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

Yeast β-glucan promotes antiviral type I interferon response via dectin-1

Pseudorabies virus (PRV), an alphaherpesvirus, is a neglected zoonotic pathogen. Dectin-1 sensing of β-glucan (BG) induces trained immunity, which can possibly form a new strategy for the prevention of viral infection. However, alphaherpesvirus including PRV have received little to no investigation in the context of trained immunity. Here, we found that BG pretreatment improved the survival rate, weight loss outcomes, alleviated histological injury and decreased PRV copy number of tissues in PRV-infected mice. Type I interferons (IFNs) including IFN-α/β levels in serum were significantly increased by BG. However, these effects were abrogated in the presence of Dectin-1 antagonist. Dectin-1-mediated effect of BG was also confirmed in porcine and murine macrophages. These results suggested that BG have effects on type I IFNs with antiviral property involved in Dectin-1. In piglets, oral or injected immunization with BG and PRV vaccine could significantly elevated the level of PRV-specific IgG and type I IFNs. And it also increased the antibody levels of porcine reproductive and respiratory syndrome virus vaccine and classical swine fever vaccine that were later immunized, indicating a broad-spectrum effect on improving vaccine immunity. On the premise that the cost was greatly reducing, the immunological effect of oral was better than injection administration. Our findings highlighted that BG induced type I IFNs related antiviral effect against PRV involved in Dectin-1 and potential application value as a feed additive to help control the spread of PRV and future emerging viruses.

Interference of pseudorabies virus infection on functions of porcine granulosa cells via apoptosis modulated by MAPK signaling pathways

BACKGROUND

Pseudorabies virus (PRV) is one of the major viral pathogens leading to reproductive disorders in swine. However, little is known about the effects of PRV infection on porcine reproductive system. Ovarian granulosa cells are somatic cells surrounding oocytes in ovary and required for folliculogenesis. The present study aimed to investigate the interference of PRV on functions of porcine ovarian granulosa cells in vitro.

METHODS

Primary granulosa cells were isolated from porcine ovaries. To investigate the PRV infectivity, transmission electron microscopy (TEM) was used to check the presence of viral particles, and the expression of viral gE gene was detected by quantitative real-time PCR (qPCR) in PRV-inoculated cells. After PRV infection, cell viability was detected by MTS assay, Ki67 for proliferative status was determined by immunofluorescence assay (IFA), cell cycle and apoptosis were detected by flow cytometry, and progesterone (P4) and estradiol (E2) were determined by radioimmunoassay. The checkpoint genes of cell cycle and apoptosis-related proteins were studied by qPCR and western blotting.

RESULTS

Virus particles were observed in the nucleus and cytoplasm of PRV-infected granulosa cells by TEM imaging, and the expression of viral gE gene increased in a time-dependent manner post infection. PRV infection inhibited cell viability and blocked cell cycle at S phase in porcine granulosa cells, accompanied by decreases in expression of Ki67 protein and checkpoint genes related to S phase. Radioimmunoassay revealed decreased levels in P4 and E2, and the expressions of key steroidogenic enzymes were also down-regulated post PRV-infection. In addition, PRV induced apoptosis with an increase in Bax expression and activation of caspase 9, and the phosphorylation of JNK, ERK and p38 MAPKs were significantly up-regulated in porcine ovarian granulosa cells post PRV infection.

CONCLUSIONS

The data indicate that PRV causes infection on porcine ovarian granulosa cells and interferes the cell functions through apoptosis, and the MAPK signaling pathway is involved in the viral pathogenesis.

