Tumor growth factor β1 (TGF-β1) and tumor necrosis factor-alpha (TNF-α) are cytokines that exert opposite effects on microglia: while TGF-β1 has an anti-inflammatory effect [59], under pathological conditions like AD, TNF-α is chronically released by activated microglia, neurons, and astrocytes, and, in a vicious cycle, increased levels of extracellular Aβ stimulate its release [60]. Here, TNF is linked to Alzheimer disease.