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Rickettsial agents are sensed by pattern recognition receptors but lack pathogen-associated molecular patterns commonly observed in facultative intracellular bacteria. Due to these molecular features, the order  Rickettsiales  can be used to uncover broader principles of bacterial immunity. Here, we used the bacterium  Anaplasma phagocytophilum , the agent of human granulocytic anaplasmosis, to reveal a novel microbial surveillance system. Mechanistically, we discovered that upon  A .  phagocytophilum  infection, cytosolic phospholipase A 2  cleaves arachidonic acid from phospholipids, which is converted to the eicosanoid prostaglandin E 2  (PGE 2 ) via cyclooxygenase 2 (COX2) and the membrane associated prostaglandin E synthase-1 (mPGES-1). PGE 2 -EP3 receptor signaling leads to activation of the NLRC4 inflammasome and secretion of interleukin (IL)-1β and IL-18. Importantly, the receptor-interacting serine/threonine-protein kinase 2 (RIPK2) was identified as a major regulator of the immune response against  A .  phagocytophilum . Accordingly, mice lacking COX2 were more susceptible to  A .  phagocytophilum , had a defect in IL-18 secretion and exhibited splenomegaly and damage to the splenic architecture. Remarkably,  Salmonella -induced NLRC4 inflammasome activation was not affected by either chemical inhibition or genetic ablation of genes associated with PGE 2  biosynthesis and signaling. This divergence in immune circuitry was due to reduced levels of the PGE 2 -EP3 receptor during  Salmonella  infection when compared to  A .  phagocytophilum . Collectively, we reveal the existence of a functionally distinct NLRC4 inflammasome illustrated by the rickettsial agent  A .  phagocytophilum .

The Prostaglandin E2-EP3 Receptor Axis Regulates Anaplasma phagocytophilum-Mediated NLRC4 Inflammasome Activation