Early Inflammation Dysregulates Neuronal Circuit Formation In Vivo via Upregulation of IL-1β

Neuroimmune interaction during development is strongly implicated in the pathogenesis of neurodevelopmental disorders, but the mechanisms that cause neuronal circuit dysregulation are not well understood. We performed in vivo imaging of the developing retinotectal system in the larval zebrafish to characterize the effects of immune system activation on refinement of an archetypal sensory processing circuit. Acute inflammatory insult induced hyperdynamic remodeling of developing retinal axons in larval fish and increased axon arbor elaboration over days. Using calcium imaging in GCaMP6s transgenic fish, we showed that these morphologic changes were accompanied by a shift toward decreased visual acuity in tectal cells. This finding was supported by poorer performance in a visually guided behavioral task. We further found that the pro-inflammatory cytokine, interleukin-1β (IL-1β), is upregulated by the inflammatory insult, and that downregulation of IL-1β abrogated the effects of inflammation on axonal dynamics and growth. Moreover, baseline branching of the retinal ganglion cell arbors in IL-1β morphant animals was significantly different from that in control larvae, and their performance in a predation assay was impaired, indicating a role for this cytokine in normal neuronal development. This work establishes a simple and powerful non-mammalian model of developmental immune activation and demonstrates a role for IL-1β in mediating the pathologic effects of inflammation on neuronal circuit development.SIGNIFICANCE STATEMENT Maternal immune activation can increase the risk of neurodevelopmental disorders in offspring; however, the mechanisms involved are not fully understood. Using a non-mammalian vertebrate model of developmental immune activation, we show that even brief activation of inflammatory pathways has immediate and long-term effects on the arborization of axons, and that these morphologic changes have functional and behavioral consequences. Finally, we show that the pro-inflammatory cytokine IL-1β plays an essential role in both the effects of inflammation on circuit formation and normal axonal development. Our data add to a growing body of evidence supporting epidemiological studies linking immune activation to neurodevelopmental disorders, and help shed light on the molecular and cellular processes that contribute to the etiology of these disorders.