Degree Final Project defended by Enni Amanda Ramirez Vuori

January 18^th, 2024

Mycobacterium tuberculosis complex (MBC) species are capable of infecting a wide range of hosts, including sheep, which can act as reservoirs. Our objective was to characterize histologically and immunohistochemically the pulmonary granulomas caused by three species of MTC (M. bovis, M. caprae and M. tuberculosis) in sheep (Ovis orientalis aries). Twelve animals experimentally inoculated via hematogenous route were included in a retrospective study. Granulomas were classified, quantified and described using hematoxylin-eosin and Masson`s trichrome stains. Histological study of the lesions revealed multiple granulomas of different histological stages (I, II and III) distributed multifocally in the pulmonary parenchyma. Stage III granulomas were not observed in sheep infected with M. tuberculosis. Stage IV granulomas were not observed in any case. M. bovis of high pathogenicity induced a higher number of granulomas followed by M. caprae, indicating higher pathogenicity. There were differences in the composition of the predominant cell types and their arrangement in ovine lung granulomas among the MTC species studied. By Ziehl-Neelsen staining, granulomas were observed to be paubacillary (≤ 5 bacilli). Deposition and connective tissue layer formation with Mason`s trichrome was higher in those mycobacterial species that were more pathogenic. Immunohistochemical study revealed a high number of T lymphocytes (CD3+) in early granulomas (stage I and II). With the exception of highly pathogenic M. bovis, B-plasma cells (Pax 5+) are scattered in the granuloma in the early stages, clustering at the periphery as the granuloma progresses. In the sheep in this study, IFN-γ expression in lung granulomas I and II, was observed in epithelioid cells, lymphocytes and Langhans-type multinucleated giant cells, being more intense in the more pathogenic species. TNF-α expression was low in general, being more evident in type I granulomas induced by low pathogenic M. bovis and M. tuberculosis. These results contribute to understanding the early stages in the development of pulmonary tuberculosis by different strains of M. tuberculosis complex in sheep.