**4. Adaptative immunity of the GIT**

Unlike the innate immune system which attacks only general threats, adaptative mucosal immune system is triggered by exposure of potentially dangerous pathogens. However, sometimes if overlaps some of their functions. The three most key roles of that system are: the induction of an efficient and appropriate immune response to pathogenic invaders, the tolerance of the commensal microorganisms of the intestine as well as the induction of the tolerance of nutrients and other environmental immunogens. Responses of the systemic immune system can originate from or be modified by the mucosa; this is exemplified by the attenuation of systemic immune responses to a protein that has first been fed orally to the animal (oral tolerance). Thus, the mucosal immune system must maintain the delicate balance between responsiveness to pathogens and tolerance to a vast array of other harmless antigens encountered at mucosal sites. This balance is achieved through the interplay of innate and adaptive (B- and T-lymphocyte) mechanisms [82].

The adaptative immune system in the GIT has features that are distinct from adaptative immune systems in other organs. The major form of adaptative immunity in the gut is humoral immunity directed at microbes in the lumen. This function is mediated mostly by dimeric IgA antibodies that are secreted into the lumen of the gut. Cellular adaptative immunity is carried out by an intraepithelial lymphocytes (IEL) in healthy adult bird includes major subsets of NK and T cells bearing the γδ or αβ form of the T cell receptor (TCR). In contrast to other tissues, B cells are almost entirely absent from the IEL and the T cells predominantly express the CD8 coreceptor with smaller populations of TCRαβ+ CD4+ and CD4 + CD8+ cells [83, 84].

*Advances in Poultry Nutrition Research*

cathelicidins [47–49].

**3.6 Host defense peptides**

post-translational modification [57, 60].

the preference of gram-negative or positive bacteria.

microscopy confirming the presence of granulated secretory cells at the base of the crypts in the chicken small intestine. The researchers also confirmed by Western blot the expression of lysozyme protein, which is specifically secreted by the Paneth cells in the small intestine [44]. Paneth cells have the morphological characteristics of a professional secretory cells, including an extensive ER, a Golgi apparatus and an internal secretory granule. The first assumption that Paneth cells had a hostdefense function emerged when lysozyme was identified as a product of these cells [45]. After that, it was discovered that Paneth cells secrete antimicrobial peptides (AMP) or host defense peptides (HDPs) which are important host-defense substances in the communication between host and microbiome. One of the most well characterized are β-defensins [46]. In addition to defensins, Paneth cells is able to secrete other AMPs including secretory phospholipase A2, Reg III, angiogenin 4 and

HDPs are generally positively charged small peptides with amphipathic properties [50]. These peptides present in the GIT display an important, but often overlooked role in the first line of defense. With the first avian HDPs identified in 1990s [51], the information about avian HDPs has increased considerably in the subsequent decades. Currently, avian β-defensins and cathelicidins are the two

HDPs were initially called antimicrobial peptides (AMPs), because they are characterized by the direct antimicrobial activities against a broad spectrum of numerous pathogens, including gram negative and positive bacteria, fungi, and even certain viruses [54–56]. Generally, the cytoplasmic membrane of pathogenic organisms is a frequent target for HDPs. The amphipathicity and cationic charge of HDPs allow the initial contact with membrane electrostatically, as most bacterial surfaces are hydrophobic and anionic. The peptides then insert into phospholipid bilayers and induce pore formation in membranes by toroidal pore formation, carpet formation and barrel-stave formation, resulting the cytoplasmic leakage and death of pathogens [54, 57–59]. Besides pore formation in membranes, some HDPs can directly penetrate into cells and interfere with intracellular molecules, interrupting cell wall formation, DNA and RNA synthesis, protein translation and

To be specific, chicken AvBD1, −2, and − 7 exhibit high efficiency against a large variety of both gram-negative (*E. coli*, S. enteritidis, S. typhimurium, C. jejuni, and *K. pneumoniae*) and gram-positive (*S. aureus*, B cereus, *L. monocytogenes*, S. haemolyticus, and S. saprophytus) bacteria [51, 61–64]. AvBD1 and − 7 also efficiently kill P. aeroginosa and E. cloaca, while AvDB2 showed reduced efficacy [61, 64]. AvBD4, −5, and − 11 protect host from invasion of S. enteritidis and S. typhimurium, however their antimicrobial activities on other bacteria species remain to be determined [63, 65, 66]. Although AvDB8, −9 and − 13 are active against *E. coli*, respectively, they exhibit a minimal activity against several other bacteria [66–69]. Based on studies of different AvBD isoforms, it seems that both structure and catholicity are important for antimicrobial activity but disparity in

All four chicken CATHs show antimicrobial capacities in the same order of magnitude against a wide range of gram-negative and positive bacteria, and fungi [70–73]. Similar to AvBDs, the structure and cationic charge are equally important for their antimicrobial activities. The presence of an alpha-helical region in N-terminal and hinge region around the center of the peptide are important for antimicrobial. Removal of N-terminal alpha-helix in CATH2 truncation or

major classes identified and extensively studied in chickens [52, 53].

