Characterization and transcriptional analysis of rainbow trout MHC class I pathway genes in response to viral infection

Abstract

Many of the genes of the class I antigen presentation pathway are encoded in the vertebrate major histocompatibility complex (MHC) where they are coordinately regulated by interferon (IFN) signaling. The contiguous arrangement of the MHC is conserved in nearly all vertebrates with the exception of bony fish. Salmonids encode separate, non-linked class Ia, Ib and II MHC regions. The organization and functional significance of the salmonid multi-locus MHC has yet to be fully determined. The aims of this study were to further define the content and transcriptional regulation of the rainbow trout MHC to test a general hypothesis that linkage is required for coordinated regulation of antigen presentation. Tapasin is an ER resident chaperone that aids in loading nascent MHC I molecules with appropriate peptide. Two rainbow trout tapasin genes (Onmy-TAPBP.a and .b) were identified along with similar, but distinct TAPBP-related genes (Onmy-TAPBP-R.a and .b) that had only been described previously in mammals. Onmy-TAPBP.a and Onmy-TAPBP.b mapped to the separate MHC class Ia and Ib regions whereas the Onmy-TAPBP-R genes were localized to two additional MHC regions on chromosomes 2 and 3. Onmy-TAPBP and Onmy-TAPBP-R were upregulated upon infection with infectious hematopoeitic necrosis virus where the timing of TAPBP induction, along with in vitro promoter-reporter assays demonstrated that TAPBP and TAPBP-R regulation is mediated by IFN regulatory factors.Trout possess a single classical MHC class I (MHC Ia) gene, UBA. Additionally, trout also encode at least eight non-classical MHC class Ib genes, some of which are located in known regions of the trout MHC. Quantitative RT-PCR analysis of MHC transcription was used to assess the coordinated regulation of these genes in response to viral infection. UBA was constitutively transcribed at higher levels than any of the MHC Ib genes. Interestingly, induction of several MHC Ib genes followed a pattern that was similar to the regulation of UBA. These results suggest conserved transcriptional regulation for the class Ib genes and the participation of these genes for anti-viral immunity.