Immune Defences of Teleost Fish - Aquaculture Nutrition: Gut Health, Probiotics and Prebiotics - page 32

intestinal epithelium and their number is increased after anal intubation with antigens (Davina
et al. 1980; Rombout et al. 1985; 1989a). Due to their Ig-binding capacity, these cells are
strongly Ig + (Rombout et al. 1993; Koumans-van Diepen et al. 1994) and can be loaded with
luminal antigens such that, after degradation, antigenic determinants can subsequently be pre-
sented at their surface (Rombout et al. 1986; 1989b). Granulocytes are also described in the
teleost intestinal mucosa. Although intermediate cells are sometimes described as separate
subpopulations, two main types predominate: neutrophils and eosinophils, the latter strongly
resembling mammalian mast cells (Rombout et al. 1989c; Urán et al. 2009) and named mast
cells or eosinophilic granule cells (Reite 1998). These eosinophils release tryptase (Dobson
et al. 2008), antimicrobial peptides such as lysozyme (Sveinbjørnsson et al. 1996; Urán et al.
2009), piscidin (Silphaduang and Noga 2001) and pleurocidin (Murray et al. 2007). They are
abundant in mucosae, and can react strongly upon inflammation via migration and granule
release (Urán et al. 2009). Fish neutrophils are phagocytic and abundant in the circulation,
which are rapidly recruited from the blood to sites of inflammation (Ellis 1977; Hamdani et al.
1998). In the healthy, homeostatic status, fish intestine in general contains fewer neutrophils
than eosinophils/basophils; however their number strongly increases under danger or stress
conditions and they frequently penetrate the intestinal epithelium (Sundh 2009).
2.6 MUCOSAL IMMUNITY
These mucosal tissues, such as the gills and gut, are under constant challenge as a conse-
quence of the antigenic nature of the aquatic environment; thus mucosal immunity is vital to the
well-being of the fish. These host mucosal defences can be thought of as existing in several lay-
ers; these layers to the mucosal immune system include (1) mucus, (2) epithelial lining and (3)
underlying lamina propria/mucosal tissue. In addition to these layers of host mucosal defence,
an additional non-host defence layer can be considered - that of the commensal microflora that
populate the real external surface (for more information, refer to Chapters4-6 focusing on fish
gut microbiota). The commensal defence layer consists of commensal non-harmful beneficial
microbes that line the gut; they play an important role in protection against pathogen inva-
sion by utilizing several mechanisms. Commensals and indeed probiotic organisms that are
obtained through fish feeds (or the rearing water) facilitate host defences by competing with
pathogens for nutrients, binding sites on the epithelial cell surface, initiating production of
defensin and other anti-microbial peptides from the epithelial cells, modulating epithelial cell
growth and apoptosis turnover, increasing epithelial barrier strength by modulating tight junc-
tion proteins, and advancing epithelial cell mucus production and modulation of underlying
innate and adaptive immune components present in the lamina propria (reviewed in Merrifield
et al. 2010) (also refer to later chapters in this topic). The second layer encountered in mucosal
defence is the mucus. This acts as a viscous barrier, preventing pathogen invasion of mucosal
tissue and facilitating the removal of potentially pathogenic antigenic material; this mechanism
may be aided by trapping of antigenic material via binding to mucosal immunoglobulin which
is itself bound to mucus by mucin binding sites. It is possible that the teleost mucosal iso-
type IgZ 2 (Ryo et al. 2010) may function in this way - facilitating neutralization of pathogen
invasion and clearance.
In addition to their capability in mounting non-specific innate responses, antigen sampling
by cells in the teleost intestinal epithelium was thought to be undertaken by specialized