Immune Defences of Teleost Fish - Aquaculture Nutrition: Gut Health, Probiotics and Prebiotics - page 39

immunity such as intracellular habitation, and escape from intracellular processing pathways
such as residence in phagosomes by prevention of phagolysosomal fusion or through escape
from the phagosome to the cytosolic compartment. Pathogens may employ immune deviation
through the expression of cytokine homologues which may change an efficient clearing
immune response to an inappropriate response resulting in pathogen retention. In addition,
many pathogens have been described as expressing superantigens, capable of inducing
immune suppression via oligo-clonal deletion of a wide array of TCR Vβ sharing T cells,
and finally perhaps inducing suppression via immunoregulatory mechanisms employing T reg
and regulatory cytokines such as IL-10 and TGFβ. Thus far, these escape mechanisms are
suggested in mammalian systems but await full characterization in teleost fish. Only with
the full characterization of both host-derived responses and pathogen counter-responses will
scientists be in a position to successfully design pathogen-specific treatments resulting in
total eradication of the infectious pathogen.
2.8 FUTURE CONSIDERATIONS
Pathogens and microbes may be used to modulate or divert immune responses to other
pathologies! The recognition of PAMPs derived from fish pathogens may indeed be harnessed
in the context of immune activation. It is well established that PAMPs such as LPS, PGN, LTA
etc. induce the expression of co-stimulatory molecules such as B7; any such up-regulation of
B7 is likely to play a role in microbial adjuvanticity to initiate more antigen-specific adaptive
responses tailored towards vaccine development against specific pathogenic infections. To
initiate the most appropriate immune responses - CMI towards intracellular pathogens and
humoral responses to extracellular pathogens - it is essential that scientists fully understand
the adjuvanticity role of innate response activators and whether the co-stimulatory molecules
up-regulated display T cell subset specificity. Teleost fish express several homologues of
the co-stimulator B7; it is possible that these isoforms will differentially regulate immune
responses, namely immune suppression/regulation and the activation of specific T cell subsets.
In addition, characterization of antigen/pathogen processing and presentation in the context
of immunodominant peptide mapping will uncover the best way to develop pathogen-specific
vaccines which mediate their effect through antigen-specific adaptive responses. It should not
be ignored, however, that the teleost fish immune system is predominated by innate immune
responses. In this antigen-rich environment of the teleosts, would it be more appropriate to
focus our attention on enhancing innate defences to a broad range of pathogens, rather than
rely on antigen-specific adaptive responses to a specific pathogen?
The overall strengthening of teleost immune responses may be enhanced or modulated by
nutrients, supplements and probiotics. Of particular interest at the moment is the role of probi-
otic bacteria in modulating the teleost mucosal immune responses of the intestines. Extensive
research has demonstrated that probiotics modulate immune defences in a similar manner as
that of human mucosal immunity. Probiotic bacteria used in fish feed have been shown to influ-
ence mucosal barrier integrity, promote commensal bacterial defences and modulate mucosal
immunity by immune deviation, activation or suppression (reviewed in Merrifield et al. 2010).
Thus, the use of probiotics in the feed of teleost fish can facilitate appropriate immune respon-
siveness to pathogenic organisms or the induction of mucosal tolerance, whereby teleost fish
tolerate non-pathogenic commensal organisms and food-derived antigens through immune