Role of viral proteins and concanavalin A in in vitro replication of pseudorabies virus in porcine peripheral blood mononuclear cells

Pseudorabies virus (PRV) strain with defects in gE, gC, and TK genes protects piglets against an emerging PRV variant

The prevalence of an emerging variant of the pseudorabies virus (PRV) has been causing serious losses to farmers in China. Moreover, the commercially available PRV vaccine often fails to provide thorough protection. Therefore, in this study, we generated a PRV-∆gC\gE∆TK strain with defects in gC, gE, and TK of PRV. Compared to the parental PRV strain and the single gene deletion strains (PRV-∆gC, PRV-∆gE, and PRV-∆TK), PRV-∆gC\gE∆TK grew slowly, and exhibited fewer and smaller plaques on swine testis (ST) cells. Furthermore, animal experiment results showed that mice that were immunized intramuscularly with PRV-∆gC\gE∆TK, survived throughout the experiment with no observed clinical symptoms, and were completely protected against PRV challenge. Additionally, deletion of the gC, gE, and TK genes significantly alleviated viral damage in the brain. Furthermore, one-day-old weaned piglets immunized intramuscularly with PRV-∆gC\gE∆TK elicited higher levels of gB antibodies against both the emerging PRV variant and the parental PRV, exhibited full protection against challenge with both variants, and showed neutralization capacity against PRV. These data suggest that PRV-∆gC\gE∆TK is a promising vaccine candidate for the control of pseudorabies.

Quantification and molecular characterization of the feline leukemia virus A receptor

Virus receptors and their expression patterns on the cell surface determine the cell tropism of the virus, host susceptibility and the pathogenesis of the infection. Feline thiamine transport protein 1 (fTHTR1) has been identified as the receptor for feline leukemia virus (FeLV) A. The goal of the present study was to develop a quantitative, TaqMan real-time PCR assay to investigate fTHTR1 mRNA expression in tissues of uninfected and FeLV-infected cats, cats of different ages, in tumor tissues and leukocyte subsets. Moreover, the receptor was molecularly characterized in different feline species. fTHTR1 mRNA expression was detected in all 30 feline tissues investigated, oral mucosa scrapings and blood. Importantly, identification of significant differences in fTHTR1 expression relied on normalization with an appropriate reference gene. The lowest levels were found in the blood, whereas high levels were measured in the oral mucosa, salivary glands and the musculature. In the blood, T lymphocytes showed significantly higher fTHTR1 mRNA expression levels than neutrophil granulocytes. In vitro activation of peripheral blood mononuclear cells with concanavalin A alone or followed by interleukin-2 led to a transient increase of fTHTR1 mRNA expression. In the blood, but not in the examined tissues, FeLV-infected cats tended to have lower fTHTR1 mRNA levels than uninfected cats. The fTHTR1 mRNA levels were not significantly different between tissues with lymphomas and the corresponding non-neoplastic tissues. fTHTR1 was highly conserved among different feline species (Iberian lynx, Asiatic and Indian lion, European wildcat, jaguarundi, domestic cat). In conclusion, while ubiquitous fTHTR1 mRNA expression corresponded to the broad target tissue range of FeLV, particularly high fTHTR1 levels were found at sites of virus entry and shedding. The differential susceptibility of different species to FeLV could not be attributed to variations in the fTHTR1 sequence.

Development of a Conditionally Replicating Pseudorabies Virus for HER-2/neu-overexpressing Bladder Cancer Therapy

Overexpression of the HER-2/neu oncogene, a frequent molecular event in a variety of cancers including bladder cancer, is associated with tumor progression and poor prognosis. Therapeutic strategies to targeting HER-2/neu-overexpressing cancer cells have shown promise. Pseudorabies virus (PrV), a herpesvirus of swine, may be exploited as an oncolytic agent for human cancer. Herein, we generated a conditionally replicating glycoprotein E-defective PrV mutant carrying glycoprotein D and herpes simplex virus type 1 thymidine kinase genes, which are essential for viral entry and replication, under the transcriptional control of the HER-2/neu promoter. The recombinant PrV, designated YP2, selectively replicated in and lysed HER-2/neu-overexpressing human bladder, mouse bladder, and hamster oral cancer cells in vitro. Notably, YP2 retarded MBT-2 bladder tumor growth in mice by more than 50% and more than half of the mice survived for over 50 days, whereas all the control mice survived less than 30 days. Taken together, our results suggest that YP2 may have therapeutic potential for the treatment of invasive bladder cancer. Furthermore, because HER-2/neu is overexpressed in a broad spectrum of cancers, this conditionally replicating PrV may be broadly applicable.

