Phylogeny and expression of the tetraspanin CD9 in salmonid cell lines in response to interferon stimulation

Abstract

Abstract CD9 is a member of the cell membrane associated tetraspanin family and has been shown to have a wide array of functions, including promotion of MHC clustering, antigen presentation, T cell activation, cell adhesion, motility, growth and differentiation, signal transduction, tumor formation and egg/sperm fusion. CD9 is ubiquitously expressed in mammalian tissues and its roles are cell type dependent. CD9 is a typical interferon stimulated gene and further associated with MHC II and the immune system and inflammation in general, as has been shown in mammals and to a lesser extent in fish. In mammals, some viruses, such as influenza, coronavirus and hepatitis C, exploit CD9 for exit of new virus particles from host cells. In contrast, increased expression of CD9 can limit HIV-I virus budding. Due the limited knowledge of the involvement of CD9 in immune system responses in fish, we explored the phylogeny and expression of this gene in salmonids. We found 6 paralogues, which can be further organized into three distinct clades. We termed these clades CD9a, CD9b and CD9c, each of which include two paralogues reflecting the salmonid specific whole genome duplication. CD9a and CD9b are closely related and have the greatest sequence homology with the mammalian single copy gene of CD9, indicative of the teleost specific whole genome duplication. The CD9c clade is very distinct to CD9a and CD9b in sequence identity and further shows little sequence homology with the mammalian CD9, therefore could be an ancestral form of CD9 that was subsequently lost in all other vertebrate classes. We investigated the expression of the different paralogues in embryonic chinook salmon cells (CHSE) stimulated with interferon type I, an inducer of the antiviral pathways in fish. The paralogues of clade CD9c were highly inducible by interferon stimulation, whilst CD9a and CD9b appeared to be non-responsive. The specific inducibility of the ancestral CD9c clade to interferon type I highlights the unique immune responses in teleost. The presence of 6 paralogues organized in three clades may also reflect the diversity of roles this gene has been implicated in. In future, we aim to explore the expression of CD9, especially the putatively immune system relevant clade CD9c, in different cell types at baseline and in response to virus stimulations. This study contributes to a better understanding of CD9 involvement in immune system responses and how the gene is related to the antiviral interferon type I response. As CD9 has been shown to be important for the replication of certain viruses in mammals, this could be explored for fish viruses and potentially used as an anti-viral target. Keywords: Tetraspanins, Salmonid, Interferon signaling, Antiviral immune response, PhylogenyCopyright © 2019