28th ANNUAL EASTERN FISH HEALTH WORKSHOP
April 21-25, 2003
An Increase In Immunoglobulin Diversity In Aeromonas salmonicida-Exposed Atlantic
Salmon (Salmo salar L); An Immunomic
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Andrew Dacanay 1,
Hannah McKenzie 1,2, Jane Osborne 1, Laura L. Brown 1,
Stewart C. Johnson 1
National Research Council Canada's Institute for Marine Biosciences,
1411 Oxford Street, Halifax, Nova Scotia, B3H 3Z1; 2 Departments of
Biology and Mathematics, Dalhousie University, Halifax, Nova Scotia, B3H 3J5
The
Atlantic salmon (Salmo salar L.), in common with all gnathostomes,
produces immunoglobulin heavy chain diversity by somatic recombination of four
genes: Variable (VH), diversity (DH), joining (JH)
and constant (CH) gene rearrangements. The totality of the VHDHJHCH
rearrangements is referred to as the immunome. The immunome of the Atlantic salmon has been catalogued in depth
but never in response to physiological perturbation such as stress or disease. In this study the antibody response of S.
salar to a virulent strain of the pathogenic fish bacterium Aeromonas
salmonicida was assessed at the immunomic level after immersion
challenge. Even though up to four VH
families were detected in the genome and at least nine VH families
are reported in the literature, pairwise comparisons of rearranged VH
showed that a single rearranged VH family dominated the immunome of
both control and challenged fish.
Further comparison with reference salmon VH sequences showed
that this family was salmon family 5 (IGVHS 5). Analysis of amino acid positional variability using entropy plots
showed that there was greater variability in both Complementarity Determining
Region 1(CDR 1) and CDR 2 in the challenged group than in the naïve group. This increased variability was generated by
both the use of a new VH gene and small sequences differences
between sequences in the challenged animals.
The likely germline VH sequence was also obtained and that
showed that these amino acid replacements were coded for by point
mutations. The CDR 3 of control and
challenged animals were equally variable when compared by entropy plots but the
CDR 3 of challenged animals was shorter than the controls. All five JH reported by others
were also observed together with a variant of JH3, again generated
by a single point mutation. The accrual
of replacement mutations in and immediately adjacent to CDRs is consistent with
somatic hypermutation; an important phenomenon in generating non-templated diversity
in mammals but not yet described in teleosts.
The role of immunoglobulin in the immune response of S. salar to A.
salmonicida is unclear. The
generation of new rearrangements and evidence for somatic hypermutation
suggests that it may be of importance in the immune response of S. salar.
