## 5 The Neuroimmunological Crosstalk in the Gut: The Central Role of the ENS  
Alternating, peristaltic contractions of the smooth muscle layer of the intestinal tract (musculars externa) are responsible for the gastrointestinal motility essential for nutrient transport and waste excretion, coordinated by the intrinsic system of the ENS. Although the neurons of the enteric ganglia receive extrinsic innervation from the sympathetic and parasympathetic fibers of the autonomic nervous system, they are able to function as a single autonomic unit, which is why the enteric nervous system is often referred to as the'second brain' [65]. While there is extensive literature on the role of mucosal macrophages in gut immunity and secondary mucosal lymphatic tissue, little has been reported for a long time on macrophages of the musculars externa. The two essential survival factors of the mononuclear cell line are FLT3 and CSF1. FLT3 is required for the survival of dendritic cells, whereas CSF1R is required for the survival of MP\({}_{\text{M}}\)-s, as evidenced by the disappearance of macrophages from the musculars externa in CSF1R--KO mice and after intraperitoneal administration of a CSF1R-blocking antibody [25]. While MP\({}_{\text{M}}\) cells show an 80% depletion in response to the CSF1R-blocking antibody, the LpM population is not drastically affected by depletion treatments [25; 66]. BMP-like growth factors also play a role in the interaction of MP\({}_{\text{M}}\)-s and the ENS. BMP2 and its receptor are well-known factors in the development and organization of embryonic smooth muscle and ENS [67; 68]; BMP2 expression is specific for MP\({}_{\text{M}}\)-s in the intestinal smooth muscle layer, and blocking BMP signaling with dosomorphin, similar to the MP\({}_{\text{M}}\)-depletion that is achieved with a CSF1R blockade, results in dysmotility and a prolonged intestinal transit time in mice [25]. In ex vivo colon cultures of mice treated with anti-CSF1R antibodies, the addition of BMP2 can reduce the motility impairment caused by MP\({}_{\text{M}}\)-depletion [25]. BMP2 produced by MP\({}_{\text{M}}\)-s in the smooth muscle layer acts on the BMPII receptor (BMPRII) of enteric neurons. Upon ligand binding of the receptor, a SMAD phosphorylation cascade is initiated [69], suggesting constitutive activation of MP\({}_{\text{M}}\)-s on enteric neurons mediated by BMP2 [25]. This study suggests that the ENS produces CSF1 as part of a macrophage-neuron crosstalk in 'exchange' for ensuring the survival of the MP\({}_{\text{M}}\) population. However, subsequent studies have suggested that MP\({}_{\text{M}}\)-s colonize the gut even in the absence of ENS, and that CSF1 is predominantly produced by mucosal cells rather than ENS neurons [54]. In addition to the chemical crosstalk between intestinal macrophages and the ENS, the presence of a luminal microbial flora is required for proper motility of the intestinal tract, as supported by the development of severe intestinal dysmotility in homozygous mice reared in a germ-free environment [70] and expressing the TLR4 (toll-like receptor 4) gene [71]. In germ-free and antibiotic-treated TLR4-KO mutant mice, alterations in the neuronal phenotype of the ganglion, in particular a reduction in the number of nitroger neurons, are thought to be responsible for reduced intestinal motility [71]. Antibiotic treatment of mice in vivo clearly reduces BMP2 expression levels, which is due to the depletion of the MP\({}_{\text{M}}\) population. As a consequence of BMP2 deficiency, the number of pSMAD1/5/8 positive enteric neurons in the intestinal tract is also reduced. A direct link between the commensal flora, macrophages and ENS is also suggested by the finding that adding LPS to primary enteric neuron cell culture (LPS, a systemic inflammatory mediator released from bacteria that dissociate during infection) increases neuronal CSF1 secretion in vitro [25].