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ABSTRACTAs one of the first lines of host defense, monocytes play important roles in clearing infected microbes. The defensive response is triggered by recognition of diverse microbial moieties, including released factors, which modulate host immune responses to establish a harsh environment for clinically important bacterial pathogens. In this study, we found that the expression of PTX3, a soluble form of pattern recognition receptor, was induced by infection with live Pseudomonas aeruginosaor treatment of cells with its supernatant. P. aeruginosaGroEL, a homolog of heat shock protein 60, was identified as one of the factors responsible for inducing the expression of PTX3in host cells. GroEL induced PTX3expression by activating the Toll-like receptor 4 (TLR4)-dependent pathway via nuclear factor-kappa B (NF-κB), while simultaneously inhibiting expression of microRNA-9, which targets the PTX3transcript. Finally, by acting as an opsonin, GroEL-induced PTX3 promoted the association and phagocytosis of Staphylococcus aureusinto macrophages. These data suggest that the host defensive environment is supported by the production of PTX3 in response to GroEL, which thus has therapeutic potential for clearance of bacterial infections.