**124**

Moreover, within the CD8+ IEL population the majority express CD8αα homodimers rather than the CD8αβ heterodimer commonly expressed on classical CD8+ T cells found at systemic sites [83–85]. The proportions of IEL belonging to each subpopulation differ according to age, genetics and environment (including infection). Numerically, B and T cells are the most common lymphocytes (~90%), the remainder being of the NK cell phenotype (CD3-Bu-1-). In contrast to the IEL population, the T cell population of the lamina propria contains a smaller proportion of γδ-T cells (~10%); the much larger αβ-T cell population is dominated by CD4+ T cells, with a less prominent CD8+ cell population [86].

#### **4.1 Secretory IgA (sIgA) and its transporter, polymeric Ig receptor (pIgR)**

The existence of sIgA in the bird has been established for quite some time, but studies are relatively limited compared with mammals. In humans, it is estimated that approximately 70% of the body's IgA-producing plasma cells (differentiated from activated B cells) reside in the lamina propria of intestinal mucosa [87–89]. Although sIgA belongs to adaptive immunity by definition, it plays an important role in the first lines of mucosal defense [87, 90]. There are three modes of defense modulated by sIgA on gut mucosal surfaces: (1) sIgA has been shown to interfere with the early steps of the infection process through directly blocking pathogens and toxins from attaching to the intestinal epithelium [91]; (2) sIgA exerts the protective immunity through immune exclusion, which is the prevention of pathogens and toxins from approaching to epithelium through the stepwise procedures involving antibody-mediated agglutination, entrapment in mucus, and clearance through intestine peristalsis [92, 93]; (3) sIgA exhibits the ability to neutralize intracellular pathogens, viruses, and toxins within intestinal epithelial cells, which requires binding of specific IgA and occupation of antigens by pIgR transportation vesicles, followed by the passage of antigens into the lumen. Notably, the intracellular neutralization of LPS by IgA indicates the potential role in anti-inflammation and deactivation of the proinflammatory pathways in epithelial cells [90, 94, 95].

T-cells generally produce high-affinity IgA antibodies. IgA has the specificity against previous exposure of the GIT by pathogens and more invasive commensal species [89, 96]. Conversely, low-affinity IgA antibodies can also be produced from T-cell-independent (TI) pathways. These low-affinity IgA antibodies act through coating commensal bacteria thereby augmenting the competitive inhibition of pathogens [88, 89, 96]. The production of both high and low-affinity production of IgA provides protective roles during an overt infection with a pathogen as well as during unchallenged/non-pathogenic bacterial exposure.

Presence of microflora in the the GIT may also regulate production of IgA. Studies with germ-free mice [99] and pigs [100, 101] have demonstrated that intestinal IgA and IgA-positive cells in the lamina propria are dramatically reduced versus conventionally reared animals. Further studies have shown that specific microflora (e.g. segmented filamentous bacteria and clostridia) when given to germ-free mice will stimulate the development of IgA-producing cells, while other microflora will have no effect or inhibit this development [97, 98]. Thus, other researchers have reported similar IgA production responses with poultry diets were supplemented with probiotics [99, 100]. Notably, IgA development in the hindgut of the bird early in life coincides with the rapidity of bacterial colonization [101].

Prior to development of IgA by the GIT, the chick is reliant upon maternal antibodies and physical defenses (such as mucins and intestinal turnover). In birds, a small amount of IgA (~ 0.3 mg) is transferred via the embryo imbibing amniotic

**127**

defenses.

*Secretory Defense Response in the Bird's Gastro-Intestinal Tract and Nutritional Strategies…*

by goblet cells, which serves as a reservoir to slowly release maternal IgA.

fluid prior to internal pipping [102, 103]. The endogenous IgA expression starts to increase after the second week post-hatch [101]. Bar-Shira et al. [103] suggested that the resistance of rapid depletion of maternal IgA may be due to unique uptake

Circulating IgA is predominantly in a monomeric form, whereas in intestinal secretions it is found in a dimeric configuration both in mammals and birds [96, 104]. IgA secreted by plasma cells accumulates in the lamina propria. To exert its protective effect, pIgR is constitutively expressed by epithelial cells to transport IgA through the epithelia from the lamina propria to intestinal lumen. During the transcytosis, IgA is bound by pIgR on the basolateral surface and transported to the apical surface. At this surface, cleavage of the extracellular portion of pIgR results in release of secretory component (SC) as part of the dimeric IgA, otherwise known as the sIgA complex [105]. In this complex, sIgA is thought to be protected against degradation by proteases and pH fluctuations in the gut [90]. Excess production of pIgR which is not utilized as an IgA chaperone is also secreted as "free SC", which may have additional bacterial scavenger properties [106]. Once secreted, the N-glycans of SC can then bind to itself, and/or sIgA in the mucin layer thereby bridging these luminal

