Background: The acute phase of mycobacterial lung infection is characterized by a nearly exponential outgrowth of mycobacteria in the alveolar airspace and lung parenchymal tissue, suggesting insufficient early protective immunity against mycobacterial challenge. In the current study, we tested the hypothesis that a CC chemokine ligand 2 (CCL2)-dependent increased mononuclear phagocyte subset accumulation in distal airspaces would improve the lungs' protective immunity to infection with Mycobacterium bovis bacille Calmette-Guérin (hereafter, "M. bovis BCG").
Methods: Wild-type mice and CCL2-overexpressing mice that exhibited increased pools of alveolar and lung mononuclear phagocytes-due to the lung-specific overexpression of human CCL2 in type-II alveolar epithelial cells-were infected intratracheally with M. bovis BCG and the developing lung inflammatory response was analyzed.
Results: CCL2-overexpressing mice demonstrated significantly decreased mycobacterial loads in the bronchoalveolar space, lung parenchymal tissue, and spleen compared with wild-type mice, when both groups of mice were infected with M. bovis BCG. Moreover, in M. bovis BCG-infected mice, later-developing, accelerated resolution of lung granuloma formation was noted, particularly in CCL2-overexpressing mice as compared with wild-type mice. In addition, CCL2-overexpressing mice demonstrated an increased trafficking of mycobacteria-loaded dendritic cells towards lung-draining lymph nodes that was found to coincide with increased mycobacterial loads in this compartment.
Conclusions: The data of the current study suggest that CCL2-dependent amplification of endogenous host-defense programs in the lung may improve the lungs' protective immunity against mycobacterial infections.