Two Modes of the Axonal Interferon Response Limit Alphaherpesvirus Neuroinvasion

Infection by alphaherpesviruses, including herpes simplex virus (HSV) and pseudorabies virus (PRV), typically begins at epithelial surfaces and continues into the peripheral nervous system (PNS). Inflammatory responses are induced at the infected peripheral site prior to invasion of the PNS. When the peripheral tissue is first infected, only the innervating axons are exposed to this inflammatory milieu, which includes the interferons (IFNs). The fundamental question is how do PNS cell bodies respond to these distant, potentially damaging events experienced by axons. Using compartmented cultures that physically separate neuron axons from cell bodies, we found that pretreating isolated axons with beta interferon (IFN-β) or gamma interferon (IFN-γ) significantly diminished the number of herpes simplex virus 1 (HSV-1) and PRV particles moving in axons toward the cell bodies in a receptor-dependent manner. Exposing axons to IFN-β induced STAT1 phosphorylation (p-STAT1) only in axons, while exposure of axons to IFN-γ induced p-STAT1 accumulation in distant cell body nuclei. Blocking transcription in cell bodies eliminated antiviral effects induced by IFN-γ, but not those induced by IFN-β. Proteomic analysis of IFN-β- or IFN-γ-treated axons identified several differentially regulated proteins. Therefore, unlike treatment with IFN-γ, IFN-β induces a noncanonical, local antiviral response in axons. The activation of a local IFN response in axons represents a new paradigm for cytokine control of neuroinvasion.

Neurons are highly polarized cells with long axonal processes that connect to distant targets. PNS axons that innervate peripheral tissues are exposed to various situations that follow infection, inflammation, and damage of the tissue. After viral infection in the periphery, axons represent potential front-line barriers to PNS infection and damage. Indeed, most viral infections do not spread to the PNS, yet the mechanisms responsible are not well studied. We devised an experimental system to study how axons respond to inflammatory cytokines that would be produced by infected tissues. We found that axons respond differentially to type I and type II interferons. The response to type I interferon (IFN-β) is a rapid axon-only response. The response to type II interferon (IFN-γ) involves long-distance signaling to the PNS cell body. These responses to two interferons erect an efficient and rapid barrier to PNS infection.