Non-immune cells autophage too

Non-immune cells can eliminate invading bacteria effectively by enveloping them with autophagic machinery, Japanese researchers report in the November 5 Science. "We found a new function of autophagy," coauthor Tamotsu Yoshimori at the National Institute of Genetics in Mishima-Shizuoka, Japan, told The Scientist. In a healthy, uninfected cell, autophagy breaks down damaged, atrophied, or excess cell parts to keep the cell functioning properly. Research groups worldwide over the last decade have observed autophagosome-like compartments that contained bacteria such as Rickettsia conorii and Listeria monocytogenes, hinting autophagy could act as an innate host defensive pathway. Still, the significance of this encapsulation remained unclear. "We could not say whether this compartmentalization was a mechanism induced by the bacteria or a reaction of the host cell towards the bug," Jean-Pierre Gorvel of the Centre d'Immunologie de Marseille-Luminy in France, who was not involved in Science study, told The Scientist. Using confocal microscopy, researchers found Streptococcus pyogenes [group A Streptococcus (GAS)] colocalized in HeLa cells with vacuole-like compartments bearing autophagosome-specific membrane marker LC3. The number of cells with GAS-containing LC3-positive autophagosome-like vacuoles, the area of these vacuoles, and the ratio of GAS trapped in them to total intracellular GAS increased over time after infection, with the vacuoles eventually trapping about 80% of intracellular GAS. Yoshimori and colleagues also used immunoblot analysis to observe LC3-II, a form of LC3 that correlates directly with the number of autophagosomes, and found LC3-II levels increased after infection. To see whether bacteria are killed or survive after entering the compartments, the researchers directly scored bacterial viability by counting colony-forming units in mouse embryonic fibroblasts. The mouse cells either were wildtype and had autophagosomes or lacked autophagosomes due to deficiency in Atg5. In wildtype cells, intracellular GAS died 4 hours after infection, while in Atg5-deficient cells, GAS remained viable. Using electron microscopy, researchers observed autophagosome-like vacuoles 4 hours after infection contained degraded cytosol and partially degraded GAS, features characteristic of autophagosomes fused with lysosomes. LC3 and LAMP-1, a lysosomal membrane protein, also colocalized 2 to 3 hours after infection, suggesting fusion with lysosomes after formation of the vacuoles, similar to what occurs in the standard autophagic pathway. When Yoshimori and colleagues used lysosomal protease inhibitors to mimic inhibition of lysosome function, wildtype cells had significantly more viable intracellular GAS than before, suggesting lysosomes need to fuse with autophagosomes to decrease GAS viability, said Gorvel. He added that he plans on investigating this autophagic mechanism using Brucella abortus. "The startling finding is that this newly discovered autophagic response is specifically induced by GAS invasion and that it selectively targets the bacterium, while autophagy has been considered to be non-selective in general," Yoshimori said. "Moreover, the size of the GAS-specific autophagosomes is over 10 times larger than that of standard autophagosomes. The distinctions between classical autophagy and GAS-induced autophagy suggest that the latter is a specific rescue program for counterattack against intracellular pathogens." Follow-up studies need to elucidate the mechanisms that recognize and envelop intracellular GAS and to uncover any other pathways that might be connected, said David Walker, chair of the University of Texas Medical Branch's Pathology Department, who did not participate in the study. Future investigations also could see whether this autophagic response occurs in a select few host cell types or in most, if it works against most invading organisms or only a few species, and what mechanisms invading bacteria may use to adapt to autophagy, said Ann Progulske-Fox, director of the University of Florida's Center for Molecular Microbiology. "Aspects of the innate immunity may be utilized for new and novel antimicrobial agents. These are especially needed for GAS," said Progulske-Fox, who did not participate in the study. Assessment of autophagic activity in individuals might assist prophylaxis of diseases, Yoshimori said.

Non-immune cells autophage too

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