Differential Regulation of Caspase-1 Activation, Pyroptosis, and Autophagy via Ipaf and ASC in Shigella-Infected Macrophages

TLDR

Shigella infection, the cause of bacillary dysentery, induces caspase‑1 activation and macrophage cell death, yet the underlying mechanisms remain poorly understood. Autophagy triggered by Shigella requires an intact bacterial type III secretion system but not the VirG protein, which is needed for autophagy in epithelial cells. The study demonstrates that Shigella activates caspase‑1 and IL‑1β through the NLR protein Ipaf and the adaptor ASC, with Ipaf essential for pyroptosis but ASC dispensable; caspase‑1 activation is flagellin‑independent, and Shigella‑induced autophagy—enhanced when caspase‑1 or Ipaf are absent—protects macrophages from pyroptosis, revealing a novel role for NLR proteins in bacterial‑host interactions.

Abstract

Shigella infection, the cause of bacillary dysentery, induces caspase-1 activation and cell death in macrophages, but the precise mechanisms of this activation remain poorly understood. We demonstrate here that caspase-1 activation and IL-1beta processing induced by Shigella are mediated through Ipaf, a cytosolic pattern-recognition receptor of the nucleotide-binding oligomerization domain (NOD)-like receptor (NLR) family, and the adaptor protein apoptosis-associated speck-like protein containing a C-terminal caspase recruitment domain (ASC). We also show that Ipaf was critical for pyroptosis, a specialized form of caspase-1-dependent cell death induced in macrophages by bacterial infection, whereas ASC was dispensable. Unlike that observed in Salmonella and Legionella, caspase-1 activation induced by Shigella infection was independent of flagellin. Notably, infection of macrophages with Shigella induced autophagy, which was dramatically increased by the absence of caspase-1 or Ipaf, but not ASC. Autophagy induced by Shigella required an intact bacterial type III secretion system but not VirG protein, a bacterial factor required for autophagy in epithelial-infected cells. Treatment of macrophages with 3-methyladenine, an inhibitor of autophagy, enhanced pyroptosis induced by Shigella infection, suggesting that autophagy protects infected macrophages from pyroptosis. Thus, Ipaf plays a critical role in caspase-1 activation induced by Shigella independently of flagellin. Furthermore, the absence of Ipaf or caspase-1, but not ASC, regulates pyroptosis and the induction of autophagy in Shigella-infected macrophages, providing a novel function for NLR proteins in bacterial-host interactions.