Immune Defences of Teleost Fish - Aquaculture Nutrition: Gut Health, Probiotics and Prebiotics - page 15

upon secondary exposure. The relative paucity of reliable, specific reagents to components
of the teleost fish immune system has resulted in slow progress in the understanding of
immune defences in these fish. Although studies have characterized the presence of immune
cells and molecules, they are rarely consolidated by functional studies and, as such, some of
our understanding of fish immunology has been obtained by matching observations in the
fish system with inferences from more comprehensively characterized mammalian immune
systems. The aim of this chapter is to review the current understanding of the immune system
in teleost fish in the context of how innate, adaptive and mucosal barrier defences deal with
the wide array of pathogenic organisms present in the fish's environment.
2.2 INNATE IMMUNITY
The aquatic environment of teleost fish, by its very nature, is rich in pathogens and anti-
genic stimuli. As a result, it is vital that the fish immune system is capable of a rapid and
robust immune response capable of protecting the host from the plethora of pathogenic insults.
To cope with this, teleosts have a well-developed innate immune system which is particu-
larly strong with respect to barrier/mucosal defences. The barriers which are constantly under
attack or challenge by such an antigen-rich environment include the skin and the mucous
membranes of the gills and the intestinal tract. Epithelial cells are at the forefront of these
barrier defences and function as both a physical barrier and an immune instructor, capable
of perpetuating either immune tolerance in homeostatic conditions or immune activation in
response to pathogen invasion. These cells can protect the host by innate defences which
include the secretion of anti-microbial molecules such as lysozyme, cathelicidins, cathepsins
and defensins. These serve to break down pathogen cell walls, hence killing them and prevent-
ing their multiplication. If this barrier defence is compromised, then the pathogen is faced with
a barrage of underlying defences which include a plethora of cellular and soluble components.
These innate defences include cells such as macrophages and neutrophils involved in phago-
cytosis, complementing production and expression of innate inflammatory cytokines; each
displays different effector functions which are dependent on the immune cell being activated
and by the pathogen activation molecules encountered.
The immediate lines of defence, prior to epithelial cells and those of the underlying innate
immune cells, consist of a physical barrier of commensal bacteria and mucus. Commensal
bacteria are integral to mucosal immunity and will be covered later in this chapter and in
following chapters. In general, commensal bacteria compete with pathogenic microbes for
nutrients, binding sites on epithelial cells, and can modulate the immune system to benefit
the host. Mucus is also vital to host protection and is produced at mucosal surfaces such
as the gills and intestinal tract as well as the external surface of the skin. Mucus exists in a
state of constant translocation, being physically removed from the skin and mucosal surfaces,
hence trapping and removing pathogens and preventing their attachment to and invasion of
host cells (reviewed in Ellis 2001). Skin mucus acts as a lubricant, is involved in locomo-
tion and osmoregulation, and plays a role in the prevention of colonization by pathogens
such as bacteria, fungi and parasites. Its biochemical properties afford it chemical defence
(Jakowska 1963). The basic components of mucus include macromolecular mucin compo-
nents (mucopolysaccharides) and glycoproteins. In general, mucus contains many different
types of secretory substances with a wide variety of functions. These secretory substances