**3.5 Paneth cells**

Paneth cells are physiologically found at the distal small intestinal crypts of Lieberkühn and contain abundant secretory granules. Their unique histomorphological features implicate special functions in cellular homeostasis as well as in the establishment and configuration of the mucosal barrier as a physical and highly organized immune interface [43]. Previous studies suggesting the existence of Paneth cells in the chicken remained controversial. However, recent research has supported Paneth cells existence in the small intestine of the chicken by electron

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 cathelicidins [47–49].

#### **3.6 Host defense peptides**

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

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 post-translational modification [57, 60].

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 the preference of gram-negative or positive bacteria.

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

**125**

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

disrupted helix formation in a-helical synthetic peptide leads to the loss of antimicrobial activity [72, 74, 75]. Although deletion of C-terminal alpha-helix in CATH2 reduces the activity against pathogens, the remaining truncation is still capable to kill bacteria [75]. The truncation of CATH2 with N-terminal alpha-helix alone shows increased antibacterial activity [76]. The hinge region plays a key role in the insertion of CATH into the bacterial membrane and pore formation [74, 77]. Disruption of the hinge region by point mutation or removal in the center of the CATHs largely decreases the antimicrobial activity [72, 74, 78]. The cationicity of CATH and AvBDs is important for the initial contact with the surface of bacteria. The higher cationic charge in CATH2 and the synthetic analogs results in the better

In addition to direct antimicrobial activity, the HDPs exhibit the immunomodulatory function, involving inflammation and chemotaxis. Chicken AvBD13 was reported as a direct TLR4 ligand [79], increases production of IFN-γ and IL-12 in mouse monocytes through activation of TLR4-NFκB axis. Combined with the evidence that AvBD13 increases serum IgG and IgM levels in chicken and induces lymphocytes proliferation in spleen after the administration of the infectious bursal disease vaccine (IBDV) [80], activation of TLR signaling by AvBD13 indicates an immune enhancement rather than a merely pro-inflammatory effect. Moreover, chicken AvBD1 fusion protein expressed by IBDV enhances CD4+, CD8+, and CD3+ T-cell proliferation, increases antibody titers, improves survival rate in in vivo experiment [81]. Additionally, HDPs have been shown chemotactic effect. Investigations about immunomodulation by avian AvBDs and CATHs are mainly limited to NF-κB activation, cytokine production, and direct immune activation. The similar findings in human and mouse studies suggest the conserved function of HDPs among species, providing the guideline for the application and future

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].

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

antimicrobial outcomes [72, 75].

research in poultry area.

**4. Adaptative immunity of the GIT**

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

disrupted helix formation in a-helical synthetic peptide leads to the loss of antimicrobial activity [72, 74, 75]. Although deletion of C-terminal alpha-helix in CATH2 reduces the activity against pathogens, the remaining truncation is still capable to kill bacteria [75]. The truncation of CATH2 with N-terminal alpha-helix alone shows increased antibacterial activity [76]. The hinge region plays a key role in the insertion of CATH into the bacterial membrane and pore formation [74, 77]. Disruption of the hinge region by point mutation or removal in the center of the CATHs largely decreases the antimicrobial activity [72, 74, 78]. The cationicity of CATH and AvBDs is important for the initial contact with the surface of bacteria. The higher cationic charge in CATH2 and the synthetic analogs results in the better antimicrobial outcomes [72, 75].

In addition to direct antimicrobial activity, the HDPs exhibit the immunomodulatory function, involving inflammation and chemotaxis. Chicken AvBD13 was reported as a direct TLR4 ligand [79], increases production of IFN-γ and IL-12 in mouse monocytes through activation of TLR4-NFκB axis. Combined with the evidence that AvBD13 increases serum IgG and IgM levels in chicken and induces lymphocytes proliferation in spleen after the administration of the infectious bursal disease vaccine (IBDV) [80], activation of TLR signaling by AvBD13 indicates an immune enhancement rather than a merely pro-inflammatory effect. Moreover, chicken AvBD1 fusion protein expressed by IBDV enhances CD4+, CD8+, and CD3+ T-cell proliferation, increases antibody titers, improves survival rate in in vivo experiment [81]. Additionally, HDPs have been shown chemotactic effect. Investigations about immunomodulation by avian AvBDs and CATHs are mainly limited to NF-κB activation, cytokine production, and direct immune activation. The similar findings in human and mouse studies suggest the conserved function of HDPs among species, providing the guideline for the application and future research in poultry area.
