Gardnerella vaginalis triggers NLRP3 inflammasome recruitment in THP-1 monocytes

Gardnerella vaginalis is a Gram-positive bacterium associated with bacterial vaginosis (BV), pelvic inflammatory disease, and preterm birth. BV is the most prevalent vaginal dysbiosis in women of childbearing age characterized by the absence of normal lactobacilli and an overgrowth of G. vaginalis and other bacteria. Although mucosal fluids from BV patients exhibit increases in proinflammatory cytokines and Toll-like receptor 2 and 4 mRNA, G. vaginalis has not been demonstrated to directly induce an inflammatory response. This study tested the hypothesis that G. vaginalis induces an inflammatory response in the human monocyte cell line, THP-1. The objectives of the study were to measure proinflammatory cytokine production, molecular mechanisms by which cytokines are produced, and whether G. vaginalis results in death of the monocytic cells. We found that G. vaginalis induced significant increases in the inflammasome-dependent cytokines IL-1β, IL-18, as well as TNF-α in treated cells. G. vaginalis caused significant cell death by 24h post-treatment compared with untreated controls, but cells remained 66% viable. Caspase-1 cleavage in treated cells confirmed the inflammatory cell death, and NLRP3 knockdown confirmed its involvement through reduction of IL-1β secretion. Using a stably expressing YFP-ASC THP-1 cell model with immunofluorescent staining, YFP-ASC colocalized with NLRP3 in G. vaginalis-treated cells and the addition of a caspase-1 inhibitor wholly ameliorated IL-1β secretion. Our study provides new insight into the role of G. vaginalis in inflammatory conditions in the genital tract.