Feline Herpesvirus Pneumonia: Investigations Into the Pathogenesis

Feline herpesvirus type 1 (FeHV-1) is one of the etiological agents of feline respiratory disease. FeHV-1 is an epitheliotropic and cytopathic virus that mainly causes rhinitis and conjunctivitis, although pneumonia is also occasionally seen. In this study, the authors investigated the pathogenesis of FeHV-1-associated pneumonia, comparing natural cases with viral infection of tracheal ring and cell cultures in vitro, using histology, immunohistology, double immunofluorescence, and transmission electron microscopy as investigative tools. The results confirm that FeHV-1 targets both respiratory epithelial cells and pneumocytes and indicate that FeHV-1 pneumonia is the consequence of continuous cell-to-cell viral spread from the upper airways via the trachea into the lungs. They provide strong evidence that FeHV-1–infected cells die primarily via apoptosis, following loss of cell-to-cell contact, rounding, and detachment. However, virus-induced lesions in vivo are dominated by marked neutrophil infiltration and extensive necrosis with less prominent apoptosis; in the airways, the tissue necrosis can extend into the submucosa. The necrosis appears to result from virus-induced neutrophil influx and release of proteolytic enzymes, such as matrix metalloproteinase-9, from the neutrophils.

Interaction of the European genotype porcine reproductive and respiratory syndrome virus (PRRSV) with sialoadhesin (CD169/Siglec-1) inhibits alveolar macrophage phagocytosis

Porcine reproductive and respiratory syndrome virus (PRRSV) is an arterivirus that shows a restricted in vivo tropism for subsets of porcine macrophages, with alveolar macrophages being major target cells. The virus is associated with respiratory problems in pigs of all ages and is commonly isolated on farms with porcine respiratory disease complex (PRDC). Due to virus-induced macrophage death early in infection, PRRSV hampers the innate defence against pathogens in the lungs. In addition, the virus might also directly affect the antimicrobial functions of macrophages. This study examined whether interaction of European genotype PRRSV with primary alveolar macrophages (PAM) affects their phagocytic capacity. Inoculation of macrophages with both subtype I PRRSV (LV) and subtype III PRRSV (Lena) showed that the virus inhibits PAM phagocytosis. Similar results were obtained using inactivated PRRSV (LV), showing that initial interaction of the virion with the cell is sufficient to reduce phagocytosis, and that no productive infection is required. When macrophages were incubated with sialoadhesin- (Sn) or CD163-specific antibodies, two entry mediators of the virus, only Sn-specific antibodies downregulated the phagocytic capacity of PAM, indicating that interaction with Sn, but not CD163, mediates the inhibitory effect of PRRSV on phagocytosis. In conclusion, this study shows that European genotype PRRSV inhibits PAM phagocytosis in vitro, through the interaction with its internalization receptor Sn. If similar events occur in vivo, this interaction may be important in the development of PRDC, as often seen in the field.

Polymicrobial respiratory disease in pigs

Respiratory disease in pigs is common in modern pork production worldwide and is often referred to as porcine respiratory disease complex (PRDC). PRDC is polymicrobial in nature, and results from infection with various combinations of primary and secondary respiratory pathogens. As a true multifactorial disease, environmental conditions, population size, management strategies and pig-specific factors such as age and genetics also play critical roles in the outcome of PRDC. While non-infectious factors are important in the initiation and outcome of cases of PRDC, the focus of this review is on infectious factors only. There are a variety of viral and bacterial pathogens commonly associated with PRDC including porcine reproductive and respiratory syndrome virus (PRRSV), swine influenza virus (SIV), porcine circovirus type 2 (PCV2), Mycoplasma hyopneumoniae (MHYO) and Pasteurella multocida (PMULT). The pathogenesis of viral respiratory disease is typically associated with destruction of the mucocilliary apparatus and with interference and decrease of the function of pulmonary alveolar and intravascular macrophages. Bacterial pathogens often contribute to PRDC by activation of inflammation via enhanced cytokine responses. With recent advancements in pathogen detection methods, the importance of polymicrobial disease has become more evident, and identification of interactions of pathogens and their mechanisms of disease potentiation has become a topic of great interest. For example, combined infection of pigs with typically low pathogenic organisms like PCV2 and MHYO results in severe respiratory disease. Although the body of knowledge has advanced substantially in the last 15 years, much more needs to be learned about the pathogenesis and best practices for control of swine respiratory disease outbreaks caused by concurrent infection of two or more pathogens. This review discusses the latest findings on polymicrobial respiratory disease in pigs.

