In fact, many studies have also shown the presence of AI-2 in humans, particularly in inflammatory bowel disease patients and major depressive disorder patients (Raut et al., 2013; Fu et al., 2020; Medina-Rodriguez et al., 2020), and suggested that AI-2 directly promotes Th17-cell differentiation and that fecal IL-17A levels are increased in patients with major depressive disorder (Medina-Rodriguez et al., 2020). This evidence concerns the gene IL17A and major depressive disorder.