Herpes simplex virus and kallikrein-related peptidases - a functional connection in Alzheimer's disease.

BACKGROUND: The viral hypothesis of Alzheimer's disease (AD) proposes that neurotropic pathogens may interact with amyloid and tau pathology, but the evidence remains controversial. We investigated whether cerebrospinal fluid (CSF) kallikrein-related peptidase 6 (KLK6) is associated with AD biomarkers and with an intrathecal anti-HSV immune response in patients with AD, and whether HSV-1 infection alters KLK6 expression in cell culture models. METHODS: CSF KLKs and AD biomarkers were analyzed in patients with AD. HSV-1 serostatus and anti-HSV-1/2 CSF-to-serum antibody index (HSV-AI) were used as markers of previous or ongoing intrathecal anti-HSV immune response. In vitro, HSV-1 infection and KLK6 knockdown were examined in human cell models. RESULTS: CSF-KLK6 correlated with tau and amyloid biomarkers. In HSV-1-seropositive patients, HSV-AI was associated with CSF-KLK6, although the effect size was modest and the association was strongly context-dependent on the Aβ42/40 ratio. In vitro, productive HSV-1 infection induced and stabilized KLK6 protein expression, and KLK6 knockdown reduced HSV-1 protein expression and infectious progeny in HaCaT cells. CONCLUSIONS: KLK6 is associated with AD biomarkers and with an intrathecal anti-HSV immune response in a subset of patients with AD. The in vitro data support a functional relationship between HSV-1 infection and KLK6, but the clinical findings are associative and do not demonstrate ongoing CNS HSV-1 replication or causality. These findings warrant independent validation. However, this research provides further evidence for a role of HSV-1 in the pathophysiology of AD.