Mild traumatic brain injury augments innate immune responses through neurokinin and cholinergic signaling
Pneumonia is the second leading cause of disability-adjusted life-years lost worldwide and the eighth leading cause of death in the United States. Traumatic brain injury (TBI) patients have classically been considered immunosuppressed, but recent research reported that mild head trauma patients have reduced incidence of pneumonia compared to blunt trauma patients. Using our mild TBI model followed by bacterial pneumonia, we investigated the effect of neuronal signaling on innate immune function. To test whether any mild injury primes host immune responses to pneumonia, we generated a mild tail trauma (TT) model. mTBI mice showed protection from bacterial pneumonia while TT mice did not. Using an FDA-approved neurokinin-1 receptor (NK1R) antagonist, aprepitant, we confirmed our previous findings that substance P (SP) is a key mediator of enhanced resistance to pneumonia. Blocking NK1R showed that mTBI-induced release of SP augments pulmonary neutrophil recruitment and microbicidal activity to pulmonary bacterial pathogens. In TT mice, NK1R agonism enhanced the same neutrophil functions, further supporting the hypothesis. No differences were found between mTBI and TT neutrophils’ ability to phagocytose, generate oxidative burst, or acidify phagosomes. However, neutrophils from mTBI mice produced more neutrophil extracellular traps in response to bacterial challenge. These studies show that neurokinin signaling in our model contributes to enhanced bacterial clearance. Cholinergic anti-inflammatory pathway signaling though the α7 nicotinic acetylcholine receptor (α7 nAChR) is also a critical component of improved survival. Blockade of α7 nAChR abrogated the mTBI survival benefit. Mimicking cholinergic signaling using α7 nAChR agonist recapitulated the mTBI reduced pro-inflammatory cytokine production and improved survival. No physiologic differences emerged within 24h following pneumonia, but mTBI and α7 agonist treated mice had significantly lower TNFα in bronchoalveolar fluid, suggesting reduced injurious pulmonary inflammation. However, replacing early TNFα during pneumonia did not increase mortality. Western blot analysis showed downregulation of HMGB1 release in mTBI mice, suggesting that vagal cholinergic signaling reduces late mediators of organ damage. Our experiments show that mTBI enhances resistance to pneumonia by activating the vagus nerve signaling through neurokinin and cholinergic pathways. Translation of these findings could be innovative solutions to fighting or preventing infections.