Evaluation of suitable reference genes for gene expression studies in porcine alveolar macrophages in response to LPS and LTA

Background

To obtain reliable quantitative real-time PCR data, normalization relative to stable housekeeping genes (HKGs) is required. However, in practice, expression levels of 'typical' housekeeping genes have been found to vary between tissues and under different experimental conditions. To date, validation studies of reference genes in pigs are relatively rare and have never been performed in porcine alveolar macrophages (AMs). In this study, expression stability of putative housekeeping genes were identified in the porcine AMs in response to the stimulation with two pathogen-associated molecular patterns (PAMPs) lipopolysaccharide (LPS) and lipoteichoic acid (LTA). Three different algorithms (geNorm, Normfinder and BestKeeper) were applied to assess the stability of HKGs.

Results

The mRNA expression stability of nine commonly used reference genes (B2M, BLM, GAPDH, HPRT1, PPIA, RPL4, SDHA, TBP and YWHAZ) was determined by qRT-PCR in AMs that were stimulated by LPS and LTA in vitro. mRNA expression levels of all genes were found to be affected by the type of stimulation and duration of the stimulation (P < 0.0001). geNorm software revealed that SDHA, B2M and RPL4 showed a high expression stability in the irrespective to the stimulation group, while SDHA, YWHAZ and RPL4 showed high stability in non-stimulated control group. In all cases, GAPDH showed the least stability in geNorm. NormFinder revealed that SDHA was the most stable gene in all the groups. Moreover, geNorm software suggested that the geometric mean of the three most stable genes would be the suitable combination for accurate normalization of gene expression study.

Conclusions

There was discrepancy in the ranking order of reference genes obtained by different analysing algorithms. In conclusion, the geometric mean of the SDHA, YWHAZ and RPL4 seemed to be the most appropriate combination of HKGs for accurate normalization of gene expression data in porcine AMs without knowing the type of bacterial pathogenic status of the animals.

Targeting the porcine immune system--particulate vaccines in the 21st century

During the last decade, the propagation of immunological knowledge describing the critical role of dendritic cells (DC) in the induction of efficacious immune responses has promoted research and development of vaccines systematically targeting DC. Based on the promise for the rational design of vaccine platforms, the current review will provide an update on particle-based vaccines of both viral and synthetic origin, giving examples of recombinant virus carriers such as adenoviruses and biodegradable particulate carriers. The viral carriers carry pathogen-associated molecular patterns (PAMP), used by the original virus for targeting DC, and are particularly efficient and versatile gene delivery vectors. Efforts in the field of synthetic vaccine carriers are focussing on decorating the particle surface with ligands for DC receptors such as heparan sulphate glycosaminoglycan structures, integrins, Siglecs, galectins, C-type lectins and toll-like receptors. The emphasis of this review will be placed on targeting the porcine immune system, but reference will be made to advances with murine and human vaccine delivery systems where information on DC targeting is available.

Experimental dual infection of specific pathogen-free pigs with porcine reproductive and respiratory syndrome virus and pseudorabies virus

