Elucidating the basis of brain control of innate immune responses

National Institute Of Allergy And Infectious Diseases

Investigators

Linked publications & trials

Abstract

We reasoned that if the brain monitors and modulates peripheral immune responses, brain neurons should be activated by peripheral immune inputs. Such immune-activated neuronal populations, if identified, would help dissect the neural control of immunity. We used lipopolysaccahride (LPS), a canonical immune stimulus derived from the outer membrane of gram-negative bacteria to elicit innate immune responses. We then scanned mices brains for induction of the immediate early gene, Fos, as a proxy for neural activity. Our results showed that among other brain regions, LPS elicits significant labeling in the dorsal vagal complex (DVC), an area that encompasses caudal nucleus of the solitary tract (cNST) and area postrema (AP). The DVC is a primary target of the vagus nerve, functioning as a nexus to broadcast interoceptive signals from the body to the brain. Given DVCs strategic position as the interface between the body and the brain, we hypothesize that the DVC interprets immune signals to constitute a crucial circuit modulating peripheral innate immune responses. To test this hypothesis, our strategy was to employ the Targeted Recombination in Active Populations (TRAP) system to target Cre-recombinase to LPS-activated neurons, and then used a Cre-dependent chemogenetic effector (silencer and activator) to examine LPS-evoked responses in control and silenced animals. Our results showed that silencing these neurons exacerbates inflammation. Conversely, activating them dampens inflammation. Together, these silencing and activation experiments showed that modulating the activity of these DVC neurons can bidirectionally regulate peripheral inflammation, substantiating that the DVC functions as a homeostatic neural control of innate immune responses. We are now using single-cell RNA sequencing (scRNA-seq) coupled with functional assay to expose the genetic identity of DVC neurons modulating inflammation. DVC is the target of the vagus nerve, a principal conduit linking the body and the brain. Therefore we employed functional imaging and manipulation to monitor and perturb the responses of vagal-DVC axis to immune signals as a way to understand how the brain is informed of the emerging immune responses. We implemented an in vivo vagal ganglion (i.e., nodose ganglion) functional imaging platform to monitor vagal responses to immune signals. While LPS itself does not directly activate vagal sensory neurons, cytokines potently activate these neurons. More importantly, pro-inflammatory and anti-inflammatory cytokines activate discrete subsets of vagal sensory neurons. We further showed that vagal neurons responding to pro-inflammatory and anti-inflammatory cytokines are defined by the expression of Calca and Trpa1, respectively. Chemogenetic activation of TRPA1-expressing vagal neurons dramatically enhances the anti-inflammatory response, while severely suppressing the levels of pro-inflammatory cytokines. By contrast, activating CALCA-expressing population significantly decreases the level of pro-inflammatory cytokines without impacting that of anti-inflammatory cytokine. How does immune information conveyed by the two vagal lines (TRPA1+ and CALCA+) converge in the DVC? To answer this, we will monitor neural dynamics in the DVC while manipulating TRPA1+ and CALCA+ vagal populations in the same mice using orthogonal recombination (Cre and Flp) and chemogenetic (DREADD and PSEM) systems. How do DVC neurons in the brain change the immune function in the periphery? We are exploring the nature of descending circuits and recipient immune cells that mediate top-down immune modulation. We anatomically traced the projections from DVC LPS-TRAPed neurons and showed that they heavily innervated the paraventricular hypothalamus, a prominent neuroendocrine center, and rostral ventralateral medulla (RVLM), a primary sympathetic nervous center. We are now testing the hypothesis that descending humoral and neural signals collaborate to drive immune modulation by optogenetically activating or silencing projections to these two areas while measuring inflammatory responses to LPS. Following the characterization of descending centers, we will identify recipient immune cells targeted by descending signals, and neuro-to-immune signaling that modifies immune function. We will focus on the spleen as the site of modulation by descending inputs because it is the primary immune reservoir mobilized during systemic inflammation. We will use scRNA-seq to identify and characterize spleen immune populations that respond to the manipulation of descending centers in the brain. We will then examine these immune populations for the presence of cognate receptors for descending signals, to pinpoint the population directly targeted by top-down modulation. Finally, to probe the role of neuro-immune signaling in immune modulation, we will selectively ablate the receptor in the target immune population using conditional knockouts, and test the resulting consequence on inflammatory responses.