Fate of Legionella pneumophila in macrophages of C57BL/6 chronic granulomatous disease mice

Colony types and virulence traits of Legionella feeleii determined by exopolysaccharide materials

Legionella feeleii is a Gram-negative pathogenic bacterium that causes Pontiac fever and pneumonia in humans. When L. feeleii serogroup 1 (ATCC 35072) was cultured on BCYE agar plates, two types of colonies were observed and exhibited differences in color, opacity and morphology. Since the two colony types are white rugose and brown translucent, they were termed as white rugose L. feeleii (WRLf) and brown translucent L. feeleii (BTLf), respectively. They exhibited different growth capacities in BYE broth in vitro, and it was found that WRLf could transform to BTLf. Under the electron microscope, it was observed that WRLf secreted materials which could be stained with ruthenium red, which was absent in BTLf. When U937 macrophages and HeLa cells were infected with the bacteria, WRLf manifested stronger internalization ability than BTLf. Intracellular growth in murine macrophages and Acanthamoeba cells was affected by the level of initial phagocytosis. WRLf was more resistant to human serum bactericidal action than BTLf. After being inoculated to guinea pigs, both organisms caused fever in the animals. These results suggest that ruthenium red-stained materials secreted in the surroundings may play a crucial role in determining L. feeleii colony morphology and virulence traits.

SQSTM1/p62/A170 regulates the severity of Legionella pneumophila pneumonia by modulating inflammasome activity

Sequestosome1/A170/p62 (SQSTM1) is a scaffold multifunctional protein involved in several cellular events, such as signal transduction, cell survival, cell death, and inflammation. SQSTM1 expression by macrophages is induced in response to environmental stresses; however, its role in macrophage-mediated host responses to environmental stimuli, such as infectious pathogens, remains unclear. In this study, we investigated the role of SQSTM1 in host responses to Legionella pneumophila, an intra-cellular pathogen that infects macrophages, in both an SQSTM1-deficient (SQSTM1(-/-) ) mouse model and macrophages from these mice. Compared with wild-type (WT) macrophages, the production and secretion of the proinflammatory cytokine IL-1β was significantly enhanced in SQSTM1(-/-) macrophages after infection with L. pneumophila. Inflammasome activity, indicated by the level of IL-18 and caspase-1 activity, was also elevated in SQSTM1(-/-) macrophages after infection with L. pneumophila. SQSTM1 may interact with nucleotide-binding oligomerization domain-like receptor family, caspase recruitment domain-containing 4 and nucleotide-binding oligomerization domain like receptor family, pyrin domain containing 3 proteins to inhibit their self-dimerization. Acute pulmonary inflammation induced by L. pneumophila and silica was enhanced in SQSTM1(-/-) mice with an increase in IL-1β levels in the bronchoalveolar lavage fluids. These findings suggest that SQSTM1 is a negative regulator of acute pulmonary inflammation, possibly by regulating inflammasome activity and subsequent proinflammatory cytokine production.

Differences in protein synthesis between wild type and intracellular growth-deficient strains of Legionella pneumophila in U937 and Acanthamoeba polyphaga

An important aspect of Legionnaires' disease is the growth of the causative agent, Legionella pneumophila, within infected host cells. Many proteins including stress proteins of L. pneumophila were strongly induced in a wild type strain that had been used to infect U937 human macrophage-like cells. In contrast, the expression of the proteins was much weaker within a protozoan host, Acanthamoeba polyphaga. The results suggested that active bacterial protein synthesis is required more within macrophages than within protozoa for adaptation of L. pneumophila to intracellular environments. The synthesis of these proteins was not observed in intracellular growth-deficient strains after infection in either type of host cells. The inability of protein synthesis in these strains is correlated with their inability of intracellular growth. Furthermore, on U937 infection, the synthesis of beta-galactosidase encoded in an inducible reporter construct immediately ceased in the in intracellular growth-deficient strains after infection, while the wild type strain was able to synthesize it during the course of infection. These results suggested that the intracellular growth of Legionella pneumophila within macrophages requires active protein synthesis from an earlier stage of bacterial infection.