From the article:
Evidence in mice suggests that the entry of a virus anywhere in the bloodstream turns on “first responder” immune cells called CX3CR1highLY6Clow monocytes, which then release the inflammatory signaling protein TNF-α. According to the authors of the study, TNF-α then travels to the brain where it blocks the formation of nerve cell connections needed to turn sensory information into memories.
Researchers also measured the levels of pro-inflammatory signaling proteins (cytokines) in mice at several time points after the injection of poly(I:C), and found a larger, longer-lasting increase in levels of TNF-α than in other cytokines. Given their findings, the team guessed that the impact of systemic immune response on brain cell connections was executed through TNF-α signaling. Indeed, mice engineered to lack TNF-α signals in white blood cells saw neither a drop in dendritic spine formation nor in motor learning ability when exposed to the viral mimetic.