Another in vivo study reported that tryptophan supplementation promoted the proliferation and activation of Treg cells, thus suppressing the infiltration of CD8+ T cells into the brain and excessive activation of microglia, thereby ameliorating LPS‐induced cognitive impairment.[50] Similar to our study, tryptophan‐rich diet alleviated inflammatory responses in brain of mouse model of chronic‐stress‐induced depression.[51] Thus, tryptophan supplementation may exert protective effect in CRD mice through multiple pathways, including the modulation on 5‐HT synthesis and neuroinflammation. Here, CD8A is linked to major depressive disorder.