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observed in the skin of different teleost species. In particular, B cells were found in common
carp (Rombout et al. 1993), rainbow trout (St Louis-Cormier et al. 1984), spotted wolffish
Anarhichas minor Olafsen (Grontvedt and Espelid 2003) and channel catfish Ictalurus punc-
tatus skin (Zhao etal. 2008). In this latter species the numbers of IgM-secreting cells increased
20-fold following immunization with the protozoan parasite Ichthyophthiriusmultiiliis ,show-
ing that it plays a critical role in host defence against surface infection (Zhao et al. 2008). In
addition the ciliate parasite Cryptocaryon irritans induced specific IgM responses in the skin
of grouper ( Epinephelus coioides ) after immunization by surface exposure or intraperitoneal
injection (Luo etal. 2007). Functional studies on the skin secretory system, however, are never-
theless scarce and always refer only to IgM levels; how sIgs interact with antigens at a mucosal
surface, which is constantly exposed to water and swimming forces, is completely unknown.
Only recently has the expression of pIgR on common carp (Rombout et al. 2008) and fugu
(Hamuro et al. 2007) skin been discovered. In particular, biochemical analysis of fugu skin
mucus showed that the pIgR is associated with fugu IgM (Hamuro et al. 2007), while in carp
it was concluded that skin epithelial cells have IgM at their surface and it was suggested that
pIgR is at least one or even the major Ig receptor playing a significant role in mucosal immunity
(Rombout et al. 2008).
2.5.4 The gill-associated lymphoid tissue
Teleost gills are large mucosal surfaces that comprise the main respiratory surfaces of the fish,
and in addition with the intestine are considered important immune organs capable of mount-
ing robust immune responses (Paulsen et al. 2001; Penissi et al. 2003; Smith and Fernandes
2009). The gills are also considered as portals of entry for fish pathogens (Ellis 2001; Holzer
et al. 2003; Grove et al. 2006; Mulero et al. 2008a) and resistance to infection and recovery
is facilitated in part by innate non-specific immunity consisting of a plethora of constitu-
tively expressed elements as well as induced components of the inflammatory response (for
review, see Dickerson 2009). The gills of modern bony fish consist of four paired arches, each
containing two rows of posteriolaterally oriented filaments with lamellae covered by respira-
tory epithelium. The filaments are supported along the proximal third of their length by an
interbranchial septum of connective and muscle tissue (Wilson and Laurent 2002). Recent
morphological studies on Nile tilapia ( Oreochromis niloticus ) have described the fine struc-
ture of the branchial filaments, suggesting that the filament's superficial layer is involved in
gill osmoregulation whereas the deep layer, through immune and neuroendocrine systems, acts
in the regeneration and defence of the tissue (Monteiro et al. 2010). Small and large lympho-
cytes (Lin et al. 1998; Grove et al. 2006), macrophages (Lin et al. 1998; Mulero et al. 2008b),
neutrophils (Lin et al. 1998), eosinophilic granulocytes (Barnett et al. 1996; Lin et al. 1998;
Mulero et al. 2007) and antibody-secreting cells (Davidson et al. 1997; dos Santos et al. 2001)
have been observed in the gill-associated lymphoid tissue of different fish species. However, in
contrast to the intestinal epithelium, the function of the various types of leukocytes in the gills
has been poorly investigated. It has been suggested that their roles are presumably to capture
foreign substances and kill infectious agents that gain entry from the water, and in particular it
has been proposed that gill phagocytes participate in the clearance of foreign substances from
the blood.
To date, very few studies have examined the distribution and role of B cells in the gill arch
and filaments of teleost fish (Davidson et al. 1997; Grøntvedt and Espelid 2003; Savan et al.
2005; Grove et al. 2006), however the presence of IgM + cells in the gill mucosa of different
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