Peripheral virus infection-induced microglial and neuronal alterations

Abstract

Infection with influenza A viruses (IAV) may lead to seasonal epidemics among all age groups mainly affecting bronchial epithelial cells of the upper respiratory tract. A viral infection is initially sensed by components of the innate immune system, leading to a swift production of pyrogenic cytokines to mount an anti-viral immune response. However, systemic inflammation is accompanied by fever and further results in altered behavioral routines, such as reduced ap-petite and social isolation, implying a disturbed brain homeostasis that may precede neurolog-ical disorders. Previous experimental studies reported about dysregulation of microglial cells, increased expression of inflammatory cytokines, and compromised hippocampal neuronal mor-phology causing impaired cognition in mice upon influenza infection. However, characteriza-tion of microglia activation and underlying processes of behavioral alterations remained un-addressed. Utilizing a murine infection model with a non-neurotropic IAV strain that mimics the disease progression in humans, this study demonstrates that an infection-induced peripheral immune response is trailed by a temporally disturbed expression of blood-brain barrier-associ-ated proteins. Although histological examination of brains from IAV-infected mice displayed no noticeable pathological effects, in-depth characterization of microglial cells via flow cytom-etry highlighted an increased surface expression of major histocompatibility complex classes I and II, cluster of differentiation (CD) 80, and F4/80 upon infection. Gene expression analysis of lysosomal proteins, scavenger receptors, and complement factors further deciphered an in-creased capacity of microglia-mediated synaptic pruning in IAV-infected mice. Finally, a novel multiplexed flow cytometry-based approach was established and applied to scrutinize previ-ously described neurological alterations. Notably, quantitative analysis of synaptosomes iso-lated from cortex and hippocampus revealed a significant reduction of the vesicular glutamate transporter 1 (VGLUT1), thereby highlighting a dysbalance in excitatory neurotransmission in IAV-infected mice. In summary, the present study highlights the disturbed CNS homeostasis indicated by distinct microglia activation and altered glutamatergic neurotransmission as a re-sult of the peripheral infection-induced systemic inflammation, further underlining the im-portance of the body-brain axis.