TNF and neurodegenerative disease: The impact of systemic infection on the progression of neurodegenerative diseases has been documented (Arai et al. 2006; Combrinck et al. 2002; Panitch 1994), and several circulating cytokines (interferon-γ and TNF-α) (Perry et al. 2003; Qin et al. 2007) have been proposed to be important mediators.