Immune Defences of Teleost Fish - Aquaculture Nutrition: Gut Health, Probiotics and Prebiotics - page 22

protection is passed on to fry as soon as possible in their development. Like mammalian
systems, this early protection is afforded to the fry passively. With regard to the ontogeny of
humoral responses, IgM and IgZ were detectable 1 to 4 days post fertilization, with the first
B cells detected from 7 days post fertilization (Romano et al. 1997a); it is probable that early
antibodies were of maternal origin. Indeed, parenteral immunization or passive immunity
may be conferred through mucus feeding of fry, where studies have demonstrated the elevated
presence of IgM-like antibodies in the skin mucus of parent fish (Buckley et al. 2010). Thus,
the humoral response of teleost fish, although slower to initiate than that of mammals/humans,
would appear to exhibit all the cellular and molecular complexities already characterized
in their warm-blooded mammalian counterparts. Further vaccination/challenge studies will
elucidate functional B cell/antibody phenotypes involved in tissue-specific protection against
a variety of fish pathogens present in such an antigen-rich environment.
2.4 CYTOKINES DRIVE IMMUNE RESPONSIVENESS
The development, activation and functionality of immune cells such as T and B lymphocytes
are characterized by specific profiles of their cytokine signalling molecules and their recep-
tors. In the relative absence of species-specific antibodies for detection of cytokine proteins,
many researchers have found mRNA expression of cytokine genes a useful tool for analysing
immune responses elicited in fish and interpreting functional outcomes through comparison
with the wealth of functional cytokine data available in mammalian systems. A wide array of
cytokine homologues have been described as being expressed by teleost fish; many of these
cytokines are likely to play an important role in differentiation, initiation, activation and reg-
ulation of both innate and adaptive immune responses (reviewed in Aoki et al. 2008). The
homologues so far described include IL-1β (Hong et al. 2004), IL-4Rα (Wang et al. 2011a),
IL-6 (Bird et al. 2005b; Iliev et al. 2007), IL-11 (Wang et al. 2005), IL-12 p40/p35 subunits
(Nascimento et al. 2007), IL-13Rα1 and IL-13Rα2 (Wang et al. 2011a), IL-15 (Wang et al.
2007), IL-17 gene products (Wang et al. 2010b), IL-18 (Zou et al. 2004), IL-20 (Wang et al.
2010c), IL-21 (Wang et al. 2011b), IL-22 (Monte et al. 2011), IL-23α chain (Holt et al. 2011),
IFNγ (Zou et al. 2005), TNFα (Laing et al. 2001) and type I antiviral interferons (Robertsen
et al. 2003). Adaptive regulatory cytokines and their signalling molecules already described
include TGFβ, IL-2 and two IL-10 gene products (Daniels and Secombes 1999; Zou et al.
2003; Bird et al. 2005a; Diaz-Rosales et al. 2009; Harun et al. 2011) and SOCS-1-3 (Wang
and Secombes 2008). By inference, IL-12, IL-18, IL-2, TNFα and IFNγ are all involved in
the development and effector functions of Th 1 cells that drive CMI responses (Tc activation
and delayed type hypersensitivity (DTH)) to intracellular-resident pathogens such as the bac-
terium Mycobacterium marinum and infectious pancreatic necrosis virus (IPNV). IL-4Rα,
IL-13Rα1 and IL-13Rα2, IL-20 and possibly IL-10 are likely to be involved in Th 2 -driven
humoral antibody-mediated immunity to extracellular pathogens. IL-1β, IL-6, IL-23 and IL-17
are implicated in Th 17 -mediated anti-fungal responses through the activation of granulocytic
cells. Additionally, the expression of IL-10, TGFβ and IL-2, in combination with FoxP3 char-
acterization, is suggestive of the presence and the suppressive regulatory function of natural
T reg . Finally, the expression of IL-15, IL-21 and IL-22 may also indicate the format of immune
response being mediated. IL-15 is a gamma-chain receptor utilizing cytokine that promotes T
cell growth, in particular CD8 + memory T cells and IELs as well as NK cells. In addition,