Immune Defences of Teleost Fish - Aquaculture Nutrition: Gut Health, Probiotics and Prebiotics - page 36

on the basolateral surface, transcytosis through the epithelial cell and secretion into the lumi-
nal compartment of the gut. In doing so, the extracellular domain of the receptor is cleaved off
with the antibody and serves as a proteolytic cleavage protective secretory component. This
polymeric immunoglobulin receptor pIgR has been described in teleost fish including fugu,
common carp, grouper and rainbow trout (Hamuro et al. 2007; Rombout et al. 2008; Feng
etal. 2009) and has been shown to bind IgM and IgT (Zhang etal. 2010). With the recent char-
acterization of the mucosal isotype IgZ 2 (Ryo et al. 2010), it is likely that this pIgR-mediated
mechanism of antibody transcytosis also exists for this isotype, making it readily available in
secretions at the mucosal membranes and relatively resistant to proteolytic cleavage.
T cells are the predominant leukocyte population present in the gut mucosa with a wide dis-
tribution throughout the intestine and with CD8 + T cells outnumbering CD4 + T cells. The gut,
however, exhibits regional immune functional specialization (Rombout etal. 1989c; Vigneulle
and Baudin-Laurencin 1991), as CD8 + T cells were found to be particularly highly prominent
in the posterior region of the intestine whereas CD4 and MHC II expression was relatively low
in this region. In contrast, CD4 and MHC IIβ were expressed at higher levels in the anterior
and middle regions of the gut. MHC Iα gene transcripts were generally constantly expressed
along the length of the gut, independent of CD8α expression (Picchietti et al. 2011). Thus,
it is clear that the intestinal mucosa can be segregated according to T cell populations and
hence functionality. The posterior intestine gut segment is dominated by cytotoxic CD8 + T c
and potentially IELs whereas the anterior and middle segments are dominated by CD4 + T h
cells; this is possibly reflective of a more tolerant T reg -driven function during homeostasis and
T h -driven CMI and humoral responses upon breakage of tolerance to pathogen invasion of the
mucosa.
From the increasing wealth of both expression and functionality studies focused on the gut
mucosa of teleost fish, it is evident that these fish possess all the immune machinery capable of
responding to pathogenic non-self, beneficial non-self and self. Anti-pathogen responses can
be mediated by aggressive non-specific innate immune defences initiated by macrophages,
neutrophils and MCs/EGCs whereas more antigen-specific responses are mediated by Th 1 ,
Th 2 ,Th 17 , Tc-driven humoral and CMI in response to intracellular and extracellular pathogens,
respectively. The selective sampling of non-danger signals in luminal contents by DCs/M cells
results in tolerance/regulation of the gut mucosa. This mechanism suppresses anti-pathogen
responses and mechanisms potentially harmful to host tissue. The complexity of and interplay
between all of the components of the mucosal immune system thus determine the desired
response (Figure 2.2). The teleost fish mounts immune defences against harmful pathogens
and tolerates non-harmful, beneficial commensal microbes and probiotics, while at the same
time tolerating food components that benefit the development of the host.
2.7 COMMON PATHOGENS INFECTING TELEOSTS: WHAT
IMMUNE RESPONSES ARE REQUIRED?
Relatively little is known with regard to the functional immune mechanisms that must be
induced in teleost fish to be able to recognize and eradicate defined pathogens. As a con-
sequence of a stronger innate immune system and the abundance of specific reagents aimed
at identifying functional cells and molecules of innate immune responses, it has become rel-
atively simple to identify innate responses, but these by their very nature are pathogen and