Microscopical and immunological features of tuberculoid granulomata and cavitary pulmonary tuberculosis in naturally infected goats.

Abstract

Caprine tuberculosis is caused by bacteria of the Mycobacterium tuberculosis complex (Mycobacterium bovis and Mycobacterium caprae). Although typical tuberculoid granulomata are usually observed in the lungs and lymph nodes of infected goats, the presence of cavitary lesions with exuberant mycobacterial growth is also a common feature in this species. The aim of this study was to characterize the immunological mechanisms that lead to liquefaction and cavity formation by comparing granulomata and cavitary lesions. Samples from animals positive by skin testing were collected for microscopical and immunohistochemical examination. Samples were also collected for analysis of cytokine gene expression in the lesions by real time polymerase chain reaction. There were marked differences between granulomata and cavitary lesions. In cavitary lesions there was a substantial population of neutrophils and a significant decrease in the number of CD4(+) T cells, with concomitant increases in other T-cell populations (CD8(+) and cells expressing the γδ form of the T-cell receptor). The enzyme iNOS was strongly expressed by macrophages in the cavitary lesions. There was no difference in the balance of pro- and anti-inflammatory cytokine gene expression in the lesions. These findings suggest that cavitary lesions are reactivation sites, where conditions are optimal for Mycobacterium proliferation and that immunological mechanisms may underlie the severe destruction of lung tissue that characterizes the cavitary pathology.