Vagal Nerve Stimulation Blocks Burn Injury-Induced Priming of Peritoneal Macrophages

Abstract

Introduction: Large surface area burn injuries lead to activation of the innate immune system. Activated, hyper-responsive macrophages are thought to be responsible for the increased susceptibility to sepsis which is seen after severe burn. This supports a two-hit model in which the macrophage inflammatory response is dependent on pre-existing levels of immune cell activation. We, and others, have shown that activation of the parasympathetic nervous system via vagal nerve stimulation (VNS) can alter the inflammatory response to injury. We hypothesized that VNS would decrease peritoneal macrophage responsiveness to an inflammatory insult after severe burn injury. Methods: Male balb/c mice were subjected to a 30% total body surface area steam burn with and without electrical stimulation to the right cervical vagus nerve. Peritoneal macrophages were collected at various time-points after injury. to assess the degree of macrophage hyper-responsiveness, peritoneal macrophages were incubated in LPS or vehicle. Flow cytometry was then used to identify F4/80+ macrophages and to analyze the phosphorylation state of the NFkb pathway mediator, p65 Rel A in this population. Results: There was a significant increase in macrophage NFkb p65 Ser 536 phosphorylation in macrophages exposed burn injury, followed by LPS treatment. Neither burn alone, nor LPS alone was capable of increasing macrophage NFkb p65 Ser 536 phosphorylation. Increased NFkb p65 Ser 536 phosphorylation was seen within 4 hours after burn injury, with phosphorylation returning to baseline by 24 hours post-burn. Treatment with VNS significantly reduced NFkb p65 Ser 536 phosphorylation in burn-primed macrophages that were exposed to LPS. Finally, VNS mediated an unexpected reduction in baseline NFkb p65 Ser 536 phosphorylation levels in sham animals. Conclusions: VNS prevented priming of peritoneal macrophages after severe burn injury, characterized by decreased phosphorylation of NFkb p65 Ser 536. Moreover, VNS limited the baseline inflammatory state of macrophages in uninjured animals. These studies suggest that VNS mediates the inflammatory response in peritoneal macrophages by affecting the set point of LPS-responsiveness.