A cell line, RTgutGC, was developed from the intestine of Oncorhynchus mykiss. RTgutGC has an epithelial-like shape, been passaged over 100 times, and cryopreserved successfully. A rainbow trout origin was confirmed by sequencing a 652 bp region of the mitochondrial cytochrome c oxidase I gene. RTgutGC is grown routinely in Leibovitz’s L15 without glutamine supplemented with 10% fetal bovine serum (FBS). Cell viability was evaluated with Alamar blue (AB) for metabolic activity and carboxyfluorescein diacetate acetoxymethyl ester (CFDA AM) for membrane integrity. Viability was unchanged by lipopolysaccharide (LPS) for cultures in FBS. For cultures at low cell densities in L15 without FBS or glutamine, cell viability declined in a LPS dose-dependent manner, allowing calculation of the concentration causing a 50% decline in viability (EC₅₀). When glutamine was present, the EC₅₀ was increased for both AB and CFDA AM. As the cell density increased, LPS became much less cytotoxic and no EC₅₀ could be calculated for very confluent cultures. Only high-density cultures had alkaline phosphatase (AP) activity. Thus, glutamine and possibly AP protect against LPS cytotoxicity. RTgutGC should be a useful in vitro tool for studying problems of nutrition and gastrointestinal health in fish.

A homologue of interleukin 18 has been identified from rainbow trout, Oncorhynchus mykiss. The trout IL‐18 gene spans 3.7 kb and consists of six exons and five introns, sharing the same gene organization with its human counterpart. The putative translated protein is 199 amino acids in length with no predicted signal peptide. Analysis of the multiple sequence alignment reveals a conserved ICE cut site, resulting in a mature peptide of 162 amino acids. The trout IL‐18 shares 41–45% similarity with known IL‐18 molecules and contains an IL‐1 family signature motif. It is constitutively expressed in a wide range of tissues including brain, gill, gut, heart, kidney, liver, muscle, skin and spleen. Transcription is not modulated by lipopolysaccharide, poly(I:C) or trout recombinant IL‐1β in primary head kidney leucocyte cultures and RTS‐11 cells, a macrophage cell line. However, expression is downregulated by lipopolysaccharide and rIL‐1β in RTG‐2 cells, a fibroblast‐like cell line. An alternatively spliced form of IL‐18 mRNA has also been found and translates into a 182 amino acid protein with a 17 amino acid deletion in the precursor region of the authentic form. This alternatively spliced form is also widely expressed although much lower than the authentic form. Interestingly, its expression is upregulated by lipopolysaccharide and poly(I:C), but is not affected by rIL‐1β in RTG‐2 cells. The present study suggests that alternative splicing may play an important role in regulating IL‐18 activities in rainbow trout.