Identification and expression analysis of lymphotoxin-beta like homologues in rainbow trout Oncorhynchus mykiss

A lymphotoxin-beta (LT-β) gene has been cloned and sequenced in rainbow trout and provides the first conclusive evidence for the existence of LT-β in teleost. Two isoforms of LT-β were isolated. LT-β1 cDNA was composed of 952 bp (with a 139 bp 5′-UTR and a 201 bp 3′-UTR) and LT-β2 cDNA was 836 bp (with a 237 bp 5′-UTR and a 197 bp 3′-UTR) both of which translated into a protein of 203 amino acid residues. Both isoforms contained a predicted transmembrane domain of 21 amino acid residues (Leu11-Val31) and the TNF family signature (Val104-Phe120). Homology and phylogenetic analysis of trout LT-β's with other known TNF family member showed good similarity to TNF-N (teleost) and other LT-β (mammals and frog). LT-β1 and TNF-α (1 and 2) genes were highly expressed in unstimulated trout head kidney, spleen, gill and intestine, whereas LT-β2 was weakly expressed only in the gill. The expression of LT-β1 and -β2 genes was not found in macrophage (RTS-11) and fibroblast (RTG-2) like cell lines, although the TNF-α2 gene was expressed in both cell lines with the TNF-α1 gene only expressed in RTS-11 cells. In head kidney cells, expression of LT-β1 and TNF-α (1, 2) genes was increased by stimulation with PHA or LPS. The discovery of trout LT-β will allow a more complete analysis of fish inflammatory responses.