Solitary Tumours Associated with Jaagsiekte Retrovirus in Sheep are Heterogeneous and Contain Cells Expressing Markers Identifying Progenitor Cells in Lung Repair

Ovine pulmonary adenocarcinoma (OPA) is a naturally occurring lung cancer of sheep caused by jaagsiekte sheep retrovirus (JSRV). This study examines immunohistochemically solitary lung nodules considered as early OPA lesions from 11 sheep infected naturally by JSRV. All 11 neoplastic nodules exhibited features of adenocarcinoma and in four of them mesenchymal growth was also observed. Both types of lesion were labelled with antibody specific for JSRV-Env. In two cases infiltrating lymphoreticular cells also contained JSRV-Env. All tumours had a high Ki67 labelling index and variably contained cells expressing CC10 (a marker of Clara cells (CCs)), SPC (a marker of type II pneumocytes), p63 and keratin 14 (markers for stem/progenitor cells of the lung airway epithelia). Tumours with mesenchymal growth had intense expression of vimentin and desmin, weak expression of smooth muscle actin and did not express pancytokeratin and p63. Both epithelial and mesenchymal proliferations did not express the stem cell markers CD90 and CD117, but some tumour infiltrating cells expressed CD133. Solitary OPA tumours can therefore be adenocarcinomas or mixed tumours and have a heterogeneous cellular composition, containing groups of cells expressing markers that characterize local progenitor cells involved in lung repair.