TNF and atrial fibrillation: Both NP and AF cells, as well as macrophages, T cells and neutrophils secrete higher levels of pro-inflammatory cytokines such as tumor necrosis factor (TNF)-α, interferon-γ (IFN-γ), IL-1β, IL-10, IL-4, IL-6, IL-17, IL-2, IL-8, and chemokines such as C-C chemokine receptor 6 (CCR6) and C-C chemokine ligand 20 (CCL20) that is known to mediate inflammation-induced disc degeneration [47,48,49].