Immune escape of equine herpesvirus 1 and other herpesviruses of veterinary importance

Equine herpesvirus (EHV)-1 is a pathogen of horses, well known for its ability to induce abortion and nervous system disorders. Clinical signs may occur despite the presence of a virus-specific immune response in the horse. The current review will summarize the research, on how, EHV-1-infected cells can hide from recognition by the immune system. Research findings on immune evasion of EHV-1 will be compared with those of other herpesviruses of veterinary importance.

A simple selection system for construction of recombinant gD-negative pseudorabies virus as a vaccine vector

We describe a simple, efficient two-step method for construction of glycoprotein D (gD)-negative pseudorabies virus (PrV) carrying transgenes inserted in place of the gD gene. The first step was the use of the thymidine kinase (TK) gene of herpes simplex virus (HSV) for insertional inactivation of the gD gene in a PrV mutant deficient in both TK and glycoprotein E (gE). The gD-negative, HSV-TK-positive mutant could be selected in HAT medium. The second step was substitution of HSV-TK with other genes of interest. The resultant gD/gE/TK-negative mutant was easily isolated by acyclovir selection. The expression of the transgene was detectable in vivo and the antibody responses against both inserted antigens and PrV were induced. The protective efficacy of the gD/gE/TK-negative PrV against lethal PrV challenge was also demonstrated. This PrV mutant carrying immunogenic proteins from unrelated porcine pathogens may be tested as a multivalent vaccine candidate for swine.

Suppression of the proliferation of mouse splenocytes by pseudorabies virus

Pseudorabies virus (PRV) infection in resistant swine caused immunosuppression which sometimes resulted in secondary infection by other viruses or bacteria. However the mechanism of the immunosuppression is not well understood. In this study, the effect of PRV on the immune system was examined in the mouse model. Splenocytes or lymphocytes prepared from the spleen of BALB/c mice were incubated in vitro with mitogen, and the ability of cells to proliferation was measured. When the cells were incubated with PRV, the ability of cells to proliferate was inhibited, although PRV did not multiply in the lymphocytes. UV-inactivated PRV also suppressed the proliferation of mice splenocyte. This result suggests that the structural component of PRV virion might cause the immunosuppression.

Time course of the porcine cellular and humoral immune responses in vivo against pseudorabies virus after inoculation and challenge: significance of in vitro antigenic restimulation

We investigated the time course of porcine cellular and humoral immune responses against pseudorabies virus (PRV) after pigs were inoculated with PRV gE(-) mutant strain M141 and challenged with wild-type virus NIA-3. Peripheral blood mononuclear cells (PBMC) were isolated from blood samples; half were used directly and half were restimulated with PRV in vitro before use in a cytolytic assay. We determined time course and extent of PRV-specific lymphoproliferative and cytolytic response. In addition, serum samples were examined for neutralizing antibodies. After inoculation, the frequency of various lymphocyte subsets in peripheral blood was determined by FACScan. One week after inoculation, T-lymphocytes proliferated abundantly and a B-lymphocyte response was observed. When PBMC were used directly without restimulation, only 15% of the PRV-infected target cells were lysed, and about 15-20% of uninfected target cells were lysed. In contrast, when PBMC were restimulated with PRV, up to 50% of the PRV-infected target cells were lysed while only 30% of the uninfected target cells were lysed. The frequency of various T-lymphocyte subsets in the circulation did not change significantly after inoculation, which indicates that the number of PRV-specific lymphocytes in circulation was very small. After challenge, the T-lymphocyte response was enhanced, but the B-lymphocyte response was not. When PBMC were used directly, only 20% of the PRV-infected and uninfected target cells were lysed after challenge. In contrast, when PBMC were restimulated with PRV, they again lysed more PRV-infected target cells than uninfected target cells. Cytolytic cells were detected for a longer period after challenge than after inoculation. Since it was only possible to clearly detect cytolysis after lymphocytes were restimulated with PRV, it may be that they do not preferentially localize in blood or that they are too few in blood to be detected without further antigenic restimulation in vitro. These lymphocytes may instead localize in other tissues, such as mucosal tissues, tonsils and draining lymph nodes. Whether such a reservoir of PRV-specific cytolytic cells is important in clearing the virus is still unknown. In this study we demonstrated PRV-specific lymphocytes in circulation after they were restimulated in vitro with PRV.

