Phosphorylated tau exhibits antimicrobial activity capable of neutralizing herpes simplex virus 1 infectivity in human neurons.

Tau is a microtubule-associated cytoskeletal protein, which, when hyperphosphorylated and aggregated, can result in a myriad of different tauopathies, including Alzheimer's disease (AD). We previously showed that the principal component of senile plaques, amyloid beta (Aβ), is an antimicrobial peptide capable of binding and entrapping microbial pathogens. Here we show that tau is hyperphosphorylated in neurons in response to viral infection and can neutralize herpes simplex virus 1 (HSV-1) infectivity by directly binding to viral capsids. Our data suggest that the 'pathogenic' characteristics of tau hyperphosphorylation, microtubule destabilization and aggregation are part of an antiviral response, in which tau serves as a host defense protein in the innate immune system of the brain. The combined antimicrobial activities of Aβ and phosphorylated tau resulting in Aβ plaques and neurofibrillary tangles, along with neuroinflammation, suggest that AD neuropathology may have evolved as an orchestrated innate immune host defense response to microbial infection in the brain.