**6.1 The gut-associated lymphoid tissues (GALT) following PN**

The Gut-Associated Lymphoid Tissues (GALT), a compartment which contains an astonishing 70–80% of all active immune cells, consists of both innate and adaptive cells residing beneath the epithelium and sampling the intestinal lumen [41]. The GALT facilitates release of sIgA on mucosal surfaces throughout the body. sIgA serves as an opsonin that can bind pathogens either specifically or non-specifically [42]. sIgA can mediate tolerance leading to attenuated inflammatory responses and induction of Treg lymphocytes [43]. One of the major detrimental effects of PN is GALT atrophy which occurs quickly after cessation of enteral feeding, and is driven by changes in blood flow, decreased expression of leukocyte binding, and decreased cellularity throughout the splanchnic bed. Grossly, PN-induced gut atrophy is observed with decreased organ wet and smaller bowel circumference approaching declines of 10% [44].

In rodent models of PN, Peyer's patch lymphocyte numbers begin to decline within 1–2 days of PN, where 75% of total cells are lost by 3 days compared with controls [45]. While total cellularity decreases, the ratios of T to B-lymphocytes, CD4+ to CD8+ cells, and relative percentages of memory, activated, and naïve cells remain stable [46]. In normal Peyer's patch function, specialized microfold cells cover Peyer's patches and sample luminal antigen to present to dendritic cells and naïve αβ+ T and B-lymphocytes within underlying germinal centers [47]. The naïve cells are localized to the Peyer's patches via expression of the integrins L-selectin and to a lesser extent α4β7. The integrins interact with mucosal addressing cellular adhesion molecule-1 (MadCAM-1) [41]. Diapedesis of the naïve cells into the Peyer's patch is facilitated by the chemokines CXCL13, CCL19, and CCL21 [48].

PN alters MAdCAM-1 expression within the Peyer's patch tissues by altering two MAdCAM-1 regulatory networks, through the lymphotoxin β receptor (LTβR) and noncanonical NFkB signaling pathways [49]. In the LTβR pathway, lymphotoxin α and β on the surface of systemic lymphocytes bind LTβR within the GALT tissues, elevating MAdCAM-1 and Th2 cytokines including IL-4 [50]. Following PN, Peyer's patch and GALT expression of LTβR rapidly declines, leading to decreased MAdCAM-1 within 4 h [51]. Mechanistically, inhibition of LTβR alone with blocking antibodies significantly decreases MAdCAM-1 expression. Conversely, providing stimulation of the LTβR under PN feeding through anti-LTβR monoclonal antibodies increase Peyer's patch lymphocyte numbers and mucosal release of sIgA in the gut and respiratory tract [44].

The second MAdCAM-1 regulatory signaling pathway is noncanonical NFkB, which is regulated in-part through lymphoid receptors and LTβR signaling described above. The canonical (or classical) NFkB pathway is stimulated in infectious and injurious insults, driving inflammatory tissue responses. In contrast, noncanonical NFkB triggers nuclear P52/RelB dimer formation and subsequently elevation of MAdCAM-1. Animal studies providing PN have demonstrated that both the canonical and noncanonical NFkB pathways are reduced during PN feeding [52]. Experimental inhibition of LTβR signaling significantly decreases nuclear P52/RelB dimerization and leads to lower MAdCAM-1, CCL19, CCL20, and CCL25 expression, but blockade of LTβR does not affect canonical NFkB protein levels [51]. Experimental stimulation of LTβR with agonists during PN drives expression of MAdCAM-1, P52/RelB, and IL-10. On the other hand, blocking ligands administered to control animals result in less MadCAM-1, L-selectin, and α4β7 [51]. These studies highlight the changes that occur in gut signaling following PN with lack of enteral stimulation. Fortunately, providing enteral stimulation drives normalization of these parameters in experimental animals within 2 days [53].

Peyer's patches serve as important induction sites for gut immune responses, where cells subsequently enter the lymphatics and circulation before returning to mucosal effector sites throughout the body. During transit through these compartments, plasma cells are activated and return producing IgA [54]. Through the same anatomical transit, T helper lymphocytes subpopulations are stimulated, including Th1, Th2, Th17, Th22, and Treg [55]. Th2 lymphocytes generate IL-4, IL-5, IL-6, IL-9, IL-10, IL-13, and IL-25, which plasma cell IgA production by plasma cells. These cytokines also have important roles in driving epithelial machinery that is required to translocate IgA to the luminal surface, such as polymeric immunoglobulin receptor (pIgR) [56]. Enterocyte pIgR bonds with a dimeric for of IgA where endocytosis moves the complex to the luminal surface before releasing secretory IgA (sIgA). The ratios of cytokines expressed in the lamina propria balance the release of sIgA production and release. For instance, IL-10, IL-17, and TGF-β drives plasma cell IgA production and pIgR expression, while IL-2, IFN-γ, and TNFα decrease pIgR expression [57, 58]. Of these cytokines, TGF-β appears to be critical,

#### *Parenteral Nutrition Modeling and Research Advances DOI: http://dx.doi.org/10.5772/intechopen.101692*

as mutant animals lacking TGF-β fail to present sIgA at mucosal surfaces, perhaps in part due to the need for this cytokine in plasma cell maturation [59].

By reducing the expression of α4β7, the integrin that binds MadCAM-1to help localize lymphocytes to the lamina propria, PN functionally results in reduced systemic lymphocytes dedicated for mucosal defense (CD4+CD25+) as well as resident lymphocytes in GALT tissues [46]. Furthermore, the activated lymphocyte population has a reduced capacity for tolerance or memory of self-antigens, as evidenced by reduced expression of Treg (CD4+CD25+Foxp3+) and memory (CD44+). The reduction in Treg cells also results in less TGFβ and IL-10 are produced, which usually support plasma cell function by counteracting the pro-inflammatory Th1 cytokine IFN-γ. PN decreases GALT IL-4 and IL-10 levels [45].

PN alters Th1:Th2 ratios by reducing the production of Th2 but not Th1 cytokines. Implications are increased neutrophil recruitment through ICAM-1 expression due to loss of IL-4 and IL-10 with stable IFN levels [60, 61]. Following PN, elevates neutrophils are observed in multiple organs, which may result in greater injury following hemorrhagic shock, ischemia, and sepsis. Experimental injury demonstrates that the percentage of activated neutrophils is significantly higher following PN, functionally resulting in greater mortality (50%) than enterally fed controls (5%).

Mucosal IgA responses are specific and vital, as has been shown in IgA mucosal vaccination studies for poliovirus and enterotoxigenic *Escherichia coli*, where specific sIgA appears at all body surfaces following mucosal exposure [62, 63].

Viral and bacterial challenges in rodent models have shown that functionally, PN leads to lower IgA-mediated immunity for antigen recognition and elimination of pathogens. Immunizing mice against *Pseudomonas (Ps) aeruginosa* leads to 90% survival when exposed to an intra-tracheal challenge compared with only 10% survival in control animals [64]. Given the dramatic effect of PN on adaptive immune responses, immunized animals provided PN survive intra-tracheal Ps at the rate of unimmunized animals. Similar results were obtained with influenza shedding studies, illustrating the loss of adaptive immunity to specific pathogens in the absence of gut feeding [65]. These findings draw a larger working schematic that PN feeding, without enteral intake, functionally alters the GALT compartment into a state that is far less protective. Unfortunately, these adaptive immune changes occur in parallel with increased pro-inflammatory neutrophil infiltrates that make any subsequent injury or infection more severe.