As one molecule of pIgR is required to bind and transport one dimeric IgA for secretion of sIgA into the intestinal lumen, pIgR's expression regulates sIgA capacity into the GIT [105, 107]. Expression regulation in mammals can be induced by numerous cytokines, including: interferon-γ (IFN-γ), tumor necrosis factor (TNF), interleukin-1β (IL-1β), and IL-4. These cytokines act through mediating a transcriptional response through activation of several transcription factor-binding sites in regulatory regions [105, 107–109]. In the chick, increases in IFN-γ, IL-1β and IL-4 expression in the second week post-hatch [110, 111] may influence subsequent increases in expression of the chicken pIgR gene [111]. Additional bacterial binding to Toll-like receptors have also been shown to increase pIgR expression in epithelial

The GIT is an extremely expensive tissue in terms of energy and nutrient needs to maintain and facilitate the full range of barrier and energy/nutrient assimilation functions it displays. Cant et al. [113] estimated that the GIT consumes approximately 20% of dietary energy with a turn-over rate of 50 to 75% per day. However, the GIT is a dynamic organ system whose maintenance needs dramatically changes based on responsive demands. Applegate [114] elucidated some of these adaptive responses, including: changes to peristaltic rate, changes to enterocyte turnover, tight junction regulation, mucin production (quantity and composition), changes to differentiation direction of undifferentiated cells and changes to secretory

While we often think of presence of microbiota as an additional barrier cost, there is some symbiotic relationships that they convey to the host. For example, the presence of the ceca contributes approximately 3% to dry matter digestibility to the bird [115] in part through 8% of energy derived from microbial fermentation

Due to limitations of space in this review, we were unable to address all nutrient impacts on the secretory immune defenses of the bird. Notably, recent reviews have published on roles of amino acids on physiological, immunological, and microbiological responses as well as quantification of changes to endogenous amino acid production in the bird [118, 119]; as well as implications of protein indigestibility in

*DOI: http://dx.doi.org/10.5772/intechopen.95952*

defenses [105].

cells [105, 107, 112].

**5. Nutrition and secretory immune response**

resulting in short-chain fatty acids [116, 117].

#### *Secretory Defense Response in the Bird's Gastro-Intestinal Tract and Nutritional Strategies… DOI: http://dx.doi.org/10.5772/intechopen.95952*

fluid prior to internal pipping [102, 103]. The endogenous IgA expression starts to increase after the second week post-hatch [101]. Bar-Shira et al. [103] suggested that the resistance of rapid depletion of maternal IgA may be due to unique uptake by goblet cells, which serves as a reservoir to slowly release maternal IgA.

Circulating IgA is predominantly in a monomeric form, whereas in intestinal secretions it is found in a dimeric configuration both in mammals and birds [96, 104]. IgA secreted by plasma cells accumulates in the lamina propria. To exert its protective effect, pIgR is constitutively expressed by epithelial cells to transport IgA through the epithelia from the lamina propria to intestinal lumen. During the transcytosis, IgA is bound by pIgR on the basolateral surface and transported to the apical surface. At this surface, cleavage of the extracellular portion of pIgR results in release of secretory component (SC) as part of the dimeric IgA, otherwise known as the sIgA complex [105]. In this complex, sIgA is thought to be protected against degradation by proteases and pH fluctuations in the gut [90]. Excess production of pIgR which is not utilized as an IgA chaperone is also secreted as "free SC", which may have additional bacterial scavenger properties [106]. Once secreted, the N-glycans of SC can then bind to itself, and/or sIgA in the mucin layer thereby bridging these luminal defenses [105].

As one molecule of pIgR is required to bind and transport one dimeric IgA for secretion of sIgA into the intestinal lumen, pIgR's expression regulates sIgA capacity into the GIT [105, 107]. Expression regulation in mammals can be induced by numerous cytokines, including: interferon-γ (IFN-γ), tumor necrosis factor (TNF), interleukin-1β (IL-1β), and IL-4. These cytokines act through mediating a transcriptional response through activation of several transcription factor-binding sites in regulatory regions [105, 107–109]. In the chick, increases in IFN-γ, IL-1β and IL-4 expression in the second week post-hatch [110, 111] may influence subsequent increases in expression of the chicken pIgR gene [111]. Additional bacterial binding to Toll-like receptors have also been shown to increase pIgR expression in epithelial cells [105, 107, 112].
