TNF and major depressive disorder: Characterization of microglia in mouse models of depression showed that microglial cells adopt an activated phenotype expressing surface markers such as major histocompatibility complex (MHC) class II, CD86 or CD54 (Wachholz et al., 2016) and producing pro-inflammatory cytokines [e.g., IL-1β, IL-6, tumor necrosis factor α (TNFα)] (Chabry et al., 2015), toxic molecules (e.g., nitric oxide) or extracellular vesicles, such as exosomes, which might be responsible for propagating inflammatory signals throughout the brain (Fruhbeis et al., 2013).