Twenty 6-week-old specific pathogen-free pigs were divided into four groups. On day 0 of the experiment, PRRSV-PRV (n = 6) and PRRSV (n = 4) groups were intranasally inoculated with porcine reproductive and respiratory syndrome virus (PRRSV) (10(5.6) TCID50). On day 7, the PRRSV-PRV and PRV (n = 6) groups were intranasally inoculated with pseudorabies virus (PRV) (10(3.6) TCID50). Control pigs (n = 4) were kept as uninoculated negative controls. Half of the pigs in each group were euthanized and necropsied on day 14 or 21. Clinical signs such as depression and anorexia were observed in the PRRSV-PRV and PRV groups after inoculation with PRV. Although febrile response was observed after virus inoculations, the duration of that response was prolonged in the PRRSV-PRV group compared with the other groups. The lungs in the PRRSV-PRV group failed to collapse and were mottled or diffusely tan and red, whereas the lungs of the pigs in the other groups were grossly normal. Histopathologically, interstitial pneumonia was present in all PRRSV-inoculated pigs, but the pneumonic lesions were more severe in the PRRSV-PRV group. Mean PRRSV titres of tonsil and lung in the PRRSV-PRV group were significantly (P < 0.05) higher than that in the PRRSV group on day 21. These results indicate that dual infection with PRRSV and PRV increased clinical signs and pneumonic lesions in pigs infected with both viruses, as compared to pigs infected with PRRSV or PRV only, at least in the present experimental conditions.

Pseudorabies virus is transmitted among vaccinated conventional pigs, but not among vaccinated SPF pigs

Whereas the reproduction ratio (R) of pseudorabies virus (PRV) in vaccinated specific pathogen free (SPF) pigs without maternally derived antibodies under experimental conditions has repeatedly been shown to be significantly below 1, R in vaccinated conventional pigs in the field with maternally derived antibodies was significantly above 1. To exclude the difference in husbandry conditions as a cause for this discrepancy, we quantified and compared the transmission of PRV in both groups under identical experimental conditions. Whereas none of the SPF sentinel pigs became infected (R=0, significantly<1), all conventional sentinel pigs did become infected (R=2.5, significantly>1). Moreover, only one SPF pigs shed virus in saliva, the mean cumulative titre being almost a 100-fold less than in conventional pigs (17 pigs, P=0.003). In addition, the mean proliferation of peripheral blood lymphocytes in response to PRV antigens was significantly higher in SPF pigs than in conventional pigs at all points studied (P<0.0001). Moreover, the virus-neutralising antibody titre after vaccination was significantly higher in SPF pigs than in conventional pigs. We conclude that the discrepancy in transmission between vaccinated SPF pigs and vaccinated conventional pigs cannot be attributed to the experimental conditions.

Experimental dual infection of cesarean-derived, colostrum-deprived pigs with Mycoplasma hyopneumoniae and pseudorabies virus

To determine whether pseudorabies virus (PRV) infection increases the severity of pneumonia by Mycoplasma hyopneumoniae, 18, 10-week-old Cesarean-derived, colostrum-deprived pigs were randomly assigned to 3 groups of 6 pigs each. Pigs in groups A and C were inoculated intranasally with M. hyopneumoniae at 10-week-old. At 11-week-old, pigs in groups B and C were inoculated intranasally with PRV. All pigs were initially seronegative for M. hyopneumoniae and PRV. Three pigs of each group were euthanized at 12-week-old, and remaining pigs at 14-week-old. At necropsy, gross lesions in the lung were observed in the pigs of groups A and C. On post-inoculation-week (PIW) 2 with M. hyopneumoniae (at 12-week-old), lung lesions were recognized in one of the 3 pigs in group A and all the pigs in group C. The mean percentage of the lung lesions were 0.1% in group A and 9.8% in group C. M. hyopneumoniae was isolated from broncho-alveolar lavage fluids (BALF) of pigs in group A with titer of 10(2) to 10(3) CCU/0.2 ml and in group C with titer of 10(5) to 10(6) CCU/0.2 ml. On PIW 4 (at 14-week-old), lung lesions were observed in all the pigs in groups A and C, and the mean percentage of the lung lesions were 8.3% in group A and 17.2% in group C. M. hyopneumoniae was isolated from BALF in group A with titer of 10(4) to 10(7) CCU/0.2 ml and in group C with titer of 10(6) to 10(7) CCU/0.2 ml. PRVs were isolated from nasal swab and tissue samples in groups B and C. After inoculation, antibody against M. hyopneumoniae was detected in groups A and C, and against PRV in groups B and C. Under the present experimental conditions, PRV infection appear to have effect on the severity of experimentally induced acute mycoplasmal pneumonia in young pigs.

