Discovery and expression of novel immunoglobulin-like transcripts (NILTs) in salmonids

Three new NILT genes were successfully cloned and characterized from rainbow trout, with one NILT alternatively spliced into a long isoform containing two Ig domains and a short isoform containing one Ig domain.  The expression of NILTs was studied in six different tissues and two different cell lines, with expression apparent in immunologically important tissues.  Furthermore, phylogenetic analysis showed that NILTs are more closely related to triggering receptor expressed on myeloid (TREM) cells and Nkp44 from humans than to NITRs from rainbow trout. The genomic organisation and structure of the multigene family of NILTs in Atlantic salmon was investigated using a BAC sequencing approach.  This revealed the presence of six novel NILT genes, which either contained one or two Ig domains and several immunoreceptors tyrosine-based inhibitory motifs (ITIMs) in the cytoplasmic region. By homology search two NILT-like genes in zebrafish (Danio rerio) located on chromosome 1 have been obtained. Chromosome 1 in zebrafish also contains the Dare-ZE genes, which are equivalent to the human MHC class I genes located on chromosome 6.  The distance between the later and the TREM genes on chromosome 6 is similar to the distance between the NILT-like genes and Dare-ZE genes on zebrafish chromosome 1.  In addition, two NILT-like Ig domains were obtained from the green spotted pufferfish (Tetraodon nigroviridis), putatively part of the same receptor. The results will contribute to our knowledge of the immune system in fish and provide useful information for the control of inflammatory processes in salmonids.