TNF and major depressive disorder: Previous studies found that plasma TNF-α level was positively correlated with cognitive impairment, depression, and disability in PD patients (Menza et al., 2010; Lindqvist et al., 2012), and it was also positively correlated with anxiety (Wang et al., 2016), age, and disease duration (Kouchaki et al., 2018; Li et al., 2018).