IDO1 and obesity due to melanocortin 4 receptor deficiency: Leptin is involved in neuroimmune interactions that link obesity, inflammation, and mood disorders, as it stimulates the release of proinflammatory cytokines such as IL-6, TNF-α, and IFN-γ, which are well-established activators of IDO1, thereby increasing the degradation of Trp through the KP [71].