Borrelia burgdorferi induces changes in the physical forces and immunity signaling pathways of endothelial cells early but not late during in vitro exposure

Borrelia burgdorferi (Bb) is the causative bacterial agent of Lyme disease which is on the rise in the USA and Europe. Once transmitted to the human host via an arthropod vector, Bb spreads through tissues by traveling along and through the vasculature, whose inner lumen is lined by a monolayer of endothelial cells (ECs). What are the precise mechanisms that Bb employs to achieve efficient dissemination through the vasculature is not well understood. Could extracellular Bb induce alterations in the EC force generation machinery, promoting its own spread by potentially weakening the EC barrier integrity? Using videomicroscopy coupled with traction force and monolayer stress microscopy, we monitored the response of ECs to Bb exposure and found a sharp, transient increase in EC traction and intercellular stresses, followed by a prolonged decrease in EC traction and monolayer stresses up to 15 hours post-exposure. After this point though all variables return to levels similar to those observed for never-exposed ECs. Using RNA-sequencing to better understand the underlying biochemical mechanisms involved, we discovered an upregulation of EC innate immune signaling pathways during early but not late exposure to Bb. In contrast to exposure of ECs to live Bb, when we exposed ECs to heat-inactivated Bb, we evidenced no sharp increase in traction forces at early exposure, neither reversal at late exposure, but found a sustained weakening of physical forces all throughout the exposure. Thus, we discovered a differential response of EC physical force generation and innate immune signaling to live versus heat-inactivated Bb. This indicates a tight interplay between the two and is suggestive of an active modulation of both processes that might be key in establishing infection.