Interaction of pseudorabies virus with immortalized porcine B cells: influence on surface class I and II major histocompatibility complex and immunoglobulin M expression

Cytolytic function for pseudorabies virus-stimulated porcine CD4+ CD8dull+ lymphocytes

We previously observed that pseudorabies (PRV) virus-specific killing in vitro was mediated by CD6+ CD8+ lymphocytes. Also a high percentage of CD4+ lymphocytes, among these CD6+ CD8+ lymphocytes, was observed. The purpose of this study was, therefore, to further characterize the killing ability of PRV-stimulated CD4+ CD8+ lymphocytes. Peripheral blood mononuclear cells (PBMC) were isolated from blood of PRV-immune pigs and were stimulated in vitro with PRV. After 6 days, the frequency of CD4+ CD8+ lymphocytes in peripheral blood was determined by flow cytometry analyses. Lymphocytes were separated using a magnet-activated cell sorter or a FACSVantage SE, and the cytolytic activity of the isolated populations was determined. Flow cytometry analyses demonstrated that PRV stimulation of immune PBMC resulted in the occurrence of 26% +/- 4% CD4+ CD8dull+ lymphocytes. We further demonstrated that killing by PRV-stimulated PBMC was mediated by CD4+ CD8dull+ T lymphocytes and CD4- CD8+ T lymphocytes (classic cytolytic T lymphocytes and natural killer cells). The CD4+ CD8dull+ T lymphocytes showed major histocompatibility complex (MHC) II-restricted PRV-specific killing. The CD4- CD8+ T lymphocytes showed both PRV-specific and natural killing. The CD4+ CD8dull+ lymphocytes, which are unique in the pig, seemed to have a more heterogeneous function than was earlier demonstrated. In conclusion, we demonstrated that PRV-specific CD4+ CD8dull+ lymphocytes are able to kill PRV-infected target cells in a MHC II-restricted manner.

Effects of a porcine reproductive and respiratory syndrome virus infection on the development of the immune response against pseudorabies virus

The aim of this study was to investigate the effects of a porcine reproductive and respiratory syndrome virus (PRRSV) infection on the development of the immune response after pseudorabies virus (PRV) vaccination in pigs. Pigs were intranasally inoculated with the European PRRSV strain, Lelystad virus ter Huurne, and were vaccinated intramuscularly with PRV 2 weeks later (LV-PRV group). Control pigs were vaccinated with PRV only (PRV group). Eight weeks after PRV vaccination, pigs from both groups were challenged intranasally with wild-type PRV. We measured the lymphoproliferative, and the cytolytic responses to PRV of peripheral blood mononuclear cells (PBMC), isolated from blood samples. In addition, serum samples were examined for antibodies against PRV and LV. One week after PRV vaccination, PBMC proliferated abundantly to PRV in both groups. However, in the LV-PRV group the lymphoproliferative response declined after 1 week, whereas, in the PRV group, the lymphoproliferative response was high for 3 weeks and declined thereafter (P<0.05). After challenge, the lymphoproliferative response was 1 week earlier and was consistently and significantly higher in the PRV group than in the LV-PRV group. The PRV-specific killing was higher at 3 weeks after PRV vaccination and 5 weeks after PRV challenge 19+/-3 and 24+/-6%, respectively, in the PRV group, compared to 7+/-4 and 6+/-9%, respectively, in the LV-PRV group (P<0.05). However, later after vaccination and challenge the cytolytic response was identical in both groups. The antibody titre against PRV developed equally in both groups. After challenge, no PRV virus was isolated from both groups. From these results we conclude that, although PRRSV infection did cause changes in the time course of the T-lymphocyte response after PRV vaccination, PRRSV infection did not inhibit the development of vaccine-induced protection after PRV.

Discrimination of different subsets of cytolytic cells in pseudorabies virus-immune and naive pigs

We previously observed that pseudorabies virus (PRV)-induced, cell-mediated cytolysis in pigs includes killing by natural killer (NK) cells. We also observed that IL-2 stimulation in vitro of naive PBMC expands porcine NK cells. The purpose of this study was to compare the phenotypes of the cytolytic subsets stimulated in vitro by PRV and by IL-2. PBMC were isolated from blood of PRV-immune and naive pigs and stimulated in vitro with PRV or IL-2. After 6 days, the frequency of various lymphocyte subsets in these cultured PBMC was determined by flow cytometry: the cells were separated with a magnet-activated cell sorter and the cytolytic activity of the separated populations was determined. When lymphocytes were separated and analysed with FACScan, the following lymphocyte subsets were discriminated: CD6(+) CD8(bright+) CD4(-) (CTL phenotype), CD6(+) CD8(dull+) CD4(+) (the fraction containing memory T helper cells), CD6(+) CD8(-) CD4(+) (T helper cell phenotype), CD6(-) CD8(dull+) CD4(-) gammadelta-T(+) ( gammadelta-T cell phenotype), CD6(-) CD8(dull+) CD4(-) gammadelta-T(-) (NK phenotype) and CD6(-) CD8(-) CD4(-) gammadelta-T(-) or gammadelta-T(+). Flow cytometry analysis demonstrated that PRV stimulation of immune PBMC resulted in the occurrence of more CD6(+) CD8(+) and CD4(+) CD8(+) and fewer CD6(-) CD8(+) and gammadelta-T(+) CD8(+) lymphocytes than IL-2 stimulation of naive PBMC (P<0.05). It was demonstrated further that killing by PRV-stimulated PBMC was mediated mainly by CD6(+) CD8(+) T lymphocytes. Killing by IL-2-stimulated PBMC was mediated mainly by CD6(-) CD8(+) T lymphocytes. These results demonstrate that both natural killing and killing by classical PRV-specific CTL were detected in PRV-immune pigs, whereas IL-2 stimulation of PBMC isolated from naive pigs mainly induced natural killing.

