Immunohistochemical characterization of type II pneumocyte proliferation after PRRSV (Type I) challenge

The study aimed to histologically and immunohistochemically characterize lung lesions after a challenge with a recently isolated PRRSV field strain in growing pigs 10 and 21 days post infection (DPI). In the first phase of the study lung lesions were evaluated microscopically on routine haematoxylin and eosin stained slides. The evaluation was performed as a blinded analysis and the lesions were scored based on the following criteria: (1) pneumocyte hypertrophy and hyperplasia, (2) septal mononuclear infiltration, (3) intraalveolar necrotic debris, (4) intraalveolar inflammatory cell accumulation and (5) perivascular inflammatory cell accumulation. For further characterization of the lung lesions, immunohistochemical stainings were performed using anti-cytokeratin, anti-Ki67, anti-TTF-1 (Thyroid Transcription Factor-1), anti-myeloid receptor (MAC387), and anti-PRRSV antibodies to identify alveolar epithelial cells, proliferating cells, type II pneumocytes, macrophages, and PRRSV antigen, respectively. The evaluation of the immunohistochemical stainings revealed that humanized anti TTF-1 antibodies can successfully identify type II pneumocytes in porcine lung tissue. Marked proliferation of these cells was confirmed by a significant (p<0.05) increase of TTF-1 positive cells in acute cases compared to the lungs of control pigs. Cytokeratin labeling marked the type I, and type II pneumocytes as well as bronchial epithelial cells, however this staining was not suitable for cell counting purposes. When the routine histological scores were compared to the number of immunohistochemically positive cells, Ki67 cell counts were found to show positive correlation (p