Study of the persistence of Aujeszky's disease (pseudorabies) virus in peripheral blood mononuclear cells and tissues of experimentally infected pigs

The presence of Aujeszky's disease virus (ADV) in peripheral blood mononuclear cells (PBMC) and tissues of experimentally infected pigs was studied. Vaccinated and unvaccinated pigs were inoculated with different doses of Aujeszky's disease NIA-3 strain. Pigs were periodically bled and PBMC were used for virus isolation and PCR detection of virus. Tissues were obtained at the time of death (8 weeks post-inoculation) and used for ADV genome detection by PCR. ADV genome was amplified from PBMC during the acute phase of infection and, in some experimental groups, up to 38 days post-inoculation (PI). The virus was sporadically detected by virus isolation performed from PBMC. In neural tissues, ADV was constantly amplified from the trigeminal ganglia and the olfactory bulb of persistently infected pigs (euthanized 8 weeks PI). In other tissues, the viral genome was rarely detected in lymph nodes and tonsils, and occasionally, in the bone marrow. Our results indicated that PBMC are not an appropriate source for detecting ADV persistence, since inconsistent results were obtained throughout the experiments. In neural tissues, the olfactory bulb turned out to be as important a target for ADV persistence as the trigeminal ganglia. Viral genome detection in the bone marrow indicated that this tissue may play a role in the establishment of a persistent infection.

Sequences of the ribonucleotide reductase-encoding genes of felid herpesvirus 1 and molecular phylogenetic analysis

The felid herpesvirus 1 (FHV-1) genes encoding the two ribonucleotide reductase (RR) subunits (RR1, large subunit and RR2, small subunit) were cloned and their nucleotide (nt) sequence determined. The RR1 open reading frame (ORF) is 2358 nts long and is predicted to encode a protein of 786 amino acids (aa). In common with herpesviruses in the Varicellovirus genus of the alphaherpesvirus subfamily, FHV-1 RR1 lacks the N-terminal serine threonine protein kinase region present in herpes simplex virus (HSV)-1 and -2. FHV-1 RR1 has a predicted aa identity of 47-64% with other alphaherpesvirus RR1 peptides, falling to 26-29% for gammaherpesviruses. The RR2 ORF is 996 nts long, predicted to encode a protein of 332 aa and has aa identities of 64-70% with alphaherpesviruses and 38-39% with gammaherpesviruses. Molecular phylogenetic analysis groups FHV-1 with equid herpesviruses 1 and 4 (EHV 1 and 4), pseudorabies virus (PRV) and bovid herpesvirus 1 (BHV 1) within the genus Varicellovirus.

The influence of maternal immunity on the development of the in vitro lymphocyte proliferation response against pseudorabies virus in pigs

In this study, the influence of maternal immunity against pseudorabies virus (PRV) on the development of a PRV-specific T-cell response in pigs was investigated. Pigs with or without maternal immunity were challenged by inoculation with wild-type PRV, or were vaccinated with 783 and subsequently inoculated. Peripheral blood lymphocytes, collected after vaccination and/or inoculation, were used for PRV-induced lymphocyte proliferation assays in vitro as a measure of a PRV-specific T-cell response. In unvaccinated pigs, the presence of maternal immunity did not inhibit the development of a T-cell response after inoculation with PRV. In contrast, maternal immunity did inhibit the development of a PRV-induced response after intramuscular vaccination. Moreover, vaccinated pigs without maternal immunity shed virus for a shorter period after inoculation than vaccinated pigs with maternal immunity. The greater T-cell response induced by the vaccine might have contributed to the more rapid clearance of PRV in these pigs.

Glycoprotein D-negative pseudorabies virus can spread transneuronally via direct neuron-to-neuron transmission in its natural host, the pig, but not after additional inactivation of gE or gI

Envelope glycoprotein D (gD) is essential for entry of pseudorabies virus (PRV) into cells but is not required for the subsequent steps in virus replication. Phenotypically complemented gD mutants can infect cells and can spread, both in vitro and in mice, by direct cell-to-cell transmission. Progeny virions released by infected cells are noninfectious because they lack gD. The aim of this study was to determine the role of gD in the neuropathogenicity of PRV in its natural host, the pig. We investigated whether gD-negative PRV can spread transneuronally via synaptically linked neurons of the olfactory and trigeminal routes. High doses of a phenotypically complemented gD mutant and gD mutants that are unable to express either gI or gI plus gE were inoculated intranasally in 3- to 5-week-old pigs. Compared with the wild-type virus, the virulence of the gD mutant was reduced. However, pigs inoculated with the gD mutant still developed fever and respiratory signs. Additional inactivation of either gI or gI plus gE further decreased virulence for pigs. Immunohistochemical examination of infected pigs showed that a PRV gD mutant could replicate and spread transneuronally into the central nervous system (CNS). Compared with the wild-type virus, the gD mutant had infected fewer neurons of the CNS on day 2. Nevertheless, on day 3, the gD-negative PRV had infected more neurons and viral antigens were present in second- and third-order neurons in the olfactory bulb, brain stem, and medulla oblongata. In contrast, gD mutants which are unable to express either gI or gI plus gE infected a limited number of first-order neurons in the olfactory epithelium and in the trigeminal ganglion and did not spread transneuronally or infect the CNS. Thus, transsynaptic spread of PRV in pigs can occur independently of gD. Possible mechanisms of transsynaptic transport of PRV are discussed.