An early pseudorabies virus protein down-regulates porcine MHC class I expression by inhibition of transporter associated with antigen processing (TAP)

The objectives of this study were to identify the mechanism(s) of pseudorabies virus (PrV)-induced down-regulation of porcine class I molecules and the viral protein(s) responsible for the effect. The ability of PrV to interfere with the peptide transport activity of TAP was determined by an in vitro transport assay. In this assay, porcine kidney (PK-15) cells were permeabilized with streptolysin-O and incubated with a library of 125I-labeled peptides having consensus motifs for glycosylation in the endoplasmic reticulum (ER). The efficiency of transport of peptides from the cytosol into the ER was determined by adsorbing the ER-glycosylated peptides onto Con A-coupled Sepharose beads. Dose-dependent inhibition of TAP activity was observed in PrV-infected PK-15 cells. This inhibition, which occurred as early as 2 h postinfection (h.p.i.), reached the maximum level by 6 h.p.i., indicating that TAP inhibition is one of the mechanisms by which PrV down-regulates porcine class I molecules. Infection of cells with PrV in the presence of metabolic inhibitors revealed that cycloheximide a protein synthesis inhibitor, but not phosphonoacetic acid a herpesvirus DNA synthesis inhibitor, could restore the cell surface expression of class I molecules, indicating that late proteins are not responsible for the down-regulation. Infection in the presence of cycloheximide followed by actinomycin-D, which results in accumulation of the immediate-early protein, failed to down-regulate class I, indicating that one or more early proteins are responsible for the down-regulation of class I molecules.

Life beyond eradication: veterinary viruses in basic science

To some, the focus of research in virology entails the search for solutions of practical problems. By definition then, attention is limited to those viruses that cause disease or to exploitation of some aspect of virology to a practical end (e.g., antiviral drugs or vaccines). Once a disease is cured, or the agent eradicated, it is time to move on to something else. To others, virology offers the opportunity to study fundamental problems in biology. Work on these problems may offer no obvious practical justification; it is an affliction of the terminally curious, perhaps with the outside hope that something "useful" will come of it. To do this so-called "basic science", one must find the most tractable system to solve the problem, not the system that has "relevance" to disease. I have found that veterinary viruses offer a variety of opportunities to study relevant problems at the fundamental level. To illustrate this point, I describe some recent experiments in my laboratory using pseudorabies virus (PRV), a swine herpesvirus.

Differential regulation of Dk and Kk major histocompatibility complex class I proteins on the cell surface after infection of murine cells by pseudorabies virus

After pseudorabies virus (PRV) infection of murine L929 cells, the cell surface expression of major histocompatibility complex (MHC) class I proteins changes such that the total amount of MHC class I molecules remains relatively constant but the levels of the individual alleles Dk and Kk vary. This is an active process involving at least three PRV gene products that act in an allele-specific manner such that cell surface expression of MHC class I Dk is decreased and that of Kk is increased. Our results indicate that an early gene product mediates the overall reduction in Dk protein and a late gene product which is mutant in the attenuated PRV strain Bartha mediates the increase in Kk protein. We provide additional evidence for a third gene product involved in the regulation of the synthesis of both the Dk and Kk proteins. In addition, we show that the early decrease in the Dk protein is not due to a block in synthesis or processing of the complex through the secretory system.

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.

Establishment and characterization of porcine cytolytic cell lines and clones

Although non-major-histocompatibility-complex-restricted cytolytic cells appear to significantly influence antiviral immunity in pigs, the phenotype and functional characteristics of these cells are not well defined. To allow a detailed analysis of these subsets, we established and characterized cell lines and clones of interleukin-2-activated (IL-2) cytolytic cells. Cell lines and clones were obtained from peripheral blood mononuclear cells of minipigs of the swine-leucocyte-antigen-complex (SLA) d/d haplotype. Cells were cultured in the presence of human recombinant IL-2 and cloned by double limiting dilution in the presence of gamma-irradiated L14 cells (a retrovirus immortalized B-lymphoblastoid cell line of the haplotype SLAd/d) or gamma-irradiated autologous peripheral blood mononuclear cells as feeder cells. Cytolytic cell lines and clones were characterized for their ability to kill different target cells and for their cell surface phenotype. All obtained clones expressed CD2 and CD8 and were negative for CD4. The following three subsets of cytolytic cells were identified: Subset 1) CD3- CD5- cells that killed K562 cells (a natural killer cell susceptible target cell line), as well as the pseudorabies virus (PRV)-infected or uninfected porcine kidney cells. These cells were considered to be typical natural killer cells. Subset 2) CD3 gamma/delta + CD5- T-cells that killed K562 cells and PRV virus-infected or uninfected porcine kidney cells, infected or uninfected L14 cells, and L14 cells constitutively expressing the PRV viral glycoprotein gB or gC. These cells were considered to be gamma/delta T-cells with natural killer activity. Subset 3) CD3 alpha/beta + CD5+ T-cells that killed L14 cells, PRV-infected L14 cells, and PRV gB- and gC-transfected L14 cells. These cells were possibly induced by the L14 feeder cells, used in the in vitro culture system. None of the cytolytic effector cells killed only MHC-matched viral infected cells. In conclusion, we describe a method to isolate, clone, and culture cytolytic cells from pigs. The clones could be cultured for 5 months, which allowed appropriate phenotypic and functional characterization of the various clones. Two of the subsets, CD3 gamma/delta T- and the natural killer cell subset may be involved in antiviral immunity in this species.