Immune Defences of Teleost Fish - Aquaculture Nutrition: Gut Health, Probiotics and Prebiotics - page 29

fish species is conflicting. In fact, IgM + cells have been detected in the gill filament epithelium
of halibut and in spotted wolffish (Grøntvedt and Espelid 2003), while in cod IgM + cells were
detected in connective tissue and blood plasma, similar to the findings in Atlantic salmon
(Schrøder et al. 1998; Haugarvoll et al. 2008). Moreover, both IgM + and IgZ + B cells were
described along the gill filaments of mandarin fish Siniperca chuatsi (Tian et al. 2009), while
IgZ + cells were also detected in the gill of fugu (Savan et al. 2005). Interestingly, in salmonid
interbranchial lymphoid tissue (Haugarvoll et al. 2008; Koppang et al. 2010), very few IgM +
cells were detected while abundant T cells were embedded in a meshwork of epithelial cells,
showing an organization that has no resemblance to that previously described in lymphoid
tissues (Koppang et al. 2010). Takizawa et al. (2011) found that in contrast to leukocytes
from blood and secondary lymphoid organs of rainbow trout, CD8α + cells were abundant in
the mucosal respiratory (gill) tissue. In the gill, CD8α + cells were present not only in the
epithelium of secondary lamellae but also in lymphoid structures located at the surface of gill
arches and primary lamellae, as well as at the basal junction between primary lamellae, which
is akin to the situation in healthy Atlantic salmon (Hetland et al. 2010) and the organization of
salmonid interbranchial lymphoid tissue (Haugarvoll et al. 2008; Koppang et al. 2010). The
high abundance of T lymphocytes in the teleost gill can be explained by considering that the
gill provides an efficient gas exchange with the aqueous environment but in fact possesses a
thin gill epithelium and the protecting mucus layer is relatively sharp, thus increasing the risk
of pathogen entry.
2.5.5 The gut-associated lymphoid tissue
The immune system of the gut is referred to as gut-associated lymphoid tissue (GALT); the
piscine gut immune system is quite different from that of mammals. In mammals, the GALT
consists of both organized lymphoid tissues, such as mesenteric lymph nodes (MLN) and
Peyer's patches (PP), and more diffusely scattered lymphocytes in the intestinal lamina pro-
pria (LP) and epithelium including large numbers of IgA + plasmablasts (Forchielli and Walker
2005). Essential to mucosal function in the follicle-associated epithelium are the M cells,
which are specialized enterocytes facilitating antigen transport through the epithelium (Beier
and Gebert 1998; Owen 1999; Kyd and Cripps 2008). M cells promote adherence and uptake
of foreign macromolecules, particles and microorganisms (Neutra etal. 2001) that are actively
transported to the underlying lymphoid tissue, resulting in IgA class switching (Cerutti 2008)
and secretion of high amounts of dimeric IgA (Brandtzaeg and Pabst 2004; Brandtzaeg et al.
2008). The secreted IgA is bound by the polymeric Ig receptor (pIgR) and transcytosed to the
intestinal lumen (or to the bile in the liver) to coat and protect mucosal surfaces against harmful
microbial invasion. In contrast to mammals, the intestinal mucosal surfaces of teleost fish are
arranged in folds rather than as villi and do not have crypts of Lieberkühn (Ng etal. 2005; Wal-
lace et al. 2005) (Figure 2.1). Although teleost fish possess an adaptive immune system, char-
acterized by the presence of key antigen receptors of the immunoglobulin superfamily (Cooper
and Alder 2006), they lack lymph nodes and Peyer's patches. They do, however, have epithelial
cells that share morphological similarities with immature mammalian M cells, whose pheno-
type may represent evolutionary early antigen-sampling enterocytes (Fuglem et al. 2010). As
far as teleost research extends, dendritic cells (DC) are not known to be well established in
the mucosae. Such cells in mammals directly take up luminal antigen (Brandtzaeg and Pabst
2004; Brandtzaeg et al. 2008); however, cells with dendritic appearance (MHC II + ) have been
identified in the second gut segment of Atlantic salmon (Koppang et al. 2003).