CD4 and major depressive disorder: For example, circulating immune cells exhibited the premature aging characteristic of not being able to generate sufficient energy.[46] Previous studies have demonstrated mitochondrial fusion dysfunction in CD4+ naive T cells in MDD patients or stressed animals.[7, 47] More importantly, the mitochondrial respiratory chain complex I activity was decreased in responders but increased in non‐responders.[48] Taken together, these findings may be useful to form an association between our antidepressant‐induced signatures with cell‐type specific responses in MDD.