The roles of extrinsic neurons in modulating neuroimmune crosstalk during acute enteric infection with Citrobacter rodentium
Authors: Tay Emmy , Cremin Michael , Reardon Colin 
Published: May 2024

Background: The intestinal tract is one of the most densely innervated organs in our body, containing extrinsic sensory and sympathetic neurons. With their cell bodies located outside of the gut, these neurons directly detect enteric pathogens and release immunomodulatory neurotransmitters or neuropeptides to communicate with immune cells in the gut. Specifically, sympathetic neurons secrete neurotransmitter norepinephrine, which activate β2 adrenergic receptors (β2ΑR) on immune cells to regulate immune functions. Recent studies from our lab, as well as others, have begun to highlight the neural regulation of immune responses during enteric infections. However, the precise pathways responsible for orchestrating these effects remain elusive. Hypothesis: We sought to determine whether extrinsic sensory and sympathetic neurons are implicated in host protective responses against enteric pathogen Citrobacter rodentium (C. rodentium). Methods: Mice were inoculated by oral gavage with C. rodentium (108 colony-forming units) or sterile luria broth. Bacterial colonization was quantified by homogenization of distal colon tissues and plating serial dilutions onto MacConkey agar. Relevant tissues were collected for qPCR, flow cytometry or histology. Genetically engineered Arc Targeted Recombination in Active Population (TRAP) mice were used to identify neurons that were activated during infection. These ArcTRAP mice received single intraperitoneal (i.p.) injection of 4-hydroxytamoxifen post-infection to “trap” active neurons, marking them tdTomato+. 6-hydroxydopamine (6OHDA) was used to chemically ablate sympathetic innervations to the colon via 3 consecutive daily i.p. injections. Results: At day 4 post-infection, there were significantly more active tdTomato+ neurons in the rostral ventrolateral medulla (RVLM) of the infected mice compared to the non-infected controls. This increase was more pronounced as the infection peaked at day 10, indicating that more neurons were being activated in the RVLM in response to C. rodentium in the gut. 6OHDA treatment significantly increased bacterial burden in distal colon at day 10 post-infection. This was associated with decreased tyrosine hydroxylase expression compared to the vehicle controls. Subsequently, qPCR revealed that there was increased pro-inflammatory cytokines expression in the colon of sympathectomized infected mice compared to the infected controls. Similarly, β2AR knockout significantly increased bacterial burden in distal colon at day 3 and 10 post-infection. Increased pro-inflammatory cytokines expression suggests an immunomodulatory role for β2AR during C. rodentium infection. Conclusions: These studies provided novel insights into the neuroimmune crosstalk that is crucial for acute enteric infections with C. rodentium. Specifically, these experiments may reveal how extrinsic neurons shape local immune responses in the gut. NIH NIAID R01AI150647; Graduate Student Support Program (UC Davis: School of Veterinary Medicine). This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.