Immunohistochemical staining of IFN-γ positive cells in porcine reproductive and respiratory syndrome virus-infected lungs

Paraffin-embedded lungs were obtained from a previous porcine reproductive and respiratory syndrome virus (PRRSV)-challenged experiment involving three groups: an uninfected control group, a low virulence (LV, Resp PRRSV/Repro™)-infected group, and a high virulence (HV, VR-2385)-infected group. Tissues were collected at 3, 7, 10, 14 or 28 days post-inoculation (DPI) (n=5). Lungs were examined to detect IFN-γ positive cells by immunohistochemical staining using polyclonal antibodies to IFN-γ. The microscopic lung lesions induced by the HV group were more severe than those in the LV group. A significant increase in number of lymphocytes in the HV group was observed at 10 DPI (24.90±9.79%), 14 DPI (22.00±11.47%) and 28 DPI (28.95±15.11%) (P<0.05). A relative decrease in macrophage numbers was observed and correlated well with the increase in lymphocyte numbers when the disease progressed. IFN-γ positive cells were demonstrated in both lymphocytes and macrophages, particularly pulmonary alveolar macrophages. A significant increase in IFN-γ positive cells was found at 7 DPI (15.90±13.65%), 10 DPI (46.95±13.79%), 14 DPI (10.90±5.13%) and 28 DPI (13.40±4.89%) in the HV group (P<0.05). The results suggested that the increase in IFN-γ positive cells in the HV group correlated well with the severity of the lung lesions, which may be because of the presence of PRRSV in the lung.