Direct cytolytic activity of pbmc from pigs infected with porcine reproductive and respiratory syndrome

Abstract - Porcine reproductive and respiratory syndrome is characterised by reproductive failure in sows and respiratory distress in pigs of all ages. The causative agent of the disease, a positive strand RNA virus named porcine reproductive and respiratory virus (PRRSV), primarily infects and destroys alveolar macrophages of the pig. Earlier, we demonstrated that an increased number of lymphocytes with a cytolytic phenotype are found in the lungs of PRRSV infected pigs. The aim of the present study was to investigate whether similar changes in leukocyte populations could be detected in the circulation of PRRSV infected pigs and correlate these findings with possible changes of leukocyte function. Either one-week-old gnotobiotic pigs or 8-week-old specific pathogen free (SPF) minipigs were inoculated intra-nasally with the LV Ter Huurne PRRSV strain or culture supernatant as a control. After several days post-infection, heparin blood was collected and the peripheral blood mononuclear cells (PBMC) were isolated. The cells were stained with monoclonal antibodies directed against lymphocyte surface antigens, analysed by flow cytometry, and a K562 lysis assay was performed. Analysis of PBMC of PRRSV infected gnotobiotic pigs showed an increase in the percentage of CD2 ⁺CD3 ^- T-cells in comparison with control pigs. These cells were mainly CD8 ⁺CD4 ^-, CD8 ⁺CD6 ^- and CD8 ⁺ -TCR ^-. In SPF minipigs a strong decrease in the percentage of CD4 ⁺ cells was observed. At day 7 post-infection a strong direct cytolytic response against K562 cells was detected in PBMC from PRRSV infected SPF minipigs which was significantly different from PBMC of control piglets. This response lasted for several weeks of infection. From these results it can be concluded that during a PRRSV infection a direct cytolytic activity of PBMC can be detected. This may be due to the increase in the percentage of CD8 ⁺ cells that was observed in the circulation. Our findings agree with the earlier finding that during PRRSV infection, an influx of cytolytic cells in the lungs occurs. Further investigation will focus on elucidating whether this cytolytic response is protective during PRRSV infection.