IL6 and major depressive disorder: In the present study, following the evidence of neuroaxonal injury in major depressive disorder, NfL as a biomarker of neuroaxonal injury in various neurological diseases, and a positive association between NfL and proinflammatory cytokines, we examined the levels of NfL and proinflammatory cytokines, including TNF-α and IL-6, and assessed the cognitive function in patients with major depressive disorder.