TNF and major depressive disorder: Gardnerella vaginalis (GV), an opportunistic pathogen in the vagina, causes vaginitis-, osteoporosis-, and depression-like symptoms through the activation of receptor activator of nuclear factor κB (RANK)/RANK ligand (RANKL)- and TNF/TNF receptor (TNFR)-mediated NF-κB signaling [14,15,16].