Immune Defences of Teleost Fish - Aquaculture Nutrition: Gut Health, Probiotics and Prebiotics - page 23

IL-21 plays a role in innate and adaptive responses (both CMI and humoral) by inducing pro-
liferation of T, B and NK cells. Both IL-17 and IL-22 are generally produced by Th 17 cells
and induce pro-inflammatory cytokine production by epithelial cells, affecting epithelial cell
turnover and mucosal barrier integrity. Thus far, little functional evidence exists in teleost fish
for the role played by these cytokine homologues. Future research investigating functional
aspects of these fish cytokines will further expand our understanding of cellular and molecular
aspects of teleost immunity against aquatic pathogens.
The evidence presented thus far suggests the existence of many of the cellular and molecular
components required to mount efficient adaptive responses to specific fish pathogens, whether
they be intracellular resident or extracellular. The existence of APCs, MHC I and MHC II,
CD8 + Tc, CD4 + Th subsets and B cell subsets expressing immunoglobulin points to teleost
fish having the full capability of mounting humoral responses as well as CMI responses such as
Tc-cytotoxicity and DTH responses. In a functional context, DTH, cell-mediated lympholysis
(CML) and graft versus host reactions have been described in teleost fish (Nakanishi etal. 1999
and reviewed in Manning and Nakanishi 1996 and Nakanishi et al. 2002). These responses
utilize many of the potential cellular and molecular mechanisms alluded to by the wealth of
cloning and expression data that exist for teleosts. In general, there is still a long way to go
in the functional characterization of teleost adaptive immune responses; however, all the basic
adaptive components are present to be able to interpret immune observations in teleost fish in
the context of the more comprehensive understanding that exists in human/mammalian sys-
tems. Studies investigating the interrelations between immune cell function, tissue expression
and co-expression with other immune cell subsets will be vital to our overall understanding of
the teleost immune system.
2.5 IMMUNE TISSUES
The fish immune system developed as a form of protection from aggressive life forms to ensure
survival of the individual to reproductive age and consequently ensure survival of the species.
It would seem that the driving force that led to the evolution of organized lymphoid struc-
tures in prehistoric fish was linked to the primary route of pathogen entry into the organisms,
a process which in general occurs through mucosal surfaces (Matsunaga and Rahman 2001;
Boehm and Bleul 2007). Looking at the modern bony fish (teleosts), the immune system was
found to be compartmentalized into specialized organs and to use specialized cell types to
defend against different pathogen challenges. Lymphoid organs are important regulators of
lymphocyte development and immune responses and it has been suggested that they evolved
to facilitate antigen-receptor gene assembly (variable-diversity-joining-type recombination)
(Boehm and Bleul 2007). Teleosts have two primary lymphoid organs, the thymus and a sug-
gested bone marrow analogue located in the head kidney, which may also serve as a secondary
lymphoid organ together with the spleen. Apart from functioning as physical barriers sepa-
rating host animals from the external environment, mucosae are also active immunological
sites that protect fish against exposure to microbes and stressors. These tissues constitute the
mucosa-associated lymphoid tissue (MALT) that, in teleosts, can be subdivided into three main
immune compartments: skin-associated lymphoid tissue, gill-associated lymphoid tissue and
gut-associated lymphoid tissue (GALT). In contrast to mammalian germinal centres, B cell
follicles and lymph nodes have not been reported in teleost fish.