Phenotypic analysis of lymphocyte populations in the lungs and regional lymphoid tissue of sheep naturally infected with maedi visna virus.

We have analysed the phenotype of lymphocytes in lung and regional lymph node of symptomatic and asymptomatic sheep infected with the ovine lentivirus, maedi visna virus (MVV). Compared to equivalent tissues from age-matched, non-infected controls, MVV-infected sheep show increased numbers of lymphocytes in the lung, both in the bronchus-associated lymphoid tissue (BALT) and in the alveolar septae. Both CD8+ and CD4+ T lymphocyte numbers in alveolar septae were increased, particularly in animals with clinical respiratory disease. The ratio of CD8+ to CD4+ lymphocytes was similar to that in normal lung. In both MVV-infected and uninfected animals a high proportion of pulmonary lymphocytes, particularly in the alveolar septae, did not express the CD5 antigen, suggesting that they were activated. The number of activated cells was higher in infected sheep. Variable numbers of alveolar macrophages containing MVV-core protein were present in alveolar lumina, the majority of positive cells showing morphological evidence of activation. In regional lymphoid tissue there were increased numbers of CD8+ and gamma delta expressing T cells in lymphoid follicles and germinal centres of infected animals. The specificity of these cells is unknown and we could find no evidence for the presence of cells productively infected with the virus in these structures. This study shows that activated T lymphocytes, particularly of the CD8 subset, play a major part in the pathogenesis of MVV-induced pulmonary and regional lymph node lesions.