CD8A and major depressive disorder: Dysregulation of the hypothalamic-pituitary-adrenal axis is a hallmark of depressive disorder, which results in the hypersecretion of cortisol and exerts inhibitory effects on the immune system by suppressing the cellular immune response and increasing inflammatory cytokines [48] A previous study investigated the specific alterations in the lymphocyte subsets of patients with MDD and found that the level of CD8+ T cells in these patients was higher than that in the healthy control population [49].