Differential immune response of rainbow trout (Oncorhynchus mykiss) at early developmental stages (larvae and fry) against the bacterial pathogen Yersinia ruckeri

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Innate immune factors play a crucial role in survival of young fish especially during early stages of life when
adaptive immunity is not fully developed. In the present study, we investigated the immune response of
rainbow trout (Oncorhynchus mykiss) larvae and fry at an early stage of development. We exposed 17
and 87 days post hatch larvae and fry (152 and 1118 degree days post hatch; avg. wt. 70 and 770 mg,
respectively) to the bacterial pathogen, Yersinia ruckeri for 4 h by bath challenge. Samples were taken at
4, 24, 72 and 96 h post exposure for qPCR and immunohistochemical analyses to elucidate the immune
response mounted by these young fish. Larvae showed no mortality although infected larvae at 48 h post
exposure showed hyperaemia in the mouth region and inflammation on the dorsal side of the body. Gene
expression studies showed an up-regulation of iNOS and IL-22 in infected larvae 24 h post exposure but
most of the investigated genes did not show any difference between infected and uninfected larvae. Immunohistochemical
studies demonstrated a high expression of IgT molecules in gills and CD8 positive cells in
thymus of both infected and uninfected larvae. Infection of rainbow trout fry with Y. ruckeri, in contrast,
induced a cumulative mortality of 74%. A high expression of cytokines (IL-1b, TNF-a, IL-22, IL-8 and IL-
10), acute phase proteins (SAA, hepcidin, transferrin and precerebellin), complement factors (C3, C5 and
factor B), antimicrobial peptide (cathelicidin-2) and iNOS was found in infected fry when compared to
the uninfected control. IgT molecules and mannose binding lectins in gills of both infected and uninfected
fry were detected by immunohistochemistry. The study indicated that early life stages (yolk-sac larvae),
merely up-regulate a few genes and suggests a limited capacity of larvae to mount an immune response
by gene regulation at the transcriptional level. Based on the observed clearance of bacteria and lack of mortality
it could be speculated that larvae may be covered by protective shield of different immune factors
providing protection against broad range of pathogens. However, the increased susceptibility of older fry
suggests that Y. ruckeri may utilize some of the immune elements to enter the naive fish. The up-regulation
of iNOS and IL-22 in the infected larvae implicates an important role of these molecules in immune
response at early developmental stages. A dense covering of surfaces of gill filaments by IgT antibody in
the young fish suggest a role of this antibody as innate immune factor at early developmental stages.
Original languageEnglish
JournalDevelopmental & Comparative Immunology
Issue number2
Pages (from-to)463-474
Number of pages12
Publication statusPublished - 2012

ID: 35243505