Molecular cloning of IgT from Atlantic salmon, and analysis of the relative expression of τ, μ, and δ in different tissues.

In the present study, IgT genes of Atlantic salmon were cloned and characterised. Analysis of our sequence data as well as ESTs reported to the databases revealed three distinct IgT heavy chain sub-variants in salmon, as opposed to two of IgM and IgD. The IgT sub-variants in salmon are 76-80% identical to each other, and 75-82% identical to the reported rainbow trout sequences, whereas the similarity to the orthologous molecules in zebrafish, grass carp, mandarin fish, and grouper is 25-41%. The heavy chains of both secreted and membrane anchored forms of salmon IgT include four constant Ig domains, τ1-τ4. This parallels the IgM heavy chains in elasmobranch fish and higher vertebrates, but differs from IgM in teleost fish where the membrane anchored form include only three constant Ig domains, μ1-μ3. The similarity between τ1 and μ1 in salmon is relatively high (52%) when compared to the remaining part of the molecules (τ2-τ4 and μ2-μ4 are 13-24% similar). To compare τ, μ and δ expressions in different tissues (head kidney, thymus, spleen, gill, skin, hind gut, brain and muscle) of Atlantic salmon, RT-qPCR assays were designed and evaluated. The analyses revealed that IgM transcripts are most abundant (up to 200 times more than IgD) followed by IgT (up to 20 times more than IgD) in most tissues. Highest expression of IgM, IgT, and IgD was in head kidney and spleen.