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and Irwin 1985; Irwin and Kaattari 1986; Zapata and Cooper 1990; Zwollo
et al.
2005; 2008)
and IgM-secreting cells are generated in LPS-activated cultures derived from splenic B cells
(Bromage
etal.
2004). In this context, analysis of Ig isotype expression demonstrated differen-
tial expression patterns between the spleen and the head kidney. In particular the head kidney
of Atlantic salmon (Tadiso
et al.
2011) showed the highest expression of IgM, IgT and IgD
followed by the spleen, while in the thymus all Ig transcript levels were much lower in accor-
dance with previous studies (Stenvik and Jorgensen 2000; Stenvik
et al.
2001; Hirono
et al.
2003; Hansen
et al.
2005; Saha
et al.
2005; Tian
et al.
2009). In particular, IgM transcripts
were most abundant followed by IgT, especially in the head kidney and spleen, as compared
to IgD, indicating that IgT is the dominant Ig next to IgM (Tadiso
et al.
2011). The expression
and tissue distribution patterns of IgT, reported simultaneously as the orthologous molecule in
zebrafish, named IgZ (Danilova
etal.
2005; Hansen
etal.
2005), however, can exhibit consider-
able variation in different species. In fact, in Atlantic salmon (Tadiso
et al.
2011) and rainbow
trout (Hansen
et al.
2005), IgT is expressed in various tissues, especially in the spleen and
head kidney; in adult zebrafish, however, the expression of IgZ is limited to primary lymphoid
organs, including the thymus (Danilova
et al.
2005). In contrast, transcripts of a newly discov-
ered zebrafish IgZ-2 isotype are widely expressed in both primary and secondary lymphoid
organs (Hu
et al.
2010).
2.5.3 The skin-associated lymphoid tissue
Skin is a metabolically active tissue (Bullock and Roberts 1974) that has been thought to play
a rather passive role in protective immunity, serving as an anatomical and physiological bar-
rier against the external environment. In addition to being a mechanical barrier, the fish skin
has other roles including the secretion of mucus, which contains innate immune factors such
as proteases, antibacterial agents and other immune molecules (Suzuki
et al.
2003; Easy and
Ross 2009), and it has been shown as having an active immunological role against parasitic
infection (Lindenstrom
et al.
2004; Sigh
et al.
2004a; 2004b; Gonzalez
et al.
2007a; 2007b;
Forlenza
et al.
2008). Its importance as a vital immune organ within the mucosal immune
system was demonstrated when transcript analysis of common carp revealed 82 orthologues
of immune-relevant genes previously described in other organisms (Gonzalez
et al.
2007a).
Likewise, microarray analysis of Atlantic salmon skin after infection with the louse
Lepeoph-
theirus salmonis
showed changes in the expression of genes belonging to immune response,
oxidative stress, protein folding and cytoskeletal/structural proteins (Skugor
et al.
2008). In
addition a recent study performed by Caipang
et al.
(2011) evaluated the transcription profiles
of selected genes involved in the cutaneous immune defence of Atlantic cod. Specifically, the
expression levels of genes related to antibacterial activity, antiviral response, cytokine pro-
duction, glucose transport, stress response and anti-apoptotic activity were quantified in the
dorsal and ventral regions of the skin of Atlantic cod and, in general, the highest expression
was found at the latter side. Analysis of skin mucosal proteomic mapping ascertained that
skin mucosa in Atlantic cod is a veritable source of several important proteins, many of them
considered immune competent molecules (galectin-1, mannan binding lectin, serpins, cystatin
B, cyclophilin A, FK-506 binding protein, proteasome subunits alpha-3 and -7, ubiquitin, and
g-type lysozyme) (Rajan
et al.
2011).
Adaptive and innate immune elements are detectable in teleost skin; in fact in addition to
secretory cells, leukocytes such as granulocytes, macrophages and lymphocytes (Iger
et al.
1988; Peleteiro and Richards 1990; Davidson
et al.
1993; Herbomel
et al.
2001) have been
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