Pulmonary immune responses during primary Mycobacterium bovis Calmette-Guerin bacillus infection in C57Bl/6 mice

Mechanisms of protective immunity to mycobacterial infection in the lung re main poorly defined. In this study, T-cell subset expansion and cytokine ex pression in bronchoalveolar spaces, lung parenchyma. and mediastinal lymph nodes of mice infected intratracheally with Mycobacterium bovis-Calmette-Gu erin bacillus (BCG) were analyzed in parallel with histopathology and bacte rial burden. M. bovis-BCG was cleared rapidly from bronchoalveolar spaces w ithout evidence for persistence. In lung parenchyma bacteria grew during th e first 4 wk followed by gradual clearance with less than 0.1% of the origi nal inoculum persisting for more than 8 mo. Clearance of M. bovis-BCG from bronchoalveolar lavage was associated with recruitment of both neutrophils and lymphocytes. Lung CD4(+), CD8(+), and gamma delta T-cell receptor-posit ive T cells expanded maximally by Week 3, and declined by Week 8 to control values despite bacterial persistence, Both CD4(+) and CD8(+) lung T cells produced interferon (IFN)-gamma in response to M. bovis-BCG. Four distinct pathologic states of lung parenchymal infection were noted. Early focal sub -bronchial inflammation with transmigration of cells into airways was follo wed by diffuse peribronchitis, perivasculitis, and alveolitis with activate d macrophages, lymphoblasts, and occasional giant cells. The latter stage c orresponded to maximal M. bovis-BCG growth. Resolving infection consisted o f small lymphocytes and foamy macrophages, which coincided with decreasing M. bovis-BCG colony-forming units, T-cell infiltration, and IFN-gamma expre ssion. A final quiescent phase consisted of residual lymphoid aggregates an d perivasculitis associated with persistent spontaneous IFN-gamma productio n. Bacterial dissemination to lymph node and spleen occurred by Week 4 and declined in parallel to lung. In contrast to lung, IFN-gamma secretion was detected only late despite early expansion of CD4(+) and CD8(+) T cells. By reverse transcriptase/polymerase chain reaction, IFN-gamma and interleukin (IL)-12 p40 messenger RNA (mRNA) in lung paralleled IFN-gamma protein prod uction. Tumor necrosis factor-alpha, IL-4 and IL-10 mRNA expression was not increased during M. bovis-BCG lung infection. Thus, protective immunity to M. bovis-BCG in the lung evolved differently in air space, lung, and lymph node.