Immunobiology of pseudorabies (Aujeszky's disease)

Aujeszky's Disease (AD), a serious illness of pigs causing significant economic losses in the pig industry, is caused by Pseudorabies Virus (PrV). PrV belongs to the alphaherpesvirus subfamily of the herpesviruses with a double-stranded DNA genome in an enveloped capsid capable of encoding approximately 70 proteins. For disease control, vaccination with live and killed vaccines is performed. Recently, 'marked' vaccines have become available for use in eradication programs based on the differentiation between infected and vaccinated animals. PrV is also used as a viral vector for the development of multivalent vaccines. Despite the effectiveness of PrV vaccines, relatively little is known about the immune response against PrV infection. Several viral envelope glycoproteins have been shown to represent targets for antibody responses, and a number of isolated glycoproteins as well as genetically engineered proteins were able to elicit protective immunity. The nature of the cellular immune response is even less defined. Using viral mutants genetically engineered to lack specific antigens, it has been shown that glycoprotein C (gC) acts as a target for cytotoxic T-lymphocytes, and gB, gC, gD, and gH appear to be involved in stimulation of in vitro proliferation of PBMC from immune animals. In addition, gB and gC have been implicated in recognition of infected cells by lymphokine-activated killer (LAK) cells. In summary, the data indicate a prominent role for viral envelope glycoproteins in eliciting humoral and cellular immune responses in the animal host. A complicating factor is the ability of PrV to productively infect cells of the hematopoietic system, which may impair immune responses and might also play a role in persistent or latent infection.

Effect of porcine circovirus infection on porcine alveolar macrophage function

The effect of porcine circovirus (PCV) infection of porcine alveolar macrophage cultures on some of the functional properties of these cells are reported. PCV infection of alveolar macrophages did not effect their ability to phagocytose and kill complement-coated yeast cells or the expression of Fc or complement receptors. A transient increase in major histocompatibility complex (MHC) class I expression in PCV-infected cells were observed 4 days after infection and a decrease in the number of cells expressing MHC class II antigens was observed 8 days after infection. Infection of alveolar macrophages with PCV also resulted in a transient decrease in their ability to act as accessory cells in mitogen-induced lymphocyte proliferation of monocyte-depleted porcine peripheral blood mononuclear cells.

Survival of pseudorabies virus on swabs maintained under standard field sample shipping conditions

Pseudorabies virus survival was compared using three different types of applicator swabs in Eagle's minimum essential medium held under shipping conditions (packed with frozen gel packs) for up to 96 hours. Virus titer decay rates for dacron-tipped applicators were not statistically different from those of controls. Titer decay rates were statistically different from controls for cotton- and calcium alginate-tipped applicators. With the lowest input virus titer, virus was detectable up to 96, 72, or 24 hours after inoculation for dacron-, cotton-, and calcium alginate-tipped applicators, respectively. Dacron-tipped applicators were chosen to evaluate pseudorabies virus survival on tonsil swabs collected from experimentally challenged or contact control pigs to simulate field sampling and shipping conditions. Virus was still detectable in 20 of 24 swab samples after 72 hours in cell culture medium under shipping conditions.

