophage phenotype as its abundance and localisation were comparable in all the breeds. Strikingly, we found that the reduced or delayed growth of PRRSV was temporally associated with high levels of tumor necrosis factor-alpha (TNF-alpha) and interleukin (IL)-8 mRNA accumulation and substantial reduction of secretion of IL-8, suggesting a key contributory role for cytokine synthesis and secretion during the innate immune response to PRRSV infection.     

African swine fever virus replication in porcine lymphocytes

Purified preparation of porcine lymphocytes were infected with three isolates of virulent African swine fever virus (ASFV). Electron microscopy showed the presence of small numbers of mature virus particles in degenerating cells. The titres of infective virus released were low and reached a maximum by 24 h after infection.