Immune Defences of Teleost Fish - Aquaculture Nutrition: Gut Health, Probiotics and Prebiotics - page 31

It is generally accepted that fish lack an organized GALT and thus contain not follicular
lymphoid structures but rather a more diffusely distributed gut-associated lymphoid tissue
containing many lymphoid cells, macrophages, eosinophilic and neutrophilic granulocytes
(Zapata and Amemiya 2000; Bernard et al. 2006a; Huttenhuis et al. 2006). Mucosal T lym-
phocytes represent the major leukocyte population within the teleost gut observed in European
sea bass (Abelli et al. 1997; Picchietti et al. 1997; Scapigliati et al. 2002) and common carp
(Rombout et al. 1998a) intestine. In addition an anti-human CD3ε antibody demonstrated
an abundance of T cells in the epithelium and lamina propria of common carp (Urán et al.
2008) and Atlantic salmon (Bakke-McKellep et al. 2007). In situ hybridization studies con-
firmed the wide distribution of T cells throughout the gut of European sea bass (Picchietti
et al. 2011) which exhibited cytotoxic function. In fact, lymphocytes purified from intestinal
mucosa exhibited significant cytotoxic activity against xenogeneic and allogeneic cell targets
(Picchietti et al. 2011). The results corroborated previous data showing intraepithelial lym-
phocytes (IEL) isolated from rainbow trout intestine to be spontaneously cytotoxic against
a mouse tumour cell line (McMillan and Secombes 1997). In addition in European sea bass
intestine, TCRβ and CD8α expressing cells exceeded CD4 + cells, indicating the predominance
of CD8α + cells in the gut, whose transcripts significantly increased in the posterior segment.
These data agree with previous findings demonstrating an increase of mucosal T cells along the
European sea bass intestine towards the anus (Abelli etal. 1997; Picchietti etal. 1997; Romano
et al. 1997b), reflecting regional immune specialization as previously reported in other teleost
species (Rombout et al. 1989b; 1989c; Vigneulle and Baudin-Laurencin 1991).
In addition to the major sources of B lymphocytes (head kidney and spleen), the teleost
GALT also contains B cells, although little is known with regard to their origin, their
activation, their differentiation into plasmablasts and plasma cells, and the distribution of Ig +
cells in different regions of the GIT. The proportion of B cells in isolated gut cell suspensions
has generally been reported to be low and variable among teleost species. In European sea
bass (dos Santos et al. 1997; Romano et al. 1997a; dos Santos et al. 2000), common carp
(Rombout et al. 1998b) and rainbow trout (Zhang et al. 2010) approximately 2-12% of the
isolated leukocytes were surface IgM-positive and these IgM + cells were detected mainly in
the lamina propria, although in Atlantic halibut IgM + cells were distributed more commonly
within the epithelium (Grove et al. 2006). In a recent report, the gene sequences of three
different Ig heavy chains, namely IgM, IgD and IgZ, were cloned from mandarin fish and the
distributions of these three kinds of Ig-producing cells were investigated in the intestine by in
situ hybridization. The data confirmed that IgM-producing cells could only be detected at the
submucosa and lamina propria of the posterior intestine, while no IgZ or IgD positive cells
were found in the gut (Tian etal. 2009). IgT, an immunoglobulin isotype of unknown function,
was described as acting like a mucosal antibody in rainbow trout. This IgT was claimed to
be specialized in mucosal immunity and, in addition, IgT responses to a gut parasite were
restricted to the intestine (Zhang et al. 2010). In this species both IgM + and IgT + cells were
detected in the lamina propria and to a lesser degree within the epithelium (Zhang et al. 2010),
while in fugu IgZ + cells could also be detected in the intestinal epithelium (Savan et al. 2005).
In contrast to T and B cells, few studies are dedicated to the intestinal macrophages of fish,
probably because of the lack of suitable markers. Although gut macrophages are morphologi-
cally described in a variety of teleosts (Georgopoulou and Vernier 1986; Temkin and McMillan
1986; Rombout et al. 1986; Inami et al. 2009) their number, size and Ig-binding capacity vary
among species. In particular, cyprinid fish have many macrophage-like cells in and under the