Interaction of bovine respiratory syncytial virus with bovine alveolar macrophages in vivo: effects of virus infection upon selected cell functions

The effect of bovine respiratory syncytial virus (BRSV) upon alveolar macrophage (AM) function was investigated using an in vivo calf inoculation model. Alveolar macrophages were collected sequentially from live calves at multiple time points during the 14 day period following viral inoculation. Alveolar macrophages from bronchoalveolar lavage fluids were purified by density gradient centrifugation (> 95% AM) prior to in vitro evaluation of cell functions. There were significant but variable and inconsistent differences in the functions of AM from the BRSV inoculated calves compared to the control calves. Fc-receptor mediated phagocytosis was either increased or unchanged by BRSV inoculation. Nonopsonized phagocytosis was decreased during the early postinoculation period and later increased. There was a variable effect on AM phagosome lysosome fusion with increased fusion activity on postinoculation days 2 through 5, 7 and 12 but reduced activity on days 6 and 10. The AM respiratory burst, as measured by nitroblue tetrazolium dye reduction, was essentially unaffected with a reduction in activity on day 10 only. In this model, BRSV inoculation of calves primarily resulted in an alteration of the membrane associated phagocytic functions of the alveolar macrophages (p < 0.05).

Rapid identification and quantitation of cells infected by recombinant herpesvirus (pseudorabies virus) using a fluorescence-based beta-galactosidase assay and flow cytometry

We recently described construction and use of a beta-galactosidase expression cassette in isolating recombinant pseudorabies virus (PrV) mutants (Mettenleiter and Rauh, 1990). We report here the identification and exact quantitation of cells infected by these mutants using an assay based on the reaction of intracellular beta-galactosidase expressed during infection by the recombinant viruses with the fluorogenic substrate fluorescein di-beta-D-galactopyranoside (FDG) followed by detection of positive cells in flow cytometry (FACS-Gal assay; Nolan et al., 1988). The detection method is fast, sensitive, and reliable, and yields quantitative results on single cell basis.

Actinobacillus pleuropneumoniae culture supernatants interfere with killing of Pasteurella multocida by swine pulmonary alveolar macrophages

The effect of Actinobacillus pleuropneumoniae culture supernatant on swine pulmonary alveolar macrophage (PAM) functions was studied. The A. pleuropneumoniae culture supernatant was toxic to PAMs when tested by MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) and lactate dehydrogenase (LDH) release assays. Biological activity of the supernatant was ascribed to cytotoxins. Both the LDH and MTT assays were used for measurement of crude A. pleuropneumoniae cytotoxin concentration with good reproducibility. A preparation containing 6,800 toxic units/mL (determined by MTT assay) was used for subsequent experiments. The objective was to study the effect of crude cytotoxin on the ability of swine PAMs to kill Pasteurella multocida. Phagocytosis of opsonized P. multocida type A by PAMs was not efficient. Only 8% of incubated organisms were ingested by noncytotoxin-treated PAMs after 30 min phagocytosis. The bactericidal effect of noncytotoxin-treated PAMs only last for 60 min, after which, the rate of growth of surviving P. multocida exceeded the rate of bacterial killing by PAMs. Complete elimination of P. multocida by PAMs was not observed in this study. A total loss of ability to kill P. multocida by PAMs was seen when the PAMs were pretreated with a high concentration (340 toxic units/mL) of A. pleuropneumoniae cytotoxin. If the PAMs were pretreated with a low concentration (3.4 toxic units/mL) of cytotoxin, a significant reduction in the killing of P. multocida was still observed. The reductions in phagocytosis, phagosome-lysosome fusion (demonstrated using yeast particles of Candida albicans), and oxidative burst (demonstrated by nitro blue tetrazolium reduction (NBT) assay) may have contributed to the impaired killing of P. multocida by PAMs.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of pseudorabies virus infection upon cytotoxicity and antiviral activities of porcine alveolar macrophages

