Inflammasome components coordinate autophagy and pyroptosis as macrophage responses to infection

When microbes contaminate the macrophage cytoplasm, leukocytes undergo a proinflammatory death that is initiated by nucleotide-binding-domain-, leucine-rich-repeat-containing proteins (NLR proteins) that bind and activate caspase-1. We report that these inflammasome components also regulate autophagy, a vesicular pathway to eliminate cytosolic debris. In response to infection with flagellate Legionella pneumophila, C57BL/6J mouse macrophages equipped with caspase-1 and the NLR proteins NAIP5 and NLRC4 stimulated autophagosome turnover. A second trigger of inflammasome assembly, K(+) efflux, also rapidly activated autophagy in macrophages that produced caspase-1. Autophagy protects infected macrophages from pyroptosis, since caspase-1-dependent cell death occurred more frequently when autophagy was dampened pharmacologically by either 3-methyladenine or an inhibitor of the Atg4 protease. Accordingly, in addition to coordinating pyroptosis, both (pro-) caspase-1 protein and NLR components of inflammasomes equip macrophages to recruit autophagy, a disposal pathway that raises the threshold of contaminants necessary to trigger proinflammatory leukocyte death.

An exciting development in the innate-immunity field is the recognition that macrophages enlist autophagy to protect their cytoplasm from infection. Nutrient deprivation has long been known to induce autophagy; how infection triggers this disposal pathway is an active area of research. Autophagy is encountered by many of the intracellular pathogens that are known to trigger pyroptosis, an inflammatory cell death initiated when nucleotide-binding-domain-, leucine-rich-repeat-containing proteins (NLR proteins) activate caspase-1 within inflammasome complexes. Therefore, we tested the hypothesis that NLR proteins and caspase-1 also coordinate autophagy as a barrier to cytosolic infection. By exploiting classical bacterial and mouse genetics and kinetic assays of autophagy, we demonstrate for the first time that, when confronted with cytosolic contamination, primary mouse macrophages rely not only on the NLR proteins NAIP5 and NLRC4 but also on (pro-)caspase-1 protein to mount a rapid autophagic response that wards off proinflammatory cell death.