Immune Defences of Teleost Fish - Aquaculture Nutrition: Gut Health, Probiotics and Prebiotics - page 19

leukocytes were found to be abundant in the intestinal tract, in particular, in the barrier of
the intestinal epithelium and in the underlying lamina propria of the mucosa (Rombout et al.
1989b; 1993; Abelli et al. 1997). Teleost fish, as a whole, display a lower level of organi-
zation of the gut-associated lymphoid tissue (GALT) than mammals; however, lymphocytes,
antigen-presenting cells (APCs) and other innate immune cells are diffusely organized along
the intestinal tract which is suggestive of both innate and adaptive system readiness to respond
to pathogens that infiltrate the mucosal barrier defences. The lack of any comprehensive data
regarding the functional role of dendritic cells (DCs) in the gut mucosa of teleost fish may sug-
gest that the predominant APC that primes adaptive responses is the macrophage cell; however,
DCs are likely to exist as the DC differentiation marker CD83 has been characterized in teleost
and elasmobranch fish (Ohta et al. 2004). Gut macrophages have been described in a number
of teleost fish (Rombout et al. 1986; Georgopoulou and Vernier 1986; Inami et al. 2009) and
have been shown to increase in density of numbers upon anal/intestinal antigenic challenge and
to present antigenic determinants at their surface upon processing (Rombout etal. 1985; 1986;
Rombout and van den Berg 1989a). These antigenic determinants, like mammalian systems,
are presented in the context of MHC molecules where MHC I gene products are expressed on
all nucleated cells in teleost fish (Hansen et al. 1996); upon vaccination, MHC II was demon-
strated to be highly inducible in the head kidney, spleen, hindgut and gills of the Atlantic
salmon (Koppang et al. 1998). In addition, the abundance of both B and T cell lymphocytes
in proximity to these antigen-presenting macrophages and their MHC antigen-presentation
molecules infers a clear presentation to and priming of both T cells and B cells involved in
MHC I-restricted T cell cytotoxic responses and MHC II-restricted T helper (Th) responses
including cell-mediated immunity (CMI) and humoral responses.
2.3.1 T cells: molecular and functional characterization
The presence of conventional T cells in teleost fish was first suggested upon observation of
graft rejection and antibody production. Immunohistochemical analysis of gut mucosal tissue
has resulted in the characterization of several T cell populations. In addition, molecular studies
have demonstrated a wide diversity and repertoire of T cell receptors expressed in teleosts
(reviewed in Castro et al. 2011). Intestinal intraepithelial lymphocytes were found to express
αβTCR, CD3, CD4 and CD8α and even γδTCR (Hansen and Strassburger 2000; Nam et al.
2003; Bernard et al. 2006a; Shang et al. 2008). By inference with mammalian systems and
some observations made in the fish mucosal immune system, these markers are expressed on
Th, cytotoxic T cells (Tc) and γδT cells; these cells are integral to cell-mediated cytotoxic
immunity, humoral immunity and homeostatic, tolerogenic regulatory responses. Functional
analyses have characterized lymphocytes from teleosts to exhibit potent cytotoxic activity to
allogeneic and xenogeneic target cells (Fischer et al. 2003; Bernard et al. 2006a; Toda et al.
2009). Thus, it would appear that functionally teleost fish have the capability to induce immune
non-responsiveness and tolerate food antigens and commensal organisms of benefit to the host
whilst, at the same time, maintaining the capability to positively respond both to pathogens
that reside within host cells and those that reside extracellularly. The possible existence of
a regulatory T cell subset has been strongly suggested by the identification of two FoxP3
genes (transcription factor associated with natural T reg development) in rainbow trout (Wang
et al. 2010a). In addition, the same group also demonstrated the sequence and expression
analysis of the developmental transcription factors T-bet and GATA-3 that are associated with
the development of the T helper subsets Th 1 and Th 2 (Wang et al. 2010b).