Alveolar macrophages (AM) infected with Pseudorabies virus (PRV) were compared to noninfected AM for cytotoxicity against foreign or transformed cells and production of interferon (IFN). Five PRV strains were used to infect AM including strains that are known to be highly virulent for pigs, i.e. strain 4892 and strain S-62 as well as strains that are regarded as mild or nonvirulent, i.e. BUK and Bartha. The multiplicity of infection ranged from 0.005 to 0.05 TCID50/cell. The target cells in the cytotoxicity assays were either chicken red blood cells, PRV-infected vero cells, or human myeloblastoma cells (K562 cell line). For the production of IFN, AM cultures were treated with polyinosinic:polycytidylic acid (Poly I:C) diluted in tissue culture media at a concentration of 5 micrograms/10(6) cells. Culture supernatants were collected at various times poststimulation and tested for antiviral activity using the Vesicular Stomatitis Virus replication inhibition test. Swine AM were able to lyse chicken red blood cells in an antibody-independent way but not in an antibody-dependent way, whereas lysis of PRV-infected vero cells was accomplished both ways. The cytotoxicity against chicken red blood cells was reduced in the PRV-infected AM as compared to noninfected cells, particularly in AM infected with virulent PRV strains. Specific 51Cr release values for AM infected with S-62 and 4892 strains were 14 and 19, while the noninfected AM had values of 36. Similarly, in the antibody-dependent cytotoxicity assay against PRV-infected vero cells there was no activity of AM against K562 cells. The production of IFN was readily stimulated with Poly I:C. The optimal time for supernatant collection was between 12 and 16 h poststimulation. The antiviral activity was abrogated by treatment of the supernatant with antiserum against human leukocyte IFN; it was therefore considered to be due to interferon-alpha (IFN alpha) released from the macrophages. The antiviral activity present in supernatants of PRV-infected AM was reduced compared to noninfected AM. The difference between AM cultures infected with virulent strains of PRV and noninfected AM cultures was statistically significant at P < or = 0.025. The results provide support to the premise that the role of AM in lung defense can be compromised by PRV infection.

Replication and immunosuppressive effects of Pseudorabies virus on swine peripheral blood mononuclear cells

The infectivity and potential immunosuppressive effects of Pseudorabies virus (PRV) was evaluated in swine peripheral blood mononuclear cells (PBMC). Virus progeny titers and viral DNA synthesis at various intervals post-inoculation revealed the replication of PRV in both peripheral blood monocytes and lymphocytes; however, replication in lymphocytes was restricted compared with monocytes. PRV infection resulted in the damage and death of monocytes. Although PRV did not appear to affect the viability of the lymphocytes, PRV infection suppressed lymphocyte functions such as proliferation and interleukin-2 (IL-2) synthesis in response to Concanavalin A. This immunosuppression was dependent upon the multiplicity of infection (MOI) of infectious PRV. UV-inactivated PRV was not immunosuppressive. There was no effect of PRV on natural killer (NK) cell activity. The reduction of lymphocyte proliferation by PRV was not reversible by the addition of supernatant containing porcine IL-2 and non-infected monocytes to the infected cultures. The results from these in vitro studies demonstrate that PRV can infect and cause immunosuppressive effects on swine PBMC. These effects may explain the potential role of PRV in predisposing infected pigs to secondary infection and support the hypothesis that PRV can spread systemically by infected PBMC in blood and lymph.

Study of the potential involvement of pseudorabies virus in swine respiratory disease

In order to investigate the potential involvement of pseudorabies virus (PRV) in swine respiratory disease, nine week old pigs were intranasally inoculated with the PRV strain 4892. Two doses of infection were used: 10(4.5) median tissue culture infectious doses (TCID50)/pig and 10(3.5) TCID50/pig, with ten pigs per group. In the group of pigs inoculated with 10(4.5) TCID50, seven out of ten pigs died within six days after inoculation. The mortality rate in the group of pigs inoculated with the lower dose was only two out of ten and, there were several pigs in this group that showed signs of respiratory distress besides some mild nervous signs. Pseudorabies virus was isolated from various tissues collected postmortem, including alveolar macrophages. Virus localization in tissues was also detected by in situ hybridization. The histopathological examination of the respiratory tract tissues revealed a pathological process that was progressing from mild pneumonia to severe suppurative bronchopneumonia. The isolation of virus from alveolar macrophages provides support to the hypothesis that replication of PRV during the course of infection produces an impairment of the defense mechanisms in the respiratory tract.