TNF and major depressive disorder: Three studies reported significant differences in inflammatory protein concentrations between MS participants with and without depression for IL-6 in serum (Ibrahim and Afifi, 2012; Koutsouraki et al., 2011), and IL-9,30 TNF-α (Andlauer et al., 2019) and CRP (Andlauer et al., 2019) in both serum and CSF (see Table 3).