**12.1 Enteric nervous system molecules**

Outside of the central nervous system, the enteric nervous system contains an enormous number of autonomous neurons and glial cells that coordinate the physiological functions of digestion, gut immune homeostasis, and diverse numbers of epithelial functions. Among the targets of ENS neuropeptides, including gastrin-releasing peptide (GRP), substance P, and VIP, are GALT immune cells. During normal feeding, ENS fibers release GRP, stimulating a cascade of digestion and immune cell responses [136]. Analogues for GRP, including bombesin (BBS), have been used to efficiently stimulate the GRP receptor and mimic gut feeding responses. Administration of BBS to mice on PN significantly elevates intestinal blood flow, Peyer's patch lymphocytes [46], activated and memory lymphocytes in the lamina propria, and elevated pIgR and luminal IgA [137–139]. BBS also drives increased expression of Paneth cell antimicrobial enzymes. Functionally these changes following BBS results in increased resistance to infectious organisms, including respiratory H1N1 and *Pseudomonas* [140], and intestinal enteroinvasive *E. coli* [141], compared with PN alone. Outside of the immune compartment, BBS stimulates GI motility and pancreatic secretions during PN that are otherwise attenuated [142].

In summary, providing rodents with exogenous neuropeptides, for example BBS, can compensate for the loss of normal enteral nutrient stimulation that drives gut physiological and immunological responses. Artificial stimulation of these gut functions may hold promise in patients where prolonged periods of PN are needed and patients may otherwise be at increased risk of mucosal immune atrophy and infectious microorganisms.

## **12.2 AHR molecules**

Given the loss of microbial community structure and function in the absence of enteral feeding, disturbances in the gut microbiota occur rapidly following PN. One such change is the loss of microbial Aryl hydrocarbon receptor (Ahr) ligands production that normally stimulate IL-22 production. IL-22 generates epithelail barrier responses, including antimicrobial molecule production [143]. Ahr deficient animals are at increased susceptibility to infectious challenge, including *Citrobacter rodentium*. Prior work demonstrates that microbial Ahr production elevates bone marrow B cell maturation [144, 145].

## **12.3 SCFAs**

Short chain fatty acids (SCFAs) Are generated by microbial fermentation of dietary carbohydrates. SCFAs phase can reach 130 mmol/kg in the distal gut where they stimulate the receptors, GRP41, GPR42, and GPR109a, shaping immune responses and modifying the release of enteroendocrine hormones. Exogenous administration of SCFAs stimulates the number of IgA+ producing plasma cells in the gut and elevates circulating immunoglobulins [146]. Functionally, the importance of these metabolites in immune stimulation have been demonstrated through experimental studies administering SCFA to mice during infection with *Citrobacter rodentium*, and enhanced IgA levels and pathogen clearance was observed. In addition to driving antibodies at mucosal surfaces, SCFAs mitigate pro-inflammatory responses and stimulate tolerance by inducing Treg cells [147]. Isolated plasma cells enhance production of IgG and IgA when exposed to SCFAs, demonstrating the direct effect of these metabolites on immune function [146]. At the mucosal barrier, through GRP receptors, SCFAs stimulate goblet cell mucin gene expression and improve tight junction protein expression [148, 149]. Dietary SCFAs administration improves diet induced metabolic complications including liver dysfunction via the G-protein coupled receptor FFAR3 and prevents *de-novo* lipogenesis in mice [150, 151]. SCFAs modulate gut microbiome-brain communication by crossing the blood-brain barrier and regulating signaling pathways involved in central nervous system (CNS) production of neurotransmitters such as dopamine and serotonin [152]. SCFAs alleviate blood-brain barrier permeability and microglia maturation and function [153]. Also, dietary administration of SCFAs increases serum plasma of GLP-1 and protects mice from diet-induced obesity [154].

#### **12.4 Polyamines**

Polyamines are a unique class of polycationic metabolites produced by many lifeforms. Diets lacking polyamines lead to slowing of intestinal development and atrophy of the mucosa [155]. By enhancing expression of occludin and E-cadherin, polyamines improve epithelial tight-junction function in addition to improving mucous glycoprotein release [156, 157]. Polyamines are also demonstrated to drive IgA levels and increase lamina propria CD4+ T cells [158, 159]. Considering the importance of these molecules in normal gut homeostasis, this class of molecules represents one area of innovation for researchers investigating PN additives for improved outcomes.
