Interleukin-1beta in immune cells of the abdominal vagus nerve: a link between the immune and nervous systems?

Abstract

Intraperitoneal administration of the cytokine interleukin-1beta (IL-1beta) induces brain-mediated sickness symptoms that can be blocked by subdiaphragmatic vagotomy. Intraperitoneal IL-1beta also induces expression of the activation marker c-fos in vagal primary afferent neurons, suggesting that IL-1beta is a key component of vagally mediated immune-to-brain communication. The cellular sources of IL-1beta activating the vagus are unknown, but may reside in either blood or in the vagus nerve itself. We assayed IL-1beta protein after intraperitoneal endotoxin [lipopolysaccharide (LPS)] injection in abdominal vagus nerve, using both an ELISA and immunohistochemistry, and in blood plasma using ELISA. IL-1beta levels in abdominal vagus nerve increased by 45 min after LPS administration and were robust by 60 min. Plasma IL-1beta levels increased by 60 min, whereas little IL-1beta was detected in cervical vagus or sciatic nerve. IL-1beta-immunoreactivity (IR) was expressed in dendritic cells and macrophages within connective tissues associated with the abdominal vagus by 45 min after intraperitoneal LPS injection. By 60 min, some immune cells located within the nerve and vagal paraganglia also expressed IL-1beta-IR. Thus, intraperitoneal LPS induced IL-1beta protein within the vagus in a time-frame consistent with signaling of immune activation. These results suggest a novel mechanism by which IL-1beta may serve as a molecular link between the immune system and vagus nerve, and thus the CNS.

Fig. 1.

Interleukin-1β content as assessed using ELISA in abdominal vagus, cervical vagus, and proximal sciatic nerve tissues (A) and blood plasma (B) 30, 45, and 60 min after intraperitoneal injection of 100 μg/kg LPS or saline. Only the abdominal vagus shows the induction of IL-1β as early as 45 min. IL-1β content in plasma is significantly elevated by 60 min. *p < 0.05; **p < 0.0002.

Fig. 2.

LPS-induced IL-1β-IR in tissue associated with the abdominal vagus nerve. A, Cells in perivagal NALC expressing IL-1β-IR (dark cytoplasmic stainingindicated with arrows) 45 min after intraperitoneal LPS administration. B–D, IL-1β-IR dendritiform cells in connective tissue adjacent to the abdominal vagus nerve 60 min after LPS. B shows a high magnification of an IL-1β-IR dendritiform cell. E, F, Darkly stained IL-1β-positive cells in perivagal NALC (E, F, arrow), most of which are round in shape. Some dendritiform IL-1β-IR cells are present between nerve fibers at edge of vagus nerve (F,arrowheads). Scale bars: A, 25 μm;B, 10 μm; C, 50 μm; D, 25 μm; E, 50 μm; F, 50 μm.NALC, Nerve-associated lymphoid/myeloid cells;VN, vagus nerve.

Fig. 3.

A–C, MHC class II-IR dendritiform cells (dark reaction product) within the abdominal vagus nerve (A), cervical vagus nerve (B), and proximal sciatic nerve (C). Note the difference in density of immunostained cells, which is highest in the abdominal vagus (A) and lowest in the proximal sciatic nerve (C). MHC class II-IR is present in both saline- and LPS-treated rats. D, E, Immune cells in abdominal vagal paraganglia constitutively expressing MHC class II-IR (D) interspersed among vagus nerve fibers and glomus cells in a paraganglion, and those with similar morphology expressing LPS-induced IL-1β-IR (E) surrounding unlabeled glomus cells. F, ED-1-IR in cells of perivagal NALC 60 min after LPS treatment, showing macrophage-like (round or oval-shaped) morphology. Counterstaining with cresyl violet provides light background staining in A–D. Scale bars, 50 μm. NALC, Nerve-associated lymphoid/myeloid cells; PG, paraganglion; VN, vagus nerve.

Fig. 4.

Colocalization of immunofluoresence for IL-1β (A′–C′), MHC-II (A, B), and ED-1 (C) in connective tissue surrounding the abdominal vagus. A, B′,Arrows indicate cells showing strong immunofluorescence for both MHC-II (A, B) and IL-1β (A′, B′). Subcellular localization of the two antigens is not always overlapping.Single arrowheads indicate strongly MHC class II-positive cells that lack prominent labeling for IL-1β.Double arrowheads (A, A′) point to cells that show clear IL-1β-IR but have weak or no MHC class II-IR. C, C′, Cells show strong dual labeling for ED-1 (C, arrows) and IL-1β (C′, arrows). Arrowheadspoint to weak ED-1-positive cells that lack IL-1β-IR. Scale bars, 25 mm.