Long-term persistence of BCG Pasteur in lungs of C57BL/6 mice following intranasal infection.

Different Mycobacterium bovis Bacillus Calmette-Guérin (BCG) vaccine substrains may vary in their efficacy. Here, we describe differences in disease progression and pathology in the lungs of female C57BL/6 mice infected intranasally with BCG Russia or BCG Pasteur and followed for 17 months. The lungs were investigated for bacillary load, histopathology and expression of cytosolic and secreted proteins by immunohistochemistry. BCG Russia was cleared from the lungs by 8 months. BCG Pasteur reached a low-level persistence at 8 months and remained at this level until the end of the experiment. BCG Pasteur induced greater pathology than BCG Russia, and there were more macrophage and lymphocyte infiltrates in animals infected with BCG Pasteur (P < 0.05). Bacterial growth correlated with cellular infiltration. All selected mycobacterial proteins were found to be expressed in the lesions by both BCG strains, and there were only minor variations between the strains. Furthermore, we identified isolated cells containing a high mycobacterial protein load in the normal-looking lung parenchyma. The infected cells in the healthy areas of the lung may represent the ability of mycobacteria to evade immune activation and thereby persist in the host. Clearance of BCG Russia indicates that a more effective and sterilizing immune response is established by this strain. On the other hand, the ability of BCG Pasteur to persist could be important for long-term protection against challenge with Mycobacterium tuberculosis.